From the Big Bang

Scientists may soon glimpse the universe’s ESTINY

D beginnings by studying the subtle ripples made by gravitational waves Echoes from the big bang

BY ROBERT R. CALDWELL AND MARC KAMIONKOWSKI

osmologists are still asking the same questions that only one part in 100,000—in the radiation’s temperature. the first stargazers posed as they surveyed the heav- These variations provide evidence of small lumps and bumps ens. Where did the universe come from? What, if in the primordial plasma. The inhomogeneities in the distrib- anything, preceded it? How did the universe arrive ution of mass later evolved into the large-scale structures of the at its present state, and what will be its future? Al- cosmos: the galaxies and galaxy clusters that exist today. though theorists have long speculated on the ori- In the late 1990s several ground-based and balloon-borne Cgin of the cosmos, until recently they had no way to probe the detectors observed the CMB with much finer angular resolu- universe’s earliest moments to test their hypotheses. In recent tion than COBE did, revealing structures in the primordial plas- years, however, researchers have identified a method for ob- ma that subtend less than one degree across the sky. (For com- serving the universe as it was in the very first fraction of a sec- parison, the moon subtends about half a degree.) The size of ond after the big bang. This method involves looking for traces the primordial structures indicates that the geometry of the uni- of gravitational waves in the cosmic microwave background verse is flat. The observations are also consistent with the the- (CMB), the cooled radiation that has permeated the universe for ory of inflation, which postulates that an epoch of phenome- nearly 15 billion years. nally rapid cosmic expansion took place in the first few mo- The CMB was emitted about 400,000 years after the big ments after the big bang. In June 2001 NASA launched the bang, when electrons and protons in the primordial plasma— Microwave Anisotropy Probe (MAP), to extend the precise ob- the hot, dense soup of subatomic particles that filled the early servations of the CMB to the entire sky [see “A Cosmic Car- universe—first combined to form hydrogen atoms. Because this tographer,” on page 74]. Currently gathering data from its radiation provides a snapshot of the universe at that time, it has deep orbit 1.5 million kilometers beyond the earth, MAP is ex- become the Rosetta stone of cosmology. After the CMB was pected to deliver its first scientific results by early 2003. The Eu- discovered in 1965, researchers found that its temperature—a ropean Space Agency’s Planck spacecraft, to launch in 2007, measure of the intensity of the black body radiation—was very will conduct an even more detailed mapping. Cosmologists ex- close to 2.7 kelvins, no matter which direction they looked in pect that the observations will unearth a treasure trove of in- the sky. In other words, the CMB appeared to be isotropic, formation about the early universe. which indicated that the early universe was remarkably uni- In particular, researchers are hoping to find direct evidence form. In the early 1990s, though, a satellite called the Cosmic of the epoch of inflation. The strongest evidence—the “smok- Background Explorer (COBE) detected minuscule variations— ing gun”—would be the observation of inflationary gravitation-

GRAVITATIONAL WAVES Although gravitational waves have never been directly observed, theory direction (b). Then the effects are reversed (c), and the distortions predicts that they can be detected because they stretch and squeeze the oscillate at the wave’s frequency (d and e). The distortions shown here space they travel through. On striking a spherical mass (a), a wave first have been greatly exaggerated; gravitational waves are usually too weak stretches the mass in one direction and squeezes it in a perpendicular to produce measurable effects.

DISTORTED UNIVERSE The fantastically rapid expansion of the universe immediately after the big bang should have produced gravitational waves. These waves would have stretched and squeezed the primordial plasma, inducing motions in the spherical surface that emitted the cosmic microwave background, or CMB. These motions, in turn, would have caused redshifts and blueshifts in the radiation’s temperature and polarized the CMB. The illustration above shows the effects of a gravitational wave traveling from pole to pole, with a wavelength that is one quarter the radius of the sphere. ILLUSTRATIONS BY SLIM FILMS

www.sciam.com THE ONCE AND FUTURE COSMOS 77 COPYRIGHT 2002 SCIENTIFIC AMERICAN, INC. al waves. In 1918 Albert Einstein predict- gravitational waves should allow re- tional waves could propagate through ed the existence of gravitational waves as searchers to view astrophysical phenom- the plasma. What is more, the theory of a consequence of his theory of general rel- ena that cannot be seen otherwise. Al- inflation predicts that the explosive ex- ativity. They are analogues of electro- though gravitational waves have never pansion of the universe 10–38 second af- magnetic waves, such as x-rays, radio been directly detected, astronomical ob- ter the big bang should have produced waves and visible light, which are moving servations have confirmed that pairs of gravitational waves. If the theory is cor- disturbances of an electromagnetic field. extremely dense objects such as neutron rect, these waves would have echoed Gravitational waves are moving distur- stars and black holes generate the waves across the early universe and, 400,000 bances of a gravitational field. Like light as they spiral toward each other. years later, left subtle ripples in the CMB or radio waves, gravitational waves can The plasma that filled the universe that can be observed today. carry information and energy from the during its first 400,000 years was opaque sources that produce them. Moreover, to electromagnetic radiation, because any Waves from Inflation gravitational waves can travel unimped- emitted photons were immediately scat- TO UNDERSTAND HOW inflation ed through material that absorbs all forms tered in the soup of subatomic particles. could have produced gravitational waves, of electromagnetic radiation. Just as x- Therefore, astronomers cannot observe let’s examine a fascinating consequence rays allow doctors to peer through sub- any electromagnetic signals dating from of quantum mechanics: empty space is stances that visible light cannot penetrate, before the CMB. In contrast, gravita- not so empty. Virtual pairs of particles are spontaneously created and destroyed COSMIC TIME LINE all the time. The Heisenberg uncertainty During the epoch of inflation—the tremendous expansion of the universe that took place in the principle declares that a pair of particles first moments after the big bang—quantum processes generated a spectrum of gravitational waves. with energy ∆E may pop into existence The waves echoed through the primordial plasma, distorting the CMB radiation that was emitted for a time ∆t before they annihilate each about 400,000 years later. By carefully observing the CMB today, cosmologists may detect the other, provided that ∆E ∆t < h/2, where plasma motions induced by the inflationary waves. h is the reduced Planck’s constant (1.055 15 billion years × 10–34 joule-second). You need not wor- ry, though, about virtual apples or ba- nanas popping out of empty space, because the formula applies only to elementary particles and not to complicated arrangements of atoms. One of the elementary particles af- fected by this process is the graviton, the quantum particle of gravitational waves (analogous to the photon for electromagnetic waves). Pairs of virtual gravi- Cosmic microwave background radiation tons are constantly popping in and out of existence. During inflation, however, the virtual gravitons would have been pulled apart much faster than they could have disappeared back into the vacuum. In 400,000 essence, the virtual particles would have years become real particles. Furthermore, the fantastically rapid expansion of the universe would have stretched the graviton wavelengths from microscopic to macro- scopic lengths. In this way, inflation would have pumped energy into the pro-

ME AFTER THE BIG BANG duction of gravitons, generating a spec- TI Inflationary trum of gravitational waves that reflect- 10–36 second gravitational waves ed the conditions in the universe in those first moments after the big bang. If inflationary gravitational waves do indeed exist, they would be the oldest relic in the Epoch of inflation universe, created 400,000 years before –38

10 second the CMB was emitted. ALFRED T. KAMAJIAN

78 SCIENTIFIC AMERICAN THE ONCE AND FUTURE COSMOS COPYRIGHT 2002 SCIENTIFIC AMERICAN, INC. RELIC IN THE RADIATION Inflationary gravitational waves would have left a distinctive imprint on the CMB. This illustration antenna to move back and forth. The depicts the simulated temperature variations and polarization patterns that would result from the motions of these electrons produce an distortions shown in the bottom illustration on page 77. The red and blue spots represent colder and electric current that the receiver records. hotter regions of the CMB, and the small line segments indicate the orientation angle of the polarization in each region of the sky. Similarly, a gravitational wave can in- duce an oscillatory stretching and squeez- Whereas the microwave radiation in beginning but became distinct 10–38 sec- ing of the space it travels through. These the CMB is largely confined to wave- ond after the big bang, and this event oscillations would cause small motions in lengths between one and five millimeters somehow triggered the sudden expansion a set of freely floating test masses. In the (with a peak intensity at two millime- of the cosmos. If the theory is correct, in- late 1950s physicist Hermann Bondi of ters), the wavelengths of the inflationary flation would have had an energy scale of King’s College London tried to convince gravitational waves would span a much 1015 to 1016 GeV. (One GeV is the ener- skeptics of the physical reality of such broader range: one centimeter to 1023 gy a proton would acquire while being waves by describing a hypothetical grav- kilometers, which is the size of the pres- accelerated through a voltage drop of one itational-wave detector. The idealized ap- ent-day observable universe. The theory billion volts. The largest particle acceler- paratus was a pair of rings hanging freely of inflation stipulates that the gravita- ators currently reach energies of 103 on a long, rigid bar. An incoming gravi- tional waves with the longest wave- GeV.) On the other hand, if inflation tational wave of amplitude h and fre- lengths would be the most intense and were triggered by another physical phe- quency f would cause the distance L be- that their strength would depend on the nomenon occurring at a later time, the tween the two rings to alternately con- rate at which the universe expanded dur- gravitational waves would be weaker. tract and expand by an amount h × L, ing the inflationary epoch. This rate is Once produced during the first frac- with a frequency f. The heat from the proportional to the energy scale of inflation of a second after the big bang, the in- friction of the rings rubbing against the tion, which was determined by the tem- flationary gravitational waves would bar would provide evidence that the perature of the universe when inflation propagate forever, so they should still be gravitational wave carries energy. began. And because the universe was running across the universe. But how can Researchers are currently building so- hotter at earlier times, the strength of the cosmologists observe them? First consid- phisticated gravitational-wave detectors, gravitational waves ultimately depends er how an ordinary stereo receiver detects which will use lasers to track the tiny moon the time at which inflation started. a radio signal. The radio waves consist of tions of suspended masses [see box on Unfortunately, cosmologists cannot oscillating electrical and magnetic fields, next page]. The distance between the test pinpoint this time, because they do not which cause the electrons in the receiver’s masses determines the band of wave- know in detail what caused inflation. Columbia University Some physicists have theorized that in- ROBERT R. CALDWELL and MARC KAMIONKOWSKI were both physics majors in the class of flation started when three of the funda- 1987 at Washington University. Caldwell earned his Ph.D. in physics at the University of mental interactions—the strong, weak Wisconsin–Milwaukee in 1992. One of the chief formulators of the theory of quintessence, and electromagnetic forces—became dis- Caldwell is now assistant professor of physics and astronomy at Dartmouth College. Kami- sociated soon after the universe’s cre- onkowski earned his doctorate in physics at the University of Chicago in 1991. Now pro- ation. According to this theory, the three fessor of theoretical physics and astrophysics at the California Institute of Technology, THE AUTHORS

COURTESY OF LEVEN WADLEY forces were one and the same at the very he received the Warner Prize in 1998 for his contributions to theoretical astronomy.

www.sciam.com THE ONCE AND FUTURE COSMOS 79 COPYRIGHT 2002 SCIENTIFIC AMERICAN, INC. to shorter wavelengths (and hence a high- er temperature). Conversely, if a gravita- Wave Hunters tional wave was squeezing a region of New detectors will soon be ready plasma away from us when the CMB was emitted, the radiation will appear redder THE GRAVITATIONAL WAVES Livingston, La. because it has shifted to longer wave- produced by quantum lengths (and a lower temperature). By processes during the surveying the blue and red spots in the inflationary epoch are by no CMB—which correspond to hotter and means the only ones believed colder radiation temperatures—research- to be traveling across the ers could conceivably see the pattern of universe. Many astrophysical plasma motions induced by the inflation- systems, such as orbiting ary gravitational waves. The universe itself binary stars, merging neutron becomes a gravitational-wave detector. stars and colliding black holes, should also emit powerful Particulars of Polarization gravitational waves. THE TASK IS NOT so simple, however. The problem with detecting As we noted at the beginning of this arti- the waves is that their strength cle, mass inhomogeneities in the early uni- fades as they spread outward while traveling hundreds of millions of light-years to the verse also produced temperature varia- earth. To measure such minuscule oscillations, researchers are preparing the Laser tions in the CMB. (For example, the grav- Interferometer Gravitational-Wave Observatory (LIGO), which consists of facilities in itational field of the denser regions of Livingston, La. (above), and Hanford, Wash. Results from the facilities will be compared plasma would have redshifted the pho- to rule out local effects that mimic gravitational waves, such as seismic activity, tons emitted from those regions, produc- acoustic noise and laser instabilities [see “Ripples in Spacetime,” on page 88]. ing some of the temperature differences Physicists are also building smaller detectors that will work in tandem with LIGO, observed by COBE.) If cosmologists look allowing researchers to triangulate the sources of gravitational waves. Examples of at the radiation temperature alone, they these observatories are TAMA (near Tokyo), Virgo (near Pisa, Italy) and GEO (near cannot tell what fraction (if any) of the Hannover, Germany). And to monitor gravitational waves with longer wavelengths, NASA variations should be attributed to gravi- and the European Space Agency are planning to launch the Laser Interferometer Space tational waves. Even so, scientists at least Antenna in 2010. Unfortunately, none of these proposed observatories will be sensitive know that gravitational waves could not enough to detect the gravitational waves produced by inflation. Only the cosmic have produced any more than the one-in- microwave background radiation can reveal their presence. —R.R.C. and M.K. 100,000 temperature differences observed by COBE and the other CMB radiation lengths that the devices can monitor. The freely floating test masses separated by detectors. This fact puts an interesting largest of the ground-based detectors, similarly large distances. Serendipitously, constraint on the physical phenomena which has a separation of four kilometers nature has provided just such an arrange- that gave rise to inflation: the energy scale between the masses, will be able to mea- ment: the primordial plasma that emit- of inflation must be less than about 1016 sure the oscillations caused by gravita- ted the CMB radiation. During the GeV, and therefore the epoch could not tional waves with wavelengths from 30 to 400,000 years between the epoch of in- have occurred earlier than 10 –38 second 30,000 kilometers; a planned space-based flation and the emission of the CMB, the after the big bang. observatory may be able to detect wave- ultralong-wavelength gravitational waves But how can cosmologists go further? lengths about 1,000 times longer. The echoed across the early universe, alter- How can they get around the uncertain- gravitational waves generated by neutron nately stretching and squeezing the plasma ty over the origin of the temperature fluc- star mergers and black hole collisions [see illustration on page 78]. Researchers tuations? The answer lies with the polar- have wavelengths in this range, so they can observe these oscillatory motions to- ization of the CMB. When light strikes a can be detected by the new instruments. day by looking for slight Doppler shifts surface in such a way that the light scat- But the inflationary gravitational waves in in the CMB. ters at nearly a right angle from the orig- this range are much too weak to produce If, at the time when the CMB was inal beam, it becomes linearly polarized— measurable oscillations in the detectors. emitted, a gravitational wave was stretch- that is, the waves become oriented in a The strongest inflationary gravitation- ing a region of plasma toward us—that is, particular direction. This is the effect that al waves are those with the longest wave- toward the part of the universe that polarized sunglasses exploit: because the Skyview Technologies lengths, comparable to the diameter of the would eventually become our galaxy— sunlight that scatters off the ground is observable universe. To detect these the radiation from that region will appear typically polarized in a horizontal direc-

waves, researchers need to observe a set of bluer to observers because it has shifted tion, the ﬁlters in the glasses reduce the ALEX DESSELLE

80 SCIENTIFIC AMERICAN THE ONCE AND FUTURE COSMOS COPYRIGHT 2002 SCIENTIFIC AMERICAN, INC. glare by blocking light waves with this late the amplitude of the ultralong-wave- orientation. The CMB is polarized as length inflationary gravitational waves. well. Just before the early universe be- Because the amplitude of the waves was came transparent to radiation, the CMB determined by the energy of inflation, re- photons scattered off the electrons in the searchers will get a direct measurement of plasma for the last time. Some of these that energy scale. This finding, in turn, photons struck the particles at large an- will help answer the question of whether gles, which polarized the radiation. inflation was triggered by the unification The key to detecting the inflationary of fundamental forces. gravitational waves is the fact that the What are the prospects for detecting plasma motions caused by the waves pro- the gentle rings and curls and swirls of the POLARIZATION PATTERNS duced a different pattern of polarization polarized CMB sky? That is the next goal The polarization of the CMB may hold important than the mass inhomogeneities did. The of CMB scientists. Although theorists are clues to the history of the early universe. idea is relatively simple. The linear po- confident that the relic radiation is polar- Density variations in the primordial plasma larization of the CMB can be depicted ized, observational verification has elud- would cause ringlike and radial patterns of polarization (top). Gravitational waves, in with small line segments that show the ed researchers. Roughly a dozen experi- contrast, would produce right- and left-handed orientation angle of the polarization in ments worldwide are striving to measure swirls (bottom). each region of the sky [see illustration on the variation of the polarization pattern, page 79]. These line segments are some- and we can expect exciting results soon. cosmos, then CMBPOL will be able to times arranged in rings or in radial pat- But theorists also predict that the strength sense the telltale signs of the squeezing terns. The segments can also appear in ro- of the curl-free polarization component and stretching of the plasma at last scat- tating swirls that are either right- or left- is much stronger than the curl compo- tering. The discovery would extend our handed—that is, they seem to be turning nent—the “smoking gun” of the infla- understanding of the universe back to the clockwise or counterclockwise [see illus- tionary gravitational waves we have de- earliest fraction of a second after the big tration at right]. scribed. So though there is a good chance bang. But if inflation was triggered by The “handedness” of these patterns is the MAP satellite or one of the ground- other physical phenomena occurring at the clue to their origin. The mass inho- or balloon-based experiments will detect later times and lower energies, the signal mogeneities in the primordial plasma the CMB’s curl-free polarization within from the gravitational waves will be far could not have produced such polariza- the next year, the curl component will re- too weak to be detected in the foreseeable tion patterns, because the dense and rar- main just out of reach. future. efied regions of plasma had no right- or Subsequent experiments may have a Because cosmologists are not certain left-handed orientation. In contrast, grav- better chance. If inflation was indeed about the origin of inflation, they cannot itational waves do have a handedness: caused by the unification of forces, its definitively predict the strength of the po- they propagate with either a right- or left- gravitational-wave signal may be strong larization signal produced by inflationary handed screw motion. The polarization enough to be detected by the Planck satel- gravitational waves. But if there is even pattern produced by gravitational waves lite, although an even more sensitive a small chance that the signal is de- will look like a random superposition of next-generation spacecraft might be tectable, then it is worth pursuing. Its de- many rotating swirls of various sizes. Re- needed. CMB scientists are already work- tection would not only provide incontro- searchers describe these patterns as hav- ing with NASA to plan such a mission: the vertible evidence of inflation but also give ing a curl, whereas the ringlike and radi- Cosmic Microwave Background Polar- us the extraordinary opportunity to look al patterns produced by mass inhomo- ization Experiment (CMBPOL), which back at the very earliest times, just 10–38 geneities have no curl. would fly sometime after 2014. If the in- second after the big bang. We could then Not even the most keen-eyed observ- flationary theory is true, and there are ul- contemplate addressing one of the most er can look at a polarization diagram, tralong-wavelength gravitational waves compelling questions of the ages: Where such as the one shown on page 79, and of primordial origin coursing through the did the universe come from? tell by eye whether it contains any patterns with curls. But an extension of MORE TO EXPLORE Fourier analysis—a mathematical tech- First Space-Based Gravitational-Wave Detectors. Robert R. Caldwell, Marc Kamionkowski and nique that can break up an image into a Leven Wadley in Physical Review D, Vol. 59, Issue 2, pages 27101–27300; January 15, 1999. series of waveforms—can be used to di- Recent observations of the cosmic microwave background are described at these Web sites: vide a polarization pattern into its con- pupgg.princeton.edu/~cmb/; www.physics.ucsb.edu/~boomerang/; cosmology.berkeley.edu/group/cmb/ stituent curl and curl-free patterns. Thus, Details of the MAP and Planck missions are available at map.gsfc.nasa.gov/; if cosmologists can measure the CMB po- astro.estec.esa.nl/astrogen/planck/mission–top.html larization and determine what fraction More information on gravitational-wave detectors is available at www.ligo.caltech.edu;

COURTESY OF ROBERT R. CALDWELL AND MARC KAMIONKOWSKI came from curl patterns, they can calcu- lisa.jpl.nasa.gov