Constants & Units · constant

CMB temperature

T₀ = 2.725 48 ± 0.000 57 K

Unit

K

Value

2.725 48 ± 0.000 57

Exact

no

Note

The most perfect black body ever measured, and the coldest thing in nature outside a laboratory.

Symbol

T₀

Category

Cosmological

Numeric

2.72548

Uncertainty

±0.000 57

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From the Constants database in Notion, last edited 2026-08-15. This page is the row, rendered by the build; nothing on it is written by hand.