Large sound speed in dense matter and the deformability of neutron stars

Brendan Reed, C. J. Horowitz

Research output: Contribution to journalArticlepeer-review

40 Scopus citations


The historic first detection of the binary neutron star merger GW170817 by the LIGO-Virgo collaboration has set a limit on the gravitational deformability of neutron stars. In contrast, radio observations of PSR J0740+6620 find a very massive neutron star. Tension between the small deformability and the large maximum mass may suggest that the pressure rises rapidly with density and thus the speed of sound in dense matter is likely a large fraction of the speed of light. We use these observations and simple constant sound-speed model equations of state to set a lower bound on the maximum speed of sound in neutron stars. If the tidal deformability of a 1.4M neutron star is less than 600, as is suggested by subsequent analyses of GW170817, then we find that the sound speed in the cores of neutron stars is likely larger than the conformal limit of c/3. Implications of this for our understanding of both hadronic and quark-gluon descriptions of dense matter are discussed.

Original languageEnglish
Article number045803
JournalPhysical Review C
Issue number4
StatePublished - Apr 2020
Externally publishedYes

Bibliographical note

Publisher Copyright:
© 2020 American Physical Society.


This work benefited from discussions at the First Frontiers Summer School supported by the National Science Foundation under Grant No. PHY-1430152 (JINA Center for the Evolution of the Elements). This material is based upon work supported by the US Department of Energy Office of Science, Office of Nuclear Physics under Awards No. DE-FG02-87ER40365 (Indiana University) and No. DE-SC0008808 (NUCLEI SciDAC Collaboration).

FundersFunder number
National Science FoundationPHY-1430152
U.S. Department of Energy
Directorate for Mathematical and Physical Sciences1430152
Nuclear PhysicsDE-FG02-87ER40365
Indiana UniversityDE-SC0008808


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