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A recent discovery in 2009 by Banados, Silk and West (BSW), which generated a lot of interest, involves the arbitrary high center-of-mass (c.m.) energies for free particle collisions at the horizon of an extreme kenblack hole when one of the free particles has a critical value of the angular momentum. In light of this we consider the rotating Ken-like black hole solution in Brans-Dicke theory and study the motion of scalar test charges in the vicinity of the black hole horizon. We show that the interaction of the test scalar charges with the background scalar field in this spacetime suppresses the c.m. energy for collisions occurring near the event horizon, and the value of the c.m. energy there, is finite inespective of whether the black hole is extreme or not and its value is also independent of the angular momenta of the colliding test charges. [ABSTRACT FROM AUTHOR]

The phenomenon discovered in 2009 by Bañados, Silk and West where particle collisions can achieve arbitrary high center-of-mass (c.m.) energies close to the event horizon of an extreme Kerr black hole, has generated a lot of interest. Although rotation seemed to be an essential requirement, it was later shown that arbitrary high energies can also be achieved for collisions between radially moving particles near the horizon of the electrically charged extreme Reissner-Nordström black hole. Recently Patil and Joshi claimed that instead of spinning up the black hole one can also crank up the c.m. energy of particle collisions by "charging up" a static black hole with a massless scalar field. In this regard they showed that infinite energies can be attained in the vicinity of the naked singularity of the Janis-Newman-Wincour (JNW) spacetime, which contains a massless scalar field that also becomes infinite at the position of the curvature singularity. In this study we show that Patil and Joshi's claim does not apply for other static black hole systems endowed with a massless scalar field. In particular we consider the well-known Bekenstein black hole and the recently discovered Martinez-Troncoso-Zanelli black hole, and show that the expression of the c.m. energy for particle collisions near the event horizons of these black holes is no different than the corresponding case with vanishing scalar field represented by the Schwarzschild solution. Moreover by studying the motion of scalar test charges that interact with the background scalar field in these black hole spacetimes we show that the resulting c.m. energies are even smaller than in the case of free particles. This shows that the infinite energies obtained by Patil and Joshi may not be due to the fact that the black hole contains a massless scalar field, but may be instead related to the geometry of the naked singularity in the JNW spacetime. An analogous case of infinite c.m. energy in the vicinity of a naked singularity is also presented. [ABSTRACT FROM AUTHOR]

Despite the great observational success of the standard cosmological model some discrepancies in the inferred parameter constraints have manifested among a number of cosmological data sets. These include a tension between the expansion rate of our Cosmos as inferred from the cosmic microwave background (CMB) and as found from local measurements, the preference for an enhanced amplitude of CMB lensing, a somewhat low quadrupole moment of the CMB fluctuations as well as a preference for a lower amplitude of matter fluctuations in large-scale structure surveys than inferred from the CMB. We analyze these observational tensions under the addition of spatial curvature and a free CMB background temperature that may deviate from its locally measured value. With inclusion of these parameters, we observe a trend in the parameter constraints from CMB and baryon acoustic oscillation data towards an open and hotter universe with larger current expansion rate, standard CMB lensing amplitudes, lower amplitude of matter fluctuations, and marginally lower CMB quadrupole moment, consistently reducing the individual tensions among the cosmological data sets. Combining this data with local distance measurements, we find a preference for an open and hotter universe beyond the 99.7% confidence level. Finally, we briefly discuss a local void as a possible source for a deviation of the locally measured CMB temperature from its background value and as mimic of spatial curvature for CMB photons. This interpretation implies a ~20 % underdensity in our local neighborhood of~10-100 Mpc in diameter, which is well within cosmic variance. [ABSTRACT FROM AUTHOR]

We examine the extent to which primordial black holes (PBHs) can constitute the observed dark matter while also giving rise to the measured matter-antimatter asymmetry and account for the observed baryon abundance through asymmetric Hawking radiation generated by a derivative coupling of curvature to the baryon-lepton current. We consider both broad and monochromatic mass spectra for this purpose. For the monochromatic spectrum we find that the correct dark matter and baryon energy densities are recovered for peak masses of the spectrum of Mpk≥1012 kg whereas for the broad case the observed energy densities can be reproduced regardless of peak mass. Adopting some simplifications for the early-time expansion history as a first approximation, we also find that the measured baryon asymmetry can be recovered within an order of magnitude. We argue furthermore that the correct value of the baryon-lepton yield can in principle be retrieved for scenarios where a significant amount of the radiation is produced by PBH decay during or after reheating, as is expected when the decaying PBHs also cause reheating, or when an early matter-dominated phase is considered. We conclude from this first analysis that the model merits further investigation. [ABSTRACT FROM AUTHOR]

Cosmic microwave background (CMB) lensing is an integrated effect whose kernel is greater than half the peak value in the range 1

We elucidate the importance of the consistent treatment of gravity-model specific nonlinearities when estimating the growth of cosmological structures from redshift space distortions (RSD). Within the context of standard perturbation theory (SPT), we compare the predictions of two theoretical templates with redshift space data from COLA (comoving Lagrangian acceleration) simulations in the normal branch of DGP gravity (nDGP) and general relativity (GR). Using COLA for these comparisons is validated using a suite of full N-body simulations for the same theories. The two theoretical templates correspond to the standard general relativistic perturbation equations and those same equations modeled within nDGP. Gravitational clustering nonlinear effects are accounted for by modeling the power spectrum up to one-loop order and redshift space clustering anisotropy is modeled using the Taruya, Nishimichi and Saito (TNS) RSD model. Using this approach, we attempt to recover the simulation's fiducial logarithmic growth parameter f. By assigning the simulation data with errors representing an idealized survey with a volume of 10 Gpc³/h³, we find the GR template is unable to recover fiducial f to within 1σ at z=1 when we match the data up to kmax=0.195h/Mpc. On the other hand, the DGP template recovers the fiducial value within 1σ. Further, we conduct the same analysis for sets of mock data generated for generalized models of modified gravity using SPT, where again we analyze the GR template's ability to recover the fiducial value. We find that for models with enhanced gravitational nonlinearity, the theoretical bias of the GR template becomes significant for stage IV surveys. Thus, we show that for the future large data volume galaxy surveys, the self-consistent modeling of non-GR gravity scenarios will be crucial in constraining theory parameters. [ABSTRACT FROM AUTHOR]

We use a set of N-body simulations employing a modified gravity (MG) model with Vainshtein screening to study matter and halo hierarchical clustering. As test-case scenarios we consider two normal branch Dvali-Gabadadze-Porrati (nDGP) gravity models with mild and strong growth rate enhancement. We study higher-order correlation functions ξn(R) up to n=9 and associated reduced cumulants Sn(R)≡ξn(R)/σ(R)2n-2. We find that the matter probability distribution functions are strongly affected by the fifth force on scales up to 50h-1 Mpc, and the deviations from general relativity (GR) are maximized at z=0. For reduced cumulants Sn, we find that at small scales R ≤ 6h-1 Mpc the MG is characterized by lower values, with the deviation growing from 7% in the reduced skewness up to even 40% in S5. To study the halo clustering we use a simple abundance matching and divide haloes into thee fixed number density samples. The halo two-point functions are weakly affected, with a relative boost of the order of a few percent appearing only at the smallest pair separations (r ≤ 5h-1 Mpc). In contrast, we find a strong MG signal in Sn(R)'s, which are enhanced compared to GR. The strong model exhibits a >3σ level signal at various scales for all halo samples and in all cumulants. In this context, we find that the reduced kurtosis to be an especially promising cosmological probe of MG. Even the mild nDGP model leaves a 3σ imprint at small scales R ≤ 3h-1 Mpc, while the stronger model deviates from a GR signature at nearly all scales with a significance of >5σ. Since the signal is persistent in all halo samples and over a range of scales, we advocate that the reduced kurtosis estimated from galaxy catalogs can potentially constitute a strong MG-model discriminatory as well as GR self-consistency test. [ABSTRACT FROM AUTHOR]

Future galaxy clustering surveys will probe small scales where nonlinearities become important. Since the number of modes accessible on intermediate to small scales is very high, having a precise model at these scales is important especially in the context of discriminating alternative cosmological models from the standard one. In the mildly nonlinear regime, such models typically differ from each other, and galaxy clustering data will become very precise on these scales in the near future. As the observable quantity is the angular power spectrum in redshift space, it is important to study the effects of nonlinear density and redshift space distortion (RSD) in the angular power spectrum. We compute nonlinear contributions to the angular power spectrum using a flat-sky approximation, and compare the results of different perturbative and nonperturbative approaches. We find that the Taruya-Nishimichi-Saito (TNS) perturbative approach is significantly closer to the comoving Lagrangian acceleration approximation than Eulerian or partially resummed-Lagrangian one-loop approximations, effective field theory of large scale structure or a halofit-inspired model. However, none of these prescriptions agree with each other in the nonlinear regime. A surprising and new result of the present analysis is that for narrow redshift bins, Δz≲0.01, the angular power spectrum acquires nonlinear contributions on all scales, right down to ℓ=2, and is hence not a reliable tool at this time. To overcome this problem, we need to model nonlinear RSD terms, for example as TNS does, but for a matter power spectrum that remains reasonably accurate well into the deeply nonlinear regime, such as halofit. [ABSTRACT FROM AUTHOR]