Measuring the Fate of the UniverseAs the Universe expands, gravity pulls on the Universe, and slows the expansion down over time. As we look to great distances, we are looking back in time. If we can measure how fast the Universe is expanding in the past, and compare it to how fast it is expanding now, we can see the total gravitational effect of all matter in the Universe.
If there is lots of
material, the Universe will be expanding much faster in the past --- it
will have slowed down a lot --- so much so that the Universe will eventually
halt in its expansion, start to contract, and eventually end in the gnaB
giB (that is the Big Bang backwards). In most models of the universe,
this type of Universe curves onto itself (like a sphere), and is finite.
A favourite model amongst theorists is for the Universe to be precariously balanced between being finite and infinite. This balanced Universe is known as a critical universe. Space neither curves away nor onto itself, it is flat, and is, for most theorists infinite. A final possibility is that the Universe has something other than gravity in it which accelerates the Universe over time. It would be a mysterious substance indeed which did this!
We use Einstein's
equations of General Relativity to understand what we see in the Universe.
In addition to assuming his theory is right (it sure seems to be everywhere
we have be able to measure so far), we do have to make a few assumptions.
The most important of these are that the universe is homogenous (that
is, the material in the Universe is, on average, evenly spread through
out the Universe) and isotropic (matter, the expansion, and everything
else is the same in all directions that we look). With these assumptions
we can predict how bright an object will be given its rate of recession
(the simple relation found by Hubble breaks down at large distances).