One of the most dramatic consequences of low-scale (~1 TeV) quantum gravity in models with large or warped extra dimension(s) is possibly copious production of mini black holes at future colliders. Hawking radiation of these black holes is expected to be constrained mainly to our three-dimensional world and results in rich phenomenology. In this talk we discuss selected aspects of mini black hole phenomenology, such as production at colliders, black-hole decay properties, and Hawking radiation as a sensitive probe of the dimensionality of extra space.

We describe a program that we have embarked on to estimate the spins of stellar-mass black holes in x-ray binaries. We fit the continuum x-ray spectrum of the radiation from the accretion disk using the standard thin disk model, and extract the dimensionless spin parameter a* = a/M of the black hole as a parameter of the fit. We have obtained results on three systems, 4U 1543–47 (a* = 0.7−0.85), GRO J1655−40 (0.65−0.8), and GRS 1915+105 (0.98−1), and have nearly completed the analysis of two additional systems. We anticipate expanding the sample of spin estimates to about a dozen over the next several years.

How did the mass of 108−1010M⊙ super-massive black holes at the center of massive galaxies in the local Universe build up? Did the bulk of the growth happen in an optically luminous AGN phase? Or did a substantial fraction of SMBH growth occur in a dusty, obscured phase, visible as a luminous infrared galaxy? Has there been substantial SMBH growth in a low luminosity or radiatively inefficient regime after the more luminous AGN phase? These are particularly important questions, given the tight relationship between the mass of galaxy bulges and their SMBHs, suggesting that the formation and evolution of galaxies and SMBHs are intimately linked. We use the multi-wavelength data in the NDWFS Boötes field to address this issue. We have performed an x-ray stacking analysis of ~20,000 red galaxies at z = 0.2−1 to show that the average nuclear accretion rates in these sources are low and decreasing with time. Given the long timescale, significant SMBH mass growth could occur in this regime. We also investigate the nature of an extreme, obscured population of AGN-dominated luminous infrared galaxies which are likely to host SMBHs undergoing a period of rapid and substantial growth.

In recent years deep x-ray and infrared surveys have provided an efficient way to find accreting supermassive black holes, otherwise known as active galactic nuclei (AGN), in the young universe. Such surveys can, unlike optical surveys, find AGN obscured by high column densities of gas and dust. In those cases, deep optical data show only the host galaxy, which can then be studied in greater detail than in unobscured AGN. Some years ago the hard spectrum of the x-ray “background” suggested that most AGN were obscured. Now GOODS, MUSYC, COSMOS, and other surveys have confirmed this picture and given important quantitative constraints on AGN demographics. Specifically, we show that most AGN are obscured at all redshifts and the amount of obscuration depends on both luminosity and redshift, at least out to redshift z ~ 2, the epoch of substantial black holes and galaxy growth. Larger-area deep infrared and hard x-ray surveys will be needed to reach higher redshifts and to fully probe the co-evolution of galaxies and black holes.

Coalescing black-hole binaries are expected to be the strongest sources of gravitational waves for ground-based interferometers, as well as the space-based interferometer LISA. Recent progress in numerical relativity now makes it possible to calculate the waveforms from the strong-field dynamical merger, and is revolutionizing our understanding of these systems. We review these dramatic developments, emphasizing applications to issues in gravitational wave observations. These new capabilities also make possible accurate calculations of the recoil or kick imparted to the final remnant black hole when the merging components have unequal masses, or unequal or unaligned spins. We highlight recent work in this area, focusing on results of interest to astrophysics.

The cosmological evolution of supermassive black holes (SMBHs) seems to be intimately linked to their host galaxies. Active galactic nuclei (AGN) can be probed by deep x-ray surveys. We review results from selected large x-ray samples including the first results from the XMM-Newton COSMOS survey. A new picture arises from the fact that high-luminosity AGN grow earlier than low-luminosity AGN. In particular, the space density of low-luminosity AGN exhibits a significant decline for redshifts above z = 1. This “anti-hierarchical” growth scenario of SMBHs can be explained by two modes of accretion with different efficiency. The population of Compton-thick sources plays a key role in our understanding of the BH growth history. Their space density and redshift distribution is relevant to estimate the SMBH mass function. A comparison with the relic SMBH mass distribution in the local Universe constrains the average radiative efficiency and Eddington ratio of the accretion. We discuss a new synthesis model of Compton-thin and Compton-thick sources that is in concordance with deep x-ray observations, and in particular predicts the right level of contribution of the Compton-thick source population observed in the Chandra Deep Field South observations as well as the first INTEGRAL and Swift catalogs of AGN. Currently, one of the most important problems is the evolution of obscuration with redshift.

A brief overview of the methods commonly used to determine or estimate the black-hole mass in quiescent or active galaxies is presented and it is argued that the use of mass-scaling relations is both a reliable and the preferred method to apply to large samples of distant quasars. The method uses spectroscopic measurements of a broad emission-line width and continuum luminosity and currently has a statistical 1σ uncertainty in the absolute mass values of about a factor of 4. Potentially, this accuracy can be improved in the future. When applied to large samples of distant quasars it is evident that the black-hole masses are very large, of order 1 to 10 billion M⊙, even at the highest redshifts of 4 to 6. The black holes must build up their mass very fast in the early universe. Yet they do not grow much larger than that: a maximum mass of ~1010M⊙ is also observed. Preliminary mass functions of active black holes are presented for several quasar samples, including the Sloan Digital Sky Survey. Finally, common concerns related to the application of the mass-scaling relations, especially for high redshift quasars, are briefly discussed.

If massive black holes (BHs) are ubiquitous in galaxies and galaxies experience multiple mergers during their cosmic assembly, then BH binaries should be common, albeit temporary, features of most galactic bulges. Observationally, the paucity of active BH pairs points toward binary lifetimes far shorter than the Hubble time, indicating rapid inspiral of the BHs down to the domain where gravitational waves lead to their coalescence. Here, we review a series of studies on the dynamics of massive BHs in gas-rich galaxy mergers that underscore the vital role played by a cool, gaseous component in promoting the rapid formation of the BH binary. The BH binary is found to reside at the center of a massive self-gravitating nuclear disk resulting from the collision of the two gaseous disks present in the mother galaxies. Hardening by gravitational torques against gas in this grand disk is found to continue down to sub-parsec scales. The eccentricity decreases with time to zero and when the binary is circular, accretion sets in around the two BHs. When this occurs, each BH is endowed with its own small-size (≲ 0.01 pc) accretion disk comprising a few percent of the BH mass. Double AGN activity is expected to occur on an estimated timescale of ≲1 Myr. The double nuclear point-like sources that may appear have typical separation of ≲ 10 pc, and are likely to be embedded in the still ongoing starburst. We note that a potential threat of binary stalling, in a gaseous environment, may come from radiation and/or mechanical energy injections by the BHs. […]

Accreting black holes often show iron line emission in their x-ray spectra. When this line emission is very broad or variable, it is likely to originate from close to the black hole. The theory and observations of such broad and variable iron lines are briefly reviewed here. In order for a clear broad line to be found, one or more of the following have to occur: high iron abundance, dense disk surface and minimal complex absorption. Several excellent examples are found from observations of Seyfert galaxies and Galactic Black Holes. In some cases there is strong evidence that the black hole is rapidly spinning. Further examples are expected as more long observations are made with XMM-Newton, Chandra and Suzaku. The x-ray spectra show evidence for the strong gravitational redshifts and light bending expected around black holes.

The massive black hole (MBH) in the Galactic Center (GC) and the stars around it form a unique stellar dynamics laboratory for studying how relaxation processes affect the distribution of stars and compact remnants and lead to close interactions between them and the MBH. Recent theoretical studies suggest that processes beyond “minimal” 2-body relaxation may operate and even dominate relaxation and its consequences in the GC. I describe loss-cone refilling by massive perturbers, strong mass segregation and resonant relaxation; review observational evidence that these processes play a role in the GC; and discuss some cosmic implications for the rates of gravitational wave emission events from compact remnants inspiraling into MBHs, and the coalescence timescales of binary MBHs.

Black holes are popping up all over the place: in compact binary x-ray sources and GRBs, in quasars, AGNs and the cores of all bulge galaxies, in binary black holes and binary black hole–neutron stars, and maybe even in the Large Hadron Collider! Black holes are strong-field objects governed by Einstein's equations of general relativity. Hence general relativistic, numerical simulations of dynamical phenomena involving black holes may help reveal ways in which black holes can form, grow, and be detected in the universe. To convey the state-of-the art, we summarize several representative simulations here, including the collapse of a hypermassive neutron star to a black hole following the merger of a binary neutron star, the magnetorotational collapse of a massive star to a black hole, and the formation and growth of supermassive black hole seeds by relativistic MHD accretion in the early universe.