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Universe expansion is slowing down

by Myslius
Tags: expansion, slowing, universe
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jimpy
#19
May6-12, 04:09 PM
P: 6
I'm confused about the standoff here.
As an observer so far of this exchange we seem to be talking at cross purposes.
The point under discussion appears to be is .51 older or younger than .49 in terms of observed phenomena.
So which is it?
Have I missed a trick here or should this be obvious?
Myslius
#20
May6-12, 05:21 PM
P: 98
It's not about age. It's about dark energy. Current theories state that space expansion was accelerating, decelerating and is accelerating again, and that we have this repulsive (i guess, source isn't described) and unknown force called dark energy.

Instead of this it could be simply explained by deceleration. And known force - gravity? It's a hypothesis, same as dark energy.
Dead Boss
#21
May6-12, 06:05 PM
P: 150
But the problem is that the acceleration is not a hypothesis, it's a measurable fact. Your hypothesis of decelerating expansion is contradicting known evidence, therefore it's not true.
Drakkith
#22
May6-12, 06:08 PM
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Quote Quote by Myslius View Post
It's not about age. It's about dark energy. Current theories state that space expansion was accelerating, decelerating and is accelerating again, and that we have this repulsive (i guess, source isn't described) and unknown force called dark energy.
Inflation is a completely separate subject than the current expansion of the universe. Our rules for the expansion only apply to non-inflation expansion. So in terms of this, the universe was decelerating up until a certain point in time at which dark energy became dominant and the expansion rate started to increase.

Instead of this it could be simply explained by deceleration. And known force - gravity? It's a hypothesis, same as dark energy.
I trust that the guys who earned the Nobel prize for proving that the expansion rate of the universe is increasing were correct, and that the expansion rate is not decreasing as you are claiming.
Myslius
#23
May6-12, 11:13 PM
P: 98
Dead Boss, what fact my hypothesis contradicts? Collected data is a fact, data interpretation isn't. You clearly don't understand what I'm saying. My hypothesis does not contradict collected data. I pointed out the mistake in dark energy interpretation.
I would appreciate if you point a mistake in my interpretation.

Drahhith, we can leave inflation alone. It doesn't change the fact of "accelerating" expansion.
Bet on Nobel price winners sure is better than on random guy. But i would like to see some reasoning.
Myslius
#24
May6-12, 11:44 PM
P: 98
Dark energy:
* Expansion is accelerating
* Velocities of distant objects will increase with acceleration over time relative to us
* Universe will end up in big rip if dark energy continues to dominate, spacetime expands faster then c.
* Time will come when CMBR and other galaxies will not be visible due to faster expansion then c.
* unknown force.

My hypothesis:
* Expansion is not accelerating, it's constant (was decelerating earlier)
* Velocities of distant objects will not increase with acceleration over time relative to us
* Universe is flat, spacetime always expands at the speed of c.
* All galaxies and CMBR will be visible all the time, at least till Planck's limit.
* No additional force.

I bet against DE due to Occam razor principle and due to the error in the interpretation.
Drakkith
#25
May6-12, 11:57 PM
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Quote Quote by Myslius View Post
I bet against DE due to Occam razor principle and due to the error in the interpretation.
Please point out the specific error you are referring to.
Myslius
#26
May7-12, 12:00 AM
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Quote Quote by Drakkith View Post
Please point out the specific error you are referring to.
reversed time scale.
Drakkith
#27
May7-12, 12:07 AM
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Quote Quote by Myslius View Post
reversed time scale.
And how did you come to that conclusion?
petm1
#28
May7-12, 12:52 AM
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Just when you see the increase in the rate of expansion that increase was billions of years ago, not now
We see the photon at the end of its duration and I would think that the increase in the red shift took place over its entire duration not just at the beginning.


reversed time scale.
So just where and when would you have to reverse the time scale?
Chronos
#29
May7-12, 02:00 AM
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Myslius, you are arguing against mainstream scientists. Show your math, or, better yet, how their math is wrong. Logic is not better, or even equivalent to math.
Ich
#30
May7-12, 05:37 AM
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Quote Quote by Myslius
I would appreciate if you point a mistake in my interpretation.
In the OP:
Quote Quote by Myslius
Standard point of view: When we look at distant galaxies, we see that redshift is higher then expected so we conclude that the universe is expanding at accelerating rate.
We see the opposite.
Chalnoth
#31
May7-12, 05:46 AM
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Quote Quote by Myslius View Post
Correct. Just when you see the increase in the rate of expansion that increase was billions of years ago, not now. If you want to know how the rate of expansion changed (or will change) over time you have to reverse the timescale.
I think you're a bit confused between the rate of expansion [itex]H[/itex] and the acceleration of the scale factor, [itex]a(t)[/itex].

The rate of expansion [itex]H[/itex] has always been decreasing. However, lately it has been decreasing more slowly, slowly enough that it leads to an accelerating expansion. How is this possible? Well, the rate of expansion [itex]H[/itex] is defined as:

[tex]H(t) = {1 \over a(t)}{da(t) \over dt}[/tex]

I think the easiest way to see why a slowly-decreasing [itex]H(t)[/itex] leads to an accelerating expansion is to consider a constant [itex]H(t) = H_0[/itex].

[tex]H_0 = {1 \over a(t)}{da(t) \over dt}[/tex]
[tex]{da(t) \over dt} = H_0 a(t)[/tex]

So a constant rate of expansion [itex]H(t)[/itex] means that change in the scale factor is proportional to the scale factor: this is exponential growth! Specifically:

[tex]a(t) = a(t=0) e^{H_0 t}[/tex]

So a constant rate of expansion means that objects within the universe are accelerating away from one another exponentially fast. This isn't the situation we're in yet, but it appears that our universe is approaching this situation.
RUTA
#32
May7-12, 07:13 PM
P: 669
Quote Quote by Drakkith View Post
I trust that the guys who earned the Nobel prize for proving that the expansion rate of the universe is increasing were correct, and that the expansion rate is not decreasing as you are claiming.
"Various alternatives to an accelerating universe have also been proposed. Whether such alternatives are viable remains to be seen, but the Nobel Committee for Physics has perhaps acted somewhat prematurely by selecting a preferred interpretation of the supernova projects’ data. The effect, intentional or not, is to bully the skeptics into silence, self-censorship, or ridicule, whereas good science proceeds with a healthy dose of skepticism and with open minds." http://www.physicstoday.org/resource...nt&bypassSSO=1

We just published an alternative in Class. Quant. Grav. last month whereby the universe is a decelerating Einstein-deSitter model (no cosmological constant, no dark energy, no acceleration) http://arxiv.org/abs/1110.3973. Our model fits the supernova data as well as the accelerating LambdaCDM (Einstein-deSitter plus cosmological constant).

Don't get me wrong, LCDM has also fit WMAP and BAO data better than alternatives (we haven't even tried that yet). So, the safe bet is accelerating expansion per LCDM. But, I wouldn't go so far as to claim the accelerating expansion has been "measured." It is a model dependent "acceleration."
Drakkith
#33
May7-12, 07:25 PM
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Interesting Ruta. I'll be sure to take a look at the paper as soon as I get a chance. Thanks for the link.
Chalnoth
#34
May7-12, 10:27 PM
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Quote Quote by RUTA View Post
We just published an alternative in Class. Quant. Grav. last month whereby the universe is a decelerating Einstein-deSitter model (no cosmological constant, no dark energy, no acceleration) http://arxiv.org/abs/1110.3973. Our model fits the supernova data as well as the accelerating LambdaCDM (Einstein-deSitter plus cosmological constant).

Don't get me wrong, LCDM has also fit WMAP and BAO data better than alternatives (we haven't even tried that yet). So, the safe bet is accelerating expansion per LCDM. But, I wouldn't go so far as to claim the accelerating expansion has been "measured." It is a model dependent "acceleration."
This is the fundamental problem. Supernova data are, today, the data that constrain the accelerating universe the least. The real tests are the WMAP and BAO data. Once you add those it's next to impossible to have anything but an accelerating universe.
Radrook
#35
May8-12, 12:57 AM
P: 334
Quote Quote by Mark M View Post
Myslius, the universe has been accelerating since approximately 7 billion years after the big bang. NOT since the big bang itself. In fact, it was decelerating until roughly the 7 billion year mark. At that time, the density of dark energy (the cosmological constant, so it probably doesn't ever lose density) overcame the density of matter/dark matter, which falls very fast.
Absolutely true!

The Expanding Universe: From Slowdown to Speed Up
Distant supernovae are revealing the crucial time when the expansion of the universe changed from decelerating to accelerating


Currently the density of dark energy is higher than that of matter, but in the distant past the density of matter should have been greater, so the expansion should have been slowing down then.
http://www.scientificamerican.com/ar...ws-then-speeds
Chronos
#36
May8-12, 01:31 AM
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According to theorists, if the universe is absolutely flat, the matter density of the universe is, and always will be, equal to its critical density since the end of the inflationary epoch.


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