Editing Neutron star (section)

=== Current constraints ===
Because equations of state for neutron stars lead to different observables, such as different mass-radius relations, there are many astronomical constraints on equations of state. These come mostly from [[LIGO]],<ref>{{Cite web |title=LIGO Lab {{!}} Caltech {{!}} MIT |url=https://www.ligo.caltech.edu/ |access-date=2024-05-10 |website=LIGO Lab {{!}} Caltech}}</ref> which is a gravitational wave observatory, and [[Neutron Star Interior Composition Explorer|NICER]],<ref>{{Cite web |title=NICER - NASA Science |url=https://science.nasa.gov/mission/nicer/ |access-date=2024-05-10 |website=science.nasa.gov |language=en-US}}</ref> which is an X-ray telescope.

NICER's observations of [[pulsar]]s in binary systems, from which the pulsar mass and radius can be estimated, can constrain the neutron star equation of state. A 2021 measurement of the pulsar [[PSR J0740+6620]] was able to constrain the radius of a 1.4 solar mass neutron star to {{val|12.33|0.76|0.8}} km with 95% confidence.<ref>{{Cite journal |last1=Raaijmakers |first1=G. |last2=Greif |first2=S. K. |last3=Hebeler |first3=K. |last4=Hinderer |first4=T. |last5=Nissanke |first5=S. |last6=Schwenk |first6=A. |last7=Riley |first7=T. E. |last8=Watts |first8=A. L. |last9=Lattimer |first9=J. M. |last10=Ho |first10=W. C. G. |date=2021-09-01 |title=Constraints on the Dense Matter Equation of State and Neutron Star Properties from NICER's Mass–Radius Estimate of PSR J0740+6620 and Multimessenger Observations |journal=The Astrophysical Journal Letters |volume=918 |issue=2 |pages=L29 |doi=10.3847/2041-8213/ac089a |doi-access=free |arxiv=2105.06981 |bibcode=2021ApJ...918L..29R |issn=2041-8205}}</ref> These mass-radius constraints, combined with [[Chiral perturbation theory|chiral effective field theory]] calculations, tightens constraints on the neutron star equation of state.<ref name=":1" />

Equation of state constraints from LIGO gravitational wave detections start with nuclear and atomic physics researchers, who work to propose theoretical equations of state (such as FPS, UU, APR, L, SLy, and others). The proposed equations of state can then be passed onto astrophysics researchers who run simulations of [[Neutron star merger|binary neutron star mergers]]. From these simulations, researchers can extract [[gravitational wave]]forms, thus studying the relationship between the equation of state and gravitational waves emitted by binary neutron star mergers. Using these relations, one can constrain the neutron star equation of state when gravitational waves from binary neutron star mergers are observed. Past [[numerical relativity]] simulations of binary neutron star mergers have found relationships between the equation of state and frequency dependent peaks of the gravitational wave signal that can be applied to [[LIGO]] detections.<ref>{{Cite journal |last1=Takami |first1=Kentaro |last2=Rezzolla |first2=Luciano |last3=Baiotti |first3=Luca |date=2014-08-28 |title=Constraining the Equation of State of Neutron Stars from Binary Mergers |url=https://link.aps.org/doi/10.1103/PhysRevLett.113.091104 |journal=Physical Review Letters |language=en |volume=113 |issue=9 |page=091104 |doi=10.1103/PhysRevLett.113.091104 |pmid=25215972 |arxiv=1403.5672 |bibcode=2014PhRvL.113i1104T |issn=0031-9007}}</ref> For example, the LIGO detection of the binary neutron star merger [[GW170817]] provided limits on the tidal deformability of the two neutron stars which dramatically reduced the family of allowed equations of state.<ref>{{Cite journal |last1=Annala |first1=Eemeli |last2=Gorda |first2=Tyler |last3=Kurkela |first3=Aleksi |last4=Vuorinen |first4=Aleksi |date=2018-04-25 |title=Gravitational-Wave Constraints on the Neutron-Star-Matter Equation of State |url=https://link.aps.org/doi/10.1103/PhysRevLett.120.172703 |journal=Physical Review Letters |language=en |volume=120 |issue=17 |page=172703 |doi=10.1103/PhysRevLett.120.172703 |pmid=29756823 |arxiv=1711.02644 |bibcode=2018PhRvL.120q2703A |issn=0031-9007}}</ref> Future gravitational wave signals with next generation detectors like [[Cosmic Explorer (gravitational wave observatory)|Cosmic Explorer]] can impose further constraints.<ref>{{Cite journal |last1=Finstad |first1=Daniel |last2=White |first2=Laurel V. |last3=Brown |first3=Duncan A. |date=2023-09-01 |title=Prospects for a Precise Equation of State Measurement from Advanced LIGO and Cosmic Explorer |journal=The Astrophysical Journal |volume=955 |issue=1 |pages=45 |doi=10.3847/1538-4357/acf12f |doi-access=free |arxiv=2211.01396 |bibcode=2023ApJ...955...45F |issn=0004-637X}}</ref>

When nuclear physicists are trying to understand the likelihood of their equation of state, it is good to compare with these constraints to see if it predicts neutron stars of these masses and radii.<ref>{{cite arXiv |last1=Lovato |first1=Alessandro |last2=Dore |first2=Travis |display-authors=1 |title=Long Range Plan: Dense matter theory for heavy-ion collisions and neutron stars |date=2022 |class=nucl-th |eprint=2211.02224}}</ref> There is also recent work on constraining the equation of state with the speed of sound through hydrodynamics.<ref>{{cite journal|last1=Hippert |first1=Mauricio |last2=Noronha |first2=Jorge |last3=Romatschke |first3=Paul |title=Upper Bound on the Speed of Sound in Nuclear Matter from Transport |journal=Physics Letters B |date=2025 |volume=860 |doi=10.1016/j.physletb.2024.139184 |arxiv=2402.14085|bibcode=2025PhLB..86039184H }}</ref>