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WMAP 5 year full sky
A basic difference between the two, i.e., a finite and an infinite universe, can be arrived at when we imagine what would happen if a photon managed to travel on either one.
A photon that follows a straight path on a finite universe may end up in the same spot as when it first started out, while a photon that would head out in the same manner on an infinite one will never find its way back (as long as it doesn’t turn back, of course!).
Is there a way to determine whether the Universe is infinite or finite? Apparently, its geometry (open, closed, or flat) is largely dependent on the density of matter in it.
That is, if the density of matter is larger than a certain value, known as the critical density, the geometry should be closed. If it is less than the said value, the geometry should be open. And if it is equal to the critical density, then it should be flat.
Measurements made on the cosmic microwave background radiation by WMAP point to a curvature that can be well considered flat. In other words, we most likely have an infinite universe.
Having an infinite universe, regardless of whether it is flat or open, somehow eliminates certain predictions as to how it will meet its own end.
Before WMAP’s discovery, there have been 4 strong predictions on the ultimate fate of the Universe: a Big Crunch, a Big Bounce, a Big Freeze, and a Big Rip. Unless any solid evidence is found that will contradict WMAP’s findings, we will have to do away with the Big Crunch and the Big Bounce models.
These two can only occur if there were possibilities of an eventual collapse. However, a collapse could only happen with a close or finite universe. In an infinite one, there should be a continuous expansion.
The main difference between the two different kinds of infinite universes, open and flat, is in the rate of expansion. In the former, the expansion will be forever and at a constant or even accelerated (by factoring in dark energy) rate. In the latter, the expansion will still be forever but at an asymptotically decelerating rate.
We have some articles in Universe Today that are related to the infinite universe. Here are two of them:
Infinite universe articles brought to you by Physics World, here are the links:
Tired eyes? Let your ears help you learn for a change. Here are some episodes from Astronomy Cast that just might suit your taste:
Source: NASA
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