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Ruth Durrer

Publications and source records attributed to Ruth Durrer.

At least 145 records · Page 8Linked to original sources
arXiv2011

Effects of biasing on the galaxy power spectrum at large scales

n this paper we study the effect of biasing on the power spectrum at large scales. We show that even though non-linear biasing does introduce a white noise contribution on large scales, the $P(k)\propto k^n$ behavior of the matter power spectrum on large scales may still be visible and above the white noise for about one decade. We show, that the Kaiser biasing scheme which leads to linear bias of the correlation function on {\em large} scales, also generates a linear bias of the {\rm power spectrum} on rather small scales. This is a consequence of the divergence on small scales of the pure Harrison-Zeldovich spectrum. However, biasing becomes k-dependent when we damp the underlying power spectrum on small scales. We also discuss the effect of biasing on the baryon acoustic oscillations.

astro-ph.CO↗
arXiv2011

Can slow roll inflation induce relevant helical magnetic fields?

We study the generation of helical magnetic fields during single field inflation induced by an axial coupling of the electromagnetic field to the inflaton. During slow roll inflation, we find that such a coupling always leads to a blue spectrum with $B^2(k) \propto k$, as long as the theory is treated perturbatively. The magnetic energy density at the end of inflation is found to be typically too small to backreact on the background dynamics of the inflaton. We also show that a short deviation from slow roll does not result in strong modifications to the shape of the spectrum. We calculate the evolution of the correlation length and the field amplitude during the inverse cascade and viscous damping of the helical magnetic field in the radiation era after inflation. We conclude that except for low scale inflation with very strong coupling, the magnetic fields generated by such an axial coupling in single field slow roll inflation with perturbative coupling to the inflaton are too weak to provide the seeds for the observed fields in galaxies and clusters.

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arXiv2011

A large scale coherent magnetic field: interactions with free streaming particles and limits from the CMB

We study a homogeneous and nearly-isotropic Universe permeated by a homogeneous magnetic field. Together with an isotropic fluid, the homogeneous magnetic field, which is the primary source of anisotropy, leads to a plane-symmetric Bianchi I model of the Universe. However, when free-streaming relativistic particles are present, they generate an anisotropic pressure which counteracts the one from the magnetic field such that the Universe becomes isotropized. We show that due to this effect, the CMB temperature anisotropy from a homogeneous magnetic field is significantly suppressed if the the neutrino masses are smaller than 0.3 eV.

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arXiv2010

The local B-polarization of the CMB: a very sensitive probe of cosmic defects

We present a new and especially powerful signature of cosmic strings and other topological or non-topological defects in the polarization of the cosmic microwave background (CMB). We show that even if defects contribute 1% or less in the CMB temperature anisotropy spectrum, their signature in the local $\tilde{B}$-polarization correlation function at angular scales of tens of arc minutes is much larger than that due to gravitational waves from inflation, even if the latter contribute with a ratio as big as $r\simeq 0.1$ to the temperature anisotropies. We show that when going from non-local to local $\tilde{B}$-polarization, the ratio of the defect signal-to-noise with respect to the inflationary value increases by about an order of magnitude. Proposed B-polarization experiments, with a good sensitivity on arcminute scales, may either detect a contribution from topological defects produced after inflation or place stringent limits on them. Even Planck should be able to improve present constraints on defect models by at least an order of magnitude, to the level of $\ep <10^{-7}$. A future full-sky experiment like CMBpol, with polarization sensitivities of the order of $1μ$K-arcmin, will be able to constrain the defect parameter $\ep=Gv^2$ to a few $\times10^{-9}$, depending on the defect model.

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arXiv2010

Model-independent cosmological constraints from the CMB

We analyse CMB data in a manner which is as model-independent as possible. We encode the effects of late-time cosmology into a single parameter which determines the distance to the last scattering surface. We exclude low multipoles $\ell<40$ from the analysis. We consider the WMAP5 and ACBAR data. We obtain the cosmological parameters $100ω_b =2.13\pm 0.05$, $ω_c=0.124\pm 0.007$, $n_s=0.93\pm 0.02$ and $θ_A=0.593\pm 0.001$ degrees (68% C.L.). The last number is the angular scale subtended by the sound horizon at decoupling. There is a systematic shift in the parameters as more low $\ell$ data is omitted, towards smaller values of $ω_b$ and $n_s$ and larger values of $ω_c$. The scale $θ_A$ remains stable and very well determined.

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arXiv2010

Gravitational waves from cosmological phase transitions

I discuss the generation of a stochastic background of gravitational waves during a first order phase transition. I present simple general arguments which explain the main features of the gravitational wave spectrum like the $k^3$ power law growth on large scales and a estimate for the peak amplitude. In the second part I concentrate on the electroweak phase transition and argue that the nucleosynthesis bound on its gravitational wave background seriously limits seed magnetic fields which may have been generated during this transition.

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arXiv2009

The stochastic gravitational wave background from turbulence and magnetic fields generated by a first-order phase transition

We analytically derive the spectrum of gravitational waves due to magneto-hydrodynamical turbulence generated by bubble collisions in a first-order phase transition. In contrast to previous studies, we take into account the fact that turbulence and magnetic fields act as sources of gravitational waves for many Hubble times after the phase transition is completed. This modifies the gravitational wave spectrum at large scales. We also model the initial stirring phase preceding the Kolmogorov cascade, while earlier works assume that the Kolmogorov spectrum sets in instantaneously. The continuity in time of the source is relevant for a correct determination of the peak position of the gravitational wave spectrum. We discuss how the results depend on assumptions about the unequal-time correlation of the source and motivate a realistic choice for it. Our treatment gives a similar peak frequency as previous analyses but the amplitude of the signal is reduced due to the use of a more realistic power spectrum for the magneto-hydrodynamical turbulence. For a strongly first-order electroweak phase transition, the signal is observable with the space interferometer LISA.

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arXiv2009

Gravitational waves from self-ordering scalar fields

Gravitational waves were copiously produced in the early Universe whenever the processes taking place were sufficiently violent. The spectra of several of these gravitational wave backgrounds on subhorizon scales have been extensively studied in the literature. In this paper we analyze the shape and amplitude of the gravitational wave spectrum on scales which are superhorizon at the time of production. Such gravitational waves are expected from the self ordering of randomly oriented scalar fields which can be present during a thermal phase transition or during preheating after hybrid inflation. We find that, if the gravitational wave source acts only during a small fraction of the Hubble time, the gravitational wave spectrum at frequencies lower than the expansion rate at the time of production behaves as $Ω_{\rm GW}(f) \propto f^3$ with an amplitude much too small to be observable by gravitational wave observatories like LIGO, LISA or BBO. On the other hand, if the source is active for a much longer time, until a given mode which is initially superhorizon ($kη_* \ll 1$), enters the horizon, for $kη\gtrsim 1$, we find that the gravitational wave energy density is frequency independent, i.e. scale invariant. Moreover, its amplitude for a GUT scale scenario turns out to be within the range and sensitivity of BBO and marginally detectable by LIGO and LISA. This new gravitational wave background can compete with the one generated during inflation, and distinguishing both may require extra information.

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arXiv2009

On Adiabatic Renormalization of Inflationary Perturbations

We discuss the impact of adiabatic renormalization on the power spectrum of scalar and tensor perturbations from inflation. We show that adiabatic regularization is ambiguous as it leads to very different results, for different adiabatic subtraction schemes, both in the range $v\equiv k/(aH) \gsim 0.1$ and in the infrared regime. All these schemes agree in the far ultraviolet, $v\gg 1$. Therefore, we argue that in the far infrared regime, $v\ll 1$, the adiabatic expansion is no longer valid, and the unrenormalized spectra are the physical, measurable quantities. These findings cast some doubt on the validity of the adiabatic subtraction at horizon exit, $v=1$, to determine the perturbation spectra from inflation which has recently advocated in the literature.

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arXiv2009

Graviton production in anti-de Sitter braneworld cosmology: A fully consistent treatment of the boundary condition

In recent work by two of us, [Durrer & Ruser, PRL 99, 071601 (2007); Ruser & Durrer PRD 76, 104014 (2007)], graviton production due to a moving spacetime boundary (braneworld) in a five dimensional bulk has been considered. In the same way as the presence of a conducting plate modifies the electromagnetic vacuum, the presence of a brane modifies the graviton vacuum. As the brane moves, the time dependence of the vacuum leads to particle creation via the so called 'dynamical Casimir effect'. In our previous work a term in the boundary condition which is linear in the brane velocity has been neglected. In this work we develop a new approach which overcomes this approximation. We show that the previous results are not modified if the brane velocity is low.

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arXiv2009

Can the observed large scale magnetic fields be seeded by helical primordial fields?

Gravitational wave production induces a strong constraint on the amplitude of a primordial magnetic field. It has been shown that the nucleosynthesis bound for a stochastic gravitational wave background implies that causally generated fields cannot have enough power on large scales to provide the seeds necessary for the observed magnetic fields in galaxies and clusters, even by the most optimistic dynamo amplification. Magnetic fields generated at inflation can have high enough amplitude only if their spectrum is very red. Here we show that helicity, which leads to an inverse cascade, can mitigate these limits. In particular, we find that helical fields generated at the QCD phase transition or at inflation with red spectrum are possible seeds for the dynamo. Helical fields generated at the electroweak phase transition are instead excluded as seeds at large scales. We also calculate the spectrum of gravitational waves generated by helical magnetic fields.

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arXiv2009

CMB anisotropies from acausal scaling seeds

We investigate models where structure formation is initiated by scaling seeds: We consider rapidly expanding relativistic shells of energy and show that they can fit current CMB and large scale structure data if they expand with super-luminal velocities. These acausally expanding shells provide a viable alternative to inflation for cosmological structure formation with the same minimal number of parameters to characterize the initial fluctuations. Causally expanding shells alone cannot fit present data. Hybrid models where causal shells and inflation are mixed also provide good fits.

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arXiv2009

General Properties of the Gravitational Wave Spectrum from Phase Transitions

In this paper we discuss some general aspects of the gravitational wave background arising from post-inflationary short-lasting cosmological events such as phase transitions. We concentrate on the physics which determines the shape and the peak frequency of the gravitational wave spectrum. We then apply our general findings to the case of bubble collisions during a first order phase transition and compare different results in the recent literature.

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arXiv2009

Graviton production in non-inflationary cosmology

We discuss the creation of massless particles in a Universe, which transits from a radiation-dominated era to any other expansion law. We calculate in detail the generation of gravitons during the transition to a matter dominated era. We show that the resulting gravitons generated in the standard radiation/matter transition are negligible. We use our result to constrain one or more previous matter dominated era, or any other expansion law, which may have taken place in the early Universe. We also derive a general formula for the modification of a generic initial graviton spectrum by an early matter dominated era.

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arXiv2009

Braneworlds, graviton production, dynamical Casimir effect

If our Universe is a 3+1 brane in a warped 4+1 dimensional bulk so that its expansion can be understood as the motion of the brane in the bulk, the time dependence of the boundary conditions for arbitrary bulk fields can lead to particle creation via the dynamical Casimir effect. In this talk I report results for the simplest such scenario, when the only particle in the bulk is the graviton and the bulk is the 5 dimensional anti-de Sitter spacetime.

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arXiv2009

Gravitational wave generation from bubble collisions in first-order phase transitions: an analytic approach

Gravitational wave production from bubble collisions was calculated in the early nineties using numerical simulations. In this paper, we present an alternative analytic estimate, relying on a different treatment of stochasticity. In our approach, we provide a model for the bubble velocity power spectrum, suitable for both detonations and deflagrations. From this, we derive the anisotropic stress and analytically solve the gravitational wave equation. We provide analytical formulae for the peak frequency and the shape of the spectrum which we compare with numerical estimates. In contrast to the previous analysis, we do not work in the envelope approximation. This paper focuses on a particular source of gravitational waves from phase transitions. In a companion article, we will add together the different sources of gravitational wave signals from phase transitions: bubble collisions, turbulence and magnetic fields and discuss the prospects for probing the electroweak phase transition at LISA.

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arXiv2009

Interactions of cosmological gravitational waves and magnetic fields

The energy momentum tensor of a magnetic field always contains a spin-2 component in its anisotropic stress and therefore generates gravitational waves. It has been argued in the literature (Caprini & Durrer \cite{CD}) that this gravitational wave production can be very strong and that back-reaction cannot be neglected. On the other hand, a gravitational wave background does affect the evolution of magnetic fields. It has also been argued (Tsagas et al. \cite{Tsagas:2001ak},\cite{Tsagas:2005ki}) that this can lead to very strong amplification of a primordial magnetic field. In this paper we revisit these claims and study back reaction to second order.

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