Primordial black holes microlensing

Primordial black holes microlensing

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Could the movement of a PBH found in the outer solar system by microlensing be totally unpredictable and impossible to track after it was first detected?

Hunting for Black Holes

We are searching for black holes in the Milky Way using gravitational lensing. Black holes are one of the most exotic phenomena in astrophysics and represent a breakdown in fundamental physics between gravity and quantum mechanics. The Galaxy likely contains 100 million stellar-mass black holes. The number and mass statistics of black holes can provide important constraints on the star formation history, the stellar mass function, supernova physics and how BHs form, the equation of state of nuclear matter, and the existence of primordial black holes. To date, isolated stellar-mass black holes have never been definitevely detected and only two dozen black holes have measured masses &ndash all in binaries.

Microlensing events, where a black hole&rsquos gravity lenses the light of a background star as observed from Earth, provide a way to detect isolated black holes and measure their masses. Black hole lensing events produce a photometric magnification that has a long-duration (>3 months) and an astrometric signature that can be >1 mas. However, the astrometric signature has only recently become detectable with technological advancements in high-resolution imaging, including adaptive optics. Only the combination of photometry and astrometry can be used to precisely measure the mass of the lensing object and determine if it is indeed a black hole or a chance slow-motion lensing event between two normal stars.

We aim to find the first isolated stellar&minusmass black holes by detecting the astrometric signature of microlensing. Finding just a few black holes would already reduce the orders of magnitude uncertainty on the total number of black holes in the Galaxy and constrain theories of black hole formation and evolution. The proposed development of astrometry techniques, needed for astrometric microlensing, will also lay a foundation for new explorations in many areas of astrophysics.

Title: A Microlensing Search for Primordial Black Holes in Dark Energy Survey Date

The search for dark matter is currently one of the most exciting fields in astronomy. One possible candidate for dark matter is primordial black holes, formed from density fluctuations at the beginning of the universe. We search for these black holes through microlensing events by creating light curves from stars in the Dark Energy Survey (DES). Microlensing occurs when a primordial black hole (lens) passes in front of a background star, briefly brightening the output from that star. This brightening, due to the increase in magnitude, becomes detectable. First, we must cut out galaxies from our sample of stars. We then vary key parameters involved in lensing to create potential light curves, and then compare them to light curves from actual events, as well as calculating errors. We then send off these curves for further analysis to determine if any real events occurred. We plan to create 106 light curves, due to the large amount of parameters being varied. If these primordial black holes are dark matter, we hope to eventually detect multiple events using the light curves we have created.

Primordial Black Holes Probably Don’t Pack a Dark Matter Punch

Should a black hole drift in front of a star, it could trigger a microlensing event, so astronomers set out to estimate the number of primordial black holes in Andromeda [Kavli IPMU]

Using the Andromeda galaxy as a huge detector, astronomers have taken a stab at seeing the unseeable — possibly disproving a hypothesis first put forward by the late Stephen Hawking 45 years ago.

According to Hawking’s work, the universe should be filled with black holes that were formed at the beginning of time, when the universe was a chaotic soup of energy just after the Big Bang. Known as “primordial” black holes, these ancient objects are hypothesized to invisibly occupy modern galaxies, including our own, boosting their dark matter mass.

These black holes aren’t the supermassive monsters that lurk in the centers of most galaxies they’re not even stellar-mass black holes, formed after massive stars go supernova. Primordial black holes are much smaller than that, having leaked most of their mass via Hawking radiation since their formation 13.8 billion years ago. They should, however, still have powerful gravitational effects on the space surrounding them and, in new research published last week in the journal Nature Astronomy, an international team of researchers have leveraged these hypothetical black holes’ space-time-warping powers to reveal their presence.

Central to this study is the effect of microlensing. This astronomical method relies on an object passing between us and a distant star. It has been used to great effect when detecting distant exoplanets, or rogue brown dwarfs wandering through interstellar space. Should one of these objects drift directly in front of a star, its gravitational field can create a magnification effect that briefly brightens the star’s light. The gravitational field creates a natural “lens” out of space-time itself, a prediction that arises from Einstein’s general relativity.

The effect of gravitational microlensing on a star in the Andromeda galaxy should a primordial black hole drift in front [Kavli IPMU]

It stands to reason that even though primordial black holes don’t generate any light themselves, if you stare at at entire galaxy for long enough, you should see a lot of twinkling stars, or microlensing events caused by the hypothetical swarm of primordial black holes the galaxy should contain. Count the number of events, and you can take a statistical stab the total number of primordial black holes in a galaxy like Andromeda, thereby providing an estimate as to how much of the universe’s missing dark matter mass is made up from these objects.

Using the power of the Subaru telescope in Hawaii, the researchers put this to the test, capturing 190 consecutive images of Andromeda over seven hours during one night with the observatory’s Hyper Suprime-Cam digital camera. If Hawking’s theory held, the telescope should have recorded approximately 1,000 microlensing events caused by primordial black holes with a mass of less than our moon drifting in front of Andromeda’s stars. Alas, only one microlensing event was detected that night. From this observation campaign alone, the researchers estimate that primordial black holes make up no more than 0.1 percent of the total dark matter mass in our universe.

Although this elegant study doesn’t necessarily disprove the existence of primordial black holes — one single event is interesting, but not compelling — it does put a wrench in the idea that they dominate the mass holed up in dark matter. So, the quest to understand the nature of dark matter grinds on and, with the help of this study, astronomers have now narrowed down the search by removing primordial black holes from the dark matter equation.

(Just can’t get enough) primordial black holes

The recent gravitational wave detections by LIGO/VIRGO of black hole mergers have brought primordial black holes (PBHs) right back into the fray as a potential candidate for dark matter. This is because the black holes detected by LIGO that were a few dozen times the mass of the Sun could well have been PBHs.

However, even more intriguingly, there is another window of opportunity for primordial black holes with much lower masses to make up all of dark matter. In today’s paper, Inomata et al. show that it’s possible to produce enough PBHs with masses of around (the mass of the asteroid Ida) to make them a viable dark matter candidate, without violating the new and very stringent constraints on the number of allowed PBHs from the Subaru Hyper Suprime-Cam (see the results here).

What is a primordial black hole?

Primordial black holes are thought to have formed in the very early universe, typically within the first second of its existence. They are a popular dark matter candidate because they don’t require any new physics beyond the standard model. After the period of exponential expansion known as inflation finished (see this astrobites article for an explanation), patches of the universe that were a lot denser than average could gravitationally collapse to form black holes. Smaller and lighter PBHs would have formed first, whilst larger and heavier PBHs would have formed later. However, since PBHs evaporate via Hawking radiation, all of the lighter ones should have completely evaporated by now. In fact, only PBHs of masses (the mass of Ida’s moon Dactyl) or higher could still be around today.

What can we learn from them?

The number and size of overdense patches capable of forming primordial black holes in the early universe depends on exactly how inflation happened. Since there are lots of viable inflation models which can be difficult to distinguish between, primordial black holes can act as a useful probe to differentiate between them. They are particularly handy because they are much smaller than the scales that the Cosmic Microwave Background can constrain, so they enable us to zoom in and see otherwise hidden detail. If we knew the number of PBHs in the observable universe, the correct inflationary model would possess distinct features according to this number. Conversely, if the inflationary model was fully specified, we would know how many PBHs to expect.

For now, the best we can do is to place constraints on the number of PBHs there can be, which in turn can help to rule out inflationary models that might produce too many.

New constraints and how to dodge them

New constraints from the Subaru Hyper Suprime-Cam have put tougher restrictions on how many PBHs there can be in the mass range that spans the mass of the asteroid Ida () to the mass of the Earth (). The telescope searched for microlensing effects (see this astrobite for an introduction to microlensing) from PBHs passing in front of stars in the Milky Way and M31. If there were enough PBHs to make up all of dark matter, there should have been many microlensing events. However, they only found one possible PBH candidate which their analysis could neither confirm nor rule out. Comparing their lack of detections with the expected number of events (if PBHs were to make up all of dark matter) meant that the constraint on the number of PBHs tightened by up to 3 orders of magnitude.

Inomata et al. show that it is still possible to produce enough asteroid-mass PBHs to make up all of dark matter despite these new constraints shown in the dark blue shaded region of Figure 1.

Figure 1: Plot of the ratio of PBHs to dark matter against PBH mass. If for a particular mass, then PBHs of that mass make up all of dark matter. The dark blue shaded region is ruled out by the newest constraints.

The black solid line in Figure 1 shows the ratio of PBHs to dark matter for a particular inflation model known as double inflation. The number of PBHs peaks at around and avoids the surrounding observational constraints indicated by the shaded regions which are ruled out.

If observational constraints tighten further, this model will have to be adjusted for asteroid-mass PBHs to remain a plausible candidate for dark matter. In particular, if observations could limit the ratio of PBHs to dark matter to being much less than 1 on all mass ranges, we will be able to successfully rule out PBHs as making up all of dark matter. However, if PBHs are directly detected, or it is confirmed that the black hole mergers detected by LIGO/VIRGO were indeed primordial, it may still be possible to confirm that dark matter is made up of primordial black holes.

A new look at the nature of dark matter

The microlensing object in the foreground galaxy could be a star (as depicted), a primordial black hole, or any other compact object. Credit: NASA/Jason Cowan (Astronomy Technology Center).

The nature of the dark matter which apparently makes up 80% of the mass of the particles in the universe is still one of the great unsolved mysteries of present day sciences. The lack of experimental evidence, which could allow us to identify it with one or other of the new elementary particles predicted by the theorists, as well as the recent discovery of gravitational waves coming from the merging of two black holes (with masses some 30 times that of the Sun) by LIGO the Laser Interferometer Gravitational Wave Observatory) have revived interest in the possibility that dark matter might take the form of primordial black holes with masses between 10 and 1000 times that of the Sun.

Primordial black holes, which would have originated in high density fluctuations of matter during the first moments of the Universe, are in principle very interesting. As opposed to those which form from stars, whose abundance and masses are limited by models of stellar formation and evolution, primordial black holes could exist with a wide range of masses and abundances. They would be found in the halos of galaxies, and the occasional meeting between two of them having masses 30 times that of the Sun, followed by a subsequent merger, might have given rise to the gravitational waves detected by LIGO.

If there were an appreciable number of black holes in the halos of galaxies, some of them intercept the light coming towards us from a distant quasar. Because of their strong gravitational fields, their gravity could concentrate the rays of light, and cause an increase in the apparent brightness of the quasar. This effect, known as "gravitational microlensing" is bigger the bigger the mass of the black hole, and the probability of detecting it would be bigger the more the presence of these black holes. So although the black holes themselves cannot be directly detected, they would be detected by increases in the brightness of observed quasars.

On this assumption, a group of scientists has used the microlensing effect on quasars to estimate the numbers of primordial black holes of intermediate mass in galaxies. The study, led by the researcher at the Instituto de Astrofísica de Canarias (IAC) and the University of La Laguna (ULL), Evencio Mediavilla Gradolph, shows that normal stars like the Sun cause the microlensing effects, thus ruling out the existence of a large population of primordial black holes with intermediate mass.

Using computer simulations, they have compared the rise in brightness, in visible light and in X-rays, of 24 distant quasars with the values predicted by the microlensing effect. They have found that the strength of the effect is relatively low, as would be expected from objects with a mass between 0.05 and 0.45 times that of the Sun, and well below that of intermediate mass black holes. In addition they have estimated that these microlenses form roughly 20% of the total mass of a galaxy, equivalent to the mass expected to be found in stars. So their results show that, with high probability, it is normal stars and not primordial intermediate mass black holes which are responsible for the observed microlensing.

"This study implies "says Evencio Mediavilla, "that it is not at all probable that black holes with masses between 10 and 100 times the mass of the Sun make up a significant fraction of the dark matter". For that reason the black holes whose merging was detected by LIGO were probably formed by the collapse of stars, and were not primordial black holes".

Astronomers participating in this research include Jorge Jiménez-Vicente and José Calderón-Infante (University of Granada) and José A. Muñoz Lozano, and Héctor Vives-Arias, (University of Valencia).

Primordial Black Holes And The Search For Dark Matter From The Multiverse

The Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) is home to many interdisciplinary projects which benefit from the synergy of a wide range of expertise available at the institute. One such project is the study of black holes that could have formed in the early universe, before stars and galaxies were born.

Such primordial black holes (PBHs) could account for all or part of dark matter, be responsible for some of the observed gravitational waves signals, and seed supermassive black holes found in the center of our Galaxy and other galaxies. They could also play a role in the synthesis of heavy elements when they collide with neutron stars and destroy them, releasing neutron-rich material. In particular, there is an exciting possibility that the mysterious dark matter, which accounts for most of the matter in the universe, is composed of primordial black holes. The 2020 Nobel Prize in physics was awarded to a theorist, Roger Penrose, and two astronomers, Reinhard Genzel and Andrea Ghez, for their discoveries that confirmed the existence of black holes. Since black holes are known to exist in nature, they make a very appealing candidate for dark matter.

The recent progress in fundamental theory, astrophysics, and astronomical observations in search of PBHs has been made by an international team of particle physicists, cosmologists and astronomers, including Kavli IPMU members Alexander Kusenko, Misao Sasaki, Sunao Sugiyama, Masahiro Takada and Volodymyr Takhistov.

To learn more about primordial black holes, the research team looked at the early universe for clues. The early universe was so dense that any positive density fluctuation of more than 50 percent would create a black hole. However, cosmological perturbations that seeded galaxies are known to be much smaller. Nevertheless, a number of processes in the early universe could have created the right conditions for the black holes to form.

One exciting possibility is that primordial black holes could form from the "baby universes" created during inflation, a period of rapid expansion that is believed to be responsible for seeding the structures we observe today, such as galaxies and clusters of galaxies. During inflation, baby universes can branch off of our universe. A small baby (or "daughter") universe would eventually collapse, but the large amount of energy released in the small volume causes a black hole to form.

An even more peculiar fate awaits a bigger baby universe. If it is bigger than some critical size, Einstein's theory of gravity allows the baby universe to exist in a state that appears different to an observer on the inside and the outside. An internal observer sees it as an expanding universe, while an outside observer (such as us) sees it as a black hole. In either case, the big and the small baby universes are seen by us as primordial black holes, which conceal the underlying structure of multiple universes behind their "event horizons." The event horizon is a boundary below which everything, even light, is trapped and cannot escape the black hole.

In their paper, the team described a novel scenario for PBH formation and showed that the black holes from the "multiverse" scenario can be found using the Hyper Suprime-Cam (HSC) of the 8.2m Subaru Telescope, a gigantic digital camera--the management of which Kavli IPMU has played a crucial role--near the 4,200 meter summit of Mt. Mauna Kea in Hawaii. Their work is an exciting extension of the HSC search of PBH that Masahiro Takada, a Principal Investigator at the Kavli IPMU, and his team are pursuing. The HSC team has recently reported leading constraints on the existence of PBHs in Niikura, Takada et. al. (Nature Astronomy 3, 524-534 (2019))

Why was the HSC indispensable in this research? The HSC has a unique capability to image the entire Andromeda galaxy every few minutes. If a black hole passes through the line of sight to one of the stars, the black hole's gravity bends the light rays and makes the star appear brighter than before for a short period of time. The duration of the star's brightening tells the astronomers the mass of the black hole. With HSC observations, one can simultaneously observe one hundred million stars, casting a wide net for primordial black holes that may be crossing one of the lines of sight.

The first HSC observations have already reported a very intriguing candidate event consistent with a PBH from the "multiverse," with a black hole mass comparable to the mass of the Moon. Encouraged by this first sign, and guided by the new theoretical understanding, the team is conducting a new round of observations to extend the search and to provide a definitive test of whether PBHs from the multiverse scenario can account for all dark matter.

Paper details
Journal: Physical Review Letters
Title: Exploring Primordial Black Holes from the Multiverse with Optical Telescopes
Authors: Alexander Kusenko (1, 2), Misao Sasaki (2, 3, 4), Sunao Sugiyama (2, 5), Masahiro Takada (2), Volodymyr Takhistov (1,2), and Edoardo Vitagliano (1)

Author affiliation:
1. Department of Physics and Astronomy, University of California, Los Angeles, Los Angeles, California 90095-1547, USA
2. Kavli Institute for the Physics and Mathematics of the Universe (WPI), UTIAS The University of Tokyo, Kashiwa, Chiba 277-8583, Japan
3. Center for Gravitational Physics, Yukawa Institute for Theoretical Physics, Kyoto University, Kyoto 606-8502, Japan
4. Leung Center for Cosmology and Particle Astrophysics, National Taiwan University, Taipei 10617, Taiwan
5. Department of Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

Where did the little black holes come from?

A black hole is a singularity, an infinitely dense point in space packed with matter. It forms when that matter gets so tightly packed that the force of gravity overwhelms everything else, and the matter collapses. It warps space-time and surrounds itself with an "event horizon," a spherical boundary region beyond which no light can escape.

The laws of general relativity allow black holes to exist at any scale crush an ant hard enough and it will collapse into a black hole just like a star it'll just be incredibly tiny.

Most PBH theories assume these objects have masses like small planets, with event horizons as small as grapefruits. It's an outlandish idea, still on the fringe of black hole and dark matter physics, said Joey Neilsen, a physicist at Villanova University who was not involved in the new study. But recently, as other dark matter theories have turned up empty, some researchers have given the PBH notion a second look.

If PBHs are out there though, they have to be very old. In the modern universe, there are only two known methods for creating new black holes from normal matter: stars much heavier than the sun colliding or exploding. So every known black hole weighs more than the entire solar system (sometimes much more).

Making small black holes requires a whole other set of mechanisms and ingredients.

Those ingredients would be "the stuff of the Big Bang, the same stuff that makes the stars and galaxies," Neilsen told Live Science.

Right after the Big Bang, the newly expanding universe was full of hot, dense largely-undifferentiated matter expanding in all directions. There were small pockets of turbulence in this morass &mdash still visible as fluctuations in the Cosmic Microwave Background (CMB), the afterglow of the Big Bang &mdash and those fluctuations gave the universe structure.

"If it's a little more dense at point A, then stuff is gravitationally attracted to point A," Neilsen said. "And over the history of the universe, that attraction causes gas and dust to fall inwards, coalesce, collapse and form stars, galaxies, and all the structures in the universe that we know of."

Most PBH theories involve very intense fluctuations in the early universe, stronger than the ones that formed galaxies.

In this new paper, the researchers place those intense fluctuations during a period known as "inflation." In the first thousand billion billion billionths of a second after the Big Bang, the universe expanded exponentially fast. That rapid early expansion gave space-time its current "flat" shape, researchers believe, and it likely prevented space from ending up curved, as Live Science has previously reported.

In a new paper published Nov. 20 to the arXiv database, researchers propose that during inflation, there might have been moments where all of space-time was intensely curved, before eventually flattening out. Those brief curvatures, however, would have produced fluctuations in the expanding universe intense enough to eventually form a large population of Earth-mass black holes.

Possible link between primordial black holes and dark matter

The first frame of this looping animation is an image from NASA’s Spitzer Space Telescope that shows an infrared view of a sky area in the constellation Ursa Major. In the second (coloured) frame, after masking out all known stars, galaxies and artifacts and enhancing what’s left, an irregular background glow appears. This is the cosmic infrared background (CIB) lighter colours indicate brighter areas. The CIB glow is more irregular than can be explained by distant unresolved galaxies, and this excess structure is thought to be light emitted when the universe was less than a billion years old. Scientists say it likely originated from the first luminous objects to form in the universe, which includes both the first stars and black holes. Image credits: NASA/JPL-Caltech/A. Kashlinsky (Goddard). AN animation by Ade Ashford. Dark matter is a mysterious substance composing most of the material universe, now widely thought to be some form of massive exotic particle. An intriguing alternative view is that dark matter is made of black holes formed during the first second of our universe’s existence, known as primordial black holes. Now a scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, suggests that this interpretation aligns with our knowledge of cosmic infrared and X-ray background glows and may explain the unexpectedly high masses of merging black holes detected last year.

“This study is an effort to bring together a broad set of ideas and observations to test how well they fit, and the fit is surprisingly good,” said Alexander Kashlinsky, an astrophysicist at NASA Goddard. “If this is correct, then all galaxies, including our own, are embedded within a vast sphere of black holes each about 30 times the Sun’s mass.”

In 2005, Kashlinsky led a team of astronomers using NASA’s Spitzer Space Telescope to explore the background glow of infrared light in one part of the sky. The researchers reported excessive patchiness in the glow and concluded it was likely caused by the aggregate light of the first sources to illuminate the universe more than 13 billion years ago. Follow-up studies confirmed that this cosmic infrared background (CIB) showed similar unexpected structure in other parts of the sky.

In 2013, another study compared how the cosmic X-ray background (CXB) detected by NASA’s Chandra X-ray Observatory compared to the CIB in the same area of the sky. The first stars emitted mainly optical and ultraviolet light, which today is stretched into the infrared by the expansion of space, so they should not contribute significantly to the CXB.

Yet the irregular glow of low-energy X-rays in the CXB matched the patchiness of the CIB quite well. The only object we know of that can be sufficiently luminous across this wide an energy range is a black hole. The research team concluded that primordial black holes must have been abundant among the earliest stars, making up at least about one out of every five of the sources contributing to the CIB.

The nature of dark matter remains one of the most important unresolved issues in astrophysics. Scientists currently favour theoretical models that explain dark matter as an exotic massive particle, but so far searches have failed to turn up evidence these hypothetical particles actually exist. NASA is currently investigating this issue as part of its Alpha Magnetic Spectrometer and Fermi Gamma-ray Space Telescope missions.

“These studies are providing increasingly sensitive results, slowly shrinking the box of parameters where dark matter particles can hide,” Kashlinsky said. “The failure to find them has led to renewed interest in studying how well primordial black holes &mdash black holes formed in the universe’s first fraction of a second &mdash could work as dark matter.”

Physicists have outlined several ways in which the hot, rapidly expanding universe could produce primordial black holes in the first thousandths of a second after the Big Bang. The older the universe is when these mechanisms take hold, the larger the black holes can be. And because the window for creating them lasts only a tiny fraction of the first second, scientists expect primordial black holes would exhibit a narrow range of masses.

On 14 September, gravitational waves produced by a pair of merging black holes 1.3 billion light-years away were captured by the Laser Interferometer Gravitational-Wave Observatory (LIGO) facilities in Hanford, Washington, and Livingston, Louisiana. This event marked the first-ever detection of gravitational waves as well as the first direct detection of black holes. The signal provided LIGO scientists with information about the masses of the individual black holes, which were 29 and 36 times the Sun’s mass, plus or minus about four solar masses. These values were both unexpectedly large and surprisingly similar.

“Depending on the mechanism at work, primordial black holes could have properties very similar to what LIGO detected,” Kashlinsky explained. “If we assume this is the case, that LIGO caught a merger of black holes formed in the early universe, we can look at the consequences this has on our understanding of how the cosmos ultimately evolved.”

Primordial black holes, if they exist, could be similar to the merging black holes detected by the LIGO team in 2014. This computer simulation shows in slow motion what this merger would have looked like up close. The ring around the black holes, called an Einstein ring, arises from all the stars in a small region directly behind the holes whose light is distorted by gravitational lensing. The gravitational waves detected by LIGO are not shown in this video, although their effects can be seen in the Einstein ring. Gravitational waves travelling out behind the black holes disturb stellar images comprising the Einstein ring, causing them to slosh around in the ring even long after the merger is complete. Gravitational waves travelling in other directions cause weaker, shorter-lived sloshing everywhere outside the Einstein ring. If played back in real time, the movie would last about a third of a second. Image credits: SXS Lensing.

In his new paper, published today in The Astrophysical Journal Letters, Kashlinsky analyses what might have happened if dark matter consisted of a population of black holes similar to those detected by LIGO. The black holes distort the distribution of mass in the early universe, adding a small fluctuation that has consequences hundreds of millions of years later, when the first stars begin to form.

For much of the universe’s first 500 million years, normal matter remained too hot to coalesce into the first stars. Dark matter was unaffected by the high temperature because, whatever its nature, it primarily interacts through gravity. Aggregating by mutual attraction, dark matter first collapsed into clumps called minihaloes, which provided a gravitational seed enabling normal matter to accumulate. Hot gas collapsed toward the minihaloes, resulting in pockets of gas dense enough to further collapse on their own into the first stars. Kashlinsky shows that if black holes play the part of dark matter, this process occurs more rapidly and easily produces the lumpiness of the CIB detected in Spitzer data even if only a small fraction of minihaloes manage to produce stars.

As cosmic gas fell into the minihaloes, their constituent black holes would naturally capture some of it too. Matter falling toward a black hole heats up and ultimately produces X-rays. Together, infrared light from the first stars and X-rays from gas falling into dark matter black holes can account for the observed agreement between the patchiness of the CIB and the CXB.

Occasionally, some primordial black holes will pass close enough to be gravitationally captured into binary systems. The black holes in each of these binaries will, over eons, emit gravitational radiation, lose orbital energy and spiral inward, ultimately merging into a larger black hole like the event LIGO observed.

“Future LIGO observing runs will tell us much more about the universe’s population of black holes, and it won’t be long before we’ll know if the scenario I outline is either supported or ruled out,” Kashlinsky said.


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In: Physical review letters , Vol. 122, No. 21, 211301, 29.05.2019.

Research output : Contribution to journal › Article › peer-review

T1 - Primordial Black Hole Dark Matter

N1 - Funding Information: We warmly thank the anonymous referee for asking about the propagation effect. We thank A. Katz for illuminating discussions on the microlensing and neutron star constraints on the PBH abundance and D. Racco for many discussions. We thank C. R. Contaldi for discussions on the GW propagation and M. Hindmarsh for discussion of Gaussianity. N. B. acknowledges partial financial support by the ASI/INAF Agreement I/072/09/0 for the Planck LFI Activity of Phase E2. He also acknowledges financial support by ASI Grant No. 2016-24-H.0. A. R. is supported by the Swiss National Science Foundation (SNSF), project The Non-Gaussian Universe and Cosmological Symmetries, Project No. 200020-178787. A. L. is supported by the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013) / ERC Grant Agreement No. [616170]. Publisher Copyright: © 2019 American Physical Society.

N2 - There has recently been renewed interest in the possibility that the dark matter in the Universe consists of primordial black holes (PBHs). Current observational constraints leave only a few PBH mass ranges for this possibility. One of them is around 10-12 M. If PBHs with this mass are formed due to an enhanced scalar-perturbation amplitude, their formation is inevitably accompanied by the generation of gravitational waves (GWs) with frequency peaked in the mHz range, precisely around the maximum sensitivity of the LISA mission. We show that, if these primordial black holes are the dark matter, LISA will be able to detect the associated GW power spectrum. Although the GW source signal is intrinsically non-Gaussian, the signal measured by LISA is a sum of the signal from a large number of independent sources suppressing the non-Gaussianity at detection to an unobservable level. We also discuss the effect of the GW propagation in the perturbed Universe. PBH dark matter generically leads to a detectable, purely isotropic, Gaussian and unpolarized GW signal, a prediction that is testable with LISA.

AB - There has recently been renewed interest in the possibility that the dark matter in the Universe consists of primordial black holes (PBHs). Current observational constraints leave only a few PBH mass ranges for this possibility. One of them is around 10-12 M. If PBHs with this mass are formed due to an enhanced scalar-perturbation amplitude, their formation is inevitably accompanied by the generation of gravitational waves (GWs) with frequency peaked in the mHz range, precisely around the maximum sensitivity of the LISA mission. We show that, if these primordial black holes are the dark matter, LISA will be able to detect the associated GW power spectrum. Although the GW source signal is intrinsically non-Gaussian, the signal measured by LISA is a sum of the signal from a large number of independent sources suppressing the non-Gaussianity at detection to an unobservable level. We also discuss the effect of the GW propagation in the perturbed Universe. PBH dark matter generically leads to a detectable, purely isotropic, Gaussian and unpolarized GW signal, a prediction that is testable with LISA.

Watch the video: Μαύρες Τρύπες και Σχετικότητα. Astronio #14 (September 2022).