Cosmological Red Shift

Yeah, but it doesn't make much sense to me since the expansion of space is homogenous and isotropic, and driven by the cosmological term in the Einstein Field Equations.

 

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And of course any person/Alien/ET anywhere in the universe, is always logically the center of his/her own observable universe, no matter where he/she resides in said universe.

 

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I should emphasize that from the point of view of the eventual receiver, all of the redshift happened when the photon was originally emitted; when we receive the photon, it has the same frequency and wavelength as it had when it was emitted. There is no stretching of the wavelength as the photon moves; it is emitted with the same wavelength (from the point of view of the receiver) as it was emitted with. This is why Russ says what he says, and why he is correct.

Their mistake is looking in between rather than at the receiver of the photon. From the receiver's point of view the photon is emitted with the redshift already present; it always had it. It's not something that happens while the photon is traveling through the space between the emitter and receiver.

Yes, that's very clear.

As long as it's not a circular firing squad we're OK.

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Seriously though, I'm poking around checking stuff out and kicking the tires because I came to the thread late. Thanks for informing me, and don't be surprised to have to do it again.

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Thanks for that: Actually cleared up the issue with Russ...appreciated.

 

I have not read all posted here, but here is why I think photons are stretched in flight by the expansion of the universe:

Now the photons that were emitted when neutral matter began to let them pass with little absorption or scttering came from an extremely hot plasma that was becoming neutral matter, before parts of it later collected into stars and was "re-ionized." These photons have now been traveling 13+ billion years and still have a basically black body spectrum, but with a much longer wave length for the peak of that spectrum. At some time, when the universe was not so old, the peak of that black body spectrum was in the UV, then later in the visible, later still the IR still later at X-ray wave lengths, etc.

They are the very same photons that were emitted from that cooling plasma (peak wave length of the black body spectrum when first emitted, was probably gamma rays - many dozens of thousands times shorter wave length than now. Again they were the very same photons as we now observed stretched thousands of times in wave length as they traveled thur space for more than 12 billion years with no interactions with matter in most case. How, in view of these well accepted facts can one deny that the Cosmological expansion of wave lengths exists? Say that the expansion of the universe is not stretching photons?

As I said I have not read all here, but skimming it some appear to being saying this stretching is not real - does not exist - that the photons still have the same wave length that they had when emitted. If they did not stretch, then Earth would not be solid, not even a gas, but a plasma, as immersed in that very hot black body spectrum.

 

Billy T,

You have forgotten two important principles:
1. Reference frames
2. Energy conservation

If the photons, or the waves, were stretched in motion, then #2 would be violated.

You have confounded the frame of reference where the photons were emitted with that where they are absorbed. As a result you're getting the wrong answer, because in order for energy conservation to apply, you must make all measurements in a single frame of reference.

Energy is not conserved across frames of reference unless they are co-moving. Here is a thought experiment that proves it:

Two spaceships. One is stationary relative to Earth, the other is moving away from Earth at a significant fraction of the speed of light, enough to redshift X-rays to visible light. Both are along the same line from Earth. Earth fires an X-ray laser at the line the ships lie along. The first, stationary to Earth, sees an X-ray beam. It doesn't matter how far away it is, it is in the same frame (or if you prefer, a co-moving frame) and otherwise energy conservation would be violated. The second, moving away, sees a beam of visible light due to redshift. Energy need not be conserved, because the second ship and Earth are not in the same frame of reference. Now, are you seriously contending that the X-rays were both X-rays and visible light at the same time, when they passed the first ship? Because that's what the result of what you're contending is.

In the moving ship's frame of reference, Earth shot out a laser beam of visible light. It was always visible light, from the time it left Earth.

In the motionless (relative to Earth) ship's frame of reference, Earth shot out an X-ray laser beam. It's always an X-ray laser beam, even when they see it hit the moving ship.

 

No energy has been conserved - the much larger universe is now filled with much lower energy photons. I don't know, it is above my pay grade, but the photons have been climbing up out of the earlier much stronger gravity field. Perhaps most (99+ %) of their original energy is now in their gravitational potential? I am quite confident energy has been conserved.

No, I don't need to speak of reference frames as I assume most of the photons have not been absorbed in any. They are still just traveling on their merry way. - Thru a universe that now has absorbers much less dense.

 

You've missed the entire point.

Let me make it clear: in what frame are you claiming this is what happened? In what frame is energy not conserved? Not in any single frame; energy conservation is a Law of Nature and a whole bunch of people on this site are going to tell you that claiming violation of conservation of energy is incorrect. And I'm going to be agreeing with them.

What "earlier much stronger gravity field?" Please present evidence to support this claim.

They're being absorbed in Earth's reference frame, and in order to maintain logical consistency you must consider them as they appear in that frame at all times from emission to absorption. Otherwise your results will be inconsistent and you'll have to come up with a totally unnecessary "mechanism" to "redshift the light." And that's exactly what you're doing, and while doing it violating conservation of energy. Yes, you do need to speak of reference frames; in fact, you are already doing so, but you are confusing them so your results are inconsistent with reality.

 

The main bone of contention in this thread is actually not that, although still an interesting question and I believe has been explained.
The main bone of contention in this thread appears to be that since science isn't able to describe exactly the mechanism that causes the cosmological redshift, the same way that cosmology is as yet unable to explain why the BB banged, or why gravity exhibits itself when spacetime is curved/warped/twisted in the presence of mass.
They [well the god anyway aided and abetted by expletive deleted] is then saying, therefor cosmological redshift does not exist. [see post 94]

 

And me too. I very clearly said: "I am quite confident energy has been conserved."

Although stars do convert some mass into energy, when the mass density of the universe was about eight times higher than it is now (Radius of universe, half its present value) the typical strength of the gravitational field, averaged over the universe's volume was 8 time greater than now - is that not obvious?

Stop this "they were absorbed in some reference frame," nonsense. Very few have been absorbed. As I recall, the black body distribution we now see does have a few absoption lines (from space's atomic hydrongen, I think). But most of those photons from the hot plasma are still just traveling along thur space, getting their wave lengths stretched.

 

I would guess that the "mechanism" is that the photons have been climbing out of gravity field that was stronger when the universe was smaller but with roughly the current mass in that smaller volume. ("roughly" because as I noted, stars do convert, quite inefficiently, some mass into energy). I. e. the cosmological red shift is just the gravitational red shift in disguise.

 

Each and every frame of reference is as valid as the other: Important postulate of SR.

 

Agreed* -so what? That is not of import for photons just merrily traveling thru space. They have the same speed in all frames.

* As I'm sure you agree, that is for inertial frames only.

 

Sure, but if you're going to do calculations you only have two choices:
1. Do them all in one frame of reference.
2. Laboriously convert every data item from other frames of reference into that one frame of reference and hope you didn't forget any.

All of physics is based on doing calculations in a single frame of reference. This is why the existence of inertial frames of reference is a postulate of special relativity.

What you must not ever do is drag data from one frame of reference into another without converting it. If you do, you will get inconsistent results.

The validity of all frames of reference doesn't mean you can drag data from one frame to another without converting it; it just means when you start your calculations, you can pick any inertial frame.
But once chosen, you have to stick to it or get results that don't agree with reality.

Let me put it this way: changing reference frames in the middle of a calculation is like starting Moby Dick and trying to find out the ending by reading War and Peace.

 

Billy, I have said exactly the same thing as you with relation to stretched/lengthened photons.....
I now recognise an interpretation that Russ put a while back and that Schneibs has picked up on and elaborated on.
Again though as the bone of contention in this thread is, is the fact that cosmological redshift does happen and is distinguished from any peculiar Doppler or gravitational effect.
This is what the forum is trying to knock into the god's head.

Agreed whole heartedly.


Anyway I must be off! things to do, places to see! backlater.

 

Ahhh, but you see, they don't all have the same frequency/wavelength/energy in all frames. And frequency/wavelength/energy is what redshift changes.

And what I'm saying is you've started with photons by talking about their frequency in the frame of a distant galaxy moving away from us, and wound up talking about their frequency in the frame of Earth. You've changed books in mid-chapter. No workie like that.

 

I must admit that the "mechanism" I suggested in post 111 for the cosmological red shift, is not really explaining it until the mechanism of the gravitational red shift is explained. What I did in post 111, is much like the doctor explaining how sleeping pills work by saying: "They contain a narcoleptic agent."

 

that is ture, but you don't need any frame.

No I spoke of the fraction of the BB's energy that was carried into space by photons as the hot plasma cooled and let the photons ceased being internally scattered / absorbed. I think most of this energy was transformed into gravitational potential energy as they climbed away in all directions from their essentially point origin - the very small early universe, when it first became transparent.

Yes you can measure different energy and wave lengths when you measure photons in different frames. So what? Not important if you are conserving the total energy released from the early plasma that is the form of photons, now greatly reduced for each photon as most of it has been converted into gravitational protential energy.

Consider two photons leaving the hot plasma sphere, A & B in exactly opposite directions. As you measure their frequency in different frames, their total energy is invarient. For example, in the frame where A is red shifted, to a lower energy by 1ev, then in that same frame, B has been blue shifted by + 1ev. That is why one need not chose any frame to consider what happens to the total energy released as photons from the initial hot plasma.

 

Then you're not doing physics, and shouldn't be talking about photons, waves, redshifts, frequencies, energy, or the expansion of the universe.

Because none of these things have any meaning unless you choose a frame of reference to describe them in, and stick to it.

That's called the CMB, or Cosmic Microwave Background; it's been redshifted that much. But I don't see how that's relevant to discussions of whether the redshift occurred by something happening to the photons in flight, or whether it's due to the relative motion of the site of the emission and the site of the absorption.

No. From here and now, Earth 2016 frame of reference, the CMB was always microwave. Only in the frame of reference of a portion of the universe that was 13.6 billion light years away from here was the background ever higher than that. Those photons, or waves if you prefer, have been traveling toward Earth for all of that 13.6 billion light years, which is why they got here now. Light doesn't stop.

So, you can't measure them being emitted in one frame and absorbed in another and claim that they changed between. Your results are invalid because you changed frames without converting the light's frequency/wavelength/energy into the new frame.

You're going to get into even worse trouble if you try to drag potential energy from one frame to another without converting.

In what frame of reference? Where they came from or where they're going?

But that can never happen; you can't measure light you can't see, and you just said they're going in opposite directions.

You're hypothesizing about light you can never see. That's unicorns. If you can't see it you can't measure it and until it hits something and does something, you won't even know it's there.

 

I agree.

But I should warn you that it is in part because I myself refer to the conservation of energy as a bedrock principle of physics, there are certain folks on this forum who take exception to it in ways like:

"gravity is not a conservative force", or
"in QM, you can borrow a little energy for a long time, or a lot of energy for a short time; so it doesn't always apply in that domain", or
"the BB and/or inflation did not need to conserve (before / after) energy at all."

Just so you know.