Cosmic Density Perturbations from Late-Decaying Scalar Condensations

TL;DR

This work explores cosmic density perturbations generated by a late-decaying scalar field $\phi$ that dominates the early universe before reheating. By tracking perturbation evolution in a multi-fluid, multi-source setting, the authors show that primordial fluctuations of $\phi$ can dominate the observed perturbations and produce correlated adiabatic and isocurvature modes, depending on whether baryons or CDM originate from $\phi$ or from a separate field $\psi$. The analysis demonstrates that the simple curvaton scenario is strongly constrained by current CMB data when entropy perturbations are correlated, and identifies a viable modified-curvaton regime with large correlated and uncorrelated isocurvature components that can still fit observations. These results have implications for inflationary model constraints and for interpreting future CMB measurements as probes of late-time entropy production and multi-source perturbations.

Abstract

We study the cosmic density perturbations induced from fluctuation of the amplitude of late-decaying scalar condensations (called φ) in the scenario where the scalar field φonce dominates the universe. In such a scenario, the cosmic microwave background (CMB) radiation originates to decay products of the scalar condensation and hence its anisotropy is affected by the fluctuation of φ. It is shown that the present cosmic density perturbations can be dominantly induced from the primordial fluctuation of φ, not from the fluctuation of the inflaton field. This scenario may change constraints on the source of the density perturbations, like inflation. In addition, a correlated mixture of adiabatic and isocurvature perturbations may arise in such a scenario; possible signals in the CMB power spectrum are discussed. We also show that the simplest scenario of generating the cosmic density perturbations only from the primordial fluctuation of φ(i.e., so-called ``curvaton'' scenario) is severely constrained by the current measurements of the CMB angular power spectrum if correlated mixture of the adiabatic and isocurvature perturbations are generated.

Figures (14)

• Figure 1: Contours of minimum values of the initial amplitude of the $\phi$ field with which the universe is once dominated by the energy density of $\phi$. The vertical axis is the mass of $\phi$ and the horizontal axis is $T_{\rm R2}$. $T_{\rm R1}$ is taken to be (A) $T_{\rm R1}=1 \times 10^{6}$GeV, (B) $T_{\rm R1}=1 \times 10^{10}$GeV and (C) $T_{\rm R1}=1 \times 10^{14}$GeV.
• Figure 2: Schematic picture of the thermal history of the universe; evolution of the energy density of various components. Here, we assumed that the $\phi$ field starts to oscillate in the RD1 epoch, but it may happen in the $\chi$D epoch. The solid lines are for scalar condensations and the dotted ones are for radiation.
• Figure 3: The angular power spectrum with correlated mixture of the adiabatic and isocurvature perturbations in the baryonic sector (solid line), in the CDM sector (long-dashed line), and in the baryonic and CDM sectors (dot-dashed line). (See Eqs. (\ref{['Sb']}), (\ref{['Sc']}), and (\ref{['Sbc']}), respectively.) We also show the CMB angular power spectrum in the purely adiabatic (short-dashed line) and isocurvature density perturbations (dotted line), i.e., $C_l^{(\delta\chi)}$ and $C_l^{(\delta\psi)}$. We consider the flat universe with $\Omega_bh^2=0.019$, $\Omega_m=0.3$, and $h=0.65$, and the initial power spectral indices for primordial density perturbations are all assumed to be 1 (i.e., we adopt scale-invariant initial power spectra). The overall normalizations are taken as $[l(l+1)C_l/2\pi]_{l=10}=1$.
• Figure 4: $\chi^2$ as a function of $\kappa_m$. We take $\Omega_bh^2 = 0.019$, $h=0.65$, and the values of $\Omega_m$ are shown in the figure. The flat universe is assumed. Notice that the $\kappa_m$ parameter is smaller than $\frac{9}{2}$ in our scenario.
• Figure 5: The CMB angular power spectrum with the mixed baryonic isocurvature and adiabatic density perturbations with $\alpha_b=0$ (solid line), 3 (long-dashed line) and 5 (short-dashed line). The cosmological parameters are the same as those in Fig. \ref{["fig:Cl's"]}. The overall normalizations are arbitrary.