# Was the cosmic microwave background (CMB) radiation originally gamma rays?

I believe that sometime after Big Bang the entire universe was filled with high energy particles. Now we fast forward to the present; most of these particles still linger and managed to avoid being absorbed, and also they lost quite a significant amount of energy and became today's "noise" which can be picked up by our TV antenna. Is this true, or am I mistaken?

• @ Rob Jeffries Earlier I took a quick glance/peek at Wiki and other sources on this topic and did notice it mentions wavelength lengthening and I wonders lengthening from which wavelength is it gamma? I am interested in the condition and process that generate this energy during the infant stage of the universe and maybe it is reproducible at much smaller scale like in a room someday. – user6760 Mar 18 '15 at 16:17

The cosmic microwave background is a result of an almost perfect blackbody emitter. That means the spectrum covers a broad wavelength range with a peak that is given by Wien's law: $$\lambda_{peak} = \frac{2.9\times 10^{-3}}{T},$$ where the wavelength is in metres and temperature in Kelvin.

The microwave background is formed at the epoch of (re)combination, when protons and electrons combine to form hydrogen atoms and the Uinverse effectively becomes transparent to the radiation field within it. This occurs at approximately 3000 K. So at that time, the cosmic microwave background had a peak wavelength of 970 nm, which in the near infrared part of the spectrum.

The epoch of (re)combination occurred at a redshift of approximately $z=1100$. What this means is that since then, the wavelength of the photons has been stretched by a factor of $1+z = 1101$, so that $\lambda_{peak} \simeq 1$ mm, and this corresponds to a temperature of $T=2.7$ K. I.e It is near infrared photons emitted at the epoch of recombination that have their wavelengths stretched by the expansion of the universe, and are seen today as microwaves.

However, prior to the epoch of recombination, the Universe would still have been full of radiation, and as we go back in time towards the big bang, that radiation field becomes hotter and hotter, and the peak wavelengths get shorter and shorter.

If we look at models for the thermal history of the Universe, then at about 1 second after the big bang, the Universe would have been at about $10^{10}$ K and the radiation field would have been in equilibrium with the matter at this temperature. The formula above then gives $\lambda_{peak}=2.9\times10^{-13}$ m, which is definitely in the gamma-ray regime. But let me emphasize that these photons are emitted then absorbed many times and we do not see them now.

The "particles" that you are talking about are photons of light.

• TLDR: No, it was not originally gamma radiation, but orange-hot light at 3000 K. – Florin Andrei Mar 18 '15 at 17:49