View source for Primordial black hole

{{Short description|Hypothetical black hole formed soon after the Big Bang}}
[[File:PBHs-formation.png|upright=2|thumb|Formation of the universe without (above) and with (below) primordial black holes]]

In [[cosmology]], '''primordial black holes''' ('''PBHs''') are hypothetical [[black hole]]s that formed soon after the [[Big Bang]]. In the [[inflationary era]] and early [[Scale factor (cosmology)#Radiation-dominated era|radiation-dominated]] universe, extremely dense pockets of [[Subatomic particle|subatomic matter]] may have been tightly packed to the point of [[gravitational collapse]], creating primordial black holes without the [[supernova]] compression typically needed to make black holes today. Because the creation of primordial black holes would pre-date the first stars, they are not limited to the narrow mass range of [[stellar black hole]]s.

In 1966, [[Yakov Zeldovich]] and [[Igor Dmitriyevich Novikov|Igor Novikov]] first proposed the existence of such black holes,<ref>{{cite journal|last1=Zel'dovitch & Novikov|title=The Hypothesis of Cores Retarded During Expansion and the Hot Cosmological Model|journal=Soviet Astronomy|date=14 March 1966|volume=10|issue=4|pages=602–603|bibcode=1966AZh....43..758Z}}</ref> while the first in-depth study was conducted by [[Stephen Hawking]] in 1971.<ref>{{cite journal |last1=Hawking |first1=Stephen W. |year=1971 |title=Gravitationally collapsed objects of very low mass |journal=Mon. Not. R. Astron. Soc. |volume=152 |page=75 |bibcode=1971MNRAS.152...75H |doi=10.1093/mnras/152.1.75 |doi-access=free}}</ref> However, their existence remains hypothetical. In September 2022, primordial black holes were proposed by some researchers to explain the unexpected very large early galaxies discovered by the [[James Webb Space Telescope]] (JWST).<ref name="jwst1" /><ref name="jwst2" />

PBHs have long been considered possibly important if not nearly exclusive components of [[dark matter]],<ref name="Frampton"/><ref>{{cite journal |last1=Villanueva-Domingo |first1=Pablo |last2=Mena |first2=Olga |last3=Palomares-Ruiz |first3=Sergio |title=A Brief Review on Primordial Black Holes as Dark Matter |journal=Frontiers in Astronomy and Space Sciences |date=2021 |volume=8 |page=87 |doi=10.3389/fspas.2021.681084 |arxiv=2103.12087 |bibcode=2021FrASS...8...87V |issn=2296-987X |doi-access=free }}</ref><ref>{{cite journal |last1=Green |first1=Anne M. |last2=Kavanagh |first2=Bradley J. |date=1 April 2021 |title=Primordial black holes as a dark matter candidate |url=https://iopscience.iop.org/article/10.1088/1361-6471/abc534 |journal=Journal of Physics G: Nuclear and Particle Physics |volume=48 |issue=4 |pages=043001 |arxiv=2007.10722 |bibcode=2021JPhG...48d3001G |doi=10.1088/1361-6471/abc534 |issn=0954-3899 |s2cid=220666201 |access-date=17 August 2023}}</ref><ref name="Lacki"/> the latter perspective having been strengthened by both [[LIGO]]/[[Virgo interferometer]] [[gravitational wave]] and JWST observations.<ref name="jwst">{{cite journal |last1=Hütsi |first1=Gert |last2=Raidal |first2=Martti |last3=Urrutia |first3=Juan |last4=Vaskonen |first4=Ville |last5=Veermäe |first5=Hardi |title=Did JWST observe imprints of axion miniclusters or primordial black holes? |journal=Physical Review D |date=2 February 2023 |volume=107 |issue=4 |pages=043502 |doi=10.1103/PhysRevD.107.043502 |arxiv=2211.02651 |bibcode=2023PhRvD.107d3502H |s2cid=253370365 }}</ref><ref name="Bird">{{cite journal |last1=Bird |first1=Simeon |last2=Albert |first2=Andrea |last3=Dawson |first3=Will |last4=Ali-Haïmoud |first4=Yacine |last5=Coogan |first5=Adam |last6=Drlica-Wagner |first6=Alex |last7=Feng |first7=Qi |last8=Inman |first8=Derek |last9=Inomata |first9=Keisuke |last10=Kovetz |first10=Ely |last11=Kusenko |first11=Alexander |last12=Lehmann |first12=Benjamin V. |last13=Muñoz |first13=Julian B. |last14=Singh |first14=Rajeev |last15=Takhistov |first15=Volodymyr |last16=Tsai |first16=Yu-Dai |title=Primordial black hole dark matter |journal=Physics of the Dark Universe |date=1 August 2023 |volume=41 |pages=101231 |doi=10.1016/j.dark.2023.101231 |arxiv=2203.08967 |bibcode=2023PDU....4101231B |s2cid=247518939 |issn=2212-6864}}</ref> Early constraints on PBHs as dark matter usually assumed most black holes would have similar or identical ("monochromatic") mass, which was disproven by LIGO/Virgo results,<ref name="Espinosa"/><ref name="Clesse"/><ref name="Kashlinsky"/> and further suggestions that the actual black hole mass distribution is broadly [[platykurtic]] were evident from JWST observations of early large galaxies.<ref name="jwst"/><ref name="Bird"/> Recent analyses agree, suggesting a broad mass distribution with a [[Mode (statistics)|mode]] around one [[solar mass]].<ref name="Carr24">{{cite journal |last1=Carr |first1=B. J. |last2=Clesse |first2=S. |last3=García-Bellido |first3=J. |last4=Hawkins |first4=M. R. S. |last5=Kühnel |first5=F. |title=Observational evidence for primordial black holes: A positivist perspective |journal=Physics Reports |date=26 February 2024 |volume=1054 |pages=1–68 |doi=10.1016/j.physrep.2023.11.005 |url=https://www.sciencedirect.com/science/article/pii/S0370157323003976 |issn=0370-1573|arxiv=2306.03903 |bibcode=2024PhR..1054....1C }} See Figure 39.</ref>

Many PBHs may have the mass of an asteroid but the size of a hydrogen atom and be travelling at enormous speeds, with one likely being within the solar system at any given time. Most likely, such PBHs would pass right through a star "like a bullet", without any significant effects on the star. However, the ones traveling slowly would have a chance of being captured by the star.<ref>{{cite web | url=https://www.livescience.com/space/black-holes/atom-size-black-holes-from-the-dawn-of-time-could-be-devouring-stars-from-the-inside-out-new-research-suggests#:~:text=Now%20we%27re%20talking%20about,system%20at%20any%20given%20time.%22 | title=Atom-size black holes from the dawn of time could be devouring stars from the inside out, new research suggests | website=[[Live Science]] | date=21 December 2023 }}</ref> [[Stephen Hawking]] proposed that our [[Sun]] may harbor such a PBH.<ref>{{cite journal | doi=10.3847/1538-4357/ad04de | doi-access=free | title=Solar Evolution Models with a Central Black Hole | date=2023 | last1=Bellinger | first1=Earl P. | last2=Caplan | first2=Matt E. | last3=Ryu | first3=Taeho | last4=Bollimpalli | first4=Deepika | last5=Ball | first5=Warrick H. | last6=Kühnel | first6=Florian | last7=Farmer | first7=R. | last8=De Mink | first8=S. E. | last9=Christensen-Dalsgaard | first9=Jørgen | journal=The Astrophysical Journal | volume=959 | issue=2 | page=113 | arxiv=2312.06782 | bibcode=2023ApJ...959..113B }}</ref>

== History ==
Depending on the model, primordial black holes could have initial masses ranging from {{val|u=kg|e=−8}}<ref>{{cite journal |last1=Carr |first1=B. J. |last2=Hawking |first2=S. W. |year=2004 |title=Black holes in the early Universe |journal=Monthly Notices of the Royal Astronomical Society |volume=168 |issue=2 |pages=399–416 |arxiv=astro-ph/0407207 |bibcode=1974MNRAS.168..399C |doi=10.1093/mnras/168.2.399 |doi-access=free}}</ref> (the so-called Planck relics) to more than thousands of solar masses. However, primordial black holes originally having masses lower than {{val|u=kg|e=11}} would not have survived to the present due to [[Hawking radiation]], which causes complete evaporation in a time much shorter than the age of the Universe.<ref>{{cite journal |last=del Barco |first=Oscar |year=2021 |title=Primordial black hole origin for thermal gamma-ray bursts |journal=Monthly Notices of the Royal Astronomical Society |volume=506 |issue=1 |pages=806–812 |arxiv=2007.11226 |bibcode=2021MNRAS.506..806B |doi=10.1093/mnras/stab1747|doi-access=free }}</ref> Primordial black holes are non-[[Baryonic dark matter|baryonic]],<ref>{{cite journal |last1=Overduin |first1=J. M. |last2=Wesson |first2=P. S. |date=November 2004 |title=Dark Matter and Background Light |journal=Physics Reports |volume=402 |issue=5–6 |pages=267–406 |arxiv=astro-ph/0407207 |bibcode=2004PhR...402..267O |doi=10.1016/j.physrep.2004.07.006 |s2cid=1634052}}</ref> and as such are plausible [[dark matter]] candidates.<ref name="Bird"/><ref name="Frampton"/><ref name="Espinosa"/><ref name="Clesse"/><ref name="Lacki"/><ref name="Kashlinsky"/><ref name="jwst"/> Primordial black holes are also good candidates for being the seeds of the [[supermassive black hole]]s at the center of massive galaxies, as well as of [[intermediate-mass black hole]]s.<ref>{{Cite journal |last1=Clesse |first1=S. |last2=Garcia-Bellido |first2=J. |year=2015 |title=Massive Primordial Black Holes from Hybrid Inflation as Dark Matter and the seeds of Galaxies |journal=[[Physical Review D]] |volume=92 |issue=2 |pages=023524 |arxiv=1501.07565 |bibcode=2015PhRvD..92b3524C |doi=10.1103/PhysRevD.92.023524 |s2cid=118672317 |hdl=10486/674729}}</ref><ref name="jwst1"/><ref name="jwst2"/>

Primordial black holes belong to the class of [[massive compact halo object]]s (MACHOs). They are naturally a good dark matter candidate: they are (nearly) collision-less and stable (if sufficiently massive), they have non-relativistic velocities, and they form very early in the history of the Universe (typically less than one second after the [[Big Bang]]).<ref>{{Cite web |last=Sokol |first=Joshua |date=2020-09-23 |title=Physicists Argue That Black Holes From the Big Bang Could Be the Dark Matter |url=https://www.quantamagazine.org/black-holes-from-the-big-bang-could-be-the-dark-matter-20200923/ |access-date=2021-09-06 |website=Quanta Magazine |language=en}}</ref> Nevertheless, critics maintain that tight limits on their abundance have been set up from various astrophysical and cosmological observations, which would exclude that they contribute significantly to dark matter over most of the plausible mass range.<ref>{{Cite journal |last1=Ali-Haïmoud |first1=Yacine |last2=Kovetz |first2=Ely D. |last3=Kamionkowski |first3=Marc |date=2017-12-19 |title=The merger rate of primordial-black-hole binaries |journal=Physical Review D |volume=96 |issue=12 |pages=123523 |arxiv=1709.06576 |bibcode=2017PhRvD..96l3523A |doi=10.1103/PhysRevD.96.123523 |issn=2470-0010 |s2cid=119419981}}</ref> However, new research has provided for the possibility again, whereby these black holes would sit in clusters with a 30-solar-mass primordial black hole at the center.<ref>{{Cite journal |last=Jedamzik |first=Karsten |date=2020-09-14 |title=Primordial Black Hole Dark Matter and the LIGO/Virgo observations |url=https://doi.org/10.1088/1475-7516/2020/09/022 |journal=Journal of Cosmology and Astroparticle Physics |language=en |volume=2020 |issue=9 |pages=022 |arxiv=2006.11172 |bibcode=2020JCAP...09..022J |doi=10.1088/1475-7516/2020/09/022 |issn=1475-7516 |s2cid=219956276}}</ref>

[[File:BBH gravitational lensing of gw150914.webm|alt=A simulation of two black holes colliding to form a super massive black hole|thumb|upright=1.2|Simulation of two black holes colliding]]

In March 2016, one month after the announcement of the detection by Advanced LIGO/VIRGO of gravitational waves emitted by the merging of two 30 solar mass black holes (about {{val|6|u=kg|e=31}}), three groups of researchers proposed independently that the detected black holes had a primordial origin.<ref>{{cite journal |last1=Bird |first1=S. |last2=Cholis |first2=I. |year=2016 |title=Did LIGO Detect Dark Matter? |journal=[[Physical Review Letters]] |volume=116 |issue=20 |pages=201301 |arxiv=1603.00464 |bibcode=2016PhRvL.116t1301B |doi=10.1103/PhysRevLett.116.201301 |pmid=27258861 |s2cid=23710177}}</ref><ref name="Clesse 2017 142–1472">{{cite journal |last1=Clesse |first1=S. |last2=Garcia-Bellido |first2=J. |year=2017 |title=The clustering of massive Primordial Black Holes as Dark Matter: Measuring their mass distribution with Advanced LIGO |journal=[[Physics of the Dark Universe]] |volume=10 |issue=2016 |pages=142–147 |arxiv=1603.05234 |bibcode=2017PDU....15..142C |doi=10.1016/j.dark.2016.10.002 |s2cid=119201581}}</ref><ref>{{Cite journal |last1=Sasaki |first1=M. |last2=Suyama |first2=T. |last3=Tanaki |first3=T. |year=2016 |title=Primordial Black Hole Scenario for the Gravitational-Wave Event GW150914 |journal=[[Physical Review Letters]] |volume=117 |issue=6 |pages=061101 |arxiv=1603.08338 |bibcode=2016PhRvL.117f1101S |doi=10.1103/PhysRevLett.117.061101 |pmid=27541453 |s2cid=7362051}}</ref><ref>{{cite web |date=June 15, 2016 |title=Did Gravitational Wave Detector Find Dark Matter? |url=http://releases.jhu.edu/2016/06/15/did-gravitational-wave-detector-find-dark-matter/ |access-date=June 20, 2015 |publisher=[[Johns Hopkins University]]}}</ref> Two of the groups found that the merging rates inferred by LIGO are consistent with a scenario in which all the dark matter is made of primordial black holes, if a non-negligible fraction of them are somehow clustered within halos such as faint [[Dwarf spheroidal galaxy|dwarf galaxies]] or [[globular cluster]]s, as expected by the standard theory of cosmic [[structure formation]]. The third group claimed that these merging rates are incompatible with an all-dark-matter scenario and that primordial black holes could only contribute to less than one percent of the total dark matter. The unexpected large mass of the black holes detected by LIGO has strongly revived interest in primordial black holes with masses in the range of 1 to 100 solar masses. It is still debated whether this range is excluded or not by other observations, such as the absence of micro-lensing of stars,<ref>{{Cite journal |last1=Khalouei |first1=E. |last2=Ghodsi |first2=H. |last3=Rahvar |first3=S. |last4=Abedi |first4=J. |date=2021-04-02 |title=Possibility of primordial black holes as the source of gravitational wave events in the advanced LIGO detector |url=https://link.aps.org/doi/10.1103/PhysRevD.103.084001 |journal=Physical Review D |volume=103 |issue=8 |pages=084001 |arxiv=2011.02772 |bibcode=2021PhRvD.103h4001K |doi=10.1103/PhysRevD.103.084001 |s2cid=226254110}}</ref> the [[cosmic microwave background]] anisotropies, the size of faint dwarf galaxies, and the absence of correlation between X-ray and radio sources toward the galactic center.

In May 2016, Alexander Kashlinsky suggested that the observed spatial correlations in the unresolved [[gamma-ray]] and [[X-ray]] [[background radiation]]s could be due to primordial black holes with similar masses, if their abundance is comparable to that of dark matter.<ref>{{Cite journal |last=Kashlinsky |first=A. |year=2016 |title=LIGO gravitational wave detection, primordial black holes and the near-IR cosmic infrared background anisotropies |journal=[[The Astrophysical Journal]] |volume=823 |issue=2 |pages=L25 |arxiv=1605.04023 |bibcode=2016ApJ...823L..25K |doi=10.3847/2041-8205/823/2/L25 |s2cid=118491150 |doi-access=free }}</ref>

In August 2019, a study was published opening up the possibility of making up all dark matter with asteroid-mass primordial black holes (3.5 × 10<sup>−17</sup> – 4 × 10<sup>−12</sup> solar masses, or 7 × 10<sup>13</sup> – 8 × 10<sup>18</sup> kg).<ref name=":12">{{Cite journal |last1=Montero-Camacho |first1=Paulo |last2=Fang |first2=Xiao |last3=Vasquez |first3=Gabriel |last4=Silva |first4=Makana |last5=Hirata |first5=Christopher M. |date=2019-08-23 |title=Revisiting constraints on asteroid-mass primordial black holes as dark matter candidates |journal=Journal of Cosmology and Astroparticle Physics |volume=2019 |issue=8 |pages=031 |arxiv=1906.05950 |bibcode=2019JCAP...08..031M |doi=10.1088/1475-7516/2019/08/031 |issn=1475-7516 |s2cid=189897766}}</ref>

In September 2019, a report by James Unwin and Jakub Scholtz proposed the possibility of a primordial black hole (PBH) with mass 5–15&nbsp;{{M+}} (Earth masses), about the diameter of a [[tennis ball]], existing in the extended [[Kuiper Belt]] to explain the orbital anomalies that are theorized to be the result of a [[Planet nine|9th planet]] in the solar system.<ref>{{cite report |title=What if Planet 9 is a Primordial Black Hole? |last1=Scholtz |first1=J. |last2=Unwin |first2=J. |date=2019 |arxiv=1909.11090 |journal=High Energy Physics – Phenomenology}}</ref><ref name="planet9ornot2">{{cite web |last1=Anderson |first1=D. |last2=Hunt |first2=B. |date=5 December 2019 |title=Why astrophysicists think there's a black hole in our solar system |url=https://www.businessinsider.com/astrophysicists-found-black-hole-in-solar-system-planet-2019-11 |access-date=7 December 2019 |website=[[Business Insider]]}}</ref>

In October 2019, Derek Inman and Yacine Ali-Haïmoud published an article in which they discovered that the nonlinear velocities that arise from the structure formation are too small to significantly affect the constraints that arise from CMB anisotropies<ref name=":2" />

In September 2021, the NANOGrav collaboration announced that they had found a low-frequency signal that could be attributed to [[gravitational wave]]s and potentially could be associated with PBHs.<ref name=":3" />

In September 2022, primordial black holes were used to explain the unexpected very large early (high [[redshift]]) galaxies discovered by the [[James Webb Space Telescope]].<ref name="jwst1">{{Cite journal |last1=Liu |first1=Boyuan |last2=Bromm |first2=Volker |date=2022-09-27 |title=Accelerating Early Massive Galaxy Formation with Primordial Black Holes |journal=The Astrophysical Journal Letters |language=en |volume=937 |issue=2 |pages=L30 |doi=10.3847/2041-8213/ac927f |arxiv=2208.13178 |bibcode=2022ApJ...937L..30L |s2cid=252355487 |issn=2041-8205 |doi-access=free }}</ref><ref name="jwst2">{{Cite journal|last1=Yuan |first1=Guan-Wen |last2=Lei |first2=Lei |last3=Wang |first3=Yuan-Zhu |last4=Wang |first4=Bo |last5=Wang |first5=Yi-Ying |last6=Chen |first6=Chao |last7=Shen |first7=Zhao-Qiang |last8=Cai |first8=Yi-Fu |last9=Fan |first9=Yi-Zhong |date=2024 |title=Rapidly growing primordial black holes as seeds of the massive high-redshift JWST Galaxies |journal=Science China Physics, Mechanics & Astronomy |volume=67 |issue=10 |doi=10.1007/s11433-024-2433-3 |arxiv=2303.09391|bibcode=2024SCPMA..6709512Y }}</ref>

On 26 November 2023, evidence, for the first time, of an [[overmassive black hole galaxy]] ([[O.B.G.]]), the result of "heavy black hole seed formation from direct collapse", an alternative way of producing a black hole other than the collapse of a dead star, was reported. This discovery was found in studies of [[UHZ1]], a [[Early galaxy|very early galaxy]] containing a [[quasar]], by the [[Chandra X-ray Observatory]] and [[James Webb Space Telescope]].<ref name="NYT-20231224">{{cite news |last=Overbye |first=Dennis |date=24 December 2023 |title=How to Create a Black Hole Out of Thin Air – Black holes were thought to arise from the collapse of dead stars. But a Webb telescope image showing the early universe hints at an alternative pathway. |url=https://www.nytimes.com/2023/12/24/science/space/astronomy-black-holes.html |url-status=live |archiveurl=https://archive.today/20231225230503/https://www.nytimes.com/2023/12/24/science/space/astronomy-black-holes.html |archivedate=25 December 2023 |accessdate=26 December 2023 |work=[[The New York Times]] |authorlink=Dennis Overbye}}</ref><ref name="ARX-20231126">{{cite journal |author=Natarajan, Priyamvada |display-authors=et al. |title=First Detection of an Over-Massive Black Hole Galaxy UHZ1: Evidence for Heavy Black Hole Seed Formation from Direct Collapse |journal=The Astrophysical Journal |date=2024 |volume=960 |issue=1 |pages=L1 |doi=10.3847/2041-8213/ad0e76 |doi-access=free |arxiv=2308.02654 |bibcode=2024ApJ...960L...1N }}</ref>

In 2024, a review by [[Bernard Carr]] and colleagues concluded that PBHs formed in the [[quantum chromodynamics]] (QCD) epoch prior to 10<sup>–5</sup> seconds after the Big Bang, resulting in a broadly platykurtic mass distribution today, "with a number of distinct bumps, the most prominent one being at around one solar mass."<ref name="Carr24" />

== Formation ==
[[File:PBH formation.png|upright=1.45|thumb|Primordial black holes were possibly formed by the collapse of overdense regions in the inflationary or early radiation-dominated universe.<ref>{{Cite journal |last1=Kawasaki |first1=Masahiro |last2=Kitajima |first2=Naoya |last3=Yanagida |first3=Tsutomu T. |date=2013-03-18 |title=Primordial black hole formation from an axionlike curvaton model |journal=Physical Review D |volume=87 |issue=6 |pages=063519 |doi=10.1103/PhysRevD.87.063519|arxiv=1207.2550 |bibcode=2013PhRvD..87f3519K |s2cid=119230374 }}</ref>]]

Primordial black holes could have formed in the very early Universe (less than one second after the Big Bang) during the [[Inflation (cosmology)|inflationary era]], or in the very early [[radiation-dominated era]]. The essential ingredient for the formation of a primordial black hole is a fluctuation in the density of the Universe, inducing its gravitational collapse. One typically requires density contrasts <math> \delta \rho / \rho \sim 0.1 </math> (where <math> \rho </math> is the density of the Universe) to form a black hole.<ref>{{Cite journal |last1=Harada |first1=T. |last2=Yoo |first2=C.-M. |last3=Khori |first3=K. |s2cid=119305036 |year=2013 |title=Threshold of primordial black hole formation |journal=[[Physical Review D]] |volume=88 |issue=8 |pages=084051 |arxiv=1309.4201 |bibcode= 2013PhRvD..88h4051H|doi=10.1103/PhysRevD.88.084051 }}</ref>

=== Production mechanisms ===
There are several mechanisms able to produce such inhomogeneities in the context of cosmic inflation (in hybrid inflation models). Some examples include:

==== Axion inflation ====

[[Axion]] inflation is a theoretical model in which the axion acts as an [[inflaton]] field. Because of the time period it is created at, the field is oscillating at its minimal potential energy. These oscillations are responsible for the energy density fluctuations in the early universe.<ref name=":4" /> For a review of production mechanisms arising from various models of inflation, see Ref.<ref>{{Cite journal|title=Inflation and primordial black holes|journal=Universe|volume=9|issue=5|pages=9050203|doi=10.3390/universe9050203|year=2023|last1=Ozsoy|first1=Ogan|last2=Tasinato|first2=Gianmassimo|doi-access=free |arxiv=2301.03600|bibcode=2023Univ....9..203O }}</ref>

==== Reheating ====

Reheating is the transitory process between the inflationary and the hot, dense, radiation-dominated period. During this time the inflaton field decays into other particles. These particles begin to interact in order to reach [[thermal equilibrium]]. However, if this process is incomplete it creates density fluctuations, and if these are big enough they could be responsible for the formation of PBH.<ref name=":6" /><ref>{{Cite journal|title=Nonperturbative dynamics of reheating after inflation: A review|journal=International Journal of Modern Physics D|volume=24|issue=1|pages=1530003|doi=10.1142/S0218271815300037|last1=Amin|first1=Mustafa A.|last2=Hertzberg|first2=Mark P.|last3=Kaiser|first3=David I.|last4=Karouby|first4=Johanna|date=2015 |arxiv=1410.3808|bibcode=2015IJMPD..2430003A }}</ref>

==== Cosmological phase transitions ====

[[Cosmological phase transitions]] may cause inhomogeneities in different ways depending on the specific details of each transition. For example, one mechanism is concerned with the collapse of overdense regions that arise from these phase transitions, while another mechanism involves highly energetic particles that are produced in these phase transitions and then go through gravitational collapse forming PBHs.<ref name=":7" />

== Implications ==
=== Dark matter problem ===
The dark matter problem, proposed in 1933 by Swiss-American astronomer [[Fritz Zwicky]], refers to the fact that scientists still do not know what form dark matter takes. PBH can solve that in a few ways. First, if PBHs accounted for all or a significant amount of the dark matter in the universe, this could explain the gravitational effects seen in galaxies and galactic clusters. Secondly, PBHs have different proposed production mechanisms. Unlike [[Weakly interacting massive particle|WIMPs]], they can emit gravitational waves that interact with regular matter. Finally, the discovery of PBHs could explain some of the observed gravitational lensing effects that couldn't arise from ordinary matter. While evidence that primordial black holes may constitute dark matter is inconclusive as of 2023, researchers such as [[Bernard Carr]] and others are strong proponents.<ref name="jwst"/><ref name="Bird"/><ref name=":22">{{cite journal |last1=Carr |first1=Bernard |last2=Kühnel |first2=Florian |date=2 May 2022 |title=Primordial black holes as dark matter candidates |url=https://scipost.org/SciPostPhysLectNotes.48/pdf |journal=SciPost Physics Lecture Notes |pages=48 |doi=10.21468/SciPostPhysLectNotes.48 |s2cid=238407875 |access-date=13 February 2023 |doi-access=free |arxiv=2110.02821 }} (See also the [https://indico.cern.ch/event/949654/contributions/4031007/attachments/2293539/3901659/Carr-Kuhnel.pdf accompanying slide presentation.]</ref><ref name="Espinosa">{{cite journal |last1=Espinosa |first1=J. R. |last2=Racco |first2=D. |last3=Riotto |first3=A. |title=A Cosmological Signature of the Standard Model Higgs Vacuum Instability: Primordial Black Holes as Dark Matter |journal=Physical Review Letters |volume=120 |issue=12 |pages=121301 |doi=10.1103/PhysRevLett.120.121301 |pmid=29694085 |date=23 March 2018|arxiv=1710.11196 |bibcode=2018PhRvL.120l1301E |s2cid=206309027 }}</ref><ref name="Clesse">{{cite journal |last1=Clesse |first1=Sebastien |last2=García-Bellido |first2=Juan |title=Seven Hints for Primordial Black Hole Dark Matter |journal=Physics of the Dark Universe |volume=22 |pages=137–146 |arxiv=1711.10458 |bibcode=2018PDU....22..137C |doi=10.1016/j.dark.2018.08.004 |year=2018 |s2cid=54594536 }}</ref><ref name="Lacki">{{cite journal |last1=Lacki |first1=Brian C. |last2=Beacom |first2=John F. |title=Primordial Black Holes as Dark Matter: Almost All or Almost Nothing |journal=The Astrophysical Journal |date=12 August 2010 |volume=720 |issue=1 |pages=L67–L71 |doi=10.1088/2041-8205/720/1/L67 |language=en |issn=2041-8205 |arxiv=1003.3466 |bibcode=2010ApJ...720L..67L |s2cid=118418220 }}</ref><ref name="Kashlinsky">{{cite journal |last1=Kashlinsky |first1=A. |title=LIGO gravitational wave detection, primordial black holes and the near-IR cosmic infrared background anisotropies |journal=The Astrophysical Journal |date=23 May 2016 |volume=823 |issue=2 |pages=L25 |doi=10.3847/2041-8205/823/2/L25 |issn=2041-8213|arxiv=1605.04023 |bibcode=2016ApJ...823L..25K |s2cid=118491150 |doi-access=free }}</ref><ref name="Frampton">{{cite journal |last1=Frampton |first1=Paul H. |last2=Kawasaki |first2=Masahiro |last3=Takahashi |first3=Fuminobu |last4=Yanagida |first4=Tsutomu T. |title=Primordial Black Holes as All Dark Matter |journal=Journal of Cosmology and Astroparticle Physics |date=22 April 2010 |volume=2010 |issue=4 |pages=023 |doi=10.1088/1475-7516/2010/04/023 |issn=1475-7516|arxiv=1001.2308 |bibcode=2010JCAP...04..023F |s2cid=119256778 }}</ref><ref name="Carneiro">{{cite journal |last1=Carneiro |first1=S. |last2=de Holanda |first2=P. C. |last3=Saa |first3=A. |date=2021 |title=Neutrino primordial Planckian black holes |journal=Physics Letters |volume=B822 |pages=136670 |bibcode=2021PhLB..82236670C |doi=10.1016/j.physletb.2021.136670 |issn=0370-2693 |s2cid=244196281 |doi-access=free |hdl-access=free |hdl=20.500.12733/1987}}</ref>

=== Galaxy formation ===
Since primordial black holes do not necessarily have to be small (they can have any size), they may have contributed to [[Galaxy formation and evolution|formation of galaxies]], such as those earlier than expected.<ref name="jwst1"/><ref name="jwst2"/><ref>{{Cite news |last=Frankel |first=Miriam |date=23 November 2024 |title=Volatile cosmology |work=[[New Scientist]] |pages=32–35}}</ref>

=== Cosmological domain wall problem ===
The cosmological domain wall problem, proposed in 1974 by Soviet physicist [[Yakov Zeldovich]], discussed the formation of domain walls during phase transitions of the early universe and what could arise from their large energy densities. PBHs could serve as a solution to this problem in various ways. One explanation could be that PBHs can prevent the formation of domain walls due to them exerting gravitational forces on the surrounding matter making it clump and theoretically preventing the formation of said walls. Another explanation could be that PBHs could decay domain walls; if these were formed in the early universe before PBHs then due to gravitational interactions these could eventually collapse into PBHs. Finally, a third explanation could be that PBHs do not violate the observational constraints; if PBHs in the 10<sup>12</sup>–10<sup>13</sup> kg mass range were to be detected then these would have the right density to make up all dark matter in the universe without violating constraints, thus the domain wall problem wouldn't arise.<ref>{{cite journal |author=Stojkovic |first1=D. |last2=Freese |first2=K. |last3=Starkman |first3=G. D. |name-list-style=amp |date=2005 |title=Holes in the walls: primordial black holes as a solution to the cosmological domain wall problem |journal=Phys. Rev. D |volume=72 |issue=4 |page=045012 |arxiv=hep-ph/0505026 |bibcode=2005PhRvD..72d5012S |doi=10.1103/PhysRevD.72.045012 |s2cid=51571886}}</ref>

=== Cosmological monopole problem ===
The Cosmological monopole problem, also proposed by Yakov Zeldovich in the late 1970s, consisted of the absence of [[magnetic monopole]]s nowadays. PBHs can also serve as a solution to this problem. To start, if magnetic monopoles did exist in the early universe these could have gravitationally interacted with PBHs and been absorbed thus explaining their absence. Another explanation due to PBHs could be that PBHs would have exerted gravitational forces on matter causing it to clump and dilute the density of magnetic monopoles.<ref>{{cite journal |author=Stojkovic |first1=D. |last2=Freese |first2=K. |date=2005 |title=A black hole solution to the cosmological monopole problem |journal=Phys. Lett. B |volume=606 |issue=3–4 |pages=251–257 |arxiv=hep-ph/0403248 |bibcode=2005PhLB..606..251S |doi=10.1016/j.physletb.2004.12.019 |s2cid=119401636}}</ref>

=== String theory ===
{{main|String theory}}

[[General relativity]] predicts the smallest primordial black holes would have evaporated by now, but if there were a [[Four-dimensional space|fourth spatial dimension]]&nbsp;– as predicted by [[string theory]]&nbsp;– it would affect how gravity acts on small scales and "slow down the evaporation quite substantially".<ref>McKee, Maggie. (2006) [http://www.newscientistspace.com/article/dn9240-satellite-could-open-door-on-extra-dimension.html NewScientistSpace.com&nbsp;– Satellite could open door on extra dimension]</ref> In essence, the energy stored in the fourth spatial dimension as a stationary wave would bestow a significant rest mass to the object when regarded in the conventional four-dimensional space-time. This could mean there are several thousand primordial black holes in our galaxy. To test this theory, scientists will use the [[Fermi Gamma-ray Space Telescope]], which was put into orbit by [[NASA]] on June 11, 2008. If they observe specific small interference patterns within [[gamma-ray burst]]s, it could be the first indirect evidence for primordial black holes and string theory.{{update inline|date=June 2019}}

== Observational limits and detection strategies ==
A variety of observations have been interpreted to place limits on the abundance and mass of primordial black holes:

''Lifetime, Hawking radiation and gamma-rays:'' One way to detect primordial black holes, or to constrain their mass and abundance, is by their [[Hawking radiation]]. [[Stephen Hawking]] theorized in 1974 that large numbers of such smaller primordial black holes might exist in the [[Milky Way]] in our [[galaxy]]'s [[Galactic halo|halo]] region. All black holes are theorized to emit Hawking radiation at a rate inversely proportional to their mass. Since this emission further decreases their mass, black holes with very small mass would experience runaway evaporation, creating a burst of radiation at the final phase, equivalent to a hydrogen bomb yielding millions of megatons of explosive force.<ref>{{cite journal |last1=Hawking |first1=Stephen W. |year=1977 |title=The quantum mechanics of black holes |journal=Scientific American |volume=236 |pages=34–40 |bibcode=1977SciAm.236a..34H |doi=10.1038/scientificamerican0177-34}}</ref> A regular black hole (of about 3 [[solar mass]]es) cannot lose all of its mass within the current age of the universe (they would take about 10<sup>69</sup> years to do so, even without any matter falling in). However, since primordial black holes are not formed by stellar core collapse, they may be of any size. A black hole with a mass of about 10<sup>11</sup> kg would have a lifetime about equal to the age of the universe. If such low-mass black holes were created in sufficient number in the Big Bang, we should be able to observe explosions by some of those that are relatively nearby in our own [[Milky Way]] [[galaxy]]. [[NASA]]'s Fermi Gamma-ray Space Telescope satellite, launched in June 2008, was designed in part to search for such evaporating primordial black holes. Fermi data set up the limit that less than one percent of dark matter could be made of primordial black holes with masses up to 10<sup>13</sup> kg. Evaporating primordial black holes would have also had an impact on the Big Bang nucleosynthesis and change the abundances of light elements in the Universe. However, if theoretical Hawking radiation does not actually exist, such primordial black holes would be extremely difficult, if not impossible, to detect in space due to their small size and lack of large gravitational influence.

''Temperature anisotropies in the cosmic microwave background:'' Accretion of matter onto primordial black holes in the early Universe should lead to energy injection in the medium that affects the [[Recombination (cosmology)|recombination]] history of the Universe. This effect induces signatures in the statistical distribution of the [[Cosmic Microwave Background|cosmic microwave background]] (CMB) anisotropies. The [[Planck (spacecraft)|Planck]] observations of the CMB exclude that primordial black holes with masses in the range 100–10<sup>4</sup> solar masses contribute importantly to the dark matter,<ref>{{Cite journal|last1=Ali-Haimoud  |first1=Y. |last2=Kamionkowski |first2=M. |s2cid=119483868 |title=Cosmic microwave background limits on accreting primordial black holes|journal=Physical Review D |volume=95 |issue=4 |pages=043534 |arxiv=1612.05644|year=2017 |doi=10.1103/PhysRevD.95.043534 |bibcode = 2017PhRvD..95d3534A }}</ref> at least in the simplest conservative model. It is still debated whether the constraints are stronger or weaker in more realistic or complex scenarios.

''Gamma-ray signatures from annihilating dark matter:'' If the dark matter in the Universe is in the form of [[weakly interacting massive particles]] or WIMPs, primordial black holes would accrete a halo of WIMPs around them in the early universe.<ref>{{cite journal |last1=Eroshenko |first1=Yuri |title=Dark Matter Density Spikes around Primordial Black Holes |arxiv=1607.00612 |year= 2016 |doi=10.1134/S1063773716060013 |bibcode=2016AstL...42..347E |journal=Astronomy Letters |volume=42 |issue=6 |pages=347–356 |s2cid=118477620 }}</ref> The [[annihilation]] of WIMPs in the halo leads to a signal in the gamma-ray spectrum which is potentially detectable by dedicated instruments such as the Fermi Gamma-ray Space Telescope.<ref>{{cite journal |last1=Boucenna |first1=Sofiane M. |last2=Kühnel |first2=Florian |last3=Ohlsson |first3=Tommy |last4=Visinelli |first4=Luca |title=Novel Constraints on Mixed Dark-Matter Scenarios of Primordial Black Holes and WIMPs |arxiv=1712.06383 |year= 2018 |doi=10.1088/1475-7516/2018/07/003 |bibcode= 2018JCAP...07..003B |journal=Journal of Cosmology and Astroparticle Physics |volume=1807 |issue=7 |pages= 003 |s2cid=119402552 }}</ref>

In the future, new limits will be set up by various observations:

* The [[Square Kilometre Array]] (SKA) radio telescope will probe the effects of primordial black holes on the [[reionization]] history of the Universe, due to energy injection into the intergalactic medium, induced by matter accretion onto primordial black holes.<ref>{{Cite journal|last1=Tashiro |first1=H. |title=The effect of primordial black holes on 21 cm fluctuations |journal=Monthly Notices of the Royal Astronomical Society |volume=435 |issue=4 |pages=3001 |last2=Sugiyama |first2=N. |s2cid=118560597 |arxiv=1207.6405|year=2012 |doi=10.1093/mnras/stt1493 |doi-access=free |bibcode = 2013MNRAS.435.3001T }}</ref>
* LIGO, VIRGO and future gravitational waves detectors will detect new black hole merging events, from which one could reconstruct the mass distribution of primordial black holes.<ref name="Clesse 2017 142–1472"/> These detectors could allow distinguishing unambiguously between primordial or stellar origins if merging events involving black holes with a mass lower than 1.4 solar mass are detected. Another way would be to measure the large orbital eccentricity of primordial black hole binaries.<ref>{{Cite journal|last1=Cholis |first1=I. |last2=Kovetz |first2=E.D. |last3=Ali-Haimoud |first3=Y. |last4=Bird |first4=S. |last5=Kamionkowski |first5=M. |last6=Munoz |first6=J. |last7=Raccanelli |first7=A. |s2cid=119236439 |title=Orbital eccentricities in primordial black hole binaries |journal=Physical Review D |volume=94 |issue=8 |pages=084013 |arxiv=1606.07437|year=2016 |doi=10.1103/PhysRevD.94.084013 |bibcode = 2016PhRvD..94h4013C }}</ref>
*  Gravitational wave detectors, such as the [[Laser Interferometer Space Antenna]] (LISA) and [[pulsar timing array]]s, will also probe the stochastic background of gravitational waves emitted by primordial black hole binaries when they are still orbiting relatively far from each other.<ref>{{cite journal|last1=Clesse |first1=Sebastien |last2=Garcia-Bellido |first2=Juan |title=Detecting the gravitational wave background from primordial black hole dark matter|journal=Physics of the Dark Universe |arxiv=1610.08479|year=2016 |volume=18 |pages=105–114 |doi=10.1016/j.dark.2017.10.001 |bibcode=2017PDU....18..105C |s2cid=73589635 }}</ref>
* New detections of faint dwarf galaxies, and observations of their central star clusters, could be used to test the hypothesis that these dark matter-dominated structures contain primordial black holes in abundance.
* Monitoring star positions and velocities within the Milky Way could be used to detect the influence of a nearby primordial black hole.
* It has been suggested<ref>{{cite journal|last1=Khriplovich|first1=I. B.|last2=Pomeransky|first2=A. A.|last3=Produit|first3=N.|last4=Ruban|first4=G. Yu.|s2cid=118604599|title=Can one detect passage of a small black hole through the Earth?|journal=Physical Review D|year=2008|volume=77|issue=6|pages=064017|arxiv=0710.3438|doi=10.1103/PhysRevD.77.064017|bibcode=2008PhRvD..77f4017K}}</ref><ref>{{cite arXiv | eprint=0801.4623 | last1=Khriplovich | first1=I. B. | last2=Pomeransky | first2=A. A. | last3=Produit | first3=N. | last4=Ruban | first4=G. Yu. | title=Passage of small black hole through the Earth. Is it detectable? | year=2008 | class=hep-ex }}</ref> that a small black hole passing through the Earth would produce a detectable acoustic signal. Because of its tiny diameter and large mass as compared to a [[nucleon]], as well as its relatively high speed, a primordial black hole would simply transit Earth virtually unimpeded with only a few impacts on nucleons, exiting the planet with no ill effects.
* Another way to detect primordial black holes could be by watching for ripples on the surfaces of stars. If the black hole passed through a star, its density would cause observable vibrations.<ref>{{cite web |url=http://www.space.com/13026-dark-matter-primordial-black-holes.html |title=Primitive Black Holes Could Shine|website=[[Space.com]]|date=26 September 2011}}</ref><ref>{{Cite journal|title=Transient Solar Oscillations Driven by Primordial Black Holes|journal=Physical Review Letters|volume=107|issue=11|pages=111101|doi=10.1103/PhysRevLett.107.111101|pmid=22026654|year=2011|last1=Kesden|first1=Michael|last2=Hanasoge|first2=Shravan|s2cid=20800215|bibcode=2011PhRvL.107k1101K|arxiv = 1106.0011 }}</ref>
* Monitoring quasars in the microwave wavelength and detection of the wave optics feature of gravitational microlensing by the primordial black holes.<ref>{{Cite journal|title=Primordial black hole detection through diffractive microlensing|journal=Physical Review D|volume=97|issue=10|pages=103507|doi=10.1103/PhysRevD.97.103507|year=2018|last1=Naderi|first1= Tayebeh|last2=Mehrabi|first2=Ahmad|last3=Rahvar|first3=Sohrab|s2cid=118889277|arxiv = 1711.06312 |bibcode=2018PhRvD..97j3507N}}</ref>
* Observing deviations in the motion of Solar System objects, such as the planet Mars, which could arise from the rapid transit of a primordial black hole through the inner Solar System.<ref>{{Cite journal|title=Close encounters of the primordial kind: A new observable for primordial black holes as dark matter|journal=Physical Review D|volume=110|issue=6|pages=063533|doi=10.1103/PhysRevD.110.063533|year=2024|last1=Tran|first1=Tung|last2=Geller|first2=Sarah|last3=Lehmann|first3=Benjamin|last4=Kaiser|first4=David|arxiv=2312.17217|bibcode=2024PhRvD.110f3533T }}</ref>

=== Facilities able to provide PBH measurement ===
{{Prose|section|date=April 2023}}

None of these facilities are focused on the direct detection of PBH due to them being a theoretical phenomenon, but the information collected in each respective experiment provides secondary data which can help provide insight and constraints on the nature of PBHs.<ref name=":5" />

'''GW-detectors'''

* [[LIGO]]/[[Virgo interferometer|VIRGO]] – These detectors already place important constraints on the limits of PBHs. However, they're always in the search for new unexpected signals; if they detect a black hole in the mass range that does not correspond to stellar evolution theory, it could serve as evidence for PBHs.
* [[Cosmic Explorer (gravitational wave observatory)|Cosmic Explorer]]/[[Einstein Telescope]] – Both of these projects serve as the next generation of LIGO/VIRGO, these would increase sensitivity around the 10–100&nbsp;Hz band and would allow to probe PBH information at higher redshifts
* [[North American Nanohertz Observatory for Gravitational Waves|NANOGrav]] – This collaboration detected a stochastic signal but it is not yet a certified gravitational wave signal since quadrupolar correlations have not been detected. But, should this be confirmed, it could serve as evidence for sub-solar mass PBHs.
* [[Laser Interferometer Space Antenna]] (LISA) – Like any GW detector, LISA has great potential to detect PBHs. The uniqueness of LISA lies with the ability to detect extreme mass ratio inspirals when low mass black holes merge with massive objects. Due to its sensitivity it will also allow for the detection and confirmation of the stochastic NANOGrav signal.
* AEDGE Atomic Experiment for Dark Matter and Gravity Exploration in Space – This proposed mid-range gravitational wave experiment has a uniqueness which lies in its detection ability of intermediate mass ratio mergers like the ones theorized during early supermassive black hole assembly, should the detection of these happen it would serve as evidence for PBHs.

'''Space telescopes'''

* [[Nancy Grace Roman Space Telescope]] (WFIRST) – As a space telescope, WFIRST will have the capacity of detecting or at least placing constraints on PBHs through different types of lensing, one of which is Astrometric Lensing. When an object passes in front of a known light source, such as a star, it slightly (to the order of microarcseconds) shifts its position and this is known as Astrometric lensing.

'''Sky Surveys'''

* [[Vera C. Rubin Observatory]] (LSST) – This will provide the capability of directly measuring the mass function of compact objects by [[Gravitational microlensing|microlensing]]. It will be able to observe both low and high-mass objects thus placing constraints on both sides of the spectrum. LSST will also have the ability to detect [[Kilonova]]e that lack gravitational wave signals which is related to the existence of PBHs.

'''Very Large Arrays'''

* ngVLA – the next generation Very Large Array will be able to improve GW bounds by a magnitude of the current contraints placed by the NANOGrav. This increased sensitivity will be able to confirm the nature of the GW signal from NANOGrav. It will also be able to discriminate a PBH explanation from other sources.

'''Fast Radio Bursts observatories'''

* Experiments like these obtain large numbers of [[Fast radio burst|Fast Radio Bursts]] and promote the statistical measures of their lensing which would allow for PBH lenses. Some examples include:
** [[Canadian Hydrogen Intensity Mapping Experiment]] (CHIME)
** [[Hydrogen Intensity and Real-time Analysis eXperiment]] (HIRAX)
** CHORD (Successor to CHIME)

'''MeV Gamma-Ray Telescopes'''

* Since the MeV gamma-ray band has yet to be explored, proposed experiments could place tighter constraints on the abundance of PBHs in the asteroid-mass range. Some examples of the proposed telescopes include:
** AdEPT
** AMEGO
** All-Sky ASTROGAM
** GECCO
** GRAMS
** MAST
** PANGU

'''GeV and TeV Gamma-Ray Observatories'''

* '''Wide field of view survey observatory'''
** PBHs in the mass range of ~ 5×10<sup>19</sup> solar masses would be producing TeV gamma-rays due to evaporation. Since these would occur in isotropic bursts across the sky, wide field survey observatories would be ideal to searching for these, a few examples could be:
*** [[Fermi Gamma-ray Space Telescope|FGST]]’s LAT
*** [[High Altitude Water Cherenkov Experiment]] (HAWC)
*** [[Southern Wide-field Gamma-ray Observatory]] (SWGO)
* '''Atmospheric Cherenkov telescopes'''
** Although these have a narrow field of view, they have great sensitivity for TeV cosmic-rays and thus can provide upper limits on the burst rate density. Some of these telescopes are:
*** [[VERITAS|Very Energetic Radiation Imaging Telescope Array System]] (VERITAS)
*** [[MAGIC (telescope)|Major Atmospheric Gamma Imaging Cherenkov Telescopes]] (MAGIC)
*** [[High Energy Stereoscopic System]] (HESS)
*** [[Cherenkov Telescope Array]] (CTA)

== Difference from direct collapse black holes ==

A [[direct collapse black hole]] is the result of the collapse of unusually dense and large regions of gas, after the [[radiation-dominated era]], while primordial black holes would have resulted from the direct collapse of energy, ionized matter, or both, during the [[Inflation (cosmology)|inflationary]] or radiation-dominated eras.<ref>{{cite journal |last1=Carr |first1=Bernard |last2=Kühnel |first2=Florian |title=Primordial Black Holes as Dark Matter: Recent Developments |journal=Annual Review of Nuclear and Particle Science |date=19 October 2020 |volume=70 |issue=1 |pages=355–394 |doi=10.1146/annurev-nucl-050520-125911 |arxiv=2006.02838 |bibcode=2020ARNPS..70..355C |s2cid=118475595 |url=https://www.annualreviews.org/doi/abs/10.1146/annurev-nucl-050520-125911 |access-date=4 September 2023 |language=en |issn=0163-8998}}</ref>

== See also ==
* [[Primordial gravitational wave]]
* [[Primordial fluctuations]]

== References ==
{{Refbegin}}
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* D. Page, [http://prola.aps.org/abstract/PRD/v13/i2/p198_1 Phys. Rev. D13 (1976) 198] : First detailed studies of the evaporation mechanism
* B. J. Carr & S. W. Hawking, [http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1974MNRAS.168..399C Mon. Not. Roy. Astron. Soc 168 (1974) 399] : Describes links between primordial black holes and the early universe
* A. Barrau et al., [https://arxiv.org/abs/astro-ph/0112486 Astron. Astrophys. 388 (2002) 676], [https://arxiv.org/abs/astro-ph/0207395 Astron. Astrophys. 398 (2003) 403], [https://arxiv.org/abs/astro-ph/0505436 Astrophys. J. 630 (2005) 1015] : Experimental searches for primordial black holes due to the emitted antimatter
* A. Barrau & G. Boudoul, [https://arxiv.org/abs/astro-ph/0212225 Review talk given at the International Conference on Theoretical Physics TH2002] : Cosmology with primordial black holes
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* P. Kanti, [https://arxiv.org/abs/hep-ph/0402168 Int. J. Mod. Phys. A19 (2004) 4899] : Evaporating black holes and extra-dimensions
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{{Refend}}
{{Reflist}}
== External link ==
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[[Category:Black holes]]
[[Category:Dark matter]]