Hot Dark Matter Research Articles

Nonthermal neutrino-like hot dark matter in light of the S8 tension

The $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ prediction of ${S}_{8}\ensuremath{\equiv}{\ensuremath{\sigma}}_{8}({\mathrm{\ensuremath{\Omega}}}_{m}/0.3{)}^{0.5}$---where ${\ensuremath{\sigma}}_{8}$ is the root mean square of matter fluctuations on an $8\text{ }\text{ }{h}^{\ensuremath{-}1}\text{ }\mathrm{Mpc}$ scale---once calibrated on Planck cosmic microwave background data is $2\ensuremath{-}3\ensuremath{\sigma}$ lower than its direct estimate by a number of weak lensing surveys. In this paper, we explore the possibility that the ``${S}_{8}$ tension'' is due to a fractional contribution of nonthermal hot dark matter (HDM) to the energy density of the Universe leading to a power suppression at small scales in the matter power spectrum. Any HDM model can be characterized by its effective mass ${m}_{\mathrm{sp}}^{\mathrm{eff}}$ and its contribution to the relativistic degrees of freedom at cosmic microwave background decoupling $\mathrm{\ensuremath{\Delta}}{N}_{\mathrm{eff}}$. Taking the specific example of a sterile particle produced from the decay of the inflaton during an early matter-dominated era, we find that the tension can be reduced below $2\ensuremath{\sigma}$ from Planck data only, but it does not favor a nonzero ${{m}_{\mathrm{sp}}^{\mathrm{eff}},\mathrm{\ensuremath{\Delta}}{N}_{\mathrm{eff}}}$. In combination with a measurement of ${S}_{8}$ from $\mathrm{KiDS}1000+\mathrm{BOSS}+2\mathrm{dfLenS}$, the ${S}_{8}$ tension would hint at the existence of a particle of mass ${m}_{\mathrm{sp}}^{\mathrm{eff}}\ensuremath{\simeq}0.6{7}_{\ensuremath{-}0.48}^{+0.26}\text{ }\text{ }\mathrm{eV}$ with a contribution to $\mathrm{\ensuremath{\Delta}}{N}_{\mathrm{eff}}\ensuremath{\simeq}0.06\ifmmode\pm\else\textpm\fi{}0.05$. However, Pantheon and BOSS $\mathrm{BAO}/f{\ensuremath{\sigma}}_{8}$ data restricts the particle mass to ${m}_{\mathrm{sp}}^{\mathrm{eff}}\ensuremath{\simeq}0.4{8}_{\ensuremath{-}0.36}^{+0.17}$ and contribution to $\mathrm{\ensuremath{\Delta}}{N}_{\mathrm{eff}}\ensuremath{\simeq}0.04{6}_{\ensuremath{-}0.031}^{+0.004}$. We discuss implications of our results for other canonical nonthermal HDM models---the Dodelson-Widrow model and a hidden sector model of a thermal sterile particle with a different temperature. We report competitive results on such hidden sector temperature that might have interesting implications for particle physics model building, in particular connecting the ${S}_{8}$ tension to the longstanding short baseline oscillation anomaly.

Radial distributions of globular clusters trace their host dark matter halo: insights from the E-MOSAICS simulations

ABSTRACT Globular clusters (GCs) are bright objects that span a wide range of galactocentric distances, and can therefore probe the structure of dark matter (DM) haloes. In this work, we explore whether the projected number density radial profiles of GCs can be used to infer the structural properties of their host DM haloes. We use the simulated GC populations in a sample of 166 central galaxies from the $(34.4~\rm cMpc)^3$ periodic volume of the E-MOSAICS project. We find that more massive galaxies host stellar and GC populations with shallower density profiles that are more radially extended. In addition, metal-poor GC subpopulations tend to have shallower and more extended profiles than metal-rich subsamples, which we relate to the preferentially accreted origin of metal-poor GCs. We find strong correlations between the power-law slopes and effective radii of the radial profiles of the GC populations and the structural properties of the DM haloes, such as their power-law slopes, Navarro–Frenk–White scale radii, and concentration parameters. Accounting for a dependence on the galaxy stellar mass decreases the scatter of the two-dimensional relations. This suggests that the projected number counts of GCs, combined with their galaxy mass, trace the density profile of the DM halo of their host galaxy. When applied to extragalactic GC systems, we recover the scale radii and the extent of the DM haloes of a sample of early-type galaxies with uncertainties smaller than $0.2~\rm dex$. Thus, extragalactic GC systems provide a novel avenue to explore the structure of DM haloes beyond the Local Group.

Cosmological forecasts on thermal axions, relic neutrinos and light elements

Abstract One of the targets of future Cosmic Microwave Background and Baryon Acoustic Oscillation measurements is to improve the current accuracy in the neutrino sector and reach a much better sensitivity on extra dark radiation in the Early Universe. In this paper we study how these improvements can be translated into constraining power for well motivated extensions of the Standard Model of elementary particles that involve axions thermalized before the QCD phase transition by scatterings with gluons. Assuming a fiducial ΛCDM cosmological model, we simulate future data for CMB-S4-like and DESI-like surveys and analyze a mixed scenario of axion and neutrino hot dark matter. We further account also for the effects of these QCD axions on the light element abundances predicted by Big Bang Nucleosynthesis. The most constraining forecasted limits on the hot relic masses are ma ≲ 0.92 eV and ∑mν ≲ 0.12 eV at 95%CL, showing that future cosmic observations can substantially improve the current bounds, supporting multi-messenger analyses of axion, neutrino and primordial light element properties.

Two Analytic Relations Connecting the Hot Gas Astrophysics with the Cold Dark Matter Model for Galaxy Clusters

Abstract Galaxy clusters are good targets for examining our understanding of cosmology. Apart from numerical simulations and gravitational lensing, X-ray observation is the most common and conventional way to analyze the gravitational structures of galaxy clusters. Therefore, it is valuable to have simple analytical relations that can connect the observed distribution of the hot, X-ray-emitting gas to the structure of the dark matter in the clusters as derived from simulations. In this article, we apply a simple framework that can analytically connect the hot gas empirical parameters with the standard parameters in the cosmological cold dark matter model. We have theoretically derived two important analytic relations, r s ≈ 3 r c and ρ s ≈ 9 β kT / 8 π Gm g r c 2 , which can easily relate the dark matter properties in galaxy clusters with the hot gas properties. This can give a consistent picture describing gravitational astrophysics for galaxy clusters by the hot gas and cold dark matter models.

The Tail of Late-forming Dwarf Galaxies in ΛCDM

Abstract We use a robust analytical model together with a high-resolution hydrodynamical cosmological simulation to demonstrate that in a Lambda cold dark matter (ΛCDM) universe, a small fraction of dwarf galaxies inhabiting dark matter (DM) halos in the mass range 3 × 109 ≲ M 200/M ⊙ ≲ 1010 form unusually late (z < 3) compared to the bulk population of galaxies. These galaxies originate from the interplay between the stochastic growth of DM halos and the existence of a time-dependent DM halo mass below which galaxies do not form. The formation epoch of the simulated late-forming galaxies traces remarkably well the time when their host DM halos first exceeded a nontrivial (but well-understood) time-dependent critical mass, thus making late-forming dwarfs attractive cosmological probes with constraining power over the past growth history of their host halos. The agreement between our model and the simulation results demonstrates that the population of simulated late-forming dwarfs is a robust cosmological outcome and largely independent of the specific galaxy formation model included in the simulations provided: (1) the universe underwent cosmic reionization before z re ∼ 8 ; (2) star formation proceeds in gas that self-gravitates; and (3) galaxy formation is largely restricted to atomic-cooling halos before z re. The scarcity of massive late-forming dwarfs expected in ΛCDM implies that the great majority of bright, metal-poor, and actively star-forming dwarfs observed in our local universe—the most obvious candidates for these late-forming galaxies—cannot be undergoing their formation for the first time at the present day in a ΛCDM universe.

A model for mixed warm and hot right-handed neutrino dark matter

We discuss a model where a mixed warm and hot keV neutrino dark matter rises naturally. We arrange active and sterile neutrinos in the same SU(3)L multiplet, with the lightest sterile neutrino being dark matter. The other two heavy sterile neutrinos, through their out-of-equilibrium decay, contribute both to the dilution of dark matter density and its population, after freeze-out. We show that this model features all ingredients to overcome the overproduction of keV neutrino dark matter, and explore the phenomenological implications for Big Bang Nucleosynthesis and the number of relativistic degrees of freedom.

A sparse regression approach to modelling the relation between galaxy stellar masses and their host haloes

ABSTRACT Sparse regression algorithms have been proposed as the appropriate framework to model the governing equations of a system from data, without needing prior knowledge of the underlying physics. In this work, we use sparse regression to build an accurate and explainable model of the stellar mass of central galaxies given properties of their host dark matter (DM) halo. Our data set comprises 9521 central galaxies from the EAGLE hydrodynamic simulation. By matching the host haloes to a DM-only simulation, we collect the halo mass and specific angular momentum at present time and for their main progenitors in 10 redshift bins from z = 0 to z = 4. The principal component of our governing equation is a third-order polynomial of the host halo mass, which models the stellar-mass–halo-mass relation. The scatter about this relation is driven by the halo mass evolution and is captured by second- and third-order correlations of the halo mass evolution with the present halo mass. An advantage of sparse regression approaches is that unnecessary terms are removed. Although we include information on halo specific angular momentum, these parameters are discarded by our methodology. This suggests that halo angular momentum has little connection to galaxy formation efficiency. Our model has a root mean square error (RMSE) of 0.167log10(M*/M⊙), and accurately reproduces both the stellar mass function and central galaxy correlation function of EAGLE. The methodology appears to be an encouraging approach for populating the haloes of DM-only simulations with galaxies, and we discuss the next steps that are required.

Estimation of the mass of dark matter using the observed mass profiles of late-type galaxies

The system of stability equations for galactic halos is under-determined in most of the models of dark matter (DM).Conventionally, the issue is resolved by taking the temperature as a constant, and the chemical potential and the mass density as position-dependent variables.In this paper, to close the under-determined set of equations, we remove the mass density using observations and leave the temperature and the chemical potential as position-dependent variables.We analyze observations of the mass profiles of 175 late-type galaxies in the Spitzer Photometry & Accurate Rotation Curves (SPARC) database as well as 26 late-type dwarfs in the Little Things database, to construct the temperature profile of their DM halos by assuming that (1) DM in the halos obeys either the Fermi-Dirac or the Maxwell-Boltzmann distribution, and (2) the halos are in the virial state.We derive the dispersion velocity of DM at the center of the halos and show that its correlation with the halo's total mass is the same as the one estimated in N-body simulations and consistent with the direct observations of visible matter.Taking the latter agreement as a validation of our analysis, we derive the mass to the temperature of DM at the edge of the halos and show that it is galaxy independent and is equal to m/T R 200 ≃ 1010 in natural units. In the thermal models of DM, such universal temperature is inherently assumed. In this paper, we derive the universal temperature from observations without imposing it by assumptions.Therefore, T R 200 in the above ratio can be expressed in terms of the temperature of the cosmic microwave background (CMB) at the time of DM decoupling.This result is used to study possible cosmological scenarios.We show that observations are at odds with (1) non-thermal DM, (2) hot DM, and (3) collision-less cold DM.If DM is warm, we estimate its mass to be in the range of keV–MeV.

Breakdown of Chiral Perturbation Theory for the Axion Hot Dark Matter Bound.

We show that the commonly adopted hot dark matter bound on the axion mass m_{a}≲1 eV is not reliable, since it is obtained by extrapolating the chiral expansion in a region where the effective field theory breaks down. This is explicitly shown via the calculation of the axion-pion thermalization rate at the next-to-leading order in chiral perturbation theory. We finally advocate a strategy for a sound extraction of the axion hot dark matter bound via lattice QCD techniques.

New cosmological bounds on hot relics: axions and neutrinos

ABSTRACT Axions, if realized in nature, can be copiously produced in the early universe via thermal processes, contributing to the mass-energy density of thermal hot relics. In light of the most recent cosmological observations, we analyse two different thermal processes within a realistic mixed hot dark matter scenario which includes also massive neutrinos. Considering the axion–gluon thermalization channel, we derive our most constraining bounds on the hot relic masses ma < 7.46 eV and ∑mν < 0.114 eV both at 95 per cent CL; while studying the axion–pion scattering, without assuming any specific model for the axion–pion interactions, and remaining in the range of validity of the chiral perturbation theory, our most constraining bounds are improved to ma < 0.91 eV and ∑mν < 0.105 eV, both at 95 per cent CL. Interestingly, in both cases, the total neutrino mass lies very close to the inverted neutrino mass ordering prediction. If future terrestrial double beta decay and/or long-baseline neutrino experiments find that the nature mass ordering is the inverted one, this could rule out a wide region in the currently allowed thermal axion window. Our results therefore, strongly support multi messenger searches of axions and neutrino properties, together with joint analyses of their expected sensitivities.

Nonthermal hot dark matter from inflaton or moduli decay: Momentum distribution and relaxation of the cosmological mass bound

Decay of the inflaton or moduli which dominated the energy density of the universe at early times leads to a matter to radiation transition epoch. We consider non-thermal sterile dark matter particles produced as decay product during such transitions. The particles have a characteristic energy distribution - that associated with decays taking place in a matter dominated universe evolving to radiation domination. We primarily focus on the case when the particles are hot dark matter, and study their effects on the Cosmic Microwave Background (CMB) and Large Scale Structure (LSS), explicitly taking into account their non-thermal momentum distribution. Our results for CMB angular power and linear matter power spectra reveal interesting features - such as an order of magnitude higher values of hot dark matter mass in comparison to the thermal case being consistent with the present data. We observe that this is related to the fact that $\Delta N_{\rm eff}$ and the hot DM energy density can be independent of each other unlike the case of thermal or non-resonantly produced sterile hot DM. We also find features in the CMB at low $\ell$ angular power potentially related to supersonic transmission of hot dark matter through the photon-baryon plasma.

The cosmic neutrino background as a collection of fluids in large-scale structure simulations

A significant challenge for modelling the massive neutrino as a hot dark matter is its large velocity dispersion. In this work, we investigate and implement a multi-fluid perturbation theory that treats the cosmic neutrino population as a collection of fluids with a broad range of bulk velocities. These fluids respond linearly to the clustering of cold matter, which may be linear and described by standard linear perturbation theory, or non-linear, described using either higher-order perturbation theory or N-body simulations. We verify that such an alternative treatment of neutrino perturbations agrees closely with state-of-the-art neutrino linear response calculations in terms of power spectrum and bispectrum predictions. Combining multi-fluid neutrino linear response with a non-linear calculation for the cold matter clustering, we find for a reference νΛCDM cosmology with neutrino mass sum ∑ m ν = 0.93 eV an enhancement of the small-scale neutrino power by an order of magnitude relative to a purely linear calculation. The corresponding clustering enhancement in the cold matter, however, is a modest ∼ 0.05%. Importantly, our multi-fluid approach uniquely enables us to identify that the slowest-moving 25% of the neutrino population clusters strongly enough to warrant a non-linear treatment. Such a precise calculation of neutrino clustering on small scales accompanied by fine-grained velocity information would be invaluable for experiments such as PTOLEMY that probe the local neutrino density and velocity in the solar neighbourhood.

The twisted dark matter halo of the Milky Way

ABSTRACT We analyse systems analogous to the Milky Way (MW) in the eagle cosmological hydrodynamics simulation in order to deduce the likely structure of the MW’s dark matter (DM) halo. We identify MW mass haloes in the simulation whose satellite galaxies have similar kinematics and spatial distribution to those of the bright satellites of the MW, specifically systems in which the majority of the satellites (8 out of 11) have nearly coplanar orbits that are also perpendicular to the central stellar disc. We find that the normal to the common orbital plane of the coplanar satellites is well aligned with the minor axis of the host DM halo, with a median misalignment angle of only 17.3°. Based on this result, we infer that the minor axis of the Galactic DM halo points towards (l, b) = (182°, −2°), with an angular uncertainty at the 68 and 95 percentile confidence levels of 22° and 43°, respectively. Thus, the inferred minor axis of the MW halo lies in the plane of the stellar disc. The halo, however, is not homologous and its flattening and orientation vary with radius. The inner parts of the halo are rounder than the outer parts and well aligned with the stellar disc (that is the minor axis of the halo is perpendicular to the disc). Further out, the halo twists and the minor axis changes direction by 90°. This twist occurs over a very narrow radial range and reflects variations in the filamentary network along which mass was accreted into the MW.

A model of light dark matter and dark radiation

We propose a model for dark matter and dark radiation, based on a strongly-coupled dark \(\mathrm{SU}(5)\) gauge theory with fundamental and decuplet dark-quarks. The model supports light dark-baryons, respecting the chiral symmetry, which are electrically neutral but have electromagnetic form factors, and also a light dark-axion. Since the coupling of dark baryons to the (electrically charged) standard model particles is inversely proportional to the square of the confinement scale, dark baryons become either hot dark matter or cold dark matter, depending on when the dark color confines. For the confinement scale \(\Lambda \sim 10\)–\(10^3~\mathrm{GeV}\) the dark baryons of mass about \( 1~\mathrm{GeV}\)–\(1~\mathrm{MeV}\) become cold dark matter with naturally small magnetic moment and give the correct relic abundance.

Cannibalism hinders growth: Cannibal Dark Matter and the S8 tension

Many models of dark matter have been proposed in attempt to ease the S8 tension between weak lensing and CMB experiments. One such exciting possibility is cannibalistic dark matter (CanDM), which has exothermal number-changing interactions allowing it to stay warm far into its non-relativistic regime. Here we investigate the cosmological implications of CanDM and how it impacts CMB anisotropies and the matter power spectrum, by implementing the model within a linear Einstein-Boltzmann solver. We show that CanDM suppresses the small scale matter power spectrum in a way very similar to light Warm Dark Matter or Hot Dark Matter. However, unlike in those models, the suppression may happen while the CanDM model still remains compatible with CMB constraints. We put strong constraints on the interaction strength of CanDM as a function of its abundance for both constant and temperature-dependent thermally-averaged cross sections. We find that the CanDM model can easily solve the S8 tension (but has no impact on the Hubble tension). Indeed, it can accommodate values of S8 of the order of 0.76 while being compatible with CMB+BAO data. However, as long as the S8 tension remains moderate, the overall χ2 improvement is relatively small given the number of extra free parameters, and the CanDM model is not significantly preferred.

Shining light on the scotogenic model: interplay of colliders and cosmology

In the framework of the scotogenic model, which features radiative generation of neutrino masses, we explore light dark matter scenario. Throughout the paper we chiefly focus on keV-scale dark matter which can be produced either via freeze-in through the decays of the new scalars, or from the decays of next-to-lightest fermionic particle in the spectrum, which is produced through freeze-out. The latter mechanism is required to be suppressed as it typically produces a hot dark matter component. Constraints from BBN are also considered and in combination with the former production mechanism they impose the dark matter to be light. For this scenario we consider signatures at High Luminosity LHC and proposed future hadron and lepton colliders, namely FCC-hh and CLIC, focusing on searches with two leptons and missing energy as a final state. While a potential discovery at High Luminosity LHC is in tension with limits from cosmology, the situation greatly improves for future colliders.

Systematic study of background cosmology in unitary Poincaré gauge theories with application to emergent dark radiation and H0 tension

We propose a one-parameter extension to $\Lambda$CDM, expected to strongly affect cosmological tensions. An effective dark radiation component in the early universe redshifts away as hot dark matter, then quintessence, tracking the dominant equation-of-state parameter and leaving a falsifiable torsion field in the current epoch. This picture results from a new Poincar\'{e} gauge theory (PGT), one of the most promising among the latest batch of 58 PGTs found to be both power-counting renormalisable and free from ghosts and tachyons. We systematically categorise the cosmologies of 33 of these PGTs, as special cases of the most general parity-preserving, Ostrogradsky-stable PGT with a purely Yang-Mills action. The theory we consider contains two propagating massless gravitons, which may be $J^P=2^+$ (long-range gravitation and gravitational waves). A conspiracy among the coupling constants eliminates the spatial curvature $k\in\{\pm 1,0\}$ from the field equations. We show that this `$k$-screening' is not restricted to conformal gravity theories. The flat Friedmann equations are then emergent, with potentially tension-resolving freedom at the early scale-invariant epoch that reliably gives way to an attractor-like state of modern $\Lambda$CDM evolution. We compare with related theories and promising special cases, such as $k$-screened theories with negative-definite effective $k$, and more traditional theories with effective $\Lambda$ and a $J^P=0^-$ massive graviton (dark matter candidate). As a bonus, we analyse similarly constrained actions in the new extended Weyl gauge theory (eWGT). We show that in cosmology, PGT and eWGT span exactly the same classical phenomenology up to a linear map between their coupling constants, hinting at a deeper relationship between the two.

Sterile neutrino dark matter in left-right theories

SU(2)L× SU(2)R gauge symmetry requires three right-handed neutrinos (Ni), one of which, N1, can be sufficiently stable to be dark matter. In the early universe, WR exchange with the Standard Model thermal bath keeps the right-handed neutrinos in thermal equilibrium at high temperatures. N1 can make up all of dark matter if they freeze-out while relativistic and are mildly diluted by subsequent decays of a long-lived and heavier right-handed neutrino, N2. We systematically study this parameter space, constraining the symmetry breaking scale of SU(2)R and the mass of N1 to a triangle in the (vR, M1) plane, with vR = (106− 3 × 1012) GeV and M1 = (2 keV–1 MeV). Much of this triangle can be probed by signals of warm dark matter, especially if leptogenesis from N2 decay yields the observed baryon asymmetry. The minimal value of vR is increased to 108 GeV for doublet breaking of SU(2)R, and further to 109 GeV if leptogenesis occurs via N2 decay, while the upper bound on M1 is reduced to 100 keV. In addition, there is a component of hot N1 dark matter resulting from the late decay of N2→ N1ℓ+ℓ− that can be probed by future cosmic microwave background observations. Interestingly, the range of vR allows both precision gauge coupling unification and the Higgs Parity understanding of the vanishing of the Standard Model Higgs quartic at scale vR. Finally, we study freeze-in production of N1 dark matter via the WR interaction, which allows a much wider range of (vR, M1).

Dark Matter searches: an overview

The INFIERI school aims at presenting some of the most exciting frontiers of physics and technology. The matter we know (baryonic matter made with protons, neutrons and electrons) is less than 5% of the energy content, and the Universe is dominated today by unknown Dark Energy (DE) and Dark Matter (DM). The existence of DM is now a mainstream concept, although at galactic scales all evidence for DM can also be explained by models with modified gravity. Evidence at large scales are however hard to reconcile with a simple modified gravity model. Cosmological Microwave Background (CMB) measurements show evidence for non-baryonic DM, which cannot be explained by the no-DM modified gravity models. Large scale N-body simulations prefer Cold Dark Matter (CDM) to Hot Dark Matter (HDM), Cold/Hot designating non-relativistic vs relativistic particles at decoupling, but the nature of DM is still unknown and many hypothesis have been suggested and explored. Some extensions of the Standard Model of Particle Physics expect particles which are neutral and interact very weakly with other forms of matter (WIMPs) In this short lecture notes, which is not aimed as a review of the field, I will summarize what is known about the existence of DM, which lead us to develop the most sensitive detectors in space, on Earth, and in underground laboratories. I will focus mainly on the Direct Detection of WIMPS and some of the experimental challenges. As the 2019 INFIERI school was held in Wuhan, China, I have mainly selected examples from the Chinese projects.

The H IX galaxy survey

Context. This paper presents the analysis of optical integral field spectra for the H I eXtreme (H IX) galaxy sample. H IX galaxies host at least 2.5 times more atomic gas (H I) than expected from their optical R-band luminosity. Previous examination of their star formation activity and H I kinematics suggested that these galaxies stabilise their large H I discs (radii up to 94 kpc) against star formation due to their higher than average baryonic specific angular momentum. A comparison to semi-analytic models further showed that the elevated baryonic specific angular momentum is inherited from the high spin of the dark matter host. Aims. In this paper we now turn to the gas-phase metallicity as well as stellar and ionised gas kinematics in H IX galaxies to gain insights into recent accretion of metal-poor gas or recent mergers. Methods. We compare the stellar, ionised, and atomic gas kinematics, and examine the variation in the gas-phase metallicity throughout the stellar disc of H IX galaxies. Results. We find no indication for counter-rotation in any of the components, the central metallicities tend to be lower than average, but as low as expected for galaxies of similar H I mass. Metallicity gradients are comparable to other less H I-rich, local star forming galaxies. Conclusions. We conclude that H IX galaxies show no conclusive evidence for recent major accretion or merger events. Their overall lower metallicities are likely due to being hosted by high spin halos, which slows down their evolution and thus the enrichment of their interstellar medium.