arXiv subjects

Krzysztof Bolejko

Publications and source records attributed to Krzysztof Bolejko.

At least 19 recordsLinked to original sources

arXiv2026

A hybrid method for reconstruction of the equation of state of dark energy and its application to Pantheon+SH0ES data

Cosmology aims to understand the physical properties of our Universe on its largest scales. One such feature is the expansion of the Universe, which currently seems to be dominated by a phenomenon referred to as dark energy. The physical nature and properties of dark energy are one of the main topics of investigation of modern cosmology. Observational cosmology aims to reconstruct the evolution and the equation of state of dark energy, while theoretical cosmology aims to provide methods for such reconstructions and models explaining the nature of dark energy. If the equation of state, defined as the ratio of pressure to density $w = p/ρ$, deviates from $-1$, i.e. $w\ne-1$, then this would imply the existence of some sort of dynamical process behind dark energy. Most investigations assume a specific parametric form of $w$, eg. $w(z) = w_{0} + w_a z/(1+z)$, with $z$ being redshift and $w_0$ and $w_a$ being constants. The analysis of the data is then reduced to fitting the model to the data. In this work, we take a different approach. Instead of imposing a predefined parametric form for $w(z)$, we reconstruct the equation of state indirectly from the dimensionless comoving distance $D(z)$ and its derivatives. This avoids assuming a specific physical parametrisation of dark energy, such as the CPL form, but still requires adopting functional representations for the distance--redshift relation itself. The method should therefore be regarded as a hybrid or semi-parametric reconstruction approach: the parametrisation is shifted from the equation of state to the observable distance function. Finally we apply the method to the Pantheon+ SH0ES data. The results are consistent with dark energy being the cosmological constant, i.e. $w = -1$. Future surveys such as LSST will provide more data and narrow down the uncertainty. This in turn will yield tighter constraints on dark energy.

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arXiv2026

Local environmental dependence on weak-lensing shear statistics

Despite the assumption that an ideal FLRW observer is not dependent on the local environment, observations are biased by the positions of the observers due to the matter correlations in the large-scale structure (LSS) of the universe. The variation of the mass distribution of the LSS of the universe implies that observers residing in different locations may suffer bias in their measurements when they look at the images of distant galaxies. Here, we assess the influence of the local environment on weak gravitational lensing (WL) shear statistics in the context of relativistic $N$-body code, \texttt{gevolution}. We derive numerical constraints on the cosmological parameters from the WL shear angular power spectrum and comment on the local environment's influence on WL shear. We find tighter constraints on the parameter $Ω_\mathrm{m}$ above redshift $z$ = 0.2, which implies over this redshift the local environment's impact is minor. We also investigate the bispectrum and conclude that on average the impact of the local environment on $f_{\rm NL}$ (a measure of non-Gaussianities) is minimal and consistent with zero effect. However, we find that within the assembly of all possible observers/locations, there will also be a few that could infer the parameter $f_{\rm NL}$ of the order 10. These results could thus be used to estimate the uncertainty in the inference of cosmological parameters such as $f_{\rm NL}$ based on WL shear bispectrum and thus may have implications for future surveys requiring precision at the percent level.

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arXiv2025

Beyond LambdaCDM: How the Hubble tension challenges early universe physics

Differences in the values of the Hubble constant obtained from the local universe and the early universe have resulted in a significant tension. This tension signifies that our understanding of cosmology (physical processes and/or cosmological data) is incomplete. Some of the suggested solutions include physics of the early Universe. In this paper we aim to investigate common features of various early universe solutions to the Hubble constant tension. The physics of the early universe affects the size of the sound horizon which is probed with the Cosmic Microwave Background (CMB) data. Within the standard model, the size of the horizon (within limits of current measurements) is affected by processes that could occur between (approximately) 1 day after the Big Bang and the last scattering instant. We focus on simple extensions incorporating Early Dark Energy (EDE) and show how such a model affects the inferred values of the Hubble constant. We compare this model to LambdaCDM models using MCMC analysis, likelihoods over the parameter space and Bayesian evidence. The MCMC analysis shows that EDE leads to a decrease in the size of the sound horizon that is consistent with H0 = 73.56 km/s/Mpc but we also show that MCMC analysis favours increasing redshift and proportion of EDE. The Bayesian evidence favours our EDE model for very narrow, finely-tuned parameter space. The LambdaCDM model used for comparison has good evidence across a wide parameter space. We interpret this as an indication that more sophisticated models are required. We conclude that if the Hubble tension were to be related to the physics of the early universe, EDE could be used as a window to explore conditions of the early universe and extend our understanding of that era.

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arXiv2025

Constraining decaying dark matter models with gravitational lensing and cosmic voids

Despite overwhelming observational evidence for dark matter, we still have no evidence of direct detection. Consequently, our knowledge about dark matter is limited, for example, we do not know if dark matter is a stable particle or if it decays. Without a theoretical particle model, the parameter space of possible decay models is highly variable, and astrophysical or cosmological means of indirectly constraining the phenomenological models are required. This paper investigates a scenario in which a dark matter decays and the dark daughter particle moves with respect to the comoving mother particle. The model is parameterised by the decay rate and the injection velocity of the dark matter particles, which can be converted to the mass ratio. In previous work, a simpler model was used to investigate the evolution of cosmic voids typified as regions with low content of galaxies and non-baryonic matter. It was found that the growth of S-type voids is modified by the dark matter decay, leading to imprints at the present day. Here we extend our study and improve the method used to model the decay. We also study the gravitational weak-lensing signal that will be able to detect or constrain the parameter space of decaying dark matter. The results of this study suggest that future weak lensing surveys may provide unique probes of the phenomological parameters of dark matter.

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arXiv2023

The Redshift Difference in Gravitational Lensed Systems: A Novel Probe of Cosmology

The exploration of the redshift drift, a direct measurement of cosmological expansion, is expected to take several decades of observation with stable, sensitive instruments. We introduced a new method to probe cosmology which bypasses the long-period observation by observing the redshift difference, an accumulation of the redshift drift, in multiple-image gravitational lens systems. With this, the photons observed in each image will have traversed through different paths between the source and the observer, and so the lensed images will show different redshifts when observed at the same instance. Here, we consider the impact of the underlying cosmology on the observed redshift difference in gravitational lens systems, generating synthetic data for realistic lens models and exploring the accuracy of determined cosmological parameters. We show that, whilst the redshift difference is sensitive to the densities of matter and dark energy within a universe, it is independent of the Hubble constant. Finally, we determine the observational considerations for using the redshift difference as a cosmological probe, finding that one thousand lensed sources are enough to make robust determinations of the underlying cosmological parameters. Upcoming cluster lens surveys, such as the Euclid, are expected to detect a sufficient number of such systems.

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arXiv2023

Detecting cosmic voids via maps of geometric-optics parameters

Curved-spacetime geometric-optics maps derived from a deep photometric survey should contain information about the three-dimensional matter distribution and thus about cosmic voids in the survey, despite projection effects. We explore to what degree sky-plane geometric-optics maps can reveal the presence of intrinsic three-dimensional voids. We carry out a cosmological $N$-body simulation and place it further than a gigaparsec from the observer, at redshift 0.5. We infer three-dimensional void structures using the watershed algorithm. Independently, we calculate a surface overdensity map and maps of weak gravitational lensing and geometric-optics scalars. We propose and implement a heuristic algorithm for detecting (projected) radial void profiles from these maps. We find in our simulation that given the sky-plane centres of the three-dimensional watershed-detected voids, there is significant evidence of correlated void centres in the surface overdensity $Σ$, the averaged weak-lensing tangential shear $\overline{γ_\perp}$, the Sachs expansion $θ$, and the Sachs shear modulus $\lvertσ\rvert$. Recovering the centres of the three-dimensional voids from the sky-plane information alone is significant given the weak-lensing shear $\overline{γ_\perp}$, the Sachs expansion $θ$, or the Sachs shear $\lvertσ\rvert$, but not significant for the surface overdensity $Σ$. Void radii are uncorrelated between three-dimensional and two-dimensional voids; our algorithm is not designed to distinguish voids that are nearly concentric in projection. This investigation shows preliminary evidence encouraging observational studies of gravitational lensing through individual voids, either blind or with spectroscopic/photometric redshifts. The former case - blind searches - should generate falsifiable predictions of intrinsic three-dimensional void centres.

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arXiv2022

The Instantaneous Redshift Difference of Gravitationally Lensed Images: Theory and Observational Prospects

Due to the expansion of our Universe, the redshift of distant objects changes with time. Although the amplitude of this redshift drift is small, it will be measurable with a decade-long campaigns on the next generation of telescopes. Here we present an alternative view of the redshift drift which captures the expansion of the universe in single epoch observations of the multiple images of gravitationally lensed sources. Considering a sufficiently massive lens, with an associated time delay of order decades, simultaneous photons arriving at a detector would have been emitted decades earlier in one image compared to another, leading to an instantaneous redshift difference between the images. We also investigate the effect of peculiar velocities on the redshift difference in the observed images. Whilst still requiring the observational power of the next generation of telescopes and instruments, the advantage of such a single epoch detection over other redshift drift measurements is that it will be less susceptible to systematic effects that result from requiring instrument stability over decade-long campaigns.

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arXiv2022

Ringing the universe with cosmic emptiness: void properties through a combined analysis of stacked weak gravitational and Doppler lensing

An essential aspect of cosmic voids is that these underdense regions provide complementary information about the properties of our Universe. Unlike dense regions, voids are avoided by matter and are less contaminated by baryonic processes. The first step to understanding the properties of cosmic voids is to correctly infer their mass profiles. In the literature, various techniques have been implemented. In this paper, we review them and implement a new technique that is based on Doppler lensing. We use a relativistic $N$-body code, \textsc{Gevolution}, to generate cosmological mass perturbations and implement a three-dimensional ray-tracing technique, which follows the evolution of a ray-bundles. We focus on the various properties of cosmic voids (e.g. void size function, 2-point correlation function, and the density profile of voids), and compare the results with their universal trends. We show that when weak-lensing is combined with the Doppler lensing we obtain even tighter constraints than weak-lensing alone. We also obtain better agreement between density profiles within central parts of voids inferred from lensing and density profiles inferred from halo tracers. The implication of the result relevant to the ongoing and prospective low-redshift spectroscopic surveys is briefly discussed.

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arXiv2022

Imprints of decaying dark matter on cosmic voids

The Standard Cosmological Model assumes that more than 85\% of matter is in the form of collisionless and pressureless dark matter. Unstable decaying dark matter has been proposed in the literature as an extension to the standard cold dark matter model. In this paper we investigate a scenario when dark matter decays and the resultant particle moves with respect to the dark matter. A covariant hydrodynamical model is developed in which the decay is modeled by the transfer of energy-momentum between two dark dust fluid components. We parameterise the model in terms of the decay rate $Γ$ and injection velocity $v_i$ of the resultant dark matter particles. We apply the framework to study the evolution of cosmic voids which are environments with low content of baryonic matter. Thus, unlike baryon-rich environments, voids provide an opportunity to measure dark matter signals that are less contaminated by complex baryonic processes. We find that the growth of S-type voids is modified by the dark matter decay, leading to imprints at the present day. This paper serves as a proof-of-concept that cosmic voids can be used to study dark mater physics. We argue that future cosmological observations of voids should focus on signs of reported features to either confirm or rule out the decaying dark matter scenario. Lack of presence of reported features could put constraints of the decay of dark matter in terms of $Γ> H_0^{-1}$ and $v_i<10$ km/s.

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arXiv2021

Mapping the cosmic mass distribution with stacked weak gravitational lensing and Doppler lensing

Dark matter halos represent the highest density peaks in the matter distribution. Conversely, cosmic voids are under-dense patches of the universe. Probing the mass distribution of the universe requires various approaches, including weak gravitational lensing that subtly modifies the shape of distant sources, and Doppler lensing that changes the apparent size and magnitude of objects due to peculiar velocities. In this work, we adopt both gravitational and Doppler lensing effects to study the underlying matter distribution in and around cosmic voids/halos. We use the relativistic $N$-body code \texttt{gevolution}, to generate the mass perturbations and develop a new ray-tracing code that relies on the design of the ray bundle method. We consider three categories of halo masses and void radii, and extract the cosmological information by stacking weak-lensing and Doppler lensing signals around voids/halos. The results of this paper show that the most optimal strategy that combines both gravitational and Doppler lensing effects to map the mass distribution should focus on the redshift range $z\approx 0.3-0.4$. The recommendation of this paper is that future spectroscopic surveys should focus on these redshifts and utilise the gravitational and Doppler lensing techniques to extract information about underlying matter distribution across the cosmic web, especially inside cosmic voids. This could provide a complimentary cosmological analysis for ongoing or future low-redshift spectroscopic surveys.

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arXiv2021

Exploring the Redshift-Space Peculiar Velocity Field and its Power Spectrum

Redshift-space distortions (RSD) generically affect any spatially-dependent observable that is mapped using redshift information. The effect on the observed clustering of galaxies is the primary example of this. This paper is devoted to another example: the effect of RSD on the apparent peculiar motions of tracers as inferred from their positions in redshift space (i.e. the observed distance). Our theoretical study is motivated by practical considerations, mainly, the direct estimation of the velocity power spectrum, which is preferably carried out using the tracer's redshift-space position (so as to avoid uncertainties in distance measurements). We formulate the redshift-space velocity field and show that RSD enters as a higher-order effect. Physically, this effect may be interpreted as a dissipative correction to the usual perfect-fluid description of dark matter. We show that the effect on the power spectrum is a damping on relatively large, quasilinear scales ($k>0.01\,h\,{\rm Mpc}^{-1}$), as was observed, though unexplained, in $N$-body simulations elsewhere. This paper presents two power spectrum models for the peculiar velocity field in redshift space, both of which can be considered velocity analogues of existing clustering models. In particular, we show that the "Finger-of-God" effect, while also present in the velocity field, cannot be entirely blamed for the observed damping in simulations. Our work provides some of the missing modelling ingredients required for a density--velocity multi-tracer analysis, which has been proposed for upcoming redshift surveys.

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arXiv2021

The density distributions of cosmic structures: impact of the local environment on weak-lensing convergence

Whilst the underlying assumption of the Friedman-Lemaître-Robertson-Walker (FLRW) cosmological model is that matter is homogeneously distributed throughout the universe, gravitational influences over the life of the universe have resulted in mass clustered on a range of scales. Hence we expect that, in our inhomogeneous universe, the view of an observer will be influenced by the location and local environment. Here we analyse the one-point probability distribution functions and angular power spectra of weak-lensing (WL) convergence and magnification numerically to investigate the influence of our local environment on WL statistics in relativistic $N$-body simulations. To achieve this, we numerically solve the null geodesic equations which describe the propagation of light bundles backwards in time from today, and develop a ray-tracing algorithm, and from these calculate various WL properties. Our findings demonstrate how cosmological observations of large-scale structure through WL can be impacted by the locality of the observer. We also calculate the constraints on the cosmological parameters as a function of redshift from the theoretical and numerical study of the angular power spectrum of WL convergence. This study concludes the minimal redshift for the constraint on the parameter $Ω_m$ ($H_0$) is $z \sim 0.2$ $(z \sim 0.6 )$ beyond which the local environment's effect is negligible and the data from WL surveys are more meaningful above that redshift. The outcomes of this study will have direct consequences for future surveys, where percent-level-precision is necessary.

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arXiv2020

Cosmological signatures of torsion and how to distinguish torsion from the dark sector

Torsion is a non-Riemannian geometrical extension of general relativity that allows including the spin of matter and the twisting of spacetime. Cosmological models with torsion have been considered in the literature to solve problems of either the very early (high redshift $z$) or the present-day Universe. This paper focuses on distinguishable observational signatures of torsion that could not be otherwise explained with a scalar field in pseudo-Riemannian geometry. We show that when torsion is present, the cosmic duality relation between the angular diameter distance, $D_{\mathrm A}$, and the luminosity distance, $D_{\mathrm L}$, is broken. We show how the deviation described by the parameter $η= D_{\mathrm L}/[ D_{\mathrm A}(1+z)^2] -1 $ is linked to torsion and how different forms of torsion lead to special-case parametrisations of $η$, including $η_0 z$, $η_0 z/(1+z)$, and $η_0 \ln (1+z)$. We also show that the effects of torsion could be visible in low-redshift data, inducing biases in supernovae-based $H_0$ measurements. We also show that torsion can impact the Clarkson-Bassett-Lu (CBL) function ${\cal C}(z) = 1 + H^2 (D D'' - D'^2) + H H' D D'$, where $D$ is the transverse comoving distance. If $D$ is inferred from the luminosity distance, then, in general non-zero torsion models, ${\cal C}(z) \ne 0$. For pseudo-Riemannian geometry, the Friedmann-Lemaitre-Robertson-Walker (FLRW) metric has ${\cal C}(z) \equiv 0$; thus, measurement of the CBL function could provide another diagnostic of torsion.

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arXiv2019

Probing the independence within the dark sector in the fluid approximation

The standard model of cosmology is based on two unknown dark components that are uncoupled from each other. In this paper we investigate whether there is evidence for an interaction between these components of cold dark matter (CDM) and dark energy (DE). In particular, we focus on a minimal extension and reconstruct the interaction history at low-redshifts non-parametrically using a variation of the commonly used principal component analysis. Although we focus on the interaction in the dark sector, any significant deviation from the standard model that changes the expansion history of the Universe, should leave imprints detectable by our analysis. Thus, detecting signatures of interaction could also be indicative of other non-standard phenomena even if they are not the results of the interaction. It is thus interesting to note that the results presented in this paper do not provide support for the interaction in the dark sector, although the uncertainty is still quite large. In so far as interaction is present but undetectable using current data, we show from a Fisher forecast that forthcoming LSST and DESI surveys will be able to constrain a DM-DE coupling at $20\%$ precision --- enough to falsify the non-interacting scenario, assuming the presence of a modest amount of interaction.

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arXiv2019

Influence of the local Universe on weak gravitational lensing surveys

Observations of the large-scale structure (LSS) implicitly assume an ideal FLRW observer with the ambient structure having no influence on the observer. However, due to correlations in the LSS, cosmological observables are dependent on the position of an observer. We investigate this influence in full generality for a weakly non-Gaussian random field, for which we derive expressions for angular spectra of large-scale structure observables conditional on a property of the large-scale structure that is typical for the observer's location. As an application, we then apply to the formalism to angular spectra of the weak gravitational lensing effect and provide numerical estimates for the resulting change on the spectra using linear structure formation. For angular weak lensing spectra we find the effect to be of order of a few percent, for instance we estimate for an overdensity of $δ=0.5$ and multipoles up to $\ell=100$ the change in the weak lensing spectra to be approximately 4 percent. We show that without accounting for correlation between the density at observer's location and the weak gravitational lensing spectra, the values of the parameters $Ω_m$ and $σ_8$ are underestimated by a few percent. Thus, this effect will be important when analysing data from future surveys such as Euclid, which aim at the percent-level precision. The effect is difficult to capture in simulations, as estimates of the number of numerical simulations necessary to quantify the effect are high.

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arXiv2019

Direct detection of the cosmic expansion: the redshift drift and the flux drift

The redshift drift, the change of cosmological redshift with time, is a direct consequence of the expansion of the Universe. Thus the measurement of the cosmological redshift drift will offer a direct test of our models of cosmology. The magnitude of the effect is very small, i.e. the spectral shift is of order of $10^{-10} - 10^{-9}$ over the period of a decade, but the next generation facilities such as ELT and SKA will be able to directly detect the expansion of our Universe by the year 2040. In this paper we focus on detectebility of this effect, including strategies of overcoming the kinematic contamination of the cosmological signal. We also show that the redshift drift directly impacts the change of flux. Thus apart from the redshift drift, measurements of the flux drift will provide an additional tool of detecting the expansion of the universe, including its acceleration. We discuss the strategies of detecting the flux drift and show that by including the flux drift signal to the redshift drift signal we boost the chances of a direct detection of the expansion of the Universe. We show that if only the stability of flux is at the level of $ΔF/F \sim 10^{-6}$ then the SKA1-mid Array should be able to detect these effects, before the ETL and the full SKA. Thus, by including the flux drift into the SKA1-mid Array's analysis pipeline, we could be able to provide by mid-2030s a direct evidence of the expansion of the universe including its accelerating phase.

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arXiv2018

The environment-dependence of the growth of the most massive objects in the Universe

This paper investigates the growth of the most massive cosmological objects. We utilize the Simsilun simulation, which is based on the approximation of the silent universe. In the limit of spatial homogeneity and isotropy the silent universes reduce to the standard FLRW models. We show that within the approximation of the silent universe the formation of the most massive cosmological objects differs from the standard background-dependent approaches. For objects with masses above $10^{15} M_\odot$ the effect of spatial curvature (overdense regions are characterized by positive spatial curvature) leads to measurable effects. The effect is analogous to the effect that the background cosmological model has on the formation of these objects (i.e. the higher matter density and spatial curvature the faster the growth of cosmic structures). We measure this by the means of the mass function and show that the mass function obtained from the Simsilun simulation has a higher amplitude at the high-mass end compared to standard mass function such as the Press-Schechter or the Tinker mass function. For comparison, we find that the expected mass of most massive objects using the Tinker mass function is $4.4^{+0.8}_{-0.6} \, \times 10^{15} M_\odot$, whereas for the Simsilun simulation is $6.3^{+1.0}_{-0.8} \, \times 10^{15} M_\odot$. We conclude that the nonlinear relativistic effects could affect the formation of the most massive cosmological objects, leading to a relativistic environment-dependence of the growth rate of the most massive clusters.

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arXiv2018

Emerging spatial curvature can resolve the tension between high-redshift CMB and low-redshift distance ladder measurements of the Hubble constant

The measurements of the Hubble constant reveal a tension between high-redshift (CMB) and low-redshift (distance ladder) constraints. So far neither observational systematics nor new physics has been successfully implemented to explain this tension away. This paper present a new solution to the Hubble constant problem. The solution is based on the Simsilun simulation (relativistic simulation of the large scale structure of the Universe) with the ray-tracing algorithm implemented. The initial conditions for the Simsilun simulation were set up as perturbations around the $Λ$CDM model. However, unlike in the Standard Cosmological Model (i.e. $Λ$CDM model + perturbations), within the Simsilun simulation relativistic and nonlinear evolution of cosmic structures leads to the phenomenon of emerging spatial curvature, where the mean spatial curvature evolves from spatial flatness of the early universe towards slightly curved present-day universe. Consqeuently, the present-day expansion rate is slightly faster compared to the spatially flat $Λ$CDM model. The results of the ray-tracing analysis show that the universe which starts with initial conditions consistent with the Planck constraints should have the Hubble constant $H_0 = 72.5 \pm 2.1$ km s$^{-1}$ Mpc$^{-1}$. When the Simsilun simulation was re-run with no inhomogeneities imposed, the Hubble constant inferred within such a homogeneous simulation was $H_0 = 68.1 \pm 2.0$ km s$^{-1}$ Mpc$^{-1}$. Thus, the inclusion of nonlinear relativistic evolution that leads to the emergence of the spatial curvature can explain why the low-redshift measurements favour higher values compared to high-redshift constraints and alleviate the tension between the CMB and distance ladder measurements of the Hubble constant.

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