Dark Radiation Research Articles

Novel theorems for a Bochner flat Lorentzian K[formula omitted]hler space-time manifold with [formula omitted]-Ricci-Yamabe solitons

In the current work, we examine η-Ricci-Yamabe solitons in a Bochner flat Lorentzian Ka¨hler space-time manifolds. We have addressed various circumstances for Ricci-Yamabe solitons to be steady, shrinking or expanding in terms of isotropic pressure, the cosmological constant, energy density and gravitational constant in different perfect fluids such as dark fluid, Stiff matter, dust fluid and radiation fluid.

Confronting interacting dark radiation scenarios with cosmological data

Dark radiation (DR) is generally predicted in new physics scenarios that address fundamental puzzles of the Standard Model or tensions in the cosmological data. Cosmological data have the sensitivity to constrain not only the energy density of DR but also whether it is interacting. In this paper, we present a systematic study of five types of interacting DR (free-streaming, fluid, decoupling, instantaneous decoupling, and recoupling DR) and their impact on cosmological observables. We modify the Boltzmann hierarchy to describe all these types of interacting DR under the relaxation time approximation. We, for the first time, robustly calculate the collision terms for recoupling scalar DR and provide a better estimation of the recoupling transition redshift. We demonstrate the distinct features of each type of DR on the cosmic microwave background and matter power spectra. We perform Markov-chain Monte Carlo scans using the Planck 2018 data and baryon acoustic oscillation data. Assuming no new physics in the standard model neutrino sector, we find no statistically significant constraints on the couplings of DR, although there is a slight preference for the fluidlike limit of all the cases. In the case of instantaneous decoupling DR, this limit corresponds to a late transition redshift around recombination. The $\mathrm{\ensuremath{\Delta}}{N}_{\mathrm{eff}}$ constraint varies marginally depending on the type of DR.

Dark matter and dark radiation from the early universe with a modulus coupled to the PQMSSM

The supersymmetrized DFSZ axion model is especially compelling in that it contains 1. the SUSY solution to the gauge hierarchy problem, 2. the Peccei-Quinn (PQ) solution to the strong CP problem and 3. the Kim-Nilles solution to the SUSY μ problem. In a string setting, where a discrete R-symmetry ( {textbf{Z}}_{24}^R for example) may emerge from the compactification process, a high-quality accidental axion (accion) can emerge from the accidental, approximate remnant global U(1)PQ symmetry where the decay constant fa is linked to the SUSY breaking scale, and is within the cosmological sweet zone. In this setup, one also expects the presence of stringy remnant moduli fields ϕi. Here, we consider the situation of a single light modulus ϕ coupled to the PQMSSM in the early universe, with mixed axion plus higgsino-like WIMP dark matter. We evaluate dark matter and dark radiation production via nine coupled Boltzmann equations and assess the severity of the cosmological moduli problem (CMP) along with dark matter and dark radiation production rates. We find that typically the light modulus mass should be mϕ ≳ 104 TeV to avoid the moduli-induced dark matter overproduction problem. If one is able to (anthropically) tune the modulus field amplitude, we find a value of ϕ0 ≲ 10–7mP would be required to solve the overall CMP.

Cosmological dark matter from a bulk black hole

We study the cosmology of a three-brane in a specific five-dimensional scalar-gravity (i.e. soft-wall) background, known as the linear dilaton background. We discover that the Friedmann equation of the brane-world automatically contains a term mimicking pressureless matter. We propose to identify this term as dark matter. This dark matter arises as a projection of the bulk black hole on the brane, which contributes to the brane Friedmann equation via both the Weyl tensor and the scalar stress tensor. The nontrivial matter-like behavior is due to an exact cancellation between the Weyl and scalar pressures. We show that the Newtonian potential only receives a mild short-distance correction going as inverse distance squared, ensuring compatibility of the linear dilaton brane-world with observed 4D gravity. Our setup can be viewed as a consistent cosmological description of the holographic theories arising in the linear dilaton background. We also present more general scalar-gravity models where the brane cosmology features an effective energy density whose behavior smoothly interpolates between dark radiation, dark matter and dark energy depending on a model parameter.

Toward a better understanding of peroxymonosulfate and peroxydisulfate activation using a nano zero-valent iron catalyst supported on graphitized carbon: Mechanisms and application to the degradation of estrogenic compounds in different water matrix

In this study, we employed nano zero-valent iron catalysts supported on graphitized carbon (GC-nZVI) catalyst to efficiently degrade BPA under circumneutral pH conditions in both dark and UVA radiation. The GC-nZVI catalyst was characterized using specific techniques (XPS, FTIR, BET, etc.), and was used to activate both PS and PMS in solution. Although no significant BPA degradation constant (kBPA) was determined using PS/PMS or GC-nZVI alone, a significant synergistic effect was observed in the system. Under dark conditions, kBPA increased from 0.01 min−1 to 0.17 when PS was used, both in the dark and under UVA. Notably, when PMS was present in the system, the improvement was even higher with kBPA reaching 0.18 and 0.26 min−1 under dark and UVA conditions, respectively. To further support the synergistic effect, it was found that 80% of mineralization was achieved within 2 h of UVA exposure.Chemical quenching experiments were conducted using selective probes and kinetic modelling showed that the activation of PS/PMS produced sulfate radicals (SO4•−), hydroxyl radicals (•OH) and ferryl ions (FeIVO2+) in both systems. Additionally, GC-nZVI/PMS system demonstrated good stability in recycling experiments, with up to 70% of BPA still being degraded after 3 cycled in 1 h under UVA.In conclusion, the degradation efficiencies of GC-nZVI/PMS and GC-nZVI/PS systems under dark and UVA radiation were assessed for three different EDCs (BPA, E2, and EE2) in both tap and sewage treatment plant waters.This study has demonstrated that the highly efficient GC-nZVI/PS/UVA or GC-nZVI/PMS/UVA system has significant potential for application in different water matrix, and our findings provide insights into the design of heterogeneous Fenton-like and photo-Fenton-like catalysts.

Stepped partially acoustic dark matter, large scale structure, and the Hubble tension

We propose a new interacting dark sector model, Stepped Partially Acoustic Dark Matter (SPartAcous), that can simultaneously address the two most important tensions in current cosmological data, the H0 and S8 problems. As in the Partially Acoustic Dark Matter (PAcDM) scenario, this model features a subcomponent of dark matter that interacts with dark radiation at high temperatures, suppressing the growth of structure at small scales and thereby addressing the S8 problem. However, in the SPartAcous model, the dark radiation includes a component with a light mass that becomes non-relativistic close to the time of matter-radiation equality. As this light component annihilates away, the remaining dark radiation heats up and its interactions with dark matter decouple. The heating up of the dark sector results in a step-like increase in the relative energy density in dark radiation, significantly reducing the H0 tension, while the decoupling of dark matter and dark radiation ensures that the power spectrum at larger scales is identical to ΛCDM.

Observational constraints on the modification of standard ΛCDM cosmology with dark radiation

We revisit a cosmological model modified by an extra “radiation-like” energy component in the Universe, which can be interpreted as many physical phenomena. Here, we derive the constraint on the energy density from this modification with current cosmological observations, including Cosmic Microwave Background, Baryon Acoustic Oscillation, Observational Hubble Parameter data and Type Ia supernovae. Performing a Bayesian analysis gives at 95% confidence level. Our work shows the dark radiation energy is small. Alleviating H 0 tension by introducing this term can also be interpreted as the systematic error caused by the numerical method.

Testing decaying dark matter models as a solution to the S8 tension with the thermal Sunyaev-Zel’dovich effect

Considering possible solutions to the S8 tension between the Planck cosmic microwave background (CMB) measurement and low-redshift probes, we extended the standard ΛCDM cosmological model by including decay of dark matter (DDM). We first tested the DDM model in which dark matter decays into a form of noninteracting dark radiation. Under this DDM model, we investigated the impacts of DDM on the Sunyaev Zel’dovich (SZ) effect by varying the decay lifetime, Γ−1, including the background evolution in cosmology and the nonlinear prescription in the halo mass function. We performed a cosmological analysis under the assumption of this extended cosmological model by combining the latest high-redshift Planck CMB measurement and low-redshift measurements of the SZ power spectrum as well as the baryonic acoustic oscillations (BAO) and luminosity distances to type Ia supernovae (SNIa). Our result shows a preference for Γ−1 ∼ 220 Gyr with a lower bound on the decay lifetime of ∼38 Gyr at 95% confidence level. Additionally, we tested the other DDM model in which dark matter decays into warm dark matter and dark radiation. This model supports Γ−1 ∼ 137 Gyr to resolve the S8 tension with a lower bound on the decay lifetime of ∼24 Gyr at 95% confidence level. Comparing these two models, we find that the second leads to slightly better reconciliation of the S8 tension.

Impact of high-scale Seesaw and Leptogenesis on inflationary tensor perturbations as detectable gravitational waves

We discuss the damping of inflationary gravitational waves (GW) that re-enter the horizon before or during an epoch, where the energy budget of the universe is dominated by an unstable right handed neutrino (RHN), whose out of equilibrium decay releases entropy. Starting from the minimal Standard Model extension, motivated by the observed neutrino mass scale, with nothing more than 3 RHN for the Seesaw mechanism, we discuss the conditions for high scale leptogenesis assuming a thermal initial population of RHN. We further address the associated production of potentially light non-thermal dark matter and a potential component of dark radiation from the same RHN decay. One of our main findings is that the frequency, above which the damping of the tensor modes is potentially observable, is completely determined by successful leptogenesis and a Davidson-Ibarra type bound to be at around 0.1 Hz. To quantify the detection prospects of this GW background for various proposed interferometers such as AEDGE, BBO, DECIGO, Einstein Telescope or LISA we compute the signal-to-noise ratio (SNR). This allows us to investigate the viable parameter space of our model, spanned by the mass of the decaying RHN {M}_1gtrsim 2.4times {10}^8textrm{GeV}cdot sqrt{2times {10}^{-7}textrm{eV}/{tilde{m}}_1} (for leptogenesis) and the effective neutrino mass parameterizing its decay width {tilde{m}}_1 < 2.9 × 10−7 eV (for RHN matter domination). Thus gravitational wave astronomy is a novel way to probe both the Seesaw and the leptogenesis scale, which are completely inaccessible to laboratory experiments in high scale scenarios.

Distinct signatures of spinning PBH domination and evaporation: doubly peaked gravitational waves, dark relics and CMB complementarity

Ultra-low mass primordial black holes (PBH), which may briefly dominate the energy density of the universe but completely evaporate before the big bang nucleosynthesis (BBN), can lead to interesting observable signatures. In our previous work, we studied the generation of a doubly peaked spectrum of induced stochastic gravitational wave background (ISGWB) for such a scenario and explored the possibility of probing a class of baryogenesis models wherein the emission of massive unstable particles from the PBH evaporation and their subsequent decay contributes to the matter-antimatter asymmetry. In this work, we extend the scope of our earlier work by including spinning PBHs and consider the emission of light relativistic dark sector particles, which contribute to the dark radiation (DR) and massive stable dark sector particles, thereby accounting for the dark matter (DM) component of the universe. The ISGWB can probe the non-thermal production of these heavy DM particles, which cannot be accessible in laboratory searches. For the case of DR, we find a novel complementarity between the measurements of ∆Neff from these emitted particles and the ISGWB from PBH domination. Our results indicate that the ISGWB has a weak dependence on the initial PBH spin. However, for gravitons as the DR particles, the initial PBH spin plays a significant role, and only above a critical value of the initial spin parameter a*, which depends only on initial PBH mass, the graviton emission can be probed in the CMB-HD experiment. Upcoming CMB experiments such as CMB-HD and CMB-Bharat, together with future GW detectors like LISA and ET, open up an exciting possibility of constraining the PBHs parameter space providing deeper insights into the expansion history of the universe between the end of inflation and BBN.

Thermal friction as a solution to the Hubble and large-scale structure tensions

Thermal friction offers a promising solution to the Hubble and the large-scale structure (LSS) tensions. This additional friction acts on a scalar field in the early universe and extracts its energy density into dark radiation, the cumulative effect being similar to that of an early dark energy (EDE) scenario. The dark radiation automatically redshifts at the minimal necessary rate to improve the Hubble tension. On the other hand, the addition of extra radiation to the Universe can mitigate the LSS tension. We explore this model in light of cosmic microwave background (CMB), baryon acoustic oscillation, and type Ia supernova data, including the SH0ES ${H}_{0}$ measurement and the Dark Energy Survey Y1 data release in our analysis. Our results indicate a preference for the regime where the scalar field converts to dark radiation at very high redshifts ($z\ensuremath{\gtrsim}{10}^{5}$), asymptoting effectively to an extra self-interacting radiation species rather than an EDE-like injection. In this limit, thermal friction can ease both the Hubble and the LSS tensions, but not resolve them. We find the source of this preference to be the incompatibility of the CMB data with the linear density perturbations of the dark radiation when injected at redshifts close to matter-radiation equality.

Millicharged relics reveal massless dark photons

The detection of massless kinetically-mixed dark photons is notoriously difficult, as the effect of this mixing can be removed by a field redefinition in vacuum. In this work, we study the prospect of detecting massless dark photons in the presence of a cosmic relic directly charged under this dark electromagnetism. Such millicharged particles, in the form of dark matter or dark radiation, generate an effective dark photon mass that drives photon-to-dark photon oscillations in the early universe. We also study the prospect for such models to alleviate existing cosmological constraints on massive dark photons, enlarging the motivation for direct tests of this parameter space using precision terrestrial probes.

The effect of stellar encounters on the dark matter annihilation signal from prompt cusps

ABSTRACT Prompt cusps are the densest quasi-equilibrium dark matter objects; one forms at the instant of collapse within every isolated peak of the initial cosmological density field. They have power-law density profiles, ρ ∝ r−1.5 with central phase-space density set by the primordial velocity dispersion of the dark matter. At late times, they account for $\sim 1~{{\ \rm per\ cent}}$ of the dark matter mass but for $\gt 90~{{\ \rm per\ cent}}$ of its annihilation luminosity in all but the densest regions, where they are tidally disrupted. Here we demonstrate that individual stellar encounters rather than the mean galactic tide are the dominant disruptors of prompt cusps within galaxies. Their cumulative effect is fully (though stochastically) characterized by an impulsive shock strength $B_* = 2\pi G\int \rho _*({\bf x}(t))\, \mathrm{d}t$ where ρ*, the total mass density in stars, is integrated over a cusp’s entire post-formation trajectory. Stellar encounters and mean tides have only a small effect on the halo annihilation luminosity seen by distant observers, but this is not true for the Galactic halo because of the Sun’s position. For a 100 GeV WIMP, Earth-mass prompt cusps are predicted, and stellar encounters suppress their mean annihilation luminosity by a factor of two already at 20 kpc, so that their annihilation emission is predicted to appear almost uniform over the sky. The Galactic centre γ-ray excess is thus unaffected by cusps. If it is indeed dark matter annihilation radiation, then prompt cusps in the outer Galactic halo and beyond must account for 20–80 per cent of the observed isotropic γ-ray background in the 1–10 GeV range.

Boosted dark matter from primordial black holes produced in a first-order phase transition

During a cosmological first-order phase transition in a dark sector, fermion dark matter particles χ can form macroscopic Fermi balls that collapse to primordial black holes (PBHs) under certain conditions. The evaporation of the PBHs produces a boosted χ flux, which may be detectable if χ couples to visible matter. We consider the interaction of χ with electrons, and calculate signals of the dark matter flux in the XENON1T, XENONnT, Super-Kamiokande and Hyper-Kamiokande experiments. A correlated gravitational wave signal from the phase transition can be observed at THEIA and μAres. An amount of dark radiation measurable by CMB-S4 is an epiphenomenon of the phase transition.

Multi-messenger constraints on Abelian-Higgs cosmic string networks

Nielsen-Olesen vortices in the Abelian-Higgs (AH) model are the simplest realisations of cosmic strings in a gauge field theory. Large-scale numerical solutions show that the dominant decay channel of a network of AH strings produced from random initial conditions is classical field radiation. However, they also show that with special initial conditions, loops of string can be created for which classical field radiation is suppressed, and which behave like Nambu-Goto (NG) strings with a dominant decay channel into gravitational radiation. This indicates that cosmic strings are generically sources of both high-energy particles and gravitational waves. Here we adopt a simple parametrisation of the AH string network allowing for both particle and gravitational wave production. With a reference to a specific model for NG-like loop distribution, this sets the basis for a “multi-messenger” investigation of this model. We find that, in order to explain the NANOGrav detection of a possible gravitational wave background, while satisfying the constraint on NG-like loop production from simulations and bounds from the cosmic microwave background, the tension of the AH string in Planck units Gμ and the fraction of the NG-like loops f NG should satisfy Gμ f NG 2.6 ≳ 3.2 × 10-13 at 95% confidence. On the other hand, for such string tensions, constraints from the diffuse gamma-ray background (DGRB) indicate that more than 97% of the total network energy should be converted to dark matter (DM) or dark radiation. We also consider joint constraints on the annihilation cross-section, the mass, and the relic abundance of DM produced by decays of strings. For example, for a DM mass of 500 GeV, the observed relic abundance can be explained by decaying AH strings that also account for the NANOGrav signal.

Resolution of Cosmological Singularity in Hořava–Lifshitz Cosmology

The standard ΛCDM model, despite its agreement with observational data, still has some issues unaddressed, such as the problem of initial singularity. Solving that problem usually requires modifications of general relativity. However, there appeared the Hořava–Lifshitz (HL) theory of gravity, in which equations governing cosmological evolution include a new term scaling similarly as the dark radiation term in the Friedmann equations, enabling a bounce of the universe instead of initial singularity. This review describes past works on the stability of such a bounce in different formulations of HL theory, an initial detailed balance scenario, and further projectable versions containing higher than quadratic terms to the original action.

Generation of neutrino dark matter, baryon asymmetry, and radiation after quintessential inflation

We construct a model explaining dark matter, baryon asymmetry, and reheating in a quintessential inflation model. Three generations of right-handed neutrinos having hierarchical masses and the light scalar field leading to the self-interaction of active neutrinos are introduced. The lightest sterile neutrino is a dark matter candidate produced by a Dodelson-Widrow mechanism in the presence of a new light scalar field, while the heaviest and next-heaviest sterile neutrinos produced by gravitational particle production are responsible for the generation of the baryon asymmetry. Reheating is realized by spinodal instabilities of the Standard Model Higgs field induced by the nonminimal coupling to the scalar curvature, which can solve the overproduction of gravitons and curvature perturbation created by the Higgs condensation.

Equality scale-based and sound horizon-based analysis of the Hubble tension

The Hubble horizon at matter-radiation equality (${k}_{\mathrm{eq}}^{\ensuremath{-}1}$) and the sound horizon at the last scattering surface [${r}_{s}({z}_{*})$] provides an interesting consistency check for the standard $\mathrm{\ensuremath{\Lambda}}$ Cold Dark Matter ($\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$) model and its extensions. It is well known that the reduction of ${r}_{s}$ can be compensated by the increase of ${H}_{0}$, while the same is true for the standard rulers ${k}_{\mathrm{eq}}$. Adding extra radiational component to the early Universe can reduce ${k}_{\mathrm{eq}}$. The addition of early dark energy (EDE), however, tends to increase ${k}_{\mathrm{eq}}$. We perform ${k}_{\mathrm{eq}}$- and ${r}_{s}$-based analyses in both the EDE model and the Wess-Zumino dark radiation (WZDR) model. In the latter case, we find $\mathrm{\ensuremath{\Delta}}{H}_{0}=0.4$ between the ${r}_{s}$- and ${k}_{\mathrm{eq}}$-based datasets, while in the former case, we find $\mathrm{\ensuremath{\Delta}}{H}_{0}=1.2$. This result suggests that the dark radiation scenario is more consistent in the fit of the two standard rulers (${k}_{\mathrm{eq}}$ and ${r}_{s}$). As a forecast analyses, we fit the two models with a mock ${k}_{\mathrm{eq}}$ prior derived from Planck best-fit $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ model. Compared with the best-fit ${H}_{0}$ in baseline $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ model, we find $\mathrm{\ensuremath{\Delta}}{H}_{0}=1.1$ for the WZDR model and $\mathrm{\ensuremath{\Delta}}{H}_{0}=\ensuremath{-}2.4$ for EDE model.

Early dark energy by a dark Higgs field and axion-induced nonthermal trapping

We propose a new scenario of early dark energy (EDE) with a dark Higgs field trapped at the origin. To keep this dark Higgs trapped until around the matter-radiation equality, we use dark photons produced nonthermally by coherent oscillations of axions, which have a much stronger trapping effect than thermal mass. When the trapping ends, the dark Higgs quickly decays into dark photons, which are then redshifted as radiation. In fact, dark photons are self-interacting dark radiation, and the dark Higgs field can also be in thermal equilibrium if its self-coupling is small enough. In some cases, these particles can act as the Wess-Zumino dark radiation. The dark Higgs EDE scenario works well for an ordinary Mexican-hat potential, and the dark Higgs naturally sits at the origin from the beginning, since it is the symmetry-enhanced point. Thus, unlike the axion EDE, there is no need for elaborate potentials or fine-tuning with respect to the initial condition. Interestingly, the axion not only produces dark photons, but also explains dark matter. We find the viable parameter region of the axion decay constant and the axion mass where dark matter and the ${H}_{0}$ tension can be simultaneously explained. We also discuss the detectability of the axion in the presence of axion-photon coupling, and show that the axion can be the QCD axion.

Primordial lepton asymmetries in the precision cosmology era: Current status and future sensitivities from BBN and the CMB

Using a new sample of extremely metal poor systems, the EMPRESS survey has recently reported a primordial helium abundance that is $3\sigma$ smaller than the prediction from the standard big bang nucleosynthesis (BBN) scenario. This measurement could be interpreted as a hint for a primordial lepton asymmetry in the electron neutrino flavor. Motivated by the EMPRESS results, we present a comprehensive analysis of the lepton asymmetry using measurements of the abundances of primordial elements, along with cosmic microwave background (CMB) data from Planck. Assuming that there is no dark radiation in our Universe, we find an electron neutrino chemical potential $\xi_{\nu_e} = 0.043 \pm 0.015$, which deviates from zero by $2.9\sigma$. If no assumption is made on the abundance of dark radiation in the Universe, the chemical potential is $\xi_{\nu_e} = 0.046 \pm 0.021$, which deviates from zero by $2.2\sigma$. We also find that this result is rather insensitive to the choice of nuclear reaction rates. If the true helium abundance corresponds to the EMPRESS central value, future CMB observations from the Simons Observatory and CMB-S4 will increase the significance for a nonzero lepton asymmetry to $4\sigma$ and $5\sigma$ respectively, assuming no dark radiation, or to $3\sigma$ when no assumption is made on the abundance of dark radiation.