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Question about the accelerating universe


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#1    Homer

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Posted 04 July 2003 - 08:47 PM

                                                      After more than 15 billion years, the universe is not only still expanding, but also accelerating.
Assuming the universe continues to expand forever, which is a popular theory, could this acceleration ever surpass the speed of light?

Does the speed of expansion follow the same rules as the universal speed limit? If the expansion were to continue to accelerate, and there is not enough gravity to slow this acceleration, will the laws governing the speed of light determine that the expansion will stop accelerating once it reaches the speed of light?

How could the speed of expansion be limited by this law?                                                      

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#2    Nethius

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Posted 05 July 2003 - 02:23 PM

                                                      cool, i love these conversations

found a little info here that is quite interesting, and you may want to have a look!

QUOTE
Conventional wisdom holds that the acceleration will proceed at a constant rate, akin to a car that moves 10 mph faster with each mile traveled. With nothing to cap the acceleration, all galaxies will eventually recede from one another at the speed of light, leaving each galaxy alone in a cold, dark universe within 100 billion years. We would not be able to see any galaxies outside our Milky Way, even with the most powerful telescopes.

That's the conventional view, remarkable as it sounds.

The Big Rip theory has dark energy's prowess increasing with time, until it's an out-of-control phantom energy. Think of our car accelerating an additional 10 mph every half mile, then every hundred yards, then every foot.

Before long, the bumpers are bound to fly off. Sooner or later, our hypothetical engine will come apart, regardless of how much we spend on motor oil.



this ties in with a post i made elsewhere, about "does time really exist?"

the link above is also talking about after the "Big Rip" after universe ends...  they state
QUOTE
"On the face of it, it would look like time ends."

Does time only end because we are no longer here to perceive it?





..                                                      


#3    Homer

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Posted 05 July 2003 - 03:56 PM

                                                      I'm familiar with that theory, and in fact, that very article was posted awhile back in a previous thread. very interesting.
According to that theory,
QUOTE
With nothing to cap the acceleration, all galaxies will eventually recede from one another at the speed of light

Which would indicate that even space itself must follow the universal speed limit of light speed.

Thanks for your input, Nethius original.gif                                                      

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#4    antares

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Posted 05 July 2003 - 06:05 PM

                                                      Here is my opinion, Homer,
As we know the Hubble's law says  that the RADIAL velocity of a galaxy (v) and its distance from us (d) are related in a simple equation, where H0 is Hubble's constant (still undetermined)
v= H0 x d

I think determining the constant will answer the question whether or not the radial velocity can be bigger than the speed of light. But as we know the total velocity is a vector sum of radial+tangential components, so even if the radial is approaching the speed of light this does not mean that the total velocity will do so...
Generally speaking Hubble's law does not put any limits on the radial velocity, the further the galaxy is, the faster it moves away from us (red shift).
But I still think the speed of light is the upper limit of propagation in our Universe...                                                      


#5    God18

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Posted 05 July 2003 - 10:32 PM

                                                      There is something that already moves faster than light. We just do not understand it yet.                                                      


#6    Homer

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Posted 06 July 2003 - 03:56 PM

                                                      Hubble's Law is an exceptional tool for getting distances from velocities, but for very distant objects(redshifts of 2 or greater), Hubble's Law gets a little fuzzy. When we're talking about the farthest reaches of the universe, we have to take the 4-dimensional curvature into account, which Hubble's Law does not. Another significant effect is that in most cosmologies, Hubble's Constant(whatever number it really is) isn't really constant. Hubble's Constant increases as you look back in time, so the value is different today than for galaxies with a redshift of 5.

There is some evidence that the universe went through a period of expansion at rate faster than the speed of light. In the early phase of the universe, it went through a phase called inflation. In that phase, the universe expanded by about 10^50 in a time scale about 10^-30 seconds.                                                      

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#7    antares

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Posted 06 July 2003 - 06:05 PM

                                                      I think you are right,
The Hubble constant measures how fast the universe expands right now. It tells nothing about how fast the universe was expanding in the past, or how old the universe is.
But nobody knows what was the Hubble constant in the early Universe...  The laws of physics as we know them right now did not exist in the first few minutes after the Big Bang. So the speed of light probably was different(larger) from its present value and that dictated the rate of expansion.
One of my favorite books dealing with this matter is
"The First Three Minutes: A Modern View of the Origin of the Universe" by  Steven Weinberg.

                                                      


#8    Saru

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Posted 07 July 2003 - 09:16 AM

                                                      Interesting discussion going on here.  original.gif

QUOTE

So the speed of light probably was different(larger) from its present value and that dictated the rate of expansion.


I'm more inclined to think that the speed of light is dictated by the rate of expansion, rather than the other way around. The laws of physics in effect during inflation would have enabled light to travel far faster than it does today, and possibly at a speed proportional to the rate of expansion at that time.

I'm not convinced that the acceleration of the expansion of the universe will slow down because of the limit of the speed of light, more that perhaps the speed of light may slowly increase as the rate of expansion begins to exceed the current value of C.

Basically the question I'm asking here would be - Is the speed of light itself limited by the current rate of expansion, or is the rate of expansion limited by the speed of light which is itself limited by something else entirely ?                                                      


#9    Homer

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Posted 07 July 2003 - 12:07 PM

                                                      Thank you for your inputs antares and SaRuMaN original.gif

SaRuMaN, the question you asked is what I was trying to ask, only you worded it better. I have been thinking about this for the past couple months, and doing some research to find something on it. Obviously nobody knows with absolute certainty, but I was surprised on how little I found regarding any thoeries about this.

Like you, I'm not convinced that the acceleration will slow down and eventually come to a stop just to satisfy current laws of physics.                                                      

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