On Tuesday, October 17, 2017 at 4:25:36 AM UTC-4, Tom Roberts wrote:   
   > Note: the current limit on the sum of neutrino masses is below 1 eV. The   
   > neutrinos we detect have energies much greater than that, so they have   
   > speeds greater than 0.99 c. I will discuss only neutrinos that have some   
   > prospect of being detected.   
   >   
   > On 10/16/17 6:45 AM, toadastronomer@gmail.com wrote:   
   > > Could neutrinos be lost behind black hole horizons   
   >   
   > Certainly.   
   >   
   > > or get trapped in accretion around them?   
   >   
   > Not likely. The phase space for orbits with speeds > 0.99 c is extremely   
   > small [#], and there's no plausible mechanism to trap neutrinos in such   
   > orbits.   
   >   
   > 	[#] Light orbits a Schwarzschild black hole at r=3M; these   
   > 	orbits would be slightly outside that. Light orbits are   
   > 	exactly circular; these are very nearly so.   
   >   
   > > Could neutrinos interact strongly with neutron star matter or perhaps get   
   > > trapped in low neutron star orbit?   
   >   
   > Note that "strongly interacting" has a technical meaning in physics, and   
   > neutrinos don't interact strongly; they are neutral leptons which   
   > interact only weakly (also a technical term) and gravitationally   
   > (ditto). Once formed, a neutron star is nearly transparent to neutrinos,   
   > due to the tiny neutrino cross-section and the colossal degeneracy of   
   > the neutron-star constituents.   
   >   
   > For an object that originates far away from a mass to enter an orbit   
   > requires the object to interact non-gravitationally near the mass, such   
   > that its speed is reduced to that of an orbit there. There is no   
   > plausible way to do that for a neutrino.   
   >   
   > > Might the neutrino flux in the direction of the galactic center (or   
   > > line-of-sight to any compact object) be slightly lower than background, due   
   > > to such gravitational interactions?   
   >   
   > Unmeasurably so. The only missing neutrinos would be those that   
   > intersect the black hole's horizon, and that subtends an extremely tiny   
   > solid angle from earth. Its gravitation will deflect neutrinos over a   
   > much larger area, which is in principle observable, but in practice our   
   > neutrino detectors cannot point back with anywhere close to enough   
   > accuracy to see it.   
   >   
   > Tom Roberts   
      
   21-OCT-2017   
      
   Your explanation resolves my error; i.e. trying   
   to impose a late time view.  The substantive   
   relations between neutrinos and say a neutron   
   star, occur in the formation of such an object.   
      
   Naively, I had suspected that something with a   
   density of maybe 10^18 kg/m^3 would be rather   
   opaque to neutrinos.   
      
   I suspect now that this absence of observable   
   effects between liberated neutrinos and pre-existing   
   neutron stars, must be complementary to the flux we   
   can detect, signalling the neutron star's birth.   
      
   But the universe itself, something of a compact   
   object, must have possessed the same "degeneracy"   
   that would have the neutrino pressure and flux   
   responsible for outward pressure and "strong"   
   interactions.  As the universe expands and cools,   
   star/planetary/life formation yield ever more   
   neutrinos to the expanding space.   
      
   Could dark matter/energy in this sense, be responsive   
   to neutrino pressure so that neutrinos, in perpetuity,   
   drive the resistance to gravitational collapse of the   
   whole thing, and hence the (accelerating) expansion?   
      
   cheers,   
   mark jonathan horn   
      
   --- SoupGate-Win32 v1.05   
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