From the Friedmann equation (5) (where henceforth we take the effects of a cosmological constant into account by including the vacuum energy density into the total density ), for any value of the Hubble parameter there is a critical value of the energy density such that the spatial geometry is flat ():
In general, the energy density will include contributions from various distinct components. From the point of view of cosmology, the relevant feature of each component is how its energy density evolves as the universe expands. Fortunately, it is often (although not always) the case that individual components have very simple equations of state of the form
The simplest example of a component of this form is a set of massive particles with negligible relative velocities, known in cosmology as “dust” or simply “matter”. The energy density of such particles is given by their number density times their rest mass; as the universe expands, the number density is inversely proportional to the volume while the rest masses are constant, yielding . For relativistic particles, known in cosmology as “radiation” (although any relativistic species counts, not only photons or even strictly massless particles), the energy density is the number density times the particle energy, and the latter is proportional to (redshifting as the universe expands); the radiation energy density therefore scales as . Vacuum energy does not change as the universe expands, so ; from (26) this implies a negative pressure, or positive tension, when the vacuum energy is positive. Finally, for some purposes it is useful to pretend that the term in (5) represents an effective “energy density in curvature”, and define . We can define a corresponding density parameter
The ranges of values of the ’s which are allowed in principle (as opposed to constrained by observation) will depend on a complete theory of the matter fields, but lacking that we may still invoke energy conditions to get a handle on what constitutes sensible values. The most appropriate condition is the dominant energy condition (DEC), which states that , and is non-spacelike, for any null vector ; this implies that energy does not flow faster than the speed of light . For a perfect-fluid energy-momentum tensor of the form (4), these two requirements imply that and , respectively. Thus, either the density is positive and greater in magnitude than the pressure, or the density is negative and equal in magnitude to a compensating positive pressure; in terms of the equation-of-state parameter , we have either positive and or negative and . That is, a negative energy density is allowed only if it is in the form of vacuum energy. (We have actually modified the conventional DEC somewhat, by using only null vectors rather than null or timelike vectors; the traditional condition would rule out a negative cosmological constant, which there is no physical reason to do.)
There are good reasons to believe that the energy density in radiation today is much less than that in matter. Photons, which are readily detectable, contribute , mostly in the cosmic microwave background [211, 87, 225]. If neutrinos are sufficiently low mass as to be relativistic today, conventional scenarios predict that they contribute approximately the same amount . In the absence of sources which are even more exotic, it is therefore useful to parameterize the universe today by the values of and , with , keeping the possibility of surprises always in mind.
One way to characterize a specific Friedmann–Robertson–Walker model is by the values of the Hubble parameter and the various energy densities . (Of course, reconstructing the history of such a universe also requires an understanding of the microphysical processes which can exchange energy between the different states.) It may be difficult, however, to directly measure the different contributions to , and it is therefore useful to consider extracting these quantities from the behavior of the scale factor as a function of time. A traditional measure of the evolution of the expansion rate is the deceleration parameter
Notice that positive-energy-density sources with cause the universe to decelerate while leads to acceleration; the more rapidly energy density redshifts away, the greater the tendency towards universal deceleration. An empty universe (, ) expands linearly with time; sometimes called the “Milne universe”, such a spacetime is really flat Minkowski space in an unusual time-slicing.
In the remainder of this section we will explore the behavior of universes dominated by matter and vacuum energy, . According to (33), a positive cosmological constant accelerates the universal expansion, while a negative cosmological constant and/or ordinary matter tend to decelerate it. The relative contributions of these components change with time; according to (28) we have
Given , the value of for which the universe will expand forever is given by
The dynamics of universes with are summarized in Figure 1, in which the arrows indicate the evolution of these parameters in an expanding universe. (In a contracting universe they would be reversed.) This is not a true phase-space plot, despite the superficial similarities. One important difference is that a universe passing through one point can pass through the same point again but moving backwards along its trajectory, by first going to infinity and then turning around (recollapse).
Figure 1 includes three fixed points, at equal to , , and . The attractor among these at is known as de Sitter space – a universe with no matter density, dominated by a cosmological constant, and with scale factor growing exponentially with time. The fact that this point is an attractor on the diagram is another way of understanding the cosmological constant problem. A universe with initial conditions located at a generic point on the diagram will, after several expansion times, flow to de Sitter space if it began above the recollapse line, and flow to infinity and back to recollapse if it began below that line. Since our universe has expanded by many orders of magnitude since early times, it must have begun at a non-generic point in order not to have evolved either to de Sitter space or to a Big Crunch. The only other two fixed points on the diagram are the saddle point at , corresponding to an empty universe, and the repulsive fixed point at , known as the Einstein–de Sitter solution. Since our universe is not empty, the favored solution from this combination of theoretical and empirical arguments is the Einstein–de Sitter universe. The inflationary scenario [113, 159, 6] provides a mechanism whereby the universe can be driven to the line (spatial flatness), so Einstein–de Sitter is a natural expectation if we imagine that some unknown mechanism sets . As discussed below, the observationally favored universe is located on this line but away from the fixed points, near . It is fair to conclude that naturalness arguments have a somewhat spotty track record at predicting cosmological parameters.
The lookback time from the present day to an object at redshift is given by[264, 149, 48], but generally the integral is straightforward to perform numerically.
In a generic curved spacetime, there is no preferred notion of the distance between two objects. Robertson–Walker spacetimes have preferred foliations, so it is possible to define sensible notions of the distance between comoving objects – those whose worldlines are normal to the preferred slices. Placing ourselves at in the coordinates defined by (2), the coordinate distance to another comoving object is independent of time. It can be converted to a physical distance at any specified time by multiplying by the scale factor , yielding a number which will of course change as the universe expands. However, intervals along spacelike slices are not accessible to observation, so it is typically more convenient to use distance measures which can be extracted from observable quantities. These include the luminosity distance,
The proper-motion distance between sources at redshift and can be computed by using along a light ray, where is given by (2). We have. Note that, for large redshifts, the dependence of the various distance measures on is not necessarily monotonic.
The comoving volume element in a Robertson–Walker universe is given by
The introduction of a cosmological constant changes the relationship between the matter density and expansion rate from what it would be in a matter-dominated universe, which in turn influences the growth of large-scale structure. The effect is similar to that of a nonzero spatial curvature, and complicated by hydrodynamic and nonlinear effects on small scales, but is potentially detectable through sufficiently careful observations.
The analysis of the evolution of structure is greatly abetted by the fact that perturbations start out very small (temperature anisotropies in the microwave background imply that the density perturbations were of order 10–5 at recombination), and linearized theory is effective. In this regime, the fate of the fluctuations is in the hands of two competing effects: the tendency of self-gravity to make overdense regions collapse, and the tendency of test particles in the background expansion to move apart. Essentially, the effect of vacuum energy is to contribute to expansion but not to the self-gravity of overdensities, thereby acting to suppress the growth of perturbations [149, 189].
For sub-Hubble-radius perturbations in a cold dark matter component, a Newtonian analysis suffices. (We may of course be interested in super-Hubble-radius modes, or the evolution of interacting or relativistic particles, but the simple Newtonian case serves to illustrate the relevant physical effect.) If the energy density in dynamical matter is dominated by CDM, the linearized Newtonian evolution equation is to yield , as well as analytic expressions for flat universes . Note that this analysis is consistent only in the linear regime; once perturbations on a given scale become of order unity, they break away from the Hubble flow and begin to evolve as isolated systems.
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