arXiv subjects

Chris Kouvaris

Publications and source records attributed to Chris Kouvaris.

At least 19 recordsLinked to original sources

arXiv2026

Imprints of energy injection by compact dark stars in the 21-cm signal

A strongly self-interacting component of asymmetric dark matter particles can form compact dark stars. The high dark matter density in these objects may allow significant dark matter annihilation into Standard Model particles, even when the portals to the visible sector are extremely weak. In this paper, we argue that compact dark stars could constitute an important source of energy injection during the cosmic dawn era in addition to that of the baryonic stars. Therefore, if dark stars annihilate into photons, the luminosity of dark stars may affect the reionization history of the Universe. We show that the evolution with the redshift of the temperature brightness of the 21-cm line could significantly deviate from the expectations of standard Cosmology, thus providing a new probe for particle dark matter.

hep-ph↗

arXiv2026

Gravitational Waves from Mergers of Asymmetric Dark Stars

A strongly self-interacting component of asymmetric dark matter (DM) particles can form compact dark stars (DSs). These objects have a broad spectrum of masses and radii, with distinct evolution histories from both neutron stars and black holes (BHs). We argue that these differences allow a population of DSs to contribute significantly to the astrophysical merger rate in unique and discernible ways. Specifically, their merger rate could dominate at low redshifts over other sources, while their mass function may populate windows outside known astrophysical processes. We investigate the structure and formation of DSs within a dissipative model, and calculate the enhancement of their merger cross-section due to tidal deformation effects. From this, we derive the present-day merger rate and its differential mass distribution. These findings open a new window to probe DM substructure and particle interactions through present and future gravitational wave (GW) observatories.

hep-ph↗

arXiv2025

Compact Stars as Portals to Extra-Dimensional Dark Matter

We investigate hydrostatic configurations of asymmetric dark matter (DM) spheres in scenarios where fermionic DM can propagate into extra spatial dimensions, while Standard Model fields remain confined to ordinary three dimensions. As the number of extra dimensions increases, the effective equation of state for non-relativistic matter softens, making even modest DM accumulation inside neutron stars susceptible to gravitational collapse into extra-dimensional black holes. These black holes are longer lived than their $3$ dimensional counterparts and can accrete enough material to consume an entire neutron star, ultimately producing solar-mass black holes. For geometric cross sections, DM with masses above $\mathcal{O}(10\,{\rm TeV})$ may already be excluded for more than two extra dimensions of size ${\mathcal{O}(\rm fm})$ -- sharply contrasting with the standard $3$ dimensional case, where comparable limits only appear for masses $\gtrsim 10^{5}$ TeV at typical halo densities of $0.3\, \rm{GeV/cm^3}$.

hep-ph↗

arXiv2025

Millicharged Particle Production in Pulsars via the Schwinger Effect

Low mass particles with small electric charges can be produced abundantly in large electric fields via the Schwinger effect. We study the production rate of such particles inside the polar gap of nearby pulsars. After production they are accelerated above MeV energies by the local electric fields. These pulsar-produced millicharged particles can be detected at Earth in low-threshold dark matter direct detection experiments. We find that the current XENONnT data constrains millicharged particles produced in the Crab pulsar to have charges less than $O(10^{-6})$ for sub-eV masses.

hep-ph↗

arXiv2025

Gravitational Waves from Magnetars

We study the emission of gravitational waves produced by the magnetosphere of magnetars. We argue that several features in the spectrum could facilitate the identification of that source. In addition, in cases of extremely large magnetic fields we demonstrate that this emission can make the braking index of such stars to be well over 3, which is the standard prediction of the magnetic dipole radiation and aligned rotator mechanisms. A similar picture arises if one focuses on the second braking index. Moreover the braking index depends on both the rotational frequency and the strength of the magnetic field in striking difference from the other mechanisms. We also show that gravitational waves can be produced by polar gap regions due to their rapid charge-discharge process that takes place in timescales from nanosec to microsec. This can provide an alternative way to probe magnetars and test the polar gap model.

astro-ph.HE↗

arXiv2025

Dark Matter Annihilation from Pulsating Dark Stars

A strongly self-interacting component of asymmetric dark matter can collapse and form compact objects, provided there is an efficient mechanism of energy evacuation. If the dark matter quantum number is not completely conserved but it is slightly violated due to some new physics e.g. at the Planck scale, dark matter particles can annihilate into Standard Model particles. Even tiny annihilation cross sections are sufficient to create observable luminosities. We demonstrate that these dark matter annihilations can trigger radial pulsations, causing a characteristic time modulation of the luminosities produced. We argue that in this scenario, the spectral features along with the properties of the oscillation can create a unique discovery signal for such objects in the sky.

hep-ph↗

arXiv2024

Gravitational Waves from Collapse of Pressureless Matter in the Early Universe

If an early matter phase of the Universe existed after inflation with the proper power spectrum, enhanced density perturbations can decouple from the Hubble flow, turn around and collapse. In contrast to what happens in a radiation dominated Universe where pressure nullifies deviations from sphericity in these perturbations, in a matter dominated Universe, the lack of pressure although on the one hand facilitates the gravitational collapse, it allows small deviations from sphericity to grow substantially as the collapse takes place. The subsequent collapse is complicated: initially as non-spherical deviations grow, the collapsing cloud takes the form of a ``Zel'dovich pancake". After that, the more chaotic and nonlinear stage of violent relaxation begins where shells of the cloud cross and the matter is redistributed within a factor of a few of the free fall timescale, reaching a spherical virialized state. During the whole process, strong gravitational waves are emitted due to the anisotropy of the collapse and the small time interval that the effect takes place. The emission of gravitational waves during the stage of the violent relaxation cannot be easily estimated with an analytical model. We perform an $N$-body simulation to capture the behaviour of matter during this stage in order to estimate the precise spectrum of gravitational waves produced in this scenario.

astro-ph.CO↗

arXiv2023

Defrosting and Blast Freezing Dark Matter

We show that the present-day dark matter abundance can be produced through a novel mechanism that involves a very rapid thermal freeze-out caused by inhomogeneous heating and successive fast cooling of small fireballs in the early Universe. The fireballs can be produced from energy deposited in small scale structure growth induced by Yukawa interactions in certain particle species. Yukawa interactions are known to cause growth of halos even during a radiation dominated era, and the same interactions facilitate cooling and collapse of the halos by the emission of scalars. Energy deposited in the Standard Model plasma at the locations of the halo collapse can heat the plasma, re-establishing thermal equilibrium. The subsequent expansion and cooling of plasma fireballs leads to freeze-out of dark matter on timescales much shorter than the Hubble time. This mechanism can produce the right abundance of dark matter for masses and annihilation cross sections previously thought to be ruled out.

hep-ph↗

arXiv2023

Proton capture in compact dark stars and observable implications

Asymmetric dark matter under certain conditions could form compact star-like objects, which can be searched either through gravitational lensing or by observation of gravitational waves from binaries involving such compact objects. In this paper we analyze possible signatures of such dark stars made of asymmetric dark matter with a portal to the Standard Model. We argue that compact dark stars could capture protons and electrons from the interstellar medium, which then accumulate in the core of the dark star, forming a very hot gas that emits X-rays or $γ$-rays. For dark matter parameters compatible with current laboratory constraints, compact dark stars could be sufficiently luminous to be detected at the Earth as point sources in the X-ray or $γ$-ray sky.

hep-ph↗

arXiv2022

Radio Signals from Axion Star-Neutron Star Binaries

Axion stars could form binaries with neutron stars. Given the extremely strong external magnetic field exhibited by individual neutron stars, there can be a substantial conversion of axions to photons in these binaries. The photon emission is doubly modulated due to the neutron star spinning and the axion star orbiting, yielding a unique discovery signal. Similar features are also generated in binaries between a neutron star and an axion-clouded black hole. Encouragingly, such binaries are found to be within the reach of ongoing and upcoming experiments (e.g., the Five hundred meter Aperture Spherical Telescope and the future Square Kilometer Array) for certain parameter regions. They thus provide a promising astronomical laboratory for detecting axions and axion dark matter.

astro-ph.HE↗

arXiv2021

Using atomic clocks to detect local dark matter halos

It is possible that bosonic dark matter forms halos around the Sun or the Earth. We discuss the possibility of probing such halos with atomic clocks. Focusing on either a Higgs portal or photon portal interaction between the dark matter and the Standard Model, we search the possible parameter space for which a clock on Earth and a clock in space would have a discernible frequency difference. Bosonic dark matter halos surrounding the Earth can potentially be probed with current optical atomic clocks.

hep-ph↗

arXiv2021

Gravitational Waves from Density Perturbations in an Early Matter Domination Era

We calculate the gravitational wave background produced from density perturbations in an early matter domination era where primordial black holes form. The formation of black holes requires perturbations out of the linear regime. Space with such perturbations reach a maximum expansion before it collapses asymmetrically forming a Zel'dovich pancake which depending on the parameters can either lead to a black hole or a virialized halo. In both cases and due to the asymmetry of the collapsing matter, a quadrupole moment generates gravitational waves which leave an imprint in the form of a stochastic background that can be detectable by near future gravitational interferometers.

astro-ph.CO↗

arXiv2020

The Photon Spectrum of Asymmetric Dark Stars

Asymmetric Dark Stars, i.e., compact objects formed from the collapse of asymmetric dark matter could potentially produce a detectable photon flux if dark matter particles self-interact via dark photons that kinetically mix with ordinary photons. The morphology of the emitted spectrum is significantly different and therefore distinguishable from a typical black-body one. Given the above and the fact that asymmetric dark stars can have masses outside the range of neutron stars, the detection of such a spectrum can be considered as a smoking gun signature for the existence of these exotic stars.

astro-ph.CO↗

arXiv2020

Probing Bosonic Stars with Atomic Clocks

Dark Matter could potentially manifest itself in the form of asymmetric dark stars. In this paper we entertain the possibility of probing such asymmetric bosonic dark matter stars by the use of atomic clocks. If the dark sector connects to the standard model sector via a Higgs or photon portal, the interior of boson stars that are in a Bose-Einstein condensate state can change the values of physical constants that control the timing of atomic clock devices. Dilute asymmetric dark matter boson stars passing through the Earth can induce frequency shifts that can be observed in separated Earth based atomic clocks. This gives the opportunity to probe a class of dark matter candidates that for the moment cannot be detected with any different conventional method.

hep-ph↗

arXiv2019

Direct Detection of Strongly Interacting Sub-GeV Dark Matter via Electron Recoils

We consider direct-detection searches for sub-GeV dark matter via electron scatterings in the presence of large interactions between dark and ordinary matter. Scatterings both on electrons and nuclei in the Earth's crust, atmosphere, and shielding material attenuate the expected local dark matter flux at a terrestrial detector, so that such experiments lose sensitivity to dark matter above some critical cross section. We study various models, including dark matter interacting with a heavy and ultralight dark photon, through an electric dipole moment, and exclusively with electrons. For a dark-photon mediator and an electric dipole interaction, the dark matter-electron scattering cross-section is directly linked to the dark matter-nucleus cross section, and nuclear interactions typically dominate the attenuation process. We determine the exclusion bands for the different dark-matter models from several experiments - SENSEI, CDMS-HVeV, XENON10, XENON100, and DarkSide-50 - using a combination of Monte Carlo simulations and analytic estimates. We also derive projected sensitivities for a detector located at different depths and for a range of exposures, and calculate the projected sensitivity for SENSEI at SNOLAB and DAMIC-M at Modane. Finally, we discuss the reach to high cross sections and the modulation signature of a small balloon- and satellite-borne detector sensitive to electron recoils, such as a Skipper-CCD. Such a detector could potentially probe unconstrained parameter space at high cross sections for a sub-dominant component of dark matter interacting with a massive, but ultralight, dark photon.

hep-ph↗

arXiv2018

Structure Formation and Exotic Compact Objects in a Dissipative Dark Sector

We present the complete history of structure formation in a simple dissipative dark-sector model. The model has only two particles: a dark electron, which is a subdominant component of dark matter, and a dark photon. Dark-electron perturbations grow from primordial overdensities, become non-linear, and form dense dark galaxies. Bremsstrahlung cooling leads to fragmentation of the dark-electron halos into clumps that vary in size from a few to millions of solar masses, depending on the particle model parameters. In particular, we show that asymmetric dark stars and black holes form within the Milky Way from the collapse of dark electrons. These exotic compact objects may be detected and their properties measured at new high-precision astronomical observatories, giving insight into the particle nature of the dark sector without the requirement of non-gravitational interactions with the visible sector.

hep-ph↗

arXiv2018

Neutron Star Stability in Light of the Neutron Decay Anomaly

A recent proposal suggests that experimental discrepancies on the lifetime of neutrons can be resolved if neutrons decay to dark matter. At the same time it has been demonstrated that such a decay mode would soften the nuclear equation of state resulting in neutron stars with a maximum mass much below currently observed ones. In this paper we demonstrate that appropriate dark matter-baryon interactions can accommodate neutron stars with mass above 2 solar masses. We also show that dark matter self-interactions could also help neutrons stars reach 2 solar masses provided that dark matter is of asymmetric nature.

hep-ph↗

arXiv2018

How blind are underground and surface detectors to strongly interacting Dark Matter?

Above a critical dark matter-nucleus scattering cross section any terrestrial direct detection experiment loses sensitivity to dark matter, since the Earth crust, atmosphere, and potential shielding layers start to block off the dark matter particles. This critical cross section is commonly determined by describing the average energy loss of the dark matter particles analytically. However, this treatment overestimates the stopping power of the Earth crust. Therefore the obtained bounds should be considered as conservative. We perform Monte Carlo simulations to determine the precise value of the critical cross section for various direct detection experiments and compare them to other dark matter constraints in the low mass regime. In this region we find parameter space where typical underground and surface detectors are completely blind to dark matter. This "hole" in the parameter space can hardly be closed with an increase in the detector exposure. Dedicated surface or high-altitude experiments may be the only way to directly probe this part of the parameter space.

hep-ph↗