Archive ouverte UNIGE | last documents for author 'Syksy Rasanen'https://archive-ouverte.unige.ch/Latest objects deposited in the Archive ouverte UNIGE for author 'Syksy Rasanen'engCan extragalactic foregrounds explain the large-angle CMB anomalies?https://archive-ouverte.unige.ch/unige:4603https://archive-ouverte.unige.ch/unige:4603We address the effect of an extended local foreground on the low-l anomalies found in the CMB. Recent X-ray catalogues point us to the existence of very massive superstructures at the 100 h^(-1) Mpc scale that contribute significantly to the dipole velocity profile. Being highly non-linear, these structures provide us a natural candidate to leave an imprint on the CMB sky via a local Rees-Sciama effect. We show that the Rees-Sciama effect of local foregrounds can induce CMB anisotropy of DeltaT/T ~ 10^(-5) and we analyse its impact on multipole power as well as the induced phase pattern on largest angular scales.Thu, 03 Dec 2009 17:03:27 +0100Photon physics in heavy ion collisions at the LHChttps://archive-ouverte.unige.ch/unige:2583https://archive-ouverte.unige.ch/unige:2583Various pion and photon production mechanisms in high-energy nuclear collisions at RHIC and LHC are discussed. Comparison with RHIC data is done whenever possible. The prospect of using electromagnetic probes to characterize quark-gluon plasma formation is assessed...Mon, 07 Sep 2009 10:50:24 +0200On hydrodynamical description of thermal photonshttps://archive-ouverte.unige.ch/unige:2582https://archive-ouverte.unige.ch/unige:2582The WA98 collaboration in the CERN SPS has reported an excess of photons over those originating from the decays of final hadrons in the lead-lead collisions. These photons can originate either from primary interactions of partons from colliding nuclei or from secondary interactions among produced particles. Photons produced in the secondary interactions, often called thermal photons, can be calculated by using thermal production rates and equilibrium hydrodynamics for the evolution of the expanding matter. I will review the main features of hydrodynamical studies for the WA98 data. The data can be reproduced both with or without a phase transition to the QGP, but high initial temperature, over the values predicted for the phase transition temperature, is required by the data. I will also show a prediction for the photon excess for central gold-gold collisions at the Brookhaven RHIC collider. In this prediction, the initial state for the hydrodynamical expansion is obtained from a perturbative QCD calculation.Mon, 07 Sep 2009 10:48:32 +0200Photon production from non-equilibrium QGP in heavy ion collisionshttps://archive-ouverte.unige.ch/unige:2579https://archive-ouverte.unige.ch/unige:2579We present a calculation of thermal photon production, i.e. photons from secondary interactions among particles produced in heavy ion collisions at collider energies. This is done within the framework of hydrodynamics. We take into account the lack of chemical equilibrium in QGP. It turns out that the main effects from the chemical non-equilibrium composition of QGP, reduction of particle number and increase in temperature, nearly cancel in the photon spectrum.Thu, 03 Sep 2009 10:37:35 +0200Predictions for low-pT and high-pT hadron spectra in nearly central Pb+Pb collisions at sqrt[sNN]=5.5 TeV tested at sqrt[sNN]=130 and 200 GeVhttps://archive-ouverte.unige.ch/unige:2578https://archive-ouverte.unige.ch/unige:2578We study the hadron spectra in nearly central A+A collisions at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) in a broad transverse momentum range. We cover the low-pT spectra using longitudinally boost-invariant hydrodynamics with initial energy and net-baryon number densities from the perturbative QCD (pQCD)+saturation model. Buildup of the transverse flow and sensitivity of the spectra to a single decoupling temperature Tdec are studied. Comparison with RHIC data at sqrt[sNN]=130 and 200 GeV suggests a rather high value Tdec=150 MeV. The high-pT spectra are computed using factorized pQCD cross sections, nuclear parton distributions, fragmentation functions, and describing partonic energy loss in the quark-gluon plasma by quenching weights. Overall normalization is fixed on the basis of p+p̅ (p) data and the strength of energy loss is determined from RHIC Au+Au data. Uncertainties are discussed. With constraints from RHIC data, we predict the pT spectra of hadrons in 5% most central Pb+Pb collisions at the LHC energy sqrt[sNN]=5500 GeV. Because of the closed framework for primary production, we can also predict the net-baryon number at midrapidity, as well as the strength of partonic energy losses at the LHC. Both at the LHC and RHIC, we recognize a rather narrow crossover region in the pT spectra, where the hydrodynamic and pQCD fragmentation components become of equal size. We argue that in this crossover region the two contributions are to a good approximation mutually independent. In particular, our results suggest a wider pT region of applicability for hydrodynamical models at the LHC than at RHIC.Thu, 03 Sep 2009 10:19:38 +0200Transverse spectra of hadrons in central $AA$ collisions at RHIC and LHC from pQCD+Saturation+Hydrodynamics and from pQCD+Energy losseshttps://archive-ouverte.unige.ch/unige:2577https://archive-ouverte.unige.ch/unige:2577We study the transverse spectra of hadrons in nearly central AA collisions at RHIC and LHC in a broad transverse momentum range [1]. Low-pT spectra are calculated by using boost-invariant hydrodynamics with initial energy and net-baryon densities from the EKRT [2] pQCD+saturation model. High-pT spectra are obtained from pQCD jet calculation [3] including the energy loss of the parton [4] in the matter prior to its fragmentation to final hadrons.Thu, 03 Sep 2009 10:18:01 +0200Backreaction of linear perturbations and dark energyhttps://archive-ouverte.unige.ch/unige:2576https://archive-ouverte.unige.ch/unige:2576A simple discussion on the backreaction of inhomogeneities in cosmology, focusing on the possibility that it could explain the present acceleration and solve the coincidence problem.Thu, 03 Sep 2009 08:59:13 +0200Hadron multiplicities, pT spectra and net-baryon number in central Pb+Pb collisions at the LHChttps://archive-ouverte.unige.ch/unige:2575https://archive-ouverte.unige.ch/unige:2575We compute the initial energy density and net baryon number density in 5% most central Pb+Pb collisions at $sqrt s=5.5$ TeV from pQCD + (final state) saturation, and describe the evolution of the produced system with boost-invariant transversely expanding hydrodynamics. In addition to the total multiplicity at midrapidity, we give predictions for the multiplicity of charged hadrons, pions, kaons and (anti)protons, for the total transverse energy and net-baryon number, as well as for the $p_T$-spectrum of charged hadrons, pions and kaons. We also predict the region of applicability of hydrodynamics by comparing these results with high-$p_T$ hadron spectra computed from pQCD and energy losses.Thu, 03 Sep 2009 08:44:19 +0200A primer on the ekpyrotic scenariohttps://archive-ouverte.unige.ch/unige:2571https://archive-ouverte.unige.ch/unige:2571This is an introduction to the ekpyrotic scenario, with an emphasis on the two contexts of brane cosmology and primordial universe scenarios. A self-contained introduction to brane cosmology and a qualitative overview and comparison of the inflationary, pre-big bang and ekpyrotic scenarios are given as background. The ekpyrotic scenario is then presented in more detail, stressing various problems.Mon, 31 Aug 2009 17:58:35 +0200On ekpyrotic brane collisionshttps://archive-ouverte.unige.ch/unige:2570https://archive-ouverte.unige.ch/unige:2570We derive the five-dimensional metrics which describe a non-singular boundary brane collision in the ekpyrotic scenario in the context of general relativity, taking into account brane tension. We show that the metrics constrain matter created in the collision to have negative energy density or pressure. In particular, the minimal field content of heterotic M-theory leads to negative energy density. We also consider bulk brane-boundary brane collisions and show that the collapse of the fifth dimension is an artifact of the four-dimensional effective theory.Mon, 31 Aug 2009 17:57:52 +0200Hubble law and brane matter after ekpyrosishttps://archive-ouverte.unige.ch/unige:2569https://archive-ouverte.unige.ch/unige:2569We study brane matter in the ekpyrotic scenario and observe that in order to obtain standard gravity on the visible brane, the tension of the visible brane should be positive. If the sizes of both the fifth dimension and the Calabi-Yau threefold are fixed, the Israel junction conditions do not allow time-dependent brane matter. Relaxing this constraint, it is possible to obtain approximately standard cosmology on the visible brane, with small corrections due to possible time-dependence of the Calabi-Yau threefold.Mon, 31 Aug 2009 17:57:02 +0200Constraints on brane and bulk ideal fluid in Randall-Sundrum cosmologieshttps://archive-ouverte.unige.ch/unige:2568https://archive-ouverte.unige.ch/unige:2568We investigate constraints for including bulk and brane matter in the Randall- Sundrum model. In static configurations with two zero thickness branes, we find that no realistic brane matter is possible. We also consider the possibility that the radion has stabilized by dissipating its energy into the bulk in the form of some unspecified matter, and find the Randall-Sundrum cosmological solutions in the presence of bulk ideal fluid. We discover that the metric is necessarily in a static configuration. We also discover that there is only one allowed equation of state for the bulk fluid, $p= ho $, corresponding to the stiff ideal fluid. We find the corresponding brane cosmologies and compare them with the Friedmann-Robertson-Walker model.Mon, 31 Aug 2009 17:55:43 +0200The effect of structure formation on the expansion of the universehttps://archive-ouverte.unige.ch/unige:2566https://archive-ouverte.unige.ch/unige:2566Observations of the expansion rate of the universe at late times disagree by a factor of 1.5-2 with the prediction of homogeneous and isotropic models based on ordinary matter and gravity. We discuss how the departure from linearly perturbed homogeneity and isotropy due to structure formation could explain this discrepancy. We evaluate the expansion rate in a dust universe which contains non-linear structures with a statistically homogeneous and isotropic distribution. The expansion rate is found to increase relative to the exactly homogeneous and isotropic case by a factor of 1.1-1.3 at some tens of billion of years. The timescale follows from the cold dark matter transfer function and the amplitude of primordial perturbations without additional free parameters.Mon, 31 Aug 2009 16:27:19 +0200Evaluating backreaction with the peak model of structure formationhttps://archive-ouverte.unige.ch/unige:2565https://archive-ouverte.unige.ch/unige:2565We evaluate the average expansion rate of a universe which contains a realistic evolving ensemble of non-linear structures. We use the peak model of structure formation to obtain the number density of structures, and take the individual structures to be spherical. The expansion rate increases relative to the FRW value on a timescale of 10-100 billion years, because the universe becomes dominated by fast-expanding voids. However, the increase is not rapid enough to correspond to acceleration. We discuss how to improve our treatment. We also consider various qualitative issues related to backreaction.Mon, 31 Aug 2009 16:25:43 +0200Lovelock inflation and the number of large dimensionshttps://archive-ouverte.unige.ch/unige:2564https://archive-ouverte.unige.ch/unige:2564We discuss an inflationary scenario based on Lovelock terms. These higher order curvature terms can lead to inflation when there are more than three spatial dimensions. Inflation will end if the extra dimensions are stabilised, so that at most three dimensions are free to expand. This relates graceful exit to the number of large dimensions.Mon, 31 Aug 2009 16:22:32 +0200Comment on "Nontrivial geometries: bounds on the curvature of the universe"https://archive-ouverte.unige.ch/unige:2563https://archive-ouverte.unige.ch/unige:2563The paper 0705.0332v1 seeks to study the effect of non-trivial spatial curvature in homogeneous and isotropic models. We note that the space considered is not homogeneous, and that the equations of motion used are inconsistent with the metric. Also, we explain why the spatial curvature of homogeneous and isotropic spacetimes always evolves like 1/a^2, contrary to the central assumption of 0705.0332v1.Mon, 31 Aug 2009 16:21:17 +0200Accelerated expansion from structure formationhttps://archive-ouverte.unige.ch/unige:2562https://archive-ouverte.unige.ch/unige:2562We discuss the physics of backreaction-driven accelerated expansion. Using the exact equations for the behaviour of averages in dust universes, we explain how large-scale smoothness does not imply that the effect of inhomogeneity and anisotropy on the expansion rate is small. We demonstrate with an analytical toy model how gravitational collapse can lead to acceleration. We find that the conjecture of the accelerated expansion being due to structure formation is in agreement with the general observational picture of structures in the universe, and more quantitative work is needed to make a detailed comparison.Mon, 31 Aug 2009 16:20:08 +0200Cosmological acceleration from structure formationhttps://archive-ouverte.unige.ch/unige:2561https://archive-ouverte.unige.ch/unige:2561We discuss the Buchert equations, which describe the average expansion of an inhomogeneous dust universe. In the limit of small perturbations, they reduce to the Friedmann-Robertson-Walker equations. However, when the universe is very inhomogeneous, the behaviour can be qualitatively different from the FRW case. In particular, the average expansion rate can accelerate even though the local expansion rate decelerates everywhere. We clarify the physical meaning of this paradoxical feature with a simple toy model, and demonstrate how acceleration is intimately connected with gravitational collapse. This provides a link to structure formation, which in turn has a preferred time around the era when acceleration has been observed to start.Mon, 31 Aug 2009 16:18:32 +0200Microwave sky and the local Rees-Sciama effecthttps://archive-ouverte.unige.ch/unige:2560https://archive-ouverte.unige.ch/unige:2560The microwave sky shows unexpected features at the largest angular scales, among them the alignments of the dipole, quadrupole and octopole. Motivated by recent X-ray cluster studies, we investigate the possibility that local structures at the 100 h^(-1) Mpc scale could be responsible for such correlations. These structures give rise to a local Rees-Sciama contribution to the microwave sky that may amount to Delta T/T ~ 10^(-5) at the largest angular scales. We model local structures by a spherical overdensity (Lemaitre-Tolman-Bondi model) and assume that the Local Group is falling toward the centre. We superimpose the local Rees-Sciama effect on a statistically isotropic, gaussian sky. As expected we find alignments among low multipoles, but a closer look reveals that they do not agree with the type of correlations revealed by the data.Mon, 31 Aug 2009 16:16:42 +0200Dark energy and decompactification in string gas cosmologyhttps://archive-ouverte.unige.ch/unige:2559https://archive-ouverte.unige.ch/unige:2559We study the stability of extra dimensions in string gas cosmology at late times. Vacuum energy and, interestingly, baryons lead to decompactification after they become dynamically important. The string gas can stabilise the effect of baryons, but not that of vacuum energy. However, we find that the interplay of baryons and strings can lead to acceleration in the visible dimensions, without the need for vacuum energy.Mon, 31 Aug 2009 16:14:29 +0200Constraints on backreaction in dust universeshttps://archive-ouverte.unige.ch/unige:2558https://archive-ouverte.unige.ch/unige:2558We study backreaction in dust universes using exact equations which do not rely on perturbation theory, concentrating on theoretical and observational constraints. In particular, we discuss the recent suggestion (in hep-th/0503117) that superhorizon perturbations could explain present-day accelerated expansion as a useful example which can be ruled out. We note that a backreaction explanation of late-time acceleration will have to involve spatial curvature and subhorizon perturbations.Mon, 31 Aug 2009 16:12:33 +0200Backreaction in the Lemaitre-Tolman-Bondi modelhttps://archive-ouverte.unige.ch/unige:2557https://archive-ouverte.unige.ch/unige:2557We study backreaction analytically using the parabolic Lemaˆıtre-Tolman-Bondi universe as a toy model. We calculate the average expansion rate and energy density on two different hypersurfaces and compare the results. We also consider the Hubble law and find that backreaction slows down the expansion if measured with proper time, but speeds it up if measured with energy density.Mon, 31 Aug 2009 16:10:56 +0200Correlated isocurvature perturbations from mixed inflaton-curvaton decayhttps://archive-ouverte.unige.ch/unige:2556https://archive-ouverte.unige.ch/unige:2556We study cosmological perturbations in the case in which present-day matter consists of a mixture of inflaton and curvaton decay products. We calculate how the curvaton perturbations are transferred to its decay products in the general case when it does not behave like dust. Taking into account that the decay products of the inflaton can also have perturbations results in an interesting mixture of correlated adiabatic and isocurvature perturbations. In particular, negative correlation can improve the fit to the CMB data by lowering the angular power in the Sachs–Wolfe plateau without changing the peak structure. We do an 11-parameter fit to the WMAP data. We find that the best fit is not the 'concordance model', and that well-fitting models do not cluster around the best fit, so cosmological parameters cannot be reliably estimated. We also find that in our model the mean quadrupole (l = 2) power is l(l+1)Cl/2π = 1081 μK2, much lower than in the pure adiabatic ΛCDM model, which gives 1262 μK2.Mon, 31 Aug 2009 16:09:20 +0200Dark energy from backreactionhttps://archive-ouverte.unige.ch/unige:2555https://archive-ouverte.unige.ch/unige:2555We consider the effect of inhomogeneities on the expansion of the Einstein–de Sitter universe. We find that the back-reaction of linear scalar metric perturbations results in apparent dark energy with a mixture of equations of state between 0 and -4/3. We discuss the possibility that back-reaction could account for present-day acceleration.Mon, 31 Aug 2009 16:03:51 +0200Structure formation as an alternative to dark energy and modified gravityhttps://archive-ouverte.unige.ch/unige:2222https://archive-ouverte.unige.ch/unige:2222Observations are inconsistent with a homogeneous and isotropic universe with ordinary matter and gravity. The universe is far from exact homogeneity and isotropy at late times, and the effect of the non-linear structures has to be quantified before concluding that new physics is needed. We explain how structure formation can lead to accelerated expansion, and discuss a semi-realistic model where the timescale of the change in the expansion rate emerges from the physics of structure formation.Fri, 10 Jul 2009 15:23:05 +0200Light propagation in statistically homogeneous and isotropic dust universeshttps://archive-ouverte.unige.ch/unige:2221https://archive-ouverte.unige.ch/unige:2221We derive the redshift and the angular diameter distance in rotationless dust universes which are statistically homogeneous and isotropic, but have otherwise arbitrary geometry. The calculation from first principles shows that the Dyer-Roeder approximation does not correctly describe the effect of clumping. Instead, the redshift and the distance are determined by the average expansion rate, the matter density today and the null geodesic shear. In particular, the position of the CMB peaks is consistent with significant spatial curvature provided the expansion history is sufficiently close to the spatially flat ΛCDM model.Fri, 10 Jul 2009 15:20:27 +0200Fitting oscillating string gas cosmology to supernova datahttps://archive-ouverte.unige.ch/unige:2220https://archive-ouverte.unige.ch/unige:2220In string gas cosmology, extra dimensions are stabilised by a gas of strings. In the matter-dominated era, competition between matter pushing the extra dimensions to expand and the string gas pulling them back can lead to oscillations of the extra dimensions and acceleration in the visible dimensions. We fit this model to supernova data, taking into account the Big Bang Nucleosynthesis constraint on the energy density of the string gas. The fit to the Union set of supernova data is acceptable, but the fit to the ESSENCE data is poor.Fri, 10 Jul 2009 15:19:27 +0200On the relation between the isotropy of the CMB and the geometry of the universehttps://archive-ouverte.unige.ch/unige:2219https://archive-ouverte.unige.ch/unige:2219The near-isotropy of the cosmic microwave background (CMB) is considered to be the strongest indication for the homogeneity and isotropy of the universe, a cornerstone of most cosmological analysis. We derive new theorems which extend the Ehlers-Geren-Sachs result that an isotropic CMB implies that the universe is either stationary or homogeneous and isotropic, and its generalisation to the almost isotropic case. We discuss why the theorems do not apply to the real universe, and why the CMB observations do not imply that the universe would be nearly homogeneous and isotropic.Fri, 10 Jul 2009 15:16:11 +0200