In Search of the Big Bang by John Gribbin

Author:John Gribbin
Language: eng
Format: mobi, epub, azw3
Publisher: ReAnimus Press
Published: 2015-01-11T22:00:00+00:00

The next 10 billion years

A little more than half an hour after t = 0 (at t = 34 minutes and 40 seconds, to be precise), almost all of the electrons and positrons have been annihilated, and the Universe has begun to resemble the empty state we find it in today. Almost, but not quite, all of the matter has gone. In addition to the billionth part of the number of photons that is present as nucleons, when the electron-positron pairs finally annihilate, just one electron in a billion is also left over, exactly the amount required to balance the positive charge on all the protons in the Universe, and to ensure that eventually the matter will settle out as stable, uncharged atoms, with every proton in every atomic nucleus matched by an electron in the cloud on the outside of the atom. Where does this tiny proportion of matter come from? Why isn’t there a perfect symmetry between particles and antiparticles, so that everything annihilates and only radiation is left as the Universe cools? The answers emerge from an understanding of the world of particle physics under conditions even more extreme than those during the epoch of the life of the Universe following the first hundredth of a second, and they are among the simplest, but also most profound, of the puzzles resolved by the discoveries described in later chapters. For now, though, let’s stick with the expanding, cooling fireball half an hour after the moment of creation.

By now, the temperature of the fireball is down to 300 million K, and the energy density of the Universe is only 10 per cent of the mass density of water. About 69 per cent of this energy is carried by photons and 31 per cent by neutrinos, and the expansion timescale appropriate at this time has stretched to 75 minutes. Although all the available neutrons have been cooked into helium nuclei, the Universe is still too hot for stable atoms to form—as soon as a positively charged proton or helium-4 nucleus latches on to a negatively charged electron, the electron is knocked out of its grip by an energetic photon. This is the ‘radiation era’ of the Universe, with no significant particle interactions to worry about, and with the remaining matter dominated by the radiation. It lasts for about 700,000 years, until the temperature drops to about 4,000 K and the nuclei and electrons are at last able to hold together against the ever-decreasing battering they receive from photons.

The time at which this occurs is not well defined. As early as 300,000 years after the moment of creation some hydrogen atoms are beginning to form and survive for a reasonable length of time without being ionized by the radiation; after t = 106 years, all of the electrons have been bound up in atoms, so efficiently that only one electron and one proton are left out on their own for every 100,000 stable atoms, and the ‘decoupling’ of matter from radiation is complete.

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