Universe

=== Age and expansion ===
{{Main|Age of the universe|Expansion of the universe}}
Assuming that the [[Lambda-CDM model]] is correct, the measurements of the parameters using a variety of techniques by numerous experiments yield a best value of the age of the universe at 13.799 [[Measurement uncertainty|±]] 0.021 billion years, as of 2015.<ref name="Planck 2015">{{cite journal|author=Planck Collaboration|year=2016|title=Planck 2015 results. XIII. Cosmological parameters|journal=Astronomy & Astrophysics|volume=594|page=A13, Table 4|arxiv=1502.01589|bibcode=2016A&A...594A..13P|doi=10.1051/0004-6361/201525830|s2cid=119262962}}</ref>

[[File:Galactic Cntr full cropped.jpg|thumb|upright=2|Astronomers have discovered stars in the [[Milky Way]] galaxy that are almost 13.6 billion years old.]]
Over time, the universe and its contents have evolved. For example, the relative population of [[quasar]]s and galaxies has changed<ref>{{cite news |url=https://www.science.org/content/article/galaxy-collisions-give-birth-quasars |work=Science News |title=Galaxy Collisions Give Birth to Quasars |date=March 25, 2010 |first=Phil |last=Berardelli |access-date=July 30, 2022 |archive-date=March 25, 2022 |archive-url=https://web.archive.org/web/20220325005200/https://www.science.org/content/article/galaxy-collisions-give-birth-quasars |url-status=live }}</ref> and the [[expansion of the universe|universe has expanded]]. This expansion is inferred from the observation that the light from distant galaxies has been [[redshift]]ed, which implies that the galaxies are receding from us. Analyses of [[Type Ia supernova]]e indicate that the [[accelerating expansion of the Universe|expansion is accelerating]].<ref name="riess">{{cite journal|author=Riess, Adam G.|year=1998|title=Observational evidence from supernovae for an accelerating universe and a cosmological constant|journal=Astronomical Journal|volume=116|issue=3|pages=1009–1038|arxiv=astro-ph/9805201 |doi=10.1086/300499|bibcode=1998AJ....116.1009R|last2=Filippenko|last3=Challis|last4=Clocchiatti|last5=Diercks|last6=Garnavich|last7=Gilliland|last8=Hogan|last9=Jha|last10=Kirshner|last11=Leibundgut|last12=Phillips|last13=Reiss|last14=Schmidt|last15=Schommer|last16=Smith|last17=Spyromilio|last18=Stubbs|last19=Suntzeff|last20=Tonry|s2cid=15640044|author-link=Adam Riess}}</ref><ref name="perlmutter">{{cite journal|author=Perlmutter, S. |journal=Astrophysical Journal|volume=517|issue=2|pages=565–586|year=1999|title=Measurements of Omega and Lambda from 42 high redshift supernovae|arxiv=astro-ph/9812133 |doi=10.1086/307221|bibcode=1999ApJ...517..565P|last2=Aldering|last3=Goldhaber|last4=Knop|last5=Nugent|last6=Castro|last7=Deustua|last8=Fabbro|last9=Goobar|last10=Groom|last11=Hook|last12=Kim|last13=Kim|last14=Lee|last15=Nunes|last16=Pain|last17=Pennypacker|last18=Quimby|last19=Lidman|last20=Ellis|last21=Irwin|last22=McMahon|last23=Ruiz-Lapuente|last24=Walton|last25=Schaefer|last26=Boyle|last27=Filippenko|last28=Matheson|last29=Fruchter|last30=Panagia|s2cid=118910636|display-authors=29|author-link=Saul Perlmutter}}</ref>

The more matter there is in the universe, the stronger the mutual [[gravitational]] pull of the matter. If the universe were ''too'' dense then it would re-collapse into a [[gravitational singularity]]. However, if the universe contained too ''little'' matter then the self-gravity would be too weak for astronomical structures, like galaxies or planets, to form. Since the Big Bang, the universe has expanded [[monotonic]]ally. [[Anthropic principle#Anthropic 'coincidences'|Perhaps unsurprisingly]], our universe has [[Critical Mass Density of the Universe|just the right mass–energy density]], equivalent to about 5 protons per cubic metre, which has allowed it to expand for the last 13.8 billion years, giving time to form the universe as observed today.<ref>{{cite book|first1=Raymond A. |last1=Serway |first2=Clement J. |last2=Moses |first3=Curt A. |last3=Moyer |title=Modern Physics |publisher=Cengage Learning |year=2004 |isbn=978-1-111-79437-8 |page=21}}</ref><ref>{{cite book |url=https://openstax.org/books/astronomy-2e/pages/29-7-the-anthropic-principle |title=Astronomy 2e |publisher=OpenStax |isbn=978-1-951-69350-3 |first1=Andrew |last1=Fraknoi |display-authors=etal |year=2022 |page=1017 |access-date=February 14, 2023 |archive-date=February 14, 2023 |archive-url=https://web.archive.org/web/20230214122906/https://openstax.org/books/astronomy-2e/pages/29-7-the-anthropic-principle |url-status=live }}</ref>

There are dynamical forces acting on the particles in the universe which affect the expansion rate. Before 1998, it was expected that the expansion rate would be decreasing as time went on due to the influence of gravitational interactions in the universe; and thus there is an additional observable quantity in the universe called the [[deceleration parameter]], which most cosmologists expected to be positive and related to the matter density of the universe. In 1998, the deceleration parameter was measured by two different groups to be negative, approximately −0.55, which technically implies that the second derivative of the cosmic [[scale factor cosmology|scale factor]] <math> \ddot{a}</math> has been positive in the last 5–6 billion years.<ref name="nobel_2011">{{cite web |url=https://www.nobelprize.org/nobel_prizes/physics/laureates/2011/ |title=The Nobel Prize in Physics 2011 |access-date=April 16, 2015 |archive-date=April 17, 2015 |archive-url=https://web.archive.org/web/20150417023358/http://www.nobelprize.org/nobel_prizes/physics/laureates/2011/ |url-status=live }}</ref><ref>{{cite news|last=Overbye|first=Dennis|title=A 'Cosmic Jerk' That Reversed the Universe|url=https://www.nytimes.com/2003/10/11/us/a-cosmic-jerk-that-reversed-the-universe.html?pagewanted=all&src=pm|newspaper=New York Times|date=October 11, 2003|access-date=February 20, 2017|archive-date=July 1, 2017|archive-url=https://web.archive.org/web/20170701114952/http://www.nytimes.com/2003/10/11/us/a-cosmic-jerk-that-reversed-the-universe.html?pagewanted=all&src=pm|url-status=live}}</ref>