Editing Dark matter (section)

== Technical definition ==
{{see also|Friedmann equations}}
In standard cosmological calculations, ''"matter"'' means any constituent of the universe whose energy density scales with the inverse cube of the [[scale factor (cosmology)|scale factor]], i.e., {{nobr|{{math|''ρ'' ∝ ''a''{{sup|−3}} }}.}} This is in contrast to ''"radiation"'', which scales as the inverse fourth power of the scale factor {{nobr|{{math|''ρ'' ∝ ''a''{{sup|−4}} }},}} and a [[cosmological constant]], which does not change with respect to {{mvar|a}} ({{nobr|{{math|''ρ'' ∝ ''a''{{sup|0}}}}}}).<ref name="Baumann lecture notes" /> The different scaling factors for matter and radiation are a consequence of radiation [[redshift]]. For example, after doubling the diameter of the observable Universe via [[cosmic expansion]], the scale, {{mvar|a}}, has doubled. The energy of the [[cosmic microwave background radiation]] has been halved (because the wavelength of each photon has doubled);<ref>{{cite news |last1=Siegel |first1=Ethan |title=Is energy conserved when photons redshift in our expanding universe? |year=2019 |work=[[Starts With a Bang]] |url=https://www.forbes.com/sites/startswithabang/2019/08/14/is-energy-conserved-when-photons-redshift-due-to-the-expanding-universe/?sh=745b3e3a3efa |access-date=5 November 2022 |language=en}}</ref> the energy of ultra-relativistic particles, such as early-era standard-model neutrinos, is similarly halved.{{efn|
However, in the modern cosmic era, this neutrino field has cooled and started to behave more like matter and less like radiation.
}} The cosmological constant, as an intrinsic property of space, has a constant energy density regardless of the volume under consideration.<ref name="Baumann lecture notes">{{cite web |first=Daniel |last=Baumann |title=Cosmology: Part&nbsp;III |department=Mathematical Tripos |publisher=Cambridge University |pages=21–22 |url=http://www.damtp.cam.ac.uk/user/db275/Cosmology/Lectures.pdf |access-date=24 January 2017 |archive-url=https://web.archive.org/web/20170202065045/http://www.damtp.cam.ac.uk/user/db275/Cosmology/Lectures.pdf |archive-date=2 February 2017 }}</ref>

In principle, "dark matter" means all components of the universe which are not visible but still obey {{nobr|{{math|''ρ'' ∝ ''a''{{sup|−3}} }}.}} In practice, the term "dark matter" is often used to mean only the non-baryonic component of dark matter, i.e., excluding "[[missing baryon problem|missing baryons]]".<ref>{{cite arXiv |author=Peter |first=Annika H. G. |date=18 Jan 2012 |title=Dark Matter: A Brief Review |class=astro-ph.CO |eprint=1201.3942 }}</ref> Context will usually indicate which meaning is intended.