Cosmic microwave background
The cosmic microwave background is faint thermal radiation filling the universe, left over from the early stages after the Big Bang.
How big is it?
2.725 K · temperature
Also -454.8 °F
Temperature · approximate
A little perspective
Cosmic microwave background vs Boiling helium: temperature comparison
The temperature of the cosmic microwave background is 1.495 K lower than the temperature of boiling helium.
The cosmic microwave background is faint thermal radiation filling the universe, left over from the early stages after the Big Bang.
Boiling helium is helium in its liquid state at its normal boiling point under standard atmospheric pressure.
Measurements, assumptions & sources +
Cosmic microwave background
Temperature: 2.725 K; approximate.
The temperature of the nearly blackbody relic radiation. It is not the temperature of every object in space.
The marked temperature is calibrated. Unmeasured illustration details are representative.
Boiling helium
Temperature: 4.22 K; approximate.
Approximate normal boiling point of helium at one atmosphere.
NIST Chemistry WebBook — Helium phase data
The marked temperature is calibrated. Unmeasured illustration details are representative.
Use the visual comparison to explore its scale, then check the measurements and sources.
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Why this measurement?
Why we use 2.725 K
We compare the temperature of the cosmic microwave background. This is an estimate; check the notes for its rounding and uncertainty.
The temperature of the nearly blackbody relic radiation. It is not the temperature of every object in space.
The picture identifies the subject; the diagram compares the recorded values. How calibrated comparisons work →
Recorded properties
Dimensions and measurements
| Property | Selected value | Recorded range | Basis |
|---|---|---|---|
| 2.725 K | Not recorded | approximate |
Around this scale
Explore nearby values.
Compare examples with lower, similar and higher values. Each card names the measurement being compared.
Nearest lower valueTemperatureAbsolute zero0 KTemperatureAbsolute zero is the lowest temperature permitted by thermodynamics, where a system has the least thermal energy available to it.Around the same scaleTemperatureSuperfluid helium lambda point2.17 KTemperatureThe superfluid helium lambda point is the temperature at which helium-4 transitions from a normal liquid to a superfluid state.Nearest higher valueTemperatureBoiling helium4.22 KTemperatureBoiling helium is helium in its liquid state at its normal boiling point under standard atmospheric pressure.Sources and method
Want to check the number?
The source records and measurement method remain available for inspection.
- NASA — COBE science resultsCosmic microwave background blackbody temperature about 2.725 K; this is radiation temperature, not the temperature of every object in space.
More ways to see it
Curated comparisons
There’s always another scale
Follow your curiosity.
TemperatureAbsolute zero0 KAbsolute zero is the lowest temperature permitted by thermodynamics, where a system has the least thermal energy available to it.TemperatureRoom temperature20 °CRoom temperature describes indoor air conditions generally considered comfortable for people and suitable for ordinary activities.
SpaceSun1,391,000 kmThe Sun is the star at the centre of the Solar System.
Fundamental physicsPlanck temperature1.417 × 10³² KThe Planck temperature is a theoretical temperature scale at which quantum gravitational effects are expected to become important.TemperatureBoiling helium4.22 KBoiling helium is helium in its liquid state at its normal boiling point under standard atmospheric pressure.TemperatureUltracold atom cloud1 × 10⁻⁷ KAn ultracold atom cloud is a collection of atoms cooled to temperatures near absolute zero, used in advanced physics experiments.TemperatureBoiling nitrogen77.36 KBoiling nitrogen is nitrogen gas produced when liquid nitrogen is heated at normal atmospheric pressure.TemperatureSuperfluid helium lambda point2.17 KThe superfluid helium lambda point is the temperature at which helium-4 transitions from a normal liquid to a superfluid state.