Solutions for the dependence of the scale factor with respect to time for universes dominated by each component can be found. At any time, the critical density ρThe evolution of all densities is shown on the left plot of Observational evidence of the Big Bang has grown steadily since Edwin Hubble's discovery in the late 1920s that the universe was expanding. “Secondary” species are those that are formed directly from chemical reactions among the primary species. It would certainly be ruled out if it implied the Yukawa-type deviations from the Newtonian potential.The non-zero mass of the graviton leads to interesting consequences for the primordial gravitational waves as well. This is given byAt high redshift, photodissociation of most molecules and molecular ions by the CMB occurs rapidly, and the photodissociation time is small. ScienceDirect ® is a registered trademark of Elsevier B.V.URL: https://www.sciencedirect.com/science/article/pii/B9780444515605500142URL: https://www.sciencedirect.com/science/article/pii/S0924809907800382URL: https://www.sciencedirect.com/science/article/pii/S0924809907800357URL: https://www.sciencedirect.com/science/article/pii/B9780123821881000098URL: https://www.sciencedirect.com/science/article/pii/B9780128001295000031Particle Physics and Cosmology: The Fabric of Spacetime]). This term originally was used as a means to determine the The first Friedmann equation is often seen in terms of the present values of the density parameters, that isThe Friedmann equations can be solved exactly in presence of a Note that this solution is not valid for domination of the cosmological constant, which corresponds to an If the matter is a mixture of two or more non-interacting fluids each with such an equation of state, then These equations are sometimes simplified by replacing In that case the exclusion of the graviton as the dark matter candidate does not necessarily rile out the model of massive gravity and massive gravitational waves may still be present at a lower level. The current energy density of the universe isStarting from this simple beginning, many other chemical species were produced as the Universe evolved. For example, consider molecular hydrogen, HWe use cookies to help provide and enhance our service and tailor content and ads. In this regime, both timescales are typically much shorter than the Hubble time, and hence the abundance of species X evolves until it reaches an equilibrium value set by the balance between formation and photodissociation (in which case (t(where γ indicates the photon emitted during this process), and its destruction by associative detachment with atomic hydrogenAt this point, the equilibrium fractional abundance of HMost of the other secondary species behave similarly to HTurning our attention to the tertiary species, we note that the four timescales discussed above are still important, but that an added complication is posed by the fact that the evolution with redshift of the tertiary species is determined in part by the evolution with redshift of the secondary species from which they form. Therefore, in general, massive gravitons may not comprise the whole of the dark matter in the Universe. The modified equation is − $$\left (\frac{H(z)}{H_0} \right)^2 = \Omega_{m,0}(1+z)^3 + \Omega_{rad,0}(1+z)^4+\Omega_{k,0}(1+z)^2+\Omega_{\wedge,0}$$ In each we also have assumed that Rees, M., Just Six Numbers, (2000) Orion Books, London, p. 81, p. 82 Here that gives the Hubble rate in terms of the total energy density ρ. The following three items describe the key bodies of evidence.Nearly every galaxy in the universe is moving away from every other galaxy and the recessional velocities display an almost linear dependence on the distance between galaxies; see The CMB represents the photons that were in thermal equilibrium with the high-temperature plasma that existed from the very first moments of the universe until it cooled down to approximately 3000 K about 380,000 years after the Big Bang.Measurements of the Hubble parameter and the COBE and WMAP measurements of the temperature and temperature fluctuations of the CMB allow a determination of the current total energy density of the universe and its composition. The first of these is the formation time, that is, the timescale on which the abundance of species X changes significantly owing to its formation in chemical reactions. Since that time a coherent standard model for the beginning of the universe has emerged. “Primary” species are those present at the end of recombination, as discussed above. Given that a RW geometry is a good description of the observable universe on a large scale, the further issue is what are the best-fit parameters that characterize it, selecting the specific universe we observe from the family of all FL models (Various parameters are used to characterize the nature of dark matter (We only obtain good estimates of these quantities by the observational relationships characterized above (It is useful here to distinguish between methods aimed at determining the properties of the background (zeroth order) FL model directly, and those aimed at determining properties of the perturbations of these models [Let us first discuss the experimental constraints on the graviton mass.