Q&A: Cosmology

Q:
How do photons lose their energy to the universe as they cross it? Do photons shed their energy by emitting it as extremely weak photons or interact with nearby particles or each other so weakly that it only appears continuous? The amount of energy lost by photons emitted at the big bang must be enormous. Similarily with a gravitational field, are photons able to lose energy by exchanging gravitons in undetectable steps until they "evaporate" when their wavelength becomes nearly infinite?

A:
The Universe is expanding in time, as Edwin Hubble discovered in 1929. As the photons travel across the Universe, the space that they travel through expands, thereby stretching the wavelength of the photon. Longer wavelengths mean less energy. This is called the cosmological redshift.

Photons can also change energy by traversing a gravitational potential well, called the gravitational redshift. As far as gravitons and photons are concerned, it is important to realize that they are both force mediators - meaning the photon is a quanta which "transmits" the electromagnetic force. So, when 2 positively charged particles to repel each other, they are exchanging photons. Similarly, when 2 masses attract one another, they exchange gravitons. To our knowledge, gravitons and photon do not interact with each other. If you have read somewhere that the gravitational field of an object, say a black hole, has an effect on the magnetic field around it, this is true but it arises from geometrical effects (the black hole warps space around it and electromagnetic fields can be compressed or diluted depending on their location) alone.

There is a useful website which may help answer your questions, here:
http://www.astronomycafe.net/qadir/acosmexp.html

The author has separated his Q&A into 3 parts - beginning, intermediate and advanced questions.