Where does the energy vanish to when light is red shifted

[ConformalGrpOp]

Referring to Chronos' comment, I'm not sure that the concept of dilution he is using is inconsistent with the concept that a photon's energy is unchanged by the expansion.

I don't suppose he is suggesting that the expansion of the universe in the FWR model causes photons to expand like dots on the surface of an inflating balloon such that the energy they carry can be thought of as being somehow diluted in local space. Regardless of the macro state of the expanding universe, it is unclear how such an expansion would have any localizable effect on the energy of photons propagating along an unobstructed path in a field free space.

 

[Drakkith]

Hmm. I had this thought after a conversation with Sophiecentaur yesterday in another post. It seems to me that if you stop thinking about light as just photons, and start thinking about light as an EM wave where photons can only be talked about during the interaction of the wave with matter, there is not really a problem. Expansion stretches out the wave as it travels, and every doubling in size of the universe causes the wavelength to double. Where has the energy gone? Nowhere. The wavefront contains the same amount of energy as it always did, but the wavefront now encompasses a much larger volume of space, spreading the energy out. That seems to match up with what Chronos was saying about the ink dot on a balloon. Chronos does what I said match up with actual physics?

 

[Tanelorn]

The energy and momentum of a photon depend only on its frequency (ν) or inversely, to its wavelength (λ)

Therefore, since the wavelength is 1090 times longer in our current frame of reference, we measure 1090 times less energy.
As an analogy isn't it as simple as the energy released due to the relative velocities of two colliding masses?

If we were able to match the relative velocity of the orignal emitting atoms then we would measure the same photon frequency and energy.
However, to do this in our current frame of reference we would have to greatly exceed the speed of light.

 

[Drakkith]

If we were able to match the relative velocity of the orignal emitting atoms then we would measure the same photon frequency and energy.
However, to do this in our current frame of reference we would have to greatly exceed the speed of light.

We would not have to exceed c to measure the original energy, but we would have to get to a very high fraction of c. Probably greater than 0.99c.

 

[Tanelorn]

Drakkith, thanks I was getting my frames of reference mixed up. That light today is in our own frame.

 

[ConformalGrpOp]

It seems to me that if you stop thinking about light as just photons, and start thinking about light as an EM wave where photons can only be talked about during the interaction of the wave with matter, there is not really a problem. Expansion stretches out the wave as it travels, and every doubling in size of the universe causes the wavelength to double. Where has the energy gone?

I've never understood this idea of the "stretching out" of a light wave as it travels across the ever increasing distances between a source and observer in an expanding universe.

Can't we state that at any local point of observation where the observer's inertial frame is that of the source, the observed wavelength at any such local point of observation anywhere in the universe would be that of the source of propagation? Isnt it the case that the red shift is "observed" because the observer's inertial frame is moving in a direction away from the source, and that the observation of a red shift can always be overcome anywhere in the universe if the observer began moving in an inertial frame that was at rest with that of the source?

If the expanding space effected a dilatation of the light wave as it propagated, wouldn't a distant observer in an inertial frame at rest with respect to the source always see a red shift? In that case, wouldn't such an observer applying Hubble's relation, end up over estimating the distance of the source to the observer's location at the time the light was propagated?

 

[Naty1]

The exact same issues and 'answers' to your questions are currently being discussed
here:

https://www.physicsforums.com/showthread.php?t=659192&page=2
How to prove the stretching of space

But there is no 'final resolution' : [from my prior post here]:

One view is that a photon does not change it's characteristics as it moves through space-time anymore than an electron changes it charge as it does so. A photon [or light wave] is emitted at a color [that is, an energy,frequency] and stays that way.

The alternative view is the one Chronos posted "...expansion dilutes energy". That also seems to be a realistic answer since the universe has cooled via CMBR observations from when it was emitted [close to 3000 K] to that which we observe today [about 2.73K].

 

[timmdeeg]

Can't we state that at any local point of observation where the observer's inertial frame is that of the source, the observed wavelength at any such local point of observation anywhere in the universe would be that of the source of propagation?

The problem is that in the non-empty universe you can define inertial frames only locally. So, the observer's inertial frame is not that of the source.

 

[Tanelorn]

timmdeeg, so it is not possible to say that we are in the same inertial frame if we use a spaceship to reach the velocity where the CMBR frequency is the same frequency it was when first emitted?

 

[Naty1]

...so it is not possible to say that we are in the same inertial frame if we use a spaceship to reach the velocity where the CMBR frequency is the frequency it was first emitted?

CMBR was first emitted at speed c; it has spend 13.7B years or so reaching us, all the time at c; it remains at see for the entire future;p tomorrows CMBR will also be traveling at c.

The CMBR was created by inflationary expansion and was not emitted by atoms; atoms had not formed at 380,000 years after the big bang when the CMBR appeared.

 

[Drakkith]

CMBR was first emitted at speed c; it has spend 13.7B years or so reaching us, all the time at c; it remains at see for the entire future;p tomorrows CMBR will also be traveling at c.

The CMBR was created by inflationary expansion and was not emitted by atoms; atoms had not formed at 380,000 years after the big bang when the CMBR appeared.

Actually the CMBR was created during recombination when the universe cooled far enough for electrons and nuclei to combine.

http://en.wikipedia.org/wiki/Recombination_(cosmology)

 

[Naty1]

Actually the CMBR was created during recombination when the universe cooled far enough for electrons and nuclei to combine.

I have read different versions...here is one:

Cosmic background radiation is well explained as radiation left over from an early stage in the development of the universe, and its discovery is considered a landmark test of the Big Bang model of the universe. When the universe was young, before the formation of stars and planets, it was denser, much hotter, and filled with a uniform glow from a white-hot fog of hydrogen plasma. As the universe expanded, both the plasma and the radiation filling it grew cooler. When the universe cooled enough, protons and electrons combined to form neutral atoms. These atoms could no longer absorb the thermal radiation, and so the universe became transparent instead of being an opaque fog. Cosmologists refer to the time period when neutral atoms first formed as the recombination epoch, and the event shortly afterwards when photons started to travel freely through space rather than constantly being scattered by electrons and protons in plasma is referred to as photon decoupling. The photons that existed at the time of photon decoupling have been propagating ever since, though growing fainter and less energetic, since the expansion of space causes their wavelength to increase over time (and wavelength is inversely proportional to energy according to Planck's relation). This is the source of the alternative term relic radiation. The surface of last scattering refers to the set of points in space at the right distance from us so that we are now receiving photons originally emitted from those points at the time of photon decoupling.

I'll see if I can find anything else interesting...

 

[Naty1]

By sheer luck: Ned Wright's Cosmology

http://www.astro.ubc.ca/people/scott/faq_basic.html

Where did the photons actually come from?
A very good question. We believe that the very early Universe was very hot and dense. At an early enough time it was so hot, ie there was so much energy around, that pairs of particles and anti-particles were continually being created and annihilated again. This annihilation makes pure energy, which means particles of light - photons. As the Universe expanded and the temperature fell the particles and anti-particles (quarks and the like) annihilated each other for the last time, and the energies were low enough that they couldn't be recreated again. For some reason (that still isn't well understood) the early Universe had about one part in a billion more particles than anti-particles. So when all the anti-particles had annihilated all the particles, that left about a billion photons for every particle of matter. And that's the way the Universe is today!

So the photons that we observe in the cosmic microwave background were created in the first minute or so of the history of the Universe. Subsequently they cooled along with the expansion of the Universe, and eventually they can be observed today with a temperature of about 2.73 Kelvin.

But this doesn't mean the wikipedia description is wrong...I'd GUESS most CMBR came as described here but photons 'busted loose' from hydrogen collisions also.

 

[Tanelorn]

Perhaps both descriptions are correct in that the original heat and light energy was already present from the BB, but the actual "last scattering" photons that we see now were emitted when the ions and electrons were able to combine, thus becoming transparent. Before this earlier photons were absorbed.

 

[Drakkith]

Perhaps it's more accurate to say that when recombination happened, the hot gas emitted radiation just like any other hot object does. At the moment of recombination the thermal radiation could suddenly travel freely throughout the universe. I'd assume that in addition to this thermal radiation, the recombination of electrons with protons emitted radiation as well that followed the pattern of spectral lines of hydrogen and added to the thermal radiation. Either way, there were definitely atoms around when recombination happened and the CMB was emitted.