Universal physical quantities, like fine-structure constant, that may vary with dark matter density.Fundamental constants are fixed numerical values that define the laws of physics, such as the speed of light, Planck's constant, and the fine-structure constant (α ≈ 1/137). They are assumed to be immutable across space and time, forming the bedrock of our physical theories. However, some theories beyond the Standard Model suggest that these constants might vary slightly, for example, in regions with high dark matter density or during cosmic evolution. The fine-structure constant, which sets the strength of electromagnetic interactions, is a prime candidate for such variation. If it changes, atomic spectra, chemical reaction rates, and even the stability of matter could shift subtly.
The concept of universal constants emerged in the 19th century with the work of Maxwell and Planck. The fine-structure constant was introduced by Arnold Sommerfeld in 1916. The idea that constants might vary was proposed by Dirac in 1937 with his Large Numbers Hypothesis, though modern tests have placed stringent limits on such variation.
Fundamental constants are not derived from deeper principles; they are measured experimentally and inserted into equations. For instance, the fine-structure constant combines the electron charge, speed of light, and Planck's constant. A variation in α would alter the energy levels of atoms, which can be probed by comparing atomic clocks over time or by looking at light from distant quasars. Dark matter could interact with ordinary matter in a way that changes the effective values of these constants, for example, through a scalar field that couples to electromagnetic fields.
Precision measurements of fundamental constants underpin metrology, including the definition of the second and the meter. Searching for their variation is a key test of string theory and other unified theories, and could reveal new physics beyond the Standard Model.
The fine-structure constant is often called 'the coupling constant for electromagnetism' and its value is so precisely known that it is used to define the kilogram in the new SI system.
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