Cosmology: The Whole Story

Image: NASA, ESA, and S. Beckwith (STScI) and the HUDF Team. The Hubble Ultra Deep Field — nearly 10,000 galaxies crowded into a patch of sky the size of a grain of sand held at arm's length. The faintest red specks are among the most distant things ever seen, shining when the universe was only a few hundred million years old. This one image is a core of cosmology: look far enough, and you are looking back toward the beginning.

Every telescope is a time machine

Light is fast but not infinitely fast, so it always arrives late. Sunlight is eight minutes old; the nearest star's light is four years old; Andromeda's is 2.5 million years old. Push that to the limit and something wonderful happens: the farther out you look, the further back in time you see. The deepest telescope images show galaxies as they were billions of years ago, when the cosmos was an infant. There is a wall to this — the cosmic microwave background, the oldest light there is — and beyond it, the universe was too hot and dense to be transparent. Cosmology is built on this single strange gift: the sky is a history book, and the pages get older as you read outward.

That's the honest spine of this whole section. Some of what follows we can see directly — the afterglow of the Big Bang, galaxies flying apart, the light of the first stars. Some of it we can only infer from its gravity, and we've given those pieces placeholder names — dark matter and dark energy — that are really admissions of ignorance. This guide keeps the two apart: here is what we know, and here is exactly how we know it.

The universe is mostly stuff we can't see

Add up everything — every star, planet, nebula, and galaxy on this entire site, every atom of every living thing — and you have accounted for about 5% of the universe. The rest is dark. Roughly a quarter is dark matter, an invisible substance we detect only by its gravity. The largest share, more than two-thirds, is dark energy, a mysterious pressure driving the cosmos to expand ever faster. These figures come from ESA's Planck mission, which measured the ancient light of the universe to remarkable precision.

Diagram by Michael Paycer, using Planck (2018) values. The label on that gold sliver is the entire visible universe — everything astronomy has ever photographed.

Where to go from here

Thirteen point eight billion years, in one paragraph

Here is the whole story in miniature, and every chapter has its own page. About 13.8 billion years ago, the universe began in an unimaginably hot, dense state and has been expanding and cooling ever since — the Big Bang. In the first minutes it forged hydrogen and helium; after 380,000 years it cooled enough to become transparent, releasing the cosmic microwave background we still detect today. Gravity, aided by invisible dark matter, pulled that gas into the first stars and galaxies, which arranged themselves along a vast cosmic web. Around six billion years ago, a mysterious dark energy took over and expansion began to accelerate. Where it goes from here — a slow freeze, a violent rip, or a collapse — is the subject of the section's final page. Everything else on this site is a detail inside that sentence.

Made of star-stuff

Cosmology can feel abstract — dark this, cosmic that — but it is the most personal science there is. The hydrogen in every glass of water is nearly as old as the universe, made in the Big Bang itself. The oxygen you're breathing and the calcium in your bones were forged inside stars that lived and died before the Sun was born, then scattered by supernovae into the cloud that became our Solar System. You are, quite literally, the universe examining itself. That's not poetry stretched over physics — it is the physics. Carl Sagan said it best, and it is the through-line of this entire section.

Big Bang · CMB · Expanding Universe · Dark Matter · Dark Energy · Cosmic Web · Fate of the Universe · Observable Universe · Galaxies · Stars · Astronomy · Glossary