Accelerating Expansion Threshhold

[VooDooX]

so in essence your saying its due to time dilation ?

 

[twofish-quant]

This is one of the reasons physics is interesting. A lot of the standard concepts and assumptions don't work. If you talk about a "distance" to a distant galaxies, you have to define exactly what you mean by "distance" and it turns out that there are about five or six perfectly reasonable definitions for distance, that give you different numbers.

 

[twofish-quant]

so in essence your saying its due to time dilation ?

It's actually more fundamental. What is "distance" and what is "time"?

For something that I can hold in my hand, 'distance" is what I'm measuring with a ruler and "time" is what I measure with a stopwatch. The trouble is that for a distant galaxy I can't take a ruler and run it all the way to the galaxy, and I can't easily take my stopwatch and measure something that is going on there.

So I have to come up with another definition of distance and another definition of "time" and it turns out that there are several different reasonable definitions that give you different numbers.

I should point out that this is completely separate from the expansion of the universe.

 

[Wallace]

so in essence your saying its due to time dilation ?

No, what I'm saying is you can't say it's beacuse of this or because of that. Let me give a more concrete example. Let's ignore gravity for a moment and imagine you've got two massless dots moving apart. If we define our co-ordinates using simple Minkowski space (which is equivalent to special relativity) then there is no curvature of space-time. Now, in these co-ordinates we can define one dot to be at rest at some chosen origin, and the other dot is moving.

At any given moment, specified by the time on the 'stationary' observers watch, we can work out the co-ordinate distance to the 'moving' dot. Since there is no gravity, this distance increases with time at a steady rate, at less than the speed of light. If we fire photons between the dots, we find they are redshifted, this is because due to time dilation, the moving dot's clock runs at a different rate than the stationary one. All very well and good.

But, we don't have to describe this situation in this way. We could instead define a co-ordinate system that itself expanded, such that both dots are at rest with respect to the co-ordinates. This is just the same as taking say a rotating reference frame in the case of something that is spinning, just in this case we take a convenient co-ordinate system for an expanding system.

Now, in this set of expanding co-ordinates, both dots are at rest. But if we fire photons between the particles, we see they are red-shifted. How does this happen? It turns out that in this new co-ordinate system, as a photon passes through spatial co-ordinates, it gets steadily streched. This is because in this co-ordinate system 'space expands', because the spatial co-ordiantes have a time dependance.

So, in the first case, the dots move apart and redshift is caused by time dilation and in the second case the dots are at rest and redshift is caused by the expansion of space. Both descriptions are correct and give you the same answer for the redshift, but if you tried to assert that something was 'caused by time dilation' someone could also choose a different co-ordinate system in which that thing was 'caused' by something else.

There are certain 'invariants' that do describe the underlying 'truth' of a given space-time, but these are not the co-ordinates.

Note that the only difference between my example and the real universe is the addition of gravity, which adds in addition effects but doesn't change the issues around co-ordinates and there interpretation.

 

[S.Vasojevic]

Wallace, very useful posts. You just answered at about a half of threads in a cosmology forum, in a very coherent fashion.

 

[VooDooX]

yup i perfectly understood that thanks wallace

 

[sylas]

Would it be inappropriate to mention in an off-hand way that the "2009 PF Member Awards" are still collecting votes, and that you can have your say for cosmology in the [thread=359217]Astro/Cosmo Award[/thread]? There are a number of very worthy nominees and at this point the winner is all but wrapped up; but it is still worth giving a nod to anyone you think appropriate.

 

[voyager4383]

Ok, total newbie here. I'm deeply involved in an IT career, however I've often thought about learning more about physics as I have a lot of ideas, but no knowledge to back them up...so I'm bouncing them off everyone else to get feedback on my thoughts.

My first thought is this: The original thread was posted about the acceleration of the expansion of the universe...which is just way to big to try to think about at this time. Apparently entire galaxies are accelerating which throws a lot of physics out the window, as that just shouldn't happen. If I had a pool table full of balls but no pockets, and the balls had no drag or gravity affecting them, the balls would bounce forever, yet by colliding into each other some would end up moving faster, and some would be slower, all by transferring motion. Even so, there is no acceleration, only transfer of motion making some objects move faster than others...some may even end up at nearly a standstill until something else whacks into it.

In an accelerating universe, the general idea is that everything must be pushed or pulled into moving in a direction, which would be the billiards theory plus gravity. So there is gravity (a force that tugs objects in a direction) and collision (which transfers motion between objects). We have a pull, we have collision, but we don't have a push which is what people are looking for in dark matter or dark force. It must be pushing objects away.

Now...my thought is truly this: We can talk about transfer of motion, and pushing and pulling all day. If you simplify a car...with an engine that moves it forward or backward...it is able to move itself. What is stopping something that is incredibly massive from accelerating itself...maybe simply because it "wants" for the lack of a better word, to be moving as close to the speed of light as possible. Maybe highly massive objects move barely perceptively faster over time without intervention, and we just can't see it because it happens sooooooo slowly that we cannot measure it at this time. Beyond highly massive objects, we have something as large as an entire galaxy full of mass...working together like an engine, that just ever so slightly accelerates ITSELF in the direction it is already going.

I dunno, maybe I'm just way wrong, or this has already been proven wrong due to other circumstances I'm not seeing...but sometimes I feel as though some people are just so intelligent that they miss the obvious...and I really enjoy theorizing about things just out of my current reach and thinking that just maybe everything is as simple and yet complicated as we think.

I look forward to any and all replies! Thank you all, I've got to go to work!