# Distribution of energy after matter/antimatter annihilation

1. Feb 26, 2015

### ft_c

Hi all

Maybe a quick question!

After the big bang and inflation, a little while later there is the mass annihilation event where 10 billion matter particles and 10 billion anti matter particles annihilate, sending out energetic photons. For each 10 billion annihilation events there is one remaining matter particle (or 2? whatever.. :)

So the contribution to the energy content of the universe from photons should be 10 (or is it 20) billion times more than that of matter. But as we're told, by like wikipedia and books and stuff, the radiation content of the universe is negligible....

What's going on there? Where have all those photons gone? Or where has their energy gone, I don't think the expanding universe/photons can cover all that!

Thanks!

2. Feb 26, 2015

### Chalnoth

Radiation dilutes faster than matter. As the universe expands by a factor of $a$, the energy density of matter drops as $1/a^3$, while the energy density of radiation drops as $1/a^4$. This is because radiation redshifts as the universe expands.

In the very early universe, radiation was the dominant energy density of our universe. But it diluted until matter had a higher energy density.

3. Feb 26, 2015

### ft_c

Ahh right yes, thanks very much!

4. Feb 26, 2015

### Garth

The energy density of the radiation (largely in the CMB) is negligible compared with matter, Dark Matter and Dark Energy, however that is because the energy of each photon (now largely in the microwave region of the spectrum) is so low.

There are still 108 photons to every nuclear particle.

Garth

5. Feb 26, 2015

### Chalnoth

This is true now. It wasn't true in the very early universe: before our universe was ~75,000 years old, radiation had a higher energy density than matter (normal + dark).

6. Feb 27, 2015

### Garth

Of course, as you said in your post #2. I was emphasizing the present day situation.

Garth

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