5.2 The origin of the 3 K radiation
In speaking of the radiation as having a cosmic origin, what do we have in mind? Essentially this:
In the violent conditions of the early evolution of the Universe, a stage was reached where the matter consisted of a plasma of electrons, protons, neutrons, and some light nuclei such as helium. There were no atoms as such for the simple reason that atoms would have been too fragile to withstand the violence of the collisions that were taking place at the temperature that then existed. As electromagnetic radiation passed through the plasma, it interacted with the matter, exchanging energy in packets or ‘quanta’ of magnitude
where f is the frequency of the radiation and h is the Planck constant.
The radiation was mainly affected by its collisions with the electrons. This is because such collisions cause much bigger energy changes to the photons than collisions with the far more massive nucleons (just as a table tennis ball may lose all its energy in a collision with another table tennis ball, but will bounce off a relatively massive billiard ball with little change in energy). Thus there is a ready exchange of energy between the photons and the electrons, in the process of which, the radiation acquires the thermal spectrum characterised by the temperature of the electrons. The radiation and the electrons tend to come into thermal equilibrium with each other, and the electrons are said to have thermalised the radiation.
As the expansion of the Universe proceeded, the temperature of the radiation progressively fell, and so did that of the matter. This fall led to important changes in behaviour. From the earliest times, the Universe had been opaque to radiation, in the sense that it could not travel far before it interacted with the electrons. But as the temperature declined and photon energies decreased, a stage was reached where electrons could be bound to nuclei to form neutral atoms – atoms that were no longer likely to be disrupted in collisions with the reduced-energy photons. Later, the energy of the radiation reduced still further to the point where it could not even excite the atomic electrons to higher energy states. At this stage, the radiation could no longer be strongly absorbed by matter. This being so, the Universe became transparent. This stage we call the decoupling of radiation from matter. (You will find that some books refer to this stage in the development of the Universe as the ‘recombination’ era rather than the decoupling epoch.) It occurs when the radiation has cooled down to the point where the most probable photon energy corresponds to a temperature of 3000 K. This occurred some 4 × 105 years after the instant of the big bang. Thus the radiation we now observe as 3 K radiation is today's cooled-down remnant of that 3000 K big bang radiation.
How confident can we be that this was indeed the origin of the 3 K radiation? There are essentially four properties that lead to this conclusion:
As we have already mentioned, the isotropy of the radiation points to some global, cosmic origin.
The spectrum of the radiation is such that it could only have been produced by a sufficiently rapid interaction of the radiation with matter for the thermal energy distribution of the particles of matter to be imprinted on the radiation. Only in the early dense stages of the Universe were particles and radiation interacting fast enough for this to have been achieved within the time available.
The present temperature of the radiation of only 3 K is lower than that of most visible matter currently in the Universe. How could it be so low? The only reasonable explanation is that it has been strongly redshifted -indicating that it has been travelling towards us over an exceedingly long period of time, i.e. it was emitted soon after the big bang.
The density of photons corresponding to the 3 K radiation is enormous. In fact there are believed to be about 109 times as many 3 K photons in any large region of the Universe as there are neutrons and protons. Clearly this radiation is no mere by-product of an obscure process; it is a ubiquitous feature of the Universe. This prompts us to ask at what stage of the Universe is radiation likely to have played a dominant role? The answer has to be: the violent early Universe.