Editing Physical cosmology (section)

===Dark energy===
{{Main|Dark energy}}
If the universe is [[Flatness (cosmology)|flat]], there must be an additional component making up 73% (in addition to the 23% dark matter and 4% baryons) of the energy density of the universe. This is called dark energy. In order not to interfere with Big Bang nucleosynthesis and the cosmic microwave background, it must not cluster in haloes like baryons and dark matter. There is strong observational evidence for dark energy, as the total energy density of the universe is known through constraints on the flatness of the universe, but the amount of clustering matter is tightly measured, and is much less than this. The case for dark energy was strengthened in 1999, when measurements demonstrated that the expansion of the universe has begun to gradually accelerate.<ref name=Perlmutter1999>{{cite journal | title=Constraining Dark Energy with Type Ia Supernovae and Large-Scale Structure | last1=Perlmutter | first1=Saul | last2=Turner | first2=Michael S. | last3=White | first3=Martin | journal=Physical Review Letters | volume=83 | issue=4 | year=1999 | pages=670–673 | doi=10.1103/PhysRevLett.83.670 | bibcode=1999PhRvL..83..670P | arxiv=astro-ph/9901052 | s2cid=119427069 | url=https://zenodo.org/record/1233929 }}</ref>

Apart from its density and its clustering properties, nothing is known about dark energy. ''[[Quantum field theory]]'' predicts a cosmological constant (CC) much like dark energy, but 120 [[orders of magnitude]] larger than that observed.<ref name=Adler1995>{{cite journal | title=Vacuum catastrophe: An elementary exposition of the cosmological constant problem | last1=Adler | first1=Ronald J. | last2=Casey | first2=Brendan | last3=Jacob | first3=Ovid C. | journal=American Journal of Physics | volume=63 | issue=7 | pages=620–626 | date=July 1995 | doi=10.1119/1.17850 | bibcode=1995AmJPh..63..620A | doi-access=free }}</ref> [[Steven Weinberg]] and a number of string theorists ''(see [[string landscape]])'' have invoked the 'weak [[anthropic principle]]': i.e. the reason that physicists observe a universe with such a small cosmological constant is that no physicists (or any life) could exist in a universe with a larger cosmological constant. Many cosmologists find this an unsatisfying explanation: perhaps because while the weak anthropic principle is self-evident (given that living observers exist, there must be at least one universe with a cosmological constant (CC) which allows for life to exist) it does not attempt to explain the context of that universe.<ref name=Siegfried2006>{{cite journal | title=A 'Landscape' Too Far? | first1=Tom | last1=Siegfried | journal=Science | date=11 August 2006 | volume=313 | issue=5788 | pages=750–753 | doi=10.1126/science.313.5788.750 | pmid=16902104 | s2cid=118891996 }}</ref> For example, the weak anthropic principle alone does not distinguish between:
* Only one universe will ever exist and there is some underlying principle that constrains the CC to the value we observe.
* Only one universe will ever exist and although there is no underlying principle fixing the CC, we got lucky.
* Lots of universes exist (simultaneously or serially) with a range of CC values, and of course ours is one of the life-supporting ones.

Other possible explanations for dark energy include [[quintessence (physics)|quintessence]]<ref name=Sahni2002>{{cite journal | title=The cosmological constant problem and quintessence | last1=Sahni | first1=Varun | journal=Classical and Quantum Gravity | volume=19 | issue=13 | pages=3435–3448 | year=2002 | doi=10.1088/0264-9381/19/13/304 | bibcode=2002CQGra..19.3435S | arxiv=astro-ph/0202076 | s2cid=13532332 }}</ref> or a modification of gravity on the largest scales.<ref name=Nojiri2007>{{cite journal | title=Introduction to Modified Gravity and Gravitational Alternative for Dark Energy | last1=Nojiri | first1=S. | last2=Odintsov | first2=S. D. | journal=International Journal of Geometric Methods in Modern Physics | volume=04 | issue=1 | pages=115–146 | year=2006  | doi=10.1142/S0219887807001928 | arxiv=hep-th/0601213 | bibcode=2007IJGMM..04..115N | s2cid=119458605 }}</ref> The effect on cosmology of the dark energy that these models describe is given by the dark energy's [[equation of state (cosmology)|equation of state]], which varies depending upon the theory. The nature of dark energy is one of the most challenging problems in cosmology.

A better understanding of dark energy is likely to solve the problem of the [[ultimate fate of the universe]]. In the current cosmological epoch, the accelerated expansion due to dark energy is preventing structures larger than [[supercluster]]s from forming. It is not known whether the acceleration will continue indefinitely, perhaps even increasing until a [[big rip]], or whether it will eventually reverse, lead to a [[Heat death of the universe|Big Freeze]], or follow some other scenario.<ref name=Jambrina2014>{{cite journal | title=Grand rip and grand bang/crunch cosmological singularities | last1=Fernández-Jambrina | first1=L. | journal=Physical Review D | volume=90 | issue=6 | page=064014 | date=September 2014 | doi=10.1103/PhysRevD.90.064014 | bibcode=2014PhRvD..90f4014F | arxiv=1408.6997 | s2cid=118328824 }}</ref>