Assessing the dependency of the fine structure constant on gravity using hot white dwarfs
White dwarf stars have recently found a new use in the fundamental
physics community, because they potentially allow us to directly observe variations in
fundamental constants at locations of high gravitational potential.
Many prospective theories of the fundamental interactions of nature allow
traditional constants to vary in some way. In case of the fine structure
constant, α, such a change would result in a small shift in wavelength
centroid of atomic transitions. This effect is larger for metals with more
protons (higher atomic number) and higher ionization energy.
Measurements of this shift have previously been conducted using optical
spectroscopy of Quasars, with the implication that α was smaller in the
past (Webb et al. 2001). Magueijo et al. (2002) showed how varying constants,
like α, can occur in the presence of strong gravitational fields, being
proportional to the dimensionless potential GM/Rc2, and hence the compactness
(M/R) of an object of mass M and radius R. This means that compact objects
with high mass, and small radius - like white dwarfs - could exhibit detectable
variations of α.
A first attempt to measure shifts in the wavelengths of highly ionized Fe and
Ni lines in the high-resolution STIS spectra of the DA white dwarf G191-B2B
was performed by Berengut et al (2013), demonstrating the validity of the method.
We are now extending this work by (1) using new (high precision) laboratory
wavelengths, (2) refining the analysis methodology (incorporating robust
techniques from previous studies towards quasars), and (3) enlarging the
sample size by studying other hot white dwarf, but also hot sub-dwarfs (this allows
us to cover a broader compactness range).
A successful detection would be the first direct measurement of a
gravitational field effect on a bare constant of nature.
This project is founded by a Leverhulme Trust Research Project Grant.
The Leverhulme Trust was established by the Will of William Hesketh Lever,
the founder of Lever Brothers. Since 1925 the Trust has provided grants and
scholarships for research and education. Today, it is one of the largest
all-subject providers of research funding in the UK, distributing
approximately £80M a year. For more information about the Trust,
please visit www.leverhulme.ac.uk.