L28. The Expanding Universe

Unit 6 · Galaxies and the Universe

R-report

L28. The Expanding Universe

Nearly every distant galaxy is moving away from us, and the farther away it is, the faster it recedes. That sounds like we are at the centre of a great explosion. We are not — and understanding why is one of the most satisfying ideas in science.

The redshift discovery

In the 1920s astronomers measured the spectra of many galaxies and found something systematic: the absorption lines were nearly always shifted toward the red end. A redshift means the source is moving away. A few nearby galaxies, including Andromeda, show blueshift, but the overwhelming majority are receding. Edwin Hubble then combined those velocities with distance measurements and found a clear relationship: the farther away a galaxy is, the faster it is moving away. Double the distance, double the recession speed. That pattern is the fingerprint of expansion. Consider a rubber band with dots drawn along it. Stretch it and every dot moves away from every other dot, and dots that started farther apart separate faster. Nothing about that requires a special dot. So the observation does not put us at the centre. Observers in any galaxy would see the same thing.

Space itself is stretching

The crucial refinement is that galaxies are not flying outward through space. Space between them is expanding, carrying galaxies along with it. A useful picture is a loaf of raisin bread rising in the oven. As the dough expands, every raisin moves away from every other raisin, and raisins farther apart separate more quickly. The raisins are not travelling through the dough; the dough itself is growing. Two consequences follow. - There is no centre of the expansion and no edge to point at. The expansion happens everywhere at once. - Objects held together by their own gravity do not expand. Your body, Earth, the solar system, and the Milky Way are all bound. Expansion shows up only over the vast distances between galaxy clusters. Running the expansion backwards leads to a moment when everything was extremely hot and dense — the beginning we call the Big Bang. Calculations put it about 13.8 billion years ago. Note what the Big Bang was not: it was not an explosion in space, and it did not happen at a particular location. It was the beginning of the expansion of space itself.

Evidence, and a surprise

Three independent lines of evidence support this picture. - Redshifts: galaxies recede at speeds proportional to distance, exactly as expansion predicts. - The cosmic microwave background: a faint microwave glow coming from every direction, discovered accidentally in 1965. It is the leftover heat of the hot early universe, cooled by expansion to about -270 °C. Its existence had been predicted beforehand. - The proportions of light elements: calculations of nuclear reactions in the first minutes predict about 75% hydrogen and 25% helium, which matches what we measure. Then, in 1998, two teams measuring distant supernovas found something nobody expected. The expansion is not slowing down under gravity, as everyone assumed. It is speeding up. The cause is called dark energy, and like dark matter the name marks a measurement rather than an explanation. It appears to make up around 68% of the total energy content of the universe, and what it actually is remains unknown.

Putting the investigation together

Galaxies recede at speeds proportional to their distance, which is what an expanding space produces. Space itself stretches, carrying galaxies with it, so there is no centre and no edge. Running the expansion backwards gives a hot dense beginning about 13.8 billion years ago.

The 1998 acceleration result is a good final note on how science works. A well-established theory was tested with better data, the data disagreed with the expectation, and rather than being explained away the disagreement opened a new question. Dark energy is currently a name for something measured but not understood — and that is a legitimate state for knowledge to be in.