Universe expansion is slowing down

[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.
I don't see the logic here.

When we talk about acceleration, we mean how velocity changes over time. If velocity over time increases - it means object is accelerating.
If velocity was higher long time ago and becomes slower over time we have to conclude that velocity decreases over time (decelerating) instead.

 

[ilhan8]

dont think about velocity.
acceleration was much higher while inflating.
current expansion acceleration is lower but it still accelerates.

 

[Myslius]

You are mixing something. If universe expansion speed is accelerating from the big bang till now it means that current expansion speed is higher then it was in the inflation. Which doesn't make sense.

 

[Mark M]

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.

 

[marcus]

When we talk about acceleration, we mean how velocity changes over time. If velocity over time increases - it means object is accelerating.
...

No, that is not what we mean by acceleration in cosmology. There is no definite velocity that characterizes the expansion. We are not talking about the change over time of some velocity.

I'll try to explain how the term is used.

First learn the idea of the Hubble rate, which is a percentage growth rate of distance (measured in the universal rest frame, in which distances grow but galaxies mostly don't move save for small random motions).

The Hubble rate has been decreasing since very early days and is expected to continue to decline, but less rapidly. It is currently about 1/140 of one percent every million years. That is on average largescale distances (between objects at rest) increase by about 1/140 of a percent every million years.

There are online calculators which can tell you, for a given redshift, what the Hubble rate was back then when the distant galaxy emitted the light that now has that redshift, that we see it with.

Then learn about the scale factor a(t) a function of time. By convention a(present)=1.
It turns out that the Hubble rate is equal to a'(t)/a(t). That is the ratio of the derivative or slope of the scalefactor to the scalefactor itself. A fractional increase per unit time. You can express a'(t)/a(t) as a percentage growth of distance per unit time.

a(t) is not a velocity. a'(t) is not a velocity. the Hubble rate is not a velocity (it is a percentage growth rate of distances between stationary objects.)

a(t) and the Hubble rate are governed by an equation or more exactly by a pair of equations. They are simple equations but out of laziness I nearly always use the readymade online calculators that solve them for you automatically.

What is meant in cosmology by the acceleration or "speeding up" of expansion is that the second derivative a"(t) of the scalefactor a(t) is positive.

Not only is the a(t) increasing with time but the slope of the a(t) curve is increasing.

Do you understand how a'(t) can be increasing and yet the Hubble rate a'(t)/a(t) can be decreasing?
It sometimes helps to think of money in the bank---say in a savings account where the bank is very gradually decreasing the percentage of interest that they pay to savers. But the total dollars in your account nevertheless keeps going up by a greater and greater amount each year. Because the bank is easing off the interest rate only very slowly.

I don't believe that thinking in terms of conventional motion or velocity will help you understand this. Distance change is convenient to think about as a change in *geometry*, not motion as we know it. Everybody's distances increase and nobody gets anywhere by it. Relative positions don't change (except for small individual random motions whose effect on the big picture is negligible).

 

[Myslius]

No, that is not what we mean by acceleration in cosmology. There is no definite velocity that characterizes the expansion. We are not talking about the change over time of some velocity.

I'll try to explain how the term is used.

First learn the idea of the Hubble rate, which is a percentage growth rate of distance (measured in the universal rest frame, in which distances grow but galaxies mostly don't move save for small random motions).

The Hubble rate has been decreasing since very early days and is expected to continue to decline, but less rapidly. It is currently about 1/140 of one percent every million years. That is on average largescale distances (between objects at rest) increase by about 1/140 of a percent every million years.

There are online calculators which can tell you, for a given redshift, what the Hubble rate was back then when the distant galaxy emitted the light that now has that redshift, that we see it with.

Then learn about the scale factor a(t) a function of time. By convention a(present)=1.
It turns out that the Hubble rate is equal to a'(t)/a(t). That is the ratio of the derivative or slope of the scalefactor to the scalefactor itself. A fractional increase per unit time. You can express a'(t)/a(t) as a percentage growth of distance per unit time.

a(t) is not a velocity. a'(t) is not a velocity. the Hubble rate is not a velocity (it is a percentage growth rate of distances between stationary objects.)

a(t) and the Hubble rate are governed by an equation or more exactly by a pair of equations. They are simple equations but out of laziness I nearly always use the readymade online calculators that solve them for you automatically.

What is meant in cosmology by the acceleration or "speeding up" of expansion is that the second derivative a"(t) of the scalefactor a(t) is positive.

Not only is the a(t) increasing with time but the slope of the a(t) curve is increasing.

Do you understand how a'(t) can be increasing and yet the Hubble rate a'(t)/a(t) can be decreasing?
It sometimes helps to think of money in the bank---say in a savings account where the bank is very gradually decreasing the percentage of interest that they pay to savers. But the total dollars in your account nevertheless keeps going up by a greater and greater amount each year. Because the bank is easing off the interest rate only very slowly.

I don't believe that thinking in terms of conventional motion or velocity will help you understand this.

Hubble "constant" decreases over time. And it describes the expansion rate (speed) and that universe expands at the speed of c (flat)

H(t) = (Mpc/c*t)*c per Mpc
H(now) = 3.08 * 10^22 m / (3*10^9 km/s * 13.7by) * 3*10^9 km/s per Mpc = 72km/s/Mpc
(Mpc/c*t)*c per Mpc = 1/t or simply scale factor
and H decreases over time because t increases. So the expansion rate is slowing down. Of course derivative is positive because the universe is expanding.

Hubble constant says nothing about accelerating expansion. It says that total expansion is flat and is c, and that local expansion decreases over time. So where is the acceleration again?

 

[marcus]

Hubble "constant" decreases over time. And it describes the expansion rate (speed) and that universe expands at the speed of c (flat)

H(t) = (Mpc/c*t)*c per Mpc
H(now) = 3.08 * 10^22 m / (3*10^9 km/s * 13.7by) * 3*10^9 km/s per Mpc = 72km/s/Mpc
(Mpc/c*t)*c per Mpc = 1/t or simply scale factor
and H decreases over time because t increases. So the expansion rate is slowing down. Of course derivative is positive because the universe is expanding.

Hubble constant says nothing about accelerating expansion. It says that total expansion is flat and is c, and that local expansion decreases over time. So where is the acceleration again?

Can't help. Thanks for responding.

 

[Mark M]

So where is the acceleration again?

We can measure the redshift of distant Type Ia Supernovae to determine that the universe is accelerating. This chart here plots distance against redshift:

Hubble's Law for an expanding universe predicts a linear relationship - as distance increases, redshift increases. Without any increase in slope. However, if we look at the chart, we see that since z = 0.5 Hubble's Law is violated, and the slope is increasing. As marcus said, the SECOND derivative is positive. Expansion has been acceleration since z = 0.51, about 7 billion years ago.

 

[Myslius]

Try to look from perspective when time was 0 and plot this graph till now.

 

[Mark M]

Try to look from perspective when time was 0 and plot this graph till now.

What? Did you not read my post? The expansion has been accelerating since 7 billion years after the big bang. NOT since t = 0.

 

[Myslius]

... We do see distant galaxies as they were few billion years after big bang. It's 7b y old information, faster expansion was 7b years ago, not now. We see no increase when we look 7b years around.

 

[Drakkith]

Am I correct in thinking that the increase in the rate of expansion would show up the most when viewing very distance galaxies since they are receding at a much faster velocity than closer ones are, and would show the greatest increase in recession velocity with any increase in expansion rate?

 

[Mark M]

... We do see distant galaxies as they were few billion years after big bang. It's 7b y old information, faster expansion was 7b years ago, not now. We see no increase when we look 7b years around.

Yes we do! Did you not look at the chart? After z = 0.51, Hubble's law is violated.

 

[Myslius]

Am I correct in thinking that the increase in the rate of expansion would show up the most when viewing very distance galaxies

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.

 

[Myslius]

Mark M, yes, i noticed.

 

[Myslius]

Increase of rate should merge with inflation period at some point.

 

[Mark M]

Increase of rate should merge with inflation period at some point.

Why do you keep getting caught up on inflation? We can't see what the universe looked like when inflation occurred. We are limited by the cosmic microwave background, the first light emitted at the recombination. This occurred 380,000 years after inflation. Nothing before this is visible to us.

 

[Myslius]

Just a though. I would rather have explained and described phenomena than something that has no math.

 

[jimpy]

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]

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]

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]

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]

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]

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]

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]

Please point out the specific error you are referring to.

reversed time scale.

 

[Drakkith]

reversed time scale.

And how did you come to that conclusion?

 

[petm1]

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]

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]

I would appreciate if you point a mistake in my interpretation.

In the OP:

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]

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.

 

[RUTA]

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/1/phtoad/v65/i2/p10_s1?view=print&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]

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]

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]

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/article.cfm?id=expanding-universe-slows-then-speeds

 

[Chronos]

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.

 

[Chalnoth]

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.

While strictly true, I'm pretty sure this isn't the case, as it requires perfect fine-tuning of the initial conditions of our universe.