arXiv subjects

Afaq Maqsood

Publications and source records attributed to Afaq Maqsood.

5 recordsLinked to original sources

arXiv2026

Derivative hierarchy as the origin of kernel-dependent trends in Gaussian process reconstructions of the hubble parameter

We perform a model independent reconstruction of the cosmic expansion history using Gaussian Process regression, investigating how the smoothness properties of covariance kernels influence the inferred Hubble parameter. Using 32 cosmic chronometer measurements, we reconstruct $H(z)$ with the Mat\'ern $3/2$, $5/2$, $7/2$, $9/2$, and Squared Exponential kernels, which form a well-defined hierarchy of differentiability. Across this hierarchy, we observe a systematic and monotonic trend in which increasing kernel smoothness is associated with progressively lower reconstructed values of the present-day Hubble constant $H_0$, accompanied by reduced statistical uncertainties. Rather than indicating a statistical preference for a specific kernel, this behaviour reflects the sensitivity of non-parametric reconstructions to the assumed smoothness prior encoded in the covariance function. The same ordering is reflected in descriptive goodness-of-fit statistics and remains stable when incorporating recent DESI DR2 BAO measurements and performing jackknife resampling tests, demonstrating robustness against individual data points and dataset variations. Additional analysis of derivative reconstructions shows that differences in kernel differentiability propagate into the local slope of the expansion history, providing a consistent interpretation of the observed ordering in $H_0$. Our results highlight that kernel smoothness plays an important role in Gaussian Process reconstructions and should be carefully accounted for when interpreting non-parametric cosmological inferences.

astro-ph.CO↗

arXiv2026

Gaussian process reconstruction of scalar field dynamics from recent cosmological data

We reconstruct the late-time expansion history and dark energy dynamics using available cosmological data. We consider dark energy as an effective minimally coupled canonical scalar field without assuming a specific form for its potential. For reconstruction, we use Gaussian Process (GP) regression with a joint dataset consisting of 32 cosmic chronometer measurements (CC32), DESI DR2 baryon acoustic oscillation data, three Type Ia supernova compilations (Pantheon+, Union3, and DES Y5), and compressed CMB distance priors. From the reconstructed Hubble parameter and its derivatives, we obtain the scalar field kinetic and potential energy densities, the equation of state $w(z)$, and the dimensionless slope parameter $|\lambda(z)|$ of the scalar field potential. The reconstructed potential is nearly constant at late times, with $\tilde{V}(0) \simeq 0.68$--$0.70\rho_{c,0}$, close to the dark-energy density in flat $\Lambda$CDM. The kinetic term remains small over $0 \lesssim z \lesssim 2.5$, while $w(z)$ remains close to $-1$ within the uncertainties. Using only CC32 and DESI DR2, we find a mild ($1\sigma$) hint of a phantom-divide crossing near $z\sim0.5$. However, this feature depends on the supernova compilation and cannot be regarded as a firm conclusion with current data. The slope parameter $|\lambda(z)|$ shows mild evolution, with present-day central values around $0.8$--$1.0$, but large uncertainties due to its dependence on higher-order derivatives of the expansion history. The curvature parameter $\Gamma(z)$ requires even higher-order derivatives and is not constrained by current data; therefore, we do not report its reconstruction. Allowing small spatial curvature, $\Omega_k=0$--$0.02$, changes the results only slightly and remains within the existing uncertainty bands.

astro-ph.CO↗

arXiv2026

Model-independent reconstruction of cosmic thermodynamics and dark energy dynamics

We perform a model-independent investigation of the thermodynamic evolution of the Universe by reconstructing the expansion history from observational data using Gaussian Process regression. We consider three independent combinations of datasets, namely CC32+DESI DR2+Pantheon+, CC32+DESI DR2+Union3, and CC32+DESI DR2+DES Y5, allowing us to assess the impact of different supernova samples on the reconstruction. From the reconstructed Hubble parameter and its derivatives over the redshift range 0 to 2, we evaluate key thermodynamic quantities associated with the apparent horizon, including the diagnostic function $P(z)$, the entropy production rate $\dot{S}_{\mathrm{tot}}$, and its second derivative $\ddot{S}_{\mathrm{tot}}$. We find that $P(z)$ remains positive across all redshifts, ensuring the validity of the generalized second law of thermodynamics. Correspondingly, $\dot{S}_{\mathrm{tot}} > 0$ throughout, while $\ddot{S}_{\mathrm{tot}} < 0$ at low redshifts, indicating that the Universe evolves toward stable thermodynamic equilibrium. To assess methodological robustness, the reconstruction is performed using multiple covariance kernels, including the Squared Exponential and Mat\'ern kernels with $\nu = 5/2, 7/2,$ and $9/2$, all of which yield consistent results within uncertainties. We also reconstruct the dark energy equation of state in a fully model-independent manner and find it to be consistent with a cosmological constant at the present epoch, with no statistically significant deviation from $\Lambda$CDM.

astro-ph.CO↗

arXiv2025

Cosmological implications of tracker scalar fields: Testing the evidence for dynamical dark energy with recent data

We investigate non phantom tracker scalar field models as dynamical dark energy scenario. These models can alleviate the cosmic coincidence problem and transition to a cosmological constant-like behaviour at late times. Focusing on the inverse axionlike and inverse steep exponential potentials, we study their background evolution and perturbations, finding a mild suppression in the matter power spectrum compared to $Λ$CDM but no distinguishing features in the bispectrum. Using combined datasets of ${\rm CMB}+{\rm BAO\; (DESI~DR1\; \&\; DR2)}+{\rm Pantheon~Plus}+{\rm Hubble\; parameter}+{\rm RSD}$, we perform a statistical comparison based on the Akaike Information Criterion (AIC) and the Bayesian Information Criterion (BIC). Our results indicate that, within the framework of non-phantom tracker models, the data show no evidence for dynamical dark energy. The $Λ$CDM model continues to provide a better fit to current observations in the non phantom regime. We emphasise, however, that our analysis does not rule out the possibility of phantom-crossing dark energy models, which have been found in other studies to provide a better fit to some datasets.

astro-ph.CO↗

arXiv2024

A comparison between axion-like and power law potentials in cosmological background

In this paper, we compare the scalar field dynamics in axion-like and power law potentials for both positive and negative values of the exponents. We find that, for positive exponents, both the potentials exhibit similar scalar field dynamics and it can be difficult to distinguish them at least at the background level. Even though the potentials are oscillatory in nature for positive exponents scaling solutions can be achieved for larger values of the exponent for which the dynamics can be different during early times. Because of the presence of this scaling nature there is a turnaround in the values of the scalar field equation of state as we increase the values of the exponent in both the potentials. This indicates the deviation from the oscillatory behaviour for the larger values of the exponent. For negative values of the exponent, the dynamics of the scalar field is distinguishable and axion-like potential can give rise to cosmologically viable tracker solutions unlike the power law potentials. For negative values of the exponent, axion-like potential can behave like a cosmological constant around its minima and the dark energy scale can be related to the potential scale. Due to the cosmological constant like behavior of the axion-like potential for negative exponent around its minima the late time dynamics can be similar to $Λ$CDM and we get similar observational constraint on the parameters for both $Λ$CDM and axion-like potential with negative exponent. So, while for positive exponents we may not distinguish the two potentials for negative exponents the dynamics of the scalar field is distinguishable.

astro-ph.CO↗