Showing papers in "Journal of Cosmology and Astroparticle Physics in 2004"

DarkSUSY: Computing Supersymmetric Dark Matter Properties Numerically

Abstract: The question of the nature of the dark matter in the Universe remains one of the most outstanding unsolved problems in basic science. One of the best motivated particle physics candidates is the lightest supersymmetric particle, assumed to be the lightest neutralino—a linear combination of the supersymmetric partners of the photon, the Z boson and neutral scalar Higgs particles. Here we describe DarkSUSY, a publicly available advanced numerical package for neutralino dark matter calculations. In DarkSUSY one can compute the neutralino density in the Universe today using precision methods which include resonances, pair production thresholds and coannihilations. Masses and mixings of supersymmetric particles can be computed within DarkSUSY or with the help of external programs such as FeynHiggs, ISASUGRA and SUSPECT. Accelerator bounds can be checked to identify viable dark matter candidates. DarkSUSY also computes a large variety of astrophysical signals from neutralino dark matter, such as direct detection in low-background counting experiments and indirect detection through antiprotons, antideuterons, gamma-rays and positrons from the galactic halo or high-energy neutrinos from the centre of the Earth or of the Sun. Here we describe the physics behind the package. A detailed manual will be provided with the computer package.

A single-field consistency relation for the three-point function

Abstract: We point out the existence of a consistency relation involving the three-point function of scalar perturbations which is valid in any inflationary model, independently of the inflaton Lagrangian under the assumption that the inflaton is the only dynamical field. The three-point function in the limit in which one of the momenta is much smaller than the other two is fixed in terms of the power spectrum and its tilt. This relation, although very hard to verify experimentally, could be easily proved wrong by forthcoming data, thus ruling out any scenario with a single dynamical field in a model independent way.

The shape of non-Gaussianities

Abstract: We study the dependence on the configuration in momentum space of the primordial three-point function of density perturbations in several different scenarios: standard slow-roll inflation, curvaton and variable decay models, ghost inflation, models with higher derivative operators and the DBI model of inflation. We define a cosine between the distributions using a measure based on the ability of experiments to distinguish between them. We find that models fall into two broad categories with fairly orthogonal distributions: models where non-Gaussianity is created on crossing the horizon during inflation and models in which the evolution beyond the horizon dominates. In the first case the three-point function is largest for equilateral triangles, while in the second the dominant contribution to the signal comes from the influence of long wavelength modes on small wavelength ones. We show that, because the distributions in these two cases are so different, translating constraints on parameters of one model to those of the other on the basis of the normalization of the three-point function for equilateral triangles can be very misleading.

The holographic dark energy in a non-flat universe

Abstract: We study the model for holographic dark energy in a spatially closed universe, generalizing the proposal in hep-th/0403127 for a flat universe. We provide independent arguments for the choice of the parameter c = 1 in the holographic dark energy model. On the one hand, c cannot be less than 1, to avoid violating the second law of thermodynamics. On the other hand, observation suggests c to be very close to 1; it is hard to justify a small deviation of c from 1 if c>1.

The case for dynamical dark energy revisited

Abstract: We investigate the behaviour of dark energy using the recently released supernova data of Riess et al 2004 and a model independent parametrization for dark energy (DE). We find that, if no priors are imposed on Ω0m and h, DE which evolves with time provides a better fit to the SNe data than ΛCDM. This is also true if we include results from the WMAP CMB data. From a joint analysis of SNe + CMB, the best fit DE model has w0 −1 at the present epoch and the transition from deceleration to acceleration occurs at zT = 0.39 ± 0.03. However, DE evolution becomes weaker if the ΛCDM based CMB results Ω0m = 0.27 ± 0.04, h = 0.71 ± 0.06 are incorporated in the analysis. In this case, zT = 0.57 ± 0.07. Our results also show that the extent of DE evolution is sensitive to the manner in which the supernova data is sampled.

Dilatonic ghost condensate as dark energy

Abstract: We explore a dark energy model with a ghost scalar field in the context of the runaway dilaton scenario in low-energy effective string theory. We address the problem of vacuum stability by implementing higher-order derivative terms and show that a cosmologically viable model of 'phantomized' dark energy can be constructed without violating the stability of quantum fluctuations. We also analytically derive the condition under which cosmological scaling solutions exist, starting from a general Lagrangian including the phantom type scalar field. We apply this method to the case where the dilaton is coupled to non-relativistic dark matter and find that the system tends to become quantum mechanically unstable when a constant coupling is always present. Nevertheless, it is possible to obtain a viable cosmological solution in which the energy density of the dilaton eventually approaches the present value of dark energy provided that the coupling rapidly grows during the transition to the scalar field dominated era.

Dark energy from mass varying neutrinos

Abstract: We show that mass varying neutrinos (MaVaNs) can behave as a negative pressure fluid which could be the origin of the cosmic acceleration. We derive a model independent relation between the neutrino mass and the equation of state parameter of the neutrino dark energy, which is applicable for general theories of mass varying particles. The neutrino mass depends on the local neutrino density and the observed neutrino mass can exceed the cosmological bound on a constant neutrino mass. We discuss microscopic realizations of the MaVaN acceleration scenario, which involve a sterile neutrino. We consider naturalness constraints for mass varying particles, and find that both eV cut-offs and eV mass particles are needed to avoid fine-tuning. In microscopic realizations of this scenario with a sterile neutrino, these considerations give the sterile neutrino a maximum mass today of order an eV, which could be detectable at MiniBooNE. Because the sterile neutrino was much heavier at earlier times, constraints from big bang nucleosynthesis on additional states are not problematic. We consider regions of high neutrino density and find that the most likely place today to find neutrino masses which are significantly different from the neutrino masses in our solar system is in a supernova. The possibility of different neutrino mass in different regions of the galaxy and the local group could be significant for Z-burst models of ultra-high energy cosmic rays. We also consider the cosmology of and the constraints on the 'acceleron', the scalar field which is responsible for the varying neutrino mass, and briefly discuss neutrino density dependent variations in other constants, such as the fine structure constant.

Formation and evolution of cosmic D strings

Abstract: We study the formation of D and F cosmic strings in D-brane annihilation after brane inflation. We show that D string formation by quantum de Sitter fluctuations is severely suppressed, due to suppression of RR field fluctuations in compact dimensions. We discuss the resonant mechanism of production of D and F strings, which are formed as magnetic and electric flux tubes of the two orthogonal gauge fields living on the world-volume of the unstable brane. We outline the subsequent cosmological evolution of the D−F string network. We also compare the nature of these strings with the ordinary cosmic strings and point out some differences and similarities.

Supernova constraints on a holographic dark energy model

Abstract: In this paper, we use the type Ia supernova data to constrain the model of holographic dark energy. For d = 1, the best fit result is Ωm0 = 0.25, the equation of the state of the holographic dark energy wΛ0 = −0.91 and the transition between the decelerating expansion and the accelerating expansion happened when the cosmological red-shift was zT = 0.72. If we set d as a free parameter, the best fit results are d = 0.21, Ωm0 = 0.46, wΛ0 = −2.67, which sounds like a phantom today, and the transition red-shift is zT = 0.28.

Could dark energy be vector-like?

Abstract: In this paper I explore whether a vector field can be the origin of the present stage of cosmic acceleration. In order to avoid violations of isotropy, the vector has be part of a 'cosmic triad', that is, a set of three identical vectors pointing in mutually orthogonal spatial directions. A triad is indeed able to drive a stage of late accelerated expansion in the universe, and there exist tracking attractors that render cosmic evolution insensitive to initial conditions. However, as in most other models, the onset of cosmic acceleration is determined by a parameter that has to be tuned to reproduce current observations. The triad equation of state can be sufficiently close to minus one today, and for tachyonic models it might be even less than that. I briefly analyse linear cosmological perturbation theory in the presence of a triad. It turns out that the existence of non-vanishing spatial vectors invalidates the decomposition theorem, i.e. scalar, vector and tensor perturbations do not decouple from each other. In a simplified case it is possible to analytically study the stability of the triad along the different cosmological attractors. The triad is classically stable during inflation, radiation and matter domination, but it is unstable during (late time) cosmic acceleration. I argue that this instability is not likely to have a significant impact at present.

Dark energy from back-reaction

Abstract: We 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.

Gravitational clustering of relic neutrinos and implications for their detection

Abstract: We study the gravitational clustering of big bang relic neutrinos onto existing cold dark matter (CDM) and baryonic structures within the flat ΛCDM model, using both numerical simulations and a semi-analytical linear technique, with the aim of understanding the neutrinos' clustering properties for direct detection purposes. In a comparative analysis, we find that the linear technique systematically underestimates the amount of clustering for a wide range of CDM halo and neutrino masses. This invalidates earlier claims of the technique's applicability. We then compute the approximate phase space distribution of relic neutrinos in our neighbourhood at Earth, and estimate the large scale neutrino density contrasts within the local Greisen–Zatsepin–Kuzmin zone. With these findings, we discuss the implications of gravitational neutrino clustering for scattering-based detection methods, ranging from flux detection via Cavendish-type torsion balances, to target detection using accelerator beams and cosmic rays. For emission spectroscopy via resonant annihilation of extremely energetic cosmic neutrinos on the relic neutrino background, we give new estimates for the expected enhancement in the event rates in the direction of the Virgo cluster.

Cosmological constraints on the dark energy equation of state and its evolution

Abstract: We have calculated constraints on the evolution of the equation of state of the dark energy, w(z), from a joint analysis of data from the cosmic microwave background, large scale structure and type-Ia supernovae. In order to probe the time evolution of w we propose a new, simple parametrization of w, which has the advantage of being transparent and simple to extend to more parameters as better data become available. Furthermore it is well behaved in all asymptotic limits. Based on this parametrization we find that w(z = 0) = −1.43−0.38+0.16 and dw/dz(z = 0) = 1.0−0.8+1.0. For a constant w we find that -1.34 ≤ w ≤ -0.79 at 95% CL. Thus, allowing for a time-varying w shifts the best fit present day value of w down. However, even though models with time variation in w yield a lower χ2 than pure ΛCDM models, they do not have a better goodness-of-fit. Rank correlation tests on supernova data also do not show any need for a time-varying w.

Axino dark matter from thermal production

Abstract: The axino is a promising candidate for dark matter in the Universe. It is electrically and colour neutral, very weakly interacting, and could be—as assumed in this study—the lightest supersymmetric particle, which is stable for unbroken R-parity. In supersymmetric extensions of the standard model, in which the strong CP problem is solved via the Peccei–Quinn mechanism, the axino arises naturally as the fermionic superpartner of the axion. We compute the thermal production rate of axinos in supersymmetric QCD. Using hard thermal loop resummation, we obtain a finite result in a gauge-invariant way, which takes into account Debye screening in the hot quark–gluon–squark–gluino plasma. The relic axino abundance from thermal scatterings after inflation is evaluated. We find that thermally produced axinos could provide the dominant part of cold dark matter, for example, for an axino mass of keV and a reheating temperature of GeV.

The XMM-LSS survey. Survey design and first results

Abstract: We have designed a medium deep large area X-ray survey with XMM - the XMM Large Scale Structure survey, XMM-LSS - with the scope of extending the cosmological tests attempted using ROSAT cluster samples to two redshift bins between 0

Nuclear reaction network for primordial nucleosynthesis: a detailed analysis of rates, uncertainties and light nuclei yields

Abstract: We analyse in detail the standard primordial nucleosynthesis scenario. In particular, we discuss the key theoretical issues which are involved in a detailed prediction of light nuclide abundances, such as the weak reaction rates, neutrino decoupling and nuclear rate modelling. We also perform a new analysis of available data on the main nuclear processes entering the nucleosynthesis reaction network, with particular stress on their uncertainties as well as on their role in determining the corresponding uncertainties on light nuclide theoretical estimates. The current status of theoretical versus experimental results for 2H, 3He, 4He and 7Li is then discussed using the determination of the baryon density as obtained from cosmic microwave background anisotropies.

Quantum gravity at astrophysical distances

Abstract: Assuming that quantum Einstein gravity (QEG) is the correct theory of gravity on all length scales, we use analytical results from nonperturbative renormalization group (RG) equations as well as experimental input in order to characterize the special RG trajectory of QEG which is realized in Nature and to determine its parameters. On this trajectory, we identify a regime of scales where gravitational physics is well described by classical general relativity. Strong renormalization effects occur at both larger and smaller momentum scales. The latter lead to a growth of Newton's constant at large distances. We argue that this effect becomes visible at the scale of galaxies and could provide a solution to the astrophysical missing mass problem which does not require any dark matter. We show that an extremely weak power law running of Newton's constant leads to flat galaxy rotation curves similar to those observed in Nature. Furthermore, a possible resolution of the cosmological constant problem is proposed by noting that all RG trajectories admitting a long classical regime automatically give rise to a small cosmological constant.

Creation of a compact topologically nontrivial inflationary universe

Abstract: If inflation can occur only at the energy density V much smaller than the Planck density, which is the case for many inflationary models based on string theory, then the probability of quantum creation of a closed or an infinitely large open inflationary universe is exponentially suppressed for all known choices of the wavefunction of the universe. Meanwhile under certain conditions there is no exponential suppression for creation of topologically nontrivial compact flat or open inflationary universes. This suggests, contrary to the standard textbook lore, that compact flat or open universes with nontrivial topology should be considered a rule rather than an exception.

Brane structure from a scalar field in warped spacetime

Abstract: We deal with a scalar field coupled to gravity in five dimensions in a warped geometry. We investigate models described by potentials that drive the system to support thick brane solutions that engender internal structure. We find analytical expressions for the brane solutions, and we show that they are all linearly stable.

Chaotic inflation on the brane with induced gravity

Abstract: We study the slow-roll inflationary dynamics in a self-gravitating induced gravity braneworld model with a bulk cosmological constant. For we find important corrections to the four-dimensional Friedmann equation which bring the standard chaotic inflationary scenario into closer agreement with recent observations. For we find five-dimensional corrections to the Friedmann equation, which give the known Randall–Sundrum results for the inflationary parameters.

Testing the running of the cosmological constant with type Ia supernovae at high z

Abstract: Within the quantum field theory context the idea of a `cosmological constant' (CC) evolving with time looks quite natural as it just reflects the change of the vacuum energy with the typical energy of the universe. In a particular frame that we have considered previously, a `running CC' at low energies may arise from generic quantum effects near the Planck scale, MP, provided there is a smooth decoupling of all massive particles below MP. In this work we further develop the cosmological consequences of a `running CC' by addressing the accelerated evolution of the universe within that model. The rate of change of the CC stays slow, without fine-tuning, and is comparable to H2MP2. It can be described by a single parameter, ν, that can be determined from already planned experiments using SNe Ia at high z. The range of allowed values for ν follows mainly from nucleosynthesis restrictions. Present samples of SNe Ia can not yet distinguish between a `constant' CC or a `running' one. The numerical simulations presented in this work show that SNAP can probe the predicted variation of the CC either ruling out this idea or confirming the evolution hereafter expected.

Neutrino signatures of supernova forward and reverse shock propagation

Abstract: A few seconds after bounce in a core-collapse supernova, the shock wave passes the density region corresponding to resonant neutrino oscillations with the 'atmospheric' neutrino mass difference. The transient violation of the adiabaticity condition manifests itself in an observable modulation of the neutrino signal from a future galactic supernova. In addition to the shock wave propagation effects that were previously studied, a reverse shock forms when the supersonically expanding neutrino-driven wind collides with the slower earlier supernova ejecta. This implies that for some period the neutrinos pass two subsequent density discontinuities, giving rise to a 'double-dip' feature in the average neutrino energy as a function of time. We study this effect both analytically and numerically and find that it allows one to trace the positions of the forward and reverse shocks. We show that the energy dependent neutrino conversion probabilities allow one to detect oscillations even if the energy spectra of different neutrino flavours are the same as long as the fluxes differ. These features are observable in the signal for an inverted and in the νe signal for a normal neutrino mass hierarchy, provided the 13-mixing angle is 'large' ().

Cosmological mass limits on neutrinos, axions, and other light particles

Abstract: The small-scale power spectrum of the cosmological matter distribution, together with other cosmological data, provides a sensitive measure of the hot dark matter fraction, leading to restrictive neutrino mass limits. We extend this argument to generic cases of low-mass thermal relics. We vary the cosmic epoch of thermal decoupling, the radiation content of the universe, and the new particle's spin degrees of freedom. Our treatment covers various scenarios of active plus sterile neutrinos or axion-like particles. For three degenerate massive neutrinos, we reproduce the well-known limit of mν<0.34 eV. In a 3+1 scenario of 3 massless and 1 fully thermalized sterile neutrinos we find mν<1.0 eV. Thermally produced QCD axions must obey ma<3.0 eV, superseding limits from a direct telescope search, but leaving room for solar eV-mass axions to be discovered by the CAST experiment.

Second order general slow-roll power spectrum

Abstract: Recent combined results from the Wilkinson microwave anisotropy probe (WMAP) and Sloan digital sky survey (SDSS) provide a remarkable set of data which requires more accurate and general investigation. Here we derive formulae for the power spectrum of the density perturbations produced during inflation in the general slow-roll approximation with second order corrections. Also, using the result, we derive the power spectrum in the standard slow-roll picture with previously unknown third order corrections.

Confronting the IR fixed point cosmology with high-redshift observations

Abstract: We use high-redshift type Ia supernova and compact radio source data in order to test the infrared (IR) fixed point model of the late Universe which was proposed recently. It describes a cosmology with a time dependent cosmological constant and Newton constant whose dynamics arises from an underlying renormalization group flow near an IR-attractive fixed point. Without any fine-tuning or quintessence field it yields ?M = ?? = 1/2. Its characteristic t4/3-dependence of the scale factor leads to a distance?redshift relation whose predictions are compared both to the supernova and to the radio source data. According to the ?2 test, the fixed point model reproduces the data at least as well as the best-fit (Friedmann?Robertson?Walker) standard cosmology. Furthermore, we extend the original fixed point model by assuming that the fixed point epoch is preceded by an era with constant G and ?. By means of a Monte Carlo simulation we show that the data expected from the forthcoming SNAP satellite mission could detect the transition to the fixed point regime provided it took place at a redshift of less than about 0.5.

Seed perturbations for primordial magnetic fields from minimally supersymmetric standard model flat directions

Abstract: We demonstrate that the minimally supersymmetric standard model (MSSM) flat directions can naturally account for the seed magnetic fields in the early Universe. The non-zero vacuum expectation value of an MSSM flat direction condensate provides masses to the gauge fields and thereby breaks conformal invariance. During inflation the condensate receives spatial perturbations and SU(2) × U(1)Y gauge currents are generated together with (hyper)magnetic fields. When these long wavelength vector perturbations re-enter our horizon they give rise to U(1)em magnetic fields with an amplitude of 10−30 G, as required by the dynamo mechanism.

Evolution of second-order cosmological perturbations and non-Gaussianity

Abstract: We present a second-order gauge-invariant formalism to study the evolution of curvature perturbations in a Friedmann–Robertson–Walker universe filled by multiple interacting fluids. We apply such a general formalism to describe the evolution of the second-order curvature perturbations in the standard one-single-field inflation, in the curvaton and in the inhomogeneous reheating scenarios for the generation of the cosmological perturbations. Moreover, we provide the exact expression for the second-order temperature anisotropies on large scales, including second-order gravitational effects and extend the well known formula for the Sachs–Wolfe effect at linear order. Our findings clarify what is the exact non-linearity parameter fNL entering in the determination of higher-order statistics such as the bispectrum of Cosmic Microwave Background temperature anisotropies. Finally, we compute the level of non-Gaussianity in each scenario for the creation of cosmological perturbations.

Ultra-high energy neutrino fluxes: new constraints and implications

Abstract: We apply new upper limits on neutrino fluxes and the diffuse extragalactic component of the GeV γ-ray flux to various scenarios for ultra-high energy cosmic rays and neutrinos. As a result we find that extragalactic top-down sources cannot contribute significantly to the observed flux of highest energy cosmic rays, except if the AGASA flux normalization is too high by at least a factor of two. The explanation of the observed ultra-high energy cosmic ray flux by the decay products of Z bosons produced in ultra-high energy neutrino interaction with relic neutrino background is ruled out by recent data from GLUE and FORTE experiments, provided cosmological limits on neutrino mass and clustering apply.

Reconstructing the primordial power spectrum—a new algorithm

Abstract: We propose an efficient and model independent method for reconstructing the primordial power spectrum from cosmic microwave background (CMB) and large scale structure observations. The algorithm is based on a Monte Carlo principle and therefore very simple to incorporate into existing codes such as the Markov chain Monte Carlo one. The algorithm has been used on present cosmological data to test for features in the primordial power spectrum. No significant evidence for features is found, although there is a slight preference for an overall bending of the spectrum, as well as a decrease in power at very large scales. We have also tested the algorithm on mock high precision CMB data, calculated from models with non-scale-invariant primordial spectra. The algorithm efficiently extracts the underlying spectrum, as well as the other cosmological parameters in each case. Finally we have used the algorithm on a model where an artificial glitch in the CMB spectrum has been imposed, like the ones seen in the WMAP data. In this case it is found that, although the underlying cosmological parameters can be extracted, the recovered power spectrum can show significant spurious features, such as bending, even if the true spectrum is scale invariant.

The role of antimatter searches in the hunt for supersymmetric dark matter

Abstract: We analyse the antimatter yield of supersymmetric (SUSY) models with large neutralino annihilation cross sections. We introduce three benchmark scenarios, featuring bino, wino and higgsino-like lightest neutralinos, respectively, and we study in detail the resulting antimatter spectral features. We carry out a systematic and transparent comparison between current and future prospects for direct detection, neutrino telescopes and antimatter searches. We demonstrate that often, in the models we consider, antimatter searches are the only detection channels that already constrain the SUSY parameter space. In particular large antiprotons fluxes are expected for wino-like lightest neutralinos, while significant antideuteron fluxes result from resonantly annihilating binos. We introduce a simple and general recipe which allows us to assess the visibility of a given SUSY model at future antimatter search facilities. We provide evidence that upcoming space-based experiments, like PAMELA or AMS, are going to be, in many cases, the unique open road towards dark matter discovery.