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

196 infinity and the nostalgia of the starswhen the universe was 380,000 years old, only 0.003 percent of its present age. Despitethe enormous number of galaxies, the voids between them are huge and the universeis essentially empty. Therefore, the CMB traveled rather undisturbed and brings to usa remarkably faithful view of the early universe, back to the time when the photonslast interacted with matter. Before then, the universe was opaque to light. This ultimatecurtain – the so-called last scattering surface – represents a physical obstacle to directobservation into earlier cosmic epochs: beyond that limit our view of the universe isblocked. 8Are there ways to get direct information from what lies beyond the last scatteringsurface? We could go a bit further, at least in principle, but not by much. In the future(well beyond what is predictable today), we might be able to detect cosmic neutrinos,which easily cross the hot plasma unimpeded and reach us directly from a universeonly one-second old. 9 If primordial gravitational waves will some day be detected, theywould get us a direct signature from the inflation era, some 10 −35 seconds of the BigBang. 10 Eventually, however, we approach the ultimate barrier of our cosmic horizon:we cannot get information from regions farther away than the distance traveled by lightin the entire lifetime of the universe. It is not a matter of improving our instrumentsand observing strategy; rather, it is a fundamental limit set by the finite speed of lightand the finite cosmic age. As a consequence, we can observationally test the validityof the cosmological principle only within a limited region of space. The extensionof isotropy and uniformity to the entire universe – something we may call a “strongcosmological principle” – is ultimately unverifiable. That would be the case for theextrapolation to the universe as a whole of any other physical property observed withinour cosmic horizon. The expanding hot Big Bang universe brings with it the notion ofa horizon that delimits the part of cosmic space we can probe: the observable universeis definitely finite.9.2 Hints of Infinity in the Primordial MusicIf we assume a strong cosmological principle, then the metric of spacetime can berepresented in the Friedmann-Lemaître-Robertson-Walker (FLRW) form, and generalrelativity describes beautifully the evolution and geometry of the universe under theaction of gravity. The theory leads to differential equations whose solutions depend onadimensional density parameters (normalized to a critical density ρ C ) that quantify the8 However, by observing in detail the properties of the CMB (its frequency spectrum, its angular distributionor anisotropy, and its polarization), we can learn a great deal of what happened in the first 380,000 years ofcosmic infancy.9 The cosmic neutrino background is expected to have a present equivalent temperature of 1.9 K.10 High-precision polarization measurements of the CMB may also lead indirectly to such a result (see, e.g.,Boyle, Steinhardt, and Turok 2006). The Planck satellite, launched in May 2009, should be able to explore thispossibility. The ultimate conceivable early phase is the Planck era, at times 10 −43 seconds from the beginningof the universe, when space and time become subject to quantum fluctuations and Einstein’s theory of gravityfails. This frontier corresponds to energies exceeding 10 19 GeV and sizes smaller than 10 −35 cm. Currentlywe have no way to describe the universe in this state. To extend physics beyond this limit, we would need aquantum theory of gravity. Superstring theory might offer an attractive approach, but it is not known whetherit would lead to successful synthesis.