arXiv subjects

David Mota

Publications and source records attributed to David Mota.

8 recordsLinked to original sources

arXiv2025

Do low-redshift observations open the doors to an open universe?

The detection of a significant deviation from a zero curvature would have profound consequences for inflationary theories and fundamental physics. Relative to high-redshift Planck's CMB measurements, indicating a $\sim 2\sigma$ evidence for a closed universe, low-redshift observations of BAO and SN Ia have the advantages of weak dependence on early universe physics, independently observational systematics, and strong redshift dependence of distances in constraining the cosmic curvature. Using the integrated observations from DESI BAO and SN Ia, we find an unexpected $2\sigma$ evidence for an open universe, regardless of the SN Ia sample employed. When considering DESI, SN Ia and the acoustic scale $\theta_\star$ data, the preference for an open universe exceeds the $3\sigma$ level, reaching $5\sigma$ for the case of DESY5 Supernovae data. Therefore, low-redshift observations favor an open universe, and this preference persists even when alternative high-redshift priors are adopted. Our results point to the existence of an additional tension between high- and low-redshift observations, present also in non-flat models beyond the minimal $\Lambda$CDM scheme, thereby challenging the standard inflationary predictions.

astro-ph.CO↗

arXiv2025

Line-of-Sight Velocity Projection Impact on the Local Group Mass

The mass of the Local Group (LG), comprising the Milky Way (MW), Andromeda (M31), and their satellites, is crucial for validating galaxy formation and cosmological models. Traditional virial mass estimates, which rely on line-of-sight (LoS) velocities and simplified infall assumptions, are prone to systematic biases due to unobserved velocity components and anisotropic kinematics. Using the TNG cosmological simulation, we examine two limiting cases: the \underline{minor infall} model -- ignoring perpendicular velocities to the LoS directions) and the \underline{major infall} model -- assuming purely radial motion towards the Center of Mass (CoM). Our simulations demonstrate that geometric corrections are vital: the minor-infall model underestimates the true mass, while the major-infall model overestimates it. By applying these calibrated corrections to observed dwarf galaxy kinematics within 1 Mpc of the LG's CoM, we derive a refined LG mass of $M_{\mathrm{LG}} = (2.99 \pm 0.60) \times 10^{12}\, M_\odot$. This finding aligns with predictions from the $\Lambda$CDM model, timing arguments, and independent mass estimates, resolving previous discrepancies. Our analysis highlights the importance of correcting for velocity anisotropy and offers a robust framework for dynamical mass estimation in galaxy groups.

astro-ph.GA↗

arXiv2025

Did DESI DR2 truly reveal dynamical dark energy?

A fundamental question in cosmology is whether dark energy evolves over time, a topic that has gained prominence since the discovery of cosmic acceleration. Recently, the DESI collaboration has reported increasing evidence for evolving dark energy using combinations of cosmic microwave background (CMB), type Ia supernova (SN), and their new measurements of baryon acoustic oscillations (BAO). However, our analysis reveals that these combinations are problematic due to clear tensions among the CMB, BAO and SN datasets. Consequently, DESI's claim of dynamical dark energy (DDE) is not robust. A more reliable approach involves constraining the evolution of dark energy using each dataset independently. Through a statistical comparison for each dataset, on average, we find that DDE is strongly preferred over the $\Lambda$CDM model. This suggests that DDE likely exists, although its real parameter space remains elusive due to weak constraints on the dark energy equation of state and inconsistencies among the datasets. Interestingly, when considering DDE, none of the individual datasets -- including CMB, DESI DR2, Pantheon+, Union3, and DESY5 -- can independently detect cosmic acceleration at a significant level. Our findings not only clarify the current understanding of the nature of dark energy but also challenge the established discovery of cosmic acceleration and the long-held notion that dark energy exerts negative pressure. Both individual and combined datasets suggest that the ultimate fate of the universe is likely to be dominated by matter rather than dark energy.

astro-ph.CO↗

arXiv2023

Exploration and Analysis of Combinations of Hamming Codes in 32-bit Memories

Reducing the threshold voltage of electronic devices increases their sensitivity to electromagnetic radiation dramatically, increasing the probability of changing the memory cells' content. Designers mitigate failures using techniques such as Error Correction Codes (ECCs) to maintain information integrity. Although there are several studies of ECC usage in spatial application memories, there is still no consensus in choosing the type of ECC as well as its organization in memory. This work analyzes some configurations of the Hamming codes applied to 32-bit memories in order to use these memories in spatial applications. This work proposes the use of three types of Hamming codes: Ham(31,26), Ham(15,11), and Ham(7,4), as well as combinations of these codes. We employed 36 error patterns, ranging from one to four bit-flips, to analyze these codes. The experimental results show that the Ham(31,26) configuration, containing five bits of redundancy, obtained the highest rate of simple error correction, almost 97\%, with double, triple, and quadruple error correction rates being 78.7\%, 63.4\%, and 31.4\%, respectively. While an ECC configuration encompassed four Ham(7.4), which uses twelve bits of redundancy, only fixes 87.5\% of simple errors.

cs.AR↗

arXiv2021

4D Gauss-Bonnet gravity: cosmological constraints, $H_0$ tension and large scale structure

We perform correct and reasonable cosmological constraints on the newly proposed 4D Gauss-Bonnet gravity. Using the joint constraint from cosmic microwave background, baryon acoustic oscillations, Type Ia supernovae, cosmic chronometers and redshift space distortions, we obtain, so far, the strongest constraint $\tilde{\alpha}=(1.2\pm5.2)\times 10^{-17}$, namely $\alpha=(2.69\pm11.67)\times10^{48}$ eV$^{-2}$, among various observational limitations from different information channels, which is tighter than previous bound from the speed of gravitational wave by at least one order of magnitude. We find that our bound is well supported by the observations of temperature and lensing potential power spectra of cosmic microwave background from the Planck-2018 final release. Very interestingly, the large $H_0$ tension between the local measurement from the Hubble Space Telescope and global derivation from the Planck-2018 final data under the assumption of $\Lambda$CDM can be greatly resolved from $4.4\sigma$ to $1.94\sigma$ level in the 4D Gauss-Bonnet gravity. In theory, we find that this model can partly relieve the coincidence problem and the rescaling Gauss-Bonnet term, which needs the help of the cosmological constant to explain current cosmic acceleration, is unable to serve as dark energy alone.

astro-ph.CO↗

arXiv2020

Can $f(T)$ gravity resolve the $H_0$ tension?

Motivated by the discrepancy in measurements of $H_0$ between local and global probes, we investigate whether teleparallel gravities could be a better model to describe the present days observations or at least to alleviate the $H_0$ tension. Specifically, in this work we study and place constraints on three popular $f(T)$ models in light of the Planck-2018 CMB data release. We find that the $f(T)$ power-law model can alleviate the $H_0$ tension from $4.4\sigma$ to $1.9\sigma$ level, while the $f(T)$ model of two exponential fail to resolve this inconsistency. Moreover, for the first time, we obtain constraints on the effective number of relativistic species $N_{eff}$ and on the sum of the neutrino masses $\Sigma m_\nu$ in $f(T)$ gravity. We find that the constraints obtained are looser than in $\Lambda$CDM. However, the introduction of massive neutrinos into the cosmological model alleviate the $H_0$ tension for the power-law model. Finally, we find that whether a viable $f(T)$ theory can mitigate the $H_0$ tension depends on the mathematical structure of the distortion factor $y(z,\,b)$. These results could provide a clue for theoreticians to write a more physical-motivated expression of $f(T)$ function.

astro-ph.CO↗

arXiv2019

Breaking cosmic degeneracies: Disentangling neutrinos and modified gravity with kinematic information

Searches for modified gravity in the large-scale structure try to detect the enhanced amplitude of density fluctuations caused by the fifth force present in many of these theories. Neutrinos, on the other hand, suppress structure growth below their free-streaming length. Both effects take place on comparable scales, and uncertainty in the neutrino mass leads to a degeneracy with modified gravity parameters for probes that are measuring the amplitude of the matter power spectrum. We explore the possibility to break the degeneracy between modified gravity and neutrino effects in the growth of structures by considering kinematic information related to either the growth rate on large scales or the virial velocities inside of collapsed structures. In order to study the degeneracy up to fully non-linear scales, we employ a suite of $N$-body simulations including both $f(R)$ modified gravity and massive neutrinos. Our results indicate that velocity information provides an excellent tool to distinguish massive neutrinos from modified gravity. Models with different values of neutrino masses and modified gravity parameters possessing a comparable matter power spectrum at a given time have different growth rates. This leaves imprints in the velocity divergence, which is therefore better suited than the amplitude of density fluctuations to tell the models apart. In such models with a power spectrum comparable to $\Lambda$CDM today, the growth rate is strictly enhanced. We also find the velocity dispersion of virialised clusters to be well suited to constrain deviations from general relativity without being affected by the uncertainty in the sum of neutrino masses.

astro-ph.CO↗

arXiv2014

Releasing scalar fields: cosmological simulations of scalar-tensor theories for gravity beyond the static approximation

Several extensions of General Relativity and high energy physics include scalar fields as extra degrees of freedom. In the search for predictions in the non-linear regime of cosmological evolution, the community makes use of numerical simulations in which the quasi-static limit is assumed when solving the equation of motion of the scalar field. In this Letter, we propose a method to solve the full equations of motion for scalar degrees of freedom coupled to matter. We run cosmological simulations which track the full time and space evolution of the scalar field, and find striking differences with respect to the commonly used quasi-static approximation. This novel procedure reveals new physical properties of the scalar field and uncovers concealed astrophysical phenomena which were hidden in the old approach.

astro-ph.CO↗