arXiv subjects

Daniel Sudarsky

Publications and source records attributed to Daniel Sudarsky.

At least 19 recordsLinked to original sources

arXiv2026

Constraints on unimodular diffusion models with latest observables

Cosmological models incorporating a time-dependent equation of state have recently been explored \cite{DESI:2025fii}, showing a preference for a dynamical dark energy component. In this work, we investigate a scenario in which an effective, time-dependent cosmological constant arises as an emergent manifestation of a violation of energy-momentum conservation. In \cite{Landau:2022mhm}, such a violation of energy conservation was studied as a diffusion mechanism affecting matter (dark and baryonic), leading to an effective dark energy component within the framework of unimodular gravity. Here, we present an updated analysis using the more recent Type Ia supernova data set from the Dark Energy Survey (DESY5) and the baryon acoustic oscillation (BAO) measurements from the Dark Energy Spectroscopic Instrument (DESI) Data Release 2 (DR2), along with the CMB temperature, polarization, and lensing data from Planck 2018. Our results identify a transition phase that occurs at intermediate times, with slight evidence in favor of the model relative to the $Λ$CDM according to the $\mathrm{ΔDIC}$ criterion. Interestingly, a non-decisive preference for an evolution corresponding to either a time-decreasing or time-increasing effective cosmological constant is found. However, slightly higher values of $H_0$ favor a time-increasing effective cosmological constant. Although the $H_0$ tension is not significantly alleviated, these results suggest that a more refined modeling of the physics of the diffusion mechanism may offer a viable route toward addressing the current discrepancy in the Hubble expansion rate, while also providing a natural framework for incorporating a dynamical dark energy and addressing the problem of vacuum energy contribution.

physics.gen-ph↗

arXiv2026

Breaking Eternal Inflation: Empirical Viability of a Spontaneous Collapse Scenario

We revisit an inflationary scenario in which primordial inhomogeneities arise from a quantum collapse, a stochastic mechanism described in the context of quantum collapse theories in its continuous version and within semiclassical gravity. The predictions of the model show a non-conventional scalar spectrum governed by two new parameters in the collapse rate, whose aim is twofold: on one side, to account for the primordial cosmic structure, and on the other to explain the suppression amplitude associated with long-wavelength modes, thereby eliminating the occurrence of eternal inflation. Furthermore, this model can contribute to accounting for the lack of power anomaly in the low $l$ angular power spectra of the Cosmic Microwave Background (CMB). Using the latest data from the Planck (2018) collaboration, we establish observational constraints on the model parameters, which produce a characteristic low-$\ell$ suppression in the cosmic microwave background spectrum. We conclude that the Planck data support the solution presented in the previous works, in other words, that the model allows us to solve simultaneously the emergence of the cosmic structure and, at the same time, avoid the eternal inflation scenario.

gr-qc↗

arXiv2026

A lesson from a small particle about quantum theory with strong implications for cosmology

The establishment of extremely strong bounds on the magnitude of the electric dipole moment of the neutron, a quantity that is of great importance for determining the level of time reversal symmetry respected by the strong interactions, offers an important lesson regarding the manner in which quantum uncertainties are interpreted in the inflationary cosmological account of the generation of the primordial inhomogeneities that give rise to the universe's structure. The identification of quantum uncertainties with actual stochastic fluctuations, a standard aspect of the current physical account for the emergence of the cosmic structure, is called into question. This opens the door for novel aspects of physics that are needed in order to provide a satisfactory account that is both conceptually clear and does not conflict with the use of quantum theory in other settings.

quant-ph↗

arXiv2026

Renormalization of the Quantum Stress Tensor Fluctuations and the Limits of Semiclassical Gravity

We analyze the expectation value of the energy-momentum tensor and its fluctuations in quantum field theory on curved spacetimes $\langle T_{ab} \rangle$. A generally accepeted condition for the conceptual consistency of semiclassical gravity, where $\langle T_{ab} \rangle$ represent the sources of the Einstein equations, is that the fluctuations of the energy momentum tensor remain small compared to its expectation value. We study the renormalization of both the energy-momentum tensor $\langle T_{ab}(x)\rangle_{\rm ren}$ and the fluctuation tensor $\langle T_{ab}(x) T_{cd}(x) \rangle_{\rm ren}$ for suitable Hadamard states, using the operator product expansion for a free scalar field on a fixed curved background. We show that states (usually referred to as `squeezed vacua') -- arising naturally in black hole evaporation and in inflationary cosmology -- fail to satisfy the natural semiclassicality criterion.

gr-qc↗

arXiv2025

Addressing the so called quantum/classical "divide" in gravitational contexts, and its implications in cosmology

The manner in which one approaches the interface between gravitation and quantum theory is influenced by one's posture regarding quantum mechanics and the issues that revolve about its interpretational problems. We discuss here the way in which these issues occur in the inflationary cosmology setting, the serious confusions that ensue, and one path that we have used to deal with the problem which perhaps surprisingly ends up, not just offering a conceptually clear physical picture, but also modifying some of the standard observational predictions of the inflationary approach.

gr-qc↗

arXiv2024

On the Quantum Uncertainty of the Neutron Electric Dipole Moment

The continued interest in placing bounds on the neutron's Electric Dipole Moment (EDM) is due to the implications regarding the characteristics of the strong interaction and, in particular, its behavior under the CP symmetry. In this work, we discuss the apparent tension resulting from the discrepancy of about 13 orders of magnitude between the current bounds and the expected quantum uncertainty in the relevant quantity. We offer a resolution of the "puzzle" in terms of the notion of a weak measurement, using a version of the corresponding formalism adapted to consideration of the nEDM experiment at the Spallation Neutron Source at the Oak Ridge National Laboratory.

hep-ph↗

arXiv2024

The Hadamard condition on a Cauchy surface and the renormalized stress-energy tensor

Given a Cauchy surface in a curved spacetime and a suitably defined quantum state on the CCR algebra of the Klein-Gordon quantum field on that surface, we show, by expanding the squared spacetime geodesic distance and the `$U$' and `$V$' Hadamard coefficients (and suitable derivatives thereof) in sufficiently accurate covariant Taylor expansions on the surface that the renormalized expectation value of the quantum stress-energy tensor on the surface is determined by the geometry of the surface and the first 4 time derivatives of the metric off the surface, in addition to the Cauchy data for the field's two-point function. This result has been anticipated in and is motivated by a previous investigation by the authors on the initial value problem in semiclassical gravity, for which the geometric initial data corresponds {\it a priori} to the metric on the surface and up to 3 time derivatives off the surface, but where it was argued that the fourth derivative can be obtained with aid of the field equations on the initial surface.

gr-qc↗

arXiv2023

A clarification on prevailing misconceptions in unimodular gravity

The traditional presentation of Unimodular Gravity (UG) consists on indicating that it is an alternative theory of gravity that restricts the generic diffeomorphism invariance of General Relativity. In particular, as often encountered in the literature, unlike General Relativity, Unimodular Gravity is invariant solely under volume-preserving diffeomorphisms. That characterization of UG has led to some confusion and incorrect statements in various treatments on the subject. For instance, sometimes it is claimed (mistakenly) that only spacetime metrics such that $|$det $g_{μν}| = 1$ can be considered as valid solutions of the theory. Additionally, that same (incorrect) statement is often invoked to argue that some particular gauges (e.g. the Newtonian or synchronous gauge) are not allowed when dealing with cosmological perturbation theory in UG. The present article is devoted to clarify those and other misconceptions regarding the notion of diffeomorphism invariance, in general, and its usage in the context of UG, in particular.

gr-qc↗

arXiv2023

On the initial value problem for semiclassical gravity without and with quantum state collapses

We explore how the initial value problem may be formulated for globally hyperbolic, Hadamard, solutions of the semiclassical Einstein-Klein-Gordon equations. Given a set of data on an initial 3-surface, consisting of the values on the surface of a spacetime metric and its first 3 time derivatives off the surface, we introduce a notion of 'surface Hadamard' state on the CCR algebra of the surface. We conjecture that, for a given such set of classical Cauchy data with a surface Hadamard state satisfying the semiclassical constraint equations, the initial value problem will be well posed. We present similar conjectures for a semiclassical scalars model and semiclassical electrodynamics. Moreover, partly inspired by work of Parker and Simon in 1993, we define semiclassical gravity `physical solutions' to be those that are (jointly smooth) functions of $\hbar$ and of coordinates continuous in $\hbar$ at $\hbar =0$. We conjecture that for such solutions the second and third time derivatives of the metric off the surface need not be specified, but rather will be determined by that continuity condition. Assuming the initial value conjecture for such physical solutions holds, and that a stochastic rule were available which leads to quantum state collapses occurring on (non-intersecting) random Cauchy surfaces, we discuss the well-posedness of semiclassical gravity with stochastic quantum state collapses. We also discuss two notions of approximate physical semiclassical solutions (both with and without collapses): Namely solutions to order $\hbar$ (first discussed by Parker and Simon in 1993) and solutions to order $\hbar^0$. We point out that the latter do not require higher derivative terms or Hadamard subtractions, but that nevertheless order $\hbar^0$ semiclassical gravity is a distinct theory from classical general relativity capable of incorporating quantum interference phenomena.

gr-qc↗

arXiv2022

Cosmological constraints on unimodular gravity models with diffusion

A discrete space-time structure lying at about the Planck scale may become manifest in the form of very small violations of the conservation of the matter energy-momentum tensor. In order to include such kind of violations, forbidden within the General Relativity framework, the theory of unimodular gravity seems as the simplest option to describe the gravitational interaction. In the cosmological context, a direct consequence of such violation of energy conservation might be heuristically viewed a "diffusion process of matter (both dark and ordinary)" into an effective dark energy term in Einstein's equations, which leads under natural assumptions to an adequate estimate for the value of the cosmological constant. Previous works have also indicated that these kind of models might offer a natural scenario to alleviate the Hubble tension. In this work, we consider a simple model for thecosmological history including a late time occurrence of such energy violation and study the modifications of the predictions for the anisotropy and polarization of the Cosmic Microwave Background (CMB). We compare the model's predictions with recent data from the CMB, Supernovae Type Ia, cosmic chronometers and Baryon Acoustic Oscillations. The results show the potential of this type of model to alleviate the Hubble tension.

astro-ph.CO↗

arXiv2022

What happens once an accelerating observer has detected a Rindler particle?

In a seminal paper, Unruh and Wald found that the detection of a right Rindler particle by a linearly uniformly accelerated detector coupled to a Klein-Gordon field in the Minkowski vacuum leads to the creation of a Minkowski particle from the inertial viewpoint. In this paper, we revisit the framework studied by Unruh and Wald, but now consider in addition what happens once the particle has been measured somewhere in the right Rindler wedge. From an orthodox point of view, the change in the field state induced by the measurement is non-local and occurs both in the left and right Rindler wedges. If one takes semiclassical gravity seriously in this context, this seems to open the possiblity for designing superluminal communication protocols between two spacelike separated observers confined to the right and left Rindler wedges respectively. We discuss the possible ways in which physics could prevent such measurement-induced, faster-than-light signaling protocols.

gr-qc↗

arXiv2022

A dialog on the fate of information in black hole evaporation

We present two alternative perspectives for the resolution of Hawking's information puzzle in black hole evaporation. The two views are deeply contrasting, yet they share several common aspects. One of them is the central role played by the existence of the interior singularity (whose physical relevance is implied by the singularity theorems of Penrose) that we expect to be replaced by a region described by a more fundamental quantum gravity formulation. Both views rely on the notion that the standard effective quantum field theoretic perspective would require some deep modifications. In this respect both of our scenarios are deeply influenced by ideas that Roger Penrose has advocated at various times and thus serves to illustrate the lasting influence that his deep thinking on these and related matters continues to have on the modern thinking about fundamental aspects of both quantum theory and gravitation. Despite that, there is of course no claim that R. Penrose would agree with any of the concrete proposals that will be discussed here.

gr-qc↗

arXiv2021

Critical review of prevailing explanations for the emergence of classicality in cosmology

There have been recent attempts at justifying, from first principles, and within the standard framework, the emergence of classical behavior in the post-inflationary cosmological context. Accounting for this emergence is an important issue, as it underlies the extraordinary empirical success of our current understanding of cosmology. In this work, we offer a critique of different efforts at explaining the emergence of classical behavior in cosmology within the standard framework. We argue that such endeavors are generically found lacking in conceptual clarity, as they invariably rely, either upon unjustified, implicit assumptions, or on circular logic. We conclude that, within the standard approach, the emergence of classical behavior in cosmology constitutes an unexplained phenomenon.

gr-qc↗

arXiv2021

Resolving the $H_0$ tension with diffusion

The tension between the value of the Hubble constant $H_0$ determined from local supernovae data and the one inferred from the cosmic microwave background based on the $Λ$CDM cosmological model may indicate the need for new physics. Here, we show that this `Hubble tension' can be resolved in models involving an effective energy flux from the matter sector into dark energy resulting naturally from a combination of unimodular gravity and an energy diffusion process. The scheme is one where dark energy has the standard equation of state $w=-1$. This proposal provides an alternative phenomenological paradigm accounting for the observations, while offering a general framework to study diffusion effects coming from novel fundamental physical processes.

astro-ph.CO↗

arXiv2020

Discussions about the landscape of possibilities for treatments of cosmic inflation involving continuous spontaneous localization models

In this work we consider a wide variety of alternatives opened when applying the continuous spontaneous localization (CSL) dynamical collapse theory to the inflationary era. The definitive resolution of many of the issues discussed here will have to await, not only for a general relativistic CSL theory, but for a fully workable theory of quantum gravity. Our concern here is to explore these issues, and to warn against premature conclusions. This exploration includes: two different approaches to deal with quantum field theory and gravitation, the identification of the collapse-generating operator and the general nature and values of the parameters of the CSL theory. All the choices connected with these issues have the potential to dramatically alter the conclusions one can draw. We also argue that the incompatibilities found in a recent paper, between the CSL parameter values and the CMB observational data, are associated with specific choices made for the extrapolation to the cosmological context of the CSL theory (as it is known to work in non-relativistic laboratory situations) which do not represent the most natural ones.

gr-qc↗

arXiv2020

Comment on "Cosmic Microwave Background Constraints Cast a Shadow On Continuous Spontaneous Localization Models"

In a recent paper [J. Martin and V. Vennin, Phys. Rev. Lett. 124, 080402 (2020)] it was argued that, for most natural choices, the direct application of the continuous spontaneous localization (CSL) theory to the inflationary case, as it is known to work in non-relativistic laboratory situations, is ruled out by cosmological observational data, thus casting a shadow on models based on CSL theory. We point out that such results are based on the consideration of a rather narrow set of choices for the application of the theory to the cosmological context and that the landscape of open and different possibilities is extremely vast.

gr-qc↗

arXiv2019

Can the quantum vacuum fluctuations really solve the cosmological constant problem?

Recently it has been argued that a correct reading of the quantum fluctuations of the vacuum could lead to a solution to the cosmological constant problem. In this work we critically examine such a proposal, finding it questionable due to conceptual and self-consistency problems, as well as issues with the actual calculations. We conclude that the proposal is inadequate as a solution to the cosmological constant problem.

gr-qc↗

arXiv2019

Black holes, Planckian granularity, and the changing cosmological `constant'

In a recent work we have argued that nosy energy momentum diffusion due to space-time discreteness at the Planck scale (naturally expected to arise from quantum gravity) can be responsible for the generation of a cosmological constant during the electro-weak phase transition era of the cosmic evolution. Simple dimensional analysis and an effectively Brownian description of the propagation of fundamental particles on a granular background yields a cosmological constant of the order of magnitude of the observed value, without fine tuning. While the energy diffusion is negligible for matter in standard astrophysical configurations (from ordinary stars to neutron stars) here we argue that a similar diffusion mechanism could, nonetheless be important for black holes. If such effects are taken into account two observational puzzles might be solved by a single mechanism: the `$H_0$ tension' and the relatively low rotational spin of the black holes detected via gravitational wave astronomy.

gr-qc↗