Non-zero Cosmological Constant Research Articles

Conformal Killing gravity: pp-waves; multiple matter sources

Abstract Recently Harada proposed a gravitational theory which is of third order in the derivatives of the metric tensor. This attracted some attention particularly as it predicts a late-time transition from cosmological deceleration to accelerated expansion without assuming the presence of dark energy or a non-zero cosmological constant. This theory has been dubbed conformal Killing gravity (CKG). The most general exact solutions of the Harada field equations are known for a number of important physical situations: homogeneous and isotropic cosmological models, static spherically symmetric vacuum and electrovac spacetimes. These are analogues of the well-known FRWL, Schwarzschild and Reissner–Nordström metrics of general relativity (GR). In this paper a subclass of pp-waves are studied and the most general exact solution obtained together with its specialization for plane waves. The generalization from GR to Harada’s theory is straightforward. The solutions have Petrov type N or 0 and the Ricci tensor is either zero or the Segré type is [(211)] with zero eigenvalue. For any metric in CKG it is shown that more than one possible matter source can generate the solution. If the metric admits one or more Killing vectors or Killing tensors, the ambiguity in the possible matter sources increases.

Impact of chaplygin-like equation of state on Joule–Thomson expansion and tidal forces of AdS black holes

This study examines a recently hypothesized black hole. We study the Joule–Thomson coefficient, the inversion temperature and also the isenthalpic curves in the Ti−Pi plane. The Joule–Thomson coefficient, the inversion curves and the isenthalpic curves are discussed in AdS black holes surrounded by Chaplygin dark fluid. In T−P plane, the inversion temperature curves and isenthalpic curves are obtained with different parameters. Next, we explore the radial time-like geodesics that leads us to explore the tidal force effects for a radially in-falling particle in such black hole spacetime. We also numerically solve the geodesic deviation equation for two nearby radial geodesics for a freely falling particle. Our analysis shows that contrary to the Schwarzschild spacetime, the tidal forces do not become zero at spatial infinity due to the lack of asymptotic flatness because of the presence of a non-zero cosmological constant. The geodesic separation profile shows an oscillating trend and depends on the dynamic spacetime parameters q,B and Λ.

Incorporating the cosmological constant in a modified uncertainty principle

This study explores the cosmological constant problem and modified uncertainty principle within a unified framework inspired by a void-dominated scenario for cosmology. In a recent paper [E. Yusofi, M. Khanpour, B. Khanpour, M. A. Ramzanpour and M. Mohsenzadeh, Mon. Not. R. Astron. Soc. 511, L82 (2022)], voids were modeled as spherical bubbles of similar average sizes, and the surface energy on the voids’ borders was calculated across various scales in a heuristic manner. We show that these result in a significant discrepancy of approximately [Formula: see text] between the cosmological constant values from the minimum to the maximum radii of bubbles. Furthermore, when considering the generalized form of the uncertainty principle with both minimum and maximum lengths, i.e. [Formula: see text], a similar order of discrepancy is observed between [Formula: see text] and [Formula: see text], indicating that [Formula: see text]. As a primary outcome of this finding, we offer a novel uncertainty principle that incorporates a nonzero cosmological constant.

Moduli dynamics in effective nested warped geometry in four dimensions and some cosmological implications

We analyze the effective four-dimensional dynamics of the extra-dimensional moduli fields in curved braneworlds having nested warping, with particular emphasis on the doubly warped model which is interesting in the light of current collider constraints on the mass of the Kaluza-Klein graviton. The presence of a non-zero brane cosmological constant (Ω) naturally induces an effective moduli potential in the four-dimensional action, which shows distinct features in dS (Ω > 0) and AdS (Ω < 0) branches. For the observationally interesting case of dS 4-branes, a metastable minimum in the potential arises along the first modulus, with no minima along the higher moduli. The underlying nested geometry also leads to interesting separable forms of the non-canonical kinetic terms in the Einstein frame, where the brane curvature directly impacts the kinetic properties of only the first modulus. The non-canonicity of the scenario has been illustrated via an explicit computation of the field space curvature. We subsequently explore the ability of curved multiply warped geometries to drive inflation with an in-built exit mechanism, by considering predominant slow roll along each modular direction on a case-by-case basis. We find slow roll on top of the metastable plateau along the first modular direction to be the most viable scenario, with the higher-dimensional moduli parametrically tuning the height of the potential without significant impact on the inflationary observables. On the other hand, while slow roll along the higher moduli can successfully inflate the background and eventually lead to an exit, consistency with observations seemingly requires unphysical hierarchies among the extra-dimensional radii, thus disfavouring such scenarios.

Thermodynamic properties of an interacting fermion-antifermion pair in a magnetized spacetime with a non-zero cosmological constant

In a magnetized three-dimensional Bonnor-Melvin spacetime with a non-zero cosmological constant, we explore the dynamics of a fermion-antifermion pair interacting through an attractive Coulomb potential. To analyze the relativistic behavior, we seek an analytical solution for the fully covariant two-body Dirac equation derived from quantum electrodynamics. The resulting equation provides a second-order wave equation that governs the relative motion of the interacting pair. Obtaining an exact solution to this wave equation seems not possible; however, we notice solubility, especially when we consider particles to be closely spaced, meaning as the distance between them approaches zero. At that rate, we determine the energy eigenvalues and wave functions utilizing well-known special functions. By employing these solutions, we determine the thermal properties of this system. Despite the divergence observed in the partition function, we effectively tackle this issue by applying a regularization technique based on the mathematical zeta Hurwitz function. This method facilitates the computation of various thermal quantities, such as free energy, total energy, entropy function, and specific heat. Consequently, we provide an in-depth analysis of the thermodynamic characteristics of the system under consideration.

Optical features of rotating quintessential charged black holes in de-Sitter spacetime

One of the most important and actual issues in relativistic astrophysics is testing gravity theories and obtaining constraint values for the parameters of black holes using observational data. In this research, we aimed to explore the optical features of a Kerr–Newman black hole model in the presence of a quintessential field, which may be a candidate for a dark-energy model with a nonzero cosmological constant. First, we obtain the equations of motion for photons using the Hamilton–Jacobi formalism. We also study the horizons and shapes of the apparent regions of the photon region around the said black hole. In various scenarios, we investigate shadows cast by the black hole using celestial coordinates. Furthermore, we analyze the effects of the quintessential field and black hole charge on the shadow radius and distortion. Furthermore, we look into the constraints on the spin and charge of supermassive black holes M87∗ and Sagittarius A∗ for different values of the quintessential field using their shadow size measured by the Event Horizon Telescope Collaboration. Finally, we study the effects of the quintessential field, black hole spin and charge on its energy emission rate by Hawking radiation and compare our results with those of the available literature.

Fermion-antifermion pairs in a magnetized space-time with non-zero cosmological constant

We analyze a fermion-antifermion pair within the (2+1)-dimensional Bonnor-Melvin magnetic (BMM) spacetime featuring a nonzero cosmological constant (Λ>0). To do this, we rigorously investigate analytical solutions to the corresponding form of a fully-covariant two-body Dirac equation. We derive a non-perturbative wave equation and obtain an exact closed-form analytical solution for the system under consideration. In this magnetic background spacetime, described by a surface with positive Gaussian curvature, we noticed that fermion-antifermion pairs display a collective behavior resembling that of a single quantum oscillator, carrying a total rest mass of the particles and oscillating with a frequency (ωo∝Λ), even in the absence of mutual interaction between the particles. This indicates a compelling correlation between the cosmological constant and the energy spectrum of fermion-antifermion systems, implying connections between quantum realms and cosmology.

Relativistic fermions and vector bosons in magnetized three-dimensional space-time with a cosmological constant

In this manuscript, we study the relativistic dynamics of fermions and vector bosons within a (2+1)-dimensional magnetized space-time. We consider the Bonnor-Melvin magnetic space-time, characterized by a homogeneous magnetic field aligned along the symmetry axis and a non-zero cosmological constant. This space-time background, featuring cylindrical symmetry, retains the invariance of quantum field dynamics under Lorentz boosts along the z-direction. This enables us to explore (2+1)-dimensional realms, where the associated 2+1-dimensional spacetime background is recognized as the Bonnor-Melvin magnetic 2+1+0-brane solution within the framework of gravity coupled with nonlinear electrodynamics. We seek exact solutions for relativistic fermions and vector bosons within this space-time background. We have managed to derive the radial wave equations in both instances, securing precise eigenvalue solutions devoid of any approximations. Our findings extend seamlessly to massless fermions and vector bosons, ensuring generality. Moreover, we observe that the ground state energy of massless vector bosons (photons) within the examined space-time background is unequivocally zero.

Black hole solutions with constant Ricci scalar in a model of Finsler gravity

Ricci scalar being zero is equivalent to the vacuum field equation in Finsler space-time. The Schwarzschild metric can be concluded from the field equation's solution if the space-time conserves spherical symmetry. This research aims to investigate Finslerian Schwarzschild-de Sitter space-time. Recent studies based on Finslerian space-time geometric models are becoming more prevalent because the local anisotropic structure of space-time influences the gravitational field and gives rise to modified cosmological relations. We suggest a gravitational field equation with a non-zero cosmological constant in Finslerian geometry and apprehend that the presented Finslerian gravitational field equation corresponds to the non-zero Ricci scalar. In Finsler geometry, the peer of spherical symmetry is the Finslerian sphere. Assuming space-time to conserve the “Finslerian sphere” symmetry, the counterpart of the Riemannian sphere (Finslerian sphere) must have a constant flag curvature (λ). It is demonstrated that the Finslerian covariant derivative of the geometric part of the gravitational field equation is preserved under a condition using the Chern connection. According to the string theory, string clouds can be defined as a pool of strings made due to symmetry breaking in the universe's early stages. We find that for λ ≠ 1, this solution resembles a black hole surrounded by a cloud of strings. Furthermore, we investigate null and time-like geodesics for λ = 1. In this regard, the photon geodesics are obtained that are the closest paths to the photon sphere of the first photons visible at the black hole shadow limit. Also, circular orbit conditions are obtained for the effective potential.

Experimental tests of dark bubble cosmology

In this paper we use α′2 corrections to the D3-brane action to obtain a nonzero and positive cosmological constant in the induced cosmology living on the dark bubble. Its measured value together with the value of the fine structure constant correspond to a dark dimension of size 5×10−5 m and a string scale at 11 TeV. We conclude that the dark bubble model predicts deviations of Newtonian gravity and stringy excitations of known particles within reach of future experiments. Published by the American Physical Society 2024

Accelerating internal dimensions and nonzero positive cosmological constant

We present a new scenario for the moduli stabilization with a very small but nonzero positive cosmological constant λ. In this scenario the complex structure moduli are still stabilized by the three-form fluxes as in the usual flux compactifications, but the Kähler modulus is not fixed by the KKLT scenario. In our case the scale factor (or the Kähler modulus) of the internal dimensions is basically allowed to change with time. But at the supergravity level it is fixed by a set of dynamical (plus constraint) equations defined on the 4D spacetime, not by the nonperturbative corrections of KKLT. Also at the supergravity level it is shown that λ is fine-tuned to zero, λ=0, by the same set of 4D equations. This result changes once we admit α′-corrections of the string theory. The fine-tuning λ=0 changes into λ=23Q, where Q is a constant representing quantum corrections of the brane and 6D action defined on the internal dimensions and its value is determined by the α′-corrections. It is also shown that this nonzero λ must be positive and at the same time the internal dimensions must evolve with time almost at the same rate as the external dimensions in the case of nonzero λ.

Dynamical emergence of SUq(2) from the regularization of (2+1)D gravity with a cosmological constant

The quantization of the reduced phase-space of the Einstein-Hilbert action for gravity in $2+1D$ has been shown to bring about the emergence, at the quantum level, of a topological quantum field theory endowed with an $SU_q(2)$ quantum group symmetry structure. We hereby tackle the same problem, but start from the kinematical $SU(2)$ (quantum) Hilbert space of the theory of $2+1D$ gravity with non-zero cosmological constant in the Palatini formalism, and subsequently impose the constraints. We hence show the dynamical emergence of the $SU_q(2)$ quantum group at the quantum level within the spin-foam framework. The regularized curvature constraint is responsible for the effective representations of $SU_q(2)$ that are recovered for any Wilson loop evaluated at the $SU(2)$ group element that encodes the discretization of the space-time curvature induced by the cosmological constant. The extension to the spin-network basis, and consequently to any transition amplitude between its generic states, enables us to derive in full generality the recoupling theory of $SU_q(2)$. We provide constructive examples for the scalar product of two loop states and spin-networks encoding trivalent vertices. We further comment on the diffeomorphism symmetry generated by the implementation of the curvature constraint, and finally derive explicitly the partition function amplitude of the Turaev-Viro model.

Spinor representation of the Hamiltonian constraint in 3D loop quantum gravity with a nonzero cosmological constant

We develop in a companion paper the kinematics of three-dimensional loop quantum gravity in Euclidean signature and with a negative cosmological constant, focusing in particular on the spinorial representation that is well known at zero cosmological constant. In this paper, we put this formalism to the test by quantizing the Hamiltonian constraint on the dual of a triangulation. The Hamiltonian constraints are obtained by projecting the flatness constraints onto spinors, as done in the flat case by the first author and Livine. Quantization then relies on $q$-deformed spinors. The quantum Hamiltonian constraint acts in the $q$-deformed spin network basis as difference equations on physical states, which are thus the Wheeler-DeWitt equations in this framework. Moreover, we study how physical states transform under Pachner moves of the canonical surface. We find that those transformations are in fact $q$ deformations of the transition amplitudes of the flat case as found by Noui and Perez. Our quantum Hamiltonian constraints, therefore, build a Turaev-Viro model at real $q$.

3D Gravity, Point Particles, and Deformed Symmetries

It is well known that gravity in 2+1 dimensions can be recast as Chern-Simons theory, with the gauge group given by the local isometry group, depending on the metric signature and the cosmological constant. Point particles are added into spacetime as (spinning) conical defects. Then, in principle, one may integrate out the gravitational degrees of freedom to obtain the effective particle action; the most interesting consequence is that the momentum space of a particle turns out to be curved. This is still not completely understood in the case of non-zero cosmological constant.

Global dynamics of the Hořava–Lifshitz cosmology in the presence of non-zero cosmological constant in a flat space

Using the qualitative theory of differential equations, the global dynamics of a cosmological model based on Hořava-Lifshitz gravity is studied in the space with zero curvature in the presence of the non-zero cosmological constant. The study shows that there may be three unstable finite equilibrium positions under the Hořava-Lifshitz cosmology model and that the final evolution of the orbits of the cosmological model in the physical region of interest may tend towards some infinite equilibrium position, which may correspond to the late-time state of the universe.

Temperature anisotropy of the CMBR and the nonzero cosmological constant

In this article, we analyze the effect of a spherically symmetrical clump on the anisotropy of the cosmic microwave background radiation temperature in the framework of the standard model of cosmology with a nonzero cosmological constant [Formula: see text] and we show that it weakens the Rees–Sciama effect.

Comments on the cosmological constant in generalized uncertainty models

The existence of a small, non-zero cosmological constant is one of the major puzzles in fundamental physics. Naively, quantum field theory arguments would imply a cosmological constant which is up to 10,120 times larger than the observed one. It is believed a comprehensive theory of quantum gravity would resolve this enormous mismatch between theory and observation. In this work, we study the ability of generalized uncertainty principle (GUP) models, which are phenomenologically motivated models of quantum gravity, to address the cosmological constant problem. In particular, we focus on how these GUP models may change the phase space of QFT, and how this affects the momentum space integration of the zero-point energies of normal modes of fields. We point out several issues that make it unlikely that GUP models, in their current form, would be able to adequately address the cosmological constant problem.

Effective quantum gravity, cosmological constant, and the Standard Model of particle physics

The renormalization group in effective quantum gravity can be consistently formulated using the Vilkovisky and DeWitt version of effective action and assuming a non-zero cosmological constant. Taking into account that the vacuum counterpart of the cosmological constant is dramatically different from the observed energy density of vacuum, the running of the last quantity in the late cosmology indicates strong constraints on the physics beyond the minimal Standard Model of particle physics.

Global dynamics for the Szekeres system with non-zero cosmological constant

The Szekeres system with cosmological constant term describes the evolution of the kinematic quantities for the Einstein field equations in dimension four. It is a Hamiltonian system (with Hamiltonian H ). We restrict the dynamics on each one of the level surfaces H = h with h ∈ R and using the Poincaré compactification on R 3 we analyze the global dynamics of the Szekeres system. It was known that the Szekeres system with cosmological constant term exhibits an attractor in the finite regime. Here we provide a new proof of the finite attractor and additionally we prove that also exhibits a repulsor in the finite regime, and that at infinity there is an attractor and a repulsor.

Charges and fluxes on (perturbed) non-expanding horizons

In a companion paper [1] we showed that the symmetry group mathfrak{G} of non-expanding horizons (NEHs) is a 1-dimensional extension of the Bondi-Metzner-Sachs group mathfrak{B} at mathcal{I} +. For each infinitesimal generator of mathfrak{G} , we now define a charge and a flux on NEHs as well as perturbed NEHs. The procedure uses the covariant phase space framework in presence of internal null boundaries mathcal{N} along the lines of [2–6]. However, mathcal{N} is required to be an NEH or a perturbed NEH. Consequently, charges and fluxes associated with generators of mathfrak{G} are free of physically unsatisfactory features that can arise if mathcal{N} is allowed to be a general null boundary. In particular, all fluxes vanish if mathcal{N} is an NEH, just as one would hope; and fluxes associated with symmetries representing ‘time-translations’ are positive definite on perturbed NEHs. These results hold for zero as well as non-zero cosmological constant. In the asymptotically flat case, as noted in [1], mathcal{I} ±are NEHs in the conformally completed space-time but with an extra structure that reduces mathfrak{G} to mathfrak{B} . The flux expressions at mathcal{N} reflect this synergy between NEHs and mathcal{I} +. In a forthcoming paper, this close relation between NEHs and mathcal{I} + will be used to develop gravitational wave tomography, enabling one to deduce horizon dynamics directly from the waveforms at mathcal{I} +.