arXiv subjects

Thomas Konstandin

Publications and source records attributed to Thomas Konstandin.

At least 37 records · Page 2Linked to original sources

arXiv2021

Effect of density fluctuations on gravitational wave production in first-order phase transitions

We study the effect of density perturbations on the process of first-order phase transitions and gravitational wave production in the early Universe. We are mainly interested in how the distribution of nucleated bubbles is affected by fluctuations in the local temperature. We find that large-scale density fluctuations ($H_* < k_* < β$) result in a larger effective bubble size at the time of collision, enhancing the produced amplitude of gravitational waves. The amplitude of the density fluctuations necessary for this enhancement is ${\cal P}_ζ(k_*) \gtrsim (β/ H_*)^{-2}$, and therefore the gravitational wave signal from first-order phase transitions with relatively large $β/ H_*$ can be significantly enhanced by this mechanism even for fluctuations with moderate amplitudes.

astro-ph.CO↗

arXiv2021

Finding sound shells in LISA mock data using likelihood sampling

We study to what extend LISA can observe features of gravitational wave spectra originating from cosmological first-order phase transitions. We focus on spectra which are of the form of double-broken power laws. These spectra are predicted by hydrodynamic simulations and also analytical models such as the sound shell model. We argue that the ratio of the two break frequencies is an interesting observable since it can be related to the wall velocity while overall amplitude and frequency range are often degenerate for the numerous characteristics of the phase transition. Our analysis uses mock data obtained from the power spectra predicted by the simplified simulations and the sound shell model and analyzes the detection prospects using $χ^2$-minimization and likelihood sampling. We point out that the prospects of observing two break frequencies from the electroweak phase transition is hindered by a shift of the spectrum to smaller frequencies for strong phase transitions. Finally, we also highlight some differences between signals from the sound shell model compared to simulations.

astro-ph.CO↗

arXiv2021

Neutrino mass bounds from confronting an effective model with BOSS Lyman-alpha data

We present an effective model for the one-dimensional Lyman-$α$ flux power spectrum far above the baryonic Jeans scale. The main new ingredient is constituted by a set of two parameters that encode the impact of small, highly non-linear scales on the one-dimensional power spectrum on large scales, where it is measured by BOSS. We show that, by marginalizing over the model parameters that capture the impact of the intergalactic medium, the flux power spectrum from both simulations and observations can be described with high precision. The model displays a degeneracy between the neutrino masses and the (unknown, in our formalism) normalization of the flux power spectrum. This degeneracy can be lifted by calibrating one of the model parameters with simulation data, and using input from Planck CMB data. We demonstrate that this approach can be used to extract bounds on the sum of neutrino masses with comparably low numerical effort, while allowing for a conservative treatment of uncertainties from the dynamics of the intergalactic medium. An explorative analysis yields an upper bound of $0.16\,$eV at $95\%$ C.L. when applied to BOSS data at $3\leq z\leq 4.2$. We also forecast that if the systematic and statistical errors will be reduced by a factor two the upper bound will become $0.1\,$eV at $95\%$ C.L., and $0.056\,$eV when assuming a $1\%$ error.

astro-ph.CO↗

arXiv2021

Is Cosmological Tuning Fine or Coarse?

The fine-tuning of the universe for life, the idea that the constants of nature (or ratios between them) must belong to very small intervals in order for life to exist, has been debated by scientists for several decades. Several criticisms have emerged concerning probabilistic measurement of life-permitting intervals. Herein, a Bayesian statistical approach is used to assign an upper bound for the probability of tuning, which is invariant with respect to change of physical units, and under certain assumptions it is small whenever the life-permitting interval is small on a relative scale. The computation of the upper bound of the tuning probability is achieved by first assuming that the prior is chosen by the principle of maximum entropy (MaxEnt). The unknown parameters of this MaxEnt distribution are then handled in such a way that the weak anthropic principle is not violated. The MaxEnt assumption is "maximally noncommittal with regard to missing information." This approach is sufficiently general to be applied to constants of current cosmological models, or to other constants possibly under different models. Application of the MaxEnt model reveals, for example, that the ratio of the universal gravitational constant to the square of the Hubble constant is finely tuned in some cases, whereas the amplitude of primordial fluctuations is not.

physics.gen-ph↗

arXiv2021

A hybrid simulation of gravitational wave production in first-order phase transitions

The LISA telescope will provide the first opportunity to probe the scenario of a first-order phase transition happening close to the electroweak scale. By now, it is evident that the main contribution to the GW spectrum comes from the sound waves propagating through the plasma. Current estimates of the GW spectrum are based on numerical simulations of a scalar field interacting with the plasma or on analytical approximations -- the so-called sound shell model. In this work we present a novel setup to calculate the GW spectra from sound waves. We use a hybrid method that uses a 1d simulation (with spherical symmetry) to evolve the velocity and enthalpy profiles of a single bubble after collision and embed it in a 3d realization of multiple bubble collisions, assuming linear superposition of the velocity and enthalpy. The main advantage of our method compared to 3d hydrodynamic simulations is that it does not require to resolve the scale of bubble wall thickness. This makes our simulations more economical and the only two relevant physical length scales that enter are the bubble size and the shell thickness (that are in turn enclosed by the box size and the grid spacing). The reduced costs allow for extensive parameter studies and we provide a parametrization of the final GW spectrum as a function of the wall velocity and the fluid kinetic energy.

astro-ph.CO↗

arXiv2021

Model-independent energy budget for LISA

We provide an easy method to obtain the kinetic energy fraction in gravitational waves, generated during a cosmological first-order phase transition, as a function of only the wall velocity and quantities that can be determined from the particle physics model at the nucleation temperature. This generalizes recent work that achieved this goal for detonations. Here we present the corresponding results for deflagrations and hybrids. Unlike for detonations, the sound speed in the symmetric phase also enters the analysis. We perform a detailed comparison between our model-independent approach and other approaches in the literature. We provide a Python code snippet to determine the kinetic energy fraction $K$ as a function of the wall velocity, the two speeds of sound and the strength parameter of the phase transition. We also assess how realistic sizable deviations in speed of sound are close to the phase transition temperature in a specific model.

astro-ph.CO↗

arXiv2021

Detecting gravitational waves from cosmological phase transitions with LISA: an update

We investigate the potential for observing gravitational waves from cosmological phase transitions with LISA in light of recent theoretical and experimental developments. Our analysis is based on current state-of-the-art simulations of sound waves in the cosmic fluid after the phase transition completes. We discuss the various sources of gravitational radiation, the underlying parameters describing the phase transition and a variety of viable particle physics models in this context, clarifying common misconceptions that appear in the literature and identifying open questions requiring future study. We also present a web-based tool, PTPlot, that allows users to obtain up-to-date detection prospects for a given set of phase transition parameters at LISA.

astro-ph.CO↗

arXiv2020

Model-independent energy budget of cosmological first-order phase transitions

We study the energy budget of a first-order cosmological phase transition, which is an important factor in the prediction of the resulting gravitational wave spectrum. Formerly, this analysis was based mostly on simplified models as for example the bag equation of state. Here, we present a model-independent approach that is exact up to the temperature dependence of the speed of sound in the broken phase. We find that the only relevant quantities that enter in the hydrodynamic analysis are the speed of sound in the broken phase and a linear combination of the energy and pressure differences between the two phases which we call pseudotrace (normalized to the enthalpy in the broken phase). The pseudotrace quantifies the strength of the phase transition and yields the conventional trace of the energy-momentum tensor for a relativistic plasma (with speed of sound squared of one third). We study this approach in several realistic models of the phase transition and also provide a code snippet that can be used to determine the efficiency coefficient for a given phase transition strength and speed of sound. It turns out that our approach is accurate to the percent level for moderately strong phase transitions, while former approaches give at best the right order of magnitude.

astro-ph.CO↗

arXiv2020

Relativistic bubble collisions -- a closer look

We study scalar bubble collisions in first-order phase transitions focusing on the relativistic limit. We propose 'trapping equation' which describes the wall behavior after collision, and test it with numerical simulations in several setups. We also examine the energy dynamics after collision and discuss its implications to gravitational wave production.

hep-ph↗

arXiv2020

Baryogenesis from a dark first-order phase transition

We present a very minimal model for baryogenesis by a dark first-order phase transition. It employs a new dark $SU(2)_{D}$ gauge group with two doublet Higgs bosons, two lepton doublets, and two singlets. The singlets act as a neutrino portal that transfer the generated asymmetry to the Standard Model. The model predicts $ΔN_\text{eff} = 0.09-0.13$ detectable by future experiments as well as possible signals from exotic decays of the Higgs and $Z$ bosons and stochastic gravitational waves.

hep-ph↗

arXiv2020

The Schrödinger-Poisson method for Large-Scale Structure

We study the Schrödinger-Poisson (SP) method in the context of cosmological large-scale structure formation in an expanding background. In the limit $\hbar \to 0$, the SP technique can be viewed as an effective method to sample the phase space distribution of cold dark matter that remains valid on non-linear scales. We present results for the 2D and 3D matter correlation function and power spectrum at length scales corresponding to the baryon acoustic oscillation (BAO) peak. We discuss systematic effects of the SP method applied to cold dark matter and explore how they depend on the simulation parameters. In particular, we identify a combination of simulation parameters that controls the scale-independent loss of power observed at low redshifts, and discuss the scale relevant to this effect.

astro-ph.CO↗

arXiv2020

The Effective Field Theory of Large Scale Structure at Three Loops

We study the power spectrum of dark matter density fluctuations in the framework of the Effective Field Theory of Large Scale Structures (EFTofLSS) up to three loop orders. In principle, several counter-terms may be needed to handle the short-distance sensitivity in perturbation theory. However, we show that a small number of extra coefficients are sufficient to match numerical simulations with percent accuracy when a generic renormalization prescription is implemented (allowing for running of the individual counter-terms). We show that the level of accuracy increases with respect to the two loop results, up to $k \simeq 0.4\, h$Mpc$^{-1}$ at redshift $z=0$, although the overall improvement is somewhat marginal. At the same time, we argue there is evidence that the behavior of the loop expansion in the EFTofLSS is typical of an asymptotic series, already on the brink of its maximum predictive power (at $z=0$). Hence, the inclusion of higher orders will likely deteriorate the matching to data, even at moderate values of $k$. Part of the reason for this behavior is due to large contributions to the (renormalized) power spectrum at three loop order from mildly non-linear scales, even after the UV counter-terms are included. In conclusion, the EFTofLSS to three loop orders provides the best approximation to the (deterministic part of the) power spectrum in the weakly non-linear regime at $z=0$, and higher loops are not expected to improve our level of accuracy.

astro-ph.CO↗

arXiv2019

Vacuum stability of Froggatt-Nielsen models

We discuss vacuum stability in Froggatt-Nielsen (FN) models. One concern in FN models is that for large flavon VEVs the running of the quartic Higgs coupling is enhanced what might lead to a more severe instability compared to the Standard Model (SM). We study this issue using the renormalization-group improved scalar potential. Another issue is that the mixing between the Higgs and the flavon can potentially destabilize the potential. However, taking current bounds on the flavon phenomenology into account, we find that both effects do not lead to an instability that is more severe than in the SM.

hep-ph↗

arXiv2018

A First-Order Electroweak Phase Transition from Varying Yukawas

We show that the dynamics responsible for the variation of the Yukawa couplings of the Standard Model fermions generically leads to a very strong first-order electroweak phase transition, assuming that the Yukawa couplings are large and of order 1 before the electroweak phase transition and reach their present value afterwards. There are good motivations to consider that the flavour structure could emerge during electroweak symmetry breaking, for example if the Froggatt-Nielsen field dynamics were linked to the Higgs field. In this paper, we do not need to assume any particular theory of flavour and show in a model-independent way how the nature of the electroweak phase transition is completely changed when the Standard Model Yukawas vary at the same time as the Higgs is acquiring its vacuum expectation value. The thermal contribution of the fermions creates a barrier between the symmetric and broken phase minima of the effective potential, leading to a first-order phase transition. This offers new routes for generating the baryon asymmetry at the electroweak scale, strongly tied to flavour models.

hep-ph↗

arXiv2018

A Fresh Look at the Calculation of Tunneling Actions in Multi-Field Potentials

The quantum decay of a metastable vacuum is exponentially suppressed by a tunneling action that can be calculated in the semi-classical approximation as the Euclidean action of a bounce that interpolates between the false and true phases. For multi-field potentials, finding the bounce is non-trivial due to its peculiar boundary conditions and the fact that the action at the bounce is not a minimum but merely a saddle point. Recently, an alternative tunneling action has been proposed that does not rely on Euclidean bounces and reproduces the standard result at its minimum. Here we generalize this new approach for several scalar fields and demonstrate how its use can significantly improve the numerical calculation of tunneling actions for multi-field potentials.

hep-th↗

arXiv2018

Bubble wall velocities in the Standard Model and beyond

We present results for the bubble wall velocity and bubble wall thickness during a cosmological first-order phase transition in a condensed form. Our results are for minimal extensions of the Standard Model but in principle are applicable to a much broader class of settings. Our first assumption about the model is that only the electroweak Higgs is obtaining a vacuum expectation value during the phase transition. The second is that most of the friction is produced by electroweak gauge bosons and top quarks. Under these assumptions the bubble wall velocity and thickness can be deduced as a function of two equilibrium properties of the plasma: the strength of the phase transition and the pressure difference along the bubble wall.

hep-ph↗

arXiv2018

Gravitational radiation from a bulk flow model

We perform simulations in a simple model that aims to mimic the hydrodynamic evolution of a relativistic fluid during a cosmological first-order phase transitions. The observable we are concerned with is hereby the spectrum of gravitational radiation produced by colliding fluid shells. We present simple parameterizations of our results as functions of the wall velocity, the duration of the phase transition and the latent heat. We also improve on previous results in the envelope approximation and compare with hydrodynamic simulations.

astro-ph.CO↗

arXiv2018

Lyman-$α$ forest constraints on interacting dark sectors

The Lyman-$α$ forest is a valuable probe of dark matter models featuring a scale-dependent suppression of the power spectrum as compared to $Λ$CDM. In this work, we present a new estimator of the Lyman-$α$ flux power spectrum that does not rely on hydrodynamical simulations. Our framework is characterized by nuisance parameters that encapsulate the complex physics of the intergalactic medium and sensitivity to highly non-linear small-scale modes. After validating the approach based on high-resolution hydrodynamical simulations for $Λ$CDM, we derive conservative constraints on interacting dark matter models from BOSS Lyman-$α$ data on large scales, k<0.02(km/s)^(-1), with the relevant nuisance parameters left free in the model fit. The estimator yields lower bounds on the mass of cannibal dark matter, where freeze-out occurs through 3-to-2 annihilation, in the MeV range. Furthermore, we find that models of dark matter interacting with dark radiation, which have been argued to address the $H_0$ and $σ_8$ tensions, are compatible with BOSS Lyman-$α$ data.

astro-ph.CO↗