George F R Ellis
2007 Philosophy of Physics  
After a survey of the present state of cosmological theory and observations, this article discusses a series of major themes underlying the relation of philosophy to cosmology. These are: A: The uniqueness of the universe; B: The large scale of the universe in space and time; C: The unbound energies in the early universe; D: Explaining the universe -the question of origins; E: The universe as the background for existence; F: The explicit philosophical basis; G: The Anthropic question: fine
more » ... g for life; H: The possible existence of multiverses; I: The natures of existence. Each of these themes is explored and related to a series of Theses that set out the major issues confronting cosmology in relation to philosophy. An initial singularity? The above are specific models: what can one say generically? When the inequality (8) is satisfied, one obtains directly from the Raychaudhuri equation (7) the Friedmann-Lemaître Universe Singularity Theorem [45, 50]: In a FL universe with Λ ≤ 0 and µ + 3p/c 2 > 0 at all times, at any instant t 0 when This is not merely a start to matter -it is a start to space, to time, to physics itself. It is the most dramatic event in the history of the universe: it is the start of existence of everything. The underlying physical feature is the non-linear nature of the EFE: going back into the past, the more the universe contracts, the higher the active gravitational density, causing it to contract even more. The pressure p that one might have hoped would help stave off the collapse makes it even worse because (consequent on the form of the EFE) p enters algebraically into the Raychaudhuri equation (7) with the same sign as the energy density µ. Note that the Hubble constant gives an estimate of the age of the universe: the time τ 0 = t 0 − t * since the start of the universe is less than 1/H 0 . This conclusion can in principle be avoided by a cosmological constant, but in practice this cannot work because we know the universe has expanded by at least a ratio of 11, as we have seen objects at a redshift 6 of 10; from (14) , the cosmological constant would have to have an effective magnitude at least 11 3 = 1331 times the present matter density to dominate and cause a turn-around then or at any earlier time, and so would be much bigger than its observed present upper limit (of the same order as the present matter density). Accordingly, no turn around is possible while classical physics holds [47] . However energy-violating matter components such as a scalar field (Sec.2.6) can avoid this conclusion, if they dominate at early enough times; but this can only be when quantum fields are significant, when the universe was at least 10 12 smaller than at present. Because T rad ∝ S −1 (eqn.(6)), a major conclusion is that a Hot Big Bang must have occurred; densities and temperatures must have risen at least to high enough energies that quantum fields were significant, at something like the GUT energy. The universe must have reached those extreme temperatures and energies at which classical theory breaks down. The hot big bang The matter and radiation in the universe gets hotter and hotter as we go back in time towards the initial quantum state, because it was compressed into a smaller volume. In this Hot Big Bang epoch in the early universe, we can 3 The Steady State universe of Bondi, Hold and Hoyle [14] utilised this metric, but was non-empty as they abandoned the EFE. 4 It is however singular in that it is geodesically incomplete; this metric covers only half the de Sitter hyperboloid [192, 110] . 5 There is also a static (non-RW) form of the metric -the first form of the metric discovered. 6 The redshift z for light emitted at t e and observed at t 0 is related to the expansion by 1 + z = S(t 0 )/S(t e ), see Sec.2.3.3. 7 The dynamically dominant Cold Dark Matter (Sec.2.3.6) obeys the same density law (6) as baryons.
doi:10.1016/b978-044451560-5/50014-2 fatcat:s4d5vlhcsndkph6yg6t4ohcfjm