By Emily Levesque, University of Washington
There are three models for the universe’s death: a runaway expansion that would rip the universe apart; a slowdown and ultimate reversal that would end with the universe collapsing in a big crunch; or a slow and gradual expansion that would, slowly and gradually, let the universe expand and cool into nothing.
An Omega of One
These three models are often described with something called the density parameter, or omega. An omega smaller than one gives us a universe with a swooping curved geometry, shaped like a bowl or a saddle; a universe like that would expand forever, potentially ripping itself apart. An omega bigger than one would give us a universal geometry similar to a sphere; its internal gravity would eventually slow down and reverse the expansion, giving us our big crunch.
However, an omega of exactly one would give us a universe with a nice, neat, flat geometry. Observations of the cosmic microwave background—the same observations that caught Robert Dicke’s attention and inspired Alan Guth to come up with the theory of inflation—have shown us that the value of omega looks to be a practically perfect one, indicating a flat universe. An omega of one tells us exactly how much matter and energy there is in the whole universe.
Mysterious Energy in the Universe
What that matter and energy looks like is a more complicated question. We know that some fraction of the universe is made of matter that experiences gravitational attraction, but this doesn’t account for all the matter and energy in the universe. The rest, well, we’re not quite sure what it is. In mathematical equations, it’s referred to as the cosmological constant. It’s also sometimes referred to as dark energy.
Despite the name, dark energy is very different from dark matter. Dark matter is a form of matter that exerts a clear gravitational influence, but doesn’t interact with light in any way. Dark energy’s name doesn’t come from how it interacts with light; it comes from how mysterious it is. We think it’s a form of energy that helps to drive, and accelerate, the expansion of the universe, but understanding what it is, how much of it there is, and what it does to the universe is still a puzzle.
By measuring the expansion of the universe and comparing them to predictions for how a flat universe, with an omega of one, should look, scientists converged on a solution. Their observations agreed extremely well with a universe that contained both matter and dark energy. Adding up to an omega of one, about 32% of the universe is composed of matter; 5% of that is normal matter, while the other 27% is the much more abundant dark matter. The remaining value gives us a universe that is 68% dark energy.
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Predicting the Universe’s Death
So, what does this mean for how the universe will end? We’re still not quite sure. Based on what we know now, most astrophysicists agree that the universe will keep expanding forever. This could give us a universe that slowly drifts apart, eventually making it impossible for stars to form. As the stars wink out and the material in the universe spreads further and further apart and cools off, we’re eventually left with a dark, frigid, motionless universe. This scenario is sometimes referred to as heat death or the big freeze.
Another possibility is the big rip, which imagines a steady increase in the universe’s expansion that pulls apart galaxies, stars, planets, and eventually even splits up atoms into elementary particles. Whatever the scenario, our best theories all maintain that the end of the universe is coming in 20 billion to 200 billion years.
The end of the universe may seem impossibly far away, and we certainly aren’t able to wait around and see what happens. This, once again, is why observations and theories are so important.
As we build these theories of how the universe begins and ends, and develop new ways to test them, we keep coming closer to answering our big fundamental questions. Where, exactly, did the universe come from? How did it start, how did it expand so quickly, and how does it look so perfect and uniform today? And, based on what we see around us today, how do we think it will end?
Common Questions about the Ultimate Death of the Universe
The three models of the universe can be described with omega or the density parameter. An omega smaller than one means that the universe has a swooping curved geometry, while an omega higher than one means that the universe’s geometry is similar to a sphere. If omega is one, it means the universe is a flat universe with a neat and flat geometry. These geometries affect our opinions on the universe’s death.
To understand what the universe’s death will look like, we have to observe the existing matter and energy in the universe. Dark matter is simply called that because it affects gravity but doesn’t interact with light, while dark energy is only called that because of its mysterious and unknown nature to scientists. It’s been proposed that dark energy is what is fueling the universe’s expansion.
Though it’s hard to tell right now, there are different scenarios for the universe’s death. One is that the universe will eventually expand so much that even atoms will be split apart into elementary particles. Another, which is what most astrophysicists think is most likely to happen, is known as a heat death or the big freeze. This means that the universe will slowly drift apart, making it impossible for stars to form, and so we’re left with a motionless, rigid universe.
