From cosmos to cosmology
For most of history the universe as a whole was the province of myth, not measurement. What changed is one audacious hypothesis that turns out to work: the same physics you test in a lab — gravity, thermodynamics, atomic spectra — governs the entire cosmos. Grant that, and the universe becomes a physics problem you can actually solve. Everything in this track rests on that bet, and the bet keeps paying off.
The working simplification is the cosmological principle: on large enough scales the universe is homogeneous (the same everywhere) and isotropic (the same in every direction). Locally the universe is gloriously lumpy — stars, galaxies, voids — but average over patches bigger than about 300 million light-years and the lumpiness washes out. That symmetry is what makes the mathematics tractable: one function of time can describe the whole thing.
One more gift comes for free: because light travels at finite speed c, to look far away is to look back in time. A galaxy a billion light-years off is seen as it was a billion years ago. The universe is its own history book, and a telescope is a time machine pointed at the past.
Everything is rushing apart
In 1929 Edwin Hubble found the single fact from which modern cosmology unfolds: distant galaxies are moving away from us, and the more distant a galaxy, the faster it recedes. Speed is proportional to distance. This is Hubble's law.
Recession speed grows linearly with distance. The slope H_0 (the Hubble constant) is about 70 km/s per megaparsec.
We measure that speed from redshift: the light of a receding source is stretched to longer (redder) wavelengths, just as a receding siren drops in pitch. The fractional stretch defines the redshift z.
Redshift measures how much a wavelength has been stretched between emission and observation.
Running the film backwards
If the universe is expanding today, then yesterday it was smaller, denser, and — because compressing a gas heats it — hotter. Run the expansion backwards far enough and you reach an epoch that was hot and dense beyond anything we can build. That extrapolation is the Big Bang: not a bang in space, but the hot dense beginning of the expansion of space itself, about 13.8 billion years ago.
The concordance model
Piecing the evidence together — Hubble expansion, the microwave background, the abundance of helium, the growth of galaxies, distant supernovae — gives one strikingly consistent picture, the standard model of cosmology, usually called \LambdaCDM. Its inventory of the present universe is humbling: only about 5% is ordinary matter (everything made of atoms, all the stars and gas), roughly 27% is dark matter, and about 68% is dark energy.
Read that inventory again: 95% of the universe is stuff we cannot see and do not understand. Yet the model that contains it is a triumph of precision — its handful of parameters are pinned down to the percent level by independent datasets that agree. The rest of this track unpacks it: how the expansion works (Guide 2), the hot early universe and its afterglow (Guide 3), the dark 95% (Guide 4), and the deepest unsolved questions at the very frontier (Guide 5).