LIGO Scientific Collaboration, Virgo Collaboration

A wide variety of astrophysical and cosmological sources are expected to contribute to a stochastic gravitational-wave background. Following the observations of GW150914 and GW151226, the rate and mass of coalescing binary black holes appear to be greater than many previous expectations. As a result, the stochastic background from unresolved compact binary coalescences is expected to be particularly loud. We perform a search for the isotropic stochastic gravitational-wave background using data from Advanced LIGO's first observing run. The data display no evidence of a stochastic gravitational-wave signal. We constrain the dimensionless energy density of gravitational waves to be $\Omega_0<1.7\times 10^{-7}$ with 95% confidence, assuming a flat energy density spectrum in the most sensitive part of the LIGO band (20-86 Hz). This is a factor of ~33 times more sensitive than previous measurements. We also constrain arbitrary power-law spectra. Finally, we investigate the implications of this search for the background of binary black holes using an astrophysical model for the background.

Ignacy Sawicki, Dept. Theor. Phys. and ASCR, Inst. Phys., Prague), Ippocratis D. Saltas, Mariele Motta, Dept. Theor. Phys.), Luca Amendola, ITP), Martin Kunz, Dept. Theor. Phys. and African Inst. Math. Sci., Cape Town)

In many generalized models of gravity, perfect fluids in cosmology give rise to gravitational slip. Simultaneously, in very broad classes of such models, the propagation of gravitational waves is altered. We investigate the extent to which there is a one-to-one relationship between these two properties in three classes of models with one extra degree of freedom: scalar (Horndeski and beyond), vector (Einstein-Aether) and tensor (bimetric). We prove that in bimetric gravity and Einstein-Aether, it is impossible to dynamically hide the gravitational slip on all scales whenever the propagation of gravitational waves is modified. Horndeski models are much more flexible, but it is nonetheless only possible to hide gravitational slip dynamically when the action for perturbations is tuned to evolve in time toward a divergent kinetic term. These results provide an explicit, theoretical argument for the interpretation of future observations if they disfavoured the presence of gravitational slip.

LIGO Scientific Collaboration, Virgo Collaboration

We employ gravitational-wave radiometry to map the gravitational waves stochastic background expected from a variety of contributing mechanisms and test the assumption of isotropy using data from Advanced LIGO's first observing run. We also search for persistent gravitational waves from point sources with only minimal assumptions over the 20 - 1726 Hz frequency band. Finding no evidence of gravitational waves from either point sources or a stochastic background, we set limits at 90% confidence. For broadband point sources, we report upper limits on the gravitational wave energy flux per unit frequency in the range $F(f, \Theta) < (0.1 - 56) \times 10^{-8}$ erg cm$^{-2}$ s$^{-1}$ Hz$^{-1}$ (f/25 Hz)$^{\alpha-1}$ depending on the sky location $\Theta$ and the spectral power index $\alpha$. For extended sources, we report upper limits on the fractional gravitational wave energy density required to close the Universe of $\Omega(f,\Theta) < (0.39-7.6) \times 10^{-8}$ sr$^{-1}$ (f/25 Hz)$^\alpha$ depending on $\Theta$ and $\alpha$. Directed searches for narrowband gravitational waves from astrophysically interesting objects (Scorpius X-1, Supernova 1987 A, and the Galactic Center) yield median frequency-dependent limits on strain amplitude of $h_0 <$ (6.7, 5.5, and 7.0) $\times 10^{-25}$ respectively, at the most sensitive detector frequencies between 130 - 175 Hz. This represents a mean improvement of a factor of 2 across the band compared to previous searches of this kind for these sky locations, considering the different quantities of strain constrained in each case.

Marius Cautun, Carlos S. Frenk

We estimate the systemic orbital kinematics of the Milky Way classical satellites and compare them with predictions from the \Lambda{} cold dark matter (\Lambda{}CDM) model derived from a semi-analytical galaxy formation model applied to high resolution cosmological N-body simulations. We find that the Galactic satellite system is atypical of \Lambda{}CDM systems. The subset of 10 Galactic satellites with proper motion measurements has a velocity anisotropy, \beta=-2.2$\pm$0.4, that lies in the 2.9% tail of the \Lambda{}CDM distribution. Individually, the Milky Way satellites have radial velocities that are lower than expected for their proper motions, with 9 out of the 10 having at most 20% of their orbital kinetic energy invested in radial motion. Such extreme values are expected in only 1.5% of \Lambda{}CDM satellites systems. This tangential motion excess is unrelated to the existence of a Galactic "disc of satellites". We present theoretical predictions for larger satellite samples that may become available as more proper motion measurements are obtained.

Marcelo Schiffer

We calculate the black hole mass distribution function that follows from the random emission of quanta by Hawking radiation and with this function we calculate the black hole mass fluctuation. From a complete different perspective we regard the black hole as quantum mechanical system with a quantized event horizon area and transition probabilities among the various energy levels and then calculate the mass dispersion. It turns out that there is a perfect agreement between the statistical and the microscopic calculations if and only if the area spectrum is linear. Accordingly, the quantum mechanical properties of the black hole which are supposedly relevant only at Planckian scales do leave an imprint in the black hole mass dispersion at much larger scales where gravity can be dealt classically, as one would expect from the correspondence principle.

Jiri Novotny

Theory known as Special Galileon has recently attracted considerable interest due to its peculiar properties. It has been shown that it represents an extremal member of the set of effective field theories with enhanced soft limit. This property makes its tree-level S-matrix fully on-shell reconstructible and representable by means of the Cachazo-He-Yuan representation. The enhanced soft limit is a consequence of new hidden symmetry of the Special Galileon action, however, until now, the origin of this peculiar symmetry has remained unclear. In this paper we interpret this symmetry as a special transformation of the coset space $GAL(D,1)/SO(1,D-1)$ and show, that there exists a three-parametric family of invariant Galileon actions. The latter family is closed under duality which appears as a natural generalization of the above mentioned symmetry. We also present a geometric construction of the Special Galileon action using $D$-dimensional brane propagating in $2D$-dimensional flat pseudo-riemannian space. Within such framework, the Special Galileon symmetry emerges as an $U(1,D-1)$ symmetry of the target space, which can be treated as a $D$-dimensional K\"ahler manifold. Such a treatment allows for classification of the higher order invariant Lagrangians needed as counterterms on the quantum level. We also briefly comment on relation between such higher order Lagrangians and the Lagrangians invariant with respect to the polynomial shift symmetry.