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I would like to think that lifebearing planets possess a similar variety.
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Wat, so it seems like the most basic model of the universe has it expanding into itself? So basically speaking, the universe creates its own space and time is just a function of how quickly it creates that space?
I try not to think of it as the universe creating space -- that provokes a whole nest of difficult questions like "how does it create space?" or "where does that new space come from?". There aren't meaningful answers to those questions, nor does describing it that way make the math any easier.
Instead, I like to think of the universe like a grid. The grid is just an imaginary thing you slap onto the space to mark locations, just like street addresses. Then to a very good approximation, the locations of things in the large scale universe (like clusters of galaxies) are fixed on that grid, since they don't move much through the space. Now the question we're interested in is how does that grid evolve with time? If the grid expands, then the physical distance between two locations gets larger, even though neither location is actually moving.
General relativity (GR) describes how the shape of space (and space-time) is affected by what the space contains -- the matter and energy. Here's where things get a little weird, and possibly even surprising if you've heard some common descriptions about how GR works.
Most people have heard of the rubber sheet or trampoline analogy for gravitation in GR. As it often goes "an empty space is like a flat rubber sheet. Add a big mass to it, and the sheet gets curved." That's sort of true, especially on a local scale, but the surprising bit is that in cosmology, empty space is actually not
flat. We call that a Milne Universe
, which is totally empty and has negative curvature, meaning straight parallel lines will spread apart and the sum of angles in a triangle is less than 180°.
That's probably pretty unintuitive. The one thing about it that is
intuitive is that such a universe will have a constant expansion rate. There isn't anything trying to pull things apart or back together again, so there's no acceleration. If it started out expanding it will stay expanding at the same rate forever.
Now let's suppose we fill the space uniformly with matter. A little bit at first, and then more and more. This will add a gravitating effect, where everything is pulling on everything else and therefore the universe is pulled back in on itself. If the universe started out static (a stationary grid), then it will start to shrink and collapse as a Big Crunch. If it started out expanding, then the expansion rate will slow down. The space also gets less and less curved -- closer and closer to "flat". Eventually we'll reach a point called the "critical density" of the universe, which happens to be just a few hydrogen atoms per cubic meter, and this makes the geometry perfectly flat (straight parallel lines remain parallel, and sum of angles in a triangle is exactly 180°).
A universe filled only with matter and exactly enough of it to make it "flat" is called an Einstein-de Sitter universe
. We don't live in such a universe.
The universe we do
live in has some matter in it (obviously), but it isn't enough to make the space flat. However, we also know that the space is
close to flat. So there must be something else. Part of it is dark matter, which is just like adding more regular matter. But the even bigger part is the dark energy.
The dark energy is kind of like adding a bit of something with antigravity everywhere. It's a property of the space itself which makes it expand more. A universe filled only
with dark energy, and just enough of it to make it flat, is a de Sitter universe
, and it expands exponentially faster.
The universe we live in seems to be somewhere between the de Sitter and Einstein-de Sitter universe type. It is flat with a mixture of matter (regular and dark) and dark energy. It started out expanding after the Big Bang, slowed down at first due to the matter, but now speeds up due to the dark energy, since the effect of matter weakens as it gets diluted while dark energy does not.
Okay, that's probably a lot to digest. What's the take-home message? The point is that in GR, matter and energy don't just cause gravity, but they also affect the shape of space itself -- the shape of the grid I was talking about. It also says that the size of the grid can change, and the way it changes depends on what the space contains. Matter will try to slow it down by pulling things together. Dark energy will try to speed it up by spreading everything apart.
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So trying to ascribe a speed to that expansion is like asking where the "center" of the universe is- the answer is undefined because in a universe which creates its own space, that point no longer exists!
It's more like the problem of comparing velocity with acceleration. The units don't work. There is definitely a "rate" to the expansion of the universe (e.g. it seems to be about 70 km/s/Mpc), but it is not a "speed". It is a "speed per distance". Something twice as far away recedes twice as fast. Because space itself expands, the farther away two things are, the faster they move away, so you can't ascribe a unique speed to the whole thing. Instead you ascribe a speed per distance to the whole thing, but that's incomparable to plain "speed" as in speed of light.
What making the expansion rate faster will do is cause the cosmological horizon to shrink closer to you. Objects may slip past the horizon and lose all further causal contact with you. This is what happened with inflation, when nearby regions of space that had
been able to "talk" to each other were suddenly driven apart so quickly that the causal connection was lost, and now they are no longer in the same observable universe as each other. This is what resolves the Horizon Problem
, which asked how distant parts of the universe could look so remarkably similar, as if they had actually been in contact with each other and established thermal equilibrium.