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AFAIK one possible explanation for the black hole issues could be primordial black holes, which are also a candidate for at least a component of dark matter.

by api 2y ago

PaulHoule 2y ago
Yep. There is the idea that you could get little primordial black holes (that maybe weigh as much as a mountain and could be evaporating now) and the idea that you could get huge primordial black holes. Also the occasional strange idea that the universe might be cyclic (not too fashionable but can fill the hole left by inflation) and that black holes can survive the crunch.
- numpy-thagoras 2y ago
  Black holes can survive a Big Crunch scenario? That can go a long way to explaining many things. Can you please provide a paper with more references to this, and potentially one with an example mechanism?
- api 2y ago
  I love the idea. It’s one of our current physics hypotheses I hope is true, because it means the universe would be full of tiny things the size of a hydrogen atom with the mass of asteroids. “The devil’s glitter?” BTW they would not suck up planets and stuff like inaccurate sci-fi. One could be going real fast and fire right though the Earth and do little, maybe cause some seismic events, but we would never know unless we knew exactly what to look for. A tiny black hole would have a tiny event horizon. If you dropped one into a planet or star with a low enough velocity that it didn’t shoot out the other side it might do a lot of damage, then come to rest in the middle and slowly grow. I recall reading that one in Earth’s core would take possibly millions of years to do much since the radiation pressure caused by accretion around it would limit the rate of matter falling into it. Earth would eventually become an Earth mass black hole but it would not happen in any human lifetime, possibly not in the lifetime of the human race.
  - Vampiero 2y ago
    If a micro black hole could cause a seismic event couldn't it also just kill random people by flying through their brains?
- Vecr 2y ago
  How could the universe be cyclic? There would need to be some strange self-consistency/teleology thing forcing it to collapse each time. Generally seems doubtful. Are black holes needed right now? If not, you can't argue off selection.
  - Vampiero 2y ago
    Conformal cyclic cosmology states that the end state of the universe is conformally equivalent to its beginning and that therefore any notion of scale loses its meaning at the end/beginning of a cycle. So it doesn't collapse. It only gets larger and larger, but since the notion of distance is encoded in the metric, and since only relative distances matter, it's "as if" it restarts once there's nothing left but energy (i.e. nothing to measure distances from or to). It doesn't actually contract and return to T=0, it's more like going to T % N and starting over. And there are theoretical gravitational artifacts that could be measured in the CMB if the theory is true.
    - raattgift 2y ago
      A couple things on conformal cyclic cosmology (CCC) for smooth 3+1 dimensional spacetimes: > it restarts once there's nothing left but energy The "nothing left but energy" part is a requirement that matter (in the broadest sense) must be conformally invariant so that its active gravitation as a source allows for the CCC conformal rescaling of the FLRW metric, which is how a predecessor "aeon" is connected mathematically to its successor. Of the Friedmann dusts, only lightlike radiation can be rescaled this way. Equivalently, all the matter left in the universe at the end of an aeon must be on null geodesics. Because local Lorentz invariance is fully baked into the Standard Model of Particle Physics (SM), and because the SM has particles with nonzero rest masses (which in a Lorentzian patch cannot couple to null geodesics), NOBODY knows how to do this in a consistent way (let alone in a way that matches actual evidence from particle physics). (If we restrict to QED we need to convert all electrons and positrons into photons and impose some unknown mechanism to suppress statmech fluctuations back to a non-conformally-invariant condition. In reverse order, two-photon physics are maaaaaaybe supressible adiabatically. However, there just aren't enough positrons to find and annihilate every electron (adiabatic expansion makes that even less likely!), and electrons on their own don't decay into photons. Of course, once we add in the weak and strong forces, we are far beyond quantum electrodynamics, with all sorts of new ways in which reaching a conformally invariant stress-energy state becomes implausible. Oh yeah, and now do dark matter.) WRT previous comments in this thread, any black hole (BH) at the end of an "aeon" must radiate only massless bosons. This puts a pretty strong lower limit on the mass of Hawking-radiating BHs crossing an boundary between "aeons", or destroys the mathematics of the hypothesized conformal rescaling by virtue of having incompatible non-conformally-invariant field theories on both sides of the Einstein Field Equations. Binary BHs don't fit cleanly into the FLRW rescaling picture either: among other things they source a metric that isn't locally isotropic and homogeneous (a dust of high mass isolated singleton Birkhoff-theorem BHs is mostly fine though, and in principle you could get through through BH mergers and a stronger cosmological coupling (some types of "fifth force"/quintessence, for instance, to break apart wide BH binaries/triples/multiples)). There are maybe escapes from some of these constraints in extra dimensions and lattices, but I've been under the impression that one of the attractions of CCC is that it's compatible with the FLRW metric of the standard cosmology. Does CCC work with small perturbations on the boundary between aeons? Who knows. However, it probably works with small perturbations near that boundary, because we do perturbative FLRW routinely these days. So maybe there's some mechanism (e.g. in dark energy) that makes small deviations from conformally invariant field configurations entirely vanish at the aeon boundary. But we have no astrophysical or particle physics evidence for that at all. Finally, the article at the top is about inhomogeneous cosomology -- i.e., Timescapes metric is not FLRW metric -- and whatever one's position on CCC, it rests on the standard view that at large scales the universe is homogeneous and that any inhomogenities, anisotropies, and backreactions vanish at smaller scales (and so admit perturbation theory).

AFAIK one possible explanation for the black hole issues could be primordial black holes, which are also a candidate for at least a component of dark matter.

by api 2y ago

AFAIK one possible explanation for the black hole issues could be primordial black holes, which are also a candidate for at least a component of dark matter.

PaulHoule 2y ago

Yep. There is the idea that you could get little primordial black holes (that maybe weigh as much as a mountain and could be evaporating now) and the idea that you could get huge primordial black holes. Also the occasional strange idea that the universe might be cyclic (not too fashionable but can fill the hole left by inflation) and that black holes can survive the crunch.

numpy-thagoras 2y ago
Black holes can survive a Big Crunch scenario? That can go a long way to explaining many things. Can you please provide a paper with more references to this, and potentially one with an example mechanism?
api 2y ago
I love the idea. It’s one of our current physics hypotheses I hope is true, because it means the universe would be full of tiny things the size of a hydrogen atom with the mass of asteroids. “The devil’s glitter?” BTW they would not suck up planets and stuff like inaccurate sci-fi. One could be going real fast and fire right though the Earth and do little, maybe cause some seismic events, but we would never know unless we knew exactly what to look for. A tiny black hole would have a tiny event horizon. If you dropped one into a planet or star with a low enough velocity that it didn’t shoot out the other side it might do a lot of damage, then come to rest in the middle and slowly grow. I recall reading that one in Earth’s core would take possibly millions of years to do much since the radiation pressure caused by accretion around it would limit the rate of matter falling into it. Earth would eventually become an Earth mass black hole but it would not happen in any human lifetime, possibly not in the lifetime of the human race.
- Vampiero 2y ago
  If a micro black hole could cause a seismic event couldn't it also just kill random people by flying through their brains?
Vecr 2y ago
How could the universe be cyclic? There would need to be some strange self-consistency/teleology thing forcing it to collapse each time. Generally seems doubtful. Are black holes needed right now? If not, you can't argue off selection.
- Vampiero 2y ago
  Conformal cyclic cosmology states that the end state of the universe is conformally equivalent to its beginning and that therefore any notion of scale loses its meaning at the end/beginning of a cycle. So it doesn't collapse. It only gets larger and larger, but since the notion of distance is encoded in the metric, and since only relative distances matter, it's "as if" it restarts once there's nothing left but energy (i.e. nothing to measure distances from or to). It doesn't actually contract and return to T=0, it's more like going to T % N and starting over. And there are theoretical gravitational artifacts that could be measured in the CMB if the theory is true.
  - raattgift 2y ago
    A couple things on conformal cyclic cosmology (CCC) for smooth 3+1 dimensional spacetimes: > it restarts once there's nothing left but energy The "nothing left but energy" part is a requirement that matter (in the broadest sense) must be conformally invariant so that its active gravitation as a source allows for the CCC conformal rescaling of the FLRW metric, which is how a predecessor "aeon" is connected mathematically to its successor. Of the Friedmann dusts, only lightlike radiation can be rescaled this way. Equivalently, all the matter left in the universe at the end of an aeon must be on null geodesics. Because local Lorentz invariance is fully baked into the Standard Model of Particle Physics (SM), and because the SM has particles with nonzero rest masses (which in a Lorentzian patch cannot couple to null geodesics), NOBODY knows how to do this in a consistent way (let alone in a way that matches actual evidence from particle physics). (If we restrict to QED we need to convert all electrons and positrons into photons and impose some unknown mechanism to suppress statmech fluctuations back to a non-conformally-invariant condition. In reverse order, two-photon physics are maaaaaaybe supressible adiabatically. However, there just aren't enough positrons to find and annihilate every electron (adiabatic expansion makes that even less likely!), and electrons on their own don't decay into photons. Of course, once we add in the weak and strong forces, we are far beyond quantum electrodynamics, with all sorts of new ways in which reaching a conformally invariant stress-energy state becomes implausible. Oh yeah, and now do dark matter.) WRT previous comments in this thread, any black hole (BH) at the end of an "aeon" must radiate only massless bosons. This puts a pretty strong lower limit on the mass of Hawking-radiating BHs crossing an boundary between "aeons", or destroys the mathematics of the hypothesized conformal rescaling by virtue of having incompatible non-conformally-invariant field theories on both sides of the Einstein Field Equations. Binary BHs don't fit cleanly into the FLRW rescaling picture either: among other things they source a metric that isn't locally isotropic and homogeneous (a dust of high mass isolated singleton Birkhoff-theorem BHs is mostly fine though, and in principle you could get through through BH mergers and a stronger cosmological coupling (some types of "fifth force"/quintessence, for instance, to break apart wide BH binaries/triples/multiples)). There are maybe escapes from some of these constraints in extra dimensions and lattices, but I've been under the impression that one of the attractions of CCC is that it's compatible with the FLRW metric of the standard cosmology. Does CCC work with small perturbations on the boundary between aeons? Who knows. However, it probably works with small perturbations near that boundary, because we do perturbative FLRW routinely these days. So maybe there's some mechanism (e.g. in dark energy) that makes small deviations from conformally invariant field configurations entirely vanish at the aeon boundary. But we have no astrophysical or particle physics evidence for that at all. Finally, the article at the top is about inhomogeneous cosomology -- i.e., Timescapes metric is not FLRW metric -- and whatever one's position on CCC, it rests on the standard view that at large scales the universe is homogeneous and that any inhomogenities, anisotropies, and backreactions vanish at smaller scales (and so admit perturbation theory).