Читать книгу Origin and Evolution of the Universe - Группа авторов - Страница 15

Observations of the CMB made since 2000 have shown a preferred angular scale of 0.8°. This scale is about 10 times smaller than the beam size of the COBE experiment. The Wilkinson Microwave Anisotropy Probe, launched by NASA in 2001, had a beam size of 0.2° and could accurately measure this preferred scale. The Planck satellite, launched by the European Space Agency (ESA) in 2009, further refined these measurements with a beam size of 0.08°. This preferred scale is related to the horizon angle discussed above. At times earlier than 400,000 years after the Big Bang, the Universe was ionized and the ionized plasma strongly scattered the photons of the CMB (cosmic microwave background). The pressure of the photons led to sound waves, or acoustic oscillations, that traveled at a large fraction of the speed of light. Thus, the two parts of a density perturbation will split up. The dark matter density perturbation will stay fixed, but the ionized gas perturbation will move away, traveling as a sound wave, due to the pressure of the CMB photons. Then, 400,000 years after the Big Bang, the Universe cools to the point where the plasma recombines into transparent gases. This leads to an interference pattern which enhances perturbations of a certain wavelength. This preferred wavelength fits 220 times around the circumference of the sky. This preferred spot size can be seen in Figure 1.5 (use the app at http://www.esa.int/Our_Activities/Space_Science/Planck/Planck_reveals_an_almost_perfect_Universe).

The acoustic scale can also be seen in the spatial distribution of galaxies. Galaxies are likely to form where the density is high, and for a given initial density peak that leads to a central spike of galaxies surrounded by a spherical shell of galaxies where the traveling sound wave ended up 400,000 years after the Big Bang. This separation can be measured by studying the correlation of galaxies: there is an enhanced probability that two galaxies are separated by 142 Mpc instead of 132 or 152 Mpc. This excess probability of galaxy separations of 142 Mpc is clearly seen in the data on galaxy clustering shown in Figure 1.6. The vertical scale shows the observed strength of clustering. Size scale increases along the horizontal axis. For the current value of the Hubble Constant, h = H₀/100 = 0.7, the red arrow shows the increased clustering at a separation of 142 Mpc.

Figure 1.5. Picture of the CMB sky seen by the ESA Planck mission.

Figure 1.6. Strength of galaxy/galaxy clustering is shown on the vertical axis, versus separation, s, on the horizontal axis. The blue points are from the galaxy survey by Blake et al. (2011); the black points are from Eisenstein et al. (2005). The red arrow shows the excess probability of galaxy clustering on a separation scale of s = 142 Mpc, as predicted from sound waves crossing the Universe in its first 400,000 years.