arXiv subjects

Asta Heinesen

Publications and source records attributed to Asta Heinesen.

At least 19 recordsLinked to original sources

arXiv2026

Cosmography for a General Spacetime Centred at Arbitrary Redshift

With upcoming surveys providing large volumes of highly precise observational data across a wide range of redshifts, it is increasingly important to have tools that can translate observational data into geometric and dynamical information without imposing a predetermined cosmological model. General cosmographic expansions centred at arbitrary redshift provide exactly such a tool. We here present the formalism for general cosmographic expansions centred at an arbitrary redshift, valid for 4-dimensional Lorentzian spacetimes. We then apply the expansion formalism to test its ability to reproduce the redshift-distance relation in two examples of large-scale cosmic structures (an underdensity and an overdensity) modelled by the Lema\^itre-Tolman-Bondi metric, where we examine the effect of choosing different redshift intervals for the cosmographic series expansions. This quantifies the extent to which cosmographic coefficients inferred from redshift-distance observations retain their interpretation as local geometric and dynamical quantities, as is expected in standard FLRW cosmology. Similarly to earlier results, we here find that in more general spacetimes the coefficients instead become effective parameters reflecting the finite observational range probed. Lastly, we discuss possible strategies for using the expansions to constrain dynamical and geometric quantities.

astro-ph.CO↗

arXiv2026

Post-Newtonian Global Conservation Laws in the Presence of Sensitive Bodies

The parameterized post-Newtonian (PPN) approach is the state of the art formalism for performing theory independent tests of weak-field gravity, and for constraining possible deviations from Einstein's theory. Within this framework, global conservation laws are useful for the calculation of dynamics and for giving meaning to parameters. In this paper we extend the concept of semi-conservative and fully-conservative theories of gravity to include situations in which compact astrophysical bodies are modeled as masses that are sensitive to their local environment, as relevant for theories that violate the strong equivalence principle. We find that globally conserved quantities can still exist in the presence of such sensitivities, and find their explicit forms when they do. We identify new ways of writing the coefficients that enter into the PPN metric when a theory of gravity admits conserved quantities in the presence of a sensitive body, and demonstrate the applicability of our approach by comparing it to known results in scalar-tensor theories of gravity.

gr-qc↗

arXiv2026

Mapping the Universe as a Bianchi I cosmology with Gaia data

Measurements of tangential drifts of distant quasars and galactic nuclei on the celestial sphere provide a novel and independent method of testing cosmological hypotheses. In this work, we employ an axisymmetric Bianchi I model as a relatively simple phenomenological model that is useful for quantifying departures from the cosmological principle. Using a quality-filtered sample of 1.2 million proper motion vectors of distant quasars from Gaia Data Release 3, we perform global fits of the position drift fields with vector spherical harmonics (VSH) to second degree for five non-overlapping subsets of the sources with redshifts from 0.5 to 3, and assess the ability of the Bianchi I model to describe the signal. We theoretically demonstrate that an axisymmetric Bianchi I model produces a signal that can be described as a single quadrupole VSH term with an eigendirection which is aligned with the axis of maximum expansion anisotropy. We estimate this preferred direction from the Gaia data and the VSH fit, and perform point-estimates of the amplitude of the signal as a function of redshift. Although a significant quadrupole signal is detected in each bin, the increase of the amplitude of the signal with redshift predicted by the Bianchi I model is not confidently confirmed. The estimated value of the local expansion shear is higher than expected. Possible advances in describing the kinematic patterns of a high-redshift Universe with more complex cosmologies accommodating time-dependent anisotropy and rotation are discussed.

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arXiv2026

Observational Quantities in Quasi-Newtonian Descriptions of Cosmological Space-Times

We investigate measures of distance and redshift in cosmological space-times that admit a shear-free foliation, which we henceforth refer to as `quasi-Newtonian'. Space expands isotropically in this description, and small-scale gravitational physics has a natural Newtonian limit, which makes it ideal for considering the physics of wide classes of cosmological models. By assuming that the energy-momentum tensor is dominated by rest-mass density, and that the 3-velocity of matter is small in the quasi-Newtonian frame, we derive fundamental results for kinematics and light propagation. Our results provide a new way of formulating general-relativistic cosmologies with non-perturbative structures in terms of quantities that can be understood from cosmological perturbation theory and post-Newtonian expansions, and allow us to quantify departures of observables from the predictions of Friedmann cosmology. It thereby provides a route to understanding inherently relativistic space-time structures, such as those that occur in Lema\^{i}tre-Tolman-Bondi, Szekeres solutions, and Bianchi cosmologies in terms of Newtonian degrees of freedom. We illustrate our results using the degenerate Kasner solution as an example, and explain how our approach can be used to provide new insights into the current cosmological tensions.

gr-qc↗

arXiv2026

Observational Tests for Distinguishing Classes of Cosmological Models

We investigate observational tests that can be used to distinguish between broad classes of cosmological models. This is achieved using curvature-consistency tests of the Friedmann-Lema\^{i}tre-Robertson-Walker (FLRW) models, which we investigate in two scenarios where they can be violated; (i) when the optical properties of the cosmology deviate from the expectations of FLRW, and also (ii) when the large-scale expansion of the cosmology is different from FLRW. We identify useful ways to determine the properties of these alternative scenarios in terms of the violation of the curvature-consistency tests, and propose a new null test that can be used to isolate cosmologies with non-FLRW observational relations. The characteristic signatures we find can be used, together with the results of recent and upcoming cosmological observations, to probe and/or rule out large classes of cosmological models. This becomes an increasingly important task as the number of proposals in the literature increases, as cosmologists attempt to explain tensions, anomalies, and the dark sector of the Universe. Our approach provides a clear route for telling apart these different proposals, and offers a new opportunity for using precision cosmological data to efficiently discriminate between cosmological models.

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arXiv2026

Cosmography with $\Lambda$-Szekeres Models

The cosmological tensions present in the $\Lambda$ cold dark matter model that have emerged and strengthened over recent years motivate model independent approaches to analysing data. Cosmography is useful for interpreting data in cosmology without imposing assumptions about the field equations of gravity or the matter content in the Universe. Some cosmography methods, denoted covariant cosmography, go even further and stay agnostic to the underlying space-time metric. Due to their high level of generality, covariant cosmography methods can incorporate the anisotropies and inhomogeneities in the observer's vicinity, and may in turn inform about the associated curvature of the relevant structures in our cosmic neighbourhood. Thus, covariant cosmography is a powerful model-independent tool for analysing cosmological data while also enabling the mapping of our local cosmic neighbourhood. In order to be able to explore the covariant cosmography framework to its fullest, it must be tested in tractable models and simulations. In this paper we derive the cosmography of luminosity distance to fourth order in redshift and investigate it in the special case of axially symmetric Szekeres models. We compare the numerical results for the distance-redshift relations of synthetic observers placed within the Szekeres structures with the predictions from the cosmography, and comment on the found level of approximation of the cosmography in relation to other results in the litterature.

astro-ph.CO↗

arXiv2025

Position drift with Gaia

The proper motion (also known as position drift) field of extragalactic sources at cosmological distances across our sky can be used to measure the acceleration of the Solar System through the aberration effect. If measured very precisely, the signal would also hold cosmological information, for instance about bulk flows of distant sources or the presence of tensor modes. In the $\Lambda$ cold dark matter ($\Lambda$CDM) model, the acceleration of the Solar System is by far the dominant contributor to the position drift signal for sources at cosmological distances, and the measurement is therefore expected to yield a constant spheroidal dipole across redshifts as long as convergence to the cosmic restframe has been reached. The aim of this paper is to test this hypothesis. We analyze data from the cosmic reference frame dataset of Gaia data release 3 focusing on constraining the dipole and quadrupole in the position drift signal, with an emphasis on redshift dependence of the signal as a consistency test of the $\Lambda$CDM model. The spheroidal dipole that we find is in mild tension, at the level of $2-3\sigma$, with the constant-in-redshift signature expected from the local acceleration of the Solar System. We also find significant quadrupole components, that however do not have any significant evolution with redshift. The most straightforward interpretation of these findings is (unknown) systematic errors related to the Gaia instrumentation, but a cosmological origin is a possibility. Our analysis remains inconclusive on the cause of the redshift dependence of the dipole and warrants further investigations with upcoming data releases. We discuss possible implications of our results and highlight the importance of proper motion measurements for rest frame determinations in cosmology. In our discussion, we highlight interesting avenues for doing cosmology with Gaia data.

astro-ph.CO↗

arXiv2025

A theoretical prediction for the dipole in nearby distances using cosmography

Cosmography is a widely applied method to infer kinematics of the Universe at small cosmological scales while remaining agnostic about the theory of gravity at play. Usually cosmologists invoke the Friedmann-Lemaitre-Robertson-Walker (FLRW) metric in cosmographic analyses, however generalised approaches allow for analyses outside of any assumed geometrical model. These methods have great promise to be able to model-independently map the cosmic neighborhood where the Universe has not yet converged to isotropy. In this regime, anisotropies can bias parameter inferences if they are not accounted for, and thus must be included for precision cosmology analyses, even when the principle aim is to infer the background cosmology. In this paper, we develop a method to predict the dipole in luminosity distances that arises due to nearby inhomogeneities. This is the leading-order correction to the standard isotropic distance-redshift law. Within a very broad class of general-relativistic universe models, we provide an interpretation of the dipole in terms of the gradients in expansion rate and density which is free from any underlying background cosmology. We use numerical relativity simulations, with improved initial data methods, alongside fully relativistic ray tracing to test the power of our prediction. We find our prediction accurately captures the dipole signature in our simulations to within ~10% for redshifts $z\lesssim 0.07$ in reasonably smooth simulations. In the presence of more non-linear density fields, we find this reduces to $z\lesssim 0.02$. This represents up to an order of magnitude improvement with respect to what is achieved by naive, local cosmography-based predictions. Our paper thus addresses important issues regarding convergence properties of anisotropic cosmographic series expansions that would otherwise limit their applicability to very narrow redshift ranges.

astro-ph.CO↗

arXiv2024

Differential age observations and their constraining power in cosmology

We derive the differential age signal valid for cosmic chronometers (passively evolving galaxies) in any space-time that satisfies the following assumptions: (i) The space-time has a metric with Lorentzian signature and the connection is the Levi-Civita connection; (ii) the cosmic chronometers are collectively well approximated as a geodesic and irrotational congruence of time-like worldlines in the space-time; (iii) light travels on null geodesics and caustics on the observer's past light cone can be ignored; (iv) the space-time is cosmological, meaning that isotropic and positive expansion degrees-of-freedom dominate over anisotropic and negative expansion degrees-of-freedom when viewed on sufficiently large scales in the frame of the cosmic chronometers. The main result of the paper is an expression for the differential age signal that is written in terms of line-of-sight averages of the expansion rate along individual null lines, thus providing a kinematic interpretation of the differential age signal applicable to cosmological space-times satisfying (i)--(iv). We explain how this result indicates that the differential age signal is a robust probe of the volume-average expansion rate in very general statistically homogeneous and isotropic space-time scenarios where other probes of the volume-average expansion rate tend to yield biased results. We argue that this unique property of the differential age signal makes it an ideal measurement for constraining the expansion history model-independently.

gr-qc↗

arXiv2024

Exploring the rich geometrical information in cosmic drift signals with covariant cosmography

Real-time measurements are becoming feasible in cosmology, where the next generation of telescopes will detect the temporal change of redshifts and sky positions of individual sources with a precision that will allow a direct detection of the cosmic expansion rate. These detections of cosmic drifts of redshifts and positions are likely to become cornerstones in modern cosmology, where one has otherwise relied on the indirect inference of cosmic expansion by estimation of the slope of the fitted distance-redshift relation. Because of their ability to directly detect the cosmic time-evolution, real-time measurements are powerful as model-independent probes. We develop a cosmographic framework for analysing cosmological redshift drift and position drift signals without knowledge of the space-time geometry. The framework can be applied to analyse data from surveys such as the Gaia observatory, the Square Kilometer Array (SKA), and the Extremely Large Telescope (ELT). The drift effects are distorted by the regional kinematics and tidal effects in the cosmic neighbourhood of the observer, giving rise to non-trivial corrections to the well known Friedmann-Lema\^ıtre-Robertson-Walker (FLRW) results. We discuss how one may concretely implement the framework in the statistical analysis of real-time data, along with assumptions and limitations that come with such an analysis. We also discuss the geometrical information that can ideally be extracted from ideal high-resolution data of cosmic drifts in combination with distance-redshift data.

astro-ph.CO↗

arXiv2024

Towards Cosmography of the Local Universe

Anisotropies in the distance-redshift relation of cosmological sources are expected due to large-scale inhomogeneities in the local Universe. When the observed sources are tracing a large-scale matter flow in a general spacetime geometry, the distance-redshift relation with its anisotropies can be described with a geometrical prediction that generalises the well-known Friedmann-Lemaître-Robertson-Walker result. Furthermore, it turns out that a finite set of multipole coefficients contain the full information about a finite-order truncation of the distance-redshift relation of a given observer. The multipoles of the distance-redshift relation are interesting new cosmological observables that have a direct physical interpretation in terms of kinematical quantities of the underlying matter flow. Using light cones extracted from $N$-body simulations we quantify the anisotropies expected in a $Λ$ cold dark matter cosmology by running a Markov chain Monte Carlo analysis on the observed data. In this observational approach the survey selection implements an implicit smoothing scale over which the effective rest frame of matter is fitted. The perceived anisotropy therefore depends significantly on the redshift range and distribution of sources. We find that the multipoles of the expansion rate, as well as the observer's velocity with respect to the large-scale matter flow, can be determined robustly with our approach.

astro-ph.CO↗

arXiv2024

Redshift drift in a universe with structure III: Numerical relativity

Measurements of the cosmic redshift drift - the change in redshift of a source over time - will enable independent detection of cosmological expansion thanks to the immense precision soon reached by new facilities such as the Square Kilometer Array Observatory and the Extremely Large Telescope. We conduct the first ever redshift drift computation in fully relativistic cosmological simulations, with the simulations performed with the Einstein Toolkit. We compute the redshift drift over the full skies of 50 synthetic observers in the simulation. We compare all-sky averages for each observer - and across all observers - to the Einstein-de Sitter (EdS) model which represents the large-scale spatially-averaged spacetime of the simulation. We find that at $z\approx0.2$ the mean redshift drift across the sky for all observers deviates from the EdS prediction at the percent level, reducing to $\sim0.1\%$ by $z\approx 1$. However, fluctuations in the redshift drift across the sky are $\sim 10-30\%$ at $z\approx 0.1$ and a few percent at $z\approx 0.5$. Such fluctuations are large enough to potentially exceed the expected precision of upcoming redshift drift measurements. Additionally, we find that along 0.48% of the light rays the redshift drift becomes temporarily positive at very low redshift of $z\lesssim 0.02$. This occurs despite our simulation data being based on a matter-dominated model universe. By including a cosmological constant, we expect a slower growth of structures than in the leading-order EdS space-time, and this may reduce the anisotropy over the observers' skies, although we generally expect our results to hold as order-of-magnitude estimates. Redshift drift is arguably one of the most important measurements to be made by next-generation telescopes. Our results collectively serve as preparation for interpreting such a measurement in the presence of realistic cosmic structures.

astro-ph.CO↗

arXiv2024

Splitting the spacetime: A systematic analysis of foliation dependence in cosmic averaging

It is a fundamental unsolved question in general relativity how to unambiguously characterize the effective collective dynamics of an ensemble of fluid elements sourcing the local geometry, in the absence of exact symmetries. In a cosmological context this is sometimes referred to as the averaging problem. At the heart of this problem in relativity is the non-uniqueness of the choice of foliation within which the statistical properties of the local spacetime are quantified, which can lead to ambiguity in the formulated average theory. This has led to debate in the literature on how to best construct and view such a coarse-grained hydrodynamic theory. Here, we address this ambiguity by performing the first quantitative investigation of foliation dependence in cosmological spatial averaging. Starting from the aim of constructing slicing-independent integral functionals (volume, mass, entropy, etc.) as well as average functionals (mean density, average curvature, etc.) defined on spatial volume sections, we investigate infinitesimal foliation variations and derive results on the foliation dependence of functionals and on extremal leaves. Our results show that one may only identify fully foliation-independent integral functionals in special scenarios, requiring the existence of associated conserved currents. We then derive bounds on the foliation dependence of integral functionals for general scalar quantities under finite variations within physically motivated classes of foliations. Our findings provide tools that are useful for quantifying, eliminating or constraining the foliation dependence in cosmological averaging.

gr-qc↗

arXiv2023

Bulk flows in the local Universe and the importance of relativistic effects

Bulk flow velocities are typically estimated in the idealised picture where observers are moving within a perfectly homogeneous and isotropic space-time. This picture is consistent within standard perturbation theory up to relativistic effects that lead to correction terms of order $v z$, where $z$ is the redshift of observation, and $v$ is the amplitude of the bulk flow. The dominant relativistic contributions at scales $z \lesssim 1$ are caused by gravitational redshift and time evolution of the velocity field. We include these effects within a broadly applicable weak-field approximation, and provide a cosmographic formula for estimating bulk flows at a high precision. Based on this formula, we judge that recent bulk flow estimates are biased toward larger values by $\sim 10\%$. This theoretical bias surpasses the measurement biases of the same estimates, and it will become still more important to account for the relativistic effects as the scales at which bulk flows are estimated to increase.

astro-ph.CO↗

arXiv2023

Reconciling a decelerating Universe with cosmological observations

Can modern cosmological observations be reconciled with a general-relativistic Universe without an anti-gravitating energy source? Usually, the answer to this question by cosmologists is in the negative, and it is commonly believed that the observed excess dimming of supernovae relative to that in the Milne model is evidence for dark energy. In this paper, we develop theorems that clarify the conditions for such an excess dimming, based on which we argue that the answer to the above question may counter-intuitively be `yes'.

gr-qc↗

arXiv2022

The averaging problem on the past null cone in inhomogeneous dust cosmologies

Cosmological models typically neglect the complicated nature of the spacetime manifold at small scales in order to hypothesize idealized general relativistic solutions for describing the average dynamics of the Universe. Although these solutions are remarkably successful in accounting for data, they introduce a number of puzzles in cosmology, and their foundational assumptions are therefore important to test. In this paper, we go beyond the usual assumptions in cosmology and propose a formalism for averaging the local general relativistic spacetime on an observer's past null cone: we formulate average properties of light fronts as they propagate from a cosmological emitter to an observer. The energy-momentum tensor is composed of an irrotational dust source and a cosmological constant -- the same components as in the $Λ$CDM model for late cosmic times -- but the metric solution is not \emph{a priori}constrained to be locally homogeneous or isotropic. This generally makes the large-scale dynamics depart from that of a simple Friedmann--Lema\^ıtre--Robertson--Walker solution through \emph{backreaction} effects. Our formalism quantifies such departures through a fully covariant system of area-averaged equations on the light fronts propagating towards an observer, which can be directly applied to analytical and numerical investigations of cosmic observables. For this purpose, we formulate light front averages of observable quantities, including the effective angular diameter distance and the cosmological redshift drift and we also discuss the backreaction effects for these observables.

gr-qc↗

arXiv2022

Redshift drift in a universe with structure I: Lemaitre-Tolman-Bondi structures with arbitrary angle of entry of light

We consider the redshift drift and position drift associated with astrophysical sources in a formalism that is suitable for describing emitters and observers of light in an arbitrary spacetime geometry, while identifying emitters of a given null-geodesic bundle that arrives at the observer worldline. We then restrict the situation to the special case of a Lemaitre-Tolman-Bondi (LTB) geometrical structure, and solve for light rays propagating through the structure with arbitrary impact parameters, i.e., with arbitrary angles of entry into the LTB structure. The redshift drift signal emitted by comoving sources and viewed by a comoving observer turns out to be dominated by Ricci curvature and electric Weyl curvature contributions as integrated along the connecting light ray. This property simplifies the computations of the redshift drift signal tremendously, and we expect that the property extends to more complicated models including Swiss-cheese models. When considering several null rays with random impact parameters, the mean redshift drift signal is well approximated by a single Ricci focusing term. This suggests that the measurement of cosmological redshift drift can be used as a direct probe of the strong energy condition in a realistic universe where photons pass through many successive structures.

astro-ph.CO↗

arXiv2022

The quadrupole in the local Hubble parameter: first constraints using Type Ia supernova data and forecasts for future surveys

The cosmological principle asserts that the Universe looks spatially homogeneous and isotropic on sufficiently large scales. Given the fundamental implications of the cosmological principle, it is important to empirically test its validity on various scales. In this paper, we use the Type Ia supernova (SN~Ia) magnitude-redshift relation, from both the Pantheon and JLA compilations, to constrain theoretically motivated anisotropies in the Hubble flow. In particular, we constrain the quadrupole moment in the effective Hubble parameter and the dipole moment in the effective deceleration parameter. We find no significant quadrupole term regardless of the redshift frame we use. Our results are consistent with the theoretical expectation of a quadrupole moment of a few percent at scales of $\sim 100 h^{-1}$ Mpc. We place an upper limit of a $\sim 10\%$ quadrupole amplitude relative to the monopole, $H_0$, at these scales. We find that we can detect a $\sim 7\%$ quadrupole moment at the 5$σ$ level, for a forecast low-$z$ sample of 1055 SNe~Ia. We find an exponentially decaying dipole moment of the deceleration parameter varies in significance depending on the redshift frame we use. In the heliocentric frame, as expected, it is detected at $\sim 3 σ$ significance. In the rest-frame of the cosmic microwave background (CMB), we find a marginal $\sim 2 σ$ dipole, however, after applying peculiar velocity corrections, the dipole is insignificant. Finally, we find the best-fit frame of rest relative to the supernovae to differ from that of the CMB.

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