Stellar remnants in a state of matter denser than neutron-degenerate

Question

When discussing the stellar life cycle, it's often stated that if the collapsing core of a star is bigger than the mass limit for a stable neutron star, it must collapse to a black hole.

However, could there be additional states of matter, denser than neutron-degenerate matter, that could form a stable stellar remnant without collapsing all the way to a black hole?

For example, I've seen a bit about hypothetical "quark matter" on Wikipedia, but there's not much info about it. Could there also be some type of "GUT matter" based on the unknown fundamental physics at the GUT scale? Or does current knowledge make this unlikely?

Answer 1

There is some debate about whether it exists or not, but there has been some research into what is called a quark star.

This article (should be a free link, but here is the arXiv version in case) suggests that

A recent calculation for cold and dense QCD strange quark matter including corrections to order $O\left(\alpha_s^2\right)$ indicates that massive compact stars with mass $\gtrsim2\,M_\odot$ up to maximal masses $\sim2.75\,M_\odot$ would be interpreted as possible candidates for strange quark stars...

where the star would consist entirely of deconfined $u$, $d$, and $s$ quarks.

The current problem is that we just don't know if the compact objects we think are quark stars are actually quark stars or "normal" neutron stars. It could be the case that quark stars would be excellent observatories of GUTs, but we just don't have enough information.