Exploring the Expanding Universe: Understanding the Concept of Space Expansion

[oldman]

...anyone else will tell me the actual way of explaining that universe is really expanding.

I don't think anyone can.

For historical reasons it was assumed, shortly after the isotropic red-shift of the Hubble law was discovered, that it is caused by the isotropic recession from us of remote galaxies. It is of course impossible to check whether this is true by direct measurement, say with rulers or radar.

Instead, cosmologists soon developed a model, based on general relativity, of how the universe can be described as "expanding". This model has proved consistent with many observations, especially with the striking discovery of the cosmic microwave background in the 1960's. The assumption of expansion on which the model is based is now accepted as a working hypothesis (or maybe a stronger truth) by nearly all professional cosmologists.

But over the years there have been problems with the model. Some have been resolved by various devices, most notably the inflationary scenario. Presently solutions include the ad-hoc invention of new imaginary substances like dark matter and dark energy. And other problems remain.

When in physics an accepted consensus persists in generating problems, it is in my view a prudent step to re-examine its foundations. In this case they include a prescription used to measure cosmological distances, called the R-W metric. And, it turns out, this prescription is ambiguous, in that the undoubted change that the universe is undergoing can only be properly described as a change in the ratio of the "metric" coefficients of its space and time coordiate differentials. This means that one cannot with certainty say which, if either -- or both--- of these coefficients are changing.

Assigning change to the space coordinates, as is done in the model that cosmologists use, is semantically convenient. It allows one to describe change as "expansion" --- a familiar concept.

But the question you ask, whether the universe "really" expands, is one that, in my opinion, can't "really" be answered. Yet.

 

[Garth]

The question is: "What do we mean by "really expanding?"

It depends how you measure expansion.

An alternative 'mass field' explanation of Hubble red shift was used by Hoyle in 1975 in order to provide an alternative origin of the CMB. In this theory red shift was caused by distant and ancient atoms having less mass than nearby and recent ones.

On the origin of the microwave background - F. Hoyle Ap.J. 196 661-670 1975 March 15

The universe was static and space-time was divided into a number of four-dimensional volumes which made plus and minus contributions to the mass field. A plus aggregate was bordered by minus aggregates and vice versa.

The BB singularity was simply our observation of the interface between our '+' and surrounding '-' aggregates at which atomic masses were zero and photons were strongly absorbed and re-emitted and were thus thermalised producing the CMB.

However if atomic masses have increased with time then their size would shrink.

Therefore an expanding universe with fixed rulers was replaced by Hoyle with a static universe with shrinking rulers.

As measurements are always that of a ratio between an object and a standard, Hoyle's scenario and the BB are the same thing!

It depends how you measure it!

Garth

 

[Thor]

The Universe is infinite and unbounded - how could it expand?

Three independent values (coordinates) are all that is necessary to uniquely specify any point of existence within the Universe - hence there are said to be three 'dimensions' (more accurately, three axes). If someone wants me to believe the universe is finite, then all they have to do is simply prove to me that at least one of those independent values has a limit (I'll provide the bus fare :).

Similarly, if anyone wishes me to believe there are more than three dimensions, then prove to me that there are locations in the cosmos which cannot be specified within those three aforementioned coordinates.

Good Luck. I don't envy you your task.

 

[marcus]

I don't think anyone can.

For historical reasons it was assumed, shortly after the isotropic red-shift of the Hubble law was discovered, that it is caused by the isotropic recession from us of remote galaxies. It is of course impossible to check whether this is true by direct measurement, say with rulers or radar.

Instead, cosmologists soon developed a model, based on general relativity, of how the universe can be described as "expanding". This model has proved consistent with many observations, especially with the striking discovery of the cosmic microwave background in the 1960's. The assumption of expansion on which the model is based is now accepted as a working hypothesis (or maybe a stronger truth) by nearly all professional cosmologists.
...

I think your picture of history, because of an inverted narrative order, does not do justice to modern cosmology.
It is really important, REALLY IMPORTANT, in Science that a theory makes predictions of a testable effect BEFORE it is observed. That's what the whole current discussion about "falsifiability" is about. Theories have to risk refutation by making predictions BEFORE the experiment or the observation.
Predictions after the fact ("post-dictions") contribute little credibility, by comparison. They could be just fudging around to fit known data.

That is why I am bothered by your account, which seems to INVERT THE HISTORICAL ORDER.

Gen Rel was 1915, at a time when Einstein thought the universe was observed to be static, and it bothered him that his theory seemed to favor dynamic large-scale change. But they went ahead and tested. GR predicted lightbending and in 1919 they checked that.

People like Slipher observed galaxy redshift before Hubble as early as 1917, but they did not formulate a "Hubble Law" distance-redshift relation.
http://en.wikipedia.org/wiki/Edwin_Hubble

Hubble formulated Hubble Law in 1929. He presented it as an EMPIRICAL LAW that he got by plotting observations---something without theoretical basis. But in fact it was a check of General Relativity. This is something that Gen Rel had, in effect, BEEN PREDICTING ALL ALONG----as one of several possibilities for expanding or contracting dynamic universe---although not everybody realized it.

that is not the whole story----Friedmann and LeMaitre had each worked out the expanding universe consequence of Gen Rel before it was clear from the observations that that was what actually happening.
In rough outline Gen Rel (which says dynamic distances) was 1915 and the empirical Hubble Law was 1929. that is a 14 year lead. One can mention Slipher and Keeler and redshifts observed in 1917, but not many people knew about that or saw a pattern, so basically there's 14 years.
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So oldman I would fault your brief historical sketch here because of a certain NUANCE it has by telling the story backwards. It makes it seem as if they FIRST observed Hubble-Law expansion and THEN cooked up a model to fit the observations.

One reason Gen Rel has prestige is that (not only many of its predictions have been exquisitely accurate but also) IT PREDICTED THINGS PEOPLE WEREN'T EVEN THINKING ABOUT OR EXPECTING.

Like the Microwave Background was predicted theoretically actually back in 1948! And not observed until 1965.
As you probably know the 1948 prediction of a CMB around 5 K was published by Gamow, Alpher, Herman. (they came darn close to the right temperature)
=========================

It has been prophetic to an uncanny degree.

And the theory is a coherent organic whole, so it is not so easy to construct an alternative that somehow makes all the OTHER predictions (which involve dynamic geometry) and yet is rigid in largescale distance.

I don't know of anyone doing it. (and it would be after-the-fact POSTdiction, in any case---contriving to fit past data---so less convincing)
==========================

Don't misunderstand me. I feel sure that Gen Rel is WRONG. I expect it to be replaced by a theory that has some quantum corrections. This will, in turn, make some remarkable predictions which people will test and test---until they think they've got the right quantum-corrected version.

but I have no reason to expect that the new improved models will have static distance. Why should they? I see no evidence of static largescale distance in nature, so it is not the sort of feature I would expect a new theory of geometry to have!

I expect OBJECTS, like pieces of metal, to have fairly stable dimensions.
I don't expect largescale DISTANCES in empty space to be stable. I see no convincing evidence of that in nature. My experience does not teach me to expect it. It would surprise me if a theory was concocted that had static geometry and my first reaction would be skeptical. I would anticipate that observational tests (say of light bending) would refute such a theory.
============================
more random thoughts on the subject:
Gen Rel is our prevailing theory of geometry and says, among other things, that distances between stationary objects should normally be constantly changing: Spatial geometry evolves under the influence of matter.

Gen Rel is one coherent theory that makes testable predictions in a lot of different arenas------in solar system (light bending, GPS clock adjustment, Mercury orbit...)-----outside solar system (binary pulsar decay, expansion or contraction of distances). And it is highly successful.

I don't know of a way to buy lightbending (with the observed degree of precision) and all the other precise predictions without buying a geometry with dynamic distances along with the rest. You would have to construct a theory in which spacetime geometry is dynamic is all the ways Gen Rel says EXCEPT the variability of distance. I don't think such a variant of Gen Rel has ever been constructed. Space would have to curve the amount required to match light-bending data, for example, but distances would NOT be able to change with time. It is not obvious to me how one could do that.

 

[MeJennifer]

Gen Rel is our prevailing theory of geometry and says, among other things, that distances between stationary objects should normally be constantly changing: Spatial geometry evolves under the influence of matter.

Define a stationary object.

Space-time, not space, evolves under the influence of matter.

An observer's view of space and time is different than what is happening in space-time.

What is defined as time and space in space-time does not have a one to one relationship with an observer view of time and space. An observer's view on space-time is dependent on it's relative speed and acceleration due to the fact that space or time (but not both) are imaginary in space-time. Furthermore his view is dependent on the amount of warping of his location in space-time.

 

[Chronos]

Define a stationary object.

Relativity insists all objects can be considered stationary within their own reference frame.

Space-time, not space, evolves under the influence of matter.

That makes no sense.

An observer's view of space and time is different than what is happening in space-time.

More nonsense..

What is defined as time and space in space-time does not have a one to one relationship with an observer view of time and space. An observer's view on space-time is dependent on it's relative speed and acceleration due to the fact that space or time (but not both) are imaginary in space-time. Furthermore his view is dependent on the amount of warping of his location in space-time

I'm very confused. What are you talking about?

 

[oldman]

It is really important, REALLY IMPORTANT, in Science that a theory makes predictions of a testable effect BEFORE it is observed. That's what the whole current discussion about "falsifiability" is about. Theories have to risk refutation by making predictions BEFORE the experiment or the observation.
Predictions after the fact ("post-dictions") contribute little credibility, by comparison. They could be just fudging around to fit known data.

I agree with these criteria, which in an ideal scientific world would always be met. One should deeply distrust theories which don't meet them (for example string theory).

I think your picture of history, because of an inverted narrative order, does not do justice to modern cosmology...That is why I am bothered by your account, which seems to INVERT THE HISTORICAL ORDER.

I'm no historian of science, so I might well have done this. I'll try and explain how I understand the sequence of early events after this:

Gen Rel was 1915, at a time when Einstein thought the universe was observed to be static, and it bothered him that his theory seemed to favor dynamic large-scale change. But they went ahead and tested. GR predicted lightbending and in 1919 they checked that.

People like Slipher observed galaxy redshift before Hubble as early as 1917, but they did not formulate a "Hubble Law" distance-redshift relation.
http://en.wikipedia.org/wiki/Edwin_Hubble

Hubble formulated Hubble Law in 1929. He presented it as an EMPIRICAL LAW that he got by plotting observations---something without theoretical basis. But in fact it was a check of General Relativity. This is something that Gen Rel had, in effect, BEEN PREDICTING ALL ALONG----as one of several possibilities for expanding or contracting dynamic universe---although not everybody realized it.

As I see it, long before the isotropic cosmological redshift was discovered as you describe, the practice was to measure red/blueshifts to establish stellar radial velocities and to detect spectroscopic binaries. Such red/blueshifts were (correctly) interpreted as a Doppler shifts caused by stellar motion. It was with this astronomically established perspective that Hubble's redshift was approached, leading to the quite natural conclusion that the universe is expanding and, later, to the GR description of expansion in terms of the R-W metric with its time-varying scale factor.

These are the "historical reasons (why I believe) it was assumed, shortly after the isotropic red-shift of the Hubble law was discovered, that it is caused by the isotropic recession from us of remote galaxies". I didn't mean to minimise what must have been the stunning impact made by the confirmation of Friedmann and LeMaitre's prediction, as deduced from Einstein's GR.

About GR itself: I agree that, as you said: "It has been prophetic to an uncanny degree". And I don't doubt that it is a correct description of how gravity behaves and of gravitational phenomena, such as tidal effects and some others you mention "(light bending, GPS clock adjustment, Mercury orbit...)-----outside solar system (binary pulsar decay..." .

Perhaps I didn't make it clear that I'm not proposing any modification to or replacement of GR itself, let alone a new theory of geometry with static distance. I think GR is the best of the best there is in physics.

But I think an opportunity is being missed for some ambitious cosmologist to consider this: is it just possible that a wider interpretation of how GR is applied to cosmology might throw light on some the problems that modern cosmology is plagued with? Specifically, the accepted interpretation of the R-W metric looks ambiguous to me, and I'm expressing the crazy thought that its present interpretation arose as a plausible historical accident without observational foundation.

...It is not obvious to me how one could do that.

Me neither. But I like discussing such matters.

 

[Garth]

But I think an opportunity is being missed for some ambitious cosmologist to consider this: is it just possible that a wider interpretation of how GR is applied to cosmology might throw light on some the problems that modern cosmology is plagued with? Specifically, the accepted interpretation of the R-W metric looks ambiguous to me, and I'm expressing the crazy thought that its present interpretation arose as a plausible historical accident without observational foundation.

One question is: "How is gravitational/cosmological red shift to be interpreted?"

You may be interested in a published alternative explanation: http://en.wikipedia.org/wiki/Self_creation_cosmology .[/quote]

"some ambitious cosmologist"

 

[oldman]

Our posts crossed, as it were. This in reply to your post #37:

The question is: "What do we mean by "really expanding?"

It depends how you measure expansion... an expanding universe with fixed rulers was replaced by Hoyle with a static universe with shrinking rulers.

As measurements are always that of a ratio between an object and a standard, Hoyle's scenario and the BB are the same thing!

It depends how you measure it!

Garth

I couldn't agree more. In cosmology, as described with general relativity, one measures distance with standards and a metric. By standards I think one means ultimately the laws of physics, provided they are accepted as ubiquitous and eternal, rather than fixed rulers. And in this case the metric is the R-W metric. I also like the idea of ratios being important.

In my previous post in this forum I have repeated to Marcus my misgivings about the accepted interpretation of this metric, which I first expressed in an old thread "How do you interpret the Robertson-Walker metric?" (last posted on: 22 May). Have you any fresh thoughts on this matter?

 

[Garth]

I couldn't agree more. In cosmology, as described with general relativity, one measures distance with standards and a metric. By standards I think one means ultimately the laws of physics, provided they are accepted as ubiquitous and eternal, rather than fixed rulers. And in this case the metric is the R-W metric. I also like the idea of ratios being important.

In my previous post in this forum I have repeated to Marcus my misgivings about the accepted interpretation of this metric, which I first expressed in an old thread "How do you interpret the Robertson-Walker metric?" (last posted on: 22 May). Have you any fresh thoughts on this matter?

As I said "It depends on how you measure it."

In order to make measurements, or any statement at all, of the universe at large we need standard units against which things can be compared. You need something that doesn't change across space and time. That is you need a Conservation Principle.

If that principle is the Conservation of Energy-Momentum, as it is in GR, guaranteed by the Equivalence Principle, then atomic masses are constant and rulers are of fixed length and clocks are 'regular'. They are defined to be so.

In this case, in GR, the the interpretation of the R-W metric is as it is normally understood, the rulers are of fixed length and the universe expands around them from a BB singularity at some fixed point of time in the past.

However if atomic masses are not constant, as in Hoyle's conformal gravity theories, or my SCC then the interpretation of the R-W metric is different.

You have to be consistent with your basic Principles.

Garth

 

[oldman]

You may be interested in a published alternative explanation: http://en.wikipedia.org/wiki/Self_creation_cosmology .

Yes, I am interested, and I downloaded the Wikipedia article some time ago. My trouble is that I am not a cosmologist, although I think I sceptically understand the present consensus and how it came about. My scepticism makes me persist in asking all the awkward questions I can think of.

I am incompetent to judge the merits of Self-creation cosmology. For instance I have not followed the development of the Brans-Dicke theory; I can barely cope with grasping the perspective of GR.

I await the judgement of observation on SCC (the Gravity-B probe experiment?) and the reaction it brings from established cosmologists, although their consensus seems to be almost, but not quite, as entrenched as string-theory dogma, possibly for similar reasons. I suppose one just has to wait and see --- a trying process for an impatient person like myself. In the meantime I am being taught many things by the contributors to this forum.

I can only comment that the Brief Overview section of SCC in the Wikipedia could do with a bit of Brian-Greene-type popularising for folk like myself.

 

[Garth]

I can only comment that the Brief Overview section of SCC in the Wikipedia could do with a bit of Brian-Greene-type popularising for folk like myself.

Have a go!

Garth

 

[oldman]

Have a go!

Garth

Thanks, but it need amplification "for folk like myself, not by such ignoramuses!

 

[J T Francis]

The expansion of the universe found to be accelerating may be explained as a continuation of the initial "Big Bang" force, wherein Matter/Energy is being converted into Space. This antigravitational force {force means acceleration} is in a process of converting antigravitational energy into K.E. and continues to accelerate until the counter- gravitational force becomes dominate and reverses the expansion. A matter of converting the potential energy stored from the antigravitational expansion into a reversing force "G".

 

[Garth]

Hi J T! Welcome to these Forums.

I'm not sure what you mean by the "initial "Big Bang" force". Forces are internal to the spatially expanding space-time manifold. This manifold is curved by the presence of the matter-energy within it and that curvature results in space-like foliations 'within it' expanding with time, also 'within it'.

However, you might want to relate cosmic acceleration with the postulated inflationary period when such acceleration was much greater. It could be caused by a false vacuum energy left over from that period.

Garth

 

[Mike2]

Hi J T! Welcome to these Forums.

I'm not sure what you mean by the "initial "Big Bang" force". Forces are internal to the spatially expanding space-time manifold. This manifold is curved by the presence of the matter-energy within it and that curvature results in space-like foliations 'within it' expanding with time, also 'within it'.

However, you might want to relate cosmic acceleration with the postulated inflationary period when such acceleration was much greater. It could be caused by a false vacuum energy left over from that period.

Garth

What do they mean "false" vacuum? The vacuum is neither true nor false. Do they mean unstable vacuum?

 

[Garth]

What do they mean "false" vacuum?

Good question! It may be just an 'epicycle' to 'save the appearances, but see: False Vacuum.

Garth

 

[Mike2]

Good question! It may be just an 'epicycle' to 'save the appearances, but see: False Vacuum.

Garth

I read somewhere that QFT in curved spacetime does not allow particle creation, that only fields are considered. Could it be that during inflation space was too curved to allow particles to be pulled out of the vacuum. And so there was no matter to prevent spacetime from expanding very rapidly during inflation. But after space flattened out enough, it became possible for the particles of QFT to be pulled out of the vacuum and slow the rate of expansion. Does this sound like a possibility? Thanks.

 

[Harry Costas]

Hello All

If you work with the BBT than the universe originated from one nothing point and expanding into space. Saying that beyond the known universe there is nothing.

For one thing, the universe is endless and when we look into deep field images we see existing galaxing billions of years old. The expanding of the universe is another point. How can an infinite universe expand. The parts withing may contract and expand as we actually observe them.

If the universe is expanding can someone tell me which part.
Our local galaxy is not expanding.
Our local cluster of galaxies is not expanding
Our local cluster of cluster galaxy is not expanding.
Our super cluster of galaxies is not expanding .
see link http://astro.uwaterloo.ca/~mjhudson/research/threed/

So can someone tell me what is expanding?

 

[Jorrie]

Hello All
If you work with the BBT than the universe originated from one nothing point and expanding into space. Saying that beyond the known universe there is nothing.

No Harry, the BB theory does not say the universe originated from ‘one nothing point and expanding into space’! It says that the universe was initially infinitely (or very near infinitely) dense and space started to expand (the BB), making the density less extreme.

My take on your final question: on large scales, we observe that space is (or was) expanding. On smaller scales (even supercluster scale) it is thought that the mutual gravity overwhelms the expansion, which is small over shorter distances. But add enough teeny-weeny bits of expanding space up over the large scales and the expansion 'overwhelms' gravity.

You haven’t asked, but why does space expand?... that's a long story...

 

[marcus]

No Harry, the BB theory does not say the universe originated from ‘one nothing point and expanding into space’! It says that the universe was initially infinitely (or very near infinitely) dense and space started to expand (the BB), making the density less extreme.
.

I agree with Jorrie. Henry also asks "what is it that expands" and I think that DISTANCES expand.

In the best theory of geometry we have, it is naturally for large-scale distances to vary. they can be mostly all expanding, or in principle could be mostly all contracting (but we don't observe that happening) and it can also be a mix depending on how matter is distributed and suchlike conditions.

small scale distances can also vary but ordinarily it is so slight that it is not detectable. brief variations in distance is what gravity waves are all about!

Anyway, I think the secret is to get away from this expectation that distances should be fixed and rigid and unchanging. It simply is not true. We should not expect it.

============
it can sound a bit mysterious to say "space expands" (although that is a fine way to think of it) because then people get the idea that space has to be something, like some material, and they say "what is space" and "how does it expand"----and they sometimes get confused.
So if you don't like that you can just observe that distances expand. It is just a fact of nature. No reason to expect them not to vary. Belief in Euclidean metric is prejudice.