# 153 Since energy is conserved, where does the energy of redshifted photons go?

## Description

This article is from the Astronomy
FAQ, by Joseph Lazio (jlazio@patriot.net) with numerous
contributions by others.

# 153 Since energy is conserved, where does the energy of redshifted photons go?

By Peter Newman <p.r.newman@uclan.ac.uk>

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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