Based on the Comisso–Asenjo mechanism, we investigate the kinematic images of plasma before and after magnetic reconnection in Kerr–Anti-de Sitter(Kerr–AdS) black holes. Following a brief review of the Comisso–Asenjo process in Kerr–AdS black holes, we introduce the hotspot model and the imaging method. Building upon these foundational theories, we obtain the trajectory of the plasma and the temporal evolution of the hotspot images. It is found that there are three flares within the observing time, which is driven by the Comisso–Asenjo mechanism. We also discuss the influence of the cosmological parameter on the hotspot imaging. The results indicate that the hotspot image enlarges as the absolute value of $$\Lambda $$ increases, demonstrating that the cosmological constant significantly affects the hotspot.

China

Xiao-Xiong Zeng

Ke Wang

Hotspot image driven by magnetic reconnection in Kerr–anti de Sitter black holes

In this study, we analyze $\textit{f}$($\textit{R}$) gravity employing the $$ f(R) = R+ \alpha _1 R^p - \alpha _2 R^{-q} $$ model within the Friedmann–Lemaître–Robertson–Walker (FLRW) background. We derive the Friedmann equations via modified gravity action and subsequently reexpress them in terms of the standard Friedmann equations. Our investigation explores the behavior of a bouncing cosmological model within this modified gravity context, offering a potential solution to the singularity problem encountered in the standard Big Bang cosmology. We represent cosmological parameters as functions of cosmic time and scrutinize the conditions for a cosmic bounce. Additionally, we reconstruct the $\textit{f}$($\textit{R}$) gravity model using the redshift parameter and graphically present cosmological parameters as functions of redshift. These plots reveal an accelerated nature of Universe expansion. Furthermore, we reconstruct $\textit{f}$($\textit{R}$) gravity models for scale factors, $$a(t) = e^{e^{\lambda t}}$$ which exhibits the bouncing behavior. Finally, we investigated the stability scenario by examining the sound speed. The resulting graph of sound speed plotted against redshift confirms late-time stability.

Pakistan

M. Ilyas

Saudi Arabia

Khalid Masood

South Korea

Nehad Shah

Dynamical analysis of bouncing cosmology in the Capozziello–De Laurentis model of $\textit{f}$($\textit{R}$) gravity

We study perturbative unitarity bounds on the field-space curvature in de Sitter spacetime, using the momentum-space entanglement approach recently proposed by Duaso Pueyo, Goodhew, McCulloch, and Pajer. As an illustration, we perform a perturbative computation of the purity in two-scalar models and compare the resulting unitarity bounds with those obtained via a flat space approximation. In particular, we find that perturbative unitarity imposes an upper bound on the field-space curvature of the Hubble scale order, in addition to a bound analogous to the flat space result. This reflects the thermal nature of de Sitter spacetime. We also discuss generalizations to higher-dimensional field spaces.

Japan

Qianhang Cai

Tomoya Inada

Masataka Ishikawa

Kanji Nishii

Toshifumi Noumi

Perturbative unitarity bounds on field-space curvature in de Sitter spacetime: purity vs scattering amplitude

In this study, we investigate a scenario based on the effects of the Loop Quantum Gravity (LQG) on the electromagnetic sector of the Standard Model of Fundamental Interactions and Particle Physics (SM). Starting from a post-Maxwellian version of electromagnetism that includes LQG effects, we derive and discuss the influence of LQG parameters on classical quantities such as the components of the stress-tensor. Furthermore, we inspect the propagation of electromagnetic waves and examine the optical properties of the QED vacuum in this scenario. Among these, we contemplate the combined effect of the LQG parameters and a homogeneous background magnetic field on the propagation of electromagnetic waves by considering issues such as group velocities and refractive indices of the QED vacuum, in detail. Finally, with the aid of the previously analyzed LQG-extended photonic dispersion relations, we re-discuss the kinematics of the Compton effect and conclude that an interesting nonlinear profile emerges in the wavelengths of the incoming and deflected photons.

Brazil

P.A.L. Mourão

G.L.L.W. Levy

J.A. Helayël-Neto

Reassessing aspects of LQG-modified dispersion relations of photons * P. A. L. Mourão is support by CAPES. G. L. L. W. L. Levy is support by the Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ), Grant No. E-26/202.437/2024.

<p>We study the tree-level scattering of massless scalars followed by an emission of a soft graviton in the small compact region inside the static patch in de Sitter space. We derive in the small cosmological constant limit the perturbative corrections to the Weinberg soft graviton theorem. Exploiting the remarkable relationship between asymptotic symmetries and soft theorems in flat space, we derive perturbative corrections to the supertranslation Ward identity. We further show that the derived supertranslation Ward identity reduces to the perturbative soft graviton theorem for the same choice of the supertranslation parameter as in flat space.</p>

Pratik Chattopadhyay

India

Divyesh N. Solanki

de Sitter corrections to supertranslation Ward identity and soft graviton theorem

The states of a single photon in four-dimensional de Sitter (dS) spacetime form a Unitary Irreducible Representation (UIR) of SO(1,4), which we call the photon UIR. While in flat spacetime photons are intimately tied to gauge symmetry, we demonstrate that in de Sitter, photon states emerge generically in any quantum field theory, even without an underlying U(1) gauge field. We derive a Källén-Lehmann representation for antisymmetric tensor two-point functions and show that numerous composite operators constructed from massive free fields can create states in the photon UIR. Remarkably, we find that some of these operators exhibit two-point functions with slower late-time and large-distance decay than the electromagnetic field strength itself, challenging the conventional notion that photons dominate the infrared regime. Using our spectral representation, we establish non-perturbative bounds on the late-time behavior of electric and magnetic fields in de Sitter, with potential implications for primordial magnetogenesis. Through one-loop calculations, we demonstrate that both the creation of photon states and the enhanced late-time large-distance behavior persist in weakly interacting theories.

Switzerland

Manuel Loparco

João Penedones

Germany

Yannis Ulrich

What is a photon in de Sitter spacetime?

The perturbative corrections to soft photon theorems with massive scalars in de Sitter spacetime were computed in (Bhatkar and Jain in J High Energy Phys 2023:55, 2023). However, the massless limit of the scalar modes is ill-defined in their work. It therefore is ambiguous to take the massless limit of the soft factors. In this paper, we derive the massless scalar modes in $\textit{d}$-dimensional de Sitter spacetime and use it to compute the perturbative corrections to the leading and sub-leading soft photon theorems. Our framework corresponds to tree level scattering of massless scalars followed by an emission of a soft photon in a compact region of the static patch of de Sitter. We show that our results are consistent with (Bhatkar and Jain in J High Energy Phys 2023:55, 2023) in the massless limit and we comment on the universality of our results.

Perturbative corrections to soft photon theorems for massless scalar QED in de Sitter spacetime

We investigate torsion-driven cosmological dynamics within the framework of Einstein–Cartan gravity using the De Donder–Weyl Hamiltonian formalism, where the tetrad and Lorentz connection act as independent variables and the Hamiltonian includes quadratic Riemann Cartan corrections. Embedding this theory in an FLRW background, we derive the corresponding torsion-modified Friedmann equations and analyze their solutions across radiation and matter-dominated epochs. The commonly assumed power law form $$a(t)=\beta t^{\alpha }$$ is shown to generate multiple solution branches, many of which can be considered to be “unphysical”. A hybrid solution, $$a(t)=Ct^{\alpha }e^{Dt^{\beta }}$$ emerges in the special case $$g_1=0$$ , where the quadratic Riemann–Cartan term vanishes. For $$g_1\not =0,$$ the equations become nonlinear, precluding closed-form analytic solutions. These findings highlight the limitations of the power-law approximation and identify the restricted conditions under which torsion can coherently drive cosmic expansion.

Aarav Shah

France

Maxim Khlopov

Russia

Maxim Krasnov

Cosmology in the De Donder–Weyl formulation of Einstein–Cartan gravity

The primary objective of the article is to investigate the symmetry and pseudosymmetry properties of the Reissner-Nordström-de Sitter (briefly, RNdS) spacetime. The secondary aim of the paper is to explore the notion of Ricci solitons in RNdS spacetimes. The study is important due to the conceding of almost Ricci soliton and almost Ricci Yamabe soliton of the RNdS spacetime. The analysis shows that this spacetime satisfies multiple types of symmetric and pseudosymmetric conditions. It is interesting to note that RNdS spacetime revealed pseudosymmetry, Weyl conformal pseudosymmetry, Weyl projective pseudosymmetry, conharmonic pseudosymmetry, and concircular pseudosymmetry. Also the RNdS spacetime is pseudosymmetric due to conformal, projective, conharmonic curvature tensors. Furthermore, in the RNdS spacetime, the second order commutator tensor <italic>R</italic>&#8239;&#183;&#8239; <italic>R</italic> is linearly dependent on <italic>Q</italic>( <italic>Ric, R</italic>) and <italic>Q</italic>( <italic>g, C</italic>), and the tensor <italic>R</italic>&#8239;&#183;&#8239; <italic>C</italic> does not commute with the tensor <italic>C</italic>&#8239;&#183;&#8239; <italic>R</italic>. The investigation also shows that RNdS spacetime is a Roter type. It is demonstrated that the RNdS spacetime is 2-quasi Einstein and an Ein(2) space with recurrent conformal 2-forms. Riemann compatibility preserved under geodesic mapping and it has a great geometric insight in determining the nature of Pontryagian forms. We derive the general form of the compatible tensors of the RNdS spacetime. The energy-momentum tensor of the RNdS spacetime is shown to be pseudosymmetric, and also the energy momentum tensor is pseudosymmetric due to conformal, conharmonic, concircular and projective curvature tensor. We note that geodesic maps do not necessarily preserve Weyl compatibility. However such kind of compatibility remains invariant under conformal maps. The presence of a Weyl-compatible vector ensures that the Weyl tensor assumes an algebraically special form, and this condition is both necessary and sufficient for the vanishing of its magnetic component. The energy momentum tensor is compatible with projective, conharmonic, concircular, Riemann and conformal curvature. It is shown that the energy momentum tensor of the RNdS spacetime is Weyl compatible and hence the Weyl tensor of a such a spacetime is algebraically special and hence its magnetic component vanishes. Consequently the Weyl tensor in RNdS spacetime is purely electric. We study a generalized notion of curvature inheritance and find that, with respect to the non-Killing vector fields &#8706;/&#8706; <italic>r</italic> and &#8706;/&#8706; <italic>&#952;</italic>, the RNdS spacetime does not satisfy such inheritance condition. However, the RNdS spacetime is shown to admit an almost Ricci soliton and an almost <italic>&#951;</italic>-Ricci Yamabe soliton with respect to the non-Killing vector field &#8706;/&#8706; <italic>r</italic>, but with respect to the non-Killing vector field &#8706;/&#8706; <italic>&#952;</italic> the spacetime does not admit such notions. Finally, we present a comparison between the RNdS and Vaidya-Bonner-de Sitter (VBdS) spacetimes.

Absos Ali Shaikh

 Kamiruzzaman

Symmetry and pseudosymmetry properties with Ricci soliton of the Reissner-Nordström-de Sitter spacetime

The representation theory of de Sitter space admits partially massless (PM) particles, but whether such particles can participate in consistent interacting theories remains unclear. We investigate the consistency of theories containing PM fields, particularly when these fields are coupled to gravity. Our strategy exploits the fact that PM fields correspond to partially conserved currents on the spacetime boundary, which generate symmetries. These symmetries place stringent constraints on correlation functions of charged operators, allowing us to test the consistency of a proposed bulk spectrum. When the assumed operator content violates these constraints, the corresponding bulk theory is ruled out. Applying this framework, we show that, in four-dimensional de Sitter space, PM fields of spin 2 or 3 (at depth 0) cannot couple consistently to gravity: such couplings necessitate additional massive fields, which are inevitably non-unitary. In higher dimensions, however, the constraints can be satisfied without violating unitarity if further PM fields are included. The resulting structure leads to additional charge conservation laws, which suggests that consistency may ultimately require an infinite tower of higher-spin PM fields, akin to the situation for ordinary higher-spin symmetries. The methods developed here provide powerful constraints on possible long-range interactions in de Sitter space and delineate the landscape of consistent quantum field theories in cosmological spacetimes.