# New space that accounts for dark energy?

Tags:
1. Aug 28, 2013

### keepit

What are the current theories as to the source of the new space that accounts for dark energy?

2. Aug 28, 2013

### bapowell

What do you mean by "new space"?

3. Aug 28, 2013

### keepit

As i understand it, since nothing can travel faster than light and since some galaxies are traveling away from us faster than light, then there must be new space coming into existence between galaxies.

4. Aug 28, 2013

### bapowell

Some galaxies are indeed receding from us at speeds surpassing that of light, but that's true of expansion in general, not only expansion resulting from dark energy. As far as space coming into existence, in what sense do you think space is a "thing" that "comes into existence"?

5. Aug 28, 2013

### marcus

Space is not believed to be a substance or material that needs a "source".
Instead, distances between stationary points increase at speeds proportional to their size, that is by a certain small percentage per unit of time. It's not like ordinary MOTION we are used to, because nobody gets anywhere by it. Nobody approaches any goal or destination, everybody just becomes farther apart. Distances grow.

So you could say that geometry is dynamic. 1915 GR told us that. The network of geometric relations can change and can interact with matter. Dynamic geometry is governed by an equation first written down in 1915. According to that equation the percentage rate of distance growth should gradually decline.

As far as we can tell from observations, the rate of distance growth has in fact been declining, the equation (called Einstein GR equation) fits the data remarkably well!

And the GR equation has a CONSTANT term in it called "cosmological constant" whose effect is to SLOW the gradual decline. Mathematically the constant is one over a certain area, an "inverse area" if you like---technically it is called a curvature (a quantity measured in units of inverse area.) This curvature constant in Einstein's GR equation was always, for no good reason, assumed to be zero and then, in 1998, was discovered not to be zero and was measured to have a small positive value.

Probably because it sounds jazzy and excites interest, the constant acquired the name "dark energy" and its effect was described as ACCELERATION. But actually all we know is there's a nonzero constant in the GR equation that slows the gradual decline in the percentage rate of distance growth.

The distance growth rate used to be 1% per million years (that was a long time ago.)
And it has always been declining, so after a while it was 1/2% per million years.
And now, as near as we can tell, it is 1/144% per million years, and still declining.
According to the standard cosmic model that is used in professional cosmology it is expected to continue tapering off, but not to zero!
Because of the effect of cosmological constant, the decline is expected to level off at around 1/173% per million years.

This is based on the latest observation results (from Planck satellite) that came out in March 2013.

We do not know of any "dark energy" underlying the cosmo constant. That part could just be a fiction.
No reason to speculate about it. Operationally, in terms of what we MEASURE, we can measure that the percentage rate of distance growth is declining on a track that suggests it will NOT go all the way to zero, but instead will taper off to around a steady 1/173% per million years.

6. Aug 28, 2013

### keepit

By "new space" i guess i mean the changes in distances that are occurring.
I understand that decrease in the rate of expansion but it is also true that a distant galaxy's distance from us is not as different last year as it will be this year. I assume that can be called acceleration.

7. Aug 28, 2013

### marcus

Sure you can talk about what happens to a particular distance in those terms. The percentage growth rate is declining, but it is declining SLOWLY. So what we see if we fix attention on one particular distance, say to some particular galaxy we choose, is like a bank account growing at (nearly) constant percentage rate of interest.

The cosmological constant (jazzy name: "dark energy") doesn't make much difference. Before 1998 we thought it was zero and we still understood that distances between CMB stationary objects were growing at a certain percent rate.

That rate now measured 1/144% per My would have been about the same rough size if the cosmo const had been zero.

So cosmo const. or "dark energy" is not the CAUSE of the geometry change governed by GR equation.

GR equation is our law of geometry that describes how distances behave (and angles, areas, volumes). It was already known in 1923 that if you started with distances expanding in a uniform way then they will CONTINUE though perhaps at a declining rate, as indeed was observed later to be the case.

Since geometry we now know is dynamic and governed by GR equation (not by Euclid) we have no right to expect that distances between CMB stationary objects will remain constant. THIS DOES NOT REQUIRE A SOURCE. Distances can simply grow according to a pattern by which, as I said earlier, nobody gets anywhere. The GR equation explains why in a weak gravity field such as we have on earth the spatial geometry is ALMOST Euclidean (and if you include space-time then almost Lorentzian i.e. according to Special Rel) So the GR equation explains why we have the kind of local geometry we see about us in which distances seem not to change. And it also describes something analogous to "momentum" by which, once a general expansion process gets started (say around big bang time) then it will CONTINUE by its own "momentum-like" rules. That's how geometry behaves, according to the modern 1915 theory of it. (which is also our accepted law of gravity, gravity and geometry are the same)

so maybe the answer to your question is THERE IS NO SOURCE because geometry is not like water or air that has a source (it's a bunch of relationships governed by its own type of law)

or maybe the answer to your question is THE SOURCE IS THE GR EQUATION the law of how geometry behaves, in conjunction with matter, once it gets started---the rule according to which geometric change continues

or maybe you would prefer to go way back to the start of expansion, which might have been a big bounce combined with a brief period of inflation (that we do not have a confirmed explanation for, only conjectures about) or some other scenario. If you like you could say that speculative mental picture is the "source".

Personally I think it is clearer to say there is no source of "new" space. I see no way of distinguishing between "new" and "old" space. I can't separate out regions and say this piece is new and this is old. So I think it best to avoid saying words like "source of new space". those are the wrong words to use because they give the wrong ideas---make people think in pictures that do not match reality.

Last edited: Aug 28, 2013
8. Aug 28, 2013

### Naty1

That's a bit like asking "What is the source of new time?"

[Don't know why nobody asks THAT!]

Nothing wrong with either question, by the way.

I sense dark energy is currently a bit less popular than when I started in these forums and "dynamic geometry" more so. Is that because 'dark energy' can't be experimentally verified [see below].

Wikipedia hasn't changed it's descriptions as far as I can tell but this snippet I found interesting:

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

Can anyone provide insights on just what is being measured? Any results?

Wiki also says:
'independent of its actual nature'...that sounds crazy...Has 'negative pressure' aka' repulsive gravity' been divorced from 'dark energy'??

In another discussion in these forums, Chalnoth answered this question:

Chalnoth replied:

[I did not record the discussion link.]

Marcus, what do you think of such 'evidence' .....seems like both cosmological expansion and this Sachs Wolfe redshift relies on some 'hand waving' [aka 'dark energy'] to explain??

9. Aug 28, 2013

### keepit

How about if i call the new space the "new distance" instead of new space or maybe better said the "new difference in distance" between galaxies which would actually be composed of many little new distances that would add up to the new distance (new difference in distances)? I just thought there needed to be an explanation of why certain galaxies are moving away from us at a faster than light speed. Won't all galaxies eventually be moving away from each other at a faster than light speed?

Last edited: Aug 28, 2013
10. Aug 28, 2013

### marcus

Well it's not ordinary motion, so it is not governed by the local speed limit. GR distance growth is not supposed to obey local special rel (SR applies only to flat non-expanding space so it is a local approximation)

But if you don't think of recession/distance growth as motion then your picture is basically RIGHT!

You should realize that MOST of the galaxies that we can see with telescope now today, the distances to them are increasing faster than c. This is normal. So it is not a big deal.

Our local group of a dozen or so galaxies is not destined to be split up. They will continue to orbit each other in a little cluster, and some will merge (like andromeda and us). Perhaps as orbits decay in very very long term all our local group of galaxies will merge.

But other galaxies, not in our local group, will continue to recede until their distances are so great (> 17.3 billion LY) that even the very slow percentage growth rate of 1/173 % per million years means that their distances will be increasing > c. That is already the case with the majority.

Not sure what your point is, or if you are trying to make any special point. Incidental fact: most galaxies we can see have redshift greater than 2. And if a galaxy redshift is > 2 then not only is the distance to it, today, increasing faster than c but also the distance to it WAS increasing faster than c at the time it emitted the light which today is reaching our telescope and allowing us to see it. I think the more precise limit is 1.6, or maybe 1.7. I forget which. Most of what we see in universe was receding faster than c at the time it emitted the light we are now getting from it. This is, of course, not like ordinary motion, just geometry change. and it is normal---the way things are, as a general rule.

11. Aug 28, 2013

### keepit

Thanks, i'm trying to get my mind around the thought that it is only geometric change. It may take a while.

12. Aug 28, 2013

### keepit

Considering the pictures describing the warping of pace in that space seems to go out of existence in locations of high gravity (motion is altered or made to seem like there is less distance than would have been expected between objects). Where does this space go to and when there is little gravity, where does this expanding space come from. Sorry but i know i'm ignoring your explanation of the geometry for the moment.

13. Aug 28, 2013

### Staff: Mentor

You can't ignore the geometry explanation, for that is the correct explanation. This "space" does not go anywhere, come from anywhere, or anything like that. Space is not a "thing". What you would call space is simply the distance between objects. If an object moves twice as far away from me as it was before that doesn't mean that more space was created, it only means that the distance between us doubled.

14. Aug 29, 2013

### Naty1

yes, it requires some different thinking. My prior post suggested one way to think in a new way... 'new distance' is no different than 'new time'.....Regarding "new distance",

doesn't fit the model so well. A major aspect of increasing distance as Marcus has explained results from the geometric assumptions of homogeneity and isotropic space. That is what leads to geometric distance change and expansion at speeds greater than the speed of light. But such a model does not apply well to 'little new distances' within galaxies when things have lumpy masses like planets and stars and black holes.
edit: so there is no easy way to extend the 'new distance' idea in a model consistent way....

[Frankly, I don't like a number of 'explanations' we have today, and although they may be absolutely, precisely correct and there is nothing more to know, I personally hope mankind will have something a lot better a 1,000 years from now. How sad if we did not understand more by then. On the other side of this 'coin', how absolutely incredible that Einstein uncovered as much as he did. It's almost unbelievable that we have the insights we do. ]

Last edited: Aug 29, 2013
15. Aug 29, 2013

### keepit

Naty1 -could it be that the 'new little distances" would only come in when gravity is weak such between galxies but not within galaxies?

16. Aug 29, 2013

### Naty1

could be possible.

Some think galaxies and galactic clusters don't expand because gravity holds them overpowering much weaker 'expansion'. Others think our FLRW cosmological model of homogeneity and isotropic space doesn't even apply at such 'small scales'.

Wikipedia says:
http://en.wikipedia.org/wiki/Integr...te-time_integrated_Sachs.E2.80.93Wolfe_effect

I'm a bit more accepting of dark energy as a possibility than expressed here by Marcus, since as far as I know accelerated expansion in the early universe, inflation [ the exponential expansion of the very early universe] has as its generally accepted source a negative pressure...a temporary higher level of vacuum energy. So why not some remaining dark energy [negative pressure] as the cosmological constant....as a corollary to spacetime. However wiki also says:

so my conclusion so far is nobody really knows.

While I am on it, Drakkith posts:
and I'd say that likely reflects the majority opinion I have seen in these forums. I prefer to instead think of it as 'something', but who knows what....that's been a subject of other discussions in these forums:

Tomstoer has one post I saved:

also, it curves!

17. Aug 29, 2013

### keepit

As i understand it, mathematics sometimes only describes relationships and changes but sometimes not the causes of the changes. If that is true, could you apply that reasoning to the cause of the geometric changes? I apologize if this is too philosophical.

18. Aug 29, 2013

### PAllen

Let me add three more observations on why distance growth is not the same as relative speed.

1) Even in SR, in standard coordinates, if object A is going left at .99c and object B is going right at .99c, the distance between them, in this frame, is growing by 1.98c. Despite this, the speed of A relative to B is .99995c. This is very analogous to the cosmology situation in that 'distance growing' is in a common CMB frame (coordintates, really), not in coordinates adapted to galaxy A or galaxy B.

2) The Milne coordinates, which model an expanding universe in flat, Minkowski space, show growth of distance (recession speed) of much greater than 2c. While the Milne universe is physically implausible (and contradicted by observation), this shows how recession speed is a function coordinates not an invariant fact. Ultimately, the only difference between the Milne 'universe' and Minkowski coordinates is choice of foliation, which can never be physically significant.

3) In GR there is no unique way to define relative velocity of distant objects. What makes it possible in SR is path independence of parallel transport. However, despite lack of uniqueness, one may ask what happens if you compare the relative speed of distant galaxies by parallel transport along various paths. The result is that while the 'relative speed' varies with path, it is always less than c.

I really think cosmologists create much confusion by calling a growth of separation, which is routinely greater than c, a recession velocity, which leads to the perception that it is like a relative velocity.

Last edited: Aug 29, 2013
19. Sep 1, 2013

### keepit

"in what sense do you think of space as a thing that comes into existence?"
I'm going to try to give a thought that will be pummeled but i'm here to learn.
Could space be like a blob of something that can be stretched or compressed? When it is stretched could space from the other dimensions (Calabai yau) move to fill the less dense space and vice versa could space from our dimensions be compressed into the Calabai Yau spaces. Could gravitons be the mediator of this stretching and compressing?

Last edited: Sep 1, 2013
20. Sep 8, 2013

### Mordred

space by itself has no substance, its simply put geometric volume. At one time the substance of space was called the ether. This is no longer supported. The changes in volume are a direct relation to the energy densities (pressure relations) between baryonic matter, dark matter and dark energy, as well as relativistic and non relativistic radiation in a small contribution. The graviton itself is a mediator boson of gravity but has never been confirmed to exist.

Last edited: Sep 8, 2013
21. Sep 10, 2013

### andrewkirk

Contrary to popular belief, science does not deal in causes, in the everyday sense of that term. Physicists sometimes talk about 'causality' but that usually means something quite different from the everyday meaning. What science does is find mathematical laws that describe how systems evolve. Einstein's GR equation is an example of that. Einstein didn't say anything about 'why' spacetime obeys that equation. He just hypothesised that it does, then did some calculations that enabled us to experimentally test whether it does, and it did.

If we ever find out the 'reason' for the GR equation, it'll probably be just another bunch of lower level equations, that have no apparent 'reason' for them either.

Science is a pragmatic discipline.