The present radiation density parameter is then:
Ω(r,0) = Ω(CMB,0)+ Ω(ν,0)≈9×10−5. (2.23) The other cosmological parameters may be obtained from measurements of the temperature anisotropies in the CMB, which were made by the Planck Telescope in 2018 [4].
Figure 2.6: Temperature anisotropies of the CMB [1]
From these measurements it was determined that the universe has an almost perfectly flat ge- ometry such that Ω(k,0) = 0.001±0.002. The density of matter has also been determined to be Ω(m,0) = 0.315±0.007, of which about 85% consists of cold dark matter Ω(dm,0) = 0.266 and only about 15% is baryonic matter Ω(bm,0) = 0.049 [4]. Using these parameters, the dark energy density Ω(Λ,0) may be determined using equation (1.33):
1 = Ω(r,0)+ Ω(m,0)+ Ω(Λ,0)+ Ω(k,0)
→Ω(Λ,0) = 1−Ω(r,0)−Ω(m,0)−Ω(k,0) = 1−9×10−5−0.315−0.001≈0.685.
(2.24)
Taking the sum of the dark matter and dark energy parameters Ω(dm,0)+ Ω(Λ,0) = 0.685 + 0.266 = 0.951 we see that 95% of the energy budget of the universe is distributed between the dark sectors.
Finally, the Planck telescope also measured the Hubble constant to be H0 = 67.4 km s−1Mpcs−1 [4]. The following cosmological parameters will thus be used throughout, unless stated otherwise:
Ω(r,0) = 9×10−5 ; Ω(m,0)= 0.315 ; Ω(Λ,0) = 0.685 ; H0= 67.4 km s−1Mpcs−1. (2.25) It should be noted that there are many other probes to measure these cosmological parameters.
For a review of the different measurements of cosmological parameters at the end of 2019, see [73].
2.5.3 Radiation, matter and dark energy dominated epochs
The ΛCDM model is a multi-fluid model, with each of the fluids evolving independently of the others. Since the energy densities of radiation, matter and the cosmological constant dilute at different rates as the universe expands (∝ a−4, ∝ a−3 and ∝ a0 respectively ), each species will dominate at different times in the history of the universe. This can be seen by plotting the energy densitiesρand density parameters Ω for radiation (2.3), matter (2.6) and dark energy (2.9) against redshift (1 +z), using the cosmological parameters (2.25) as the present initial conditions.
Figure 2.7: Evolution of energy densitiesρ with redshift (1 +z) (ΛCDM)
Figure 2.8: Evolution of density parameters Ωwith redshift (1 +z) (ΛCDM)
From Figures 2.7 and 2.8 we can see that there was an early period (a large redshift z equates to small a, and thus the distant past) of radiation domination, which was followed by periods of matter and dark energy domination respectively. The exact redshift where the radiation-matter equalityzr=m and the matter-dark energy equalityzm=Λ happens is calculated in AppendixD.1as:
zr=m = Ω(m,0)
Ω(r,0) −1 = 0.315
9×10−5 −1 = 3499 (whenρr=ρm≈10.93 Joule m−3), zm=Λ=
Ω(Λ,0) Ω(m,0)
1/3
−1 =
0.685 0.315
1/3
−1 = 0.3 (whenρm=ρΛ≈5.54×10−10 Joule m−3).
(2.26) The radiation, matter and dark energy dominated epochs will each leave their mark on the expansion history of this model. This can be seen by considering the three fluids in the model as a single fluid with a dynamical effective equation of state ωeff. This effective equation of state is given by the ratio of the sum of each fluid’s pressure Ptot over the total energy densities ρtot:
ωeff = Ptot
ρtot = Pr+Pm+PΛ
ρr+ρm+ρΛ = ωrρr+ωmρm+ωΛρΛ
ρr+ρm+ρΛ =
1
3ρr−ρΛ
ρr+ρm+ρΛ =
1
3Ωr−ΩΛ
Ωr+ Ωm+ ΩΛ. (2.27)
Substituting the density parameters for radiation (2.3), matter (2.6) and dark energy (2.9) gives:
ωeff =
1
3Ω(r,0)(1 +z)4−Ω(Λ,0)
Ω(r,0)(1 +z)4+ Ω(m,0)(1 +z)3+ Ω(Λ,0). (2.28) This effective equation of state ωeff may be used alongside the deceleration parameter (2.18) with parameters (2.25) to see the different phases of the expansion history.
Figure 2.9: Evolution of the effective equa- tion of stateωeff with redshift (1 +z) (ΛCDM)
Figure 2.10: Evolution of deceleration pa- rameter qwith redshift (1 +z) (ΛCDM)
From Figure 2.9 it can be seen that the ΛCDM model mimics the behaviour of radiation fluid in the distant past, since the effective equation of state is equal to that of radiation ωeff =ωr= 1/3.
This behaviour coincides with the deceleration parameter q ≈ 1 in the distant past in Figure 2.10, mimicking that of a flat radiation-dominated universe (which is indicated by the blue dashed line). Similarly, as the universe expands it later mimics matter fluid ωeff = ωm = 0 and finally a cosmological constant ωeff = ωΛ = −1. These epochs, in turn, have a deceleration parameter q = 0.5 andq=−1, which mimic a flat matter (Einstein-de Sitter indicated by the dashed red line) and flat cosmological constant (de Sitter indicated by the dashed green line) universe, respectively.
It can thus be seen that the expansion of the ΛCDM model decelerates during radiation and matter domination before accelerating when dark energy becomes dominant enough. This transition from deceleration to acceleration (called a cosmic jerk) occurs whenq= 0, which happens if Ωdm≈2ΩΛ D.8 or the effective equation of state is ωeff = 1/3. The exact transition redshift zt where this cosmic jerk occurs is calculated in AppendixD.1, giving:
zt=
2Ω(Λ,0) Ω(m,0)
1/3
−1 =
2(0.685) 0.315
1/3
−1 = 0.63. (2.29)
Direct evidence from Type Ia supernovae for this cosmic jerk was discovered by one of the teams who first discovered the cosmic acceleration [74,75].
2.5.4 Origin, evolution and fate of ΛCDM model
As seen in the previous section, the ΛCDM model should initially have a period of decelerating expansion followed by accelerated expansion. This should be reflected in the way the scale factor evolves with time. The Friedmann equation for the ΛCDM model (2.19) cannot be solved analyti- cally in any simple manner and therefore numerical methods will be resorted to instead. The 4th order Runge-Kutta method was adopted [76] which is generally recommended for numerical inte- gration of first-order differential equations (This method is described in AppendixA.4). To ensure the numerical integration works for the ΛCDM model, we will also use the numerical method to reproduce some of the previous analytical solutions from Figures2.3,2.4 and2.5.
Figure 2.11: Origin, evolution and fate ofΛCDM model
From Figure2.11the expansion history of the ΛCDM model (solid purple line) can be seen, along- side that of a de-Sitter universe (dashed purple line), Einstein-de Sitter universe (dashed red line), closed matter-dominated universe (solid red line) and a flat radiation-dominated universe (dashed blue line). The age of each model is calculated by numerical integration of the Friedmann equation (2.19), which may be rewritten as [9]:
Z t
0
dt= Z a
0
1
H(a)da= Z a
0
1 H0q
(Ω(r,0)a−2+ Ω(m,0)a−1+ Ω(Λ,0)a2+ Ω(k,0))
da. (2.30)
The resulting plots and ages of each of the single-fluid models can be seen to coincide with the analytical solutions obtained in Figures 2.3, 2.4 and 2.5. Therefore, we may conclude that the numerical method is an accurate approximation of the true behaviour of the ΛCDM model.
It may therefore be seen that the ΛCDM model started from a big bang approximately 13.80 billion years ago [4]. The universe then started with an initial period of decelerating expansion (where the plot is convex), followed by a recent period of accelerating expansion (where the plot is concave), which should continue indefinitely into the future. This behaviour coincides with Figures 2.7, 2.8, 2.9 and 2.10, where there is an initial period of deceleration q > 0 during radiation and matter domination, followed by an infinite period of accelerating expansion q <0 during the final era of dark energy domination. It can thus be seen that the ΛCDM model behaves qualitatively similar to an Einstein-de Sitter universe in the distant past while behaving like a de-Sitter universe at later times. It may also be seen that the Hubble parameter converges to a constant value as a→ ∞ which corresponds to (1 +z)→0, thus from equation (2.14) we have:
H=H0
q
Ω(r,0)(0)4+ Ω(d,0)(0)3+ Ω(Λ,0)+ Ω(k,0)(0)2 =H0
q
Ω(Λ,0) ≈56 km/s/Mpc. (2.31) Since we now know how the scale factor evolves with time, and since the scale factor is related to redshift by the relationa= 1/(1 +z), we can determine at which redshiftz and timet important cosmic events in the ΛCDM model happened. Using the equations which relate energy density to redshift (2.3,2.6and 2.9), we may also find the energy density of radiationρr, matterρmand dark energy ρΛ at these events. These events include the big bang, the radiation-matter and matter- dark energy equalities (2.26) as well as acceleration at the cosmic jerk (2.29). These results are summarised in Table 2.6below.
Table 2.6: Important events in the ΛCDM model (using CMB data [4])
Event Redshiftz Time (Gyr) ρr ρm ρΛ (J/m3)
Big bang singularity ∞ 13.80 ∞ ∞ ∞
Radiation-matter equality 3499 13.80 10.9 10.9 5.5e-10 Cosmic jerk (q = 0) 0.63 6.12 5.2e-13 1.2e-9 5.5e-10 Matter-dark energy equality 0.30 3.50 2.1e-13 1.1e-9 5.5e-10
with Λ≈1.09×10−53 m−2 from (2.9). From Table2.6it is seen that the radiation-matter equality occurred soon after the big bang, leading to a period of matter domination that lasted approxi- mately 10 billion years before dark energy became dominant. The universe then started accelerating at approximately half its present age, when ρm≈2ρΛ. This accelerated expansion then continues indefinitely until the universe cools with a slow heat death over billions of years.
These conclusions and observations all rely on the ΛCDM model being an accurate description of the universe we live in. To know if this is the case, we should also consider it’s shortcomings.