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The observations described in Chapter 17 show our universe to be approximately homogeneous and isotropic on spatial distance scales above several hundred megaparsecs. The simplest cosmological models enforce these symmetries exactly as a first approximation. For instance, the matter in galaxies and the radiation are approximated by smooth density distributions that are exactly uniform in space. Similarly, the geometry of spacetime incorporates the homogeneity and isotropy of space exactly. These simplifying assumptions define the Friedman–Robertson–Walker (FRW) family of cosmological models, which are the subject of this chapter.
To test which of these models applies to our universe, one needs to extend redshift measurements to large distances, out to several Giga-light years. The most successful approach has been to use white-dwarf supernovae (SN type Ia) as very luminous standard candles. One of the greatest surprises of modern astronomy is that the expansion of the universe must be accelerating! This implies there must be a positive, repulsive force that pushes galaxies apart, in opposition to gravity. We dub this force “dark energy.”
This chapter explores what is known as the Cosmic Microwave Background (CMB), what it is, how it was discovered and our recent efforts to measure and map it. In general, the analysis finds remarkably good overall agreement with predictions of the now-standard “lambda CDM” model of a universe, in which there is both cold dark matter (CDM) to spur structure formation, as well as dark-energy acceleration that is well-represented by a cosmological constant, lambda. From this we can infer 13.8 Gyr for the age of the universe.
According to many physicists, several aspects of the laws of nature, the constants, and the cosmic boundary conditions are fine-tuned for life: had they been slightly different, life would not have existed. Here I review the claimed instances of fine-tuning and some of the criticism that has been levelled against the fine-tuning considerations. I also discuss in which sense, if any, fine-tuned parameters may qualify as improbable. Finally, I review the naturalness criterion of theory choice and discuss how violations of naturalness may be regarded as relevant to the discussion about fine-tuning for life.
This chapter turns to the prospects for empirically testing specific cosmological multiverse theories such as the landscape multiverse scenario or cyclic multiverse models. The most commonly pursued strategy to extract concrete empirical consequences from specific multiverse theories is to regard them as predicting what typical multiverse inhabitants observe if the theories are correct, where "“typical” is spelled out as “randomly selected from some suitably chosen reference class.” I scrutinize a proposal by Srednicki and Hartle to treat the self-sampling assumption and the reference class to which it is applied as matters of empirical fact that are themselves amenable to empirical tests. Unfortunately, this proposal turns out to be incoherent. A much better idea, which coheres well with the intuitive motivation for the self-sampling assumption, is that we should make this assumption with respect to some reference class of observers precisely if our background information is consistent with us being any of those observers and neutral between them. I call this principle the “background information constraint” (BIC) and point out that it at least formally solves the problem of selecting the appropriate observer reference class.
The first chapter contains a räsumä of the cosmology treating the homogeneous and isotropic universe. The Friedmann equations are derived and the thermal history of the Universe is discussed in some detail. Special emphasis is laid on the process of recombination and the decoupling of photons from the cosmic uid. Nucleosynthesis and cosmic in ation are also discussed.
The standard model of cosmology called LCDM has its origins in the work of great scientists including Einstein, Friedmann, Slipher, Hubble, Lemaitre, and Gamow. Lemaitre’s 1930s “Cosmic Egg” or “Primeval Nucleus” was the basis for the Big Bang model. In its new variant called LCDM, “L” represents the cosmological constant Lambda and “CDM” represents Cold Dark Matter. These two components, L and CDM, account for 95 percent of the mass–energy content of the Universe. Edwin Hubble correctly showed in the 1930s that galaxies are distributed on the largest scales in a homogeneous and isotropic way, but on a more local scale of 300 million light-years Hubble failed to recognize significant inhomogeneities. Hubble and Humason validated the velocity–distance relation for galaxies and galaxy clusters demonstrating the expansion of the Universe. They did not call out how significant the velocity–distance relationship would become in our effort to determine the 3D structure in the galaxy distribution.
With the development of general relativity, Einstein realised that he had a theory which for the first time could be used to create fully self-consistent cosmological models. In 1917, he introduced the cosmological constant to create a static closed Universe. The standard world models were discovered by Friedman in 1922 and 1924 and rediscovered by Lemaitre a few years later. The expansion of the Universe was discovered by Hubble in 1929. A key discovery was that of the cosmic microwave background radiation by Penzias and Wilson in 1965. The resulting hot big bang scenario for the large-scale structure and evolution of the Universe became the preferred cosmological model. With the development of precision cosmology through precise measurements of the cosmic microwave background radiation, it was established that the cosmological constant has a finite value and that the Universe is geometrically flat.
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