The Expanding Universe

The Expanding Universe

Split the light from a distant galaxy and you see the same pattern of dark lines that the Sun produces, in the same order and the same spacing, but shifted bodily towards the red. The pattern matching is what makes it evidence rather than a curiosity. Because we know exactly which wavelengths those lines should sit at, we can measure precisely how far they have moved, and from that work out how fast the galaxy is receding. Do it for enough galaxies and the answer is not merely that they are moving. It is that the universe has a history.

Shifted light

Five terms, and the second one is the reason this is evidence at all:

The galaxy coming towards us

The Andromeda galaxy, our nearest large neighbour, is blue-shifted: it is approaching us at about 110 km/s. Does that undermine the claim that the universe is expanding?

• No. Andromeda is close enough that the gravitational attraction between it and our own galaxy outweighs the expansion over that distance. Expansion only dominates on the far larger scales between clusters of galaxies, which is where the red-shift pattern is found
• Yes. A single galaxy moving the wrong way contradicts the pattern, so the theory must be wrong
• No, because the Andromeda measurement is almost certainly a measurement error
• No, because Andromeda is on the opposite side of the universe from us and so appears to move differently

Plot the galaxy

This graph plots a galaxy's distance (across) against its recession speed (up). The two points already marked show that speed rises in direct proportion to distance. Plot the point for a galaxy at distance 3.

Two theories, one observation

Red-shift alone does not settle the question. For decades two models both accounted for an expanding universe, and they made one prediction that differed:

The glow from everywhere

In 1964 two engineers found a faint microwave signal coming uniformly from every direction in the sky, which they could not eliminate however carefully they cleaned their antenna. Why does this cosmic microwave background support the Big Bang rather than the Steady State model?

• It is the leftover radiation from the hot early universe, stretched by billions of years of expansion from intense heat down into faint microwaves. The Big Bang predicts it must exist; a universe with no hot beginning has nothing to produce a uniform glow at all
• It is the combined light of all the distant galaxies, red-shifted so far that it has become microwaves
• It comes from the point where the Big Bang happened, so it shows us where the centre of the universe is
• It is radiation from the earliest stars, which formed shortly after the universe began

We are not at the centre

Everything distant is moving away from us, in every direction we look. It sounds exactly like being at the centre of an explosion, and it is not. The raisin loaf. Picture raisins in dough as it rises. Every raisin gets further from every other raisin, and the ones furthest apart separate fastest, because there is more dough between them to expand. Sit on any raisin and you would see all the others receding from you. None of them is the centre, and there is no raisin the expansion started from. And the galaxies are not flying through space. It is space itself that is stretching, carrying them with it, which is why the light stretches on the way as well. That is also why the expansion has no edge to expand into: there is no outside. An astronomer in a galaxy a billion light years away sees precisely what we do, and would be equally entitled to think themselves at the middle of it.

The view from elsewhere

An astronomer in a distant galaxy carries out the same survey we have: measuring the red-shift of every galaxy they can see. What do they find?

• Exactly what we find. Everything distant receding from them, with the furthest receding fastest, because in an expanding universe every observer sees the same thing and no position is special
• Everything moving towards them, since they are further out than we are and the expansion runs the other way from there
• Everything receding except our galaxy, which would appear to be at the centre from their viewpoint
• A mixture, with roughly half the galaxies approaching and half receding

From observation to origin

Put the reasoning in order: from what is observed to what it implies about the universe's beginning.

• The spectral lines of distant galaxies are shifted towards the red
• So those galaxies are receding from us
• The more distant the galaxy, the greater the shift, so the faster it recedes
• That pattern is what an expansion of space itself produces, seen from anywhere within it
• Running the expansion backwards, everything was once packed into a tiny, hot, dense region
• The microwave background is the leftover heat of that region, which confirms it was hot as well as small

The result nobody expected

By the 1990s the question seemed to be how fast the expansion was slowing down. Gravity pulls everything together, so it should be putting a brake on the expansion, and two teams set out to measure the deceleration by observing very distant exploding stars. They found the opposite. The most distant supernovae were fainter than expected, meaning they were further away than a steadily slowing expansion could account for. The expansion is not slowing at all: it is accelerating. Nothing in the theory predicted this and nothing yet explains it. The name given to whatever is driving it is dark energy, which is a label for an unknown rather than an explanation of it, and it appears to make up the majority of the universe. It is worth knowing that a well-established theory can still have a hole this size in it. The Big Bang model is the best explanation we have, and best is not the same as finished.

What does the evidence support?

Select the THREE statements that are genuinely supported by the observations described in this module.

• The universe is expanding, and has been for a very long time
• The early universe was hot as well as dense
• The rate of expansion is currently increasing rather than slowing
• Our galaxy is close to the centre of the universe, since everything recedes from us
• The universe is expanding into an empty space that lies beyond its edge
• Dark energy has been detected and its nature is now understood

What does each observation show?

Defend the theory

Three challenges from a sceptical classmate.

• "Red-shift just means the galaxies are flying away from us through space, like shrapnel from a bomb." What is the better description?
• "If red-shift is such strong evidence, why did anyone need the microwave background?" What does the CMB add that red-shift cannot?
• "Scientists just believe the Big Bang because it is what they were taught." What single kind of observation would genuinely put the theory in trouble?

Tour summary

The spectral lines of distant galaxies are shifted to longer wavelengths, showing they are receding, and the more distant ones recede _____. That pattern looks the same from every galaxy, which is why it shows an expansion of space rather than putting us at the _____. Red-shift alone shows only that the universe is expanding; the cosmic microwave background is the evidence that it began _____ as well as dense. And distant supernovae show the expansion is currently _____, which nothing in the theory predicted.