dark energy (not so) obviously

[alexro]

Could anyone please explain to me how can an accelerated expansion take place in a homogenous & isotropic universe? I really can't figure that out - at all.

 

Not only cannot happen radial, but not even linear. Unless I'm terribly wrong - and there's where you'll probably come in.

 

If you even take the time for an (simple !) illustration, I would thank you even more.

 

By the way, not only I cannot figure out dark energy, but I cannot imagine expansion even before DE - how could even a constant expansion, or a deccelerating one, could take place in a homogenous & isotropic three-dimensional universe? As soon as I think of several points in space and try to imagine what would happen (the observations should be the same, but they can't be!), well...

 

I'm really waiting for your input. Thank you so much.

 

[Arch2008]

Welcome alexro!

Accelerated expansion actually creates a universe that is homogeneous (the same everywhere) and isotropic (a universe that can give rise to sentient creatures like us). Alan Guth proposed that the very early universe underwent inflation on an epic scale. This created a homogeneous effect that gave rise to stars and star stuff like planets and us. It is reflected in the Cosmic Microwave Background Radiation. The CMBR displays only a minute variation in the remaining background temperature throughout the observable universe.

Inflation has met some predictions, so even if you cannot figure it out, it does not mean that it cannot happen. Take a Schwartzschild singularity at the center of a blackhole. It's a point because gravity has crushed everything, to include spacetime into a point. So too much gravity gets you no dimensions. Now take the opposite point. The predominance of anti-gravitational inflationary stuff, like Dark Energy, causes the original spacetime singularity to expand into three separate dimensions plus time. As the universe continued to inflate after about 7 billion years, the effect of gravity was supposed to slow the expansion. However, as the distance between super galaxy clusters increased, the effect of gravity in the regions in between was again overcome by the predominance of DE. The universe is undergoing accelerated inflation...again.

Edited February 19, 2013 by Arch2008

[Spyman]

alexro, try to imagine that you are in a very large building that has various objects spread out on the floor in a homogenous and isotropic pattern. You have a clock and a ruler that you use to measure and record distances between the objects on regular intervals.

After several sets of measurements, you will notice that you, your ruler and all other objects inside the building appears to be shrinking, because all distances between each object increases at the same scalar way, over the duration of a certain time.

You get worried and start making more accurate measurements, which reveal that the rate of change slowly increases with time.

Next day when you meet up with another observer and compare measurements, you notice that while ranges and durations match, the other person thinks that it is the building that is growing and bringing the objects apart instead of objects shrinking.

 

In Einstein's theory of general relativity the geometry of space is dynamic, it can expand or contract, and when applying this to cosmological models the scale of space in the Universe can change relative the observer's ruler, such that distances between objects can increase without actual movement. Acceleration means that the rate of expansion is speeding up with time and happens faster.

Follow the link under the quote and you will find much more to read:

"The metric expansion of space is the increase of the distance between two distant parts of the universe with time. It is an intrinsic expansion whereby the scale of space itself is changed. That is, a metric expansion is defined by an increase in distance between parts of the universe even without those parts "moving" anywhere. This is not the same as any usual concept of motion, or any kind of expansion of objects "outward" into other "preexisting" space, or any kind of explosion of matter which is commonly experienced on earth."
http://en.wikipedia.org/wiki/Metric_expansion_of_space

[alexro]

@Arch2008:Thank you for both your welcome & reply, but what I asked has nothing to do with inflation.Im puzzled however that you said:Alan Guth proposed that the very early universe underwent inflation on an epic scale.Well, to my knowledge, it wasnt epic at all. When it ended, the universe wasnt larger than a golf ball. If Im wrong, please correct me.You also said:Inflation has met some predictions, so even if you cannot figure it out, it does not mean that it cannot happen.Mute, since I didnt talk about inflation at all.Then, what you say in the last part of your comments, in my view, cancels evolution of stars, planets, etc. How could they have formed? Because, remember, the universe is STILL expanding at an accelerated rate,So, Im still puzzled.Thanks for the comments though, you were kind to reply.Now I think its your turn to understand what I meant: in an accelerated expansionary universe, 2 separate observers simply cannot see the same thing. Thus, although for one of them the universe may look isotropic & homogeneous, for the other it wont.For the first observer (if his universe is isotropic & homogeneous), wherever he looks (any line of sight) he would detect accelerated expansion (outwards from his location). Another observer would see very different things: inward expansion along the line of sight towards the first observer, and in all other directions varying according to the angle to that first line of sight.

 

@SpymanThank you for your reply. But you stick to only one point of observation. What I meant was multiple observers. Once you introduce another observer in the universe (different location), he cant possibly observe what we observe. Remember, the further away, the faster things appear to move from the observer. So the second observer not only will not see WHEREVER he looks the universe appearing isotropic (as the first observer), but also looking straight towards the first observer (along the line uniting them) will see something very different.How can that be?See also my last comments for Arch2008.Thanks.

[ACG52]

When it ended, the universe wasnt larger than a golf ball

 

Yes, but when it began, it was smaller than a proton. (10^-26 cm) And it all happened between 10^-43 and 10^-35 seconds.

 

That's pretty epic.

[Spyman]

@Spyman Thank you for your reply. But you stick to only one point of observation. What I meant was multiple observers. Once you introduce another observer in the universe (different location), he cant possibly observe what we observe. Remember, the further away, the faster things appear to move from the observer. So the second observer not only will not see WHEREVER he looks the universe appearing isotropic (as the first observer), but also looking straight towards the first observer (along the line uniting them) will see something very different. How can that be? See also my last comments for Arch2008. Thanks.

Actually my analogy did allow for several different locations of observations since the observer was moving around inside the building, while measuring distances with a ruler, so there was access to more than one view and I even included a meeting with another observer to compare results, but I think I understand your question better now - so let's try again.

(I am not going to do this in full 3D, so I hope that this 2D visualization is enough.)

 

Here are two images of evenly distributed objects, painted as black dots, but to distinguish two of them from the others, they have been colored red and blue. Space has been expanded by about a quarter between images:

 

If I take those two images and put them above each others with the red dot in the center, to show how the red observers views the expansion, then it looks like this:

 

And likewise, the same two images above each others but with the blue dot in the center, to show how the blue observers views the expansion, then it looks like this:

 

Finally we can turn the image with the blue dot in the center around and place them next to each others, to show that their views are identical regarding space expansion:

Edited February 20, 2013 by Spyman

[Arch2008]

Alexro, everything you posted is about inflation. That is what the accelerated expansion is called. You are asking about inflation and that is what you are not understanding.

Two scientists will observe the same thing. A prediction has to match the result no matter who does the test. That makes it science. Physics is the same no matter where you are. An observer somewhere else in the universe will see inflation occuring in every direction as well. The universe is not expanding here and contracting there. In between every super galaxy cluster the effect of gravity is overcome by DE as space inflates the way it did after the Big Bang.

However, the matter within the gravity of a galaxy is not expanding. Within galaxies stars and planets are still forming. After a period of exponential (some would say epic) inflation, the universe expanded at a slower rate that allowed hydrogen to form into clouds and collapse into massive stars. These massive stars erupted after only a few million years releasing the elements that became planets and eventually you and me.

It's not easy to percieve just from some posts. Would you prefer links to more detailed explanations?

[alexro]

@Arch2008:You say:"everything you posted is about inflation."Actually, nothing of what I said is about inflation. Accelerated expansion is dark energy, not inflation. Compared to inflation, accelerated expansion is nothing.You say:"Two scientists will observe the same thing."Actually, I don't know of any scientists who would claim they can observe inflation. Not only was so short in time (EXTREMELY), but it was long ago and far away...You say:"However, the matter within the gravity of a galaxy is not expanding."Actually, there are theories whether the expansion of matter also takes place or not.If you know for sure they settled it, I would like the source, please.You say:"Within galaxies stars and planets are still forming."Well, for me it's obvious THEY CAN'T FORM in the conditions of even a constant expansion that pulls everything away form all the rest, let alone an accelerated expansion. That's why I'm puzzled.Thank you.@SpymanI really want to thank you for taking the time with those illustrations. You are very nice indeed.I tried to figure it out, but I suppose I'm too tired right now. I'll come back on this. At first look, seems it might work, but I'm still not convinced about it in 3d. Things get very complicated in real space.Even more with an accelerated expansion. At that time, I think I read something like "the further away, the faster it moves away", but I will have to check that.So, see you tomorrow, and thank you so much again.

 

Spyman: My reply to you is above. I really don't know what happens, and all my sentences are pulled together (even different paragraphs) almost gravitationally...

[ACG52]

Actually, there are theories whether the expansion of matter also takes place or not

Please provide a citation for this.

 

Well, for me it's obvious THEY CAN'T FORM in the conditions of even a constant expansion that pulls everything away form all the rest, let alone an accelerated expansion. That's why I'm puzzled

 

Gravity is overwhelmingly stronger than expansion, out to distances of 200 million lys (the size of the local galactic supergroup). We don't see expansion closer than that because gravity holds things together. That's why we don't see the distance between the stars in our galaxy increasing. That's also why Andromeda is moving towards the Milky Way.

[pwagen]

Ignoring inflation for a while, are you simply asking why the expansion of the universe is expanding? Because to me, the answer would have nothing to do with inflation, as you point out, since the expansion slowed down after it was over (didn't it?), but has been picking up the pace after the event. I'm sure better knowledgeable will correct me about the details of that description, but the question is; is the original question about the accelerated expansion AFTER inflation?

[Arch2008]

_{I believe that someone (quite) obviously is just trolling.}

Edited February 21, 2013 by Arch2008

[alexro]

I don't know what happens, lately I can't even use citations. Please excuse me.@ACG52You say:"Please provide a citation for this."In the late years I read quite a few theories of expansion including the matter.I suppose you won't expect me to remember where exactly I read it, would you?Anyway, you can search online by yourself, I suppose.Now, about gravity, you may have a point there, but there's a point even larger: think about dark matter.@pwagenYou ask:"is the original question about the accelerated expansion AFTER inflation?"Obviously not, since after inflation the expansion could ONLY decelerate (compared to inflation). No, I'm talking about what they're currently seeing. They were even awarded a Nobel prize, you know...@Arch2008You say:"I believe that someone (quite) obviously is just trolling."An why exactly would you think that?@SpymanThanks again for your illustrations, but I don't think they work. I don't think that's what they would see. They might see that independently, but the question is: will they see that AT THE SAME TIME? Because expansion (apparent movement) FROM observer 1 is TOWARD observer 2.And an accelerated expansion detected by observer 1 will be perceived by observer 2 (at least on the line of sight directly towards observer 1) as deceleration.Thanks to all.If again all my posts will be joined together, please be aware that I replied above to everybody. Thanks.

 

If again all my posts will be joined together, please be aware that I replied above to everybody. Thanks.

[ACG52]

"Please provide a citation for this." In the late years I read quite a few theories of expansion including the matter. I suppose you won't expect me to remember where exactly I read it, would you? Anyway, you can search online by yourself

The only 'theories' incuding the expansion of matter are from cranks.

 

Because expansion (apparent movement) FROM observer 1 is TOWARD observer 2.

 

This is not the case. It's been very clearly explained, and I'm tending to agree with Arch.

[pwagen]

@pwagen You ask: "is the original question about the accelerated expansion AFTER inflation?" Obviously not, since after inflation the expansion could ONLY decelerate (compared to inflation). No, I'm talking about what they're currently seeing. They were even awarded a Nobel prize, you know...

I think you'll find that the universe is expanding at an increasing rate, which is what they're actually seeing. Which Nobel prize, specifically, was given to someone who showed the universe's expansion is decelerating?

 

http://en.wikipedia.org/wiki/Accelerating_universe

[alexro]

The only 'theories' incuding the expansion of matter are from cranks.

 

 

This is not the case. It's been very clearly explained, and I'm tending to agree with Arch.

 

 

@ACG52You say:"The only 'theories' incuding the expansion of matter are from cranks."Really? Well, a lot of guys in the quantum are looking for this. Even Dirac asked himself that question at some point.

 

@ACG52 You say: "This is not the case." Actually, this is the case. But if that's not the case, please explain acceleration then.

 

@ACG52 You say: "I'm tending to agree with Arch." You mean you tend to agree with the fellow who doesn't even know the topic of the discussion? Then you also don't know the topic. Why don't you find out what you're taking about before actually talking. This is supposed to be a science forum, isn't it?

 

@pwagen: "Which Nobel prize, specifically, was given to someone who showed the universe's expansion is decelerating?"I disn't say that (on the contrary). You arrived at your own conclusion.

[pwagen]

@pwagen: "Which Nobel prize, specifically, was given to someone who showed the universe's expansion is decelerating?" I disn't say that (on the contrary). You arrived at your own conclusion.

Really?

 

@pwagen You ask: "is the original question about the accelerated expansion AFTER inflation?" Obviously not, since after inflation the expansion could ONLY decelerate (compared to inflation). No, I'm talking about what they're currently seeing. They were even awarded a Nobel prize, you know...

Please clarify which Nobel prize you're talking about, and why they got it.

Edited February 22, 2013 by pwagen

[ACG52]

Because expansion (apparent movement) FROM observer 1 is TOWARD observer 2.

This is not what is observed. What is observed is that the expansion is between all points in space.

 

But if that's not the case, please explain acceleration then.

 

 

 

You seem to be having a very difficult time with a very simple concept. After the initial Inflationary period ended, the expansion of the universe gradually slowed down. This is what would be expected because of gravity. But then, about 7 billion years ago, the rate of expansion began to increase. This was not expected, and it was for discovering the ACCELERATED EXPANSION that the 2011 Nobel prize in Physics was awarded to Saul Perlmutter, Brian Schmidt and Adam Riess

 

I disn't say that (on the contrary). You arrived at your own conclusion.

 

On the contrary?

This is what you said.

Obviously not, since after inflation the expansion could ONLY decelerate (compared to inflation). No, I'm talking about what they're currently seeing. They were even awarded a Nobel prize, you know...

Not much room to weasle out of what you said. So what Nobel prize was given for currently seeing deceleration?

 

 

 

 

You say: "I'm tending to agree with Arch."

 

 

Yes, I'm tending to agree that you're a trolling crank. Prove me wrong.