# Does Space Expand?

by Wallace
Tags: expand, space
 Sci Advisor P: 1,253 The problem, or I guess the trick is the rate of expansion is irrelevant. It is the acceleration of the expansion that tells you what happens. So in a contracting universe the particle could move away, or in an expanding universe the particle could comes towards you. You don't intuitively expect this behavior if you think of the universe as a loaf of rising bread filled with raisins!
 P: 33 So, it falls under the category of 'hey, that's cool', rather than 'something's amiss'. Got it, thanks.
 Sci Advisor P: 1,253 Right, but there is something amiss in that the devices used to explain to people how the universe works leads the intuition astray. It's a question of pedagogy rather than physics admittedly.
 P: 2,043 I think the problem is in the teaching. Those people who teach that things like "cosmological time", "preferred space-time hyperplanes", "spatial distance", "time", "preferred coordinate systems", "preferred metrics (even with cross terms)", "expansion", "Hubble flows" etc are absolute properties of the universe as modeled by general relativity. Only, IMHO, to confuse even more. These things are dissections or space-time, very useful for analysis, but once these things start living a life of their own and represent "The Universe" the recipients of all this will completely miss the point. Furthermore: "An expanding balloon with coins stuck on it that do not expand themselves", is about the worst model I have ever encountered. "A wavelength stretching apart due to the expansion of space", is a close second. Anyway I am sure many will completely disagree with me.
Astronomy
PF Gold
P: 22,808
 Quote by MeJennifer Those people who teach that things like "cosmological time", "preferred space-time hyperplanes", "spatial distance", "time", "preferred coordinate systems", "preferred metrics (even with cross terms)", "expansion", "Hubble flows" etc are absolute properties of the universe as modeled by general relativity. .
I havent encountered such people. Would you allow that it is possible you may be misunderstanding others and imagining that they are saying what they are not?
The properties you mention seem to be by and large to be properties of a particular solution to the Einstein equation. They are not absolute.

Once one has narrowed down to a particular metric, which is a particular solution, then there may well be a preferred time, an idea of being at rest with respect to Hubble flow, a preferred foliation or spatial slicing etc. These things are not absolute, but depend on one's choice of metric---and hopefully the metric will be a reasonably good fit to observation.
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You said something about "background independence" which I think needs clarifying. Many people use this term to describe GR and other theories which can be constructed without using a prior-choice of background metric
QFT is NOT background independent because at the very start, in constructing it, you have to commit to some rigid geometry---can be curved but typically is just flat Minkowski space.
GR IS background independent because you start with a continuum with no fixed geometry. You can define the theory without resorting to a background metric. Background independence is a fairly unusual property for theories to have. Quantum gravitists generally want their theories to have this property because they are aiming at getting a quantum theory with the main features of GR.

But once you HAVE a solution to the Einstein equation, a metric, say like the FRW metric-----or the flat Minkowski metric (also a solution, just a different solution obtained with zero matter)----then there is no more expectation that there will be background independence!

Solutions to the Einstein equation typically do not have Poincaré symmetry either. The flat (empty universe) solution DOES have the global symmetry one learns about in Special Relativity. But generic solutions do not
P: 24
 Quote by MeJennifer "An expanding balloon with coins stuck on it that do not expand themselves", is about the worst model I have ever encountered.
What is wrong about thinking of a "balloon", if you think of it as an HyperSphere and not a 3D balloon ?
P: 2,043
 Quote by Ballon What is wrong about thinking of a "balloon", if you think of it as an HyperSphere and not a 3D balloon ?
"Expansion of space" is a completely wrong terminology.
It implies that space is some sort of a substance that can expand and contract.

That an observer measures a change in distance is perfectly valid in relativity but it has nothing to do with an expansion or contraction of space.

"Photon's being streched by exanding space" is another one these absurd phrases.

That an emitter and an absorber of a photon measures a different frequency is perfectly valid in relativity but it has nothing to do with a change in the state of the photon.
PF Gold
P: 3,273
 Quote by MeJennifer "Expansion of space" is a completely wrong terminology. It implies that space is some sort of a substance that can expand and contract. That an observer measures a change in distance is perfectly valid in relativity but it has nothing to do with an expansion or contraction of space. "Photon's being streched by exanding space" is another one these absurd phrases. That an emitter and an absorber of a photon measures a different frequency is perfectly valid in relativity but it has nothing to do with a change in the state of the photon.
The problem is observers do not measure a change in distance, we are not paying out a tape measure stretching to the Virgo cluster, for example.

What we do measure is red shift, primarily, and then the angular diameter of 'standard rulers' and the apparent magnitude of 'standard candles' etc. and use GR to interpret such observations as the expansion of space.

Using a metre metal rule as the standard of length measurement, a red-shifted photon has increased in wavelength and it has 'lost' energy.
As $\lambda(t) \propto$ a(t), where $\lambda(t)$ is the wavelength of a cosmological photon such as one sampled from the peak intensity of the CMB at cosmological time t, it might be said that "Photon's are being stretched by expanding space"

Those statements are theory dependent, i.e. dependent on the theory of GR.

Garth
P: 2,043
 Quote by Garth Those statements are theory dependent, i.e. dependent on the theory of GR.
A photon's frequency is a eigenstate of a photon.

Observing a redshift phenomenon is due to a different position in curved space-time of the emitter and the observer, it has nothing to do with the eigenstates of the photon.

I think this is a good example on how the FRW metric can confuse coordinate effects with physical effects!

If you think I am wrong, could you please direct me to a publication that claims that the space-time curvature of a particle traveling on a geodesic path can influence that particle's properties.
Because I believe this would be completely counter to the principle of equivalence.
PF Gold
P: 3,273
 Quote by MeJennifer A photon's frequency is an eigenstate of a photon.
Is not a photon a supposition of the eigenstates of the electron that emitted the photon? And a measurement of its wavelength a determination of the eigenstate of the electron that absorbed the photon?
 Observing a redshift phenomenon is due to a different position in curved space-time of the emitter and the observer, it has nothing to do with the eigenstates of the photon. I think this is a good example on how the FRW metric can confuse coordinate effects with physical effects!
Does the photon itself have eigenstates?

Or are the only eigenstates involved those of emitter and absorber?

What about the Doppler shift of the photons measuring my speed in a radar speed trap? How do they gain/lose frequency?

Garth
P: 2,043
 Quote by Garth What about the Doppler shift of the photons measuring my speed in a radar speed trap? How do they gain/lose frequency?
They don't!
Absolutely nothing happens to the photons.

The relative motion between the emitter and absorber causes the effect, it has nothing to do with the photons.
PF Gold
P: 3,273
 Quote by MeJennifer They don't! Absolutely nothing happens to the photons. The relative motion between the emitter and absorber causes the effect, it has nothing to do with the photons.
Absolutely, the only eigenstates involved are those of emitter and absorber.

Garth
P: 24
 Quote by MeJennifer "Expansion of space" is a completely wrong terminology. It implies that space is some sort of a substance that can expand and contract.
If one accept the term of "time dilatation" and "space contraction" and since space and time are the so-called "spacetime", it is not absurd to think about "relax" as the opposite of the contraction, of course relatively, not as "adding" new matter.
 Astronomy Sci Advisor PF Gold P: 22,808 the amount of energy in the CMB photons helps to determine the evolution of the universe along with all other matter CMB photons constantly interact gravitationally with other matter in mainstream cosmology, CMB photons have a well-defined wavelength distribution during the billions of years between their emission and their absorption
P: 2,043
 Quote by marcus CMB photons constantly interact gravitationally with other matter
Indeed, photons are massless but they do have energy so obviously they do interact gravitationally.

However this gravitational interaction is not the same thing as the incorrect idea that the energy of photons changes between an emitting and absorbing event due to the expansion of space.

All forms of redshift have nothing to do with a change in the state of the photon. Instead it is related to the relative position and orientation of the emitter and absorber in curved space-time.

On the idea of loosing energy, it should be noted that energy is not a Lorentz invariant property.
PF Gold
P: 3,273
 Quote by MeJennifer On the idea of loosing energy, it should be noted that energy is not a Lorentz invariant property.
Absolutely correct; therefore it is important to choose a conservation convention by which something is defined to be constant across cosmological space and time and by which mass, and therefore energy, length and time can be measured.

The standard convention is that of the conservation of energy-momentum, (GR), which results in fundamental particles having constant mass. Therefore, atoms are defined to provide regular clocks and fixed rulers by which the universe can be measured.

Photons are measured by those atoms, as the frequency of emission, determined in the laboratory, is compared to the frequency of absorption, the result is they are found to lose energy, i.e. red-shifted.

If another convention is chosen, such as the conservation of energy it is the photon that remains constant in energy and hence frequency and the masses of atoms, and therefore atomic clocks and steel rulers that change over cosmological time. There are theories that take such an approach, such as here.

Garth
 P: 2,043 You seem to have left out the obvious, and that is that a measurement depends on an observer's relative orientation and location in curved space-time. Redshift has everything to do with that and nothing with a change in the frequency of photons. The issue is simple, and directly related to the principle of equivalence in general relativity. The assumption that a photon changes due to the curvature of space-time is a direct violation of this principle. So "stretching photons" is, as Pauli could have said: "not even wrong".
PF Gold
P: 3,273
 Quote by MeJennifer It may be a convention fo some but that does not make it any way true. The issue is simple and directly related to the principle of equivalence in general relativity. The assumption that a photon changes due to the curvature of space-time is a direct violation of the principle of equivalence. So "stretching photons" is, as Pauli could have said: "not even wrong".
The question is "How do we measure mass, length and time across cosmological distances?"

To be able to make cosmological measurements we need something that stays constant when transported across the universe as a standard, therefore a conservation principle is absolutely essential, not just a vague "assumption", but the question is which principle?

As we cannot prove that anything remains constant over cosmological space-time all we can do is define a conservation principle and test to see whether it is concordant and internally consistent.

At the heart of GR is the conservation of energy-momentum, which leads to the atom, atomic 'regular' clocks and 'rigid' steel rulers, being the standard by which to measure the universe. GR has been tested, and is still being tested and so far has not be falsified. Perhaps Saturday will tell!

Garth

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