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SpaceDifficulty 3-5

Cosmic expansion

Cosmic expansion is the stretching of space that carries distant galaxies away from one another, so that the farther away a galaxy is, the faster it recedes (Hubble's law). Traced back, it implies a hot, dense beginning about 13.8 billion years ago, whose afterglow we still detect. Since 1998 we have known the expansion is speeding up, driven by an unexplained 'dark energy' — and today's best measurements of how fast it is expanding do not agree, an open problem called the Hubble tension.

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Expansion History and the Age of the Universe

If the expansion rate had always been the same, the age would be the Hubble time 1/H₀, about 14 billion years for H₀ = 70 km/s/Mpc. After the big bang gravity slowed the expansion; about nine billion years later it began to speed up. These effects nearly cancel, so WMAP's age, 13.77 billion years to within half a percent, is close to 1/H₀.

T0≈1H0T_0 \approx \dfrac{1}{H_0}

Hubble time: the age the universe would have if it had always expanded at today's rate.

Acceleration was found in 1998 because more than 50 distant type Ia supernovae — explosions of old, compact stars as heavy as the Sun but as small as Earth — looked fainter than expected. Dark energy, about 68% of the universe's contents, is distinct from dark matter, about 27%.

Info: One candidate for dark energy is Einstein's cosmological constant, an energy of empty space. Theory predicts either far too much vacuum energy for stars and galaxies to form, or none at all — the unsolved 'cosmological constant problem'.
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What it is

In the 1910s Vesto Slipher found that the light from spiral galaxies is redshifted — stretched to longer wavelengths — meaning they are moving away from us. In 1927 Georges Lemaître showed that Einstein's equations allow an expanding universe, and in 1929 Edwin Hubble, using redshifts measured by Milton Humason and distances from Cepheid stars, showed that more distant galaxies recede faster.

v=H0×dv = H_0 \times d

Hubble's law: recession velocity v is proportional to distance d; H₀ is the Hubble constant.

Space itself is stretching

Picture raisin bread rising: the dough is space and the raisins are galaxies. As the loaf rises, every raisin moves away from every other, and a raisin twice as far away moves away twice as fast — yet the raisins themselves do not grow and none is the centre. The analogy has limits: a loaf rises into the surrounding oven and has an edge, whereas the universe is not expanding into anything; there is simply more space.

Worked example

Take H₀ = 22 km/s per million light-years (an illustrative value, roughly 72 km/s per megaparsec). A galaxy 100 million light-years away then recedes at 22 × 100 = 2,200 km/s. Reversing the logic, measuring a galaxy's redshift gives its speed, and Hubble's law then gives its distance (our own calculation).

Open questions

About 68% of the universe appears to be dark energy and 27% dark matter, leaving only about 5% as ordinary matter; what dark energy is remains unknown. The expansion rate itself is disputed. Planck's measurements of the microwave background, interpreted with the standard model of cosmology, imply H₀ = 67.4 ± 0.5 km/s/Mpc, while the SH0ES team's Cepheid-and-supernova measurement gives 73.04 ± 1.04 — a 5-sigma difference. A team using red-giant stars finds about 70.4, consistent with the standard model. Whether this 'Hubble tension' signals new physics or hidden measurement errors is not settled, and 2025 DESI results hint — not yet at discovery level — that dark energy may weaken over time.

Common misconception: A common misconception is that the big bang was an explosion at one point in space, with galaxies flying out from a centre. Expansion happens everywhere at once: space itself stretches, every galaxy sees the others receding, and there is no centre.

Where it connects

Cosmic expansion is measured using galaxies (Galaxies) and distance markers studied with large telescopes (Telescopes), and it is described by Einstein's curved, stretching space-time (Space-time).

How we know

  • Galaxy redshifts grow in proportion to distance, exactly as uniform expansion predicts.
  • The cosmic microwave background, first detected in 1964, is light released about 380,000 years after the big bang and stretched by expansion to about 2.7 K today.
  • In 1998 two teams found distant type Ia supernovae fainter than expected: the expansion is accelerating (2011 Nobel Prize in Physics).
  • Combining such measurements, WMAP put the age of the universe at 13.77 billion years, to within half a percent.

Assumptions and limits

  • The Planck value of H₀ is inferred within the standard ΛCDM model, so the Hubble tension tests the model as well as the measurements.
  • Hubble's law in the simple form v = H₀d works for nearby galaxies; over very large distances the expansion history has to be modelled.
  • Galaxies themselves do not expand; it is the space between them that grows, as in the raisin-bread picture.

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Sources and methodology

  • In 1929 Edwin Hubble published 'A relation between distance and radial velocity among extra-galactic nebulae' in the Proceedings of the National Academy of Sciences. (awaiting scientific review)
  • In 1927 Georges Lemaître showed that Einstein's equations support an expanding universe, and in 1929 Edwin Hubble and Milton Humason found that galaxies move away from us faster the farther away they are — a relation now called Hubble's law, or the Hubble–Lemaître law. (awaiting scientific review)
  • Around the 1910s Vesto Slipher used a spectrograph to make the first measurements of how quickly distant spiral galaxies are moving away from us, from the redshift of their light. (awaiting scientific review)
  • The expansion of the universe can be pictured as raisin bread rising: space itself expands and carries the galaxies apart while the matter itself does not expand, so each galaxy's speed of recession is proportional to its distance. (awaiting scientific review)
  • The redshifts of distant galaxies are not exactly Doppler shifts but 'expansion redshifts', caused by the expansion of space itself while the light travels. (awaiting scientific review)
  • Planck's final measurements of the cosmic microwave background, interpreted with the standard ΛCDM model, infer a Hubble constant of 67.4 ± 0.5 km/s per megaparsec. (awaiting scientific review)
  • The SH0ES team's 2022 measurement using Cepheid variables and type Ia supernovae gives a Hubble constant of 73.04 ± 1.04 km/s per megaparsec, a 5-sigma difference from the value predicted by Planck with ΛCDM. (awaiting scientific review)
  • The mismatch between the expansion rate measured in the nearby universe and the rate predicted from Planck's early-universe data — the 'Hubble tension' — is unresolved; it is not yet known whether it requires new physics or reflects measurement errors. (awaiting scientific review)
  • In 2024 Webb observations of Cepheid variables confirmed earlier Hubble measurements, which the SH0ES team says rules out stellar crowding and other measurement errors as the cause of the Hubble tension with very high confidence. (awaiting scientific review)
  • A 2025 Chicago-Carnegie Hubble Program analysis using tip-of-the-red-giant-branch stars found a Hubble constant of 70.39 ± 1.22 (statistical) ± 1.33 (systematic) ± 0.70 (supernova) km/s per megaparsec and reported its results consistent with the standard ΛCDM model without new physics. (awaiting scientific review)
  • In 1998 two research teams found that more than 50 distant type Ia supernovae were fainter than expected, a sign that the expansion of the universe is accelerating; the discovery won the 2011 Nobel Prize in Physics. (awaiting scientific review)
  • After the big bang, gravity slowed the expansion of the universe, but about nine billion years after the universe began the expansion started to speed up. (awaiting scientific review)
  • Whatever is driving the accelerating expansion is called dark energy; its nature is unknown, and it makes up approximately 68.3 to 70 percent of the universe. (awaiting scientific review)
  • The universe is estimated to be made up of about 5 percent ordinary matter, about 27 percent dark matter and about 68 percent dark energy. (awaiting scientific review)
  • The cosmic microwave background was released about 380,000 years after the big bang, when the universe had cooled to about 3,000 K; the expansion of space has since stretched its light to an effective temperature of about 2.7 K, and it was first detected, by accident, by Arno Penzias and Robert Wilson in 1964. (awaiting scientific review)
  • Measurements by NASA's WMAP mission determined the universe to be 13.77 billion years old to within half a percent. (awaiting scientific review)
  • Theoretical calculations of the energy of empty space ('vacuum energy') give either far too much to allow stars and galaxies to form or none at all, a discrepancy with the observed dark energy known as the cosmological constant problem. (awaiting scientific review)
  • In 2025 the DESI survey reported that, combined with other data, its results prefer dark energy that weakens over time at 2.8 to 4.2 sigma, below the 5-sigma threshold for a discovery, while DESI's data alone remain consistent with the standard ΛCDM model. (awaiting scientific review)

Claims marked “awaiting scientific review” cite the sources listed but have not yet been signed off by a scientific reviewer.

Content status: published 1 October 2026.

  • Scientific review: this version has not yet been signed off by a scientific reviewer.
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