Editing Cosmological constant (section)

=== Density parameter Ω ===
The dimensionless density parameter Ω represents the ratio of the actual density of a component of the universe to the critical density. The total density parameter for a flat universe (concluded by [https://wmap.gsfc.nasa.gov/universe/uni_shape.html WMAP]), can be expressed as:

<math display="block">\Omega_\text{total} = \Omega_m + \Omega_{\Lambda} + \Omega_k = 1</math>

Where:

* Ω<sub>''m''</sub> is the matter density parameter (including both baryonic and dark matter);
* Ω<sub>Λ</sub> is the density parameter for dark energy (cosmological constant); and
* Ω<sub>''k''</sub> describes the curvature of the universe which is 0 in a flat universe.

Instead of the cosmological constant itself, cosmologists often refer to the ratio between the energy density due to the cosmological constant and the [[Friedmann equations#Density parameter|critical density]] of the universe, the tipping point for a sufficient density to stop the universe from expanding forever (the dark energy density parameter). This ratio is estimated to be 0.714, according to results published by the [[Planck Collaboration]] in 2018 and clearly mentioned on [https://wmap.gsfc.nasa.gov/universe/uni_matter.html WMAP].<ref name="Planck-20182">{{harvp|The Planck Collaboration|2020}}.</ref> More intuitively, this parameter could be described as the fraction of the universe that is made up of dark energy. Note that this value changes over time: The critical density changes with [[cosmological time]] but the energy density due to the cosmological constant remains unchanged throughout the history of the universe, because the amount of dark energy increases as the universe grows but the amount of matter does not.<ref>{{Cite web |last=Siegel |first=Ethan |title=Dark Energy May Not Be A Constant, Which Would Lead To A Revolution In Physics |url=https://www.forbes.com/sites/startswithabang/2019/01/31/dark-energy-may-not-be-a-constant-which-would-lead-to-a-revolution-in-physics/ |access-date=2023-09-10 |website=Forbes |language=en}}</ref>{{sfnp|Peebles|Ratra|2003}}<ref>{{Cite book |last=Davies |first=Paul |title=What's Eating the Universe?: And Other Cosmic Questions |publisher=Penguin Books Australia |year=2021 |isbn=9780141993720 |language=en}}</ref>