- #36
Drakkith
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MainFragger said:As best as I can tell, you are saying that even before the universe as we knew it existed there was a void, but technically that void is the universe.
Not really. Here's where it's important to understand the implications of what's in the title of the thread. The backwards extrapolation of a model built up by observations. We observe that there are galaxies and stars and other bits of matter in all directions. We also observe that there is a redshift associated with practically all observable galaxies. This redshift, along with other observations, such as the makeup of the particular spectra of different galaxies and their brightness, size, and shape support the idea that almost all of the matter in the universe is receding from us, and the further away it is, the faster it recedes. So we build up a model of the universe in which this expansion is a fundamental part of.
From this it makes sense to extrapolate backwards, in which case all matter was closer together in the past. In other words, if we run the clock backwards, all matter gets closer together.
But, we run into a problem. Our observations can only see out to a certain distance, both in terms of physical distance and in terms of time since light travels at a finite speed. In other words, it takes time for light to travel from one point to another. There is a 'barrier' of sorts that we can't see past using light or other forms of electromagnetic radiation. This barrier is called the surface of last scattering and it represents the point in time in which the universe transitioned from being filled with a hot, ionized plasma to a cooler, un-ionized gas. Because plasma strongly absorbs EM radiation, there is simply no light in existence from before this time, as any light emitted from before this time would have been absorbed. The first EM radiation that was emitted that was able to freely travel through space for large distances without getting absorbed has been heavily redshifted and is now in the microwave region of the spectrum. It's called the Cosmic Microwave Background, or CMB. The CMB is the oldest thing we can observe using EM radiation.
But, this hasn't stopped cosmologists from speculating on what the universe might have been like before this time. If we take our model and continue running the clock backwards we will find that the universe continues to increase in density and temperature as all matter and all the ambient radiation gets closer and closer together. As this matter gets closer together, the gravity between them gets greater too, and at a certain point the density becomes so great that the equations of General Relativity start producing infinities as solutions. These solutions are called singularities in math, and it is from this that the idea of a 'real' singularity comes from. 'Real' singularities probably don't exist, as history has shown that any time you get singularities in math they are either meaningless, or you're math isn't quite right for those conditions.
There are many different ideas about what the very early universe might have been like, and they range from an infinitely long history of being hot and dense, to being utterly devoid of all matter and radiation as we know them, to not existing at all in terms we are familiar with. It's very important to understand that none of these are supported by observations at this time, as we literally cannot make observations beyond the patterns and structure we see in the CMB.
MainFragger said:At some point or other, SOMETHING at some random location in the void heated up really intensley and spewed out our universe (as we know it)...
Sort of. One of the possibilities is that some section of our universe underwent some sort of exotic phase change in a fundamental field, giving rise to matter, radiation, and possibly an extremely quick 'inflation' phase. But that's a very speculative idea at the moment.
MainFragger said:then it cooled down a bit, slowed down a bit, and then at some point sped up again.
Yes, the cooling and slowing down part is correct. The accelerating expansion you've probably heard about is more aptly described as a particular way the expansion is slowing. Basically, it's not slowing down as quickly as we thought. If the rate of deceleration is high enough, the universe eventually stops and collapses back in on itself. If it's 'just right' the expansion continues forever, but it gradually slows, tending to zero as time tends to infinity. If the deceleration is not high enough, then eventually the expansion will actually speed up again and we run into an exponentially accelerating expansion, known as the 'big rip' scenario.
MainFragger said:And All the galaxies are spreading out relative to each other, and as they spread out, the density of the universe gets redistributed. When the universe stops expanding..does that mean the density has equalized across the universe?
No, there can still be small-scale density fluctuations. What matters is the large scale. There needs to be enough matter and radiation for gravitation to counter both the inertia of the expansion and the repulsive effects of dark energy. Small-scale density changes don't matter.