Todays lecture was held by Professor Paul Boynton, who will also be the lecturer on Monday, July 8.
Our Universe is evolving. Three aspects are:
Todays Universe is matter dominated. This means that the mass density is much larger than the the equivalent matter density of the radiation in the Universe. In the past, the Universe was radiation dominated for the following reason. Imagine you put a sub-region of the Universe in a box (of side length d) that expands with the Universe. The density of matter will decrease proportional to 1/d3, because the mass inside the box will stay the same but the volume will increase proportional to d3. The density of photons that make up the radiation in this box will also decrease the same way, but in addition to the decrease of their number density proportional to 1/d3, they also lose energy because their wavelength increases. The energy of an individual photon is proportional to 1/d. So the equivalent matter density (=(E(photon)/c2)*n(photon)) of the radiation is proportional to 1/d 4 and therefore decreases faster than the mass density. There was an instant when mass and radiation density of the Universe were the same. Before, the Universe was radiation dominated, afterwards, it was matter dominated.
When the Univere was younger than 0.1 milliseconds, there were no stable atomic nuclei. The temperature was greater than 1013 K which corresponds to the energy equivalent to the mass of a proton or a neutron. Pairs of protons and antiprotons and neutrons and antineutrons were continuously created and annihilated. When the temperature dropped below 1013 K, almost all the protons/neutrons combined with antiprotons/antineutrons and converted their mass into radiation. For quantum menchanical reasons, there was a small excess of matter over antimatter (for every billion protons annihilated by antiprotons, one survived with no antiparticles to destroy it). That's why we live in a world of normal matter.
When the Universe was about 100 s old, its temperature was about 109K, its density 10-6 g/cm3. This temperature corresponds to the binding energy of the deuteron (H2) - the atomic nucleus of deuterium which consists of one proton and one neutron. Before, such nuclei were not stable. They were formend and then quickly destroyed by high-energy photons. Other nuclei, such as H3 (tritium, 1 proton + 2 neutrons), and He4 (helium, 2 protons, 2 neutrons) formed as well. All these processes stopped when the temperature dropped below about 108K for 2 reasons: 1. The speed of the individual particles became too low to overcome the Coulomb repulsion, and 2. the density decreased, so the probability for collisions of the particles decreased.
When the Universe was about 100,000 to 1,000,000 years old, the temperature had dropped to about 3000 K. This temperature corresponds to the binding energy between an electron and a proton. Before, any electron-proton pair which was formed was quickly destroyed by a photon. Since electrons carry a negative and protons carry a positive charge and charged particle strongly interact with electromagnetic radiation, matter and radiation were strongly coupled. Photons were constantly emitted and then reabsorbed by the charged particles. The Universe was opaque. After the temperature had dropped below about 3000 K, the Universe became neutral. Radiation and matter decoupled and the Universe became transparent. This is referred to as the era of recombination. It's a bit of a misnomer because electrons and protons didn't "re"-combine to form hydrogen. They combined for the first time!
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