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 Frost

Ever 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.

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 an **attractive** force. Starting from a hot, dense, expanding Universe, you can easily imagine three different cases for its fate.

- 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 atom*in 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.

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 measurements.

Whether they’re formed by a white dwarf accreting matter until it passes above a critical threshold (above), or by two white dwarfs merging with one another (below), one of the most useful objects in the whole Universe for determining great distances are Type Ia supernovae.

Type Ia supernovae are so useful because their light-curves — how their brightnesses evolve over time — are so well-understood. If you watch a type Ia supernova over a long enough time period, you can determine what the **intrinsic** brightness of this event was.

And because you also observed the **apparent brightness** of the supernova, you can determine how far away it is! Combine that information with the observed redshift (i.e., how fast it’s expanding away from us), and that’s what it takes to determine how the Universe has expanded throughout its history. And as far back as you can accurately measure these supernovae, that’s as far back as you can know the Universe’s expansion history.

So these two teams, using the Hubble Space Telescope, set out to measure these distant supernovae as accurately as possible. In the graph below — from the Supernova Cosmology Project — there are three black lines: the top one corresponds to a “coasting” Universe, the middle one to a “critical” Universe, and the lowest one to a “recollapsing” Universe. So what’s our fate?

The disturbing answer is **none of them**! What both teams found in 1998 was that the expansion rate will *not* approach zero, even an infinite time into the future, but will always remain some **significant positive number**!

In other words, as objects expand ever farther away from us, the speed that they move away from us *increases*, rather than slowing down!

This result, when it first came out, was met with a great deal of skepticism, because it would mean that the Universe is full of not just matter (both baryonic and dark) and radiation, but **a new type of energy intrinsic to spacetime itself**! (This is now known by many names, including dark energy and a cosmological constant.)

But the results have not only held up, but have only grown stronger over time, while alternative explanations — however clever and interesting — have fallen flat.

And when you put together the results of these supernova teams with the other great cosmological observations — that of large-scale-strcture (BAO, above) and the cosmic microwave background (CMB, above) — you find that, in fact, the Universe **is dominated** by this dark energy. Around **70-75%** of the total energy density in the Universe today is given by this dark energy!

What does this mean for the expansion history — and fate — of the Universe?

It means we live in an **accelerating** Universe, one in which the objects which are not gravitationally bound to us *right now* (i.e., not in the local group) will eventually speed away from us and accelerate out of the Universe we can observe.

The most distant galaxies and clusters are already doing this! And it was the supernova data collected by these two teams that allowed us to discover the fate of our Universe.

So when I woke up this morning to read that the leaders of these teams — Saul Perlmutter from the Supernova Cosmology Project and (jointly) Adam Riess and Brian Schmidt from the High-z Supernova Search — were awarded the 2011 Nobel Prize in Physics, I couldn’t have been happier. It’s hard to argue that there’s any discovery in physics over the last 15 years that’s been more profound and deserving of this award.

Congratulations to all involved, including all past and current members of these two teams (not just the leaders), for discovering the fate of the Universe, *scientifically*!

So while our Sun eventually boiling our oceans, becoming a red giant and frying the Earth may be the fate of our planet, for the Universe I suppose Frost had it entirely right after all.

But if it had to perish twice

I think I know enough of hate

That for destruction ice

Is also great

And would suffice.

Congratulations once again, and for a little more info, don’t hesitate to read Sean’s, Steinn’s or Peter’s interesting takes, also, on the 2011 Nobel Prize in Physics!