How can the expansion of space cause light to loose energy?

[Garth]

Be careful movingfinger time dilation is cosmological red shift.

One mistake would be to argue that on top of the red shift of expansion there is also the gravitational red shift caused by the universe being denser in the past and therefore the photon has escaped from a high curvature powerful gravitational potential well into a lesser curvature weak gravitational well. If you do that you predict twice the actual red shift observed.

Cosmological red shift of recession is cosmological gravitational red shift, they are two perspectives on the same thing.

In GR: From a (3+1)D, Space + Time, perspective the universe is expanding, the galaxies are receeding and red shift is doppler in nature.

From a 4D, space-time, perspective the view is static. World-lines of representative cosmological particles diverge in the positive time-like direction and red shift is gravitational time dilation in nature, i.e. the world lines of two adjacent peaks of the wave form diverge in the positive time-like direction.

However if fundamental particle masses are to be explained in string theory by the energy of vibrating strings then should not the particle mass also be subject to the same time dilation and therefore such red shift not be observed?

Garth

 

[moving finger]

Be careful movingfinger time dilation is cosmological red shift.

One mistake would be to argue that on top of the red shift of expansion there is also the gravitational red shift caused by the universe being denser in the past and therefore the photon has escaped from a high curvature powerful gravitational potential well into a lesser curvature weak gravitational well. If you do that you predict twice the actual red shift observed.

Thanks for the warning Garth, but I'm not talking about gravitational time-dilation.

I'm talking about the so-called time-dilation in the intensity-time distribution curves observed from distant SNe casued by their recession speeds (well reported and documented) which has to do with the "spreading" of the intensity-time distribution of received energy, and nothing (directly) to do with either gravitation or redshifts.
MF

 

[Garth]

moving finger As the recession speeds are cosmological and not local velocities I think you are talking about gravitational time dilation. By gravitational I am referring to the gravitational field of the average density of the whole universe, smeared out homogeneously, and not some local "Schwarzschild solution" gravitational red shift. Of course quasars and possibly S/N Ia will have some of that as well.

Garth

 

[Billy T]

A very interesting thread. Both Garth and MF are better informed than I am, so it is with some hesitation that I suggest anything. However let me try making some statements.
(1)The z=2 SN is moving away from us at twice the speed of the z=1 SN, because of the expansion of the universe.
(2)Thus if they are in their own local frames duplicate events, with time T for the duration of 90% of the energy released in photons, the z=2 one will travel twice as far during the interval T.
(3)These “duplicate events” will deliver these photons to the surface of a sphere (on which we are located), but the delivery will occur over a longer period for the z=2 SN than the z=1 one for the reason MF is suggesting, that is my Statement (2).
(4) The total energy arriving /crossing their respective spheres will be essentially twice larger for the z=1 SN than the z=2 one as each photon is half as energetic when arriving at the sphere of the z=2 SN and they do not magically give birth to any more inflight to compensate.
(5) There does indeed seem to be lost energy.
(6) The time of the Z=2 event was twice as far back in the history of the universe, but as the rate of expansion is not constant this does not imply it was exactly half the size and eight times more dense. It certainly does imply, as Garth is suggesting, that they climbed up from a deeper gravitational well to get to their sphere. I will accept his statement that this is as valid a way to explain the energy loss of my Statement (5) as the more common “Doppler shift” view. (I am not qualified to argue the point, even if I were so inclined.)

Please correct any of the above.

Two ideas seem reasonable to me, but may well be false:
(1) There is no loss of energy because the red shifted photons are now “sitting on a higher gravity hill.” - Very much like a brick thrown up in the Earth’s gravity field appears to have lost energy when its vertical motion stops has not because it to is “sitting on a higher gravity hill.” Or stating this in other terms: If one of the escaping photons were approaching a black hole it would recover it original frequency (undo the red shift) at some gravitational point near the BH. If this is false, then perhaps:
(2) A third way to look at the loss of my Statement (5) is that radiation pressure exists. This pressure in an expanding chamber (the universe) does work. That is (and deterministic MF will like to toy with this idea, I bet) the expansion of the universe could be considered (at least in part) to be the effect, not the cause, of the lost of photon energy.

Any comments?

 

[moving finger]

moving finger As the recession speeds are cosmological and not local velocities I think you are talking about gravitational time dilation. By gravitational I am referring to the gravitational field of the average density of the whole universe, smeared out homogeneously, and not some local "Schwarzschild solution" gravitational red shift. Of course quasars and possibly S/N Ia will have some of that as well.

Garth

The so-called time dilation I am talking about (it is not really time-dilation, but this unfortunately is how most people refer to it, including in the scientific literature) has nothing whatsoever to do with gravity.

Take an isolated supernova (SN) event in an otherwise empty universe. Take also an observer.

Assume that there is no relative motion between SN and observer, and the observer sees the duration of the SN event to be 10 days (for example) - in other words, from the onset of increasing brightness at the start of the SN event, to the fall in brightness at the end of the SN event, 10 days elapses (we will be idealistic and assume a square-wave intensity-time distribution for the SN emission).

Now take the same SN, but assume that there is a relative motion of v (where v << c) between observer and SN (it matters not whether this motion is cosmological expansion or peculiar motion). The apparent duration of the SN event, seen by the observer, will now be longer by (10 x v)/c days, ie the total duration of the event will appear to be 10 x (1 + v/c) days.
(why? because by the time the SN event has ended, the SN will be futher away than it was when the SN event started, hence the total duration of the event will appear to the observer to be longer, or "dilated").

Generalising, similar effects will occur also at relativitistic speeds.

Since the total energy-flux emitted is unchanged, the moving SN will appear to be less bright than the static SN (because the energy from the moving SN is spread out over a greater time).

This is exactly what is observed with type 1a SNe, the effect has to be (and is) corrected for in plotting apparent magnitude vs redshift (Hubble's law plots), and it has nothing to do with gravity or spatial curvature.

MF

 

[Garth]

moving finger The question is whether the velocity of recession is due to cosmological expansion, or a local peculiar motion superimposed on the cosmological recession for an object at that distance.

If it is the former then the time dilation you refer to, and the red shift by which it is observed, is caused by cosmological gravitation, i.e. the average curvature of the homogeneous and isotropic universe at the observed epoch.

The two effects, doppler red shift and cosmological gravitational red shift, are two different ways, or perspectives, of interpreting the same Hubble red shift. One interpretation is made from the 3D + time perspective, and the other from the 4D space-time 'static' perspective.

Garth

 

[moving finger]

moving finger The question is whether the velocity of recession is due to cosmological expansion, or a local peculiar motion superimposed on the cosmological recession for an object at that distance.

If it is the former then the time dilation you refer to, and the red shift by which it is observed, is caused by cosmological gravitation, i.e. the average curvature of the homogeneous and isotropic universe at the observed epoch.

The two effects, doppler red shift and cosmological gravitational red shift, are two different ways, or perspectives, of interpreting the same Hubble red shift. One interpretation is made from the 3D + time perspective, and the other from the 4D space-time 'static' perspective.

Garth

I believe we are talking about completely different things here.

In an attempt to find out why we appear to be not communicating :

You keep referrring to curvature of the universe - exactly what do you mean by this please?

MF

 

[Garth]

If we are talking about the universe at large, at cosmological distances, then, assuming homogeneity and isotropy, the Robinson-Walker metric describes the metric of space-time. That metric describes curvature in that the elements of the Riemann tensor are not all zero, except in the case of the Milne model that is empty. With any mass in the universe at all there would be an associated amount of curvature. This is curvature that causes gravitational fields. The whole universe can be described as having 'an average gravitational field'. As it is homogeneous and isotropic it has no affect on test particles within the universe except time dilation. This time dilation is observed as Hubble red shift, which can be interpreted as a doppler red shift of recession in an (spatially) expanding universe.

I hope this helps.

Garth