I just had a thought.
Since universe expands because of the energy inside it, would the universe not simply expand further if you reached some sort of theoretical ‘border’?
Edit: I’m really having trouble imagining the ‘border’ issue in flat and open universes now, for some reason.
Well at some point in the far future the universe will have grown so big that gravity no longer has affect on the speed of growth, at this point the universe will grow exponentially which could lead the the doom of the universe (the “Big Rip”) but that ofcourse is just a theory.
This also means that at some point if there were civilizations like humanity in a galaxy they would no longer be able to receive the light from other galaxies. These aliens would never be able to detect whether the universe is growing (thus never figure out how it was born) and their science would suggest their galaxy is the only one and that it makes up the entire universe (which is obviously false because there are hundreds of billions of galaxies today).
Whatever happens in the faaaaaaaaaaaaaaar future of the universe, none of it is good, because one thing is for certain and that is that one day stars will stop forming because the hydrogen will have depleted (which automatically means no more new planets). All living stars will die and (possibly) explode and then the universe will be completely dark, filled with countless of trillions of gigantic black holes.
To me this is comparable to a draw in a paper wondering what would happen if it left the paper. It can just leave the paper if someone pulls him out. If you want something to pull you out, ok, but you will only know the result if something pulls you out.
Sorry, I don’t think I am viewing this the wrong way. You can go in a “straight” line, except the line does not look straight. In the example I gave, the straight lines are actually ellipses.
If you are in the empty space, no mass, and you have some velocity and no accelerations involved, you move in a straight line. If the universe consists of you and, say, the Earth, then you will also move in a straight line, but, in this case, the straight line is an ellipse, or a parabola, because the mass of the Earth curved the space and transformed lines into ellipses and parabolas.
I’m not getting this view from my magical hat. The straight lines being transformed into ellipses is the explanation a professor in Physics explained me (and the class) about how the gravity changes the space. He actually gave the same example, but, in his explanations, the mass of the Earth was collapsed to a point, which is then removed from the space (as a point where you could not go) and all straight lines would be ellipses and parabolas, depending on your speed and distance. So there were no ellipses degenerated into line segments like in my example.
Yes, I understand that.
Actually, gravity will always change your velocity, either increasing, decreasing or changing direction. Except if something stands in the way (like the floor behind you) or if you use other forces (like an airplane), but then there are other forces involved that fights against the gravity.
Unless you consider the space to be curved by gravity. In this case the velocity would not be changing, it would just be following the straight line in the curved space.
Ok, there is no center of gravity. In my example the curve was made by a central point, yes, but in the actual universe the curve is made by many bodies spread everywhere. So actual straight lines might look much stranger than pretty ellipses in the example, but the central point still remains: the curve in space is the effect of the acceleration caused by a mass, not a magical distortion that looks like a mirror or a Pacman game like many people here suggested. Just like you said below: you just can’t reach the “border” without violating the laws of Physics.
Well, I can consider the boundary of the universe being very up high from the Earth in my example, why not? For instance, the end of the atmosphere (is there such a thing?), or the maximum distance you can go if you are thrown at, say, a thousand miles per hour (which would then, according to my intuition, be comparable to the speed of light in the real universe) up from the center of the Earth.
In the actual universe, the boundary of the universe is farther that the first photon of light that was emitted in the Big Bang. As far as I know, this is very far away from any kind of mass in the universe, though I know little about this.
No, this something would not escape because of the gravity. That’s my point.
By escaping I was meaning go as far away as you want from the center, not leaving the universe. Like a rocket launched from Earth that escapes from Earth.
One last thing, by the way: I see the theory of curved space as just a different way to intuitively understand gravity and the effects it has on matter and light. The question was why gravity changes the direction of the light as if it had a mass, because it doesn’t, so this looks counter-intuitive. The answer that makes it more comprehensible is this: the mass curves the space, so the light still follows a straight line.
Sorry, I completely misunderstood your post then.
Yeah, that’s pretty much what I meant.
How exactly is the boundary of the universe defined? And do we actually know the current size of the universe?
Sorry, misunderstood again.
I actually meant: if the boundary of the universe exists, it is farther then that photon. But I know nothing about the universe boundary.
EDIT:
Why will the universe grow exponentially? In my mind it would grow at an approximately constant speed. But that, of course, would lead to the same result, only slower.
I believe that, in this far future, humans will be able to transform matter into energy like if they were playing a game for kids. Maybe even matter from black holes. And the amount of matter in the universe is very big, so there would be plenty of time. That’s just science fiction, of course.
I edited to “How exactly is the boundary of the universe defined? And do we actually know the current size of the universe?”
But I don’t think we know the size of the universe. At least, wikipedia doesn’t mention it :3
I edited to “How exactly is the boundary of the universe defined? And do we actually know the current size of the universe?”
But I don’t think we know the size of the universe. At least, wikipedia doesn’t mention it :3
Edit:
I just reread your posts, gugamilare, and I understand what you were saying now, and I agree. I hadn’t considered your point of being pulled back by gravity when you run out of energy that pushes you away.
I think I understand how to imagine a flat universe and its ‘borders’ now - whether or not it has a border, you can never go very far away from matter because of gravity. The shape of the universe and any practical way of imagining the universe depends on the matter inside it, anyway. Unless I’m mistaken.
Also, gravity theoretically reaches infinitely far, doesn’t it? In that case, no matter how far away from any matter you go, you’ll always get pulled back eventually.
I think I have this figured out a bit better than before now.
In a flat universe, the repulsive energy that is responsible for the expansion of the universe is equal to the attractive energy responsible for slowing down that expansion.
If they are equal, then they would theoretically cancel each other out when collapsed into a single point, resulting in zero energy. This opens the possibility of the universe having started from nothing, since you always get something from nothing in quantum physics.
edit: oops, double post
God I love this site…
Awesome folks
Believe me, at that time WE will be making stars!
And how would you do that when you have no hydrogen? You do realize that a star could easily be a million times bigger than the Earth?
Oh my god. You’re still being a pedant.
Like I said in the post you quoted, we’re not talking about the logistics of ‘leaving’ the universe.
with SCIENCE![/SIZE] sorry guys i had to do it… 
Actually, gravity reaches infinity far away, but it also tends to zero. Therefore it can diminish your speed without actually nullifying and reverting it. As much as I understood about the differences of closed, open and flat universe, in a closed universe the gravity is so big it will pull you back eventually. In a open universe, you can get as far away as you can with a velocity greater than some positive constant. In a flat universe you can go as far away either, but your speed would tend to zero as you get farther.
I might be wrong about this, but, to me, if you say that the closed universe has a border, then the border would be the maximum distance you can go from the center of the universe by using any method Physics allow you to, unless, perhaps, using something from outside. If you can reach the border, it is not a border at all.
Yes, I was referring to the greater universe as in an all encompassing one. I consider the multiverse to be parallel universes.
And as far as I was aware, the end of smaller particles into energy is just speculatory theory, and nowhere near being proved. We don’t have the means yet to zoom in far enough to prove or disprove that idea, and at this moment, I stand by my initial statement.
Technically speaking, if one were to get beyond the confines of the universe, you would just be expanding and distorting that one little portion of the universe. Remember, by definition, the universe is “all that is” so you couldn’t actually escape the universe; you’d just change its shape.
That’s really just a semantic argument. From my current understanding, since space is distorted you’d just end up curving back really slowly until you ended up in the universe again.
:3
Like I said, I think that’s a purely semantic argument. Since gravitational force would carry on infinitely, it’s a moot point.
Why bother about what’s outside the universe, if there’s no space-time you simply can’t exist. Try explaining that to religious people, what a nightmare that is. :jizz:
If the universe is limited, so is numbers and numbers are not limited from what we say. So I think the universe is unlimited space out there filled with unimaginable activities
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