Pub Date : 2026-01-09DOI: 10.1016/j.dark.2026.102211
Abdul Malik Sultan , Abdul Jawad , Hamood Ur Rehman , N. Myrzakulov , Sanjar Shaymatov
In this article, we investigate the gravitational baryogenesis mechanism in the framework of Extended Proca-Nuevo gravity, a theory where a massive vector field is non-minimally coupled to the curvature. This analysis is carried out for an early universe, encompassing three separate cosmological scenarios defined by power-law, exponential, and modified exponential scale factors. By deriving the modified field equations from the EPN action, we obtain precise solutions for each scale factor, including the influence of the vector field. We compute the baryon-to-entropy ratio using the gravitational baryogenesis formalism, where the baryon asymmetry arises from a dynamical coupling of the baryon current with the derivative of the Ricci scalar. Moreover, we discuss the stability of the assumed cosmological model in two different ways, the dynamical system analysis and squared speed of sound parameter. Our findings demonstrate that the baryon-to-entropy ratio is consistent with observational constraints in all scenarios, highlighting the potential of EPN gravity as a viable theory to explain the matter-antimatter asymmetry in the early universe. Moreover, stability analysis through dynamical system leads to a deSitter stability which is a trivial case while squared speed of sound gives that system is stable for all three scale factor models. The study further stresses the contribution of anisotropy and vector field dynamics to the cosmological evolution within modified gravity models.
{"title":"Feasible regions of GB phenomenon in extended Proca-Nuevo gravity","authors":"Abdul Malik Sultan , Abdul Jawad , Hamood Ur Rehman , N. Myrzakulov , Sanjar Shaymatov","doi":"10.1016/j.dark.2026.102211","DOIUrl":"10.1016/j.dark.2026.102211","url":null,"abstract":"<div><div>In this article, we investigate the gravitational baryogenesis mechanism in the framework of Extended Proca-Nuevo gravity, a theory where a massive vector field is non-minimally coupled to the curvature. This analysis is carried out for an early universe, encompassing three separate cosmological scenarios defined by power-law, exponential, and modified exponential scale factors. By deriving the modified field equations from the EPN action, we obtain precise solutions for each scale factor, including the influence of the vector field. We compute the baryon-to-entropy ratio using the gravitational baryogenesis formalism, where the baryon asymmetry arises from a dynamical coupling of the baryon current with the derivative of the Ricci scalar. Moreover, we discuss the stability of the assumed cosmological model in two different ways, the dynamical system analysis and squared speed of sound parameter. Our findings demonstrate that the baryon-to-entropy ratio is consistent with observational constraints in all scenarios, highlighting the potential of EPN gravity as a viable theory to explain the matter-antimatter asymmetry in the early universe. Moreover, stability analysis through dynamical system leads to a deSitter stability which is a trivial case while squared speed of sound gives that system is stable for all three scale factor models. The study further stresses the contribution of anisotropy and vector field dynamics to the cosmological evolution within modified gravity models.</div></div>","PeriodicalId":48774,"journal":{"name":"Physics of the Dark Universe","volume":"51 ","pages":"Article 102211"},"PeriodicalIF":6.4,"publicationDate":"2026-01-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"146022828","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-01-08DOI: 10.1016/j.dark.2026.102213
Taishi Katsuragawa , Shin’ichi Nojiri , Sergei D. Odintsov
We study wormhole geometries embedded in an expanding universe within a four-scalar non-linear σ model, where the target-space metric is identified with the spacetime Ricci tensor. In this framework, wormholes can remain stable even when conventional energy conditions are violated. However, once cosmological expansion is included, the effective energy density and pressure are modified by the cosmological fluid, enabling the energy conditions to be satisfied. We further present intriguing geometries in which a finite future singularity appears in our universe but not in another universe connected by the wormhole. Near the throat, the hypersurface becomes timelike, allowing trajectories to traverse to the other universe before the singularity and return afterwards. We also construct wormhole solutions motivated by galactic dark-matter halo profiles, where the required non-vanishing pressure arises naturally from the four-scalar non-linear σ model.
{"title":"Wormhole spacetimes in an expanding universe: Energy conditions and future singularities","authors":"Taishi Katsuragawa , Shin’ichi Nojiri , Sergei D. Odintsov","doi":"10.1016/j.dark.2026.102213","DOIUrl":"10.1016/j.dark.2026.102213","url":null,"abstract":"<div><div>We study wormhole geometries embedded in an expanding universe within a four-scalar non-linear <em>σ</em> model, where the target-space metric is identified with the spacetime Ricci tensor. In this framework, wormholes can remain stable even when conventional energy conditions are violated. However, once cosmological expansion is included, the effective energy density and pressure are modified by the cosmological fluid, enabling the energy conditions to be satisfied. We further present intriguing geometries in which a finite future singularity appears in our universe but not in another universe connected by the wormhole. Near the throat, the hypersurface becomes timelike, allowing trajectories to traverse to the other universe before the singularity and return afterwards. We also construct wormhole solutions motivated by galactic dark-matter halo profiles, where the required non-vanishing pressure arises naturally from the four-scalar non-linear <em>σ</em> model.</div></div>","PeriodicalId":48774,"journal":{"name":"Physics of the Dark Universe","volume":"51 ","pages":"Article 102213"},"PeriodicalIF":6.4,"publicationDate":"2026-01-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145977318","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-01-07DOI: 10.1016/j.dark.2025.102204
Adeela Afzal
In this work, we propose a novel realization of type-I cosmic strings arising from the spontaneous breaking of an extended gauge symmetry in the context of a low-scale split seesaw mechanism for neutrino mass generation. We demonstrate that the split seesaw framework, which explains the smallness of neutrino masses, naturally motivates a small scalar self-coupling λ. This intrinsically links the neutrino mass generation mechanism to the formation of type-I cosmic strings, where the gauge coupling dominates over the scalar self-coupling (β ≡ λ/(2g2) < 1). We explore the cosmological implications of these strings, including their gravitational wave signatures that are testable in current and future experiments. Our findings establish a compelling and testable connection between neutrino mass generation and cosmic string phenomenology in an underexplored region of parameter space.
{"title":"Gravitational waves from type-I strings in a neutrino mass model","authors":"Adeela Afzal","doi":"10.1016/j.dark.2025.102204","DOIUrl":"10.1016/j.dark.2025.102204","url":null,"abstract":"<div><div>In this work, we propose a novel realization of <em>type-I</em> cosmic strings arising from the spontaneous breaking of an extended gauge symmetry <span><math><mrow><mi>S</mi><mi>U</mi><msub><mrow><mo>(</mo><mn>2</mn><mo>)</mo></mrow><mi>R</mi></msub><mo>×</mo><mi>U</mi><msub><mrow><mo>(</mo><mn>1</mn><mo>)</mo></mrow><mrow><mi>B</mi><mo>−</mo><mi>L</mi></mrow></msub></mrow></math></span> in the context of a low-scale split seesaw mechanism for neutrino mass generation. We demonstrate that the split seesaw framework, which explains the smallness of neutrino masses, naturally motivates a small scalar self-coupling <em>λ</em>. This intrinsically links the neutrino mass generation mechanism to the formation of <em>type-I</em> cosmic strings, where the gauge coupling dominates over the scalar self-coupling (<em>β</em> ≡ <em>λ</em>/(2<em>g</em><sup>2</sup>) < 1). We explore the cosmological implications of these strings, including their gravitational wave signatures that are testable in current and future experiments. Our findings establish a compelling and testable connection between neutrino mass generation and cosmic string phenomenology in an underexplored region of parameter space.</div></div>","PeriodicalId":48774,"journal":{"name":"Physics of the Dark Universe","volume":"51 ","pages":"Article 102204"},"PeriodicalIF":6.4,"publicationDate":"2026-01-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145977317","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-01-05DOI: 10.1016/j.dark.2025.102205
Anna Chiara Alfano , Orlando Luongo
The cosmic distance duality relates angular-diameter and luminosity distances ensuring the cosmological principle, null geodesic motion of photons, and photon number conservation. Hence, any violations would challenge any metric theory of gravity. On the other hand, although independent from the specific cosmological model, it is particularly relevant to revisit the cosmic distance duality in view of recent DESI results, favoring dynamical dark energy over a cosmological constant standard paradigm. To do so, we take into account possible violations by considering four different parameterizations, namely: a Taylor expansion around z ≃ 0, a slightly-departing logarithmic correction, a (1;2) Padé rational series to heal the convergence problem and a Chebyshev polynomial expansion, reducing de facto the systematic errors associated with the analysis. We test each of them in a model-independent (-dependent) way, by working out Monte-Carlo Markov chain analyses, employing the Bézier interpolation of the Hubble rate H(z) for the model-independent approach while assuming the flat (non-flat) ΛCDM and ω0ω1CDM models, motivating the latter paradigm in view of the DESI findings. Subsequently, we explore two analyses, employing observational Hubble data, galaxy clusters from the Sunyaev-Zeldovich effect and type Ia supernovae, investigating the impact of the DESI data catalog, first including then excluding the entire data set. Afterwards, we adopt statistical model selection criteria to assess the statistically favored cosmological model. Our results suggest no violation of the cosmic distance duality as the contour plots suggest, where the parameter used to address a possible violation is independent of the background cosmological models adopted. Moreover, from our analyses we conclude that the Taylor parametrization is the worst performing and that the largest deviations from 0 at z ≈ 2 are found when the DESI sample is removed. In particular, for the flat case the cosmic distance duality is less than 2.1% (Bézier), 1.7% (ΛCDM) and 2.1% (ω0ω1CDM) with DESI versus 5.1% (Bézier), 3.3% (ΛCDM) and 7.0% (ω0ω1CDM) without DESI. This is even more accentuated when the curvature is accounted for, when we consider DESI the cosmic distance duality is less 4.0%, 1.2% and 2.1% while without the BAO it is less than 22.0%, 2.2% and 11.0% for Bézier, the concordance and ω0ω1CDM scenario, respectively. Finally, while a slight spatial curvature cannot be entirely excluded, the preferred cosmological model remains the flat ΛCDM background, even when incorporating DESI data.
{"title":"Cosmic distance duality after DESI 2024 data release and dark energy evolution","authors":"Anna Chiara Alfano , Orlando Luongo","doi":"10.1016/j.dark.2025.102205","DOIUrl":"10.1016/j.dark.2025.102205","url":null,"abstract":"<div><div>The cosmic distance duality relates angular-diameter and luminosity distances ensuring the cosmological principle, null geodesic motion of photons, and photon number conservation. Hence, any violations would challenge any metric theory of gravity. On the other hand, although independent from the specific cosmological model, it is particularly relevant to revisit the cosmic distance duality in view of recent DESI results, favoring dynamical dark energy over a cosmological constant standard paradigm. To do so, we take into account possible violations by considering four different parameterizations, namely: a Taylor expansion around <em>z</em> ≃ 0, a slightly-departing logarithmic correction, a (1;2) Padé rational series to heal the convergence problem and a Chebyshev polynomial expansion, reducing <em>de facto</em> the systematic errors associated with the analysis. We test each of them in a model-independent (-dependent) way, by working out Monte-Carlo Markov chain analyses, employing the Bézier interpolation of the Hubble rate <em>H</em>(<em>z</em>) for the model-independent approach while assuming the flat (non-flat) ΛCDM and <em>ω</em><sub>0</sub><em>ω</em><sub>1</sub>CDM models, motivating the latter paradigm in view of the DESI findings. Subsequently, we explore two analyses, employing observational Hubble data, galaxy clusters from the Sunyaev-Zeldovich effect and type Ia supernovae, investigating the impact of the DESI data catalog, first including then excluding the entire data set. Afterwards, we adopt statistical model selection criteria to assess the statistically favored cosmological model. Our results suggest <em>no violation</em> of the cosmic distance duality as the contour plots suggest, where the parameter used to address a possible violation is independent of the background cosmological models adopted. Moreover, from our analyses we conclude that the Taylor parametrization is the worst performing and that the largest deviations from 0 at <em>z</em> ≈ 2 are found when the DESI sample is removed. In particular, for the flat case the cosmic distance duality is less than 2.1% (Bézier), 1.7% (ΛCDM) and 2.1% (<em>ω</em><sub>0</sub><em>ω</em><sub>1</sub>CDM) with DESI versus 5.1% (Bézier), 3.3% (ΛCDM) and 7.0% (<em>ω</em><sub>0</sub><em>ω</em><sub>1</sub>CDM) without DESI. This is even more accentuated when the curvature is accounted for, when we consider DESI the cosmic distance duality is less 4.0%, 1.2% and 2.1% while without the BAO it is less than 22.0%, 2.2% and 11.0% for Bézier, the concordance and <em>ω</em><sub>0</sub><em>ω</em><sub>1</sub>CDM scenario, respectively. Finally, while a slight spatial curvature cannot be entirely excluded, the preferred cosmological model remains the flat ΛCDM background, even when incorporating DESI data.</div></div>","PeriodicalId":48774,"journal":{"name":"Physics of the Dark Universe","volume":"51 ","pages":"Article 102205"},"PeriodicalIF":6.4,"publicationDate":"2026-01-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145926341","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
This paper presents a comprehensive analysis of the Joule–Thomson (J-T) expansion and thermodynamic properties of charged AdS black holes (BHs) coupled to nonlinear electrodynamics and embedded within a string cloud with particular emphasis on elucidating the profound role of thermal fluctuations. By formulating the equation of state and analyzing the J-T coefficient, we distinguish the heating and cooling phases of the BH and demonstrate how the mass, string cloud parameter and deviation parameter κ influence the transition between them. The isenthalpic curves in the plane reveal that larger BH masses enhance cooling efficiency while nonlinear electrodynamics suppresses it. Incorporating logarithmic corrections due to thermal fluctuations, we study the corrected entropy and various thermodynamic potentials, including the Helmholtz free energy, internal energy, enthalpy, Gibbs free energy and heat capacity. Evidence from the analysis suggests that quantum thermal corrections significantly alter the stability profile of AdS BHs. Configurations of small horizon radius exhibit negative heat capacity, thereby entering an unstable regime, whereas sufficiently large BHs undergo a transition to positive capacity, restoring stability and equilibrium. The radiative spectrum, moreover, is characterized by a pronounced peak at intermediate frequencies, echoing the profile of Hawking emission yet distinctly modified through nonlinear electrodynamic contributions. Collectively, these insights establish that nonlinear electrodynamics, the presence of a string cloud, and quantum fluctuations are fundamental determinants in shaping the phase structure, equilibrium properties, and radiative dynamics of AdS BHs.
{"title":"Phase structure, thermal fluctuations and emission spectrum of nonlinear electrodynamics AdS black holes surrounded by cloud of strings","authors":"Faisal Javed , A. Eid , Abdelmalek Bouzenada , Arfa Waseem , N. Mustapha , Munisbek Akhmedov , Yunus Turaev , Ertan Güdekli","doi":"10.1016/j.dark.2025.102207","DOIUrl":"10.1016/j.dark.2025.102207","url":null,"abstract":"<div><div>This paper presents a comprehensive analysis of the Joule–Thomson (J-T) expansion and thermodynamic properties of charged AdS black holes (BHs) coupled to nonlinear electrodynamics and embedded within a string cloud with particular emphasis on elucidating the profound role of thermal fluctuations. By formulating the equation of state and analyzing the J-T coefficient, we distinguish the heating and cooling phases of the BH and demonstrate how the mass, string cloud parameter and deviation parameter <em>κ</em> influence the transition between them. The isenthalpic curves in the <span><math><mrow><mi>T</mi><mo>−</mo><mi>P</mi></mrow></math></span> plane reveal that larger BH masses enhance cooling efficiency while nonlinear electrodynamics suppresses it. Incorporating logarithmic corrections due to thermal fluctuations, we study the corrected entropy and various thermodynamic potentials, including the Helmholtz free energy, internal energy, enthalpy, Gibbs free energy and heat capacity. Evidence from the analysis suggests that quantum thermal corrections significantly alter the stability profile of AdS BHs. Configurations of small horizon radius exhibit negative heat capacity, thereby entering an unstable regime, whereas sufficiently large BHs undergo a transition to positive capacity, restoring stability and equilibrium. The radiative spectrum, moreover, is characterized by a pronounced peak at intermediate frequencies, echoing the profile of Hawking emission yet distinctly modified through nonlinear electrodynamic contributions. Collectively, these insights establish that nonlinear electrodynamics, the presence of a string cloud, and quantum fluctuations are fundamental determinants in shaping the phase structure, equilibrium properties, and radiative dynamics of AdS BHs.</div></div>","PeriodicalId":48774,"journal":{"name":"Physics of the Dark Universe","volume":"51 ","pages":"Article 102207"},"PeriodicalIF":6.4,"publicationDate":"2026-01-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145926345","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
In this paper, we focus on the influence of effective quantum gravity corrections on neutrino oscillations around static black holes. Following a covariant Hamiltonian, which maintains diffeomorphism invariance, we consider a black hole metric from semiclassical quantum gravity and look at how the parameter ξ/M, which describes the deviation from the Schwarzschild solution affects the neutrino flavor transitions. The analysis includes both radial and non-radial propagation of neutrinos with emphasis on their gravitational lensing. The analysis is performed with two- and three-flavor oscillation models with normal and inverted mass hierarchy configurations and varying effective neutrino mass values. The calculation of the energy deposition rate due to neutrino annihilation is also presented.
{"title":"Gravitational lensing effects on neutrino oscillations around static black hole in effective quantum gravity","authors":"Sojida Mannobova , Bakhtiyor Narzilloev , Farruh Atamurotov , Ahmadjon Abdujabbarov , Ibrar Hussain , Bobomurat Ahmedov","doi":"10.1016/j.dark.2025.102194","DOIUrl":"10.1016/j.dark.2025.102194","url":null,"abstract":"<div><div>In this paper, we focus on the influence of effective quantum gravity corrections on neutrino oscillations around static black holes. Following a covariant Hamiltonian, which maintains diffeomorphism invariance, we consider a black hole metric from semiclassical quantum gravity and look at how the parameter <em>ξ</em>/<em>M</em>, which describes the deviation from the Schwarzschild solution affects the neutrino flavor transitions. The analysis includes both radial and non-radial propagation of neutrinos with emphasis on their gravitational lensing. The analysis is performed with two- and three-flavor oscillation models with normal and inverted mass hierarchy configurations and varying effective neutrino mass values. The calculation of the energy deposition rate due to neutrino annihilation is also presented.</div></div>","PeriodicalId":48774,"journal":{"name":"Physics of the Dark Universe","volume":"51 ","pages":"Article 102194"},"PeriodicalIF":6.4,"publicationDate":"2026-01-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145883993","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
We examine the shadow properties of the rotating Fang-Wang black hole (RFWBH)-a regular black hole solution constructed via the Newman-Janis algorithm from a spherically symmetric solution in general relativity with nonlinear electrodynamics–described by the parameters mass, spin, and a deviation parameter M, a, and l respectively. In turn, we analyse the horizon structure and the ergoregion, demonstrating how l modifies these features compared to the Kerr solution. The shadow of a black hole is studied using celestial coordinates, revealing that increasing l reduces the shadow size and distorts its shape. By employing shadow observables - oblateness D and area A—we estimate the parameters a and l. We further constrain the RFWBH parameter space using recent Event Horizon Telescope (EHT) observations of Sgr A* and M87*, particularly through measurements of the angular shadow diameter. For M87*, at inclination , (a/M, l/M) ≤ [0.59, 0.07953], with l/M upper bounded for 0.59 ≤ a ≤ 0.8428 lowering with spin; at , (a/M, l/M) ≤ [0.39, 0.09869] with similar upper bounds on l/M for 0.39 ≤ a ≤ 0.8164. In the case of Sgr A*, the parameter space is constrained as follows: at inclination , the allowed range for a/M and l/M are [0,0.6692] and [0,0.0305], respectively; and for 90∘, the allowed ranges for a/M and l/M are [0,0.759] and [0,0.03075], respectively. We also analyse the energy emission rate, finding that the deviation parameter l subdues high-energy emission. Our results indicate that RFWBHs are consistent with current EHT observations and represent viable alternatives to astrophysical black hole models.
我们研究了旋转Fang-Wang黑洞（RFWBH）的阴影特性，这是一个由广义相对论中具有非线性电动力学的球对称解通过Newman-Janis算法构造的规则黑洞解，分别由参数质量、自旋和偏差参数M、a和l描述。反过来，我们分析了视界结构和遍历区域，演示了与Kerr解决方案相比，l如何修改这些特征。利用天体坐标研究了黑洞的阴影，发现增加l会减小阴影的大小并扭曲其形状。通过使用阴影观测值——扁率D和面积a，我们估计了参数a和l。我们利用事件视界望远镜（EHT）最近对Sgr a *和M87*的观测，特别是通过测量角阴影直径，进一步限制了RFWBH参数空间。对于M87*，在倾角θ=17°时，（a/M, l/M） ≤ [0.59,0.07953]，l/M上限为0.59 ≤ a ≤ 0.8428随自旋降低；θ=90°，（a/M, l/M） ≤ [0.39,0.09869]，l/M的上界为0.39 ≤ a ≤ 0.8164。在Sgr A*的情况下，参数空间受如下限制：在倾角θ=50°时，A /M和l/M的允许范围分别为[0,0.6692]和[0,0.0305]；对于90°，a/M和l/M的允许范围分别为[0,0.759]和[0,0.03075]。我们还分析了能量发射速率，发现偏差参数l抑制了高能发射。我们的研究结果表明，RFWBHs与目前的EHT观测结果一致，并代表了天体物理学黑洞模型的可行替代方案。
{"title":"Observational constraints on the rotating Fang-Wang black hole from EHT shadow imaging of M87* and Sgr A*","authors":"Himanshi Gulia , J.K. Singh , Farruh Atamurotov , Sushant G. Ghosh","doi":"10.1016/j.dark.2025.102203","DOIUrl":"10.1016/j.dark.2025.102203","url":null,"abstract":"<div><div>We examine the shadow properties of the rotating Fang-Wang black hole (RFWBH)-a regular black hole solution constructed via the Newman-Janis algorithm from a spherically symmetric solution in general relativity with nonlinear electrodynamics–described by the parameters mass, spin, and a deviation parameter <em>M, a</em>, and <em>l</em> respectively. In turn, we analyse the horizon structure and the ergoregion, demonstrating how <em>l</em> modifies these features compared to the Kerr solution. The shadow of a black hole is studied using celestial coordinates, revealing that increasing <em>l</em> reduces the shadow size and distorts its shape. By employing shadow observables - oblateness <em>D</em> and area <em>A</em>—we estimate the parameters <em>a</em> and <em>l</em>. We further constrain the RFWBH parameter space using recent Event Horizon Telescope (EHT) observations of Sgr A* and M87*, particularly through measurements of the angular shadow diameter. For M87*, at inclination <span><math><mrow><mi>θ</mi><mo>=</mo><msup><mn>17</mn><mo>∘</mo></msup></mrow></math></span>, (<em>a</em>/<em>M, l</em>/<em>M</em>) ≤ [0.59, 0.07953], with <em>l</em>/<em>M</em> upper bounded for 0.59 ≤ <em>a</em> ≤ 0.8428 lowering with spin; at <span><math><mrow><mi>θ</mi><mo>=</mo><msup><mn>90</mn><mo>∘</mo></msup></mrow></math></span>, (<em>a</em>/<em>M, l</em>/<em>M</em>) ≤ [0.39, 0.09869] with similar upper bounds on <em>l</em>/<em>M</em> for 0.39 ≤ <em>a</em> ≤ 0.8164. In the case of Sgr A*, the parameter space is constrained as follows: at inclination <span><math><mrow><mi>θ</mi><mo>=</mo><msup><mn>50</mn><mo>∘</mo></msup></mrow></math></span>, the allowed range for <em>a</em>/<em>M</em> and <em>l</em>/<em>M</em> are [0,0.6692] and [0,0.0305], respectively; and for 90<sup>∘</sup>, the allowed ranges for <em>a</em>/<em>M</em> and <em>l</em>/<em>M</em> are [0,0.759] and [0,0.03075], respectively. We also analyse the energy emission rate, finding that the deviation parameter <em>l</em> subdues high-energy emission. Our results indicate that RFWBHs are consistent with current EHT observations and represent viable alternatives to astrophysical black hole models.</div></div>","PeriodicalId":48774,"journal":{"name":"Physics of the Dark Universe","volume":"51 ","pages":"Article 102203"},"PeriodicalIF":6.4,"publicationDate":"2025-12-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145926339","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-12-30DOI: 10.1016/j.dark.2025.102209
Long-Yue Li , Xia-Yuan Liu , Rong-Gen Cai , Yungui Gong , Wenting Zhou
We study images of spacetimes containing continuous photon spheres (CPS). For a self-gravitating, isotropic, spherically symmetric spacetime with CPS, we find that a thin accretion disk produces images that closely resemble those of a Schwarzschild black hole, despite significant differences in photon dynamics. More generally, for any static, spherically symmetric spacetime with a luminous CPS core, the image profile is universal: members of this class produce identical image shapes, differing only by an overall normalization factor. This universality is, however, sensitive to the nature of the accretion flow and breaks down for spherically symmetric infalling accretion, where Doppler shifts and non-static emission introduce image features that depend on the flow dynamics and the metric. Finally, we investigate photon regions in a rotating CPS spacetime and find that unlike in Kerr spacetime, the photon region appears as one or two angular sectors in a constant-ϕ cross section. These distinctive photon region properties could produce observable signatures that distinguish rotating CPS spacetimes from the Kerr one.
{"title":"Images and photon regions of continuous photon sphere spacetime","authors":"Long-Yue Li , Xia-Yuan Liu , Rong-Gen Cai , Yungui Gong , Wenting Zhou","doi":"10.1016/j.dark.2025.102209","DOIUrl":"10.1016/j.dark.2025.102209","url":null,"abstract":"<div><div>We study images of spacetimes containing continuous photon spheres (CPS). For a self-gravitating, isotropic, spherically symmetric spacetime with CPS, we find that a thin accretion disk produces images that closely resemble those of a Schwarzschild black hole, despite significant differences in photon dynamics. More generally, for any static, spherically symmetric spacetime with a luminous CPS core, the image profile is universal: members of this class produce identical image shapes, differing only by an overall normalization factor. This universality is, however, sensitive to the nature of the accretion flow and breaks down for spherically symmetric infalling accretion, where Doppler shifts and non-static emission introduce image features that depend on the flow dynamics and the metric. Finally, we investigate photon regions in a rotating CPS spacetime and find that unlike in Kerr spacetime, the photon region appears as one or two angular sectors in a constant-<em>ϕ</em> cross section. These distinctive photon region properties could produce observable signatures that distinguish rotating CPS spacetimes from the Kerr one.</div></div>","PeriodicalId":48774,"journal":{"name":"Physics of the Dark Universe","volume":"51 ","pages":"Article 102209"},"PeriodicalIF":6.4,"publicationDate":"2025-12-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145926340","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-12-30DOI: 10.1016/j.dark.2025.102208
Wasif Husain
In this study, the impact of neutron decay into dark matter and various dark matter self-interaction strengths on neutron star properties have been explored. Using the quark-meson coupling (QMC) model for nucleon-only equations of state (EoSs), the effects of different matter compositions have been compared, including strange matter and self-interacting dark matter. The results demonstrate that increasing DM-DM self-repulsion stiffens the EoS, influencing the mass-radius relationship and stability of neutron stars. Furthermore, fundamental mode (f-mode) oscillations have been analyzed, which serve as a diagnostic tool for probing neutron star interiors. The f-mode frequencies follow universal relations, reinforcing their applicability for constraining dense matter properties. It has been shown that neutron stars composed of nucleons-only and self-interacting dark matter exhibit a universal behavior in damping time and angular frequency, whereas strange matter and non-self-interacting dark matter deviate from this trend. Importantly, it has been shown that for a GW energy release of E ∼ 1052 erg and a source distance of 25 Mpc, the characteristic strain and signal-to-noise ratio exceed the ET-D sensitivity threshold below ∼ 2.1 kHz for all models except the non-interacting DM case, demonstrating that neutron-to-dark matter decay scenarios, including the role of DM self-interactions, can be tested through next-generation gravitational-wave asteroseismology, offering a new probe of DM physics and the neutron lifetime anomaly.
{"title":"F-mode oscillations of neutron stars with dark matter from neutron decay: Implications for gravitational-wave detectability","authors":"Wasif Husain","doi":"10.1016/j.dark.2025.102208","DOIUrl":"10.1016/j.dark.2025.102208","url":null,"abstract":"<div><div>In this study, the impact of neutron decay into dark matter and various dark matter self-interaction strengths on neutron star properties have been explored. Using the quark-meson coupling (QMC) model for nucleon-only equations of state (EoSs), the effects of different matter compositions have been compared, including strange matter and self-interacting dark matter. The results demonstrate that increasing DM-DM self-repulsion stiffens the EoS, influencing the mass-radius relationship and stability of neutron stars. Furthermore, fundamental mode (f-mode) oscillations have been analyzed, which serve as a diagnostic tool for probing neutron star interiors. The f-mode frequencies follow universal relations, reinforcing their applicability for constraining dense matter properties. It has been shown that neutron stars composed of nucleons-only and self-interacting dark matter exhibit a universal behavior in damping time and angular frequency, whereas strange matter and non-self-interacting dark matter deviate from this trend. Importantly, it has been shown that for a GW energy release of <em>E</em> ∼ 10<sup>52</sup> erg and a source distance of 25 Mpc, the characteristic strain and signal-to-noise ratio exceed the ET-D sensitivity threshold below ∼ 2.1 kHz for all models except the non-interacting DM case, demonstrating that neutron-to-dark matter decay scenarios, including the role of DM self-interactions, can be tested through next-generation gravitational-wave asteroseismology, offering a new probe of DM physics and the neutron lifetime anomaly.</div></div>","PeriodicalId":48774,"journal":{"name":"Physics of the Dark Universe","volume":"51 ","pages":"Article 102208"},"PeriodicalIF":6.4,"publicationDate":"2025-12-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"146022829","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-12-30DOI: 10.1016/j.dark.2025.102210
Shan-Ping Wu , Yu-Xiao Liu , Shao-Wen Wei
The generalized free energy landscape plays a pivotal role in understanding black hole thermodynamics and phase transitions. In general relativity, one can directly derive the generalized free energy from the contributions of black holes exhibiting conical singularities. In this work, we extend this idea to general covariant theories. By employing Noether’s second theorem, we present an alternative formulation of the Lagrangian, which can elucidate the role of conical singularities. We demonstrate that, in general, the contribution from conical singularities depends on the specific implementation of the regularization scheme and is not uniquely determined; this feature is explicitly exhibited and confirmed in three-dimensional new massive gravity. Nevertheless, these ambiguities can be absorbed into the second-order (and higher) corrections induced by conical singularities when the gravitational theory is described by the Lagrangian L(gab, Rabcd). Moreover, for certain theories such as general relativity and Bumblebee gravity, this contribution simplifies to a well-defined result. However, the interpretation of the generalized free energy in Bumblebee gravity is somewhat different, with its extrema corresponding to the geometry of conical singularities. Our results uncover the particular properties of the generalized free energy beyond general relativity.
{"title":"Generalized free energy landscapes from Iyer-Wald formalism","authors":"Shan-Ping Wu , Yu-Xiao Liu , Shao-Wen Wei","doi":"10.1016/j.dark.2025.102210","DOIUrl":"10.1016/j.dark.2025.102210","url":null,"abstract":"<div><div>The generalized free energy landscape plays a pivotal role in understanding black hole thermodynamics and phase transitions. In general relativity, one can directly derive the generalized free energy from the contributions of black holes exhibiting conical singularities. In this work, we extend this idea to general covariant theories. By employing Noether’s second theorem, we present an alternative formulation of the Lagrangian, which can elucidate the role of conical singularities. We demonstrate that, in general, the contribution from conical singularities depends on the specific implementation of the regularization scheme and is not uniquely determined; this feature is explicitly exhibited and confirmed in three-dimensional new massive gravity. Nevertheless, these ambiguities can be absorbed into the second-order (and higher) corrections induced by conical singularities when the gravitational theory is described by the Lagrangian <em>L</em>(<em>g<sub>ab</sub>, R<sub>abcd</sub></em>). Moreover, for certain theories such as general relativity and Bumblebee gravity, this contribution simplifies to a well-defined result. However, the interpretation of the generalized free energy in Bumblebee gravity is somewhat different, with its extrema corresponding to the geometry of conical singularities. Our results uncover the particular properties of the generalized free energy beyond general relativity.</div></div>","PeriodicalId":48774,"journal":{"name":"Physics of the Dark Universe","volume":"51 ","pages":"Article 102210"},"PeriodicalIF":6.4,"publicationDate":"2025-12-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145926342","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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