Almost everything is running away
In the last rung you learned that the spiral nebulae are not clouds inside the Milky Way but separate galaxies, each a city of stars far beyond ours, their distances pinned down one Cepheid at a time. That settled *where* galaxies are. This rung asks a stranger question: what are they all *doing*? When astronomers in the 1920s took the spectra of galaxy after galaxy, a pattern surfaced that nobody had ordered. Almost every galaxy's light was redshifted — its spectral lines all slid bodily toward the red, long-wavelength end of the spectrum.
Recall from the light rung how this is read. A galaxy's spectrum carries the same dark absorption lines — the fingerprints of hydrogen, calcium, and other elements — that we measure in the lab. If those whole patterns appear shifted to longer wavelengths, the source is, in the simplest reading, receding. A redshift is the cosmic cousin of the falling pitch of a siren racing away from you. The shock was not that a few galaxies were redshifted, but that nearly *all* of them were. Whatever was going on, it was happening to the universe as a whole, not to a handful of stray objects.
Hubble's law: farther means faster
Edwin Hubble — the same man who had just proven galaxies external — took the next step around 1929. He had distances to a few dozen galaxies, measured the hard way with Cepheids and other rungs of the [[cosmic-distance-ladder|distance ladder]], and he paired each distance against the galaxy's recession speed read from its redshift. Plotting one against the other, he found something beautifully simple: the farther away a galaxy lies, the faster it is receding, and the relationship is roughly a straight line. Double the distance, and you double the speed. This is [[hubbles-law|Hubble's law]] — arguably the single most important graph in the history of cosmology.
v = H0 x d v recession speed (km/s) d distance (megaparsecs, Mpc) H0 Hubble constant ~ 70 km/s per Mpc 1 Mpc away -> ~70 km/s 2 Mpc away -> ~140 km/s 10 Mpc away -> ~700 km/s
The slope of that line is the [[hubble-constant|Hubble constant]], written H-naught, and today it sits near 70 kilometres per second per megaparsec. A parsec is about 3.26 light-years, and a megaparsec is a million of those — about 3.26 million light-years, a comfortable galaxy-to-galaxy hop. So the constant says: for every megaparsec of distance, a galaxy's recession speed climbs by about 70 km/s. There is a deep honesty to flag here. Hubble's own first value was around 500 — seven times too large — because his Cepheid distances were badly miscalibrated. The *pattern* he found was right; the *number* took decades of careful work to nail down.
It is space itself that stretches
Now comes the idea that everyone gets wrong at first, so go slowly. It is tempting to picture the galaxies as shrapnel flying outward through space from some central blast, with us near the middle since everything runs from us. That picture is wrong, and the giveaway is the very symmetry of the recession. An observer in *any* galaxy would see exactly the same thing — everyone else fleeing, faster with distance. There is no special centre, no edge we are sitting beside. The honest description is that galaxies are not mostly moving *through* space at all; rather, [[expansion-of-space|space itself is expanding]], and the galaxies are carried apart on it like marks on an inflating surface.
The classic picture is a balloon with dots inked on its skin. Inflate it, and every dot moves away from every other dot. From any one dot, the neighbours drift off slowly and the far dots drift off fast — Hubble's law, automatically. Crucially, no dot is the centre; the centre of the expansion is not anywhere *on* the surface at all. Picture the balloon's two-dimensional skin as a stand-in for our three-dimensional space, and you have the right intuition. The dots themselves do not grow — galaxies, held together by their own gravity, keep their size. It is the rubber *between* them that stretches.
This recasts the redshift too. The light from a distant galaxy is not reddened mainly because the galaxy is rushing through space like a fleeing siren — it is reddened because space *itself* stretched while the light was in flight, lengthening the light's wavelength along with it. That is why cosmologists call it [[cosmological-redshift|cosmological redshift]], not a Doppler shift. For nearby galaxies the two descriptions give almost the same answer, so the loose phrase 'recession velocity' is fine as shorthand. But for very distant light, the stretched-space picture is the one that stays honest, and the everyday Doppler intuition quietly breaks down.
The same everywhere: the cosmological principle
Why should the balloon picture apply to the *real* universe and not just to us? Because of a working assumption called the [[cosmological-principle|cosmological principle]]: on large enough scales, the universe looks the same everywhere (it is *homogeneous*) and the same in every direction (it is *isotropic*). No place is special; no direction is special. We do not sit at any centre, because there is no centre to sit at. This is exactly what lets the recession look identical from every galaxy — and it is the reason the balloon analogy, where no dot is privileged, is the right one.
Be precise about what 'the same everywhere' means, because it is plainly false up close. Your room is not like the space outside the window; the Earth is not like the empty gulf beyond the Milky Way; galaxies clump into clusters and the great filaments of the cosmic web, threaded with vast near-empty voids. The principle only claims to hold once you smear the universe out over enormous scales — hundreds of millions of light-years — where lumps and voids average away and one huge box of cosmos looks much like the next. It is an idealisation, well supported by surveys, but an idealisation all the same.
Running the film backward: the Hubble time
Here is the payoff that makes Hubble's law so much more than a curiosity. If everything is flying apart today, then run the film backward and everything was closer together yesterday — and closer still the day before. Keep rewinding and you reach a moment when all of it was crammed together, the instant we will later call the hot beginning. The Hubble constant even hands you a rough timer for it. Since speed equals distance divided by time, the ratio of distance to speed for any galaxy is roughly the same number — about one over H-naught — and that quantity is the [[hubble-time|Hubble time]], a crude estimate of how long the expansion has been running.
Plug in H-naught near 70 and the arithmetic spits out roughly 14 billion years — astonishingly close to the carefully measured age, about 13.8 billion years. That nearness is no accident, but do not oversell it. The Hubble time assumes the expansion rate has been *constant*, which it has not: gravity slowed it for billions of years, and a mysterious push (dark energy, met later in this rung) has been speeding it up since. Those two effects roughly cancel for our universe, which is why the crude estimate lands so near the truth. Treat the Hubble time as a back-of-envelope age, not the final word.
Three things this does not mean
Expansion is so easy to mis-picture that it is worth nailing down what it is *not*, before the next guides build on it. None of these are nitpicks; each is a misconception that trips up almost everyone, and getting them straight is most of what it means to truly understand the expanding universe.
- It is not an explosion in space with a centre. The hot beginning happened everywhere at once; there is no point you could fly to and say 'the bang went off here.' Space did not expand *into* anything — there is no outside.
- It does not stretch you, your ruler, or the galaxies themselves. Anything held together by gravity or by atomic forces simply resists the pull. Only the unbound gulfs between distant galaxies grow; rulers and atoms and stars stay put.
- Faraway galaxies receding faster than light do not break relativity. That apparent speed is space stretching between us, not anything moving *through* space — and the rule that nothing outruns light applies only to motion through space, not to the swelling of space itself.
Hold these three together and a clean picture stands: a centreless universe, the same everywhere on the grandest scale, with space gently swelling and floating the galaxies apart. Rewind that swelling and you are pointed straight at a hot, dense beginning. That beginning, and the faint relic glow it left across the whole sky, is exactly where the next guide picks up.