Red-shift and the Expanding Universe

Point a big telescope at a faint smudge of a galaxy, billions of times too far away to ever visit, and split its light into a rainbow. Hidden in that rainbow is a set of sharp dark lines — a barcode printed into the light by the atoms inside the galaxy's stars. We know exactly where those lines should sit, because we make the very same atoms glow in the lab.

And here is the astonishing thing: in the light from almost every distant galaxy, the whole barcode has slid along — every line shifted towards the red (long-wavelength) end of the spectrum. This sliding is called red-shift, and once you understand what causes it you have stumbled onto one of the biggest facts about everything: the entire Universe is expanding. This page follows that clue from a shifted line of colour all the way back to the Big Bang.

First, a sound you already know: the Doppler effect

Stand on a pavement as an ambulance races past. As it speeds towards you the siren sounds high and urgent; the instant it passes and starts speeding away, the pitch suddenly drops to a lower, sadder note. The driver hears no change at all — so what happened?

Sound is a wave. As the ambulance chases after its own sound waves, it squashes the ones in front into a shorter wavelength (higher pitch), and stretches the ones behind into a longer wavelength (lower pitch). A wave source moving away from you has its waves stretched out. That change in wavelength caused by motion is the Doppler effect, and it happens to every kind of wave — including light.

Because light isn't sound — for light, a stretched wavelength doesn't change a pitch, it changes a colour. Our eyes read a wave's wavelength as its colour: short waves look blue/violet, long waves look red. So when a galaxy's light waves are stretched by its motion away from us, the colours slide towards red — exactly the same physics as the ambulance's dropping note, just played out in light instead of sound. A source rushing towards us would instead have its light squashed to shorter wavelengths: a blue-shift.

Red-shift: the light from distant galaxies is stretched

Now apply that to the galaxies. The dark barcode lines in a galaxy's spectrum are shifted towards longer wavelengths — towards the red end. The only sensible explanation is that the galaxy is moving away from us, stretching the light waves on their long journey here, just like the ambulance stretched its siren. The faster it recedes, the more the waves are stretched, and the bigger the red-shift.

We can even put a number on it. If a line that should sit at wavelength \lambda arrives shifted by an extra \Delta\lambda, the amount of red-shift is

z = \frac{\Delta\lambda}{\lambda} \approx \frac{v}{c},

where v is the galaxy's recession speed and c = 3\times10^{8}\ \text{m/s} is the speed of light. A bigger z means a bigger stretch, which means a faster-receding galaxy. Play with the figure below to see the barcode slide.

See it for yourself

The top bar is a spectrum from a source sitting still in the lab, with three dark barcode lines marking where they belong. The bottom bar is the same set of lines arriving from a distant galaxy. Drag the recession speed slider up: the galaxy slides further away, and its lines shift bodily towards the red end. Turn the speed back to zero and the two barcodes line up perfectly — no motion, no red-shift.

Hubble's discovery: further means faster

In the 1920s, Edwin Hubble measured the red-shift of galaxy after galaxy — and also worked out roughly how far away each one was. When he lined the two measurements up, a stunning pattern jumped out:

Almost every galaxy is red-shifted, so almost every galaxy is moving away from us.
The more distant a galaxy is, the bigger its red-shift — so the faster it is receding.

More distant galaxies race away faster, in near-perfect proportion to their distance. That simple rule — recession speed grows with distance — is the fingerprint of a Universe that is expanding.

Light from distant galaxies is red-shifted: shifted to longer wavelengths, so the galaxies are moving away from us.
The red-shift is bigger for more distant galaxies, meaning recession speed increases with distance (v \propto d).
This is evidence that the whole Universe is expanding, and that it began in a hot, dense state — the Big Bang.

It isn't galaxies flying through space — it's space itself stretching

Here is the subtle, mind-bending part. It is tempting to picture the galaxies as shrapnel blown outward from an explosion, hurtling through space away from some central point. That picture is wrong. What is really happening is that space itself is stretching, and the galaxies are carried along with it, drifting apart like raisins in a rising loaf.

Because every bit of space is stretching, an observer in any galaxy sees all the others rushing away from them — the further ones fastest. There is no special centre that everything is fleeing from; the expansion looks the same from everywhere. We are not at the middle of the Universe. There is no middle.

Imagine raisins dotted through a lump of bread dough, and slide it into the oven. As the dough rises, it swells everywhere at once, carrying every raisin further from every other raisin. Sit on any one raisin and look around: nearby raisins drift away slowly, while distant raisins — with more swelling dough between you and them — race away fast. That is exactly Hubble's law, and no raisin is the "centre". A balloon with dots inked on its surface works the same way: blow it up and every dot moves away from every other dot, the far ones fastest.

Rewinding the film: the Big Bang

If the Universe is expanding — everything getting further apart — then run the film backwards and everything crowds closer and closer together. Keep rewinding and the whole Universe collapses to something unimaginably small, hot and dense. That starting point, about 13.8 billion years ago, is the Big Bang: not an explosion in space, but the moment space itself began expanding. Everything — every galaxy, star, atom, and the very space between them — has been spreading out from that hot dense beginning ever since.

So a shifted line of colour in a faint galaxy's spectrum turns out to be a message about the origin of the whole cosmos. Red-shift is the single most important piece of evidence that the Universe had a beginning and has been expanding for billions of years.

Red-shift isn't the only clue. If the early Universe was blindingly hot, it should have been flooded with radiation — and that radiation should still be around today, stretched by 13.8 billion years of expansion into feeble microwaves. In 1965 two engineers found exactly that: a faint hiss of microwave radiation coming from every direction in the sky, the cosmic microwave background (CMB). It is the afterglow of the Big Bang itself — the most ancient light there is — red-shifted all the way from searing visible light down to cold microwaves. Finding it turned the Big Bang from a bold idea into the accepted story of our Universe.

Red-shift means moving AWAY, not towards. Red is the long-wavelength (stretched) end, and you only stretch a wave by receding. A galaxy heading towards us would show a blue-shift (squashed, shorter wavelengths) instead. Red = away, blue = approaching.
It is space stretching, not galaxies flying out from a centre. The galaxies aren't debris rushing away from one special explosion point. Space itself is expanding and carrying them along, so there is no centre to the expansion — it looks the same from every galaxy.
A bigger red-shift does not mean "older" or a different "colour of light". A bigger red-shift means the galaxy is further away and receding faster — a bigger stretch of the same lines, not a change in what the atoms are or how old the light "looks".
The Big Bang was not an explosion in space. There was no empty stage it went off in the middle of. The Big Bang is the moment space and time themselves started expanding — it happened everywhere at once.