International Journal of Modern Physics D

Some remarkable examples of alternative cosmological theories are reviewed here, ranging from a compilation of variations on the Standard Model through the more distant quasi-steady-state cosmology, plasma cosmology, or universe models as a hypersphere, to the most exotic cases including static models.

The present-day Standard Model of cosmology, $\mathrm{\Lambda }$CDM, gives us a representation of a cosmos whose dynamics is dominated by gravity (Friedmann equations derived from general relativity) with a finite lifetime, large scale homogeneity, expansion and a hot initial state, together with other elements necessary to avoid certain inconsistencies with observations (inflation, nonbaryonic dark matter, dark energy, etc.). There are however some models with characteristics that are close to those of the Standard Model but differing in some minor aspects; we call these “variations on the Standard Model”. Many of these models are indeed investigated by some mainstream cosmologists: different considerations on CP violation, inflation, number of neutrino species, quark-hadron phase transition, baryonic or nonbaryonic dark-matter, dark energy, nucleosynthesis scenarios, large-scale structure formation scenarios; or major variations like a inhomogeneous universe, Cold Big Bang, varying physical constants or gravity law, zero-active mass (also called “$R_h=\mathrm{\text{ct}}$”), Milne, and cyclical models.

At the most extreme distance from the Standard Model, the static models, a noncosmological redshift includes “tired-light” hypotheses, which assume that the photon loses energy owing to an intrinsic property or an interaction with matter or light as it travels some distance, or other nonstandard ideas. Our impression is that none of the alternative models has acquired the same level of development as $\mathrm{\Lambda }$CDM in offering explanations of available cosmological observations. One should not, however, judge any theory in terms of the number of observations that it can successfully explain (ad hoc in many cases) given the much lower level of development of the alternative ones, but by the plausibility of its principles and its potential to fit data with future improvements of the theories. A pluralist approach to cosmology is a reasonable option when the preferred theory is still under discussion.

In this paper, we explore anisotropic compact stellar models in the framework of general theory of relativity with $g_{rr}$ component governed by Buchdahl ansatz. We choose the anisotropic factor such as to render the solution physically compatible for stellar modeling. We have analyzed the solution for different values of $K$ to demonstrate the profiles of thermodynamical variables. The stability of the proposed models has been verified using different stability criteria. The hydrostatic equilibrium condition is well retained by the models studied in this work. The Buchdahl limit is well obeyed by the proposed models. The models of four compact stars viz., PSR J1614-2230, Vela X-1, Cen X-3 and SMC X-4 are extensively analyzed in close agreement with their observational details.

The latest findings of static and spherically symmetric black hole solution give a potential platform to investigate the novel four-dimensional Einstein Gauss–Bonnet gravity. In order to obtain a rotating black hole solution, we first adopt the Newman Janis algorithm and study the structure of its horizons. To analyze the said black hole shadow, we move forward to compute expressions of the celestial coordinates using the geodesic equations. Furthermore, we provide a detailed analysis of the shadow size and its distortion parameter, adopting the Hioki and Maeda method, together with the applications of a supermassive black hole shadow in the center of nearby galaxy Messier 87 and obtained constraints on the relationships of spin and charge. From the obtained results, we demonstrate that both spin and coupling parameters of the black hole have a substantial influence on shadow structure. The increment in the values of these parameters diminishes the shadow radius. We also study the energy emission rate using the Hawking temperature. Furthermore, it is shown that whenever the collision of two electrically neutral test particles takes place in the vicinity of the black hole horizon, the Gauss–Bonnet black hole may serve as a particle accelerator with an infinitely large center of mass energy.

In the background dynamics of a spatially flat FLRW model of the universe, we investigate an interacting dark energy (DE) model in the context of Lyra’s geometry. Pressure-less dust is considered as dark matter, mass of which varies with time via scalar field in the sense that decaying of dark matter particles reproduces the scalar field. Here, quintessence scalar field is adopted as DE candidate which evolves in exponential potential. Mass of the dark matter particles is also considered to be evolved in exponential function of the scalar field. Cosmological evolution equations are studied in the framework of dynamical systems analysis. Dimension-less variables are chosen properly so that the cosmological evolution equations are converted into an autonomous system of ordinary differential equations. Linear stability is performed to find the nature of critical points by perturbing the system around the critical points in the phase-space. Classical stability is also executed by finding out the speed of sound. Dynamical systems explore several viable results which are physically interested in some parameter regions. Late-time scalar field-dominated attractors are found by critical points, corresponding to the accelerating universe. Scalar field-displacement vector field scaling solutions are realized that represent late-time decelerated universe. Dark energy-dark matter scaling solutions are also exhibited by critical points which correspond to accelerated attractors possessing similar order of energy densities of dark energy and dark matter, that provides the possible solutions of coincidence problem.

In the context of scalar–tensor theories, the inclusion of new degrees of freedom coupled nonminimally to the gravitational sector might produce some appealing effects on the cosmic expansion history. We investigate this premise by including a canonical SU(2) Yang–Mills field to the total energy budget of the universe coupled to the standard quintessential field by a disformal transformation.From the dynamical system analysis, we study three cases of cosmological interest that span most of the physical phase space of the model: the uncoupled limit, the isotropic and the Abelian cases. New scaling solutions with a nonvanishing gauge field are found in all cases which can be interesting for early cosmological scenarios. Some of these scaling solutions even exhibit anisotropic features. Also, the background evolution of the universe is studied by means of numerical analysis. As an interesting result, the disformal coupling changes the equation of state of the gauge field from radiation to matter at some stages of the evolution of the universe, thereby the gauge field can contribute to some fraction of the total dark matter. We have also quantified the redshift-dependent contribution of the gauge field in the form of dark radiation during the radiation era to the effective number of relativistic species. This depends essentially on the initial conditions and, more importantly, on the disformal coupling function.

In this paper, we study a cosmological model governed by a generalized scalar field nonminimally coupled to scalar torsion in the framework of teleparallel gravity. By applying the Noether symmetry approach, we attempt to explore the explicit forms of the coupling function and the scalar field potential in the Friedmann–Lemaitre–Robertson–Walker (FLRW) flat spacetime. The existence of such symmetry permits the equations of motion to take a simpler form and provides an exponential form cosmological solution for both the scale factor and scalar field. It is shown that the tachyon field behaves as the origin of inflation and contributes significantly to the accelerating expansion of the universe in the early times. While, in the late times, gravitational sources are a pressureless matter field together with the tachyon field, which plays the essential role of dark energy and is the origin of deceleration-acceleration phase transition. Later on, it is also shown that the present model contains a cosmological degeneracy in the sense that different coupling parameters and tachyonic potentials can contribute to the same cosmological behaviors such as the cosmic acceleration, age, equation of state and mean Hubble of FLRW universe.

The properties of proto neutron star (PNS) PSR J0740+6620 are studied with the relative mean field theory. In our calculations, we find that five nucleon coupling parameters (DD-MEI, NL1, NL2, TW99 and GM1) can give the mass of the PNS PSR J0740+6620. The radius of PNS PSR J0740+6620 calculated by GM1 is the smallest, $R=14.63$–$13.44$km, and that calculated by TW99 is the largest, $R=17.46$–$17.07$km. The radius calculated by other nucleon coupling parameters is between the two. We find that properties of PNS PSR J0740+6620 calculated by different nucleon coupling parameters are different. The central baryon density ${\rho }_c$, the central field strength ${\sigma }_{0c}$, ${\omega }_{0c}$, ${\sigma }_{0c}^{\ast }$ and ${\varphi }_{0c}$ of mesons $\sigma$, $\omega$, ${\sigma }^{\ast }$ and $\varphi$, the central energy density ${𝜀}_c$, and the central pressure $p_c$ calculated by GM1 are all the largest among the five sets of nucleon coupling parameters. The calculations with DD-MEI, TW99 and GM1 show that eight baryons (n, p, $\mathrm{\Lambda }$, ${\mathrm{\Sigma }}^{-}$, ${\mathrm{\Sigma }}^0$, ${\mathrm{\Sigma }}^{+}$, ${\mathrm{\Xi }}^{-}$ and ${\mathrm{\Xi }}^0$) appear inside PNS PSR J0740+6620 whereas those with NL1 and NL2 show that hyperon ${\mathrm{\Sigma }}^{+}$ does not appear. The relative particle density of baryons inside PNS PSR J0740+6620 is also very different as the PNS is calculated with different nucleon coupling constants.

Hořava–Lifshitz gravity is a very fascinating proposal of modified gravity for explaining the current acceleration and other cosmic issues. In continuation to this discussion, we explore the consequences of Tsallis dark energy with Hubble and particle horizons in the framework of deformed Hořava–Lifshitz gravity for flat FRW universe. The interaction between Tsallis holographic dark energy and dark matter is also taken into account to discuss the underlying scenario. It is found that fractional dark energy density (${\mathrm{\Omega }}_D$) approaches to $0.70_{-0.03}^{+0.03}$ as redshift goes to zero (present epoch) for both cases of horizons. The cosmic parameter (${\omega }_D$) shows phantom crossing behavior in both models, i.e. it evolutes the universe from quintessence phase to phantom phase by crossing the vacuum horizon. In both cases of horizons, the deceleration supports the cosmic acceleration by exhibiting the negative values. The perturbation sound speed also favors the viability of our models through positive behavior. The coincidence parameter supports the exchange of dark matter to dark energy phenomenon through positivity. We also discuss the behavior of growth rate and growth index in the growth of matter density perturbations for both horizons. These parameters mimic the $\mathrm{\Lambda }$CDM behavior of the universe.

Spherical energy shells in General Relativity tend to collapse due to gravitational effects and/or due to tension effects. Shell stabilization may be achieved by modifying the gravitational properties of the background spacetime. Thus, gravastars consist of stiff matter shells with an interior deSitter space and an exterior Schwarzshild spacetime whose attractive gravity balances the interior repulsive gravity of the interior deSitter spacetime leading to a stable stiff matter shell. Similar stabilization effects may be achieved by considering rotating shells. Here we study the stability of slowly rotating fluid shells. We show that the angular velocity of the shell has stabilizing properties analogous to the repulsive deSitter gravity of the interior of a gravastar. We thus use the Israel junction conditions [W. Israel, Nuovo Cim. B 44S10 (1966) 1, Erratum: Nuovo Cim. B 48 (1967) 463; N. Deruelle, M. Sasaki and Y. Sendouda, Prog. Theor. Phys. 119 (2008) 237, arXiv:0711.1150 [gr-qc]] and the fluid equation of state of the rotating shell to construct the dynamical equations that determine the evolution of the rotating shell radius. These dynamical equations depend on the parameters of the background spacetime and on the angular velocity of the shell. Assuming a rotating interior and a Schwarzschild exterior spacetime we show that the angular velocity of the shell has interesting stabilizing properties on the evolution of its radius R. Thus, rotating matter (or vacuum) shells can imitate black holes while avoiding the presence of a singularity and without the presence of an interior deSitter space.

The baryon acoustic oscillation (BAO) peak, seen in the cosmic matter distribution at redshifts up to $\sim$3.5, reflects the continued expansion of the sonic horizon first identified in temperature anisotropies of the cosmic microwave background. The BAO peak position can now be measured to better than $\sim$1% accuracy using galaxies and $\sim$1.4–1.6% precision with Ly-$\alpha$ forests and the clustering of quasars. In conjunction with the Alcock–Paczyński (AP) effect, which arises from the changing ratio of angular to spatial/redshift size of (presumed) spherically-symmetric source distributions with distance, the BAO measurement is viewed as one of the most powerful tools to use in assessing the geometry of the Universe. In this paper, we employ five BAO peak measurements from the final release of the Sloan Digital Sky Survey IV, at average redshifts $〈z〉=0.38$, $0.51$, $0.70$, $1.48$ and $2.33$, to carry out a direct head-to-head comparison of the standard model, $\mathrm{\Lambda }$CDM, and one of its principal competitors, known as the $R_{\mathrm{\text{h}}}=ct$ universe. For completeness, we complement the AP diagnostic with a volume-averaged distance probe that assumes a constant comoving distance scale $r_{\mathrm{\text{d}}}$. Both probes are free of uncertain parameters, such as the Hubble constant, and are therefore ideally suited for this kind of model selection. We find that $R_{\mathrm{\text{h}}}=ct$ is favored by these measurements over the standard model based solely on the AP effect, with a likelihood $\sim$75% versus $\sim$25%, while Planck-$\mathrm{\Lambda }$CDM is favored over $R_{\mathrm{\text{h}}}=ct$ based solely on the volume-averaged distance probe, with a likelihood $\sim 80\%$ versus $\sim$20%. A joint analysis using both probes produces an inconclusive outcome, yielding comparable likelihoods to both models. We are therefore not able to confirm with this work that the BAO data, on their own, support an accelerating Universe.

We study the properties of the null geodesics of four-dimensional Einstein–Gauss–Bonnet (4D EGB) black holes surrounded by quintessence matter, recently derived by Shah et al. [Phys. Lett. B 818 (2021) 136383]. Due to the quintessence correction, we discuss the radial and nonradial geodesics. For the case of nonradial geodesics we obtain the effective potential, photon sphere and impact parameter associated to the null geodesics in detail. We analyze the different possible orbits of massless particles (i.e. photons) such as unstable circular orbits and unbounded orbits and investigate the role of the Gauss–Bonnet coupling $\alpha$ and the quintessence parameter q on the null geodesic trajectories. Moreover, we study the effects of the model parameters on the shadow cast by such black holes. The results are compared to that of Schwarzschild black hole with and without quintessence matter and 4D EGB black holes without quintessence. As a physical application of null geodesics, the deflection angle is calculated and the effect of the quintessence matter on it is investigated.