Cosmology

What this part of the map covers

Cosmology is the study of the Universe as a whole: how we picture it and how we investigate it.^[1] The part of the Universe we can see, inside our cosmological horizon, is called the observable Universe.^[5] This page covers the evidence that space is expanding, the Big Bang model, the oldest light in the Universe and the questions that are still open.

An expanding Universe

In 1929 the American astronomer Edwin Hubble noticed that distant galaxies appear to be moving away from us, and that the farther away they are, the faster they appear to recede.^{[7, 9]} This link between distance and speed became known as Hubble’s Law.^{[2, 13]}

The evidence is in the colours of galaxy light. As space expands, the light travelling through it is stretched and becomes redder, an effect called cosmological redshift.^[2] The more distant the galaxy, the greater its redshift.^[2] Scientists later understood that galaxies do not recede because they travel through space; the space between them is what expands.^[7]

The idea of an expanding Universe is a key support of the Big Bang theory.^[13] The standard model of cosmology also assumes that, on very large scales, the Universe is much the same everywhere and has no special direction.^[3]

The first moments, in outline

Cosmologists’ current theories describe a sequence of early stages.^[1] In the first few minutes, in an era called nucleosynthesis, protons and neutrons collided to make the first elements: hydrogen, helium and small traces of lithium and beryllium.^[1] About 380,000 years after the Big Bang, the Universe had cooled to around 3,000 kelvin, cool enough for electrons to join atomic nuclei and form atoms; astronomers call this recombination.^{[1, 3]} A haze that remained was mostly cleared by reionisation, finished about one billion years after the Big Bang.^[8]

WMAP’s results, together with other results, support inflation, the idea that the Universe went through a dramatic period of expansion.^[6] What came before inflation, and what drove it, is still unknown.^[1]

The oldest light

The cosmic microwave background (CMB) is radiation left over from the Big Bang that fills the whole Universe.^[3] It is the oldest light we can observe, and the furthest back in time that light lets us explore.^{[1, 3]} Its temperature varies by tiny fractions of a degree, and these ripples mark small differences in density that were present just after the Universe formed: the seeds where galaxies would later grow.^{[3, 6]}

Three space missions have mapped it: NASA’s COBE, launched in 1989, NASA’s WMAP in 2001 and ESA’s Planck in 2009, each in more detail than the last.^[3] Because different conditions in the early Universe would leave different patterns in the CMB, these maps let astronomers work out the numbers that describe the young Universe.^[3]

How old is the Universe?

Astronomers estimate the age of the Universe in two ways: by finding the oldest stars, and by measuring how fast the Universe is expanding and working back to the Big Bang.^[4] WMAP’s measurements gave 13.77 billion years, plus or minus 0.059 billion, to within half a percent.^{[4, 6]} ESA’s Planck spacecraft refined this to 13.8 billion years.^[4] Before the Hubble Space Telescope sharpened the expansion rate, estimates ranged from 9.7 to 19.5 billion years.^[9]

What the Universe is made of

From its census of the Universe, WMAP estimated that ordinary atoms make up only about 5 percent of it, dark matter about 25 percent and dark energy about 70 percent.^[6] ESA’s Planck pages point out that, to reconcile theory with observation, cosmologists added dark matter and dark energy, which lack experimental confirmation.^[3]

Astronomers detect dark matter through the pull of its gravity on the matter we can see, but no one knows exactly what it is.^{[11, 16]} Dark energy is the name for whatever is thought to be accelerating the expansion, and it remains a complete mystery.^{[11, 3]} The acceleration itself was discovered in 1998, when distant supernovae turned out fainter than expected.^[11] Cosmologists suggest the Universe will probably keep expanding forever.^[1]

The Hubble tension

The Hubble constant measures how fast the Universe is expanding today.^[4] Cepheid variable stars are the first rung of the cosmic distance ladder: because we know how bright they really are, comparing that with how bright they look tells us how far away they are.^{[14, 9]} The Hubble telescope still uses them to refine the expansion rate.^[15] Space telescope measurements give roughly 70–76 kilometres per second per megaparsec, while values worked out from the CMB are about 67–68.^[9] This mismatch is called the Hubble tension, and when our source was written it was still unexplained; exotic particles, other theories of gravity and early forms of dark energy have all been proposed.^[9]

The math (optional)

Hubble’s Law says that a galaxy’s speed of recession is the Hubble constant times its distance.^{[9, 2]}

speed = Hubble constant × distance

Common misconceptions

“Galaxies are rushing apart through space.” The Universe appears to expand because the space between galaxies expands, not because the galaxies move through it.^[7]

“We see distant galaxies as they are today.” Their light takes so long to reach us that the more redshifted an object is, the further back in time we see it.^[2]

“Dark matter and dark energy are the same kind of thing.” Dark matter is detected by its gravity and there are ideas about what particles it might be, while dark energy remains a complete mystery.^[11]

Going further

Start with Light & Waves and Gravity, then see Galaxies, Stars and Space Telescopes. The inflation mechanism and the long-term fate of the Universe are left for deeper pages.