arXiv subjects

Christoph Schmid

Publications and source records attributed to Christoph Schmid.

27 records · Page 2Linked to original sources

arXiv1999

Clumping of CDM from the cosmological QCD transition

The cosmological QCD transition affects primordial density perturbations. If the QCD transition is first order, the sound speed vanishes during the transition and density perturbations fall freely. For scales below the Hubble radius at the transition the primordial Harrison-Zel'dovich spectrum of density fluctuations develops large peaks and dips. These peaks grow with wave number for both the radiation fluid and for cold dark matter (CDM). The peaks in the radiation fluid are wiped out during neutrino decoupling. For cold dark matter that is kinetically decoupled at the QCD transition (e.g. axions) these peaks lead to the formation of CDM clumps of masses $10^{-20} M_\odot< M < 10^{-10} M_\odot$.

astro-ph↗

arXiv1998

Amplification of Cosmological Inhomogeneities by the QCD Transition

The cosmological QCD transition affects primordial density perturbations. If the QCD transition is first order, the sound speed vanishes during the transition and density perturbations fall freely. For scales below the Hubble radius at the transition the primordial Harrison-Zel'dovich spectrum of density fluctuations develops large peaks and dips. These peaks grow with wave number for both the hadron-photon-lepton fluid and for cold dark matter. At the horizon scale the enhancement is small. This by itself does not lead to the formation of black holes at the QCD transition. The peaks in the hadron-photon-lepton fluid are wiped out during neutrino decoupling. For cold dark matter that is kinetically decoupled at the QCD transition (e.g., axions or primordial black holes) these peaks lead to the formation of CDM clumps of masses $10^{-20} M_\odot< M_{\rm clump} < 10^{-10} M_\odot$.

astro-ph↗

arXiv1998

Primordial Black Holes from the QCD Transition?

Can a violent process like sudden reheating after supercooling at the onset of a first-order QCD transition improve the possibility of primordial black hole formation? Underdensities reheat earlier than overdensities, there is a short period of huge pressure differences, hence fluid acceleration. Density perturbations on scales far below the Hubble radius $λ\ll R_{\rm H} $ get an amplification which grows quadratically in wavenumber, the amplifications at the horizon scale are small. Primordial black hole formation cannot be sufficiently amplified by the QCD transition unless the initial spectrum is fine tuned.

astro-ph↗

arXiv1998

Primordial black hole formation caused by the QCD transition ?

We consider the evolution of cosmological perturbations at the QCD transition, in particular the sudden reheating from a supercooled universe to the transition temperature. Sudden reheating happens at a specific temperature, hence density, and singles out one specific hypersurface. Underdensities reach the reheating earlier than overdensities, there is a short period of huge pressure differences which leads to a jump in the fluid velocity. Density perturbations of scales far below the Hubble radius $λ\ll R_H $ get an amplification which grows quadratically in wavenumber. Primordial black hole formation will not be sufficiently amplified by the QCD transition unless the initial spectrum is fine tuned.

astro-ph↗

arXiv1996

Peaks above the Harrison-Zel'dovich spectrum due to the Quark-Gluon to Hadron Transition

The quark-gluon to hadron transition affects the evolution of cosmological perturbations. If the phase transition is first order, the sound speed vanishes during the transition, and density perturbations fall freely. This distorts the primordial Harrison-Zel'dovich spectrum of density fluctuations below the Hubble scale at the transition. Peaks are produced, which grow at most linearly in wavenumber, both for the hadron-photon-lepton fluid and for cold dark matter. For cold dark matter which is kinetically decoupled well before the QCD transition clumps of masses below $10^{-10} M_\odot$ are produced.

astro-ph↗

arXiv1996

Deviations from the Harrison-Zel'dovich spectrum due to the Quark-Gluon to Hadron Transition

We investigate the effect of the quark-gluon to hadron transition on the evolution of cosmological perturbations. If the phase transition is first order, the sound speed vanishes during the transition, and density perturbations fall freely. The primordial Harrison-Zel'dovich density fluctuations for scales below the Hubble radius at the transition develop peaks, which grow linearly with the wavenumber, both for the hadron-photon-lepton fluid and for cold dark matter. The large peaks in the spectrum produce cold dark matter clumps of $10^{-8}$ to $10^{-11} M_\odot$.

astro-ph↗

arXiv1994

Energy Density Fluctuations in Inflationary Cosmology

We analyze the energy density fluctuations contributed by scalar fields $Φ$ with vanishing expectation values, $\langleΦ\rangle=0$, which are present in addition to the inflaton field. For simplicity we take $Φ$ to be non--interacting and minimally coupled to gravity. We use normal ordering to define the renormalized energy density operator $ρ$, and we show that any normal ordering gives the same result for correlation functions of $ρ$. We first consider massless fields and derive the energy fluctuations in a single mode $\vk$, the two--point correlation function of the energy density, the power spectrum, and the variance of the smeared energy density, $\ddR$. Mass effects are investigated for energy fluctuations in single modes. All quantities considered are scale invariant at the second horizon crossing (Harrison--Zel'dovich type) for massless and for unstable massive fields. The magnitude of the relative fluctuations $\deρ/\rt$ is of order $(\Hi/\Mp)^2$ in the massless case, where $\Hi$ is the Hubble constant during inflation. For an unstable field of mass $m_Φ\ll\Hi$ with a decay rate $Γ_Φ$ the magnitude is enhanced by a factor $\sqrt{m_Φ/Γ_Φ}$. Finally, the prediction for the cosmic variance of the average energy density in a sample is given in the massless case.

gr-qc↗

arXiv1994

Non--Gaussian Primordial Fluctuations

We analyze the non--Gaussian primordial fluctuations which are inescapably contributed by scalar fields $Φ$ with vanishing expectation values, $\langleΦ\rangle=0$, present during inflation in addition to the inflaton field. For simplicity we take $Φ$ to be non--interacting and minimally coupled to gravity. $Φ$ is a Gaussian variable, but the energy density fluctuations contributed by such a field are $χ^2$--distributed. We compute the three--point function $\xxxT$ for the configuration of an equilateral triangle (with side length $\ell$) and the skewness $\dddRR$, {\it i.e.} the third moment of the one--point probability distribution of the spatially smeared energy density contrast $\de_R$, where $R$ is the smearing scale. The relative magnitudes of the non--Gaussian effects, $[ξ^{(N)}]^{1/N}/[ξ^{(2)}]^{1/2}$, do not grow in time. They are given by numerical constants of order unity, independent of the scale $\ell$. The "bi--skewness" $\dddRS$ is positive. For smearing lengths $R\ll S$ this shows that in our model (in contrast to Gaussian models) voids are more quiet than high--density regions.

gr-qc↗

arXiv1994

Energy Fluctuations Generated by Inflation

The energy density correlation function $C(x,y)=<ρ(x)ρ(y)>-<ρ>^2$ and its Fourier transform generated by the gravitational tidal forces of the inflationary de Sitter expansion are derived for a massless or light ($m< =0$. Every observationally relevant mode (which has today $λ_{phys} H$ we have the universal law $k^3\dk\sim k^8$.

gr-qc↗