arXiv subjects

Dominik J. Schwarz

Publications and source records attributed to Dominik J. Schwarz.

At least 37 records · Page 2Linked to original sources

arXiv2021

Lepton Flavour Asymmetries and the Mass Spectrum of Primordial Black Holes

We study the influence of lepton flavour asymmetries on the formation and the mass spectrum of primordial black holes. We estimate the detectability of their mergers with LIGO/Virgo and show that the currently published gravitational wave events may actually be described by a primordial black hole spectrum from non-zero asymmetries. We suggest to use gravitational-wave astronomy as a novel tool to probe how lepton flavour asymmetric the Universe has been before the onset of neutrino oscillations.

astro-ph.CO↗

arXiv2021

Extreme-Value Distributions and Primordial Black-Hole Formation

We argue that primordial black-hole formation must be described by means of extreme-value theory. This is a consequence of the large values of the energy density required to initiate the collapse of black holes in the early Universe and the finite duration of their collapse. Compared to the Gaussian description of the most extreme primordial density fluctuations, the holes' mass function is narrower and peaks towards larger masses. Secondly, thanks to the shallower fall-off of extreme-value distributions, the predicted abundance of primordial black holes is boosted by $10^{7}$ orders of magnitude when extrapolating the observed nearly scale-free power spectrum of the cosmic large-scale structure to primordial black-hole mass scales.

astro-ph.CO↗

arXiv2020

One- and Two-point Source Statistics from the LOFAR Two-metre Sky Survey First Data Release

The LOFAR Two-metre Sky Survey (LoTSS) will map the complete Northern sky and provide an excellent opportunity to study the distribution and evolution of the large-scale structure of the Universe. We study the completeness of the LoTSS first data release (DR1) and find a point-source completeness of 99 % above flux densities of 0.8 mJy and define a suite of quality cuts. We determine the count-in-cell statistics and differential source counts statistic and measure the angular two-point correlation function of the LoTSS radio sources. The counts-in-cell statistic reveals that the distribution of radio sources cannot be described by a spatial Poisson process. Instead, a good fit is provided by a compound Poisson distribution. The differential source counts are in good agreement with previous findings in deep fields at low radio frequencies and with simulated catalogues from the SKA design study sky and the Tiered Radio Extragalactic Continuum Simulation. The angular two-point correlation is $<10^{-2}$ at angular scales $> 1$ deg. Restricting the value added source catalogue to low-noise regions and a flux density threshold of 2 mJy provides our most reliable estimate of the angular two-point correlation. For smaller flux density thresholds systematic issues are identified, most likely related to the flux density calibration of the individual pointings. Based on the distribution of photometric redshifts of LoTSS sources and the Planck 2018 best-fit cosmological model, the theoretically predicted angular two-point correlation between 0.1 deg and 6 deg agrees with the measured clustering for a subsample of radio sources with redshift information. We find agreement with the expectation of large-scale statistical isotropy of the radio sky at the per cent level. The angular two-point correlation agrees well with the expectation of the cosmological standard model. (abbreviated)

astro-ph.CO↗

arXiv2020

Cross-correlating radio continuum surveys and CMB lensing: constraining redshift distributions, galaxy bias and cosmology

We measure the harmonic-space auto-power spectrum of the galaxy overdensity in the LOFAR Two-metre Sky Survey (LoTSS) First Data Release and its cross correlation with the map of the lensing convergence of the cosmic microwave background (CMB) from the Planck collaboration. We report a $\sim5σ$ detection of the cross-correlation. We show that the combination of the clustering power spectrum and CMB lensing cross-correlation allows us to place constraints on the high-redshift tail of the redshift distribution, one of the largest sources of uncertainty in the use of continuum surveys for cosmology. Our analysis shows a preference for a broader redshift tail than that predicted by the photometric redshifts contained in the LoTSS value added catalog, as expected, and more compatible with predictions from simulations and spectroscopic data. Although the ability of CMB lensing to constrain the width and tail of the redshift distribution could also be valuable for the analysis of current and future photometric weak lensing surveys, we show that its performance relies strongly on the redshift evolution of the galaxy bias. Assuming the redshift distribution predicted by the Square Kilometre Array Design simulations, we use our measurements to place constraints on the linear bias of radio galaxies and the amplitude of matter inhomogeneities $σ_8$, finding $σ_8=0.69^{+0.14}_{-0.21}$ assuming the galaxy bias scales with the inverse of the linear growth factor, and $σ_8=0.79^{+0.17}_{-0.32}$ assuming a constant bias.

astro-ph.CO↗

arXiv2020

Searching for axion-like particles under strong gravitational lenses

We establish strong gravitational lens systems as robust probes of axion-like particles (ALPs) -- a candidate for dark matter. A tiny interaction of photons with ALPs induces birefringence. Multiple images of gravitationally lensed polarised objects allow differential birefringence measurement, alleviating systematics and astrophysical dependencies. We apply this novel method to the lens system CLASS B1152+199 and constrain ALP-photon coupling $\le 9.2\times 10^{-11}\, {\rm GeV}^{-1} \textrm{ to } 7.7\times 10^{-8}\, {\rm GeV}^{-1}$ ($95\%$ C.L.) for ALP mass between $3.6\times 10^{-21} \,{\rm eV}$ and $4.6 \times 10^{-18} \,{\rm eV}$. A larger sample will improve the constraints.

astro-ph.CO↗

arXiv2020

Geometric optics in the presence of axion-like particles in curved space-time

We present a concise derivation of geometric optics in the presence of axionic fields in a curved space-time. Whenever light can be described via geometric optics (the eikonal approximation), the only difference to the situation without axionic field is the phenomenon of achromatic birefringence. Consequently, redshift of light and distance estimates based on propagating light rays, as well as shear and magnification due to gravitational lensing are not affected by the interaction of light with an axionic field.

hep-ph↗

arXiv2020

A LOFAR Observation of Ionospheric Scintillation from Two Simultaneous Travelling Ionospheric Disturbances

This paper presents the results from one of the first observations of ionospheric scintillation taken using the Low-Frequency Array (LOFAR). The observation was of the strong natural radio source Cas A, taken overnight on 18-19 August 2013, and exhibited moderately strong scattering effects in dynamic spectra of intensity received across an observing bandwidth of 10-80MHz. Delay-Doppler spectra (the 2-D FFT of the dynamic spectrum) from the first hour of observation showed two discrete parabolic arcs, one with a steep curvature and the other shallow, which can be used to provide estimates of the distance to, and velocity of, the scattering plasma. A cross-correlation analysis of data received by the dense array of stations in the LOFAR "core" reveals two different velocities in the scintillation pattern: a primary velocity of ~30m/s with a north-west to south-east direction, associated with the steep parabolic arc and a scattering altitude in the F-region or higher, and a secondary velocity of ~110m/s with a north-east to south-west direction, associated with the shallow arc and a scattering altitude in the D-region. Geomagnetic activity was low in the mid-latitudes at the time, but a weak sub-storm at high latitudes reached its peak at the start of the observation. An analysis of Global Navigation Satellite Systems (GNSS) and ionosonde data from the time reveals a larger-scale travelling ionospheric disturbance (TID), possibly the result of the high-latitude activity, travelling in the north-west to south-east direction, and, simultaneously, a smaller--scale TID travelling in a north-east to south-west direction, which could be associated with atmospheric gravity wave activity. The LOFAR observation shows scattering from both TIDs, at different altitudes and propagating in different directions. To the best of our knowledge this is the first time that such a phenomenon has been reported.

astro-ph.EP↗

arXiv2019

Flavour composition and entropy increase of cosmological neutrinos after decoherence

We propose that gravitational interactions of cosmic neutrinos with the statistically homogeneous and isotropic fluctuations of space-time leads to decoherence. This working hypothesis, which we describe by means of a Lindblad operator, is applied to the system of two- and three-flavour neutrinos undergoing vacuum oscillations and the consequences are investigated. As result of this decoherence we find that the neutrino entropy would increase as a function of initial spectral distortions, mixing angles and CP-violation phase. Subsequently we discuss the chances to discover such an increase observationally (in principle). We also present the expected flavour composition of the cosmic neutrino background after decoherence is completed. The physics of two- or three-flavour oscillation of cosmological neutrinos resembles in many aspects two- or three-level systems in atomic clocks, which were recently proposed by Weinberg for the study of decoherence phenomena.

astro-ph.CO↗

arXiv2019

CMB foreground measurements through broad-band radio spectro-polarimetry: prospects of the SKA-MPG telescope

Precise measurement of the foreground synchrotron emission, which contaminates the faint polarized cosmic microwave background radiation (CMB), is a major challenge for the next-generation of CMB experiments. To address this, dedicated foreground measurement experiments are being undertaken at radio frequencies between 2 and 40 GHz. Foreground polarized synchrotron emission measurements are particularly challenging, primarily due to the complicated frequency dependence in the presence of Faraday rotation, and are best recovered through broad fractional-bandwidth polarization measurements at frequencies $\lesssim 5$ GHz. A unique opportunity for measuring the foreground polarized synchrotron emission will be provided by the 15-m SKA-MPG telescope operating in the frequency range 1.7 to 3.5~GHz (S-Band). Here, we present the scope of a Southern sky survey in S-Band at 1 degree angular resolution and explore its added advantage for application of powerful techniques, such as, Stokes $Q$, $U$ fitting and RM-synthesis. A full Southern-sky polarization survey with this telescope, when combined with other on-going efforts at slightly higher frequencies, will provide an excellent frequency coverage for modeling and extrapolating the foreground polarized synchrotron emission to CMB frequencies ($\gtrsim80$~GHz) with rms brightness temperature better than 10~nK per 1 degree$^2$. We find that this survey will be crucial for understanding the effects of Faraday depolarization, especially in low Galactic latitude regions. This will allow better foreground cleaning and thus will contribute significantly in further improving component separation analyses and increase usable sky area for cosmological analysis of the \textit{Planck} data, and the \textit{LiteBIRD} mission in the future.

astro-ph.GA↗

arXiv2019

How to relax the cosmological neutrino mass bound

We study the impact of non-standard momentum distributions of cosmic neutrinos on the anisotropy spectrum of the cosmic microwave background and the matter power spectrum of the large scale structure. We show that the neutrino distribution has almost no unique observable imprint, as it is almost entirely degenerate with the effective number of neutrino flavours, $N_{\mathrm{eff}}$, and the neutrino mass, $m_ν$. Performing a Markov chain Monte Carlo analysis with current cosmological data, we demonstrate that the neutrino mass bound heavily depends on the assumed momentum distribution of relic neutrinos. The message of this work is simple and has to our knowledge not been pointed out clearly before: Cosmology allows that neutrinos have larger masses if their average momentum is larger than that of a perfectly thermal distribution. Here we provide an example in which the mass limits are relaxed by a factor of two.

astro-ph.CO↗

arXiv2019

Linking multipole vectors and pseudoentropies for CMB analysis

Multipole vectors and pseudoentropies provide powerful tools for a numerically fast and vivid investigation of possible statistically anisotropic, respectively non-Gaussian signs in CMB temperature fluctuations. After reviewing and linking these two conceptions we compare their application to data analysis using the Planck 2015 NILC full sky map.

astro-ph.CO↗

arXiv2019

Gravitational waves from inspiralling compact binaries in conformal gravity

We investigate the production of gravitational waves during the inspiral of compact binaries close to their merger in the context of a conformal gravity model. The model incorporates five massive polarization degrees of freedom, besides the two massless gravitational wave polarizations of general relativity. For small graviton mass, we find that the amplitude of the gravitational waves is strongly suppressed as compared to general relativity and decreases as coalescence is approached, which contradicts the observational fact. We conclude that this model with small graviton mass, including a regime that can explain galaxy rotation curves without dark matter, cannot describe the observed gravitational wave events. For a large graviton mass, the modifications to the waveform, compared to the one from general relativity, are negligible on the relevant distance scales and hence a conformal gravity model with a large graviton mass is in agreement with LIGO/VIRGO observations and leads to chirp mass and distance estimates that agree with those from general relativity.

gr-qc↗

arXiv2019

Testing the standard model of cosmology with the SKA: the cosmic radio dipole

The dipole anisotropy seen in the {cosmic microwave background radiation} is interpreted as due to our peculiar motion. The Cosmological Principle implies that this cosmic dipole signal should also be present, with the same direction, in the large-scale distribution of matter. Measurement of the cosmic matter dipole constitutes a key test of the standard cosmological model. Current measurements of this dipole are barely above the expected noise and unable to provide a robust test. Upcoming radio continuum surveys with the SKA should be able to detect the dipole at high signal to noise. We simulate number count maps for SKA survey specifications in Phases 1 and 2, including all relevant effects. Nonlinear effects from local large-scale structure contaminate the {cosmic (kinematic)} dipole signal, and we find that removal of radio sources at low redshift ($z\lesssim 0.5$) leads to significantly improved constraints. We forecast that the SKA could determine the kinematic dipole direction in Galactic coordinates with an error of $(Δl,Δb)\sim(9^\circ,5^\circ)$ to $(8^\circ, 4^\circ)$, depending on the sensitivity. The predicted errors on the relative speed are $\sim 10\%$. These measurements would significantly reduce the present uncertainty on the direction of the radio dipole, and thus enable the first critical test of consistency between the matter and CMB dipoles.

astro-ph.CO↗

arXiv2018

Cosmic QCD Epoch at Nonvanishing Lepton Asymmetry

We investigate how a lepton asymmetry impacts the cosmic trajectory in the QCD phase diagram. We study the evolution of chemical potentials during the QCD epoch of the early Universe using susceptibilities from lattice QCD to interpolate between an ideal quark gas and an ideal hadron resonance gas. The lepton asymmetry affects the evolution of all chemical potentials. The standard cosmic trajectory is obtained assuming tiny lepton and baryon asymmetries. For larger lepton asymmetry, the charge chemical potential exceeds the baryon chemical potential before pion annihilation.

hep-ph↗

arXiv2018

Cosmology with Phase 1 of the Square Kilometre Array; Red Book 2018: Technical specifications and performance forecasts

We present a detailed overview of the cosmological surveys that will be carried out with Phase 1 of the Square Kilometre Array (SKA1), and the science that they will enable. We highlight three main surveys: a medium-deep continuum weak lensing and low-redshift spectroscopic HI galaxy survey over 5,000 sqdeg; a wide and deep continuum galaxy and HI intensity mapping survey over 20,000 sqdeg from z = 0.35 - 3; and a deep, high-redshift HI intensity mapping survey over 100 sqdeg from z = 3 - 6. Taken together, these surveys will achieve an array of important scientific goals: measuring the equation of state of dark energy out to z ~ 3 with percent-level precision measurements of the cosmic expansion rate; constraining possible deviations from General Relativity on cosmological scales by measuring the growth rate of structure through multiple independent methods; mapping the structure of the Universe on the largest accessible scales, thus constraining fundamental properties such as isotropy, homogeneity, and non-Gaussianity; and measuring the HI density and bias out to z = 6. These surveys will also provide highly complementary clustering and weak lensing measurements that have independent systematic uncertainties to those of optical surveys like LSST and Euclid, leading to a multitude of synergies that can improve constraints significantly beyond what optical or radio surveys can achieve on their own. This document, the 2018 Red Book, provides reference technical specifications, cosmological parameter forecasts, and an overview of relevant systematic effects for the three key surveys, and will be regularly updated by the Cosmology Science Working Group in the run up to start of operations and the Key Science Programme of SKA1.

astro-ph.CO↗

arXiv2018

Multipole vectors of completely random microwave skies for $l \leq 50$

The statistical cosmological principle states that observables on the celestial sphere are sampled from a rotationally invariant distribution. Previously certain large scale anomalies which violate this principle have been found, for example an alignment of the lowest multipoles with the cosmic dipole direction. In this work we continue the search for possible anomalies using multipole vectors which represent a convenient tool for this purpose. In order to study the statistical behavior of multipole vectors, we revisit several construction methods. We investigate all four full-sky foreground-cleaned maps from the Planck 2015 release with respect to four meaningful physical directions using computationally cheap statistics that have a simple geometric interpretation. We find that the full-sky SEVEM map deviates from all the other cleaned maps, as it shows a strong correlation with the Galactic Pole and Galactic Center. The other three maps COMMANDER, NILC and SMICA show a consistent behavior. On the largest angular scales, $l \leq 5$, as well as on intermediate scales, $l = 20, 21, 22, 23, 24$, all of them are unusually correlated with the cosmic dipole direction. These scales coincide with the scales on which the angular power spectrum deviates from the Planck 2015 best-fit ΛCDM model. In the range $2 \leq l \leq 50$ as a whole there is no unusual behavior visible globally. We do not find abnormal intramultipole correlation, i.e. correlation of multipole vectors inside a given multipole without reference to any outer direction.

astro-ph.CO↗

arXiv2018

SKA and the Cosmic Radio Dipole

We study the prospects to measure the cosmic radio dipole by means of continuum surveys with the Square Kilometre Array. Such a measurement will allow a critical test of the cosmological principle. It will test whether the cosmic rest frame defined by the cosmic microwave background at photon decoupling agrees with the cosmic rest frame of matter at late times.

astro-ph.CO↗

arXiv2018

Astrophysical Gravitational Waves in Conformal Gravity

We investigate the gravitational radiation from binary systems in conformal gravity (CG) and massive conformal gravity (MCG). CG might explain observed galaxy rotation curves without dark matter, and both models are of interest in the context of quantum gravity. Here we show that gravitational radiation emitted by compact binaries allows us to strongly constrain both models. We work in Weyl gauge, which fixes the rescaling invariance of the models, and derive the linearized fourth-order equation of motion for the metric, which describes massless and massive modes of propagation. In the limit of a large graviton mass, MCG reduces to general relativity (GR), whereas CG does not. Coordinates are fixed by Teyssandier gauge to show that for a conserved energy-momentum tensor the gravitational radiation is due to the time-dependent quadrupole moment of a non-relativistic source and we derive the gravitational energy-momentum tensor for both models. We apply our findings to the case of close binaries on circular orbits, which have been used to indirectly infer the existence of gravitational radiation prior to the direct observation of gravitational waves. As an example, we analyze the binary system PSR J1012+5307, chosen for its small eccentricity. When fixing the graviton mass in CG such that observed galaxy rotation curves could be explained without dark matter, the gravitational radiation of a binary system is much smaller than in GR. The same holds for MCG for small masses of the graviton. Thus gravitational radiation cannot explain the orbital decay of binary systems and replace dark matter simultaneously. We also analyse MCG for large graviton masses and conclude that MCG can describe the orbital periods of compact binaries in agreement with data, as it reduces to GR in that limit.

gr-qc↗