Dark Matter Constraints from a Cosmic Index of Refraction

Understand

The dark-matter candidates of particle physics invariably possess electromagnetic interactions, if only via quantum fluctuations.

• Taken en masse, dark matter can thus engender an index of refraction which deviates from its vacuum value.
• Its presence is signaled through frequency-dependent effects in the propagation and attenuation of light.
• We discuss theoretical constraints on the expansion of the index of refraction with frequency, the physical interpretation of the terms, and the particular observations needed to isolate its coefficients.