Sterrekunde

Kan swaartekraggolwe naby samesmeltende swart gate self tot 'n swart gat ineenstort?

Kan swaartekraggolwe naby samesmeltende swart gate self tot 'n swart gat ineenstort?


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Ek het verstaan ​​dat samesmeltende swart gate 'n aansienlike hoeveelheid van hul massa as gravitasiegolwe (tot 5-10%) uitstraal. Op grond van wat ek van gravitasiegolwe verstaan, hoe nader swart gate aan samesmelting is, word die intensiteit (beide frekwensie en amplitude) sterker.

Ek neem dus aan dat baie van die totale GW-uitset tydens die laaste "baan" van die inspirasie plaasvind, en dat die oorsprong van hierdie golwe binne "die finale baan" geleë is, dws regtig naby die horison van beide swart gate, my vraag is dit in wese, is dit moontlik dat sulke GW self in 'n swart gat kan ineenstort (iets soos 'n Kugelblitz, glo ek), aangesien daar net soveel energie in GW's (sê maar 'n paar sonmassas) in so 'n klein hoeveelheid ruimte (sê maar tien) van km's in blokkies gesny)?

Ek glo dit kan afhang van die grootte van die samesmelting van swart gate, aangesien beide die intensiteit van GW's en die grootte van die geleentheidshorisonne afhang van die massa. Ek het die gevoel dat kleiner swart gate nader aan hierdie limiet kan wees, maar miskien hang die intensiteit en die grootte van die horisonlyn lineêr af van die massa, en dit maak glad nie saak nie.


U sal dalk na enkele simulasies van die SXS-samewerking wil kyk. In wese is die swart gate en die swaartekraggolwe aspekte van dieselfde ding - die kromming van die ruimte. In die besonder lyk die manier waarop swart gate in hierdie simulasies saamsmelt, dat die twee swart gate baie naby kom en die ruimte rondom hulle meer en meer geboë word (en op meer ingewikkelde maniere). Sommige van die kromming versprei in die vorm van gravitasiegolwe, word minder intens soos dit verder wegkom, maar sommige daarvan, baie naby aan die twee swart gate, bereik die "kritieke" vlak van kromming waarop 'n gebeurtenishorison verskyn (wat strek die gebeurtenishorisonne van die twee swart gate) wat dan "afklink" en bolvormig word en die saamgevoegde swart gat vorm.


Navorsers vind die oorsprong en maksimum massa van massiewe swart gate waargeneem deur swaartekraggolfdetektore

Deur simulasies van 'n sterwende ster het 'n span teoretiese fisika-navorsers die evolusie-oorsprong en die maksimum massa swart gate gevind wat ontdek word deur die opsporing van gravitasiegolwe, soos getoon in Figuur 1.

Die opwindende opsporing van gravitasiegolwe met LIGO (laser-interferometer gravitasiegolfwaarneming) en VIRGO (Maagd-interferometriese gravitasiegolfantenne) het getoon dat swart gate in noue binêre stelsels saamsmelt.

Figuur 1: Skematiese diagram van die binêre vorming van swart gate vir GW170729. 'N Ster onder 80 sonmassa ontwikkel en ontwikkel tot 'n kern-ineenstorting-supernova. Die ster ervaar nie pare-onstabiliteit nie, dus is daar geen beduidende massa-uitwerping deur pulsasie nie. Nadat die ster 'n massiewe ysterkern gevorm het, stort dit in deur sy eie swaartekrag en vorm dit 'n swart gat met 'n massa onder die 38 sonmassa. 'N Ster tussen 80 en 140 sonmassa ontwikkel en ontwikkel in 'n pulserende supernova vir paar-onstabiliteit. Nadat die ster 'n massiewe koolstof-suurstofkern vorm, ervaar die kern katastrofiese elektron-positron-parskepping. Dit prikkel sterk pulsasie en gedeeltelike uitwerping van die sterre materiaal. Die uitgestote materiale vorm die sirkelvormige materie rondom die ster. Daarna gaan die ster voort om te ontwikkel en vorm 'n massiewe ysterkern wat op 'n manier ineenstort soos die gewone kernval-supernova, maar met 'n hoër finale swartgatmassa tussen 38 en # 8211 52 sonmassa. Hierdie twee paaie kan die oorsprong van die bespeurde binêre swartgatmassas van die gravitasiegolfgebeurtenis GW170729 verklaar. Krediet: Shing-Chi Leung et al. / Kavli IPMU

Die massas van die waargenome swart gate voor samesmelting is gemeet en het blykbaar 'n veel groter as voorheen verwagte massa van ongeveer tien keer die massa van die son (sonmassa). In een van hierdie gebeurtenisse, GW170729, is die waargenome massa van 'n swart gat voor samesmelting eintlik so groot soos ongeveer 50 sonmassa. Maar dit is nie duidelik watter ster so 'n massiewe swart gat kan vorm nie, of wat die maksimum swart gate is wat deur gravitasiegolfdetektore waargeneem word nie.

Om hierdie vraag te beantwoord, het 'n navorsingspan by die Kavli-instituut vir die fisika en wiskunde van die heelal (Kavli IPMU) bestaande uit destydse projeknavorser Shing-Chi Leung (tans aan die California Institute of Technology), senior wetenskaplike Ken'ichi Nomoto en besoekende senior wetenskaplike Sergei Blinnikov (professor aan die Instituut vir Teoretiese en Eksperimentele Fisika in Mosow) het die finale fase van die evolusie van baie massiewe sterre ondersoek, in die besonder 80 tot 130 sonmassa-sterre in noue binêre stelsels. Hul bevinding word getoon in Illustrasies (a & # 8211 e) en Figure (1 & # 8211 4).

Simulasie: Pulsasionele paar-onstabiliteit supernova evolusionêre proses. Krediet: Shing-Chi Leung et al.

In noue binêre stelsels verloor aanvanklik 80 tot 130 sonmassa-sterre hul waterstofryke omhulsel en word dit heliumsterre van 40 tot 65 sonmassa. Wanneer die aanvanklik 80 tot 130 sonmassa-sterre suurstofryke kern vorm, ondergaan die sterre dinamiese pulsasie (Illustrasies a & # 8211 b en Figuur 2), omdat die temperatuur in die sterre binnekant hoog genoeg word om fotone om te skakel in elektron- positron pare. Sodanige 'parskepping' maak die kern onstabiel en versnel die inkrimping tot in duie stort (Illustrasie b).

In die oormatige ster brand suurstof plofbaar. Dit veroorsaak 'n weiering van ineenstorting en dan 'n vinnige uitbreiding van die ster. 'N Gedeelte van die sterre buitenste laag word uitgegooi, terwyl die binneste deel afkoel en weer inmekaar stort (Illustrasie c). Die polsing (ineenstorting en uitbreiding) herhaal totdat suurstof uitgeput is (illustrasie d). Hierdie proses word 'pulsational pair-instability' (PPI) genoem. Die ster vorm 'n ysterkern en stort uiteindelik in 'n swart gat in, wat die ontploffing van die supernova sal veroorsaak (Illustrasie e), wat PPI-supernova (PPISN) genoem word.

Figuur 2: Die rooi lyn toon die tydsontwikkeling van die temperatuur en digtheid in die middel van die aanvanklik 120 sonmassa-ster (PPISN: pulserende paar-instabiliteit supernova). Die pyle wys die rigting van die tyd. Die ster pols (d.w.s. samentrekking en uitbreiding twee keer) deur op # 1 en # 2 te bons en stort uiteindelik in 'n lyn ineen as die van 'n ster van 25 sonmassa (dun blou lyn: CCSN (kern-ineenstorting supernova)). Die dik blou lyn toon die inkrimping en finale uitbreiding van die ster van die 200 sonmassa, wat heeltemal ontwrig word sonder dat daar 'n swart gat agterbly (PISN: supernova vir paar-instabiliteit). Die gebied links bo, omring deur die swart, effense lyn, is die gebied waar 'n ster dinamies onstabiel is. Krediet: Shing-Chi Leung et al.

Deur verskeie sulke pulsasies en gepaardgaande massa-uitwerping te bereken totdat die ster ineenstort om 'n swart gat te vorm, het die span bevind dat die maksimum massa van die swart gat wat gevorm word uit 'n pulserende paar-instabiliteit-supernova 52 sonmassa is (Figuur 3).

Sterre wat aanvanklik massiewer is as 130 sonmassa (wat heliumsterre massiewer as 65 sonmassa vorm) ondergaan 'n paar onstabiliteitsupernova's as gevolg van die ontploffing van suurstof wat die ster heeltemal ontwrig sonder om 'n swart gat oor te bly. Sterre bo 300 sonmassa val inmekaar en kan 'n swart gat vorm wat massiewer is as ongeveer 150 sonmassa.

Figuur 3: Die rooi lyn (wat die rooi simulasiepunte verbind) toon die massa van die swart gat wat oorbly na die pulserende paar-instabiliteitsupernova (PPISN) teenoor die aanvanklike stermassa. Die rooi en swart stippellyne toon die massa van die heliumkern wat in die binêre stelsel oorbly. Die rooi lyn is laer as die stippellyn omdat 'n mate van massa deur die pulserende massaverlies uit die kern verlore gaan. (Supernova vir paar-onstabiliteit, PISN, ontplof heeltemal sonder dat daar nog 'n oorblyfsel oor is.) Die piek van die rooi lyn gee die maksimum massa, 52 sonmassa, van die swart gat wat deur gravitasiegolwe waargeneem kan word. Krediet: Shing-Chi Leung et al.

Bogenoemde resultate voorspel dat daar 'n "massagaping" bestaan ​​in die swartgatmassa tussen 52 en ongeveer 150 sonmassa. Die resultate beteken dat die swart sonkrag van 50 sonkragmassa in GW170729 heel waarskynlik 'n oorblyfsel is van 'n pulserende paar-instabiliteit-supernova, soos getoon in Figuur 3 en 4.

Figuur 4: Die massas van 'n paar swart gate (aangedui deur dieselfde kleur) waarvan die samesmelting gravitasiegolwe (GW) opspoor deur gevorderde LIGO en VIRGO (samesmeltingsgebeurtename GW150914 tot GW170823 dui jaar-maand-dag aan). Die houer wat deur die 38 & # 8211 52-sonmassa ingesluit is, is die oorblywende massa-reeks wat deur PPISNe vervaardig word. Swartgatmassas wat binne hierdie boks val, moet die oorsprong van PPISN hê voordat dit ineenstort. Onder die 38 sonmassa is die swart gat wat gevorm word deur 'n massiewe ster wat CCSN ondergaan. Benewens GW170729, is GW170823 'n kandidaat van 'n PPISN in die onderste massalimietkant. Krediet: Shing-Chi Leung et al.

Die resultaat voorspel ook dat 'n massiewe sirkelvormige medium gevorm word deur die pulserende massaverlies, sodat die supernova-ontploffing wat verband hou met die vorming van die swart gat, die botsing van die uitgestote materiaal met die sirkelvormige materie sal veroorsaak om 'n superlichtende supernova te word. Toekomstige gravitasiegolfseine sal 'n basis bied waarop hul teoretiese voorspelling getoets sal word.

Verwysing: & # 8220Pulsational Pair-instability Supernovae. I. Evolusie en pulserende massa-uitwerping voor ineenstorting & # 8221 deur Shing-Chi Leung, Ken & # 8217ichi Nomoto en Sergei Blinnikov, 11 Desember 2019, Die Astrofisiese Tydskrif.
DOI: 10.3847 / 1538-4357 / ab4fe5


Die samesmelting van swart gate ontplof lig ... maar nie om die rede wat u dink nie

As twee swart gate mekaar opeet, gee dit baie energie vry.

A baie. 'N Beduidende fraksie van die massa van die swart gate word omgeskakel in energie, wat uitstraal as swaartekraggolwe, rimpels in die weefsel van die ruimtetyd. Dit kan soveel energie hê as - en ek maak nie 'n grap met jou hier nie - tienduisende kere soveel as wat die son sal uitstraal oor sy hele leeftyd... en hulle blaas weg binne 'n paar sekondes. Anders gestel: gedurende daardie tyd straal die swart gate honderdmiljoene keer meer energie uit as al die sterre in die sterrestelsel saam.

Hierdie soort krag is skrikwekkend. Mind crushing. Tog is die gebeurtenis op sigself heeltemal onsigbaar en gee dit glad nie lig nie. Dit is moontlik onder sommige spesifieke omstandighede sommige lig sal uitgestraal word, maar oor die algemeen smelt die twee swart gate saam, skree hul swaartekraggolwe uit en word 'n enkele, ietwat groter swart gat ... en dit alles sonder dat 'n foton uitgestraal word.

Geen lig word dus direk geproduseer nie. Maar indirek, dit blyk dat dinge anders kan wees.

Kunswerke wat twee swart gate om mekaar uitbeeld, kort voordat dit saamsmelt en swaartekraggolwe ontplof. Krediet: LIGO / Caltech / MIT / Sonoma State (Aurore Simonnet)

Op 21 Mei 2019 het die LIGO- en Maagd-gravitasiegolfwaarnemings 'n paar swart gate opgespoor wat êrens in die heelal saamsmelt - die geleentheid het die benaming S190521g gekry. Deur die sein te driehoekig, is gevind dat dit kom vanaf 'n kol in die lug in die rigting van die sterrebeeld Coma Berenices. Binne enkele oomblikke is 'n outomatiese alarm gestuur na teleskope regoor die planeet om na 'n soort optiese sein te soek.

Geen ligflits is gesien nie. Ten minste nie dadelik nie.

Op Mt. Daar is 'n 1,2-meter-teleskoop in Palomar in Kalifornië met 'n 600 megapixel-kamera wat 'n yslike 47 vierkante grade lug tegelyk kan sien, 'n groot gesigsveld. Dit word die Zwicky Transient Facility genoem, en dit skandeer die lug op soek na dinge wat snags flits (of net die helderheid verander).

34 dae na die opsporing van die gravitasiegolf van LIGO-Virgo, het dit na Coma Berenices gekyk en gesien hoe 'n fakkel, 'n helderder, kom uit die verre sterrestelsel AGN J124942.3 +344929 (laat ons dit kortweg J1249 noem). Dit is 'n aktiewe sterrestelsel, een met 'n supermassiewe swart gat in sy hart (met 'n massa van 100 miljoen keer die son, so ordentlik groot) wat aktief materiaal eet. Soos dit binneval, word hierdie materiaal baie warm en straal heelwat lig uit, wat hierdie sterrestelsel sigbaar maak, selfs vanaf sy verbode afstand van 4,6 miljard ligjare.

Die fakkel wat opgespoor is, het ongeveer 50 dae geduur. Gedurende die tyd was die hoeveelheid energie wat dit vrygestel het, groot, ongeveer 10 44 Joule - meer energie as wat die son gedurende sy hele leeftyd sal uitstraal!

'N Maand nadat 'n paar swart gate saamgesmelt het, het 'n groot liggloed van die sterrestelsel AGN J124942.3 +344929 (pyl) losgelaat, moontlik omdat die saamgevoegde swart gat met materiaal rondom 'n derde supermassiewe swart gat saamgewerk het. Krediet: Aladin / DSS

Daar is bekend dat aktiewe sterrestelsels vlam, maar hierdie spesifieke een was stil, of het ten minste 'n bestendige helderheid gehad, rondom 'n jaar. Deur die geskiedenis van die sterrestelsel (en die geskiedenis van ander sterrestelsels) na te gaan en statistieke toe te pas, het sterrekundiges die waarskynlikheid gevind dat hierdie fakkel te wyte is aan 'n mate van intrinsieke veranderlikhede in die sterrestelsel as minder as 0,01% - een kans uit tienduisend. Dus, baie laag.

Hulle het ook na ander potensiële bronne gekyk, soos supernovas, 'n ster wat deur 'n swart gat (!) Verskeur word, en selfs mikrolensering, maar niks pas by die profiel nie.

Wat doen fit is egter 'n "klein" swart gat wat vinnig deur die ruimte sorg en in die skyf materiaal om die supermassiewe swart gat in die middel van J1249 klap. 'N Gebeurtenis soos hierdie kan groot skokgolwe op die skyf veroorsaak, wat daartoe lei dat dit aansienlik verhit en straling weke lank uitblaas. Gas wat in die kleiner swart gat val, sal ook daaromheen ophoop, verhit en ook baie lig gee. Hierdie is uiters kragtige gebeure, wat gedurende die hele leeftyd ongeveer soveel energie vrystel as wat die son sal doen.

'N Binêre paar swart gate wat op die punt staan ​​om saam te smelt, naby die groot skyf materiaal wat om 'n supermassiewe swart gat in 'n aktiewe sterrestelsel draai. Krediet: Caltech / R. Seer (IPAC)

OK, jy dink moontlik, so wat? Wat het dit te doen met S190521g, die samesmelting van swart gate wat 'n maand tevore plaasgevind het?

Ag, hier is die baie lekker deel. As twee swart gate met ongelyke massa saamsmelt, word die ontploffing van swaartekraggolfenergie op 'n asimmetriese manier vrygestel: meer daarvan word na die een kant gestuur as die ander. Dit lewer 'n groot krag op die oorblywende saamgevoegde swart gat, wat dit 'n uiters kragtige skop gee. Onthou dat die energieë waaroor ons hier praat, is groot - en dit kan tot hoë snelhede deur die ruimte versnel word.

En as die samesmelting van swart gate naby die middelpunt van 'n sterrestelsel plaasgevind het, sê een met 'n supermassiewe swart gat en 'n groot skyf materie wat daar rondom draai, dan kan dit teen 'n hoë spoed kanonkol deur daardie skyf beweeg, wat skokgolwe kan skep wat die maak materiaal op en skep 'n reuse-flits van twee maande lank, een wat helder genoeg is om meer as 4 miljard ligjare te beweeg en steeds deur 'n teleskoop in Kalifornië gesien kan word.

Ongelooflik werk die wiskunde uit. As die gravitasiegolfgebeurtenis S190521g afkomstig was van twee swart gate in die aktiewe sterrestelsel J1249 wat saamgevoeg het tot 'n enkele groter swart gat van ongeveer 100 keer die massa van die son, sou die skop van die kragtige golwe dit kon versnel tot 'n verbysterende snelheid van 200 kilometer per sekonde. - meer as 700 000 kilometer per uur! Dan, 'n maand of wat later, het hierdie swart gat deur die skyf om die supermassiewe swart gat gedompel, en die bogenoemde gebeure volg. Die hoeveelheid energie wat uit so 'n katastrofe uitgestraal word, sou ongeveer wees wat gesien is, en sou duur so lank as wat die fakkel uit die sterrestelsel was.

Dit word beter. U het 'n paar binêre swart gate nodig om saam te smelt, en swart gate vorm wanneer massiewe sterre ontplof. Maar dit is moeilik om swart gate te maak met ongeveer 50 keer die son se massa elk (of sê 80 en 20). U het 'n ster groter nodig as wat ons ooit gesien het. So miskien het die twee swart gate afsonderlik gevorm, van twee onafhanklike sterre, mettertyd gegroei deur dinge om hulle te eet, en dan op een of ander manier bymekaar gekom om later 'n binêre stelsel te vorm. 'N Goeie idee, maar dit is amper onmoontlik om dit in 'n leë ruimte uit te voer. Dit is ongelooflik onwaarskynlik, en selfs dan is dit moeilik om hulle om mekaar te laat draai.

Daar is 'n plek in die heelal waar so iets makliker is: naby 'n supermassiewe swart gat! Daar kan baie massiewe sterre wees wat in 'n wentelbaan draai wat in swart gate verander, en genoeg om hulle te voed. Dit is makliker om hulle om mekaar te laat draai as daar nog baie ander goed is wat hulle veilig kan saamstamp. In werklikheid is dit waarskynlik dat swart gate so groot die gevolg is van die samesmelting van vorige swart gate. Selfs as hulle een van die skoppe buite die middel kry, sal die erns van die veel groter supermassiewe swart gat in die omgewing hulle verhinder om uit die sterrestelsel te skiet.

Hierdie verhaal hang dus saam van die begin tot die einde. Dit kan eintlik korrek wees.

Let wel, dit is alles omstandighede. Die wiskunde werk, en die fisika ook, maar dit beteken nie dat dit is wat gebeur het nie. Ek was baie skepties toe ek die persverklaring sien. Maar as ek die koerant lees, moet ek dink dat hierdie reeks gebeure in Rube Goldberg eintlik die waarskynlikste scenario is.

Ek sou sê "ongelooflik", maar dit is letterlik geloofwaardig. Verbasend, beslis.

Die samesmelting het dus nie direk 'n ligflits opgelewer nie, maar moontlik 'n reeks gebeurtenisse, wat 'n yslike groot flits 'n maand later tot gevolg gehad het. Ongelooflik. En as dit waar is, beteken dit dat dit baie keer in die Heelal kan gebeur, wat beteken dat ons nie net onmiddellik na samesmeltings kan soek nie, maar ook na weke of maande later.


Skokkende nuwe waarneming: samesmelting van swart gate kan regtig lig uitstraal

Op 14 September 2015 is geskiedenis gemaak toe die NSF se tweeling LIGO-detektore die eerste swaartekraggolf van die mensdom direk waargeneem het. Van meer as 'n miljard ligjare weg het twee swart gate van 36 en 29 sonmassas elk saamgevoeg en die rimpels in die ruimtetyd geskep wat op daardie noodlottige dag aangebreek het. In 'n onverwagse draai het NASA se Fermi-satelliet net 0,4 sekondes 'n swak gammastraal sein vanaf 'n onbekende plek waargeneem.

In die daaropvolgende vyf jaar is LIGO opgegradeer en bygevoeg deur Maagd, waar sommige

Daar is 50 addisionele samesmeltings met swart gate en swart gate gesien. In al hierdie gebeure het nie een gammastrale, X-strale, radiogolwe of enige ander swaartekraggolfsein uitgestuur nie. Tot 21 Mei 2019 toe die Zwicky-verbygaande fasiliteit 'n elektromagnetiese fakkel gesien het wat saamval met een van die samesmeltings. As dit waar is, kan dit veroorsaak dat ons alles heroorweeg. Miskien smelt swart gate tog wel lig uit.

As u nadink oor wat 'n swart gat is, sal u dadelik verstaan ​​waarom dit nie lig moet uitstraal as twee van hulle bots nie. 'N Swart gat is nie 'n vaste, fisiese voorwerp soos die ander vorms van materie in ons heelal nie. Hulle is nie saamgestel uit identifiseerbare deeltjies nie, hulle reageer nie of reageer nie met die deeltjies in hul omgewing nie; hulle sal nie lig uitstraal as 'n ander voorwerp met hulle bots nie.

Die rede hiervoor is natuurlik dat swart gate gedefinieer word as gebiede van die ruimte wat so erg geboë is - met soveel materie en energie in so 'n klein volume - dat niks, selfs nie lig, daaruit kan ontsnap nie. As u twee swart gate het wat om mekaar wentel, sal gravitasiestraling die bane laat verval. Wanneer die twee swart gate saamsmelt, smelt hul gebeurtenishorisonne saam, maar daar is nog geen manier waarop die lig kan kan ontsnap nie.

Dit is in skrille kontras met die samesmelting van omtrent elke ander klas astrofisiese voorwerpe. As twee sterre saamsmelt, sal hulle 'n helder, fakkelende verskynsel skep wat bekend staan ​​as 'n helderrooi nova, as gevolg van die wisselwerking tussen die materie in die verskillende lae van die twee sterre terwyl hulle saamsmelt. Twee wit dwerge wat saamsmelt, sal lei tot 'n selfs meer skouspelagtige verskynsel: 'n tipe Ia-supernova, waar die daaropvolgende ontploffing sal lei tot die vernietiging van beide wit dwergvaders.

En soos ons die eerste keer in 2017 ontdek het, wanneer twee neutronsterre saamsmelt, kan hulle 'n kilonova-gebeurtenis skep: 'n helder, heftige gammastraal wat lei tot die sentrale skepping van 'n nuwe neutronster of 'n swart gat, terwyl dit genereer en 'n groot hoeveelheid swaar elemente in die heelal uitwerp.

Vir swart gate moet dit egter nie die geval wees nie. Sodra u bo 'n spesifieke kritieke massadrempel uitkom - iewers tussen 2,5 en 2,75 sonmassas - kan u nie meer 'n digte, ontaarde voorwerp van konvensionele deeltjies hê nie. Enigiets wat 'n wit dwerg of 'n neutronster sou wees, kan nie meer bestaan ​​nie, dit moet onvermydelik ineenstort om 'n swart gat te vorm.

Wit dwerge word onderdruk deur die degeneratiedruk tussen elektrone: die feit dat geen twee identiese fermione (een van die twee klasse fundamentele deeltjies) dieselfde kwantumtoestand kan inneem nie. Neutronsterre word aangehou deur dieselfde verskynsel, maar tussen neutrone: hulle kan ook nie dieselfde kwantumtoestand inneem nie. As die materie wat hierdie voorwerpe saamstel te dig word, veroorsaak dit 'n stel kernreaksies wat die elektromagnetiese straling (dws lig) voortbring wat ons dan waarneem.

Geen sulke reaksies is moontlik as twee swart gate saamsmelt nie. Dit is omdat die interne struktuur wat hulle het - as 'n punt-singulariteit vir (onrealistiese) nie-roterende swart gate en 'n sirkelvormige ring-singulariteit vir (realistiese) roterende - verskuil is agter die gebeurtenishorison. Niks wat oorsteek na die binnekant van 'n gebeurtenishorison kan ooit ontsnap nie, dus sal reaksies wat binne die gebeurtenishorison voorkom nooit uitkom nie.

Met ander woorde, selfs al is daar 'n interne, nie-triviale struktuur aan swart gate, sal alles wat tydens 'n botsing tussen twee daarvan voorkom nooit uitkom nie. Daar sal nooit deeltjies, lig of enige ander sein uit hulle samesmeltings uitgestuur word wat voortspruit uit enigiets wat binne die horison voorkom nie.

Die enigste hoop wat ons het om alles te sien, moet kom uit interaksies buite die gebeurtenishorison self.

Dit is die enigste aanneemlike meganisme waardeur samesmelting van swart gate 'n elektromagnetiese (liggebaseerde) sein kan genereer: as die aangeleentheid rondom dit saamwerk tydens die eindstadia van die samesmeltingsproses. Daar is baie bekende voorbeelde in die sterrekunde waar materie met swart gate in wisselwerking is om lig te produseer:

  • tydens getyversteuringsgebeurtenisse, waar 'n ster uitmekaar geskeur word en naby 'n swart gat gaan,
  • in X-straal-binaries, waar 'n reuse-ster massa op sy swart metgesel in 'n wentelbaan gesif het,
  • in 'n aktiewe sterrestelsel of kwasar, waar aangelegde materiaal in en om die swart gat vloei,

en so aan. In al hierdie gevalle is dit nie so dat materiaal van binne die gebeurtenishorison besig is om uit te kom nie, maar dat materiaal van buite die swart gat in wisselwerking is met die eksterne omgewing, wat lig uitstraal in die proses.

Wat kan dus gebeur om die vrystelling van lig te veroorsaak as twee swart gate inspirerend is en uiteindelik saamsmelt? Dit kan net wees as gevolg van die teenwoordigheid van materie buite die horison van beide swart gate. Alhoewel die meeste modelle van swartgatomgewings tydens 'n samesmelting slegs baie klein hoeveelhede energie-oordrag na die omliggende materiaal voorspel, is dit moontlik - ten minste in sommige uiterste gevalle - dat samesmeltings met swart gat-swartgate 'n lig-emitterende gebeurtenis kan veroorsaak.

Vir die heel eerste swart gat-swart gat samesmelting wat deur LIGO gesien is, was die sein wat by NASA se Fermi-teleskoop aangekom het, swak en het dit sonder rigtinggewende inligting aangekom. Dit was slegs 'n 2.9-sigma-sein: potensieel 'n vals positiewe opsporing, die kanse van 0,22% vir 'n 'vals alarm' is volgens fisika standaarde baie hoog. Die gammastraal-burst-kandidaat het plaasgevind toe die detektor swak georiënteerd was ten opsigte van die gebeurtenis, en die aanvullende INTEGRAL-satelliet van ESA het geen tekens van hoë energie-uitstoot gesien nie.

Van die tientalle samesmeltings van swart gate en swart gate wat daarna opgespoor is, het Fermi van NASA presies geen tekens gesien van 'n ander kandidaat vir gammastraal. Miskien was dit tog bloot 'n nie-verwante toeval.

Tot 21 Mei 2019. Op die datum het die LIGO-databasis vir superevent 'n yslike drie kandidaatgebeurtenisse aangeteken, waaronder een wat aanvanklik as 'n waarskynlike samesmelting van swart gat-swart gat met 97% waarskynlikheid beskou is. Die sein daarvan is in al drie operasionele detektors gesien: LIGO Livingston, LIGO Hanford en Maagd. Dit was gelokaliseer in 'n taamlike noue ruimte (net

2% van die lug met 90% vertroue), en dit lyk asof dit baie massief is (ongeveer 150 sonmassas in totaal) en baie ver (miskien 10-15 miljard ligjare weg) in vergelyking met die meer tipiese samesmeltings van swart gat-swart gate ons het gesien.

Maar die grootste nuus daaroor is dat die Zwicky-verbygaande fasiliteit blykbaar 'n kort elektromagnetiese fakkel bespeur het wat in tyd en ruimte saamval met wat ons swaartekraggolfverklikkers gesien het. Wat baie opwindend is, is dat daarbinne

In die 2% gebied van die lug het hulle die bron van die kortstondige emissie gevind, geïdentifiseer en gemeet en 'n skouspelagtige moontlike skuldige gevind: 'n aktiewe galaktiese kern. Dit het soos normaal saamgejaag en in die dae na die gravitasiegolfgebeurtenis agterdogtig opgehelder en in die loop van 'n maand stadig verdwyn.

Die beste wetenskaplike verklaring is die volgende: die samesmelting van swart gat-swart gate kon plaasgevind het in die sentrale, gasryke streek van 'n sterrestelsel waarvan die supermassiewe swart gat tans materie voed. Die fakkel is waarskynlik aangedryf deur 'n aanwasstert en was sigbaar in die optiese deel van die spektrum: die eerste en enigste samesmelting met swart gat-swart gate wat tot dusver 'n optiese eweknie gehad het. Die kleur is relatief konstant, en dit moet een van die helderste seine wees wat die samesmelting van swart gate kan oplewer: groot massas, relatief lae spoed-skoppe, in digte gasomgewings.

Alhoewel die hoop aanvanklik groot was dat die samesmelting van swart gate ligseine kon oplewer, het die entoesiasme die afgelope paar jaar verdwyn omdat die samesmelting na die samesmelting glad nie daarin kon slaag nie. Met hierdie nuwe gebeurtenis word opgewondenheid nou weer aangewakker: miskien het swart gate net die regte omstandighede nodig om te vlam wanneer dit saamsmelt, en dat toekomstige waarnemings uiteindelik die verband tussen die samesmelting van swart gate en die emissie van lig sal openbaar.

Soos dr Eric Burns - wat aan die 2015-opsporing gewerk het as deel van die NASA Fermi-span - dit stel:

As dit waar is, sal dit ons 'n ander soort gesamentlike GW-EM-opsporings gee, wat baie verder in die heelal opgespoor kan word en steeds 'n magdom multiboodskapwetenskap moontlik maak. Ek dink dat hierdie werk, GW150914-GBM, en soortgelyke waarnemingsondersoeke belangrik is om te verseker dat ons verwagtinge voldoen aan die werklikheid. Toekomstige studies behoort hierdie vraag in die volgende paar jaar op te los.

Die toekoms van die samesmelting van swart gate was nog nooit so helder nie.


'N Swartgatwoestyn

Die samesmeltingssein, genaamd GW190521, het slegs 'n tiende van 'n sekonde geduur - maar wetenskaplikes het onmiddellik besef dat dit buitengewoon was in vergelyking met die eerste opsporing van LIGO in 2015.

"Dit lyk nie veel soos 'n 'tjirp & # x27 nie, wat ons gewoonlik opspoor', het die lid van die Maagd, Nelson Christensen, in LIGO se persverklaring gesê. "Dit is meer soos iets wat 'bang' word, en dit is die mees massiewe sein wat LIGO en Maagd gesien het."

Dit is nie verbasend nie dat hierdie vreemde sein geproduseer is deur die samesmelting van twee ewe vreemde swart gate met massas van ongeveer 66 en 85 sonmassas, wat 'n paar vrae laat ontstaan ​​rakende die vorming daarvan.

Gedurende 'n tipiese sterre-leeftyd kan sterre hul gewig dra, omdat interne versmelting 'n uitwaartse krag genereer wat die inwendige druk van swaartekrag balanseer. Maar as 'n ster massief genoeg is, kan die gravitasie-ineenstorting nie meer beveg word as dit brandstof raak nie. Uiteindelik stort die kern van so 'n ster onder sy eie gewig in duie voordat dit weer as 'n dramatiese supernova terugspring.

Maar enige ster wat teoreties 'n swart gat tussen 65 en 120 sonmassas kan vorm, soos enige stamvader van hierdie unieke samesmelting, ontplof nie as supernova nie. Dit beteken dat daar nie swart gate uit sterre wat in duie stort in hierdie massa-reeks gebore mag wees nie.

In plaas daarvan begin 'n verskynsel wat bekend staan ​​as 'paar onstabiliteit' wanneer 'n ster so groot begin, en die ster word onstabiel tot op die punt dat dit swaartekrag vermy - ten minste 'n rukkie. En as dit uiteindelik ontplof, laat dit niks agter nie. (Aan die ander kant van die spektrum gaan sterre bo 120 sonmassas nooit supernova nie omdat hulle direk in swart gate ineenstort.)

"Verskeie scenario's voorspel die vorming van swart gate in die sogenaamde massa-gaping vir paar-onstabiliteit: dit kan voortspruit uit die samesmelting van kleiner swart gate," het Michela Mapelli, lid van die Maagd-samewerking, in die persverklaring van Virgo gesê. "Dit is egter ook moontlik dat ons ons huidige begrip van die finale stadiums van die ster se lewe moet hersien."


'N Swart gat wat 'n wurmgat omring, sal vreemde gravitasiegolwe uitstraal

'N Wurmgat is 'n tonnel deur die ruimtetyd wat verskillende dele van die kosmos verbind (geïllustreer). Wetenskaplikes rapporteer dat 'n swart gat wat deur 'n wurmgat sirkel, 'n teken van gravitasiegolwe sal uitstraal.

estt / iStock / Getty Images Plus

Deel dit:

Swaartekraggolfverklikkers het al geheimsinnige swart gate opgemerk. Maar iets vreemder kan volgende wees: wurmgate.

'N Swart gat wat in 'n wurmgat draai, sal 'n vreemde patroon van rimpeling in die ruimtetyd skep wat die LIGO- en Maagd-gravitasiegolfwaarnemings in staat sal wees om op te tel, berig natuurkundiges op 17 Julie op arXiv.org. Die golwe knip aan en aan terwyl die swart gat deur die wurmgat beweeg en dan terugkom.

Wurmgate is hipotetiese voorwerpe waarin ruimtetyd gebuig word in 'n tonnel wat verre kosmiese lokale of potensieel verskillende heelalle verbind (SN: 8/5/13). Van buite kan wurmgate soos swart gate lyk. Maar terwyl 'n voorwerp wat in 'n swart gat val, daar vasgevang is, kan iets wat in 'n wurmgat val, daardeur na die ander kant beweeg.

Geen bewyse is gevind dat wurmgate bestaan ​​nie. "Dit is beslis spekulatief, met 'n hoofletter S," sê fisikus William Gabella van die Vanderbilt-universiteit in Nashville. Maar as dit wel bestaan, het navorsers die kans om die wurmgate op te spoor via swaartekraggolwe.

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Gabella en kollegas het 'n swart gat met 'n massa van vyf keer die son beskou, wat ongeveer 1,6 miljard ligjare van die aarde af wentel. Soos die swart gat om die wurmgat swaai, het die navorsers bereken, sou dit begin deur na binne te draai asof dit om 'n ander swart gat wentel. Aanvanklik sou die gravitasiegolwe daaruit lyk soos 'n standaardhandtekening vir twee swart gate, 'n patroon van golwe wat mettertyd in frekwensie toeneem, wat 'n getjirp genoem word.

But when it reaches the wormhole’s center, called the “throat,” the black hole would pass through. The researchers considered what would happen if the black hole emerged in a distant realm, such as another universe. In that case, the gravitational waves in the first universe would abruptly die off. In the second universe, the black hole would shoot outward before spiraling back in. Then, it would pass back through the wormhole to the first universe.

As the black hole returns, it would initially spiral outward from the wormhole, perhaps producing an “anti-chirp,” a pattern of gravitational waves opposite to a chirp’s, before plunging back in with a chirp. The black hole would continue bouncing between the two universes, causing repeated bursts of gravitational waves punctuated by silence. Once the black hole lost enough energy to gravitational waves, its journey would end as it settled down in the wormhole’s throat.

“You can’t reproduce that with two black holes, so it’s a clear-cut signal of a wormhole,” says physicist Dejan Stojkovic of the University at Buffalo in New York, who was not involved with the research. The waves “should be sticking [out] like a sore thumb,” he says.

According to the general theory of relativity, which describes gravity as the result of the curvature of spacetime, wormholes are possible. But actually detecting one would imply that there exists a strange type of matter that physicists don’t understand. That’s because a substance with negative mass would be necessary to prop open a wormhole’s throat to prevent it from collapsing, and no known type of material fits the bill.

The United States–based Advanced LIGO, or Laser Interferometer Gravitational-Wave Observatory, and Advanced Virgo in Italy detect ripples from black holes or dense stellar corpses called neutron stars. Those massive objects orbit around one another before they merge.

Scientists are now skilled at spotting such mergers, having confirmed more than a dozen since 2015, with more awaiting confirmation. But at some point, physicists will need to start focusing on more unusual possibilities, says physicist Vítor Cardoso of Instituto Superior Técnico in Lisbon, Portugal. “We need to look for strange but exciting signals.”

Vrae of opmerkings oor hierdie artikel? Stuur 'n e-pos aan ons na [email protected]

A version of this article appears in the August 29, 2020 issue of Wetenskapnuus.


A “bang” in LIGO and Virgo detectors signals most massive gravitational-wave source yet

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For all its vast emptiness, the universe is humming with activity in the form of gravitational waves. Produced by extreme astrophysical phenomena, these reverberations ripple forth and shake the fabric of space-time, like the clang of a cosmic bell.

Now researchers have detected a signal from what may be the most massive black hole merger yet observed in gravitational waves. The product of the merger is the first clear detection of an “intermediate-mass” black hole, with a mass between 100 and 1,000 times that of the sun.

They detected the signal, which they have labeled GW190521, on May 21, 2019, with the National Science Foundation’s Laser Interferometer Gravitational-wave Observatory (LIGO), a pair of identical, 4-kilometer-long interferometers in the United States and Virgo, a 3-kilometer-long detector in Italy.

The signal, resembling about four short wiggles, is extremely brief in duration, lasting less than one-tenth of a second. From what the researchers can tell, GW190521 was generated by a source that is roughly 5 gigaparsecs away, when the universe was about half its age, making it one of the most distant gravitational-wave sources detected so far.

As for what produced this signal, based on a powerful suite of state-of-the-art computational and modeling tools, scientists think that GW190521 was most likely generated by a binary black hole merger with unusual properties.

Almost every confirmed gravitational-wave signal to date has been from a binary merger, either between two black holes or two neutron stars. This newest merger appears to be the most massive yet, involving two inspiraling black holes with masses about 85 and 66 times the mass of the sun.

The LIGO-Virgo team has also measured each black hole’s spin and discovered that as the black holes were circling ever closer together, they could have been spinning about their own axes, at angles that were out of alignment with the axis of their orbit. The black holes’ misaligned spins likely caused their orbits to wobble, or “precess,” as the two Goliaths spiraled toward each other.

The new signal likely represents the instant that the two black holes merged. The merger created an even more massive black hole, of about 142 solar masses, and released an enormous amount of energy, equivalent to around 8 solar masses, spread across the universe in the form of gravitational waves.

“This doesn’t look much like a chirp, which is what we typically detect,” says Virgo member Nelson Christensen, a researcher at the French National Centre for Scientific Research (CNRS), comparing the signal to LIGO’s first detection of gravitational waves in 2015. “This is more like something that goes ‘bang,’ and it’s the most massive signal LIGO and Virgo have seen.”

The international team of scientists, who make up the LIGO Scientific Collaboration (LSC) and the Virgo Collaboration, have reported their findings in two papers published today. One, appearing in Physical Review Letters, details the discovery, and the other, in The Astrophysical Journal Letters, discusses the signal’s physical properties and astrophysical implications.

“LIGO once again surprises us not just with the detection of black holes in sizes that are difficult to explain, but doing it using techniques that were not designed specifically for stellar mergers,” says Pedro Marronetti, program director for gravitational physics at the National Science Foundation. “This is of tremendous importance since it showcases the instrument’s ability to detect signals from completely unforeseen astrophysical events. LIGO shows that it can also observe the unexpected.”

In the mass gap

The uniquely large masses of the two inspiraling black holes, as well as the final black hole, raise a slew of questions regarding their formation.

All of the black holes observed to date fit within either of two categories: stellar-mass black holes, which measure from a few solar masses up to tens of solar masses and are thought to form when massive stars die or supermassive black holes, such as the one at the center of the Milky Way galaxy, that are from hundreds of thousands, to billions of times that of our sun.

However, the final 142-solar-mass black hole produced by the GW190521 merger lies within an intermediate mass range between stellar-mass and supermassive black holes — the first of its kind ever detected.

The two progenitor black holes that produced the final black hole also seem to be unique in their size. They’re so massive that scientists suspect one or both of them may not have formed from a collapsing star, as most stellar-mass black holes do.

According to the physics of stellar evolution, outward pressure from the photons and gas in a star’s core support it against the force of gravity pushing inward, so that the star is stable, like the sun. After the core of a massive star fuses nuclei as heavy as iron, it can no longer produce enough pressure to support the outer layers. When this outward pressure is less than gravity, the star collapses under its own weight, in an explosion called a core-collapse supernova, that can leave behind a black hole.

This process can explain how stars as massive as 130 solar masses can produce black holes that are up to 65 solar masses. But for heavier stars, a phenomenon known as “pair instability” is thought to kick in. When the core’s photons become extremely energetic, they can morph into an electron and antielectron pair. These pairs generate less pressure than photons, causing the star to become unstable against gravitational collapse, and the resulting explosion is strong enough to leave nothing behind. Even more massive stars, above 200 solar masses, would eventually collapse directly into a black hole of at least 120 solar masses. A collapsing star, then, should not be able to produce a black hole between approximately 65 and 120 solar masses — a range that is known as the “pair instability mass gap.”

But now, the heavier of the two black holes that produced the GW190521 signal, at 85 solar masses, is the first so far detected within the pair instability mass gap.

“The fact that we’re seeing a black hole in this mass gap will make a lot of astrophysicists scratch their heads and try to figure out how these black holes were made,” says Christensen, who is the director of the Artemis Laboratory at the Nice Observatory in France.

One possibility, which the researchers consider in their second paper, is of a hierarchical merger, in which the two progenitor black holes themselves may have formed from the merging of two smaller black holes, before migrating together and eventually merging.

“This event opens more questions than it provides answers,” says LIGO member Alan Weinstein, professor of physics at Caltech. “From the perspective of discovery and physics, it’s a very exciting thing.”

“Something unexpected”

There are many remaining questions regarding GW190521.

As LIGO and Virgo detectors listen for gravitational waves passing through Earth, automated searches comb through the incoming data for interesting signals. These searches can use two different methods: algorithms that pick out specific wave patterns in the data that may have been produced by compact binary systems and more general “burst” searches, which essentially look for anything out of the ordinary.

LIGO member Salvatore Vitale, assistant professor of physics at MIT, likens compact binary searches to “passing a comb through data, that will catch things in a certain spacing,” in contrast to burst searches that are more of a “catch-all” approach.

In the case of GW190521, it was a burst search that picked up the signal slightly more clearly, opening the very small chance that the gravitational waves arose from something other than a binary merger.

“The bar for asserting we’ve discovered something new is very high,” Weinstein says. “So we typically apply Occam’s razor: The simpler solution is the better one, which in this case is a binary black hole.”

But what if something entirely new produced these gravitational waves? It’s a tantalizing prospect, and in their paper the scientists briefly consider other sources in the universe that might have produced the signal they detected. For instance, perhaps the gravitational waves were emitted by a collapsing star in our galaxy. The signal could also be from a cosmic string produced just after the universe inflated in its earliest moments — although neither of these exotic possibilities matches the data as well as a binary merger.

“Since we first turned on LIGO, everything we’ve observed with confidence has been a collision of black holes or neutron stars,” Weinstein says. “This is the one event where our analysis allows the possibility that this event is not such a collision. Although this event is consistent with being from an exceptionally massive binary black hole merger, and alternative explanations are disfavored, it is pushing the boundaries of our confidence. And that potentially makes it extremely exciting. Because we have all been hoping for something new, something unexpected, that could challenge what we’ve learned already. This event has the potential for doing that.”


The universe teems with weird black holes, gravitational wave hunters find

Less than 5 years ago, physicists rocked the scientific world when they first spotted gravitational waves—fleeting ripples in space and time—set off when two gargantuan black holes billions of light-years away swirled into each other. Since then, scientists have detected a scad of similar events, mostly reported event by event. Today, however, researchers with a global network of gravitational wave detectors announced the first major statistical analyses of their data so far, 50 events in all. Posted online in four papers, the analyses show that black holes—ghostly ultraintense gravitational fields left behind when massive stars collapse—are both more common and stranger than expected. They also shed light on mysteries such as how such black holes pair up before merging.

The new studies, posted on the physics preprint server arXiv, “are superimportant,” says Carl Rodriguez, an astrophysicist at Carnegie Mellon University who was not involved in the work. “With an individual event, there’s only so much you can do in comparing to astrophysics models. But with a catalog you can not only begin to constrain the theory, you can start to understand the landscape.” Selma de Mink, an astrophysicist at Harvard University, says she and her colleagues have been waiting to do their own analyses of the data trove. “There will definitely be a flurry of papers that are rushing to take the first stabs at the data.”

The observations come from three huge L-shaped optical instruments called interferometers that can measure the infinitesimal stretching of space itself by a passing gravitational wave. Two of those detectors belong to the Laser Interferometer Gravitational-Wave Observatory (LIGO), a pair of detectors with arms 4 kilometers long in Louisiana and Washington that spotted the first gravitational waves in 2015. The third detector is Virgo, an interferometer near Pisa, Italy, that has 3-kilometer-long arms and joined the hunt for gravitational waves in 2017.

LIGO and Virgo had already spotted 11 events, including one merger of neutron stars, an event that may shed light on how the universe forges heavy elements. Now, the team has cataloged 37 additional black hole mergers, one likely neutron star merger, and one possible merger of a black hole and neutron star from the first half of its third observing run, from April through September 2019.

Analyses of all 50 events show that when it comes to black holes, “the diversity is surprisingly large,” says Frank Ohme, a gravitational wave astronomer at the Max Planck Institute for Gravitational Physics. From details of the mergers’ chirplike signals, scientists can calculate the masses of the colliding black holes. They expected to find a “mass gap” between about 45 and 135 solar masses—the result of particle physics processes that should blow apart stars within a certain mass range before they can collapse into black holes.

However, LIGO and Virgo have now spotted mergers involving black holes squarely within the gap, including one with a mass of roughly 85 solar masses. De Mink, who models the evolution of black hole pairs from binary star systems, says accounting for the interlopers will be challenging. The mass gap is “such a clear prediction from the models that it’s hard to believe that there’s not a feature there” in the mass spectrum, she says.

Similarly, scientists expected another forbidden range below five solar masses, based on previous observations of individual black holes peacefully orbiting normal stars. But at least one hole in the catalog appears to fall below that limit. “How do you describe the boundaries of this population?” Ohme asks. “It’s not such a clear picture anymore.”.

Their new ability to take a census of black holes has also enabled researchers to probe whether black holes in a merging pair point in the same direction as they orbit each other—a potential clue to how the pair came together in the first place. If the spins align with the orbital axis, the black holes might have formed from a pair of stars that were born together, naturally acquired matching spins, and remained companions after they collapsed. If the spins point in different directions, the black holes might have formed first and then somehow paired later. Which formation channel dominates is a subject of intense debate.

In particular, if one of the black holes spins in the opposite sense of the orbit, the pair would more likely come from the mingling of black holes that had already formed. But it’s very hard to tell for sure whether that’s happening from the warble of a single signal, says Maya Fishbach, an astrophysicist and LIGO member from Northwestern University. However, by analyzing the events en masse, scientists have teased out evidence that at least some of the mergers involve reversed spins. That result in turn suggests that black hole pairs form in more than one way, Fishbach says. “It seems like there might be multiple things going on.”

Rodriguez notes that the overall rate of black hole mergers that LIGO and Virgo see seems to roughly match the rate he predicted in his model, in which already formed black holes find each other and pair in knots of old stars called globular clusters. “I shouldn’t toot my own horn—but I totally am going to,” he says. But, he adds, the data are also consistent with such a mechanism producing just onequarter of the mergers.

Researchers have even been able to probe how the number of black hole mergers may have changed over cosmic time, Fishbach says. The rate is expected to be higher in the early universe, when the pace of star formation was also higher. But previous data allowed that rate to be up to 100,000 times higher than it is now. Now, scientists have seen enough far-flung events to say that the rate of mergers 8 billion years ago was no more than 10 times what it is now, Fishbach says.

LIGO and Virgo scientists owe their scientific bounty to the increasing sensitivity of their detectors, which has enabled them to spot ever fainter and more distant events. Now, they are eager to build up their catalog even further. With more events, they find a correlation between spin alignment and the masses of the black holes that could help reveal whether the heaviest might themselves have formed through mergers. (If the two black holes’ spins aren’t aligned, then they may not have formed from an isolated pair of stars, and theorists wouldn’t necessarily have to explain how a collapsing star could produce such a heavy black hole.) “We’ve answered a lot of questions we didn’t even know we had,” Fishbach says, “but we raised even more. This is just the beginning of the science.”


Inhoud

GW190521 is a significant discovery due to the masses of the resulting large black hole and of one or both of the smaller constituent black holes. Stellar evolution theory predicts that a star cannot collapse itself into a black hole of more than about 65 M , leaving a black hole mass gap above 65 M . The 85 +21
−14 M [note 3] and 142 +28
−16 M black holes observed in GW190521 are conclusively in the mass gap, indicating that it can be populated by the mergers of smaller black holes. [4]

Only indirect evidence for intermediate mass black holes, those with between 100 and 100,000 solar masses, had been observed earlier, and it was unclear how they had formed. [13] Researchers hypothesize that they form from a hierarchical series of mergers, in which each black hole is the result of successive mergers involving smaller black holes. [8]

According to discovery team member Vassiliki Kalogera of Northwestern University, "this is the first and only firm/secure mass measurement of an intermediate mass black hole at the time of its birth . Now we know reliably at least one way [such objects can form], through the merger of other black holes." [9]

In June 2020, astronomers reported observations of a flash of light that might be associated with GW190521. The Zwicky Transient Facility (ZTF) reported a transient optical source within the region of the GW190521 trigger, though as the uncertainty in sky position was hundreds of square degrees the association remains uncertain. If the two events are actually linked, the event is claimed to be the first finding of an electromagnetic source related to the merger of two black holes. [2] [3] [6] [14] Mergers of black holes do not typically emit any light. The researchers suggest that it could be explained if the merging of the two smaller black holes sent the newly formed intermediate mass black hole on a trajectory that hurtled through the accretion disk of an unrelated but nearby supermassive black hole, disrupting the disk material and producing a flare of light. The newly formed black hole would have traveled at 200 km/s (120 mi/s) through the disk, according to the astronomers. [15] If this explanation is correct, the flare should repeat after about 1.6 years [3] when the intermediate mass black hole again encounters the accretion disk. [15]

According to Matthew Graham, lead astronomer for the study, "This supermassive black hole was burbling along for years before this more abrupt flare. The flare occurred on the right timescale, and in the right location, to be coincident with the gravitational-wave event. In our study, we conclude that the flare is likely the result of a black hole merger, but we cannot completely rule out other possibilities." [15]

While the original LIGO/Virgo data analysis assumed a quasi-circular inspiral waveform model, subsequent publications claimed that this source could have been significantly eccentric. Romero-Shaw et al. showed that the data is better described by a non-precessing eccentric waveform with e 10 H z ≥ 0.1 >geq 0.1> than a spin-precessing quasi-circular model. [16] Using eccentric waveforms based on numerical relativity, Gayathri et al. 2020 found a best fit with e 10 H z = 0.67 >=0.67> and source masses 102 +7
−11 M for both merging BHs. [17]


An unexpected origin story for a lopsided black hole merger

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A lopsided merger of two black holes may have an oddball origin story, according to a new study by researchers at MIT and elsewhere.

The merger was first detected on April 12, 2019 as a gravitational wave that arrived at the detectors of both LIGO (the Laser Interferometer Gravitational-wave Observatory), and its Italian counterpart, Virgo. Scientists labeled the signal as GW190412 and determined that it emanated from a clash between two David-and-Goliath black holes, one three times more massive than the other. The signal marked the first detection of a merger between two black holes of very different sizes.

Now the new study, published today in the journal Physical Review Letters, shows that this lopsided merger may have originated through a very different process compared to how most mergers, or binaries, are thought to form.

It’s likely that the more massive of the two black holes was itself a product of a prior merger between two parent black holes. The Goliath that spun out of that first collision may have then ricocheted around a densely packed “nuclear cluster” before merging with the second, smaller black hole — a raucous event that sent gravitational waves rippling across space.

GW190412 may then be a second generation, or “hierarchical” merger, standing apart from other first-generation mergers that LIGO and Virgo have so far detected.

“This event is an oddball the universe has thrown at us — it was something we didn’t see coming,” says study coauthor Salvatore Vitale, an assistant professor of physics at MIT and a LIGO member. “But nothing happens just once in the universe. And something like this, though rare, we will see again, and we’ll be able to say more about the universe.”

Vitale’s coauthors are Davide Gerosa of the University of Birmingham and Emanuele Berti of Johns Hopkins University.

A struggle to explain

There are two main ways in which black hole mergers are thought to form. The first is known as a common envelope process, where two neighboring stars, after billions of years, explode to form two neighboring black holes that eventually share a common envelope, or disk of gas. After another few billion years, the black holes spiral in and merge.

“You can think of this like a couple being together all their lives,” Vitale says. “This process is suspected to happen in the disc of galaxies like our own.”

The other common path by which black hole mergers form is via dynamical interactions. Imagine, in place of a monogamous environment, a galactic rave, where thousands of black holes are crammed into a small, dense region of the universe. When two black holes start to partner up, a third may knock the couple apart in a dynamical interaction that can repeat many times over, before a pair of black holes finally merges.

In both the common envelope process and the dynamical interaction scenario, the merging black holes should have roughly the same mass, unlike the lopsided mass ratio of GW190412. They should also have relatively no spin, whereas GW190412 has a surprisingly high spin.

“The bottom line is, both these scenarios, which people traditionally think are ideal nurseries for black hole binaries in the universe, struggle to explain the mass ratio and spin of this event,” Vitale says.

Black hole tracker

In their new paper, the researchers used two models to show that it is very unlikely that GW190412 came from either a common envelope process or a dynamical interaction.

They first modeled the evolution of a typical galaxy using STAR TRACK, a simulation that tracks galaxies over billions of years, starting with the coalescing of gas and proceeding to the way stars take shape and explode, and then collapse into black holes that eventually merge. The second model simulates random, dynamical encounters in globular clusters — dense concentrations of stars around most galaxies.

The team ran both simulations multiple times, tuning the parameters and studying the properties of the black hole mergers that emerged. For those mergers that formed through a common envelope process, a merger like GW190412 was very rare, cropping up only after a few million events. Dynamical interactions were slightly more likely to produce such an event, after a few thousand mergers.

However, GW190412 was detected by LIGO and Virgo after only 50 other detections, suggesting that it likely arose through some other process.

“No matter what we do, we cannot easily produce this event in these more common formation channels,” Vitale says.

The process of hierarchical merging may better explain the GW190412’s lopsided mass and its high spin. If one black hole was a product of a previous pairing of two parent black holes of similar mass, it would itself be more massive than either parent, and later significantly overshadow its first-generation partner, creating a high mass ratio in the final merger.

A hierarchical process could also generate a merger with a high spin: The parent black holes, in their chaotic merging, would spin up the resulting black hole, which would then carry this spin into its own ultimate collision.

“You do the math, and it turns out the leftover black hole would have a spin which is very close to the total spin of this merger,” Vitale explains.

If GW190412 indeed formed through hierarchical merging, Vitale says the event could also shed light on the environment in which it formed. The team found that if the larger of the two black holes formed from a previous collision, that collision likely generated a huge amount of energy that not only spun out a new black hole, but kicked it across some distance.

“If it’s kicked too hard, it would just leave the cluster and go into the empty interstellar medium, and not be able to merge again,” Vitale says.

If the object was able to merge again (in this case, to produce GW190412), it would mean the kick that it received was not enough to escape the stellar cluster in which it formed. If GW190412 indeed is a product of hierarchical merging, the team calculated that it would have occurred in an environment with an escape velocity higher than 150 kilometers per second. For perspective, the escape velocity of most globular clusters is about 50 kilometers per second.

This means that whatever environment GW190412 arose from had an immense gravitational pull, and the team believes that such an environment could have been either the disk of gas around a supermassive black hole, or a “nuclear cluster” — an incredibly dense region of the universe, packed with tens of millions of stars.

“This merger must have come from an unusual place,” Vitale says. “As LIGO and Virgo continue to make new detections, we can use these discoveries to learn new things about the universe.”