Hubble's constant

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Definition
  1. Noun:
    • (Cosmology) The Hubble Constant: A fundamental parameter in cosmology representing the rate at which the universe is expanding. It is defined as the ratio of the speed at which a distant galaxy is receding from an observer (due to the expansion of space) to its distance from that observer. Its value provides an estimate for the age and scale of the universe. Critically, it is called a "constant" as a historical convention, but its value is not constant over cosmic time; it refers specifically to the expansion rate at the present epoch.
Usage Examples
  • Noun:
    • Astronomers use Hubble's constant to estimate the age of the universe.
    • A precise measurement of Hubble's constant is crucial for understanding cosmic evolution.
    • The discrepancy between different methods of calculating Hubble's constant is a major topic in modern cosmology.
Advanced Usage
  • "The Hubble tension": Refers to the persistent and statistically significant discrepancy between the value of Hubble's constant measured 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).
    • Resolving the Hubble tension may require new physics beyond the standard cosmological model.
Variants and Related Words
  • Hubble parameter (H) : The more general term, where Hubble's constant (H₀) is the present-day value of this time-varying parameter.
    • The Hubble parameter decreases over time in most cosmological models.
  • Hubble flow : The motion of galaxies due solely to the expansion of the universe, described by Hubble's law.
    • Galaxies not bound by gravity follow the Hubble flow.
  • Hubble time : A rough estimate for the age of the universe, given by the reciprocal of Hubble's constant.
    • Hubble time provides an upper limit to the universe's age.
Synonyms
  • H₀ (pronounced "H-naught"): The standard astronomical symbol for Hubble's constant.
  • Expansion rate (of the universe) : A more descriptive phrase for the concept quantified by Hubble's constant.
Related Phrases and Terms
  • Hubble's law : The observational principle that the velocity of a galaxy's recession is proportional to its distance, with Hubble's constant being the proportionality constant.
    • Hubble's law, v = H₀d, is a cornerstone of modern cosmology.
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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