Friedmann cosmology
3. A QUALITATIVE ANALYSIS OF THE FRIEDMANN DYNAMICS 21 t
a k=1 k=0 k=−1
Figure 7. A sketch of solutions of the Friedmann equations for the spherical (k = 1), flat (k = 0) and hyperbolic (k =−1) case for w >−13.
is zero. This singularity is referred to as the big bang. Then, for the open (flat and hyperbolic) Friedmann cosmologies a is strictly monotonically increasing for all t > 0. In contrast for the closed (spherical) Friedmann cosmologies a is at first strictly monotonically increasing until it attains a maximum. After that, a is strictly monotonically decreasing until it vanishes again. Hence the open Friedmann cosmologies describe forever expanding universes, while the closed Friedmann cos- mologies describe universes which expand until a state of maximal spatial volume, after which they contracts again down to ‘zero size’. This recollapse to another singular state that occurs in the evolution of the spherical Friedmann cosmologies is referred to as the big crunch.
3. A qualitative analysis of the Friedmann dynamics
Although the Friedmann equations can be solved exactly for perfect fluids with linear equation of state, many of the interesting qualitative features of the evolu- tion of these models can be read out from the Friedmann equations (13) and the conservation equation (14) without solving; cf. [5, p 98–100]. This section deals with such a qualitative analysis, however using a method following the discussion in [7, section 2.3], that can readily be generalised for the analysis of more general spatially homogenous cosmologies, which will be discussed in chapter 5:
In order to rewrite the evolution equations in the desired way, one defines the variables (15) Hubble scalar, deceleration parameter and expansion-normalised energy density H := ˙a a, q :=− ¨ aa ˙a2 and Ω := ρ 3H2.
H is called the Hubble scalar, and gives a measure of the rate of expansion of the underlying universe. q is called the deceleration parameter, and gives a measure of the deceleration of the expansion. Ω is simply the expansion-normalised (or Hubble- normalised) energy density, which is often referred to as the density parameter. H has the dimension of time−1 while both q and Ω are dimensionless. Differentiating H with respect to time and using q yields ˙H =−(1+q)H2as an evolution equation for H. The considerations shall again be restricted to a perfect fluid with linear
22 4. FRIEDMANN COSMOLOGY
k=−1 k=0 k=1
Ω
0 1
Figure 8. Flow diagram for the qualitative evolution of the Friedmann cosmologies for perfect fluids with p = wρ and w >−13.
equation of state; p = wρ. Then by differentiating Ω with respect to time and using the conservation equation (14), one analogously arrives at ˙Ω = (−3(1 + w) + 2(1 + q))ΩH as an evolution equation for Ω. Finally using the second Friedmann equation (13), one finds that q is related to Ω by q = 1+3w
2 Ω.
rescaled time With this, and
after defining a rescaled, dimensionless time variable τ through d dτ :=
1 H
d
dt the two evolution equations above become
(16) rescaled evolution equations H0=−1 + 1 + 3w 2 Ω H and Ω0= (1 + 3w)(Ω− 1)Ω,
where here and henceforth the prime denotes derivatives with respect to rescaled time τ . These equations can be seen as the rescaled evolution equations, being equivalent to the Friedmann equations (13). However unlike the system (13), the system (16) is decoupled; there is no H appearing in the equation for Ω. Also, the equation for H has dimension time−1 while the equation for Ω is dimensionless.
To solve for the metric components exactly, one would need to solve the full system (16). However it suffices to focus on the equation for Ω when one is merely interested in the major qualitative features of the solutions to this system. In fact for this, one does not even have to solve this equation exactly. A qualitative analysis is sufficient. This can best be seen by visualising the qualitatively different solutions of this equation in a flow diagram, which is shown in figure8:
the state space Assuming a non-negative energy density ρ, the possible values that Ω can attain is the non-negative part of the real line. This is also called the state space of the differential equation.
fixed point solutions Restricting again to the case w > −13 one then finds that this equation has two static solutions, Ω = 0 and Ω = 1, for which Ω0 = 0. The corresponding points in the diagram are called fixed points, since for instance if Ω = 1 at one instant of time during the evolution, then Ω = 1 during the whole evolution. Static solutions are therefore also called fixed point solutions.
generic solutions Solutions
for which Ω0 6= 0 are called generic solutions. This is because for a solution of this type, an initial value can be out of an open subset of the state space, and does not need to be fine tuned as it has to be for the fixed point solutions. For Ω∈ (0, 1) one has Ω0 < 0. Hence for a solution with initial value in this range, Ω is strictly monotonically decreasing with (rescaled and coordinate) time. By the uniqueness theorems of the theory of ordinary differential equations, these solutions approach the value Ω = 1 asymptotically into the past (τ → −∞) and Ω = 0 asymptotically into the future (τ → ∞), however never reach these values. Hence for these solutions the matter appears to thin out with the evolution, and solutions of this type therefore seem to correspond to the open, forever expanding Friedmann cosmologies. In contrast, for Ω∈ (1, ∞) one has Ω0> 0 so that the matter density appears to diverge with the evolution. Accordingly solutions of this kind seem to correspond to the closed, recollapsing Friedmann cosmologies. However one has to take care with these assumptions, since Ω only denotes the rescaled, Hubble- normalised energy density. To check how the actual energy density ρ evolves, one would also have to bring in information from the equation for H. Yet one can also convince oneself more easily that the above claims are indeed true by observing that the first Friedmann equation (13) can be expressed in terms of Ω as
Ω− 1 = k H2a2,