Hubble Constant and Expansion of the Observable Universe

Scientific summary

The Astronoo article presents the Hubble constant and the expansion of the observable Universe. It traces the observations of Edwin Hubble and Vesto Slipher, who showed that galaxies recede at a speed proportional to their distance, as well as the cosmic distance ladder that makes this measurement possible. The Hubble constant deduced from the Cosmic Microwave Background by the Planck satellite is 67.4 km/s/Mpc, while the distance ladder method (Cepheids and type Ia supernovae, SH0ES team) gives about 73 km/s/Mpc. This persistent discrepancy, called the Hubble tension, now exceeds 5 standard deviations and remains one of the unresolved enigmas of modern cosmology. The article also explains why expansion does not affect gravitationally bound systems, such as the solar system.

What is the Hubble constant and what does the expansion of the Universe mean?

This article is devoted to the Hubble constant, denoted H₀, and to the expansion of the Universe. But what does this mean exactly? The Hubble constant is the rate at which the Universe expands. More precisely, it describes the speed at which a galaxy recedes from us as a function of its distance. For example, a galaxy located one megaparsec (≈3.26 million light-years) away recedes at a speed between 67 and 73 kilometres per second depending on the measurement method used. This relation, discovered by Edwin Hubble in 1929, revealed that the Universe is not static, but expanding. Be careful: this is not a movement of galaxies through space, but a dilation of space-time itself. Expansion applies only to very large distances, between galaxy clusters. Systems bound by gravity, such as the solar system or our Galaxy, do not expand. The exact value of the Hubble constant remains debated, between 67 and 73 km/s/Mpc depending on whether one starts from the primordial Universe or the nearby Universe — a persistent disagreement, the "Hubble tension", which today stimulates research in cosmology.

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The expansion of the universe

An expanding universe

When we look into the past, the universe is smaller, denser and hotter. In other words, all the objects of the Universe come closer to one another when we watch the film backwards, going back in time.

Edwin Hubble's observations

In 1923 Edwin Hubble (1889-1953) used the 250 cm Hooker telescope, the most powerful telescope of the time. While observing a variable star in the Andromeda nebula (M31), he identified a Cepheid — a star whose brightness varies with a period directly related to its actual luminosity, according to the period-luminosity relation discovered by Henrietta Leavitt in 1912. This method allowed him to estimate the distance of M31 at about 900,000 light-years, placing it largely outside our Galaxy. Thus, Hubble put an end to the long debate about the nature of the diffuse objects that would henceforth be called galaxies.

A little later, in 1929, Hubble analysed the radial velocities of galaxies, previously measured by Vesto Slipher (1875-1969), from the redshifts of spectral lines. He first limited himself to galaxies located less than 6 million light-years away and noticed that the velocity-distance relation is approximately linear. Together with Milton Humason (1891-1972), he extended his study to distant galaxies up to 100 million light-years away; the relation remained linear.

Hubble's law and the expansion rate

Hubble then stated his famous Hubble law, "Galaxies recede from one another at a speed proportional to their distance". In other words, the farther a galaxy is from us, the faster it appears to recede. He thus created the concept of the expansion of the Universe. Be careful: galaxies recede, but this is not a real movement of galaxies; it is the entire Universe that swells and gives this apparent velocity to galaxies. It is the space between galaxies that increases; in reality, it is space-time that dilates.

The expansion rate of galaxies relative to one another corresponds to the Hubble constant (H₀) calculated by Edwin Hubble and Georges Lemaître in the 1930s. The inverse of the Hubble constant is called the Hubble time; it corresponds to the time since the Big Bang, and therefore to the age of the universe. The fossil radiation, discovered in 1965, is a true "Rosetta Stone" for cosmologists, because it allows the thermal history of the Universe since the Big Bang to be deciphered.

N.B.: The distance of 900,000 light-years calculated by Hubble in 1923-1925 for M31 greatly underestimated reality (today established at about 2.5 million light-years). The error came from a confusion between two different populations of Cepheids, with distinct period-luminosity relations, corrected only in 1952 by the astronomer Walter Baade — who suddenly doubled the estimated size of the observable Universe.

The cosmic distance ladder: from Cepheids to supernovae

A chain of measurements calibrated on one another

Measuring a recession velocity is relatively simple, thanks to the redshift of the light spectrum. Measuring an intergalactic distance is, however, much more delicate, and this is precisely the main source of uncertainty on the Hubble constant. Astronomers use a cosmic distance ladder, a succession of measurement standards, each calibrated on the previous one: parallax for stars close to our Galaxy, Cepheids for nearby galaxies (their actual luminosity being deduced from their pulsation period), then type Ia supernovae for more distant galaxies, whose maximum brightness is remarkably standardised and serves as a "standard candle".

An alternative method: the tip of the red giant branch

Another method, called the Tip of the Red Giant Branch (TRGB), relies on the fact that red giant stars all reach a very similar maximum luminosity before starting helium fusion in their cores. It offers a calibration independent of Cepheids and, carried out with the James Webb Space Telescope, gives values of the Hubble constant intermediate between those obtained by the two main methods — a serious lead for understanding the origin of the disagreement described below.

What is the value of the Hubble constant?

Hubble Constant and expansion of the Universe — These small temperature fluctuations are the first glimmers of the observable universe seen by the Planck mission (March 2013). Since 2013, European, Canadian and American astrophysicists have refined the composition of the Universe. This false-colour composite image represents the traces of the first moments of creation, about 380,000 years after the Big Bang, 13.8 billion years ago (≈1%). The observable universe is composed of 69.4% dark energy, 25.8% cold dark matter and 4.8% atoms, that is, ordinary matter.
Image source: ESA (new window).

The Cosmic Microwave Background

The map of the Cosmic Microwave Background represents the electromagnetic radiation from the first moments of the observable Universe. These photons, detectable today in the radio wave range, have kept the traces of the large nascent structures that will be carried by inflation.

Measurements of the Hubble constant

The value of the Hubble constant (H₀) is not known with perfect precision, and above all, it differs depending on the method used. In March 2013, a first analysis of Planck satellite data gave a value of 67.8 km/s/Mpc; the final analysis of the mission, published in 2018, refined it to 67.4 ± 0.5 km/s/Mpc, deduced from the Cosmic Microwave Background. For its part, the SH0ES team, by combining Cepheids and type Ia supernovae on the distance ladder, obtains a significantly higher value, about 73.0 ± 1.0 km/s/Mpc (2022 results). A bubble of 1 Mpc therefore swells today, depending on the method used, by 67 to 73 km every second; a bubble of 10 Mpc, ten times more; a bubble of 100 Mpc, a hundred times more…

The Hubble tension: an anomaly that persists

Two methods, two incompatible results

This discrepancy between the two great families of measurements — 67.4 km/s/Mpc from the Cosmic Microwave Background, against about 73 km/s/Mpc from the local distance ladder — has a name: the Hubble tension. It is not a simple margin of error: the discrepancy now exceeds 5 standard deviations (5σ) according to the SH0ES team, which makes it highly improbable that it is a statistical coincidence. Yet both methods rest on extremely well-understood physics — the standard ΛCDM cosmological model for the Cosmic Microwave Background, and the well-established calibration of Cepheids and type Ia supernovae for the distance ladder.

Independent measurements that complicate the picture

Other methods, totally independent of the two previous ones, feed the debate without settling it. Baryon acoustic oscillations (BAO), measured in 2024 by the DESI instrument, give a low value of about 68.4 km/s/Mpc, close to that of Planck. Conversely, recent measurements based on time delays of gravitationally lensed quasars (TDCOSMO collaboration, 2025) give high values, around 70 to 78 km/s/Mpc, rather reinforcing the local measurements camp. In between, the TRGB method carried out with the James Webb Space Telescope gives intermediate values, around 69 to 70 km/s/Mpc, which could indicate a lead for partial resolution — without the scientific community being unanimous today.

New physics, or simple systematic error?

Two main hypotheses confront each other to explain the Hubble tension. The first assumes the existence of a systematic error not yet identified in one of the measurement methods — a bias in the calibration of Cepheids, or a misunderstanding of a parameter of the ΛCDM model. The second, more speculative, envisages a genuine new physics: a form of early dark energy, a modification of gravity, or a physics of neutrinos different from that accepted today. Future missions, such as the Nancy Grace Roman Space Telescope or ESA's Euclid mission, should provide additional data in the coming years to try to decide between these hypotheses.

What is the effect of the expansion of the Universe on the Earth-Sun distance?

Large-scale expansion

The expansion of the universe applies only over very large spaces, that is, between entities such as galaxy clusters or superclusters.

Gravitationally bound systems

In the solar system, objects are "connected" by the gravitational force of the Sun, and the whole can be considered a compact system.

Objects with mass whose velocity relative to one another is less than their escape velocity are part of a gravitational system. As long as the system is gravitationally bound, the space between the objects cannot swell under the effect of an antigravitational force. The entire system is decoupled from the expansion of the observable Universe. The same is true for the Sun inside the Galaxy.

Table: Equivalences between distance units

This table presents the equivalences between the main distance units used in astronomy: the parsec (pc), the light-year (ly), the astronomical unit (au) and the kilometre (km).

Astronomical distance units
Distance units	pc	ly	au	km
pc	1	3.26	206265	3.09x10¹³
ly	0.307	1	63242	9.46x10¹²
au	4.85x10^-6	1.58x10^-5	1	1.50x10⁸
km	3.24x10^-14	1.06x10^-13	6.68x10^-9	1

References

FAQ: Everything you need to know about the Hubble constant and the expansion of the Universe

What is the Hubble constant?

The Hubble constant is the expansion rate of the Universe. It measures the speed at which a galaxy recedes as a function of its distance. Its value lies between 67 and 73 km/s/Mpc depending on the measurement method used.

What is the current value of the Hubble constant?

The Planck satellite, from the Cosmic Microwave Background, measured 67.4 km/s/Mpc, while the SH0ES team, from Cepheids and type Ia supernovae, obtains about 73 km/s/Mpc. This difference, statistically very significant, is called the Hubble tension.

What is the Hubble tension?

The Hubble tension refers to the persistent disagreement, now greater than 5 standard deviations, between the value of H₀ deduced from the primordial Universe (Cosmic Microwave Background) and that measured in the nearby Universe (distance ladder). Its origin remains debated: unidentified systematic error, or new physics beyond the standard model of cosmology.

How are distances between galaxies measured?

Astronomers use a cosmic distance ladder: Cepheids, variable stars whose actual luminosity is deduced from their pulsation period, allow the calibration of type Ia supernovae, used as standard candles for more distant galaxies. The Tip of the Red Giant Branch (TRGB) method offers an independent calibration.

Why does the expansion of the Universe not affect the solar system?

Expansion applies only to very large distances, between galaxy clusters. Systems bound by gravity, such as the solar system or our Galaxy, are decoupled from expansion. Their gravitational cohesion is much stronger than the expansion of space.

What is the Hubble time?

The Hubble time is the inverse of the Hubble constant. It gives an estimate of the age of the Universe, about 13.8 billion years.

What is the Cosmic Microwave Background?

The Cosmic Microwave Background (CMB) is a fossil radiation emitted about 380,000 years after the Big Bang. It constitutes a photograph of the primordial Universe and allows the Hubble constant to be measured.

Is the expansion of the Universe accelerating?

Yes, since 1998, it has been known that the expansion of the Universe is accelerating. This acceleration is attributed to dark energy, a mysterious component that represents about 69% of the Universe.

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