2019-07-01_Australian_Sky_&_Telescope

18 AUSTRALIAN SKY & TELESCOPE July 2019

mismatchbeingsomestatisticalflukeis about1 in7,000.

Clearly,there’ssomethingamiss.

Hutereragrees.“TheHubbletensionis real,”hesays.

Nowwhat?

Whilesomescientistsstillthinktheremaybean

undiscoverederrorineitheroneofthetwoapproaches(or

maybeinboth!),mostbelievethattheresultsaresolid.But

thatdoesn’tmeantheyknowhowtoexplainthediscrepancy.

EvenverycreativetheoristslikeHarvard’sLoebarestumped.

“I triedtocomeupwitha solutiontopresentat the

symposium,”hesays,“butI havenothingnewtoreport.It’s

nota simpleproblemtosolve.”

Onething’sforsure:ThesuggestionthatourMilkyWay

Galaxymaysitina hugelocalvoid,whichwouldhavea

higher-than-averageexpansionrate,doesn’twork.“That

effectis muchtoosmall,”saysLoeb.“Moreover,it wouldalso

leaveanimprintonthecosmicmicrowavebackground.”

Riessagrees.“Ifcausedbya localvoid,thedifference

betweenthetwoestimatesofH 0 wouldbelessthana percent,

not9%,”hesays.Riessis veryconfidentaboutthehigh-H 0

resultsoftheSH0ESteam,partlybecausesomealternative

methodsofdeterminingtheHubbleconstantarriveat a

similarvalue.“Maybewe’rejustnotcreativeenough”tosolve

theriddle,hesays.

Onefar-fetchedpossibilitymightbethatdarkmatterhas

beendestroyingitselfovertime,weakeningitsabilitytoslow

theuniverse’sexpansion.Butcomingupwithviablemodels

ofdecayingdarkmatterhasturnedouttobedifficult.In

Berlin,theoreticalphysicistLisaRandall(HarvardUniversity)

presentedherverypreliminaryideasonwhatshecallsa

“quintessentialsolutiontotheHubblepuzzle,”inwhichdark

matterwouldbecomelessmassiveovertime.“But,”shesaid

at theconference,“ifthegapremainsaslargeas9%,this

can’tbethefinalsolution.It’sa verychallengingproblem to

address.”Sincethenshe’sbeenabletorevisethemodelto

provide“arathergoodfittoexistingdata”.

Anotherpotentialsolutionwouldbea slowchangein

the‘density’ofdarkenergyovercosmologicaltime.Recent

X-rayobservationsof1,600distantquasarsappearedto

indicatejustthat.However,“thejuryis definitelystillout,”

acknowledgesteamleaderGuidoRisaliti(Universityof

Florence,Italy).“We’llhavetolookat manymoremodels in

greatdetailbeforewecansolvethiscosmicconundrum.”

BothRandallandLoebarereluctanttoconsider

speculativetheoriesonmodifiedgravitythatmightsomehow

explainawaytheproblem.“Thatwouldbeunwarrantedby

commonsense,”saysLoeb.“Itwouldbelikekillinga flywith

anatomicbomb.”

Still,accordingtoFreedman,aninformalvoteamongst

scientistsduringa cosmologyconferenceinChicagoin

earlyOctober 2018 revealedthatthevastmajoritythought

thatsomeformof‘newphysics’willbeneededtosolve

themystery.“It’swhatcosmologistsreallyhope,”says

Huterer.“Whoknows,”commentsSchmidt,“theremightbe

somethingfundamentallywrongwithourinterpretationof

thecosmicmicrowavebackground.”

Temperature

fluctuations

(microkelvin

2 )

Angularseparation

STEP 1: Astronomers measure the patchiness

of the cosmic microwave background. (Data

from Planck shown.)

How astronomers calculate H 0 from the CMB

STEP 2: The map gives them the power spectrum,

which plots how large the temperature differences are

between two locations on the sky, depending on how

far apart those locations are.

H^0

FROM CMB: GREGG DINDERMAN /

S&T

, PLANCK DATA (2): ESA / PLANCK COLLABORATION, GALAXY

WEDGE: LEAH TISCIONE /

S&T

;^ EXPLOSION GRAPHIC: TATIANAZAETS / ISTOCK / GETTY IMAGES PLUS

COSMIC EXPANSION PUZZLE

“That would be unwarranted by common

sense,” says Loeb. “It would be like killing

a fly with an atomic bomb.”