Dark Matter Component Research Articles

Dark Matter–admixed Rotating White Dwarfs as Peculiar Compact Objects

Discoveries of anomalous compact objects challenge our understanding of the standard theory of stellar structures and evolution, so they serve as an excellent laboratory for searching for new physics. Earlier studies on spherically symmetric dark matter (DM)–admixed compact stars could explain a handful of anomalies. In this paper, we investigate the observational signatures of DM-admixed rotating white dwarfs, and make connections to observed peculiar compact objects. We compute the equilibrium structures of DM-admixed rotating white dwarfs using a self-consistent, two-fluid method, with the DM component being a nonrotating degenerate Fermi gas. We find that admixing DM to rotating white dwarfs could: (1) account for some peculiar white dwarfs that do not follow their usual mass–radius relation; (2) allow stable rapid-rotating white dwarfs that are free from thermonuclear runaway to exist, which could explain some soft gamma-ray repeaters/anomalous X-ray pulsars; and (3) produce universal I (moment of inertia)–Love (tidal Love number)–Q (quadrupole moment) relations that span bands above those without DM admixture, thus providing an indirect way of searching for DM in white dwarfs through gravitational-wave detection. To conclude, DM-admixed rotating white dwarfs can account for some peculiar compact objects. Our results suggest a systematic approach to accounting for the unusual compact objects that upcoming surveys could discover.

Is the Core-cusp Problem a Matter of Perspective? Jeans Anisotropic Modeling against Numerical Simulations

Mock member stars for 28 dwarf galaxies are constructed from the cosmological auriga simulation, which reflects the dynamical status of realistic stellar tracers. Axisymmetric Jeans Anisotropic Multi-Gaussian Expansion (jam) modeling is applied to 6000 star particles for each system to recover the underlying matter distribution. The stellar or dark matter component individually is poorly recovered, but the total profile is constrained more reasonably. The mass within the half-mass radius of tracers is recovered the tightest, and the mass between 200 and 300 pc, M(200–300 pc), is an unbiasedly constrained ensemble, with a scatter of 0.167 dex. If using 2000 particles and only line-of-sight velocities with typical errors, the scatter in M(200–300 pc) is increased by ∼50%. Quiescent Saggitarius dSph–like systems and star-forming systems with strong outflows show distinct features, with M(200–300 pc) mostly underestimated for the former, and likely overestimated for the latter. The biases correlate with the dynamical status, which is a result of contraction motions due to tidal effects in quiescent systems or galactic winds in star-forming systems, driving them out of equilibrium. After including Gaia DR3 proper motion errors, we find proper motions can be as useful as line-of-sight velocities for nearby systems at < ∼60 kpc. By extrapolating the actual density profiles and the dynamical constraints down to scales below the resolution, we find the mass within 150 pc can be an unbiasedly constrained ensemble, with a scatter of ∼0.255 dex. We show that the contraction of member stars in nearby systems is detectable based on Gaia DR3 proper motion errors.

Extended Bose–Einstein condensate dark matter in viscous Gauss–Bonnet gravity

In this paper, we study the [Formula: see text] gravity model with an interacting model by flat-FRW metric in a viscous fluid. We consider that the universe dominates with components of dark matter and dark energy. This means that the dark matter component derives from Extended Bose–Einstein Condensate (EBEC) and the components of dark energy arise from the [Formula: see text] gravity. After obtaining the Einstein equation, the energy density and the pressure of dark energy are written in terms of the geometry of the curvature and the Gauss–Bonnet terms, and components of dark matter and viscous fluid. Also, the corresponding continuity equations are written with the presence of interaction terms. In what follows, we employ the EBEC regime instead of the normal dark matter by the dark matter Equation of State (EoS) as [Formula: see text], which arises from the gravitational form. The EoS can be expressed from the perspective of the virial expansion, in which the first and second terms represent normal dark matter and quantum ground state. Next, the corresponding Friedmann equations reconstruct in terms of the redshift parameter, then by using the scenario of the power-law cosmology for the scale factor, we fit the present model with the Hubble amounts of 51 supernova data by the likelihood analysis. In that case, we acquire the cosmological parameters of dark energy in terms of the redshift parameter, and by plotting these graphs, we see that the universe is currently undergoing an accelerated expansion phase. Finally, we investigate the stability of the present model with the sound speed parameter.

WIMP and FIMP dark matter in the inert doublet plus singlet model

We consider multicomponent dark matter in a model where one dark matter component is feebly interacting (FIMP) while the second is weakly interacting (WIMP). The model contains an inert scalar doublet and a complex scalar singlet and features a discrete ${Z}_{4}$ symmetry. Using new features of micromegas that allow us to compute the relic density for multicomponent dark matter including WIMPs and FIMPs, we determine the parameter space that satisfies theoretical constraints, collider constraints, relic density as well as direct and indirect detection limits. We discuss the possibility to probe the model through collider and astrophysical searches. We find that it is possible that the FIMP forms the dominant dark matter (DM) component, in this case the astrophysical signatures of the WIMP are much suppressed. We also explore the case where the doublet can decay into pairs of singlets leaving only one DM component, the FIMP. These scenarios are constrained by cosmology, notably big bang nucleosynthesis, and can be explored at colliders while astrophysical signatures are suppressed.

Newtonian fractional-dimension gravity and the external field effect

We expand our analysis of Newtonian Fractional-Dimension Gravity (NFDG), an extension of the classical laws of Newtonian gravity to lower dimensional spaces, including those with fractional (i.e., non-integer) dimension. We apply our model to four rotationally supported galaxies (NGC 5033, NGC 6674, NGC 5055, NGC 1090), in addition to other three galaxies (NGC 7814, NGC 6503, NGC 3741) which were analyzed in previous studies. NFDG is able to fit the rotation curves of all these galaxies without any dark matter component. We also investigate the possible violation of the strong equivalence principle, in relation to the External Field Effect (EFE), i.e., the dependence of the internal motion of a self-gravitating system under freefall on an external gravitational field. This effect is not present in Newtonian or Einstein gravity, but is predicted by some alternative theories of gravity. On the contrary, we show that NFDG does not imply the EFE, at least for values of the fractional dimension in the range $1 \leq D \leq 3$. Using improved NFDG numerical computations, we analyze the rotation curves of the aforementioned galaxies and obtain perfect fits to the experimental data, by using a fractional-dimension function $D\left (R\right )$ which characterizes each individual galaxy. In the galactic sample studied here with NFDG methods, we do not detect any significant differences between galaxies that are supposed to show/not show the EFE according to other alternative theories. A larger sample of galaxies will be needed to fully determine the absence of any external field effect in NFDG.

Gravitational lensing effects of supermassive black holes in cluster environments

ABSTRACT This study explores the gravitational lensing effects of supermassive black holes (SMBHs) in galaxy clusters. While the presence of central SMBHs in galaxies is firmly established, recent work from high-resolution simulations predict the existence of an additional population of wandering SMBHs. Though the masses of these SMBHs are a minor perturbation on the larger scale and individual galaxy scale dark matter components in the cluster, they can impact statistical lensing properties and individual lensed image configurations. Probing for these potentially observable signatures, we find that SMBHs imprint detectable signatures in rare, higher order strong lensing image configurations although they do not manifest any statistically significant detectable evidence in either the magnification distribution or the integrated shear profile. Investigating specific lensed image geometries, we report that a massive, near point-like, potential of an SMBH causes the following detectable effects: (i) image splitting leading to the generation of extra images; (ii) positional and magnification asymmetries in multiply imaged systems; and (iii) the apparent disappearance of a lensed counter image. Of these, image splitting inside the cluster tangential critical curve, is the most prevalent notable observational signature. We demonstrate these possibilities in two cases of observed giant arcs in SGAS J003341.5+024217 and RX J1347.5−1145, wherein specific image configurations seen can be reproduced with SMBHs. Future observations with high-resolution instrumentation (e.g. MAVIS-Very Large Telescope, MICADO-Extremely Large Telescope, and the upgraded ngVLA, along with data from the Euclid and Nancy Grace Roman Space Telescopes and the Rubin LSST Observatory are likely to allow us to probe these unique yet rare SMBHs lensing signatures.

Growth of fluctuations in Chaplygin gas cosmologies: A nonlinear Jeans scale for unified dark matter

Unified dark matter cosmologies economically combine missing matter and energy in a single fluid. Of these models, the standard Chaplygin gas is theoretically motivated, but faces problems in explaining large scale structure if linear perturbations are directly imposed on the homogeneous fluid. However, early formation of a clustered component of small halos is sufficient (and necessary) for hierarchical clustering to proceed in a cold dark matter (CDM) component as in the standard scenario, with the remaining homogeneous component acting as dark energy. We examine this possibility. A linear analysis shows that a critical Press-Schechter threshold for collapse can generally only be reached for generalized Chaplygin gas models that mimic $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$, or ones where superluminal sound speeds occur. However, the standard Chaplygin gas case turns out to be marginal, with overdensities reaching order one in the linear regime. This motivates a nonlinear analysis. A simple infall model suggests that collapse is indeed possible for perturbations of order 1 kpc and above; for, as opposed to standard gases, pressure forces decrease with increasing densities, allowing for the collapse of linearly stable systems. This suggests that a cosmological scenario based on the standard Chaplygin gas may not be ruled out from the viewpoint of structure formation, as often assumed. On the other hand, a ``nonlinear Jeans scale,'' constricting growth to scales $R\ensuremath{\gtrsim}\mathrm{kpc}$, which may be relevant to the small scale problems of CDM, is predicted. Finally, the background dynamics of clustered Chaplygin gas cosmologies is examined and confronted with observational datasets. It is found to be viable (at 1-sigma), with a mildly larger ${H}_{0}$ than $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$, if the clustered fraction is larger than 90%.

Primordial black holes and mirror dark matter

If mirror matter exists but cannot comprise all of the dark matter (DM) in the universe, we can expect that the additional DM component may only interact with the other sectors gravitationally. One of the natural candidates of a gravitationally interacting component is a primordial black hole (PBH). Therefore, if mirror matter exists but cannot comprise all of the DM in the universe, the existence of PBH may be expected as a candidate of the additional DM component. In this case, the remaining DM components may be PBHs or [Formula: see text] singlet particles from PBH. We show constraints on PBH with the mirror DM. Particularly, the initial PBH mass is estimated to be [Formula: see text], if the DM comprises mirror baryons and PBHs.

Self-interacting dark baryons

Using results from lattice QCD, it is possible to quantitatively design models of dark baryons leading to velocity-dependent self-interaction cross sections that match the values needed for solving small-scale structure problems of standard cold dark matter. However it is not obvious that the main dark matter component in such models will be nucleons rather than large nuclei, or dark pions or atoms, whose scattering properties would be different. We first identify the parameters of a dark SU(3) sector analogous to QCD---the confinement scale $\mathrm{\ensuremath{\Lambda}}$ and pion mass ${m}_{\ensuremath{\pi}}$---needed to reproduce desired self-interaction cross sections. Then we show that these values can generically be compatible with the absence of a sufficiently stable deuteron bound state, and hence leading to no heavier nuclei, thus establishing the consistency of the scenario for self-interacting dark nucleons. The range of dark photon masses needed to avoid dominant pion or atomic dark matter is determined, as well as allowed values for the kinetic mixing parameter. The dark proton might be detected directly in future searches, by dark photon exchange.

Rescuing constraints on modified gravity using gravitational redshift in large-scale structure

The distribution of galaxies provides an ideal laboratory to test for deviations from General Relativity. In particular, redshift-space distortions are commonly used to constrain modifications to the Poisson equation, which governs the strength of dark matter clustering. Here, we show that these constraints rely on the validity of the weak equivalence principle, which has never been tested for the dark matter component. Relaxing this restrictive assumption leads to modifications in the growth of structure that are fully degenerate with modifications induced by the Poisson equation. This in turns strongly degrades the constraining power of redshift-space distortions. Such degeneracies can however be broken and tight constraints on modified gravity can be recovered by measuring gravitational redshift from the galaxy distribution, an effect that will be detectable by the coming generation of large-scale structure surveys.

Multi-component dark sectors: symmetries, asymmetries and conversions

We study the relic abundance of several stable particles from a generic dark sector, including the possible presence of dark asymmetries. After discussing the different possibilities for stabilising multi-component dark matter, we analyse the final relic abundance of the symmetric and asymmetric dark matter components, paying special attention to the role of the unavoidable conversions between dark matter states. We find an exponential dependence of the asymmetries of the heavier components on annihilations and conversions. We conclude that having similar symmetric and asymmetric components is a natural outcome in many scenarios of multi-component dark matter. This has novel phenomenological implications, which we briefly discuss.

Observational constraint on axion dark matter with gravitational waves

Most matter in the Universe is invisible and unknown, which is called dark matter. A candidate of dark matter is axion, which is an ultra-light particle motivated as a solution for the CP problem. Axions form clouds in a galactic halo, amplify, and delay a part of gravitational waves propagating in the clouds. The Milky Way is surrounded by the dark matter halo composed of a number of axion patches. Thus, the characteristic secondary gravitational waves are always expected right after the reported gravitational-wave signals from compact binary mergers. In this paper, we derive a realistic amplitude of the secondary gravitational waves. Then we search the gravitational waves having the characteristic time delay and duration with a method optimized for them. We find no significant signal. Assuming the axions are dominant component of dark matter, we obtain the constraints on the axion coupling to the parity violating sector of gravity for the mass range, [$1.7 \times 10^{-13}, 8.5 \times 10^{-12}$]$\mathrm{eV}$, which is at most $\sim 10$ times stronger than that from Gravity Probe B.

Manifesting hidden dynamics of a sub-component dark matter

We emphasize the distinctive cosmological dynamics in multi-component dark-matter scenarios and their impact on probing a sub-dominant component of dark matter. The dynamics originate from the conversion among different dark-matter components. We find that the temperature of the self-interacting sub-component dark matter is significantly enhanced by the dark-matter annihilation into the sub-component. The same annihilation sharply increases the required annihilation cross section for the sub-component as we consider a smaller relative abundance fraction among the dark-matter species. Because of the enhanced temperature and couplings of the sub-component, contrary to a naive expectation, the sub-component with smaller abundance fractions tends to be disfavored by dark-matter direct/indirect-detection experiments and cosmological observations. We demonstrate this by taking a dark photon portal scenario for the sub-component at the sub-GeV mass scale. For the abundance fraction ≳ 10 %, the enhanced temperature of the sub-component is subject to warm dark matter constraints which are complementary to the parameter space probed by accelerator-based experiments. Smaller abundance fractions tend to be disfavored by accelerator-based experiments due to the enhanced coupling to Standard Model particles.

Testing strong lensing subhalo detection with a cosmological simulation

ABSTRACT Strong gravitational lensing offers a compelling test of the cold dark matter paradigm, as it allows for subhaloes with masses of ∼109 M⊙ and below to be detected. We test commonly used techniques for detecting subhaloes superposed in images of strongly lensed galaxies. For the lens we take a simulated galaxy in a ∼1013 M⊙ halo grown in a high-resolution cosmological hydrodynamical simulation, which we view from two different directions. Though the resolution is high, we note the simulated galaxy still has an artificial core which adds additional complexity to the baryon dominated region. To remove particle noise, we represent the projected galaxy mass distribution by a series of Gaussian profiles which precisely capture the features of the projected galaxy. We first model the lens mass as a (broken) power-law density profile and then search for small haloes. Of the two projections, one has a regular elliptical shape, while the other has distinct deviations from an elliptical shape. For the former, the broken power-law model gives no false positives and correctly recovers the mass of the superposed small halo; however, for the latter we find false positives and the inferred halo mass is overestimated by ∼4–5 times. We then use a more complex model in which the lens mass is decomposed into stellar and dark matter components. In this case, we show that we can capture the simulated galaxy’s complex projected structures and correctly infer the input small halo.

Gravitational Stability of Vortices in Bose-Einstein Condensate Dark Matter

We investigate a simple model for a galactic halo under the assumption that it is dominated by a dark matter component in the form of a Bose-Einstein condensate involving an ultra-light scalar particle. In particular we discuss the possibility if the dark matter is in superfluid state then a rotating galactic halo might contain quantised vortices which would be low-energy analogues of cosmic strings. Using known solutions for the density profiles of such vortices we compute the self-gravitational interactions in such halos and place bounds on the parameters describing such models, such as the mass of the particles involved.

Study of a Viscous ΛWDM Model: Near-Equilibrium Condition, Entropy Production, and Cosmological Constraints

Extensions to a ΛDM model have been explored in order to face current tensions that occur within its framework, which encompasses broadening the nature of the dark matter (DM) component to include warmness and a non-perfect fluid description. In this paper, we investigated the late-time cosmological evolution of an exact solution recently found in the literature, which describes a viscous warm ΛDM model (ΛWDM) with a DM component that obeys a polytropic equation of state (EoS), which experiences dissipative effects with a bulk viscosity proportional to its energy density, with proportionality constant ξ0. This solution has the particularity of having a very similar behavior to the ΛCDM model for small values of ξ0, evolving also to a de Sitter type expansion in the very far future. We explore firstly the thermodynamic consistences of this solution in the framework of Eckart’s theory of non-perfect fluids, focusing on the fulfillment of the two following conditions: (i) the near-equilibrium condition and (ii) the positiveness of the entropy production. We explore the range of parameters of the model that allow to fulfill these two conditions at the same time, finding that a viscous WDM component is compatible with both ones, being in this sense, a viable model from the thermodynamic point of view. Furthermore, we constrained the free parameters of the model with the observational data coming from supernovae Ia (SNe Ia) and the observational Hubble parameter data (OHD), using these thermodynamics analyses to define the best priors for the cosmological parameters related to the warmness and the dissipation of the DM, showing that this viscous ΛWDM model can describe the combined SNe Ia+OHD data in the same way as the ΛCDM model. The cosmological constraint at 3σ CL gives us an upper limit on the bulk viscous constant of order ξ0∼106 Pa·s, which is in agreement with some previous investigations. Our results support that the inclusion of a dissipative WDM, as an extension of the standard cosmological model, leads to a both thermodynamically consistent and properly fitted cosmological evolution.

Phenomenology of a two-component dark matter model

We study a two-component dark matter model consisting of a Dirac fermion and a complex scalar charged under new U(1) gauge group in the hidden sector. The dark fermion plays the dominant component of dark matter which explains the measured DM relic density of the Universe. It has no direct coupling to ordinary standard model particles, thus evading strong constraints from the direct DM detection experiments. The dark fermion is self-interacting through the light dark gauge boson and it would be possible to address that this model can be a resolution to the small scale structure problem of the Universe. The light dark gauge boson, which interacts with the standard model sector, is also stable and composes the subdominant DM component. We investigate the model parameter space allowed by current experimental constraints and phenomenological bounds. We also discuss the sensitivity of future experiments such as SHiP, DUNE and ILC, for the obtained allowed parameter space.

Merger Effects on the Spin and Shape Alignments of Galaxy Stellar, Cold Gas, Hot Gas, and Dark Matter Components

We present a numerical evidence supporting the scenario that the peculiar alignments of the galaxy stellar spins with the major principal axes of the local tidal tensors are produced during the quiescent evolution period when the galaxies experience no recent merger events. Analyzing the merger tree from the TNG300-1 simulation of the IllustrisTNG project, we find the latest merger epochs, a(z m ), of the galaxies, and create four a(z m )-selected samples that are controlled to share the identical mass and density distributions. For each sample, we determine the spin and shape vectors of the galaxy stellar, cold and hot gas, and dark matter components separately, and compute the average strengths of their alignments with the principal directions of the local tidal fields as well as their mutual alignment tendencies. It is found that the stellar (cold gas) spin axes of the galaxies whose latest merger events occur at earlier epochs are more strongly aligned (weakly antialigned) with the major principal axes of the tidal fields. It is also shown that, although the mass-dependent transition of the galaxy DM spins have little connection with the merger events, the morphologies, spin–shape, and shape-shear alignment strengths of the four components of the galaxies sensitively depend on a(z m ). Noting that the stellar components of the galaxies that undergo long quiescent evolution have distinctively oblate shapes and very strong spin–shape alignments, we suggest that the local tidal field might be traced by using the stellar shapes of galaxies without signatures of mergers as a proxy of their stellar spins.

Hierarchical merger of primordial black holes in dwarf galaxies

We study the merger history of primordial black holes (PBHs) in a scenario where they represent the dominant dark matter component of a typical dwarf galaxies' core. We investigate the possibility of a sequence of collisions resulting in a hierarchical merger of black holes, and look at the final mass spectrum in such clusters, which initially present a monochromatic (single-mass) PBH population. Our study shows that the merging process results in the transfer of about 40% of the total mass of the core to the merger products regardless of the initial mass of PBHs, with about 5% of energy radiated out in the form of gravitational waves. We find that, in the lighter mass limit, black holes up to eight times more massive than the original population can be formed within a Hubble time.

Dark Matter Admixed Neutron Star Properties in the Light of X-Ray Pulse Profile Observations

The distribution of the dark matter (DM) in DM-admixed neutron stars (DANSs) is supposed to result in either a dense dark core or an extended dark halo, subject to the DM fraction of the DANS (f χ ) and the DM properties, such as the mass (m χ ) and the strength of the self-interaction (y). In this paper, we perform an in-depth analysis of the formation criterion for dark cores/dark halos, and point out that the relative distribution of these two components is essentially determined by the ratio of the central enthalpy of the DM component to that of the baryonic matter component inside the DANSs. For the critical case where the radii of the DM and the baryonic matter are the same, we further derive an analytical formula to describe the dependence of on m χ and y for a given DANS mass. The relative distribution of the two components in DANSs can lead to different observational effects. We here focus on the modification of the pulsar pulse profile, due to the extra light-bending effect in the case of a dark halo existence, and conduct the first investigation into the dark halo effects on the pulse profile. We find that the peak flux deviation is strongly dependent on the ratio of the halo mass to the radius of the DM component. Last, we perform Bayesian parameter estimation on the DM particle properties, based on the recent X-ray observations of PSR J0030+0451 and PSR J0740+6620 by the Neutron Star Interior Composition Explorer.