In 1984 Edward Witten, in a famous paper, formulated the hypothesis that the absolute ground state of matter is not 56Fe, but a cluster of up, down and strange quarks, an idea which became known as the “strange quark matter hypothesis”. In that seminal paper he also speculated that strange quark matter can constitute dark matter and that it can exist in chunks of all sizes, from small ones containing maybe just a few hundred quarks up to objects having the mass of the Sun and called strange quark stars.
After almost 40 years we are still investigating Witten’s hypothesis, which, if true, would have consequences in nuclear physics, in astrophysics, in cosmology and, potentially, could even open a new path to producing energy.
I will clarify what Witten’s hypothesis is and how it relates to a variety of observables, ranging from properties of compact stars, to features of explosive astrophysical processes, to properties of dark matter and I will discuss how strange quark matter can be searched for.
In the last years a few observations have suggested that some specific objects can be identified as strange quark stars. In particular, one of the two objects in the merger GW190814 has a mass too large to be an ordinary neutron star [1,2], while the masses of SAX1808.4-3658 [3] and of HESS J1731-347 [4] are too small. We have shown instead that all of those objects can easily be interpreted as strange quark stars [5,6,7].
[1] Abbott et al. ApJ 896:L44 (2020)
[2] F. J. Fattoyev et al., Phys. Rev. C 102, 065805
[3] T. Di Salvo et al., MNRAS 483, 767–779 (2019)
[4] G.Pühlhofer & A. Santangelo, Nature Astronomy doi.org/10.1038/s41550-022-01800-1
[5] I. Bombaci, A. Drago, D. Logoteta, G. Pagliara and I. Vidaña, PRL 126, 162702 (2021)
[6] F. Di Clemente, A. Drago, P. Char, G. Pagliara, 2207.08704
[7] F. Di Clemente, A. Drago, G. Pagliara, 2211.07485