http://www.skyandtelescope.com.au 17and‘cold’spotsintheCMB(infact,temperaturedifferences
oflessthana ten-thousandthofa degree),asobservedin
finedetailbytheEuropeanPlanckspacecraft,canonlybe
understoodif theuniverseis largelymadeupofdarkenergy
anddarkmatter.Fromthe 2018 finalPlanckdatarelease,
cosmologistsconcludethat68.4%ofthematter-energy
densityofouruniverseis accountedforbydarkenergy;26.5%
bydarkmatter(probablysomeas-yet-undiscoveredtypeof
elementaryparticle),andnomorethan4.9%byordinary
matter,consistingofatomsandmolecules.(Thesefractions
don’tquiteaddupto100%becauseofroundingandthestill-
uncertainneutrinomass,amongotherreasons.)
AsAntonyLewis(UniversityofSussex,UK)toldthe
Berlinaudience,thesecosmologicalparametershavenow
beenderivedsopreciselythatit’seasytodeducewhatthe
currentvalueoftheHubbleconstantshouldbe:67.4km/s/
Mpc,withanerrormarginoflessthan1%.(Thisdeduction
takesintoaccountthefactthatthecosmicexpansionrate
firstsloweddownbecauseoftheuniverse’sself-gravitybutis
nowspeedingupagain,becausedarkenergystartedtoprevail
some6 billionyearsago.)Andthereappearstobeverylittle
wiggleroom,saysColless.“It’shardtogetridof[thisresult]
withoutrunningintoallkindsofotherproblems.”
Thesamevalueis arrivedat bycompletelyindependent
resultsfromtheDarkEnergySurvey(DES).Carriedout
at theCerroTololoInter-AmericanObservatoryinChile,
DESlookedat theclusteringpropertiesofgalaxiesandat
weaklensing— thetiny‘shape-shifting’ofremotegalaxies
duetothelight-bendinggravityofforegroundgalaxiesand
clusters.Theseresultshaveanerrormarginofabout2%,says
DEStheoreticalcosmologistDraganHuterer(Universityof
Michigan).
Butthelackofwiggleroomintheseresultsis causing
a problemofitsown:Theydon’tjibewithupdated‘local’
measurementsofH 0 fromCepheidsandsupernovae.The
Laboratoryreference Redshift
Distantgalaxy
400 450 500 550 600 650 700 750 800
Distance(megaparsecs)H 0 = 73.5km/s/MpcVelocity(km/second)RATE: A plot of the galaxies’ distances versus
redshifts shows that farther galaxies recede faster.
(Simulated data shown.) The slope of the line is the
universe’s expansion rate.VELOCITY: The expansion of the universe shifts the
standard candle’s light to longer, redder wavelengths.
The amount of redshift tells astronomers the galaxy’s
apparent recession velocity.
2001 result from Freedman’s Hubble Key Project had a large
enough range of uncertainty that, at first, there didn’t seem
to be cause for concern. But over the past years, a team led
by Riess has arrived at a much more precise calibration of
the cosmological distance ladder, and at a correspondingly
much more precise value for the Hubble constant. The result:
73.5 km/s/Mpc, with an uncertainty of just 2.2%. “The value
hasn’t changed very much,” he says, “but the uncertainty has
come down significantly.”
To achieve this high level of precision, Riess’s ‘Supernova,
H 0 , for Equation of State of Dark Energy’ (SH0ES) teamdetermined the parallax, and thus the distance, of 50 Cepheid
variables in our own Milky Way Galaxy — a necessary step
in accurately calibrating the Leavitt Law. Subsequently, they
studied Cepheids in 37 galaxies in which Type Ia supernovae
had also been observed. Using the Cepheid distances of these
37 galaxies, the team then calibrated the standard candle
properties of Type Ia’s. Finally, they derived the Hubble
constant from observations of some 300 supernovae in more
distant galaxies, for which the redshift is a reliable measure
of the cosmological recession velocity.
“Our data set has been re-analysed by many, many
independent groups,” Riess says, “and they all arrive at the
same value” of 73.5 km/s/Mpc. That’s about 9% higher than
the value obtained by Planck. Given the precision of both
estimates, the statistical significance of this discrepancy is
3.8 sigma, according to Riess. That means the chance of the=
No one realised it at the time, but the discovery
of the accelerated expansion of the universe
germinated the current crisis.