Critical Surface for Explosions of Rotational Core-Collapse Supernovae

The effect of rotation on the explosion of core-collapse supernovae is investigated systematically in three-dimensional simulations. In order to obtain the critical conditions for explosion as a function of mass accretion rate, neutrino luminosity, and specific angular momentum, rigidly rotating matter was injected from the outer boundary with an angular momentum, which is increased every 500 ms. It is found that there is a critical value of the specific angular momentum, above which the standing shock wave revives, for a given combination of mass accretion rate and neutrino luminosity, i.e. an explosion can occur by rotation even if the neutrino luminosity is lower than the critical value for a given mass accretion rate in non-rotational models. The coupling of rotation and hydrodynamical instabilities plays an important role to characterize the dynamics of shock revival for the range of specific angular momentum that are supposed to be realistic. Contrary to expectations from past studies, the most rapidly expanding direction of the shock wave is not aligned with the rotation axis. Being perpendicular to the rotation axis on average, it can be oriented in various directions. Its dispersion is small when the spiral mode of the standing accretion shock instability (SASI) governs the dynamics, while it is large when neutrino-driven convection is dominant. As a result of the comparison between 2D and 3D rotational models, it is found that m=!0 modes of neutrino-driven convection or SASI are important for shock revival around the critical surface.Comment: First revised version, submitted to ApJ, 14 pages, 13 figures, 2 table

Criticality and Big Brake singularities in the tachyonic evolutions of closed Friedmann universes with cold dark matter

The evolution of a closed Friedmann universe filled by a tachyon scalar field with a trigonometric potential and cold dark matter (CDM) is investigated. A subset of the evolutions consistent to 1$\sigma$ confidence level with the Union 2.1 supernova data set is identified. The evolutions of the tachyon field are classified. Some of them evolve into a de Sitter attractor, while others proceed through a pseudo-tachyonic regime into a sudden future singularity. Critical evolutions leading to Big Brake singularities in the presence of CDM are found and a new type of cosmological evolution characterized by singularity avoidance in the pseudo-tachyon regime is presented.Comment: 7 pages, 5 figures, to be published in Phys. Rev.

Tachyons, Scalar Fields and Cosmology

We study the role that tachyon fields may play in cosmology as compared to the well-established use of minimally coupled scalar fields. We first elaborate on a kind of correspondence existing between tachyons and minimally coupled scalar fields; corresponding theories give rise to the same cosmological evolution for a particular choice of the initial conditions but not for any other. This leads us to study a specific one-parameter family of tachyonic models based on a perfect fluid mixed with a positive cosmological constant. For positive values of the parameter one needs to modify Sen's action and use the sigma process of resolution of singularities. The physics described by this model is dramatically different and much richer than that of the corresponding scalar field. For particular choices of the initial conditions the universe, that does mimick for a long time a de Sitter-like expansion, ends up in a finite time in a special type of singularity that we call a "big brake". This singularity is characterized by an infinite deceleration.Comment: 7 figures. Enlarged discussion of the big brake cosmology. Continuation of the model clarified. References adde