OT - Physics Question on Anti-Matter

[Raapys]

Yeah, but when that 'energy' is depleted and everything has 'stopped', gravity will still be there( as far as we know ). What is to stop it from pulling everything together again?

I read the Last Answer. It was a nice read, thanks.

[aegisx]

Gravity would dissipate to pretty much nothing I suspect. Gravity is a force and spread over theoretical particles called gravitons. So as stars, planets, elements, atoms and particles break down, so does the gravitational force. All these things are held together by very strong forces compared to gravity, so if those are breaking, gravity probably isn't a big issue.

Of course, this is just based on my basic understanding and I could be wrong.

[Raapys]

Well, I wont claim to know any better, but I would have thought that if gravity was still there, however 'weak', the particles would slowly start to pull together again; unless something was actually exerting force on them( stronger than the gravity ), keeping them apart.

Anyway, if indeed the expansion is speeding up, that means energy is being continusly applied to the mass in the universe( thus providing the speed increase). Since energy = mass ( right? ), that means further that the amount of mass in the universe is increasing as well? And since all mass has gravity, as far as we know, that also means gravity increases? Thus eventually becoming strong enough to counter whatever other force is pulling the stuff apart...?

My non-existing education got cut short, so I'm a little out of the loop. Please correct me if something's wrong with my understanding.

Also, wont the whole 'expansion speeding up' thing also mean the entire mass of the universe will be traveling at light-speed in a 'few years'?

[Suicide Junkie]

Its not the objects accelerating (IE Kinetic energy), but the underlying spacetime itself expanding.

BTW, if something is at escape velocity, it will never fall back (barring interference from other space objects)... that's because even though gravity is always there tugging on it, gravity is weakening faster than the speed is decreasing.
On a parabolic "orbit", the speed approaches zero as time approaches infinity... but it never quite manages to go negative and loop back. No matter how long you wait, you've always got just a little speed left, and gravity is *just* slightly too weak to stop you.

[Suicide Junkie]

As far as the accelerating expansion, sort of.

Consider the balloon analogy. You've got a superstretchy rubbery surface which is being inflated, at a rate such that two centimeters stretches to four centimeters every minute.

Lets add some starry anthills to the surface, such that most of them are emitting ants, all of whom crawl along the surface towards our hill. Ants crawling at the speed of light, which we will make a quite leisurely pace, say 8 centimeters a minute.

So the little photon ants crawling towards us are hustling along, but the space is expanding so their trips take longer and longer.

For the ants that start a centimeter away, no problem. They arrive before there is much noticable expansion.
For the ants that start 8 cm away, they suffer. The distance between them and their goal is expanding at 8cm/minute at the start, so although they move away from their starting point quickly, they never quite reach the destination.

The ants at 7.9 cm distant make a teensy bit of progress at each step, and as they slowly close the distance, things get easier. All the ants from that hill will eventually make it, although it will take a long time.

So, with this rate of expansion, we can see everything closer than 8cm. Ants that started just short of 8cm will have taken the lifetime of the universe to reach us.

Now, if the expansion accelerates, then the ants from the 7.9cm distant hills can no longer reach us.
As such, the observable part of the universe has shrunk from 8cm radius to 7.9cm or less.

For "small" things like superclusters of galaxies, gravity's pull is plenty strong enough to keep things snug for now. They pull apart a little, but at the same time, keep moving towards each other. If the expansion keeps accelerating then gravity starts losing at smaller and smaller scales... all the way down to star systems. And then eventually electromagnetic forces lose, and rocks get pulled apart... and then nuclear forces lose and atoms get pulled apart. But by then, nobody will care.

[Raapys]

Thanks for the explanation, SJ.

They haven't figured out how the whole 'dark matter' and 'anti-stuff' fits in, have they? And let's say we had a super-fast spaceship and travelled to the edge of the spacetime, do they have any idea what would happen if we tried to move past it?

[Suicide Junkie]

If one of the ants got into a drop of caffeine and started travelling in clear defiance of the speed limits imposed, it would go round and round... there is no edge.

And that's pretty much what we've probably got, in 3D.
As far as it is possible to see, in all directions... you have stuff, more stuff, older stuff, and eventually the microwave background.

Whether it is closed like a balloon shape, or flat or open (saddle-shaped), it is uniform in all directions to the edge of the visible universe. (7.999... cm on the balloon analogy)
So, no evidence for edges.

If it helps, you could think of a closed expanding universe as a game of asteroids... year by year, you keep increasing the resolution of the screen, but it is still a finite size (320x200 way back when, and 1280x1024 nowadays)... there are no edges to fall off of, since the left side is connected to the right side and the top to bottom.
Its not spherical, but rather doughnut shaped, but still finite with no edges, and a decent example.

Also, if your ship can only travel at 1 pixel per second, and you increase the screen rez fast enough, you won't be able to reach objects that happen to be too far away when you start.

[Renegade 13]

Raapys said:
They haven't figured out how the whole 'dark matter' and 'anti-stuff' fits in, have they?

Nope, they basically have no real idea what dark matter is. An interesting side point about antimatter is that "normal" (baryonic) matter and antimatter should have been created in equal quantity at the big-bang. However, we take a look around and we can't detect any antimatter out there, but there's lots of normal matter. They still haven't figured that one out

[capnq]

If you're finding this discussion interesting, you might also enjoy the most recent Irregular Webcomic podcast (The word "podcast" is a second link.) You can download Episode #4, or read the transcript.

This is one of my favorite Webcomics. The author is a professional astrophysicist, and is quite good at explaining things in layman's terms. Podcast #4 explains why the night sky is black, which turns out to be relevant to this thread.

[Randallw]

I was a tad dissapointed with the latest IWC podcast. It goes for about 9 minutes but after about 30 seconds when he started going on about giant red lights in the sky I thought "Do I really need to listen to this when he will obviously start explaining why the night sky is black". I mean ok he's got a Phd in Physics, what can you expect, but I already know that sort of stuff. When is he going to start explaining M-branes?

What do I know of brane cosmology?. Easy, I wrote half of one of the scenarios .

Edit: oh and capnq, just if you're interested, I am one of the writers for his new comic project.