Scalar field could unlock supernova mystery

A new component of gravity, the scalar gravitational field, may explain the mechanism that allows the immense explosions of type II supernovae to take place. However, this could happen only through a dynamic process – parametric instability – that dates back to work by Lord Rayleigh in the 1880s.

When the central core of a massive star runs out of nuclear fuel (having been converted mainly into iron), it collapses under its own weight in less than a second into an extremely dense neutron star, releasing an enormous amount of gravitational energy. A supernova results, but only a small fraction of the total energy released appears as electromagnetic radiation (light) of the “new star”. The kinetic energy of the exploding stellar envelope is 10 times greater, but the greatest part of the energy by far is carried away by neutrinos, which can more easily escape the dense material of the core.

Detection of neutrinos from supernova SN1987A did much to verify this picture. During core-collapse, the density at the centre of the star rapidly increases, finally forming dense nuclear matter that is extremely difficult to compress. The collapsing material rebounds from this nuclear matter, resulting in an outgoing pressure wave, which soon becomes a huge shock wave.

Extensive studies have attempted to decide whether this “prompt shock” travels all of the way out and ejects the outer part of the star. Indeed, simulations suggest that it stalls at distances of about 300 km from the centre because of the immense energy required to dissociate iron and other nuclei. However, further simulations have found that the shock could restart if the electrons could absorb about 1% of the energy carried by neutrinos. In the neutrino-plasma coupling model, collective interactions between the neutrinos and the plasma could initiate the required energy transfer. Alternatively, recent research suggests that the solution to re-energizing the shock may lie in a fundamental field that takes the simple form of a scalar (like the Higgs field).

Gravity containing a scalar field (originally proposed by Carl Brans and Robert Dicke in the 1960s as an additional component of the gravitational field) has been considered as a promising extension to Einstein’s general relativity in connection with quantum gravity and grand unification. The theory of Brans and Dicke was based on a relatively simple linear coupling to the scalar gravitational field. A few years ago, this linear coupling was shown to be negligible, using radio signals transmitted from the Cassini spacecraft when it was near Saturn.

Now, researchers in the UK and Portugal have analysed the nonlinear coupling to a scalar gravitational field. They find that under extreme conditions with strong time-varying gravity such as may be found in the interior of a newly born neutron star, the scalar gravitational field may be stimulated via parametric instability. The resulting emission of scalar gravitational waves from the neutron core of a collapsing heavy star may be sufficient to re-energize the stalled shock, thus providing a 19th-century solution to a 20th-century problem.