What causes red shift in the universe?

[presto]

I see you have a great problem with the energy conservation, because of the gravitational redshift effect.

There is none problem with this!

Just an emitter placed lower - in a stronger gravity (lower potential),
radiates lower energy at start already, thus we get the lower energy,
and it's exactly what was emitted by the source, not less!
Eout = Ein = inv.

 

[presto]

Here's an excellent explanation by Michael Weiss and John Baez: http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html

This looks like a proof of the energy conservation in the expanding universe.
Therefore this is automatically a disproof of any light stretching in a fly, wrongly presupposed by the astro...logists. :)

 

[Drakkith]

This looks like a proof of the energy conservation in the expanding universe.
Therefore this is automatically a disproof of any light stretching in a fly, wrongly presupposed by the astro...logists. :)

Then you should re-read the article again, as it most definitely does not prove that energy conservation exists in GR in all circumstances. In fact, the very first line gives us an idea of the difficulties when it says, "In general — it depends on what you mean by "energy", and what you mean by "conserved". It also does not say that light does not stretch in an expanding universe. In fact, one of the main sections of the article deals with this redshift. I recommend you actually make an attempt to read and understand the article completely before trying to use it as evidence for your own point of view.

Note that it's not accurate to say that energy isn't conserved in GR. The fact is that energy conservation in GR is a complicated issue and you may or may not have energy conservation depending on what you mean by energy and by conservation, along with how you do the math.

 

[Drakkith]

I see you have a great problem with the energy conservation, because of the gravitational redshift effect.

That is incorrect. Gravitational redshift and cosmological redshift are two different things.

 

[ricardo81]

Well :) I came across something that had mentioned that the maths is well understood, though physicists have a hard time agreeing on the wording of it. I can relate to that being a programmer.

Perhaps one day I'll brush of my understanding of mathematical notation and the symbols for well-known constants to try and have a stab for my own understanding.

It is fascinating, for me I think the answers are towards the Planck length and as a Layman it seems to be that there's two separate scales, down there, one larger than the other. Relativity is sometimes described as a matrix of clocks with their own time frame... I like the idea of it actually being a matrix of processors processing the information across space-time. It seems like things like redshift perhaps hint towards a computational ceiling. Anyways, that's just some blurry thoughts from an amateur :) (the two scales would be one processor sized one and the smaller one would be 1 bit of information within)

 

[presto]

Then you should re-read the article again, as it most definitely does not prove that energy conservation exists in GR in all circumstances. In fact, the very first line gives us an idea of the difficulties when it says, "In general — it depends on what you mean by "energy", and what you mean by "conserved". It also does not say that light does not stretch in an expanding universe. In fact, one of the main sections of the article deals with this redshift. I recommend you actually make an attempt to read and understand the article completely before trying to use it as evidence for your own point of view.

Note that it's not accurate to say that energy isn't conserved in GR. The fact is that energy conservation in GR is a complicated issue and you may or may not have energy conservation depending on what you mean by energy and by conservation, along with how you do the math.

You too much improvise, instead just calculate this exactly.
Energy is explicitly conserved; this is not any confidential principle, nor top secret.

 

[presto]

That is incorrect. Gravitational redshift and cosmological redshift are two different things.

Of course. The one is measured and explained very well, and the second just not exists at all in the model based on the space expansion.

The scale change described in the Friedman metric:

ds^2 = dt^2 - a(t)^2 dR^2

when Friedman is scaling the space in this way,
then he automatically scales the vielocitis in the same way, therefore c' = a(t)c;
thus the light freq. is preserved also. :)

 

[timmdeeg]

I see you have a great problem with the energy conservation, because of the gravitational redshift effect.

There is none problem with this!

Just an emitter placed lower - in a stronger gravity (lower potential),
radiates lower energy at start already, thus we get the lower energy,
and it's exactly what was emitted by the source, not less!
Eout = Ein = inv.

No, the emitter doesn't radiate lower energy at start already, instead the lower energy is measured by the observer. In this static case you are talking about the energy is conserved. Think of a box containing source and observer, its energy doesn't depend on the actual position of the photon.

In the dynamic case, say the expanding universe, the photon's energy isn't lower at the source (at start) as well. So, what's the difference? Again, pick a box and look what happens. As the box expands the energy of the photons inside decreases due to the work done on the walls. Now you can argue, well the lost energy is gained back outside the box. But this is obviously wrong as there is no preferred box.

 

[presto]

Do not improve: the measured energy is lower, because a transmitted energy was just lower at start!

And in the analogy, with the expanding box, the whole energy is conserved also,
because there is impossible any work on a boundary of the universe;
this is just the main error in the idea of losing energy in the expanding universe - there are no bounds, where photons could to bounce!

 

[PeterDonis]

the measured energy is lower, because a transmitted energy was just lower at start!

No, that's not correct. At least, if you place a measuring device next to the transmitter, at rest relative to it, it will not measure a lower energy being transmitted. (Which just illustrates that "energy" is observer-dependent; what energy is measured depends on what is doing the measuring, and how its worldline through spacetime relates to the worldline of the transmitter. There is no absolute notion of "energy" that can be used as a standard. That seems to be the main point that you are not getting.)

this is just the main error in the idea of losing energy in the expanding universe - there are no bounds, where photons could to bounce!

The "box" timdeeg is talking about is not the entire universe. It's any volume within the universe that contains a fixed number of photons. As the universe expands, the volume of such a box increases, and the energy per photon decreases. One way of interpreting this is that the photons are doing work on the boundary of the box. It's true that there isn't a physical wall there, but the interpretation still works.

It is an interpretation, however. Other interpretations could be constructed that do not involve the box or photons doing work on its boundary. However, any interpretation must match the actual observation, which is that the energy per photon in an expanding universe, as measured by "comoving" observers in that universe (note how I'm specifying explicitly what is doing the measuring), decreases with time. This is why, for example, the temperature of the CMBR is 2.7 degrees Kelvin, instead of the several thousand degrees it was when it was formed (because that's the temperature at which electrons and ions recombine into atoms).

 

[presto]

No, that's not correct. At least, if you place a measuring device next to the transmitter, at rest relative to it, it will not measure a lower energy being transmitted. (Which just illustrates that "energy" is observer-dependent; what energy is measured depends on what is doing the measuring, and how its worldline through spacetime relates to the worldline of the transmitter. There is no absolute notion of "energy" that can be used as a standard. That seems to be the main point that you are not getting.)

This very correct and even accepted by the standard science.
You just try to improvise with the naked facts!

Your presumption about the dependence of energy on the frame is wrong.
Even in the Doppler effect the energy is conserved,
but it is more hard to notice, than in the case of the gravitational redshift,
where the situation is completely fixed - stationary, therefore simpler - the simplest possible.

The "box" timdeeg is talking about is not the entire universe. It's any volume within the universe that contains a fixed number of photons. As the universe expands, the volume of such a box increases, and the energy per photon decreases. One way of interpreting this is that the photons are doing work on the boundary of the box. It's true that there isn't a physical wall there, but the interpretation still works.

Wrong! The energy in the box is conserved!
There is nothing above, external of the box, what can absorb the energy.
The Universe is just the everything already, nothing more exists!

It is an interpretation, however. Other interpretations could be constructed that do not involve the box or photons doing work on its boundary. However, any interpretation must match the actual observation, which is that the energy per photon in an expanding universe, as measured by "comoving" observers in that universe (note how I'm specifying explicitly what is doing the measuring), decreases with time. This is why, for example, the temperature of the CMBR is 2.7 degrees Kelvin, instead of the several thousand degrees it was when it was formed (because that's the temperature at which electrons and ions recombine into atoms).

You should avoid these interpretations.
The actual temperature of the CMB is perfectly consistent with the energy conservation principle.

 

[PeterDonis]

This very correct and even accepted by the standard science.

Good, I'm glad we agree on that. However, the rest of your post is inconsistent with what you have just said is correct. See below.

You just try to improvise with the naked facts!

I'm afraid I fail to see why concentrating on the facts is a problem. ;)

Your presumption about the dependence of energy on the frame is wrong.

But you just agreed that "energy is observer-dependent" and "there is no absolute notion of energy" were correct. Now you're saying those statements are incorrect. Which is it?

Even in the Doppler effect the energy is conserved

Same comment here; if "energy is observer-dependent" is correct, then energy changes in the Doppler effect, obviously, because the whole point is that the receiver is moving relative to the emitter, so his notion of "energy" is different. So you're saying two inconsistent things here.

but it is more hard to notice, than in the case of the gravitational redshift,
where the situation is completely fixed - stationary, therefore simpler - the simplest possible.

No, the stationary situation is in fact the only situation where we can define an invariant notion of "energy" that works globally, instead of just locally. (Note that we can always define an invariant local notion of "energy", namely the stress-energy tensor, which obeys a well-defined local conservation law. The issue we are discussing here is about how to define a single notion of "energy" that applies at different, separated events in spacetime.) This is because, to define an invariant notion of energy that works globally, we need a timelike Killing vector field, and that is only present in a stationary spacetime.

All of this is very basic GR, by the way, and it is not at all controversial. If you want to continue arguing against it, you're going to need to give mainstream references and show your math explicitly. First, as I requested above, you're going to need to take one consistent position and stick to it.

The actual temperature of the CMB is perfectly consistent with the energy conservation principle.

Please show explicitly, with math, how this is true.

 

[presto]

All of this is very basic GR, by the way, and it is not at all controversial. If you want to continue arguing against it, you're going to need to give mainstream references and show your math explicitly. First, as I requested above, you're going to need to take one consistent position and stick to it.

Against what?

There are many papers about this wrong interpretation of the gravitational redshift.
This is just a so-called clock-effect, nothing more.
The energy of light do not change during fly at all,
and it isn't equal everywhere what you try to imply.

You probably use some highly special version of the relativity principle: everything is invariant,
because all is equal, permanently constant, nothing can change. :)

And in the space expansion concept there is an error... ever on a computational stage:
people compute these stretched wavelengths, but never check how a speed transforms under such scaled metric.
l -> a l -> OK.
but:
c -> a c => f -> f = ac/al = c/l = inv.
and the end of the story about a space expansion - the idea doesn't work.

Scale is and was always arbitrarily - this trick changes nothing in the physics.

 

[Drakkith]

The original question was answered on page one and the rest of the thread is just responses to misinformation and misunderstanding. Thread locked.