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