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Hoe bepaal u die ouderdom van 'n ou sterrestelsel?

Hoe bepaal u die ouderdom van 'n ou sterrestelsel?


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Onlangs is 'n sterrestelsel A1689B11 ontdek met 'n ouderdom van 11 miljard jaar. Hoe is daardie ouderdom bepaal? Nuusbronne het berig dat van die nuutste tegnologie met gravitasie-lens gebruik is. Kan iemand die presiese prosedure verduidelik?


As die artikel oor die "era" praat, beteken dit dat ons daardie sterrestelsel sien soos lank gelede. Ons sien dit as 'n baie jong sterrestelsel. Die sterrestelsel is baie ver weg, en dit het dus lank geneem om ons vanaf die sterrestelsel te bereik. Die lig van die sterrestelsel is 11 miljard jaar oud. Ons meet die ouderdom van die sterrestelsel deur die afstand vanaf ons te vind.

Om die afstand te vind, gebruik ons ​​die Hubble-wet, wat sê dat hoe verder 'n sterrestelsel is, hoe vinniger beweeg dit weg van ons, as gevolg van die uitbreiding van die heelal. En ons kan die spoed meet wat die sterrestelsel van ons af wegbeweeg, want as 'n voorwerp baie vinnig beweeg, word die lig vanaf die voorwerp na langer golflengtes verskuif (rooi verskuif). Deur lig te meet waarvan bekend is dat dit 'n vaste golflengte het as dit nie rooi verskuif nie, kan ons die spoed van die sterrestelsel akkuraat vind en dan die Hubble-wet gebruik om die afstand en dus die ouderdom van die sterrestelsel te bepaal. (bron)

A1689B11 het 'n rooi verskuiwing z = 2.54 wat ooreenstem met 'n ligte reistyd van 11,1 miljard jaar

Gravitasie-lenswerk maak die sterrestelsel helderder, sodat dit op hierdie groot afstande waargeneem kan word. Dit word nie direk gebruik om die afstand te vind nie, maar sonder gravitasielensing sou die sterrestelsel nie sigbaar wees nie.


Hoe ken ons die ouderdom van die heelal?

'N Onlangse Forbes-artikel, geskryf deur die astrofisikus Ethan Siegel, het die uitdagende titel, "How Do We Know the Age of the Universe?" Alhoewel daar niks nuuts in hierdie artikel is nie, is dit 'n goeie bespreking op lae vlak van die huidige begrip van die ouderdom van die heelal binne die oerknal-model. Aangesien Forbes 'n goed geleesde bron is, kan baie mense wat die Answers in Genesis-webwerf gereeld besoek, wonder wat die Bybelse reaksie is. Dit is 'n goeie geleentheid om 'n bietjie gedagtes te bedink.

Siegel begin sy stuk met die opmerking dat, in 'n vraag van hierdie aard, die beste sou wees om '' 'n ongelooflike aantal onafhanklike bewyse te hê, wat almal na dieselfde antwoord konvergeer. Maar in werklikheid is daar net twee goeies, en die een is beter as die ander. ' Dit is alte dikwels 'n verfrissende openhartige waarneming, hierdie soort artikels het veel meer sekerheid as wat geregverdig word.


Rainer C. Gaitzsch

Werkgroep "EAAE Summerschools"

Onderwysersakademie van Beiere (Duitsland)

Opsomming

Voorbeelde sal gewys word hoe om spektrums van helder sterre te kry, maklik gemaak met gewone materiaal wat by die skool beskikbaar is. Om 'n Hertzsprung-Russell-diagram (HRD) van sekere sterre te verkry, moet spektra ontleed word.

Deur middel van databanke van verskeie sterretrosse op gegewe referate, sal die deelnemers self 'n HRD van 'n sekere sterregroep uitwerk. In hierdie diagram word die skynbare grootte geteken teen die oppervlaktemperatuur (resp. Die kleurindeks B-V). Die groep sal dan hul resultate bespreek. Deur hul eie HRD's met die standaard hoofreeks sterre in absolute groottes te vergelyk, sal die deelnemers die afstand van hul eie groep aflei en leer hoe om die ouderdom daarvan te skat.

Spesiale rekenaarsimulasies met die deelnemers wat betrokke is, sal 'n paar vrae aan verskynsels rakende die evolusie van sterre illustreer, dws: Wat gebeur in die kern van die ster? Waar skuif die trossterre hul plek in die MVD tydens die verouderingsproses? Waarom versprei alle oop sterretrosse uiteindelik?

Sterre versprei oor die lug. Sommige van hulle staan ​​alleen en ander is in sterretrosies ingebed. Maar nie een ster begin sy bestaan ​​afsonderlik nie, elke ster is in 'n groep gebore. Aan die nagskyn kan ons twee verskillende soorte trosse sien: die oop sterretrosse met honderd lede wat in die skyfgedeelte van ons sterrestelsel geleë is, meestal in die spiraalarms, en die bolvormige trosse wat honderdduisende sterre bevat, geleë in die stralekrans van ons sterrestelsel. Hoe kan ons iets weet van daardie verre voorwerpe? Ons kry al ons inligting deur hul spektra te lees.

Fig. 1a: Oop sterretros M45 ("Plejades") in die Taurus, 400 ly weg. Fig. 1 b: Bolvormige tros M13 in Hercules, 25000 ly weg.

Dit is moontlik om spektra's van 'n paar helder sterre te kry, slegs met gewone toerusting wat in elke fisika-afdeling op hoërskool beskikbaar is. 'N Voorbeeld word aan die deelnemers getoon waar 'n student, 18 jaar oud, 'n eenvoudige spektrograaf gebou het en 'n paar mooi spektra gekry het wat hy dan kon ontleed. Die ster skep sy spektrum op 'n natuurlike manier deur sy eie gewone beweging oor die nightky.

Fig. 2a: montering van prisma en kamera. Fig. 2 b: Spectrum van Wega met H-lyne.

Om 'n Hertzsprung-Russell-Diagram (HRD) van sekere sterre of 'n sterreswerm te verkry, moet spektra professioneel ontleed word. Een van die belangrikste gegewens wat 'n spektrum kan openbaar, is die oppervlaktemperatuur van die ster. Dit kan nie maklik op skool gemaak word nie. Die visuele skynbare grootte van die ster kan ook nie op 'n maklike manier op skool gemeet word nie. Maar ons kan 'n databank van 'n sekere sterregroep gebruik wat ons wil ondersoek. In plaas van die ster se temperatuur word die kleurindeks (B-V) dikwels in die diagram gebruik. Daar is 'n duidelike korrelasie tussen temperatuur T en kleurindeks (B-V). (W.J. Kaufmann: "Universe", of Gondolatsch u.a .: "Astronomie II"). As die ster warm is, is dit blouerig met 'n (B-V) indeks van minder as nul. As 'n ster koel is, is sy (B-V) indeks positief. Die Sun (B-V) -indeks is ongeveer +0,62. Nadat 'n ster se B- en V-mag. Gemeet is, kan 'n sterrekundige die ster se temperatuur skat vanaf 'n grafiek soos hier.

Fig. 3: Swartliggaam temperatuur teenoor kleurindeks.

Voordat ons ons eie MH van 'n oop sterregroep sal uitwerk, moet ons iets weet oor die betekenis van 'n MHB in die algemeen. HRD's toon dat daar verskillende soorte sterre is. In die diagramme van sterretrosse word die skynbare grootte geteken teenoor die oppervlaktemperatuur, onderskeidelik die kleurindeks (B-V). Elke kolletjie verteenwoordig 'n ster waarvan die helderheid en die tempo gemeet is. Die feit dat die data in drie verskillende streke val, beteken dat daar drie verskillende soorte sterre aan die hemel is: gewone hoofreekssterre, rooi reuse en wit dwerge.

Fig. 4: Die tipiese MVS. Fig. 5: Die evolusionêre spoor van 'n sontipe ster.

In die binnekern van elke hoofreeks-ster word waterstof in helium omskep, dus werk die kernfusie soos volg: H ® He.

Al daardie sterre gehoorsaam ongeveer die massa-helderheidsverhouding L

Die massiewe sterre ontwikkel baie vinniger in vergelyking met ons son as wat die minder massiewe sterre doen. Vir die gemiddelde tyd van die evolusie (t) kan ons stel: t

M (berging van brandstof), maar ook t

1 / L (verlies aan energie deur bestraling). Ten slotte:

1 / M 2 (ongeveer die waarde vir ons son is ongeveer t = 1 × 10 10 jaar).

As ons dus die afdraaipunt in die HRD van 'n sterregroep kan vind waar die hoofreekssterre hul eerste fase van 'n ster se lewe beëindig en in die streek van die rooi reuse opspoor, kan ons die ouderdom daarvan skat.

Fig. 6 a: 'n Jong oop sterretros ('Hyades'). Fig. 6 b: 'n Ou bolvormige sterretros (M3).

Die vorm van die hoofreeks is dieselfde vir alle sterretrosse, ongeag die ouderdom, met slegs klein variasies. Hierdie feit is 'n baie belangrike manier om die afstand van 'n sterreswerm te vind wat andersins onbekend is. Ons moet net die kleur-grootte-diagram wat ons uit ons groep verkry het, vergelyk met die standaard hoofreeks, waar die absolute grootte M teenoor die kleurindeks gestip word.

Die verskil tussen oënskynlike mag. m en absolute mag. M (die sogenaamde afstandsmodulus) word gegee deur: m - M = 5 × log (r / 10 st), waar r die afstand is.

Die afstandsmodus is die skuif in die vertikale as wat nodig is om sy kleur-mag-diagram in ooreenstemming met die standaard hoofreeks te bring. Die groep kan, weens ouderdom, sy helderste sterre verloor het, maar sterre wat tot die onderste deel van die hoofreeks behoort, is altyd teenwoordig.

Oefening

Bepaal die afstand en skat die ouderdom van die oop tros NGC 6025.

- Teken die kleur-mag-diagram uit die gegewens in tabel 1 in die vorm van skynbare mag. m (= V) teenoor (B-V). (bylaag 1)

- Teken die standaard hoofreeks uit die gegewens in tabel 2 in die vorm van absolute mag. M teen (B-V).

- Kry die verskil m - M en bereken die afstand r van die tros NGC 6025.

- Gebruik die afdraaipunt vanaf u NGC 6025-diagram as ouderdomsindikator en skat die ouderdom van die sterregroep. (Vergelyk ook met tabel 3)
Wenk: Aangesien die gemiddelde verlenging van die temperatuurbereik ongeveer 3000 K tot 30000 K is en die gemiddelde verlenging van die helderheidsgebied (in eenhede van die sonligsterkte) ongeveer 0,01 tot 10000 is, is dit 'n goeie skatting vir L

T 6 (sien ook Fig. 4). Met die wet van Stefan-Boltzmann L

R 2 × T 4 en die massa-helderheidsverhouding L

T 2 en vir die tyd van evolusie t

- Probeer ook om waardes te kry vir die ouderdomme van die twee trosse wat in Fig. 6a (Hyades) en Fig. 6b (M3) getoon word.

Bykomende oefening

Die bolvormige groep M13 (Fig. 1b) het 'n skynbare straal van 5.0 'en die skynbare grootte m = 5.7. M13 bevat veranderlike sterre van RR Lyr-tipe (abs. Mag. M = 0,0), gegee met hul oënskynlike mag. m = 14,9.

a) Bereken die afstand en die radius van hierdie groep M13 in ly.

b) Bepaal die abs. mag. van M13 en skat die massa daarvan in eenhede van sonmassas op voorwaarde dat alle sterlede sonvormige sterre is (M = 4.8).

Fig. 7: HRD-simulasieprogram van die evolusie van 'n sterre-trosstyd: 300 miljoen. jare. Fig. 8: HRD, wat die afdraaipunt van verskeie sterretrosse aandui. Op daardie stadium verlaat die mees massiewe sterre die hoofreeks vir die rooi-winsgewende streek.


Skatting van die ouderdom van die melkweg

Waarnemings deur 'n internasionale span sterrekundiges met die UVES-spektrometer op die ESO en die Very Large Telescope by die Paranal Observatory (Chili) het nuwe lig gewerp op die vroegste tydvak van die Melkwegstelsel.

Die allereerste meting van die Beryllium-inhoud in twee sterre in 'n bolvormige groep (NGC 6397) en die huidige astronomiese tegnologie wat tot die uiterste beweeg, het dit moontlik gemaak om die vroeë fase tussen die vorming van die eerste generasie sterre in die Melkweg en dié van hierdie sterregroepering. Hierdie tydsinterval is 200 & # 8211 300 miljoen jaar.

Die ouderdom van die sterre in NGC 6397, soos bepaal deur middel van sterre-evolusiemodelle, is 13 400? 800 miljoen jaar. As u die twee tydsintervalle byvoeg, word die ouderdom van die Melkweg 13.600? 800 miljoen jaar.

Die huidige skatting van die ouderdom van die heelal, soos afgelei, byvoorbeeld uit metings van die Kosmiese Mikrogolfagtergrond, is 13.700 miljoen jaar. Die nuwe waarnemings dui dus aan dat die eerste generasie sterre in die Melkwegstelsel kort na die einde van die

200 miljoen jaar lange & # 8220Dark Ages & # 8221 wat die oerknal opgevolg het.

Die ouderdom van die Melkweg
Hoe oud is die Melkweg? Wanneer het die eerste sterre in ons sterrestelsel ontvlam?

'N Behoorlike begrip van die vorming en evolusie van die Melkwegstelsel is van kardinale belang vir ons kennis van die heelal. Nietemin is die verwante waarnemings die moeilikste, selfs met die kragtigste teleskope wat beskikbaar is, aangesien dit 'n uitvoerige studie van ou, afgeleë en meestal flou hemelvoorwerpe behels.

Globale trosse en die ouderdomme van sterre

Moderne astrofisika kan die ouderdomme van sekere sterre meet, dit is die tyd wat verloop het sedert dit gevorm is deur kondensasie in groot interstellêre wolke van gas en stof. Sommige sterre is astronomies baie & # 8220young & # 8221; net 'n paar miljoen jaar oud soos dié in die nabygeleë Orionnevel. Die son en sy planetêre stelsel is ongeveer 4,560 miljoen jaar gelede gevorm, maar baie ander sterre het baie vroeër gevorm. Sommige van die oudste sterre in die Melkweg kom voor in groot sterreswerms, veral in & # 8220globular clusters & # 8221 (PR Photo 23a / 04), sogenaamd vanweë hul bolvormige vorm.

Sterre wat aan 'n bolvormige groep behoort, is saam uit dieselfde wolk en terselfdertyd gebore. Aangesien sterre van verskillende massas teen verskillende tempo's ontwikkel, is dit moontlik om die ouderdom van bolvormige trosse met 'n redelike goeie akkuraatheid te meet. Die oudste is meer as 13 000 miljoen jaar oud.

Hierdie trossterre was nogtans nie die eerste sterre wat in die Melkweg gevorm is nie. Ons weet dit, want hulle bevat klein hoeveelhede van sekere chemiese elemente wat in 'n vroeëre generasie massiewe sterre gesintetiseer moes word wat na 'n kort en energieke lewe as supernovas ontplof het. Die verwerkte materiaal is neergelê in die wolke waaruit die volgende generasies sterre gemaak is, vgl. ESO PR 03/01.

Ondanks intensiewe soektogte, was dit tot nou toe nie moontlik om minder massiewe sterre van hierdie eerste generasie te vind wat vandag nog kan skyn nie. Ons weet dus nie wanneer hierdie eerste sterre gevorm is nie. Ons kan voorlopig net sê dat die Melkweg ouer moet wees as die oudste bolvormige trossterre.

Beryllium tot die redding
Wat astrofisici graag wil hê, is dus 'n metode om die tydsinterval te meet tussen die vorming van die eerste sterre in die Melkweg (waarvan baie vinnig supernovas geword het) en die oomblik toe die sterre in 'n bolvormige groep van bekende ouderdom gevorm is. Die som van hierdie tydsinterval en die ouderdom van die sterre is dan die ouderdom van die Melkweg.

Nuwe waarnemings met die VLT by ESO & # 8217 s Paranal Observatory het nou 'n deurbraak in hierdie rigting opgelewer. Die towerelement is & # 8220Beryllium & # 8221!

Berillium is een van die ligste elemente [2] & # 8211 die kern van die mees algemene en stabiele isotoop (Beryllium-9) bestaan ​​uit vier protone en vyf neutrone. Slegs waterstof, helium en litium is ligter. Maar terwyl hierdie drie tydens die oerknal geproduseer is, en terwyl die meeste van die swaarder elemente later in die binnekant van sterre geproduseer is, kan Beryllium-9 slegs vervaardig word deur & # 8220cosmic spallation & # 8221. Dit wil sê deur fragmentasie van vinniger swaarder kerne en # 8211 wat ontstaan ​​in die genoemde supernovae-ontploffings en na verwys word as energieke & # 8220galaktiese kosmiese strale & # 8221 & # 8211 wanneer hulle met ligte kerne bots (meestal protone en alfa-deeltjies, dws waterstof en heliumkerne) in die interstellêre medium.

Galaktiese kosmiese strale en die Beryllium-horlosie
Die galaktiese kosmiese strale het deur die vroeë Melkweg gereis, gelei deur die kosmiese magnetiese veld. Die gevolglike produksie van Beryllium was redelik eenvormig in die sterrestelsel. Die hoeveelheid Beryllium het mettertyd toegeneem en daarom kan dit as 'n & # 8220kosmiese horlosie & # 8221 dien.

Hoe langer die tyd verloop het tussen die vorming van die eerste sterre (of, meer korrek, hul vinnige ondergang in supernovae-ontploffings) en die vorming van die bolvormige trossterre, hoe hoër was die Beryllium-inhoud in die interstellêre medium waaruit hulle gevorm is . As ons dus aanvaar dat hierdie Beryllium in die steratmosfeer bewaar word, hoe meer Beryllium in so 'n ster aangetref word, hoe langer is die tydsinterval tussen die vorming van die eerste sterre en van hierdie ster.

Die Beryllium kan ons dus unieke en belangrike inligting verskaf oor die duur van die vroeë stadiums van die Melkweg.

'N Baie moeilike waarneming
So ver so goed. Die teoretiese grondslae vir hierdie dateringsmetode is gedurende die afgelope drie dekades ontwikkel, en al wat nodig is, is dan om die Beryllium-inhoud in sommige bolvormige trossterre te meet.

Maar dit is nie so eenvoudig soos dit klink nie! Die grootste probleem is dat Beryllium vernietig word by temperature bo enkele miljoene grade. Wanneer 'n ster in die rigting van die ligte reusefase ontwikkel, begin gewelddadige beweging (konveksie), die gas in die boonste steratmosfeer raak in kontak met die warm binnegas waarin al Beryllium vernietig is en die aanvanklike Beryllium-inhoud in die steratmosfeer is dus aansienlik verdun. Om die Beryllium-klok te gebruik, is dit dus nodig om die inhoud van hierdie element in minder massiewe, minder ontwikkelde sterre in die bolvormige groep te meet. En hierdie sogenaamde & # 8220turn-off (TO) sterre & # 8221 is intrinsiek flou.

Trouens, die tegniese probleem wat u moet oorkom, is drievoudig: eerstens is alle bolvormige trosse redelik ver weg en aangesien die sterre wat gemeet moet word intrinsiek flou is, lyk dit redelik flou in die lug. Selfs in NGC6397, die tweede naaste bolvormige groep, het die TO-sterre 'n visuele grootte van

16, of 10000 keer flouer as die vaagste ster wat sigbaar is vir die blote oog. Tweedens is daar slegs twee Beryllium-handtekeninge (spektrale lyne) sigbaar in die stertspektrum, en aangesien hierdie ou sterre relatief min Beryllium bevat, is die lyne baie swak, veral in vergelyking met naburige spektrale lyne van ander elemente. En derdens, die twee Beryllium-lyne is geleë in 'n bietjie verkenne spektraalgebied op golflengte 313 nm, dws in die ultraviolet gedeelte van die spektrum wat sterk beïnvloed word deur absorpsie in die aardse atmosfeer naby die afsnyding by 300 nm, waaronder waarnemings vanaf die grond is nie meer moontlik nie.

Dit is dus geen wonder dat sulke waarnemings nog nooit voorheen gedoen is nie; die tegniese probleme was eenvoudig onoorkomelik.

VLT en UVES doen die werk
Met behulp van die hoëprestasie-UVES-spektrometer op die 8,2 m-Kuyen-teleskoop van ESO en # Very Very Large Telescope by die Paranal Observatory (Chili) wat besonder sensitief is vir ultraviolet lig, het 'n span ESO en Italiaanse sterrekundiges [1] daarin geslaag eerste betroubare metings van die Beryllium-inhoud in twee TO-sterre (aangedui & # 8220A0228 & # 8221 en & # 8220A2111 & # 8221) in die bolvormige tros NGC 6397 (PR Foto 23b / 04). Geleë op 'n afstand van ongeveer 7 200 ligjare in die rigting van 'n ryk sterveld in die suidelike sterrebeeld Ara, is dit een van die twee naaste sterregroepe van hierdie soort, die ander is Messier 4.

Die waarnemings is gedurende 'n paar nagte gedurende 2003 gedoen. Die totale blootstelling aan meer as tien uur blootstelling aan elk van die sterre van die 16de sterkte het die VLT en UVES na die tegniese limiet gedruk. As gevolg van die tegnologiese vooruitgang, is die leier van die span, ESO-sterrekundige Luca Pasquini, verheug: & # 8220Jou 'n paar jaar gelede sou enige waarneming soos hierdie onmoontlik gewees het en net 'n sterrekundige gebly het! & # 8221

Die resulterende spektra (PR Foto 23c / 04) van die dowwe sterre toon die swak handtekeninge van Beryllium-ione (Be II). Deur die waargenome spektrum met 'n reeks sintetiese spektra met verskillende Beryllium-inhoud te vergelyk (in astrofisika: & # 8220abundance & # 8221), kon die sterrekundiges die beste pas vind en sodoende die baie klein hoeveelheid Beryllium in hierdie sterre meet: vir elke Beryllium-atoom. daar is ongeveer 2 224 000 000 000 waterstofatome.

Berilliumlyne word ook gesien in 'n ander ster van dieselfde tipe as hierdie sterre, HD 218052, vgl. PR Foto 23c / 04. Dit is egter nie 'n lid van 'n groep nie en sy ouderdom is verreweg nie so bekend soos dié van die sterre nie. Die Beryllium-inhoud daarvan stem baie ooreen met dié van die trossterre, wat daarop dui dat hierdie veldster ongeveer dieselfde tyd as die tros gebore is.

Van die oerknal tot nou toe
Volgens die beste huidige teorieë vir spallasie, moet die gemete hoeveelheid Beryllium in die loop van 200 & # 8211 300 miljoen jaar opgehoop het. Die Italiaanse sterrekundige Daniele Galli, 'n ander lid van die span, doen die berekening: & # 8220So weet ons nou dat die ouderdom van die Melkweg soveel meer is as die ouderdom van daardie bolvormige groep & # 8211 ons sterrestelsel dus 13.600 moet wees? 800 miljoen jaar oud. Dit is die eerste keer dat ons 'n onafhanklike bepaling van hierdie fundamentele waarde verkry! & # 8221.

Binne die gegewe onsekerhede pas hierdie getal ook baie goed by die huidige skatting van die ouderdom van die Heelal, 13 700 miljoen jaar, dit is die tyd wat verloop het sedert die oerknal. Dit blyk dus dat die eerste generasie sterre in die Melkwegstelsel gevorm is teen die tyd dat die & # 8220Dark Ages & # 8221 geëindig het, wat nou vermoedelik ongeveer 200 miljoen jaar na die oerknal was.

Dit wil voorkom asof die stelsel waarin ons woon inderdaad een van die & # 8220stounding & # 8221 lede van die sterrestelselbevolking in die heelal kan wees.

Meer inligting
Die navorsing wat in hierdie persverklaring aangebied word, word bespreek in 'n referaat getiteld & # 8220Be in turn-off stars of NGC 6397: early Galaxy spallation, cosmochronology and cluster formation & # 8221 deur L. Pasquini en mede-outeurs wat in die Europese publikasie gepubliseer sal word. navorsingstydskrif & # 8220Astronomy & amp Astrophysics & # 8221 (astro-ph / 0407524).


Hallo, ek is verward oor hoe om die ouderdom van die heelal met rooi verskuiwing te bereken
sê byvoorbeeld

Die ouderdom van die heelal is nou 13 miljard jaar oud (en 'n kritieke heelal).
Hoe vind ek die ouderdom van die heelal as dit 'n rooi verskuiwing was op sê 10 ??

Moet ek eers die skaalfaktor vind?
Ek is nie baie seker nie, help asseblief !!

twee goeie aanlyn kosmologiese sakrekenaars:

Siobahn Morgan s'n
http://www.earth.uni.edu/

tuisblad vir Siobahn as u wil sien wie sy is
http://www.earth.uni.edu/smm.html
tuisblad vir Ned vir ingeval u wil sien wie hy is
http://www.astro.ucla.edu/


jy het gevra oor rooi verskuiwing z = 10

Die antwoord wat u moet kry, as u die beste ramings van vandag se parameters opstel, is 0,48 miljard jaar

dit wil sê as jy lig uit 'n sterrestelsel sien en daardie lig rooi verskuiwing is 10
toe is dit deur die sterrestelsel uitgestraal toe die heelal net ongeveer die helfte was
'n miljard jaar oud

Hallo, ek is verward oor hoe om die ouderdom van die heelal met rooi verskuiwing te bereken
sê byvoorbeeld

Die ouderdom van die heelal is nou 13 miljard jaar oud (en 'n kritieke heelal).
Hoe vind ek die ouderdom van die heelal as dit 'n rooi verskuiwing was op sê 10 ??

Moet ek eers die skaalfaktor vind?
Ek is nie baie seker nie, help asseblief !!

Sê net as u hulp nodig het om die sakrekenaars te gebruik.

hulle word albei deur astronomieprofessore opgestel om hul studente te help.

die maklikste om te gebruik is Ned Wright's

gaan net daarheen, plaas 10 in die z-boks, moenie iets anders verander nie
en druk & quotgeneral & quot
dit sal die antwoord 0,482 miljard jaar gee

Siobhan Morgan's is egter lekker om mee te speel, want sy gee ook resessiesnelhede, wat nie doen nie.
met haar sakrekenaar moet u 0.27 tik vir Omega (materie-breuk) en 0.73 vir Lambda (kosmologiese konstante of donker-energie-breuk) en 71 vir die Hubble-parameter. sit dan z = 10 in.

ned wright het al hierdie standaardwaardes van die kosmologiese parameters vir u ingestel, sodat hy u minder werk laat doen.
albei sakrekenaars gee dieselfde antwoord, soos u sou verwag


Dit is hoe sterrekundiges die ouderdom van die heelal ken (en jy ook kan)

Ons hele kosmiese geskiedenis word teoreties goed verstaan, maar slegs omdat ons die. [+] gravitasieteorie wat daaraan ten grondslag lê, en omdat ons weet wat die heelal se huidige uitbreidingstempo en energiesamestelling is. Lig sal altyd voortgaan om voort te plant deur hierdie uitbreidende Heelal, en ons sal daardie lig na willekeur bly ontvang tot in die toekoms, maar dit sal mettertyd beperk wees tot wat ons bereik. Ons het nog steeds onbeantwoorde vrae oor ons kosmiese oorsprong, maar die ouderdom van die heelal is bekend.

Nicole Rager Fuller / National Science Foundation

Konseptueel lyk dit na die eenvoudigste idee om die ouderdom van die Heelal te bepaal. Sodra u agterkom dat die Heelal uitbrei, hoef u slegs die uitbreidingstempo van vandag te meet en die wette van die fisika te gebruik om te bepaal hoe die uitbreidingstempo met verloop van tyd moes verander. In plaas daarvan om vorentoe te ekstrapoleer om die lot van die Heelal te bepaal, doen u die berekening in plaas daarvan en gaan u terug tot u die voorwaardes van die warm oerknal self bereik.

Hierdie vanselfsprekende metode werk nie net nie, maar dit bly die beste manier om die wêreld se ouderdom te bereken, selfs vandag nog. Tog is dit baie maklik om verkeerd te gaan, want daar is baie vereenvoudigende aannames wat u kan gee om u 'n maklike antwoord te gee wat nie noodwendig korrek is nie, insluitend foute wat selfs 'n Nobelpryswenner vroeër vanjaar gemaak het. Hier is hoe u ook die ouderdom van die heelal kan vasstel.

Standaard kerse (L) en standaard liniale (R) is twee verskillende tegnieke wat sterrekundiges gebruik om te meet. [+] die uitbreiding van die ruimte op verskillende tye / afstande in die verlede. Op grond van hoe groothede soos helderheid of hoekgrootte met afstand verander, kan ons die uitbreidingsgeskiedenis van die heelal aflei. Die gebruik van die kersmetode is deel van die afstandsleer en lewer 73 km / s / Mpc. Die gebruik van die liniaal is deel van die vroeë seinmetode en lewer 67 km / s / Mpc.

Die eerste plek om te begin is met die uitbreidende heelal self en die een parameter wat ons langer as enige ander probeer meet: die Hubble-konstante. Op die grootste skaal gehoorsaam die sterrestelsels wat ons in die heelal vind, 'n baie eenvoudige verband tussen die twee waarneembare hoeveelhede afstand en rooi verskuiwing, hoe verder 'n voorwerp van ons af is, hoe groter sal die gemete rooi verskuiwing wees.

Opvallend is dat die wet wat dit verband hou uiters eenvoudig is: die resessiesnelheid wat u van die rooiverskuiwing van 'n sterrestelsel sou aflei, is gelyk aan die afstand tot daardie sterrestelsel vermenigvuldig met die Hubble-konstante. Nog merkwaardiger is dat die konstante dieselfde waarde het vir bykans elke sterrestelsel wat ons meet, veral vir sterrestelsels binne enkele miljard ligjare van ons. Al is daar ekstra kosmiese bewegings wat inherent is aan elke sterrestelsel wat deur swaartekrag-effekte veroorsaak word, bly hierdie wet waar as u al die sterrestelsels wat u kan vind, gemiddeld gebruik.

Die verhouding tussen rooi verskuiwing en afstand vir sterrestelsels in die verte. Die punte wat nie presies op die. [+] lyn het die effense wanverhouding te danke aan die verskille in eienaardige snelhede, wat slegs geringe afwykings bied van die algehele waargenome uitbreiding. Die oorspronklike data van Edwin Hubble, wat die eerste keer gebruik word om aan te dui dat die heelal besig was om uit te brei, pas alles in die klein rooi blokkie links onder.

Robert Kirshner, PNAS, 101, 1, 8-13 (2004)

Wat meet ons dan die Hubble-konstante? Dit hang af van hoe u dit meet, aangesien:

  • as u dit meet deur seine te gebruik wat in die vroegste stadiums van die oerknal ingeprent is, kry u 'n waarde vir die Hubble-konstante van 67 km / s / Mpc, met 'n onsekerheid van 1-2%,
  • maar as u dit meet deur individuele ligbronne te meet wat eers arriveer voordat die heelal miljarde jare oud is, verkry u 'n waarde vir die Hubble-konstante van 73 km / s / Mpc, met 'n onsekerheid van net 2-3% .

Waarom hierdie twee waardes nie ooreenstem nie - en waarom hulle sulke verskillende, onderling teenstrydige antwoorde gee - is een van die belangrikste raaisels van die moderne kosmologie.

'N Reeks verskillende groepe wat die uitbreidingstempo van die Heelal wil meet, tesame met hul. [+] kleurgekodeerde resultate. Let op hoe daar 'n groot verskil is tussen vroeë (top twee) en laat (ander) resultate, met die foutbalke baie groter vir elk van die laat-tydopsies. Die enigste waarde wat onder skoot gekom het, is die CCHP-waarde, wat herontleed is en gevind is dat dit 'n waarde nader aan 72 km / s / Mpc as 69,8 het.

L. Verde, T. Treu en A.G. Riess (2019), arXiv: 1907.10625

Die skerpsinnigste onder u sal egter iets opmerk oor die Hubble-konstante self: dit kom in eenhede wat 'n snelheid (km / s) per eenheidseenheid het (Mpc, waar 1 megaparsek ongeveer 3,26 miljoen ligjare is). As u na 'n sterrestelsel kyk wat 100 Mpc weg is, sou u verwag dat dit tien keer vinniger sou wegtrek as een slegs 10 Mpc daarvandaan, maar slegs een tiende so vinnig soos 'n sterrestelsel van 1 000 Mpc verder. Dit is die eenvoudige krag van die verhouding tussen rooi skuif en afstand.

Maar daar is 'n ander manier om die Hubble-konstante te manipuleer: om te herken dat 'n snelheid (afstand per tyd) per (gedeel deur) eenheidsafstand (afstand) dieselfde is as eenhede van omgekeerde tyd. Waarmee kan die fisiese betekenis van daardie 'omgekeerde tyd' ooreenstem? Miskien kan u dit redelikerwys voorstel dat dit ooreenstem met die ouderdom van die Heelal.

Die verskillende moontlike lotgevalle van die Heelal, met ons werklike, vinnige lot aan die regterkant. . [+] Die besonderhede van die samestelling van die Heelal beïnvloed die ouderdom van die Heelal, soos u kan sien deur te kyk na die 'beginpunt' wat in verskillende kosmologieë op verskillende waardes in die verlede voorkom, selfs met presies dieselfde uitbreidingsnelheid vandag.

Daar is ongeveer 3,1 × 10 19 kilometer in een megaparsek, wat beteken dat as u die Hubble-konstant in 'n omgekeerde tyd verander, u fassinerende dinge vind.

  • Die 'tyd' waarmee 'n waarde van 67 km / s / Mpc ooreenstem, is gelyk aan 14,6 miljard jaar.
  • Die 'tyd' waarmee 'n waarde van 73 km / s / Mpc ooreenstem, is gelykstaande aan 13,4 miljard jaar.

Dit is albei amper gelyk aan die aanvaarde ouderdom van die heelal, maar nie heeltemal nie. Daarbenewens is hulle albei byna gelyk aan mekaar, maar verskil ongeveer dieselfde hoeveelheid as wat die twee ramings vir die Hubble-konstante verskil met: 9% of so.

U kan egter nie net die ouderdom van die heelal verander deur die Hubble-konstante te verander nie, en daar is 'n subtiele, maar belangrike rede waarom dit so is.

'N Foto van my by die American Astronomical Society se hipermuur in 2017, saam met die eerste. [+] Friedmann-vergelyking regs. Die eerste Friedmann-vergelyking beskryf die Hubble-uitbreidingsnelheid aan die linkerkant, wat die evolusie van die ruimtetyd beheer. Die regterkant bevat al die verskillende vorme van materie en energie, tesame met ruimtelike kromming (in die laaste kwartaal), wat bepaal hoe die Heelal in die toekoms sal ontwikkel. Dit word die belangrikste vergelyking in die hele kosmologie genoem, en is in 1922 deur Friedmann afgelei in sy moderne vorm.

Perimeter Institute / Harley Thronson

Die waarde van die Hubble-konstante van vandag is nie net die omgekeerde van die waarde van die ouderdom van die Heelal nie, alhoewel die eenhede u probeer meet. In plaas daarvan moet die uitbreidingsnelheid wat u meet - die Hubble-konstante van vandag - die somtotaal balanseer van elke vorm van energie wat bydra tot die samestelling van die Heelal, insluitend:

  • normale saak,
  • donker materie,
  • neutrino's,
  • bestraling,
  • donker energie,
  • ruimtelike kromming,
  • en enigiets anders wat jy kan gaarmaak.

Die vergelyking wat die uitbreidende heelal beheer (hierbo getoon) kan in enkele eenvoudige gevalle presies opgelos word.

Die skaal van die heelal, op die y-as, word as 'n funksie van tyd op die x-as geteken. Of. [+] die heelal bestaan ​​uit materie (rooi), straling (blou) of energie inherent aan die ruimte self (geel), dit neem af na 'n grootte / skaal van 0 as u agtertoe ekstrapoleer in die tyd. Die ouderdom van die heelal vermenigvuldig met die Hubble-konstante sal verskillende waardes vir universums wat uit verskillende komposisies bestaan, gelyk wees.

If your Universe is exclusively made up of radiation, you find that the Hubble constant multiplied by the age of the Universe since the Big Bang equals ½, exactly. If your Universe is exclusively made up of matter (normal and/or dark), you find that the Hubble constant multipled by the age of the Universe equals ⅔, exactly. And if your Universe is entirely made of dark energy, you'll find that there is no exact answer the value of the Hubble constant multiplied by the age of the Universe always continues to increase (towards infinity) as time goes on.

This means that if we want to accurately calculate the age of the Universe, we can do it, but the Hubble constant alone isn't enough. In addition, we also need to know what the Universe is made out of. Two imagined Universes with the same expansion rate today but made out of different forms of energy will have different expansion histories and, therefore, different ages from one another.

Measuring back in time and distance (to the left of "today") can inform how the Universe will evolve . [+] and accelerate/decelerate far into the future. We can learn that acceleration turned on about 7.8 billion years ago with the current data, but also learn that the models of the Universe without dark energy have either Hubble constants that are too low or ages that are too young to match with observations. If dark energy evolves with time, either strengthening or weakening, we will have to revise our present picture. This relationship enables us to determine what's in the Universe by measuring its expansion history.

Saul Perlmutter of Berkeley

So, to find out how old the Universe actually is since the onset of the hot Big Bang, all we have to do is determine the expansion rate of the Universe and what the Universe is made out of. There are a variety of methods that we can use to make this determination, but there's one vital thing we have to remember: many of the ways we have of measuring one parameter (like the expansion rate) are dependent on our assumptions about what the Universe is made out of.

In other words, we cannot assume that the Universe is made out of a certain amount of matter, a certain amount of radiation, and a certain amount of dark energy in a way that's independent of the expansion rate itself. Perhaps the most powerful way to illustrate this is to look at the leftover glow from the Big Bang itself: the Cosmic Microwave Background.

The leftover glow from the Big Bang, the CMB, isn't uniform, but has tiny imperfections and . [+] temperature fluctuations on the scale of a few hundred microkelvin. While this plays a big role at late times, after gravitational growth, it's important to remember that the early Universe, and the large-scale Universe today, is only non-uniform at a level that's less than 0.01%. Planck has detected and measured these fluctuations to better precision than ever before, and can use the fluctuation patterns that arise to place constraints on the Universe's expansion rate and composition.

ESA and the Planck collaboration

This, above, is a map of the fluctuations in the Cosmic Microwave Background. Overall, every direction in the Universe displays the same average temperature as every other direction: approximately 2.725 K. When you subtract that mean value out, you get the pattern that you see above: the fluctuations, or departures from the average temperature.

Where you see dark blue or dark red spots, those are regions where the temperature fluctuations are largest: approximately 200 microkelvin colder (for blue) or hotter (for red) than the mean value. These fluctuations exhibit particular patterns in their magnitude on a variety of angular scales, with the fluctuations rising in magnitude down to some particular angular scale of about 1 degree, then decreasing and increasing in an oscillatory fashion. Those oscillations tell us some vital statistics about the Universe.

Four different cosmologies lead to the same fluctuation patterns in the CMB, but an independent . [+] cross-check can accurately measure one of these parameters independently, breaking the degeneracy. By measuring a single parameter independently (like H_0), we can better constrain what the Universe we live in has for its fundamental compositional properties. However, even with some significant wiggle-room remaining, the age of the Universe isn't in doubt.

Melchiorri, A. & Griffiths, L.M., 2001, NewAR, 45, 321

What's most important to realize is that there are many possible combinations of values that can fit any particular graph. For example, given the fluctuations we see, we can have a Universe with:

  • 4% normal matter, 21% dark matter, 75% dark energy and a Hubble constant of 72,
  • 5% normal matter, 30% dark matter, 65% dark energy and a Hubble constant of 65,
  • or 8% normal matter, 47% dark matter, 49% dark energy, -4% curvature and a Hubble constant of 51.

You will notice a pattern here: you can have a larger Hubble constant if you have less matter and more dark energy, or a smaller Hubble constant if you have more matter and less dark energy. What's remarkable about these combinations, however, is that they all lead to almost exactly the same age for the Universe since the Big Bang.

There are many possible ways to fit the data that tells us what the Universe is made of and how . [+] quickly it's expanding, but these combinations all have one thing in common: they all lead to a Universe that's the same age, as a faster-expanding Universe must have more dark energy and less matter, while a slower-expanding Universe requires less dark energy and greater amounts of matter.

Planck Collaboration (maps and graphs), E. Siegel (annotations)

The reason that we can claim the Universe is 13.8 billion years old to such enormous precision is driven by the full suite of data that we have. A Universe that expands more quickly needs to have less matter and more dark energy, and its Hubble constant multiplied by the age of the Universe will have a larger value. A slower-expanding Universe requires more matter and less dark energy, and its Hubble constant multiplied by the age of the Universe gets a smaller value.

However, in order to be consistent with what we observe, the Universe can be no younger than 13.6 billion years and no older than 14.0 billion years, to more than 95% confidence. There are many properties of the Universe that are indeed in doubt, but its age isn't one of them. Just make sure you take the Universe's composition into account, or you'll wind up with a naive — and incorrect — answer.


How do scientists determine the ages of stars? Is the technique really accurate enough to use it to verify the age of the universe?

"Astronomers usually cannot tell the age of an individual star. There are certain stars that we know are very young, and others that are very old, but for most stars we cannot tell. When we have a large group of stars, however, we can tell its age. This is possible because all of the stars in a cluster are presumed to have begun their life at approximately the same time. After a relatively brief time (in 'star time,' that is--we are talking thousands to millions of years here) stars reach the adult phase of their life, which we call the main sequence phase. The length of time a star spends in the main sequence phase depends on its mass.

"Constructing a plot, called the HR diagram, of the stars in the cluster, scientists can determine the mass of the stars that are just ending this phase and moving on to the next phase of their life, the red giant phase. Computer models allow us to predict how old a star of that mass must be to be at that juncture of its life, and hence to estimate the age of the cluster. Recently, this procedure has come under close scrutiny because that age it gives for the oldest star clusters in our Milky Way seems to be older than the age of the universe derived from the most recent Hubble Space Telescope data."

Peter B. Stetson, senior research officer at the Dominion Astrophysical Observatory in Victoria, British Columbia, provides a more detailed reply:

"It is impossible to determine the age of a single star all by itself. The only real means we have to determine stellar ages is through the study of star clusters. In our galaxy, the Milky Way, there are two basic types of star cluster. Clusters of the first type are called 'globular clusters' because they appear as huge, round globs containing anywhere from a few thousand to a few million stars. Globular clusters are very old, and they are scattered around (not just within) the Milky Way these clusters seem to have originated near the time our galaxy started to form, when the universe was quite young. Clusters of the second type used to be called 'galactic clusters' because we see them inside the body of our galaxy, but now it is more common to refer to them as 'open clusters' because they are much looser and their stars more spread out on the sky than are those in globular clusters. Open clusters can contain anywhere from a few dozen to a few thousand stars, and they come in a wide range of ages. Apparently our galaxy started making open clusters soon after it settled down to its present size and continues making them even today.

"The stars in either type of star cluster were all formed at the same time and out of the same material. The essential feature of a star cluster that lets us estimate its age is that each cluster contains stars with a range of masses. When a cluster is born, it will contain many stars of about the same size and mass as our sun, but there will also be numerous stars more massive than our sun and many other stars less massive than our sun. For about 90 percent of its lifetime, a star shines because nuclear reactions are converting hydrogen to helium in the star's center, releasing vast amounts of energy. This energy works its way from the center of the star to the surface and escapes the star in the form of light. The more massive a star is, the bigger the furnace in the center, and the brighter and the hotter the star is in this stable stage of its life. The most massive stars are very bright and blue-hot a less massive star is somewhat fainter and white-hot a star like our sun is a bit fainter still and is yellow-hot and the least massive stars are very faint and merely red-hot. During this period of its life, a star hardly changes either in brightness or in temperature.

"The duration of the stable, or 'main sequence,' phase depends on a star's mass. A star 10 times as massive as the sun contains, clearly, 10 times as much fuel. It consumes that fuel roughly 10,000 times faster than the sun, however. As a result, it has a total lifetime 1,000 times shorter than that of our sun. When the hydrogen fuel in the center of a massive star is exhausted--'the center' representing about 10 percent of the star's total mass--it becomes increasingly unstable. The star remains bright, but it quickly switches from being comparatively small and hot to being huge and red for a while, then it briefly becomes smaller and bluer, then even larger and even redder, and finally explodes as a supernova, spewing its nuclear ashes as well as its unburned fuel back into space. Similarly, a star five times more massive than the sun has a lifetime roughly 100 times shorter than the sun before it becomes unstable and ends its active life. A star like our sun is calculated to have a total stable life-span of around 10 billion years the sun is now a bit less than half that age (this age is very accurately determined from radioactive elements in meteorites), so we have another five billion years or so before we have to start looking for a new home.

"In the case of a single star, its brightness and temperature don't tell us much. Because these properties stay fairly constant for 90 percent of its lifetime, the star could be fairly young or fairly old, and we wouldn't be able to tell the difference. In a star cluster, we have the advantage that stars of all masses formed at about the same time. So all we have to do is look at the cluster and determine how hot and how massive is the hottest, bluest, most massive star that has not yet entered the late, unstable period of its life. The star's mass tells us how much fuel the star had when it was born, and the star's brightness tells us how fast it is burning that fuel. We know that the star is just about to start becoming unstable--after all, the stars that are more massive have already started to become unstable. We also know that its fuel is just about exhausted. The ratio of how much fuel the star had in the beginning to how fast it has been burning that fuel tells us how long the star has been alive. (By analogy, if we know how much kerosene our hurricane lamp contained when we lit it and how fast it consumes the kerosene, and if the lamp is just now starting to go out, then we can deduce how long it has been lit.) Because all the stars in the cluster are the same age, the age of that one star tells us the age of the entire cluster.

"The basic physics of how hydrogen is converted to helium in the centers of stars and the amount of energy generated by this process is comparatively simple and well understood. For much of the 20th century, the main limitation to our knowledge of stellar ages has been due to the difficulty of measuring the distances to the clusters--especially the distances to the oldest clusters, the globulars, which are comparatively far away. (We know how bright a star looks, but to know how bright it really is, you have to know how far away it is: is it like a headlight a mile away or an airport beacon 10 miles away? In the dark of the nighttime sky with no reference points, it's pretty hard to tell.) Technical advances, such as the introduction of charge-coupled devices to replace photographic plates for the measuring of stellar distances and brightnesses, are making our observations more secure.

"Distance measurements have improved to the point at which other details needed to determine the ages of star clusters--such as the fine details of how a star converts nuclear energy to visible light--can no longer be ignored. How exactly does the energy get from the center of the star, where it is generated, to the surface, where it becomes the light that we see? How important is convection as a means of transporting energy, and how efficient is the convection? The answer to these questions has some effect on the inferred relationship between mass and surface temperature. Just how much oxygen is in the stars, along with the hydrogen and helium? The relative amount of oxygen present has a modest effect on the efficiency of the central furnace, affecting the relation between mass and brightness and, hence, age.

"Taken together, the uncertainty in the observations and the uncertainty in the relevant theoretical physics probably lead to an uncertainty of 10 percent to 20 percent in our estimate of the absolute ages of the globular clusters. According to our best available estimates, stars having about 90 percent of the sun's mass are just now starting to die in the globulars. These stars are most probably around 15 billion years old, but they could conceivably be as young as 12 billion years or as old as 18 billion years. It is very unlikely that most of them could be either younger or older than this range. This estimate is already accurate enough to place some very interesting limits on the age and life history of the universe."


Inleiding

Astronomers and geologists have determined that the universe and Earth are billions of years old. This conclusion is not based on just one measurement or one calculation, but on many types of evidence. Here we will describe just two types of evidence for an old Earth and two types of evidence for an old universe more types can be found under further reading. These methods are largely independent of each other, based on separate observations and arguments, yet all point to a history much longer than 10,000 years. As Christians, we believe that God created the world and that the world declares his glory, so we can’t ignore what nature is telling us about its history.


How do we really know how old the universe is?

Once you figure out how fast distant galaxies are moving and how far away they are, then you can calculate when the Big Bang occurred.

After a fortnight gallivanting around Europe and being more creative in my modes of transport than expected thanks to an unpronounceable mountain in Iceland, I’m back in Hawaii. On the flight to LA I ended up chatting to a bloke who works for a large US computer firm about various geeky things. The “what do you do?” question came up, and given he seemed worth talking to I opted for astronomer rather than physicist.

Then at a lull in the conversation he volunteered the question, “how do they know how old the universe is?” We’ve been planning to add a “How do we know?” category to the blog so this seems like the perfect place for me to start.

The simplest measure of the age of the universe is known as the Hubble Time. The universe is expanding, we know this because we can see that light from distant galaxies is Doppler Shifted towards redder wavelengths, indicating they are moving away from us. The further away the galaxy, the faster it moves away. This is known as Hubble’s Law.

The rate at which the recession velocity of a galaxy increases with its distance for us is known as the Hubble Constant. If we know how fast the universe is expanding, we can extrapolate back and see when the universe would have a size of zero, ie. when the big bang happened. Of course if we know how long ago the big bang was, we know roughly how old the universe is.

So all we need is the Hubble Constant, easy yeah, erm not really. The value of the Hubble Constant was for half a century the subject of great dispute. This period was known as the Hubble Wars which conjures up massed ranks of Welsh longbowmen cutting down the flower of French chivalry to establish domination over the fundamental constants of the universe. In reality it was a debate about measuring the distance to far off galaxies.

Getting recession velocities of galaxies is pretty easy, but to find the Hubble constant, you’ve got to know the distance of each galaxy too. Measuring distances in astronomy is pretty hard, something we might deal with later in this series, so various novel techniques must be used.

When Edwin Hubble first worked on the recession velocities and distance of galaxies in the 1920s and 30s he used a fortuitously odd type of star as a “standard candle”. In astronomy, if you know how much light a star or galaxy puts out in total and how much we receive on Earth, you can combine these to get how far away it is. Hubble used unstable stars which have finished their main life as a normal star, known as Cepheid variables. They pulsate, and so vary in brightness, and the really lucky bit is that the pulsation rate is related to the total light emitted by the star.

So the pulsation period gives the total light emitted - combine this with the apparent brightness and you get the distance. The problem is that you need a pretty powerful telescope to resolve individual stars in distant galaxies. Even with the largest telescope available in the middle of the last century, the 5m Hale Telescope on Palomar, only relatively nearby galaxies can have their Cephieids resolved from the mass of other stars. So astronomers had to get creative.

This doesn’t mean they went off and played guitar in Queen, Coldplay or, (as rumoured in the case of one astronomy blogger) the opening act for The Velvet Underground. Science itself is a creative process, trying to dream up innovative solutions to work around the limitations of the available technology and data. The work mostly rested on calibrating a myriad of new distance indicators using local galaxies at known distances and applying these new estimates to more distant objects.

The results fell into two broad camps, one side led by the American astronomer Allan Sandage claimed a value of about 50 (I won’t go into the slightly obtuse units used for this measurement) and another led by the French cosmologist Gerard de Vaucouleurs claimed a value of about 100. For decades they fought over seeming minor points that shifted one particular rung on the intricate astronomical distance ladder up or down. From how dust in our own Galaxy affects the measured brightnesses of distant galaxies to subtle biases in samples of galaxies to the brightness of exploding stars, no point in the other group’s work was too minor to pick apart.

Fast forward to the end of the last century and say hello to the now 20-year-old Hubble Space Telescope. One of its key projects was to pick up where its namesake left off and find the Hubble Constant using Cepheid variables in more distant galaxies. After a huge amount of effort it came out with a result of about 72, giving an Hubble Time of roughly 13.8 billion years.

This fits in fairly well with the estimated ages of the oldest stars. More recent measurements, such those from the WMAP study of ripples in the cosmic microwave background and more up to date supernovae studies have supported a value of roughly 70. However they also predict the expansion of the universe is accelerating, meaning our simple extrapolation, assuming constant expansion won’t give exactly the right answer.

I didn’t say all this to the bloke on the plane, we were about to land so I didn’t have much time, but I hope I got it across fairly well both to him and you.


How AstronomySupports Evolution

A recent Pew survey has found that one third of Americans believe that humans and other living things have existed in their present form since the dawn of time. That’s one third of the adult population who reject evolution, which is the bedrock theory of biology. Indirectly, they also reject the foundations of geology, physics and astronomy. Much of the commentary about this survey has focused on the religious and political correlations, but let’s look at the science behind the ideas. If evolution is correct (and it is) then it must have occurred over billions of years, not a mere 10,000 or so. So how do we know — really, really know — that the Universe is billions of years old? It all comes down to a bit of astronomy.

/>NASA It&rsquos taken 10,000 years just for the light in the yellow circle to reach us.

One way we determine the age of the Universe is through cosmic distances. Since light travels at a finite speed, the light from distant objects takes time to reach us. The more distant the objects we can see, the older the Universe must be. So how far does 10,000 years get you? Not very far, as you can see in the figure above. For anything outside the yellow circle, the light has taken longer than 10,000 years to reach us. If the Universe was only 10,000 years old, we wouldn’t yet see anything beyond that circle. The faint glow of the Milky Way in a dark sky? Most of it would be missing. The Large Magellanic Cloud? Totally gone. The Andromeda galaxy? Not a chance. The night sky of a young Universe would be darker, and not nearly as interesting.

So how do we know our distances are correct? There are actually several methods to determine cosmic distances, and these are combined to create what is known as the cosmic distance ladder. The most direct method uses the property of parallax. Parallax occurs when you look at an object from two slightly different positions. You probably use it every day, because it is what gives humans depth perception. When you look at an object, each of your eyes has a slightly different point of view. Your brain uses this information to determine which objects are close and which are farther away. This is also why you have to wear special glasses when you go to see a 3D movie. The glasses ensure that your eyes each get a slightly different perspective, which gives the movie the illusion of depth. If you take off the glasses during the movie, it will look slightly blurry. Without the glasses, your eyes see both points of view blurred together.

/>NASA, ESA, and A. Feild Determining the parallax of a star.

You can see the effect of parallax with a simple experiment. Hold up your thumb at arm’s length, and look at it with only one eye. Without moving your thumb, switch eyes, and you will see that your thumb appears to move relative to more distant objects. This shift is known as a parallax shift. If you bring your thumb closer and do the experiment again, you’ll see that the parallax shift is larger. If it is farther away, the parallax shift is smaller.

With a little bit of trigonometry, you can calculate the distance to an object by measuring its parallax. This is how astronomers can measure the distances to nearby stars, using the motion of the Earth to their advantage. The radius of the Earth’s orbit about the Sun is 150 million kilometers. By observing the position of a star on a particular night, and then on a night months later, astronomers can measure the parallax shift of the star from two points of view. The bigger the parallax shift, the closer the star. The recently launched Gaia spacecraft can measure parallax with a precision of a few microarcseconds, which gives us the ability to measure stellar distances up to 30,000 light years away with an accuracy of 10%.

Beyond that distance parallax is too small to be of use, so we can use another method looking at a type of star known as a cepheid variable. Cepheid variables are stars that vary in brightness over a period of days. The first such star to be observed was Delta Cephei in 1784 (the fourth brightest star in the constellation of Cepheus), hence the name. For nearby Cepheids, we can determine their distance via parallax. We can also determine their apparent magnitude (how bright they appear), and given their distance we can determine their absolute magnitude (how bright they actually are) using the fact that the brightness of an object decreases with distance following what is known as an inverse square law.

/>NASA / JPL-Caltech / Carnegie The period brightness relation for Cepheids.

In the early 1900s astronomer Henrietta Leavitt analyzed more than 1700 variable stars to discover the luminosity-period relation for Cepheid variables. By looking at Cepheids in a particular Magellanic cloud she was able to demonstrate a linear relationship between absolute brightness (luminosity) and period, such as seen in the figure above. This meant Cepheids could be used as “standard candles”. By observing their variable period, we can determine their absolute brightness. Comparing this to their apparent brightness, we can determine their distance. From the Hubble telescope we have observations of Cepheid variables in lots of nearby galaxies, for which we can measure galactic distances out to about 100 million light years.

Beyond this distance, Cepheid variables are too faint to use accurately, so we need another method. This is often done with another class of standard candle known as a Type Ia Supernova. This type of supernova can often occur when two white dwarfs are in close orbit with each other. A white dwarf is formed when a Sun-sized star begins to run out of hydrogen to fuse in its core. The star fuses helium for a while, causing it to swell into a red giant. Depending on its mass, a star will fuse some higher elements in its core, and the resulting heat and light drives off much of the outer material of the star, but there comes a point where the star simply can’t keep fusing higher elements. After this, what remains of the star compresses down to a white dwarf. In a white dwarf it isn’t the heat and pressure of fusion that balances against the weight of gravity, but the pressure of the electrons pushing against each other. Type Ia Supernova are typically caused by a collision or merger of two white dwarfs. If the two stars are in a close binary orbit, particularly with a third star orbiting at as part of a trinary system, the orbits of the white dwarfs can degrade to the point where they collide, resulting in a supernova explosion.

What makes these type of supernovae particularly interesting is that they always have about the same brightness. We’ve observed Type Ia Supernovae in galaxies whose distance was already known from the Cepheid variables. We can observe how bright the supernovae appear, and knowing their distance we can determine how bright they actually are. What we find is that Type Ia Supernovae always have the same luminosity.

This property means we can use them as a standard candle as well. If we observe a Type Ia Supernova in a distant galaxy, we can observe how bright it appears. Since we know how bright it actually is, we can calculate the distance to the galaxy, since the more distant a light source is, the dimmer it appears. We can therefore use this type of supernova to measure the distance to its galaxy. This allows us to measure cosmic distances of billions of light years.

Now, as a skeptic you might point out that all I’ve done is shown that the Universe is large, not that it is old. Sure, the light of distant galaxies might take billions of years to reach us now, but what if the speed of light were much faster in the past? How do we know that the speed of light hasn’t changed over time?

One of the things we can do is look at the emission and absorption spectra of atoms and molecules in distant stars, nebulae and galaxies. The patterns of these spectra allow us to identify these atoms and molecules, like a kind of fingerprint. But they also allow us to test whether physical constants have changed over time. Not just the speed of light, but the charge of the electron, Planck’s constant and others. If any of these constants had changed over time, the lines in a spectrum would shift relative to each other. The pattern would spread apart in some areas and scrunch together in others. When we look at distant objects, we find no such shift in any of them. Given the limits of our equipment, this means the speed of light can have changed no more than one part in a billion over the past 7 billion years. As far as we can observe, the speed of light has always been the same.

So this gives us confidence in a wonderful aspect of observational astronomy. When you look at more and more distant objects, you are also looking further back into time. But we can take that idea one step further, because not only do we know the Universe is old, we know just how old it is using the Doppler effect. The observed color of light can be affected by the relative motion of its source. If a light source is moving toward us, the light we see is more bluish than we would expect (blueshifted). If a light source is moving away from us, the light is more reddish (redshifted). The faster the source is moving, the greater the shift.

/>Right: Robert P. Kirshner Left: Edwin Hubble The Hubble relation for galaxies.

We’ve measured this color shift for lots of stars, galaxies and clusters, and when we plot a graph of the distance of galaxies versus their redshift we find an interesting relation, seen above. The greater a galaxy’s distance, the greater its redshift. This means galaxies are not simply moving at random, as you would expect in a stable, uniform Universe. Instead, the more distant the galaxy the faster it is moving away from us. This relation between distance and speed is the same in all directions, which means the Universe seems to be expanding in all directions. Of course if the Universe is expanding, then it must have been smaller in the past. In other words, the Universe has a finite age, and it began very small, very dense (and therefore very hot). We call that starting point the Big Bang. If you do the math, you get an age of about 13.8 billion years.

Of course the story I’ve told here is just one path to the age of the Universe. We have lots of other observational evidence such as the cosmic microwave background, stellar evolution, baryon acoustic oscillations, and the hydrogen/helium ratio, to say nothing of planetary science, geology, and biology. This confluence of evidence points to a Universe that is not thousands, but billions of years old.

There was a time when the idea of a small, young Universe seemed reasonable. We now know that it is far older and far more wondrous than we ever expected. 1

This post was originally written as a guest post for Ethan Siegel’s Starts With A Bang! ↩︎