This is a profound question and has been studied by many eminent cosmologists. First let's get some basics straight.
First, one should call it mass-equivalent
and not simply mass
because light has no rest mass
i.e. the mass, usually denoted by mo
when a moving object is brought to rest. Since light always travels at the speed of light, it cannot be brought to rest. The mass-equivalent
is the energy of a photon or collection of photons divided by the square of the velocity of light, c
Second, we should not consider the total amount of light or matter since the universe being infinite in extent the total amount is infinite. So what you do instead is to consider the density of light or matter i.e. the amount per cubic meter. That is a well defined quantity.
There is a straightforward relationship between the density of light and the average temperature of the universe. You can actually measure this temperature by observing the cosmic micro wave background. It is about 2.7 degrees C above absolute zero. The mass-equivalent density of this radiation amounts to approximately one ten-thousandth of the total mass density that we can observe in the form of galaxies and stars
, a factor of 10 smaller when you add in the so-called dark matter. Things get more complicated when you extrapolate back to a few seconds after the Big Bang
when the temperature of the universe was 10,000 times or more than what it is now. At that epoch the ratio of light-density to matter-density was about one to one.
So you see, it's a very complicated and interesting question. Astronomers and cosmologists are still looking for answers. There is a web site athttp://library.thinkquest.org/C0126626/evo/evolution%20of%20the%20universe.after%20the%20big%20bang.density%20of%20matter%20and%20radiation.htm
(I know, it's a long address) that has some explanations. Another web site at:http://www.astro.virginia.edu/~jh8h/Foundations/Foundations_1/chapter13.html
has more than information than you probably want. It's fun to browse though.
(published on 09/15/09)