Hubble Time

Hubble time assumes the universe has always expanded at its CURRENT rate, but the actual expansion history includes an early period slowed by gravity from matter and a more recent period accelerated by dark energy. Because the rate has changed over time, the simple reciprocal 1/H₀ is only an approximation -- the currently accepted age of the universe, about 13.8 billion years, comes from a full cosmological model rather than this one-line formula.

It expresses how much faster a galaxy appears to recede for every additional megaparsec of distance from us -- a galaxy twice as far away recedes at roughly twice the speed, in the simplest expanding-universe picture. That mixed unit (a speed per a distance) is why converting H₀ into a Hubble time in years requires the unit reconciliation this calculator performs internally.

A higher H₀ shortens the Hubble time -- the two are inversely proportional, since Hubble time is defined as 1/H₀. A universe expanding faster today implies, under the constant-rate assumption, that less time has passed since everything was concentrated at a point.

It refers to a persistent, several-percent disagreement between H₀ values measured from the early universe (cosmic microwave background) versus values measured from nearby supernovae and other "local" distance indicators. Because Hubble time depends directly on which H₀ value is used, that disagreement is one reason this calculator's result is best read as an order-of-magnitude estimate rather than a precise figure.