In this paper, we investigate the properties of relativistic stars made of isotropic matter within the framework of the minimal Standard Model Extension, where a Bumblebee field (BF) coupled to spacetime induces spontaneous Lorentz symmetry breaking. We adopt analytic equations-of-state describing either condensate dark stars or strange quark stars. We solve the structure equations numerically, and we compute the mass-to-radius relationships. The influence of the Bumblebee parameter <bold>l</bold> is examined in detail, and an upper bound is obtained using the massive pulsar (PSR) J0740+6620 and the strangely light High Energy Stereoscopic System (HESS) J1731-347 compact object.

Chile

Grigoris Panotopoulos

Türkiye

Ali Övgün

Strange quark stars and condensate dark stars in Bumblebee gravity

We investigate the structure and radial oscillations of anisotropic compact stars composed of dark energy, using the vanishing complexity factor formalism within general relativity. This novel approach establishes a direct link between the energy density and anisotropic factor, providing a robust framework for studying these exotic stellar objects. Employing an Extended Chaplygin Gas equation of state, we numerically compute interior solutions for both isotropic and anisotropic stars, revealing distinct differences in their properties. Additionally, we examine the oscillation modes and frequencies of these stars, highlighting the impact of anisotropy on their pulsational behaviour. Our results reveal distinct differences in the stellar properties, such as the metric potentials, pressure, speed of sound, and relativistic adiabatic index, between the two cases. Furthermore, we calculate the large frequency separation for the fundamental and first excited modes, offering insights relevant for future asteroseismology studies. Our findings shed light on the complex interplay of gravity, matter, and anisotropy in compact stars, providing a new perspective on dark energy's role in their structure and dynamics.

Spain

Ángel Rincón

Portugal

Ilídio Lopes

Anisotropic dark energy stars within vanishing complexity factor formalism: Hydrostatic equilibrium, radial oscillations, and observational implications

We investigate the peculiar nature of strange stars through an analysis of different quark models, i.e. vBag model and CFL model equation of states at different parameter sets, and focus on understanding the equation of state governing the intriguing central compact object (CCO) within the supernova remnant HESS J1731-347, with a mass and radius of  $$M = 0.77^{+0.20}_{-0.17} M_{\odot }$$  <math> <mrow> <mi>M</mi> <mo>=</mo> <mn>0</mn> <mo>.</mo> <msubsup> <mn>77</mn> <mrow> <mo>-</mo> <mn>0.17</mn> </mrow> <mrow> <mo>+</mo> <mn>0.20</mn> </mrow> </msubsup> <msub> <mi>M</mi> <mo>⊙</mo> </msub> </mrow> </math>   and  $$R = 10.4^{+0.86}_{-0.78}$$  <math> <mrow> <mi>R</mi> <mo>=</mo> <mn>10</mn> <mo>.</mo> <msubsup> <mn>4</mn> <mrow> <mo>-</mo> <mn>0.78</mn> </mrow> <mrow> <mo>+</mo> <mn>0.86</mn> </mrow> </msubsup> </mrow> </math>   km, respectively. Additionally, we compare the radial oscillations of two models to determine the frequency of the HESS J1731-347 compact object at its maximum mass. The frequencies of radial oscillations are computed for each of the four EoSs considered. In total, the 10 lowest radial frequencies for each of those EoSs have been computed. By delving into these aspects, we aim at deepening our understanding of strange stars and their connection to the observed HESS J1731-347 mass-radius relationship.

Ishfaq Rather

Quark models and radial oscillations: decoding the HESS J1731-347 compact object’s equation of state

Within Einstein’s General Relativity we study exotic stars made of dark energy assuming an extended Chaplygin gas equation-of-state. Taking into account the presence of anisotropies, we employ the formalism based on the complexity factor to solve the structure equations numerically, obtaining thus interior solutions describing hydrostatic equilibrium. Making use of well-established criteria we demonstrate that the solutions are well behaved and realistic. A comparison with another, more conventional approach, is made as well.

Anisotropic stars made of exotic matter within the complexity factor formalism

We study cosmological inflation and its dynamics in the framework of the Randall–Sundrum II brane model. In particular, we analyze in detail four representative small-field inflationary potentials, namely Natural inflation, Hilltop inflation, Higgs-like inflation, and Exponential SUSY inflation, each characterized by two mass scales. We constrain the parameters for which a viable inflationary Universe emerges using the latest PLANCK results. Furthermore, we investigate whether or not those models in brane cosmology are consistent with the recently proposed Swampland Criteria, and give predictions for the duration of reheating as well as for the reheating temperature after inflation. Our results show that (i) the distance conjecture is satisfied, (ii) the de Sitter conjecture and its refined version may be avoided, and (iii) the allowed range for the five-dimensional Planck mass,  $$M_5$$  <math> <msub> <mi>M</mi> <mn>5</mn> </msub> </math>  , is found to be  $$10^5\,\text {TeV}\lesssim M_5\lesssim 10^{12}\,\text {TeV}$$  <math> <mrow> <msup> <mn>10</mn> <mn>5</mn> </msup> <mspace width="0.166667em"></mspace> <mtext>TeV</mtext> <mo>≲</mo> <msub> <mi>M</mi> <mn>5</mn> </msub> <mo>≲</mo> <msup> <mn>10</mn> <mn>12</mn> </msup> <mspace width="0.166667em"></mspace> <mtext>TeV</mtext> </mrow> </math>  . Our main findings indicate that non-thermal leptogenesis cannot work within the framework of RS-II brane cosmology, at least for the inflationary potentials considered here.

Constanza Osses

Nelson Videla

Reheating in small-field inflation on the brane: the swampland criteria and observational constraints in light of the PLANCK 2018 results

In the present paper, we treat the problem of the existence of quark stars (QSs) for selected homogeneous and unpaired charge-neutral 3-flavor interacting quark matter with <math><mi>O</mi><mo>(</mo><msubsup><mrow><mi>m</mi></mrow><mrow><mi>s</mi></mrow><mrow><mn>4</mn></mrow></msubsup><mo>)</mo></math> corrections equations of state (EoS). Using the EoS combined with the Tolmann-Oppenheimer-Volkoff (TOV) structure equations, the properties of stars are explored by obtaining their mass-radius relations. All calculations are carried out within the framework of the R-squared gravity defined by <math><mi>f</mi><mo>(</mo><mi>R</mi><mo>)</mo><mo>=</mo><mi>R</mi><mo>+</mo><mi>a</mi><msup><mrow><mi>R</mi></mrow><mrow><mn>2</mn></mrow></msup></math>. Our main goal is to discuss the effect for a wide range of the R-squared gravity parameter, a, on the mass-radius and the mass-central mass density <math><mo>(</mo><mi>M</mi><mo>−</mo><msub><mrow><mi>ϵ</mi></mrow><mrow><mi>c</mi></mrow></msub><mo>)</mo></math> relation of QSs. Furthermore, we investigate the dynamical stability condition for those stars, and we show that their dynamical stability depends on the anisotropic parameters <math><msub><mrow><mi>B</mi></mrow><mrow><mo>⊥</mo></mrow></msub></math> and <math><msubsup><mrow><mi>a</mi></mrow><mrow><mn>4</mn></mrow><mrow><mo>⊥</mo></mrow></msubsup></math> coming from the respective EoS. In such a scenario our results provide circumstantial evidence in favor of super-massive pulsars.

Thailand

Takol Tangphati

South Africa

Ayan Banerjee

Oman

M.K. Jasim

Anisotropic quark stars in Gravity

We obtain well behaved interior solutions describing hydrostatic equilibrium of anisotropic relativistic stars in scale-dependent gravity, where Newton’s constant is allowed to vary with the radial coordinate throughout the star. Assuming (1) a linear equation-of-state in the MIT bag model for quark matter, and (2) a certain profile for the energy density, we integrate numerically the generalized structure equations, and we compute the basic properties of the strange quark stars, such as mass, radius and compactness. Finally, we demonstrate that stability criteria as well as the energy conditions are fulfilled. Our results show that a decreasing Newton’s constant throughout the objects leads to slightly more massive and more compact stars.

Interior solutions of relativistic stars with anisotropic matter in scale-dependent gravity

We compute the lowest frequency radial oscillation modes of boson stars. It is assumed that the object is made of pseudo-Goldstone bosons subjected to a scalar potential that leads to a repulsive self-interaction force, and which is characterized by two unknown mass scales m (mass of the particle) and F (decay constant). First we integrate the Tolman-Oppenheimer-Volkoff equations for the hydrostatic equilibrium of the star, and then we solve the Sturm-Liouville boundary value problem for the perturbations using the shooting method. The effective potential that enters into the Schrödinger-like equation as well as several associated eigenfunctions are shown as well. Moreover, we found that the large frequency separation, i.e. the difference between consecutive modes, is proportional to the square root of the mass of the star and the cube of the mass scale defined by <math><mi>Λ</mi><mo>≡</mo><msqrt><mrow><mi>m</mi><mi>F</mi></mrow></msqrt></math>.

Radial oscillations of boson stars made of ultralight repulsive dark matter

We study three interacting dark energy models within the framework of four-dimensional General Relativity and a spatially flat Universe. In particular, we first consider two vacuum models where dark energy interacts with dark matter, while relativistic matter as well as baryons are treated as non-interacting fluid components. Secondly, we investigate a third model where the gravitational coupling is assumed to be a slowly-varying function of the Hubble rate and dark energy and dark matter interact as well. We compute the statefinders parameters versus red-shift as well as the critical points and their nature applying dynamical systems methods. In the case of only an interaction term, our main findings indicate that (i) significant differences between the models are observed as we increase the strength of the interaction term, and (ii) all the models present an unique attractor corresponding to acceleration. On the other hand, when we allow for a variable gravitational coupling, we find that (i) the deviation from the concordance model depends of both the strength of gravitational coupling parameter and the interaction term, and (ii) there is an unique attractor corresponding to acceleration.

Giovanni Otalora

Dynamical systems methods and statender diagnostic of interacting vacuum energy models

We study chaotic inflation with a Galileon-like self-interaction $$G(\phi ,X)\Box \phi $$ <math><mrow><mi>G</mi><mo>(</mo><mi>ϕ</mi><mo>,</mo><mi>X</mi><mo>)</mo><mo>□</mo><mi>ϕ</mi></mrow></math> , where $$G(\phi ,X)\propto X^{n}$$ <math><mrow><mi>G</mi><mrow><mo>(</mo><mi>ϕ</mi><mo>,</mo><mi>X</mi><mo>)</mo></mrow><mo>∝</mo><msup><mi>X</mi><mi>n</mi></msup></mrow></math> . General conditions required for successful inflation are deduced and discussed from the background and cosmological perturbations under slow-roll approximation. Interestingly, it is found that in the regime where the Galileon term dominates over the standard kinetic term, the tensor-to-scalar ratio becomes significantly suppressed in comparison to the standard expression in General Relativity (GR). Particularly, we find the allowed range in the space of parameters characterizing the chaotic quadratic and quartic inflation models by considering the current observational data of Planck from the $$n_{\mathcal {S}}-r$$ <math><mrow><msub><mi>n</mi><mi>S</mi></msub><mo>-</mo><mi>r</mi></mrow></math>  plane. Finally, we discuss about the issue if the Galileon term is dominant by the end of inflation, this can affect the field oscillation during reheating.