3.2 Cosmic Expansion
We know that our universe has been expanding since the Big Bang.
Astronomer Edwin Hubble found the evidence for the expansion of the universe. In the 1920s, he measured that other galaxies were moving away from us; the farther they were, the faster they were receding.
Einstein's general theory of relativity described this in an amazing way—the galaxies weren't moving, but rather, space was expanding uniformly and the galaxies were being carried along. Uniform expansion explained why the further galaxies appeared to be moving faster, since they were being carried greater distances.
There is no "center" of the universe. Each galaxy appears to be at the center because from its perspective, all other galaxies are moving away from it.
Because of uniform expansion, we can describe the evolution of the universe as simply a 'scaling-up' with time.
The size of the universe increases with time, with gravity slowing this expansion.
With enough mass in the universe, gravity would eventually slow the expansion to a halt, and then the universe would collapse on itself. Without enough mass in the universe, expansion would continue forever. With just the right amount of mass, expansion would keep slowing and slowing under the force of gravity, and an infinite amount of time after the Big Bang, expansion would stop.
In a sense, if we could determine the rate at which the expansion of the universe was slowing, we could know the ultimate destiny of the universe.
And so, attempts to measure the expansion rate and the "shape" of the universe became significant goals in cosmology.
To study cosmic expansion, we can compare Hubble's law to galactic data.
Hubble showed a linear relationship between a galaxy's distance and its recession speed, given by $$v_0=H_{0}d$$
where $v_0$ is the velocity today.
The general theory of relativity tells us that if we measure galaxy distances and apparent velocities today, we should see a straight line because the expansion of the universe is just a stretching of space.
But we can't measure the velocities today, because when we look at distant galaxies, we're measuring their speeds from long ago.
Let's suppose the universe is slowing down. And we know that when we measure velocities, we're really measuring them from many years ago. So, the data points should show a higher velocity way back when—higher than the straight line—since the universe is slowing down. And so cosmologists have been trying to study the deviation of Hubble's law from a straight line, and measure its slight upward curvature.
However, it is difficult to study this, because the distances of galaxies are very hard to measure. To do so, we can use a standard candle.
Standard candles allow for the measurement of galactic distances.
A standard candle is a type of cosmic body, such as a Type Ia supernova, which has a known luminosity. If a galaxy is near a standard candle, we can calculate the galaxy's distance based on this luminosity.
However, these supernovae don't explode very often—only about once in a hundred years per galaxy. Technology (large CCD cameras) has allowed us to look at thousands and thousands of galaxies at once, enabling us to plot galaxies on the Hubble line without waiting hundreds of years.
Remarkably, it turns out that the velocities of galaxies long ago are lower than the Hubble line—meaning the expansion of the universe is speeding up, not slowing down.
For this discovery, the 2011 Nobel Prize in Physics was awarded to Saul Perlmutter, Brian P. Schmidt, and Adam G. Riess.
The data is only consistent if there is something driving this accelerated expansion—something like Dark Energy.