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Astute.

In fact, photons do lose energy as they travel over intergalactic distance. In fact, that's one perspective on the redshift---a redshifted photon has less energy than one viewed in the "rest frame" of the object emitting the photons.

The sci.astro FAQ addresses this point more specifically:

Subject: I.17 Since energy is conserved, where does the energy of redshifted photons go?

Author: Peter Newman

The energy of a photon is given by

`E = hc/lambda`

, where`h`

is Planck's constant,`c`

is the speed of light, and`lambda`

is its wavelength. The cosmological redshift indicates that the wavelength of a photon increases as it travels over cosmological distances in the Universe. Thus, its energy decreases.One of the basic conservation laws is that energy is conserved. The decrease in the energy of redshifted photons seems to violate that law. However, this argument is flawed. Specifically, there is a flaw in assuming Newtonian conservation laws in general relativistic situations. To quote Peebles (Principles of Physical Cosmology, 1995, p. 139):

Where does the lost energy go? ... The resolution of this apparent paradox is that while energy conservation is a good local concept ... and can be defined more generally in the special case of an isolated system in asymptotically flat space, there is not a general global energy conservation law in general relativity theory.

In other words, on small scales, say the size of a cluster of galaxies, the notion of energy conservation is a good one. However, on the size scales of the Universe, one can no longer define a quantity

`E_total`

, much less a quantity that is conserved.

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