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Heisenberg L.,KTH Royal Institute of Technology | Heisenberg L.,The Oskar Klein Center
Classical and Quantum Gravity | Year: 2015

We compute the one-loop quantum corrections to the interactions between the two metrics of the ghost-free massive bigravity. When considering gravitons running in the loops, we show how the structure of the interactions gets destabilized at the quantum level, exactly in the same way as in its massive gravity limit. A priori one might have expected a better quantum behavior, however, the broken diffeomorphism invariance out of the two initial diffeomorphisms in bigravity has similar consequences at the quantum level as the broken diffeomorphism in massive gravity. From lessons of the generated quantum corrections through matter loops we propose yet other types of effective composite metrics to which the matter fields can couple. Among these new effective metrics there might be one or more that could provide interesting phenomenology and important cosmological implications. © 2015 IOP Publishing Ltd. Source

Schmidt-May A.,The Oskar Klein Center
Journal of Cosmology and Astroparticle Physics | Year: 2015

In this paper we study the ghost-free bimetric action extended by a recently proposed coupling to matter through a composite metric. The equations of motion for this theory are derived using a method which avoids varying the square-root matrix that appears in the matter coupling. We make an ansatz for which the metrics are proportional to each other and find that it can solve the equations provided that one parameter in the action is fixed. In this case, the proportional metrics as well as the effective metric that couples to matter solve Einstein's equations of general relativity including a matter source. Around these backgrounds we derive the quadratic action for perturbations and diagonalize it into generalized mass eigenstates. It turns out that matter only interacts with the massless spin-2 mode whose equation of motion has exactly the form of the linearized Einstein equations, while the field with Fierz-Pauli mass term is completely decoupled. Hence, bimetric theory, with one parameter fixed such that proportional solutions exist, is degenerate with general relativity up to linear order around these backgrounds. © 2015 IOP Publishing Ltd and Sissa Medialab srl . Source

Kuhnel F.,The Oskar Klein Center
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2014

General types of Bose-Einstein condensates are considered. The formation of black-hole analogues is examined for both short- and long-range interactions for arbitrary spatial dimensions greater than 2. The former case includes nonlinear derivative terms plus an inevitable external potential, while the latter one consists solely of gravitylike self-interactions for which intrinsic formation of analogue Schwarzschild-type background space-times is possible. The corresponding geometries are studied, and it is shown how they can be made stable. Their Hawking temperature is estimated, and it is found that in certain setups it can be significantly increased, thus providing better detectability. © 2014 American Physical Society. Source

Bergstrom L.,The Oskar Klein Center
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012

Recently, an interesting indication for a dark matter signal in the form of a narrow line, or maybe two lines and/or an internal bremsstrahlung feature, has been found in analyses of public data from the Fermi-LAT satellite detector. As recent analyses have also shown that there is little sign of extra contributions to continuum photons, it is natural to investigate leptophilic interacting massive particle models. We show that a model of radiatively generated neutrino masses may have the properties needed to explain the Fermi-LAT structure around 130GeV. This model was proposed some 10 years ago, and predicted a clearly observable γ-ray signal in the Fermi-LAT (then GLAST) detector. Here, we update and improve that analysis, and show as an example that a right-handed neutrino of mass 135GeV should give rise to three conspicuous effects: a broad internal bremsstrahlung bump with maximum around 120GeV, a 2γ line around 135GeV, and a Zγ line at 119.6GeV (neglected in the previous work). These features together give a good fit to the 130GeV structure, given the present energy resolution of the Fermi-LAT data. An attractive feature of the model is that the particle physics properties are essentially fixed, once the relic density and the mass of the right-handed neutrino dark matter particle have been set. Puzzling features of the data at present are a slight displacement of the signal from the galactic center, and a needed boost factor of order 5-15. This presents interesting challenges for numerical simulations including both baryons and dark matter on scales of 100pc, and perhaps a need to go beyond the simplest halo models. With upcoming experiments having better energy resolution, or with future Fermi-LAT data, the double-peak structure with a definite predicted ratio of the strengths of the two lines and the internal bremsstrahlung feature should be seen, if this model is correct. With the planned satellite GAMMA-400, a striking fingerprint of this dark matter candidate should then appear. © 2012 American Physical Society. Source

Bergstrom L.,The Oskar Klein Center
Annalen der Physik | Year: 2012

The problem of the darkmatter in the universe is reviewed. A short history of the subject is given, and several of the most obvious particle candidates for dark matter are identified. Particular focus is given to weakly interacting, massive particles (WIMPs) of which the lightest supersymmetric particle is an interesting special case and a useful template. The three detection methods: in particle accelerators, by direct detections of scattering in terrestrial detectors, and indirect detection of products from dark matter particle annihilation in the galactic halo, are discussed and their complementarity is explained. Direct detection experiments have revealed some possible indications of a dark matter signal, but the situation is quite confusing at the moment. Very recently, also indirect detection has entered a sensitivity region where some particle candidates could be detectable. Indeed, also here there are some (presently non-conclusive) indications of possible dark matter signals, like an interesting structure at 130 GeV γ-ray energy found in publicly available data from the Fermi-LAT space detector. The future of the field will depend on whether WIMPs are indeed the dark matter, something that may realistically be probed in the next few years. If this exciting scenario turns out to be true, we can expect a host of other, complementary experiments in the coming decade. If it is not true, the time scale and methods for detection will be much more uncertain. © 2012 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Source

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