Atomic and nuclear clocks, space–time variation of the fundamental constants and dark matter

Fundamental constants—such as the fine-structure constant α, the strong-interaction scale, and particle masses—may vary in an expanding Universe. A spatial variation could help explain apparent fine tuning: we inhabit a region where the values permit life. Hints from quasar absorption spectra suggest a gradient in α, but decisive confirmation requires laboratory tests. Atomic clocks provide such tests and, through their exquisite stability, enable sensitive searches for new physics.

Interactions between dark matter and ordinary matter can induce temporal variation of constants. For low-mass bosonic dark matter produced after the Big Bang, the field behaves classically, yielding first-order effects in the coupling—an enormous advantage over traditional second-order responses. Using clock comparisons, existing bounds on scalar dark-matter couplings to photons, electrons, quarks, and the Higgs can be tightened dramatically; our analyses improved previous limits by up to 15 orders of magnitude.

We assess several promising clock candidates with enhanced sensitivity to α variation while offering accessible cooling E1 lines and small systematic effects.

Highly charged-ion clocks offer reduced systematics due to their compact size, with α -variation and dark-matter responses enhanced by 1 - 2 orders of magnitude.

The isomeric 8.4 eV nuclear transition in ²²⁹Th, recently laser-excited by multiple groups, opens a path to a nuclear clock with accuracy potentially exceeding the best optical atomic clocks. Because the nucleus is well shielded from environmental perturbations, systematic shifts can be intrinsically small; however, the surrounding electrons strongly mediate excitation and decay via the electronic-bridge mechanism and can modify both transition frequency and lifetime by orders of magnitude. The ²²⁹Th transition is exceptionally sensitive to physics beyond the Standard Model, with four orders of magnitude enhancement.