We know that the photons from the big bang are continually being red shifted and losing more and more energy. In terms of the graviton view, how would you explain that? Where is the energy going?

Are the photons emitting or absorbing gravitons?

What is a good explanation without using the phrase "expansion of space"?

  • $\begingroup$ The energy is not going anywhere. It is lost. Have a look at Is the Universe leaking energy? (which is behind a paywall, but can be found elsewhere). $\endgroup$ – pela Jul 16 '18 at 12:59

There is no explanation without "the phrase" expansion of space. Such an explanation would compete with the explanation by the expansion of space.

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    $\begingroup$ Why can't there be two mathematically equivalent explanations for a single phenomenon? For example, why couldn't there be something analogous to the Heisenberg & Schrödinger pictures in quantum mechanics? $\endgroup$ – Michael Seifert Jul 16 '18 at 13:12
  • $\begingroup$ There can be, but there isn't, so far. $\endgroup$ – my2cts Jul 16 '18 at 14:04
  • $\begingroup$ I'd disagree, from a GR point of view the expansion of space comes from the choice of spatial coordinates and from a physical point of view the justification for me is more a matter of convenience than absolute fact. For example cosmological red shift could be described by some variation of the 'tired light' hypothesis, but spatial expansion is more convenient. $\endgroup$ – John Davis Jul 16 '18 at 16:38
  • $\begingroup$ @John Davis Your statements conflict with standard theory so let's see the peer reviewed references to back it up. $\endgroup$ – my2cts Jul 16 '18 at 17:36
  • $\begingroup$ In what way do they conflict with standard theory? They don't. $\endgroup$ – John Davis Jul 16 '18 at 17:48

To quote Mark Whittle (see his online lecture notes for lecture 16, Cosmology):

"You may ask: Where does the energy go to (photons) come from (vacuum)? The answer is rather subtle:

"In a Newtonian framework: it goes to (comes from) the gravitational binding energy of the Universe.

"In a GR framework: it appears as modifications to the geometry terms in Gmn"


You don’t.

Let’s state an underlying assumption of your question:

The excitations of the force carrying fields are quantised and can be thought of as particles.

This is one of the basic assumptions of particle physics and is the key to understanding and predicting the behaviour connected with electromagnetism and the weak and strong nuclear forces.

The quantum field theory approach to gravity is not nearly as successful. Arguably it just doesn’t work. One of the crucial difficulties stems from the following fact. Quantum field theories are defined on a background geometry, but the gravitational field is the geometry.

A partial work around is to treat the problem pertubatively: assume a background geometry, and treat the small variations around it as a separate field. This perturbative approach is already hard in the classical field theory of gravity (General Relativity) and is non-renormalisable in the quantum version (someone correct me if I’m wrong).

But even if it did work, we still wouldn’t be able to answer your question i think. One would still have to assume the background geometry to be that of our universe (expanding, isotropic, homogeneous) and thus the photon redshift would still be an effect of geometry, not of particle interactions.

In conclusion, to my knowledge, there is no way to understand gravity in the form of gravitons in a way that is self-consistent and would be able to answer questions like yours.

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    $\begingroup$ I don't think this is quite correct, the only condition imposed on the background spacetime by the physical spacetime is the topology (see Wald). So assuming a topology of R^4, it's quite possible to describe an expanding Universe using a Minkowski background with a perturbation. Of course as you mention such an approach to QG is non-renormalizable. $\endgroup$ – John Davis Jul 16 '18 at 13:11
  • $\begingroup$ Don’t you need that the perturbation on the background metric be, and stay, “small” in some appropriate sense? Then I don’t see how you could treat FLRW metric as a perturbation on a static metric, given the drastic change in the metric components. $\endgroup$ – Andrea Jul 16 '18 at 15:51
  • $\begingroup$ You need the perturbation to be 'small' for linearized gravity to allow you to ignore the higher order terms, but for the peturbative approach to QG you're not specifically looking to ignore these terms as any QGT needs to encompass the nonlinear nature of general relativity. If you have a copy of Wald he discusses this briefly when he discusses quantum gravity. $\endgroup$ – John Davis Jul 16 '18 at 16:21
  • $\begingroup$ @JohnDavis I’ll check wald, I have that with me! $\endgroup$ – Andrea Jul 16 '18 at 16:27
  • $\begingroup$ @JohnDavis but, to the point of the question, can then those non-linear perturbation terms shed light on the photon redshift in terms of interactions with gravitons? $\endgroup$ – Andrea Jul 16 '18 at 16:32

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