Answer to Question #134121 in Mechanics | Relativity for Adil

Question

If I throw a photon into space, over time its energy will decrease because its wavelength is stretched by the expansion of space.
We canbthen ask ourselves "where is this lost energy going?" or this energy just disappear?

Answer 1

Solution

Energy of photon is given by

$E=h\nu=\frac{hc}{\lambda}$

This clearly tells that by expansion of space the wavelength is stretched whereby its energy will decreased.

Actually this decreased energy of photon is used to expanding the space.

We all know energy conservation is always valid.