Cosmological Properties Research Articles

The power of general relativity

We study the cosmological and weak-field properties of theories of gravity derived by extending general relativity by means of a Lagrangian proportional to ${R}^{1+\ensuremath{\delta}}$. This scale-free extension reduces to general relativity when $\ensuremath{\delta}\ensuremath{\rightarrow}0$. In order to constrain generalizations of general relativity of this power class, we analyze the behavior of the perfect-fluid Friedmann universes and isolate the physically relevant models of zero curvature. A stable matter-dominated period of evolution requires $\ensuremath{\delta}>0$ or $\ensuremath{\delta}<\ensuremath{-}1/4$. The stable attractors of the evolution are found. By considering the synthesis of light elements (helium-4, deuterium and lithium-7) we obtain the bound $\ensuremath{-}0.017<\ensuremath{\delta}<0.0012$. We evaluate the effect on the power spectrum of clustering via the shift in the epoch of matter-radiation equality. The horizon size at matter-radiation equality will be shifted by $\ensuremath{\sim}1%$ for a value of $\ensuremath{\delta}\ensuremath{\sim}0.0005$. We study the stable extensions of the Schwarzschild solution in these theories and calculate the timelike and null geodesics. No significant bounds arise from null geodesic effects but the perihelion precession observations lead to the strong bound $\ensuremath{\delta}=2.7\ifmmode\pm\else\textpm\fi{}4.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19}$ assuming that Mercury follows a timelike geodesic. The combination of these observational constraints leads to the overall bound $0\ensuremath{\le}\ensuremath{\delta}<7.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19}$ on theories of this type.

Is Our Universe Brany?

In brane-worlds, our universe is assumed to be a submanifold, or brane, embedded in a higher-dimensional bulk spacetime. Focusing on scenarios with a curved five-dimensional bulk spacetime, I discuss their gravitational and cosmological properties.

Crossing thew=−1barrier in the D3-brane dark energy model

We explore a possibility for the Universe to cross the $w=\ensuremath{-}1$ cosmological constant barrier for the dark energy state parameter. We consider the Universe as a slowly decaying D3-brane. The D3-brane dynamics are approximately described by a nonlocal string tachyon interaction and the backreaction of gravity is incorporated in the closed string tachyon dynamics. In a local effective approximation this model contains one phantom component and one usual field with a simple polynomial interaction. To understand cosmological properties of this system we study toy models with the same scalar fields but with modified interactions. These modifications admit polynomial superpotentials. We find restrictions on these interactions under which it is possible to reach $w=\ensuremath{-}1$ from below at large time. Explicit solutions with the dark energy state parameter crossing or not crossing the barrier $w=\ensuremath{-}1$ at large time are presented.

Host galaxies of gamma-ray bursts and their cosmological evolution

We use numerical simulations of large-scale structure formation to explore the cosmological properties of gamma-ray burst (GRB) host galaxies. Among the different subpopulations found in the simulations, we identify the host galaxies as the most efficient star-forming objects, i.e. galaxies with high specific star-formation rates. We find that the host candidates are low-mass, young galaxies with low to moderate star-formation rate. These properties are consistent with those observed in GRB hosts, most of which are subluminous, blue galaxies. Assuming that host candidates are galaxies with high star-formation rates would have given conclusions inconsistent with the observations. The specific star-formation rate, given a galaxy mass, is shown to increase as the redshift increases. The low mass of the putative hosts makes them difficult to detect with present-day telescopes and the probability density function of the specific star-formation rate is predicted to change depending on whether or not these galaxies are observed.

Cosmology of intersecting brane-world models inGauss–Bonnet gravity

We study the cosmological properties of a codimension two braneworld that sits at the intersection between two four-branes, inthe framework of six-dimensional Einstein–Gauss–Bonnetgravity. Due to contributions of the Gauss–Bonnet terms, thejunction conditions require the presence of localized energydensity on the codimension two defect. The induced metric onthis surface assumes a FRW form, with a scale factor associatedwith the position of the brane in the background; we can embed inthe codimension two defect the preferred form of energy density. We present the cosmological evolution equations forthe three-brane, showing that, for the case of pure AdS6 backgrounds, they acquire the same form as the ones for theRandall–Sundrum II model. When the background is different frompure AdS6, the cosmological behaviour is potentially modified in respect to the typical one of codimension one braneworlds. We discuss, in a particular model embedded in anAdS6 black hole, the conditions one should satisfy in order to obtain standard cosmology at late epochs.

Brane gases in the early universe: thermodynamics andcosmology

We consider the thermodynamic and cosmological properties of branegases in the early universe. Working in the low energy limit ofM-theory we assume the universe is a homogeneous but anisotropic10-torus containing wrapped 2-branes and a supergravity gas. Wedescribe the thermodynamics of this system and estimate a Hagedorntemperature associated with excitations on the branes. We investigatethe cross-section for production of branes from the thermal bath andderive Boltzmann equations governing the number of wrapped branes. Abrane gas may lead to decompactification of three spatial dimensions.To investigate this possibility we adopt initial conditions in whichwe fix the volume of the torus but otherwise assume all states areequally likely. We solve the Einstein–Boltzmann equationsnumerically, to determine the number of dimensions with no wrappedbranes at late times; these unwrapped dimensions are expected todecompactify. Finally we consider holographic bounds on the initialvolume, and find that for allowed initial volumes all branes typicallyannihilate before freeze-out can occur.

Trans-Planckian dark energy?

It has recently been proposed by Mersini et al. 01, Bastero-Gil and Mersini 02 that the dark energy could be attributed to the cosmological properties of a scalar field with a non-standard dispersion relation that decreases exponentially at wave-numbers larger than Planck scale (k_phys > M_Planck). In this scenario, the energy density stored in the modes of trans-Planckian wave-numbers but sub-Hubble frequencies produced by amplification of the vacuum quantum fluctuations would account naturally for the dark energy. The present article examines this model in detail and shows step by step that it does not work. In particular, we show that this model cannot make definite predictions since there is no well-defined vacuum state in the region of wave-numbers considered, hence the initial data cannot be specified unambiguously. We also show that for most choices of initial data this scenario implies the production of a large amount of energy density (of order M_Planck^4) for modes with momenta of order M_Planck, far in excess of the background energy density. We evaluate the amount of fine-tuning in the initial data necessary to avoid this back-reaction problem and find it is of order H/M_Planck. We also argue that the equation of state of the trans-Planckian modes is not vacuum-like. Therefore this model does not provide a suitable explanation for the dark energy.

Global monopole in general relativity

We consider the gravitational properties of a global monopole on the basis of the simplest Higgs scalar triplet model in general relativity. We begin with establishing some common features of hedgehog-type solutions with a regular center, independent of the choice of the symmetry-breaking potential. There are six types of qualitative behaviors of the solutions; we show, in particular, that the metric can contain at most one simple horizon. For the standard Mexican hat potential, the previously known properties of the solutions are confirmed and some new results are obtained. Thus, we show analytically that solutions with the monotonically growing Higgs field and finite energy in the static region exist only in the interval 1 < λ < 3, where λ is the squared energy of spontaneous symmetry breaking in Planck units. The cosmological properties of these globally regular solutions apparently favor the idea that the standard Big Bang might be replaced with a nonsingular static core and a horizon appearing as a result of some symmetry-breaking phase transition at the Planck energy scale. In addition to the monotonic solutions, we present and analyze a sequence of families of new solutions with the oscillating Higgs field. These families are parametrized by n, the number of knots of the Higgs field, and exist for λ < γn=6/[(2n + 1)(n + 2)]; all such solutions possess a horizon and a singularity beyond it.

Size of the neutralino–nucleon cross-section in the light of a new determination of the pion–nucleon sigma term

We discuss the implications of a new determination of the pion–nucleon sigma term for the evaluation of the neutralino–nucleon cross-section, and, in turn, for detection rates for relic neutralinos in WIMP direct experiments and in some of the indirect searches. It is shown that the new range for the pion–nucleon sigma term, taken at its face value, favours values of the neutralino–nucleon cross-sections which are sizeably larger than some of the current estimates. Implications for neutralino cosmological properties are derived.

Blazars from the CLASS Survey

AbstractThis paper presents preliminary results from the CLASS blazar sample; the aim is to study the cosmological properties of radio selected BL Lacs and their closest relatives, the other low radio luminosity flat spectrum sources.

Probing the supersymmetric parameter space by weakly interacting massive particle direct detection

We discuss to what extent the present experiments of direct search for weakly interacting massive particles (WIMPs), when interpreted in terms of relic neutralinos, probe interesting regions of the supersymmetric parameter space, which are also being progressively explored at accelerators. Our analysis is performed in a number of different supersymmetric schemes. We derive the relevant neutralino cosmological properties, locally and on the average in the universe. We prove that part of the supersymmetric (SUSY) configurations probed by current WIMP experiments entails relic neutralinos of cosmological interest. The main astrophysical and particle physics uncertainties, relevant for a proper comparison between theory and experimental data, are stressed and taken into account.

Brane world linearized cosmic string gravity

The gravitational properties of cosmic strings in the Randall–Sundrum brane world scenario are investigated. Using a gauge in which the brane remains straight, the leading order corrections to the metric on the brane are determined. In contrast to their non-brane equivalents, these strings have an attractive 1/ r 2 potential and a radially dependent deficit angle. These two effects alter many cosmological properties of the string, such as the formation of double images. As a result of the attractive force the string will collect matter as it moves through space.

Properties of cosmologies with dynamical pseudo Nambu-Goldstone bosons

We study observational constraints on cosmological models with a quintessence field in the form of a dynamical pseudo Nambu-Goldstone boson. After reviewing the properties of the solutions, from a dynamical systems phase space analysis, we consider the constraints on parameter values imposed by luminosity distances from the 60 type Ia supernovae published by Perlmutter et al., and also from gravitational lensing statistics of distant quasars. In the case of the type Ia supernovae we explicitly allow for the possibility of evolution of the peak luminosities of the supernovae sources, using simple empirical models which have been recently discussed in the literature. We find weak evidence to suggest that the models with supernovae evolution fit the data better in the context of the quintessence models in question. If source evolution is a reality then the greatest challenge facing these models is the tension between the current value of the expansion age, ${H}_{0}{t}_{0}$, and the fraction of the critical energy density, ${\ensuremath{\Omega}}_{\ensuremath{\varphi}0}$, corresponding to the scalar field. Nonetheless there are ranges of the free parameters which fit all available cosmological data.

Relic neutralinos — update on neutralino-nucleon scalar cross-section

We discuss the effect induced on the neutralino-nucleon scalar cross-section by the present uncertainties in the values of the quark masses and of the quark scalar densities in the nucleon. We examine the implications of this aspect on the determination of the neutralino cosmological properties, as derived from measurements of WIMP direct detection. We show that, within current theoretical uncertainties, the DAMA annual modulation data are compatible with a neutralino as a major dark matter component, to an extent which is even larger than the one previously derived.

Implications for relic neutralinos of the theoretical uncertainties in the neutralino–nucleon cross section

We discuss the effect induced on the neutralino–nucleon cross section by the present uncertainties in the values of the quark masses and of the quark scalar densities in the nucleon. We examine the implications of this aspect on the determination of the neutralino cosmological properties, as derived from measurements of WIMP direct detection. We show that, within current theoretical uncertainties, the DAMA annual modulation data are compatible with a neutralino as a major dark matter component, to an extent which is even larger than the one previously derived. We also comment on implications of the mentioned uncertainties for experiments of indirect dark matter detection.

Supersymmetric dark matter — direct searches

The sensitivities of the experiments of direct search for WIMPs have remarkably improved in recent years, allowing now the exploration of sizeable regions of the physical parameter space of specific particle candidates for dark matter. This is the case of the neutralino for which some direct detection experiments are already capable of investigating significant features in domains of the supersymmetric parameter space. Some of these configurations, which are also under current exploration at LEP2, show interesting astrophysical and cosmological properties of the neutralino as the dark matter component of our Galaxy.

The sedentary multifrequency survey - I. Statistical identification and cosmological properties of high-energy peaked BL Lacs

We have assembled a multifrequency data base by cross-correlating the NRAO VLA Sky Survey (NVSS) catalogue of radio sources with the RASSBSC list of soft X-ray sources, obtaining optical magnitude estimates from the Palomar and UK Schmidt surveys as provided by the Automated Plate Measurement (APM) and COSMOS on-line services. By exploiting the nearly unique broad-band properties of high-energy peaked BL Lacs (HBL), we have statistically identified a sample of 218 objects that is expected to include about 85 per cent of BL Lacs and that is therefore several times larger than all other published samples of HBLs. Using a subset (155 objects) that is radio-flux-limited and statistically well-defined, we have derived the V/Vm distribution and the log N—log S of extreme HBLs (fx/fr3×10−10 erg cm−2 s−1 Jy−1) down to 3.5 mJy. We find that the log N—log S flattens around 20 mJy and that 〈V/Vm〉=0.42±0.02. This extends to the radio band earlier results, based on much smaller X-ray-selected samples, about the anomalous cosmological observational properties of HBLs. A comparison with the expected radio log N—log S of all BL Lacs (based on a beaming model) shows that extreme HBLs make up roughly 2 per cent of the BL Lac population, independently of radio flux. This result, together with the flatness of the radio log N—log S at low fluxes, is in contrast with the predictions of a recent model that assumes an anticorrelation between peak frequency and bolometric luminosity. This scenario would in fact result in an increasing dominance of HBLs at lower radio fluxes; an effect that, if at all present, must start at fluxes fainter than our survey limit. The extreme fx/fr flux ratios and high X-ray fluxes of these BL Lacs makes them good candidate TeV sources; some of the brighter (and closer) ones are possibly detectable with the current generation of Cerenkov telescopes. Statistical identification of sources based on their location in multiparameter space, of the kind described here, will have to become commonplace with the advent of the many large, deep surveys at various frequencies currently scheduled or under construction.

Gravitating brane systems: Some general theorems

Multidimensional gravity interacting with intersecting electric and magnetic p-branes is considered for fields depending on a single variable. Some general features of the system behavior are revealed without solving the field equations. Thus, essential asymptotic properties of isotropic cosmologies are indicated for different signs of spatial curvature; a no-hair-type theorem and a single-time theorem for black holes are proved (the latter makes sense in models with multiple time coordinates). The validity of the general observations is verified for a class of exact solutions known for the cases when certain vectors, built from the input parameters of the model, are either orthogonal in minisuperspace, or form mutually orthogonal subsystems. From the nonexistence of Lorentzian wormholes, a universal restriction is obtained, applicable to orthogonal or block-orthogonal subsystems of any p-brane system.

Neutralino properties from a possible modulation signal in WIMP direct search

We report the main results of a paper by A. Bottino, F. Donato, N. Fornengo and S. Scopel, where the properties of neutralino are analysed under the hypothesis that some preliminary experimental results of the DAMA/NaI Collaboration may be indicative of a yearly modulation effect. The relevant supersymmetric configurations are analysed and the possibility of their investigation by indirect search for relic neutralinos and by accelerator measurements are discussed. It is shown that some of the configurations singled out by the DAMA/NaI results would have cosmological properties compatible with a neutralino as a dominant component of cold dark matter (on the average in the Universe and in our galactic halo).

The REX Survey: A Search for Radio‐emitting X‐Ray Sources

We present the scientific goals, strategy, and first results of the REX project, an effort aimed at creating a sizable and statistically complete sample of radio-emitting X-ray sources (REXs) using the available data from a VLA survey (NVSS) and the ROSAT PSPC archive. Through a positional cross-correlation of the two data sets, we have derived a sample of about 1600 REXs. Among the 393 REXs identified so far (either from literature or from our own spectroscopic observations) a high fraction are active galactic nuclei (AGNs; about 60%-80%), typically radio-loud QSOs, and BL Lac objects. The remaining sources are galaxies, typically radio galaxies isolated or in clusters. Thanks to the low flux limits in the radio (5 mJy at 1.4 GHz) and in the X-ray band (~5×10−14 ergs s-1 cm-2, 0.5-2.0 keV) and the large area of sky covered by the survey (2183 deg2), we intend to derive a new complete and unbiased sample of BL Lac objects that will contain both radio-selected (RBL) and X-ray-selected (XBL) objects. In this way, the apparent dichotomy resulting from the current samples of BL Lac objects can be directly analyzed in a unique sample. Moreover, the high number of BL Lac objects expected in the REX sample (~200) will allow an accurate estimate of their statistical properties, such as the X-ray, radio, and optical luminosity functions and the cosmological evolution. For these reasons, the REX sample will be a powerful tool to accurately test the current theoretical models proposed for BL Lac objects. To date, we have discovered 15 new BL Lac objects and 11 BL Lac candidates with optical properties intermediate between those of a typical elliptical galaxy and those of a typical BL Lac object. These objects could harbor sources of nonthermal continuum in their nuclei, and if confirmed they could represent the faint tail of the BL Lac population. The existence of such weak BL Lac objects has been matter of discussion in recent literature and could lead to a reassessment of the defining criteria of a BL Lac object, and consequently to a revision of their cosmological and statistical properties. Finally, the sample of ~800 emission-line AGNs generated by the REX survey will be useful in addressing many of the open questions regarding AGN phenomenology, such as the relationship between radio-loud and radio-quiet AGNs.