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A capital H with a nought subscript, the current expansion rate.
Also known as: Hubble constant, Hubble parameter, Cosmic expansion rate, H with a zero subscript
The Hubble constant is how fast the universe is expanding now, and the nought subscript matters: it is not constant over time, only over space at a given moment. Two independent methods of measuring it disagree by more than their stated errors, which is the Hubble tension.
Denotes the present-day rate of cosmic expansion, relating a galaxy's recession speed to its distance.
Contexts: Laboratory
Carries a nought subscript because it changes with cosmic time; the general quantity is the Hubble parameter.
Contexts: Laboratory
Is measured to incompatible values by early-universe and local-distance methods, a discrepancy known as the Hubble tension that remains unresolved.
Contexts: Laboratory
A capital letter H with a small numeral zero set below and to the right.