# Big Bang energy density

## Summary:

How close to the Big Bang can modern particle accelerators get?

## Main Question or Discussion Point

How many nanoseconds after the Big Bang was its energy density that achievable in modern particle accelerators?

mfb
Mentor
In the range of many nanoseconds, but without a specific number - if you match the density you don't match the temperature and vice versa, and every collision is different.

Thanks. I had however imagined that temperature and energy density were the same thing. What is the difference?

mfb
Mentor
Roughly: Temperature is the energy per particle, density is how many particles you have per volume. [Here is a sketch of a phase diagram](https://en.wikipedia.org/wiki/File:Phases_of_Nuclear_Matter.JPG).

More detailed discussion - figure 11 has the profile for the early universe and dots corresponding to experiments. The x axis is the "chemical" potential of baryons instead of density, but they are related quantities.

Proton-proton collisions can reproduce processes at higher energy but they don't reach thermal equilibrium so temperature and density become a bit ill-defined, but they can study what happens at higher temperatures.

• • pinball1970 and DrClaude
Thanks. From fig.11 I see that temperature is given in baryon-Mev, which is a unit of energy, rather than degrees C. So they are different ways of expressing the same thing?

PeterDonis
Mentor
2019 Award
From fig.11 I see that temperature is given in baryon-Mev, which is a unit of energy, rather than degrees C. So they are different ways of expressing the same thing?
No. Energy per particle and energy density are not the same thing. Baryon-Mev is a unit of energy per particle; it's just Boltzmann's constant times degrees C (or more precisely degrees K). Expressing temperature in units of energy per particle instead of degrees is common in physics.

Thanks. I thing energy-per-particle/temperature was what I was really after.

• berkeman