Hubble parameter

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Definition
  1. Noun:
    • (Cosmology): The Hubble parameter is 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. Although often called the Hubble constant, its value can change over cosmic time.
Usage Examples
  • Noun:
    • The precise measurement of the Hubble parameter is a major goal of modern cosmology.
    • Scientists use distant supernovae to calculate the Hubble parameter.
    • A discrepancy in measurements of the Hubble parameter is known as the "Hubble tension."
Advanced Usage
  • "Hubble parameter" vs. "Hubble constant": In strict usage, the Hubble parameter, denoted as H(t), describes the expansion rate as a function of time. The Hubble constant, H₀, specifically refers to the present-day value of the Hubble parameter.
    • While the Hubble parameter changes over the life of the universe, the Hubble constant gives its current value.
Variants and Related Words
  • Hubble constant (H₀) (n): The present-day value of the Hubble parameter.
    • The latest measurements of the Hubble constant suggest the universe is expanding faster than some models predict.
  • Hubble law (n): The observational correlation that a galaxy's recessional velocity is proportional to its distance, with the Hubble parameter as the constant of proportionality.
    • Edwin Hubble's discovery of the Hubble law provided evidence for an expanding universe.
Synonyms
  • Hubble constant (specifically for its present-day value)
  • Expansion rate (of the universe)
Related Terms and Concepts
  • Cosmological redshift (n): The increase in wavelength of light from distant galaxies due to the expansion of space, directly related to recessional velocity.
    • The cosmological redshift of a galaxy is used to determine its recessional velocity for the Hubble parameter calculation.
  • Scale factor (n): A function in cosmology describing how the size of the universe changes with time; the Hubble parameter is defined by the rate of change of the scale factor.
    • The Hubble parameter is mathematically related to the time derivative of the scale factor.
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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