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One of the important parameters in cosmology is the parameter characterizing the equation of state (EoS) of the sources driving the cosmic expansion. Epochs that are dominated by radiation, matter, or scalar fields, whether they are probed either directly or indirectly, can be characterised by a unique value of this parameter. However, the EoS parameter during reheating---a phase succeeding inflation which is supposed to rapidly defrost our universe---remains to be understood satisfactorily. In order to circumvent the complexity of defining an instantaneous EoS parameter during reheating, an effective parameter $w_\mathrm{eff}$, which is an average of the EoS parameter over the duration of reheating, is usually considered. The value of $w_\mathrm{eff}$ is often chosen arbitrarily to lie in the range $-1/3 \leq w_\mathrm{eff} \leq 1$. In this work, we consider the time evolution of the EoS parameter during reheating and relate it to inflationary potentials $V(\phi)$ that behave as $\phi^p$ around the minimum, a proposal which can be applied to a wide class of inflationary models. We find that, given the index $p$, the effective EoS parameter $w_{\rm eff}$ is determined uniquely. We discuss the corresponding effects on the reheating temperature and its implications., Comment: v1: 10 pages, 5 figures; v2: 13 pages, 7 figures, discussions and references added, version to apprear in Phys. Rev. D

We propose a two-phase reheating scenario where the initial preheating dynamics is described by an effective dynamics followed by the standard perturbative reheating. Some of the important universal results of lattice simulation during preheating have been considered as crucial inputs in our two-phase dynamics. In this framework, detailed phenomenological constraints have been obtained on the inflaton couplings with reheating fields, and dark matter parameters in terms of CMB constrained inflationary scalar spectral index. It is observed that the conventional reheating scenario generically predicts the maximum reheating temperature $T_{re}^{max} \simeq 10^{15}$ GeV, corresponding to an almost instantaneous transition from the end of inflation to radiation domination. This fact will naturally lead to the problem of non-perturbative inflaton decay, which is in direct conflict with the perturbative reheating itself. Taking into account this by incorporating effective non-perturbative dynamics as the initial phase, our model of two-phase reheating scenarios also predicts model-independent maximum reheating temperature, which does not correspond to the instantaneous process. Further, $T_{re}^{max}$ is predicted to lie within $(10^{13}, 10^{10})$ GeV if CMB constraints on inflaton couplings with different reheating field are taken into account. We have further studied in detail the dark matter phenomenology in a model-independent manner and show how dark matter parameter space can be constrained through CMB parameters via the inflaton spectral index. Considering dark matter production during reheating via the Freeze-in mechanism, its parameter space has been observed to be highly constrained by our two-phase reheating than the constraints predicted by the conventional reheating scenarios, which are believed to theoretically incomplete., 63 pages, 29 figures, An expanded version of this paper arxiv number: 1709.00251

Understanding the properties of dark matter has proved to be one of the most challenging problems of particle phenomenology. In this paper, we have tried to understand the phenomenology of dark matter in light of very well understood properties of cosmic microwave background (CMB) anisotropy. To connect these two, inflation and its subsequent evolution known as the reheating phase play the important role. Following the previous analysis, we first established a one-to-one correspondence between the CMB power spectrum and the reheating temperature assuming the perturbative reheating scenario. Further by incorporating a possible dark matter candidate through the radiation annihilation process during reheating and the current value of dark matter abundance, we constrain the dark matter parameter space through the inflationary power spectrum for different inflationary models., Added discussions, matches published version