Phenomenology of Dark Matter via a Bimetric Extension of General Relativity

We propose a relativistic model of dark matter reproducing at once the concordance cosmological model $\Lambda$-Cold-Dark-Matter ($\Lambda$-CDM) at cosmological scales, and the phenomenology of the modified Newtonian dynamics (MOND) at galactic scales. To achieve this we postulate a non-standard form of dark matter, consisting of two different species of particles coupled to gravity via a bimetric extension of general relativity, and linked together through an internal vector field (a "graviphoton") generated by the mass of these particles. We prove that this dark matter behaves like ordinary cold dark matter at the level of first order cosmological perturbation, while a pure cosmological constant plays the role of dark energy. The MOND equation emerges in the non-relativistic limit through a mechanism of gravitational polarization of the dark matter medium in the gravitational field of ordinary matter. Finally we show that the model is viable in the solar system as it predicts the same parametrized post-Newtonian parameters as general relativity.Comment: 34 pages, matches published versio

Classroom 4.0

Artificial intelligence, once the stuff of futuristic books and films, is finding its way onto college campuses. How is American higher education adapting to the digital age

Scattering of scalar, electromagnetic and gravitational waves from binary systems

The direct detection of gravitational waves crowns decades of efforts in the modelling of sources and of increasing detectors' sensitivity. With future third-generation Earth-based detectors or space-based observatories, gravitational-wave astronomy will be at its full bloom. Previously brushed-aside questions on environmental or other systematic effects in the generation and propagation of gravitational waves are now begging for a systematic treatment. Here, we study how electromagnetic and gravitational radiation is scattered by a binary system. Scattering cross-sections, resonances and the effect of an impinging wave on a gravitational-bound binary are worked out for the first time. The ratio between the scattered-wave amplitude and the incident wave can be of order $10^{-5}$ for known pulsars, bringing this into the realm of future gravitational-wave observatories. For currently realistic distribution of compact-object binaries, the interaction cross-section is too small to be of relevance.Comment: 19 pages, 3 figures, to appear in PR

Partially Massless Graviton on Beyond Einstein Spacetimes

We show that a partially massless graviton can propagate on a large set of spacetimes which are not Einstein spacetimes. Starting from a recently constructed theory for a massive graviton that propagates the correct number of degrees of freedom on an arbitrary spacetime, we first give the full explicit form of the scalar constraint responsible for the absence of a sixth degree of freedom. We then spell out generic conditions for the constraint to be identically satisfied, so that there is a scalar gauge symmetry which makes the graviton partially massless. These simplify if one assumes that spacetime is Ricci symmetric. Under this assumption, we find explicit non-Einstein spacetimes allowing for the propagation of a partially massless graviton.Comment: 17 pages. v2 error in eq. 31 and its propagation correcte

Do You Need a New Donor Management System? A Step-by-step Decision Making Workbook

Workbook provides guidance when considering a switch to a new donor management system. Worksheets and questionnaires help you assess your needs, compare them with what you have, and pinpoint the benefits and costs of migrating to a new system. Includes resources for more information

Dynamics of compact binary systems in scalar-tensor theories: I. Equations of motion to the third post-Newtonian order

Scalar-tensor theories are one of the most natural and well-constrained alternative theories of gravity, while still allowing for significant deviations from general relativity. We present the equations of motion of nonspinning compact binary systems at the third post-Newtonian (PN) order in massless scalar-tensor theories. We adapt the Fokker action of point particles in harmonic coordinates in general relativity to the specificities of scalar-tensor theories. We use dimensional regularisation to treat both the infrared and ultraviolet divergences, and we consistently include the tail effects that contribute by a non-local term to the dynamics. This work is crucial in order to compute the scalar gravitational waveform and the energy flux at 2PN order.Comment: 24 pages, matches the published versio