By the 1990s, we knew an awful lot about the Universe. You can check out what the greatest discoveries were (in my opinion) from the 1910s, 1920s, 1930s, 1940s, 1950s, 1960s, 1970s, and 1980s. By this point, we understood the origin of the Universe as well as we understand it today, knew about dark matter and normal matter, and were trying to figure out what the fate of the Universe was. It started with a bang (of course), with a supremely hot (~10^20 degrees), dense, and rapidly expanding state (image courtesy of Stephen van Vuuren):
We knew that there were (and are) two things going on, fighting each other for supremacy. On one hand, everything — each single particle — in the young Universe had a tremendous amount of energy. Each particle was flying apart from every other particle as fast as they possibly could. On the other hand, you have gravity, fighting to pull everything back together, and trying to recollapse matter. It’s pretty easy to see that on (relatively) small scales, gravity can win, as gravitational collapse has led to the formation of all the stars, galaxies, and clusters of galaxies we observe today.
But what happens on the largest scale? What happens on the scale of the entire Universe? You’ve got this rapidly expanding Universe, and gravity fighting the expansion. As we saw it, there were three options.
- On one extreme, perhaps gravity can do it. Perhaps it’s so powerful that it can take this expanding Universe and pull it back together, causing it to recollapse and end in a big crunch.
- On the other far extreme, perhaps the expansion is just too powerful. Perhaps gravity can slow the expansion, but never stop it, and certainly never reverse it. In this case, the Universe freezes to death, and simply coasts off into infinity.
- And there’s the “Goldilocks” situation. The first option gets too hot, the second option gets too cold, but maybe the Universe is balanced just right. Perhaps gravity can slow the expansion rate down towards zero, but will never cause it to recollapse. This borderline case, on the cusp between coasting forever and recollapsing, is called a critical Universe.
Illustrated, the three possibilities look like this:
At the same time that scientists were considering these three options, technology had improved to the point where we — for the first time — could seriously consider answering this question. The dawn of the 1990s brought us to the launch of the first high-quality space telescope, Hubble:
We could now see farther and clearer than ever before, and through a brilliant trick, we could finally hope to measure the entire expansion history of the Universe. Learning what the Universe did in the past would allow us to figure out what it was going to do in the future. So how do we figure it out, and what’s the answer?
Imagine you have a 100 Watt light bulb. You know how intrinsically bright it is, right?
You also know that the farther away it is, the dimmer it appears. In fact, all you have to do is measure its observed brightness and you can figure out how far away it has to be!
Now, there aren’t 100 Watt bulbs in space, but there is something close enough that operates under the same principle: Type Ia supernovae. All you need is a white dwarf star that orbits another, larger star:
The white dwarf starts to steal mass from the larger star. When the white dwarf reaches a certain mass — about 1.4 times the mass of our Sun — the white dwarf becomes unstable, collapses, and then explodes!
This process is common enough that it happens all over the Universe, producing a Type Ia supernova. These are also standard enough that we can use these to measure distances the same way we would measure the distance to a 100 Watt light bulb by measuring its brightness.
By also measuring the speed that the supernova is moving away from us (remember Hubble’s Law and the expanding Universe?), we then know both how far away it is and how fast it’s moving. Combine those two pieces of information for enough supernovae, and you learn the expansion history of the Universe.
So, what was it doing? Was it going to recollapse? Was it going to coast off into infinity? Was it right on the edge?
To everyone’s surprise, it was doing none of those things. In 1998, two separate teams — one led by Saul Perlmutter and the other by Adam Riess — announced that the Universe looked like it was a critical Universe, and then all of a sudden, the expansion rate refused to slow down anymore. Instead of recollapsing, coasting, or riding on the brink of both, the Universe was and is accelerating!
By far, this was the greatest discovery of the 1990s, and one of the most revolutionary discoveries of the entire 20th century. The Universe won’t end with a crunch nor with a coast, but with an expansion rate that continues to push things away from us farther and faster. If we lived another 100 billion years, we would find that — after we merged with Andromeda — every other galaxy in the Universe was gone from our sight, pushed away by this accelerated expansion.
These early 1998 results were, of course, questioned, but they have held up, and subsequent data overwhelmingly supports an accelerating Universe. Enjoy the night sky while we have it, because someday everything beyond our own galaxy will be hopelessly out of reach.