The Universe as a Cosmic String


Niedermann, F. (2014). The Universe as a Cosmic String. Perimeter Institute. https://pirsa.org/14110140


Niedermann, Florian. The Universe as a Cosmic String. Perimeter Institute, Nov. 13, 2014, https://pirsa.org/14110140


          @misc{ pirsa_14110140,
            doi = {},
            url = {https://pirsa.org/14110140},
            author = {Niedermann, Florian},
            keywords = {Cosmology, Particle Physics},
            language = {en},
            title = {The Universe as a Cosmic String},
            publisher = {Perimeter Institute},
            year = {2014},
            month = {nov},
            note = {PIRSA:14110140 see, \url{https://pirsa.org}}


We are investigating modifications of general relativity that are operative at the largest observable scales. In this context, we are investigating the model of brane induced gravity in 6D, a higher dimensional generalization of the DGP model. As opposed to different claims in the literature, we have proven the quantum stability of the theory in a weakly coupling regime on a Minkowski background. In particular, we have shown that the Hamiltonian of the linear theory is bounded from below. This result opened a new window of opportunity for consistent modified Friedmann cosmologies. In our recent work it is shown that a brane with FRW symmetries necessarily acts as a source of cylindrically symmetric gravitational waves, so called Einstein-Rosen waves. Their existence essentially distinguishes this model from its codimension-one counterpart and necessitates to solve the non-linear system of bulk and brane-matching equations. A numerical analysis is performed and two qualitatively different and dynamically separated classes of cosmologies are derived: degravitating solutions for which the Hubble parameter settles to zero despite the presence of a non-vanishing energy density on the brane and super-accelerating solutions for which Hubble grows unbounded. The parameter space of both the stable and unstable regime is derived and observational consequences are discussed: It is argued that the degravitating regime does not allow for a phenomenologically viable cosmology. On the other hand, the super-accelerating solutions are potentially viable, however, their unstable behavior questions their physical relevance.