Origin and Evolution of the Universe. Группа авторов

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      Penzias and Wilson (1965) reported the discovery of a microwave background with a brightness at a wavelength of 7 cm equivalent to that radiated by an opaque, nonreflecting object with a temperature of 3.7 ± 1 degree Kelvin. Further observations at many wavelengths from 0.05 to 73 cm show the brightness of the sky is equivalent to the brightness of an opaque, nonreflecting object (a blackbody) with a temperature of T₀ = 2.725 ± 0.001 K. The spectrum of the sky as a function of wavelength differs from an exact blackbody spectrum by less than ±60 parts per million. This shows that the Universe was once very nearly opaque and very nearly isothermal (the same temperature everywhere). By contrast, the Universe now has galaxies scattered about and separated by vast stretches of transparent space. Because the conditions necessary to produce the microwave background radiation are so different from the current conditions, we know that the Universe has evolved a great deal over its history. Since the steady-state model predicted that the Universe did not evolve, its predictions are not consistent with the observed microwave background.

      Observations of the microwave background toward different parts of the sky show a small variation in the temperature, with one side of the sky being 3.36 mK (0.00336 Centigrade degrees) hotter and the opposite side of the sky being 3.36 mK colder than the average. The pattern of a hot pole and a cold pole is called a dipole. It is a measure of the peculiar velocity of our solar system at 369 ± 1 km/s relative to the Hubble law. The velocity is the sum of the motions caused by the revolution of the Sun around the Milky Way, the orbit of the Milky Way around the center of mass of the local group of galaxies, and the motion of the local group caused by the gravitational forces from the Virgo Supercluster, the Great Attractor, and other clumps of matter. The local group contains about 30 galaxies, of which the Milky Way and the Andromeda nebula are the biggest; the Virgo Supercluster contains thousands of galaxies. The reader will find more details on galaxies and their clusters in Alan Dressler’s Chapter 2.

      After the dipole pattern is accounted for, the remaining temperature fluctuations are very small, only 11 parts per million. These tiny temperature differences were detected by NASA’s Cosmic Background Explorer (COBE) satellite. This implies that the initial density fluctuations in the Universe were also very small.

      The current energy density of the microwave background is quite small, as might be expected for the thermal radiation from something that is colder than liquid helium. The number density of microwave photons is 410 cm⁻³, and the average energy per photon is 0.00063 electron volt (eV). Thus, the energy density is only 0.26 eV/cm³. This is 20,000 times less than the critical density. But when the Universe was very young (e.g., t = 0.5 × 10⁻⁶ t₀, about 7,000 years after the Big Bang) and the scale factor was very small, a(t) = 10⁻⁴, the number density of photons was much greater, 4.1 × 10¹⁴ cm⁻³, and the energy per photon was also much greater, 6.3 eV. The photon density and average energy per photon correspond to a hotter blackbody with a temperature T = T₀/a(t) = 27,250 K. As the Universe expands, it also cools. Thus, the energy density of the background when the Universe was small, dense, and hot was very large, 2.6 × 10¹⁵ eV/cm³ when a(t) = 10⁻⁴, and dominated the density of the Universe for all times less than 50,000 years after the Big Bang.

      The very small temperature fluctuations indicate corresponding density fluctuations of about 33 parts per million 10,000 years after the Big Bang. Once the energy density of background radiation becomes less than the density of matter, the fluctuations grow as the denser regions gravitationally attract more material. The process of gravitational collapse makes the fluctuations grow in proportion to the scale factor a(t). Thus, in the case of a Universe with the critical density described before, the Universe was no longer radiation-dominated when a = 0.0003, so the fluctuations grew from 33 parts per million to 11%. This is just enough to explain the observed clustering of galaxies that we see in the Universe now. But if the Universe had no dark matter, then a = 10⁻³ when the Universe stopped being radiation-dominated. Furthermore, ordinary matter interacts with light and could not move through the background radiation until the Universe was cold enough for neutral hydrogen atoms to be stable. This happened approximately 400,000 years after the Big Bang, when the temperature of the microwave background fell to 3000 K, which, coincidentally, is when a = 10⁻³. Thus, if only ordinary matter had been present, the fluctuations implied by the COBE observations would have grown only to 3.3% at the current epoch. Such small density contrasts would be completely inconsistent with the far higher density contrasts we currently observe on many scales, such as galaxy clusters with amplitudes of well over 100%.

       Light Element Abundances

      Although the energy density of the microwave background dominated the Universe for the first 50,000 years after the Big Bang, it is even more significant during the first 3 minutes. One second after the Big Bang, the average energy of a photon was 3 million electron volts (MeV), which is a gamma ray. Gamma rays destroy any atomic nucleus; thus, 1 second after the Big Bang there were only protons (p or hydrogen nuclei), free neutrons (n), electrons (e), and positrons (e+), neutrinos (v_e− ν_μ and ν_τ), and their antiparticle counterparts, the antineutrinos. The three types of neutrinos correspond to the three “families” of elementary particles, but, except for the neutrinos, all the particles in the second and third families (such as muons and tauons), are so heavy and unstable that they decay during the first second after the Big Bang. Weak nuclear interactions such as

      determined the ratio of neutrons to protons. The neutron is heavier than the proton; the neutron-to-proton ratio declines as the temperature falls. But eventually, at about 1 second after the Big Bang, the density of electrons and neutrinos falls to such a low level that reaction (9) is no longer effective. After this time, the neutron-to-proton ratio gradually falls because of the radioactive decay of the neutron,

      which has a half-life of 615 seconds.

      As neutrons decay, the Universe expands and grows colder. Eventually the temperature falls to the point where the simplest nucleus, the heavy hydrogen or deuterium nucleus (d, the nucleus having both one proton and one neutron), is stable. This occurs when the temperature is about 10⁹K, which occurs about 100 seconds after the Big Bang. At this point, the reaction

      very quickly converts all neutrons into deuterium nuclei. Once deuterium is formed, it is quickly converted into helium through a network of interactions, with the net effect

      Because almost all neutrons that survive until T is less than 10⁹K end up bound in helium nuclei, the helium abundance in the Universe provides a measurement of the time it takes for the Universe to cool to 10⁹K. If the Universe cools rapidly, there is a large helium abundance, but slow cooling gives low helium abundance because more of the neutrons decay. The standard Big Bang model, with three types of neutrinos, predicts a helium abundance that is correct to within the 1% margin of uncertainty of current observations. Reaction (12) requires collisions between two nuclei, and if the density of atomic nuclei is low, then a fraction of the deuterium will not react. Thus, the residual fraction of deuterium in the Universe is a sensitive measure СКАЧАТЬ