Gravitational Waves (GW's) can determine the luminosity distance of the progenitor directly from the amplitude of the wave, without assuming any specific cosmological model. Thus, it can be considered as a standard siren. The coalescence of binary neutron stars (BNS) or neutron star-black hole pair (NSBH) can generate GW's as well as the electromagnetic counterpart, which can be detected in a form of Gamma-Ray Bursts (GRB) and can be used to determine the redshift of the source. Consequently, such a standard siren can be a very useful probe to constrain the cosmological parameters. In this work, we consider an interacting Dark Matter-Dark Energy (DM-DE) model. Assuming some fiducial values for the parameters of our model, we simulate the luminosity distance for a “realistic” and “optimistic” GW+GRB events , which can be detected by the third-generation GW detector Einstein Telescope (ET). Using these simulated events, we perform a Monte Carlo Markov Chain (MCMC) to constrain the DM-DE coupling constant and other model parameters in 1σ and 2σ confidence levels. We also investigate how GW's can improve the constraints obtained by current cosmological probes.

The lack of objects between 2 and 5 M ⊙ in the joint mass distribution of compact objects has been termed the “mass gap,” and attributed mainly to the characteristics of the supernova mechanism precluding their birth. However, recent observations show that a number of candidates reported to lie inside the “gap” may fill it, suggesting instead a paucity that may be real or largely a result of small number statistics. We quantify in this work the individual candidates and evaluate the joint probability of a mass gap. Our results show that an absolute mass gap is not present, to a very high confidence level. It remains to be seen if a relative paucity of objects stands in the future, and how this population can be related to the formation processes, which may include neutron star mergers, the collapse of a neutron star to a black hole, and others.

Binary population synthesis provides a direct way of studying the effects of different choices of binary evolution models and initial parameter distributions on present‐day compact binary merger populations, which can then be compared to empirical properties such as observed merger rates. Samples of zero‐age main sequence binaries to be evolved by such codes are typically generated from a universal initial mass function (IMF) and simple, uniform, distributions for orbital period P, mass ratio q, and eccentricity e. More recently, however, mounting observational evidence has suggested the non‐universality of the IMF and the existence of correlations between binary parameters. In this study, we implement a metallicity‐ and redshift‐dependent IMF alongside correlated distributions for P, q, and e to generate representative populations of binaries at varying redshifts, which are then evolved with the COMPAS code in order to study the variations in merger rates and overall population properties.

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