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Showing votes from 2016-02-19 12:30 to 2016-02-23 11:30 | Next meeting is Friday May 8th, 11:30 am.
In recent work we proposed a novel theory of dark matter (DM) superfluidity that matches the successes of the LambdaCDM model on cosmological scales while simultaneously reproducing MOdified Newtonian Dynamics (MOND) phenomenology on galactic scales. The agents responsible for mediating the MONDian force law are superfluid phonons that couple to ordinary (baryonic) matter. In this paper we propose an alternative way for the MOND phenomenon to emerge from DM superfluidity. The central idea is to use higher-gradient corrections in the superfluid effective theory. These next-to-leading order terms involve gradients of the gravitational potential, and therefore effectively modify the gravitational force law. In the process we discover a novel mechanism for generating the non-relativistic MOND action, starting from a theory that is fully analytic in all field variables. The idea, inspired by the symmetron mechanism, uses the spontaneous breaking of a discrete symmetry. For large acceleration, the symmetry is unbroken and the action reduces to Einstein gravity. For small acceleration, the symmetry is spontaneously broken and the action reduces to MONDian gravity. Cosmologically, however, the universe is always in the Einstein-gravity, symmetry-restoring phase. The expansion history and linear growth of density perturbations are therefore indistinguishable from LambdaCDM cosmology.
Cosmology experiments at mm-wavelengths may be able to detect Planet Nine if it is the size of Neptune, has an effective temperature of 40 K, and is 700 AU from the Sun. It would appear as a ~30 mJy source at 1 mm (or ~8 mJy at 150 GHz) with a parallax of ~5 arcmin. The challenge will be to distinguish it from the ~4000 foreground asteroids brighter than 30 mJy. Fortunately, asteroids can by identified by looking for sources that move across a resolution element in a matter of hours, orders of magnitude faster than Planet Nine. If Planet Nine is smaller, colder, and/or more distant than expected, then it could be as faint as 1 mJy at 1 mm. There are approximately 1E6 asteroids this bright, making many cosmology experiments confusion limited for moving sources. Nonetheless, it may still be possible to find the proverbial needle in the haystack using a matched filter. This would require mm telescopes with high angular resolution and high sensitivity in order to alleviate confusion and to enable the identification of moving sources with relatively short time baselines. Regardless of its mm flux density, searching for Planet Nine would entail obtaining frequent radio measurements for large swaths of the sky, including the ecliptic and Galactic plane. Even if Planet Nine had already been detected by other means, measuring its mm-flux would constrain its internal energy budget, and therefore help resolve the mystery of Uranus and Neptune, which have vastly different internal heat.
Recently, an interesting mechanism [Phys.Rev.Lett.106:231101,2011] has been proposed which produces all nonlinear terms in massive gravity to all orders . In this work, we reproduce these results in M-theory and consider the process of birth and growth of nonlinear gravity during the construction of Mp-branes. It has been shown that Mp brane are built up of p- M1-branes which each of them are connected to M1-branes of other Mp-brane through a wormhole. In this model, by increasing the number of dimensions, the number of nonlinear terms in relevant action of branes enhances and some theories like lovelock and nonlinear gravity are raised. By compacting M-branes, graviton fields in nonlinear gravity converts to photon fields and thus nonlinear electrodynamics are produced.