dark energy not

8/9/2019 Dark Energy Not

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48 Scientific AmericAn A l 2009

I s, h gds voluos oggd by h slls dsps. I h16h uy, bsd o wh suk y o

hs opos s h so u o -lsl oos, Copus suggsd h Ehws o, , h o h uvs. Iou ow , oh voluo bg o uold11 ys go wh h dsovy o h l-g uvs. A y dvo h bghss oxplodg ss ld soos o olud hhy hd o d wh 70 p o h osososss o. All hy ould ll ws h sp slld wh subs ulk y ohoh pushs log h xpso o h uvsh h holdg bk. Ths subs b- kow s dk gy.

I s ow ov dd l, d h xso dk gy s sll so puzzlg h so os-

ologss vsg h udl posu-ls h ld h o ddu s xs hs pl. O o hs s h podu o hl voluo: h Cop ppl, hEh s o l o ohws spl po-so h uvs. I w dsd hs bsppl, supsgly d pu o whould ou o h obsvos gs.

Mos o us vy l wh h d hou pl s ohg o h y spk o-bg ypl s, sowh h dg o ohws uowohy glxy. I h ds

o uvs populd by bllos o glxsh sh ou o ou os hozo, w ldo blv h h s ohg spl o uqubou ou loo. Bu wh s h vd ohs os huly? Ad how would w b blo ll w were spl pl? Asoosyplly gloss ov hs qusos, ssugou ow yply suly obvous o w- o uh dsusso. To h o-o h w y, , hv spl loo h uvs s, o y, uhkbl. Nv-hlss, h s xly wh so sll goups ophysss oud h wold hv ly bosdg.

Iolly, ssug ouslvs o b sg- hs gd osologss g xploypow. I hs llowd us o xpol owh w s ou ow os ghbohood o

h uvs lg. Hug os hv bd osug s-o-h- odls oh uvs bsd o h osologl p-pl glzo o h Cop pplh ss h y o ll posd dos sp look h s. Cobdwh ou od udsdg o sp, d , h osologl ppl plsh sp s xpdg, h h uvs s g-g ool d h s populd by ls os ho bggpdos h ll boou by obsvos.

Key conceptS

The universe appears to be

expanding at an accelerat-

ing rate, implying the exis-

tence o a strange new

orm o energydark ener-

gy. The problem: no one is

sure what dark energy is.

Cosmologists may not ac-

tually need to invoke exotic

orms o energy. I we live

in an emptier-than-average

region o space, then the

cosmic expansion rate

varies with position,which could be mistaken

or a variation in time,

or acceleration.

A giant void strikes most

cosmologists as highly un-

likely but so or that matter

does dark energy. Observa-

tions over the coming years

will dierentiate between

the two possibilities.

The Editors

cosmology

Does

Really Exist?Maybe not.The observations that led astronomers todeduce its existence could have anotherexplanation: that our galaxy lies at the centerof a giant cosmic void

By Timothy Clifton andPedro G. Ferreira

2009 SCIENTIFIC AMERICAN, INC.


2/9www.S A. Scientific AmericAn 49

yu a h

uneven expansion o space, d t

th dt mtt l,

ld d th t tht tm

tll ttt t dk g.



3/950 Scientific AmericAn A l 2009

wh, w ld o h oluso h h u-vs us b pd by xo o o -gy, dk gy, h xs pulsv o.

Nohg g h dspo o dk -gy pps physss Sdd Modl oudl pls d os. I s sub-

s h hs o s y b sud dly,hs pops ulk yhg w hv v sd hs gy dsy so 10120 s lssh w y hv vly xpd. Physsshv ds o wh gh b, bu hy spulv [s Th Qussl Uvs,by Jh P. Osk d Pul J. Shd;Scientiic American, Juy 2001]. I sho,w vy uh h dk bou dk gy.Rshs wokg o ub o b-ous d xpsv goud- d sp-bsdssos o d d hz dk gy,

whv y b. To y, s h gshllg g od osology.

A Lighter Alternat iveCood wh sohg so sg dsgly so pobbl, so shs vsg h sog h ld h o . Oo h py ssupos hy qusogs whh w lv psv p o huvs. Could h vd o dk gy boud o oh wys w w o dowy wh h osologl ppl?

I h ovol pu, w lk bou hxpso o h uvs o h whol. I s vyuh lk wh w lk bou blloo blowgup: w dsuss how bg h blloo gs,o how uh h dvdul ph o h bl-loo fs. Bu w ll hv hd xp whhos oyg py blloos h f u-vly. O g shs qukly, d h dks whl o h up. I lv vwo h uvs, o h jsos h osolog-l ppl, sp, oo, xpds uvly. Ao oplx pu o h osos gs.

Cosd h ollowg so, s suggs-

d by Gog Ells, Chls Hllby d NzMusph, ll h Uvsy o Cp Tow Souh A, d subsquly ollowd up byM-Noll Cl o h Ps-Mudo Ob-svoy F. Suppos h h xpso s dlg vywh, s ugs osp d slows dow. Suppos, uh,h w lv ggu os vodo oplly py go, bu o whh hvg dsy o s oly hl o yb hd o h dsy lswh. Th p ph o sp s, h lss os o

Asoos d, o xpl, h h lgho ds glxs s dd h h o -by glxs. Ths phoo, kow s d-sh, s ly xpld s shg o lghwvs by h xpso o sp. Also, o-wv dos vl los ply sooh

u o do g o vy lys: h os owv bkgoud, lo h podl bll. I s o sy hhs susss p sul o ou ow hu-lyh lss w ssu bou ou ow sg-, h o w sy bou h uvs.

Darkness Closes InSo why ok h bo? I h osologl p-pl s so sussul, why should w quso ?Th oubl s h sool obsv-os hv b podug so vy sg

suls. Ov h ps dd soos hvoud h o gv dsh, ds supo-v xplosos look d h xpd. Rd-sh sus h ou h sp hsxpdd. By sug how uh h lgho ds supov hs dshd, osol-ogss h how uh sll h u-vs ws h o h xploso s o-pd wh s sz ody. Th lg h dsh,h sll h uvs ws wh h supovoud d h h o h uvs hsxpdd bw h d ow.

Th obsvd bghss o supov po-vds su o s ds o us, whh u vls how uh hs lpsd s oud. I supov wh gv dshlooks d h xpd, h h supo-v us b h wy h sooshough. Is lgh hs k log o h us,d h h uvs us hv k log ogow o s u sz [see box on opposite

page]. Cosquly, h xpso o huvs us hv b slow h ps hpvously xpd. I , h ds supo-v d ough h h xpso o h u-

vs us hv ld o hv ugh upwh s u xpso [s SuvygSp wh Supov, by Cg J. Hog,Rob P. Ksh d Nhols B. Suz; Sci-entiic American, Juy 1999].

Ths lg xpso s h bg su-ps h d h u voluo osol-ogy. M h uvs should ug h b- o sp, slowg dow h xpso,bu h supov d suggs ohws. I os-ologss p h osologl ppl dssu h hs lo hpps vy-

copernicuss

legacy

The Copernican principle holds thatEarth does not occupy a special placein the universe. The universe has a

uniorm density (homogeneity) andlooks the same in every direction(isotropy).

Though powerul, the principleapplies only on scales much largerthan a galaxy. Ater all, i the cosmoswere completely uniorm, it would bea thin gruel o atoms rather than aconstellation o galaxies. Also, theprinciple applies in space but not intime. We live in a special era long

enough ater the big bang thatcomplex lie can orm but not so longthat stars have all died o.

Copernicus is commonly associat-ed with a dethroning o humanityrom any position o importance. Butas historian Dennis Danielson o theUniversity o British Columbia argues,although pre-Copernican Europeansplaced Earth at the center o the

universe, they did not consider thecenter a position o importance butquite the oppositeas Galileo put it,the sump where the universes lthand ephemera collect.



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scenario 1: expansion is acceLeraTinGIn the usual interpretation o supernova observations, the rate o cosmic expansion used to be slower than it isnow. Consequently, the universe has taken longer to grow to it s present size and supernova light has had moretime to spread out, so that it appear s dimmer to us. To drive this acceleration requires dark energy.

oLD view: expansion is DeceLeraTinGPrior to 1998, most cosmologists assumed that cosmic expansion was slowing down over time. In each timeincrement, the region o space increases in size by a diminishing actor. They based their expec tations osupernova brightness on this assumption.

scenario 2: universe is inHoMoGeneousAlternatively, perhaps expansion is decelerating but at dierent rates in dierent places. I our neighborhoodis emptier than other areas, it has less matter to retard the expansion and decelerates less quickly.

Region o space

Astronomers have ound that distant supernovae explosions are dimmer

than expected. To see what this discovery means or cosmic expansion,

consider a region o space that encompasses a supernova and our Milky

Way galaxy. Over time this region gets bigger as the abric o space stretch-

es like a rubber sheet. The supernova goes o when the universe is about

hal its current size (which occurs at dierent times depending on whether

the expansion is decelerating or accelerating). Light rom the explosion

spreads out and eventually reaches us on the outskirts o the Milky Way.

Mlk w

s

rdldgg

slldgg

[THe basics]

Th w edg u

As light rom a supernova spreads out, it enters zones o increasingly rapid expansion which has the same eect as cosmic acceleration but without any need or dark energy.

Lght m

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o vl g ds h would uoly xpdg uvs, whh s hsupov hs o b h wy d hopp d.

Aoh wy o pu s h vo o x-pso wh poso s vo . I hs wy, osologss xpl huxpd supov obsvos whou -vokg dk gy. Fo suh lv x-plo o wok, w would hv o lv vod o uly os popoos. Th supovobsvos xd ou o bllos o lgh-ys,

sg o o h obsvbluvs. A vod would hv o b o sl sz.Eoous by (los) yos sdds.

A Far-fetched PossibilitySo how ouldsh s hs os vod? A sgl, vy. I would s o fy h oh os owv bkgoud, whh suo o o p 100,000, o o oh pply uo dsbuo o glxs[s Rdg h Blups o Co, byMhl A. Suss; Scientiic American, Fb-

slow dow h xpso o sp; odgly,h lol xpso s s wh h vodh s lswh. Th xpso s ss h vy o h vod d dshs o-wd h dg, wh h hgh-dsy xobgs o k sl l. A y gv d- ps o sp wll xpd ds, lk h uvly fd py blloo.

Now g supov xplodg d- ps o hs hoogous uvs, solos o h o h vod, ohs hdg d so ousd h vod. I w

h o h vod d supov s hou, sp xpds s ou vy h dos h loo o h supov. As lgho h supov vls owd us, pssshough gos h xpdg v s- s. Eh go shs h lgh by - ou s psss hough, d h uu-lv podus h dsh w obsv.Lgh vlg gv ds s dshd bylss h would b h whol uvs x-pdd ou lol . Covsly, o hv dsh suh uvs, h lgh hs

In his Hitchhikers Guide to the Galaxyseries o novels, Douglas Adams

imagines a torture device that drives people insane by showing them the utter

insignicance o their place in the universe. One would-be victim emerges

unscathed when it turns out that the universe does, in act, revolve around

him. In a case o lie imitating art, many cosmologists are investigating wheth-

er our planet indeed has a special place within the grand scheme o things.

HoMoGeneous universe: our LocaTion is TypicaLIn the standard view, galaxies are lined up in a spidery pattern, but overallspace looks much the same everywhere, and Earths position is nothing special.

inHoMoGeneous universe: our LocaTion is speciaLAlternatively, the density o matter could vary on large scales, and Earthmay lie at or near the center o a relatively less dense region, or void.

[THe biG picTure]

a sl pl u

ob

serv

ableunivers

e

y h

vd




nasa/esa,

huBBleKeyProjectteama

ndhigh-ZsuPernov

asearchteam

ou obsvbl uvs would s o b y.Sll, h s possbl loophol. I h ly

1990s o o h uhos o wh s ow h s-dd odl o h ly uvs, Ad Ld,d hs ollboos Sod Uvsyshowd h lhough g vods , hy

xpd s ly o d o o do hvolu o h uvs. Th pobbly o ob-svs dg hslvs suh suuy o b so y ll. Ths sul shows hh osologl ppl (h w do o lv spl pl) s o lwys h s hg s hppl o doy (h w ypl ob-svs). O , ss, b boh ypl dlv spl pl.

Testing the VoidWh obsvos ould ll whh h xp-

so o h uvs s dv by dk gy owhh w lvg spl pl, suh s h o g vod? To s o h ps- o vod, osologss d wokgodl o how sp, d shouldbhv s vy. Jus suh odl ws o-uld 1933 by Abb Gogs L,dpdly dsovd y l by Rh-d Tol d uh dvlopd WoldW II by H Bod. Th uvs hyvsgd hd xpso s h dpddo oly o bu lso o ds o sp- po, jus s w ow hypohsz.

Wh h L-Tol-Bod odl hd, osologss k pdos o

uy 2004]. O los spo, howv, hsvd y o b so olusv.

Th uoy o h l do lyqus h uvs o look ly h s vy do. I vod s oughly sphl d w l sobly los o s , hs ob-

svos do o ssly plud . I d-do, h os owv bkgoud hsso olous us h ould pollyb xpld by lg-sl hoogy [seebox on next page].

As o h glxy dsbuo, xsg su-vys do o xd ough o ul ou vodo h sz h would dk gy. Thydy sll vods, ls o doh suus hudds o llos o lgh-ys sz, bu h puv vod s od ogud lg. A lvly db s ow ud

wy sooy s o whh glxy suvysoobo h osologl ppl. A lyss by Dvd Hogg o Nw Yok Uvsyd hs ollboos ds h h lgssuus h uvs bou 200 llolgh-ys sz; o lg sls, p-ps soohly dsbud, od whh ppl. Bu Fso Sylos Lb o hEo F C Ro d hs ollgusgu h h lgs suus dsovd so ld oly by h sz o h glxy suvysh oud h. Sll lg suus ghsh byod h sop o h suvys.

By logy, suppos you hd p showg go 10 ls wd, o whh od shdo o sd o h oh. I would b sko olud h h logs possbl od s 10ls log. To d h lgh o h logsod, you would d p h lly showdh d pos o ll ods, so h you wouldkow h ull x. Slly, soosd glxy suvy h s lg h h bg-gs suus h uvs hy o povh osologl ppl. Whh suvys bg ough y s h subj o h db.

Fo hoss, oo, olossl vod s dulo soh. All vlbl vd suggss hglxs d lg suus suh s lsd vods gw o osop quu sdsh os xpso lgd o soolpopoos, d osologl hoy ks pdos o how y suus shouldxs wh sz. Th lg suus, h should b. Th pobbly o vod bg ough o dk gy s lssh o p 10100. G vods y wll x-s ou h, bu h h o ou dg o

no a-voiding it

althgh m d mm

dk g, th mth t -

t. umg t ll

lk tlltl d.

Additional supernova observations

will pin down the expansion rate

and check whether it varies withposition, as a void model predicts.

Galaxy clusters refect light and, in

eect, let us view our cosmic neigh-borhood in the mirror. I we live ina void, we should be able to see it.

Galaxies and galaxy clusters evolve

at a pace that depends on the ex-pansion rate at their location andthereore on the presence o a void.

Neutrinos let over rom the primor-

dial universe could reveal a void.

supernova 1994D (arrow) d ml l

d t m .




lo, bu h lk o poyss sh hy do o podu exactly h s -suls s dk gy. I pul, h pp o lo vs wh dsh ll-l wy. I pp wh K Ld, h hUvsy o Oxod, w showd h svl

hudd w supov, o op o h w hu-dd w uly hv, should b ough o s-l h ssu. Supov-obsvg ssos sd vy good h o hvg hs gol soo.

Supov o h oly obsvblsvlbl. Jy Good o Po U-vsy suggsd oh possbl s 1995usg h owv bkgoud do. Ah , h bs vd o dk gy hdo y gd, d Good ws o skg xplo o y uxpld phobu poo o h Cop ppl sl. Hs

d ws o us ds luss o glxs sos o look h uvs o d po-sos, lk lsl dssg oo. Glxyluss f sll o o h owvdo h hs h. By ully sugh spu o hs do, osologssould so sps o wh h uvswould look lk vwd o o o h. I sh o vwpo hgd how h uvslookd, would b powul vd o vodo sl suu.

Two s o osologss ly pu hsd o h s. Rob Cldwll o Douh

g o obsvbl qus. To bg, osd- h supov h s ld o h odk gy. Th o supov h soo-s obsv, h o uly hy -osu h xpso hsoy o h uvs.Sly spkg, hs obsvos o v

ul ou h vod odl, bus osologssould - y s o supov d byhoosg subly shpd vod. Y o vod ob oplly dsgushbl o dk -gy, would hv o hv so vy sg pop-s dd.

Th so s h h puv lg x-pso ous gh up o h ps o.Fo vod o xly, h xpso us ds shply wy o us d vydo. Tho, h dsy o dgy us s shply wy o us v-

y do. Th dsy pol us look lk upsd-dow whs h, h p o whh o-spods o wh w lv. Suh pol wouldgo gs ll ou xp o wh suus h uvs look lk: hy usully sooh,o poy. Ev wos, Al Vdvld d - Flg, boh h Coll Uvsy,showd h h p o h h, wh w lv,would hv o b sguly, lk h uldsgo h o blk hol.

I, howv, h vod hs o ls,sooh dsy pol, h ds obsv-ol sgu pss sl. Sooh vods sllpodu obsvos h ould b sk o

[THe auTHors]

Tmth clt d pd G. mlgt t thut od. bth td

th h th l

d ttl mdft t

et gl th lt-

t. clt, k hl,

h t tt l bgd

. th th

l-ll tm k, The

State of the Universe,

gm tt d t

od, d tt

jt t t d-t a.

sd t th vdMost suggestions that we live in a cosmic void place us at its

center, but what i we lived away rom the middle? The

universe would then look slightly lopsided. Hvard Alnes and

Morad Amarzguioui, both at the University o Oslo, have

shown that the cosmic microwave background radiation

would look slightly hotter in one direction than in the other.

Such an asymmetry, called a dipole, has indeed been observed in

the microwave background. It is usually attributed to our solar systems

motion through space but could also be a sign o a lumpy universe.Furthermore, small fuctuations in the microwave background appear to align in a

specic directiondubbed the axis o evil by Joo Magueijo and Kate Land, both then at Imperial College

London [see Is the Universe Out o Tune? by Glenn D. Starkman and Dominik J. Schwarz; ScientificAmericAn,

August 2005]. This alignment picks out a preerred direction in the sky, which, though hard to imagine in a

Copernican universe, might be explained in terms o our displacement rom the center o a void. A preerred

direction would also have other eects, such as large-scale coherent motions o galaxies and galaxy clusters.

Several researchers have claimed to have detected such a dark fow, but it remains controversial.

Although it is tempting to attribute these anomalies to a giant void, this explanation does not really hold

together. For a start, these eects each pick out dierent directions. Furthermore, the strength o the cosmic

dipole would suggest that we are only about 50 million light-years rom the center, which is only a very small

raction o the total size o the putative void. T.C. and P.G.F.

axis o eviL, lgmt

t th m m-

kgd dt,

ld g tht l

hmg .

Alignmeo eatur




esa/aoesmedialaB

hg us, d h vos h xpso wk h bghss d dsh. So ,howv, h d dos o look vy posg.O o us (Clo), ogh wh Josph Zuzo Oxod, ly showd h podug hs o dk gy would k los o vods o

vy low dsy, dsbud spl wy.Aoh possbly s h dk gy s o h hl ppoxosh osologss ouly us. To lul hos xpso , w yplly ou uphow uh go o sp os, d-vd by h volu o h go d v hvg gy dsy. W h s hs v-g dsy o Ess quos o gvyd d h vgd xpso oh uvs. Alhough h dsy vs opl o pl, w hs s s sll fu-

uos bou h ovll vg.Th pobl s h solvg Ess qu-os o vgd dsbuo s noth s s solvg o h l dsbu-o d h vgg h sulg goy.I oh wods, w vg d h solv, whlly w should solv d h vg.

Solvg h ull s o quos o yhgv vguly ppoxg h l uvs suhkbly dul, d so os o us so oh spl ou. Thos Buh o h U-vsy o Lyo F hs k up h sko dg how good ppoxo lly s. H hs odud x s o so h osologl quos o ou oh o odud by vgg bo solvg.I hs s pov o b sll, h h ppox-o s good; hy lg, s o. Thsuls so olusv. So shshv suggsd h h x s y bough o ou o dk gy ly,whs ohs l hy glgbl.

Obsvol ss o dsgush bwdk gy d h vod odls s o b -d ou h vy uu. Th Supov

Lgy Suvy, ld by P As o h Uv-sy o Ps, d h Jo Dk Egy Msso,uly ud dvlop, should p dowh xpso hsoy o h uvs. Th PlkSuvyo sll d vy o goud-bsdd blloo-bo sus wll p ou howv bkgoud v g dl.Th Squ Klo Ay, gg dolsop pld o 2020, wll supply us wh suvy o ll h glxs wh ou obsvblhozo. Ths voluo osology bg dd go, d s o ov.

Collg d Alb Sbbs o h F No-l Alo Lbooy Bv, Ill., suddps sus o dsoos h o-wv bkgoud, d Ju G-Blldo oh Uvsy o Mdd d Tols Hugbllo h Uvsy o Ahus Dk lookd

dvdul luss dly. Nh goup d-d vod; h bs h shs ould dows o ow dow h pops h suh vod ould hv. Th Plk Suvyo sll,shduld o luh hs oh, should b blo pl sog ls o h vod popsd yb ul ou vod logh.

A hd ppoh, dvod by Bu Bss,Chs Clkso d Ts Lu, ll h Uv-sy o Cp Tow, s o k dpd -sus o h xpso d lo-os. Asoos usully su xpso

s s o dsh, whh s h uulv o h xpso o ll gos o sp b-w lsl body d us. By lupg llhs gos ogh, dsh o ds-gush vo o xpso sp o vo . I would b b o suh xpso sp spl loos,spg ou h s o xpso ohloos. Th s dul poposo, hough,d hs y o b do. O possbly s o ob-sv how suus o d pls.Th oo d voluo o glxs d gl-xy luss dpd, lg p, o h lol o xpso. By sudyg hs objs d loos d oug o oh -s h ply ol h voluo, soo-s y b bl o p ou subl ds xpso .

A Not So Special PlaceTh possbly h w lv h ddl o g os vod s x jo o hosologl ppl, bu h gl pos-sbls. Th uvs ould oby h osolog-l ppl o lg sls, bu h sll vods

d ls h glxy suvys hv dsov-d gh ollvly h s o dkgy. Thb Bsws d Alsso No,boh MGll Uvsy, s wll s VloM d hs ollboos, h h Uv-sy o Pdu Ily d h Uvsy o Ch-go, hv sudd hs d. I h odls, huvs looks lk Swss hsuo o hwhol bu ddld wh hols. Cosquly, hxpso vs slghly o pl o pl.Rys o lgh d by ds supov v-l hough ulud o hs sll vods bo

More toexplore

Gtm rmd. JeremyGoodman in Physical Review D,Vol. 52, No. 4, pages 18211827;March 15, 1995. htt://.g//t-h/9506068

Th stt th u: a pm-

Md cmlg. Pedro G.Ferreira. Phoenix, 2007.

cmlg: pth slt.

G.F.R. Ellis in Nature, Vol. 452, pages158161; March 12, 2008.

Lg vd: Ttg th c-

pl th Dtt

s. Timothy Cliton, PedroG. Ferreira and Kate Land in PhysicalReview Letters, Vol. 101, Paper No.131302; September 26, 2008. htt://

.g//0807.1443

walking

the planck

The latest spacecrat to measurethe cosmic microwave backgroundradiation, the European Space Agen-cys Planck Surveyor, is scheduled tolaunch this month.

Planck should provide a complete

inventory o fuctuations in thetemperature o the microwave

background, thereby completing anobservational eort that began in the1960s. These fuctuations revealwhat the universe looked like at thetender age o 400,000 years and howit has grown since then. It could tellus whether we live in a giant void.

Planck will also measure fuctua-

tions in the polarization (or direction-ality) o the radiation, which couldreveal whether gravitational wavescoursed through the ancient universeas a result o high-energy processes araction o a second ater the bigbangor even beore it.



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