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Sirthomas42
2015-Sep-25, 05:02 AM
Forgive me if this has been asked and answered elsewhere, my (brief) searching didn't readily give me a satisfying answer...

If the universe is not just expanding, but expanding at an accelerated rate, shouldn't distant galaxies appear to be moving away from us *slower* than nearby galaxies? I feel like there's something I'm missing... but, since looking at a distant object, say, 5 billion light years away, we see it as it was 5 billion years ago... so if the expansion rate of the universe is accelerating, that is, it's expanding faster today than yesterday, and faster tomorrow than today, etc, then shouldn't it have been expanding more slowly in the past, and a distant object appear to be moving away more slowly to reflect that?

antoniseb
2015-Sep-25, 01:22 PM
Your thought has some of the tricky stuff right, but acceleration is the 2nd derivative of distance, not the first, so the amount they are receding per unit distance is increasing, but it is always the case that of two galaxy clusters the more distant one is more red-shifted. It is also worth noting both: that the rate of acceleration is quite small, and that there are other factors involved complicating this relationship from any view we get before the universe was six billion years old.

Ken G
2015-Sep-25, 10:00 PM
Forgive me if this has been asked and answered elsewhere, my (brief) searching didn't readily give me a satisfying answer...

If the universe is not just expanding, but expanding at an accelerated rate, shouldn't distant galaxies appear to be moving away from us *slower* than nearby galaxies? I feel like there's something I'm missing... but, since looking at a distant object, say, 5 billion light years away, we see it as it was 5 billion years ago... That's what you are missing, the redshift we see of a distant galaxy does not depend on how it was moving 5 billion years ago, it only depends on what has happened to the expansion since then. This is because of the "cosmological principle", which asserts that anything that has happened to the expansion since then is happening everywhere and to everything-- including that galaxy! So for example if the expansion ground to a sudden halt a second after 5 billion years ago, then the galaxy we see 5 billion light years away will not be redshifted at all-- even though we are seeing it as it was at a time prior to the halting of the expansion!

The way to understand this is to recognize that cosmological redshifts should not be thought of as being due to the motion of the galaxy when its light was emitted. In fact, it's better to imagine that none of the galaxies are moving at all! Instead, imagine they are being "carried along with" the "expansion of space", like non-moving pieces on a rubber chessboard that is being stretched. Now imagine that the redshift we see is nothing other than the amount that wavelengths are stretched during propagation to us-- which is just the factor by which space has expanded as the light propagated. The exact same thing would happen to the stretching of a line of ants moving between the chess pieces on that rubber board. That's why if the expansion ground to a sudden halt, we would not see redshifts in light that was emitted not only after the halting, but even slightly prior to it-- no stretching during propagation, no redshift. So the key point to get is, all redshifts give us a record of the total amount of universal expansion since the light was emitted, not the conditions of the expansion when the light was emitted. It's not saying that space "really is expanding", because we don't actually have a model of space, it's merely a convenient picture that one can use when applying the cosmological principle to general relativity.

ShinAce
2015-Sep-25, 10:52 PM
Just to reiterate, the redshift of an observed galaxy tells you the size of the universe (when the light was emitted) divided by the size of the universe (when that light is detected). The history of the expansion is irrelevant. Whether it has been slowing down, coasting at constant speed, or accelerating makes no difference. It could even be slowing down for a week, then speeding up the next, and so forth.

It's by plotting redshift versus distance that we get the expansion history. Measuring distance, however, is quite tricky. Hence why we resort to 'standard candles' to get a baseline luminosity(or flux) that we can use confidently.

GarethMeredith
2015-Oct-03, 06:25 PM
Forgive me if this has been asked and answered elsewhere, my (brief) searching didn't readily give me a satisfying answer...

If the universe is not just expanding, but expanding at an accelerated rate....

Might be interesting for you to know that we now think the universe is expanding at least at a steady rate. It seems we got enthusiastic and wrong.

Exposed
2015-Oct-03, 07:08 PM
Might be interesting for you to know that we now think the universe is expanding at least at a steady rate. It seems we got enthusiastic and wrong.

Can you elaborate on this further? Are you saying we no longer believe the expansion of the universe is accelerating?

GarethMeredith
2015-Oct-03, 07:11 PM
Can you elaborate on this further? Are you saying we no longer believe the expansion of the universe is accelerating?

That is what I am saying. It will take a while for the evidence to soil properly, but yes, we are now thinking it isn't any more.

http://uanews.org/story/accelerating-universe-not-as-fast

Exposed
2015-Oct-03, 07:19 PM
That is what I am saying. It will take a while for the evidence to soil properly, but yes, we are now thinking it isn't any more.

http://uanews.org/story/accelerating-universe-not-as-fast

What I got from that link is that the acceleration of expansion may not be as what we thought, but there is still acceleration in the expansion of the universe.

What you said first is that expansion was at a "Steady state", which means no acceleration. I don't think that is mainstream. Plus this paper you linked still needs to be peer reviewed.

GarethMeredith
2015-Oct-03, 07:31 PM
I should have said ''at least a steady state'' but far slower than previously expected.

Give it time, a lot of investigation has went into this, it will be peer reviewed no problem. Watch this space, ''imma good at this stuff momma!''

ShinAce
2015-Oct-03, 08:19 PM
Yet space is independently measured as being flat. So if we must decrease the amount of dark energy in the energy we must then increase the estimate for the amount of matter/dark matter.

Look at the wiki for dark energy and you'll realize there's already quite a bit of wiggle room:
"he best current measurements indicate that dark energy contributes 68.3% of the total energy"
"Recent observations of supernovae are consistent with a universe made up 71.3% of dark energy"
"The Wilkinson Microwave Anisotropy Probe (WMAP) spacecraft seven-year analysis estimated a universe made up of 72.8% dark energy"

That paper requires follow-up observations and corroborating evidence. A prince may one day be king, but there are steps that prince must follow before ascending the throne.

Reality Check
2015-Oct-06, 12:01 AM
What you said first is that expansion was at a "Steady state", which means no acceleration. I don't think that is mainstream. Plus this paper you linked still needs to be peer reviewed.
"Steady State" has a specific meaning in cosmology - no expansion at all. It has been ruled out for decades.
Accelerating Universe: Not As Fast? (http://uanews.org/story/accelerating-universe-not-as-fast) is about papers that find that type Ia supernovae fall into 2 populations rather than 1. The authors speculate that this will have an effect on the measurement of the rate of acceleration.
The papers have been published, e.g. THE CHANGING FRACTIONS OF TYPE IA SUPERNOVA NUV–OPTICAL SUBCLASSES WITH REDSHIFT (http://iopscience.iop.org/article/10.1088/0004-637X/803/1/20/meta;jsessionid=165BE5B9FC2A01188D2638C84DC0CFF0.c 1)

...Not accounting for this effect should thus produce a distance bias that increases with redshift and could significantly bias measurements of cosmological parameters.

StupendousMan
2015-Oct-06, 02:00 AM
"Steady State" has a specific meaning in cosmology - no expansion at all.

Actually, the "Steady State" model proposed by Hoyle does, in fact, expand. This article, published in 1954,

http://adsabs.harvard.edu/abs/1954Obs....74..253H

has the title "The steady-state theory of the homogeneous expanding universe".

In this model, the universe expands, but new matter is created in the vacancies
left between the retreating galaxies, causing the overall density to remain
uniform on large scales.

GarethMeredith
2015-Oct-06, 08:31 AM
Actually, the "Steady State" model proposed by Hoyle does, in fact, expand. This article, published in 1954,

http://adsabs.harvard.edu/abs/1954Obs....74..253H

has the title "The steady-state theory of the homogeneous expanding universe".

In this model, the universe expands, but new matter is created in the vacancies
left between the retreating galaxies, causing the overall density to remain
uniform on large scales.

That's right.

Cougar
2015-Oct-06, 11:44 AM
Might be interesting for you to know that we now think the universe is expanding at least at a steady rate.

That's wrong.

GarethMeredith
2015-Oct-06, 11:46 AM
That's wrong.

It's already been pointed out.

I chose my words wrong. It's not accelerating anywhere near as fast as we once thought.

Cougar
2015-Oct-06, 12:53 PM
It's not accelerating anywhere near as fast as we once thought.

"Anywhere near" is kind of imprecise. Can you quantify that?

GarethMeredith
2015-Oct-06, 12:58 PM
"Anywhere near" is kind of imprecise. Can you quantify that?

Why would I be able to quantify it? I don't study the area, I ONLY quote the scientists who do.

GarethMeredith
2015-Oct-06, 12:59 PM
http://www.dailygalaxy.com/my_weblog/2015/04/new-light-on-our-accelerating-universe-not-as-fast-as-thought.html

GarethMeredith
2015-Oct-06, 01:01 PM
Besides, there are so many variables to consider, universal acceleration remains a theory. In my opinion, there are things that are neglected, such as there being a natural Hubble law stating the further away objects are, the faster they seem to accelerate away.

Why this has never been an acceptable answer is unclear and I've never had an acceptable answer.

mkline55
2015-Oct-06, 01:56 PM
Besides, there are so many variables to consider, universal acceleration remains a theory. In my opinion, there are things that are neglected, such as there being a natural Hubble law stating the further away objects are, the faster they seem to accelerate away.

Why this has never been an acceptable answer is unclear and I've never had an acceptable answer.

I believe I questioned the terms before. In normal parlance, "expansion is accelerating" would mean that if you drew a chart with a straight line representing average expansion of say 70 km/s/kpc, then nearby objects, should appear to be above that line (above 70km/s/kpc), and farther objects should be below that line. But I believe that is a misunderstanding of what astronomers mean by "accelerating expansion".

WayneFrancis
2015-Oct-07, 12:27 AM
Besides, there are so many variables to consider, universal acceleration remains a theory. In my opinion, there are things that are neglected, such as there being a natural Hubble law stating the further away objects are, the faster they seem to accelerate away.

Why this has never been an acceptable answer is unclear and I've never had an acceptable answer.

This is a problem where not understanding or even knowing how they determined that scientists have determined the acceleration is increasing is a big problem.
First you have the speed at which things are receding. That does increase with distance because of Hubbles law. So IF the acceleration of expansion was always the same we'd see something different then we do.
Scientist can determine the rates at different points. Sure add them all up and they do increase but it isn't as simple as that. We happen to be looking back in time so we don't see a "universal now" if there even could be one.
Very simplistically we observe 2 supernova and determine their distance to us as some distances. The 2nd super nova 2x the distance as the first.
If the expansion was constant we'd expect the 2nd super nova's light curve to be stretched out 2x more then the first. But we don't see that We see that that it is different which indicates that the expansion rate has changed.

For the lay person out there. If you have a question about main stream science and think the answer is not acceptable there is a good chance you just don't understand the answer. You may not have been provided the full answer because, honestly, it would go over the heads of most people and/ or make most peoples eyes glaze over. So if you want an "acceptable answer" your first objective is to fully understand the current mainstream answer. Then, and only then, you can start attacking the current mainstream answer. If you don't understand the current mainstream answer then how can you complain that it is wrong. It is like saying "I don't understand how the engine in my car works but I know the mechanic doesn't know what they are saying when they tell me the fuel injection system is clogged. I mean there are so many other things under that hood that they haven't considered...."