arXiv subjects

Denitsa Staicova

Publications and source records attributed to Denitsa Staicova.

At least 19 recordsLinked to original sources

arXiv2026

White dwarfs in minimal dilatonic gravity

We study static, spherically symmetric white dwarfs in minimal dilatonic gravity (MDG) -- a Brans-Dicke theory with fixed coupling $\alpha^2=1/3 $ and free Compton length $\lambda_{\Phi}$. Solving the full relativistic structure equations we find that MDG white dwarfs are strictly sub-Chandrasekhar for every $\lambda_{\Phi}$: the maximum mass falls from $1.425\,M_\odot$ in GR to $1.27\,M_\odot$ at $200$ km and $1.09\,M_\odot$ at $500$ km. The ratio by which MDG reduces the maximum mass is robust against the equation of state. Including rigid rotation near mass shedding, consistency with the most massive observed white dwarfs restricts $\lambda_{\Phi} \lesssim 300$ km. An independent gravitational-redshift bound gives $\lambda_{\Phi} \lesssim 720$ km. Because the dilaton mass is density-independent the same coupling produces a percent-level altitude dependence of the Kepler-inferred $ GM_\oplus$, constraining it from the Solar-System side. To tackle the problem, we introduce a free-boundary collocation method with a linearized-exterior Robin condition that eliminates the exponential stiffness of shooting methods and gives access to the screened regime in double precision.

gr-qc↗

arXiv2026

Reconstructing the Type Ia Supernova Absolute Magnitude with Two-Probe Physics-Informed Neural Networks

We apply two variants of Physics-Informed Neural Networks (PINNs) to reconstruct the Type~Ia supernova absolute magnitude $M_B(z)$ from joint BAO and supernova data under four cosmological models ($\Lambda$CDM, CPL, GEDE, $\Lambda_s$CDM) and two DESI~DR2 fiducial sets. A heteroscedastic single-network method tested across four constraint configurations establishes that the Etherington distance duality relation is a more fundamental constraint than cosmological model priors, reducing internal inconsistencies by up to an order of magnitude. Under full constraints all models recover $M_B \approx -19.3$~mag with biases below 0.05~mag. A Fisher information-weighted two-network variant trains independent networks on BAO and SN data, providing clean probe separation; it finds no significant pointwise $M_B$ evolution in $z \in [0.3, 1.5]$, but reveals a systematic separation of redshift-binned $M_B$ distributions. The heteroscedastic method identifies a persistent $2$--$3\sigma$ residual at $z \sim 0.4$--$0.5$ that is consistent across all four models and both fiducials, implying the same underlying tension. While the origin of this feature remains ambiguous, its model-independence and cross-method consistency warrant further investigation with forthcoming data.

astro-ph.CO↗

arXiv2025

Statistical Nuances in BAO Analysis: Likelihood Formulations and Non-Gaussianities

We present a systematic comparison of statistical approaches to Baryon Acoustic Oscillation (BAO) analysis using DESI DR2 data. We evaluate four methods for handling the nuisance parameter $\beta=1/(H_0 r_d)$: marginalization, profiling, Taylor expansion, and full likelihood analysis across multiple cosmological models. Our results demonstrate that while these methods yield consistent constraints for $\Lambda$CDM and $\Omega_K$CDM models, they produce notable differences for models with dynamical dark energy parameters. Through eigenvalue decomposition of Fisher matrices, we identify extreme parameter degeneracies in $ww_a$CDM and $\Omega_Kww_a$CDM models that explain these statistical sensitivities. Surprisingly, $\Omega_K$CDM shows the highest information content across datasets, suggesting BAO measurements are particularly informative about spatial curvature. We further use skewness and kurtosis analysis to identify deviations from Gaussianity, highlighting limitations in Fisher approximations in the dark energy models. Our analysis demonstrates the importance of careful statistical treatment when extracting cosmological constraints from increasingly precise measurements.

astro-ph.CO↗

arXiv2025

Electromagnetic Waves in Cosmological Space-Time II. Luminosity Distance

In this article, we continue our investigation on how the electromagnetic waves propagate in the Friedman-Lemaitre-Robertson-Walker spacetime. Unlike the standard approach, which relies on null geodesics and geometric optics approximation, we derive explicit solutions for electromagnetic waves in expanding spacetime and examine their implications for cosmological observations. In particular, our analysis reveals potential modifications to the standard luminosity distance formula. Its effect on other cosmological parameters, e.g., the amount of cold dust matter in the Universe, is considered and estimated from Type Ia supernovae data. We see that this alternative model is able to fit the supernova data, but it gives a qualitatively different Universe without a cosmological constant but with stiff or ultra-stiff matter.

gr-qc↗

arXiv2025

Modern Bayesian Sampling Methods for Cosmological Inference: A Comparative Study

We present a comprehensive comparison of different Markov Chain Monte Carlo (MCMC) sampling methods, evaluating their performance on both standard test problems and cosmological parameter estimation. Our analysis includes traditional Metropolis-Hastings MCMC, Hamiltonian Monte Carlo (HMC), slice sampling, nested sampling as implemented in dynesty, and PolyChord. We examine samplers through multiple metrics including runtime, memory usage, effective sample size, and parameter accuracy, testing their scaling with dimension and response to different probability distributions. While all samplers perform well with simple Gaussian distributions, we find that HMC and nested sampling show advantages for more complex distributions typical of cosmological problems. Traditional MCMC and slice sampling become less efficient in higher dimensions, while nested methods maintain accuracy but at higher computational cost. In cosmological applications using BAO data, we observe similar patterns, with particular challenges arising from parameter degeneracies and poorly constrained parameters.

astro-ph.CO↗

arXiv2024

Late-Time constraints on Interacting Dark Energy: Analysis independent of $H_0$, $r_d$ and $M_B$

We investigated a possible interaction between cold dark matter and dark energy, corresponding to a well-known interacting dark energy model discussed in the literature within the context of resolving the Hubble tension. We put constraints on it in a novel way, by creating new likelihoods with an analytical marginalization over the Hubble parameter $H_0$, the sound horizon $r_d$, and the supernova absolute magnitude $M_B$. Our aim is to investigate the impacts on the coupling parameter of the interacting model, $ξ$, and the equation of state of dark energy $w$ and the matter density parameter $Ω_{m,0}$. The late-time cosmological probes used in our analysis include the PantheonPlus (calibrated and uncalibrated), cosmic chronometers, and baryon acoustic oscillation samples and the Pantheon for comparison. Through various combinations of these datasets, we demonstrate hints of an up to $2σ$ deviation from the standard $Λ$ cold dark matter model.

astro-ph.CO↗

arXiv2024

Studying the Supernova Absolute Magnitude Constancy with Baryonic Acoustic Oscillations

In this proceeding we review and expand on our recent work investigating the constancy of the absolute magnitude $M_B$ of Type Ia supernovae. In it, we used baryonic acoustic oscillations (BAO) to calibrate the supernova data and to check whether the resulting $M_B$ is constant. We used non-parametric methods like Gaussian processes and artificial neural networks to reconstruct $M_B(z)$. Here we elaborate on the results by putting them in the context of other studies investigating possible non-constant $M_B$ and the impact of the distance-duality relation. We also present some numerical details on the calculations in the original paper and new non-parametric reconstructions, including a conservative model-independent fit, confirming its main results. Notably, we see that $M_B$ remains constant within $1σ$, with a possible jump around $z = 0.01 - 0.15$. Furthermore, the observed distribution of $M_B(z)$ cannot be described by a single Gaussian, displaying multiple peaks and tails. The choice of the only remaining parameter -- the sound horizon $r_d$ leads to a tension in the $M_B-r_d$ plane. Fitting different non-constant $M_B(z)$ models does not significantly improve the fit and there is no preference for any of the models by the statistical measures we employ.

astro-ph.CO↗

arXiv2024

Probing for Lorentz Invariance Violation in Pantheon Plus Dominated Cosmology

The Hubble tension in cosmology is not showing signs of alleviation and thus, it is important to look for alternative approaches to it. One such example would be the eventual detection of a time delay between simultaneously emitted high-energy and low-energy photons in gamma-ray bursts (GRB). This would signal a possible Lorentz Invariance Violation (LIV) and in the case of non-zero quantum gravity time delay, it can be used to study cosmology as well. In this work, we use various astrophysical datasets (BAO, Pantheon Plus and the CMB distance priors), combined with two GRB time delay datasets with their respective models for the {\em {intrinsic time delay}}. Since the intrinsic time delay is considered the largest source of uncertainty in such studies, finding a better model is important. Our results yield as quantum gravity energy bound $E_{QG}\ge 10^{17}$ GeV and $E_{QG}\ge 10^{18}$ GeV respectively. The difference between standard approximation (constant intrinsic lag) and the extended (non-constant) approximations is minimal in most cases we coincide. However, the biggest effect on the results comes from the prior on the parameter $\frac{c}{H_0 r_d}$, emphasizing once again that at current precision, cosmological datasets are the dominant factor in determining the cosmology. We estimate the energies at which cosmology gets significantly affected by the time delay dataset

gr-qc↗

arXiv2024

Dark Energy as a Critical Period in Binary Motion: Bounds from Multi-scale Binaries

The two-body problem under the influence of both dark energy and post-Newtonian modifications is studied. In this unified framework, we demonstrate that dark energy plays the role of a critical period with $T_Λ = 2π/c \sqrtΛ \approx 60~\text{Gyr}$. We also show that the ratio between orbital and critical period naturally emerges from the Kretschmann scalar, which is a quadratic curvature invariant characterizing all binary systems effectively represented by a de Sitter-Schwarzschild spacetime. The suitability of a binary system to constrain dark energy is determined by the ratio between its Keplerian orbital period $T_\text{K}$ and the critical period $T_Λ$. Systems with $T_\text{K} \approx T_Λ$ are optimal for constraining the cosmological constant $Λ$, such as the Local Group and the Virgo Cluster. Systems with $T_{\text{K}} \ll T_Λ$ are dominated by attractive gravity (which are best suited for studying modified gravity corrections). Systems with $T_{\text{K}} \gg T_Λ$ are dominated by repulsive dark energy and can thus be used to constrain $Λ$ from below. We use our unified framework of post-Newtonian and dark-energy modifications to calculate the precession of bounded and unbounded astrophysical systems and infer constraints on $Λ$ from them. Pulsars, the solar system, S stars around Sgr A*, the Local Group, and the Virgo Cluster, having orbital periods of days to gigayears, are analyzed. The results reveal that the upper bound on the cosmological constant decreases when the orbital period of the system increases, emphasizing that $Λ$ is a critical period in binary motion.

astro-ph.CO↗

arXiv2023

Impact of Cosmology on Lorentz Invariance Violation Constraints from GRB Time-Delays

Putting constraints on a possible Lorentz Invariance Violation (LIV) from astrophysical sources such as gamma-ray bursts (GRBs) is essential for finding evidences of new theories of quantum gravity (QG) that predict an energy-dependent speed of light. This search has its own difficulties, so usually, the effect of the cosmological model is understudied, with the default model being a fixed-parameters $Λ$CDM. In this work, we use various astrophysical datasets to study the effect of a number of dark energy models on LIV constraints. To this end, we combine two public time-delay GRB datasets with the supernovae Pantheon dataset, several measurements of angular baryonic acoustic oscillations (BAO), the cosmic microwave background (CMB) distance prior and an optional GRB or quasars dataset. For the LIV parameter $α$, we find the expected from previous works average value of $α\sim 4 \times 10^{-4}$, corresponding to $E_{QG}\ge 10^{17}$ GeV for both time-delay (TD) datasets, with the second one being more sensitive to the cosmological model. The cosmology results in a minimum 20\% deviation in our constraints on the energy. Interestingly, adding the TD points makes the DE models less-preferable statistically and shifts the value of the parameter $c/(H_0 r_d)$ down, making it smaller than the expected value. We observe that possible LIV measurements critically depend on the transparency of the assumptions behind the published data concerning cosmology, and taking this into account may be an important contribution in the case of possible detection.

gr-qc↗

arXiv2023

Model selection results from different BAO datasets -- DE models and $Ω_K$CDM

The use of the baryonic acoustic oscillations (BAO) datasets offers a unique opportunity to connect the early universe and the late one. In this proceeding, we discuss recent results that used a marginalised likelihood to remove the $H_0-r_d $ degeneracy and then tested it on different dark energy (DE) models. It was found that this approach which does not rely on calibration on $r_d$ or $H_0$, allows us to obtain results, comparable to the ones calculated with standard likelihoods. Here we emphasize on the major differences that we observed for the two different BAO datasets that we employed -- a transversal one, containing only angular BAO measurements, and a mixed one, containing both angular and radial BAO measurements. We see that the two datasets have different statistical preferences for DE models and also different preference for the curvature of the universe.

astro-ph.CO↗

arXiv2022

Special cases of the Multi-Measure Model -- understanding the prolonged inflation

The multi-measure model (MMM), in which one modifies the action to include both the Riemannian measure and a non-Riemannian one, has proven to be able to produce viable Universe evolution scenarios. In this article we consider two special cases of the multi-measure model, in which we first decouple the two kinetic terms in the Lagrangian entirely and later remove the dark charge of the model. We show numerically that those special cases still possess the needed evolutionary stages of the Universe and furthermore, for them one can obtain a sufficient number of e-folds of the early inflation. In the first case, the inflaton still moves backwards on the effective potential during inflation, while in the second, it does not, meaning that this behavior comes from the dark charge. We connect the model with hyperinflationary models and investigate how the different epochs are born from the interplay between the two scalar fields. We demonstrate that there is a dynamically induced slow-roll epoch, which is prolonged by the complicated movement of the two scalars in the field space. Finally, we show that while the adiabatic speed of sound can become imaginary, the phase speed of sound remains real.

gr-qc↗

arXiv2022

DE models with combined $H_0 \cdot r_d $ from BAO and CMB dataset and friends

It has been theorized that Dynamical Dark Energy (DDE) could be a possible solution to the Hubble tension. To avoid the degeneracy between the Hubble parameter $H_0$ and the sound horizon scale $r_d$, in this article we use their multiplication as one parameter $c/\left(H_0 r_d\right)$ and we use it to infer cosmological parameters for 6 different models - $Λ$CDM and 5 DDE parametrizations -- the Chevallier-Polarski-Linder (CPL), the Barboza-Alcaniz (BA), the Low correlation (LC), the Jassal-Bagla-Padmanabhan (JBP) and the Feng-Shen-Li-Li model. We choose a dataset that treats this combination as one parameter, that includes the Baryon Acoustic Oscillation (BAO) data $0.11 \le z \le 2.40$ and additional points from the Cosmic Microwave Background (CMB) Peaks ($z \approx 1090$). To them, we add the marginalized Pantehon dataset and GRB dataset. We see that the tension is moved from $H_0$ and $r_d$ to $c/\left(H_0 r_d\right)$ and $Ω_m$. There is only one model that satisfies the Planck 2018 constraints on both parameters and this is LC with a huge error. The rest cannot fit into both constraints. $Λ$CDM is preferred with respect to the statistical measures.

astro-ph.CO↗

arXiv2022

On the Robustness of the Constancy of the Supernova Absolute Magnitude: Non-parametric Reconstruction \& Bayesian approaches

In this work, we test the robustness of the constancy of the Supernova absolute magnitude $M_B$ using Non-parametric Reconstruction Techniques (NRT). We isolate the luminosity distance parameter $d_L(z)$ from the Baryon Acoustic Oscillations (BAO) data set and cancel the expansion part from the observed distance modulus $μ(z)$. Consequently, the degeneracy between the absolute magnitude and the Hubble constant $H_0$, is replaced by a degeneracy between $M_B$ and the sound horizon at drag epoch $r_d$. When imposing the $r_d$ value, this yields the $M_B(z) = M_B + δM_B(z)$ value from NRT. We perform the respective reconstructions using the model independent Artificial Neural Network (ANN) technique and Gaussian processes (GP) regression. For the ANN we infer $M_B = -19.22\pm0.20$, and for the GP we get $M_B = -19.25\pm0.39$ as a mean for the full distribution when using the sound horizon from late time measurements. These estimations provide a $1\,σ$ possibility of a nuisance parameter presence $δM_B(z)$ at higher redshifts. We also tested different known nuisance models with the Markov Chain Monte Carlo (MCMC) technique which showed a strong preference for the constant model, but it was not possible not single out a best fit nuisance model.

astro-ph.CO↗

arXiv2022

Constraining the dark energy models using Baryon Acoustic Oscillations: An approach independent of $H_0 \cdot r_d$

The $H_0$ tension and the accompanying $r_d$ tension are a hot topic in current cosmology. In order to remove the degeneracy between the Hubble parameter $H_0$ and the sound horizon scale $r_d$ from the Baryon Acoustic Oscillations (BAO) datasets, we redefine the likelihood by marginalizing over the $H_0 \cdot r_d$ parameter and then we perform full Bayesian analysis for different models of dark energy (DE). We find that our uncalibrated by early or late physics {datasets cannot} constrain the DE models properly without further assumptions. By adding the type IA supernova dataset, the models are constrained better with smaller errors on the DE parameters. The two BAO datasets we use -- one with angular measurements and one with angular and radial ones with their covariances, show statistical preferences for different models, with $Λ$CDM being the best model for one of them. Adding the Pantheon SnIA dataset with its covariance matrix boosts the statistical preference for $Λ$CDM.

astro-ph.CO↗

arXiv2021

Hints of the $H_0-r_d$ tension in uncorrelated Baryon Acoustic Oscillations dataset

Baryon Acoustic Oscillations (BAO) datasets use very precise measurements of the spatial distribution of large-scale structures as a distance ladder to help constrain cosmological parameters. In a recent article \cite{Benisty:2020otr}, we combined 17 uncorrelated BAO measurements in the effective redshift range $0.106 \le z \le 2.36$ with the Cosmic Chronometers data, the Pantheon Type Ia supernova and the Hubble Diagram of Gamma Ray Bursts and Quasars to obtain that the $Λ$CDM model fit infers for the Hubble constant: $69.85 \pm 1.27km/sec/Mpc$ and for the sound horizon distance: $146.1 \pm 2.15Mpc$. Beyond the $Λ$CDM model we test $Ω_k$CDM and wCDM and we get $Ω_k = -0.076 \pm 0.012$, $w = -0.989 \pm 0.049$ accordingly. In this proceeding we present elaborate on our findings and we compare them to other recent results in the literature.

astro-ph.CO↗

arXiv2021

The role of the slope in the the multi-measure cosmological model

In this work, we report some results on the numerical exploration of the model of Guendelman-Nissimov-Pacheva. This model has been previously applied to cosmology, but there were open questions regarding its parameters. Here we demonstrate the existence of families of solutions on the slope of the effective potential which preserve the duration of the inflation and its power. For this solutions, one can see the previously reported phenomenon of the inflaton scalar field climbing up the slope, with the effect more pronounced when starting lower on the potential slope. Finally we compare the dynamical and the potential slow-roll parameters for the model and we find that the latter describe the numerically observed inflationary period better.

gr-qc↗

arXiv2020

Testing Late Time Cosmic Acceleration with uncorrelated Baryon Acoustic Oscillations dataset

Baryon Acoustic Oscillations (BAO) involve measuring the spatial distribution of galaxies to determine the growth rate of cosmic structure. We derive constraints on cosmological parameters from $17$ uncorrelated BAO measurements that were collected from $333$ published data points in the effective redshift range $0.106 \leq z \leq 2.36$. We test the correlation of the subset using random covariance matrix. The $Λ$CDM model fit yields the cosmological parameters: $Ω_m = 0.261 \pm 0.028$ and $Ω_Λ= 0.733 \pm 0.021$. Combining the BAO data with the Cosmic Chronometers data, the Pantheon Type Ia supernova and the Hubble Diagram of Gamma Ray Bursts and Quasars, the Hubble constant yields $ 69.85 \pm 1.27 km/sec/Mpc$ and the sound horizon distance gives: $ 146.1 \pm 2.15 Mpc$. Beyond the $Λ$CDM model we test $Ω_K$CDM and wCDM. The spatial curvature is $Ω_k = -0.076 \pm 0.012$ and the dark energy equation of states: $w = -0.989 \pm 0.049$. {We perform AIC test to compare the 3 models and see that $Λ$CDM scores best.

astro-ph.CO↗