Physicists propose new theories of black holes from the very early universe | Hacker News Reader

Physicists propose new theories of black holes from the very early universe

phys.org

55 pointsdnetesn12 comments

When that happens, Kusenko said, the primordial black hole consumes the neutron star from the inside, a process that takes about 10,000 years. As the neutron star shrinks, it spins even faster, eventually causing small fragments to detach and fly off. Those fragments of neutron-rich material may be the sites in which neutrons fuse into heavier and heavier elements, Kusenko said. However, the probability of a neutron star capturing a black hole is rather low, said Kusenko, which is consistent with observations of only some galaxies being enriched in heavy elements. The theory that primordial black holes collide with neutron stars to create heavy elements also explains the observed lack of neutron stars in the center of the Milky Way galaxy, a long-standing mystery in astrophysics.

Very interesting, and amazing when you consider the odds of something as minuscule as a neuton star and primordial black hole colliding. Without the papers, it's hard to tell if that aspect of the conjecture is valid, could occur with the needed frequency to match observations.

For what it's worth, there are friction-like processes that allow black hole and neutron stars to get captured into each other's orbits and then spiral in toward each other. BH and NS mergers (such as seen by LIGO) are not direct head-on chance collisions between freely propagating objects in the Galaxy.

Are these processes gravitationally mediated, like tidal energy transfer?

Binary systems radiate out gravitational waves. Google image search for "binary system gravitational waves" and you will pretty easily identify the canonical graphical representation. While there are lots of problems with the "sheet" analogy for general relativity, there is still value in considering gravitational radiation to be waves in the sheet. From a layman perspective, the analogy is irresistible.

The mental picture of the spiraling-in process is pretty easy to grasp, but the process of gravitational capture is a little more difficult. Gravitational radiation is extremely dependent on the distance of the mutual orbit, so the inward spiral progresses rapidly at short distances, but can take a very long time at greater distances. So I imagine the research comes down to the general statistics of the formation of binary systems, how often they will involve a neutron star with a black hole, and how often they form at distances close enough to create the collision.

This only works when the bodies are very close together, right before they coalesce. For most of the lifetime of binary BHs and NSs, the gravitational radiation is negligible.

Yes, mostly, but it takes the form of significant gravitational wave emission only when the bodies are very close together. (Much less than an AU, I think, although don't quote me on that.) For larger distances, it's mostly a matter of ejecting dust and other matter in the vacinity.

https://en.wikipedia.org/wiki/Dynamical_friction

Given that how common it is to find massive stars in multiple systems, it would be surprising not to find systems of compact massive objects that are remnants of these.

OTOH, who knows about intermediate mass BHs and beyond. Aggregation of these multiple systems of CMOs? Let's look!

- --

sec 3.5 and table 1, Duchêne & Kraus, https://arxiv.org/abs/1303.3028

multiplicity counts are all over the map:

Eggleton & Tokovinin 2008 Guerrero et al. 2014 Lada 2006

but pretty much nobody thinks they're extremely rare.

The universe was once a much smaller and stranger place. These things happened when black holes and massive stars were a dime a dozen. The needle on LIGO would have twitched like crazy back then.

They could at least link to the papers they mention...

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.11...

I don't see how this process clears the center of the galaxy of neutron stars, especially given the fact that only some galaxies have heavy elements. How does our galaxy get cleaned out while others are left alone?

This was poorly explained. My assumption is that it changes the population spreads of early stars that would have been expected to populate the galactic center: we now need fewer supernovae to produce the heavy element distribution observed, so we'd expect to see fewer supernova remnants.

Which then begs the question of why there is a dearth of neutron stars in the galactic center in particular. This effect would reduce the number of old neutron stars everywhere, which I guess we don't observe? Or maybe it's just that very old neutron stars are hard to find in isolation (i.e. once their nebulae have dispersed) and that in the center we could see them due to gravitation because of the star density there?

Reply on news.ycombinator.com