Table Of ContentAstronomers’ Universe
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Other titles in this series
Origins: How the Planets, Stars, Galaxies,
and the Universe Began F
Steve Eales
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Calibrating the Cosmos: How Cosmology Explains Our
Big Bang Universe O
Frank Levin
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The Future of the Universe
A. J. Meadows
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It’s Only Rocket Science: An Introduction in Plain English
Lucy Rogers
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Rejuvenating the Sun and Avoiding Other
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Global Catastrophes
Martin Beech
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The Universe Before the Big Bang
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Maurizio Gasperini
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Amedeo Balbi
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The Music of the Big Bang
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The Cosmic Microwave Background
and the New Cosmology
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Dr.AmedeoBalbi
UniversitàRoma,TorVergata
Dipto.Fisica
ViadellaRicercaScientfica,1
00133Roma
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Italy
[email protected]
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TranslationfromtheItalianlanguageedition:
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LamusicadelBigBangbyAmedeoBalbi
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To my parents
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Preface
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Ever since its infancy, humankind has been seeking answers to
some very basic and profound questions. DiPd the Universe begin?
If it did, how old is it, and where did it come from? What is its
shape? What is it made of? Fascinating m yths and brilliant intu-
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itions attempting to solve such enigmas can be found all through
the history of human thought. Every culture has its own legends,
itsownworldcreationtales,itsphiloEsophicalspeculations,itsreli-
giousbeliefs.Modernscience,however,cannotcontentitselfwith
fanciful explanations, no matter Thow suggestive they are. Nowa-
days, our theories about the Universe, built upon rational deduc-
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tion,have tosurvivethe hardtestofexperimentand observation.
Cosmology, the science which studies the origin and evolu-
tionoftheUniverse,hadtoEovercomeenormousdifficultiesbefore
it could achieve the same level of dignity as other physical disci-
plines.Atfirst,ithadnoRseriousphysicalmodelandmathematical
toolsthatcouldbeusedtoaddressthecomplexityoftheproblems
it had to face. Then, iRt suffered from a chronic lack of experimen-
tal data, which made it almost impossible to test the theoretical
speculations.GiveOn this situation,answeringrigorouslythemany
questionsonthenatureoftheUniverseseemednothingmorethan
a delusion.
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Today, however, things have changed. We live in the golden
ageofcosmology:anexcitingmoment,when,forthefirsttime,we
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areable to scientificallyunderstandour Universe.
One of the greatest living cosmologists, James Peebles, once
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compared the situation of a cosmologist to that of Tantalus.
According to the myth, Tantalus was punished by Zeus for hav-
ing discovered the secret of ambrosia, the food which made the
godsimmortal.Sentencedtosufferfromhungerandthirst,hewas
immersedinwater,buthecouldnotdrink,becausewhenhetried,
the waterreceded;juicy fruits were suspendedabovehis head,but
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viii Preface
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when he raised his hands to grasp them, the branches rose away
from him. O
Similarly, cosmologists can look at celestial objects as long
as they wish, but they cannot “touch” them. This is one of the
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facts that puts cosmology in a more difficult situation than other
fields of physics. If we want to study the properties of a material,
we can analyze a certain quantityof it in a laboRratory,given some
carefullycontrolledconditions.Wecanalsorepeattheexperiment
for as long as we like. Cosmologists cannotParrange anything like
that.TheyjusthaveoneUniversetostudy,theycannotdecidehow
to set up its conditions. Furthermore, th e subject of investigation
is, by its nature, very difficult to grasp.DDelving into the furthest
reaches of the Cosmos and observing the feeble light reaching us
from unimaginable distances, requirEes sophisticated instruments
which have not been available to us until very recently. Much
more than for any other science,Tthe history of cosmology is also
a history of the tools we use to observe the world. Our idea of the
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Universe has been shaped by the extent of what we could observe
of it. Starting from Galileo’s telescope, the Cosmos became larger
and strangeras we wereableEto look into it in moredetail.
Asastrangesortofcompensation,however,thevastityofthe
Cosmos is also an unexpRected tool that we can use to investigate
its properties. The Universe is so large that it takes an enormous
amountoftimeforligRhttotravelthroughit(evenatthemaximum
speed allowed by the laws of physics,about 300 thousandkilome-
terspersecond).WhenwelookattheSun,weseeitasitwasabout
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8 minutes ago; when we look at the closest star to the Sun, Alfa
Centauri, we see it as it was about 4 years ago; the closest galaxy
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to our own, M31 in Andromeda, looks as it was about two and a
halfmillionyearsago;andsoon.Cosmologistsmeasuresuchhuge
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distancesin terms of light-years—thedistance traveledby light in
one year. One light-year corresponds to a distance of about 9,460
billion kiUlometers.
This fact provides cosmologists with a sort of time machine.
They can look at the Universe at different phases of its evolution
and reconstruct its history, much as an archaeologist can look at
fossilsfromdifferentepochs.Usingthisextraordinaryopportunity,
and perfecting it over the years, cosmology slowly abandoned the
status of immature science it had at the beginning of the 20th
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century,startinga toughpath which, in the last few years,has led
to its becoming one of the most advanced and succesOsful fields of
scientificresearch.
We now know, for example, that the Universe expands and
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evolves, and that it reached its current conditions starting from a
much simpler state,when it was much smaller and denserthan it
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istoday.PushingourphysicaldescriptionoftheUniversetoearlier
andearliertimes,weeventuallyreachastateofpracticallyinfinite
temperature and density, taking place abouPt 14 billion years ago,
which is popularly known by the name of “Big Bang”. The cos-
mological model based on Big Bang is ex traordinarily effective in
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describing the evolution of the entire Universe—and, at the same
time, surprisingly simple. It only takes a handful of parametersto
characterizethe physical state of theECosmos from its very begin-
ningtothepresent.ThemainfeaturesoftheBigBangcosmological
modelare describedin Chapter1Tofthis book.
At some point, during their incessant research about the ori-
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ginsofourUniverse,cosmologistsstartedaskinghowfarinspace—
andthenbackintime—theycouldgowiththeirobservations.Was
it perhaps possible to look dEirectly at the moment when the Uni-
versebegan?Initsearliestphases,theentireUniversewasextraor-
dinarily hot and bright.RAt first, all this light could not travel a
long way, because of the dense fog of matter which pervaded the
Cosmos. But after a feRw hundred thousand years the Universe be-
came transparent, so that light could finally stream unimpeded
throughspace.TodOay,morethan13billion yearslater,a dim trace
of the immense primordial glow keeps coming to us from the far-
thestreachesoftimeandspace.Althoughonlycoldcindersremain
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ofthattremendousprimordialincandescence,wecanstillmeasure
its presence.It pervadesthe entire space,all around us. If we tune
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our radio on an empty channel, about one percent of the noise we
hearismadeupofthiscosmicsignal,themostdistantandancient
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in the Universe. This fossil relic of the Big Bang is called the cos-
mic microwave background (or CMB for short), and it is the real
protagonist of this book. By observing it, cosmologists collected a
numberofimportantcluesaboutthephysicalstateoftheUniverse
in its earliest phases. Chapter 2 deals with a detailed explanation
of its origin,and tellsthe fascinatingstoryof its discovery.
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The primordial Cosmos was a realm of simplicity. A sort of
uniform,thickfogpervadedtheentirespace,andeveryOpointinthe
Universehadalmostidenticaldensityandtemperature.Butthisex-
tremely uniform situation was slightly altered by the presence of
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tiny clumps. Matter then started collapsing around these clumps,
in a slow but inexorable process. The gigantic cosmic structure
we observe in the present Cosmos—galaxies, cRlusters of galaxies,
clusters of clusters and so on—was formed by a gradual process
of aggregation around those ancient cosmicPseeds. Our own exis-
tence, after all, is due to those slight imperfections existing in the
primordialUniverse.InChapter3,Iwill saymoreabouttheorigin
of cosmic seeds, and will explain howDwe finally measured their
existence from the imprints they left in the cosmic background
radiation. E
Cosmic structure formation has been a tug of war between
opposing forces. Dense regions teTnded to grow denser because of
self-gravity,butinternalpressureopposedthisgrowth—justasagas
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resists compression—forcing matter to re-expand. A strange kind
of dance then took place, a series of alternating compressions and
rarefactions of the cosmic flEuid. Those periodic oscillations were
identical to the ones passing through air when sounds propagate.
In other words, acousticRwaves traveled across space in the early
Universe.Byanalysingtheripplesthesewavesleftimprintedinthe
cosmic background raRdiation, cosmologists can today reconstruct
their complicatedoverlapping,which encodescrucialinformation
onthephysicalstateoftheprimordialCosmos.Justasanymusical
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instrument produces a characteristic spectrum of frequencies, so
the physical parameters which define the nature of our Universe
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manifest themselves through the specific timbre of those primor-
dialacousticwaves.Thehuntforthis“music”oftheBigBangkept
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cosmologists busy for decades. Chapters 4 and 5 tell the story of
this fascinating endeavor, which was finally successful in recent
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Throughout this book, then, we will see how the careful in-
vestigationofthecosmicbackgroundradiationgaveustheanswers
tomanyfundamentalquestionsaboutourCosmos.Wenowknow
that the Universe has been expanding for almost 14 billion years
andthat,perhaps,itwillkeepexpandingforever.Weknowthatthe
majesticarchitectureofgalaxiesformedoverthecourseofbillions
