arXiv subjects

Martin Kunz

Publications and source records attributed to Martin Kunz.

At least 19 recordsLinked to original sources

arXiv2026

Nonlinear velocity power spectrum: modeling the cosmological dependence on the Hubble constant and cold dark matter density

In this paper we present a semi-analytical model for the velocity power spectrum in $\La$CDM cosmology for wave numbers $k<1/$Mpc. We mainly concentrate on the dominant divergence part but also present some results on the vorticity contribution. We divide cosmological parameters into evolution and shape parameters and model the dependence of the evolution parameter $h$ and of the shape parameter $\om_{\rm cdm}$ with an accuracy better than 2.5\%. A surprising finding of our study is that the velocity power spectrum becomes independent of $\om_{\rm cdm}$ on nonlinear scales. A python implementation of the model is publicly available.

astro-ph.CO↗

arXiv2026

Signatures of kinetic gravity braiding in cosmological probes of the gravitational field

We study the observational signatures of kinetic gravity braiding (KGB) models in relativistic cosmological probes constructed along the past light cone. Using the relativistic $N$-body code KGB-evolution, we generate light-cone outputs and compute several observables that directly probe the gravitational field, including weak gravitational lensing convergence, Shapiro time delay, the integrated Sachs-Wolfe and Rees-Sciama (ISW-RS) effects, and gravitational redshift. Full-sky maps and angular power spectra of these quantities are constructed and compared with $k$-essence models and predictions from linear perturbation theory. We find that the derivative coupling between the scalar field and the metric modifies both the amplitude and the time evolution of the gravitational potentials, producing scale-dependent deviations ranging from a few percent to tens of percent. In particular, the ISW-RS signal exhibits the largest fractional response, as the slower decay of the Weyl potential suppresses the KGB signal in the ISW-dominated regime, whereas nonlinear evolution reverses this trend at higher multipoles, producing differences of tens of percent relative to $k$-essence. Weak gravitational lensing also provides a strong complementary probe and, for the model considered here, exhibits clear deviations from the $k$-essence prediction at small scales with enhancements up to $\sim 10$-$12\%$ at multipoles $\ell \sim 10^2$-$10^3$. Our results show that linear perturbation theory accurately describes the large-scale behaviour, while nonlinear effects become important at smaller scales, particularly for the ISW-RS signal and, more moderately, for the convergence, and must therefore be included for reliable theoretical predictions.

gr-qc↗

arXiv2026

Dish Assembly Precision for HIRAX

The Hydrogen Intensity and Real-time Analysis eXperiment (HIRAX) is a radio interferometer array that is being deployed at the South African Radio Astronomy Observatory (SARAO) Square Kilometer Array (SKA) site in South Africa. Mapping the southern sky, its aim is to observe neutral hydrogen (HI) through intensity mapping (IM) across the redshift range of 0.78-2.55. The observation of HI makes it possible to tomographically probe large cosmological volumes, enabling constraints on, for example, the dark energy equation of state. Systematics are a significant concern in deriving cosmological constraints from HI IM due to the presence of strong foreground signals. Instrumental effects such as dish surface deviations and feed placement errors need to be carefully controlled and monitored to preserve the sensitivity of HIRAX's redundant array configuration. These instrumental systematics cause bright foreground power to leak into the faint cosmological signal. The 6 m parabolic dishes are made from fiberglass with an embedded aluminum mesh that acts as the reflector. The feed is held in place at the prime focus (f/D = 0.21) by four fiberglass legs. This paper presents the dish surface and feed placement precision of the first 28 dishes, derived from photogrammetry metrology measurements taken during the dish fabrication, as well as from measurements in the field. While surface deviations are predominantly well within requirements, feed placement satisfies all accuracy criteria but fails precision limits; however, precision is expected to improve with a larger sample size. These results establish a baseline for modeling primary beam effects, ultimately enabling the characterization and mitigation of systematic errors in the HI IM signal with the full HIRAX array.

astro-ph.IM↗

arXiv2026

Tellurium sublattice instability driven amorphization in the chalcogenide AgSbTe2 under pressure

Pressure provides a powerful thermodynamic route to access hidden structural states in functional materials, yet the microscopic origin of pressure-induced amorphization remains elusive in many complex chalcogenides. Here we report a detailed high-pressure structural study of AgSbTe2,combining synchrotron X-ray diffraction with density functional theory and molecular dynamics calculations up to 60 GPa. We uncover a pressure-driven transformation from the ambient R-3m phase to a fully disordered cubic Im3m phase, through an extended intermediate amorphous state. Enthalpy calculations reveal a near-degeneracy between the R3m and Im3m structures over a broad pressure range, dictating amorphization. Contrary to previously speculated cation vacancies, the amorphization is governed by a pronounced displacement instability of the Te sublattice. Remarkably, the time dependent decompression pathway controls the final structural state, resulting in either amorphous (slow decompression) or fully crystalline (fast decompression) states, indicative of a strong counterintuitive kinetic effect.

cond-mat.mtrl-sci↗

arXiv2026

Inflationary interpretation of the gravitational-wave signal in the European Pulsar Timing Array DR2 with constraints

The second data release of the European Pulsar Timing Array (EPTA) collaboration provides evidence for the presence of a gravitational-wave (GW) background. In this work, we explore a potential cosmological interpretation of this signal in terms of inflationary scenarios. We parametrize the tensor power spectrum in terms of the tensor-to-scalar ratio $r$, the tensor spectral index $n_t$, the reheating temperature $T_{\text{rh}}$, and the cut-off frequency $f_{\text{end}}$. We incorporate all relevant observational constraints, including those from the Cosmic Microwave Background, Big Bang Nucleosynthesis, and LIGO-Virgo-KAGRA observations. We demonstrate that imposing these constraints consistently reduces the region of parameter space that provides a viable interpretation of the EPTA signal, to $-11.66 \lesssim \log_{10}r \lesssim -1.45$, $1.32 \lesssim n_t \lesssim 2.47$, $1.78\text{ MeV} \lesssim T_{\text{rh}} \lesssim 28.2\text{ GeV}$, and $75.86\text{ nHz} \lesssim f_{\text{end}} \lesssim 14.45\text{ Hz}$ at the 95% confidence level. This favours the scenario in which the GW spectrum in the EPTA frequency band originates from tensor modes that re-entered the Hubble radius during the radiation-dominated era, allowing for a higher $r$ and a flatter spectrum. However, $T_{\text{rh}}$ must take very low values, which are challenging to explain theoretically.

astro-ph.CO↗

arXiv2026

Late-time reconstruction of non-minimally coupled gravity with a smoothness prior

We present a non-parametric, model-independent reconstruction of the cosmological background and perturbation dynamics in non-minimally coupled theories of gravity. Within the Effective Field Theory of dark energy framework, we reconstruct the time-dependent cosmological constant, $\Lambda(t)$, and the non-minimal coupling function, $\Omega(t)$, from cosmological data. To ensure stability, we apply a correlated smoothness prior that restricts the reconstruction to the space of sufficiently smooth functions. Using CMB, DESI BAO, Type Ia supernovae, CMB-ISW lensing cross-correlations, and large-scale 3x2pt DES Year 3 data, we find a $2.8\sigma$ hint for a non-minimal coupling. For the dark energy equation of state, our results indicate a preference for the existence of crossing of the phantom divide, $w_{DE}=-1$, at $z<0.8$. The non-minimal coupling effect stabilizes dark energy perturbations, providing a viable physical interpretation of the phantom crossing scenario. Our work paves the way for model-agnostic searches for signatures of modified gravity in cosmological data.

astro-ph.CO↗

arXiv2026

Cosmic strings, domain walls and environment-dependent clustering

Recent cosmological data favour phantom-crossing dark energy, motivating models with non-minimal couplings that induce a fifth force on structure formation. Reconciling these models with local tests often requires strong screening, leading to environment-dependent clustering. We investigate such effects via a late-time structure-induced phase transition driven by a non-minimally coupled scalar field. For this purpose, we introduce norns, a fully relativistic cosmological particle-mesh code that self-consistently evolves a complex scalar field - a generalisation of the symmetron producing global U(1) strings rather than domain walls. Using simulations, we compare string and wall-forming models, quantifying impacts on the matter power spectrum, halo mass function, and defect dynamics. Strong environment-dependent effects can generate significant departures from LCDM in underdense regions while keeping the overall power spectrum changes modest (~ 4-15% at k~0.3-0.5 h Mpc^-1, sub-percent for z > 0.2). We find that an attractive fifth force can locally suppress structure growth in voids while enhancing it in surrounding overdense regions by driving outflows from the voids. These effects leave distinctive signatures in the matter density probability density function and in marked halo power spectra, which are likely detectable in low-redshift data.

astro-ph.CO↗

arXiv2025

KGB-evolution: a relativistic $N$-body code for kinetic gravity braiding models

We present KGB-evolution, a relativistic $N$-body simulation code that extends the $k$-evolution code by incorporating an effective field theory parameterization of kinetic gravity braiding, while also including the $k$-essence model as a limiting case. As a first step, we implement the linearized dark energy stress-energy tensor and scalar field equations, providing the groundwork for a future full Horndeski theory extension. We validate KGB-evolution by comparing its power spectra against linear predictions from hi$\_$class, finding excellent agreement on large scales at low redshifts and over all scales at high redshifts. We demonstrate that nonlinear growth of matter and metric perturbations on small scales drives the linearized dark energy field into a nonlinear clustering regime, which in turn feeds back on the growth of cosmic structure. In contrast to the $k$-essence limit, a nonzero braiding considerably amplifies this backreaction, producing a significantly stronger alteration of structure formation in the kinetic gravity braiding model.

astro-ph.CO↗

arXiv2025

High pressure synthesis and structural study of AuGa2 intermetallic compound

We report the synthesis of the AuGa2 intermetallic compound, using a direct reaction of the relevant elements at room temperature and at very low pressure. The pressure needed to synthesize the AuGa2 compound is below 1 GPa, that is at the lower limit of modern large volume presses, routinely used to synthesize other commercially available materials. This study presents a new method of synthesizing AuGa2, which is much more cost efficient and environmentally friendly than the previously used high-temperature synthesis techniques, and will open new possibilities of synthesizing other intermetallic compounds using high-pressure athermal techniques.

cond-mat.mtrl-sci↗

arXiv2025

One-loop kernels in scale-dependent Horndeski theory

We investigate the nonlinear evolution of cosmological perturbations in theories with scale-dependent perturbation growth, first in general and then focusing on Horndeski gravity. Within the framework of standard perturbation theory, we derive the second- and third-order kernels and show that they are fully determined by two effective functions, $ h_1 $ and $ h_c $, which parametrize deviations from general relativity. Using the Wronskian method, we obtain solutions for the nonlinear growth functions and present explicit expressions for the resulting kernels, including bias and redshift space distortions, valid in the limit in which the $k$-dependent part is subdominant. We show that the kernels are entirely dependent on the linear growing mode: once this is calculated, the kernels are analytic up to a time integral. We also include redshift-space distortions (RSD) and scale-dependent bias. Our approach provides a physically motivated framework for evaluating the one-loop galaxy power spectrum in scale-dependent theories, suitable for the forecasts and actual data analysis.

astro-ph.CO↗

arXiv2025

Anchors no more: Using peculiar velocities to constrain $H_0$ and the primordial Universe without calibrators

We develop a novel approach to constrain the Hubble parameter $H_0$ and the primordial power spectrum amplitude $A_\mathrm{s}$ using type Ia supernovae (SNIa) data. By considering SNIa as tracers of the peculiar velocity field, we can model their distance and their covariance as a function of cosmological parameters without the need of calibrators like Cepheids; this yields a new independent probe of the large-scale structure based on SNIa data without distance anchors. Crucially, we implement a differentiable pipeline in JAX, including efficient emulators and affine sampling, reducing inference time from years to hours on a single GPU. We first validate our method on mock datasets, demonstrating that we can constrain $H_0$ and $\log 10^{10}A_\mathrm{s}$ within $10\%$ and $15\%$, respectively, using $\mathcal{O}(10^3)$ SNIa. We then test our pipeline with SNIa from an $N$-body simulation, obtaining $6\%$-level unbiased constraints on $H_0$ with a moderate noise level. We finally apply our method to Pantheon+ data, constraining $H_0$ at the $15\%$ level without Cepheids when fixing $A_\mathrm{s}$ to its $\it{Planck}$ value. On the other hand, we obtain $20\%$-level constraints on $\log 10^{10}A_\mathrm{s}$ in agreement with $\it{Planck}$ when including Cepheids in the analysis. In light of upcoming observations of low redshift SNIa from the Zwicky Transient Facility and the Vera Rubin Legacy Survey of Space and Time, surveys for which our method will develop its full potential, we make our code publicly available.

astro-ph.CO↗

arXiv2025

Reconstructing the dark energy density in light of DESI BAO observations

In light of the evidence for dynamical dark energy (DE) found from the most recent Dark Energy Spectroscopic Instrument (DESI) baryon acoustic oscillation (BAO) measurements, we perform a non-parametric, model-independent reconstruction of the DE density evolution. To do so, we develop and validate a new framework that reconstructs the DE density through a third-degree piece-wise polynomial interpolation, allowing for direct constraints on its redshift evolution without assuming any specific functional form. The strength of our approach resides in the choice of directly reconstructing the DE density, which provides a more straightforward relation to the distances measured by BAO than the equation of state parameter. We investigate the constraining power of cosmic microwave background (CMB) observations combined with supernovae (SNe) and BAO measurements. In agreement with results from other works, we find a preference for deviations from $\Lambda$CDM, with a significance of $2.4\sigma$ when using the Dark Energy Survey Year 5 (DESY5) SNe data, and $1.3\sigma$ with PantheonPlus. In all the cases we consider, the derived DE equation of state parameter presents evidence for phantom crossing. By investigating potential systematic effects in the low-redshift samples of DESY5 observations, we confirm that correcting for the offset in apparent magnitude with respect to PantheonPlus data, as suggested in previous studies, completely removes the tension. Furthermore, we assess the risk of potentially overfitting the data by changing the number of interpolation nodes. As expected, we find that with lesser nodes we get a smoother reconstructed behavior of the DE density, although with similar overall features. The pipeline developed in this work is ready to be used with future high-precision data to further investigate the evidence for a non-standard background evolution.

astro-ph.CO↗

arXiv2025

Modified gravity constraints with Planck ISW-lensing bispectrum

We present updated constraints on modified gravity by including the Integrated Sachs-Wolfe (ISW) effect from CMB lensing-CMB temperature cross-correlations, based on the latest Planck PR4 maps. Utilizing the Effective Field Theory of dark energy approach and adopting the $w_0w_a$CDM background cosmological model, we find that including the CMB ISW lensing cross-correlations tighten constraints on the modified gravity parameters by approximately $20\%$, reducing the viable parameter space by $40-80\%$. We derive constraints from Planck CMB, Planck and ACT CMB lensing, DESI DR1 BAO, CMB ISW-lensing, and type Ia supernovae (SN Ia) data. The constraints on the EFT parameters controlling the kinetic braiding and non-minimal coupling are consistent with General Relativity (GR) at the $95\%$ CL. In particular, we obtain a bound on the kinetic braiding parameter, $c_B < 1.2$ at $95\%$ CL. In the $w_0$-$w_a$ parameter space, our results imply a crossing of the phantom divide, $w=-1$. The modified gravity model shows a mild preference over $\Lambda$CDM at the $1.8\sigma$, $2.6\sigma$ and $3.2\sigma$ levels for the combinations with Pantheon+, Union3 and DESY5 supernova datasets. We find that using the latest $\texttt{HiLLiPoP}+\texttt{LoLLiPoP}$ likelihoods alleviates the departure of modified gravity parameters from the GR-values compared to results using {\it Planck} 2018 data. This paper underlines the importance of the ISW lensing probe in constraining late-time modifications of gravity.

astro-ph.CO↗

arXiv2025

Comparative high-pressure structural and electrical transport properties study of thermoelectric (Bi1-xSbx)2Te3 compounds

Thermoelectric (Bi1-x Sbx)2Te3 (BST-x) compounds with x=0.2, 0.7 and 0.9 have been studied using synchrotron angle-dispersive powder x-ray diffraction in a diamond anvil cell up to 25 GPa (at room temperature). The results clearly indicate that all compounds of this study follow a similar structural evolution with the one of pure Bi2Te3 and Sb2Te3 under pressure. From the comparison between the critical pressures of the corresponding phase transitions, a clear trend of increasing critical pressure for the transition to the disordered solid-solution BCC phase was observed with the increase of Sb concentration. In the case of the BST-0.7, an extended stability of the solid-solution BCC phase up to, at least, 180 GPa was observed. Finally, electrical transport properties measurements under pressure for BST-0.7, document a reversible pressure-induced metallization above 12 GPa.

cond-mat.mtrl-sci↗

arXiv2025

High pressure structural and lattice dynamics study of {\alpha}-In$_2$Se$_3$

Layered $\alpha$-In$_2$Se$_3$has been studied using a concomitant in-situ synchrotron angle dispersive powder x-ray diffraction and Raman spectroscopy study in a diamond anvil cell up to 60+ GPa, at room temperature. Helium, that remains fairly hydrostatic up to the highest pressure in this study, was used as the pressure-transmitting medium. The results from both experimental methods reveal a pressure-induced structural phase transition from $\alpha$-In$_2$Se$_3$ to a monoclinic $\beta$'-In2Se3 structure at $\approx$1 GPa, in agreement with previous studies. Based on our detailed measurements using both experimental techniques and F-f formalism, the $\beta$'-In$_2$Se$_3$ structure remains stable up to 45 GPa, without a clear indication of a phase transition towards the previously reported $\beta$-In2Se3 phase. Above this pressure, In$_2$Se$_3$ adopts a disordered solid-solution-like orthorhombic structure, phase IV. The results are discussed in comparison with the relevant previous studies of $\alpha$-In$_2$Se$_3$ under pressure.

cond-mat.mtrl-sci↗

arXiv2025

TABASCAL: Removing multi-satellite interference from radio interferometry observations

In the first trajectory-based radio frequency interference (RFI) subtraction and calibration (TABASCAL) paper, we showed how to calibrate radio interferometers in the presence of RFI sources by simultaneously isolating the trajectories and signals of the RFI sources. In this paper, we show that we can accurately remove RFI (i.e. recover the astronomical signal) from simulated MeerKAT radio interferometry target data. We are able to do so for a single frequency channel, corrupted by up to nine simultaneous satellites, with average RFI amplitudes varying from weak to very strong (1-1000 Jy). Additionally, TABASCAL also manages to leverage the signal-to-noise ratio (S/N) of the RFI to phase-calibrate the astronomical signal. TABASCAL, effectively performs a suitably phased up fringe filter for each RFI source, which essentially allows for an ideal removal of RFI across all RFI strengths. As a result, TABASCAL is able to reach image noises equivalent to the uncorrupted, no-RFI, case. For larger RFI amplitudes, the resulting image noise is 10x - 100x smaller than those from traditional RFI flagging methods such as AOFLAGGER. As a specific application, we show that point-source science with TABASCAL almost matches the no-RFI case with near perfect completeness for all RFI amplitudes. In contrast, the completeness of AOFLAGGER and idealised $3\sigma$ flagging drops below 40% for strong RFI amplitudes, where recovered flux errors are approximately 10x - 100x worse than those from TABASCAL. Finally, we note that TABASCAL works for astronomical sources with both static and varying fluxes.

astro-ph.IM↗

arXiv2024

High Pressure Structural Behavior of Silicon Telluride (Si2Te3) Nanoplates

The high-pressure behavior of silicon telluride (Si2Te3), a two-dimensional (2D) layered material, was investigated using synchrotron X-ray powder diffraction in a diamond anvil cell to 11.5 GPa coupled with first-principles theory. Si2Te3 undergoes a phase transition at < 1 GPa from a trigonal to a hexagonal crystal structure. At higher pressures (> 8.5 GPa), X-ray diffraction showed the appearance of new peaks possibly coincident with a new phase transition, though we suspect Si2Te3 retains a hexagonal structure. Density functional theory calculations of the band structure reveal metallization above 9.1 GPa consistent with previous measurements of the Raman spectra and disappearance of color and transparency at pressure. The theoretical Raman spectra reproduce the prominent features of the experiment, though a deeper analysis suggests that the orientation of Si dimers dramatically influences the vibrational response. Given the complex structure of Si2Te3, simulation of the resulting high-pressure phase is complicated by disordered vacancies and the initial orientations of Si-Si dimers in the crushed layered phase.

cond-mat.mtrl-sci↗

arXiv2024

An Independent Measure of the Kinematic Dipole from SDSS

We utilize the Sloan Digital Sky Survey (SDSS) extended Baryon Oscillation Spectroscopic Survey (eBOSS) and Baryon Oscillation Spectroscopic Survey (BOSS) catalogs with precise spectroscopic redshifts to estimate the kinematic redshift dipole caused by the proper motion of the Solar system. We find that the velocity extracted from the kinematic dipole is consistent with Cosmic Microwave Background inferred values. Although the small sky coverage and limited number density of the SDSS sources constrain us from obtaining precise and robust measurements, we leverage the redshift dipole method to estimate the kinematic dipole. The velocity measurements in this study are insensitive to intrinsic clustering, associated with the source count dipole. The kinematic dipole measured in this work and its consistency with CMB values do not guarantee isotropy at large scales. The anisotropy (excess dipole) measured with the NRAO VLA Sky Survey (NVSS) and the WISE Catalog (CatWISE) could be due to the intrinsic distribution of galaxies. The results in this work focus solely on the kinematic dipole term.

astro-ph.CO↗