International Journal of Modern Physics D

The Laser Interferometer Space Antenna (LISA) mission, scheduled for launch in the mid-2030s, is a gravitational wave space observatory designed to detect sources emitting in the millihertz band. LISA is an European Space Agency flagship mission, currently entering the Phase-B development phase. It is expected to help us improve our understanding of our Universe by measuring gravitational wave sources of different types, with some of the sources being at very high redshifts ($z\sim 20$). On February 23, 2022, we organized the first LISA in Greece Workshop. This Workshop aimed to inform the Greek scientific and tech industry communities about the possibilities of participating in LISA science and the LISA mission, with the support of the Hellenic Space Center. In this White Paper, we summarize the outcomes of the Workshop, the most important aspect of it being the inclusion of $15$ Greek researchers to the LISA Consortium, raising our total number to $22$. At the same time, we present a road map with the future steps and actions of the Greek Gravitational Wave community with respect to the LISA mission.

We study inflation in scalar–tensor perturbative quantum gravity driven by a one-loop effective potential. We consider effective potentials generated by three models. The first model describes a single scalar field with a nonvanishing mass. The second model describes a massless scalar field with nonminimal coupling to the Einstein tensor. The third model generalizes the Coleman–Weinberg model for the gravitational case. The first model can be consistent with the observational data for $N\sim 70$ e-foldings. The second model can be consistent with the observational data for $N\sim 40$ e-foldings. We did not find parameters that make the generalized Coleman–Weinberg model consistent with the observational data. We discuss the implications of these results and ways to improve them with other terms of effective action.

An explanation of the nature of dark energy has been treated in extra dimensions within the scheme of string theory. One of the most successful models is inspired by the Dvali–Gabadadze–Porrati (DGP) model, in which the universe is a 4-dimensional brane embedded in a 5-dimensional Minkowski spacetime. In this landscape, the study of the evolution of the normal branch has led us to different kinds of dark energy, where the most simple case is the cosmological constant $\mathrm{\Lambda }$. Moreover, other viable cosmological solutions are related to agegraphic dark energy, which allows a late cosmic acceleration within an interacting mechanism. To explore the viability of these solutions and possible gravitational leakage, in this paper, we present constraints on such models using recent standard sirens forecasting in addition to local observables such as Pantheon (SNIa), $H(z)$ measurements, baryonic acoustic oscillations (BAO). Our results show that the value associated with the species of quantum fields n in these models is strongly restricted for supernovae observations to $n=20$, and for GW standard sirens mock data prefers a value of $n=1$.

In this paper, we compute the black hole entropy and the entanglement entropy of Hawking radiations due to photons during the evaporation of a 4D asymptotically flat Kerr black hole. The Page curve for the Kerr black hole is obtained in the original way à la Page, and it qualitatively mimics the curve for the Schwarzschild black hole but has some new features.

In this work, we have proposed a simple parametrization for the pressure component $p(z)$ of the dark energy model and have studied the cosmological implications of this model in the framework of $f(Q)$ modified gravity theory, aka, the symmetric teleparallel gravity theory, where Q is known as the nonmetricity scalar. By considering a particular parametric form of $p(z)$, we obtained the Hubble solution for the $f(Q)$ modified gravity model. In order to see whether this model is consistent with or challenges the $\mathrm{\Lambda }$CDM limits, we tried to put constraints on the model parameters using the recent observational datasets like Hubble data, Cosmic Chronometer data, Type Ia Supernovae (SNIa) data, baryonic acoustic oscillations (BAO) data. We have employed the ${\chi }^2$ minimization technique and have carried out the Markov Chain Monte Carlo (MCMC) analysis using emcee package. We have found that the deceleration parameter shows a smooth transition from positive to negative value in recent past which is essential for the structure formation of the Universe. It has been found that the parametric form of the dark energy pressure parameter is consistent with current cosmological scenario.

In this work, we study the dynamical evolution of a homogeneous and anisotropic, noncommutative (NC) Bianchi I (BI) model coupled to a radiation perfect fluid. Our first motivation is determining if the present model tends to a homogeneous and isotropic NC Friedmann–Robertson–Walker (FRW) model, during its evolution. In order to simplify our task, we use the Misner parametrization of the BI metric. In terms of that parametrization the BI metric has three metric functions: the scale factor $a(t)$ and the two parameters ${\beta }_{\pm }(t)$, which measure the spatial anisotropy of the model. Our second motivation is trying to describe the present accelerated expansion of the universe using noncommutativity (NCTY). The NCTY is introduced by two nontrivial Poisson brackets between some geometrical as well as matter variables of the model. We recover the description in terms of commutative variables by introducing some variables transformations that depend on the NC parameter. Using those variables’ transformations, we rewrite the total NC Hamiltonian of the model in terms of commutative variables. From the resulting Hamiltonian, we obtain the dynamical equations for a generic perfect fluid. In order to solve these equations, we restrict our attention to a model in which the perfect fluid is radiation. We solve, numerically, these equations and compare the NC solutions to the corresponding commutative ones. The comparison shows that the NC model may be considered as a possible candidate for describing the accelerated expansion of the universe. Finally, we obtain estimates for the NC parameter and compare the main results of the NC BI model coupled to radiation with the same NC BI model coupled to other perfect fluids. As our main result, we show that the solutions, after some time, produce an isotropic universe. Based on that result, we can speculate that the solutions may represent an initial anisotropic stage of our Universe, that later, due to the expansion, became isotropic.