We show that the modulated reheating mechanism can naturally account for dark radiation, whose existence is hinted by recent observations of the cosmic microwave background radiation and the primordial Helium abundance. In this mechanism, the inflaton decay rate depends on a light modulus which acquires almost scale-invariant quantum fluctuations during inflation. We find that the light modulus is generically produced by the inflaton decay and therefore a prime candidate for the dark radiation. Interestingly, an almost scale-invariant power spectrum predicted in the modulated reheating mechanism gives a better fit to the observation in the presence of the extra radiation. We discuss the production mechanism of the light modulus in detail taking account of its associated isocurvature fluctuations. We also consider a case where the modulus becomes the dominant component of dark matter.
We study perturbative reheating at the end of fibre inflation where the inflaton is a closed string modulus with a Starobinsky-like potential. We first derive the spectral index $n_s$ and the tensor-to-scalar ratio $r$ as a function of the number of efoldings and the parameter $R$ which controls slow-roll breaking corrections. We then compute the inflaton couplings and decay rates into ultra-light bulk axions and visible sector fields on D7-branes wrapping the inflaton divisor. This leads to a reheating temperature of order $10^{10}$ GeV which requires $52$ efoldings. Ultra-light axions contri…
After inflation the Universe presumably undergoes a phase of reheating which in effect starts the thermal big bang cosmology. However, so far we have very little direct experimental or observational evidence of this important phase of the Universe. In this letter, we argue that measuring the spectrum of freely propagating relativistic particles, i.e. dark radiation, produced during reheating may provide us with powerful information on the reheating phase. To demonstrate this possibility we consider a situation where the dark radiation is produced in the decays of heavy, non-relativistic partic…
We derive an anthropic bound on the extra neutrino species, $ΔN_{\rm eff}$, based on the observation that a positive $ΔN_{\rm eff}$ suppresses the growth of matter fluctuations due to the prolonged radiation dominated era, which reduces the fraction of matter that collapses into galaxies, hence, the number of observers. We vary $ΔN_{\rm eff}$ and the positive cosmological constant while fixing the other cosmological parameters. We then show that the probability of finding ourselves in a universe satisfying the current bound is of order a few percents for a flat prior distribution. If $ΔN_{\rm …
Reheating is a process where the energy density of a dominant component of the universe other than radiation, such as a matter component, is transferred into radiation. It is usually assumed that the temperature of the universe decreases due to cosmic expansion even during the reheating process, in which case the maximal temperature of the universe is much higher than the reheat temperature. We point out that the temperature of the universe during reheating may in fact increase in well-motivated scenarios. We derive the necessary conditions for the temperature to increase during reheating and …
We discuss the gravitational creation of superheavy particles $χ$ in an inflationary scenario with a quartic potential and a non-minimal coupling between the inflaton $φ$ and the Ricci curvature: $ξφ^2 R/2$. We show that for large constants $ξ>> 1$, there can be abundant production of particles $χ$ with masses largely exceeding the inflationary Hubble rate $H_{infl}$, up to $(a~few) \times ξH_{infl}$, even if they are conformally coupled to gravity. We discuss two scenarios involving these gravitationally produced particles $χ$. In the first scenario, the inflaton has only gravitational intera…
We consider the direct $s$-channel gravitational production of dark matter during the reheating process. Independent of the identity of the dark matter candidate or its non-gravitational interactions, the gravitational process is always present and provides a minimal production mechanism. During reheating, a thermal bath is quickly generated with a maximum temperature $T_{\rm max}$, and the temperature decreases as the inflaton continues to decay until the energy densities of radiation and inflaton oscillations are equal, at $T_{\rm RH}$. During these oscillations, $s$-channel gravitational pr…
In order to describe inflation in general relativity, scalar fields must inevitably be used, with all the setbacks of that description. On the other hand, $f(R)$ gravity and other modified gravity theories seem to provide a unified description of early and late-time dynamics without resorting to scalar or phantom theories. The question is, can modified gravity affect directly the mysterious radiation domination era? Addressing this question is the focus in this work, and we shall consider the case for which in the early stages of the radiation domination era, namely during the reheating era, t…
We demonstrate that light spectator fields can source sizeable CMB anisotropies through modulated reheating even in the absence of direct couplings to the inflaton. The effect arises when the phase space of the inflaton decay is modulated by the spectator which generates masses for the decay products. We call the mechanism \textit{indirect modulation} and show that it can source perturbations even four orders of magnitude larger than the observed. Importantly, the indirect mechanism is present in the Standard Model extended with right-handed neutrinos. For a minimally coupled Higgs boson this …
We perform a systematic analysis of dark matter production during post-inflationary reheating. Following the period of exponential expansion, the inflaton begins a period of damped oscillations as it decays. These oscillations and the evolution of temperature of the thermalized decay products depend on the shape of the inflaton potential $V(Φ)$. We consider potentials of the form $Φ^k$. Standard matter-dominated oscillations occur for $k=2$. In general, the production of dark matter may depend on either (or both) the maximum temperature after inflation, or the reheating temperature, where the …
We investigate how sterile neutrinos with a range of masses influence cosmology in MeV-scale reheating temperature scenarios. By computing the production of sterile neutrinos through the combination of mixing and scattering in the early Universe, we find that light sterile neutrinos, with masses and mixings as inferred from short-baseline neutrino oscillation experiments, are consistent with big-bang nucleosynthesis (BBN) and cosmic microwave background (CMB) radiation for the reheating temperature of ${\cal O}(1)$ MeV if the parent particle responsible for reheating decays into electromagneti…
Weakly Interacting Massive Particles (WIMPs) are among the best-motivated dark matter candidates. In the standard scenario where the freeze-out occurs well after the end of inflationary reheating, they are in tension with the severe experimental constraints. Here, we investigate the thermal freeze-out of WIMPs occurring {\it during} reheating, while the inflaton $ϕ$ coherently oscillates in a generic potential $\propto ϕ^n$. Depending on the value of $n$ and the spin of the inflaton decaying products, the evolution of the radiation and inflaton energy densities can show distinct features, ther…
Gravitational ultra-relativistic freeze-out during general reheating
Université Paris-Saclay（巴黎萨克雷大学）；CNRS/IN2P3, IJCLab（法国国家科学研究中心/欧洲核子研究委员会粒子物理研究所，IJCLab）；William I. Fine Theoretical Physics Institute, School of Physics and Astronomy, University of Minnesota（明尼苏达大学物理与天文学院威廉·I·芬理论物理研究所）；Theoretical Physics Division, Institute of High Energy Physics, Chinese Academy of Sciences（中国科学院高能物理研究所理论物理部）；China Center of Advanced Science and Technology（中国高等科学技术中心）
The purely gravitational dark matter (PGDM) which interacts with the standard model particles only by gravitational interaction has recently been discussed. Due to its feeble interaction, PGDM may be produced mainly by the gravitational particle creation, which plays an important role in the reheating after kinetically driven inflation and some potential-driven inflation without subsequent inflaton oscillating phase. Therefore, we consider the possibility of the gravitational reheating model which can also explain the present PGDM density at the same time. We consider a model where two massive…
