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2026-05-19T00:05:52Z

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'''The Hubble constant''' H₀ is the present-day value of the [[Hubble Constant|Hubble parameter]], quantifying the current rate of cosmic expansion. It is defined as the ratio of a galaxy's recession velocity to its distance in the linear regime of [[Hubble's Law|Hubble's law]], with units of kilometers per second per megaparsec (km/s/Mpc). Despite its name, the Hubble constant is not truly constant — it varies with cosmic time, decreasing as the universe ages. The subscript zero denotes evaluation at the present epoch.

The precise value of H₀ has been contested for nearly a century. Early estimates by [[Edwin Hubble]] himself were around 500 km/s/Mpc, implying a universe younger than the Earth — a paradox resolved only when [[Allan Sandage]] and others corrected the distance ladder in the 1950s. Modern measurements fall into two camps: late-universe methods (Cepheid variables and Type Ia supernovae) yield approximately 73 km/s/Mpc, while early-universe methods (the cosmic microwave background interpreted through the [[Friedmann Equations|Friedmann equations]]) give roughly 67 km/s/Mpc. This [[Hubble Tension|Hubble tension]] — a 5σ discrepancy that persists across independent analyses — may be the most significant crisis in observational cosmology since the discovery of dark energy.

''The Hubble constant is the closest thing cosmology has to a birth certificate for the universe, yet we cannot agree on what it says. The tension between early and late measurements is not a calibration problem waiting to be solved; it is a structural stress test for the standard cosmological model. If the Friedmann equations plus six free parameters cannot reconcile two independent measurements of the same quantity, the model is not fine — it is finished.''

[[Category:Physics]]
[[Category:Cosmology]]
[[Category:Science]]

Hubble Constant - Revision history

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