Theories unifying gravity with other interactions suggest temporal and spatial variation of the fundamental ‘constants’ in an expanding Universe. The spatial variation can explain fine tuning of the fundamental constants which allows humans (and any life) to appear. We appeared in the area of the Universe where the values of the fundamental constants are consistent with our existence. Another possible effect is dependence of the fundamental constants on the gravitational potential which leads to the violation of local position invariance.

Our group (Elizabeth Angstmann, Julian Berengut, Vladimir Dzuba and Victor Flambaum) in collaboration with American, Russian and German theorists and UNSW astrophysicists (John Webb’s group) performed a number of theoretical works devoted to the variation of the fine structure constant alpha (a combination of the electron electric charge, speed of light and quantum Plank constant), strong interaction, and fundamental masses (Higgs vacuum). There are some hints for the variation in quasar absorption spectra, Big Bang nucleosynthesis, and the Oklo natural nuclear reactor data.

A very promising method to search for the variation consists in comparing different atomic clocks. Huge enhancement of the variation effects happens in transitions between very close atomic and molecular energy levels. A new idea is to build a ‘nuclear’ clock based on the 7 eV transition in a Thorium nucleus. This may allow us to improve the sensitivity to the variation of up to 10 orders of magnitude! Huge enhancement of the variation effects is also possible in cold atomic and molecular collisions near resonance.

How may changing physical constants and violation of local position invariance occur? Light scalar fields very naturally appear in modern cosmological models, affecting  parameters of the Standard Model (e.g. a). Cosmological variations of these scalar fields should occur because of drastic changes in the composition of matter in the Universe. The latest such event is rather recent (about 5 billion years ago), from matter to dark energy domination. Massive bodies (stars or galaxies) can also affect physical constants. They have a large scalar charge S proportional to the number of particles, which produces a Coulomb-like scalar field U=S/r. This leads to a variation of the fundamental constants proportional to the gravitational potential. We compared different manifestations of this effect. The strongest limits are obtained from the measurements of dependence of atomic frequencies on the distance from the Sun (the distance varies due to the ellipticity of the Earth's orbit).

Victor Flambaum