The chapters, in order
The big bang theory. Not an explosion into empty space: the stretching of space itself, from a state hotter and denser than anything since. Everything we can see today, every galaxy, was once inside a region smaller than an atom. Scroll to run the clock.
Inflation. In the leading version of the theory, space doubles in size about a hundred times in 10⁻³² seconds, growing by a factor of 10²⁶ or more. It smooths the universe flat and plants tiny quantum ripples that will become every galaxy. When it ends, its energy dumps into a searing soup of particles.
The quark soup freezes. At ten trillion kelvin quarks and gluons roam free. Below it they lock into protons and neutrons, and the universe becomes a plasma of nuclei, electrons, photons and neutrinos, so dense that light cannot travel a hair’s width before scattering.
Ten billion kelvin. Neutrinos stop interacting and fly free; they are still passing through you, 300 per cubic centimetre, too feeble to catch. Over the next few seconds electrons and positrons annihilate, and only a one-in-a-billion surplus of matter is left to make everything that follows.
The first nuclei. Cool enough, at last, for protons and neutrons to stick. In about twenty minutes the universe makes its light elements: three quarters hydrogen, one quarter helium by mass, with traces of deuterium and lithium. Gas clouds that have never been touched by stars still show exactly these proportions.
The fog clears. At 3,000 kelvin electrons settle onto nuclei and the plasma becomes a transparent gas. Light flies free for the first time, carrying a snapshot of the ripples. That light is still travelling; stretched 1,100 times since, it reaches us as the cosmic microwave background, 2.7 kelvin in every direction.
The dark ages end. The glow has cooled into the infrared and the sky is black. Gravity, meanwhile, has been pulling gas into the ripples, drawing a web of filaments across space. Where the strands cross, the gas collapses and the first stars ignite.
Galaxies. Stars gather into galaxies and galaxies into clusters along the web. Between the clusters space keeps stretching: every galaxy recedes from every other, at a speed proportional to the distance between them. This is Hubble’s law, and it holds from any galaxy you choose to stand on.
Today. The expansion rate is about 67 km/s per megaparsec: a galaxy a hundred megaparsecs away recedes at 6,700 km/s. The background glow is 2.7 kelvin. And for the last five billion years the expansion has been speeding up, driven by the dark energy that makes up two thirds of everything.
The long dark. Space doubles every twelve billion years and never stops. Galaxies beyond our local group pass out of reach, their light redshifted to nothing; the microwave background cools below anything we could measure. What remains is a bound island of a few galaxies in an otherwise empty sky.
Where the numbers come from
A flat ΛCDM universe with the Planck 2018 parameters: H₀ = 67.4 km/s per megaparsec, 31.5% matter, 68.5% dark energy, and a photon temperature today of 2.725 K. The age, temperature, scale factor, redshift and expansion rate in the corner are computed from it as you scroll. Before three minutes the clock uses the radiation-era relation between time and temperature, with the energy density stepping across electron–positron annihilation; before a microsecond the temperature is only an extrapolation, and before inflation nobody knows.
The picture is a metaphor in places. The turbulence stands for a plasma that was, in truth, almost perfectly smooth; the ripples at the end of the plasma era are its sound waves; the mottled sky at 380,000 years is drawn with the right character but not the real map; and the galaxies live in two dimensions on a web that tiles. The expansion is real: from the first stars to the far future the view zooms with the true scale factor, so the galaxies recede at the speeds Hubble’s law gives.