Читать книгу Origin and Evolution of the Universe - Группа авторов - Страница 12

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%.