Beamline Journal

7/25/2019 Beamline Journal

1/67

ST A N F O RD L I N EA R A C C EL ERA T O R C EN T ER

Fall 1997, Vol. 27, No.3


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FEATU RES

2 INN ER SPAC E & OU TER SPAC ECosmol ogy is in th e m id st of a Golden A ge

tr iggered i n part by ideas about th e early

m om ent s of the U niv erse based upon th e uni -

ficati on of t he forces and part icl es of N atu re.

Mi chael S. Turner

14 WAS CO SMIC INFLATION THE BANG OFTHE BIG BANG ?

Tw o thousand years after Lucreti us

proclaim ed t hat n othi ng can be created

fr om not hi ng, inflat i onar y cosmol ogy assert s

that he was wrong.

Alan H. G u th

22 Th e Sloan D igi t al Sk y Surv eyPI ON THE SKY

Heidi Jo New berg

29 The D eep Extragalacti c Evolut ionary ProbePRO BING THE EVOLVING U N IVERSEAndrew Phillips and Nicole Vogt

36 THE FATE OF TH E U N IVERSEWi th the i r d iscovery of t he most d is tant

supernov as ever observed, an i nt ern ati onal

scient ific t eam led b y r esearchers from

Law rence Berk eley Nat ional L aboratory hope

to learn the ult i m ate fate of our U ni verse.

G erson G oldhaber and Judith G oldhaber

A PERIOD ICAL OF PARTICLE PH YSIC S

FALL 1997 VOL. 27, N U M BER 3

Editors

REN E D ONALDSON , BILL KIRK

Contributing Editor

M ICHAEL R IORDAN

Editorial Advisory BoardJAMES BJORKEN , G EORGE BROWN ,

ROBERT N . C AH N , D AVID H ITLIN ,

JOEL PRIMACK , N ATALIE ROE,

ROBERT SIEMANN

Illustrat ions

TERRY ANDERSON

Distribut ion

C RYSTAL TILGHMAN

The Beam Li neis published qua rterly bythe Sta nford Linear Accelerator Cent er,P O Box 4349, Stanford, C A 94309.Telephone: (415) 926-2585INTERNET: beam [email protected] : (415) 926-4500Issues of the Beam Li neare accessible electronically onth e World Wide Web at ht tp://w w w.sl ac.st an ford.edu/pubs/beam line. SLAC is operated by Stanford Universityunder contract wit h the U .S. D epartment of Energy. Theopinions of the authors do not necessarily refl ect thepolicy of the St anford Linear Accelerator C enter.

C over: Electronic p hoto m ontag e d esigned by Beam Linegrap hic artist Terry A nd erson. The Einstein im ag e is cour-tesy of the A rchives, C alifornia Institute of Technology. Thephotograph of the K eck telescop es w as taken by A ndyPerala. The H ubble D eep Field im ag e is courtesy ofA U R A /STS ci.

C

opyright1997

N

ationalG

eographic

Society.A

llrightsreserved.)


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CONTENTS

43 SEARC HIN G FOR D ARK MATTER AXIONSThe search is on for one of t he dark m att er

candi dat es so eagerly sought by

astr ophysici st sa conjectu red relic parti cle

from th e ti m e of the Big Bang cal led the axion.

Leslie J Rosenberg and Karl A. Van Bibber

49 ASTRO N OM Y & THE IN TERN ETThe aut hor toured the H arvard-Sm it hsoni an

Center for A strophysics for OMNI Online i n

November 1996. Th is piece led t he series

and is reprint ed from O M N I Online.

Fred Hapgood

DEPARTMENTS

52 TH E U N IVERSE AT LARG ECosmology: Where in the $**%Universe

Are You?

Virginia Trimble

61 CONTRIBUTORS

DATES TO REMEMBER

A

U

R

A

/STScI

D

an

Long


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2 FA LL 1997

CO S M O L O G IS TS TA C K LE th e big quest i ons. What

is the size and shape of the U niverse? What is it m ade

of? How did it all begin? How w ill it all end? Not sur-

prisingly, progress has com e in f i t s an d sta r ts . Sometim es i t

is a key event th at advances our understan ding. For exam pleEdw in H ubbles discovery of t he expansion of t he U niverse in

1929 w as the f i rs t ind ica t ion t ha t i t began f rom a B ig Bang

and Arno P enzias an d Robert Wilsons happening upon the m i

crow ave echo of the big bang in 1964 established t hat the be-

ginning w as very hot and dense. Other t im es conceptual break

throughs have advanced cosmology, such as Albert Einsteins

introduction of the G eneral Theory of Relativit y in 1916 which

allow ed the fi rst m athem atical description of the U niverse, and

G e or ge G a m o w s l a t e 1940s sugges t ion th a t t he ea rly U ni-

verse w as a nuclear furnace tha t cooked th e periodic table oelements, w hich w as the fi rst application of physics to the study

of its origin an d evolution.

G olden ages usually com e at t he conjunction of conceptual

and observational adva nces. C osmology is in the m idst of such

an age today. The conceptual breakth rough ca m e in t he 1980s

w i th t he rea l iza t ion t ha t uni f ied par t ic le theories have im-

por tant consequences for the earl ies t m oment s and m ay be

IN N E R

b y M I C H A E L S . T U R N E R

Cosm ology i s in th e

m id st of a Gol den A get ri ggered i n part by i deas

about t he earl i est

m oment s of the

U ni verse based upon

th e uni ficat i on of t he

forces and part i cles

of Nature.

SPACE

&O U T E RSPACE


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BEAM LIN E 3

crucial to answ ering some of t he m ost pressing ques-

tion s in cosmology. And furthermo re, part icle ac-

ce lera tors a re a new k ind o f t e lescope tha t a l low

the recreation of t he earliest m oments in t he colli-

sions of very high-energy part ic les . The inner

space/outer space connection has led t o a remarkable

extension of the Big Bang model w hich, if correct ,

w il l extend our understanding of the Un iverse to

w i t h i n 1032 sec of the beginnin g. This paradigm ,

know n as infl ation and cold dark matt er, holds that

the bulk of the ma tter in the U niverse exists in theform of slow ly m oving elem entary particlescold dark m att erthat

rem ain from the earl iest m om ents and tha t a l l the st ructures w e

see in th e U niversegalaxies, clust ers of galaxies, superclusters,

voids, great w alls and so ongrew from quantum mechanical fl uc-

tuations occurring on the subatomic scales.

On t he observation al side, w e are in the m idst

of a t echnologica l revolut ion t ha t can be t raced

back to the comm issioning of the 200-inch Hale

telescope on Mt. Palom ar in 1948, w hich a l low ed

cosmological pioneers Hubble, Milton H um ason,

and Allan Sandage to begin the serious study of th eU niv erse. The int roduct ion of cha rge-coupled de-

vices in astronomy in the 1970s increased th e light-

gathering pow er of photon detectors a hun dred-

fold, m aking the Hale the equivalent of a 2000-inch

telescope. The increase in com puter pow er over

the past forty years by truly astronomical factors

w as equa l ly c ruc ia l . New w indows on the Uni-

verse w ere opened w ith space-based telescopes for

the ul t raviolet , in frared, X-ray, and gam m a-ray

bands as w ell as the fi rst optical t elescope above

Earths blurring at m osphere. Other new instrum ents w ere developed:long-baseline radio interferom eters w ith m illiarcsecond resolution,

the 10-m eter Keck telescopes w ith their advanced instrum ents, and

sensitive receivers using high electron m obility transist ors and bolome-

ters to study the microw ave rem nant of t he Big Bang.

This issue of t he Beam Li neillustrat es w ell the close relationship

betw een particle physics and cosmology t hat has developed over the

pas t tw o decades . Al l the a r t ic les rela te to t es t ing in f la t ion an d

cold dark ma tt er. Man y of the protagonist s (and aut hors) start ed their


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4 FA LL 1997

careers as high-energy physicists. Alan G uth

author of the art icle on infl ation and inventor

of infl ation, w as trained as a high-energy t he

orist and w as a SLAC visitor w hen he did his

seminal w ork. Indeed, much of in f la t ionary

cosmology w as developed by high-energy t he

orists w ho have become part-tim e or full-tim e

cosmologists like G uth a nd I (m y graduate ca

reer began at SLAC ). The art icle on th e search

for axionsone of th e leading candidates for

the constituents of cold dark mat teris w ritten by tw o experiment alists, Leslie Rosenberg

and Karl van Bibber, w ho began th eir careers

in high-energy and m edium -energy physics respectiv ely. The Fate

of the U niverse w hich describes the quest t o measure its geom e

try and is w ritten by G erson G oldhaber, know n for his role in the dis

covery of charm, and his w ife Judith . Pi on the Sky, w ri t ten by

Heidi N ew berg, is about m apping the large-scale structure of the U ni

verse by determin ing th e three-dimensional posit ions of a m illion

galaxies. One of the ma jor partners in this collaborat ive effort, know n

of f icia l ly a s t he S loan D ig i t a l Sky Survey or S D S S , is the Ferm i

N ation al Accelerator Laboratory in Illinois. While the article w hichconcerns the study o f the origin and evolution of galaxies by bring

ing to bear tw o of the m ost powerful astronomical instrumentsthe

Hubble Space Telescope and the twin Keck 10-m eter telescopesis

w rit ten by t w o astronomers, Andrew Phillips and N icole Vogt , one

of th e key participants in this project is high-energy t heorist turn ed

astronomer and form er SLAC graduate student P rofessor Joel Prim ack

of the U niversity of C alifornia, Santa C ruz. Finally, Virginia Trimble

a frequent Beam Li necontributor and bicoastal astronom er, has con

tributed an overview of the U niverse, from t he sma llest to t he bigges

and th e shortest to t he longest , to put cosm ology in to i t s proper

perspective.

TALL SH OU LD ERSTH E H O T BIG -BANG (stan dard) cosmology w ill likely be view edas one of the intellectual t riumphs of the tw entieth century. It pro

vides a tested account of the U niverse from a fraction of a second af

ter the beginning until the present 10 t o 15 billion years later, as w el

The m ain ring of the w orlds m ost pow er-

ful accelerator, Ferm ilabs Tevatron. C ol-

lisions recreate conditions that last

existed 1012 seconds after the B ig Bang.

(C ourtesy Ferm ilab Visual M ed ia S ervices)


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BEAM LIN E 5

as a fi rm base for speculations about m uch earlier tim es. According

to t h is cosmology, the U niverse began as a hot , very dense form-

less soup of the fundam enta l part iclesqua rks, leptons, gauge bosons,

and possibly oth er elem entary particles. As it expanded and cooled,

layer upon layer of structu re built up. At around 105 sec, neutrons an d

proton s formed from q uarks. In a series of w ell understood nu clear re-

actions t hat t ook place betw een a fraction of a second and several hun-

dred seconds, the nuclei of th e light elements D , 3He, 4He and 7Li were

form ed. (The element s beyond 7Li w ere formed much lat er by nuclear

reactions w ithin st ars.) By a few hu ndred th ousand years the Un iverse

had cooled suf f icient ly so tha t a t oms could form from t he nucleiand free electrons present . Over t he next 10 t o 15 billion years all

the cosmic st ructure seen to-

day, from individual stars to su-

perclusters and great wal ls ,

developed th rough t he at t rac-

tive action of gravit y.

There is a w eal th of obser-

vational data th at support the

standard cosm ology; four ob-

servations provide the corner-

ston es. They a re the expansionof the U niverse; the microw ave

echo of the Big Bang (know n as

the cosm ic background radia-

tion or C BR ); the abun dance of

D , 3He, and 4He and 7Li; and

the t iny variat ions (about one

part in 105) in th e intensit y of

the C BR betw een different di-

rections on the sky.

Hubble present ed the first

evidence for a l inear rela t ionbetw een th e distances and the

veloci t ies of ga laxies . Now

know n as H ubbles Law, t his relation is w ell established (see sidebar

on page 30). The n otorious proportiona lity constan tHubbles con-

stant or H0, w hose reciprocal sets t he t im e back to th e bangis fi -

nally being pinned dow n to a precision of around 10 percent, thank s

to observat ions being m ade by t he H ubble Space Telescope and clever

techniques t hat exploit X-ray an d m icrow ave observat ions. Hubbles

B ig-bang prod uction of the light elem ents

dep ends up on the density of ordinary

m atter and agrees w ith the m easured

ab undances for densities ind icated by thew hite band, w hich falls far short of the critical

density. (C ourtesy K enneth N ollett)

101

103

105

107

109

1032

1031

1030

1029

Abundan

ceRelativetoHydrogen

Density of Ordinary Matter

(bar yo ns) (g/cc)

ConsistencyInterval C

riticalDensityfor

H 0 =70km/s/Mpc

4H e

3H e

7Li

D


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6 FA LL 1997

Law supports t he idea of an expanding universe and

provides the fundamenta l m eans of determining dis

tances to ga laxies: the m easured redshif t t imes c/H0is the distance to t he ga laxy .

U n a w a r e o f G a m ow s pr ed ic t i on o f a m ic r ow a v e

afterglow of the Big Bang, Penzia s and Wilson discovered th is ra-

diat ion serendipit iously. The Far Infrared Absolute Spectropho-

tom eter on the C O B E sa tel l i te , launched in 1989, has shown t ha t

the cosmic background radia t ion is black

b od y r a d i a t i on t o ex t r a o rd in a r y pr ec i

s ion , b et t e r t h a n 0.01 p er c en t , w i t h atem perature of 2.7277 0.002K. (All o b-

jects emit radia t ion charac t erist ic

o f the i r t empera ture ; for a fea-

tureless black body t he spectrum

only depends upon th e temperature and

is of a form first described by Pla nck.)

Since the C BR provides a snapshot of t he U niverse

at the t ime t hese photons last scat t ered, around 300,000

years after the bang w hen the Un iverse was about one thou-

sandth i t s present s ize; i t has been scrut inized for in tensi ty

(tem perature) variat ions t hat can reveal the distribution of m at-ter a t th is early t ime. In the 1970s a dipolar variat ion of about

3 m K w as d iscovered ; i t s s implest in t erpre t a t ion i s t ha t our

G alaxy m oves a t a veloci ty of 620 km /sec w ith respect t o the

cosmic rest f rame. In 1992 the Dif ferent ia l Microwave Radiome-

ter (D M R ) on t he C O B E sa tel l i te d iscovered much sm aller var ia

t ion s in the temperat ure: 30 m icroKelvin tem perature differences

betw een directions separated by an gles of around 10 degrees on th e

sky. This discovery tells us tw o thin gs: the early U niverse w as very

smoot h , and there were sm a l l va r ia t ions in the dens i t y o f m a t -

ter, about one part in 105.

The t iny varia t ions in t he C BR tem perature validat e a key ele-m en t o f t h e B ig B a n g t h eor y , t h e id ea t h a t a l l t h e s t r u c t u r e w e

see arose from small var ia t ions in t he mat ter densi ty w hich grew

under the influence of gravit y over th e past t en billion or so years

Fu r t h er , t h ey a l l ow u s t o b eg in t o q u a n t i f y t h e n a t u r e o f t h e

prim eva l lum piness and tes t ideas (inc lud ing G uth s and m ine

about t heir origin.

C O BE D M R m ap of the C B R (bright band across

m iddle is em ission from our ow n galaxy). The tw o

blow -up s show a sim ulation of the ne detail that

exists w ithin the C O B E 7 beam w hich the M A P

and Planck satellites should reveal. The d if-

ference betw een an open universe m odel

( = 0.1) on the left and a at universe m od el

( = 1) on the right is very striking.

(C ourtesy M artin W hite)


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BEAM LIN E 7

Finally, the abundan ce patt ern of the light elem ents D , 3He, 4H e

and 7Li seen in the m ost prim itive sam ples of the cosmos conform t o

the predictions of big-bang nucleosynt hesis. This tests t he Big Bang

theory t o w ithin a f rac t ion of a second of the beginning, and pro-

vides tw o bonuses: the yields of these elements depend upon t he m ass

density cont ributed by ordinary m att er (baryons) and t he num ber of

light n eutrino species. Since the early 1980s it has been known t hat

thi s agreement holds only i f the num ber of neutrino species is less

tha n four and the baryon density is betw een about 1 percent an d 10

percent of t he critical density. The cosmological prediction of the

num ber of neutr ino species m ade by D avid Schram m and his col-laborators was confi rmed in 1989 by precision m easurements of t he

properties of th e Zboson w ith the SLC at SLAC and w ith LEP at CERN ,

the European particle physics laboratory. This determin ation of th e

baryon dens i t y i s the l inchpin in the a rgument tha t most o f the

m ass in the U niverse exists in the form of elementa ry particles left

over from earliest m om ents.

The hot big-bang cosmology is not complete, nor is it likely to be

the w hole story. There are importan t properties of the Un iverse yet t o

be determ ined: the precise value of the expansion rate and t im e back

to t he bang, the fract ion of th e crit ical density contributed by m at-

ter, the geometric shape of the U niverse (fl at , curved like th e surfaceof a ball , or curved like th e surface of a saddle), th e value of Ein-

steins cosmological constant, a better understanding of how galax-

ies form and evolv e, a m ore precise description of t he large-scale

distribution of m att er, and the nat ure of the ubiquitous dark m att er.

(While the tota l am ount of m att er is not know n precisely, only a sm all

fraction of it exists in the form of stars and sim ilar visible m att er.)

And, fundam ental questions remain unansw ered. Why is t he U niverse

m ade of mat ter and not equal amount s of matt er and antim att er? What

is the origin of the m att er lumpiness that seeded all structure in the

U niverse? Why w as the early U niverse so smooth? What w ent bang?

Thanks t o the current explosion in observational cosm ology, w e arerapidly c losing in on m any of the f irst set of quest ions. The m ea-

surem ents being ma de can strengthen the case for the sta ndard cos-

m ology as w ell as help to t est ideas put forth to extend it. And, of course,

they could bring some surprises. The second set of questi ons points t o

th e existence of a grander theory, w hich is w here the inner space/out er

space connection comes in.

The LE P tunnel atC E R N w hich houses

the m ost pow erful electron/positron

collider and w ill eventually house the

Large H adron C ollider, w hich w ill be

able to recreate conditions that last

existed 1015 second s after the B ig

B ang. (C ourtesy C E R N )


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8 FA LL 1997

The B aB ar detector w ill be used to

stud y the m atter-antim atter asym m etry

puzzle and could help cosm olog ists to

understand better the origin of ordinary

m atter in the U niverse.

TH E IN N ER SPAC E/OU TER SPAC E C ONN EC TIONIN ITS IN FAN C Y TH E U NIVERSE w as a hot soup of the fundam ental part iclesquarks, leptons, and gauge bosons. The further

back in ti m e, the higher th e tem perature and energy per partic le. Par

ticle accelerators creat e a soup of high-energy q uarks, leptons, an d

related particles w hen they collide proton s and antiprotons, electron

and positrons, or positrons and protons. Therefore, like telescopes

they are cosm ic t im e machines that a llow cosmologists to explore

the earliest m om ents. C onversely, the early U niverse is a very powerful cosm ic accelerator t hat allow s particle physicists to probe deepe

into inner space than they can with terrestrial accelerators.

The unificat ion of the forces and part icles of Nat ure is the holy

gra i l of part ic le physics . There is a genera l bel ief and som e evi

dence (for example, the uni f ica t ion of the

w eak and electromagnetic forces) tha t t he ful

simplicity of N ature is only m anifest at high

energies and tem perat ures. This m akes th e

early U niverse a testing ground for th e grand

est ideas of particle physics, including uni-

f ica t ion o f t he s t rong , w eak , and e lec t rom agnetic forces, supersym m etry, and super-

strings. C onversely, the unifi cation of the par

ticles and forces certainly has consequences

for the earliest evolution of the U niverse.

The 1980s w ere the go-go days of early U ni

verse cosmology. Ma ny excit ing ideas about

the earl iest m om ents based upon specula

tions about the unifi cation of the forces w ere

put forth. An at tract ive explanation for the

origin of the asym metry betw een m atter and

ant im at ter know n as baryogenesis developed. Baryogenesis holdsthat a slight excess of matterquarks and leptonsarose early on

due to the sam e force that leads to t he instability of the proton and

the sl ight preference for m at t er in the law s of physics (C P v io la

tion). (See the article, The Myst ery of the Ma tt er Asymm etry by

Eric Sather in t he Spring/ Sum m er 1996 issue of the Beam L i ne

Vol. 26, No. 1.) After all the an tim att er annihilated w ith m att er (around

105 sec) only th e mat ter w e see today rem ained. The m ajor scien-

t i f ic goa l o f the SLAC B Factory is a bet ter understanding of C P


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BEAM LIN E 9

violation and possibly of baryogenesis (see the article by N ata lie Roe

and M ichael Riordan, Why Are We Building B Factories? in t heSpring/Sum m er 1996 issue, Vol. 26, No. 1.)

As the U niverse evolved it sho uld have gone through a series of

phase transit ions a s th e different forces of Nat ure developed their

ow n charact er and the forces became less unifi ed. These phase tran-

sit ions, not unlike the m ore fam iliar phase transit ion from steam

to w ater, could w ell have left t heir imprint on the future evolution

of the U niverse. If a phase transition did not proceed smoot hly an d

uniformly, topological defects m ay have been formed that could

still be w ith u s today. They include superheavy m agnetic m onopoles

and cosmic strings, w hich are very thin con centration s of false vac-

uum energy. These defects could act as t he seeds for the form ationof s t ructure or have ot her in terest ing consequences such as pro-

ducing extremely high-energy cosm ic rays. Som e theorists ha ve spec-

ula ted tha t even t he m ost w ell understood phase t ransi t ion , f rom

quark-gluon plasm a to ordinary ha drons, may h ave left macroscop-

ic nuggets of quark m att er.

B

EYO N D TH E STANDARD C O S M O L O G Y:

IN FLATION AN D C O LD D AR K M ATTER

THE MO S T CO M P ELLING and expansive idea to come from th einner space/outer space connection is infl ation and cold dark m att er.

It addresses essentially all of th e fundam ental q uestions being asked

by cosm ologists and has m oved a generat ion of observers and ex-

periment ers to go out an d disprove it! As G uth describes, the smoot h-

ness and f la t ness of the observed Un iverse ar ises because of an

enorm ous burst of expansion ca used by false-vacuu m energy. The

eventual decay of this energy produced all the heat of the Big Bang

PaulJ.Stom

ski,Jr.

The tw in 10-m eter K eck telescopes on

M auna K ea in H aw aii.

(C ourtesy Paul Stom ski)


12/67

10 FA LL 1997

and eventually all t he

mat ter w e see today

The m ost st riking pre

d ic t i on i s t h a t t h e

primeval lumpiness

arises due to quant um

mechan ica l f luc tua-

t ions on suba tomic

sca les , wh ich were

s t re t ched to cosmic

size by the enormousburst of expansion.

A f la t universe

has t he critical density and expands forever. Big-Bang nucleosynt hesi

tells us that ordinary m att er accounts for slightly less than 10 per-

cent of the crit ical density; this m eans that something m ore exotic

m ust t ake up the slack. Pa rticle physics provides several prom ising

particle candidates: the axion, a hypoth etical particle that is supposed

to w eigh a million, m illion t im es less than the electron (discussed

in t he art icle by va n Bibber and Rosen berg on page 43 of th is issue)

the neutralino, a h ypothetical part icle predicted to exist if N ature

is supersym m etric and is supposed to be betw een ten and fi ve hundred t im es heavier than the proton; and ordinary neutrinos if they

have m ass, as most u nified theories predict . All th ree candida tes

should have been present during the earliest fi ery m om ents in great

abundan ce and according to calculat ions should be present t oday

in about t he right n um bers to account for the critical density. Axion

and neut ra l inos m ove s lowly and for th is reason a re ca l led co ld

dark matt er; neutrinos move fast and are known as hot dark mat ter

Hav ing th e bu lk o f the ma t ter in the form of re l ic elementa ry

pa r t i c le s fi t s n i c ely w i t h t h e f a c t t h a t m os t o f t h e m a t t e r in t h e

U niverse is know n to be dark. C osmologists are more interested in

cold dark m at t er tha n neutr inos because they a re conf ident tha tthe developm ent of s t ructure in a hot dark ma t t er universe leads

to a universe radically different from ours.

The idea that t he bulk of the dark mat ter is cold dark ma tt er par

t ic les and tha t the pr imeva l lumpiness a rose f rom quan tum

m echanical fl uctua tions provide the basis for a com prehensive theory

for how cosmic structure formed and thereby a powerful w ay of test

ing inf la t ion i t sel f . The cold dark m at ter (or C D M ) th eory hol ds

tha t C D M part icles provide the cosmic infrastructure as it is th eir

The H ubble Space Telescop e, w hich

has prod uced the highest resolution and

deepest im ag es of the U niverse.

(C opyright1997 N ational G eograp hic

Society. A ll rights reserved.)


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BEA M LIN E 1

gravity t hat h olds all cosmic struct ures from galaxies to superclusters

together; and that structure forms from the bottom up. G alaxies form

fi rst w hen the U niverse was one-third t o one-half its present size; clus-

ters of galaxies form n ext ; and fina lly superclusters are just form -

ing today. The bringing together of m att er leaves voids. This picture

is generally consistent w ith a broad base of observations, from m ea-

surem ents of t iny variat ions in

the C BR tem perat ure to the deep-

est im ages of the Un iverse from

th e H ubble Space Telescope.

The C D M theory is being fur-th er tested by a f lood of obser-

vat ions and experiment s ; four

key t ests are described in thi s is-

sue of the Beam Li ne. New berg

describes how the SDSS w ill map

cosmic structure by determinin g

the positions of a m illion galax-

ies. Phillips and Vogt discuss th e

DEEP Projec t w hose sc ien t i f ic

goal is the study of t he origin and

evolution of galaxies. Van Bibberand Rosenberg tel l about their

search for the axions, w hich m ay be th e C D M that holds our G alaxy

together. Finally, the G oldhabers discuss the quest for om ega an d the

test ing of the infl ationary predict ion of a fl at universe.

Many other important t ests are underw ay; som e a t part ic le ac-

celerators. While too m uch hot da rk mat ter is a bad thing, a little bit

may be just w hat the C D M theory needs to agree w ith exist ing ob-

servations. Neut rino experiments at Fermilab and CERN are test-

ing this possibility. The neutralino, t he other prom ising C D M can-

d ida te , i s be ing hunted a round the wor ld , both w i th par t ic le

accelerators and w ith cryogenic detectors designed to detect the neu-tralinos that m ay be the dark m atter in our ow n galaxy.

Finally, let m e mention w hat I believe is the most pow erful test

o f infl a t ion + C D M as w ell as a precision probe of the stan dard cos-

m ology it self . Encoded in t he t iny variat ions of the t emperature of

the C BR across the sky a re both th e va lues o f the cosmologica l

parameters and th e deta ils of infl ation . A precision, high-resolution

ma p of the C BR has the potential to determine accurately the Hub-

ble constant , baryon density, tot al density (om ega), the value of the

r < 17.55, d > 2, 6Slice

Redshift Space

62295 G alaxies

200 h1

M pc

400 h1

M pc

A sim ulation of the galaxy distribution

that the S loan Survey m ight nd . W e sit

at the ap ex of the w ed ge.

(C ourtesy D avid W einberg)


14/67

12 FA LL 1997

cosmological constant , the value of the vacuum energy t hat drove

inflat ion, the spectrum a nd am plitude of the prim eval lumpiness

that arose from q uantum m echanica l f luctuat ions, and other im-portant cosmological parameters.

The C O B E satellite m apped the full C BR sky w ith angular reso

lution of seven degrees. G round-based and balloon-borne instrum ents

are mapping sm all patches w ith resolution of bett er than one degree

NASA plans to launch t he Microw ave Anisotropy P robe in August

2000 w hich wil l map the C BR sky w ith a beam of 0.2 degree. In 2005

the European Space Agency plans to la unch t he Plan ck sa t el l i te

w hich wil l map w ith a beam of less than 0.1 degree. Over the next

decade, ground-based, balloon-borne, an d satellit e-borne inst rum ents

should reap the full scient ifi c potent ial of this snapshot of th e ado

lescent U niverse. In process, in f la t ion + C D M w il l be pu t to thetest an d the details of infl ation m ay be revealed. In addition, preci-

sion checks of the st andard cosm ology w ill be made.

B EYOND IN FLATION AND C OLD D AR K M ATTER

The arcs seen in the left panel are

produced by gravitational lensing of

distant galaxies by a closer cluster of

galaxies. Analysis of the gravitational

lensing led to the m ap of the cluster

m ass d istribution show n on the right.

Such m aps have helped to establish the

existence of large am ounts of dark

m atter in the U niverse.

(C ourtesy Tony Tyson)

IF IN FLATIO N AN D C O L D D A RK M ATTER prove correct, itw ill be a great trium ph for the in ner space/outer space connection

The know n h is tory o f t he U niverse w i l l be extended to t imes a s

early as 1032

sec and a w indow t o the unifi cation of the fundamental forces and part icles of Na ture w ill have been opened. Our con-

ception of t he U niverse and our place in it w ill have been changed

profoundly. As G uth describes, infl ation a nsw ers w hat ban ged: Wha

w e refer to as the Big Bang and beginning of the U niverse w as sim-

ply the inflat ionary event that w as our beginning. Andrei Linde o

Stanford U niversi ty has em phasized, i f in f la t ion h as occurred a t


15/67

all, it ha s occurred countless times

in the past and w ill continue to oc-

cur forever. Each infl ationa ry event

spaw ns a region large enough so tha t

it never comm unicates with others

and can be rightfully called a sub-

universe. Sub-universes m ay be very

dif ferent as the rea l iza t ion of t he

law s of physics, including the num-

ber of spatial dimensions, may de-

pend upon historical accidents as-sociated w ith th e loss of symm etry

betw een the forces.

These are excitin g tim es in cos-

m ology. We should f ind out soon

w hether or not our most promising

and expansive ideas about the ori-

gin of t he U niverse are correct. Even

if current t hinkin g proves w rong, or

only partially correct, there are in-

teresting tim es ahead as w e search

for new ideas. They w ill alm ost cer-tain ly involve t he inn er space/outer

space connection.

BEA M LIN E 13

The cosm ic p icture. H ere and now is at the bottom ; as w e look

out into space w e look back in tim e.(C ourtesy Scott D od elson)

High-Energy

Physics Era

Radiat ion Era

Matt er Era

Here

Looking Out into th e Universe,

Back in Time

Age of Univ erse Increasing,

Temperature of

CMBR Decreasing

N o w t= 15 billion years, T= 2.73 K

G alaxies and Large-

Scale Structure

Un seen U niverse

Our Causal Horizon

Inflat ion

Helium Forms

D ecoupling Occurs

Atoms Form

Matt er Clumping Begins

First Stars and G alaxies Form

t = 1 billion years,

T= 16 K

t = 15 million years,

T= 300 K

t = 150,000 years,

T= 4,000 K

t= 100 seconds,

T= 1 billion K


16/67

14 FA LL 1997

WH EN AN O BSC U R E RU SSIAN METEOROLOG IST namedAlexander Friedman n proposed, in 1922, th at th e U niversem ight be expanding, Albert Einstein w as sure that he w aswrong. Five years earlier Einstein had published a static model of the Universe,

and he w as still convinced that it w as correct. In a rare but drama tic blunder,

Einstein bolstered his unfounded beliefs w ith an erroneous calculation, and

fi red off a note to the Z eit schri f t fr Physikclaim ing that Friedm anns theory

violated th e conservation of energy. Eight m onth s later, how ever, after a visit

from a colleague of Friedmann s, Einstein a dmit ted his m istake an d published

a retraction. The equat ions of general relativity do, he conceded, allow for the

possibility of an expand ing univ erse.

Today t he Big Bang th eory, w hich began w ith Friedm anns calculations

in 1922, has become th e accepted view of cosm ology. The expansion of th e

U niv erse w as fi rst observed in t he early 1920s by Vesto Melvi n Slipher, and in

1929 w as codifi ed by Edw in Hubble into w hat w e now know as Hubbles

Law : on average, each distan t galaxy is receding from us w ith a velocity t hat

is proportional t o it s distance. In 1965 Arno Penzias a nd R obert Wilson detect-

ed a background of m icrowa ve radiation arriving at Earth from all directions

the aft erglow of the prim ordial hot , dense fi reball. Today w e know, based on

Wa s C o sm i c In f l a t i on t hTwo th ousand years after Lucret iu s proclai m ed

th at n othi ng can be created from nothi ng,

i nflat i onar y cosm ology assert s th at he was w rong.


17/67

BEA M LIN E 15

data from the Cosmi c Background Explorer (COBE) satellite (see Beam Line,

Vol. 23, N o. 3, Fall/Wint er 1993), th at t he spectrum of t his ba ckgroun d radia -

tion agrees w ith exquisit e precisionto 1/30 of 1 percent w ith th e therm al

spectrum expected for the glow of hot m att er in t he early U niverse. In addi-

tion, calculations of nucleosynt hesis in th e early universe show tha t t he Big

Bang theory can correctly account for the cosmic abun dance of the light nu-

clear isotopes: hydrogen, deuterium, h elium -3, helium -4, and lit hium -7.

(Heavier elem ents, w e believe, w ere synt hesized m uch lat er, in the in terior of

sta rs, and w ere th en explosively ejected in to int erstellar space.)

D espite t he strikin g successes of the Big Ban g theory, there is good

reason to believe th at t he theory in it s traditional form is incomplete.

Although it is called the Big Bang theory, it is not really the th eory of a bang

at all . It is only th e theory of the af termath of a ban g. It elegant ly describes

how th e early U niverse expanded and cooled, and how m att er clumped to form

galaxies and sta rs. But t he theory says n othin g about t he underlying physics of

the prim ordial explosion. It gives not even a clue about w hat banged, w hat

caused it to bang, or w hat happened before it ban ged. The infl ationa ry univ erse

th eory, on t he oth er han d, is a description of the ban g itself, and provides plau-

sible answ ers to t hese questions and m ore.

B a n g o f t h e B ig B a n g?


18/67

16 FA LL 1997

To see how diffi cult it is to explain

th is un i fo rmi ty a s the resu l t o f an

ordinary explosion, w e need to kn ow

a lit t le about t he history of the cos-

m ic background radiati on. The early

U niverse w as so hot tha t e lectrons

w ould have been ripped aw ay from

a toms, resu l t ing in a p la sma tha t

f i l led space. Such a plasm a is very

opaque , so the pho tons tha t now

ma ke up the cosmic background ra-

diation w ere constantly absorbed and

re-emit ted. After about 300,000 years,

how ever, the U niverse cooled enough

for the plasma to form a gas of neu-

tra l a tom s, which is very t ranspar-

ent. The photons of the cosm ic back-

ground rad ia t ion have t rave led on

straight lines ever since, so they pro-

vide today an im age of the U niverse

at an age of 300,000 years, just as t he

pho tons reach ing your eye a t t h ism oment provide an im age of the page

in front o f y ou. Thus, th e observa-

t ions of th e cosmic background ra-

dia t ion show tha t t he Universe w as

uniform in temperature, to one part

in 100,000, a t an age of several hun-

dred thousand years.

U nder m any circumstan ces such

uniformity w ould be easy to under-

A VERY SPEC IAL BANG

C ould the Big Bang have been caused

by a colossal stick of TNT, or perhaps

a therm onuclear explosion? Or may -

be a gigantic ball of ma tt er collided

w ith a gigantic ball of antimatt er, re-

leasing an untold am ount of energy

in a pow erful cosmic blast .

In fact, none of t hese scenarios can

plausibly account for the Big Bang

that started our Universe, w hich hadtw o very specia l fea tures dist in-

guishing it from any t ypical explosion.

First , the Big Bang wa s far more

hom ogeneous, on large scales, th an

can be exp la ined by an o rd ina ry

explosion. In discussin g hom ogene-

ity, how ever, I must fi rst clarify that

the Universe i s in m any w ays con-

sp icuously inhomogeneous . Pa lo

Alto is very different from San Fran-

c isco , and th e s t a rs , ga lax ies , and

clusters o f ga laxies are sca t teredthrough space in a lum py, complex

pat tern . Cosmologica l ly speaking,

how ever, all this structure is small-

sca le . We can focus on the large

scales, for exam ple, by dividi ng space

into cubes of 300 m illion light-years

or more on a side. We w ould fi nd that

each such cube closely resembles th e

others in a ll it s average properties,

such as m ass density, galaxy density,

and l igh t ou tpu t . This l a rge-sca le

uniformit y can be seen in galaxy sur-

v e ys , b u t t h e m o s t d r a m a t i c e v i -

dence com es from t he cosmic back-

g ro u n d r a d i a t i o n . D a t a f ro m t h e

C OB E satellite, confi rmed by subse-

quent ground-based observat ions,

show t hat this radiation has the same

tem perat ure in all directions (after

correc t ing fo r the mot ion o f the

Earth) to an accuracy of one part in

100,000.

s t and , s ince anyth ing w i l l come to

a uniform temperature if left undis

turbed for a long enough tim e. In th e

standard Big Bang theory, how ever

the U niverse evolves so quickly that

there is no t ime for the uniformity

to be established. One ca n pretend

for the sake of discussion, tha t the

U niverse is populated by lit t le pur

ple creatures, each equipped w ith a

furnace and a refrigerator, and each

dedicated to th e cause of creating auniform tem peratu re . Those l i t t le

crea tures, how ever, w ould have to

communica te a t roughly 100 t imes

the speed o f l igh t i f they a re to

achieve their goal of creating a uni

form temperature across the v isible

U niverse by 300,000 years a f ter the

Big Bang. Since neither st icks of

d yn a m i t e n o r b a ll s o f m a t t e r a n d

antim atter can t ransmit their energy

faster than light, they cann ot accoun

for the uniform ity. The classical formof the B ig Bang th eory requ ires us

to pos tu la te , w i thou t exp lana t ion

tha t the pr imord ia l f i reba l l f i l led

space from the beginning. The tem

perature was t he same everyw here

by assum ption, not as a consequence

of any physical process. This short-

comin g i s know n as the horizon

problem, since cosmologists use t he

w o rd h o r iz o n t o in d ic a t e t h e

largest distance that informat ion or

energy could ha ve traversed since the

ins t an t o f the B ig Bang , g iven th e

restriction of t he speed of light.

The second special featu re of the

Big Bang, w hich is very dif f icult to

ima gine arising from a st andard ex

plosion, is a rem arkable coincidence

cal led the fl a tness problem. This

problem concerns the pinpoint

precision w ith w hich the mass den

si t y o f the ea r ly Un iverse must be

I nflat i on

is a w il dfire

tha t w i l l inev i tab l y

t ake over t he forest,

as l ong as

t here is som e

chance th at

i t w i l l sta r t .


19/67

BEA M LIN E 17

specified for the Big Bang t heory to

agree w ith reality.

Fi rs t , w e need to rev iew a l i t t le

vocabulary. If the m ass density of the

Universe exceeds a value called the

criti cal densit y, t hen grav i t y w i l l be

strong enough t o reverse the expan-

sion eventua lly, causing the U niverse

to recollapse into w hat is sometimes

called the bi g crunch. If the mass den-

sity is less than t he critical value, the

U niverse wil l go on expanding for-ever. The ra t io o f th e ac tua l m ass

density t o the critical value is know n

to cosmologists by the G reek letter

omega (). G eneral relativity implies

tha t the geomet ry o f th e Universe

is Eucl idean on ly i f om ega i s one ,

so an = 1 universe is ca l led fl a t

(see box on th e right ).

Om ega is very difficult to deter-

m ine, but it is safe to say t hat it s pre-

sen t va lue l ies somew here in the

range of0.1

t o2

. That seems like abroad range, but consideration of t he

t im e developm ent o f the Universe

leads to a spect acu la r ly d i f fe ren t

po in t o f v iew. = 1 i s an unst ab le

equil ibrium point o f cosmologica l

evo lu t ion , w h ich means tha t i t re-

sembles the situat ion of a pencil bal-

ancing on i t s sharpened t ip . The

phrase equilibrium point im plies that

if omega is ever exactly equal t o one,

i t w i l l rema in exact ly equa l t o one

foreverjust as a pencil bala nced pre-

cisely on end w ill , according to the

law s of classical physics, remain for-

ever ver t ica l . The w ord unst ab le

means tha t any devia t ion f rom t he

equilibrium point, in either direction,

wi l l rap id ly g row. I f the va lue o f

omega in the early U niverse w as just

a l i t t le above one , i t wou ld have

rapidly risen tow ard infinity ; if it w as

just a sm idgen below one, it w ould

Critical Mass Density

and Flatness

THE CRITICAL MASS densityc is related to the Hubble con-stant H by

c= 3H2___

8G

where Gis Newtons gravitation-al constant. The quantity isdefined by /c, where isthe actual mass density. It is of-ten assumed that the cosmolog-ical constant introduced byEinstein is zero, in which casethe Universe will recollapse ifand only if > 1. If is non-zero, the condition for recol-lapse is more complicated, butthe equation above is still takenas the definition of c.

The spatial geometry of the

Universe is determined by thequantity

+ ___3H2

.

If this quantity exceeds one, theUniverse curves back on itselfto form a closed space of finitevolume, but without boundary.In such a space the sum of theangles in a triangle would ex-ceed 180 degrees, and a star-ship traveling on a straight line

would eventually return to itspoint of origin. If the quantityabove is less than one, the Uni-verse is an open space in whichtriangles contain less than 180degrees. If the quantity is exact-ly one, the space is Euclidean,which is also called flat.

hav e rapidly fallen tow ard zero. For

omega to be anyw here nea r one

today, it m ust hav e been extraordi-

narily close to one at early t imes. For

example, consider one second after

the Big Bang, the t im e at w hich the

processes related t o Big Bang nuc leo-

synt hesis w ere just beginning. For

omega to be anyw here in the allowed

range today, at that t ime omega must

have equaled one to an accuracy of

15 decimal places!A sim ple explosion gives no ex-

planation for this razor-sharp fine-

tunin g, and indeed no explanat ion

can be found in the tradit ional ver-

sion of t he Big Bang theory. The ini-

t ia l va lues of the mass density and

expansion rate are not predicted by

the theory, but must be postulated.

U nless w e pos tu la te tha t the ma ss

d e n s i t y a t o n e s e c o n d j u s t h a p -

p en e d t o h a v e a v a l u e b e t w e en

0.999999999999999and

1.000000000000001

t im es the crit ical density, how ever,

the theory w i l l no t describe a un i-

verse tha t resembles the one in

w hich w e l ive .

TH E IN FLATION ARY U N IVERSE

Although t he propert ies o f the Big

Bang are very special, w e now know

tha t the law s o f phys ics provide a

m echan ism th a t p roduces exac t ly

this sort of a bang. The mechanism

is know n as cosmic infl at ion.

The crucial property of physical

law tha t m akes infl at ion possible is

the existence of states of mat ter that

have a high energy density tha t can-

not be rapidly low ered. Such a stat e

is called a false vacuum, w here the

w ord vacuum ind ica tes a s t a te o f

low est possible energy density, an d

t h e w o r d fa ls e i s u se d t o m e a n


20/67

18 FA LL 1997

In f la t ion i s a w i ld f i re tha t w i l l in

evitably t ake over the forest , as long

as there is some chance tha t i t w il

start .

Once a patch of t he early U niverse

is in the false vacuum stat e, the re

pulsive gravitational effect drives the

patch int o an inflat ion ary period o

exponenti al expansio n. To produce a

universe w ith the special features o

th e Big Bang discussed abov e, the ex

pansion factor must be at least abou1025. There is no upper limit to t he

am ount of expansion. Eventually the

false vacuum decays, and th e energy

that had been locked in it is released

This energy produces a hot, uniform

soup of particles, w hich is exactly t he

assumed start ing point of the tradi-

t ional Big Bang theory. At this point

the infl ationary th eory joins onto the

older theory, mainta ining all the suc

cesses for w hich t he Big Bang theory

is believed.In the inflat ionary t heory the U ni

verse begins incredibly sm all, perhap

as small as 1024 cm , a hundred bil

lion tim es smaller than a proton. The

expansion ta kes place w hile the false

vacuum m aintains a nearly constant

energy density, which m eans that the

to t a l energy increases by the cube

of the linear expansion factor, or at

least a factor of 1075. Although this

sounds like a blatant violat ion of en

ergy conservat ion, i t is in fact con-

sistent w ith physics as w e know i t .

The resolution t o th e energy para

dox l ies in the subt le behavior o f

g rav i t y . Al though i t ha s no t been

wid e ly a p p re c i a t e d , N e wt o n i a n

physics unambiguously implies tha

the energy o f a g rav i t a t iona l f ie ld

i s a lw a y s n e ga t i v e , a f a c t w h i c h

h o l d s a l s o i n g en e r a l r e l a t i v i t y

Th e N e wt o n i a n a rg u m e n t c lo sel y

temporary . For a period th at can be

long by the s t an dards o f the ea rly

U niverse, the false vacuum acts as if

the energy densi t y canno t be low-

ered, s ince the low ering of the en-

ergy is a slow process. The underly-

ing physics of the false vacuum st ate

is described in t he box on th e left.

The peculiar properties of t he false

vacuum s tem f rom i t s p ressure ,

w hich is large and negative (see box

on the right). Mechanica l ly such anegati ve pressure corresponds to a

suct ion, w hich does not sound l ike

something that w ould drive the U ni-

verse into a period of rapid expansion .

The m echan ical effects of pressure,

how ever, depend on pressure differ-

ences, so they are unimportant if the

pressure is reasonabl y un iform . Ac-

cording to general relat ivity, how ev-

er, there is a gravitat ional effect th at

i s very im port an t under these c ir-

cum stan ces. Pressures, like energydensities, create gravita tional fi elds,

and in part icular a positiv e pressure

crea tes an a t t rac t ive g rav i t a t iona l

field. The negat ive pressure of the

false vacuum , th erefore, creates a re-

pulsive gravitat ional fi eld, w hich is

the driving force behind infl ation.

There a re m any vers ions o f in-

flat ionary theories, but generically

they assume that some small patch

of the early U niverse somehow cam e

to be in a fa lse vacuum sta te . Vari-

ous possibilities have been discussed,

including supercooling during a

phase t rans i t ion in t he ea r ly U ni-

verse, or a purely random fl uctuat ion

of the f ie lds. A chance f luctuat ion

seems reasonable even if the proba-

bil i ty is low , since the inf la t ing re-

gion w ill enlarge by m any orders of

magnitude, wh ile the non-infl at ing

regions w i l l rem a in m icroscopic .

Physics

of the

False Vacuum

THE FALSE VACUUM arises nat-

urally in any theory that containsscalar fields, that is, fields thatresemble electric or magneticfields except that they have nodirection. The Higgs fields of theStandard Model of particlephysics or the more speculativegrand unified theories are exam-ples of scalar fields. It is typical ofHiggs fields that the energy den-sity is minimal not when the fieldvanishes, but instead at some

nonzero value of the field. Forexample, the energy density dia-gram might look like

The energy density is zero if= t, so this condition corre-sponds to the ordinary vacuum ofempty space. In this context it isusually called the true vacuum.The state in which the scalar fieldis near = 0, at the top of theplateau, is called the falsevacuum. If the plateau of theenergy density diagram is flatenough, it can take a very long

time, by early Universe stan-dards, for the scalar field to rolldown the hill of the energy den-sity diagram so that the energycan be lowered. For short timesthe false vacuum acts like a vac-uum in the sense that the energydensity cannot be lowered.

00

ufFalse Vacuum(metastable)

TrueVacuum

EnergyDensity


21/67

BEA M LIN E 19

parallels th e derivat ion of t he ener-

gy density o f an e lectrosta t ic f ie ld ,

except tha t the answ er has the op-

posite sign because the force law has

the opposite sign: tw o positive m ass-

es attract, w hile tw o positive charges

repel. The possibility tha t t he neg-

ativ e energy of gravity could bala nce

the posit ive energy for the ma tt er of

the U niverse was suggested as ear-

l y a s 1932 by Rich ard Tolm an , a l-

though a viable mechanism for theenergy t ransfer was not know n.

D uring in f la t ion , w h i le the en-

ergy of m att er increases by a factor

of 10 75 or more , the energy o f the

gravitat ional fi eld becomes more and

more negative to compensate. The

to t a l energymat t e r p lus g rav i t a -

t ionalremains constan t and very

sma l l , and cou ld even be exac t ly

zero. Con servation of energy places

no l imit on how much the U niverse

can infl ate, as there is no limit t o theam ount of negativ e energy tha t can

be stored in the gravitat ional fi eld.

This borrow ing o f energy f rom

the gravita t ional f ie ld g ives the in-

fl a t ionary paradigm an ent irely di f-

ferent perspective from the classic al

Big Bang theory, in w hich all t he par-

t ic les in t he U niverse (o r a t lea s t

their precursors) w ere assumed to be

in place from t he start . Infl ation pro-

vides a mechanism by w hich the en-

tire U niverse can develop from just

a few ounces of primordia l ma t ter .

Inflat ion is radically at odds w ith the

o ld d ictum o f Dem ocr itus and Lu-

c ret iu s , N o t h in g c a n b e c re a t e d

from nothing. I f in fla t ion is r ight ,

everyt hing can be created from not h-

ing, or at least from v ery lit t le. If in-

f la t ion i s r igh t , the Universe can

properly be called t he ult im ate free

lunch.

Pressure

of the

False Vacuum

THE PRESSURE OF THE FALSE VACUUM can be determinedby a simple energy-conservation argument. Imagine a chamberfilled with false vacuum, as shown in the diagram below.

For simplicity, assume that the chamber is small enough so thatgravitational effects can be ignored. Since the energy density of thefalse vacuum is fixed at some value uf, the energy inside the cham-ber is U= ufV, where Vis the volume. Now suppose the piston is

quickly pulled outward, increasing the volume by dV. If any familiarsubstance were inside the chamber, the energy density woulddecrease. The false vacuum, however, cannot rapidly lower its ener-gy density, so the energy density remains constant and the total en-ergy increases. Since energy is conserved, the extra energy mustbe supplied by the agent that pulled on the piston. A force is re-quired, therefore, to pull the piston outward, implying that the falsevacuum creates a suction, or negative pressure p. Since the changein energy is dU= ufdV, which must equal the work done, dW= pdV,the pressure of the false vacuum is glven by

p = uf .

The pressure is negative, and extremely large. General relativitypredicts that the gravitational field which slows the expansion of theuniverse is proportional to uf+ 3p, so the negative pressure of thefalse vacuum overcomes the positive energy density to produce anet repulsive gravitational field.

Before:

After:

False Vacuumu= uf, p= ?

True Vacuumu= 0, p= 0

dV

dW= p dV= uf dV

F


22/67

20 FA LL 1997

encompass the ent ire observed

Universe . The un i fo rmi ty i s p re-

served by this expansion, because the

law s of physics are (w e assume) the

same everyw here.

The infl ationary model also pro

vides a simple resolution for the fl at

ness problem, the f ine-tuning re-

qu ired o f the mass densi t y o f the

ea r ly U niverse . Reca l l tha t the ra

t io of the actual m ass density t o the

crit ical density is ca lled omega, andtha t t he problem arose because the

condit ion = 1 is unstable: om ega i

a lw ays driven aw ay from one as the

U niverse evolves, m aking it diffi cul

to unders t and how i t s va lue today

can be in the vicinity of one.

D uring the inflat ionary era, how

ever, th e peculiar nat ure of the false

v a c u u m s t a t e resu l t s in so m e im -

portant sign changes in the equa tions

tha t describe the evo lu t ion o f th e

U niverse. D uring this period, as w ehave discussed, the force of gravit y

acts t o accelera te t he expansion of

the U niverse ra ther than t o re t a rd

it . I t turns out that the equation gov

erning the evolution of omega also

has a crucia l chan ge of sign: during

the inflat ionary period the U niverse

is driven very q uickly and very pow

erfully towards a crit ical ma ss den-

sity. This effect can be understood i

one accepts from general relat ivit y

the rela t ionship betw een a cri t ica l

ma ss density and th e geometric fl at

ness of space. The huge expansion

factor of infl ation drives the Un iverse

tow ard fl atness for the same reason

tha t the Ear th appears f l a t , even

though i t i s rea l ly round . A sm a l

piece of any c urved space, i f m ag

nifi ed suffi cient ly, w il l appear fl a t .

Thus, a sh ort period of inflat ion

can drive th e va lue of omega very

INFLATION AND TH E VERY

SPECIAL BANG

Onc e infl ation h as been described, it

is not ha rd to see how it produces just

the special kind of bang that w as dis-

cussed earlier.

Consider first the horizon prob-

lem, the diffi culty of understanding

the large-scale homogeneity of the

U niverse in the context o f the t ra-

ditional Big Bang theory. Suppose w etrace back through tim e the observed

region of the Universe, w hich has a

radius today of about 10 billion light-

years. As w e trace its history back t o

the end of the infl ationary period, our

descr ipt ion i s iden t ica l t o w ha t i t

w ould be in the tradit ional Big Bang

theory, since the tw o th eories agree

exactly for all t im es after the end of

infl at ion. In t he infl at ionary theory,

how ever, th e region undergoes a t re-

mendous spurt of expansion duringthe inf la t ionary era . It fo l low s tha t

the region w as incredibly sm all before

the spurt of expansion began1025 or

more t im es sma ller in radius than in

the traditiona l theory. (N ote that I am

not saying that Universe as a whole

was very sma l l . The in f la t ionary

model makes no statem ent about the

size of the U niverse as a w hole, which

might in fact be infi nite.)

Because the region w as so small,

there was p len ty o f t ime fo r i t t o

come to a uniform tem perature, by

the same mundane processes by

w hich a cup o f ho t co f fee coo ls to

room tem perature as it sits on a table.

So in the infl ationary m odel, the uni-

form tempera ture w as es t ab l i shed

be fore in f la t ion took p lace , in an

ex t remely sm a l l reg ion . The pro-

cess of inflat ion th en stretched this

reg ion to become la rge enough to

1040

1020

100

1020

1040

1060

1040 1020 100 1017.5

Time (seconds)

RadiusofObservedUniverse

(m)

Present

Standard Theory

Inflationary Theory

Inf la tionary Period

10

The solution to the horizon prob lem . The

green line show s the radius of the region

that evolves to b ecom e the presently

ob servab le U niverse, as d escribed by

the traditional B ig B ang theory. The

black line show s the corresponding

curve for the in ationary theory. D ue to

the spectacular grow th spurt duringinfl ation, the infl ationary curve show s a

m uch sm aller U niverse than in the

standard theory for the p eriod before

infl ation. The uniform ity is established at

this early tim e, and the region is then

stretched by infl ation to b ecom e large

enough to encom pass the ob served

U niverse. N ote that the num bers

describing in ation are illustrative, as the

range of possibilities is very large.


23/67

BEA M LIN E 2

accurately to one, no mat ter w here it

start s out. There is no longer any need

to a ssume tha t the in i t i a l va lue o f

omega w as incredibly close to one.

Furtherm ore, there is a predict ion

that arises from this behavior. The

mechanism t hat drives omega to one

a lm o s t a lw a ys o v e rsh o o t s, w h ic h

means that even today the mass den-si ty should be equal t o the cri t ica l

val ue to a h igh degree of accu racy. (If

Einstein s cosmologica l constant

is nonzero, this prediction is m odi-

fi ed to become + /3H2= 1, w here

His H ubbles const ant .) Thus, t he de-

terminat ion of the mass density o f

the U niverse could be a very impor-

tant test o f the infla t ionary m odel.

U nfortunately, it is very difficult to

reliably estimate t he ma ss density of

the U niverse, since most of the m at-te r in the Univ erse i s da rk ,

de tec ted on ly t h rough i t s g rav i t a -

tional pull on visible matt er. C urrent

estim ates of omega range from 0.2 t o

1.1. Nonetheless, it is likely that this

issue can be set t led in t he near fu-

ture . The high precision m easure-

ments of t he microw ave background

rad ia t ion tha t w i l l be made by the

Microw ave Anisotropy Probe, sched-

uled for launch in a bout 2001, are ex-

pected to p in down the va lue o f

omega to about 5 percent accuracy.

THE C U RRENT PICTURE

Wh i l e i t m a y b e t o o e a r l y t o s a y

that inflat ion is proved, I claim tha t

the case for inflat ion is com pelling.

I t i s h a rd t o e v e n c o n c e iv e o f a n

a l t e rn a t i v e t h e o ry t h a t c o u ld e x-

p la in the bas ic fea tures o f th e ob-

served U niverse. Not only does in-

f l a t i o n p ro d u c e j u s t t h e k in d o f

specia l bang tha t m a tches the ob-served Univ erse, but quantum fluc-

tuations during inflat ion could have

p ro d u c e d n o n u n i f orm i t i e s w h ic h

served as the seeds of cosmic st ruc-

ture. These nonuniform ities can be

o b se rv e d d i re c t l y in t h e c o sm ic

background rad ia t ion , wi t h an am -

plitude of about one part in 100,000.

S o f a r t h e m e a s u r e m e n t s o f t h e

sp e c t ru m h a v e b e e n b e a u t i f u l l y

consistent w ith t he predictions of in-

f la t ion, a l though it m ust be admit -

ted that nonuniformit ies created by

cosmic s t r ings a re a lso cons is ten t

w ith th e observations. C osmic strings,how ever, cannot expla in t he large-

sca le homogene i t y o r the f l a tness

of the U niverse.

Whi le the ca se fo r in f la t ion i s

strong, it should be stressed that in-

fl ation is really a paradigm a nd not a

theory. The statement th at th e Uni-

verse arose from infl ation, if it is t rue,

i s no t t he end o f the s tudy o f cos-

mic originsit is in fact closer to the

beginning. The deta i ls o f in f la t ion

depend upon the deta i ls o f t he un-

derlying part icle physics, so cos-

mology an d particle physics become

intim ately lin ked together. While I

cannot see any viable alternative to

the genera l idea of in f la t ion, th ere

is st ill much w ork to be done before

a detailed picture is established. And

I suspect that there is room for ma ny

new important ideas.

The expanding sphere illustrates the

solution to the atness problem in

infl ationary cosm olog y. A s the sphere

becom es larger, its surface b ecom es

atter and atter. Sim ilarly the in ation of

space causes it to becom e geom etri-

cally at, and g eneral relativity im pliesthat the m ass density of a at universe

m ust equal the critical value.


24/67

22 FA LL 1997

WH EN TH Eoriginat ors of theSloan D igital SkySurvey (SDSS) m et at O Hare

International a irport in t he fall

of 1988, their intent w as to

form a collaborat ion t hat

w ould measure the size of thelargest st ructures of galaxies in

th e U niverse. Previous galaxy

surveys had show n that the

largest st ructures w ere at least

400 m illion light years in

extent as la rge as t he largest

structures that could have been

found by th ese surveys. In par-

ticular, the results of the C fA

Redshift Survey w ere astound-

ing. From t he spectra of on e

thousand galaxies, the re-

searchers w ere able to depict a

slice of our Un iverse w ith large

voids (w here the galaxy den-

sity w as very low ) surrounded

by dense w alls of galaxies.

heSloa

nDigit

alSk

ySur

vey

Pion th

eSky

y HEIDI

JONE

WBE

RG


25/67

BEA M LIN E 23

C osmology ha d already seen t he demise of the perfect cosmological prin-

ciple in 1929 w ith Edw in Hubbles discovery t hat the U niverse is expanding,

and t herefore chan ges over tim e. We contin ue to believe, how ever, in th e

cosm ological principleth at t he U niv erse is hom ogeneous and isot ropic. To

demonstrat e th e validity of this basic assumption about our U niverse, w e

m ust be able to fi nd some volum e of the U niverse that is representa tive of

the w hole. It is clear that a m uch larger galaxy redshift survey is required to

fi nd a represent ative sam ple of the U niverse.With projects to m easure ten t o one hundred thou sand galaxi es already

plann ed or underw ay, th e early organizers of th e Sloan D igital Sky Survey

(SDSS) proposed a redshift survey of on e milli on galaxies. Rat her tha n look-

ing at a slice of the U niverse, th is new survey w ould measure the position of

every galaxy in a patch of sky steradian s (one qu arter of th e sky) in siz e.

The survey w ould m easure the distan ces to galaxies th ree ma gnitudes

fainter (about four t im es farther aw ay) tha n t hose observed in th e C fA

Redshift Survey. In addition, t hese million galaxies w ould not be chosen

from the photographic sky surveys already in existence. Rather, they w ould

be selected from a new , carefully con trolled survey of th e sky using a largecha rge-coupled dev ice (C C D ) cam era. By im aging th e sky in several optical

passbands, including an ult ra-violet passband, t he data from this sky survey

w ould also be used to select qua si-stellar objects (Q SO s). As the m ost distan t

collapsed objects ever observed in our U niv erse, these w ould give us infor-

m ation about the st ructure of th e U niverse on t he largest possible scales.

The survey w as projected to t ake fi ve years to build, w ith a n additional fi ve

years of operation to complete the scientifi c objectives.


26/67

24 FA LL 1997

and th e budget w ere optim istic. But

i f w e w e re n o t a t t e m p t in g t h e im -poss ib le , we would no t be on the

forefront of research.

THE SLOAN DIG ITAL SKYSurvey has a t t racted the ac

tiv e partic ipation of over one

hundred scient ist s , engineers, and

softw are professionals from eight as

tronomy groups and departm ents, in

cluding: Princeton University, The

U nivers i t y o f C h icago , The JohnsHopkins U niversi ty , th e Japan Par-

t ic ipa t ion G roup (sc ien t is t s a t t he

U niversities of Tokyo a nd Kyot o), the

U nited States Na val Observatory, the

U niversity of Washington, the Insti

tute for Advanced Study, and Ferm

N ationa l Accelerator Laboratory. The

survey is being carried out under

t h e a u spic es o f t h e A st ro ph ys i c a

Research Consortium (AR C ) and h as

received signifi cant funding, totaling

about 54 mil l ion do l la rs , f rom t he

Alfred P. Sloan Foundation (N Y), fro m

the Na t iona l Sc ience Founda t ion

and from each of the mem ber insti

tutions.

The goals a nd sc ope of the projec

have changed on ly s l igh t ly f rom

t h o se pu t f o r t h b y t h e O H a re

group. Since the main surv ey area

is not observable during part of t he

year, three extra st r ips o f sky have

Al though i t may no t have been

recognized at t he t im e, the addit ion

of the ima ging survey t ransformed

the SD SS project from an am bitious

att empt to t race the large scale struc-

tures in the Un iverse into a plan to

stat ist ically sample everything in a

large corner of the visible U niverse.

What these planners had dreamed up

w as an im aging survey covering tenthousa nd squ are degrees of the sky

in four fi lters; a catalog of the 70 mil-

l ion stars , 50 m ill ion ga laxies, and

one million QSO s visible in t he imag-

ing survey; an d a spectroscopic sur-

vey of more than a million of these

objectsall rolled int o one enorm ous

project . This stat ist ical sam ple w ill

have a tremendous impact not only

on our un derstandin g of th e largest

s t ruc tures , bu t on every a spect o f

astronomy.

Any one of th ese three projects

(the im aging survey, the cata log, or

the spectroscopic survey) w ould have

been considered large by the sta n-

dards of ground-based ast ronomy.

Any one of th e three could be sci-

entifi cally justifi ed on its ow n merit .

All together, th e project is a s colos-

sal as its impact w ill be on astrono-

my. Okaythe goals, the t imeline,

A bove: The A pache Point O bservatory

in Sunspot, N ew M exico. The SD SS 2.5

m eter telescope (left) is show n w ith its

protective building rolled aw ay, as for

nightly operation. A lso show n is the

m onitor telescope dom e (top right).

(C ourtesy A pache P oint O bservatory)

M iddle: Prototype ber plug -plate for the

SD SS sp ectrog rap h. D uring op eration,all 640 bers w ill be inserted into the

plug-plates by hand. Several plates w ill

be plug ged during the day in

preparation for a nights observations.

Each ber w ill guide the light from a

galaxy, Q SO , or star into the

spectrog rap h cam era. (C ourtesy

Ferm ilab Visual M ed ia Services)

K

urt

A

nderson


27/67

BEA M LIN E 25

been added to fi ll in t he gap. Also, w e

have added one passband to t he ima g-ing survey, for a tota l of fi ve fi lters.

Most ly, w e have made t remendous

progress designing and bu ilding t he

hardw are and softw are necessary to

assure our success.

The a s t ronomica l da t a fo r the

SDSS w ill be obtained from t w o ded-

icated telescopes located at Apache

Point O bservatory in Sunspot, New

Mexico . The data w il l be part ia l ly

processed at the observatory before

being sent to Fermi N ational Accel-erator Laboratory, w here the m ajor-

i ty o f the da t a processing, s torage,

and distributi on w ill take place. The

m a in SDSS telescope has a prim ary

mirror 2.5 meters in diameter and a

fi eld of view three degrees in diam-

eter. It w ill support tw o instruments:

a photometric camera containing 54

C C D s and a spect rograph w i th 640f ibers . A fu l ly au tom a ted 24-inch

diam eter te lescope w il l opera te si-

m ultaneously. The survey softw are

is designed to opera te these te le-

scopes, p lan imaging and spectro-

scopic observa t ions so a s t o m in i-

miz e th e survey t ime-to-completion,

acquire the data from all survey in-

strum ents, process ima ging data into

catalogs of astronomical objects and

their associated parameters, calibrate

the positions and luminosit ies of the

m easured objec t s , m erge the da t a

from dif ferentC C D

s and dif ferentnights int o one large catalog, select

f rom th is ca t a log the sources fo r

w hich w e w il l obta in spectra , orga-

nize th e targeted objects int o sepa-

rate spectroscopic exposures, reduce

spectroscopic exposures into l is t s

o f objec t s w i th c la ss i f ica t ions and

redshifts, and st ore the results of all

of these steps in a large databa se.

TH E SD SS is aggressivelychar t ing new te rri to ry bo th

in t he design of the telescope

and ins t ruments , and in the pro-

cessing and acq u is i t ion o f th e sc i-

ent i f ic da t a . The spectrograph w il l

be capable of observing m ore objects

a t o n e t im e t h a n a n y o t h e r in t h e

w orld. The photom etric camera w ill

have m ore pixels in the focal plane

t h a n a n y o t h e r C C D c a m e r a i n

Telescope engineer C harlie H ull poses w ith the

skeleton of the Sloan D igital Sky Surveys 2.5

m eter telescope shortly after it w as installed.

D

an

Long


28/67

26 FA LL 1997

tractable. By building a camera wit h

30 la rge C C D s and us ing a dr i f t

scanning technique, w e w ill not only

be able to survey large areas of sky

but also to obtain the im ages simul

taneously in fi ve fi l ters .

In addit ion to the array of 30 pho

tometric C C D s, the focal plane con

ta ins 22 smaller ast rometric C C D s

w hich are used to ca l ibra te the po

sitions of the survey objects. These

C C D s image bo th the a s t romet r icstandard stars (which are saturated

on th e photometric C C D s) and also

some of the brighter stars tha t w il

be unsatura ted on th e photometric

C C D s. This allow s us to t ie our data

to a coordinate system that is fi xed

on the sky. Also, it allow s us to m ea

sure m ore accura te ly t he rela t ive

positions of th e objects found in sep

arate C C D s in the photom etric array

By comparing the same stars as im-

aged at the beginning and end of theC C D array, w e can assess how w el

the telescope is tracking its trajec

tory in th e sky.

The photometric a ccuracy of our

catalogs w ill be limit ed by our abil

ity to characterize the atmospheric

condi t ions dur ing the n igh t . Even

though photom etry w ill be at t empt

ed only on th e clearest , m ost stable

n igh t s , we w i l l measure the t rans

parency of t he atm osphere as a func

t ion of t im e, f i l ter, and posit ion in

the sky . The su i t ab i l i t y o f a g iven

night for photometry will be deter

mined w ith da ta from a w eather sta

t ion w hich logs the t empera ture

w ind speed and direction, hum idity

and dust leve l . In add i t ion , the

w ea ther st a t ion inc ludes a camera

w h ic h im a g es t h e wh o le sk y a t 10

microns every 20 minut es . At th is

w ave leng th , c louds s t and ou t very

existence. Our cat alog of objects w ill

be the largest , and w il l have bet ter

posit ional and photometric accura-

cy th an any other catalog of its kind.

In order to assure th e astromet ric and

photometric uniformity w e require

for describing our statistica l sam ples

of the sky, w e have included in the

des ign severa l nove l ins t rum ents

w hich w il l a l low us to eva luate and

ca l ibra te the da t a be t t e r than any

previous survey. I w ill discuss hereonly a few of the innovat ions w hich

ma ke the survey possible.

The 2. 5 m eter te lescope is spe-

cially designed to reduce dom e see-

ing, the distortion of images caused

by t urbulence in the air very close to

the telescope. To reduce dist ort ion

caused by disruption of th e lamina r

fl ow of air over the observatory, the

SDSS telescope is cant ilevered over

the edge of a cliff in th e direction of

the prevailing wind. In addit ion, w euse a roll-off building w hich elim i-

nat es the telescope building as a po-

tential cause of heat, w hich also con-

tributes to image distort ion. D uring

operation , th e telescope is protected

f rom w ind and s t ray l igh t by a ba f-

f le tha t i s m echan ica l ly separa ted

from the te lescope, but t ha t moves

and t racks w ith i t .

In all areas of optical astronom y

except surveys, data from C C D cam-

eras has supplant ed data from pho-

tographic plates. C C D cameras, un-

like th e plates, have linear response

functions and much higher effi cien-

cy for detecting light , w hich m akes

possible more accura te photomet-

ric (lumin osity) m easurements. U n-

til now , these cam eras w ere not used

for surveys because i t w as not pos-

sible to cover enough sky w ith one

cam era for a large area survey to be

Project Scientist Jim G unn m ounts the

quartz corrector for the SDSS photo-

m etric cam era.

This 24-inch telescope w ill be used to

de ne the SDSS lter system and w ill

also m onitor the atm osphere during

operations.

D

an

Long

M

ike

C

arr


29/67

BEA M LIN E 27

THE SDSS CAMERA will drift-scan the sky, rather thanusing the more common point-and-shoot method, in order

to increase the fraction of the time that the imaging cam-era is integrating light from the sky and to reduce the number ofimages that must be pieced together. Point-and-shoot observa-tions are obtained by tracking the apparent motion of the targetobject in the sky, and opening and closing a shutter to expose thedetector. Drift-scanning is usually done by fixing the position ofthe telescope and moving the photo-sensitive material to trackthe target object. The Sloan Digital Sky Survey will have to moveboth the telescope and the detector to image the survey area.

Before explaining how the SDSS camera works, let me firstdescribe a much simpler drift-scanning camera. Imagine a tele-scope in a fixed position on the Earths equator, and pointingdirectly overhead. The telescope focuses light from the sky

onto a single charged coupled device (CCD) in the focal plane.As the Earth turns, stars will enter the field-of-view of the CCD,travel across it at constant speed, and then disappear fromview. To drift-scan, the two dimensional array of pixel detectorson the CCD must be aligned so that the crossing star travelsexactly along one column. As light from the star moves fromone row of the CCD to the next, all of the accumulated photo-electrons are also moved to the next row. This leaves oneempty row of pixels at the beginning of the CCD, ready to startexposing a new part of the sky. The photoelectrons in the lastrow are read out and digitally stored in a computer. The cam-era accumulates data continuously along one equatorial stripof sky, without stopping to read the data out of the CCD whilethe shutter is closed. The effective exposure time of the data isthe crossing time of a star across the camera.

To scan across the sky in a direction that is not along con-stant latitude or that is far from the equator requires the tele-scope to track and (in most cases) the CCD to rotate. Trackingis also required to drift at arbitrary rate (which sets the expo-sure time) across the sky. The first telescope and CCD combi-nation capable of driven drift scans in arbitrary directions wasthe Fermilab Drift-Scan Camera mounted on the ARC 3.5 metertelescope at Apache Point Observatory, adjacent to the SDSStelescope site. The camera, which was commissioned in 1994,was built as a prototype for testing SDSS data-acquisition

software.The SDSS camera puts 30 large, 20482048 pixel CCDs inthe focal plane of the telescopesix columns of five CCDs. Thesix columns each scan a separate strip of sky while the camerais imaging. Each of the five CCDs in a given column images thesame strip of sky, but using a different filter. The effective expo-sure time in each filter is about 55 seconds. A given object willfirst traverse the CCD with the r filter, then the i , u , z , andg filters, in succession. It takes about 5.8 minutes to traverseall five filters. Each column images a strip of sky 13.7 arc min-utes wide which increases in length by 15 degrees per hour. A

similar camera operated in point-and-shoot mode would spendabout as much time exposing a 55-second image as it did read-ing out the CCD. Also, we would need to piece together 250,000individual images rather than about 1000 long, continuous strips.

In addition to the photometric CCDs, there are 22 astromet-ric chips and two focus chips in the focal plane. Since theseCCDs are the same width and pixel scale as the photometricchips, each row is read out with the same frequencyproducing the same 9.5 megabytes per minute per CCD. Fewerrows in the astrometric and focus CCDs produce shorter expo-sure times rather than lower data rates. The data from thefocus chips will be used to automatically adjust the focus in

real time. Twelve of the astrometric chips, those at the leadingand trailing edge of each column of photometric CCDs, will beused to assure that the rotation of the camera is aligned withthe transit of the sky across the camera and to measure theuniformity of the tracking rate. The ten interleaving astrometricCCDs tie together the positions of the objects found in adjacentcolumns of CCDs.

The SDSS photometric camera will image 164 square de-grees of sky on an average night. Including overhead andoverlaps between strips of sky, we will be able to cover theSDSS survey area in ninety dark, photometric nights.

Drift-Scanning the Sky

The focal plane of the SDSS cam era. The cam era contains 30

photom etric C C D s (shaded squares),22 astrom etric C C D s

(shad ed rectangles), and 2 focus C C D s (topm ost and

bottom m ost shaded rectangles). (C ourtesy M ike C arr)

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28 FA LL 1997

m onitor t e lescope and reduces the

ima ges; the softw are that selects sev

eral dozen different t ypes of spectroscopic targets w ith th eir individual

selection criteria; and on our ability

to effectively monitor the process.

Building the dat a processing and

storage system s necessary to run the

S D S S requires a higher level o f in-

frastructure than is available at any

of the universities involved w ith t his

project. In im plem enting the dat a ac

quisit ion system , the infrastructure

for the data processing softw are, and

the mechanism s for data storage, w ehave benefi ted from Fermilabs many

years of experience w ith high energy

physics experiment s. Like high en-

ergy phy sics experim ents, t he scien

t i f ic objec t ives , ins t ruments , an d

sof tw are are provided by scient ists

at Fermilab an d at each of the insti

tut ions in the collaboration . The staf

at Ferm ilab supplies the expertise in

m anaging la rge scient i f ic projects

and the accompanyin g infrastructure

that bring the project together

Fermilab staff have been instrumen

ta l in inst i tut ing coding standards

mainta ining code

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