Cosmologies with variable parameters and dynamical cosmon: implications on the cosmic coincidence problem

Dynamical dark energy (DE) has been proposed to explain various aspects of the cosmological constant (CC) problem(s). For example, it is very difficult to accept that a strictly constant Lambda-term constitutes the ultimate explanation for the DE in our Universe. It is also hard to acquiesce in the idea that we accidentally happen to live in an epoch where the CC contributes an energy density value right in the ballpark of the rapidly diluting matter density. It should perhaps be more plausible to conceive that the vacuum energy, is actually a dynamical quantity as the Universe itself. More generally, we could even entertain the possibility that the total DE is in fact a mixture of vacuum energy and other dynamical components (e.g. fields, higher order terms in the effective action etc) which can be represented collectively by an effective entity X (dubbed the ``cosmon''). The ``cosmon'', therefore, acts as a dynamical DE component different from the vacuum energy. While it can actually behave phantom-like by itself, the overall DE fluid may effectively appear as standard quintessence, or even mimic at present an almost exact CC behavior. Thanks to the versatility of such cosmic fluid we can show that a composite DE system of this sort (``LXCDM'') may have a key to resolving the mysterious coincidence problem.Comment: LaTeX, 13 pages, 5 figure

Constraining the evolutionary history of Newton's constant with gravitational wave observations

Space-borne gravitational wave detectors, such as the proposed Laser Interferometer Space Antenna, are expected to observe black hole coalescences to high redshift and with large signal-to-noise ratios, rendering their gravitational waves ideal probes of fundamental physics. The promotion of Newton's constant to a time-function introduces modifications to the binary's binding energy and the gravitational wave luminosity, leading to corrections in the chirping frequency. Such corrections propagate into the response function and, given a gravitational wave observation, they allow for constraints on the first time-derivative of Newton's constant at the time of merger. We find that space-borne detectors could indeed place interesting constraints on this quantity as a function of sky position and redshift, providing a {\emph{constraint map}} over the entire range of redshifts where binary black hole mergers are expected to occur. A LISA observation of an equal-mass inspiral event with total redshifted mass of 10^5 solar masses for three years should be able to measure $\dot{G}/G$ at the time of merger to better than 10^(-11)/yr.Comment: 11 pages, 2 figures, replaced with version accepted for publication in Phys. Rev. D

Non-monotonic orbital velocity profiles around rapidly rotating Kerr-(anti-)de Sitter black holes

It has been recently demonstrated that the orbital velocity profile around Kerr black holes in the equatorial plane as observed in the locally non-rotating frame exhibits a non-monotonic radial behaviour. We show here that this unexpected minimum-maximum feature of the orbital velocity remains if the Kerr vacuum is generalized to the Kerr-de Sitter or Kerr-anti-de Sitter metric. This is a new general relativity effect in Kerr spacetimes with non-vanishing cosmological constant. Assuming that the profile of the orbital velocity is known, this effect constrains the spacetime parameters.Comment: 9 pages, 4 figures, accepted for Class. Quant. Gra

Dark Viscous Fluid coupled with Dark Matter and future singularity

We study effects of viscous fluid coupled with dark matter in our universe. We consider bulk viscosity in the cosmic fluid and we suppose the existence of a coupling between fluid and dark matter, in order to reproduce a stable de Sitter universe protected against future-time singularities. More general inhomogeneous fluids are studied related to future singularities.Comment: 11 page

What is there in the black box of dark energy: variable cosmological parameters or multiple (interacting) components?

The coincidence problems and other dynamical features of dark energy are studied in cosmological models with variable cosmological parameters and in models with the composite dark energy. It is found that many of the problems usually considered to be cosmological coincidences can be explained or significantly alleviated in the aforementioned models.Comment: 6 pages, 1 figure, talk given at IRGAC2006 (Barcelona, July 11-15, 2006), to appear in J. Phys.

XMM-{\em Newton} and FUSE Tentative Evidence for a WHIM filament along the Line of Sight to PKS~0558-504

We present a possible OVIII X-ray absorption line at $z=0.117 \pm 0.001$ which, if confirmed, will be the first one associated with a broad HI Ly$\beta$ (BLB: FWHM=$160^{+50}_{-30}$ km s$^{-1}$) absorber. The absorber lies along the line of sight to the nearby ($z=0.1372$) Seyfert 1 galaxy PKS~0558-504, consistent with being a WHIM filament. The X-ray absorber is marginally detected in two independent XMM-Newton spectra of PKS~0558-504, a long $\sim 600$ ks Guest-Observer observation and a shorter, $\sim 300$ ks total, calibration observation, with a combined single line statistical significance of 2.8$\sigma$ (2.7$\sigma$ and 1.2$\sigma$ in the two spectra, respectively). When fitted with our self-consistent hybrid-photoionization WHIM models, the combined XMM-{\em Newton} spectrum is consistent with the presence of OVIII K$\alpha$ at $z=(0.117 \pm 0.001)$. This model gives best fitting temperature and equivalent H column density of the absorber of log$T=6.56_{-0.17}^{+0.19}$ K, and logN$_H=(21.5 \pm 0.3) (Z/Z_{0.01\odot})^{-1}$ cm$^{-2}$. The statistical sigificance of this single X-ray detection is increased by the detection of broad and complex HI Ly$\beta$ absorption in archival FUSE spectra of PKS~0558-504, at redshifts $z=0.1183 \pm 0.0001$ consistent with the best-fitting redshift of the X-ray absorber. The single line statistical significance of this line is 4.1$\sigma$ (3.7$\sigma$ if systematics are considered), and thus the combined (HI+OVIII) statistical significance of the detection is of 5.0$\sigma$. The detection of both metal and H lines at a consistent redshift, in this hot absorbing system, allows us to speculate on its metallicity. By associating the bulk of the X-ray absorber with the BLB line detected in the FUSE spectrum at $z_{BLB}=0.1183 \pm 0.0001$, we obtain a metallicity of 1-4\% Solar.Comment: 37 pages, 12 figures, 3 Tables. Accepted for publication by the ApJ

Long-lived Charginos in the Focus-point Region of the MSSM Parameter Space

We analyse the possibility to get light long-lived charginos within the framework of the MSSM with gravity mediated SUSY breaking. We find out that this possibility can be realized in the so-called focus-point region of parameter space. The mass degeneracy of higgsino-like chargino and two higgsino-like neutralinos is the necessary condition for a long lifetime. It requires the fine-tuning of parameters, but being a single additional constraint in the whole parameter space it can be fulfilled in the Constrained MSSM along the border line where radiative electroweak symmetry breaking fails. In a narrow band close to the border line the charginos are long-lived particles. The cross-sections of their production and co-production at the LHC via electroweak interaction reach a few tenth of pb.Comment: LaTeX, 11 pages, 11 eps figure

Simulations of Cold Electroweak Baryogenesis: Finite time quenches

The electroweak symmetry breaking transition may supply the appropriate out-of-equilibrium conditions for baryogenesis if it is triggered sufficiently fast. This can happen at the end of low-scale inflation, prompting baryogenesis to occur during tachyonic preheating of the Universe, when the potential energy of the inflaton is transfered into Standard Model particles. With the proper amount of CP-violation present, the observed baryon number asymmetry can be reproduced. Within this framework of Cold Electroweak Baryogenesis, we study the dependence of the generated baryon asymmetry on the speed of the quenching transition. We find that there is a separation between ``fast'' and ``slow'' quenches, which can be used to put bounds on the allowed Higgs-inflaton coupling. We also clarify the strong Higgs mass dependence of the asymmetry reported in a companion paper (hep-ph/0604263).Comment: 18 pages, 20 figure

A New Cosmological Model of Quintessence and Dark Matter

We propose a new class of quintessence models in which late times oscillations of a scalar field give rise to an effective equation of state which can be negative and hence drive the observed acceleration of the universe. Our ansatz provides a unified picture of quintessence and a new form of dark matter we call "Frustrated Cold Dark Matter" (FCDM). FCDM inhibits gravitational clustering on small scales and could provide a natural resolution to the core density problem for disc galaxy halos. Since the quintessence field rolls towards a small value, constraints on slow-roll quintessence models are safely circumvented in our model.Comment: Revised. Important new results added in response to referees comment

Stability of a vacuum nonsingular black hole

This is the first of series of papers in which we investigate stability of the spherically symmetric space-time with de Sitter center. Geometry, asymptotically Schwarzschild for large $r$ and asymptotically de Sitter as $r\to 0$, describes a vacuum nonsingular black hole for $m\geq m_{cr}$ and particle-like self-gravitating structure for $m < m_{cr}$ where a critical value $m_{cr}$ depends on the scale of the symmetry restoration to de Sitter group in the origin. In this paper we address the question of stability of a vacuum non-singular black hole with de Sitter center to external perturbations. We specify first two types of geometries with and without changes of topology. Then we derive the general equations for an arbitrary density profile and show that in the whole range of the mass parameter $m$ objects described by geometries with de Sitter center remain stable under axial perturbations. In the case of the polar perturbations we find criteria of stability and study in detail the case of the density profile $\rho(r)=\rho_0 e^{-r^3/r_0^2 r_g}$ where $\rho_0$ is the density of de Sitter vacuum at the center, $r_0$ is de Sitter radius and $r_g$ is the Schwarzschild radius.Comment: 18 pages, 8 figures, submitted to "Classical and Quantum Gravity