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7.1 Lessons from homogeneous solutions

Which features should we focus on when thinking about the dynamics of forever expanding cosmological models? Consider for a moment the Kasner solution
- dt2 + t2p1dx2 + t2p2dy2 + t2p3dz2, (3)
where p1 + p2 + p3 = 1 and p21 + p22 + p23 = 1. These are the first and second Kasner relations. They imply that not all pi can be strictly positive. Taking the coordinates x, y, and z to be periodic gives a vacuum cosmological model whose spatial topology is that of a three-torus. The volume of the hypersurfaces t = const. grows monotonically. However, the geometry does not expand in all directions, since not all pi are positive. This can be reformulated in a way which is more helpful when generalizing to inhomogeneous models. In fact the quantities pi are the eigenvalues of the second fundamental form divided by the mean curvature. The statement then is that the second fundamental form is not negative definite. Looking at other homogeneous models indicates that this behaviour of the Kasner solution is not typical of what happens more generally. On the contrary, it seems reasonable to conjecture that in general the second fundamental form eventually becomes negative definite, at least in the presence of matter.

Some examples will now be presented. The following discussion makes use of the Bianchi classification of homogenous cosmological models (see, e.g., [344Jump To The Next Citation Point]). If we take the Kasner solution and add a perfect fluid with equation of state p = (g - 1)r, 1 < g < 2, maintaining the symmetry (Bianchi type I), then the eigenvalues ci of the second fundamental satisfy ci/ trk --> 1/3 in the limit of infinite expansion. The solution isotropizes. More generally this does not happen. If we look at models of Bianchi type II with non-tilted perfect fluid, i.e. where the fluid velocity is orthogonal to the homogeneous hypersurfaces, then the quantities pi = ci/trk converge to limits that are positive but differ from 1/3 (see [344Jump To The Next Citation Point], Page 138.) There is partial but not complete isotropization. The quantities pi just introduced are called generalized Kasner exponents, since in the case of the Kasner solution they reduce to the p i in the metric form (3View Equation). This kind of partial isotropization, ensuring the definiteness of the second fundamental form at late times, seems to be typical.

Intuitively, a sufficiently general vacuum spacetime should resemble gravitational waves propagating on some metric describing the large-scale geometry. This could even apply to spatially homogeneous solutions, provided they are sufficiently general. Hence, in that case also there should be partial isotropization. This expectation is confirmed in the case of vacuum spacetimes of Bianchi type VIII [313]. In that case the generalized Kasner exponents converge to non-negative limits different from 1/3. For a vacuum model this can only happen if the quantity ^R = R/(tr k)2, where R is the spatial scalar curvature, does not tend to zero in the limit of large time. Detailed asymptotics for these spacetimes has been obtained in [314].

The Bianchi models of type VIII are the most general indefinitely expanding models of class A. Note, however, that models of class VIh for all h together are just as general. The latter models with perfect fluid and equation of state p = (g- 1)r sometimes tend to the Collins model for an open set of values of h for each fixed g (cf. [344], Page 160). These models do not in general exhibit partial isotropization. It is interesting to ask whether this is connected to the issue of spatial boundary conditions. General models of class B cannot be spatially compactified in such a way as to be locally spatially homogeneous while models of Bianchi type VIII can. See also the discussion in [33].

Another issue is what assumptions on matter are required in order that it have the effect of (partial) isotropization. Consider the case of Bianchi I. The case of a perfect fluid has already been mentioned. Collisionless matter described by kinetic theory also leads to isotropization (at least under the assumption of reflection symmetry), as do fluids with almost any physically reasonable equation of state [293]. There is, however, one exception. This is the stiff fluid, which has a linear equation of state with g = 2. In that case the generalized Kasner exponents are time-independent, and may take on negative values. In a model with two non-interacting fluids with linear equation of state the one with the smaller value of g dominates the dynamics at late times [120], and so the isotropization is restored. Consider now the case of a magnetic field and a perfect fluid with linear equation of state. A variety of cases of Bianchi types I, II, and VI0 have been studied in [221Jump To The Next Citation Point222223], with a mixture of rigorous results and conjectures being obtained. The general picture seems to be that, apart from very special cases, there is at least partial isotropization. The asymptotic behaviour varies with the parameter g in the equation of state and with the Bianchi type (only the case g > 1 will be considered here). At one extreme, Bianchi type I models with g < 4/3 isotropize. At the other extreme, the long time behaviour resembles that of a magnetovacuum model. This occurs for g > 5/3 in type I, for g > 10/7 in type II and for all g > 1 in type VI0. In all these cases there is partial isotropization.

Under what circumstances can a spatially homogeneous spacetime have the property that the generalized Kasner exponents are independent of time? The strong energy condition says that a b Rabn n > 0 for any causal vector a n. It follows from the Hamiltonian constraint and the evolution equation for trk that if the generalized Kasner exponents are constant in time in a spacetime of Bianchi type I, then the normal vector na to the homogeneous hypersurfaces gives equality in the inequality of the strong energy condition. Hence the matter model is in a sense on the verge of violating the strong energy condition and this is a major restriction on the matter model.

A further question that can be posed concerning the dynamics of expanding cosmological models is whether 2 ^r = r/(trk) tends to zero. This is of cosmological interest since r^ is (up to a constant factor) the density parameter _O_ used in the cosmology literature. Note that it is not hard to show that trk and r each tend to zero in the limit for any model with /\ = 0 which exists globally in the future and where the matter satisfies the dominant and strong energy conditions. First, it can be seen from the evolution equation for trk that this quantity is monotone increasing and tends to zero as t-- > oo. Then it follows from the Hamiltonian constraint that r tends to zero.

A reasonable condition to be demanded of an expanding cosmological model is that it be future geodesically complete. This has been proved for many homogeneous models in [290Jump To The Next Citation Point].

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