47 research outputs found
A Sun-like star orbiting a boson star
The high-precision astrometric mission GAIA recently reported the remarkable
discovery of a Sun-like star closely orbiting a dark object, with a semi-major
axis and period of and days respectively. While the
plausible expectation for the central dark object is a black hole, the
evolutionary mechanism leading to the formation of such a two-body system is
highly challenging. Here, we challenge the scenario of a central black hole and
show that the observed orbital dynamics can be explained under fairly general
assumptions if the central dark object is a stable clump of bosonic particles
of spin-0, or spin-1, known as a boson star. We further explain how future
astrometric measurements of similar systems will provide an exciting
opportunity to probe the fundamental nature of compact objects and test compact
alternatives to black holes.Comment: 11 pages, 4 figures. Comments are very welcom
Asymptotically safe Starobinsky inflation
We revisit Starobinsky inflation in a quantum gravitational context, by means of the exact renormalization group (RG). We calculate the nonperturbative beta functions for Newton’s “constant” G and the dimensionless R^2 coupling, and show that there exists an attractive UV fixed point where the latter one vanishes but not the former one, and we provide the corresponding beta functions. The smallness of the R^2 coupling, required for agreement with inflationary observables, is naturally ensured by its vanishing at the UV fixed point, ensuring the smallness of the primordial fluctuations, as well as providing a theoretical motivation for the initial conditions needed for successful inflation in this context. We discuss the corresponding RG dynamics, showing both how inflationary and classical observations define the renormalization conditions for the couplings, and also how the UV regime is connected with lower energies along the RG flow. Finally, we discuss the consistency of our results when higher-order curvature corrections are included, and show that they are robust to the inclusion of R^3 corrections
The dynamical equivalence of modified gravity revisited
We revisit the dynamical equivalence between different representations of
vacuum modified gravity models in view of Legendre transformations. The
equivalence is discussed for both bulk and boundary space, by including in our
analysis the relevant Gibbons-Hawking terms. In the f(R) case, the Legendre
transformed action coincides with the usual Einstein frame one. We then
re-express the R+f(G) action, where G is the Gauss-Bonnet term, as a second
order theory with a new set of field variables, four tensor fields and one
scalar and study its dynamics. For completeness, we also calculate the
conformal transformation of the full Jordan frame R+f(G) action. All the
appropriate Gibbons-Hawking terms are calculated explicitly.Comment: 17 pages; v3: Revised version. New comments added in Sections 3 & 5.
New results added in Section 6. Version to appear in Class. Quantum Gravit