2019-07-01_Australian_Sky_&_Telescope

16 AUSTRALIAN SKY & TELESCOPE July 2019

observedperiodtellsyouhowluminous

it is,andthestar’sapparentbrightness

thenrevealsthegalaxy’sdistance.

Usingtheeagle-eyedvisionofthe

HubbleSpaceTelescope(HST),which

wasdesignedinpartforthiswork,a

teamledbyWendyFreedman(nowat

theUniversityofChicago)succeededin

identifyingCepheidvariablesinspiral

galaxiesat distancesofhundredsof

millionsoflight-years.“Thefinalresults

ofourKeyProject,publishedin2001,

yieldeda Hubbleconstantof 72 km/s/

Mpc,”shesays,“buttheuncertainty

inthatvaluewassome10%”.Still,this

wasa hugeachievement:Beforethe

launchoftheHSTinApril1990,the

bestestimatesforH 0 rangedfrom 50 to

100 km/s/Mpc.Moreover,theHubble

resultsenabledastronomerstocalibrate

otherdistanceindicatorsthatcould

beusedfartherout,whereindividual

Cepheidscan’tbeseenanymore.

Oneofthosestandardcandlesare

TypeIa supernovae— thecatastrophic

detonationsofwhitedwarfstarsthat

becometoomassivetoresisttheirown

gravity,eitherbyaccretingmatter

froma companionstarorbymerging

withanotherwhitedwarf.Sincethe

temperatureanddensityat whicha

whitedwarfsuccumbstogravitational

collapseis usuallythesame,TypeIa

supernovaeexplodeandfadeina

standardpattern,andfromtheselight

curvesit’sprettystraightforwardto

deducetheirtrueluminosity.Once

calibrated,a comparisonwiththe

observedapparentbrightnessyieldsa

distanceestimate.(Someotherways

ofdeterminingcosmicdistancesare

describedintheboxesonpages20–21.)

UsingTypeIa supernovaeasstandard

candles,twoindependentteamsof

astronomersmadea startlingdiscovery

in1998:EventhoughthevalueofH 0

wasnotyetknowntoa highlevelof

precision,theobservationsofreally

remotegalaxiesrevealedthatthe

cosmicexpansionrateisn’tslowing

down,ashadalwaysbeenassumed,

butis actuallyspeedingup,despitethe

mutualgravitationalattractionofall

matterintheuniverse.Thismomentous

ery,forwhichSaulPerlmutter

rsityofCalifornia,Berkeley),

Riess(JohnsHopkinsUniversity)

ianSchmidtreceivedthe 2011

PrizeinPhysics,is nowseenas

evidencefora mysteriousdarkenergy,

hetruenatureofwhichis oneofthe

biggestmysteriesinscience.

Cosmologycrisis

Noonerealisedit at thetime,butthe

discoveryoftheacceleratedexpansion

oftheuniversegerminatedthecurrent

crisisincosmologythatwasthetopic

oftheBerlinsymposium.Notbecause

theconceptofdarkenergyis somehow

deficient,butbecauseit workstoowell.

Overthepast 20 years,astronomers

havecometorealisethatwelive

ina weirduniverse,dominatedby

darkenergy(denotedbytheGreek

letterlambda,Λ) and(cold)dark

matter(CDM),whichis everybit

asmysterious.Butalthoughwe

don’tknowthetruenatureofthese

enigmaticcomponents,theΛCDM

modeloftheuniversesuccessfully

accountsforallkindsofcosmological

observations,includingthelarge-scale

clusteringpropertiesofgalaxies.

Inparticular,theΛCDMmodel

appearstobetheonlyonethatis

compatiblewiththeobservedproperties

ofthecosmicmicrowavebackground.

Thestatisticaldistributionofthe‘hot‘

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T DOES T

E EXPAND

velo

from

imag

space itself is expanding, thereby

pushing the galaxies away from one

another. As a result, the distance

between two galaxies increases

over time. The more space there

is between the two galaxies, the

faster their mutual distance is

growing, in kilometres per second.

In other words, there’s no single

‘velocity value’ for the expansion of

the whole universe – it all depends

on the scale you consider. It’s like

monetary inflation: The inflation rate

for 2018 cannot be expressed in

dollars (that would only work for a

particular sum of money), but must

always be given as a percentage, or

a proportionality constant.

Not that the universe is expanding

very quickly. In fact, cosmic

distances increase by only some

0.01% in 1.4 million years. In other

words, if the current distance to

a remote galaxy is 100 million

light-years, it will increase by one

light-year every 140 years or so. A

‘recession’ velocity of one light-

year per 140 years corresponds to

about 2,150 kilometres per second

(7.8 million kph). But a galaxy at

a distance of 200 million light-

years appears to ‘recede’ twice

as fast, at some 4,300 kilometres

per second. Thus, the ‘recession’

velocity is growing by 21.

kilometres per second for every

additional million light-years, or

by some 70 kilometres per second

for every additional megaparsec

(Mpc; 1 parsec equals 3.26 light-

years). There’s your proportionality

constant, H0: 70 km/s/Mpc.

DISTANCE: Astronomers spot a

‘standard candle’ star or supernova in

a distant galaxy. They calculate how far

away the galaxy has to be in order for the

star to look as faint as it does.

How astronomers calculate

H 0 from supernovae

H^0

FROM SUPERNOVAE: GREGG DINDERMAN /

S&T

, SPECTRA: VOLKER GAIBLER/HITS, SDSS, ESO;

EXPLOSION GRAPHIC: TATIANAZAETS / ISTOCK / GETTY IMAGES PLUS

COSMIC EXPANSION PUZZLE