Supermassive black holes or boson stars? Hair counting with gravitational wave detectors

The evidence for supermassive Kerr black holes in galactic centers is strong and growing, but only the detection of gravitational waves will convincingly rule out other possibilities to explain the observations. The Kerr spacetime is completely specified by the first two multipole moments: mass and angular momentum. This is usually referred to as the ``no-hair theorem'', but it is really a ``two-hair'' theorem. If general relativity is the correct theory of gravity, the most plausible alternative to a supermassive Kerr black hole is a rotating boson star. Numerical calculations indicate that the spacetime of rotating boson stars is determined by the first three multipole moments (``three-hair theorem''). LISA could accurately measure the oscillation frequencies of these supermassive objects. We propose to use these measurements to ``count their hair'', unambiguously determining their nature and properties.Comment: 8 pages. This essay received an honorable mention in the Gravity Research Foundation Essay Competition, 200

Higgs Mass and Unnatural Supersymmetry

Assuming that supersymmetry exists well above the weak scale, we derive the full one-loop matching conditions between the SM and the supersymmetric theory, allowing for the possibility of an intermediate Split-SUSY scale. We also compute two-loop QCD corrections to the matching condition of the Higgs quartic coupling. These results are used to improve the calculation of the Higgs mass in models with high-scale supersymmetry or split supersymmetry, reducing the theoretical uncertainty. We explore the phenomenology of a mini-split scenario with gaugino masses determined by anomaly mediation. Depending on the value of the higgsino mass, the theory predicts a variety of novel possibilities for the dark-matter particle.Comment: 36 pages, 13 pdf figures; v2: matches version published in JHE

Infrared excesses in stars with and without planets using revised WISE{\it WISE} photometry

We present an analysis on the potential prevalence of mid infrared excesses in stars with and without planetary companions. Based on an extended database of stars detected with the ${\it WISE}$ satellite, we studied two stellar samples: one with 236 planet hosts and another with 986 objects for which planets have been searched but not found. We determined the presence of an excess over the photosphere by comparing the observed flux ratio at 22 $\mu$m and 12 $\mu$m ($f_{22}/f_{12}$) with the corresponding synthetic value, derived from results of classical model photospheres. We found a detection rate of 0.85$\%$ at 22 $\mu$m (2 excesses) in the sample of stars with planets and 0.1$\%$ (1 detection) for the stars without planets. The difference of the detection rate between the two samples is not statistically significant, a result that is independent of the different approaches found in the literature to define an excess in the wavelength range covered by ${\it WISE}$ observations. As an additional result, we found that the ${\it WISE}$ fluxes required a normalisation procedure to make them compatible with synthetic data, probably pointing out a revision of the ${\it WISE}$ data calibration.Comment: 10 pages, 6 figures, 3 tables. Accepted for publication in MNRA

Ultralight boson cloud depletion in binary systems

Ultralight scalars can extract rotational energy from astrophysical black holes through superradiant instabilities, forming macroscopic boson clouds. This process is most efficient when the Compton wavelength of the boson is comparable to the size of the black hole horizon, i.e. when the "gravitational fine structure constant" $\alpha\equiv G \mu M/\hbar c\sim 1$. If the black hole/cloud system is in a binary, tidal perturbations from the companion can produce resonant transitions between the energy levels of the cloud, depleting it by an amount that depends on the nature of the transition and on the parameters of the binary. Previous cloud depletion estimates considered binaries in circular orbit and made the approximation $\alpha\ll 1$. Here we use black hole perturbation theory to compute instability rates and decay widths for generic values of $\alpha$, and we show that this leads to much larger cloud depletion estimates when $\alpha \gtrsim 0.1$. We also study eccentric binary orbits. We show that in this case resonances can occur at all harmonics of the orbital frequency, significantly extending the range of frequencies where cloud depletion may be observable with gravitational wave interferometers.Comment: 12 pages, 6 figures. v2: references added, matches published versio

Slowly Rotating Anisotropic Neutron Stars in General Relativity and Scalar-Tensor Theory

Some models (such as the Skyrme model, a low-energy effective field theory for QCD) suggest that the high-density matter prevailing in neutron star interiors may be significantly anisotropic. Anisotropy is known to affect the bulk properties of nonrotating neutron stars in General Relativity. In this paper we study the effects of anisotropy on slowly rotating stars in General Relativity. We also consider one of the most popular extensions of Einstein's theory, namely scalar-tensor theories allowing for spontaneous scalarization (a phase transition similar to spontaneous magnetization in ferromagnetic materials). Anisotropy affects the moment of inertia of neutron stars (a quantity that could potentially be measured in binary pulsar systems) in both theories. We find that the effects of scalarization increase (decrease) when the tangential pressure is bigger (smaller) than the radial pressure, and we present a simple criterion to determine the onset of scalarization by linearizing the scalar-field equation. Our calculations suggest that binary pulsar observations may constrain the degree of anisotropy or even, more optimistically, provide evidence for anisotropy in neutron star cores.Comment: 19 pages, 7 figures, 1 table. Matches version in press in CQG. Fixed small typo

Quantum Reduced Loop Gravity

Quantum Reduced Loop Gravity provides a promising framework for a consistent characterization of the early Universe dynamics. Inspired by BKL conjecture, a flat Universe is described as a collection of Bianchi I homogeneous patches. The resulting quantum dynamics is described by the scalar constraint operator, whose matrix elements can be analytically computed. The effective semiclassical dynamics is discussed, and the differences with Loop Quantum Cosmology are emphasized

Superkicks in ultrarelativistic encounters of spinning black holes

We study ultrarelativistic encounters of two spinning, equal-mass black holes through simulations in full numerical relativity. Two initial data sequences are studied in detail: one that leads to scattering and one that leads to a grazing collision and merger. In all cases, the initial black hole spins lie in the orbital plane, a configuration that leads to the so-called "superkicks". In astrophysical, quasicircular inspirals, such kicks can be as large as ~3,000 km/s; here, we find configurations that exceed ~15,000 km/s. We find that the maximum recoil is to a good approximation proportional to the total amount of energy radiated in gravitational waves, but largely independent of whether a merger occurs or not. This shows that the mechanism predominantly responsible for the superkick is not related to merger dynamics. Rather, a consistent explanation is that the "bobbing" motion of the orbit causes an asymmetric beaming of the radiation produced by the in-plane orbital motion of the binary, and the net asymmetry is balanced by a recoil. We use our results to formulate some conjectures on the ultimate kick achievable in any black hole encounter.Comment: 10 pages, 6 figures, 2 table