Stars as Laboratories for Fundamental Physics - MPP Theory Group

116 Chapter 3
Finally, the matter of the galaxy would exert a leptonic force on
neutrinos propagating, say, from a distant supernova to us. This effect
would cause an energy-dependent dispersion of the measurable neutrino
burst (Sect. 13.3.3). However, when e L ∼ > 10 −20 , which is necessary to
cause an interesting effect on supernova neutrinos, then the galactic
leptonic charge is completely screened over the relevant length scales,
even if the cosmic background neutrinos are relativistic.
3.7 Graviton Emission from Stars
The one nonelectromagnetic long-range force that is actually known
to exist is gravity. Whatever the ultimate quantum theory of gravity,
there is little doubt that there will be quantized wave excitations, the
gravitons. They would be massless spin-2 particles. In fact, gravity is
the only possible force that can be mediated by a massless spin-2 boson
because as a source it needs a conserved rank-2 tensor. The energymomentum
tensor, which acts as a source for the gravitational field, is
the only example.
Classical gravitational waves are an inevitable consequence of Einstein’s
theory of general relativity. The orbital decay of the binary
pulsar PSR 1913+16 (Hulse and Taylor 1975) yields firm evidence for
their emission (Taylor and Weisberg 1989; Damour and Taylor 1991).
Gravitons can be produced in hot plasmas in analogy to axions or
neutrino pairs; typical processes are bremsstrahlung e + p → e + p + g
(graviton g) and the Primakoff effect (gravitons have a two-photon
coupling). Because of their weak interaction gravitons can freely escape
once produced in the interior of stars. Early calculations of the
emission rates were summarized by Papini and Valluri (1977). More
recent discussions include Schäfer and Dehnen (1983), Gould (1985),
and del Campo and Ford (1988). As the graviton coupling involves the
inverse of the Planck mass (1.2×10 19 GeV) the graviton luminosity of
stars is inevitably small. For the Sun it is about 10 15 erg s −1 ≈ 10 −19 L ⊙ ,
much too small to be of any observational relevance. The same conclusion
holds for other stars.
Therefore, gravity itself illustrates that the mediation of long-range
forces is a far more important effect of low-mass bosons than their
thermal emission from stellar plasmas. Unless, of course, they only
couple to the fermion spins rather than to a “charge.” Pseudoscalars
such as axions are in that category.