Heller M, Woodin W.H. (eds.) Infinity. New research frontiers (CUP, 2011)(ISBN 1107003873)(O)(327s)_MAml_

vast universe and physical infinity 195making you and me, our planet, stars, single galaxies, and even clusters of galaxies,are only second-order perturbations to an otherwise uniform distribution of matter andenergy. The cosmological principle is an advanced form of Copernican outlook: notonly does the universe have no physical center, but it also lacks any structure if welook at it as a whole.Perhaps owing to its immense benefit to theoreticians, the cosmological principlewas widely accepted well before observations could effectively verify its validity.Remarkably, recent data confirm the gradual tendency toward uniformity at largedimensions. 3 If we look at portions of the universe of sizes >100 Mpc or so, 4 wefind that the general statistical distribution is maintained, whereas the details changefrom region to region, suggesting that the cosmological principle is indeed a goodapproximation of the real universe – at least within the limits of our current data. Buthow far can we verify its validity with observation?Since the discovery of cosmic expansion in the late 1920s, our view of the universehas changed drastically (Hubble 1929; Hubble and Humason 1931). 5 Change andevolution are not only characteristics of biological life, of our planet, of stars andgalaxies, but also of the universe as a whole. We live in a historical, contingent universe:no instant of time, at cosmic scale, is the same as any another. Expansion also meansthat back in the past the universe was smaller, hotter, and denser than it is today. Thepresent estimates of the age of the universe 6 tell us that back in cosmic history some13.7 billion years ago the temperature and energy density reached fantastic valueseverywhere in space.The Hubble Ultra Deep Field image (Beckwith et al. 2006), taken with the AdvancedCamera for Surveys on board the Hubble Space Telescope (HST), shows some of thefarthest known galaxies, whose light has traveled for about 13 billion years beforehitting the HST mirrors. Those galaxies belong to our universe when it had less than10 percent of its present age. Even at those distances, if we account for evolution, thedata are consistent with cosmic uniformity. Can we look further back? Remarkably,the answer is yes. In 1965 Arno Penzias and Robert Wilson serendipitously discovereda prodigious fossil light (Penzias and Wilson 1965), named cosmic microwavebackground (CMB), 7 a sea of photons pervading the universe and released in an earlyphase (redshift z ∼ = 1100) of cosmic history. The great isotropy of the CMB, better thanone part in 10,000, provides further strong support for the cosmological principle. TheCMB photons were released when the expansion cooled the temperature below 3000 Kand allowed the first atoms to form from electrons and light nuclei. This took place3 The current most precise data come from large galaxy surveys such as the Sloan Digital Sky Survey (SDSS) andthe Two Degree Field Galaxy Redshift Survey (2dFGRS). In addition, on very large scales we have compellingevidence of high isotropy from CMB observations.4 1 Mpc (megaparsec) = 10 6 pc (parsec); 1 pc = 3.26 light-years.5 Vesto Slipher, working at Lowell Observatory, back in 1914 announced the discovery of “nebular redshifts,”and he contributed the radial velocity data used by Hubble in his 1929 discovery paper.6 The WMAP data in the standard CDM model give a cosmic age of 13.73 + 0.13/ − 0.17 billion years (Spergelet al. 2007). This is in agreement with estimates based on globular clusters (Chaboyer and Krauss 2002) andwhite dwarfs (Richer et al. 2004).7 The word “microwave” reflects the fact that the radiation spectral range, as observed today, is in the millimetermicrowaverange.