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Ruth Durrer

Publications and source records attributed to Ruth Durrer.

At least 127 records · Page 7Linked to original sources
arXiv2013

A longitudinal gauge degree of freedom and the Pais Uhlenbeck field

We show that a longitudinal gauge degree of freedom for a vector field is equivalent to a Pais-Uhlenbeck scalar field. With the help of this equivalence, we can determine natural interactions of this field with scalars and fermions. Since the theory has a global U(1) symmetry, we have the usual conserved current of the charged fields, thanks to which the dynamics of the scalar field is not modified by the interactions. We use this fact to consistently quantize the theory even in the presence of interactions. We argue that such a degree of freedom can only be excited by gravitational effects like the inflationary era of the early universe and may play the role of dark energy in the form of an effective cosmological constant whose value is linked to the inflation scale.

hep-th↗
arXiv2012

Explosive particle production in non-commutative inflation

We consider a model of inflation which has recently been proposed in the literature and where inflation is induced by corrections to the energy density coming from the non-commutativity of spacetime. We show that the very rapid inflationary expansion typical of this model is responsible for a burst of particle production which ends inflation and leads to a radiation-dominated phase. We analytically estimate the energy density of these particles and we confront the results with more precise numerical calculations. We estimate the number of inflationary e-folds before the back-reaction of the radiation energy density overcomes the non-commutative effects and terminate inflation naturally.

gr-qc↗
arXiv2012

Do the cosmological observational data prefer phantom dark energy?

The dynamics of expansion and large scale structure formation of the Universe are analyzed for models with dark energy in the form of a phantom scalar field which initially mimics a $Λ$-term and evolves slowly to the Big Rip singularity. The discussed model of dark energy has three parameters -- the density and the equation of state parameter at the current epoch, $Ω_{de}$ and $w_0$, and the asymptotic value of the equation of state parameter at $a\rightarrow\infty$, $c_a^2$. Their best-fit values are determined jointly with all other cosmological parameters by the MCMC method using observational data on CMB anisotropies and polarization, SNe Ia luminosity distances, BAO measurements and more. Similar computations are carried out for $Λ$CDM and a quintessence scalar field model of dark energy. It is shown that the current data slightly prefer the phantom model, but the differences in the maximum likelihoods are not statistically significant. It is also shown that the phantom dark energy with monotonically increasing density in future will cause the decay of large scale linear matter density perturbations due to the gravitational domination of dark energy perturbations long before the Big Rip singularity.

astro-ph.CO↗
arXiv2012

A new method for the Alcock-Paczynski test

We argue that from observations alone, only the transverse power spectrum $C_\ell(z_1,z_2)$ and the corresponding correlation function $ξ(θ,z_1,z_2)$ can be measured and that these contain the full three dimensional information. We determine the two point galaxy correlation function at linear order in perturbation theory. Redshift space distortions are taken into account for arbitrary angular and redshift separations. We discuss the shape of the longitudinal and the transversal correlation functions which are very different from each other and from their real space counterpart. We then go on and suggest how to measure both, the Hubble parameter, $H(z)$, and the angular diameter distance, $D_A(z)$, separately from these correlation functions and perform an Alcock-Paczynski test.

astro-ph.CO↗
arXiv2012

Vector and Tensor Contributions to the Luminosity Distance

We compute the vector and tensor contributions to the luminosity distance fluctuations in first order perturbation theory and we expand them in spherical harmonics. This work presents the formalism with a first application to a stochastic background of primordial gravitational waves.

astro-ph.CO↗
arXiv2012

What galaxy surveys really measure

In this paper we compute the quantity which is truly measured in a large galaxy survey. We take into account the effects coming from the fact that we actually observe galaxy redshifts and sky positions and not true spatial positions. Our calculations are done within linear perturbation theory for both the metric and the observer velocities but they can be used for non-linear matter power spectra. We shall see that the complications due to the fact that we only observe on our background lightcone and that we do not truly know the distance of the observed galaxy, but only its redshift is not only an additional difficulty, but even more a new opportunity for future galaxy surveys.

astro-ph.CO↗
arXiv2012

Magnetic fields from inflation: the transition to the radiation era

We compute the contribution to the scalar metric perturbations from large-scale magnetic fields which are generated during inflation. We show that apart from the usual passive and compensated modes, the magnetic fields also contribute to the constant mode from inflation. This is different from the causal (post inflationary) generation of magnetic fields where such a mode is absent and it might lead to significant, non-Gaussian CMB anisotropies.

astro-ph.CO↗
arXiv2012

Mode Spectrum of the Electromagnetic Field in Open Universe Models

We examine the mode functions of the electromagnetic field on spherically symmetric backgrounds with special attention to the subclass which allows for the foliation as open Friedmann-Lemaitre (FL) spacetime. It is well-known that in certain scalar field theories on open FL background there can exist so-called supercurvature modes, their existence depending on parameters of the theory. Looking at specific open universe models, such as open inflation and the Milne universe, we find that no supercurvature modes are present in the spectrum of the electromagnetic field. This excludes the possibility for superadiabatic evolution of cosmological magnetic fields within these models without relying on new physics or breaking the conformal invariance of electromagnetism.

gr-qc↗
arXiv2012

Generation of helical magnetic fields from inflation

The generation of helical magnetic fields during single field inflation due to an axial coupling of the electromagnetic field to the inflaton is discussed. We find that such a coupling always leads to a blue spectrum of magnetic fields during slow roll inflation. Though the helical magnetic fields further evolve during the inverse cascade in the radiation era after inflation, we conclude that the magnetic fields generated by such an axial coupling can not lead to observed field strength on cosmologically relevant scales.

astro-ph.CO↗
arXiv2012

Back Reaction from Walls

We study the distance-redshift relation in a universe filled with 'walls' of pressure-less dust separated by under dense regions. We show that as long as the density contrast of the walls is small, or the diameter of the under dense regions is much smaller than the Hubble scale, the distance-redshift relation remains close to what is obtained in a Friedmann universe. However, when arbitrary density contrasts are allowed, every prescribed distance-redshift relation can be reproduced with such models.

astro-ph.CO↗
arXiv2011

The Kolmogorov-Smirnov test for the CMB

We investigate the statistics of the cosmic microwave background using the Kolmogorov-Smirnov test. We show that, when we correctly de-correlate the data, the partition function of the Kolmogorov stochasticity parameter is compatible with the Kolmogorov distribution and, contrary to previous claims, the CMB data are compatible with Gaussian fluctuations with the correlation function given by standard Lambda-CDM. We then use the Kolmogorov-Smirnov test to derive upper bounds on residual point source power in the CMB, and indicate the promise of this statistics for further datasets, especially Planck, to search for deviations from Gaussianity and for detecting point sources and Galactic foregrounds.

astro-ph.CO↗
arXiv2011

Fluctuations of the luminosity distance

We derive an expression for the luminosity distance in a perturbed Friedmann universe. We define the correlation function and the power spectrum of the luminosity distance fluctuations and express them in terms of the initial spectrum of the Bardeen potential. We present semi-analytical results for the case of a pure CDM universe. We argue that the luminosity distance power spectrum represents a new observational tool which can be used to determine cosmological parameters. In addition, our results shed some light into the debate whether second order small scale fluctuations can mimic an accelerating universe.

astro-ph↗
arXiv2011

Testing Superstring Theories with Gravitational Waves

We provide a simple transfer function that determines the effect of an early matter dominated era on the gravitational wave background and show that a large class of compactifications of superstring theory might be tested by observations of the gravitational wave background from inflation. For large enough reheating temperatures $> 10^9 \GeV$ the test applies to all models containing at least one scalar with mass $< 10^{12}\GeV$ that acquires a large initial oscillation amplitude after inflation and has only gravitational interaction strength, i.e., a field with the typical properties of a modulus.

gr-qc↗
arXiv2011

What do we really know about Dark Energy?

In this paper I discuss what we truly know about dark energy. I shall argue that up to date our single indication for the existence of dark energy comes from distance measurements and their relation to redshift. Supernovae, CMB anisotropies and observations of baryon acoustic oscillations, they all simply tell us that the observed distance to a given redshift is larger than the one expected from a Friedmann Lemaitre universe with matter only and the locally measured Hubble parameter.

astro-ph.CO↗
arXiv2011

Observational constraints on scalar field models of dark energy with barotropic equation of state

We constrain the parameters of dynamical dark energy in the form of a classical or tachyonic scalar field with barotropic equation of state jointly with other cosmological ones using the combined datasets which include the CMB power spectra from WMAP7, the baryon acoustic oscillations in the space distribution of galaxies from SDSS DR7, the power spectrum of luminous red galaxies from SDSS DR7 and the light curves of SN Ia from 2 different compilations: Union2 (SALT2 light curve fitting) and SDSS (SALT2 and MLCS2k2 light curve fittings). It has been found that the initial value of dark energy equation of state parameter is constrained very weakly by most of the data while the rest of main cosmological parameters are well constrained: their likelihoods and posteriors are similar, have the forms close to Gaussian (or half-Gaussian) and their confidential ranges are narrow. The most reliable determinations of the best fitting value and $1σ$ confidence range for the initial value of dark energy equation of state parameter were obtained from the combined datasets including SN Ia data from the full SDSS compilation with MLCS2k2 fitting of light curves. In all such cases the best fitting value of this parameter is lower than the value of corresponding parameter for current epoch. Such dark energy loses its repulsive properties and in future the expansion of the Universe will change into contraction. We also perform an error forecast for the Planck mock data and show that they narrow essentially the confidential ranges of cosmological parameters values, moreover, their combination with SN SDSS compilation with MLCS2k2 light curve fitting may exclude the fields with initial equation of state parameter $>-0.1$ at 2$σ$ confidential level.

astro-ph.CO↗
arXiv2011

An analytic approach to baryon acoustic oscillations

The fitting formula for the location of the first acoustic peak in the matter power spectrum is revised. We discuss the physics that leads to baryon acoustic oscillations: the recombination history, the tight coupling approximation and the velocity overshoot effect. A new fitting formula is proposed, which is in accordance within 5% with numerical results for a suitable range of cosmological parameters, whereas previous results yield deviations of up to 20%. The crucial improvement turns out to be the accuracy of the recombination history.

astro-ph.CO↗
arXiv2011

On infrared and ultraviolet divergences of cosmological perturbations

We study a consistent infrared and ultraviolet regularization scheme for the cosmological perturbations. The infrared divergences are cured by assuming that the Universe undergoes a transition between a non-singular pre-inflationary, radiation-dominated phase and a slow-roll inflationary evolution. The ultraviolet divergences are eliminated via adiabatic subtraction. A consistent regularization of the field fluctuations through this transition is obtained by performing a mode matching for both the gauge invariant Mukhanov variable and its adiabatic expansion. We show that these quantities do not generate ultraviolet divergences other than the standard ones, when evolving through the matching time. We also show how the de Witt-Schwinger expansion, which can be used to construct the counter-terms regularizing the ultraviolet divergences, ceases to be valid well before horizon exit of the scales of interest. Thus, such counter-terms should not be used beyond the time of the horizon exit so it is unlikely that the observed power spectrum is modified by adiabatic subtraction as claimed in the literature. However, the infrared regularization might have an impact on the observed spectrum, and we briefly discuss this possibility.

astro-ph.CO↗