N-body Simulations of cosmologies with Light Massive Relics

TL;DR

This paper addresses the nonlinear impact of light massive relics (LiMRs) on cosmic structure by developing a first fully nonlinear three-fluid N-body framework that evolves CDM+baryons, SM neutrinos, and LiMRs. Building on the RePS formalism, the authors introduce Py-RePS to generate accurate initial conditions at $z=99$, incorporating LiMRs with either fermionic or bosonic distributions, and evolve them with Gadget-3 including LiMR short-range forces. Their results showLiMRs produce a suppression of the total matter power spectrum and halo statistics that strengthens with LiMR mass and, crucially, with LiMR temperature (since $ ho_l$ scales as $T_{l,0}^3$), highlighting a stronger sensitivity to thermal properties than mass. They also demonstrate the framework’s ability to handle bosonic LiMRs and validate the pipeline with consistency checks, establishing a versatile tool to forecast LiMR signatures and help break degeneracies with SM neutrinos in upcoming cosmological surveys.

Abstract

The presence of additional relativistic particles at the time of recombination can be inferred through their contribution to $ΔN_{\rm eff}$. If these species have a finite but low mass (Light Massive Relics - LiMRs), they act as a hot subcomponent of dark matter and impact late-time structure formation. Understanding these effects will be crucial to pin down the underlying particle physics properties of any future $ΔN_{\rm eff}$ detection. While their impact has been well-studied on linear scales, this work develops the framework for and presents results from the first set of cosmological N-body simulations that can track the effects of LiMRs, as a function of their mass and temperature, down to fully nonlinear scales. Importantly, our simulations model the impact of both the massive Standard Model neutrinos and LiMRs, which will be crucial in disentangling possible degeneracies. We systematically explore the effects of LiMR properties such as mass, temperature, and initial distribution, on various cosmological observables, including the total matter power spectrum, Halo Mass Functions (HMF), Mass-Concentration relation, radial halo profiles, and weak lensing signals around massive clusters. The framework and simulations developed here will enable detailed follow-up of the rich phenomenology of LiMR cosmologies.

Figures (11)

• Figure 1: This is a 2D projected density field generated by taking the mean along the z-axis for a slice of width 10 $h^{-1}\,\mathrm{Mpc}$ for 3 different simulations. The top leftmost panel shows the density field generated for a Only CDM(1-fluid) with total $\Omega_m = \Omega_c = 0.3175$. In the middle panel is the density field of a simulation with CDM and SM neutrinos(2-fluid) with total $\Omega_m = (\Omega_c + \Omega_{\nu}) = 0.3175$ for $\Sigma m_{\nu} = 0.60$ eV. Similarly, the top right-most panel corresponds to a simulation having CDM, SM neutrinos, and LiMRs(3-fluid) with total $\Omega_m = (\Omega_c + \Omega_{\nu} + \Omega_l) = 0.3175$ for $\Sigma m_{\nu} = 0.60$ eV and a single species of LiMR particle having mass 5 eV. The lower 3 plots are zoomed-in plots for the boxes marked in the corresponding density plots above them.
• Figure 2: 2D projected density fields obtained by averaging along the z-axis over a slice of width 10 $h^{-1}\,\mathrm{Mpc}$ for the three particle species in the 3-fluid simulation containing CDM, SM neutrinos, and LiMRs. The total matter density parameter is $\Omega_m = \Omega_c + \Omega_\nu + \Omega_l = 0.3175$, with $\Sigma m_\nu = 0.30$ eV and a single LiMR species of mass 1.5 eV. The top row shows the individual projected density fields of CDM (left), SM neutrinos (center), and LiMRs (right), while the bottom row presents zoomed-in views of the regions marked above. LiMRs are seen to cluster less efficiently than CDM but more strongly than SM neutrinos, which show minimal clustering due to their large thermal velocities.
• Figure 3: The top-left panel shows the total matter power spectrum for five different cases: the black(solid) curve corresponds to a 1-fluid model (CDM only), the cyan(long dashed) curve to a 2-fluid model (CDM + SM neutrinos), and the red(dashed), blue(dash-dotted), and green(dotted) curves to 3-fluid models (CDM + SM neutrinos + LiMRs) with LiMR masses of 0.7 eV, 1.1 eV, and 1.5 eV, respectively. The inset displays the LiMR power spectrum($P_l(k)$) for the three LiMR masses, with the black curve showing the CDM power spectrum in the 1-fluid case for comparison. The bottom-left panel compares the total matter power spectrum in the 3-fluid LiMR cosmologies with the total matter power spectrum in the 2-fluid case. On the right, the top panel fixes the LiMR mass at 1.1 eV and varies the LiMR temperature (0.91 K in red (dashed), 1.20 K in blue (dash-dotted), and 1.50 K in green (dotted)), while the bottom panel shows the residuals relative to the 2-fluid case. The corresponding inset highlights the LiMR power spectrum for the three different temperatures. Overall, these results demonstrate that the suppression in the power spectrum becomes stronger with increasing LiMR mass or temperature.
• Figure 4: Halo mass function for the 3-fluid model. The black solid line corresponds to a 1-fluid cosmology (CDM only, without SM neutrinos or LiMRs) and is compared to the 3-fluid case (CDM + SM neutrinos + LiMRs). In the left column, the red dashed, blue dash–dotted, and green dotted lines represent LiMR masses of 0.7 eV, 1.1 eV, and 1.5 eV, respectively (assuming Dirac fermions). In the right column, the LiMR mass is fixed at 1.1 eV, while the temperature is varied: 0.91 K (red dashed), 1.20 K (blue dash–dotted), and 1.50 K (green dotted). The shaded regions indicate the $1\sigma$ uncertainties. The bottom row shows the ratio of the 3-fluid halo mass function to that of the 1-fluid case.
• Figure 6: Comparison of 3D density profiles for the CDM-only (1-fluid) case and various 3-fluid models. In the left column, the red dashed, blue dash–dotted, and green dotted lines correspond to LiMR temperatures fixed at 0.91 K with LiMR masses of 0.7 eV, 1.1 eV, and 1.5 eV, respectively. In the right column, the LiMR mass is fixed at 1.1 eV, while the temperatures are varied: 0.91 K (red dashed), 1.20 K (blue dash–dotted), and 1.50 K (green dotted). The total 3D profiles are shown in the 3-fluid case. The bottom panels present the relative comparison with the CDM-only (1-fluid) case.