# Dark energy before radiation or after?

In the cosmic history of the universe, does the dark energy comes before the radiation epoch, or only now, in the 'matter' universe (matter dominated era)?

Because, we now know that like 75% of our visible universe is made up of dark energy, about 20% is dark matter and the rest is the 'ordinary matter', but does that mean that this dark energy came after the radiation and matter domination of the universe or was it there in the beginning, at the BB, and then was just spread out via inflation, while radiation era and matter domination era followed?

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See papers on "early dark energy" for discussion of this issue. – Mitchell Porter Jan 27 '13 at 12:34
Care to give some good links? – dingo_d Jan 27 '13 at 12:40
I don't know the literature. – Mitchell Porter Jan 29 '13 at 8:19

## 1 Answer

The Universe today is believed to be dominated by dark energy. In fact, it is believed that the dark energy may take the form of a cosmological constant, in which case its energy density has been a constant throughout the history of the Universe. Radiation and matter are also present today and have been present for most of the history of the Universe. However, as the Universe expands, matter and radiation are diluted; their densities decrease at later times. If you know about the scale-factor of the Universe, $a$, we can say that $\rho_{\gamma}\propto a^{-4}$, $\rho_{m}\propto a^{-3}$, and $\rho_{\Lambda} = constant$, where $\rho_{\gamma}$ is the density of radiation, $\rho_{m}$ is the density of matter, $\rho_{\Lambda}$ is the density of the cosmological constant. Tracing back from out dark-energy dominated Universe, we get to a matter dominated Universe, and then further back still, we get radiation domination.

However, it is also believed that the very early Universe went through a period of accelerated expansion called inflation. Here, the Universe expands as if it were dark-energy dominated; its volume increases rapidly and it is supercooled. This process doesn't last long. Also, when inflation comes to an end, the energy is used to "reheat" the Universe.

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So basically the inflation is due to dark energy? And therefore dark energy was there before the radiation era? – dingo_d Jan 27 '13 at 12:39
Not exactly. When we say "dark energy", we can often be a bit imprecise and just mean 'something that causes accelerated expansion'. But, there are many different things that can cause accelerated expansion, only some of which may be called dark energy, and even those come in all sorts of varieties. It gets a bit complicated the more you look into it. – user12345 Jan 27 '13 at 12:46
If this answer was useful, or at least interesting, consider upvoting and marking it as the accepted answer. – user12345 Jan 28 '13 at 17:30