Re: Special relativity, the expanding universe and dark energy
From: Bjoern Feuerbacher (feuerbac_at_thphys.uni-heidelberg.de)
Date: 06/07/04
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Date: Mon, 07 Jun 2004 16:52:21 +0200
Ace wrote:
> I thought the big bang theory argued that the universe originated as a
> point,
That's a popular misconception. The theory says only that the universe
started from an infinitely dense state. That doesn't had to be a point.
And even that statement of the theory is accepted by essentially no one
as probably true - people think that for describing the very earliest
stages of the universe, one needs a working Quantum Theory of Gravity.
> and is forever expanding
Probably true - although there is still a faint possibility that it
might recollapse again.
> (BTW, the brane theory, involving a
> collision between dimensions, seems more coherent to me. The
> inflation part of the big bang theory doesn't seem rational.)
Why not?
And, BTW, "collision between dimensions" does not make much sense.
You probably mean a collision between branes, i.e. the ekpyrotic model,
right?
> If the universe has no outer bound, then what does it mean for it to
> be expanding?
That all distances in it increase.
> Trying to simplify my thought, imagine two objects in orbit around
> each other. Both objects are ejecting energetic particles at high
> speed, and they are slowly converting mass into energy. So their
> mass is going down over time, and the density of mass (and energy) at
> a distance is going up. Won't they drift apart from each other as a
> result?
Depends on the details of the process, I would say.
For example, if the energy is radiated off in a spherical symmetric
way, there will be only a spherically symmetric energy distribution
around the bodies - and such a distribution does not influence the
motion of the bodies inside it.
> Or does the symmetry of the change in mass/energy
> distribution result in no net change in the forces of gravity acting
> on the objects?
Sorry, I don't understand what symmetry you are talking about here.
> I need to learn more about general relativity.
What is your background? Can you understand the math, or do you
want to have a non-technical introduction?
> I know about
> gravitational lensing, but I've never read a concise summary of it
> that made any sense to me.
What are your problems with it?
> Does it come down to the fact that energy
> also has gravity?
No, mass alone is sufficient to explain it.
> Can anyone summarize GR in a few sentences for me,
Energy curves space, and objects move in that curved space following
the curvature. That's the essence.
> or point me to a good source to read about it?
Sorry, I know of no good pop science source... :-(
For the Big Bang theory, you could try "Kosmologie fuer
die Westentasche" by Kippenhahn, if you understand German...
> On a related note, I was reading about a dark energy study that's
> looking for the disappearance of gravitons as potential proof of
> multiple dimensions.
I never heard of that. Where did you read that?
> I thought gravitons were still theoretical.
Better word here: hypothetical.
> Have they actually been observed?
No, AFAIK.
> How much is known about them?
They are hypothetical... There isn't even a working theory
which would predict them and tell us more about their
properties!
[snip sig]
Bye,
Bjoern
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