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Isocurvature, non-gaussianity and the curvaton model

Maria Beltrán

TL;DR

This paper investigates whether a large local non-Gaussianity signal in the CMB can be reconciled with isocurvature bounds within the curvaton framework. By linking the non-Gaussianity parameter $f_{\\rm NL}$ to the curvaton energy-density ratio at decay $r$, and comparing with stringent isocurvature constraints, the authors show that a sizable $f_{\\rm NL}$ generally implies an accompanying isocurvature signal unless CDM is created after curvaton decay. They derive a set of relationships among the inflationary scale $H_*$, the curvaton parameters, and the CDM creation temperature $T_{\\rm cdm}$, including a robust bound $T_{\\rm cdm} \lesssim 1.6\times10^4 M_{Pl}^{-3/2} H_*^{5/2} (100/f_{\\rm NL})$ for thermal relics and a stricter limit for quadratic inflationary potentials. The work therefore connects early-universe dynamics to dark matter phenomenology, offering clear observationally testable constraints on curvaton scenarios capable of producing large local non-Gaussianity.

Abstract

Recent analyses of the statistical distribution of the temperature anisotropies in the CMB do not rule out the possibility that there is a large non-gaussian contribution to the primordial power spectrum. This fact motivates the re-analysis of the curvaton scenario, paying special attention to the compatibility of large non-gaussianity of the local type with the current detection limits on the isocurvature amplitude in the CMB. We find that if the curvaton mechanism generates a primordial power spectrum with an important non-gaussian component, any residual isocurvature imprint originated by the curvaton, would have an amplitude too big to be compatible with the current bounds. This implies that the isocurvature mode should be equal to zero in this scenario and we explore the consequences of this inference. In order to prevent the generation of a such a signal, the CDM must be created at a late stage, after the curvaton decays completely. This requirement is used to constrain the nature of the CDM in this scenario, arriving at a general relation between the temperature of the universe at CDM creation and the scale of inflation.

Isocurvature, non-gaussianity and the curvaton model

TL;DR

This paper investigates whether a large local non-Gaussianity signal in the CMB can be reconciled with isocurvature bounds within the curvaton framework. By linking the non-Gaussianity parameter to the curvaton energy-density ratio at decay , and comparing with stringent isocurvature constraints, the authors show that a sizable generally implies an accompanying isocurvature signal unless CDM is created after curvaton decay. They derive a set of relationships among the inflationary scale , the curvaton parameters, and the CDM creation temperature , including a robust bound for thermal relics and a stricter limit for quadratic inflationary potentials. The work therefore connects early-universe dynamics to dark matter phenomenology, offering clear observationally testable constraints on curvaton scenarios capable of producing large local non-Gaussianity.

Abstract

Recent analyses of the statistical distribution of the temperature anisotropies in the CMB do not rule out the possibility that there is a large non-gaussian contribution to the primordial power spectrum. This fact motivates the re-analysis of the curvaton scenario, paying special attention to the compatibility of large non-gaussianity of the local type with the current detection limits on the isocurvature amplitude in the CMB. We find that if the curvaton mechanism generates a primordial power spectrum with an important non-gaussian component, any residual isocurvature imprint originated by the curvaton, would have an amplitude too big to be compatible with the current bounds. This implies that the isocurvature mode should be equal to zero in this scenario and we explore the consequences of this inference. In order to prevent the generation of a such a signal, the CDM must be created at a late stage, after the curvaton decays completely. This requirement is used to constrain the nature of the CDM in this scenario, arriving at a general relation between the temperature of the universe at CDM creation and the scale of inflation.

Paper Structure

This paper contains 16 sections, 54 equations, 2 figures.

Table of Contents

  1. Introduction
  2. The curvaton model
  3. Details of the model
  4. Non-gaussianity in the curvaton scenario
  5. Isocurvature in the curvaton scenario
  6. The CDM is created after the curvaton decays completely
  7. The CDM is created before the curvaton decays and while $f\ll 1$
  8. The CDM is created while the curvaton decays
  9. Data Analysis
  10. Non-gaussianity
  11. Isocurvature
  12. Implications for CDM
  13. The value of $\sigma_*$
  14. Constraints coming from the power spectrum
  15. Mass of the CDM candidate particle
  16. ...and 1 more sections

Figures (2)

  • Figure 1: Upper bound on the mass of a CDM candidate as a function of the inflationary scale. We used the two bounding values for the mass of the curvaton: $m^{max}=0.1H_*$ (dashed), and $m^{min}=7\cdot 10^{-3}H_*$ (solid). The dotted line corresponds to $m_{\rm cdm}=2.4\times 10^5$GeV, the value above which the bounds apply to the temperature of DM creation. The vertical dot-dashed line shows the tighter constraint on the scale of inflation coming from assuming a quadratic potential for the inflaton. The value of $f_{\rm NL}$ has been fixed to $100$.
  • Figure 2: Upper bound on the mass of a CDM candidate as a function of non-gaussian fraction. We used three different fixed values for the scale of inflation: $10^9$ GeV (solid), $10^{10}$ GeV (dashed), $10^{12}$ GeV (dotted). The value of $b$ used is $b=0.1$