Universe Warning: You are not logged in. Your IP address will be publicly visible if you make any edits. If you log in or create an account, your edits will be attributed to your username, along with other benefits.Anti-spam check. Do not fill this in! == Chronology and the Big Bang == {{Main|Big Bang|Chronology of the universe}} {{Nature timeline}} The prevailing model for the evolution of the universe is the Big Bang theory.<ref>{{cite book|first=Joseph|last=Silk|title=Horizons of Cosmology|publisher=Templeton Pressr|date=2009|page=208}}</ref><ref>{{cite book|first=Simon|last=Singh|title=Big Bang: The Origin of the Universe|publisher=Harper Perennial|date=2005|page=560|bibcode=2004biba.book.....S}}</ref> The Big Bang model states that the earliest state of the universe was an extremely hot and dense one, and that the universe subsequently expanded and cooled. The model is based on [[general relativity]] and on simplifying assumptions such as the [[homogeneity (physics)#Translation invariance|homogeneity]] and [[isotropy]] of space. A version of the model with a [[cosmological constant]] (Lambda) and [[cold dark matter]], known as the [[Lambda-CDM model]], is the simplest model that provides a reasonably good account of various observations about the universe. The Big Bang model accounts for observations such as the correlation of distance and [[redshift]] of galaxies, the ratio of the number of hydrogen to helium atoms, and the microwave radiation background. [[File:CMB Timeline300 no WMAP.jpg|thumb|upright=1.5|In this schematic diagram, time passes from left to right, with the universe represented by a disk-shaped "slice" at any given time. Time and size are not to scale. To make the early stages visible, the time to the afterglow stage (really the first 0.003%) is stretched and the subsequent expansion (really by 1,100 times to the present) is largely suppressed.]] The initial hot, dense state is called the [[Planck epoch]], a brief period extending from time zero to one [[Planck time]] unit of approximately 10<sup>−43</sup> seconds. During the Planck epoch, all types of matter and all types of energy were concentrated into a dense state, and [[gravity]]—currently the weakest by far of the [[fundamental interactions|four known forces]]—is believed to have been as strong as the other fundamental forces, and all the forces may have been [[grand unification|unified]]. The physics controlling this very early period (including [[quantum gravity]] in the Planck epoch) is not understood, so we cannot say what, if anything, happened [[Big Bang#Pre–Big Bang cosmology|before time zero]]. Since the Planck epoch, [[expansion of the universe|the universe has been expanding]] to its present scale, with a very short but intense period of [[cosmic inflation]] speculated to have occurred within the first [[Scientific Notation|10<sup>−32</sup>]] seconds.<ref name="Sivaram">{{cite journal |author=Sivaram |first=C. |date=1986 |title=Evolution of the Universe through the Planck epoch |journal=Astrophysics and Space Science |volume=125 |issue=1 |pages=189–199 |bibcode=1986Ap&SS.125..189S |doi=10.1007/BF00643984 |s2cid=123344693}}</ref> This initial period of inflation would explain why space appears to be [[Flatness problem|very flat]], and is [[Horizon problem|uniform on scales much larger]] than light could otherwise travel since the start of the universe. Within the first fraction of a second of the universe's existence, the four fundamental forces had separated. As the universe continued to cool from its inconceivably hot state, various types of [[subatomic particles]] were able to form in short periods of time known as the [[quark epoch]], the [[hadron epoch]], and the [[lepton epoch]]. Together, these epochs encompassed less than 10 seconds of time following the Big Bang. These [[elementary particle]]s associated stably into ever larger combinations, including stable [[proton]]s and [[neutron]]s, which then formed more complex [[atomic nuclei]] through [[nuclear fusion]].<ref name="Johnson 474–478">{{Cite journal |last=Johnson |first=Jennifer A. |date=February 2019 |title=Populating the periodic table: Nucleosynthesis of the elements |journal=Science |language=en |volume=363 |issue=6426 |pages=474–478 |doi=10.1126/science.aau9540 |pmid=30705182 |bibcode=2019Sci...363..474J |s2cid=59565697 |issn=0036-8075|doi-access=free }}</ref><ref name="durrer"/> This process, known as [[Big Bang nucleosynthesis]], lasted for about 17 minutes and ended about 20 minutes after the Big Bang, so only the fastest and simplest reactions occurred. About 25% of the [[proton]]s and all the [[neutron]]s in the universe, by mass, were converted to [[helium]], with small amounts of [[deuterium]] (a [[isotope|form]] of [[hydrogen]]) and traces of [[lithium]]. Any other [[chemical element|element]] was only formed in very tiny quantities. The other 75% of the protons remained unaffected, as [[hydrogen]] nuclei.<ref name="Johnson 474–478"/><ref name="durrer">{{cite book|last=Durrer |first=Ruth |author-link=Ruth Durrer |title=The Cosmic Microwave Background |publisher=Cambridge University Press |year=2008 |isbn=978-0-521-84704-9}}</ref>{{rp|27–42}} After nucleosynthesis ended, the universe entered a period known as the [[photon epoch]]. During this period, the universe was still far too hot for matter to form neutral [[atom]]s, so it contained a hot, dense, foggy [[Plasma (physics)|plasma]] of negatively charged [[electron]]s, neutral [[neutrino]]s and positive nuclei. After about 377,000 years, the universe had cooled enough that electrons and nuclei could form the first stable [[atom]]s. This is known as [[recombination (cosmology)|recombination]] for historical reasons; electrons and nuclei were combining for the first time. Unlike plasma, neutral atoms are [[Opacity (optics)|transparent]] to many [[wavelength]]s of light, so for the first time the universe also became transparent. The photons released ("[[photon decoupling|decoupled]]") when these atoms formed can still be seen today; they form the [[cosmic microwave background]] (CMB).<ref name="durrer"/>{{rp|15–27}} As the universe expands, the [[energy density]] of [[electromagnetic radiation]] decreases more quickly than does that of [[matter]] because the energy of each photon decreases as it is [[cosmological redshift|cosmologically redshifted]]. At around 47,000 years, the [[energy density]] of matter became larger than that of photons and [[neutrino]]s, and began to dominate the large scale behavior of the universe. This marked the end of the [[radiation-dominated era]] and the start of the [[matter-dominated era]].<ref name="steane">{{cite book|first=Andrew M. |last=Steane |title=Relativity Made Relatively Easy, Volume 2: General Relativity and Cosmology |isbn=978-0-192-89564-6 |publisher=Oxford University Press |year=2021}}</ref>{{rp|390}} In the earliest stages of the universe, tiny fluctuations within the universe's density led to [[filament (cosmology)|concentrations]] of [[dark matter]] gradually forming. Ordinary matter, attracted to these by [[gravity]], formed large gas clouds and eventually, stars and galaxies, where the dark matter was most dense, and [[Void (astronomy)|voids]] where it was least dense. After around 100–300 million years,<ref name="steane"/>{{rp|333}} the first [[star]]s formed, known as [[Population III]] stars. These were probably very massive, luminous, [[metallicity|non metallic]] and short-lived. They were responsible for the gradual [[reionization]] of the universe between about 200–500 million years and 1 billion years, and also for seeding the universe with elements heavier than helium, through [[stellar nucleosynthesis]].<ref>{{cite news |work=Scientific American |title=The First Stars in the Universe |first1=Richard B. |last1=Larson |first2=Volker |last2=Bromm |name-list-style=amp |date=March 2002 |url=http://www.scientificamerican.com/article/the-first-stars-in-the-un/ |access-date=June 9, 2015 |archive-date=June 11, 2015 |archive-url=https://web.archive.org/web/20150611032732/http://www.scientificamerican.com/article/the-first-stars-in-the-un/ |url-status=live }}</ref> The universe also contains a mysterious energy—possibly a [[scalar field]]—called [[dark energy]], the density of which does not change over time. After about 9.8 billion years, the universe had expanded sufficiently so that the density of matter was less than the density of dark energy, marking the beginning of the present [[dark-energy-dominated era]].<ref>[[Barbara Ryden|Ryden, Barbara]], "Introduction to Cosmology", 2006, eqn. 6.33</ref> In this era, the expansion of the universe is [[accelerating expansion of the universe|accelerating]] due to dark energy. Summary: Please note that all contributions to Christianpedia may be edited, altered, or removed by other contributors. If you do not want your writing to be edited mercilessly, then do not submit it here. You are also promising us that you wrote this yourself, or copied it from a public domain or similar free resource (see Christianpedia:Copyrights for details). Do not submit copyrighted work without permission! Cancel Editing help (opens in new window) Discuss this page