How to prove the stretching of space

ConformalGrpOp

Tim, I believe it is assumed that a fundamental property of EM is that the wavelength of light emitted from a distant source propagating along an unobstructed, field free path of fixed distance to an observer with no relative motion with respect to the source will exhibit no spectral shift.

In other words, it is assumed that the "doppler" and "cosmological" red shifts are not the result of the "non constancy of the wavelength", but a result of the relative motion of the observer with respect to the source (doppler), and the additional distance the light traveled from the time it was emitted to the time it was observed due to the expansion of the universe (cosmological), which looks like the same thing insofar as the expansion imparts a de facto relative motion between the source and the observer. In both cases, it is assumed that the wavelength of the light remains unchanged from the point of emission to the point of observation, and it is only the relative motion of the observer with respect to the source that results in the observer detecting a shift in the wavelength. At least, that is my understanding of the current state of knowledge with respect to the behavior of light. [Note: Some theorists contend that "space stretching", a term I believe is a misnomer, imparts a stretching of the wavelength analogous to the stretching of a dot on the surface of an inflating balloon. But, most knowledgeable scientists would view this conception as little more than a second rate, and certainly misleading pedagogical device. The more accurate explanation is that as the universe expands, the successive photons being emitted from the source travel further than the preceding ones, and this, like the doppler effect, accounts for the observed spectral shift.]

I am inclined to view the problem slightly differently; that Maxwell's equations admit a solution that permits light to "travel" within a metric that is not locally Minkowskian. Therefore, not inconsistent with Milne's view, the observed spectral shift, or perhaps a component of it, is quite likely to be the result of an intrinsic characteristic of light which is not velocity dependent, but rather is time/distance dependent.

Verification of the behavior of light over distances where Hubble's law becomes relevant is within our ability to ascertain by experiment, a circumstance wasnt even a fathomable possibility at the time when Eddington and others announced that the relation between red shift and distance codified in Hubble's law provided compelling evidence that the universe was expanding as predicted by the de Sitter and LaMaitre models.

I don't know of any experiments such as those hypothesized by Poincare, Milne and Whitrow (doing away with rigid rods and synched clocks, and using calibrated light signals), that have been proposed or carried out to specifically test the behavior of light over cosmologically relevant distances.

One might ponder the idea of a massively scaled up version of the Michelson-Morley experiment, (or its modern analog), and wonder would the experiment still yield a null result?

From a historical standpoint, its a bit intriguing to think about what might have happened if the late 19th Century experimentalists had access to the technology necessary to perform such a scaled up version of the M-M experiment and the experiment had yielded a positive result! Would Einstein have been able to successfully convince all those who were held to the theory that light could only propagate if there existed an omnipresent aether that: "NO! The positive results demonstrate a previously unknown property of EM allowed by Maxwell's equations, not proof of our motion through the aether!" ? (Rendering the theory of aether obsolete is, in my view, one of the most significant advances resulting from Einstein's theory of Special Relativity; which is a bit ironic for Einstein's SR was developed out of, and informed by the work of Lorentz and Poincare in attempting to explain the results of the M-M experiment with respect to the aether). Anyway, just a thought. Thanks for taking the time to respond.

Naty1

That captures the alternative perspectives for me....

Chalnoth

I would say this is completely wrong. Most cosmologists use an entirely accurate picture of the expanding universe. That there are other, equally-accurate pictures is irrelevant to this fact, and doesn't mean most cosmologists are misunderstanding something.

ConformalGrpOp

Hi Chalnoth, I'm not one to quibble about such things, and certainly not with the attribution of my comment to " a large percentage" cosmologists (as opposed to "most"). The comment might have been better taken if I made no such particular attribution and simple noted that the misconception is prominent in the field. But, here too, I was speaking to the idea that empty space has physical properties (a new "force-like" characteristic), which is implied in the notion that "space" stretches. This is distinct from the concept of an "expanding" universe, which is perfectly consistent with the physical implications of the standard model.

In this regard, I would simply quote Weinberg on this subject:

"[H]ow is it possible for space, which is utterly empty, to expand? How can nothing expand? The answer is: space does not expand. Cosmologists sometimes talk about expanding space, but they should know better."

But then again, I could be "completely wrong."

Cheers!

ConformalGrpOp

Naty, in general, I would agree that, in some sense, we are talking about a distinction without a difference in cosmological terms. The distinction is made to emphasize the nature of the red shift due to the Hubble relation. But, there are galactic systems which are in relative motion to us separate and apart from that due to "Hubble flow". Accordingly, the spectra of the light observed from these bodies will exhibit doppler shifts distinct from the red shift due to the Hubble relation.

Chalnoth

I'd have to see a lot more context to know exactly what he was saying here. But suffice it to say that the description of the evolution of the universe as expanding space is quite accurate. There are other accurate descriptions, but that is one. And in practice, that way of describing how the universe has changed over time doesn't seem to lead to many practical errors (except for the occasional person that doesn't realize that the expansion is an average effect that does not occur in gravitationally-bound systems).

ConformalGrpOp

It appears that we are working with a common understanding of what cosmologists mean by expanding space. That is, you don't seem to be suggesting that the increase in the "volume" of the "expanding" Universe is the result of (caused by) a physical (force-like) property of empty space.

I take Weinstein's comment to arise from the insight that it is the presence of matter/energy in space which defines the universe. It is only meaningful to state that it is matter/energy that is "flowing" in the models of an expanding universe, and that the statement that "empty space" is "flowing" and expanding in all directions has no physical meaning. Empty space doesn't flow and we do not attempt to measure any characteristic of empty space's nothingness to prove that this nothingness is the source of some physically relevant effect.

It is meaningful to say, in a physical sense, that the distances (the space) between celestial bodies is increasing over time based on our observation of physical events involving matter and energy. It is in this sense that we can say that such "space" is expanding.

In this regard, the analogy of the rising bread dough (expanding space) which "carries" the raisins (matter) out in all directions (before the whole thing is placed in the oven) can be misleading.

timmdeeg

Let's have a look on the empty FRW-Universe. Though being empty it's metric is expanding, regardless whether or not one imagines massless testparticles, whose distances are increasing. The particles are helpful to have a vision of the "flow", but they don't create it.

One can moreover imagine a large non-expanding (due to internal forces) ruler. Then you can visualize the increasing distances between the particles travelling along the ruler. The volume of the cube defined by the ruler isn't interpretable, but the increasing distances are (expansion vs. motion), as pointed out several times by Chalnoth. Admittedly in the case of the empty universe it seems much easier to interpret the recession of "galaxies" as being due to a motion, because in the Milne model, which is equivalent to the empty FRW model, the redshift is purely a Doppler shift.

ConformalGrpOp

I suppose one might be able to say that, in the FRW model, the particles are "of" the flow, and to the extent they arise out of the original conditions and instability that produced the flow, in a sense, they are material to it. How their existence in some sense might "affect" the flow is something that is beyond my pay grade to consider, if its worth considering at all.

I've been reading Relativity Gravitation and World Structure (along with Cunningham's The Principle of Relativity. Last night, by happenstance, I come across a comment by Milne on the question of the "flow" which is pertinent to the discussion. He writes,

"In assigning structure to space, in restoring structure to the structurelessness, mathematical physicists had in effect reintroduced an ether."

Then in a footnote, he remarked:

"This is illustrated by Sir James Jean's remark that the great nebulae are 'mere straws floating in the stream of space', which 'ought to show us in what way, if any, its currents are flowing'."

Just as an aside, in my estimation, the value of this forum would be inestimably diminished but for the willingness of contributors such as Chalnoth and others to offer their knowledge and insights for the benefit of everyone.

Naty1

Conformal:

The idea of 'matter/energy' flowing seems ok.....

but "empty space doesn't flow"....well, yeah, but who says that?? If you are saying 'empty space' is a lousy descriptive term, ok. but few people take the view space is 'empty'........space isn't empty, so whether you like the word 'space' or 'matter/energy' seems a matter of semantics....

second, the cosmological GR model is based on the flow of a 'perfect fluid' flow with varying pressures and densities....[or matter/energy in your terms] so what is described via the model should not be viewed as 'empty' anything.

good discussion!!

Chalnoth

Let me try to be more clear: most cosmologists simply understand the mathematical way of describing the expanding universe, and do not concern themselves with any worries over how to translate that into words. That is, they only care that their equations work out, and consider it to be a pointless exercise to quibble over semantic arguments. And most cosmologists do, indeed, correctly apply the math in most cases.

ConformalGrpOp

This makes perfect sense to me.

ConformalGrpOp

This is consistent with my understanding of the explanation of the red shift phenomenon in the FRW model; to wit, "..cosmological red shift is not, as is often implied, a gradual process caused by the stretching of the space a photon is travelling through. Rather, cosmological red shift is caused by the photon being observed in a different frame to that which it is emitted."

The key point, in terms of interpreting the results from the model is that what is relevant is that "the difference between frames relates to a changing background metric rather than a differing velocity."

As I understand the subject, this insight gets to the root of a fundamental issue about the red shift; that is, if light propagates with respect to the source with a metric that is Minkowskian, then the red shift can be understood as a characteristic time dilation with respect to the frame of the observer in an expanding universe.

This would seem to justify the proper distinction that is made between a doppler shift and a cosmological shift. That is to say, in the case of a doppler shift, the scale of the distance between the source and the observer in relative motion to one another is irrelevant to the red shift observed. In contrast, it is precisely the existence of the dependence of the observed red shift on distance that defines the Hubble relation.

(Note: all quotes in this post are from "Expanding space: Root of all Evil")

wdevlin

When trying to image an expanding timespace frame, let's not neglect that matter itself occupies space and this space too would be accelerating into the 4th dimension. Matter's volume would increase concurrently with surrounding space. It would be virtually undetectable in any give temporal frame of reference. Gravity and time would be the only perceived effects.

The 4th dimension battle is between entropy and inertia.

This tendency towards entropy would be inflation's driving force. Gravity is thus a fictitious force as matter coalesces due to non-uniform expansion rates. Distortions in the rate of inflation at the quantum level will culminate away from space with higher inflation rates that are devoid of other matter and cause itself to spatially move through 3 dimensional space towards higher matter density regions.

As matter still expands and gathers, more inertia induced drag to inflation is created. Matter itself is setting the rate towards entropy (localized time). Culminations of matter will continue to slow entropy until a singularities is achieved.

Drakkith

The entirety of your post sounds like nonsense to me. Do you have some sort of reference that supports your view on expanding matter and the rest of your post?

ConformalGrpOp

I just sat down on a fictitious chair to look at my collection of fictitious Chrondites, Octahedrites, Howardites and Tektites, all of which prove that gravity is a fictitious force arising as matter coalesces due to non-uniform expansion rates.

When it gets to the point where it is permissible to label as a fiction a physical phenomenon which, using the analytical apparatus of scientific investigation, we are able to (a) measure as a force encountered in nature and (b) predict the effects of that force on matter, one might suppose that physics will cease to be a useful intellectual endeavor.

wdevlin

Sounds like you need to begin here... http://www.lifesci.sussex.ac.uk/home....htm#Inflation

And try a different reference frame.