arXiv subjects

Erick Pastén

Publications and source records attributed to Erick Pastén.

9 recordsLinked to original sources

arXiv2026

Redshift Dipoles from Non-Geodesic Observer Congruences in Covariant Cosmology

Recent analyses of large-scale structure and redshift surveys have reported significant dipolar anisotropies in the local Universe that are not straightforwardly attributable to a kinematical boost. When interpreted within standard frameworks, these signals may correspond to coherent bulk flows that have been reported to exhibit tension with $Λ$CDM expectations. On the other hand, signals inferred from different astrophysical probes are not always consistent with the Cosmic Microwave Background (CMB) dipole, challenging the assumption of dipoles that are pure kinematical in origin. In more general cosmological scenarios, the congruence associated with cosmological observers need not be geodesic. Within a fully covariant framework, we show that a non-geodesic observer congruence introduces an additional contribution to the propagation of redshift along the past light cone, proportional to the line-of-sight projection of the observer four-acceleration. This generates an anisotropic modulation in the redshift itself, which could propagate to any observable defined in redshift space. We discuss different physical mechanisms that may produce a non-geodesic observer congruence, including interactions within the dark sector, modified theories of gravity, and relative motion between cosmological frames in tilted cosmologies.

astro-ph.CO↗

arXiv2026

Cosmology without the cosmological principle: A study of the large-scale structure effects in the background universe

Recent studies strongly suggest that the $Λ$CDM model may no longer serve as the definitive standard model in cosmology, given the increasing tensions between different cosmological observables. Some researchers have described this situation as a "crisis in cosmology." The $Λ$CDM model relies on two fundamental assumptions: the isotropy of the universe (supported by observational evidence) and the Copernican principle (a philosophical postulate). Together, these assumptions lead to the well-known Cosmological Principle, with the homogeneity of the universe emerging as a direct consequence. In this thesis, I review the cosmological consequences of relaxing some of these assumptions, exploring inhomogeneous and anisotropic universe models, as well as tilted cosmologies, which consider observers in motion relative to the Hubble flow. We examine methods to study these effects using existing cosmological data. First, we explore a variety of models that describe inhomogeneous and anisotropic universes, including different metric theories, perturbative analyses, averaging effects, tilted scenarios, and cosmographic approaches. We then apply this theoretical framework to analyze SNIA data and the local peculiar velocity field, aiming to constrain key parameters.

astro-ph.CO↗

arXiv2026

On the geometrical and dynamical distinction between Unimodular and General Relativistic wormholes

We characterize traversable wormholes in Unimodular Gravity (UG) and investigate what distinguishes them from their General Relativistic (GR) counterparts. For a class of static and spherically symmetric solutions, we analyze their embedding and timelike geodesics, showing that the geodesic structure is entirely determined by the metric and is therefore identical in both theories. To identify the physical origin of their differences, we compare the source sectors required to sustain the same wormhole geometry. We find that preserving a fixed UG geometry while imposing energy-momentum conservation requires restricting the equation of state, whereas relaxing this condition introduces an effective inhomogeneous vacuum contribution. We further show that the degree of exoticity is preserved even when energy-momentum is not conserved, while departures from the GR sector are encoded in an effective inhomogeneous vacuum structure whose asymptotic behavior resembles that of a cosmological constant. Our results reinforce that UG is geometrically equivalent to GR, while its distinctive features emerge in the dynamical interpretation of the source sector. More generally, our analysis illustrates that different gravitational theories may give rise to identical spacetime geometries while requiring different sources to sustain them.

gr-qc↗

arXiv2026

Reconstructing the kinematics of Laniakea using Type Ia Supernovae

We develop a kinematic framework that relates the monopole, dipole, and quadrupole of the luminosity distance to an ellipsoidal peculiar velocity field describing the dynamics of the Laniakea supercluster. By properly accounting for the transformations between the CMB and Laniakea reference frames and selecting Type Ia supernovae within the volume associated with the superstructure, we show that luminosity distance multipoles encode the expansion and shear of the local velocity field. This allows the ellipsoidal kinematics of Laniakea to be inferred directly from supernova observations. Our results provide a physically motivated interpretation of luminosity distance anisotropies within the volume dynamically dominated by Laniakea as signatures of its large-scale kinematics. The framework developed here suggests that future supernova observations can enable the independent reconstruction of large-scale structures in the local Universe

astro-ph.CO↗

arXiv2026

Null Raychaudhuri Equation and the Impossibility of Traversable Wormholes in Unimodular Gravity

We formulate the traversability of wormhole throats as a local and covariant null defocusing condition derived from the Raychaudhuri equation. Since unimodular gravity preserves the local geometric structure of spacetime, the null focusing properties of geodesic congruences are unchanged with respect to general relativity. We show that any genuinely traversable wormhole in unimodular gravity necessarily violates the null energy condition, establishing a local no--go theorem for wormholes supported by ordinary matter in this framework.

gr-qc↗

arXiv2026

The reason peculiar velocities grow faster in general relativity than in Newtonian gravity

An increasing number of surveys has been reporting large-scale peculiar motions with sizes and speeds in excess of those allowed by the concordance cosmological model. These are the so called bulk flows, the presence of which has come to be treated as a problem for the $Λ$CDM paradigm. However, the limits of the $Λ$CDM model are based on Newtonian studies, which predict the mediocre $v\propto t^{1/3}$ growth-rate for the peculiar-velocity field ($v$). Recently, a few fully relativistic treatments have appeared in the literature, arguing for a much stronger velocity growth that could explain the reported fast and deep bulk flows. What separates the Newtonian from the relativistic studies is the gravitational input of the peculiar flux, namely of the kinetic energy triggered by the moving matter. The latter has no direct gravitational contribution in Newtonian theory, but it does so in general relativity. This drastically changes the driving agent of the peculiar-velocity field and boosts its linear growth. The aim of this work is to directly compare the two treatments, as well as identify and discuss the reasons for their different results. In the process, we also demonstrate how one could recover the relativistic growth-rate from a Newtonian setup by selectively including certain (typically ignored) source-free terms into the Poisson equation. This way, we provide a unified covariant comparison of the Newtonian, the quasi-Newtonian and the fully relativistic studies.

gr-qc↗

arXiv2023

Testing $Λ$CDM cosmology in a binned universe: anomalies in the deceleration parameter

We study the reconstructed deceleration parameter splitting the data in different redshift bins, fitting both a cosmographic luminosity distance and also assuming a flat $Λ$CDM model, using the Pantheon+ sample of type Ia supernova data (SNIA). We observe tensions $\sim 2σ-3σ$ for different redshift and distance indicators if the full sample is used. However, those tensions disappear when the SNIA at $z<0.008$ are removed. If the data is splitted in 2 hemispheres according to our movement w.r.t CMB, a strange $3.8 σ$ tension appears in one of the samples between particular redshift bins. Finally, considering posterior distribution as Gaussian, general linear model prefers a positive slope for $q_0$ across redshift bins opposed to a zero slope expected in a $Λ$CDM universe. We discuss possible explanations for our results and the influence of lowest redshift SNIA data in cosmological analysis.

astro-ph.CO↗

arXiv2023

Approximations for the divergence of the local large-scale structure velocity field and its implications for Tilted Cosmology

We characterize the peculiar velocity field of the local large-scale structure reconstructed from the $2M++$ survey, by treating it as a fluid, extracting the divergence via different approximations over a range pf averaged scales. This reconstructed field is important for cosmology, since it was used to correct the peculiar redshifts of the last SNIA compilation Pantheon+. The results have intriguing implications for the LLSS fluid dynamics and particularly for the ``Tilted Cosmology'' model, although those results have to be taken carefully as the velocity field could contain significant bias due to the reconstruction procedure. Those possible bias and its influence in our results are discussed. Representative values of the apparent deceleration parameter ($\Tilde{q}$) are computed, in order to compare our results with the theoretical predictions of the tilted-universe scenario. We conclude that better velocity field reconstructions are necessary in order to constrain the parameters implied in LLSS research and alternative cosmologies.

astro-ph.CO↗

arXiv2023

A fractal LTB model cannot explain Dark Energy

We revisited the problem of describing, on average, a fractal distribution of matter using a Lemaitre-Tolman-Bondi (LTB) solution. Here we study the fractal structure of our local universe having a fractal dimension and a scale transition, ensuring an homogeneous bang-time function. We test our model with the latest type Ia supernova data, the Pantheon compilation, and discuss problems and possible improvements for it, concluding that a fractal transition in LTB cosmology cannot be used to explain the effects of dark energy without requiring an inhomogeneous big-bang, but it can be useful to study structures at low scales.

astro-ph.CO↗