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Reconciling Planck results with low redshift astronomical measurements

Z. Berezhiani, A. D. Dolgov, I. I. Tkachev

TL;DR

The paper addresses the tension between Planck CMB-derived cosmological parameters and low-redshift measurements such as $H_0$ and $\sigma_8$. It proposes a two-component dark matter model with a dominant stable component and a subdominant unstable fraction decaying after recombination, parameterized by the initial decaying fraction $F$ and decay width $\Gamma$, while preserving Planck's recombination constraints ($\omega_{sdm}+\omega_{ddm}=0.1198$) and fixing the sound-horizon angle via $100\,\theta_s$. Using the CLASS Boltzmann solver and Monte Carlo sampling, the authors show that nonzero $F$ with finite $\Gamma$ can reproduce Planck high-$l$ spectra while accommodating SN, HST, BAO, and cluster data, yielding $h\approx0.716\pm0.02$. The results suggest an unstable DM fraction of order $F\sim0.1$ at recombination and indicate that BAO results remain dataset-dependent, but the same DDM parameters can alleviate both the $H_0$ and $\sigma_8$ tensions, within current systematic uncertainties.

Abstract

We show that emerging tension between the direct astronomical measurements at low redshifts and cosmological parameters deduced from the Planck measurements of the CMB anisotropies can be alleviated if the dark matter consists of two fractions, stable part being dominant and a smaller unstable fraction. The latter constitutes $\sim 10$ per cent at the recombination epoch if decays by now.

Reconciling Planck results with low redshift astronomical measurements

TL;DR

The paper addresses the tension between Planck CMB-derived cosmological parameters and low-redshift measurements such as and . It proposes a two-component dark matter model with a dominant stable component and a subdominant unstable fraction decaying after recombination, parameterized by the initial decaying fraction and decay width , while preserving Planck's recombination constraints () and fixing the sound-horizon angle via . Using the CLASS Boltzmann solver and Monte Carlo sampling, the authors show that nonzero with finite can reproduce Planck high- spectra while accommodating SN, HST, BAO, and cluster data, yielding . The results suggest an unstable DM fraction of order at recombination and indicate that BAO results remain dataset-dependent, but the same DDM parameters can alleviate both the and tensions, within current systematic uncertainties.

Abstract

We show that emerging tension between the direct astronomical measurements at low redshifts and cosmological parameters deduced from the Planck measurements of the CMB anisotropies can be alleviated if the dark matter consists of two fractions, stable part being dominant and a smaller unstable fraction. The latter constitutes per cent at the recombination epoch if decays by now.

Paper Structure

This paper contains 7 sections, 1 equation, 4 figures.

Table of Contents

  1. Introduction
  2. Decaying Dark Matter
  3. Planck constraints.
  4. Adding supernova and HST constraints.
  5. DDM and BAO.
  6. DDM and cluster counts.
  7. Conclusions

Figures (4)

  • Figure 1: Hubble parameter $h$ as a function of DM decay width $\Gamma$ for different values of the DDM fraction $F$.
  • Figure 2: One and two sigma likelihood contours for our model parameters. Solid and dashed lines correspond to a dataset consisting of JLA sample of SN Ia and HST measurements of $h$, on top of the best fit Planck model parameters. Addition of Planck cluster data results in much narrower shaded area.
  • Figure 3: Hubble parameter $h(z)$ (left panel) and angular diameter distance $D_A$ (right panel). Model curves are presented for fixed $\Gamma = 2000$ and several values of $F$. Points at non-zero redshift $z$ are the SDSS BAO data. HST measurement at $z=0$ is also shown with the symbol size comparable to the errorbars.
  • Figure 4: $\Omega_m$ and $\sigma_8$ derived from cluster counts and from CMB. Line marked DDM shows trend of these parameters when $F$ and $\Gamma$ are varied in our model. White circle represents a model with $F=0.1$ and $\Gamma = 2000$ as an example.