Group items tagged

Lima and Vanzella joined with George Matsas of São Paulo State University in their latest work to examine a model of the highly-curved spacetime that appears during formation of an ultradense neutron star. For some reasonable values of the mass and size of the star, they predict that the vacuum energy will grow within milliseconds for some values of the coupling parameter. At this point the vacuum energy would begin to induce additional gravitational effects, which they haven't yet calculated, so they don't know how the star would be affected. If further research shows such a neutron star to be unstable, the existence of stable neutron stars of particular sizes could rule out the existence fields of the type they modeled.

The Orbit Determination Program (ODP) of NASA's Jet Propulsion Laboratory, which was used in the data analysis, is based on an expression for the gravitational delay which differs from the standard formula; this difference is of second order in powers of $m$ -- the sun's gravitational radius -- but in Cassini's case it was much larger than the expected order of magnitude $m^2/b$, where $b$ is the ray's closest approach distance. Since the ODP does not account for any other second-order terms, it is necessary, also in view of future more accurate experiments, to systematically evaluate higher order corrections and to determine which terms are significant.

The Copenhagen group discovered that the redshifts agreed with the predictions of both general relativity and f(R) gravity, the theory that tries to avoid dark energy. However, the error bars on the redshifts excluded MOND and TeVeS, the theories that try to avoid dark matter.