Editing Hubble's law (section)

=== Ultimate fate and age of the universe ===
[[Image:Friedmann universes.svg|thumb|upright=1.9|The [[age of the universe|age]] and [[ultimate fate of the universe]] can be determined by measuring the Hubble constant today and extrapolating with the observed value of the deceleration parameter, uniquely characterized by values of density parameters ({{math|Ω<sub>M</sub>}} for [[matter]] and {{math|Ω<sub>Λ</sub>}} for dark energy).<br>A '''closed universe''' with {{math|Ω<sub>M</sub> > 1}} and {{math|1= Ω<sub>Λ</sub> = 0}} comes to an end in a [[Big Crunch]] and is considerably younger than its Hubble age.<br>An '''open universe''' with {{math|Ω<sub>M</sub> ≤ 1}} and {{math|1= Ω<sub>Λ</sub> = 0}} expands forever and has an age that is closer to its Hubble age. For the accelerating universe with nonzero {{math|Ω<sub>Λ</sub>}} that we inhabit, the age of the universe is coincidentally very close to the Hubble age.]]

The value of the Hubble parameter changes over time, either increasing or decreasing depending on the value of the so-called [[deceleration parameter]] {{mvar|q}}, which is defined by

<math display="block">q = -\left(1+\frac{\dot H}{H^2}\right).</math>

In a universe with a deceleration parameter equal to zero, it follows that {{math|1= ''H'' = 1/''t''}}, where {{mvar|t}} is the time since the Big Bang. A non-zero, time-dependent value of {{mvar|q}} simply requires [[integral|integration]] of the Friedmann equations backwards from the present time to the time when the [[particle horizon|comoving horizon]] size was zero.

It was long thought that {{mvar|q}} was positive, indicating that the expansion is slowing down due to gravitational attraction. This would imply an age of the universe less than {{math|1/''H''}} (which is about 14 billion years). For instance, a value for {{mvar|q}} of 1/2 (once favoured by most theorists) would give the age of the universe as {{math|2/(3''H'')}}. The discovery in 1998 that {{mvar|q}} is apparently negative means that the universe could actually be older than {{math|1/''H''}}. However, estimates of the [[age of the universe]] are very close to {{math|1/''H''}}.