# Universe is made of baryonic matter

1. Dec 24, 2008

### koolmodee

So they say the universe is made of baryonic matter, dark matter and dark energy.

2. Dec 24, 2008

### xmavidis

Re: photons?

Photons are the quanta of the EM force.

3. Dec 24, 2008

### AstroRoyale

Re: photons?

Photons do indeed contribute to the comoving density, its just that their contribution is currently quite small and can be neglected in most calculations. At high redshift, in the early universe, their contribution was non-negligible and you have to take them into account.

4. Dec 24, 2008

### koolmodee

Re: photons?

But I'm not quite satisfied yet. Was their contribution non-negligible only in the early universe? Is it present universe negligible?

5. Dec 25, 2008

### koolmodee

Re: photons?

Alright, when I think of it, converting only a tenth of the baryonic mass into electromagnetic radiation, that would add incredible many more photons to the universe, so I guess they are negligible.

6. Dec 26, 2008

### stevebd1

Re: photons?

This cosmic calculator might be of interest which includes for Omega radiation in the early universe (anything with a redshift above 16, about 13.4 billion years ago)-

http://www.geocities.com/alschairn/cc_e.htm

7. Dec 26, 2008

### George Jones

Staff Emeritus
Re: photons?

Let $a ( t )$ be the time-dependent scale factor of the universe. In an expanding universe, $a ( t )$ increases as $t$ increases. Assume that dark energy is vacuum energy, so that, in terms of energy/mass density, the three main components of the universe are radiation, matter, and vacuum energy.

As the universe expands, the densities of radiation and matter decrease. The density of matter is inversely proportional to $a ( t )^3$, one factor of $a ( t )$ for each dimension of space.

As the universe expands, the number density of photons is inversely proportional to the same factor, $a ( t )^3$. The energy density of radiation includes an additional factor of $a ( t )$ because the wavelengths of radiation scale as $a ( t )$ (wavelengths expand along with the universe), and energy of radiation is inversely proportional to wavelength, so that the density of radiation is inversely proportional to $a ( t )^4$.

Since the expansion of space is, roughly, the addition of more of the same vacuum, the vacuum energy density is constant in time.

Comparing the time-evolution properties of the three components shows that there is a time $t_1$ before which radiation dominated, and a time $t_2 > t_1$ after which the vacuum dominates. Relative values of the densities for our universe are such that between $t_1$ and $t_2$ matter dominates.

8. Dec 26, 2008

### koolmodee

Re: photons?

thanks! I remember I read something like that in Barbara Ryden cosmology book, but forgot it all. many thanks