Could black holes resemble cold neutrino stars? A question about falling into a "neutrino star" led me to think about the physics of such an object, and to discover that there are already papers on the subject.(2000 2001)
I was struck by the way in which a cold neutrino star resembles a slowly evaporating black hole. The neutral-current (Z boson) interactions between the neutrinos should lead to a very slow production rate of other particle species, and when charged particles recombine as a photon, it will radiate away. It reminds me of Hawking radiation. 
So now I am wondering just how far the resemblance goes. Could the end-state of gravitational collapse be a cold gas of fermions, held apart by Pauli exclusion, and dominated by the fermion species that interacts the least? I'm not sure how to think about the event horizon in this picture, but maybe the "fuzzball" hypothesis applies. 
edit: Another thing that struck me, is that falling into a cold neutrino star is like crossing the event horizon of a large black hole; nothing happens at first, because interactions are so weak. So it simulates the equivalence principle at the horizon, yet also has an enormous entropy. 
 A: This is an odd question. I have not read the referenced papers. I will agree that in principle it is possible to have a condensate of neutrinos. It is not at all clear to me how this can evolve into being. 
This is related to the question on whether one could stand on a neutrino star. First off neutrinos only interact by the weak interaction and gravity. They do not interact by the nuclear strong force or the electromagnetic force. This means that neutrinos do not interact with things readily, and one has to have lots of detector material to get a signature of a weak interacting neutrino out of many trillions that pass through every few seconds. Largely this would suggest that virtually anything would pass through a neutrino star, light or material bodies. Based on that it would mean if you tried to stand on a surface of a neutrino star you would start to fall through it to the center and emerge out the other side. 
A condensate of neutrinos would at first detection by its gravitational influence appear similar to a black hole. It would be this invisible object that has certain gravitational influences. If it is dense enough it might even have an accretion disk from another star. However, something odd would be noticeable. The material in the accretion disk would enter this object and either fly out or remain in it. So the neutrino star would be a bunch of cold neutrinos with a lot of hot matter where the two systems are very weakly interacting with each other. At this point the difference with a black hole would be noticeable. 
A black hole has an event horizon, or as Hawking now sees it an apparent horizon that is close to looking like an event horizon, where stuff crosses it and it removed from the outside world. All the outside world can see of an object approaching a black hole is an immense time dilated and red shifted signature from the object as it approaches the horizon. In effect anything approaching a black hole is "winked out" by red shift and it is lost. This is very different from what a putative neutrino star might look like.
