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The Disappearing Universe
StartsWithABang writes: "If everything began with the Big Bang — from a hot, dense, expanding state — and things have been cooling, spreading out, but slowing down ever since, you might think that means that given enough time (and a powerful enough space ship), we'll eventually be able to reach any other galaxy. But thanks to dark energy, not only is that not the case at all, but most of the galaxies in our Universe are already completely unreachable by us, with more leaving our potential reach all the time. Fascinating, terrifying stuff."
Physics graduate student here, and I'd just like to bring something into context before any ./ readers begin an existential crisis.
We don't *KNOW* anything about the dark matter/energy hypothesis yet. They are not well accepted theories like (classical) gravity or electromagnetism, but rather the best answer to questions we don't have any other way of approaching.
Warning: if you subscribe too heavily to these ideas now, you'll be way, way off base later when science starts finding better answers to the accelerating universe and other open questions. This stuff is great for discussion about philosophy and science fiction, but it is far from well accepted science.
Actually this theory says the number of things that could terrify you is disappearing fast. So instead of being comforted by this fact, they are being terrified of running out of things that could terrify them. Universe does seem to be weirder than what you can imagine, indeed!
sed -e 's/Chuck Norris/Rajnikant/g' joke > fact
It's not really that fascinating or terrifying, though. "Based on our current understanding, we will never be able to reach certain galaxies." Ok, that's cool. We can't even reach another star system within our own galaxy at the moment, so traveling to other galaxies is a bit moot as is. We also know our understanding isn't complete, so it's entirely possible that something we don't know will allow us to travel to those galaxies.
Seriously, this doesn't feel like news. We've been working at the whole science and technology thing for what...ten thousand years or so? I say give it a million more and see where we are then, instead of cranking out sensationalist doom and gloom articles. Of course, all the doom and gloom articles tell you that we're not going to make it another decade, let alone a hundred thousand decades, so if you really feed into such things, then I'd say your outlook on the universe is far more terrifying than the article at hand.
Space appears flat on a global scale, but locally, it is highly curved around massive objects, especially around objects like black holes. Nothing we've observed so far strictly prohibits our universe having some sort of locally nontrivial topology like a wormhole. Keep in mind, also, that our observable universe is what appears globally flat. If cosmic inflation is right (and it's looking like it probably is), the actual extent of the universe could easily be 20-30 orders of magnitude larger than what we see, in which case, the universe could be highly curved on those scales and still appear quite flat to the best ability of our observations.
Seriously. Dark energy is hypothetical.
Heard it all before. Earth is flat, humans flying is impossible, break the sound barrier and you die, yadda, yadda...
If you don't understand the difference between a line of uninformed idiots who kept saying "You can't have a rocket in space because there would be nothing to push against", displaying complete ignorance of Newton's laws, and the limits which are the consequence of well-reasoned scientific models such as C being an absolute limit of material acceleration, then you flat out don't understand the difference between a scientific approach and simply drawing limits out of your butt.
It's a bit more complicated than that. General relativity allows you to pick any reference frame, even one that is bent, stretched or distorted in some other way, and do your calculations in that reference frame.
You could pick a "normal" reference frame that obeys the special theory of relativity: speed of light constant everywhere, nothing can go faster, etcetera. Nothing wrong with that, but this turns out to be impractical: we have to pick some place to consider as the center of the universe (for example some place in our immediate neighborhood), and then find that the rest of the universe is moving away at very high speeds, approaching (but not exceeding) the speed of light. This means those galaxies are shrunk in the direction of their motion (Lorentz contraction) and time passes more slowly for them (time dilation). The further you "look" (the infinitely quick kind of looking which you can only do inside a theoretical model, not having to wait for light to get here so we can actually see stuff), the more things are shrunk and the slower time is ticking. At a distance of the speed of light ("c") times the age of the universe, things approach the speed of light and time is passing so slowly that the Big Bang is only just happening right now. In this way of describing the universe, with these coordinates the universe actually fits in a finite sphere around us.
That's a perfectly valid set of coordinates, but I think you'll agree it's not very practical. So physicists invented the cosmological model: imagine a bunch of clocks everywhere in the universe, flying at the same speed as the expansion of the universe (i.e. the same speed as average galaxies in that neighborhood) and ticking at whatever rate the local clocks are ticking at (not synchronized to ours). We define time at any place in the universe as being whatever is indicated by those clocks, not ours. So in effect we change the very definition of simultaneity, moving things from the future into today simply by changing the labeling. Also, imagine measuring sticks available everywhere in the universe, but just like the clocks flying at the same speed as the local expanding universe. To measure distances, we use those sticks instead of our own.
If we now measure everything using local (Lorentz-contracted) sticks and local (time-dilated) clocks, the universe looks completely different. It is truly infinite, the same age everywhere, and distant objects are no longer flat Lorentz-contracted pancakes but look the same as objects in our neighbourhood. Note that this is not a different universe, it's the same one but with different labels stuck onto objects.
Now, with this set of coordinates, it turns out that rays of light don't travel at a fixed speed "c" relative to us, but relative to the local clocks and sticks we used to define the coordinate system. It is still true that nothing can go faster than (local) light, i.e. you cannot overtake a ray of light, but a distant object certainly can move away from us faster than the speed of a ray of light in our neighbourhood. And if some alien over there were to try and shoot a laser beam our way, that light would never reach us because it is traveling towards us at the speed of light relative to the local "space" which is moving away from us faster, like a cosmic conveyor belt. Note that this conveyor belt is not real, it's just a product of our mathematical trickery refefining distances and times.
Of course you might wonder what happens to that alien laser beam in the first coordinate system, where rays of light all travel at the same speed relative to us. Well, in that system, the aliens don't exist yet because time in that part of space is moving very slowly (and has been moving slowly ever since the big bang). And since that part of space is still accelerating away from us ever faster and closer to the speed of light, local time comes to an asymptotic halt before the aliens ever get a chance to shoot that laser.
"Space itself" is just whatever we define it to be. By changing coordinates, we can move things from the past into the future or even into "never". It doesn't matter, it's just math(s), the end result is that we will never see that laser and we will never be able to reach that galaxy either.