What's the average density of the universe? One look out at the night sky will confirm that space isn't a dense place — but still, its actual density could affect everything from the way it's shaped to how it will end.
What Einstein Told Us
We used to see the universe as a big empty space that blobs of various sizes floated around in. Sometimes the blobs were small, and sometimes they were large, but no matter what, they had less effect on the universe around them than boats have on water. Water is matter, and can be affected by other matter. Space is not matter, and not affected by anything.
Then Einstein came along, and after many years of work, trotted out the theory of general relativity. This showed us that, while space might not be the equivalent of matter, it can be affected by it. Space, and time, can be pulled out of shape by matter the way a trampoline can be pulled out of shape by a bowling ball. That stretched-out shape affects the other objects in space. We fall towards planets for the same reason one bowling ball would slide towards the other on the trampoline. Matter also affects space itself, and it may yet decide the fate of the universe.
The Critical Density of the Universe
Imagine setting up two incredibly powerful lasers so they are parallel to each other, and shooting off parallel laser beams into the universe. We know that even the most powerful beams disperse and fade away over time, and we know that's it's very possible that the beams will hit an object or be swayed off course by the gravity of a galaxy or a black hole, but let's pretend that we happened to point the beams in the direction that gives them clear skies all the way through to the other side of the universe. How will the twin laser beams move through space?
That depends on the shape of the universe. And the shape of the universe depends on how many objects there are bending it out of shape, and how they are distributed. The overall density of the universe will determine how the universe is shaped, and what will happen to those two beams of light.
If the universe is especially dense, it will be shaped like a globe, and two parallel beams that get sent out will eventually converge on each other like lines of longitude. We will live in spherical space. If the universe is not very dense, then space is shaped a bit like a horse's saddle. This is called hyperbolic space, and in order to construct a model from, say, a piece of paper, you need to flare it a bit around the edges. Two parallel beams of light in hyperbolic space will separate more and more.
If the universe is balanced just between high and low density, space will look like a flat piece of paper, and two parallel beams of light will stay parallel. This density, which produces a flat universe, is called the critical density. It's about three hydrogen atoms per cubic meter.
The Fate of the Universe
What kind of universe we live in matters a great deal, and not just when it comes to beams of light. The density and shape of the universe could determine how it will evolve over time. Two factors come into play when figuring out the critical density of the universe. There is also the Hubble constant, which determines how fast the universe is flying apart. There is the gravitational constant, which goes all the way back to Newton. It determines how much force there is pulling things together. Exactly how dense the universe is - how much matter is in play over what area of space - will decide which of these forces will win out. Will we crunch back together, or fly apart? Or will we, at the critical density, expand more and more slowly, stopping after an infinite amount of time?
The latest results from the Wilkinson Microwave Anisotropy Probe, or WMAP, indicate that we are right on the critical density. This means we live in a flat universe, and that its expansion should, slowly, come to a stop. Unfortunately, there is a spanner in the works, and it is called dark energy. Despite the fact that more and more detailed surveys of the density and shape of the universe indicate that expansion should be slowing, the universe's expansion is speeding up. So while we have an idea of what the universe is shaped like, what it will do to the light that travels through it, and how it should end, we don't quite know why its motion isn't in line with its density.
That makes us feel pretty dense.
Top Image: Forest Wander
Galaxy Image: NASA and European Space Agency