Universe expansion is slowing down

by Myslius
Tags: expansion, slowing, universe
 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?
 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.
 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.
PF Gold
P: 10,765
 Quote by Myslius 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.
 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.
 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.
PF Gold
P: 10,765
 Quote by Myslius 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.
P: 98
 Quote by Drakkith Please point out the specific error you are referring to.
reversed time scale.
PF Gold
P: 10,765
 Quote by Myslius reversed time scale.
And how did you come to that conclusion?
PF Gold
P: 366
 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?
 Sci Advisor PF Gold P: 9,088 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.
P: 1,883
 Quote by Myslius I would appreciate if you point a mistake in my interpretation.
In the OP:
 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.
P: 4,678
 Quote by Myslius 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 $H$ and the acceleration of the scale factor, $a(t)$.

The rate of expansion $H$ 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 $H$ is defined as:

$$H(t) = {1 \over a(t)}{da(t) \over dt}$$

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

$$H_0 = {1 \over a(t)}{da(t) \over dt}$$
$${da(t) \over dt} = H_0 a(t)$$

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

$$a(t) = a(t=0) e^{H_0 t}$$

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.
P: 640
 Quote by Drakkith 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."
 PF Gold P: 10,765 Interesting Ruta. I'll be sure to take a look at the paper as soon as I get a chance. Thanks for the link.