One notable difference between axions (probably) and the Higgs is that, while the Higgs has a really huge mass, by particle physics standards, the axion is (probably) quite tiny. The difficulty in detecting the Higgs is mostly just a difficulty in creating it, because that requires a really powerful particle accelerator like the LHC. The difficulty in detecting the axion, however, is almost all down to it having only very slight interactions with other particles.
Wouldn’t such tiny black holes have evaporated away as Hawking radiation by now, according to our current understanding of physics?
But it needs new physics to explain why Hawking was wrong about Hawking radiation.
I couldn’t begin to guess which strains Occam’s Razor more, though.
Evaporation via Hawking radiation is exactly what drives the lower mass limits on PBHs. The still-viable mass ranges are high enough that Hawking radiation is slow enough to not cause trouble.
Gotcha, makes sense - what threw me off was the framing of “asteroid sized” but I guess that actually covers quite a range of possible sizes. A dino-killer or Everest sized asteroid isn’t quite what I was thinking of, but haphazardly plugging those in to an online calculator does give lifespans longer than 13.8 billion years.
Am I right in thinking that there’s a balance point between PBH size and the cosmic black body radiation (ie if a PBH is large enough that the CBR is warmer than the PBH is, the PBH can’t evaporate?). Or is that crazy talk…
That’s much larger than the size needed for “evaporating, but just hasn’t finished evaporating yet”. I’m pretty sure that black holes large enough to feed on the CMB are also large enough to be ruled out as dark matter by the MACHO gravitational lensing searches.
Yep, that’s right. The balance point is around the mass of the moon, with heavier black holes absorbing more than radiating.
That’s also right. A nuance: For some masses in this range, PBHs could be as much as 10% of the dark matter. But the last thing we need is a solution that requires two independent dark matter sources, so this is a pretty compelling limit anyway.
On the other hand it’s quite possible that exotic dark matter* is more than one thing. Assuming it’s one thing is just Occam’s Razor, starting with the simplest possibility until the facts rule it out.
- As opposed to dark matter that’s plain old conventional matter that’s just, well, too dark to see. Which is definitely some of it. Just not most of it.
I mean, we wouldn’t like it because it would be conceptually messy, but then, reality often is. The simplest solution might be that there are around ten different things that contribute to the mass content of the Universe, and matter that interacts electromagnetically is just one of them. Heck, we already know that there are multiple sources of non-baryonic dark matter, because we know that neutrinos exist. We can put bounds on that say that they’re at most only a small fraction of the dark matter, but they’re not zero.
Also, while there is a black hole mass that would put it in thermal equilibrium with the CMB, and such a hole would neither emit nor receive radiation on net, that’s even less relevant than I said before, because the temperature of the Universe is changing. Any black hole of such a mass is already radiating slowly enough that it’s less than a rounding error in its lifetime for it to just wait for the Universe to cool off enough first.
Not sure if this warrants another thread, but I’ve heard this fact mentioned in numerous science shows and videos. Can someone explain what the issue was, and how GR resolved it, in layman’s terms? I always wonder how precise they could even measure it (including exact distance) back in those days.
Another fence around primordial black holes is neutron stars.
Whilst an encounter with a teensy black hole by a star or planet is essentially uneventful, with the PBH just passing through and going on its way, an encounter with a PBH will kill a neutron star,
So the simple fact that we observe neutrons stars, and observe some with reasonable age, places limits on the spatial density of PBHs in a galaxy, and thus the size they must be in order to account for dark matter. This pushes their required size up, and up against other limits.
(Or maybe the universe is littered with trillions of roughly solar mass black holes created from neutron stars that had a bad day with a PBH. And that accounts for all the dark matter. But gravitational lensing would have found them. Wouldn’t fit other aspects of dark matter distribution anyway.)
You would not want to be betting on PBHs.
In the grand scheme of things, “larger than a mountain but smaller than the moon” seems like a rather specific size range. And they would need to be spread out enough that they don’t fall into each other and form bigger black holes, right?
Is there any theory of the universe’s formation that explains how all these very specifically sized black holes would have formed everywhere?
I was wondering about this. Why can’t dark matter simply be non-luminous interstellar conventional matter that is bound to a galaxy by gravity? This would include normal matter from less than dust-sized to boulder-sized to asteroid-sized to planet-sized all the way up to sub-stellar-sized.
In other words, why are the leading dark matter candidates exotic forms of matter instead of conventional matter?
The supersimple summary with some lies-to-laymen handwaving thrown in:
Mercury’s orbit was measured. The orbital size & period didn’t agree, which is impossible; if those don’t agree Mercury would be spiraling in or out, not orbiting. Something was clearly fishy.
Pre- GR that something had to be another as-yet undiscovered planet.
When GR came along it included frame dragging. Gravity wasn’t just simple attraction, but included a rotating component if the sources were rotating. Once those additional factors were added in, Mercury’s orbit balanced perfectly.
Which strongly suggested that frame dragging was completely real, not just an Einsteinian hallucination of higher math.
Where’s that from? I thought frame dragging wasn’t a big part of the solution to Mercury’s orbit - I would have said that as near the Sun as Mercury is, the space(-time) is distorted enough that a whole orbit around the sun isn’t exactly 360 degrees, and this angle deficit leads to the unexpected motion of Mercury (perihelion advance)
Exactly. And that narrow bite is only left because it’s observationally hard and thus not well constrained, not because there is some observational pressure keeping that region alive over other mass ranges.
No. There’s no clear explanation for how PBHs would form, much less a narrow mass range of them. It’s actually rather “fine tuned” to use almost-conventional physics to form PBHs at all, since you will readily kill off free baryonic matter in the very early universe if you’re not super delicate in how you aim your model.
It wouldn’t be particularly messy, but rather inexplicably tuned. As you say, we know there are multiple things astrophysical and particulate that are technically part of dark matter, but they are on the roster only because they are known to exist incontrovertibly from other arenas but (importantly) they are present at whatever random abundance they happen to be at. We are clearly missing something big, but the presence of all those other items doesn’t substantively suggest that there would be two new unrelated things nearly identical in fractional contribution, given that each random new thing’s fractional contribution could be at any order of magnitude.
As with the still-allowed PBH sub-lunar mass range, the narrow super-lunar mass range that hasn’t been ruled out below “10% max contribution” is still on the table not because there is observational pressure keeping that 10% afloat. It’s just another observationally hard region, and those measurements have only gotten to that level of precision so far. There’s nothing suggesting that that region is somehow special otherwise.
So, rewording my point about not needing two things: The statement “We could inject two unrelated new dark matter sources and have them both not show up yet in any other data and have them coincidentally contribute at similar levels” is technically true, but that solves no problems we currently face, so this is not needed, in the most literal sense.
Is a black hole big enough to feed on the CMB effectively “immortal”, or does the CMB get weaker as it redshifts meaning that eventually even the largest black holes will evaporate (in many trillions and trillions of years)?
And presuming the latter, does this mean that black holes that are small enough to be shrinking now were potentially big enough to be growing in the past when the CMB was “hotter”? And if so, does that change our plausible size range at all?
@Andy_L is correct; frame dragging is less than a 10-4 part of the general relativistic correction for this system.
Mercury’s orbit is slightly elliptical, and the orientation of that ellipse relative to the distant stars can be tracked over long periods. The orientation is expected to slowly rotate (“precess”) due to Newtonian effects from the solar system’s other bodies pulling it to and fro. The observed precession rate was off by 7% relative to that expectation.
It turns out that Newtonian physics predicts that, in isolation, an elliptical orbit around a spherical mass undergoes no precession. But general relativity does not. An elliptical orbit around even a boring, isolated spherical star will precess. And for Mercury, the extra precession due to relativistic effects was precisely the missing 7%.
The evidence for dark matter comes from many independent observations measuring very different things. They all agree on the total amount of dark matter. And, while some of these cannot distinguish what type of stuff the dark matter would be, many can. In particular, certain lines of evidence are informative on how much “baryonic” (i.e., conventional) matter there can be. Among these:
- The cosmic microwave background radiation that is left over from when atoms first formed is exquisitely measured. The angular sizes (the size “on the sky”) of temperature fluctuations in that radiation has many features in it that are modeled very well by the presence of dark matter. In the linked image, the curve that so perfectly passes through the data points would look drastically different (and thus wrong) with even mild changes to the ratio of baryonic to non-baryonic matter.
- The generation of light isotopes of hydrogen, helium, and lithium during the early universe is highly sensitive to the fraction of matter that is baryonic.
- The large-scale structure of the universe (superclusters, galaxy filaments; for a visual, see here) has a distinct appearance that is a result of fluctuations in the primordial plasma, whose evolution included massive waves of over- and under-densities moving through. The patterns seen suggest that only a small part of the wave-supporting matter could be baryonic.
- Galaxy cluster collisions, seen even as only a mid-collision snapshot given the timescales involved, can reveal where baryonic matter and non-baryonic matter live within the clusters, and at what fractions.
There are more examples. A key is that all these independent pieces agree on the relative levels of baryonic (conventional) and non-baryonic matter.
Correct. Possible PBH growth – from any source, including just swallowing up baryonic matter – is folded in as needed, which can be particularly relevant for those working on theoretical models for PBH generation. It doesn’t affect the observational limits, as the most stringent limits on that end come not from the raw PBH lifetime but rather from the fact that we don’t see the death throes of evaporating PBHs out there.
Thank you for a much better, and better-informed, answer.