Where do high energy background photons go?

[fxdung]

The temperature of expanding Universe is cooler and cooler.The most contribution of energy of background photons(CMB) are of photons having energy ~3kT.Then where have the high energy background photons gone?

 

[Bandersnatch]

They haven't gone anywhere - they were redshifted (i.e. lost energy) and now we observe them as the 2.7K blackbody spectrum.

 

[Chronos]

I presume the next queistion is - given energy cannot be created or destroyed, where did it go?

 

[fxdung]

I think that it changes to gravitation energy. Do the curved degree of space-time(or the metric) change while the Universe expanding?But I do not understand the problem because when the Universe expands the matter density decrease,then the space-time is flater and flater, then why we have the gravity energy increase to conserve total energy?

 

[Orodruin]

Energy is not conserved in an expanding universe. In fact, it is not necessarily true that it is possible to define global energy conservation in GR.

 

[PeterDonis]

Obligatory link to Sean Carroll's article on this very topic:

http://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/

 

[fxdung]

What is Dark Matter(and Dark Energy? Does it contribute to Einstein equation or not?Is there any expansion in GR when we consider Dark Matter(and Dark Energy)?

 

[Orodruin]

What is Dark Matter(and Dark Energy? Does it contribute to Einstein equation or not?Is there any expansion in GR when we consider Dark Matter(and Dark Energy)?

Those are way too general questions if you want more than yes/no answers and also off-topic in this thread.

We do not know. Yes. Yes.

 

[nikkkom]

I presume the next queistion is - given energy cannot be created or destroyed, where did it go?

It did not go anywhere. The photons did not lose energy. In some sense, for the observer who saw them emitted, they still have the same high energy they had when they were emitted - they are just very far away from that observer now.
They have much lower energy in _our_ coordinate system. IOW: the redshift is an *apparent* loss of energy, caused by photons being observed in constantly changing choice of coordinate system.

 

[Orodruin]

In some sense, for the observer who saw them emitted, they still have the same high energy they had when they were emitted - they are just very far away from that observer now.

No, this is not true in any way. It is not even clear what would be meant by such a statement in a curved space-time.

The photons did not lose energy.

They did if you consider the comoving frame, which is the one most commonly used. It is a frame dependent issue though. The total energy of the universe is not well defined.

They have much lower energy in _our_ coordinate system. IOW: the redshift is an *apparent* loss of energy, caused by photons being observed in constantly changing choice of coordinate system.

You seem to be thinking of Doppler shift. Although it is the same basic idea, you cannot view what happens in a general space time as a Doppler shift due to relative motion of the observer and source other than locally. Also, observations have nothing to do with your choice of coordinate system. Observables are invariants.

 

[Chronos]

We live in a universe that is often bizarre observationally, although it always seems to prove to be logically consistent. I am a big fan of the logically consistent part because it offers hope we may someday figure out exactly how it works. I deeply suspect it is very simple on a basic level. All those 'out there' theories are mostly mathematical artifacts, IMO.