1. Dec 20, 2015

### wolram

Radiation dominated the universe 4,700yrs to 378,000yrs, do the facts in the literature mean there was no baryonic matter between those yrs or was there still plasma or some sort of mass?

2. Dec 20, 2015

### Jorrie

The changeover from radiation to matter domination happened around z=3400, time around 50,000 years. At that stage the matter was still in the form of plasma and the universe not transparent. At ~378,000 years it cooled enough to become transparent, but it happened over a period of time.

3. Dec 20, 2015

### Chalnoth

During the radiation-dominated era, the baryonic particles had kinetic energies much greater than their mass-energies, so that they acted like radiation.

4. Dec 20, 2015

### Drakkith

Staff Emeritus
Is that where the name "Radiation Era" comes from?

5. Dec 21, 2015

### Jorrie

But was there not a significant amount of photon energy around as well?
I understand that relativistic particles had lots of kinetic energy and that this portion decayed just like photons (de Broglie wavelength increasing inversely with scale factor a).

6. Dec 21, 2015

### bapowell

Yes, and in fact most of the energy was carried by photons. After matter/antimatter annihilation, there were about a billion photons for every baryon: this fact is often expressed via the tiny baryon-photon ratio, $\eta \approx 6 \times 10^{-10}$.

7. Dec 21, 2015

### Chalnoth

Yes. When the energies were very high, each species of particle would have occurred in approximately equal numbers, as when the typical photon energy is much higher than, say, the electron mass, then interactions are, for the most part, just as likely to produce a pair of photons as an electron/positron pair.

There are some differences due to the fact that photons and electrons/positrons are not actually the same particle and have different interactions, but these differences don't change the relative abundance by much more than about 50% or so.

As the temperature fell and the masses of the protons and electrons started to make a difference for the interactions, the number of protons and electrons fell precipitously as matter/anti-matter pairs annihilated and were not replaced by new matter/anti-matter pairs. This eventually led to the imbalance mentioned by bapowell above: once all of the matter/anti-matter pairs had annihilated and we just had matter left behind, there were about a billion times as many photons as protons/electrons.