In this talk, I will introduce our work interpreting the PTA background as originating from SMBH binaries. Focusing on its interesting statistical properties, the implications for the environmental effects, possible eccentricity signatures and anisotropies in the background that could lead to differentiate it from a cosmological source.
Anisotropies play a central role in distinguishing between cosmological and astrophysical sources of the GWB, as detectable anisotropies are expected for a GWB from a population of supermassive black hole binaries (SMBHBs) but not for cosmological sources. We perform searches for anisotropies on simulated PTA datasets, showing that null detections for anisotropies in both current and...
The evidence for a stochastic gravitational wave background at nHz frequencies by Pulsar Timing Array (PTA) observations offers an opportunity to discover cosmological signals and threatens the observability of other subdominant gravitational waves (GWs). We explore prospects to constrain scalar-induced GWs associated with enhanced curvature perturbations in the primordial universe,...
Strong first-order phase transitions offer a compelling explanation for the stochastic gravitational wave background in the nano-Hertz range measured by pulsar timing arrays (PTA). In this talk, I will consider a classically conformal dark sector in which the breaking of a dark U(1) gauge symmetry gives rise to a gravitational wave background that can fit the PTA data and additionally sources...
In the NANOGrav 15-year New Physics analysis (arXiv:2306.16219), a log-uniform prior on $G\mu$ was imposed to determine upper limits on the tension of stable cosmic strings. Here, we examine the prior dependence of this bound through comparison with new upper limits obtained using a uniform prior on $G\mu$. New posterior distributions and upper limits on stable cosmic string tension were...
Ultra-light dark matter (ULDM) is a promising candidate for cosmological dark matter. If ULDM exists and interacts directly with ordinary matter, it could produce characteristic signals in the timing data of pulsars in binary systems.
In this talk, we review the latest results on how binary pulsars can constrain the ULDM parameter space through pulsar timing data and signal modeling. Our...
A finite axion-nucleon coupling enables the production of axions in stellar environments via the thermal excitation and subsequent de-excitation of $^{57}$Fe isotopes.
Given its low-lying excited state at 14.4 keV, $^{57}$Fe can be efficiently excited in the hot cores of supergiant stars, leading to axion emission. If these axions convert into photons in the Galactic magnetic field, they...
The hot interiors of massive stars in the later stages of their evolution provide an ideal place for the production of heavy axion-like particles (ALPs) with mass up to O(100 keV) range. We show that a fraction of these ALPs could stream out of the stellar photosphere and subsequently decay into two photons that can be potentially detected on or near the Earth. In particular, we estimate the...
Dark matter can be captured in stars and annihilate, providing the star with a new energy source in addition to nuclear fusion. This significantly changes stellar evolution at the Galactic Center, where the dark matter density is extremely high. As dark matter burning replaces nuclear fusion partially or completely, stars become longer-lived, as they use up hydrogen more conservatively, or...
Primordial black holes (PBHs) in the asteroid-mass range remain a viable and until now unconstrained dark matter (DM) candidate. If these PBHs exist, they could be captured by stars in DM-dominated environments such as dwarf galaxies. The capture probability increases with the stellar mass, and captured PBHs would rapidly destroy their host stars. Using photometric observations from the Hubble...
Neutrinos, despite their weak interactions, play an important role in core-collapse supernova evolution. In the supernova core, the neutrino number density is so high that the coherent forward scattering among neutrinos leads to flavor conversion, a phenomenon that can alter both the supernova explosion dynamics and nucleosynthesis. In this talk, I will discuss how to identify the necessary...
We search for an excess of electrons and positrons in the interplanetary space from the decays of heavy neutrinos produced in nuclear reactions in the Sun. Using measurements of the electron spectra in the MeV range from the Ulysses and SOHO satellites, we report the strongest direct upper bound to date on the mixing between heavy neutral leptons with MeV masses and electron neutrinos,...
The Hubble Tension is the discrepancy between the measured value of the Hubble parameter $H_0$ and its $\Lambda$CDM model prediction using CMB data. New Early Dark Energy (NEDE) addresses this tension using a triggered phase transition in the dark sector. In this work we constrain the properties of NEDE using recent datasets. We study the equation of state parameter, characterizing the...
We employ Maximum Likelihood Estimators to examine the Pantheon+ catalogue of Type Ia supernovae for large scale anisotropies in the expansion rate of the Universe. The analyses are carried out in the heliocentric frame, the CMB frame, as well as the Local Group frame. In all frames, the Hubble expansion rate in the redshift range 0.023 < z < 0.15 is found to have a statistically significant...
The observation of galaxies and gravitational waves (GW) emitted by dark sirens provides two different and complementary measures of distance, respectively redshift and luminosity distance. Under the assumption that both dark sirens and galaxies trace the distribution of matter up to some bias parameters, it is possible to infer cosmological parameters by cross-correlating their density...
We briefly present the late-time cosmological effects of a modified theory of gravity with a non-minimal coupling between curvature and matter. By evolving the cosmological parameters that match the cosmic microwave background data until their values from direct late-time measurements, we can obtain an agreement between different experimental methods without disrupting their individual...
The ultralight dark photon is a well-motivated, hypothetical dark matter candidate. In a dilute plasma, they can resonantly convert into photons, and heat up the intergalactic medium between galaxies. In this talk, we explore the dark photon dark matter parameter space by comparing synthetic Lyman-α forest data from cosmological hydrodynamical simulations to observational data from VLT/UVES of...
While searches for neutrino isocurvature usually constrain a specific linear combination of isocurvature perturbations, this talk focusses on realistic cosmological scenarios giving rise to neutrino isocurvature - mostly in the form of dark radiation. In general both, neutrino and dark matter isocurvature perturbations are generated, whose ratio can be parameterised by a newly introduced...
