View source for Hubble's law

{{Cosmology|expansion}}
'''Hubble's law''', also known as the '''Hubble–Lemaître law''',<ref>{{cite press |date=29 October 2018 |title=IAU members vote to recommend renaming the Hubble law as the Hubble–Lemaître law |url=https://www.iau.org/news/pressreleases/detail/iau1812/?lang |publisher=[[International Astronomical Union]] |accessdate=2018-10-29}}</ref> is the observation in [[physical cosmology]] that:
# Objects observed in [[Extragalactic astronomy|deep space]]—extragalactic space, 10 mega[[parsec]]s (Mpc) or more—are found to have a [[redshift]], interpreted as a relative velocity away from [[Earth]];
# This [[Doppler effect|Doppler shift]]-measured velocity of various [[galaxy|galaxies]] receding from the Earth is approximately [[Proportionality (mathematics)|proportional]] to their distance from the Earth for galaxies up to a few hundred megaparsecs away.<ref name="riess99" /><ref name="perlmutter99" />

Hubble's law is considered the first observational basis for the [[expansion of the universe]] and today serves as one of the pieces of evidence most often cited in support of the [[Big Bang]] model.<ref name="NYT-20170220">{{cite news |last=Overbye |first=Dennis |authorlink=Dennis Overbye |title=Cosmos Controversy: The Universe Is Expanding, but How Fast? |url=https://www.nytimes.com/2017/02/20/science/hubble-constant-universe-expanding-speed.html |date=20 February 2017 |work=[[New York Times]] |accessdate=21 February 2017 }}</ref><ref name=Coles>
{{cite book
 |editor-last=Coles |editor-first=P.
 |date=2001
 |title=Routledge Critical Dictionary of the New Cosmology
 |url=https://books.google.com/?id=BgNGWVr5yhIC&pg=PA202
 |page=202
 |isbn=978-0-203-16457-0
 |publisher=[[Routledge]]
}}</ref>
The motion of astronomical objects due solely to this expansion is known as the '''Hubble flow'''.<ref>
{{cite web
 |title=Hubble Flow
 |url=http://astronomy.swin.edu.au/cosmos/h/hubble+flow
 |website=The Swinburne Astronomy Online Encyclopedia of Astronomy
 |publisher=[[Swinburne University of Technology]]
 |accessdate=2013-05-14
}}</ref>

Although widely attributed to [[Edwin Hubble]],<ref>
{{cite journal
 |last1=van den Bergh |first1=S.
 |date=2011
 |title=The Curious Case of Lemaitre's Equation No. 24
 |chapter=<!-- -->
 |journal=[[Journal of the Royal Astronomical Society of Canada]]
 |volume=105 |issue=4
 |pages=151
 |arxiv=1106.1195 
 |bibcode=2011JRASC.105..151V
 }}</ref><ref>
{{cite journal
 |last1=Nussbaumer |first1=H.
 |last2=Bieri |first2=L.
 |date=2011
 |title=Who discovered the expanding universe?
 |chapter=<!-- -->
 |journal=[[The Observatory (journal)|The Observatory]]
 |volume=131 |issue=6
 |pages=394–398
 |arxiv=1107.2281 
 |bibcode=2011Obs...131..394N
 }}</ref><ref>
{{cite journal
 |last1=Way |first1=M.J.
 |date=2013
 |title=Dismantling Hubble's Legacy?
 |chapter=<!-- -->
 |journal=[[ASP Conference Proceedings]]
 |volume=471 |pages=97–132
 |arxiv=1301.7294 
 |bibcode=2013ASPC..471...97W
}}</ref> the notion of the universe expanding at a calculable rate was first derived from the [[general relativity]] equations in 1922 by [[Alexander Friedmann]].  Friedmann published a set of equations, now known as the [[Friedmann equations]], showing that the universe might expand, and presenting the expansion speed if this was the case.<ref>{{cite journal |last1=Friedman |first1=A. |title=Über die Krümmung des Raumes |journal=Zeitschrift für Physik |date=December 1922 |volume=10 |issue=1 |pages=377–386 |bibcode=1922ZPhy...10..377F |doi=10.1007/BF01332580}}. (English translation in {{cite journal |last1=Friedman |first1=A. |title=On the Curvature of Space |journal=General Relativity and Gravitation |date=December 1999 |volume=31 |issue=12 |pages=1991–2000 |doi=10.1023/A:1026751225741|bibcode=1999GReGr..31.1991F }})</ref>  Then [[Georges Lemaître]], in a 1927 article, independently derived that the universe might be expanding, observed  the proportionality between recessional velocity of and distance to distant bodies, and suggested an estimated value of the proportionality constant, which when corrected by Hubble became known as the '''Hubble constant'''.<ref name="NYT-20170220" /><ref>
{{cite journal
 |last=Lemaître |first=G. |author-link=Georges Lemaître
 |title=Un univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extra-galactiques
 |date=1927
 |journal=Annales de la Société Scientifique de Bruxelles A
 |volume= 47 |pages=49–59
 |bibcode=1927ASSB...47...49L
}} Partially translated in {{Cite journal
 |last=Lemaître |first=G.
 |title=Expansion of the universe, A homogeneous universe of constant mass and increasing radius accounting for the radial velocity of extra-galactic nebulae
 |date=1931
 |journal=[[Monthly Notices of the Royal Astronomical Society]]
 |volume=91 |issue= 5|pages=483–490
 |bibcode=1931MNRAS..91..483L
 |doi=10.1093/mnras/91.5.483
}}</ref><ref name = Livio>
{{cite journal
 |last=Livio |first=M.
 |date=2011
 |title=Lost in translation: Mystery of the missing text solved
 |journal=[[Nature (journal)|Nature]]
 |volume=479 |issue=7372 |pages=171–3
 |bibcode=2011Natur.479..171L
 |doi=10.1038/479171a
 |pmid=22071745
}}</ref><ref>
{{cite journal
 |last1=Livio |first1=M.
 |last2=Riess |first2=A.
 |date=2013
 |title=Measuring the Hubble constant
 |journal=[[Physics Today]]
 |volume=66 |issue=10 |pages=41
 |bibcode=2013PhT....66j..41L
 |doi=10.1063/PT.3.2148
}}</ref> Though the Hubble constant <math>H_0</math> is roughly constant in the velocity-distance space at any given moment in time, the '''Hubble parameter''' <math>H</math>, which the Hubble constant is the current value of, varies with time, so the term 'constant' is sometimes thought of as somewhat of a misnomer.<ref name="NYT-20190225">{{cite news |last=Overbye |first=Dennis |authorlink=Dennis Overbye |title=Have Dark Forces Been Messing With the Cosmos? - Axions? Phantom energy? Astrophysicists scramble to patch a hole in the universe, rewriting cosmic history in the process. |url=https://www.nytimes.com/2019/02/25/science/cosmos-hubble-dark-energy.html |date=25 February 2019 |work=[[The New York Times]] |accessdate=26 February 2019 }}</ref> Moreover, two years later Edwin Hubble confirmed the existence of cosmic expansion, and determined a more accurate value for the constant that now bears his name.<ref>
{{cite journal
 |last=Hubble |first=E.
 |date=1929
 |title=A relation between distance and radial velocity among extra-galactic nebulae
 |url=http://www.pnas.org/cgi/reprint/15/3/168
 |journal=[[Proceedings of the National Academy of Sciences]]
 |volume=15 |issue=3 |pages=168–73
 |bibcode=1929PNAS...15..168H
 |doi=10.1073/pnas.15.3.168
 |pmid=16577160
 |pmc=522427
}}</ref>
Hubble inferred the recession velocity of the objects from their [[redshift]]s, many of which were earlier measured and related to velocity by [[Vesto Slipher]] in 1917.<ref>{{cite journal|last1=Slipher|first1=V.M.|title=Radial velocity observations of spiral nebulae|journal=The Observatory|date=1917|volume=40|pages=304–306|bibcode = 1917Obs....40..304S }}</ref><ref name=MS_Longair>
{{cite book
 |last=Longair |first=M. S.
 |date=2006
 |title=The Cosmic Century
 |url=https://books.google.com/?id=z0vlYHQZHJcC&pg=RA2-PA109
 |page=109
 |publisher=[[Cambridge University Press]]
 |isbn=978-0-521-47436-8
}}</ref><ref>{{cite book|last1=Nussbaumer|first1=Harry|title='Slipher's redshifts as support for de Sitter's model and the discovery of the dynamic universe' In Origins of the Expanding Universe: 1912-1932|date=2013|publisher=Astronomical Society of the Pacific|pages=25–38 |arxiv=1303.1814}}</ref><ref>{{cite book|last1=O'Raifeartaigh|first1=Cormac|title=The Contribution of V.M. Slipher to the discovery of the expanding universe in 'Origins of the Expanding Universe'|date=2013|publisher=Astronomical Society of the Pacific|pages=49–62|arxiv=1212.5499}}</ref>

The law is often expressed by the equation {{nowrap|''v'' {{=}} ''H''<sub>0</sub>''D''}}, with ''H''<sub>0</sub> the constant of proportionality—Hubble constant—between the "proper distance" ''D'' to a galaxy, which can change over time, unlike the [[Comoving and proper distances|comoving distance]], and its velocity ''v'', i.e. the [[derivative]] of proper distance with respect to [[Chronology of the universe|cosmological time]] coordinate. (See ''[[Comoving and proper distances#Uses of the proper distance|uses of the proper distance]]'' for some discussion of the subtleties of this definition of 'velocity'.) Also, the SI unit of ''H''<sub>0</sub> is s<sup>−1</sup>, but it is most frequently quoted in ([[kilometre|km]]/[[second|s]])/Mpc, thus giving the speed in km/s of a galaxy {{convert|1|Mpc|km|sigfig=3}} away. The Hubble constant is about {{nobr|70 (km/s)/Mpc}}. The reciprocal of ''H''<sub>0</sub> is the [[#Hubble time|Hubble time]].

==Discovery==
[[File:Three steps to the Hubble constant.jpg|thumb|Three steps to the Hubble constant.<ref>{{cite web|title=Three steps to the Hubble constant|url=https://www.spacetelescope.org/images/opo1812a/|website=www.spacetelescope.org|accessdate=26 February 2018}}</ref>]]

A decade before Hubble made his observations, a number of [[physicists]] and [[mathematician]]s had established a consistent theory of an expanding universe by using [[Einstein's field equations]] of [[general relativity]]. Applying the most [[Cosmological principle|general principles]] to the nature of the [[universe]] yielded a [[Dynamics (mechanics)|dynamic]] solution that conflicted with the then-prevailing notion of a [[static universe]].

===Slipher's observations===
In 1912, [[Vesto Slipher]] measured the first [[Doppler shift]] of a "[[spiral nebula]]" (spiral nebula is the obsolete term for spiral galaxies), and soon discovered that almost all such nebulae were receding from Earth. He did not grasp the cosmological implications of this fact, and indeed at the time it was [[Shapley–Curtis debate|highly controversial]] whether or not these nebulae were "island universes" outside our [[Milky Way]].<ref>
{{cite journal
 |last=Slipher |first=V. M.
 |date=1913
 |title=The Radial Velocity of the Andromeda Nebula
 |journal=[[Lowell Observatory Bulletin]]
 |volume=1 |pages=56–57
 |bibcode=1913LowOB...2...56S
 |ref=harv
}}</ref><ref>
{{Cite journal
 |last=Slipher |first=V. M.
 |date=1915
 |title=Spectrographic Observations of Nebulae
 |journal=[[Popular Astronomy (US magazine)|Popular Astronomy]]
 |volume=23 |pages=21–24
 |bibcode=1915PA.....23...21S
 |ref=harv
}}</ref>

===FLRW equations===
In 1922, [[Alexander Friedman]]n derived his [[Friedmann equations]] from [[Einstein field equations|Einstein's field equations]], showing that the universe might expand at a rate calculable by the equations.<ref>
{{Cite journal
 |last=Friedman |first=A.
 |date=1922
 |title=Über die Krümmung des Raumes
 |journal=[[Zeitschrift für Physik]]
 |volume=10 |issue=1 |pages=377–386
 |bibcode=1922ZPhy...10..377F
 |doi=10.1007/BF01332580
}} Translated in {{Cite journal
 |last1=Friedmann |first1=A.
 |date=1999
 |title=On the Curvature of Space
 |journal=[[General Relativity and Gravitation]]
 |volume=31 |issue=12 |pages=1991–2000
 |bibcode=1999GReGr..31.1991F
 |doi=10.1023/A:1026751225741
}}</ref> The parameter used by Friedmann is known today as the [[Scale factor (cosmology)|scale factor]] which can be considered as a [[Scale invariance|scale invariant]] form of the [[Proportionality (mathematics)|proportionality constant]] of Hubble's law. [[Georges Lemaître]] independently found a similar solution in 1927. The Friedmann equations are derived by inserting the [[Friedmann–Lemaître–Robertson–Walker metric|metric for a homogeneous and isotropic universe]] into Einstein's field equations for a fluid with a given [[density]] and [[pressure]]. This idea of an expanding spacetime would eventually lead to the [[Big Bang]] and [[Steady State Theory|Steady State]] theories of cosmology.

===Lemaître's equation===
In 1927, two years before Hubble published his own article, the Belgian priest and astronomer [[Georges Lemaître]] was the first to publish research deriving what is now known as Hubble's law. According to the Canadian astronomer [[Sidney van den Bergh]], "The 1927 discovery of the expansion of the universe by Lemaître was published in French in a low-impact journal. In the 1931 high-impact English translation of this article a critical equation was changed by omitting reference to what is now known as the Hubble constant."<ref>{{cite journal|last1=van den Bergh|first1=Sydney|title=The Curious Case of Lemaître's Equation No. 24|journal=Journal of the Royal Astronomical Society of Canada|volume=105|issue=4|pages=151|arxiv=1106.1195|year=2011|bibcode=2011JRASC.105..151V}}</ref> It is now known that the alterations in the translated paper were carried out by Lemaître himself.<ref name = Livio /><ref>{{cite book|last1=Block|first1=David|title='Georges Lemaitre and Stigler's law of eponymy' in Georges Lemaître: Life, Science and Legacy|date=2012|publisher=Springer|pages=89–96|edition=Holder and Mitton}}</ref>

===Shape of the universe===
Before the advent of [[Big Bang|modern cosmology]], there was considerable talk about the size and [[shape of the universe|shape]] of the [[universe]]. In 1920, the [[Great Debate (astronomy)|Shapley–Curtis debate]] took place between [[Harlow Shapley]] and [[Heber Doust Curtis|Heber D. Curtis]] over this issue. Shapley argued for a small universe the size of the [[Milky Way galaxy]] and Curtis argued that the universe was much larger. The issue was resolved in the coming decade with Hubble's improved observations.

===Cepheid variable stars outside of the Milky Way===
Edwin Hubble did most of his professional astronomical observing work at [[Mount Wilson Observatory]], home to the world's most powerful telescope at the time. His observations of [[Cepheid variable]] stars in “''spiral [[nebula]]e''” enabled him to calculate the distances to these objects. Surprisingly, these objects were discovered to be at distances which placed them well outside the [[Milky Way]]. They continued to be called “nebulae” and it was only gradually that the term “galaxies” replaced it.

===Combining redshifts with distance measurements===

[[File:Hubble constant.JPG|thumb |250px |Fit of [[#Redshift velocity|redshift velocities]] to Hubble's law.<ref name=Keel>{{cite book
 |last=Keel |first=W. C.
 |date=2007
 |title=The Road to Galaxy Formation
 |url=https://books.google.com/?id=BUgJGypUYF0C&pg=PA7
 |pages=7–8 |edition=2nd
 |publisher=[[Springer (publisher)|Springer]]
 |isbn=978-3-540-72534-3
}}</ref> Various estimates for the Hubble constant exist. The HST Key ''H''<sub>0</sub> Group fitted type Ia supernovae for redshifts between 0.01 and 0.1 to find that ''H''<sub>0</sub> = 71 ± 2 (statistical) ± 6 (systematic) km s<sup>−1</sup>Mpc<sup>−1</sup>,<ref name=Freedman2001/> while Sandage ''et al.'' find ''H''<sub>0</sub> = 62.3 ± 1.3 (statistical) ± 5 (systematic) km s<sup>−1</sup>Mpc<sup>−1</sup>.<ref name=Weinberg>
{{cite book
 |last=Weinberg |first=S.
 |date=2008
 |title=Cosmology
 |url=https://books.google.com/?id=nqQZdg020fsC&pg=PA28
 |page=28
 |publisher=[[Oxford University Press]]
 |isbn=978-0-19-852682-7
}}</ref>]]

The parameters that appear in Hubble's law, velocities and distances, are not directly measured. In reality we determine, say, a supernova brightness, which provides information about its distance, and the redshift ''z'' = ∆''λ''/''λ'' of its spectrum of radiation. Hubble correlated brightness and parameter ''z''.

Combining his measurements of galaxy distances with [[Vesto Slipher]] and [[Milton Humason]]'s measurements of the [[redshifts]] associated with the galaxies, Hubble discovered a rough proportionality between redshift of an object and its distance. Though there was considerable [[variance|scatter]] (now known to be caused by [[peculiar velocity|peculiar velocities]]—the 'Hubble flow' is used to refer to the region of space far enough out that the recession velocity is larger than local peculiar velocities), Hubble was able to plot a trend line from the 46 galaxies he studied and obtain a value for the Hubble constant of 500&nbsp;km/s/Mpc (much higher than the currently accepted value due to errors in his distance calibrations). (See [[cosmic distance ladder]] for details.)

At the time of discovery and development of Hubble's law, it was acceptable to explain redshift phenomenon as a [[Doppler shift]] in the context of special relativity, and use the Doppler formula to associate redshift ''z'' with velocity. Today, in the context of general relativity, velocity between distant objects depends on the choice of coordinates used, and therefore, the redshift can be equally described as a Doppler shift or a cosmological shift (or gravitational) due to the expanding space, or some combination of the two.<ref>
{{cite journal
 |last=Bunn |first=E. F.
 |year=2009
 |title=The kinematic origin of the cosmological redshift
 |journal=[[American Journal of Physics]]
 |volume=77 |issue=8 |pages=688–694
 |doi=10.1119/1.3129103
|arxiv=0808.1081|bibcode=2009AmJPh..77..688B}}</ref>

====Hubble diagram====
Hubble's law can be easily depicted in a "Hubble diagram" in which the velocity (assumed approximately proportional to the redshift) of an object is plotted with respect to its distance from the observer.<ref>
{{cite journal
 |last=Kirshner |first=R. P.
 |date=2003
 |title=Hubble's diagram and cosmic expansion
 |journal=[[Proceedings of the National Academy of Sciences]]
 |volume=101 |issue=1 |pages=8–13
 |bibcode=2003PNAS..101....8K
 |doi=10.1073/pnas.2536799100
 |pmid=14695886
 |pmc=314128
}}</ref> A straight line of positive slope on this diagram is the visual depiction of Hubble's law.

===Cosmological constant abandoned===
{{main|Cosmological constant}}
After Hubble's discovery was published, [[Albert Einstein]] abandoned his work on the [[cosmological constant]], which he had designed to modify his equations of general relativity to allow them to produce a static solution, which he thought was the correct state of the universe. The Einstein equations in their simplest form model generally either an expanding or contracting universe, so Einstein's cosmological constant was artificially created to counter the expansion or contraction to get a perfect static and flat universe.<ref name=mapcc>
{{cite web
 |title=What is a Cosmological Constant?
 |url=http://map.gsfc.nasa.gov/universe/uni_accel.html
 |publisher=[[Goddard Space Flight Center]]
 |accessdate=2013-10-17
}}</ref> After Hubble's discovery that the universe was, in fact, expanding, Einstein called his faulty assumption that the universe is static his "biggest mistake".<ref name=mapcc /> On its own, general relativity could predict the expansion of the universe, which (through [[Tests of general relativity|observations]] such as the [[Gravitational lens|bending of light by large masses]], or the [[Perihelion precession of Mercury|precession of the orbit of Mercury]]) could be experimentally observed and compared to his theoretical calculations using particular solutions of the equations he had originally formulated.

In 1931, Einstein made a trip to [[Mount Wilson Observatory|Mount Wilson]] to thank Hubble for providing the observational basis for modern cosmology.<ref>
{{cite book
 |last=Isaacson |first=W.
 |date=2007
 |title=Einstein: His Life and Universe
 |url=https://books.google.com/books?id=cdxWNE7NY6QC&pg=PA354
 |page=354
 |publisher=[[Simon & Schuster]]
 |isbn=978-0-7432-6473-0
}}</ref>

The cosmological constant has regained attention in recent decades as a hypothesis for [[dark energy]].<ref>
{{cite web
 |date=28 November 2007
 |title=Einstein's Biggest Blunder? Dark Energy May Be Consistent With Cosmological Constant
 |url=https://www.sciencedaily.com/releases/2007/11/071127142128.htm
 |website=[[Science Daily]]
 |accessdate=2013-06-02
}}</ref>

{{anchor|redshift}}

==Interpretation==
[[File:Velocity-redshift.JPG|thumb |250px |A variety of possible recessional velocity vs. redshift functions including the simple linear relation ''v'' = ''cz''; a variety of possible shapes from theories related to general relativity; and a curve that does not permit speeds faster than light in accordance with special relativity. All curves are linear at low redshifts. See Davis and Lineweaver.<ref name=D&L>
{{cite journal
 |last1=Davis |first1=T. M.
 |last2=Lineweaver |first2=C. H.
 |date=2001
 |title=Superluminal Recessional Velocities
 |chapter=<!-- -->
 |journal=[[AIP Conference Proceedings]]
 |volume=555 |pages=348–351
 |arxiv=astro-ph/0011070
 |bibcode=2001AIPC..555..348D
 |doi=10.1063/1.1363540
|citeseerx=10.1.1.254.1810
 }}</ref>]]
The discovery of the linear relationship between [[redshift]] and distance, coupled with a supposed linear relation between [[recessional velocity]] and redshift, yields a straightforward mathematical expression for Hubble's law as follows:

:<math>v = H_0 \, D</math>

where
* <math>v</math> is the recessional velocity, typically expressed in km/s.
* ''H''<sub>0</sub> is Hubble's constant and corresponds to the value of <math>H</math> (often termed the '''Hubble parameter''' which is a value that is [[time-variant system|time dependent]] and which can be expressed in terms of the [[Scale factor (cosmology)|scale factor]]) in the Friedmann equations taken at the time of observation denoted by the subscript ''0''. This value is the same throughout the universe for a given [[comoving time#Comoving coordinates|comoving time]].
* <math>D</math> is the proper distance (which can change over time, unlike the [[comoving distance]], which is constant) from the [[galaxy]] to the observer, measured in [[mega-|mega]] [[parsec]]s (Mpc), in the 3-space defined by given [[cosmological time]]. (Recession velocity is just ''v'' = ''dD/dt'').

Hubble's law is considered a fundamental relation between recessional velocity and distance. However, the relation between recessional velocity and redshift depends on the cosmological model adopted and is not established except for small redshifts.

For distances ''D'' larger than the radius of the [[Hubble sphere]] ''r''<sub>HS</sub>&nbsp;, objects recede at a rate faster than the [[speed of light]] (''See'' [[Comoving distance#Uses of the proper distance|Uses of the proper distance]] for a discussion of the significance of this):

:<math>r_{HS} = \frac{c}{H_0} \ . </math>

Since the Hubble "constant" is a constant only in space, not in time, the radius of the Hubble sphere may increase or decrease over various time intervals. The subscript '0' indicates the value of the Hubble constant today.<ref name=Keel/> Current evidence suggests that the expansion of the universe is accelerating (''see'' [[Accelerating universe]]), meaning that, for any given galaxy, the recession velocity dD/dt is increasing over time as the galaxy moves to greater and greater distances; however, the Hubble parameter is actually thought to be decreasing with time, meaning that if we were to look at some ''fixed'' distance D and watch a series of different galaxies pass that distance, later galaxies would pass that distance at a smaller velocity than earlier ones.<ref>{{cite web|title=Is the universe expanding faster than the speed of light?|url=http://curious.astro.cornell.edu/question.php?number=575|website=Ask an Astronomer at Cornell University|accessdate=5 June 2015|archiveurl=https://web.archive.org/web/20031123150109/http://curious.astro.cornell.edu/question.php?number=575|archivedate=23 November 2003}}</ref>

===Redshift velocity and recessional velocity===
[[Redshift]] can be measured by determining the wavelength of a known transition, such as hydrogen α-lines for distant quasars, and finding the fractional shift compared to a stationary reference. Thus redshift is a quantity unambiguous for experimental observation. The relation of redshift to recessional velocity is another matter. For an extensive discussion, see Harrison.<ref name=Harrison>{{cite journal |last=Harrison |first=E. |date=1992 |title=The redshift-distance and velocity-distance laws |journal=[[The Astrophysical Journal]] |volume=403 |issue= |pages=28–31 | bibcode=1993ApJ...403...28H |doi=10.1086/172179}}</ref>

====Redshift velocity====
The redshift ''z'' is often described as a ''redshift velocity'', which is the recessional velocity that would produce the same redshift ''if'' it were caused by a linear [[Doppler effect]] (which, however, is not the case, as the shift is caused in part by a [[Metric expansion of space|cosmological expansion of space]], and because the velocities involved are too large to use a non-relativistic formula for Doppler shift). This redshift velocity can easily exceed the speed of light.<ref name=Madsen>{{cite book
 |last=Madsen |first=M. S.
 |date=1995
 |title=The Dynamic Cosmos
 |url=https://books.google.com/?id=_2GeJxVvyFMC&pg=PA35
 |page=35
 |publisher=[[CRC Press]]
 |isbn=978-0-412-62300-4
}}</ref> In other words, to determine the redshift velocity ''v<sub>rs</sub>'', the relation:

:<math> v_{rs} \equiv cz,</math>

is used.<ref name=Dekel>{{cite book |last1=Dekel |first1=A.  |last2=Ostriker |first2=J. P. |date=1999  |title=Formation of Structure in the Universe
 |url=https://books.google.com/?id=yAroX6tx-l0C&pg=PA164 |page=164  |publisher=[[Cambridge University Press]] |isbn=978-0-521-58632-0 }}</ref><ref name=Padmanabhan>{{cite book |last=Padmanabhan |first=T. |date=1993 |title=Structure formation in the universe | url=https://books.google.com/?id=AJlOVBRZJtIC&pg=PA58 |page=58 |publisher=[[Cambridge University Press]] |isbn=978-0-521-42486-8 }}</ref> That is, there is ''no fundamental difference'' between redshift velocity and redshift: they are rigidly proportional, and not related by any theoretical reasoning. The motivation behind the "redshift velocity" terminology is that the redshift velocity agrees with the velocity from a low-velocity simplification of the so-called [[Relativistic Doppler effect|Fizeau-Doppler formula]].<ref name=Sartori>
{{cite book
 |last=Sartori |first=L.
 |date=1996
 |title=Understanding Relativity
 |page=163, Appendix 5B
 |publisher=[[University of California Press]]
 |isbn=978-0-520-20029-6
}}</ref>

:<math>z = \frac{\lambda_o}{\lambda_e}-1 = \sqrt{\frac{1+\frac{v}{c}}{1-\frac{v}{c}}}-1 \approx \frac{v}{c}.</math>

Here, ''λ<sub>o</sub>'', ''λ<sub>e</sub>'' are the observed and emitted wavelengths respectively. The "redshift velocity" ''v<sub>rs</sub>'' is not so simply related to real velocity at larger velocities, however, and this terminology leads to confusion if interpreted as a real velocity. Next, the connection between redshift or redshift velocity and recessional velocity is discussed. This discussion is based on Sartori.<ref name=L_Sartori>{{cite book|last=Sartori |first=L. |date=1996 |title=Understanding Relativity |pages=304–305 |publisher=[[University of California Press]] |isbn=978-0-520-20029-6
}}</ref>

====Recessional velocity====

Suppose ''R(t)'' is called the ''[[Scale factor (cosmology)|scale factor]]'' of the universe, and increases as the universe expands in a manner that depends upon the [[Physical cosmology|cosmological model]] selected. Its meaning is that all measured proper distances ''D(t)'' between co-moving points increase proportionally to ''R''. (The co-moving points are not moving relative to each other except as a result of the expansion of space.) In other words:

:<math>\frac {D(t)}{D(t_0)} = \frac{R(t)}{R(t_0)},</math> <ref>"Introduction to Cosmology", Matts Roos</ref>

where ''t''<sub>0</sub> is some reference time. If light is emitted from a galaxy at time ''t<sub>e</sub>'' and received by us at ''t''<sub>0</sub>, it is redshifted due to the expansion of space, and this redshift ''z'' is simply:

:<math>z = \frac {R(t_0)}{R(t_e)} - 1. </math>

Suppose a galaxy is at distance ''D'', and this distance changes with time at a rate ''d<sub>t</sub>D''. We call this rate of recession the "recession velocity" ''v<sub>r</sub>'':

:<math>v_r = d_tD = \frac {d_tR}{R} D. </math>

We now define the Hubble constant as

:<math>H \equiv \frac{d_tR}{R}, </math>

and discover the Hubble law:

:<math> v_r = H D. </math>

From this perspective, Hubble's law is a fundamental relation between (i) the recessional velocity contributed by the expansion of space and (ii) the distance to an object; the connection between redshift and distance is a crutch used to connect Hubble's law with observations. This law can be related to redshift ''z'' approximately by making a [[Taylor series]] expansion:

:<math> z = \frac {R(t_0)}{R(t_e)} - 1 \approx \frac {R(t_0)} {R(t_0)\left(1+(t_e-t_0)H(t_0)\right)}-1 \approx (t_0-t_e)H(t_0), </math>

If the distance is not too large, all other complications of the model become small corrections and the time interval is simply the distance divided by the speed of light:

:<math> z \approx (t_0-t_e)H(t_0) \approx \frac {D}{c} H(t_0), </math>

or

:<math> cz \approx D H(t_0) = v_r. </math>

According to this approach, the relation ''cz'' = ''v<sub>r</sub>'' is an approximation valid at low redshifts, to be replaced by a relation at large redshifts that is model-dependent. See [[#redshift|velocity-redshift figure]].

===Observability of parameters===
Strictly speaking, neither ''v'' nor ''D'' in the formula are directly observable, because they are properties ''now'' of a galaxy, whereas our observations refer to the galaxy in the past, at the time that the light we currently see left it.

For relatively nearby galaxies ([[redshift]] ''z'' much less than unity), ''v'' and ''D'' will not have changed much, and ''v'' can be estimated using the formula <math>v = zc</math> where ''c'' is the [[speed of light]]. This gives the empirical relation found by Hubble.

For distant galaxies, ''v'' (or ''D'') cannot be calculated from ''z'' without specifying a detailed model for how ''H'' changes with time. The redshift is not even directly related to the recession velocity at the time the light set out, but it does have a simple interpretation: ''(1+z)'' is the factor by which the universe has expanded while the photon was travelling towards the observer.

===Expansion velocity vs. relative velocity===
In using Hubble's law to determine distances, only the velocity due to the expansion of the universe can be used. Since gravitationally interacting galaxies move relative to each other independent of the expansion of the universe,<ref name="AM-20170818">{{cite web |last=Scharping |first=Nathaniel |title=Gravitational Waves Show How Fast The Universe is Expanding |url=http://www.astronomy.com/news/2017/10/gravitational-waves-show-how-fast-the-universe-is-expanding |date=18 October 2017 |website=[[Astronomy (magazine)|Astronomy]] |accessdate=18 October 2017 }}</ref> these relative velocities, called [[Peculiar velocity|peculiar velocities]], need to be accounted for in the application of Hubble's law.

The [[Redshift-space distortions|Finger of God]] effect is one result of this phenomenon. In [[Virial theorem|systems that are gravitationally bound]], such as galaxies or our planetary system, the expansion of space is a much weaker effect than the attractive force of gravity.

=== Time-dependence of Hubble parameter ===

The parameter <math>H</math> is commonly called the “''Hubble constant''”, but that is a misnomer since it is constant in space only at a fixed time; it varies with time in nearly all cosmological models, and all observations of far distant objects are also observations into the distant past, when the “constant” had a different value. The “''Hubble parameter''” is a more correct term, with <math>H_0 </math> denoting the present-day value.

Another common source of confusion is that the [[accelerating universe]] does ''not'' imply that the Hubble parameter is actually increasing with time; since <math> H(t) \equiv \dot{a}(t)/a(t) </math>, in most accelerating models <math>a</math> increases relatively faster than <math>\dot{a}</math>, so H decreases with time. (The recession velocity of one chosen galaxy does increase, but different galaxies passing a sphere of fixed radius cross the sphere more slowly at later times.)

On defining the dimensionless [[deceleration parameter]] 
:<math>  q \equiv - \frac {\ddot{a}\, a} {\dot{a}^2} </math>, it follows that 
:<math> \frac{dH}{dt} = -H^2 (1+q) </math>

From this it is seen that the Hubble parameter is decreasing with time, unless <math> q < -1 </math>; the latter can only occur if the universe contains [[phantom energy]], regarded as theoretically somewhat improbable.

However, in the standard [[Lambda-CDM]] model, <math> q </math> will tend to &minus;1 from above in the distant future as the cosmological constant becomes increasingly dominant over matter; this implies that <math> H </math> will approach from above to a constant value of <math> \approx 57 </math> km/s/Mpc, and the scale factor of the universe will then grow exponentially in time.

===Idealized Hubble's law===
The mathematical derivation of an idealized Hubble's law for a uniformly expanding universe is a fairly elementary theorem of geometry in 3-dimensional [[Cartesian coordinate system|Cartesian]]/Newtonian coordinate space, which, considered as a [[metric space]], is entirely [[Cosmological principle|homogeneous and isotropic]] (properties do not vary with location or direction). Simply stated the theorem is this:
{{bquote|Any two points which are moving away from the origin, each along straight lines and with speed proportional to distance from the origin, will be moving away from each other with a speed proportional to their distance apart.}}

In fact this applies to non-Cartesian spaces as long as they are locally homogeneous and isotropic; specifically to the negatively and positively curved spaces frequently considered as cosmological models (see [[shape of the universe]]).

An observation stemming from this theorem is that seeing objects recede from us on Earth is not an indication that Earth is near to a center from which the expansion is occurring, but rather that ''every'' observer in an expanding universe will see objects receding from them.

===Ultimate fate and age of the universe===
[[Image:Friedmann universes.svg|thumb|400px|The [[age of the universe|age]] and [[ultimate fate of the universe]] can be determined by measuring the Hubble constant today and extrapolating with the observed value of the deceleration parameter, uniquely characterized by values of density parameters (Ω<sub>M</sub> for [[matter]] and Ω<sub>Λ</sub> for [[dark energy]]). A "closed universe" with Ω<sub>M</sub> > 1 and Ω<sub>Λ</sub> = 0 comes to an end in a [[Big Crunch]] and is considerably younger than its Hubble age. An "open universe" with Ω<sub>M</sub> ≤ 1 and Ω<sub>Λ</sub> = 0 expands forever and has an age that is closer to its Hubble age. For the [[accelerating universe]] with nonzero Ω<sub>Λ</sub> that we inhabit, the age of the universe is coincidentally very close to the Hubble age.]]

The value of the Hubble parameter changes over time, either increasing or decreasing depending on the value of the so-called [[deceleration parameter]] <math>q</math>, which is defined by

:<math>q = -\left(1+\frac{\dot H}{H^2}\right).</math>

In a universe with a deceleration parameter equal to zero, it follows that ''H'' = 1/''t'', where ''t'' is the time since the Big Bang. A non-zero, time-dependent value of <math>q</math> simply requires [[integral|integration]] of the Friedmann equations backwards from the present time to the time when the [[particle horizon|comoving horizon]] size was zero.

It was long thought that ''q'' was positive, indicating that the expansion is slowing down due to gravitational attraction. This would imply an age of the universe less than 1/''H'' (which is about 14 billion years). For instance, a value for ''q'' of 1/2 (once favoured by most theorists) would give the age of the universe as 2/(3''H''). The discovery in 1998 that ''q'' is apparently negative means that the universe could actually be older than 1/''H''. However, estimates of the [[age of the universe]] are very close to 1/''H''.

===Olbers' paradox===
{{Main|Olbers' paradox}}
The expansion of space summarized by the Big Bang interpretation of Hubble's law is relevant to the old conundrum known as [[Olbers' paradox]]: If the universe were [[Infinity|infinite]] in size, [[static universe|static]], and filled with a uniform distribution of [[star]]s, then every line of sight in the sky would end on a star, and the sky would be as [[brightness|bright]] as the surface of a star. However, the night sky is largely dark.<ref name=Chase_etal_2004>{{cite web |last=Chase |first=S. I. |last2=Baez |first2=J. C. |date=2004 |title=Olbers' Paradox |url=http://math.ucr.edu/home/baez/physics/Relativity/GR/olbers.html |website=The Original Usenet Physics FAQ |accessdate=2013-10-17}}</ref><ref name=Asimov1974>{{cite book |last=Asimov |first=I. |date=1974 |chapter=The Black of Night |title=Asimov on Astronomy |publisher=[[Doubleday (publisher)|Doubleday]] |isbn=978-0-385-04111-9 |chapter-url-access=registration |chapter-url=https://archive.org/details/asimovonastronom00isaa }}</ref>

Since the 17th&nbsp;century, astronomers and other thinkers have proposed many possible ways to resolve this paradox, but the currently accepted resolution depends in part on the [[Big Bang]] theory, and in part on the Hubble expansion: In a universe that exists for a finite amount of time, only the light of a finite number of stars has had enough time to reach us, and the paradox is resolved. Additionally, in an expanding universe, distant objects [[recessional velocity|recede]] from us, which causes the light emanated from them to be [[redshift]]ed and diminished in brightness by the time we see it.<ref name=Chase_etal_2004/><ref name=Asimov1974/>

===Dimensionless Hubble parameter===
Instead of working with Hubble's constant, a common practice is to introduce the '''dimensionless Hubble parameter''', usually denoted by ''h'', and to write the Hubble's parameter ''H''<sub>0</sub> as ''h''&nbsp;×&nbsp;100&nbsp;km&nbsp;[[second|s]]<sup>−1</sup>&nbsp;[[Parsec|Mpc]]<sup>−1</sup>, all the relative uncertainty of the true value of ''H''<sub>0</sub> being then relegated to ''h''.<ref>
{{cite book
 |last=Peebles |first=P. J. E.
 |date=1993
 |title=Principles of Physical Cosmology
 |publisher=[[Princeton University Press]]
 |isbn=
}}</ref> Occasionally a reference value other than 100 may be chosen, in which case a subscript is presented after ''h'' to avoid confusion; e.g. h<sub>70</sub> denotes <math> H_0 = 70 \, h_{70} </math> km&nbsp;s<sup>−1</sup>&nbsp;Mpc<sup>−1</sup>, which implies <math> h_{70} = h / 0.7 </math>.

This should not be confused with the [[Dimensionless physical constant|dimensionless value]] of Hubble's constant, usually expressed in terms of [[Planck units]], with current  of ''H''<sub>0</sub>×[[Planck time|''t''<sub>P</sub>]]&nbsp;=&nbsp;1.18&nbsp;×&nbsp;10<sup>−61</sup>.{{citation needed|reason=A citation might let the reader know which value of the Hubble Constant is being used. Alternatively and/or in addition, the reader should be told in our article text which value is being used.|date=February 2019}}

==Determining the Hubble constant==
[[File:Recent Hubble's Constant Values.png|thumb|right|Value of the Hubble Constant including measurement uncertainty for recent surveys.<ref name="planck_overview"/>]]
The value of the Hubble constant is estimated by measuring the [[redshift]] of distant galaxies and then [[cosmic distance ladder|determining the distances to the same galaxies]] (by some other method than Hubble's law). Uncertainties in the physical assumptions used to determine these distances have caused varying estimates of the Hubble constant.<ref name="NYT-20170220" />
 
The observations of astronomer [[Walter Baade]] led him to define distinct "[[stellar population|populations]]" for stars (Population I and Population II). The same observations led him to discover that there are two types of [[Cepheid variable]] stars. Using this discovery he recalculated the size of the known universe, doubling the previous calculation made by [[Edwin Hubble|Hubble]] in 1929.<ref>Baade W (1944) The resolution of Messier 32, NGC 205, and the central region of the Andromeda nebula. ApJ 100 137-146</ref><ref>Baade W (1956) The period-luminosity relation of the Cepheids. PASP 68 5-16</ref><ref>{{cite web|last=Allen|first=Nick|title=Section 2: The Great Debate and the Great Mistake: Shapley, Hubble, Baade|url=http://www.institute-of-brilliant-failures.com/section2.htm|website=The Cepheid Distance Scale: A History|accessdate=19 November 2011}}</ref> He announced this finding to considerable astonishment at the 1952 meeting of the [[International Astronomical Union]] in Rome.

In October 2018, scientists presented a new third way (two earlier methods, one based on [[redshift]]s and another on the [[cosmic distance ladder]], gave results that do not agree), using information from [[gravitational wave]] events (especially those involving the [[Neutron star merger|merger of neutron stars]], like [[GW170817]]), of determining the Hubble constant.<ref name="PHYS-20181022">{{cite web |last=Lerner |first=Louise |title=Gravitational waves could soon provide measure of universe's expansion |url=https://phys.org/news/2018-10-gravitational-universe-expansion.html |date=22 October 2018 |work=[[Phys.org]] |accessdate=22 October 2018 }}</ref><ref name="NAT-20181017">{{cite journal |last1=Chen |first1=Hsin-Yu |last2=Fishbach |first2=Maya |last3=Holz |first3=Daniel E. |title=A two per cent Hubble constant measurement from standard sirens within five years |date=17 October 2018 |journal=[[Nature (journal)|Nature]] |volume=562 |issue=7728 |pages=545–547 |doi=10.1038/s41586-018-0606-0 |pmid=30333628 |bibcode=2018Natur.562..545C |arxiv=1712.06531 }}</ref>

In July 2019, astronomers reported that a new method to determine the Hubble constant, and resolve the discrepancy of earlier methods, has been proposed based on the mergers of pairs of [[neutron star]]s, following the detection of the neutron star merger of GW170817.<ref name="EA-20190708">{{cite news |author=National Radio Astronomy Observatory |title=New method may resolve difficulty in measuring universe's expansion - Neutron star mergers can provide new 'cosmic ruler' |url=https://www.eurekalert.org/pub_releases/2019-07/nrao-nmm070819.php |date=8 July 2019 |work=[[EurekAlert!]] |accessdate=8 July 2019 |author-link=National Radio Astronomy Observatory }}</ref><ref name="NRAO-20190708">{{cite news |last=Finley |first=Dave |title=New Method May Resolve Difficulty in Measuring Universe's Expansion |url=https://public.nrao.edu/news/new-method-measuring-universe-expansion/ |date=8 July 2019 |work=[[National Radio Astronomy Observatory]] |accessdate=8 July 2019 }}</ref> Their measurement of the Hubble constant is {{val|70.3|+5.3|-5.0}} (km/s)/Mpc.<ref name="NAT-20190708">{{cite journal |author=Hotokezaka, K. |display-authors=et al. |title=A Hubble constant measurement from superluminal motion of the jet in GW170817 |date=8 July 2019 |journal=[[Nature Astronomy]] |volume=3 |issue=10 |pages=940–944 |doi=10.1038/s41550-019-0820-1 |bibcode=2019NatAs.tmp..385H |arxiv=1806.10596 }}</ref>

Also, in July 2019, astronomers reported another new method, using data from the [[Hubble Space Telescope]], and based on distances to [[Red giant|red giant stars]] calculated using the [[tip of the red-giant branch]] (TRGB) distance indicator. Their measurement of the Hubble constant is {{val|69.8|+1.9|-1.9}} (km/s)/Mpc.<ref name="EA-20190716" /><ref name="SCI-20190719" /><ref name="The Carnegie-Chicago Hubble Program">{{cite journal|arxiv=1907.05922|author1=Sidney van den Bergh|title=The Carnegie-Chicago Hubble Program. VIII. An Independent Determination of the Hubble Constant Based on the Tip of the Red Giant Branch|journal=The Astrophysical Journal|volume=882|issue=1|pages=34|year=2019|last2=Madore|first2=Barry F.|last3=Hatt|first3=Dylan|last4=Hoyt|first4=Taylor J.|last5=Jang|first5=In-Sung|last6=Beaton|first6=Rachael L.|last7=Burns|first7=Christopher R.|last8=Lee|first8=Myung Gyoon|last9=Monson|first9=Andrew J.|last10=Neeley|first10=Jillian R.|last11=Phillips|first11=Mark M.|last12=Rich|first12=Jeffrey A.|last13=Seibert|first13=Mark|doi=10.3847/1538-4357/ab2f73|bibcode=2019ApJ...882...34F}}</ref>

===Earlier measurement and discussion approaches===
For most of the second half of the 20th century the value of <math>H_0</math> was estimated to be between 50 and {{nowrap|90 (km/s)/Mpc}}.

The value of the Hubble constant was the topic of a long and rather bitter controversy between [[Gérard de Vaucouleurs]], who claimed the value was around 100, and [[Allan Sandage]], who claimed the value was near 50.<ref name="Overbye"/> In 1996, a debate moderated by [[John N. Bahcall|John Bahcall]] between [[Sidney van den Bergh]] and [[Gustav Andreas Tammann|Gustav Tammann]] was held in similar fashion to the earlier [[Great Debate (astronomy)|Shapley–Curtis debate]] over these two competing values.

This previously wide variance in estimates was partially resolved with the introduction of the [[Lambda-CDM model|ΛCDM model]] of the universe in the late 1990s. With the ΛCDM model observations of high-redshift clusters at X-ray and microwave wavelengths using the [[Sunyaev–Zel'dovich effect]], measurements of anisotropies in the [[cosmic microwave background]] radiation, and optical surveys all gave a value of around 70 for the constant.{{Citation needed|date=July 2009}}

More recent measurements from the [[Planck (spacecraft)|Planck mission]] published in 2018 indicate a lower value of {{val|67.66|0.42}}% although, even more recently, in March 2019, a higher value of {{val|74.03|1.42}}% has been determined using an improved procedure involving the [[Hubble Space Telescope]].<ref name="SA2019">{{Citation|url=https://www.scientificamerican.com/article/best-yet-measurements-deepen-cosmological-crisis/|title=Best-Yet Measurements Deepen Cosmological Crisis|author=Anil Ananthaswamy|date=22 March 2019|access-date=23 March 2019|publisher=Scientific American}}</ref> The two measurements disagree at the 4.4[[Standard deviation|σ]] level, beyond a plausible level of chance.<ref name="Riess2019"/> The resolution to this disagreement is an ongoing area of research.<ref>{{Cite arxiv|last=Millea|first=Marius|last2=Knox|first2=Lloyd|date=2019-08-10|title=The Hubble Hunter's Guide|language=en|eprint=1908.03663v1|class=astro-ph.CO}}</ref>

''See [[Hubble's law#Measured values of the Hubble constant|table of measurements]] below for many recent and older measurements.''

===Acceleration of the expansion===
{{Main|Accelerating universe}}
A value for <math>q</math> measured from [[standard candle]] observations of [[Type Ia supernova]]e, which was determined in 1998 to be negative, surprised many astronomers with the implication that the expansion of the universe is currently "accelerating"<ref>
{{cite journal
 |last=Perlmutter |first=S.
 |date=2003
 |title=Supernovae, Dark Energy, and the Accelerating Universe
 |url=http://www.supernova.lbl.gov/PhysicsTodayArticle.pdf
 |journal=[[Physics Today]]
 |volume=56 |issue=4 |pages=53–60
 |bibcode= 2003PhT....56d..53P
 |doi=10.1063/1.1580050
|citeseerx=10.1.1.77.7990
 }}</ref> (although the Hubble factor is still decreasing with time, as mentioned above in the [[Hubble's law#Interpretation|Interpretation]] section; see the articles on [[dark energy]] and the [[ΛCDM model]]).

==Derivation of the Hubble parameter==
{{More citations needed section|date=March 2014}}

Start with the [[Friedmann equations|Friedmann equation]]:

:<math>H^2 \equiv \left(\frac{\dot{a}}{a}\right)^2 = \frac{8 \pi G}{3}\rho - \frac{kc^2}{a^2}+ \frac{\Lambda c^2}{3},</math>

where <math>H</math> is the Hubble parameter, <math>a</math> is the [[scale factor (universe)|scale factor]], ''G'' is the [[gravitational constant]], <math>k</math> is the normalised spatial curvature of the universe and equal to −1, 0, or 1, and <math>\Lambda</math> is the [[cosmological constant]].

===Matter-dominated universe (with a cosmological constant)===

If the universe is [[Matter-dominated era|matter-dominated]], then the mass density of the universe <math>\rho</math> can just be taken to include matter so

:<math>\rho = \rho_m(a) = \frac{\rho_{m_{0}}}{a^3},</math>

where <math>\rho_{m_{0}}</math> is the density of matter today. We know for nonrelativistic particles that their mass density decreases proportional to the inverse volume of the universe, so the equation above must be true. We can also define (see [[density parameter]] for <math>\Omega_m</math>)

:<math>\rho_c = \frac{3 H_0^2}{8 \pi G};</math>

:<math>\Omega_m \equiv \frac{\rho_{m_{0}}}{\rho_c} = \frac{8 \pi G}{3 H_0^2}\rho_{m_{0}};</math>

therefore:

:<math>\rho=\frac{\rho_c \Omega_m}{a^3}.</math>

Also, by definition,

:<math>\Omega_k \equiv \frac{-kc^2}{(a_0H_0)^2}</math>
:<math>\Omega_{\Lambda} \equiv \frac{\Lambda c^2}{3H_0^2},</math>

where the subscript nought refers to the values today, and <math>a_0=1</math>. Substituting all of this into the Friedmann equation at the start of this section and replacing <math>a</math> with <math>a=1/(1+z)</math> gives

:<math>H^2(z)= H_0^2 \left( \Omega_m (1+z)^{3} + \Omega_k (1+z)^{2} + \Omega_{\Lambda} \right).</math>

=== Matter- and dark energy-dominated universe ===

If the universe is both [[Matter-dominated era|matter-dominated]] and [[dark energy]]-dominated, then the above equation for the Hubble parameter will also be a function of the [[equation of state (cosmology)|equation of state of dark energy]]. So now:

:<math>\rho = \rho_m (a)+\rho_{de}(a),</math>

where <math>\rho_{de}</math> is the mass density of the dark energy. By definition, an equation of state in cosmology is <math>P=w\rho c^2</math>, and if this is substituted into the fluid equation, which describes how the mass density of the universe evolves with time, then

:<math>\dot{\rho}+3\frac{\dot{a}}{a}\left(\rho+\frac{P}{c^2}\right)=0;</math>
:<math>\frac{d\rho}{\rho}=-3\frac{da}{a}(1+w).</math>

If ''w'' is constant, then

:<math>\ln{\rho}=-3(1+w)\ln{a};</math>

implying:

:<math>\rho=a^{-3(1+w)}.</math>

Therefore, for dark energy with a constant equation of state ''w'', <math>\rho_{de}(a)= \rho_{de0}a^{-3(1+w)}</math>. If this is substituted into the Friedman equation in a similar way as before, but this time set <math>k=0</math>, which assumes a spatially flat universe, then (see [[Shape of the universe]])

:<math>H^2(z)= H_0^2 \left( \Omega_m (1+z)^{3} + \Omega_{de}(1+z)^{3(1+w)} \right).</math>

If the dark energy derives from a cosmological constant such as that introduced by Einstein, it can be shown that <math>w=-1</math>. The equation then reduces to the last equation in the matter-dominated universe section, with <math>\Omega_k </math> set to zero. In that case the initial dark energy density <math>\rho_{de0}</math> is given by<ref>{{cite book|last1=Carroll|first1=Sean|title=Spacetime and Geometry: An Introduction to General Relativity|edition=illustrated|date=2004|publisher=Addison-Wesley|location=San Fraancisco|isbn=978-0-8053-8732-2|page=328|url=https://books.google.com/books?id=1SKFQgAACAAJ}}</ref>

:<math>\rho_{de0} =  \frac{\Lambda c^2}{8 \pi G} </math> and <math> \Omega_{de}=\Omega_{\Lambda}.</math>

If dark energy does not have a constant equation-of-state w, then

:<math>\rho_{de}(a)= \rho_{de0}e^{-3\int\frac{da}{a}\left(1+w(a)\right)},</math>

and to solve this, <math>w(a)</math> must be parametrized, for example if <math>w(a)=w_0+w_a(1-a)</math>, giving

:<math>H^2(z)= H_0^2 \left( \Omega_m a^{-3} + \Omega_{de}a^{-3\left(1+w_0 +w_a \right)}e^{-3w_a(1-a)} \right).</math>

Other ingredients have been formulated recently.<ref>{{cite journal
 |last1=Tawfik |first1=A.
 |last2=Harko |first2=T.
 |date=2012
 |title=Quark-hadron phase transitions in the viscous early universe
 |journal=[[Physical Review D]]
 |volume=85 |issue=8 |pages=084032
 |arxiv=1108.5697
 |bibcode=2012PhRvD..85h4032T
 |doi=10.1103/PhysRevD.85.084032
}}</ref><ref>{{cite journal
 |last1=Tawfik |first1=A.
 |date=2011
 |title=The Hubble parameter in the early universe with viscous QCD matter and finite cosmological constant
 |journal=[[Annalen der Physik]]
 |volume=523 |issue=5 |pages=423–434
 |arxiv=1102.2626
 |bibcode=2011AnP...523..423T
 |doi=10.1002/andp.201100038
}}</ref><ref>
{{cite journal
 |last1=Tawfik |first1=A.
 |last2=Wahba |first2=M.
 |last3=Mansour |first3=H.
 |last4=Harko |first4=T.
 |year=2011
 |title=Viscous quark-gluon plasma in the early universe
 |journal=[[Annalen der Physik]]
 |volume=523 |issue=3 |pages=194–207
 |arxiv=1001.2814
 |bibcode=2011AnP...523..194T
 |doi=10.1002/andp.201000052
}}</ref>

==Units derived from the Hubble constant==
{{More citations needed section|date=March 2014}}

===Hubble time===
The Hubble constant <math>H_0</math> has units of inverse time; the '''Hubble time''' ''t<sub>H</sub>'' is simply defined as the inverse of the Hubble constant,<ref>{{cite book|last1=Hawley|first1=John F.|last2=Holcomb|first2=Katherine A.|title=Foundations of modern cosmology|date=2005|publisher=Oxford Univ. Press|location=Oxford [u.a.]|isbn=978-0-19-853096-1|pages=304|edition=2nd}}</ref> i.e.

:<math>t_H \equiv \frac{1}{H_0} = \frac{1}{67.8\textrm{(km/s)/Mpc}} = 4.55\cdot 10^{17}\textrm{s}  = 14.4 \text{ billion years}.</math>

This is slightly different from the age of the universe which is approximately 13.8 billion years. The Hubble time is the age it would have had if the expansion had been linear, and it is different from the real age of the universe because the expansion is not linear; they are related by a dimensionless factor which depends on the mass-energy content of the universe, which is around 0.96 in the standard [[Lambda-CDM model]].

We currently appear to be approaching a period where the expansion of the universe is exponential due to the increasing dominance of [[vacuum energy]]. In this regime, the Hubble parameter is constant, and the universe grows by a factor [[E (mathematical constant)|''e'']] each Hubble time:

:<math>H \equiv \frac{\dot a}{a} = \textrm{constant} \quad \Longrightarrow \quad a \propto e^{Ht} = e^{\frac{t}{t_H}}</math>

Over long periods of time, the dynamics are complicated by [[general relativity]], [[dark energy]], [[Inflation (cosmology)|inflation]], etc., as explained above.

===Hubble length===
The Hubble length or Hubble distance is a unit of distance in cosmology, defined as <math>cH_0^{-1}</math> — the speed of light multiplied by the Hubble time. It is equivalent to 4,550 million parsecs or 14.4 billion light years. (The numerical value of the Hubble length in light years is, by definition, equal to that of the Hubble time in years.) The Hubble distance would be the distance between the Earth and the galaxies which are ''currently'' receding from us at the speed of light, as can be seen by substituting <math>D=cH_0^{-1}</math> into the equation for Hubble's law, {{nowrap|''v'' {{=}} ''H''<sub>0</sub>''D''}}.

===Hubble volume===
{{main|Hubble volume}}
The Hubble volume is sometimes defined as a volume of the universe with a [[Comoving and proper distances|comoving]] size of <math>cH_0^{-1}.</math> The exact definition varies: it is sometimes defined as the volume of a sphere with radius <math>cH_0^{-1},</math> or alternatively, a cube of side <math>cH_0^{-1}.</math> Some cosmologists even use the term Hubble volume to refer to the volume of the [[observable universe]], although this has a radius approximately three times larger.

== Measured values of the Hubble constant==
Multiple methods have been used to determine the Hubble constant, with results that differ significantly. {{As of|2019}}, the cause of the discrepancy is not understood. In April 2019, astronomers reported further substantial discrepancies, depending on the measurement method used, in determining the Hubble constant, suggesting a realm of physics currently not well understood in explaining the workings of the [[universe]].<ref name="Riess2019" /><ref name="NASA-20190425">{{cite news |author=[[NASA]]/[[Goddard Space Flight Center]] |title=Mystery of the universe's expansion rate widens with new Hubble data |url=https://www.eurekalert.org/pub_releases/2019-04/nsfc-mot042519.php |date=25 April 2019 |work=[[EurekAlert!]] |accessdate=27 April 2019 }}</ref><ref name="SPC-20190425">{{cite news |last=Wall |first=Mike |title=The Universe Is Expanding So Fast We Might Need New Physics to Explain It |url=https://www.space.com/universe-expanding-fast-new-physics.html |date=25 April 2019 |work=[[Space.com]] |accessdate=27 April 2019 }}</ref><ref name="GIZ-20190425">{{cite news |last=Mandelbaum |first=Ryan F. |title=Hubble Measurements Confirm There's Something Weird About How the Universe Is Expanding |url=https://gizmodo.com/hubble-measurements-confirm-theres-something-weird-abou-1834339830 |date=25 April 2019 |work=[[Gizmodo]] |accessdate=26 April 2019 }}</ref><ref name="NAT-20190313">{{cite journal |author=Pietrzyński, G |display-authors=et al. |title=A distance to the Large Magellanic Cloud that is precise to one per cent |date=13 March 2019 |journal=[[Nature (journal)|Nature]] |volume=567 |issue=7747 |pages=200–203 |doi=10.1038/s41586-019-0999-4 |pmid=30867610 |bibcode=2019Natur.567..200P |arxiv=1903.08096 }}</ref> This discrepancy is often called the '''Hubble tension'''.<ref>{{Cite journal|last=Poulin|first=Vivian|last2=Smith|first2=Tristan L.|last3=Karwal|first3=Tanvi|last4=Kamionkowski|first4=Marc|date=2019-06-04|title=Early Dark Energy can Resolve the Hubble Tension|journal=Physical Review Letters|volume=122|issue=22|pages=221301|doi=10.1103/PhysRevLett.122.221301|pmid=31283280|arxiv=1811.04083|bibcode=2019PhRvL.122v1301P}}</ref><ref name="LS-20190826">{{cite news |last=Mann |first=Adam |title=One Number Shows Something Is Fundamentally Wrong with Our Conception of the Universe - This fight has universal implications. |url=https://www.livescience.com/hubble-constant-discrepancy-explained.html |date=26 August 2019 |work=[[Live Science]] |accessdate=26 August 2019 }}</ref>
 
{| class="wikitable sortable" style="width:100%; font-size:96%;"
|-
! Date published
! Hubble constant <br/> (km/s)/Mpc
! Observer
! class="unsortable"| Citation
! class="unsortable" | Remarks / methodology
<!-- Add entries in reverse chronological order (newest at the top). Do not remove older entries unless retracted by publisher. Add notes in remarks column as needed. -->
|-
|2019-10-14
|{{val|74.2|2.7|3.0}}
| STRIDES
|<ref>{{cite arxiv |last1=Shajib |first1=A. J. |last2=Birrer |first2=S. |last3=Treu |first3=T. |last4=Agnello |first4=A. |last5=Buckley-Geer |first5=E. J. |last6=Chan |first6=J. H. H. |last7=Christensen |first7=L. |last8=Lemon |first8=C. |last9=Lin |first9=H. |last10=Millon |first10=M. |last11=Poh |first11=J. |last12=Rusu |first12=C. E. |last13=Sluse |first13=D. |last14=Spiniello |first14=C. |last15=Chen |first15=G. C.-F. |last16=Collett |first16=T. |last17=Courbin |first17=F. |last18=Fassnacht |first18=C. D. |last19=Frieman |first19=J. |last20=Galan |first20=A. |last21=Gilman |first21=D. |last22=More |first22=A. |last23=Anguita |first23=T. |last24=Auger |first24=M. W. |last25=Bonvin |first25=V. |last26=Meylan |first26=G. |last27=Wong |first27=K. C. |last28=Abbott |first28=T. M. C. |last29=Annis |first29=J. |last30=Avila |first30=S. |last31=Bechtol |first31=K. |last32=Brooks |first32=D. |last33=Brout |first33=D. |last34=Burke |first34=D. L. |last35=Rosell |first35=A. Carnero |last36=Kind |first36=M. Carrasco |last37=Carretero |first37=J. |last38=Castander |first38=F. J. |last39=Costanzi |first39=M. |last40=da Costa |first40=L. N. |last41=De Vicente |first41=J. |last42=Desai |first42=S. |last43=Dietrich |first43=J. P. |last44=Doel |first44=P. |last45=Drlica-Wagner |first45=A. |last46=Evrard |first46=A. E. |last47=Finley |first47=D. A. |last48=Flaugher |first48=B. |last49=Fosalba |first49=P. |last50=García-Bellido |first50=J. |last51=Gerdes |first51=D. W. |last52=Gruen |first52=D. |last53=Gruendl |first53=R. A. |last54=Gschwend |first54=J. |last55=Gutierrez |first55=G. |last56=Hollowood |first56=D. L. |last57=Honscheid |first57=K. |last58=Huterer |first58=D. |last59=James |first59=D. J. |last60=Jeltema |first60=T. |last61=Krause |first61=E. |last62=Kuropatkin |first62=N. |last63=Li |first63=T. S. |last64=Lima |first64=M. |last65=MacCrann |first65=N. |last66=Maia |first66=M. A. G. |last67=Marshall |first67=J. L. |last68=Melchior |first68=P. |last69=Miquel |first69=R. |last70=Ogando |first70=R. L. C. |last71=Palmese |first71=A. |last72=Paz-Chinchón |first72=F. |last73=Plazas |first73=A. A. |last74=Romer |first74=A. K. |last75=Roodman |first75=A. |last76=Sako |first76=M. |last77=Sanchez |first77=E. |last78=Santiago |first78=B. |last79=Scarpine |first79=V. |last80=Schubnell |first80=M. |last81=Scolnic |first81=D. |last82=Serrano |first82=S. |last83=Sevilla-Noarbe |first83=I. |last84=Smith |first84=M. |last85=Soares-Santos |first85=M. |last86=Suchyta |first86=E. |last87=Tarle |first87=G. |last88=Thomas |first88=D. |last89=Walker |first89=A. R. |last90=Zhang |first90=Y. |title=STRIDES: A 3.9 per cent measurement of the Hubble constant from the strongly lensed system DES J0408-5354 |date=14 October 2019 |eprint=1910.06306 |class=astro-ph.CO }}</ref>
| Modelling the mass distribution & time delay of the lensed quasar DES J0408-5354.
|-
|2019-09-12
|{{val|76.8|2.6}}
| SHARP/H0LiCOW
|<ref>{{cite arxiv |last1=Chen |first1=G.C.-F. |last2=Fassnacht |first2=C.D. |last3=Suyu |first3=S.H. |last4=Rusu |first4=C.E. |last5=Chan |first5=J.H.H. |last6=Wong |first6=K.C. |last7=Auger |first7=M.W. |last8=Hilbert |first8=S. |last9=Bonvin |first9=V. |last10=Birrer |first10=S. |last11=Millon |first11=M. |last12=Koopmans |first12=L.V.E. |last13=Lagattuta |first13=D.J. |last14=McKean |first14=J.P. |last15=Vegetti |first15=S. |last16=Courbin |first16=F. |last17=Ding |first17=X. |last18=Halkola |first18=A. |last19=Jee |first19=I. |last20=Shajib |first20=A.J. |last21=Sluse |first21=D. |last22=Sonnenfeld |first22=A. |last23=Treu |first23=T. 
|title=A SHARP view of H0LiCOW: H0 from three time-delay gravitational lens systems with adaptive optics imaging |date=12 September 2019 |language=en|eprint=1907.02533|class=astro-ph.GA }}</ref>
| Modelling three galactically lensed objects and their lenses using ground-based adaptive optics and the Hubble Space Telescope
|-
|2019-08-15
|{{val|73.5|1.4}}
| M. J. Reid, D. W. Pesce, A. G. Riess
|<ref>{{cite arxiv |last1=Chen |first1=G.C.-F. |last2=Fassnacht |first2=C.D. |last3=Suyu |first3=S.H. |last4=Rusu |first4=C.E. |last5=Chan |first5=J.H.H. |last6=Wong |first6=K.C. |last7=Auger |first7=M.W. |last8=Hilbert |first8=S. |last9=Bonvin |first9=V. |last10=Birrer |first10=S. |last11=Millon |first11=M. |last12=Koopmans |first12=L.V.E. |last13=Lagattuta |first13=D.J. |last14=McKean |first14=J.P. |last15=Vegetti |first15=S. |last16=Courbin |first16=F. |last17=Ding |first17=X. |last18=Halkola |first18=A. |last19=Jee |first19=I. |last20=Shajib |first20=A.J. |last21=Sluse |first21=D. |last22=Sonnenfeld |first22=A. |last23=Treu |first23=T. 
|title=An Improved Distance to NGC 4258 and its Implications for the Hubble Constant |date=15 August 2019 |language=en|eprint=1908.05625|class=astro-ph.GA }}</ref>
| Measuring the distance to [[Messier 106]] using its supermassive black hole, combined with measurements of eclipsing binaries in the Large Magellanic Cloud.
|-
|2019-07-16
|{{val|69.8|1.9}}
|[[Hubble Space Telescope]]
|<ref name="EA-20190716">{{cite news |author=Carnegie Institution of Science |title=New measurement of universe's expansion rate is 'stuck in the middle' - Red giant stars observed by Hubble Space Telescope used to make an entirely new measurement of how fast the universe is expanding |url=https://www.eurekalert.org/pub_releases/2019-07/cifs-nmo071619.php |date=16 July 2019 |work=[[EurekAlert!]] |accessdate=16 July 2019 |author-link=Carnegie Institution of Science }}</ref><ref name="SCI-20190719">{{cite journal |last=Sokol |first=Joshua |title=Debate intensifies over speed of expanding universe |url=https://www.sciencemag.org/news/2019/07/debate-intensifies-over-speed-expanding-universe |date=19 July 2019 |journal=[[Science (journal)|Science]] |doi=10.1126/science.aay8123 |accessdate=20 July 2019 }}</ref><ref name="The Carnegie-Chicago Hubble Program"/>
|Distances to [[Red giant|red giant stars]] are calculated using the [[tip of the red-giant branch]] (TRGB) distance indicator.
|-
|2019-07-10
|{{val|73.3|+1.7|-1.8}}
|[[H0LiCOW]] collaboration
|<ref name="H0LiCOW2019">{{cite arxiv|eprint=1907.04869|author1=Sidney van den Bergh|title=H0LiCOW XIII. A 2.4% measurement of ''H''<sub>0</sub> from lensed quasars: 5.3σ tension between early and late-Universe probes|class=astro-ph.CO|year=2019}}</ref>
|Updated observations of multiply imaged [[quasar]]s, now using six quasars, independent of the cosmic distance ladder and independent of the cosmic microwave background measurements.
|-
|2019-07-08
|{{val|70.3|5.3|5.0}}
|[[LIGO]] and [[Virgo interferometer|Virgo]] detectors
|<ref name="NAT-20190708" />
|Uses radio counterpart of [[GW170817]], combined with earlier [[gravitational wave]] (GW) and [[electromagnetic]] (EM) data.
|-
|2019-03-28
|{{val|68.0|4.2|4.1}}
|[[Fermi Gamma-ray Space Telescope|Fermi-LAT]]
|<ref>{{cite arxiv|eprint=1903.12097v1|first1=Alberto|last1=Domínguez|first2=Radoslaw|last2=Wojtak|displayauthors=1|title=A new measurement of the Hubble constant and matter content of the Universe using extragalactic background light γ-ray attenuation|date=28 March 2019|class=astro-ph.CO}}</ref>
|Gamma ray attenuation due to extragalactic light. Independent of the cosmic distance ladder and the cosmic microwave background.
|-
|-
|2019-03-18
|{{val|74.03|1.42}}
|[[Hubble Space Telescope]]
|<ref name="Riess2019">{{cite journal|arxiv=1903.07603|first1=Adam G.|last1=Riess|first2=Stefano|last2=Casertano|first3=Wenlong|last3=Yuan|first4=Lucas M.|last4=Macri|first5=Dan|last5=Scolnic|title=Large Magellanic Cloud Cepheid Standards Provide a 1% Foundation for the Determination of the Hubble Constant and Stronger Evidence for Physics Beyond LambdaCDM |journal=The Astrophysical Journal|volume=876|issue=1|pages=85|date=18 March 2019|doi=10.3847/1538-4357/ab1422|bibcode=2019ApJ...876...85R}}</ref>
|Precision HST [[Photometry (astronomy)|photometry]] of [[Cepheid variable|Cepheids]] in the [[Large Magellanic Cloud|Large Magellanic Cloud (LMC)]] reduce the uncertainty in the distance to the LMC from 2.5% to 1.3%. The revision increases the tension with [[Cosmic microwave background|CMB]] measurements to the 4.4[[Standard deviation|σ]] level (P=99.999% for Gaussian errors), raising the discrepancy beyond a plausible level of chance. Continuation of a collaboration known as Supernovae, <math>H_0</math>, for the Equation of State of Dark Energy (SHoES).
|-
|2019-02-08
|{{val|67.78|0.91|0.87}}
|[[Joseph Ryan (astrophysicist)|Joseph Ryan]] et al.
|<ref name="Ryan2019">{{Citation|title=Baryon acoustic oscillation, Hubble parameter, and angular size measurement constraints on the Hubble constant, dark energy dynamics, and spatial curvature|journal=Monthly Notices of the Royal Astronomical Society|volume=488|issue=3|pages=3844–3856|first1=Joseph|last1=Ryan|first2=Yun|last2=Chen|first3=Bharat|last3=Ratra |arxiv=1902.03196|date=8 February 2019|doi=10.1093/mnras/stz1966|bibcode=2019MNRAS.tmp.1893R}}</ref>
|[[Quasar]] angular size and [[baryon acoustic oscillations]], assuming a flat LambdaCDM model. Alternative models result in different (generally lower) values for the Hubble constant.
|-
|2018-11-06
|{{val|67.77|1.30}}
|[[Dark Energy Survey|DES]] Collaboration
|<ref name="DES">{{cite journal|arxiv=1811.02376|last1=Macaulay|first1=E|last2=Nichol|first2=R.C|display-authors=1|title=First Cosmological Results using Type Ia Supernovae from the Dark Energy Survey: Measurement of the Hubble Constant|journal=Monthly Notices of the Royal Astronomical Society|volume=486|issue=2|pages=2184–2196|collaboration=DES collaboration|year=2018|doi=10.1093/mnras/stz978}}</ref>
|Supernova measurements using the ''inverse distance ladder'' method based on [[baryon acoustic oscillations]].
|-
|2018-09-05
|{{val|72.5|+2.1|-2.3}}
|[[H0LiCOW]] collaboration
|<ref name="H0LiCOW">{{cite journal|arxiv=1809.01274|last1=Birrer|first1=S|title=H0LiCOW - IX. Cosmographic analysis of the doubly imaged quasar SDSS 1206+4332 and a new measurement of the Hubble constant|journal=Monthly Notices of the Royal Astronomical Society|volume=484|issue=4|pages=4726–4753|last2=Treu|first2=T|last3=Rusu|first3=C. E|last4=Bonvin|first4=V|last5=Fassnacht|first5=C. D|last6=Chan|first6=J. H. H|last7=Agnello|first7=A|last8=Shajib|first8=A. J|last9=Chen|first9=G. C. -F|last10=Auger|first10=M|last11=Courbin|first11=F|last12=Hilbert|first12=S|last13=Sluse|first13=D|last14=Suyu|first14=S. H|last15=Wong|first15=K. C|last16=Marshall|first16=P|last17=Lemaux|first17=B. C|last18=Meylan|first18=G|year=2018|doi=10.1093/mnras/stz200|bibcode=2018arXiv180901274B}}</ref>
|Observations of multiply imaged [[quasar]]s, independent of the cosmic distance ladder and independent of the cosmic microwave background measurements.
|-
|2018-07-18
|{{val|67.66|0.42}}
|[[Planck (spacecraft)|Planck Mission]]
|<ref name="2018planckcosmos">{{cite journal |title=Planck 2018 results. VI. Cosmological parameters |bibcode=2018arXiv180706209P |author1=Planck Collaboration |last2=Aghanim |first2=N. |displayauthors=etal |year=2018 |arxiv=1807.06209 }}</ref>
|Final Planck 2018 results. 
|-
|2018-04-27
|{{val|73.52|1.62}}
|[[Hubble Space Telescope]] and [[Gaia (spacecraft)|Gaia]]
|<ref name="gaiariess2018">{{cite journal |last1=Riess |first1=Adam G. |last2=Casertano |first2=Stefano |last3=Yuan |first3=Wenlong |last4=Macri |first4=Lucas |last5=Bucciarelli |first5=Beatrice |last6=Lattanzi |first6=Mario G. |last7=MacKenty |first7=John W. |last8=Bowers |first8=J. Bradley |last9=Zheng |first9=WeiKang |last10=Filippenko |first10=Alexei V. |last11=Huang |first11=Caroline |last12=Anderson |first12=Richard I. |title=Milky Way Cepheid Standards for Measuring Cosmic Distances and Application to Gaia DR2: Implications for the Hubble Constant |arxiv=1804.10655|journal=The Astrophysical Journal |date=2018 |volume=861 |issue=2 |pages=126 |doi=10.3847/1538-4357/aac82e |language=en |issn=0004-637X|bibcode=2018ApJ...861..126R }}</ref><ref name="guardianhubbleconstant">{{cite news|last1=Devlin|first1=Hannah|title=The answer to life, the universe and everything might be 73. Or 67|url=https://www.theguardian.com/science/2018/may/10/the-answer-to-life-the-universe-and-everything-might-be-73-or-67|accessdate=13 May 2018|work=the Guardian|date=10 May 2018|language=en}}</ref> 
|Additional HST [[Photometry (astronomy)|photometry]] of galactic [[Cepheid variable|Cepheids]] with early Gaia [[parallax]] measurements. The revised value increases tension with [[Cosmic microwave background|CMB]] measurements at the 3.8[[Standard deviation|σ]] level. Continuation of the SHoES collaboration.
|-
|2018-02-22
|{{val|73.45|1.66}}
|Hubble Space Telescope
|<ref>{{Cite journal |last1=Riess |first1=Adam G. |last2=Casertano |first2=Stefano |last3=Yuan |first3=Wenlong |last4=Macri |first4=Lucas |last5=Anderson |first5=Jay |last6=MacKenty |first6=John W.|last7=Bowers |first7=J. Bradley |last8=Clubb |first8=Kelsey I. |last9=Filippenko |first9=Alexei V. |last10=Jones |first10=David O. |last11=Tucker |first11=Brad E. |title=New parallaxes of galactic Cepheids from spatially scanning the Hubble Space Telescope: Implications for the Hubble constant |journal=The Astrophysical Journal |volume=855 |issue=2 |pages=136 |url=http://imgsrc.hubblesite.org/hvi/uploads/science_paper/file_attachment/308/finalpass5.pdf |date=22 February 2018 |access-date=23 February 2018|bibcode=2018ApJ...855..136R |arxiv=1801.01120 |doi=10.3847/1538-4357/aaadb7 }}</ref><ref name="NASA-20180222">{{cite web |last1=Weaver |first1=Donna |last2=Villard |first2=Ray |last3=Hille |first3=Karl |title=Improved Hubble Yardstick Gives Fresh Evidence for New Physics in the Universe |url=https://www.nasa.gov/feature/goddard/2018/improved-hubble-yardstick-gives-fresh-evidence-for-new-physics-in-the-universe |date=22 February 2018 |website=[[NASA]] |accessdate=24 February 2018 }}</ref>
|Parallax measurements of galactic Cepheids for enhanced calibration of the [[Cosmic distance ladder|distance ladder]]; the value suggests a discrepancy with CMB measurements at the 3.7σ level. The uncertainty is expected to be reduced to below 1% with the final release of the Gaia catalog. SHoES collaboration.
|-
|2017-10-16
|{{val|70.0|+12.0|-8.0}}
|The [[LIGO Scientific Collaboration]] and The [[Virgo interferometer|Virgo]] Collaboration
|<ref>{{Cite journal|last=The LIGO Scientific Collaboration and The Virgo Collaboration|last2=The 1M2H Collaboration|last3=The Dark Energy Camera GW-EM Collaboration and the DES Collaboration|last4=The DLT40 Collaboration|last5=The Las Cumbres Observatory Collaboration|last6=The VINROUGE Collaboration|last7=The MASTER Collaboration|date=2017-10-16|title=A gravitational-wave standard siren measurement of the Hubble constant|url=http://www.nature.com/nature/journal/vaap/ncurrent/full/nature24471.html|journal=Nature|language=en|volume=advance online publication|issue=7678|pages=85–88|doi=10.1038/nature24471|pmid=29094696|issn=1476-4687|arxiv=1710.05835|bibcode=2017Natur.551...85A}}</ref>
| Measurement was independent of a cosmic ‘distance ladder'; the [[Gravitational wave|gravitational-wave]] analysis of a binary [[neutron star]] (BNS) merger [[GW170817]] directly estimated the luminosity distance out to cosmological scales. An estimate of fifty similar detections in the next decade may arbitrate tension of other methodologies.<ref>{{cite journal |author-link= |arxiv=1802.03404 |title= Prospects for resolving the Hubble constant tension with standard sirens|journal=Physical Review Letters |volume=122 |issue=6 |pages=061105 |last1= Feeney|first1= Stephen M|last2= Peiris|first2= Hiranya V|last3= Williamson|first3= Andrew R|last4= Nissanke|first4= Samaya M|last5= Mortlock|first5= Daniel J|last6= Alsing|first6= Justin|last7= Scolnic|first7= Dan|year= 2019|bibcode=2019PhRvL.122f1105F |doi=10.1103/PhysRevLett.122.061105 |pmid=30822066 }}</ref> Detection and analysis of a neutron star-black hole merger (NSBH) may provide greater precision than BNS could allow.<ref name="VitaleChen2018">{{cite journal |last1=Vitale |first1=Salvatore |last2=Chen |first2=Hsin-Yu |title=Measuring the Hubble Constant with Neutron Star Black Hole Mergers |journal=Physical Review Letters |date=12 July 2018 |volume=121 |issue=2 |pages=021303 |doi=10.1103/PhysRevLett.121.021303  |pmid=30085719 |arxiv=1804.07337|bibcode=2018PhRvL.121b1303V |hdl=1721.1/117110 }}</ref>
<!-- Add entries in reverse chronological order (newest at the top). Do not remove older entries unless retracted by publisher. Add notes in remarks column as needed. -->
|-
|2016-11-22
|{{val|71.9|+2.4|-3.0}}
|Hubble Space Telescope
|<ref>{{Cite journal |first1=Vivien |last1=Bonvin |first2=Frédéric |last2=Courbin |first3=Sherry H. |last3=Suyu |display-authors=etal |date=2016-11-22 |title=H0LiCOW – V. New COSMOGRAIL time delays of HE 0435−1223: ''H''<sub>0</sub> to 3.8 per cent precision from strong lensing in a flat ΛCDM model |journal=[[MNRAS]] |volume=465 |issue=4 |pages=4914&ndash;4930 |doi=10.1093/mnras/stw3006 |arxiv=1607.01790
|bibcode = 2017MNRAS.465.4914B }}</ref>
| Uses time delays between multiple images of distant variable sources produced by [[strong gravitational lensing]]. Collaboration known as <math>H_0</math> Lenses in COSMOGRAIL's Wellspring (H0LiCOW).
|-
|2016-08-04
|{{val|76.2|+3.4|-2.7}}
| Cosmicflows-3
| <ref name="Cosmicflows-3">{{cite journal |last1=Tully |first1=R. Brent |last2=Courtois |first2=Hélène M. |last3=Sorce |first3=Jenny G. |title=COSMICFLOWS-3 |journal=The Astronomical Journal |date=3 August 2016 |volume=152 |issue=2 |pages=50 |doi=10.3847/0004-6256/152/2/50 |arxiv=1605.01765 }}</ref>
| Comparing redshift to other distance methods, including [[Tully–Fisher relation|Tully–Fisher]], Cepheid variable, and Type Ia supernovae. A restrictive estimate from the data implies a more precise value of {{val|75|2}}.
|-
|2016-07-13
|{{val|67.6|+0.7|-0.6}}
|[[Sloan Digital Sky Survey|SDSS-III Baryon Oscillation Spectroscopic Survey]]
|<ref>{{Cite journal|last=Grieb|first=Jan N.|last2=Sánchez|first2=Ariel G.|last3=Salazar-Albornoz|first3=Salvador|date=2016-07-13|title=The clustering of galaxies in the completed SDSS-III Baryon Oscillation Spectroscopic Survey: Cosmological implications of the Fourier space wedges of the final sample |arxiv=1607.03143|doi=10.1093/mnras/stw3384|journal=Monthly Notices of the Royal Astronomical Society|volume=467|issue=2|page=stw3384
|bibcode = 2017MNRAS.467.2085G }}</ref>
|[[Baryon acoustic oscillations]]. An extended survey (eBOSS) began in 2014 and is expected to run through 2020. The extended survey is designed to explore the time when the universe was transitioning away from the deceleration effects of gravity from 3 to 8 billion years after the Big Bang.<ref name="ebossinfo">{{cite web|title=The Extended Baryon Oscillation Spectroscopic Survey (eBOSS)|url=http://www.sdss.org/surveys/eboss/|website=SDSS|accessdate=13 May 2018}}</ref>
|-
|2016-05-17
|{{val|73.24|1.74}}
|Hubble Space Telescope
|<ref>{{Cite journal|last=Riess|first=Adam G.|last2=Macri|first2=Lucas M.|last3=Hoffmann|first3=Samantha L.|last4=Scolnic|first4=Dan|last5=Casertano|first5=Stefano|last6=Filippenko|first6=Alexei V.|last7=Tucker|first7=Brad E.|last8=Reid|first8=Mark J.|last9=Jones|first9=David O.|date=2016-04-05|title=A 2.4% Determination of the Local Value of the Hubble Constant|arxiv=1604.01424|doi=10.3847/0004-637X/826/1/56|volume=826|issue=1|journal=The Astrophysical Journal|page=56|bibcode = 2016ApJ...826...56R }}</ref>
|[[Type Ia supernova]], the uncertainty is expected to go down by a factor of more than two with upcoming Gaia measurements and other improvements. SHoES collaboration.
|-
| 2015-02
| {{val|67.74|0.46}}
| [[Planck (spacecraft)#2015 data release|Planck Mission]]
| <ref name="planckesa2015">{{cite web |url=http://www.cosmos.esa.int/web/planck/publications |title=Planck Publications: Planck 2015 Results |publisher=European Space Agency |date=February 2015 |accessdate=9 February 2015}}</ref><ref name="nature20141202">{{cite news |url=http://www.nature.com/news/european-probe-shoots-down-dark-matter-claims-1.16462 |title=European probe shoots down dark-matter claims |work=[[Nature (journal)|Nature]] |first1=Ron |last1=Cowen |first2=Davide |last2=Castelvecchi |date=2 December 2014 |accessdate=6 December 2014 |doi=10.1038/nature.2014.16462}}</ref>
| Results from an analysis of ''Planck''{{'s}} full mission were made public on 1 December 2014 at a conference in [[Ferrara]], Italy. A full set of papers detailing the mission results were released in February 2015.
|-
| 2013-10-01
| {{val|74.4|3.0}}
| Cosmicflows-2
| <ref name="cosmicflows2">{{cite journal|last1=Tully|first1=R. Brent|last2=Courtois|first2=Helene M.|last3=Dolphin|first3=Andrew E.|last4=Fisher|first4=J. Richard|last5=Héraudeau|first5=Philippe|last6=Jacobs|first6=Bradley A.|last7=Karachentsev|first7=Igor D.|last8=Makarov|first8=Dmitry|last9=Makarova|first9=Lidia|last10=Mitronova|first10=Sofia|last11=Rizzi|first11=Luca|last12=Shaya|first12=Edward J.|last13=Sorce|first13=Jenny G.|last14=Wu|first14=Po-Feng|title=Cosmicflows-2: The Data|journal=The Astronomical Journal|date=5 September 2013|volume=146|issue=4|pages=86|doi=10.1088/0004-6256/146/4/86|issn=0004-6256|arxiv=1307.7213|bibcode=2013AJ....146...86T}}</ref>
| Comparing redshift to other distance methods, including [[Tully–Fisher relation|Tully–Fisher]], Cepheid variable, and Type Ia supernovae.
|-
| 2013-03-21
| {{val|67.80|0.77}}
| [[Planck (spacecraft)#2013 data release|Planck Mission]]
| <ref name="planck_overview">
{{cite journal
 |last=Bucher |first=P. A. R.
 |author2=''et al.'' ([[Planck Collaboration]])
 |date=2013
 |title=Planck 2013 results. I. Overview of products and scientific Results
 |arxiv=1303.5062
 |doi=10.1051/0004-6361/201321529
 |volume=571
 |journal=Astronomy & Astrophysics
 |page=A1
 |bibcode=2014A&A...571A...1P
}}</ref><ref name="ESA-20130321">
{{cite web
 |date=21 March 2013
 |title=Planck reveals an almost perfect universe
 |url=http://www.esa.int/Our_Activities/Space_Science/Planck/Planck_reveals_an_almost_perfect_Universe
 |publisher=[[European Space Agency|ESA]]
 |accessdate=2013-03-21
}}</ref><ref name="NASA-20130321">
{{cite web
 |title=Planck Mission Brings Universe Into Sharp Focus
 |url=http://www.jpl.nasa.gov/news/news.php?release=2013-109&rn=news.xml&rst=3739
 |date=21 March 2013
 |publisher=[[Jet Propulsion Laboratory|JPL]]
 |accessdate=2013-03-21
}}</ref><ref name="NYT-20130321">
{{cite news
 |last=Overbye |first=D.
 |title=An infant universe, born before we knew
 |url=https://www.nytimes.com/2013/03/22/science/space/planck-satellite-shows-image-of-infant-universe.html
 |date=21 March 2013
 |work=[[New York Times]]
 |accessdate=2013-03-21
}}</ref><ref name="NBC-20130321">
{{cite web
 |last=Boyle |first=A.
 |date=21 March 2013
 |title=Planck probe's cosmic 'baby picture' revises universe's vital statistics
 |url=http://cosmiclog.nbcnews.com/_news/2013/03/21/17397298-planck-probes-cosmic-baby-picture-revises-universes-vital-statistics
 |website=[[NBC News]]
 |accessdate=2013-03-21
}}</ref>
| The [[European Space Agency|ESA]] [[Planck (spacecraft)|Planck Surveyor]] was launched in May 2009. Over a four-year period, it performed a significantly more detailed investigation of cosmic microwave radiation than earlier investigations using [[High-electron-mobility transistor|HEMT]] [[radiometer]]s and [[bolometer]] technology to measure the CMB at a smaller scale than WMAP. On 21 March 2013, the European-led research team behind the Planck cosmology probe released the mission's data including a new CMB all-sky map and their determination of the Hubble constant.
|-
| 2012-12-20
| {{val|69.32|0.80}}
| [[Wilkinson Microwave Anisotropy Probe|WMAP]] (9 years), combined with other measurements.
| <ref>
{{cite journal
 |last1=Bennett |first1=C. L.
 |display-authors=etal
 |date=2013
 |title=Nine-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: Final maps and results
 |journal=[[The Astrophysical Journal Supplement Series]]
 |volume=208 |issue=2 |pages=20
 |arxiv=1212.5225
 |bibcode=2013ApJS..208...20B
 |doi=10.1088/0067-0049/208/2/20
}}</ref>
|
|-
| 2010
| {{val|70.4|+1.3|-1.4}}
| WMAP (7 years), combined with other measurements.
| <ref name="wmap7parameters">
{{cite journal
 |last1=Jarosik |first1=N.
 |display-authors=etal
 |date=2011
 |title=Seven-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: Sky maps, systematic errors, and basic results
 |journal=[[The Astrophysical Journal Supplement Series]]
 |volume=192 |issue=2 |pages=14
 |arxiv=1001.4744
 |bibcode=2011ApJS..192...14J
 |doi=10.1088/0067-0049/192/2/14
}}</ref>
| These values arise from fitting a combination of WMAP and other cosmological data to the simplest version of the ΛCDM model. If the data are fit with more general versions, ''H''<sub>0</sub> tends to be smaller and more uncertain: typically around {{val|67|4|u=(km/s)/Mpc}} although some models allow values near {{val|63|u=(km/s)/Mpc}}.<ref>Results for ''H''<sub>0</sub> and other cosmological parameters obtained by fitting a variety of models to several combinations of WMAP and other data are available at the [[NASA]]'s [http://lambda.gsfc.nasa.gov/product/map/current/parameters.cfm LAMBDA website].</ref>
|-
| 2010
| {{val|71.0|2.5}}
| WMAP only (7 years).
| <ref name="wmap7parameters"/>
|
|-
| 2009-02
| {{val|70.5|1.3}}
| WMAP (5 years), combined with other measurements.
| <ref name="WMAP2009">{{cite journal
 |last=Hinshaw |first=G.
 |author2=et al. (WMAP Collaboration)
 |year=2009
 |title=Five-year Wilkinson Microwave Anisotropy Probe observations: Data processing, sky maps, and basic results
 |journal=[[The Astrophysical Journal Supplement]]
 |volume=180 |issue=2 |pages=225–245
 |arxiv=0803.0732
 |bibcode=2009ApJS..180..225H
 |doi=10.1088/0067-0049/180/2/225
}}</ref>
|
|-
| 2009-02
| {{val|71.9|2.6|-2.7}}
| WMAP only (5 years)
| <ref name="WMAP2009"/>
|
|-
| 2007
| {{val|70.4|1.5|-1.6}}
| WMAP (3 years), combined with other measurements.
| <ref>
{{Cite journal
 |last=Spergel |first=D. N.
 |author2=et al. (WMAP Collaboration)
 |year=2007
 |title=Three-year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Implications for cosmology
 |journal=[[The Astrophysical Journal Supplement Series]]
 |volume=170 |issue=2 |pages=377–408
 |arxiv=astro-ph/0603449
 |bibcode=2007ApJS..170..377S
 |doi=10.1086/513700
}}</ref>
|
|-
| 2006-08
| {{val|76.9|+10.7|-8.7}}
| [[Chandra X-ray Observatory]]
| <ref>
{{cite journal
 |last1=Bonamente |first1=M.
 |last2=Joy |first2=M. K.
 |last3=Laroque |first3=S. J.
 |last4=Carlstrom |first4=J. E.
 |last5=Reese |first5=E. D.
 |last6=Dawson |first6=K. S.
 |date=2006
 |title=Determination of the cosmic distance scale from Sunyaev–Zel'dovich effect and Chandra X‐ray measurements of high‐redshift galaxy clusters
 |journal=[[The Astrophysical Journal]]
 |volume=647 |issue= 1|pages=25
 |arxiv=astro-ph/0512349
 |bibcode=2006ApJ...647...25B
 |doi=10.1086/505291
}}</ref>
| Combined [[Sunyaev–Zel'dovich effect]] and Chandra X-ray observations of [[galaxy cluster]]s. Adjusted uncertainty in table from Planck Collaboration 2013.<ref name="planck2013_parameters">{{cite journal |author1=Planck Collaboration |date=2013 |title=Planck 2013 results. XVI. Cosmological parameters |arxiv=1303.5076 |doi=10.1051/0004-6361/201321591 |volume=571 |journal=Astronomy & Astrophysics |page=A16 |bibcode=2014A&A...571A..16P}}</ref>
|-
| 2001-05
| {{val|72|8}}
| [[Hubble Space Telescope#Key projects|Hubble Space Telescope Key Project]]
| <ref name=Freedman2001>
{{cite journal
 |last=Freedman |first=W. L.
 |display-authors=etal
 |year=2001
 |title=Final results from the Hubble Space Telescope Key Project to measure the Hubble constant
 |journal=[[The Astrophysical Journal]]
 |volume=553 |issue=1 |pages=47–72
 |arxiv=astro-ph/0012376
 |bibcode=2001ApJ...553...47F
 |doi=10.1086/320638
}}</ref>
| This project established the most precise optical determination, consistent with a measurement of ''H''<sub>0</sub> based upon Sunyaev–Zel'dovich effect observations of many galaxy clusters having a similar accuracy.
|-
|before 1996
| 50&ndash;90 (est.)
|
| <ref name="Overbye">
{{cite book
 |last=Overbye |first=D.
 |date=1999
 |chapter=Prologue
 |title=Lonely Hearts of the Cosmos
 |page=1''ff'' |edition=2nd
 |publisher=[[HarperCollins]]
 |isbn=978-0-316-64896-7
}}</ref>
|
|-
| early 1970s
| ~55 (est.)
| Allan Sandage and [[Gustav Andreas Tammann|Gustav Tammann]]
| <ref name="cfa"/>
|
|-
| 1958
| 75 (est.)
| [[Allan Sandage]]
| <ref>
{{cite journal
 |last=Sandage |first=A. R.
 |title=Current problems in the extragalactic distance scale
 |date=1958
 |journal=[[The Astrophysical Journal]]
 |volume=127 |issue=3 |pages=513–526
 |bibcode=1958ApJ...127..513S
 |doi=10.1086/146483
}}</ref>
| This was the first good estimate of ''H''<sub>0</sub>, but it would be decades before a consensus was achieved.
|-
| 1956
| 180
| [[Milton L. Humason|Humason]], [[Nicholas Mayall|Mayall]] and Sandage
|<ref name="cfa">{{cite web
|url=https://www.cfa.harvard.edu/~dfabricant/huchra/hubble
|title=The Hubble Constant
|author=John P. Huchra
|website=Harvard Center for Astrophysics
|date=2008
}}</ref>
|
|-
| 1929
| 500
| [[Edwin Hubble]], [[Hooker telescope]]
|<ref>Edwin Hubble, ''A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae'', Proceedings of the National Academy of Sciences, vol. 15, no. 3, pp. 168-173, March 1929</ref><ref name="cfa"/><ref name="wwu">{{cite web
|url=https://www.wwu.edu/depts/skywise/hubble_constant.html
|title=Hubble's Constant
|website=Skywise Unlimited - Western Washington University
}}</ref>
|
|-
| 1927
| 625
| [[Georges Lemaître]]
|<ref>{{cite journal|first=Georges|last=Lemaître|title=Un Univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extra-galactiques|journal=Annales de la Société Scientifique de Bruxelles|volume=A47|pages=49–59|date=1927|bibcode=1927ASSB...47...49L|language=fr}}</ref>
|First measurement and interpretation as a sign of the [[expansion of the universe]]
|}

[[File:Hubbleconstants color.png|centre|upright=4|thumb|Estimated values of the Hubble constant, 2001-2019. Estimates in black represent calibrated distance ladder measurements, red represents early universe CMB/BAO measurements with ΛCDM parameters while blue are independent measurements.]]

==See also==
* [[Accelerating expansion of the universe]]
* [[Cosmology]]
* [[Dark matter]]
* [[Tests of general relativity]]

==Notes==
{{reflist|30em|refs=

<ref name="riess99">{{cite journal | author=Riess, A. | display-authors=et al. | title=Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant |year=1998 | journal=The Astronomical Journal | volume=116 | pages=1009–1038 | doi=10.1086/300499 | bibcode=1998AJ....116.1009R | issue=3|arxiv = astro-ph/9805201 }}</ref>

<ref name="perlmutter99">{{cite journal | author=Perlmutter, S. | display-authors=etal | title=Measurements of Omega and Lambda from 42 High-Redshift Supernovae |year=1999 | journal=The Astrophysical Journal | volume=517 | issue=2 | pages=565–586 | doi=10.1086/307221 | bibcode=1999ApJ...517..565P|arxiv = astro-ph/9812133 }}</ref>
}}

==References==
* {{Cite book
 |last=Hubble |first=E. P.
 |date=1937
 |title=The Observational Approach to Cosmology
 |publisher=[[Clarendon Press]]
 |lccn=38011865
}}
* {{Cite book
 |last=Kutner |first=M.
 |date=2003
 |title=Astronomy: A Physical Perspective
 |publisher=[[Cambridge University Press]]
 |isbn=978-0-521-52927-3
}}
* {{Cite book
 |last=Liddle |first=A. R.
 |date=2003
 |title=An Introduction to Modern Cosmology
 |edition=2nd
 |publisher=[[John Wiley & Sons]]
 |isbn=978-0-470-84835-7
}}

==Further reading==
* {{Cite journal
 |last1=Freedman | first1=W. L.
 |last2=Madore | first2=B. F.
 |date=2010
 |title=The Hubble Constant
 |journal=[[Annual Review of Astronomy and Astrophysics]]
 |volume=48 |issue= |pages=673–710
 |arxiv=1004.1856
 |bibcode=2010ARA&A..48..673F
 |doi=10.1146/annurev-astro-082708-101829
}}

==External links==
* [http://map.gsfc.nasa.gov/universe/bb_tests_exp.html NASA's WMAP - Big Bang Expansion: the Hubble Constant]
* [http://www.ipac.caltech.edu/H0kp/H0KeyProj.html The Hubble Key Project]
* [http://cas.sdss.org/dr3/en/proj/advanced/hubble/ The Hubble Diagram Project]
* [https://www.forbes.com/sites/startswithabang/2019/05/03/cosmologys-biggest-conundrum-is-a-clue-not-a-controversy/ Coming to terms with different Hubble Constants] ([[Forbes]]; 3 May 2019)
* {{cite web|last=Merrifield|first=Michael|title=Hubble Constant|url=http://www.sixtysymbols.com/videos/hubble.htm|website=Sixty Symbols|publisher=[[Brady Haran]] for the [[University of Nottingham]]|date=2009}}

{{Scientists whose names are used in physical constants}}

{{DEFAULTSORT:Hubble's Law}}
[[Category:Edwin Hubble|Law]]
[[Category:Large-scale structure of the cosmos]]
[[Category:Physical cosmology]]