# Q & A: How much mass-equivalent light in the universe?

Q:
Has anyone calculated the amount of light existing in the Universe? I say amount in terms of matter if light qualifies as matter. If no one can or has done it then that may be the missing force or matter above and beyond dark matter. Light energy is traveling through the Universe at all times from all sources so how much energy is out there. Dumb question, but what do I know. Thanks Wade
Phoenix, AZ USA
A:
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 at

(I know, it's a long address) that has some explanations.   Another web site at:

has more than information than you probably want.  It's fun to browse though.

LeeH

(published on 09/15/2009)

## Follow-Up #1: radiation content of universe

Q:
I did and approximation of this question myself, I made some huge assupmtions including how much energy in terms of light hit the Earth every second (3 kg^s-1) which I heard somewhere. Then I multiplied it up to work out the total energy given out by the Sun. I assumed that all stars were the size of our Sun and would last as long as our Sun (about 10bn years) and I calculated that the amount of mass turned into light in the universe so far was about 1/2000 the total mass of the universe. Is this anything like a reasonable answer?
- jason (age 16)
England
A:
That's a very nice calculation you've done there. It's great to see how you've got the idea of making rough calculations to get approximate answers in what might first sound like inaccessible problems.

As for the total radiation content of the universe, there's another term besides what's been given off by stars. That's the microwave background, left over from the Big Bang. Although this background is very weak compared to starlight here,  most of the universe is farther from stars.  I believe that this component of the radiation is, on the average, bigger than the component left from starlight.

To compare with the total mass density of the universe, it matters whether we use the ordinary matter, include dark matter, or include dark energy as part of the total. Let's say we include all the usual mass, dark or not, but leave off the dark energy, which doesn't quite behave like ordinary mass. Then the current mass density of the universe is around 3x10-27 kg/m3. The background radiation density is roughly 1/10000 of that.

Since stars themselves are a small fraction (roughly a tenth) of the regular mass density, this looks a bit bigger than  the term you calculated for starlight.

We'll try to get more accurate numbers to spruce this up, but this should get you started.

Mike W.

(published on 10/21/2010)