Hubble's law facts for kids
Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In other words, the farther they are, the faster they are moving away from Earth. The velocity of the galaxies has been determined by their redshift, a shift of the light they emit toward the red end of the visible spectrum.
Hubble's law is considered the first observational basis for the expansion of the universe, and today it serves as one of the pieces of evidence most often cited in support of the Big Bang model. The motion of astronomical objects due solely to this expansion is known as the Hubble flow. It is described by the equation v = H0D, with H0 the constant of proportionality—the Hubble constant—between the "proper distance" D to a galaxy, which can change over time, unlike the comoving distance, and its speed of separation v, i.e. the derivative of proper distance with respect to the cosmological time coordinate. (See Comoving and proper distances#Uses of the proper distance § Notes for discussion of the subtleties of this definition of velocity.)
The Hubble constant is most frequently quoted in (km/s)/Mpc, thus giving the speed in km/s of a galaxy 1 megaparsec (3.09×1019 km) away, and its value is about 70 (km/s)/Mpc. However, crossing out units reveals that H0 is a unit of frequency (SI unit: s−1) and the reciprocal of H0 is known as the Hubble time. The Hubble constant can also be interpreted as the relative rate of expansion. In this form H0 = 7%/Gyr, meaning that at the current rate of expansion it takes a billion years for an unbound structure to grow by 7%.
Although widely attributed to Edwin Hubble, the notion of the universe expanding at a calculable rate was first derived from 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 be expanding, and presenting the expansion speed if that were the case. 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 for the proportionality constant; this constant, when Edwin Hubble confirmed the existence of cosmic expansion and determined a more accurate value for it two years later, came to be known by his name as the Hubble constant. Hubble inferred the recession velocity of the objects from their redshifts, many of which were earlier measured and related to velocity by Vesto Slipher in 1917. Though the Hubble constant H0 is constant at any given moment in time, the Hubble parameter H, of which the Hubble constant is the current value, varies with time, so the term constant is sometimes thought of as somewhat of a misnomer.
Discovery
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 nebulae" 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.
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 scatter (now known to be caused by 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 (km/s)/Mpc (much higher than the currently accepted value due to errors in his distance calibrations).
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. A straight line of positive slope on this diagram is the visual depiction of Hubble's law.
21st century measurements
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More recent measurements from the Planck mission published in 2018 indicate a lower value of 67.66±0.42 (km/s)/Mpc, although, even more recently, in March 2019, a higher value of 74.03±1.42 (km/s)/Mpc has been determined using an improved procedure involving the Hubble Space Telescope. The two measurements disagree at the 4.4σ level, beyond a plausible level of chance. The resolution to this disagreement is an ongoing area of active research.
| Date published | Hubble constant (km/s)/Mpc |
Observer | Citation | Remarks / methodology |
|---|---|---|---|---|
| 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 (Preprint). | |
| 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  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, , 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 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 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 <  < 0.05 Type Ia supernovae used as standard candle distance indicators. The estimate is corrected for the effects of peculiar velocities in the supernova environments, as estimated from the galaxy density field. The result assumes Ωm = 0.3, ΩΛ = 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 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,  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, , 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 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, 10x more calibration methods, 2x more secondary indicators, and 3x 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. |
Related pages
See also
In Spanish: Ley de Hubble-Lemaître para niños
- Accelerating expansion of the universe
- Cosmology
- Dark matter
- List of scientists whose names are used in physical constants
- Tests of general relativity
