Hubble's law

Hubble's law, also called the Hubble–Lemaître law, tells us that galaxies are moving away from Earth. The farther a galaxy is, the faster it moves away. Scientists learn how fast a galaxy is moving by looking at changes in the color of its light, called redshift.

An analogy for explaining Hubble's law, using raisins in a rising loaf of bread in place of galaxies. If a raisin is twice as far away from a place as another raisin, then the farther raisin would move away from that place twice as quickly.

This idea was first shared by Edwin Hubble in 1929. Others had noticed similar patterns before him. Georges Lemaître studied this in 1927. Hubble used measurements of stars’ brightness made by Henrietta Swan Leavitt to learn distances to galaxies. With earlier work by Vesto Slipher, he confirmed the universe is expanding.

Hubble's law helps us understand that the universe is expanding. It supports the Big Bang model. The law is shown by the equation v = H₀D, where H₀_ is the Hubble constant. This number helps scientists learn about the age of the universe, which is about 14.4 billion years old.

Discovery

Many scientists thought the universe might be growing. They used ideas about space and gravity to understand this.

In 1912, an astronomer named Vesto Slipher saw that faraway objects called "spiral nebulae" were moving away from Earth. Later, in 1927, a scientist named Georges Lemaître used math to show how the universe might grow.

In the 1920s, Edwin Hubble measured how far away these objects were. He found that the farther they were, the faster they moved away. This idea is called Hubble's law.

Main article: Friedmann–Lemaître–Robertson–Walker metric

Main article: Cosmological constant

Interpretation

Hubble's law shows that galaxies are moving away from us. The farther they are, the faster they move. We learn this by looking at how the light from galaxies looks "redder" than it should. This change in color, called redshift, helps us know how quickly galaxies are moving away.

The law can be written as a simple rule: the speed a galaxy moves away (its recessional velocity) is a constant number (Hubble's constant) times the distance to that galaxy. This helps scientists learn about how the universe is growing. Even galaxies very far away move from us faster than the speed of light, because space itself is expanding.

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.

Derivation of the Hubble parameter

Start with the Friedmann equation:

H² ≡ ( ȧ / a )² = 8 π G / 3 ρ − k c² / a² + Λ c² / 3,

where H is the Hubble parameter, a is the scale factor, G is the gravitational constant, k is the normalised spatial curvature of the universe and equal to −1, 0, or 1, and Λ is the cosmological constant.

If the universe is matter-dominated, the mass density of the universe ρ should be taken to include just matter. This means the density changes as the universe expands. We can describe this using a special value called the density parameter. By putting all this information into the Friedmann equation, we get a way to understand how the Hubble parameter changes with time.

When we also consider dark energy, the equation becomes more complex. Dark energy can affect how the universe expands. If dark energy behaves in a simple way, the equation simplifies. But if it behaves more complicatedly, we need more details to describe it. This helps scientists understand the overall expansion of the universe.

Units derived from the Hubble constant

The Hubble constant has units of inverse time. This means we can use it to find a special time called the Hubble time. This time tells us how old the universe would be if it had been expanding at a steady rate. In simple terms, the Hubble time is about 14.4 billion years.

Another important idea from the Hubble constant is the Hubble length. This is a distance we can calculate by multiplying the speed of light by the Hubble time. It works out to be about 14.4 billion light years. This distance marks how far away galaxies are that are moving away from us at the speed of light right now.

Determining the Hubble constant

The Hubble constant, H₀, shows how fast the universe is expanding. We can't measure it directly, but we can figure it out by looking at stars and other objects in space.

Early scientists, like Edwin Hubble, used bright stars called Cepheids to measure distances to faraway galaxies. Later, astronomer Walter Baade discovered that there are two types of Cepheid stars. This changed the earlier estimates of the universe’s size.

Today, there are two main ways to measure the Hubble constant. One looks at the universe as it is now, using bright explosions called Type Ia supernovae. The other looks back to the early universe using light from the time the universe began, called the cosmic microwave background. These methods give slightly different answers, and scientists are still working to understand why.

Measurements of the Hubble constant

The Hubble constant tells us how fast galaxies move away from us. Scientists measure this by watching how the light from galaxies changes, which is called redshift. The farther a galaxy is, the faster it seems to move. This helps us learn about how the universe is growing.

Date published	Hubble constant (km/s)/Mpc	Observer	Remarks / methodology
2026-04-01	73.50±0.81	H0DN Collaboration	The Local Distance Network: A community consensus report
2025-05-27	70.39±1.94	W. Freedman et al	Tip of the Red Giant Branch (TRGB) method (values from J-Region Asymptotic Giant Branch (JAGB) and Cepheids also reported)(JWST and HST data)
2025-01-14	75.7+8.1 −5.5	Pascale et al.	Timing delay of gravitationally lensed images of Supernova H0pe. Independent of cosmic distance ladder or the CMB. JWST data. (2023-05-11 cell and this one are the only 2 values with this method so far)
2024-12-01	72.6±2.0	SH0ES+CCHP JWST	JWST, 3 methods, Cepheids, TRGB, JAGB, 2 groups data
2023-07-19	67.0±3.6	Sneppen et al.	Due to the blackbody spectra of the optical counterpart of neutron-star mergers, these systems provide strongly constraining estimators of cosmic distance.
2023-07-13	68.3±1.5	SPT-3G	CMB TT/TE/EE power spectrum. Less than 1σ discrepancy with Planck.
2023-05-11	66.6+4.1 −3.3	P. L. Kelly et al.	Timing delay of gravitationally lensed images of Supernova Refsdal. Independent of cosmic distance ladder or the CMB.
2022-12-14	67.3+10.0 −9.1	S. Contarini et al.	Statistics of cosmic voids using BOSS DR12 data set.
2022-02-08	73.4+0.99 −1.22	Pantheon+	SN Ia distance ladder (+SH0ES)
2022-06-17	75.4+3.8 −3.7	T. de Jaeger et al.	Use Type II supernovae as standardisable candles to obtain an independent measurement of the Hubble constant—13 SNe II with host-galaxy distances measured from Cepheid variables, the tip of the red giant branch, and geometric distance (NGC 4258).
2021-12-08	73.04±1.04	SH0ES	Cepheids-SN Ia distance ladder (HST+Gaia EDR3+"Pantheon+"). 5σ discrepancy with planck.
2021-09-17	69.8±1.7	W. Freedman	Tip of the red-giant branch (TRGB) distance indicator (HST+Gaia EDR3)
2020-12-16	72.1±2.0	Hubble Space Telescope and Gaia EDR3	Combining earlier work on red giant stars, using the tip of the red-giant branch (TRGB) distance indicator, with parallax measurements of Omega Centauri from Gaia EDR3.
2020-12-15	73.2±1.3	Hubble Space Telescope and Gaia EDR3	Combination of HST photometry and Gaia EDR3 parallaxes for Milky Way Cepheids, reducing the uncertainty in calibration of Cepheid luminosities to 1.0%. Overall uncertainty in the value for H₀ is 1.8%, which is expected to be reduced to 1.3% with a larger sample of type Ia supernovae in galaxies that are known Cepheid hosts. Continuation of a collaboration known as Supernovae, H₀, for the Equation of State of Dark Energy (SHoES).
2020-12-04	73.5±5.3	E. J. Baxter, B. D. Sherwin	Gravitational lensing in the CMB is used to estimate H₀ without referring to the sound horizon scale, providing an alternative method to analyze the Planck data.
2020-11-25	71.8+3.9 −3.3	P. Denzel et al.	Eight quadruply lensed galaxy systems are used to determine H₀ to a precision of 5%, in agreement with both "early" and "late" universe estimates. Independent of distance ladders and the cosmic microwave background.
2020-11-07	67.4±1.0	T. Sedgwick et al.	Derived from 88 0.02 H₀ estimate is corrected for the effects of peculiar velocities in the supernova environments, as estimated from the galaxy density field. The result assumes Ω_m = 0.3, Ω_Λ = 0.7 and a sound horizon of 149.3 Mpc, a value taken from Anderson et al. (2014).
2020-09-29	67.6+4.3 −4.2	S. Mukherjee et al.	Gravitational waves, assuming that the transient ZTF19abanrh found by the Zwicky Transient Facility is the optical counterpart to GW190521. Independent of distance ladders and the cosmic microwave background.
2020-06-18	75.8+5.2 −4.9	T. de Jaeger et al.	Use Type II supernovae as standardisable candles to obtain an independent measurement of the Hubble constant—7 SNe II with host-galaxy distances measured from Cepheid variables or the tip of the red giant branch.
2020-02-26	73.9±3.0	Megamaser Cosmology Project	Geometric distance measurements to megamaser-hosting galaxies. Independent of distance ladders and the cosmic microwave background.
2019-10-14	74.2+2.7 −3.0	STRIDES	Modelling the mass distribution & time delay of the lensed quasar DES J0408-5354.
2019-09-12	76.8±2.6	SHARP/H0LiCOW	Modelling three galactically lensed objects and their lenses using ground-based adaptive optics and the Hubble Space Telescope.
2019-08-20	73.3+1.36 −1.35	K. Dutta et al.	This H 0 {\displaystyle H_{0}} is obtained analysing low-redshift cosmological data within ΛCDM model. The datasets used are type-Ia supernovae, baryon acoustic oscillations, time-delay measurements using strong-lensing, H(z) measurements using cosmic chronometers and growth measurements from large scale structure observations.
2019-08-15	73.5±1.4	M. J. Reid, D. W. Pesce, A. G. Riess	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	69.8±1.9	Hubble Space Telescope	Distances to red giant stars are calculated using the tip of the red-giant branch (TRGB) distance indicator.
2019-07-10	73.3+1.7 −1.8	H0LiCOW collaboration	Updated observations of multiply imaged quasars, now using six quasars, independent of the cosmic distance ladder and independent of the cosmic microwave background measurements.
2019-07-08	70.3+5.3 −5.0	The LIGO Scientific Collaboration and The Virgo Collaboration	Uses radio counterpart of GW170817, combined with earlier gravitational wave (GW) and electromagnetic (EM) data.
2019-03-28	68.0+4.2 −4.1	Fermi-LAT	Gamma ray attenuation due to extragalactic light. Independent of the cosmic distance ladder and the cosmic microwave background.
2019-03-18	74.03±1.42	Hubble Space Telescope	Precision HST photometry of Cepheids in the 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 CMB measurements to the 4.4σ level (P=99.999% for Gaussian errors), raising the discrepancy beyond a plausible level of chance. Continuation of a collaboration known as Supernovae, H₀, for the Equation of State of Dark Energy (SHoES).
2019-02-08	67.78+0.91 −0.87	Joseph Ryan et al.	Quasar angular size and baryon acoustic oscillations, assuming a flat ΛCDM model. Alternative models result in different (generally lower) values for the Hubble constant.
2018-11-06	67.77±1.30	Dark Energy Survey	Supernova measurements using the inverse distance ladder method based on baryon acoustic oscillations.
2018-09-05	72.5+2.1 −2.3	H0LiCOW collaboration	Observations of multiply imaged quasars, independent of the cosmic distance ladder and independent of the cosmic microwave background measurements.
2018-07-18	67.66±0.42	Planck Mission	Final Planck 2018 results.
2018-04-27	73.52±1.62	Hubble Space Telescope and Gaia	Additional HST photometry of galactic Cepheids with early Gaia parallax measurements. The revised value increases tension with CMB measurements at the 3.8σ level. Continuation of the SHoES collaboration.
2018-02-22	73.45±1.66	Hubble Space Telescope	Parallax measurements of galactic Cepheids for enhanced calibration of the 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	70.0+12.0 −8.0	The LIGO Scientific Collaboration and The Virgo Collaboration	Standard siren measurement independent of normal "standard candle" techniques; the 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. Detection and analysis of a neutron star-black hole merger (NSBH) may provide greater precision than BNS could allow.
2016-11-22	71.9+2.4 −3.0	Hubble Space Telescope	Uses time delays between multiple images of distant variable sources produced by strong gravitational lensing. Collaboration known as H₀ Lenses in COSMOGRAIL's Wellspring (H0LiCOW).
2016-08-04	76.2+3.4 −2.7	Cosmicflows-3	Comparing redshift to other distance methods, including Tully–Fisher, Cepheid variable, and Type Ia supernovae. A restrictive estimate from the data implies a more precise value of 75±2.
2016-07-13	67.6+0.7 −0.6	SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS)	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.
2016-05-17	73.24±1.74	Hubble Space Telescope	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	67.74±0.46	Planck Mission	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	74.4±3.0	Cosmicflows-2	Comparing redshift to other distance methods, including Tully–Fisher, Cepheid variable, and Type Ia supernovae.
2013-03-21	67.80±0.77	Planck Mission	The ESA 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 HEMT radiometers 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	69.32±0.80	WMAP (9 years), combined with other measurements
2010	70.4+1.3 −1.4	WMAP (7 years), combined with other measurements	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, H₀ tends to be smaller and more uncertain: typically around 67±4 (km/s)/Mpc although some models allow values near 63 (km/s)/Mpc.
2010	71.0±2.5	WMAP only (7 years).
2009-02	70.5±1.3	WMAP (5 years), combined with other measurements
2009-02	71.9+2.6 −2.7	WMAP only (5 years)
2007	70.4+1.5 −1.6	WMAP (3 years), combined with other measurements
2006-08	76.9+10.7 −8.7	Chandra X-ray Observatory	Combined Sunyaev–Zeldovich effect and Chandra X-ray observations of galaxy clusters. Adjusted uncertainty in table from Planck Collaboration 2013.
2003	72±5	WMAP (First year) only
2001-05	72±8	Hubble Space Telescope Key Project	This project established the most precise optical determination, consistent with a measurement of H₀ based upon Sunyaev–Zel'dovich effect observations of many galaxy clusters having a similar accuracy.
before 1996	50 — 90 (est.)
1994	67±7	Supernova 1a Light Curve Shapes	Determined relationship between luminosity of SN 1a's and their Light Curve Shapes. Riess et al. used this ratio of the light curve of SN 1972E and the Cepheid distance to NGC 5253 to determine the constant.
mid 1970's	100±10	Gérard de Vaucouleurs	De Vaucouleurs believed he had improved the accuracy of Hubble's constant from Sandage's because he used 5x more primary indicators, 10× more calibration methods, 2× more secondary indicators, and 3× as many galaxy data points to derive his 100±10.
early 1970s	55 (est.)	Allan Sandage and Gustav Tammann
1958	75 (est.)	Allan Sandage	This was the first good estimate of H₀, but it would be decades before a consensus was achieved.
1956	180	Humason, Mayall and Sandage
1929	500	Edwin Hubble, Hooker telescope
1927	625	Georges Lemaître	First measurement and interpretation as a sign of the expansion of the universe.