Stars as Laboratories for Fundamental Physics - MPP Theory Group

546 Chapter 15
vacuum expectation value of a physical field was more plausibly subject
to variations than dimensionless constants such as gauge and Yukawa
couplings.
Type Ia supernovae are thought to represent the nuclear deflagration
of a white dwarf pushed beyond its Chandrasekhar limit by accretion
(e.g. Woosley and Weaver 1986b). Therefore, their light curves are
very reproducible and indeed serve as standard candles in an attempt
to improve measurements of the cosmic expansion rate. The shape of
the lightcurve is determined by radioactive heating of the SN remnant
by the decay of 56 Co; its lifetime is proportional to G −2
F . A change of
order 10% in the slope of type Ia SN lightcurves in neighboring galaxies
(Leibundgut et al. 1991) would be readily observable so that G F must
be constant within at least 5% over 30 Mpc distances.
In addition, Scherrer and Spergel showed that the agreement between
the observed primordial light element abundances and big-bang
nucleosynthesis calculations imply that G F lay within −1% and +9%
of its standard value at this early epoch.
A stringent constraint on the local 93 time variation of G F obtains
from the analysis of ores from the Oklo uranium mine where a natural
fission reactor is thought to have operated about 2 Gyr ago (e.g.
Maurette 1976). A shift of the ground state difference between 150 Sm
and 149 Sm by more than 0.02 eV is inconsistent with the isotope ratios
at Oklo (Shlyakhter 1976, 1983). According to this author the
contribution of the weak interaction to the nuclear binding energy is
about 200 eV, excluding a change of G F by more than 0.1% over the
past 2 Gyr.
15.2 Constancy of Newton’s Constant
15.2.1 Present-Day Constraints from Celestial Mechanics
Another “constant of nature” that might vary in time is Newton’s constant
G N . Indeed, there exist self-consistent alternative theories to
general relativity which actually predict a temporal variation of G N on
cosmological time scales—see Will (1993) for a summary of such theories
and detailed references. A typical scale for the rate of change
is the cosmic expansion parameter H so that it is natural to write
Ġ N /G N = σH with σ a dimensionless model-dependent number. Some
93 The Earth moves with the galaxy and the local group relative to the cosmic
microwave background (CMB). Taking 600 km s −1 for this peculiar velocity the
Earth moves by about 1.2 Mpc in 2 Gyr relative to a frame defined by the CMB.