Astronomija

Kozmička karta pozadine mikrovalnih pećnica

Kozmička karta pozadine mikrovalnih pećnica


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Kada satelit vrši merenja zračenja iz različitih nebeskih delova kako bi konstruisao mapu toplotnog zračenja svemira na površini kugle, on bi takođe sadržao i zračenje iz Mlečnog puta. Na velikom dijelu karte nalazit će se buka zbog objekata na Mliječnom putu ili iz drugih izvora.

Kako se zna gdje i koliko šuma ukloniti prilikom izrade karte?


Iznad sam ucrtao nekoliko Planckovih doprinosa zračenju: Ljubičica je zaslužna za CMBR, zelena zbog Galaksije Mliječni put, a plava zbog oboje.

Jedan od načina da se riješi CMBR je modeliranje mliječnog puta od veće frekvencije grafikona, jer je doprinos CMBR-a zanemariv kod veće frekvencije. U određenom smjeru može se odrediti temperatura zbog Mliječnog puta postavljanjem visoke frekvencije prolaska. Tada se može oduzeti ovaj temperaturni doprinos na nižim frekvencijama gdje je doprinos CMBR-a značajan. Ovo je jedan od načina koji vjerojatno nije najbolji, ali je izvediv.

Napomena1: Grafikon nije iz stvarnih podataka. To je samo za objašnjenje kako to učiniti.

Napomena2: Dobro bi bilo pitanje koja bi trebala biti visokopropusna frekvencija. Uz dobre argumente može se odlučiti koja bi trebala biti visokofrekventna frekvencija. I uvijek postoji način za to.


Astronomska slika dana

Otkrijte kosmos! Svakog dana predstavljena je drugačija slika ili fotografija našeg fascinantnog svemira, zajedno sa kratkim objašnjenjem koje je napisao profesionalni astronom.

2013. 25. marta
Planck mapira mikrotalasnu pozadinu
Image Credit: Evropska svemirska agencija, Planck Collaboration

Objašnjenje: Od čega je sačinjen naš svemir? Da bi to lakše saznao, ESA je lansirala Planckov satelit kako bi, bez presedana detalja, mapirao male temperaturne razlike na najstarijoj poznatoj površini - pozadinsko nebo napustilo se milijardama godina kada je naš svemir prvi put postao proziran za svjetlost. Vidljiva u svim pravcima, ova kosmička mikrotalasna pozadina složena je tapiserija koja može pokazati samo uočene tople i hladne obrasce da je svemir sastavljen od specifičnih vrsta energije koja se razvijala na određene načine. Rezultati objavljeni prošle sedmice ponovo potvrđuju da se većina našeg svemira uglavnom sastoji od tajanstvene i nepoznate tamne energije, te da je čak i većina preostale energije materije neobično tamna. Uz to, Planckovi podaci impresivno pripisuju starost svemira na oko 13,81 milijarde godina, nešto stariju od one koja se procjenjuje na razne druge načine, uključujući NASA-in WMAP satelit, i brzinu širenja na 67,3 (+/- 1,2) km / s / Mpc, malo niže od prethodnih procjena. Neke značajke gornje mape neba ostaju nepoznate, na primjer zašto se čini da su fluktuacije temperature na jednoj polovini neba nešto veće od druge.


KOZMIČKI MIKROVALNI POZADINA

Zašto, međutim, vidimo Kozmičku MIKROTALAVNU pozadinu? U eri rekombinacije (vrijeme kada su se formirali neutralni atomi i svemir je postao proziran), temperatura je bila T = 3000 Kelvina (približno površinska temperatura M zvijezde, kao što je Betelgeuse). Talasna dužina maksimalne emisije tada je bila 970 nanometara, u infracrvenoj mreži. Kad je svemir prvi put postao proziran, posmatrači bi, dakle, vidjeli Kozmičku INFRARNU pozadinu. Međutim, od ere rekombinacije, svemir se proširio za faktor od 1100, protežući se lambdamaks od 970 nanometara do 1 milimetra (to jest, 1.000.000 nanometara). To je ekvivalentno hlađenju temperature svemirske pozadine sa 3000 Kelvina na 2,725 Kelvina.

  • Noćno je nebo mračno: To nam govori da svemir nije beskrajno velik i beskrajno star.
  • Galaksije imaju radijalnu brzinu proporcionalnu njihovoj udaljenosti: To nam govori da se svemir jednoliko širi.
  • Svemir je ispunjen pozadinom kozmičke mikrovalne pećnice: Ovo nam govori da je svemir nekada bio dovoljno vruć i gust da bi mogao biti neproziran, a proširio se više od hiljadu puta.

(2) Žarišta u kosmičkoj mikrotalasnoj pozadini rezultat su fluktuacija gustine u ranom svemiru.

Gledajući Kozmičku mikrotalasnu pozadinu, gledamo površinu neprozirne jonizovane „magle“ koja je ispunjavala rani svemir. Kozmička mikrotalasna pozadina sadrži zanimljive informacije o tome kakav je bio svemir u nežnoj dobi od 300.000 godina (samo 1/50.000 od njegove današnje starosti).

Da biste dobro pogledali Kozmičku mikrotalasnu pozadinu, potrebno je postaviti satelit iznad vlažne atmosfere Zemlje. (Voda upija mikrovalne pećnice, ovo je prikladno ako želite zagrijavati hranu u mikrovalnoj pećnici, ali velika je smetnja ako želite otkrivati ​​mikrotalasne pećnice iz svemira.) Trenutno je (mart 2003.) sonda Wilkinson mikrotalasne anizotropije na Zemljinoj L2 tačka, daleko iznad Mjeseca, mapiranje neba u mikrotalasnim pećnicama. Prvo objavljivanje podataka (koje pokriva jednogodišnje mapiranje) dogodilo se u februaru 2003.

Dobar pogled na cijelo nebo otkriva neke zanimljive nepravilnosti u pozadini kozmičke mikrovalne pećnice. Na primjer, temperatura kosmičke mikrotalasne pozadine vrlo je malo viša na jednoj polovini neba (prema sazviježđu Lav) nego na drugoj polovini (prema Vodenjaku). Ovo je rezultat doplerovog pomaka. Sonda mikrotalasne anizotropije kreće se oko Sunca Sunce kruži oko središta naše galaksije naša galaksija pada prema galaksiji Andromeda Lokalna grupa (koja sadrži i našu galaksiju i galaksiju Andromeda) povlači se prema jatu Djevice, lokalnom superjaku ( koja sadrži i Lokalnu grupu i jato Djevica) vuče se prema superklasteru Hydra-Centaurus. Kao neto rezultat svih ovih pokreta, sonda mikrotalasne anizotropije kreće se u smjeru Lava brzinom od nekoliko stotina kilometara u sekundi. Mikrovalne pećnice iz smjera Lava blago su pomaknute prema višim temperaturama, mikrovalne pećnice iz suprotnog smjera blago su pomaknute na niže temperature.

Nakon oduzimanja Doplerovog pomaka uslijed kretanja satelita kroz svemir, Kozmička mikrotalasna pozadina i dalje pokazuje žarišta i hladne tačke, širine oko jednog stepena (dvostruku širinu punog Mjeseca). Ova uočena kolebanja temperature su rezultat gustina fluktuacije u ranom svemiru, u eri rekombinacije kada je svemir postao proziran. Regije koje su u to vrijeme bile malo komprimirane imale su više gustina i više temperatura. (Zapamtite, plin postaje topliji dok se komprimira. To se odnosi na plin napravljen od fotona, kao i na plin izrađen od atoma.)

Da je svemir imao VELIKE fluktuacije gustine prije milion godina, tada bi kozmička mikrotalasna pozadina pokazivala VELIKE fluktuacije temperature danas. Međutim, zapažanja sonde za mikrotalasnu anizotropiju otkrivaju da su žarišta samo 0,0001 Kelvina vruća od hladnih tačaka (to jest, samo nekoliko dijelova na 100 000). To ukazuje na to da su fluktuacije gustine bile male amplitude kada je svemir postao proziran. Vruće tačke, koje predstavljaju regije vrlo malo gušće od njihove okoline, s vremenom su narasle u ogromne guste super nakupine, poput onih koje danas vidimo oko sebe. Sjetite se univerzalnog zakona: Bogati postaju bogatiji. Samo regije malo gušće od prosjeka tada su rasle pod utjecajem gravitacije dok nisu mnogo gušće od prosjeka.


29.4 Kozmička mikrotalasna pozadina

Opis prvih nekoliko minuta svemira zasnovan je na teorijskim proračunima. Ključno je, međutim, da naučna teorija bude provjerljiva. Kakva predviđanja daje? I pokazuju li zapažanja tačna predviđanja? Jedan od uspjeha teorije prvih nekoliko minuta svemira je tačno predviđanje količine helijuma u svemiru.

Drugo predviđanje je da se značajna prekretnica u istoriji svemira dogodila oko 380.000 godina nakon Velikog praska. Naučnici su direktno promatrali kakav je svemir bio u ovoj ranoj fazi, a ta zapažanja nude jednu od najsnažnijih potpora teoriji Velikog praska. Da bismo saznali koja je to bila prekretnica, pogledajmo koja nam teorija govori o onome što se dogodilo tijekom prvih nekoliko stotina hiljada godina nakon Velikog praska.

Fuzija helijuma i litijuma završena je kad je svemir bio star oko 4 minute. Svemir je zatim nastavio na neki način podsjećati na unutrašnjost zvijezde još nekoliko stotina hiljada godina. Ostao je vruć i neproziran, a zračenje se raspršivalo od jedne do druge čestice. Još je bilo prevruće da bi se elektroni "slegli" i povezali s određenom jezgrom, takvi slobodni elektroni su posebno učinkoviti u rasipanju fotona, osiguravajući tako da nijedno zračenje nikada nije stiglo daleko u ranom svemiru, a da mu se put nije promijenio. Na neki način, svemir je bio poput ogromne gužve odmah nakon popularnog koncerta ako se odvojite od prijatelja, čak i ako on nosi trepćuće dugme, nemoguće je vidjeti kroz gustu gomilu da ga uoči. Tek nakon što se gomila raščisti, postoji put do koga svjetlost s njegovog dugmeta može doći.

Univerzum postaje transparentan

Tek nekoliko stotina hiljada godina nakon Velikog praska, kada je temperatura pala na oko 3000 K, a gustina atomskih jezgara na oko 1000 po kubnom centimetru, elektroni i jezgre uspjeli su se kombinirati da bi stvorili stabilne atome vodonika i helija ([veza]). Bez slobodnih elektrona koji bi raspršili fotone, svemir je postao proziran prvi put u kosmičkoj istoriji. Od ovog trenutka, materija i zračenje su mnogo rjeđe međusobno komunicirali, mi kažemo da oni razdvojeni jedan od drugog i evoluirali su odvojeno. Odjednom, elektromagnetno zračenje zaista može putovati i od tada putuje svemirom.

Otkriće kosmičkog pozadinskog zračenja

Ako je model svemira opisan u prethodnom odjeljku točan, tada - budući da u svemiru gledamo daleko, a time i u prošlost - prvi „naknadni sjaj“ vrelog, ranog svemira i dalje treba biti uočljiv. Njegova zapažanja bila bi vrlo snažan dokaz da su naši teorijski proračuni o tome kako se svemir razvijao tačni. Kao što ćemo vidjeti, zaista smo otkrili zračenje koje se emituje na ovome foton vrijeme razdvajanja, kada je zračenje počelo slobodno strujati svemirom bez interakcije s materijom (slika 1).

Usporedba kosmičke mikrotalasne pozadine i oblaka.

Slika 1. (a) Rano u svemiru, fotoni (elektromagnetna energija) rasipali su se iz prepunih, vrućih, naelektrisanih čestica i nisu mogli daleko stići bez sudara sa drugom česticom. Ali nakon što su se elektroni i fotoni naselili u neutralne atome, bilo je daleko manje raspršenja i fotoni su mogli putovati na velike udaljenosti. Univerzum je postao proziran. Dok gledamo u svemir i unatrag u vrijeme, ne možemo vidjeti dalje od ovog vremena. (b) Ovo je slično onome što se događa kada vidimo oblake u Zemljinoj atmosferi. Kapljice vode u oblaku vrlo efikasno rasipaju svetlost, ali čist vazduh omogućava svetlosti da putuje na velike udaljenosti. Dok gledamo u atmosferu, našu viziju blokiraju slojevi oblaka i ne možemo vidjeti dalje od njih. (zasluga: modifikacija rada NASA-e)

Otkrivanje ovog naknadnog sjaja u početku je bila nesreća. Krajem 1940-ih, Ralph Alpher i Robert Herman , rad sa Georgeom Gamow , shvatio je da je neposredno prije nego što je svemir postao proziran morao zračiti poput crnog tijela na temperaturi od oko 3000 K - temperaturi na kojoj su atomi vodonika mogli početi stvarati. Da smo mogli vidjeti to zračenje neposredno nakon stvaranja neutralnih atoma, nalikovalo bi zračenju crvenkaste zvijezde. Bilo je to kao da je džinovska vatrena kugla ispunila čitav svemir.

Ali to je bilo prije skoro 14 milijardi godina, a u međuvremenu se razmjeri svemira povećali tisuću puta. Ovo proširenje povećalo je valnu dužinu zračenja za faktor 1000 (vidi Proširenje i Redshift slika [veza]). Prema Wien-ovom zakonu, koji povezuje valnu duljinu i temperaturu, širenje je odgovarajuće snizilo temperaturu za faktor 1000 (vidi poglavlje O zračenju i spektrima). Kozmička pozadina ponaša se poput crnog tijela i zato bi trebala imati spektar koji se pokorava Bečkom zakonu.

Alpher i Herman su predviđali da bi sjaj vatrene kugle sada trebao biti na radiotalasnim duljinama i trebao bi nalikovati zračenju crnog tijela na temperaturi od samo nekoliko stepeni iznad apsolutne nule. Budući da je vatrena kugla bila svugdje u svemiru, zračenje koje je ostalo od nje također bi trebalo biti posvuda. Da su naše oči osjetljive na radiotalasne dužine, činilo bi se da cijelo nebo svijetli vrlo slabo. Međutim, naše oči ne mogu vidjeti na ovim valnim duljinama, a u vrijeme kada su Alpher i Herman prognozirali, nije bilo instrumenata koji bi mogli detektirati sjaj. S godinama su njihova predviđanja bila zaboravljena.

Sredinom 1960-ih, u Holmdelu u New Jerseyu, Arno Penzias i Robert Wilson iz AT & ampT-ovog Bell Laboratories izgradili su osjetljivu mikrotalasnu antenu (slika 2) za mjerenje astronomskih izvora, uključujući ostatke supernova poput Kasiopeje A (vidi poglavlje o Smrti Zvijezde). Mučila ih je neka neočekivana pozadinska buka, baš poput slabe statike na radiju, koje se nisu mogli riješiti. Zbunjujuće kod ovog zračenja bilo je to što se činilo da dolazi iz svih pravaca odjednom. Ovo je vrlo neobično u astronomiji: uostalom, većina zračenja ima određeni pravac tamo gde je najjače - na primer smer Sunca, ostatka supernove ili diska Mliječnog puta.

Robert Wilson (lijevo) i Arno Penzias (desno).

Slika 2. Ova dvojica naučnika stoje ispred antene u obliku roga kojom su otkrili kosmičko pozadinsko zračenje. Fotografija je snimljena 1978. godine, netom nakon što su dobili Nobelovu nagradu za fiziku.

Penzias i Wilson u početku su mislili da svako zračenje koje dolazi iz svih pravaca mora poticati iz njihovog teleskopa, pa su sve razdvojili kako bi potražili izvor buke. Otkrili su čak da su se neki golubovi zapeli unutar velike antene u obliku roga i ostavili (kako je Penzias delikatno rekao) "sloj bijele, ljepljive, dielektrične supstance koja prekriva unutrašnjost antene." Međutim, ništa što su naučnici učinili nije moglo smanjiti pozadinsko zračenje na nulu i nevoljko su prihvatili da ono mora biti stvarno i da mora dolaziti iz svemira.

Penzias i Wilson nisu bili kosmolozi, ali dok su o svom zagonetnom otkriću počeli razgovarati s drugim naučnicima, brzo su stupili u kontakt sa grupom astronoma i fizičara sa Univerziteta Princeton (na kratkoj vožnji). Ovi astronomi su - kao što se dogodilo - ponovili proračune Alphera i Hermana iz četrdesetih godina, a takođe su shvatili da bi zračenje iz vremena razdvajanja trebalo da bude uočljivo kao slabo naknadno sjaj radio talasa. Različiti proračuni kolike bi za to bile promatrane temperature kosmička mikrotalasna pozadina (CMB) 1 je bio neizvjestan, ali svi su predviđali manje od 40 K.

Penzias i Wilson utvrdili su da raspodjela intenziteta na različitim radio valnim duljinama odgovara temperaturi od 3,5 K. Ovo je vrlo hladno - bliže apsolutnoj nuli od većine drugih astronomskih mjerenja - i svjedoči o tome koliko prostora (i valova unutar njega) se protegao. Njihova mjerenja ponovljena su s boljim instrumentima, koji nam daju očitanje od 2,73 K. Tako su se Penzias i Wilson vrlo približili. Zaokružujući ovu vrijednost, naučnici se često pozivaju na „mikrotalasnu pozadinu od 3 stepena“.

Mnogi drugi eksperimenti na Zemlji i u svemiru ubrzo su potvrdili Penziasovo i Wilsonovo otkriće: Zračenje je zaista dolazilo iz svih pravaca (bilo je izotropno) i izvanredno precizno odgovaralo je predviđanjima teorije Velikog praska. Penzias i Wilson nehotice su opazili sjaj iskonske vatrene kugle. Za svoj rad dobili su Nobelovu nagradu 1978. I neposredno pred smrt 1966. Lemaître je saznao da je njegov „nestali sjaj“ otkriven i potvrđen.

Osobine kozmičke mikrotalasne pozadine

Jedno pitanje koje je zabrinulo astronoma je da su Penzias i Wilson mjerili pozadinsko zračenje koje ispunjava prostor kroz Zemljinu atmosferu. Šta ako je ta atmosfera izvor radio valova ili je nekako utjecala na njihova mjerenja? Bilo bi bolje izmjeriti nešto tako važno iz svemira.

Prva tačna mjerenja CMB-a izvršena su satelitom koji kruži oko Zemlje. Nazvan Cosmic Background Explorer (COBE), NASA ga je lansirala u novembru 1989. Podaci koje je dobio brzo su pokazali da se CMB u potpunosti podudara s onim što se očekivalo od crnog tijela s temperaturom od 2,73 K (slika 3). To je upravo rezultat koji se očekuje ako je CMB doista bio crveno pomaknuto zračenje koje emituje vrući plin koji je ispunio sav prostor ubrzo nakon početka svemira.

Kozmičko pozadinsko zračenje.

Slika 3. Puna linija pokazuje kako bi se intenzitet zračenja trebao mijenjati s valnom duljinom za crno tijelo s temperaturom od 2,73 K. Kutije prikazuju intenzitet zračenja kosmičke pozadine izmjeren na različitim talasnim dužinama COBE-ovim instrumentima. Fit je savršen. Kada je ovaj grafikon prvi put prikazan na sastanku astronoma, oni su mu dali ovacije.

Stoga je prvi važan zaključak iz mjerenja CMB-a taj da je svemir koji danas imamo zaista evoluirao iz vrućeg, jednolikog stanja. Ovo zapažanje takođe pruža direktnu podršku opštoj ideji da živimo u svemiru koji se razvija, jer je svemir danas hladniji nego što je bio na početku.

Male razlike u CMB-u

Još prije lansiranja COBE-a znalo se da je CMB izuzetno izotropno. Zapravo, njegova ujednačenost u svakom pravcu jedna je od najboljih potvrda kosmološkog principa - da je svemir homogen i izotropan.

Prema našim teorijama, međutim, temperatura nije mogla biti savršeno uniforma kada je emitiran CMB. Napokon, CMB je zračenje koje je bilo raspršeno iz čestica u svemiru u vrijeme razdvajanja. Ako je zračenje bilo potpuno glatko, onda bi sve te čestice morale biti ravnomjerno raspoređene kroz svemir. Ipak, upravo te čestice koje su postale sve galaksije i zvijezde (i studenti astronomije) sada naseljavaju kosmos. Da su čestice bile potpuno glatko raspoređene, ne bi mogle formirati sve strukture velikih razmjera koje su sada prisutne u svemiru - nakupine i super nakupine galaksija o kojima je bilo riječi u posljednjih nekoliko poglavlja.

Rani svemir je morao imati male fluktuacije gustine iz kojih bi takve strukture mogle evoluirati. Regioni veće od prosječne gustine privukli bi dodatnu materiju i na kraju prerasli u galaksije i jata koja danas vidimo. Ispostavilo se da bi nam se ovi gusti krajevi činili hladnijim mjestima, odnosno imali bi temperature niže od prosjeka.

Razlog povezanosti temperature i gustine može se objasniti na ovaj način. U vrijeme razdvajanja, fotoni u nešto gušćem dijelu prostora morali su trošiti dio svoje energije da bi izbjegli gravitacijsku silu koju vrši okolni plin. Gubljenjem energije, fotoni su postali malo hladniji od ukupne prosječne temperature u vrijeme razdvajanja. Obrnuto, fotoni koji su se nalazili u nešto manje gustom dijelu prostora izgubili su manje energije napuštanjem istog od ostalih fotona, čineći tako nešto vruće od prosjeka. Stoga, ako je sjeme današnjih galaksija postojalo u vrijeme kad je CMB emitovan, trebali bismo vidjeti neke male varijacije u temperaturi CMB-a dok gledamo u različitim smjerovima na nebu.

Naučnici koji rade s podacima sa satelita COBE zaista su otkrili vrlo suptilne temperaturne razlike - oko 1 dio na 100 000 - u CMB-u. Regije temperature niže od prosječne imaju razne veličine, ali čak i najmanje od hladnijih područja koja je COBE otkrio previše je veliko da bi moglo biti preteča pojedine galaksije ili čak superklaster galaksija. To je zato što je instrument COBE imao „zamagljen vid“ (loša rezolucija) i mogao je mjeriti samo velike dijelove neba. Trebali su nam instrumenti sa „oštrijim vidom“.

Najdetaljnija mjerenja CMB-a dobila su dva satelita lansirana nedavno od COBE-a. Rezultati prvog od ovih satelita, sonde mikrotalasne anizotropije Wilkinson (WMAP) svemirske letjelice, objavljene su 2003. Godine 2015. mjerenja sa satelita Planck proširila su mjerenja WMAP-a na još veću prostornu rezoluciju i nižu buku (slika 4).

CMB zapažanja.

Slika 4. Ova usporedba pokazuje koliko se detalja može vidjeti u promatranjima tri satelita koja se koriste za mjerenje CMB-a. CMB je snimak najstarije svjetlosti u našem svemiru, utisnut na nebo kad je svemir bio star otprilike 380 000 godina. Prva svemirska letelica, lansirana 1989. godine, je NASA-in Cosmic Background Explorer ili COBE. WMAP je pokrenut 2001., a Planck 2009. godine. Tri panela prikazuju zakrpe mapa neba od 10 kvadratnih stepeni. Ova kosmička pozadinska slika zračenja (dno) je karta CMB-a na čitavom nebu, kako je posmatrala misija Planck. Boje na karti predstavljaju različite temperature: crvena za toplije i plava za hladnije. Ove male fluktuacije temperature odgovaraju regionima malo drugačije gustine, predstavljajući sjeme svih budućih struktura: zvijezda, galaksija i današnjih jata galaksija. (vrh kredita: izmjena rada NASA-e / JPL-Caltech / ESA kreditno dno: izmjena djela ESA-e i Planckova suradnja)

Teoretski proračuni pokazuju da veličine vrućih i hladnih tačaka u CMB-u ovise o geometriji svemira, a time i o njegovoj ukupnoj gustini. (Uopće nije očito da bi to trebalo činiti, a za uspostavljanje veze potrebni su prilično otmjeni izračuni - daleko iznad nivoa našeg teksta, ali takva ovisnost je vrlo korisna.) Ukupna gustina o kojoj ovdje raspravljamo uključuje i količinu mase u svemiru i maseni ekvivalent tamne energije. Odnosno, moramo sabrati masu i energiju: običnu materiju, tamnu materiju i tamnu energiju koja ubrzava širenje.

Da biste vidjeli zašto ovo funkcionira, sjetite se (iz poglavlja o Crnim rupama i zakrivljenom svemirskom vremenu) toga s njegovim teorija opšte relativnosti , Einstein je pokazao da materija može zakriviti prostor i da količina zakrivljenosti ovisi o količini prisutne materije. Stoga ukupna količina materije u svemiru (uključujući tamnu materiju i ekvivalentan doprinos materije tamnom energijom) određuje ukupnu geometriju prostora. Baš kao što geometrija prostora oko crne rupe ima zakrivljenost, tako i čitav svemir može imati zakrivljenost. Pogledajmo mogućnosti (slika 5).

Ako je gustina materije veća od kritične, svemir će se na kraju srušiti. U tako zatvorenom svemiru na kraju će se sresti dva paralelno paralelna zraka svetlosti. Ova vrsta geometrije naziva se sferna geometrija. Ako je gustina materije manje od kritične, svemir će se zauvijek širiti. Dvije početne paralelne zrake svjetlosti razići će se, a to se naziva hiperboličkom geometrijom. U svemiru kritične gustine dvije paralelne zrake svjetlosti nikada se ne susreću, a širenje se zaustavlja tek u neko vrijeme beskrajno daleko u budućnosti. Ovo nazivamo a ravni svemir, a vrsta euklidske geometrije koju ste naučili u srednjoj školi primjenjuje se u ovoj vrsti svemira.

Prikazivanje zakrivljenosti svemira za čitav svemir.

Slika 5. Gustina materije i energije određuje ukupnu geometriju prostora. Ako je gustina svemira veća od kritične gustine, tada će se svemir na kraju srušiti i kaže se da je svemir zatvoreno poput površine kugle. Ako je gustina tačno jednaka kritičnoj gustini, onda je prostor stan poput lista papira, svemir će se zauvijek širiti, a brzina širenja beskrajno će se zaustaviti u budućnosti. Ako je gustoća manja od kritične, tada će se širenje nastaviti zauvijek i kaže se da je takav prostor otvoren i negativno zakrivljena poput površine sedla (gdje se otvara više prostora nego što očekujete dok se udaljavate). Imajte na umu da crvene linije na svakom dijagramu pokazuju šta se događa u svakoj vrsti prostora - u početku su paralelne, ali slijede različite staze, ovisno o zakrivljenosti prostora. Zapamtite da ovi crteži pokušavaju pokazati kako je prostor za čitav svemir "iskrivljen" - to se lokalno ne može vidjeti u maloj količini prostora koju mi ​​ljudi zauzimamo.

Ako je gustina svemira jednaka kritičnoj gustini, tada topla i hladna tačka u CMB-u obično trebaju biti veličine oko jednog stepena. Ako je gustoća veća od kritične, tada će tipične veličine biti veće od jednog stepena. Ako svemir ima gustoću manju od kritične, tada će strukture izgledati manje. Na slici 6 možete lako vidjeti razlike. WMAP i Planckova zapažanja CMB-a potvrdila su ranije eksperimente da zaista živimo u ravnom univerzumu kritične gustine.

Poređenje CMB posmatranja sa mogućim modelima svemira.

Slika 6. Kozmološke simulacije predviđaju da će, ako naš svemir ima kritičnu gustinu, tada će CMB slikama dominirati vruće i hladne tačke veličine oko jednog stepena (dolje u sredini). Ako je, s druge strane, gustoća veća od kritične (i svemir će se u konačnici urušiti), tada će se vruće i hladne točke slika pojaviti veće od jednog stupnja (dolje lijevo). Ako je gustoća svemira manja od kritične (a širenje će se nastaviti zauvijek), tada će strukture izgledati manje (dolje desno). Kao što pokazuju mjerenja, svemir je kritične gustine. Prikazana mjerenja izvršena su pomoću balonskog instrumenta nazvanog BOOMERanG (balonska posmatranja milimetrijskog ekstragalaktičkog zračenja i geofizike), koji je letio na Antarktiku. Naknadna satelitska promatranja WMAP-a i Plancka potvrđuju BOOMERanG rezultat. (zasluga: modifikacija rada NASA-e)

Ključni brojevi iz analize Planckovih podataka daju nam trenutno najbolje vrijednosti za neka od osnovnih svojstava svemira:

  • Starost svemira: 13,799 ± 0,038 milijardi godina (Napomena: To znači da znamo starost svemira unutar 38 miliona godina. Neverovatno!)
  • Hubbleova konstanta: 67,31 ± 0,96 kilometara / sekundi / milion parseka
  • Udio svemirskog sadržaja koji je "tamna energija": 68,5% ± 1,3%
  • Udio svemira koji je materija: 31,5% ± 1,3%

Imajte na umu da je ova vrijednost za Hubble-ova konstanta je nešto manja od vrijednosti od 70 kilometara / sekundi / milion parseka koju smo usvojili u ovoj knjizi. Zapravo, vrijednost izvedena mjerenjem crvenih pomaka iznosi 73 kilometra / sekundu / milion parseka. Moderna kosmologija danas je toliko precizna da naučnici naporno rade na rješavanju ove razlike. Činjenica da je razlika između ova dva neovisna mjerenja tako mala zapravo je izvanredno postignuće. Prije samo nekoliko decenija, astronomi su se prepirali oko toga je li Hubbleova konstanta oko 50 kilometara / sekundu / milion parseka ili 100 kilometara / sekundu / milion parseka.

Analiza Planckovih podataka takođe pokazuje da obična materija (uglavnom protoni i neutroni) čini 4,9% ukupne gustine. Tamna materija i normalna materija čine 31,5% ukupne gustine. Preostalih 68,5% doprinosi tamna energija. Starost svemira pri razdvajanju - odnosno kada je emitiran CMB - bila je 380 000 godina.

Možda je iznenađujući rezultat visoko preciznih mjerenja WMAP-a i još preciznijih Planckovih mjerenja što nije bilo iznenađenja. Model kozmologije s uobičajenom materijom oko 5%, tamnom materijom oko 25% i tamnom energijom oko 70% preživio je od kasnih 1990-ih kada su podaci o supernovima u tom smjeru bili prisiljeni na kozmologe. Drugim riječima, vrlo čudan svemir koji smo opisali, sa samo oko 5% svog sadržaja koji čine vrste materije koje su nam poznate ovdje na Zemlji, zaista izgleda kao svemir u kojem živimo.

Nakon emitiranja CMB-a, svemir se nastavio širiti i hladiti. Do 400 do 500 miliona godina nakon Velikog praska, već su se stvorile prve zvijezde i galaksije. Duboko u unutrašnjosti zvijezda materija se podgrijavala, nuklearne reakcije su se zapalile i započela je postupnija sinteza težih elemenata o kojima smo raspravljali u ovoj knjizi.

Podsjetnikom završavamo ovaj brzi obilazak našeg modela ranog svemira. Na Veliki prasak ne smijete misliti kao na lokalizirano eksplozija u svemiru, poput eksplozije superzvijezde. Nije bilo granica i nije bilo nijednog mjesta na kojem se dogodila eksplozija. Bila je to eksplozija prostora (i vrijeme i materija i energija) koja su se dogodila svuda u svemiru. Sva materija i energija koja danas postoje, uključujući čestice od kojih ste stvoreni, došle su iz Velikog praska. Bili smo i još uvijek smo usred Velikog praska svuda oko nas.

Ključni pojmovi i sažeci

Kada je svemir postao dovoljno hladan da formira neutralne atome vodonika, svemir je postao proziran za zračenje. Naučnici su otkrili zračenje kosmičke mikrotalasne pozadine (CMB) iz ovog doba tokom vrućeg, ranog svemira. Mjerenja sa satelitom COBE pokazuju da CMB djeluje poput crnog tijela s temperaturom od 2,73 K. Sitne fluktuacije u CMB-u pokazuju nam sjeme velikih struktura u svemiru. Detaljna mjerenja ovih kolebanja pokazuju da živimo u svemiru kritične gustine i da se kritična gustina sastoji od 31% materije, uključujući tamnu tvar, i 69% tamne energije. Obične materije - vrste elementarnih čestica koje nalazimo na Zemlji - čine samo oko 5% kritične gustine. CMB merenja takođe pokazuju da je svemir star 13,8 milijardi godina.

Fusnote

Rječnik

Za dalja istraživanja

Grupne aktivnosti u saradnji

  1. Ovo poglavlje bavi se prilično velikim pitanjima i idejama. Neki nas sustavi vjerovanja uče da postoje pitanja na koja "nismo trebali znati". Drugi ljudi smatraju da ako su naši umovi i instrumenti sposobni istražiti pitanje, ono postaje dio našeg prvorodnog prava kao mislećih ljudskih bića. Neka vaša grupa razgovara o vašim ličnim reakcijama na rasprave o pitanjima kao što su početak vremena i prostora i konačna sudbina svemira. Da li vas nervira kada čujete o naučnicima koji raspravljaju o ovim pitanjima? Ili je uzbudljivo znati da sada možemo prikupiti naučne dokaze o postanku i sudbini kosmosa? (In discussing this, you may find that members of your group strongly disagree try to be respectful of others’ points of view.)
  2. A popular model of the universe in the 1950s and 1960s was the so-called steady-state cosmology. In this model, the universe was not only the same everywhere and in all directions (homogeneous and isotropic), but also the same at all times. We know the universe is expanding and the galaxies are thinning out, and so this model hypothesized that new matter was continually coming into existence to fill in the space between galaxies as they moved farther apart. If so, the infinite universe did not have to have a sudden beginning, but could simply exist forever in a steady state. Have your group discuss your reaction to this model. Do you find it more appealing philosophically than the Big Bang model? Can you cite some evidence that indicates that the universe was not the same billions of years ago as it is now—that it is not in a steady state?
  3. One of the lucky accidents that characterizes our universe is the fact that the time scale for the development of intelligent life on Earth and the lifetime of the Sun are comparable. Have your group discuss what would happen if the two time scales were very different. Suppose, for example, that the time for intelligent life to evolve was 10 times greater than the main-sequence lifetime of the Sun. Would our civilization have ever developed? Now suppose the time for intelligent life to evolve is ten times shorter than the main-sequence lifetime of the Sun. Would we be around? (This latter discussion requires considerable thought, including such ideas as what the early stages in the Sun’s life were like and how much the early Earth was bombarded by asteroids and comets.)
  4. The grand ideas discussed in this chapter have a powerful effect on the human imagination, not just for scientists, but also for artists, composers, dramatists, and writers. Here we list just a few of these responses to cosmology. Each member of your group can select one of these, learn more about it, and then report back, either to the group or to the whole class.
    • The California poet Robinson Jeffers was the brother of an astronomer who worked at the Lick Observatory. His poem “Margrave” is a meditation on cosmology and on the kidnap and murder of a child: http://www.poemhunter.com/best-poems/robinson-jeffers/margrave/.
    • In the science fiction story “The Gravity Mine” by Stephen Baxter, the energy of evaporating supermassive black holes is the last hope of living beings in the far future in an ever-expanding universe. The story has poetic description of the ultimate fate of matter and life and is available online at: http://www.infinityplus.co.uk/stories/gravitymine.htm.
    • The musical piece YLEM by Karlheinz Stockhausen takes its title from the ancient Greek term for primeval material revived by George Gamow. It tries to portray the oscillating universe in musical terms. Players actually expand through the concert hall, just as the universe does, and then return and expand again. See: http://www.karlheinzstockhausen.org/ylem_english.htm.
    • The musical piece Supernova Sonata http://www.astro.uvic.ca/

The Baby Picture of the Universe

The CMB looks almost exactly the same, no matter what part of the sky we look at. The term for that in cosmology is “isotropic”, and the small deviations from perfect sameness are called anisotropies. Measuring the larger-sized anisotropies reveals how much dark energy, dark matter, and ordinary matter are contained in the universe.

The smaller anisotropies reveal the tiny fluctuations in density that gave rise to the pattern of galaxies and galaxy clusters we see today, which astronomers call the large-scale structure of the universe. Without those small irregularities, there wouldn’t be any galaxies, and we wouldn’t be here to observe them. Likewise, larger anisotropies wouldn’t produce the universe we see.

The overwhelming sameness of the CMB also tells us something about the early universe. Two points on the CMB on opposite sides of the sky shouldn’t have almost exactly the same temperature, since they weren’t close together at recombination. The most popular explanation for this is “inflation”: a tiny fraction of a second after the Big Bang, quantum fluctuations caused the universe to expand at an extreme rate. Points that were far apart at recombination today were neighbors before inflation, so they have nearly the same temperature.

According to theory, inflation left its mark on the CMB in the form of the twisting of light known as polarization. Astronomers use modern telescopes to look for that polarization, in hopes of seeing the behavior of the universe when it was only a fraction of a second old.

The Center for Astrophysics | Harvard & Smithsonian is home to the Kovac Lab, which has developed the BICEP program in collaboration with NASA’s Jet Propulsion Laboratory and other institutions. The various BICEP telescopes measure CMB polarization to a high degree of precision, with the goal of identifying the physical processes at work in the first instants of the cosmos. The current iteration of the project includes the BICEP3 telescope and the Keck Array, both located at the South Pole.


ESA's Planck Reveals the Most Detailed Map Ever Created of the Cosmic Microwave Background

The most detailed map ever created of the cosmic microwave background -- the relic radiation from the Big Bang - was recently released by ESA. Acquired by the Planck space telescope, the map reveals the existence of features that challenge the foundations of our current understanding of the Universe.

The image is based on the initial 15.5 months of data from Planck and is the mission's first all-sky picture of the oldest light in our Universe, imprinted on the sky when it was just 380,000 years old.

At that time, the young Universe was filled with a hot dense soup of interacting protons, electrons, and photons at about 2700 degrees Celsius. When the protons and electrons joined to form hydrogen atoms, the light was set free. As the Universe has expanded, this light today has been stretched out to microwave wavelengths, equivalent to a temperature of just 2.7 degrees above absolute zero.

This "cosmic microwave background" or CMB, shows tiny temperature fluctuations that correspond to regions of slightly different densities at very early times, representing the seeds of all future structure: the stars and galaxies of today.

According to the standard model of cosmology, the fluctuations arose immediately after the Big Bang and were stretched to cosmologically large scales during a brief period of accelerated expansion known as inflation.

ESA's Planck was designed to map these fluctuations across the whole sky with greater resolution and sensitivity than ever before. By analyzing the nature and distribution of the seeds in Planck's CMB image, we can determine the composition and evolution of the Universe from its birth to the present day.

Overall, the information extracted from Planck's new map provides an excellent confirmation of the standard model of cosmology at an unprecedented accuracy, setting a new benchmark in our understanding of the contents of the Universe.

But because precision of Planck's map is so high, it also made it possible to reveal some peculiar unexplained features that may well require new physics to be understood.

"The extraordinary quality of Planck's portrait of the infant Universe allows us to peel back its layers to the very foundations, revealing that our blueprint of the cosmos is far from complete. Such discoveries were made possible by the unique technologies developed for that purpose by European industry," says Jean-Jacques Dordain, ESA's Director General.

"Since the release of Planck's first all-sky image in 2010, we have been carefully extracting and analyzing all of the foreground emissions that lie between us and the Universe's first light, revealing the cosmic microwave background in the greatest detail yet," adds George Efstathiou of the University of Cambridge in the UK.

One of the most surprising findings is that the fluctuations in the CMB temperatures at large angular scales do not match those predicted by the standard model - their signals are not as strong as expected from the smaller scale structure revealed by Planck.

Another is an asymmetry in the average temperatures on opposite hemispheres of the sky. This runs counter to the prediction made by the standard model that the Universe should be broadly similar in any direction we look.

Furthermore, a cold spot extends over a patch of sky that is much larger than expected.

The asymmetry and the cold spot had already been hinted at with Planck's predecessor, NASA's WMAP mission, but were largely ignored because of lingering doubts about their cosmic origin.

"The fact that Planck has made such a significant detection of these anomalies erases any doubts about their reality. It can no longer be said that they are artifacts of the measurements. They are real and we have to look for a credible explanation," says Paolo Natoli of the University of Ferrara in Italy.

"Imagine investigating the foundations of a house and finding that parts of them are weak. You might not know whether the weaknesses will eventually topple the house, but you'd probably start looking for ways to reinforce it pretty quickly all the same," adds François Bouchet of the Institut d'Astrophysique de Paris in France.

One way to explain the anomalies is to propose that the Universe is in fact not the same in all directions on a larger scale than we can observe. In this scenario, the light rays from the CMB may have taken a more complicated route through the Universe than previously understood, resulting in some of the unusual patterns observed today.

"Our ultimate goal would be to construct a new model that predicts the anomalies and links them together. But these are early days. So far, we don't know whether this is possible and what type of new physics might be needed. And that's exciting," says Professor Efstathiou.

Beyond the anomalies, however, the Planck data conform spectacularly well to the expectations of a rather simple model of the Universe, allowing scientists to extract the most refined values yet for its ingredients.

Normal matter that makes up stars and galaxies contributes just 4.9 percent of the mass/energy density of the Universe. Dark matter, which has thus far only been detected indirectly by its gravitational influence, makes up 26.8 percent, nearly a fifth more than the previous estimate.

Conversely, dark energy, a mysterious force thought to be responsible for accelerating the expansion of the Universe, accounts for less than previously thought.

Finally, the Planck data also set a new value for the rate at which the Universe is expanding today, known as the Hubble constant. At 67.15 kilometers per second per megaparsec, this is significantly less than the current standard value in astronomy. The data imply that the age of the Universe is 13.82 billion years.

"With the most accurate and detailed maps of the microwave sky ever made, Planck is painting a new picture of the Universe that is pushing us to the limits of understanding current cosmological theories," says Jan Tauber, ESA's Planck Project Scientist.

"We see an almost perfect fit to the standard model of cosmology, but with intriguing features that force us to rethink some of our basic assumptions.

"This is the beginning of a new journey and we expect that our continued analysis of Planck data will help shed light on this conundrum."


Ključni pojmovi i sažeci

When the universe became cool enough to form neutral hydrogen atoms, the universe became transparent to radiation. Scientists have detected the cosmic microwave background (CMB) radiation from this time during the hot, early universe. Measurements with the COBE satellite show that the CMB acts like a blackbody with a temperature of 2.73 K. Tiny fluctuations in the CMB show us the seeds of large-scale structures in the universe. Detailed measurements of these fluctuations show that we live in a critical-density universe and that the critical density is composed of 31% matter, including dark matter, and 69% dark energy. Ordinary matter—the kinds of elementary particles we find on Earth—make up only about 5% of the critical density. CMB measurements also indicate that the universe is 13.8 billion years old.


Discovery of the Cosmic Background Radiation

If the model of the universe described in the previous section is correct, then&mdashas we look far outward in the universe and thus far back in time&mdashthe first &ldquoafterglow&rdquo of the hot, early universe should still be detectable. Observations of it would be very strong evidence that our theoretical calculations about how the universe evolved are correct. As we shall see, we have indeed detected the radiation emitted at this photon decoupling vrijeme, when radiation began to stream freely through the universe without interacting with matter (Figure (PageIndex<1>)).

Figure (PageIndex<1>) Cosmic Microwave Background and Clouds Compared. (a) Early in the universe, photons (electromagnetic energy) were scattering off the crowded, hot, charged particles and could not get very far without colliding with another particle. But after electrons and photons settled into neutral atoms, there was far less scattering, and photons could travel over vast distances. The universe became transparent. As we look out in space and back in time, we can&rsquot see back beyond this time. (b) This is similar to what happens when we see clouds in Earth&rsquos atmosphere. Water droplets in a cloud scatter light very efficiently, but clear air lets light travel over long distances. So as we look up into the atmosphere, our vision is blocked by the cloud layers and we can&rsquot see beyond them.

The detection of this afterglow was initially an accident. In the late 1940s, Ralph Alpher and Robert Herman, working with George Gamow, realized that just before the universe became transparent, it must have been radiating like a blackbody at a temperature of about 3000 K&mdashthe temperature at which hydrogen atoms could begin to form. If we could have seen that radiation just after neutral atoms formed, it would have resembled radiation from a reddish star. It was as if a giant fireball filled the whole universe.

But that was nearly 14 billion years ago, and, in the meantime, the scale of the universe has increased a thousand fold. This expansion has increased the wavelength of the radiation by a factor of 1000 (see Figure (29.2.6) in Section 29.2). According to Wien&rsquos law, which relates wavelength and temperature, the expansion has correspondingly lowered the temperature by a factor of 1000 (see the chapter on Radiation and Spectra).

Alpher and Herman predicted that the glow from the fireball should now be at radio wavelengths and should resemble the radiation from a blackbody at a temperature only a few degrees above absolute zero. Since the fireball was everywhere throughout the universe, the radiation left over from it should also be everywhere. If our eyes were sensitive to radio wavelengths, the whole sky would appear to glow very faintly. However, our eyes can&rsquot see at these wavelengths, and at the time Alpher and Herman made their prediction, there were no instruments that could detect the glow. Over the years, their prediction was forgotten.

In the mid-1960s, in Holmdel, New Jersey, Arno Penzias and Robert Wilson of AT&T&rsquos Bell Laboratories had built a delicate microwave antenna (Figure (PageIndex<2>)) to measure astronomical sources, including supernova remnants like Cassiopeia A (see the chapter on The Death of Stars). They were plagued with some unexpected background noise, just like faint static on a radio, which they could not get rid of. The puzzling thing about this radiation was that it seemed to be coming from all directions at once. This is very unusual in astronomy: after all, most radiation has a specific direction where it is strongest&mdashthe direction of the Sun, or a supernova remnant, or the disk of the Milky Way, for example.

Figure (PageIndex<2>) Robert Wilson (left) and Arno Penzias (right). These two scientists are standing in front of the horn-shaped antenna with which they discovered the cosmic background radiation. The photo was taken in 1978, just after they received the Nobel Prize in physics.

Penzias and Wilson at first thought that any radiation appearing to come from all directions must originate from inside their telescope, so they took everything apart to look for the source of the noise. They even found that some pigeons had roosted inside the big horn-shaped antenna and had left (as Penzias delicately put it) &ldquoa layer of white, sticky, dielectric substance coating the inside of the antenna.&rdquo However, nothing the scientists did could reduce the background radiation to zero, and they reluctantly came to accept that it must be real, and it must be coming from space.

Penzias and Wilson were not cosmologists, but as they began to discuss their puzzling discovery with other scientists, they were quickly put in touch with a group of astronomers and physicists at Princeton University (a short drive away). These astronomers had&mdashas it happened&mdashbeen redoing the calculations of Alpher and Herman from the 1940s and also realized that the radiation from the decoupling time should be detectable as a faint afterglow of radio waves. The different calculations of what the observed temperature would be for this cosmic microwave background (CMB) 1 were uncertain, but all predicted less than 40 K.

Penzias and Wilson found the distribution of intensity at different radio wavelengths to correspond to a temperature of 3.5 K. This is very cold&mdashcloser to absolute zero than most other astronomical measurements&mdashand a testament to how much space (and the waves within it) has stretched. Their measurements have been repeated with better instruments, which give us a reading of 2.73 K. So Penzias and Wilson came very close. Rounding this value, scientists often refer to &ldquothe 3-degree microwave background.&rdquo

Many other experiments on Earth and in space soon confirmed the discovery by Penzias and Wilson: The radiation was indeed coming from all directions (it was isotropic) and matched the predictions of the Big Bang theory with remarkable precision. Penzias and Wilson had inadvertently observed the glow from the primeval fireball. They received the Nobel Prize for their work in 1978. And just before his death in 1966, Lemaître learned that his &ldquovanished brilliance&rdquo had been discovered and confirmed.

You may enjoy watching Three Degrees, a 26-minute video from Bell Labs about Penzias and Wilson&rsquos discovery of the cosmic background radiation (with interesting historical footage).


29.4 The Cosmic Microwave Background

The description of the first few minutes of the universe is based on theoretical calculations. It is crucial, however, that a scientific theory should be testable. What predictions does it make? And do observations show those predictions to be accurate? One success of the theory of the first few minutes of the universe is the correct prediction of the amount of helium in the universe.

Another prediction is that a significant milestone in the history of the universe occurred about 380,000 years after the Big Bang. Scientists have directly observed what the universe was like at this early stage, and these observations offer some of the strongest support for the Big Bang theory. To find out what this milestone was, let’s look at what theory tells us about what happened during the first few hundred thousand years after the Big Bang.

The fusion of helium and lithium was completed when the universe was about 4 minutes old. The universe then continued to resemble the interior of a star in some ways for a few hundred thousand years more. It remained hot and opaque, with radiation being scattered from one particle to another. It was still too hot for electrons to “settle down” and become associated with a particular nucleus such free electrons are especially effective at scattering photons, thus ensuring that no radiation ever got very far in the early universe without having its path changed. In a way, the universe was like an enormous crowd right after a popular concert if you get separated from a friend, even if he is wearing a flashing button, it is impossible to see through the dense crowd to spot him. Only after the crowd clears is there a path for the light from his button to reach you.

The Universe Becomes Transparent

Not until a few hundred thousand years after the Big Bang, when the temperature had dropped to about 3000 K and the density of atomic nuclei to about 1000 per cubic centimeter, did the electrons and nuclei manage to combine to form stable atoms of hydrogen and helium (Figure 29.14). With no free electrons to scatter photons, the universe became transparent for the first time in cosmic history. From this point on, matter and radiation interacted much less frequently we say that they decoupled from each other and evolved separately. Suddenly, electromagnetic radiation could really travel, and it has been traveling through the universe ever since.

Discovery of the Cosmic Background Radiation

If the model of the universe described in the previous section is correct, then—as we look far outward in the universe and thus far back in time—the first “afterglow” of the hot, early universe should still be detectable. Observations of it would be very strong evidence that our theoretical calculations about how the universe evolved are correct. As we shall see, we have indeed detected the radiation emitted at this photon decoupling time , when radiation began to stream freely through the universe without interacting with matter (Figure 29.15).

The detection of this afterglow was initially an accident. In the late 1940s, Ralph Alpher and Robert Herman , working with George Gamow , realized that just before the universe became transparent, it must have been radiating like a blackbody at a temperature of about 3000 K—the temperature at which hydrogen atoms could begin to form. If we could have seen that radiation just after neutral atoms formed, it would have resembled radiation from a reddish star. It was as if a giant fireball filled the whole universe.

But that was nearly 14 billion years ago, and, in the meantime, the scale of the universe has increased a thousand fold. This expansion has increased the wavelength of the radiation by a factor of 1000 (see Figure 29.7). According to Wien’s law, which relates wavelength and temperature, the expansion has correspondingly lowered the temperature by a factor of 1000 (see the chapter on Radiation and Spectra). The cosmic background behaves like a blackbody and should therefore have a spectrum that obeys Wien’s Law.

Alpher and Herman predicted that the glow from the fireball should now be at radio wavelengths and should resemble the radiation from a blackbody at a temperature only a few degrees above absolute zero. Since the fireball was everywhere throughout the universe, the radiation left over from it should also be everywhere. If our eyes were sensitive to radio wavelengths, the whole sky would appear to glow very faintly. However, our eyes can’t see at these wavelengths, and at the time Alpher and Herman made their prediction, there were no instruments that could detect the glow. Over the years, their prediction was forgotten.

In the mid-1960s, in Holmdel, New Jersey, Arno Penzias and Robert Wilson of AT&T’s Bell Laboratories had built a delicate microwave antenna (Figure 29.16) to measure astronomical sources, including supernova remnants like Cassiopeia A (see the chapter on The Death of Stars). They were plagued with some unexpected background noise, just like faint static on a radio, which they could not get rid of. The puzzling thing about this radiation was that it seemed to be coming from all directions at once. This is very unusual in astronomy: after all, most radiation has a specific direction where it is strongest—the direction of the Sun, or a supernova remnant, or the disk of the Milky Way, for example.

Penzias and Wilson at first thought that any radiation appearing to come from all directions must originate from inside their telescope, so they took everything apart to look for the source of the noise. They even found that some pigeons had roosted inside the big horn-shaped antenna and had left (as Penzias delicately put it) “a layer of white, sticky, dielectric substance coating the inside of the antenna.” However, nothing the scientists did could reduce the background radiation to zero, and they reluctantly came to accept that it must be real, and it must be coming from space.

Penzias and Wilson were not cosmologists, but as they began to discuss their puzzling discovery with other scientists, they were quickly put in touch with a group of astronomers and physicists at Princeton University (a short drive away). These astronomers had—as it happened—been redoing the calculations of Alpher and Herman from the 1940s and also realized that the radiation from the decoupling time should be detectable as a faint afterglow of radio waves. The different calculations of what the observed temperature would be for this cosmic microwave background (CMB) 2 were uncertain, but all predicted less than 40 K.

Penzias and Wilson found the distribution of intensity at different radio wavelengths to correspond to a temperature of 3.5 K. This is very cold—closer to absolute zero than most other astronomical measurements—and a testament to how much space (and the waves within it) has stretched. Their measurements have been repeated with better instruments, which give us a reading of 2.73 K. So Penzias and Wilson came very close. Rounding this value, scientists often refer to “the 3-degree microwave background.”

Many other experiments on Earth and in space soon confirmed the discovery by Penzias and Wilson: The radiation was indeed coming from all directions (it was isotropic) and matched the predictions of the Big Bang theory with remarkable precision. Penzias and Wilson had inadvertently observed the glow from the primeval fireball. They received the Nobel Prize for their work in 1978. And just before his death in 1966, Lemaître learned that his “vanished brilliance” had been discovered and confirmed.

Link do učenja

You may enjoy watching Three Degrees, a 26-minute video from Bell Labs about Penzias and Wilson’s discovery of the cosmic background radiation (with interesting historical footage).

Properties of the Cosmic Microwave Background

One issue that worried astronomers is that Penzias and Wilson were measuring the background radiation filling space through Earth’s atmosphere. What if that atmosphere is a source of radio waves or somehow affected their measurements? It would be better to measure something this important from space.

The first accurate measurements of the CMB were made with a satellite orbiting Earth. Named the Cosmic Background Explorer ( COBE ), it was launched by NASA in November 1989. The data it received quickly showed that the CMB closely matches that expected from a blackbody with a temperature of 2.73 K (Figure 29.17). This is exactly the result expected if the CMB was indeed redshifted radiation emitted by a hot gas that filled all of space shortly after the universe began.

The first important conclusion from measurements of the CMB, therefore, is that the universe we have today has indeed evolved from a hot, uniform state. This observation also provides direct support for the general idea that we live in an evolving universe, since the universe is cooler today than it was in the beginning.

Small Differences in the CMB

It was known even before the launch of COBE that the CMB is extremely isotropic. In fact, its uniformity in every direction is one of the best confirmations of the cosmological principle— that the universe is homogenous and isotropic.

According to our theories, however, the temperature could not have been perfectly uniform when the CMB was emitted. After all, the CMB is radiation that was scattered from the particles in the universe at the time of decoupling. If the radiation were completely smooth, then all those particles must have been distributed through space absolutely evenly. Yet it is those particles that have become all the galaxies and stars (and astronomy students) that now inhabit the cosmos. Had the particles been completely smoothly distributed, they could not have formed all the large-scale structures now present in the universe—the clusters and superclusters of galaxies discussed in the last few chapters.

The early universe must have had tiny density fluctuations from which such structures could evolve. Regions of higher-than-average density would have attracted additional matter and eventually grown into the galaxies and clusters that we see today. It turns out that these denser regions would appear to us to be colder spots, that is, they would have lower-than-average temperatures.

The reason that temperature and density are related can be explained this way. At the time of decoupling, photons in a slightly denser portion of space had to expend some of their energy to escape the gravitational force exerted by the surrounding gas. In losing energy, the photons became slightly colder than the overall average temperature at the time of decoupling. Vice versa, photons that were located in a slightly less dense portion of space lost less energy upon leaving it than other photons, thus appearing slightly hotter than average. Therefore, if the seeds of present-day galaxies existed at the time that the CMB was emitted, we should see some slight variations in the CMB temperature as we look in different directions in the sky.

Scientists working with the data from the COBE satellite did indeed detect very subtle temperature differences—about 1 part in 100,000—in the CMB. The regions of lower-than-average temperature come in a variety of sizes, but even the smallest of the colder areas detected by COBE is far too large to be the precursor of an individual galaxy, or even a supercluster of galaxies. This is because the COBE instrument had “blurry vision” (poor resolution) and could only measure large patches of the sky. We needed instruments with “sharper vision.”

The most detailed measurements of the CMB have been obtained by two satellites launched more recently than COBE. The results from the first of these satellites, the Wilkinson Microwave Anisotropy Probe ( WMAP ) spacecraft, were published in 2003. In 2015, measurements from the Planck satellite extended the WMAP measurements to even-higher spatial resolution and lower noise (Figure 29.18).

Theoretical calculations show that the sizes of the hot and cold spots in the CMB depend on the geometry of the universe and hence on its total density. (It’s not at all obvious that it should do so, and it takes some pretty fancy calculations—way beyond the level of our text—to make the connection, but having such a dependence is very useful.) The total density we are discussing here includes both the amount of mass in the universe and the mass equivalent of the dark energy. That is, we must add together mass and energy: ordinary matter, dark matter, and the dark energy that is speeding up the expansion.

To see why this works, remember (from the chapter on Black Holes and Curved Spacetime) that with his theory of general relativity , Einstein showed that matter can curve space and that the amount of curvature depends on the amount of matter present. Therefore, the total amount of matter in the universe (including dark matter and the equivalent matter contribution by dark energy), determines the overall geometry of space. Just like the geometry of space around a black hole has a curvature to it, so the entire universe may have a curvature. Let’s take a look at the possibilities (Figure 29.19).

If the density of matter is higher than the critical density, the universe will eventually collapse. In such a closed universe, two initially parallel rays of light will eventually meet. This kind of geometry is referred to as spherical geometry. If the density of matter is less than critical, the universe will expand forever. Two initially parallel rays of light will diverge, and this is referred to as hyperbolic geometry. In a critical-density universe, two parallel light rays never meet, and the expansion comes to a halt only at some time infinitely far in the future. We refer to this as a flat universe , and the kind of Euclidean geometry you learned in high school applies in this type of universe.

If the density of the universe is equal to the critical density, then the hot and cold spots in the CMB should typically be about a degree in size. If the density is greater than critical, then the typical sizes will be larger than one degree. If the universe has a density less than critical, then the structures will appear smaller. In Figure 29.20, you can see the differences easily. WMAP and Planck observations of the CMB confirmed earlier experiments that we do indeed live in a flat, critical-density universe.

Key numbers from an analysis of the Planck data give us the best values currently available for some of the basic properties of the universe:

  • Age of universe: 13.799 ± 0.038 billion years (Note: That means we know the age of the universe to within 38 million years. Amazing!)
  • Hubble constant: 67.31 ± 0.96 kilometers/second/million parsecs (in the units we’ve been using, 20.65 kilometers/second/million light-years)
  • Fraction of universe’s content that is “dark energy”: 68.5% ± 1.3%
  • Fraction of the universe’s content that is matter: 31.5% ± 1.3%

Note that this value for the Hubble constant is slightly smaller than the value of 70 kilometers/second/million parsecs that we have adopted in this book. In fact, the value derived from measurements of redshifts is 73 kilometers/second/million parsecs. So precise is modern cosmology these days that scientists are working hard to resolve this discrepancy. The fact that the difference between these two independent measurements is so small is actually a remarkable achievement. Only a few decades ago, astronomers were arguing about whether the Hubble constant was around 50 kilometers/second/million parsecs or 100 kilometers/second/million parsecs.

Analysis of Planck data also shows that ordinary matter (mainly protons and neutrons) makes up 4.9% of the total density. Dark matter plus normal matter add up to 31.5% of the total density. Dark energy contributes the remaining 68.5%. The age of the universe at decoupling—that is, when the CMB was emitted—was 380,000 years.

Perhaps the most surprising result from the high-precision measurements by WMAP and the even higher-precision measurements from Planck is that there were no surprises. The model of cosmology with ordinary matter at about 5%, dark matter at about 25%, and dark energy about 70% has survived since the late 1990s when cosmologists were forced in that direction by the supernovae data. In other words, the very strange universe that we have been describing, with only about 5% of its contents being made up of the kinds of matter we are familiar with here on Earth, really seems to be the universe we live in.

After the CMB was emitted, the universe continued to expand and cool off. By 400 to 500 million years after the Big Bang, the very first stars and galaxies had already formed. Deep in the interiors of stars, matter was reheated, nuclear reactions were ignited, and the more gradual synthesis of the heavier elements that we have discussed throughout this book began.

We conclude this quick tour of our model of the early universe with a reminder. You must not think of the Big Bang as a localized explosion in space, like an exploding superstar. There were no boundaries and there was no single site where the explosion happened. It was an explosion of space (and time and matter and energy) that happened everywhere in the universe. All matter and energy that exist today, including the particles of which you are made, came from the Big Bang. We were, and still are, in the midst of a Big Bang it is all around us.


Cosmic Microwave Background: WMAP (first year)

" Wilkinson Microwave Anisotropy Probe, WMAP, is a NASA Explorer mission measuring the temperature of the cosmic background radiation over the full sky with unprecedented accuracy. This map of remnant heat from the Big Bang provides answers to fundamental questions about the origin and fate of our universe. " — From the NASA/WAMP website The probe is over 930,000 miles from Earth and effectively scans the entire sky every six months.

Temperature fluctuations displayed here are 13.7 billion years old, from the time when the Big Bang was thought to have occurred. Essentially, it is a detailed, all-sky display of the young universe developed from three years of WMAP data. The blue areas are cooler while the red areas are warmer. The temperature range on this map is ± 200 microKelvin, which is incredibly small. The temperature range is so small because it doesn’t measure absolute temperature but anisotropy. Anisotropy is the difference between two measurements taken in opposite directions. This is much more accurate than simply measuring the absolute temperature in one direction. This data is used to support the Big Bang theory using inflation. The concept is that the universe expanded many trillion times its size in less than a trillionth of a second at the beginning of the Big Bang. This is a map of the remnant heat left from the big bang. According to NASA, the measurements reveal size, matter content, age, geometry, and the fate of the universe.

There are two versions of the WMAP data. This dataset is from the first year of data collected by WMAP and is lower resolution. The other available dataset is from the third year of data collected and is polarized and has a higher resolution. In the third year dataset, the formation site of the Milky Way galaxy is visible in the red band.


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