hubble constant

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hubble constant

An astronomer points to a chart showing the Hubble constant in a lecture.

Definition
  1. Noun:
    • (Cosmology) The Hubble constant: A fundamental cosmological parameter representing the ratio of the speed at which a galaxy is receding from an observer (due to the expansion of the universe) to its distance from that observer. It is a measure of the current expansion rate of the universe. It is called a "constant" for historical reasons, but its value is not constant over cosmic time; it is the present-day value of the Hubble parameter.
Usage Examples
  • Noun:
    • Astronomers are working to measure the Hubble constant with greater precision.
    • A discrepancy in measurements of the Hubble constant is a major puzzle in modern cosmology.
    • The value of the Hubble constant is often expressed in kilometers per second per megaparsec.
Advanced Usage
  • "Hubble tension": Refers to the persistent discrepancy between the measured value of the Hubble constant from observations of the local universe (e.g., using Cepheid variable stars and supernovae) and the value inferred from observations of the early universe (e.g., the cosmic microwave background radiation).
    • The Hubble tension suggests there may be new physics beyond our current cosmological models.
Variants and Related Words
  • Hubble parameter (H(t)) (n): The more general time-dependent function describing the expansion rate of the universe at any epoch. The Hubble constant (H₀) is the present-day value of the Hubble parameter.
  • Hubble's law (n): The observational principle in cosmology that the velocity of a galaxy's recession is proportional to its distance, with the Hubble constant being the constant of proportionality in its simplest form.
  • Hubble time (n): An estimate of the age of the universe, given by the reciprocal of the Hubble constant.
Synonyms
  • Hubble parameter (present-day value): The Hubble constant is specifically the current value of the Hubble parameter.
  • Expansion rate (of the universe): A more descriptive term for what the Hubble constant quantifies.
Related Phrases
  • To measure the Hubble constant: The act of determining its numerical value through observational data.
    • Several space telescopes have been launched to help measure the Hubble constant more accurately.
  • Value of the Hubble constant: Refers to the specific numerical measurement, typically around 70 km/s/Mpc, though the exact figure is under active research.
    • The latest studies have refined the value of the Hubble constant.
hubble constant

An astronomer points to a chart showing the Hubble constant in a lecture.

Noun
  1. (cosmology) the ratio of the speed of recession of a galaxy (due to the expansion of the universe) to its distance from the observer; the Hubble constant is not actually a constant, but is regarded as measuring the expansion rate today

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