Some say the world will end in fire, Some say in ice. From what I've tasted of desire I hold with those who favor fire. But if it had to perish twice, I think I know enough of hate To say that for destruction ice Is also great And would suffice. -Robert FrostEver since the discovery of the radiation glow left over from the initial hot, dense state of the Universe -- the cosmic microwave background -- the Big Bang has proven to be the best description of the early Universe.
(Image credit: Stephen van Vuuren.)
This hot and dense initial state has, for the past 14 billion years, been expanding, cooling, and slowing down. This last bit is often overlooked, but it's incredibly important. Because what the Big Bang model doesn't tell us, on its own, is what the final state of the Universe is.
Because your intuition tells you that, sure, the Universe is expanding now, but gravity is anattractive force. Starting from a hot, dense, expanding Universe, you can easily imagine three different cases for its fate.
(Illustration credit: NASA.)
- Perhaps the Universe begins expanding quickly, but there's a tremendous amount of matter in it! If there's enough matter, perhaps your Universe will expand initially, with all the galaxies moving farther apart for some time, but gravity is dominant enough to halt the expansions, and even reverse it! In this case, the Universe will recollapse on itself, ending in a fiery demise known as the Big Crunch.
- Perhaps the opposite is true; perhaps the Universe begins expanding quickly but there isn't nearly enough matter to halt and reverse the expansion. In this case, the bound structures in our Universe -- galaxies, clusters of galaxies, and everything contained within them -- will all continue to expand away from one another into an infinite abyss of space. Although the expansion rate continues to drop and slow, it never reaches zero, and can never reverse itself. This coasting Universe case is known as either the Big Freeze or the heat death of the Universe; an isolated, icy fate.
- Or, I suppose, you could imagine the Goldilocks case, where putting just one more atomin the Universe would give it enough gravitational mass to stop its expansion and recollapse, but instead the expansion rate asymptotes towards zero, never quite getting there.
Each of these cases assumes that the Universe contains matter and radiation, and the geometry of the Universe is simply determined by their presence, and of course by the laws of general relativity.
What's interesting, astrophysically, is that each of these cases corresponds to a specific spatial curvature of the Universe! What do we mean by spatial curvature, and how would we measure it? Let's give you a conceptual example.
If you had a flat sheet of paper, and drew a triangle on it, any triangle, you would find that the sum of its three angles is always 180 degrees. This is true for the Universe as well; if you summed the angles between any three points in the Universe, if its geometry is flat, those three angles would indeed sum to 180 degrees as well. This is what we expect to happen for a critical Universe.
But if your Universe were positively curved like a globe, your triangle would always have its angles sum up to more than 180 degrees. Try it if you don't believe me! If you put one point at the North pole and the other two somewhere on the equator, it's very easy to see, as each of the base angles are 90 degrees. For the Universe, this corresponds to the case of a recollapsing fate.
And your Universe could also be negatively curved, like the surface of a saddle. In this case, the angles always sum up to less than 180 degrees. And this corresponds to a coasting Universe.
(Image credit: NASA.)
Each of these cases for the Universe would have a different expansion history, so that if we looked at faraway objects (and hence also looked back in time), we could measure just exactly how the Universe has expanded over its lifetime, and hence what its fate was. And the tool for doing this was none other than the Hubble Space Telescope, capable of making incredible, precise measurements farther away than any other instrument.
In the late 1990s, there were two teams -- the High-z Supernova Search and the Supernova Cosmology Project -- that went out and made the crucial
