A Light in the Dark: Looking for Bright Black Holes as Dark Matter

Title: Refining Galactic primordial black hole evaporation constraints

Authors: Pedro De la Torre Luque, Jordan Koechler, and Shyam Balaji

First Author’s Institution: Instituto de Física Teórica, IFT UAM-CSIC, Departamento de Física Teórica, Universidad Autónoma de Madrid, ES-28049 Madrid, Spain

Status: Published in Physical Review D [open access]

Of all the potential solutions to the question “what is dark matter?”, primordial black holes (PBHs)—black holes potentially formed in the very early universe—surely constitute one of the strangest. Just imagine for a moment: black holes, once thought to be an abstract, theoretical artifact of Einstein’s Theory of General Relativity, turn out to not only exist, but to theoretically constitute 85% of the mass of the universe! Even stranger still, some physicists believe these black holes could be tiny and mind-bendingly numerous. The difficulty in putting this idea to the test, however, comes from the fact that these “asteroid mass” PBHs would be nearly impossible to observe directly (or even indirectly). But, of course, that hasn’t stopped scientists from trying.

Searches for asteroid mass PBHs put your typical game of needle-in-a-haystack to shame. A single asteroid mass PBH can range in size from the width of a human hair to smaller than an atomic nucleus, and we would expect there to be at most a (very heavy) handful of them within our solar system at any given moment. Previous Astrobites have covered a few possible ways of teasing out their presence, but the one at the center of today’s paper makes use of a particularly odd feature of low-mass black holes: they’re hot!

Hawking in a Haystack

To belabor the needle-in-a-haystack metaphor just a bit longer, imagine you’re crawling through a solar-system-sized haystack (presumably intended to feed an interstellar herd of… space horses?) looking for a needle. Sounds pretty hard, right? Now, imagine that needle was 2 billion degrees C°—you’d probably notice at some point that you’re in a pretty hot haystack! This is the magic of Hawking radiation. While black holes are—you guessed it—usually pretty dark, Stephen Hawking demonstrated in 1974 that some of the subatomic particles produced in the strong gravitational field around the event horizon of the black hole can actually escape to the outside world, rather than remaining trapped within the black hole itself. While the details are a bit complicated, this result indicates that a black hole has a sort of thermodynamic temperature—in this case, called its Hawking temperature. This is somewhat analogous to how normal objects will radiate photons in a spectrum that depends on the matter’s temperature (a blackbody spectrum), but notably black holes can radiate a spectrum of many different particles depending on its mass. Because this whole effect depends on the amount of curvature at the black hole’s event horizon, and smaller/less massive black holes curve spacetime more dramatically than big ones, smaller black holes have higher Hawking temperatures and will radiate more particles into the surrounding environment. Although this effect should eventually cause black holes to shrink and evaporate, black holes at the lower end of the asteroid-mass range (say, above but below g) should be able to radiate appreciable amounts of light particles like photons, neutrinos, electrons, and positrons for very long periods of time without going away.

Okay, so our search for these particularly low mass PBHs seems a lot more promising at this point, right? Well, it turns out that regardless of this very high temperature, these black holes are so small that the effect of their presence can still be hard to notice on the vast scale of large astrophysical environments. As these white-hot primordial pot-holes spit out a zoo of particles, these particles stream outward in all directions and interact with the gas, dust, and plasma that fills interstellar and intergalactic space. This means that understanding the large-scale observable signals from a population of hot, low-mass PBHs requires a great deal of complicated particle physics simulation. Today’s paper tackles this very problem, and uses updated modeling of various particle propagation processes to help understand the maximum amount of PBHs there can be.

Propagating Plenty of Positrons

At the core of today’s paper is an attempt to more accurately model the propagation of electrons and positrons throughout the galaxy as they stream away from the PBHs that would theoretically produce them. Three main scenarios are explored, each providing different observational signals we can look for with existing experiments. 

In the first scenario, the authors consider electrons and positrons that are able to directly propagate all the way from the PBHs to some charged particle detector—in this case, the Voyager 1 probe. Voyager 1, having left our sun’s heliosphere in 2012, is uniquely equipped to obtain a clean measurement of electron and positron flux in our local astrophysical environment. In the paper, the authors model the effects of gas, magnetic fields, and even stray photons on either slowing or accelerating electrons and positrons as they diffuse into the broader environment. While this has been done before, the authors of this paper are able to place more stringent constraints on PBHs between about and g due to improved modeling of electron and positron propagation outside of the galactic disk.

The second observable considered is diffuse x-ray emission caused by a process known as inverse Compton scattering. In inverse Compton scattering ambient photons from either the CMB, light from stars, or other sources interact with charged particles and become accelerated to higher energies. The authors combine these various effects to derive a predicted spectrum of x-rays that should result from a population of PBHs in our galaxy and compare it against measurements made by XMM-Newton of the spectrum of x-rays seen close to the center of the galaxy.

Finally, there is an observable associated specifically with electron-positron pairs known as the 511 KeV line. When electrons and positrons slow down sufficiently during their travels through the galaxy, they can eventually come together and form a bound state known as positronium. As you’d probably imagine, two particles of opposite electric charge orbiting each other is not a particularly stable configuration, so the positronium state will quickly undergo particle-antiparticle annihilation, leaving behind two photons with energies exactly equivalent to the rest mass of an electron — 511 KeV. Because electrons and positrons are emitted by a whole host of astrophysical sources, we already observe a 511 KeV emission line signal throughout our galaxy. However, if some of the 511 KeV line is sourced by DM candidates like PBHs, we should see this feature wherever we expect DM to be. The authors compare their predictions for the spatial distribution of the 511 KeV line against data from ESA’s INTEGRAL space telescope and find that there is much less emission at high galactic longitudes (see Figure 2) than their PBH model suggests there should be, once again putting tight constraints on the abundance of particularly low-mass PBHs.

Conclusions

By more carefully examining both the primary and secondary effects of placing radiating PBHs throughout our galaxy, the authors of today’s paper are able to put the tightest constraints yet on the abundance of PBHs in this particular mass range. The updated constraints this paper provides are summarized in Fig 3. The 511 KeV line in particular pushes the maximum abundance of PBHs between and g up to three orders of magnitude smaller than was previously known. Of course, all of these estimates are subject to some caveats, which the authors take time to explore. For example, the authors note that PBHs with higher angular momentum would tend to produce higher energy particles, which could tighten some of these constraints even further. Modeling uncertainties surrounding galactic plasma physics and the shape of the DM distribution in our galaxy also contribute to uncertainties around these results—but even in the most optimistic (or pessimistic, depending on who you ask) case, it is still reasonable to conclude that these PBHs are not numerous enough to fully explain the origin of dark matter. Looks like we’ll have to keep searching that galactic haystack a bit longer!

Astrobite edited by Caroline von Raesfeld

Featured image credit: adapted by the author using photograph from Torben Hansen and graphics from NASA.