Primordial Black Holes

Frontier If dark matter is made of black holes left over from the first second after the Big Bang — which no observation has yet shown, and section 7 says what would — then the thirty-year, multi-billion-dollar hunt for a new dark-matter particle has been looking for the wrong kind of object. A primordial black hole is not a new particle. It is ordinary general relativity plus a large density fluctuation in the early universe; every ingredient is already in Einstein’s equations and the known cosmology. That is the whole appeal, and it is why this candidate cannot be switched off the way a particle can: there is no coupling to send to zero.

Established Fifty years of searching has produced a constraint map with exactly one clean gap in it. Evaporation gamma-rays close the abundance below roughly 10¹⁷ grams; microlensing and gas accretion close it above roughly 10²³ grams; between them lies the asteroid-mass window, about 10¹⁷ to 10²³ grams — some 5 × 10⁻¹⁷ to 5 × 10⁻¹¹ solar masses, the mass of a small mountain in an object narrower than an atom — where a population of such holes can be the whole of the dark matter and break nothing. Six orders of magnitude of mass, and no rival candidate carries a survivor this clean.

Frontier The prize, if the window is occupied, is that dark matter becomes an astronomy problem rather than a particle-physics one. The objects would be found with telescopes, X-ray timing and spacecraft ranging rather than with tonne-scale cryogenic detectors a kilometre underground, and the deciding measurements cost tens of millions rather than the billions already committed to the next collider and the next xenon experiment. The upside also accrues unusually widely: the confirming data — planetary ephemerides, X-ray pulsar light curves, an all-sky MeV map — are public and shared, not the property of one collaboration behind one embargo.

Speculative The condition the whole case rests on is that a signal exists inside the window, not merely an absence of signal on either side of it. The window today is defined entirely by what fails to appear, and an empty box and a full one look identical until someone reaches in; the bottleneck list in section 5 is largely about why reaching in is so much harder than fencing the box off from the outside.

Established A primordial black hole forms when a region of the early universe is dense enough to collapse under its own gravity before it can re-expand. In the standard picture, first written down by Zeldovich, Novikov, Hawking and Carr in the early 1970s, a density contrast of order unity on a scale entering the cosmological horizon collapses directly to a black hole of roughly the mass contained in that horizon at that time. Form at 10⁻²³ seconds and you get an asteroid-mass hole; form at one second and you get a hole of order a solar mass. No new field, particle or force is required — only a fluctuation spectrum with more power on small scales than the one we measure on large scales in the cosmic microwave background.

Established Once formed, a black hole radiates, and the rate is fixed by its mass alone. Hawking’s 1974 result gives a temperature inversely proportional to mass and an evaporation lifetime that scales as the cube of the mass; a convenient form is a lifetime of order 10⁶⁴ (M/M_sun)³ years. Setting that equal to the 13.8-billion-year age of the universe picks out a critical mass of about 5 × 10¹⁴ grams: anything lighter has already evaporated, anything heavier is still here. This single fact is why the dark-matter discussion begins at 10¹⁵ grams and not below.

Established The asteroid-mass window sits a little above that evaporation cliff and runs up to the point where lensing surveys begin to bite. Its lower edge, near 10¹⁷ grams, is set by the fact that holes only a few times heavier than the evaporation mass still emit a faint but detectable trickle of gamma-rays and positrons today. Its upper edge, near 10²³ grams, is set by the smallest lens whose bending of starlight a survey can register. A hole of 10²⁰ grams has a Schwarzschild radius of about 10⁻¹³ metres — smaller than an atomic nucleus — and a number density, if it were all the dark matter, of roughly one per volume the size of the Earth’s orbit. It is dark not because it is faint but because it is small and rare.

Established The mass a hole is born with is set by the amount of matter inside the cosmological horizon when it forms. The horizon mass grows with cosmic time, so the formation clock and the hole mass are the same variable read two ways; asteroid-mass holes, at 10¹⁷ to 10²³ grams, correspond to formation between roughly 10⁻²³ and 10⁻¹⁵ seconds after the Big Bang, deep in the radiation era and long before nucleosynthesis. This is what makes a PBH mass function a fossil of sub-second cosmology rather than of any later epoch.

Frontier The mass function need not be a single value, and that flexibility is both the model’s strength and its escape hatch. Real formation scenarios — a spike in the inflationary power spectrum, a QCD-era softening of the equation of state, the collapse of cosmic-string loops or colliding vacuum bubbles — generically produce extended mass distributions, not delta functions. An extended function that places most of its mass inside the window but a tail outside it must satisfy every constraint the tail touches, which is why a claim of “all the dark matter” is a statement about a whole distribution and not about one number.

Speculative Producing enough holes in the window is a demand for large small-scale power that the standard inflationary picture does not naturally supply. The perturbation amplitude measured on CMB scales is about 10⁻⁵; making f_PBH = 1 in the window wants an amplitude near 10⁻² on a scale seven orders of magnitude smaller, and building that spike without spoiling the large-scale spectrum is where most of the model-building difficulty lives.

Established No primordial black hole has ever been detected; every result in this field is an upper limit on abundance. The quantity everyone reports is f_PBH, the fraction of the dark-matter density in primordial holes of a given mass, and the entire published record is a set of curves saying “f_PBH is below this line here.” The asteroid-mass window is not a place where a signal has been seen; it is a place where no method has yet been able to draw the line below one. That distinction governs everything that follows.

Established The lower edge of the window is drawn by the radiation from holes just heavy enough to have survived. A hole a few times above 5 × 10¹⁴ grams still evaporates slowly, emitting gamma-rays and electron-positron pairs whose integrated flux would show up in the diffuse cosmic gamma-ray background, in the 511-keV positron-annihilation line from the Galactic centre, and in the low-energy positron spectrum measured by Voyager 1 beyond the heliopause. Green and Kavanagh’s review compiles the resulting bound as f(M) below about 2 × 10⁻⁸ at the evaporation mass, rising steeply with mass; the Voyager positron data constrain M below 10¹⁶ grams to f below 0.001, and the Galactic 511-keV line constrains the 10¹⁶ to 10¹⁷ gram range. Above roughly 10¹⁷ grams the evaporation signal falls below every current instrument, and the constraint stops.

Established Above the window, the classic stellar-scale microlensing surveys long ago excluded holes as all the dark matter across the sub-stellar to stellar range. EROS excluded compact halo objects from about 6 × 10⁻⁸ to 15 solar masses, OGLE added limits over roughly 0.1 to 20 solar masses, and the MACHO survey concluded that objects near half a solar mass contribute at most about a tenth of the halo. These are decades old, replicated and undisputed, and they are why the microlensing conversation for dark matter has moved down to the asteroid scale where the surveys run out.

Established The upper edge is drawn by microlensing, and the deepest single probe is a seven-hour stare at Andromeda. Niikura and collaborators used the Subaru Hyper Suprime-Cam in 2019 to monitor tens of millions of stars in M31 at dense cadence for one night, searching for the brief brightening a foreground lens would cause. They found a single candidate event and set stringent limits over the mass range M_PBH of about 10⁻¹¹ to 10⁻⁶ solar masses, or roughly 10¹⁹ to 10²⁴ grams. That one survey excludes f_PBH = 1 across most of the upper decade of the window.

Frontier How far down in mass that microlensing wall reaches is genuinely contested, because wave optics blunts it. When the Schwarzschild radius of the lens shrinks toward the wavelength of the light being lensed, the point-lens magnification is washed out by diffraction and the finite angular size of the source star further dilutes it. Sugiyama, Kobayashi and Takada argued in 2020 that these effects lift the honest low-mass floor of the optical search from about 10⁻¹¹ to nearer 10⁻¹⁰ solar masses — a factor of ten of window that the raw survey appeared to close but wave optics reopens. This is why the window’s upper edge is quoted as anywhere from 10²² to 10²⁴ grams depending on whose diffraction treatment you accept.

Frontier The same instrument may now be seeing candidates rather than only setting limits. A 2026 reanalysis of the HSC Andromeda data by Sugiyama and collaborators reports twelve microlensing candidates with timescales shorter than five hours, four of them high-significance, at inferred masses near 10⁻⁷ to 10⁻⁶ solar masses — above the asteroid window. Read as a null result the analysis still bounds f_PBH below about 10⁻³ there; read as detections the events would imply f_PBH of order 10⁻¹. Which reading survives scrutiny is unsettled, and it sits just outside the window rather than inside it.

Established Above the window, gas accretion onto holes in the early universe closes the stellar and heavier mass range through the microwave background. An accreting hole in the recombination era injects energy that alters the ionization history and distorts the CMB anisotropy spectrum; the Planck data, analysed by Ali-Haimoud and Kamionkowski and others, exclude f_PBH = 1 across roughly 10³ to 10¹² solar masses. This is the constraint that shuts the door the LIGO black-hole mergers seemed to open, and it is why the 10 to 100 solar-mass “stellar” window is not a rival to the asteroid one for supplying all the dark matter.

Frontier Two constraints that once appeared to close part of the window were withdrawn, which is the reason the window exists in its current width. Capture of a passing PBH by a white dwarf or a neutron star in a globular cluster was argued to destroy the star and so to forbid asteroid-mass holes; Montero-Camacho and collaborators showed in 2019 that the globular-cluster dark-matter densities assumed were far too high and that hydrodynamic simulations leave the mass range allowed. The disappearance of those limits between 2018 and 2020 is what turned a contested sliver into the six-decade gap the field now quotes.

Established Astrometric microlensing with Gaia is a real and sensitive method, but it probes the wrong masses for this window. Gaia measures the tiny wobble a foreground lens imprints on a background star’s apparent position; because the Einstein radius that sets the effect grows as the square root of the lens mass, the technique is sensitive to lenses from about 0.4 to 5 × 10⁷ solar masses, peaking near 10 solar masses, where it can exclude f_PBH as small as 3 × 10⁻⁴. Its lower reach, 0.4 solar masses or about 8 × 10³² grams, sits nine orders of magnitude above the asteroid window. Astrometric lensing is a superb tool aimed at the stellar and intermediate mass range, and it does not and cannot enter the band where PBHs can be all the dark matter — a point worth stating plainly, because it is listed in some surveys of what might probe the window.

Established Taken together, the published record is best understood as an atlas of exclusion, and exclusion does not expire. A microlensing limit set in 2019 still constrains a model written in 2026, and the accumulation of these curves over fifty years is a durable scientific asset in its own right — it is the map that makes the single remaining gap visible and worth attacking. The value already banked is the map; the value still outstanding is a signal inside the one blank square on it.

Frontier The claim that the window is the sole surviving band for f_PBH = 1 is now itself under revision, from the light-mass side. The memory-burden effect, developed by Dvali and collaborators from 2020 and applied to PBHs from 2024, argues that a black hole’s stored quantum information back-reacts on its own evaporation once it has shed about half its mass, sharply slowing the process. If real, holes far below 10¹⁵ grams need not have evaporated, and a second dark-matter window opens at masses as low as 10⁴ to 10⁹ grams. This is a live, contested piece of theory, not an established rescue — but it means the tidy statement “exactly one open window” is true only under standard Hawking evaporation.

Frontier Where exactly are the window’s two edges? The lower edge depends on how well the diffuse MeV gamma-ray background and the Galactic 511-keV line can be modelled and on whether any residual after astrophysical foregrounds could be evaporation; the upper edge depends on the wave-optics and finite-source treatment of microlensing. Both edges move by an order of magnitude between careful papers, so the honest width of the window is a range, roughly 10¹⁷ to 10²³ grams, and not two sharp numbers.

Frontier Is the window one window or two? Standard evaporation gives a single asteroid-mass band; the memory-burden effect, if it holds, adds a lower band near 10⁴ to 10⁹ grams and changes the entire lower-edge argument, because the gamma-ray bounds that draw today’s edge assume unmodified Hawking radiation. Settling whether memory burden is physics or artefact is now a frontier question for dark matter, not only for quantum gravity.

Speculative Can any early-universe mechanism actually put the required power on asteroid-mass scales without over-producing holes elsewhere? Producing f_PBH = 1 in the window needs a density-perturbation amplitude around 10⁻² on the relevant scale, seven orders of magnitude above the 10⁻⁵ measured in the CMB, and it needs that spike narrow enough not to spill mass into the excluded bands. Whether inflationary models that do this are natural or fine-tuned is unresolved and model-dependent.

Frontier Do the 2026 Subaru candidates mean anything? Four high-significance short-timescale events could be the first microlensing detection of compact objects near 10⁻⁶ solar masses, or they could be stellar variability, asteroids or systematics; the mass sits above the window, so even a confirmed detection would not by itself establish window-mass dark matter, but it would validate the method that must eventually reach into the window.

Speculative Could a window-mass hole ever be a laboratory rather than a nuisance? A captured asteroid-mass hole inside a star or planet would accrete and eventually consume its host, which is destructive rather than useful; there is no proposed route by which such an object does work for anyone, and the scientific return is entirely in the detection, not the object.

Established The defining bottleneck is that the window is bounded by non-detections on both sides, and closing in from outside gets exponentially harder as you approach the middle. Evaporation flux falls as a steep power of mass, so pushing the lower edge up from 10¹⁷ grams by one decade demands far more than one decade of gamma-ray sensitivity; microlensing sensitivity dies against the wave-optics wall, so pushing the upper edge down from 10²³ grams runs into diffraction that no exposure time can beat at optical wavelengths. The two frontiers are receding from each other, not converging.

Established Optical microlensing hits a hard wall of physics near 10²² grams. Once the lens’s Schwarzschild radius approaches the roughly half-micron wavelength of visible light, diffraction smears the magnification and the finite size of the source star dilutes it further. This is a wavelength limit, not an engineering one; the only way through it is to lens shorter-wavelength light, which is why X-ray sources become the interesting targets.

Frontier Direct dynamical detection is limited by an appallingly low event rate. If the local dark matter is all asteroid-mass holes at about 200 kilometres per second, the number crossing the inner Solar System is of order one per decade — and heavier holes, though individually more perturbing, are rarer still in inverse proportion to their mass. A method that waits for a transit is betting on a Poisson process with a decade-scale mean, so it needs continuous, uninterrupted, decades-long baselines to have any chance.

Frontier The transit rate trades off against perturbation size in a way that leaves no easy mass to catch. At fixed dark-matter density the number of holes scales inversely with their mass, so light window-mass holes are common but perturb feebly, while heavier ones tug harder but arrive centuries apart; the close-encounter method sits in a valley where neither the rate nor the signal is generous, which is the deep reason the dynamical route is a decade-scale wait rather than a quick check.

Established Astrometric microlensing cannot be pushed down to help, because its signal shrinks as the square root of the mass. The apparent-position shift Gaia measures is set by the Einstein angle, which for a 10²⁰-gram lens is roughly a billionth of the shift a stellar lens produces — far below any conceivable astrometric precision. No improvement in astrometry reaches the window; the scaling forbids it.

Established The upper and lower probes cannot be traded against each other, because they are sensitive to different physics. Microlensing responds to the total gravitating mass and is agnostic about what the hole emits; evaporation and neutrino bounds respond to the emission and are agnostic about lensing. A window closed by one method but not the other is not closed, which is why the field needs a full suite of in-window probes rather than one deeper survey.

Frontier The MeV gamma-ray band that draws the lower edge is the worst-instrumented in astronomy. Between the hard-X-ray regime and the GeV band covered by Fermi lies the “MeV gap,” where no current mission has the sensitivity to resolve a faint evaporation component against the Galactic diffuse emission. Improving the lower edge is bottlenecked less by theory than by the absence of a flown MeV telescope.

Speculative The memory-burden lower window, if pursued, is bottlenecked by a theory nobody can yet calculate reliably. The suppression exponent that decides whether the reopened band sits at 10⁴ or 10¹⁰ grams is a parameter of a quantum-gravity effect with no agreed derivation, so the observational targets it implies are only as firm as the underlying calculation, which is not firm.

Frontier The lower edge waits on a sensitive MeV gamma-ray mission. Proposed instruments such as AMEGO-X and the GECCO concept target exactly the 0.1-to-10-MeV band where an evaporation signal from holes near 10¹⁷ grams would appear; without one, the lower edge cannot be pushed up and the memory-burden reopening cannot be tested in gamma-rays. This is a dependency on a specific, currently unflown class of telescope.

Established The upper edge waits on wave-optics-correct microlensing pipelines and on short-wavelength sources. Reaching below 10²² grams requires either an X-ray microlensing survey of bright compact sources or a rigorous diffraction treatment of existing optical data; both are analysis-and-instrument dependencies rather than fundamental unknowns, and the X-ray route depends on missions that time bright pulsars at high cadence.

Established The dynamical route depends on the continued precision and continuity of Solar System ephemerides. The distance to Mars is now known to about 10 centimetres from decades of spacecraft ranging, and the close-encounter method depends entirely on that archive continuing without gaps; a break in the ranging record is a break in the only baseline long enough to catch a once-per-decade transit.

Frontier Every edge depends on better foreground and systematics models. The 511-keV line has astrophysical positron sources; the diffuse MeV background has unresolved sources; microlensing has stellar variables and asteroids; ephemerides have unmodelled asteroid masses. In each case the PBH signal is a residual after a foreground subtraction, so the dependency is as much on the model being subtracted as on the raw sensitivity.

Established The neutrino route depends on detectors that already exist and were built for something else. The IceCube archive that now bounds the memory-burden window was assembled for astrophysical neutrino astronomy, so the light-window programme inherits a decade of data at no marginal instrument cost; its dependency is on analysis and on the theoretical emission spectrum, not on new construction.

Speculative The memory-burden window depends on a settled calculation of the suppressed evaporation spectrum. The high-energy neutrino constraints already being drawn from IceCube data assume a particular late-stage emission spectrum; if the underlying quantum-gravity treatment changes, the neutrino targets move with it, so the observational programme is downstream of a theoretical result that is not yet converged.

Established The question that decides everything is simple: is there a signal inside 10¹⁷ to 10²³ grams, or only the two silences that bound it? Every constraint on the field to date fences the window from outside; the experiments that matter now are the ones that reach in. Several exist, they are cheap by the standards of dark-matter physics, and they use instruments that are flying or funded.

Frontier The decisive near-term test is gravitational microlensing of bright Galactic X-ray pulsars, which reaches most of the asteroid-mass window with an instrument that already exists. Tamta, Raj and Sharma showed in 2025 that because X-rays have wavelengths thousands of times shorter than visible light, the wave-optics wall that stops optical microlensing at 10²² grams moves down, letting X-ray microlensing probe roughly 10⁻¹⁶ to 10⁻¹¹ solar masses — about 2 × 10¹⁷ to 2 × 10²² grams, nearly the entire window. They identify the pulsar SMC X-1 as a workable target and estimate that NICER could probe the window with two months of exposure and a STROBE-X-class mission in a few weeks. This is the rare in-window test that needs no new physics and, for its first reach, no new hardware.

Frontier The most model-independent test is the Solar System itself, read as a gravitational detector. Tran, Geller, Lehmann and Kaiser argued in 2024 that if asteroid-mass holes are all the dark matter, at least one crosses the inner Solar System per decade at about 200 kilometres per second, perturbing planetary orbits by an amount that decades of ranging could register; with Mars ranged to about 10 centimetres, the existing ephemeris archive is already a search apparatus. The method assumes nothing about Hawking radiation, wave optics or the formation mechanism — only Newtonian gravity and a passing mass — which is why it is the cleanest cross-check, and its weakness is equally plain: the event rate is roughly one per decade, so a null result accumulates slowly.

Speculative A transit could leave a permanent mark on a solid body. Santarelli, Caplan and Smith proposed in 2025 that a hole passing through the Moon, Mercury or an icy moon would deposit energy along its path and punch two craters, one on entry and one on exit, with metre-scale radii and a morphology distinct from that of rocky impactors, completely independent of Hawking radiation. Their flux estimates — of order 1 gram per second of dark matter crossing the Moon under halo assumptions — imply that a systematic search of cratered surfaces could constrain holes up to about 10¹⁸ grams, improving existing limits by up to an order of magnitude, though the signature has never been searched for and its uniqueness is not yet demonstrated.

Speculative Lunar and planetary seismology is a second dynamical channel. A hole threading a planet or moon would deposit momentum and could ring the body seismically; the Apollo seismometers and any future lunar network are in principle sensitive to such an event, but the predicted amplitudes and rates are uncertain enough that this remains a proposal rather than a live search.

Frontier For the reopened light-mass window, the test is high-energy neutrinos. Chianese and collaborators showed in 2024 that memory-burdened holes emitting at late times would produce a high-energy neutrino flux; comparing to the IceCube extremely-high-energy bound of about 5.9 × 10⁻⁹ GeV per square centimetre per second per steradian already constrains f_PBH = 1 above roughly 2 × 10⁵ grams for one choice of the suppression parameter, and a 2026 dwarf-spheroidal analysis using ten years of IceCube muon tracks excludes the dominant-dark-matter case up to about 10¹⁰ grams. KM3NeT, IceCube-Gen2 and GRAND would extend this reach.

Frontier Pushing the lower edge of the standard window is a job for an MeV gamma-ray telescope. An AMEGO-X or GECCO-class mission resolving the Galactic 511-keV line and the diffuse MeV background against foregrounds could raise the lower edge above 10¹⁷ grams or, more interestingly, find the residual that an evaporation component would leave; this is the one test that attacks the window from below and simultaneously tests the memory-burden reopening.

Speculative Astrometric microlensing is often listed as a window probe and is not one. Because its signal falls as the square root of the lens mass, Gaia astrometry reaches only to about 0.4 solar masses; naming it as a decisive in-window experiment, as some sweeps do, mistakes a genuinely powerful stellar-mass tool for one that can enter the asteroid band, which it cannot. The correction matters because it removes a false sense that the window is already being probed astrometrically.

Established The X-ray route needs high-throughput, high-cadence soft-X-ray timing. NICER on the International Space Station already delivers microsecond timing of bright X-ray sources and can begin the pulsar-microlensing search now; reaching the full window at high significance wants a larger effective area, which is the design case for a STROBE-X-class observatory. The engineering ask is collecting area and timing stability, not a new detector principle.

Frontier The lower edge needs an MeV telescope, which is a genuine instrument-development problem. Compton and pair-tracking telescopes in the 0.1-to-10-MeV band suffer from large instrumental backgrounds and modest angular resolution; AMEGO-X and GECCO are attempts to close the “MeV gap,” and the engineering frontier is background rejection and effective area in a regime where photons are hard to focus. This is shared with every other science case that lives in that band, including parts of the programme in Hidden-Sector Searches.

Established The dynamical route is an exercise in metrology and data continuity, not new hardware. Spacecraft transponders, radio ranging and the ephemeris-fitting software already deliver the roughly 10-centimetre Mars distance the close-encounter method relies on; the engineering requirement is to keep ranging spacecraft at the planets and to maintain the fit without gaps over decades, so the deliverable is an uninterrupted archive rather than a device.

Established NICER shows that the first-reach instrument is already in orbit. Its soft-X-ray timing of bright sources at microsecond resolution is exactly the capability the pulsar-microlensing search needs, so the engineering gap to a first result is exposure time and analysis rather than a build; the STROBE-X case is about depth and all-window coverage, not about feasibility in principle.

Frontier Every route is ultimately a pipeline problem. A wave-optics-correct microlensing pipeline, a foreground-subtracted MeV map, an ephemeris residual analysis tuned for impulsive perturbations, and a neutrino-spectrum fit for suppressed evaporation are all software instruments, and in a field bounded by residuals the analysis pipeline is as much the apparatus as the telescope is.

Frontier Gravitational-wave astronomy is the adjacent field that would settle the heavier PBH question and inform this one. Sub-solar-mass compact-object searches in LIGO-Virgo-KAGRA data test for holes lighter than any star can make, and the space-based detector LISA would probe both mergers and a stochastic background from PBH formation; the techniques and populations overlap with Gravitational Wave Engineering and Black Hole Physics Applications.

Frontier Neutrino astronomy became a PBH probe the moment memory burden reopened the light window. The same cubic-kilometre detectors built for astrophysical neutrinos in Neutrino Astronomy now set the leading limits on 10⁵-to-10¹⁰-gram holes, a use nobody designed them for.

Established Deep-space navigation is quietly the enabling infrastructure for the dynamical route. The transponder ranging and Doppler tracking that keep interplanetary spacecraft on course produce, as a by-product, the centimetre-scale planetary distances the close-encounter search reads for perturbations; the PBH search is a free rider on an operational capability maintained for entirely practical reasons.

Established MeV gamma-ray astronomy and precision planetary radar are the enabling instrument fields. The evaporation edge rides on the same detector development that serves nuclear astrophysics and line spectroscopy, and the dynamical route rides on the deep-space navigation and radar infrastructure that also serves Mega-Telescopes and interplanetary missions.

Speculative The topic is adjacent to, but distinct from, the non-black-hole dark-matter programme. Because a PBH needs no new particle, it is the natural fallback if the searches in Dark Matter Alternatives and the particle hunts keep returning nulls; the two programmes are competitors for the same explanatory slot rather than collaborators.

Established The dynamical route needs an institution willing to guarantee continuity for decades. A once-per-decade transit can only be caught by an unbroken ranging and ephemeris archive, which means the relevant commitment is not a grant cycle but a multi-decade custodial promise across changing agencies and missions — the kind of long-baseline data stewardship that is chronically underfunded because no single result justifies it.

Frontier The MeV telescope is a mission-selection problem more than a science problem. AMEGO-X and GECCO compete for the same limited number of astrophysics mission slots; the PBH lower-edge case is one of several riding on the same launch, so the institutional question is whether the combined science justifies filling the MeV gap this decade.

Established The field is unusually cross-disciplinary and therefore poorly owned. The window is bounded by particle astrophysics, planetary science, X-ray timing and quantum gravity, and no single review panel or funding line covers all four; the strongest recent moves — cratering, X-ray microlensing, close encounters — came from small groups reaching across those boundaries rather than from a coordinated programme.

Frontier The strongest recent progress came from outside any dedicated programme, which is both encouraging and fragile. The x-ray-microlensing, close-encounter and cratering ideas were produced by small theory-led groups repurposing existing data and instruments; that keeps the cost low, but it also means the in-window search advances by opportunistic proposal rather than by a funded roadmap, and an opportunistic field can stall when the proposers move on.

Speculative A confirmed detection would create governance questions only in the mildest sense. There is no dual-use concern and no proprietary asset; the main institutional consequence of a detection would be reallocation of dark-matter funding away from particle searches, which is a scientific-priority dispute rather than a policy one.

Established The direct risks of this research are essentially nil. The experiments are observations of natural light, ranging data and archival craters; nothing is built that could harm anyone, and no material is produced. On the usual ledger of frontier science this is among the most benign programmes available.

Frontier The real ethical content is opportunity cost inside the dark-matter budget. Deciding to fund an MeV mission or an X-ray timing observatory partly for PBHs is a decision not to fund something else, and because PBHs need no new particle they are a genuine competitor to the particle-detector programmes; an honest accounting treats the choice as a bet on where the dark matter actually is, not as free additional science.

Established The public-safety framing that occasionally attaches to “a black hole passing through the Solar System” is unfounded and should be stated as such. An asteroid-mass hole has the gravity of an asteroid; a close pass perturbs orbits at the centimetre level, which is exactly why it is detectable and exactly why it is harmless. The detectability and the safety are the same fact.

Frontier A detection would reallocate serious money, and that is the honest stake. The next generation of underground particle detectors and the next collider are billion-dollar commitments premised on dark matter being a particle; a confirmed asteroid-mass population would not make those instruments worthless, but it would weaken the central justification for the largest of them, and the field has an interest in not learning that too late.

Speculative The distributional story is unusually clean. The confirming datasets are public, the instruments serve many sciences, and no firm or nation captures the result; if there is an equity concern it is only the general one that space-based astronomy is done by a handful of wealthy agencies.

Frontier Confirming that dark matter is primordial black holes would close a question open since the 1930s and do it without extending the Standard Model. That is a large intellectual result — it would mean the dark sector is not a new kind of matter at all but an old kind of object — and it would redirect a generation of experimental effort away from underground particle detectors toward telescopes and spacecraft.

Established The practical civilizational footprint, by contrast, is small. There is no energy source, no material and no device at the end of this; a window-mass hole cannot be captured, stored or used, and no serious proponent claims otherwise. The gain is knowledge and the reallocation of scientific resources, not a technology.

Speculative A nearby asteroid-mass hole is a curiosity, not a hazard or an asset. Even the closest plausible pass leaves the Earth’s orbit unchanged at any level that matters to life; the object is interesting precisely because it does almost nothing, and any narrative of danger or exploitation misreads the physics.

Frontier A confirmed detection would also sharpen, rather than settle, the tensions catalogued elsewhere in cosmology. A primordial population would feed into the small-scale structure and reionization histories that bear on the disagreements discussed in Cosmological Tensions, adding a source of early nonlinearity without adding a particle. The effect is real but modest, and it changes emphasis rather than overturning the standard model of cosmology.

Handwave On the longest horizons, a confirmed primordial population would anchor cosmology’s account of the first second. Because the holes’ masses encode the horizon size at their formation, a measured mass function would be a direct fossil of sub-second physics — but turning a detection into a reconstruction of the early-universe power spectrum is an inference chain long enough that its reliability is, today, an assertion rather than a result.

These horizons track when each route into the window could plausibly return a result, given instruments that fly or are funded, and they lean on the same instrument fields as Hidden-Sector Searches.

• 10 yr: Frontier X-ray pulsar microlensing with NICER and, if selected, a STROBE-X-class mission surveys most of the window; ongoing ephemeris analysis continues a slow, model-independent close-encounter search; IceCube tightens the memory-burden light window.
• 25 yr: Speculative An MeV gamma-ray mission, if flown, raises the lower edge and tests the memory-burden reopening in gamma-rays; a systematic cratering and lunar-seismic search either exists or has been abandoned as too ambiguous.
• 50 yr: Speculative The window is either populated by a confirmed detection or has been squeezed from both sides to f_PBH well below one across its full width, effectively closing the all-of-dark-matter case.
• 100 / 250+ yr: Handwave If a population is confirmed, its mass function is used as a fossil record of sub-second cosmology; if not, primordial black holes take their place among the elegant dark-matter ideas that the data declined.

• Depends on the constraint atlases and instrument programmes catalogued in Dark Matter Alternatives and Hidden-Sector Searches, and on the black-hole and gravitational-wave physics in Black Hole Physics Applications and Gravitational Wave Engineering; nothing else on the map blocks the concept, because the physics of formation is already in hand.
• Requires (not on this map) a microlensing limit that actually reaches below 10²³ grams rather than stopping at the wave-optics wall; a Galactic 511-keV photon budget clean enough to isolate any evaporation component at the lower edge; a STROBE-X-class soft-X-ray timing observatory to survey the full window at high significance; an unbroken, decades-long archive of planetary ranging and spacecraft Doppler to catch a once-per-decade transit; and a funded MeV gamma-ray mission line to close the MeV gap.
• Enables a resolution of the dark-matter question that adds no particle to the Standard Model, and a fossil measurement of the sub-second density-perturbation spectrum if a mass function is ever reconstructed.
• Adjacent to Neutrino Astronomy, Cosmological Tensions and Mega-Telescopes, which share its instruments and its early-universe stakes.

Established “Primordial black holes have been ruled out as dark matter.” They have been ruled out as all the dark matter across most masses, but not in the asteroid-mass window, where no method has yet pushed f_PBH below one; the honest statement is that one six-decade band remains open, not that the idea is dead.

Established “We have seen Hawking radiation from primordial black holes.” We have not. Hawking radiation from an asteroid-mass hole is far too faint to detect directly; every “evaporation constraint” is a non-detection of an integrated background, and Hawking radiation remains observationally unconfirmed for any black hole.

Frontier “The LIGO black holes are the dark matter.” The 10-to-100-solar-mass mergers LIGO detects are in a band where CMB accretion constraints exclude f_PBH = 1; a sub-dominant primordial contribution to those mergers is possible and actively debated, but the stellar-mass range is not where PBHs can be all the dark matter.

Speculative “Planet Nine might be a primordial black hole.” Scholtz and Unwin noted in 2019 that an unseen outer-Solar-System mass could formally be a grapefruit-sized hole of a few Earth masses; it is a legitimate and testable curiosity — a captured hole would emit a faint annihilation halo — but a few-Earth-mass object is far heavier than the asteroid window and would be a captured interloper, not the dark matter, so the two ideas are often wrongly merged.

Established “A passing black hole would swallow the Earth.” An asteroid-mass hole has an event horizon smaller than an atomic nucleus and the gravity of an asteroid; it would pass through matter almost without interacting, and its only detectable effect is a centimetre-scale tug on orbits. The threat narrative inverts the physics that makes the object hard to find.

Established “Big Bang nucleosynthesis rules out primordial black holes.” BBN constrains holes light enough to evaporate during or before nucleosynthesis, below about 10⁹ to 10¹³ grams; asteroid-mass holes form after that physics is fixed and inject nothing into it, so BBN says nothing about the window. The constraint is real but aimed far below the dark-matter band.

Frontier “Dark matter must be a particle.” The particle hypothesis is well motivated and heavily searched, but nothing in the evidence for dark matter — galaxy rotation, lensing, the CMB, structure formation — requires a particle rather than a population of compact objects; the primordial-black-hole case is precisely the demonstration that the gravitational evidence underdetermines the microphysics.

Frontier “The window is a single, settled band.” Its edges move by an order of magnitude between careful analyses, and the memory-burden effect, if real, opens a second window near 10⁴ to 10⁹ grams; “exactly one open window” is true only under standard Hawking evaporation and standard diffraction treatments.