What is the Densest Known Material?

[dm4b]

This is a question about something in Sean Carrol's Spacetime and Geometry text. I'll quote here:

" ... current estimates of the maximum possible neutron-star mass are around 3-4 Ms, the Oppenheimer-Volkofflimit. Since a fluid of neutrons is the densest material we know about (apart from some very speculative suggestions), it is believed that the outcome of such a collapse is a black hole"

I was just curious about the part in bold. Anybody familar with these "speculative suggestions".

Thanks
dm4b

 

[bcrowell]

Try googling on some of the following: black stars, gravastars, fuzzballs, quark stars, boson stars, Q-balls, and electroweak stars.