## The Disappearing Universe 358

Posted
by
Soulskill

from the and-for-my-next-trick dept.

from the and-for-my-next-trick dept.

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."
## Wormholes + a flat universe (Score:5, Interesting)

wormhole could poke through the hypersphere to create a shorter distancethan even a line segment from Point A to Point B.http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question35.html

http://en.wikipedia.org/wiki/Shape_of_the_universe

But measurements are

looking like the universe is flat.You never know what scientific discoveries the distant future could hold, but at the moment it looks

bleak for the concept of wormholessince the universe doesn't seem to be a hypersphere at all.## Re:Wormholes + a flat universe (Score:5, Interesting)

observableuniverse 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.## Re:FTL or Wormhole Travel (Score:5, Interesting)

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

thoseclocks, 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

thisset of coordinates, it turns out that rays of light don't travel at a fixed speed "c" relative tous, 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.

## Re:Not so quick (Score:5, Interesting)

From every description I've heard of "dark energy" it sounds like a kind of place-filler variable for

something--as in, "This equation only works if we put in X, but we have no idea what X is."Physicists brought us the dark energy hypothesis, not mathematicians. This is an important distinction: dark energy is not used to solve an equation, rather it is a phenomenon that we can indirectly observe.

Black holes, Dark energy, Zero point energy -- there are so many nascent concepts that hint at great disruption to our theories but that have not had the time to sort themselves out. Humanity rigorously worked on the concept of gravity for several hundred years before we had our Einstein.

## Re:terrifying? (Score:2, Interesting)

We already proved what dark energy is. Dark energy is the force generated from scientists who were not getting their math homework to add up which pulls on additional grants and funding by making things up like dark matter and dark energy.

This is how most advancements in physics have been made no different than leveraging of conservation laws to probe what must be gaps in our knowledge. When energies of particles in a reaction don't add up you know there is something missing you don't understand needing further analysis to resolve. This is how progress is made.