Institute of Cosmology and Gravitation, University of Portsmouth

About: Institute of Cosmology and Gravitation, University of Portsmouth is a based out in . It is known for research contribution in the topics: Galaxy & Redshift. The organization has 297 authors who have published 1207 publications receiving 76919 citations.

Spherical collapse model with shear and angular momentum in dark energy cosmologies

Abstract: We study, for the first time, how shear and angular momentum modify typical parameters of the spherical collapse model, in dark-energy-dominated universes. In particular, we study the linear density threshold for collapse δc and the virial overdensityV for several dark energy models and its influence on the cumulative mass function. The equations of the spherical collapse are those obtained in Pace et al., who used the fully non-linear differential equation for the evolution of the density contrast derived from Newtonian hydrodynamics, and assumed that dark energy is present only at the background level. With the introduction of the shear and rotation terms, the parameters of the spherical collapse model are now mass dependant. The results of the paper show, as expected, that the new terms considered in the spherical collapse model oppose the collapse of perturbations on galactic scale giving rise to higher values of the linear overdensity parameter with respect to the non-rotating case. We find a similar effect also for the virial overdensity parameter. For what concerns the mass function, we find that its high-mass tail is suppressed, while the low-mass tail is slightly affected except in some cases, e.g. the Chaplygin gas case.

The local bias model in the large-scale halo distribution

Abstract: We explore the biasing in the clustering statistics of haloes as compared to dark matter (DM) in simulations. We look at the second- and third-order statistics at large scales of the (intermediate) MICEL 1536 simulation and also measure directly the local bias relation h = f(δ) between DM fluctuations, δ, smoothed over a top-hat radius R s at a point in the simulation and its corresponding tracer h (i.e. haloes) at the same point. This local relation can be Taylor expanded to define a linear (b 1 ) and non-linear (b 2 ) bias parameter. The values of b 1 and b 2 in the simulation vary with R s approaching a constant value around R s > 30-60 Mpc h ―1 . We use the local relation to predict the clustering of the tracer in terms of the one of DM. This prediction works very well (about per cent level) for the halo 2-point correlation ξ(r 12 ) for r 12 > 15 Mpc h ―1 , but only when we use the biasing values that we found at very large smoothing radii R s > 30-60 Mpc h ―1 . We find no effect from stochastic or next-to-leading-order terms in the f(δ) expansion. However, we do find some discrepancies in the 3-point function that needs further understanding. We also look at the clustering of the smoothed moments, the variance and skewness which are volume-average correlations and therefore include clustering from smaller scales. In this case, we find that both next-to-leading-order and discreteness corrections (to the local model) are needed at the 10-20 per cent level. Shot-noise can be corrected with a term σ 2 e /n, where σ 2 e < 1, that is, always smaller than the Poisson correction. We also compare these results with the peak-background split predictions from the measured halo mass function. We find 5-10 per cent systematic (and similar statistical) errors in the mass estimation when we use the halo model biasing predictions to calibrate the mass.

Observational constraints on the spin of the most massive black holes from radio observations

Abstract: We use recent progress in simulating the production of magnetohydrodynamic jets around black holes to derive the cosmic spin history of the most massive black holes. Our work focusses on black holes with masses ∼> 108 M⊙. Under the assumption that the efficiency of jet production is a function of spin ˆa, as given by the simulations, we can approximately reproduce the observed ‘radio loudness’ of quasars and the local radio luminosity function.Using the X-ray luminosity function and the local mass function of supermassive black holes, SMBHs, we can reproduce the individual radio luminosity functions of radio sources showing high- and low-excitation narrow emission lines. We find that the data favour spin distributions that are bimodal, with one component around spin zero and the other close to maximal spin. The ‘typical’ spin is therefore really the expectation value, lying between the two peaks. In the low-excitation galaxies, the two components have similar amplitudes, meaning approximately half of the sources have very high spins, and the other have very low spins. For the high-excitation galaxies, the amplitude of the high-spin peak is typically much smaller than that of the low-spin peak, so that most of the sources have low spins. However, a small population of near maximally spinning high-accretion rate objects is inferred. A bimodality should be seen in the radio loudness of quasars, although there are a variety of physical and selection effects that may obfuscate this feature. We predict that the low-excitation galaxies are dominated by SMBHs with masses ∼> 108 M⊙, down to radio luminosity densities ∼1021 W Hz−1 sr−1 at 1.4 GHz. Under reasonable assumptions, our model is also able to predict the radio luminosity function at z=1, and predicts it to be dominated by radio sources with high-excitation narrow emission lines above luminosity densities ∼> 1026 W Hz−1 sr−1 at 1.4 GHz, and this is in full agreement with the observations. From our parametrisation of the spin distributions of the high- and low-accretion rate SMBHs, we derive an estimate of the spin history of SMBHs, which shows a weak evolution between z=1 and 0. A larger fraction of low-redshift SMBHs have high spins compared to high-redshift SMBHs. Using the best fitting jet efficiencies there is marginal evidence for evolution in spin: the mean spin increases slightly from hˆai∼0.25 at z=1 to hˆai∼0.35 at z=0, and the fraction of SMBHs with ˆa>0.5 increases from 0.16±0.03 at z=1 to 0.24±0.09 at z=0. Our inferred spin history of SMBHs is in excellent agreement with constraints from the mean radiative efficiency of quasars, as well as the results from recent simulations of growing SMBHs. We discuss the implications in terms of accretion and SMBH mergers. We also discuss other work related to the spin of SMBHs as well as work discussing the spin of galactic black holes and their jet powers.

Van der Waals quintessence stars

Abstract: The van der Waals quintessence equation of state is an interesting scenario for describing the late universe, and seems to provide a solution to the puzzle of dark energy, without the presence of exotic fluids or modifications of the Friedmann equations. In this work, the construction of inhomogeneous compact spheres supported by a van der Waals equation of state is explored. These relativistic stellar configurations shall be denoted as van der Waals quintessence stars. Despite of the fact that, in a cosmological context, the van der Waals fluid is considered homogeneous, inhomogeneities may arise through gravitational instabilities. Thus, these solutions may possibly originate from density fluctuations in the cosmological background. Two specific classes of solutions, namely, gravastars and traversable wormholes are analyzed. Exact solutions are found, and their respective characteristics and physical properties are further explored.

Escaping the Big Rip

Abstract: We discuss dark energy models which might describe effectively the actual acceleration of the universe. More precisely, for a 4-dimensional Friedmann-Lema\^{\i}tre-Robertson-Walker (FLRW) universe we consider two situations: First of them, we model dark energy by phantom energy described by a perfect fluid satisfying the equation of state $P=(\beta-1)\rho$ (with $\beta<0$ and constant). In this case the universe reaches a ``Big Rip'' independently of the spatial geometry of the FLRW universe. In the second situation, the dark energy is described by a phantom (generalized) Chaplygin gas which violates the dominant energy condition. Contrary to the previous case, for this material content a FLRW universe would never reach a ``big rip'' singularity (indeed, the geometry is asymptotically de Sitter). We also show how this dark energy model can be described in terms of scalar fields, corresponding to a minimally coupled scalar field, a Born-Infeld scalar field and a generalized Born-Infeld scalar field. Finally, we introduce a phenomenologically viable model where dark energy is described by a phantom generalized Chaplygin gas.