Universe

40 pages, 796 KiB

Open AccessReview

Energy-Momentum Squared Gravity: A Brief Overview

by Ricardo A. C. Cipriano, Nailya Ganiyeva, Tiberiu Harko, Francisco S. N. Lobo, Miguel A. S. Pinto and João Luís Rosa

Universe 2024, 10(9), 339; https://doi.org/10.3390/universe10090339 - 23 Aug 2024

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Abstract

In this work, we present a review of Energy-Momentum Squared Gravity (EMSG)—more specifically,

f (R, T_{μ ν} T^{μ ν})

gravity, where R represents the Ricci scalar and

T_{μ ν}

denotes the energy-momentum tensor. The inclusion of quadratic [...] Read more.

In this work, we present a review of Energy-Momentum Squared Gravity (EMSG)—more specifically,

f (R, T_{μ ν} T^{μ ν})

gravity, where R represents the Ricci scalar and

T_{μ ν}

denotes the energy-momentum tensor. The inclusion of quadratic contributions from the energy-momentum components has intriguing cosmological implications, particularly during the Universe’s early epochs. These effects dominate under high-energy conditions, enabling EMSG to potentially address unresolved issues in General Relativity (GR), such as the initial singularity and aspects of big-bang nucleosynthesis in certain models. The theory’s explicit non-minimal coupling between matter and geometry leads to the non-conservation of the energy-momentum tensor, which prompts the investigation of cosmological scenarios through the framework of irreversible thermodynamics of open systems. By employing this formalism, we interpret the energy-balance equations within EMSG from a thermodynamic perspective, viewing them as descriptions of irreversible matter creation processes. Since EMSG converges to GR in a vacuum and differences emerge only in the presence of an energy-momentum distribution, these distinctions become significant in high-curvature regions. Therefore, deviations from GR are expected to be pronounced in the dense cores of compact objects. This review delves into these facets of EMSG, highlighting its potential to shed light on some of the fundamental questions in modern cosmology and gravitational theory. Full article

(This article belongs to the Collection Modified Theories of Gravity and Cosmological Applications)

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14 pages, 296 KiB

Open AccessArticle

Gravitational Particle Production and the Hubble Tension

by Recai Erdem

Universe 2024, 10(9), 338; https://doi.org/10.3390/universe10090338 - 23 Aug 2024

Viewed by 163

Abstract

The effect of gravitational particle production of scalar particles on the total effective cosmic energy density (in the era after photon decoupling till the present) is considered. The effect is significant for heavy particles. It is found that gravitational particle production results in [...] Read more.

H_{0}

, while it does not affect the value of the Hubble constant in the calculation of the number density of baryons at the present time that is used to calculate recombination redshift. This may explain why the Hubble constants determined by local measurements and non-local measurements (such as CMB) are different. This suggests that gravitational particle production may have a non-negligible impact on

H_{0}

tension. Full article

(This article belongs to the Special Issue Current Status of the Hubble Tension)

26 pages, 9390 KiB

Open AccessArticle

Minute-Cadence Observations of the LAMOST Fields with the TMTS: IV—Catalog of Cataclysmic Variables from the First 3-yr Survey

by Qichun Liu, Jie Lin, Xiaofeng Wang, Zhibin Dai, Yongkang Sun, Gaobo Xi, Jun Mo, Jialian Liu, Shengyu Yan, Alexei V. Filippenko, Thomas G. Brink, Yi Yang, Kishore C. Patra, Yongzhi Cai, Zhihao Chen, Liyang Chen, Fangzhou Guo, Xiaojun Jiang, Gaici Li, Wenxiong Li, Weili Lin, Cheng Miao, Xiaoran Ma, Haowei Peng, Qiqi Xia, Danfeng Xiang and Jicheng Zhang Show full author list Hide full author list

Universe 2024, 10(9), 337; https://doi.org/10.3390/universe10090337 - 23 Aug 2024

Viewed by 248

Abstract

The Tsinghua University–Ma Huateng Telescopes for Survey (TMTS) started to monitor the LAMOST plates in 2020, leading to the discovery of numerous short-period eclipsing binaries, peculiar pulsators, flare stars, and other variable objects. Here, we present the uninterrupted light curves for a sample of 64 cataclysmic variables (CVs) observed/discovered using the TMTS during its first three-year observations, and we introduce new CVs and new light-variation periods (from known CVs) revealed through the TMTS observations. Thanks to the high-cadence observations of TMTS, diverse light variations, including superhumps, quasi-periodic oscillations, large-amplitude orbital modulations, and rotational modulations, are able to be detected in our CV samples, providing key observational clues for understanding the fast-developing physical processes in various CVs. All of these short-timescale light-curve features help further classify the subtypes of CV systems. We highlight the light-curve features observed in our CV sample and discuss further implications of minute-cadence light curves for CV identifications and classifications. Moreover, we examine the Hα emission lines in the spectra from our nonmagnetic CV samples (i.e., dwarf novae and nova-like subclasses) and find that the distribution of Hα emission strength shows significant differences between the sources with orbital periods above and below the period gap, which agrees with the trend seen from the SDSS nonmagnetic CV sample. Full article

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16 pages, 2282 KiB

Open AccessArticle

Hybrid Isentropic Twin Stars

by Juan Pablo Carlomagno, Gustavo A. Contrera, Ana Gabriela Grunfeld and David Blaschke

Universe 2024, 10(9), 336; https://doi.org/10.3390/universe10090336 - 23 Aug 2024

Viewed by 182

Abstract

We present a study of hybrid neutron stars with color superconducting quark matter cores at a finite temperature that results in sequences of stars with constant entropy per baryon,

s / n_{B} = const

. For the quark matter equation of state, [...] Read more.

We present a study of hybrid neutron stars with color superconducting quark matter cores at a finite temperature that results in sequences of stars with constant entropy per baryon,

s / n_{B} = const

. For the quark matter equation of state, we employ a recently developed nonlocal chiral quark model, while nuclear matter is described with a relativistic density functional model of the DD2 class. The phase transition is obtained through a Maxwell construction under isothermal conditions. We find that traversing the mixed phase on a trajectory at low

s / n_{B} ≲ 2

in the phase diagram shows a heating effect, while at larger

s / n_{B}

the temperature drops. This behavior may be attributed to the presence of a color superconducting quark matter phase at low temperatures and the melting of the diquark condensate which restores the normal quark matter phase at higher temperatures. While the isentropic hybrid star branch at low

s / n_{B} ≲ 2

is connected to the neutron star branch, it becomes disconnected at higher entropy per baryon so that the “thermal twin” phenomenon is observed. We find that the transition from connected to disconnected hybrid star sequences may be estimated with the Seidov criterion for the difference in energy densities. The radii and masses at the onset of deconfinement exhibit a linear relationship and thus define a critical compactness of the isentropic star configuration for which the transition occurs and which, for large enough

s / n_{B} ≳ 2

values, is accompanied by instability. The results of this study may be of relevance for uncovering the conditions for the supernova explodability of massive blue supergiant stars using the quark deconfinement mechanism. The accretion-induced deconfinement transition with thermal twin formation may contribute to explaining the origin of eccentric orbits in some binary systems and the origin of isolated millisecond pulsars. Full article

(This article belongs to the Special Issue Studies in Neutron Stars)

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Universe, Volume 10, Issue 9 (September 2024) – 4 articles

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