arXiv subjects

Gansukh Tumurtushaa

Publications and source records attributed to Gansukh Tumurtushaa.

At least 19 recordsLinked to original sources

arXiv2026

Quantum Suppression of Mass Inflation in Reissner-Nordström Interiors via Wheeler-DeWitt Equation

We construct a canonical quantization, the Wheeler-DeWitt equation, of the interior geometry of static and spherically symmetric black holes in Einstein-Maxwell-$Λ$ framework, focusing on Reissner-Nordström. The wave function of the Wheeler-DeWitt equation for the Reissner-Nordström black hole is set to be on-shell and exhibiting exponential damping away from the classical locus. Horizon boundary conditions for the wave function generate two regimes: a single inward mode from event horizon yields monotonic decay, while superpositions produce either a quantum bounce (single time arrow) or interference-driven annihilation-to-nothing (two time arrows). We show that these are generic features of static black hole interiors. Furthermore, the wave function of the Schwarzschild black hole, obtained as the charge-neutral limit of the Reissner-Nordström black hole, is monotonically decaying and no longer unbounded. Moreover, this framework unifies classical and quantum interiors, suggests a quantum gravitational suppression to the mass inflation, and motivates extensions to Kerr and regular black holes.

gr-qc↗

arXiv2025

Implications of pulsar timing arrays for Gauss-Bonnet Inflation

Correlated time-of-arrival measurements by pulsar timing arrays (PTAs) have provided a new means of constraining astrophysical or cosmological models that produce a gravitational wave (GW) background. For this work, we discuss the implications of PTA observations for Gauss-Bonnet (GB) inflationary models through the production and propagation of inflationary GWs. We show that our GB inflationary scenario is consistent with present PTA and cosmological data. A blue-tilted tensor power spectrum supported by PTAs can be naturally accommodated in GB inflation. Using observational constraints, we derive general conditions for the inflaton potential and the GB coupling function, suggesting that in GB inflation, the inflaton must climb up the potential before rolling downhill and reaching the end of inflation. We provide two concrete GB inflationary models to demonstrate the viability of this mechanism.

astro-ph.CO↗

arXiv2024

Higgs Inflation with a Gauss-Bonnet term

Higgs inflation with a Gauss-Bonnet term is studied in the Einstein frame. Our model features two coupling functions, $Ω^2(ϕ)$ and $ω(ϕ)$, coupled to the Ricci scalar and Gauss-Bonnet combinations. We found a special relation $Ω^2 \propto ω$ sets the system a lot more simplified; therefore, we take it for granted in our analytical studies. As a result of a Weyl transformation to the Einstein frame, we notice the emergence of new interactions: a non-minimal kinetic coupling between the scalar field and gravity and a derivative self-interaction of the scalar field. In the Einstein frame, we investigate the cosmological implications of these interactions by deriving the background equation of motion and observable quantities. Our numerical result on $n_S$ vs. $r$ suggests our model is consistent with the observational data for a wide range of the model parameter, $-1.4\times 10^4\lesssim α\equiv \fracω{Ω^2} \lesssim 8\times 10^3$, where both the positive and negative values of $α$ are allowed. As the Gauss-Bonnet contributions decay away with time after inflation, the propagation speed of gravitational waves turned out to be consistent with the recent constraints on the propagation speed of gravitational waves (GWs) without inducing ghost instability.

gr-qc↗

arXiv2024

Existence of gradient CKV and gradient conformally stationary LRS spacetimes

In this work, we study the existence of gradient (proper) CKVs in locally rotationally symmetric spacetimes (LRS), those CKVs in the space spanned by the tangent to observers' congruence and the preferred spatial direction, allowing us to provide a (partial) characterization of gradient conformally static (GCSt) LRS solutions. Irrrotational solutions with non-zero spatial twist admit an irrotational timelike gradient conformal Killing vector field and hence are GCSt. In the case that both the vorticity and twist vanish, that is, restricting to the LRS II subclass, we obtain the necessary and sufficient condition for the spacetime to admit a gradient CKV. This is given by a single wave-like PDE, whose solutions are in bijection to the gradient CKVs on the spacetime. We then introduce a characterization of these spacetimes as GCSt using the character of the divergence of the CKV, provided that the metric functions of the spacetimes obey certain inequalities.

gr-qc↗

arXiv2023

Dynamical Compactification with Matter

In this work, we study cosmological solutions of the 8-dimensional Einstein Yang-Mills theory coupled to a perfect-fluid matter. A Yang-Mills instanton of extra dimensions causes a 4-dimensional expanding universe with dynamical compactification of the extra dimensions. To construct physically reliable situations, we impose the null energy condition on the matter. This energy condition is affected by the extra dimensions. Then, we consider cosmological constant to grasp the structure of the solution space. Even in this simple case, we find several interesting solutions, such as bouncing universes and oscillatory solutions, eventually arriving at a de Sitter universe with stabilized compact dimensions. In addition, we consider a class of matters whose energy density depends on the volume of the extra dimensions. This case shows another set of bouncing universes. Also, a real scalar with potential is taken into account. The scalar field model admits de Sitter solutions due to the choice of potential, and we demonstrate how potentials can be constructed using flow equations. {\color{black}Thus, what we discuss in this work is based on the 8-dimensional Einstein frame, which corresponds to the 4-dimensional Jordan frame by dimensional reduction. Consequently, the results are derived in the 4-dimensional Jordan frame, not in the 4-dimensional Einstein frame.

hep-th↗

arXiv2023

Wheeler-DeWitt equation beyond the cosmological horizon: Annihilation to nothing, infinity avoidance, and loss of quantum coherence

We investigate the Schwarzschild-(anti) de Sitter spacetime with the anisotropic metric ansatz. The Wheeler-DeWitt equation for such a metric is solved numerically. In the presence of the cosmological constant $Λ$, we show that two classical wave packets can be annihilated inside the black hole horizon, i.e., the annihilation-to-nothing scenario. It is interesting that the Wheeler-DeWitt equation can be extended to the asymptotic de Sitter spacetime outside the cosmological horizon. Surprisingly, the only bounded nontrivial wave function beyond the cosmological horizon satisfies the DeWitt boundary condition, i.e., the wave function must vanish at a certain finite radius. This might be an alternative explanation to the classicalization of quantum fluctuations in the de Sitter space, where this topic is also related to decoherence.

gr-qc↗

arXiv2022

Dynamical analysis in regularized $4D$ Einstein-Gauss-Bonnet gravity with non-minimal coupling

We investigate the regularized four-dimensional Einstein-Gauss-Bonnet ($4D$ EGB) gravity with a non-minimal scalar coupling function, which is an extension of the regularized $4D$ EGB theory. By introducing non-minimal coupling to the Gauss-Bonnet term, we demonstrate the additional contribution to the dynamical equations which is otherwise absent in the dimensionally-regularized theory. Furthermore, we analyze the stability of the system by using the dynamical system approach based on fixed points. Then, we consider the time evolution to investigate the history of the universe and constraint with observational data to obtain the cosmological parameters of the model.

gr-qc↗

arXiv2021

The reheating constraints to natural inflation in Horndeski gravity

For the subclass of Horndeski theory of gravity, we investigate the effects of reheating on the predictions of natural inflation. In the presence of derivative self-interaction of a scalar field and its kinetic coupling to the Einstein tensor, the gravitational friction to inflaton dynamics is enhanced. As a result, the tensor-to-scalar ratio $r$ is suppressed. We place the observational constraints on a natural inflation model and show that the model is now consistent with the observational data for some plausible range of the model parameter $Δ$, mainly due to the suppressed tensor-to-scalar ratio. To be consistent with the data at the $1σ$ ($68\%$ confidence) level, a slightly longer $N_k\gtrsim60$ duration of inflation than usually assumed is preferred. Since the duration of inflation, for any specific inflaton potential, is related to reheating parameters, including the duration $N_{re}$, temperature $T_{re}$, and equation-of-state $ω_{re}$ parameter during reheating, we imposed the effects of reheating to the inflationary predictions to put further constraints. The results show that the duration of inflation $N_k$ is affected by considerations of reheating, mainly by the $ω_{re}$ and $T_{re}$ parameters. If reheating occurs instantaneously for which $N_{re}=0$ and $ω_{re}=1/3$, the duration of inflation is estimated to be $N_k\simeq57$, where the exact value is less sensitive to the model parameter $Δ$ compatible with the CMB data. The duration of inflation is longer (or shorter) than $N_k\simeq57$ for the equation of state larger (or smaller) than 1/3 hence $N_{re}\neq0$. The maximum temperature at the end of reheating is $T_{re}^\text{max}\simeq3\times 10^{15}$ GeV, which corresponds to the instantaneous reheating. The low reheating temperature, as low as a few MeV, is also possible when $ω_{re}$ is closer to $1/3$.

astro-ph.CO↗

arXiv2021

Primordial black holes and induced gravitational waves from inflation in the Horndeski theory of gravity

We investigate the production of primordial black holes (PBHs) and scalar-induced gravitational waves (GWs) for cosmological models in the Horndeski theory of gravity. The cosmological models of our interest incorporate the derivative self-interaction of the scalar field and the kinetic coupling between the scalar field and gravity. We show that the scalar power spectrum of the primordial fluctuations can be enhanced on small scales due to these additional interactions. Thus, the formation of PBHs and the production of induced GWs are feasible for our model. Parameterizing the scalar power spectrum with a local Gaussian peak, we first estimate the abundance of PBHs and the energy spectrum of GWs produced in the radiation-dominated era. Then, to explain the small-scale enhancement in the power spectrum, we reconstruct the inflaton potential and self-coupling functions from the power spectrum and their spectral tilt. Our results show that the small-scale enhancement in the power spectrum can be explained by the local feature, either a peak or dip, in the self-coupling function rather than the local feature in the inflaton potential.

gr-qc↗

arXiv2021

Observational Constraints on the Cosmology with Holographic Dark Fluid

We consider the holographic Friedman-Robertson-Walker (hFRW) universe on the 4-dimensional membrane embedded in the 5-dimensional bulk spacetime and fit the parameters with the observational data. In order to fully account for the phenomenology of this scenario, we consider the models with the brane cosmological constant and the negative bulk cosmological constant. The contribution from the bulk is represented as the holographic dark fluid on the membrane. We derive the universal modified Friedmann equation by including all of these effects in both braneworld and holographic cutoff approaches. For three specific models, namely, the pure hFRW model, the one with the brane cosmological constant, and the one with the negative bulk cosmological constant, we compare the model predictions with the observations. The parameters in the considered hFRW models are constrained with observational data. In particular, it is shown that the model with the brane cosmological constant can fit data as well as the standard $Λ$CDM universe. We also find that the $σ_8$ tension observed in different large-structure experiments can be effectively relaxed in this holographic scenario.

astro-ph.CO↗

arXiv2020

Generalized Holographic Cosmology: low-redshift observational constraint

Four-dimensional cosmological models are studied on a boundary of a five-dimensional Anti-de Sitter (AdS_5) black hole with AdS Reissner-Nordstrom and scalar charged Reissner- Nordstrom black hole solutions, where we call the former a "Hairless" black hole and the latter a "Hairy" black hole. To obtain the Friedmann-Robertson-Walker (FRW) spacetime metric on the boundary of the AdS_5 black hole, we employ Eddington-Finkelstein (EF) coordinates to the bulk geometry. We then derive modified Friedmann equations on a boundary of the AdS_5 black hole via AdS/CFT correspondence and discuss its cosmological implications. The late-time acceleration of the universe is investigated in our models. The contributions coming from the bulk side is treated as a dark energy source, and we perform MCMC analyses using observational data. Compared to the LCDM model, our models contain additional free parameters; therefore, to make a fair comparison, we use the Akaike information criterion (AIC) and the Bayesian information criterion (BIC) to analyze our results. Our numerical analyses show that our models can explain the observational data as reliable as the LCDM model does for the current data.

gr-qc↗

arXiv2020

Euclidean wormholes in Gauss-Bonnet-dilaton gravity

We investigate Euclidean wormholes in Gauss-Bonnet-dilaton gravity to explain the creation of the universe from nothing. We considered two types of dilaton couplings (i.e., the string-inspired model and the Gaussian model) and we obtained qualitatively similar results. There can exist Euclidean wormholes that explain the possible origin of our universe, where the dilaton field is located over the barrier of dilaton potential. This solution can exist even if dilaton potential does not satisfy slow-roll conditions. In addition, the probability is higher than that of the Hawking-Moss instanton with the same final condition. Therefore, Euclidean wormholes in Gauss-Bonnet-dilaton gravity are a possible and probable scenario, which explains the origin of our universe.

gr-qc↗

arXiv2020

Constraints on dark energy models from the Horndeski theory

In light of the cosmological observations, we investigate dark energy models from the Horndeski theory of gravity. In particular, we consider cosmological models with the derivative self-interaction of the scalar field and the derivative coupling between the scalar field and gravity. We choose the self-interaction term to have an exponential function of the scalar field with both positive and negative exponents. For the function that has a positive exponent, our result shows that the derivative self-interaction term plays an important role in the late-time universe. On the other hand, to reproduce the right cosmic history, the derivative coupling between the scalar field and gravity must dominate during the radiation-dominated phase. However, the importance of such a coupling in the present universe found to be negligible due to its drastic decrease over time. Moreover, the propagation speed of gravitational waves estimated for our model is within the observational bounds, and our model satisfies the observational constraints on the dark energy equation of state.

gr-qc↗

arXiv2020

The viable f(G) gravity models via reconstruction from the observations

We reconstruct the viable f(G) gravity models from the observations and provide the analytic solutions that well describe our numerical results. In order to avoid unphysical challenges that occur during the numerical reconstruction, we generalize f(G) models into f(GA), which is the simple extension of f(G) models with the introduction of a constant A parameter. We employ several observational data together with the stability condition, which reads d2f/dG2 > 0 and must be satisfied in the late-time evolution of the universe, to give proper initial conditions for solving the perturbation equation. As a result, we obtain the analytic functions that match the numerical solutions. Furthermore, it might be interesting if one can find the physical origin of those analytic solutions and its cosmological implications.

astro-ph.CO↗

arXiv2020

Trans-Planckian censorship and single-field inflaton potential

It was recently proposed that a field theory cannot be consistent with quantum gravity if it allows a mode shorter than the Planck length to exit the Hubble horizon. This is called the Trans-Planckian Censorship Conjecture (TCC). We discuss the implications of the TCC on the possible shape of the inflaton potential in single-field slow-roll inflation. We point out that (1) there is generically an initial condition in which the total e-folding number $N_\text{total}$ is doubled or more compared to the e-folds necessary for the cosmic microwave background fluctuations, and (2) a sizable negative running of spectral index is generically expected to make $N_\text{total}$ small. In concrete setups, we find a stringent constraint on the inflationary energy scale, $V_\text{inf}^{1/4} < \mathcal{O}(10) \, \text{TeV}$ with $r < \mathcal{O}(10^{-50})$, and the running parameter is bounded above as $α_\text{s} \lesssim - 4 \times 10^{-3}$.

hep-th↗

arXiv2019

Inflation with Derivative Self-interaction and Coupling to Gravity

We consider a subclass of Horndeski theories for studying cosmic inflation. In particular, we investigate models of inflation in which the derivative self-interaction of the scalar field and the non-minimal derivative coupling to gravity are present in the action and equally important during inflation. In order to control contributions of each term as well as to approach the single-term limit, we introduce a special relation between the derivative interaction and the coupling to gravity. By calculating observable quantities including the power spectra and spectral tilts of scalar and tensor perturbation modes, and the tensor-to-scalar ratio, we found that the tensor-to-scalar ratio is suppressed by a factor of $(1+1/γ)$, where $γ$ reflects the strength of the derivative self-interaction of the inflaton field with respect to the derivative coupling gravity. We placed observational constraints on the chaotic and natural inflation models and showed that the models are consistent with the current observational data mainly due to the suppressed tensor-to-scalar ratio.

gr-qc↗

arXiv2019

Quantum creation of traversable wormholes ex nihilo in Gauss-Bonnet-dilaton gravity

We investigate a nucleation of a Euclidean wormhole and its analytic continuation to Lorentzian signatures in Gauss-Bonnet-dilaton gravity, where this model can be embedded by the type-II superstring theory. We show that there exists a Euclidean wormhole solution in this model by choosing a suitable shape of the dilaton potential. After the analytic continuation, this explains a quantum creation of a time-like traversable wormhole. Finally, we discuss relations to the information loss problem and the current literature.

hep-th↗

arXiv2019

Emergent Dark Universe and the Swampland Criteria

We study a model of the emergent dark universe, which lives on the time-like hypersurface in a five-dimensional bulk spacetime. The holographic fluid on the hypersurface is assumed to play the role of the dark sector, mainly including the dark energy and apparent dark matter. Based on the modified Friedmann equations, we present a Markov-Chain-Monte-Carlo analysis with the observational data, including type Ia Supernova and the direct measurement of the Hubble constant. We obtain a good fitting result and the matter component turns out to be small enough, which matches well with our theoretical assumption that only the normal matter is required. After considering the fitting parameters, an effective potential of the model with a dynamical scalar field is reconstructed. The parameters in the swampland criteria are extracted, and they satisfy the criteria at the present epoch but are in tension with the criteria if the potential is extended to the future direction. The method to reconstruct the potential is helpful to study the swampland criteria of other models without an explicit scalar field.

hep-th↗