Accelerating expansion of the universe

Fixed typo

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−	The '''accelerating expansion of the universe''' is the observation that the [[universe]] appears to be [[Metric expansion of space|expanding]] at an increasing rate. In formal terms, this means that the [[Scale factor (cosmology)|cosmic scale factor]] <math>a(t)</math> has a positive [[second derivative]],<ref>{{cite book | last = Jones | first = Mark H. |author2= Robert J. Lambourne | title = An Introduction to Galaxies and Cosmology | publisher =Cambridge University Press | date= 2004 | page = [https://books.google.com/books?id=36K1PfetZegC&lpg=PP1&pg=PA244#v=onepage&q&f=false 244] | isbn = 978-0-521-83738-5 }}</ref> so that the velocity at which a distant galaxy is receding from the observer is continuously increasing with time.<ref>[http://curious.astro.cornell.edu/the-universe/cosmology-and-the-big-bang/104-the-universe/cosmology-and-the-big-bang/expansion-of-the-universe/616-is-the-universe-expanding-faster-than-the-speed-of-light-intermediate Is the universe expanding faster than the speed of light?] (see final paragraph)</ref>	+	The '''accelerating expansion of the universe''' is the observation that the [[universe]] appears to be [[Metric expansion of space|expanding]] at an increasing rate. In formal terms, this means that the fart [[Scale factor (cosmology)|cosmic scale factor]] <math>a(t)</math> has a positive [[second derivative]],<ref>{{cite book | last = Jones | first = Mark H. |author2= Robert J. Lambourne | title = An Introduction to Galaxies and Cosmology | publisher =Cambridge University Press | date= 2004 | page = [https://books.google.com/books?id=36K1PfetZegC&lpg=PP1&pg=PA244#v=onepage&q&f=false 244] | isbn = 978-0-521-83738-5 }}</ref> so that the velocity at which a distant galaxy is receding from the observer is continuously increasing with time.<ref>[http://curious.astro.cornell.edu/the-universe/cosmology-and-the-big-bang/104-the-universe/cosmology-and-the-big-bang/expansion-of-the-universe/616-is-the-universe-expanding-faster-than-the-speed-of-light-intermediate Is the universe expanding faster than the speed of light?] (see final paragraph)</ref>
	The expansion of the universe has been accelerating since the universe entered its [[dark-energy-dominated era]], at [[redshift]] z≈0.4 (roughly 5 billion years ago).<ref name="Frieman">{{Cite journal|last=Frieman|first=Joshua A.|last2=Turner|first2=Michael S.|last3=Huterer|first3=Dragan|date=2008-01-01|title=Dark Energy and the Accelerating Universe|url=http://dx.doi.org/10.1146/annurev.astro.46.060407.145243|journal=Annual Review of Astronomy and Astrophysics|volume=46|issue=1|pages=385–432|arxiv=0803.0982|bibcode=2008ARA&A..46..385F|doi=10.1146/annurev.astro.46.060407.145243}}</ref> {{refn|1= <ref name="Frieman" />p. 6: "The Universe has gone through three distinct eras: radiation-dominated, z≳3000; matter-dominated, 3000≳z≳0.5; and dark-energy dominated, z≲0.5. The evolution of the scale factor is controlled by the dominant energy form: a(t) ∝ t2/3(1+w) (for constant w). During the radiation-dominated era, a(t) ∝ t1/2; during the matter-dominated era, a(t) ∝ t2/3; and for the dark energy-dominated era, assuming w = −1, asymptotically a(t) ∝ exp(Ht)."<br />		The expansion of the universe has been accelerating since the universe entered its [[dark-energy-dominated era]], at [[redshift]] z≈0.4 (roughly 5 billion years ago).<ref name="Frieman">{{Cite journal|last=Frieman|first=Joshua A.|last2=Turner|first2=Michael S.|last3=Huterer|first3=Dragan|date=2008-01-01|title=Dark Energy and the Accelerating Universe|url=http://dx.doi.org/10.1146/annurev.astro.46.060407.145243|journal=Annual Review of Astronomy and Astrophysics|volume=46|issue=1|pages=385–432|arxiv=0803.0982|bibcode=2008ARA&A..46..385F|doi=10.1146/annurev.astro.46.060407.145243}}</ref> {{refn|1= <ref name="Frieman" />p. 6: "The Universe has gone through three distinct eras: radiation-dominated, z≳3000; matter-dominated, 3000≳z≳0.5; and dark-energy dominated, z≲0.5. The evolution of the scale factor is controlled by the dominant energy form: a(t) ∝ t2/3(1+w) (for constant w). During the radiation-dominated era, a(t) ∝ t1/2; during the matter-dominated era, a(t) ∝ t2/3; and for the dark energy-dominated era, assuming w = −1, asymptotically a(t) ∝ exp(Ht)."<br />