Physics Journal Workshop
Physics Journal Workshop PHY 250
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Date Created: 10/21/15
By Wayne Hu and Martin White New observations of the cosmic microwave background radiation show that the early unlverse resounded Wlth harmonlous osc1113tions n the beginning there was light Under the intense condi tions of the early universe ionized matter gave off radia tion that was trapped within it like light in a dense fog But as the universe expanded and cooled electrons and protons came together to form neutral atoms and matter lost its abili ty to ensnare light Today some 14 billion years later the pho tons from that great release of radiation form the cosmic mi crowave background CMB Tune a television set between channels and about 1 percent of the static you see on the screen is fromthe CMB When as tronomers scan the sky for these microwaves they nd that the signal looks almost identical in every direction The ubiquity and constancy of the CMB is a sign that it comes from a sim pler past long before structures such as planets stars and galax ies formed Because of this simplicity we can predict the prop erties of the CMB to exquisite accuracy And in the past few years cosmologists have been able to compare these predictions with increasingly precise observations from microwave tele scopes carried by balloons and spacecraft This research has brought us closer to answering some ageold questions What is the universe made of How old is it And where did objects in the universe including our planetary home come from Arno Penzias and Robert Wilson of ATSCT Bell Laborato ries detected the CMB radiation in 1965 while trying to nd the source of a mysterious background noise in their radio anten na The discovery rmly established the big bang theory which states that the early universe was a hot dense plasma of charged particles and photons Since that time the CMB has been cooled by the expansion of the universe and it is extremely cold todayicomparable to the radiation released by a body at a 44 SCIENTIFIC AMERICAN temperature of 27 kelvins that is 27 degrees Celsius above absolute zero But when the CMB was released its tempera ture was nearly 3000 kelvins or about 2727 degrees C In 1990 a satellite called COBE for Cosmic Background Explorer measured the spectrum of the CMB radiation show ing it to have exactly the expected form Overshadowing this impressive achievement however was COBE s detection of slight variationsiat the level of one part in 1000007in the temperature of the CMB from place to place in the sky Ob servers had been diligently searching for these variations for more than two decades because they hold the key to under standing the origin of structure in the universe how the pri mordial plasma evolved into galaxies stars and planets Since then scientists have employed ever more sophisti cated instruments to map the temperature variations of the CMB The culmination of these efforts was the launch in 2001 of the Wilkinson Microwave Anisotropy Probe WMAP which travels around the sun in an orbit 15 million kilome ters beyond Earth s The results from WMAP reveal that the CMB temperature variations follow a distinctive pattern pre dicted by cosmological theory the hot and cold spots in the ra diation fall into characteristic sizes What is more researchers have been able to use these data to precisely estimate the age composition and geometry of the universe The process is anal ogous to determining the construction of a musical instrument by carefully listening to its notes But the cosmic symphony is produced by some very strange players and is accompanied FEBRUARY 2004 by even stranger coincidences that cry out for explanation Our basic understanding of the physics behind these obser vations dates back to the late 1960s when P James E Peebles of Princeton University and graduate student Jer Yu realized that the early universe would have contained sound waves At almost the same time Yakov B Zel dovich and Rashid A Sun yaev of the Moscow Institute of Applied Mathematics were coming to very similar conclusions When radiation was still trapped by matter the tightly coupled system of photons elec trons and protons behaved as a single gas with photons scat tering off electrons like ricocheting bullets As in the air a small disturbance in gas density would have propagated as a sound wave a train of slight compressions and rarefactions The com pressions heated the gas and the rarefactions cooled it so any disturbance in the early universe resulted in a shifting pattern of temperature uctuations Sounding Out Origins WHEN D ISTAN CBS in the universe grew to one thousandth of their current sizeiabout 380000 years after the big bangithe temperature of the gas decreased enough for the protons to cap ture the electrons and become atoms This transition called re combination changed the situation dramatically The photons were no longer scattered by collisions with charged particles so for the rst time they traveled largely unimpeded through space Photons released from hotter denser areas were more en ergetic than photons emitted from rare ed regions so the pat tern of hot and cold spots induced by the sound waves was frozen into the CMB At the same time matter was freed of the radiation pressure that had resisted the contraction of dense clumps Under the attractive in uence of gravity the denser ar 44 391 aala ie In fact tl i 00000 variations observed in the CMB are of exactly the right ampli tude to formthe largescale structures we see today see Reading the Blueprints of Creation by Michael A Strauss on page 54 Yet what was the prime mover the source of the initial dis turbances that triggered the sound waves The question is trou bling Imagine yourself as an observer witnessing the big bang I Inflation the rapid expansion ofthe universe in the first moments afterthe big bangtriggered sound waves that alternatelg compressed and rarefied regions ofthe primordial plasma I Afterthe universe had cooled enoughto allowthe formation ofneutral atoms the pattern ofdensitg variations caused bgthe sound waveswas frozeninto the cosmic microwave background CMB radiation I Bu studging the acoustic signals in the CMB cosmologists have estimated the age composition and geometrg oftheuniverseButtheresults suggestthat the biggest component ofthe modern cosmos is a mgsterious entitg called dark energg 48 SCIENTIFIC AMERICAN and the subsequent expansion At any given point you will see only a nite region of the universe that encompasses the distance light has traveled since the big bang Cosmologists call the edge of this region the horizon the place beyond which you cannot see This region continuously grows until it reaches the radius of the observable universe today Because information cannot be conveyed faster than light the horizon de nes the sphere of in uence of any physical mechanism As we go backward in time to search for the origin of structures of a particular physi cal size the horizon eventually becomes smaller than the struc ture see illustration on opposite page Therefore no physical process that obeys causality can explain the structure s origin In cosmology this dilemma is known as the horizon problem Fortunately the theory of in ation solves the horizon prob lem and also provides a physical mechanism for triggering the primordial sound waves and the seeds of all structure in the uni verse The theory posits a new form of energy carried by a eld dubbed the in aton which caused an accelerated expansion of the universe in the very rst moments after the big bang As a result the observable universe we see today is only a small fraction of the observable universe before in ation Further more quantum uctuations in the in aton eld magni ed by the rapid expansion provide initial disturbances that are ap proximately equal on all scalesithat is the disturbances to small regions have the same magnitude as those affecting large regions These disturbances become uctuations in the energy density from place to place in the primordial plasma Evidence supporting the theory of in ation has now been found in the detailed pattern of sound waves in the CMB Be cause in ation produced the density disturbances all at once in essentially the rst moment of creation the phases of all the sound waves were synchronized The result was a sound spec trum with overtones much like a musical instrument s Consid er blowing into a pipe that is open at both ends The fundamen tal frequency of the sound corresponds to a wave also called a mode of vibration with maximum air displacement at either end and minimum displacement in the middle see top illustration in box on page 48 The wavelength of the fundamental mode is twice the length of the pipe But the sound also has a series of overtones corresponding to wavelengths that are integer fractions of the fundamental wavelength one half one third one fourth and so on To put it another way the frequencies of the overtones are two three fu In or 4 high as 39 I J quency Overtones are what distinguish a Stradivarius from an ordinary violin they add richness to the soun The sound waves in the early universe are similar except now we must imagine the waves oscillating in time instead of space see bottom illustration in box on page 48 In this anal ogy the length of the pipe represents the nite duration when sound waves traveled through the primordial plasma the waves start at in ation and end at recombination about 380000 years later Assume that a certain region of space has a maximum positive displacementithat is maximum temperatureiat in ation As the sound waves propagate the density of the region will begin to oscillate rst heading toward average tempera 39fre FEBRUARY 2004 ammumnrnrmn TIMELINE OF THE UNIVERSE photons L i drlEl 39 0 0quot Photon Neu ron Eledron nu dIIUlIIEI Helium nucleus CMB radiation ern FIIS stars any M d galaxles galaxies ii ii i perature maximum negative displacement The wave that i i i 1y one billion lightryears across This rst and highest peak in the power spectrumis evidence of the fundamental wave which i i i i i i i i 39 39 39 39 39 39 39 39 39 unie versel The overtones have wavelengths that are integer fractions ofthe fundamental Wavelength Oscillating two three or more i i i i i i 1 smaller regions of space to reach maximum displacement eie ther positive or negative at recombination do cosmologists deduce this pattern fromthe CMB They plot the magnitude ofthe temperature variations against the sizes of the hot and cold spots in a graph called a power spectrum see box 071 page 51L The results show that the re gions with the greatest variations subtend about one degree acrossthes y 39 39 39 39 39 quot these regions ha t h t d one mi11ion lightryears but because of the 10007fold expanr www scmm com 39 39 39 39 The subsequent peaks in the power spectrum represent the tem caused by the overtonesl The series of peaks strongly supports the theory that in ation triggered all the sound waves at the same time If the perturbations had been continuously genere at d over time the power spectrum would not be so harmoe niously orderedl To return to our pipe analogy considerthe Car cophony that would result from blowing into a pipe that has ho1es drilled randomly along its length he theory of in ation also predicts that the sound waves shou1d have nearly the same amplitude on all scales The powe PrnPnrnmthvPr 39Klr quot 391 p 39 39 39 39 39 39K This discrepanr cy can be explained by the fact that ound waves with s ort i ii 139 i iii u SCIENTIFICAMERICAN 4 niere wa elenoth Maximum compression SOUNDWAVESINAPIPE Maximumrarefaclion p quotquot Wmm wave L v 1 any universe AfterinflationY the fundamentalwave compressed recombinationThe overtones oscillated twoY three or more ran 39notha time mlirklll 39 my u 39 39 39 ACOUSTIC OSCILLATIONS IN THE EARLYUNIVERSE J m SCIENTIFICAMERICAN FEBRUARYZDIM awvmumngnww of particles in gas or plasma a wave cannot propagate if its wavelength is shorter than the typical distance traveled by par ticles between collisions In air this distance is a negligible 10 5 centimeter But in the primordial plasma just before recombi nation a particle would typically travel some 10000 lightyears before striking another The universe at this stage was dense only in comparison with the modern universe which is about a billion times as rare ed As measured today after its 1000 fold expansion that scale is about 10 million lightyears There fore the amplitudes of the peaks in the power spectrum are damped below about 10 times this scale Just as musicians can distinguish a worldclass violin from an ordinary one by the richness of its overtones cosmologists can elucidate the shape and composition of the universe by ex make up the bulk of socalled ordinary matter and cold dark matter which exerts gravity but has never been directly ob served because it does not interact with ordinary matter or light in any noticeable way Both ordinary matter and dark matter supply mass to the primordial gas and enhance the gravitational pull but only ordinary matter undergoes the sonic compres sions and rarefactions At recombination the fundamental wave is frozen in a phase where gravity enhances its compres sion of the denser regions of gas see box on page 52 But the rst overtone which has half the fundamental wavelength is caught in the opposite phaseigravity is attempting to compress the plasma while gas pressure is trying to expand it As a result the temperature variations caused by this overtone will be less pronounced than those caused by the fundamental wave c symphony is producedl players and is accompanied anger coincidences amining the fundamental frequency of the primordial sound waves and the strength of the overtones The CMB reveals the angular size of the most intense temperature variationsihow large these hot and cold spots appear across the skyiwhich in turn tells us the frequency of the fundamental sound wave Cos mologists can precisely estimate the actual size of this wave at the time of recombination because they know how quickly sound propagated in the primordial plasma Likewise re searchers can determine the distance CMB photons have trav eled before reaching Earthiabout 45 billion lightyears Al though the photons have traveled for only about 14 billion years the expansion of the universe has elongated their route So cosmologists have complete information about the tri angle formed by the wave and can check whether its angles add up to 180 degreesithe classic test of spatial curvature They do so to high precision showing that aside from the overall ex pansion the universe obeys the laws of Euclidean geometry and must be very close to spatially at And because the geometry of the universe depends on its energy density this nding im plies that the average energy density is close to the socalled crit ical densityiabout 10 29 gram per cubic centimeter The next thing cosmologists would like to know is the ex act breakdown of the universe s matter and energy The am plitudes of the overtones provide the key Whereas ordinary sound waves are driven solely by gas pressure the sound waves in the early universe were modi ed by the force of gravity Gravity compresses the gas in denser regions and depending on the phase of the sound wave can alternately enhance or counteract sonic compression and rarefaction Analyzing the modulation of the waves reveals the strength of gravity which in turn indicates the matterenergy composition of the medium As in today s universe matter in the early universe fell into two main categories baryons protons and neutrons which wwwsciamcom This effect explains why the second peak in the power spec trum is lower than the rst And by comparing the heights of the two peaks cosmologists can gauge the relative strengths of grav ity and radiation pressure in the early universe This measure ment indicates that baryons had about the same energy density as photons at the time of recombination and hence constitute about 5 percent of the critical density today The result is in spec tacular agreement with the number derived from studies of light element synthesis by nuclear reactions in the infant universe The general theory of relativity however tells us that mat ter and energy gravitate alike So did the gravity of the photons in the early universe also enhance the temperature variations It did in fact but another effect counterbalanced it After re combination the CMB photons from denser regions lost more energy than photons from less dense areas because they were climbing out of deeper gravitationalpotential wells This pro cess called the SachsWolfe effect reduced the amplitude of the temperature variations in the CMB exactly negating the en hancement caused by the gravity of the photons For regions of the early universe that were too big to undergo acoustic oscil lationsithat is regions stretching more than one degree across the skyitemperature variations are solely the result of the WAYNE HU and MARTIN WHITE are tryingto unveilthe history of the universe Hu is associate professorof astronomy and astro I n v v Chicagonu r y Dinphys ics from the University ofCaIiFornia Berkeley in 1995 His re search pursuits include the investigation of dark energy dark matter and the formation ofcosmological structure White pro fessorof astronomy and physics at Berkeley earned his PhD in physics fromYaIe University in 1992 In addition to exploring how structure in the universe came to be he is interested in the con nections between astrophysics and fundamental physics THE AUTHORS SCIENTIFIC AMERICAN 49 NOTES DF DISCDRD w y mmzed gas 2 mmma rmHmn gears zckn emper urevzrmmns mm mm mm Ms W de cm may weHhe 2 mmm Tmre zhv g zrge zcvnms 2 s m sea puma mum surpnsefrnrmhe mmng mm unwerse WH andM w wsv ms remmzedwesurrnundmz22s he gmm he p zsmz New Sachsr w n h n 25 puccm mm cnuczl dznsuywdny H 1m dAsnvuwcd an comm bu u wnsxcsuncnc m s wh m an s m a an museum upun ncshuwcdchn Remarkable Concord 1h xpmslun um unwtrscxszccdtunngha meSluwr UNFORTUNATELYJ am czlcuhuuns um mudzm unr ammspmaupquothymm G km and MachulSTumn a a a h h a Ouxuth mamaspr n v w v w v 1 h v m SD mmnmmmw mmmnno muwmpecimrrrm see man EHRlXTlEDESlBN graph THE POWER SPECTRUM 1mm mhs nf kalvln iampararum Dwiuinn nm Avaiaga OBSERVATIONS OF THE EMB provide a map of temperature searchers owt erature ofthe radiation varies at different scales The variations are barely noticeable atiarge scales regions about a tenth ofa degree across 2 But the tempe ture differences are quite distinctforregions about cross 39 werspectrum 1DD Angulai quunncg invain radians plasma would have randomized their direction But on the 11 i L i i i atively few scatterings so they retain quot 39 39 39 that is imprinted as a polarization of the CMB This acoustic 39 39 39 1 1 oh n 39 39 i i ll J RPV r 39 of the universe it quot 39 t with the I ale n th 39t t39 I e e39 i I I i i 1 gets a boost in energy as it falls into the potential well but be iiin pkkb r erometer an instrument operated at the A t South Pole Station in Antarctica and later by WMAP the vale a ue was In beautiful agreement with predictionsr WMAP Furthermnre 39 quot quot www sclam com cause Lu u I out it loses less energy than it previously gainedr This phenom enon called the integrated sachsWo fe effect causes largeescale mm i i i lL39 I39I I surveys with the WMAP data The amount of dark energy need i i i i SCIENTIFICAMERICAN 51 INFLUEN V V J L CE the EMB After inflation concentration Gravitational attraction um ahmlt r 39 39 39 luguulul 39 a v v v m L L I I lurur allu Iuwul FIRST PEAK Gravity and sonic motion worktogether Darkmuer AT and un at mid A SECOND PEAK Gravity counteracts sonic motion This tugofwar umnuuuriu t rk matter U1 N SCIENTIFIC AMERICAN FEBRUARY 2004 ammummgnzmu with the amount inferred fromthe acoustic peaks and the distant supernovae As the data from the galaxy surveys improve and other tracers of the largescale structure of the universe become available the integrated SachsWolfe effect could become an im portant source of information about dark energy No Requiem Yet THE CMB MAY ALSO provide crucial new evidence that could explain what happened during the very rst moments af ter the big bang Few aspects of cosmology are more bizarre than the period of in ation Did the universe really in ate and if so what was the nature of the in aton the theoretical eld that caused the rapid expansion Current measurements of the CMB have dramatically strengthened the case for the simplest models of in ation which assume that the amplitudes of the ter in density now and apparently only now To answer these questions researchers can take advantage of the fact that CMB photons illuminate structures across the entire observable uni verse By showing the amplitude of density uctuations at dif ferent points in cosmic history the CMB can reveal the tugof war between matter and dark energy Measurements of two CMB phenomena could be particu larly useful The rst called the SunyaevZel dovich effect oc curs when CMB photons are scattered by the hot ionized gas in galaxy clusters This effect allows galaxy clusters to be identi ed during the crucial period about ve billion years ago when dark energy began to accelerate the expansion of the universe The number of galaxy clusters in turn indicates the amplitude of density uctuations during this time The second phenome non gravitational lensing happens when CMB photons pass 7e lecll degrees to an improbable conclusion most of the universe today is composed of n le Clark matter and dark energy initial density uctuations were the same at all scales But if more detailed observations of the CMB reveal that the ampli tudes varied at different scales the simple in ation models would be in trouble More baroque alternatives would need to be invoked or altogether different paradigms adopted Another exciting possibility is that we could learn about the physics of in ation by determining the energy scale at which it took place For example physicists believe that the weak nu clear force and tL i of a single electroweak force when the universe was hotter than 1015 kelvins If researchers determine that in ation occurred at this energy scale it would strongly imply that the in aton had something to do with electroweak uni cation Alternatively in ation could have occurred at the much higher temperatures at which the electroweak force merges with the strong nuclear force In this case in ation would most likely be associated with the grand uni cation of the fundamental forces A distinctive signature in the CMB could allow researchers to settle this issue In addition to spawning density perturba tions in ation created uctuations in the fabric of spacetime it self These uctuations are gravitational waves whose wave lengths can stretch across the observable universe The ampli tude of these gravitational waves is proportional to the square of the energy scale at which in ation took place If in ation oc curred at the high energies associated with grand uni cation the effects might be visible in the polarization ofthe CMB Last further observations of the CMB could shed some light on the physical nature of dark energy This entity might be a form of vacuum energy as Einstein had hypothesized but its value would have to be at least 60 and perhaps as much as 120 orders of magnitude as small as that predicted from parti cle physics And why is dark energy comparable to dark mat ir force a pert wwwsciamCom by a particularly massive structure that bends their trajectories and hence distorts the pattern of temperature and polarization variations The degree of lensing reveals the amplitude of the mass density uctuations associated with these structures To conduct these investigations of in ation and dark ener gy however researchers will need a new generation of CMB telescopes that can observe the radiation with even greater sen sitivity and resolution In 2007 the European Space Agency plans to launch the Planck spacecraft a microwave observa tory that will be placed in the same orbit as WMAP Planck will be able to measure CMB temperature differences as small as ve millionths of a kelvin and detect hot and cold spots that subtend less than a tenth of a degree across the sky Such mea surements will enable scientists to glimpse the full range of acoustic oscillations in the CMB and thus sharpen their picture of the in ationary spectrum A multitude of groundbased ex periments are also under way to study CMB effects associated with structure in the current epoch of accelerated expansion though the standard cosmological model appears to work remarkably well as a phenomenological description of the uni verse a deeper understanding of its mysteries awaits the ndings of these experiments It seems clear that the cosmic symphony will continue to enchant its listeners for some time to come ED MORE TO EXPLORE Wrinkles in Time George Smoot and Keay Davidson William Morrow 1994 3K The Cosmic Microwave Background Radiation R B Partridge Cambridge University Press 1995 The Inflationary Universe The Quest for a New Theory ofCosmic Origins Alan H Guth andAIan P Lightman Perseus 1998 39 39 v mnr auu 39 39 L 39 a can L 4 0 fr m a Ilrhirngn mm SCIENTIFIC AMERICAN 53
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