OT - Physics Question on Anti-Matter

[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.

[PvK]

The Expanding Universe theory is that gravity is too weak to slow down something moving quickly away that is already an astronomical distance away. Gravity gets weaker and weaker the farther something is away. There is a limit to how much its force will ever add up to beyond a certain distance, so if the distance is increasing, and effect of gravity decreasing, things can escape gravity. The question then became exactly how massive is the universe, and how far is it apart, how fast does it seem to be expanding?

However, after many years of trying to figure that out based on all sorts of assumptions (which made sense, but are still just theories), cosmologists are starting to decide that some even more basic assumptions seem to likely be not correct, such as that the rules of the universe remain constant over time and distance...

The idea that we are seeing the universe all receding at increasing speeds at great distance is all based on assumptions about how to interpret the light we see based on a mountain of theories. It's an extremely sophisticated mountain of theories, but it's still an active volcano, so to speak.

PvK

[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.

[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.

[shinigami]

Random thoughts:

I often find it funny how some scientists deride religion because it fails to meet scientific standards but will stand firm on accepted theories that truly can not be proven right or wrong. Hmmm, sounds like faith to me.

If the Big Bang Theory is correct then everything in the universe was created in that instant, albeit in different forms of energy and matter. Therefor, infinite energy is an impossibility because we live in a finite universe. However, if you throw in the infinite universes theory you could get infinite energy by tapping into other universes. Last I heard scientists think that they could accomplish this, but would need an infinite energy source to do so.

How much is our understanding of physics colored by our own perception? Our perception of "now" is an infinitely small slice of time and by the time our brain receives sensory input the data is already old because travel time for that signal is not instant. Looked at from this point of view we are actually out of step with our environment because we can only perceive the past.

[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

[Renegade 13]

Suicide Junkie said:
Just because there is a lot of mass there dosen't mean it is infinite in any way. Where "a lot" is only a handful of stellar masses in many cases.

A handful of solar masses? The central blackhole of the central galaxy of a galactic cluster is estimated to have up to several hundred billion solar masses. It'd take a while to use up that amount of mass/energy.