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-   -   Conflicting viewpoints? (http://www.physicsforums.com/showthread.php?t=243728)

Holocene Jul6-08 05:52 PM

Conflicting viewpoints?
 
I have read that the present expansion of the univesre is slowing down, due to the gravitational attraction of the matter in contains.

Other times, I read that the expansion of the univesre is actually speeding up. For instance, light from the more distant galaxies are redshifted more than closer galaxies?

Which idea is thought to be true?

waht Jul6-08 06:20 PM

Re: Conflicting viewpoints?
 
I think the general consensus is that the universe is speeding up due to an unknown force which we call "dark matter."

Holocene Jul6-08 06:35 PM

Re: Conflicting viewpoints?
 
Quote:

Quote by waht (Post 1792938)
I think the general consensus is that the universe is speeding up due to an unknown force which we call "dark matter."

Wouldn't matter, even "dark" matter, only further halt the expansion?

Maybe you meant dark energy?

jonmtkisco Jul6-08 07:43 PM

Re: Conflicting viewpoints?
 
Hi Holocene, yes waht meant dark energy.

An easy way to think about it is that the universe is both decelerating and accelerating at the same time. In the past the deceleration dominated the acceleration; in late times the acceleration dominates the deceleration.

The gravitational deceleration parameter remains constant over time: gravity continues to assert a slowing force. But the total amount of dark energy increases in direct proportion to the amount of volume in the universe, since every cubic meter of "new" space adds to the cosmological constant. Since effective gravity decreases at a rate of r-2 while total dark energy increases at a rate of r3, it's only a matter of time until the effect of dark energy comes to overwhelmingly dominate the effect of gravity. That's the phase we are in now.

Jon

thenewmans Jul6-08 08:29 PM

Re: Conflicting viewpoints?
 
Holocene, I hope to get you to ask more questions. Generally, a galaxy sufficiently far away has been moving away from us at the same speed for the entire life of the universe. There are exceptions to this. The three biggies are inflation, acceleration and shape. Inflation affected the early universe and has very little effect today.

The shape of the universe is often described as very flat and may be perfectly flat. Flat means galaxies move away from each other at a steady speed. If it’s not flat, it’s either curved in like a ball and will someday collapse in on itself or it’s curved out like a potato chip or saddle. If it’s curved in, that other galaxy will slow down at a steady rate and eventually stop and then head towards us. When we collide, we will collide with every other galaxy. It’s the opposite of the Big Bang.

So you might thing that a saddle shaped universe would explain the acceleration. But no. General Relativity predicts a smooth, consistent acceleration. And that’s not what’s observed. Instead, it looks like the acceleration kicked in a little more recently.

robheus Jul6-08 09:20 PM

Re: Conflicting viewpoints?
 
Quote:

Quote by Holocene (Post 1792909)
I have read that the present expansion of the univesre is slowing down, due to the gravitational attraction of the matter in contains.

Other times, I read that the expansion of the univesre is actually speeding up. For instance, light from the more distant galaxies are redshifted more than closer galaxies?

Which idea is thought to be true?

The acceleration is speeding up.

Chronos Jul7-08 12:28 AM

Re: Conflicting viewpoints?
 
Acceleration is the consensus view these days. The Permutter supernova papers [e.g., http://arxiv.org/abs/astro-ph/9812133 and http://arxiv.org/abs/astro-ph/9812473] are the gold standard to date.


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