Editing Astronomy (section)

=== Physical cosmology ===
{{Nature timeline}}
{{Main|Physical cosmology}}

[[Cosmology]] (from the Greek {{lang|grc|κόσμος}} ({{transliteration|grc|kosmos}}) "world, universe" and {{lang|grc|λόγος}} ({{transliteration|grc|logos}}) "word, study" or literally "logic") could be considered the study of the Universe as a whole.{{cn|date=March 2025}}

[[File:Hubble Extreme Deep Field (full resolution).png|thumb|[[Hubble Extreme Deep Field]]]]

Observations of the [[large-scale structure of the Universe]], a branch known as [[physical cosmology]], have provided a deep understanding of the formation and evolution of the cosmos. Fundamental to modern cosmology is the well-accepted theory of the [[Big Bang]], wherein our Universe began at a single [[point in time]], and thereafter [[metric expansion of space|expanded]] over the course of 13.8 billion years<ref>{{cite web
|title = Cosmic Detectives
|url = http://www.esa.int/Our_Activities/Space_Science/Cosmic_detectives
|publisher = The European Space Agency (ESA)
|date = 2 April 2013
|access-date = 15 April 2013
|archive-date = 11 February 2019
|archive-url = https://web.archive.org/web/20190211204726/http://www.esa.int/Our_Activities/Space_Science/Cosmic_detectives
|url-status = live
}}</ref> to its present condition.<ref name=Dodelson2003/> The concept of the Big Bang can be traced back to the discovery of the [[Cosmic microwave background radiation|microwave background radiation]] in 1965.<ref name=Dodelson2003>{{cite book|last=Dodelson|first=Scott|title=Modern cosmology|date=2003|isbn=978-0-12-219141-1|publisher=[[Academic Press]]|pages=1–22}}</ref>

In the course of this expansion, the Universe underwent several evolutionary stages. In the very early moments, it is theorized that the Universe experienced a very rapid [[cosmic inflation]], which homogenized the starting conditions. Thereafter, [[Big Bang nucleosynthesis|nucleosynthesis]] produced the elemental abundance of the early Universe.<ref name=Dodelson2003/> (See also [[nucleocosmochronology]].){{cn|date=March 2025}}

When the first neutral [[atom]]s formed from a sea of primordial ions, space became transparent to radiation, releasing the energy viewed today as the microwave background radiation. The expanding Universe then underwent a Dark Age due to the lack of stellar energy sources.<ref name="cosmology 101">{{cite web|last = Hinshaw|first = Gary|date = 13 July 2006|url=http://map.gsfc.nasa.gov/m_uni.html|title = Cosmology 101: The Study of the Universe|publisher = NASA WMAP|access-date =10 August 2006| archive-url= https://web.archive.org/web/20060813053535/http://map.gsfc.nasa.gov/m_uni.html| archive-date= 13 August 2006 | url-status= live}}</ref>

A hierarchical structure of matter began to form from minute variations in the mass density of space. Matter accumulated in the densest regions, forming clouds of gas and the earliest stars, the [[Population III stars]]. These massive stars triggered the [[reionization]] process and are believed to have created many of the heavy elements in the early Universe, which, through nuclear decay, create lighter elements, allowing the cycle of nucleosynthesis to continue longer.<ref>Dodelson, 2003, pp. 216–61</ref>

Gravitational aggregations clustered into filaments, leaving voids in the gaps. Gradually, organizations of gas and dust merged to form the first primitive galaxies. Over time, these pulled in more matter, and were often organized into [[Galaxy groups and clusters|groups and clusters]] of galaxies, then into larger-scale superclusters.<ref>{{cite web|url=http://www.damtp.cam.ac.uk/user/gr/public/gal_lss.html|title = Galaxy Clusters and Large-Scale Structure|publisher = University of Cambridge|access-date =8 September 2006| archive-url= https://web.archive.org/web/20061010041120/http://www.damtp.cam.ac.uk/user/gr/public/gal_lss.html| archive-date= 10 October 2006 | url-status= live}}</ref>

Fundamental to the structure of the Universe is the existence of [[dark matter]] and [[dark energy]]. These are now thought to be its dominant components, forming 96% of the mass of the Universe. For this reason, much effort is expended in trying to understand the physics of these components.<ref>{{cite web|last = Preuss|first = Paul|url=http://www.lbl.gov/Science-Articles/Archive/dark-energy.html|title = Dark Energy Fills the Cosmos|publisher = U.S. Department of Energy, Berkeley Lab|access-date =8 September 2006| archive-url= https://web.archive.org/web/20060811215815/http://www.lbl.gov/Science-Articles/Archive/dark-energy.html| archive-date= 11 August 2006 | url-status= live}}</ref>