arXiv subjects

Steffen Hahn

Publications and source records attributed to Steffen Hahn.

7 recordsLinked to original sources

arXiv2025

Reconstruction of the depth of the shower maximum of air showers with the SD-750 surface detector of the Pierre Auger Observatory using neural networks

The origin of ultra-high-energy cosmic rays (UHECRs) is one of the intriguing mysteries in astroparticle physics. In order to identify their sources, we need precise knowledge of the mass composition of UHECRs. The direct detection of UHECRs is not feasible at energies above 0.1 PeV, necessitating the use of mass-sensitive observables of extended air showers induced by UHECRs interacting with the atmosphere. One way to achieve high statistics for these mass-sensitive observables is to use ground-based detector arrays, such as the Surface Detector (SD) of the Pierre Auger Observatory. The SD consists of three sub-arrays of independent detector stations arranged in triangular grids with different spacings. Recently, it has been shown that neural networks (NNs) can extract mass-sensitive observables from data taken by the SD-1500, the largest sub-detector of the SD. In this contribution, we demonstrate the feasibility of using NNs to reconstruct a high-level shower observable, the depth of the shower maximum, from data simulated for and observed by the SD-750, the second-largest detector array nested within the SD-1500. A simulation study shows that the SD-750 NN exhibits behavior similar to that of an SD-1500 NN and outperforms the latter in the energy range [1, 10) EeV. Moreover, we show that, after performing a correction and calibration procedure, the predictions of the SD-750 NN are consistent with the measurement of the depth of the shower maximum obtained by the Fluorescence Detector of the Pierre Auger Observatory.

astro-ph.IM↗

arXiv2025

Ideas and Requirements for the Global Cosmic-Ray Observatory (GCOS)

After a successful kick-off meeting in 2021. two workshops in 2022 and 2023 on the future Global Cosmic-Ray Observatory (GCOS) focused mainly on a straw man design of the detector and science possibilities for astro- and particle physics. About 100 participants gathered for in-person and hybrid panel discussions. In this report, we summarize these discussions, present a preliminary straw-man design for GCOS and collect short write-ups of the flash talks given during the focus sessions.

astro-ph.IM↗

arXiv2023

Computing Challenges for the Einstein Telescope project

The discovery of gravitational waves, first observed in September 2015 following the merger of a binary black hole system, has already revolutionised our understanding of the Universe. This was further enhanced in August 2017, when the coalescence of a binary neutron star system was observed both with gravitational waves and a variety of electromagnetic counterparts; this joint observation marked the beginning of gravitational multimessenger astronomy. The Einstein Telescope, a proposed next-generation ground-based gravitational-wave observatory, will dramatically increase the sensitivity to sources: the number of observations of gravitational waves is expected to increase from roughly 100 per year to roughly 100'000 per year, and signals may be visible for hours at a time, given the low frequency cutoff of the planned instrument. This increase in the number of observed events, and the duration with which they are observed, is hugely beneficial to the scientific goals of the community but poses a number of significant computing challenges. Moreover, the currently used computing algorithms do not scale to this new environment, both in terms of the amount of resources required and the speed with which each signal must be characterised. This contribution will discuss the Einstein Telescope's computing challenges, and the activities that are underway to prepare for them. Available computing resources and technologies will greatly evolve in the years ahead, and those working to develop the Einstein Telescope data analysis algorithms will need to take this into account. It will also be important to factor into the initial development of the experiment's computing model the availability of huge parallel HPC systems and ubiquitous Cloud computing; the design of the model will also, for the first time, include the environmental impact as one of the optimisation metrics.

gr-qc↗

arXiv2018

SU(2)$_{\rm CMB}$ and the cosmological model: angular power spectra

Driven by the CMB temperature-redshift ($T$-$z$) relation as demanded by deconfining SU(2) Yang-Mills thermodynamics, an according cosmological model is proposed and analysed. This model -- SU(2)$_\CMB$ -- exhibits a dark sector, representing $Λ$CDM with a certain late-time dark-matter density which transitions to a reduced (present-day) density parameter at high $z$. We statistically analyse constraints on cosmological parameters directly imposed by the values of the standard co-moving ruler $r_d$ and the angular size of the sound horizon $θ_*$. Compared to the $Λ$CDM best fit to 2015 Planck data, we require an increased (present-day) dark matter density when $r_d\cdot H_0=$\,const and a value $H_0\sim 73.5$ km\,s$^{-1}$Mpc$^{-1}$ -- typical for local extractions -- are used. The ratio between the density parameters of primordial and late-time dark matter ranges between 0.5 and 0.7. We confirm this trend by fitting the predictions of SU(2)$_\CMB$, obtained from a modified CLASS code, to the angular power spectra TT, TE, and EE. We consider adiabatic, scalar primordial curvature perturbations and distinguish two treatments of thermal quasi-particles in the perturbation equations. Best fits predict a red-tilted primordial power spectrum. Moreover, a low baryon density is obtained compared with the coincidence value of BBN, the $Λ$CDM best fit of the 2015 Planck data, and the observed deuterium abundance. Our derived values of $H_0$ support the results of local cosmological observations. Also, there is a tendency for late reionisation.

physics.gen-ph↗

arXiv2018

Exact determination of asymptotic CMB temperature-redshift relation

Based on energy conservation in a Friedmann-Lemaitre-Robertson-Walker (FLRW) Universe, on the Legendre transformation between energy density and pressure, and on nonperturbative asymptotic freedom at high temperatures we derive the coefficient $ν_{\rm CMB}$ in the high-temperature ($T$) -- redshift ($z$) relation, $T/T_0=ν_{\rm CMB}(z+1)$, of the Cosmic Microwave Background (CMB). Theoretically, our calculation relies on a deconfining SU(2) rather than a U(1) photon gas. We prove that $ν_{\rm CMB}=\left(1/4\right)^{1/3}=0.629960(5)$, representing a topological invariant. Interestingly, the relative deviation of $ν_{\rm CMB}$ from the critical exponent associated with the correlation length $l$ of the 3D Ising model, $ν_{\rm Ising}=0.629971(4)$, is less than $2\times 10^{-5}$. We are not yet in a position to establish a rigorous theoretical link between $ν_{\rm CMB}$ and $ν_{\rm Ising}$ as suggested by the topological nature of $ν_{\rm CMB}$ and the fact that both theories share a universality class. We do, however, line out a somewhat speculative map from the physical Ising temperature $θ$ to a fictitious SU(2) Yang-Mills temperature $T$, the latter continuing the asymptotic behavior of the scale factor $a$ on $T/T_0$ for $T/T_0\gg 1$ down to $T=0$, and an exponential map from $a$ to $l$ to reproduce critical Ising behavior.

physics.gen-ph↗

arXiv2017

Cosmic Microwave Background as a thermal gas of SU(2) photons: Implications for the high-z cosmological model and the value of $H_0$

Presently, we are facing a 3$σ$ tension in the most basic cosmological parameter -- the Hubble constant $H_0$. This tension arises when fitting the Lambda-cold-dark-matter model ($Λ$CDM) to the high-precision temperature-temperature (TT) power spectrum of the Cosmic Microwave Background (CMB) and to local cosmological observations. We propose a resolution of this problem by postulating that the thermal photon gas of the CMB obeys an SU(2) rather than U(1) gauge principle, suggesting a high-$z$ cosmological model which is void of dark matter. Observationally, we rely on precise low-frequency intensity measurements in the CMB spectrum and on a recent model independent (low-$z$) extraction of the relation between the comoving sound horizon $r_s$ at the end of the baryon drag epoch and $H_0$ ($r_s H_0 = \text{const}$). We point out that the commonly employed condition for baryon-velocity freeze-out is imprecise, judged by a careful inspection of the formal solution to the associated Euler equation. As a consequence, the above mentioned 3$σ$ tension actually transforms into a 5$σ$ discrepancy. To make contact with successful low-$z$ $Λ$CDM cosmology we propose an interpolation based on percolated/depercolated vortices of a Planck-scale axion condensate. For a first consistency test of such an all-$z$ model we compute the angular scale of the sound horizon at photon decoupling.

physics.gen-ph↗

arXiv2017

SU(2)$_{\tiny\mbox{CMB}}$ at high redshifts and the value of $H_0$

We investigate a high-$z$ cosmological model to compute the co-moving sound horizon $r_s$ at baryon-velocity freeze-out towards the end of hydrogen recombination. This model assumes a replacement of the conventional CMB photon gas by deconfining SU(2) Yang-Mills thermodynamics, three flavours of massless neutrinos ($N_ν=3$), and a {\sl purely baryonic} matter sector (no cold dark-matter (CDM)). The according SU(2) temperature-redshift relation of the CMB is contrasted with recent measurements appealing to the thermal Sunyaev-Zel'dovich effect and CMB-photon absorption by molecular rotations bands or atomic hyperfine levels. Relying on a realistic simulation of the ionization history throughout recombination, we obtain $z_*=1693.55\pm 6.98$ and $z_{\rm drag}=1812.66\pm 7.01$. Due to considerable widths of the visibility functions in the solutions to the associated Boltzmann hierarchy and Euler equation we conclude that $z_*$ and $z_{\rm drag}$ over-estimate the redshifts for the respective photon and baryon-velocity freeze-out. Realistic decoupling values turn out to be $z_{{\rm lf},*}=1554.89\pm 5.18$ and $z_{\rm lf,drag}=1659.30\pm 5.48$. With $r_s(z_{\rm lf,drag})=(137.19\pm 0.45)\,$Mpc and the essentially model independent extraction of $r_s\cdot H_0=\mbox{const}$ from low-$z$ data in arXiv:1607.05617 we obtain a good match with the value $H_0=(73.24\pm 1.74)\,$km\,s$^{-1}$\,Mpc$^{-1}$ extracted in arXiv:1604.01424 by appealing to Cepheid calibrated SNe~Ia, new parallax measurements, stronger constraints on the Hubble flow, and a refined computation of distance to NGC4258 from maser data. We briefly comment on a possible interpolation of our high-$z$ model, invoking percolated and unpercolated U(1) topological solitons of a Planck-scale axion field, to the phenomenologically successful low-$z$ $Λ$CDM cosmology.

physics.gen-ph↗