Editing Physical cosmology (section)

==Energy of the cosmos==
The lightest [[chemical elements]], primarily [[hydrogen]] and [[helium]], were created during the Big Bang through the process of [[nucleosynthesis]].<ref name=Burles2001/> In a sequence of [[stellar nucleosynthesis]] reactions, smaller atomic nuclei are then combined into larger atomic nuclei, ultimately forming stable [[iron group]] elements such as [[iron]] and [[nickel]], which have the highest nuclear [[binding energies]].<ref name=B2FH>{{cite journal | last1=Burbidge | first1=E. M. | last2=Burbidge | first2=G. R. | last3=Fowler | first3=W. A. | last4=Hoyle | first4=F. | year=1957 | title=Synthesis of the Elements in Stars | journal=[[Reviews of Modern Physics]] | volume=29 | issue=4 | pages=547–650 | bibcode=1957RvMP...29..547B | doi=10.1103/RevModPhys.29.547 | doi-access=free }}</ref> The net process results in a ''later energy release'', meaning subsequent to the Big Bang.<ref name=Frautschi1982>{{cite journal | title=Entropy in an expanding universe | last1=Frautschi | first1=S. | journal=Science | volume=217 | issue=4560 | pages=593–599 | date=13 August 1982 | doi=10.1126/science.217.4560.593 | pmid=17817517 | bibcode=1982Sci...217..593F | s2cid=27717447 }}</ref> Such reactions of nuclear particles can lead to ''sudden energy releases'' from [[cataclysmic variable star]]s such as [[nova]]e. Gravitational collapse of matter into [[black hole]]s also powers the most energetic processes, generally seen in the nuclear regions of galaxies, forming ''[[quasar]]s'' and ''[[active galaxies]]''.

Cosmologists cannot explain all cosmic phenomena exactly, such as those related to the [[Accelerating universe|accelerating expansion of the universe]], using conventional [[Energy forms|forms of energy]]. Instead, cosmologists propose a new form of energy called [[dark energy]] that permeates all space.<ref>{{Cite journal |doi = 10.1126/science.1086879|pmid = 12817141|bibcode = 2003Sci...300.1914K|title = Throwing Light on Dark Energy|year = 2003|last1 = Kirshner|first1 = R. P.|journal = Science|volume = 300|issue = 5627|pages = 1914–1918|s2cid = 43859435}}</ref> One hypothesis is that dark energy is just the [[vacuum energy]], a component of empty space that is associated with the [[virtual particle]]s that exist due to the [[uncertainty principle]].<ref name=Frieman2008>{{cite journal | title=Dark Energy and the Accelerating Universe | last1=Frieman | first1=Joshua A. | last2=Turner | first2=Michael S. | last3=Huterer | first3=Dragan | journal=[[Annual Review of Astronomy & Astrophysics]] | volume=46 | issue=1 | pages=385–432 | year=2008 | doi=10.1146/annurev.astro.46.060407.145243 | bibcode=2008ARA&A..46..385F | arxiv=0803.0982 | s2cid=15117520 }}</ref>

There is no clear way to define the total energy in the universe using the most widely accepted theory of gravity, general relativity. Therefore, it remains controversial whether the total energy is conserved in an expanding universe. For instance, each [[photon]] that travels through intergalactic space loses energy due to the [[redshift]] effect. This energy is not transferred to any other system, so seems to be permanently lost. On the other hand, some cosmologists insist that energy is conserved in some sense; this follows the law of [[conservation of energy]].<ref>e.g. {{cite book | author = Liddle, A. | title = An Introduction to Modern Cosmology | publisher = Wiley | isbn =978-0-470-84835-7 |year=2003 }} This argues cogently "Energy is always, always, always conserved."</ref>

Different forms of energy may dominate the cosmos—[[relativistic particle]]s which are referred to as [[radiation]], or non-relativistic particles referred to as matter. Relativistic particles are particles whose [[rest mass]] is zero or negligible compared to their [[kinetic energy]], and so move at the speed of light or very close to it; non-relativistic particles have much higher rest mass than their energy and so move much slower than the speed of light.

As the universe expands, both matter and radiation become diluted. However, the [[energy density|energy densities]] of radiation and matter dilute at different rates. As a particular volume expands, mass-energy density is changed only by the increase in volume, but the energy density of radiation is changed both by the increase in volume and by the increase in the [[wavelength]] of the photons that make it up. Thus the energy of radiation becomes a smaller part of the universe's total energy than that of matter as it expands. The very early universe is said to have been 'radiation dominated' and radiation controlled the deceleration of expansion. Later, as the average energy per photon becomes roughly 10 [[electronvolt|eV]] and lower, matter dictates the rate of deceleration and the universe is said to be 'matter dominated'. The intermediate case is not treated well [[analytic solution|analytically]]. As the expansion of the universe continues, matter dilutes even further and the cosmological constant becomes dominant, leading to an acceleration in the universe's expansion.