Editing Penrose–Hawking singularity theorems (section)

=== Modified gravity ===
In modified gravity, the Einstein field equations do not hold and so these singularities do not necessarily arise. For example, in [[Infinite Derivative Gravity]], it is possible for <math>{E[\vec{X}]^a}_a</math> to be negative even if the Null Energy Condition holds.<ref>{{Cite journal|arxiv=1605.02080 |title=Defocusing of Null Rays in Infinite Derivative Gravity|journal=Journal of Cosmology and Astroparticle Physics|volume=2017|pages=017|last1=Conroy|first1=Aindriú|last2=Koshelev|first2=Alexey S|last3=Mazumdar|first3=Anupam|year=2016|issue=1|doi=10.1088/1475-7516/2017/01/017|bibcode=2017JCAP...01..017C|s2cid=115136697}}</ref><ref>{{Cite journal|arxiv=1705.02382 |title=Newtonian Potential and Geodesic Completeness in Infinite Derivative Gravity|journal=Physical Review D|volume=96|issue=4|last1=Conroy|first1=Aindriú|last2=Edholm|first2=James|year=2017|page=044012|doi=10.1103/PhysRevD.96.044012|bibcode=2017PhRvD..96d4012E|s2cid=45816145}}</ref>