As yet another paper lays out the path to putting final nails in the coffin of primordial black holes (PBHs) as a significant source of dark matter phenomena, the significance of the search for PBHs in evolving. The observations and analysis that the paper proposes will almost surely be done in the medium term future, perhaps a decade or so, and I have little doubt that it will rule out PBHs as a significant source of dark matter phenomena, because it will found that there just aren't enough of them.
Ruling out PBH dark matter is notable, because it was pretty much the last dark matter particle candidate that didn't require new physics in the form of a beyond the Standard Model particle. This would have been one of the most conservative possible dark matter explanations. (The only other non-BSM dark matter particle proposal, heavy stable hadrons which are bosons, such as hexaquarks, has even less of a sound foundation.) We need either new particles, or new forces, or new insights into general relativity that have been overlooked for more than a century to explain dark matter phenomena. Establishing even more firmly that we have new physics to discover is good news for physicists.
The possibility that some PBHs exist at all, however, is much less strongly disfavored by theory and observation than the possibility that PBHs are a significant source of dark matter phenomena, which demands that there be a lot of them. Indeed, even discovering just one PBH would have immense scientific significance.
Removed as a significant dark matter candidate, the existence, if any, and size distribution and frequency, if they do exist, now matters more for purposes of cosmology and astrophysics:
* the size and frequency of PBHs existing today places significant bounds on the nature of the very early post-Big Bang period;
* sufficiently large PBHs could challenge or even falsify the Hawking radiation concept; and,
* the non-existence of PBHs in certain size ranges could add suggestive evidence to support a physical maximum mass-energy density in the Universe that is less rigorous than the cutoff for black hole formation. This limit would be supported if there were no PBHs below a certain threshold mass and that mass was significantly in excess of minimum PBH mass possible today in light of Hawking radiation (which causes smaller PBHs to evaporate more quickly than larger PBHs, with all PBHs below a certain size expected to have evaporated by now).
A maximum mass-energy density that is less rigorous than the black hole threshold could provide asymptotic safety to a variety of gravity and especially quantum gravity proposals, by eliminating the need for theories like the Standard Model and quantum gravity to be mathematically coherent to arbitrarily high energy-densities/temperatures in a running with energy scale. It could also point to whether sphalerons which are theoretically possible in the Standard Model are actually physically possible.
If PBHs exist at all, their size and frequency are basically a function of how clumpy v. homogeneous the universe was shortly after the Big Bang (because mass-energy needs to form clumps much more dense than a neutron star or the highest energy densities found in the Large Hadron Collider, to form a PBH), the extent to which nuclear density matter and hadrons can resist further compression, whether the formula for PBH evaporation due to Hawking radiation is accurate, and how quickly PBHs could make up for losses in size from Hawking radiation by absorbing matter and energy from their environment.
Essentially, the more rare PBHs are today, the more homogeneous the universe was at that time. We can also establish a minimum extent to which the early universe was not homogeneous from the large scale structure of the universe, the magnitude of variations in the Cosmic Microwave Background radiation, and the red shifts at which we can first observe galaxies with telescopes like the James Webb Space Telescope. And we can also estimate this directly (but not with great precision or confidence since the energy scales involved are so much greater and because of the assumptions required to estimate it) from the magnitude of quantum fluctuations seen in high energy physics experiments.
Primordial black holes (PBHs) in the mass range 10^−16 − 10^−11 M⊙ may constitute all the dark matter. We show that gravitational microlensing of bright x-ray pulsars provide the most robust and immediately implementable opportunity to uncover PBH dark matter in this mass window.
As proofs of concept, we show that the currently operational NICER telescope can probe this window near 10^−14 M⊙ with just two months of exposure on the x-ray pulsar SMC-X1, and that the forthcoming STROBE-X telescope can probe complementary regions in only a few weeks. These times are much shorter than the year-long exposures obtained by NICER on some individual sources.
We take into account the effects of wave optics and the finite extent of the source, which become important for the mass range of our PBHs. We also provide a spectral diagnostic to distinguish microlensing from transient background events and to broadly mark the PBH mass if true microlensing events are observed.
In light of the powerful science case, i.e., the imminent discovery of dark matter searchable over multiple decades of PBH masses with achievable exposures, we strongly urge the commission of a dedicated large broadband telescope for x-ray microlensing. We derive the microlensing reach of such a telescope by assuming sensitivities of detector components of proposed missions, and find that with hard x-ray pulsar sources PBH masses down to a few 10^−17 M⊙ can be probed.
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