Electromagnetic radiation is generated by the acceleration of an electric charge, e.g. electrons in an antenna. Electromagnetic radiation can be considered to be composed of waves or particles, since it displays properties of both. Electromagnetic waves are composed of oscillating electric and magnetic fields lying at right angles to each other and to the direction of travel. Energy is passed via a changing electric field producing a changing magnetic field; a changing magnetic field producing a changing electric field, etc. Maxwell’s Equations (1865) describe this action and include a constant ‘c‘, the speed at which the waves move through space. This constant was found to be precisely the speed of light.
In 1859 Kirchhoff posed the problem: how does the intensity of the electromagnetic radiation emitted by a black body depend on the frequency of the radiation? In 1900 Max Ernst Karl Ludwig Planck (1858-1947) announced that he had derived the relationship based on the idea that the energy was emitted in definite units or quanta. This laid the foundation for quantum theory. In 1905 Einstein suggested that electromagnetic radiation can be regarded as made up of individual particles, now called photons.
If the moment when the Universe emerged from the Big Bang (about 13.6×109 years ago) is set at time zero, then at 10-43 second all the matter in the Universe existed as a ball of hot and dense gas, little more than a singularity, at a temperature of 1034K. This very hot early Universe was dominated by radiation and elementary particles. Under these conditions the photons carried so much energy that they were interchangeable with particles, in line with E=mc2. When two photons collide they create particle-antiparticle pairs, and when these pairs collide they annihilate and convert back to photons. Thus in this early Universe, radiation was constantly being turned into matter and matter was constantly being turned into radiation in a maelstrom of activity.
As a proton has about 1800 times the mass of an electron it takes more energy (higher temperature) to create a proton-antiproton pair than it does to create an electron-positron pair. During the very early Universe the energy-matter interchange included these ‘heavy’ particles. At 10-4 second after time zero the temperature had fallen to 1012K and the heavy-particle era had ended. The density of nuclear matter was 1017kg/m3 (the density of water is 103kg/m3).
Because of a tiny asymmetry in the way the reactions work, first appreciated by Andrei Dmitrievich Sakharov (1921-1989), slightly more particles were produced than antiparticles – about one in a thousand million – and protons and electrons interacted to create neutrons. After 0.01 of a second the temperature of the Universe had dropped to 1011K and only electron-positron pairs were still being produced. At that time there were as many neutrons as protons but as time passed the balance shifted to protons.
At 1.1 seconds the temperature of the Universe was 1010K and the photons could no longer create electron-positron pairs and this marked the end of the light-particle era. Neutrons were no longer produced and there were now 24 neutrons for every 76 protons. After 13.8 seconds the temperature of the Universe had fallen to 3×106K and neutrons and protons had reacted to form nuclei of simple elements, first of deuterium (a hydrogen isotope) and then helium.
Three minutes after time zero of the Universe had cooled to 109K. During the fourth minute of this radiation or photon epoch the remaining neutrons were locked up in nuclei of helium-4, each of which contains two neutrons and two protons. Just under twenty-five percent of the nuclear material was converted into helium nuclei and the rest left behind as lone protons – hydrogen nuclei.
After about half an hour the positrons had annihilated almost all of the electrons with about one in a thousand million left over – matching the number of protons and thus producing the background radiation. The temperature had fallen to 3×108K but this was still too hot for stable atoms to form. When a nucleus caught an electron, the electron was knocked away by a photon of the background radiation.
After 300,000 years the radiation temperature was about 3000K and the protons were becoming too weak to knock electrons off atoms. Nuclei began to capture electrons to form hydrogen atoms. As the free electrons were now bound up in atoms, the interaction of protons with electrons ended. With the scattering of photons removed, the Universe (which had been opaque until now, filled with plasma in thermal equilibrium with the radiation) became transparent. Light could now travel large distances before being absorbed. Matter and radiation were no longer locked together. The radiation era ended and the matter era began. As the Universe expanded, the radiation expanded with it and cooled to become the 2.7K cosmic background radiation of today.
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