E INSTEIN’S L EGACY S PECIAL REVIEW Astrophysical Observations: S Lensing and Eclipsing Einstein’s Theories ECTION

Charles L. Bennett

Albert Einstein postulated the equivalence of energy and mass, developed the theory of interstellar gas, including molecules with , explained the photoelectric effect, and described Brownian motion in molecular sizes of È1 nm, as estimated by five papers, all published in 1905, 100 years ago. With these papers, Einstein provided Einstein in 1905 (6). Atoms and molecules the framework for understanding modern astrophysical phenomena. Conversely, emit spectral lines according to Einstein_s astrophysical observations provide one of the most effective means for testing quantum theory of radiation (7). The con- Einstein’s theories. Here, I review astrophysical advances precipitated by Einstein’s cepts of spontaneous and stimulated emission insights, including gravitational , gravitational lensing, gravitational waves, the explain astrophysical masers and the 21-cm Lense-Thirring effect, and modern cosmology. A complete understanding of cosmology, hydrogen line, which is observed in emission from the earliest moments to the ultimate fate of the , will require and absorption. The interstellar gas, which is developments in physics beyond Einstein, to a unified theory of and quantum heated by starlight, undergoes Brownian mo- physics. tion, as also derived by Einstein in 1905 (8). Two of Einstein_s five 1905 papers intro- Einstein_s 1905 theories form the basis for how stars convert mass to energy by nuclear duced relativity (1, 9). By 1916, Einstein had much of modern physics and astrophysics. In burning (3, 4). Einstein explained the photo- generalized relativity from systems moving 1905, Einstein postulated the equivalence of electric effect by showing that light quanta with a constant velocity (special relativity) to mass and energy (1), which led Sir Arthur are packets of energy (5), and he received accelerating systems (). Eddington to propose (2) that stars shine by the 1921 Nobel Prize in physics for this Space beyond Earth provides a unique converting their mass to energy via E 0 mc2, work. With the photoelectric effect, astron- physics laboratory of extreme pressures and and later led to a detailed understanding of omers determined that ultraviolet photons temperatures, high and low energies, weak emitted by stars impinge on interstellar dust and strong magnetic fields, and immense and overcome the work function of the grains dimensions that cannot be reproduced in Department of Physics and , The Johns Hopkins University, 3400 North Charles Street, to cause electrons to be ejected. The photo- laboratories or under terrestrial conditions. Baltimore, MD 21218, USA. E-mail: [email protected] electrons emitted by the dust grains excite the The extreme astrophysical environments

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enable us to test Einstein_s theories, and the of Mercury from the Earth is 5599 arc-sec distinguish between a local gravitational general (10) has been per century, of which 5025 arc-sec per cen- field and an equivalent uniform acceleration

ECTION tested and applied almost exclusively in the tury are caused by the precession of Earth’s in a sufficiently small region of space-time.

S astrophysical arena. equinoxes. The remaining 574 arc-sec per The gravitational follows directly century do not match the Newtonian predic- from this . Light that Gravity as Geometry tion that the axial direction of Mercury’s propagates through a gravitational field 0 Newton formulated the first universal laws elliptical orbit about the Sun should precess, changes wavelength according to l / l0

PECIAL of gravity and motion. Using Newton’s laws, advancing by 531 arc-sec per century, caused DF/c2,wherel is the shifted wavelength, S astronomers can predict how moons will by perturbations from the other planets. In l0 is the rest wavelength, c is the speed of orbit planets. Newton, however, could not 1855, Le Verrier hypothesized the existence light, and DF is the change in the Newto- understand how a moon would sense the of an undetected planet, Vulcan, between nian gravitational potential (valid in the presence of a planet and wrote, ‘‘I have not Mercury and the Sun, to explain this discrep- weak gravitational field limit). In strong been able to discover the cause of those ancy. Instead, general relativity accounts for gravitational fields that are a radial distance properties of gravity from phenomena, and the remaining 43 arc-sec per century and for r from a mass M (where r is defined as a I 0 I frame no hypotheses ’’ (11). Einstein’s the more constraining recent data acquired circumference divided by 2p), l / l0 (1 – theory of general relativity answered Newton’s from radio tracking of the Mars landers. 2GM/c2r)1/2, where G is the gravitational question: Mass causes space-time curvature. A During the 1930s, Fritz Zwicky deter- constant. The observed wavelength goes to 0 planet bends the space around it, and its mined that clusters of have 10 times zero at the Schwarzschild radius, RS moon moves under the influ- 2GM/c2 (the horizon). The ence of that bent space. was con- Einstein encouraged astrono- firmed to 10% accuracy in 1960 mers to measure the positions of by using the transmission of a stars near the Sun to see how very narrow 14.4-keV transition much the light was deflected. from the isotope 57Fe across the During an eclipse, it is possible 22.6-m height of the Jefferson to see stars near the occulted Physical Laboratory building at solar disk. An accurate measure- Harvard (18). Later refinements ment of the apparent position of of these Pound-Rebka-Snider one of these stars would provide experiments reached 1% accura- a test of Einstein’s idea (Fig. 1). cy. Vessot et al.(19) used a Eddington traveled to Principe hydrogen maser clock on a rocket Island in western Africa for the launched to an altitude of 104 km 29 May 1919 eclipse, and he to measure the gravitational red- claimed to have seen evidence of shift to an accuracy of 7 10j5. the starlight shift predicted by Einstein. Today, we no longer Gravitational Waves are constrained to viewing opti- Maxwell’s equations explain cal light during eclipses; high- how an accelerating charged par- resolution radio interferometers, Fig. 1. A mass like our Sun bends space. On the right, light loses energy ticle produces electromagnetic for example, can measure the as it climbs the gravitational potential, undergoing a gravitational radiation. Analogously, general position of quasar light that redshift. On the left, the path of distant starlight is deflected (or relativity predicts that an accel- passes near the Sun. These recent gravitationally lensed) by the curved space. Because of the relativistic erating mass produces gravita- measurements (12) provide more effects of the curved space, the deflection angle is twice that predicted tional radiation. This radiation by Newtonian calculation. The advance of the perihelion of the orbit of compelling support for relativi- the planet Mercury (seen in blue) is slightly altered by relativistic effects, has not yet been measured di- ty. Irwin Shapiro recognized that as discussed in the text. [Figure by Theophilus Britt Griswold] rectly, but it has been detected when light traverses curved indirectly. From 1974 to 1983, space, such as near the Sun, it Hulse and Taylor observed the will arrive at its destination at a later time more mass than their light suggests, and (20) PSR1913þ16, which orbits a than light that passes through unperturbed termed the unseen mass ‘‘.’’ He 1.4–solar mass (MR) star with a period of flat space (13). Shapiro used radar ranging suggested that the matter would cause space to only 7.75 hours (21). to planets and spacecraft to determine that the curve and act as a , and that According to general relativity, these two Sun’s bending of space adds about 200 ms the clusters could be ‘‘weighed’’ by the massive bodies in rapid motion produce to the round-trip travel time of a radio signal lensing effects (15, 16). The suggestion was gravitational waves, and the emission of these passing near the edge of the Sun. The not to test Einstein’s definition of gravity, but waves should cause the binary system to lose was recently confirmed to use it as a tool. Gravitationally lensed energy. Indeed, the period of the pulsar’s by tracking the Cassini spacecraft during its systems confirm Zwicky’s assertion that orbit was observed to be increasing at a rate flight to Saturn (14). The measurement clusters are dominated by dark matter (Fig. of 76 millionths of a second per year, con- agreed with relativity to 1 part in 105. 2). Observations of the rotation curves of sistent (within 0.02%) with the expected en- Of the planets in the solar system, Mer- individual spiral galaxies also require large ergy loss caused by gravitational waves (22). cury’s orbit is most affected by the Sun’s amounts of dark matter in halos around the Considerable effort is now underway to gravity (Fig. 1). The radius of Mercury’s galaxies (17). make direct measurements of 10- to 1000-Hz elliptical orbit (with an eccentricity of 0.206) gravitational radiation with the Laser Inter- varies from 46 to 70 million km. The mea- Gravitational Redshifts ferometer Observatory surement of the advance of the perihelion A central tenet of relativity is Einstein’s (LIGO). The Laser Interferometer Space (the closest approach of an orbit to the Sun) equivalence principle: It is impossible to Antenna (LISA), a planned three-spacecraft

880 11 FEBRUARY 2005 VOL 307 SCIENCE www.sciencemag.org E INSTEIN’S L EGACY S PECIAL constellation, will measure gravitational waves tempts to measure the Lense-Thirring effect scope and the spinning Earth (frame drag- in the range from 0.0001 to 1 Hz. LIGO and around black holes are often confounded ging) and the combination of the effects of LISA (Fig. 3) will seek to detect gravitational by the complexities of the dynamics of the spin-orbit coupling and space-time curvature waves from orbits, coalescences, and collisions hot gas in their accretion disks. Earth’s known as geodetic (or Thomas) precession. S of strong-field objects such as neutron stars gravitational field, although much weaker, ECTION and black holes. Their objective is to probe should also cause frame dragging. A de- Nucleosynthesis gravity in systems where the ratio of the grav- tection of the Lense-Thirring effect has been Big Bang nucleosynthesis calculations de- itational binding energy to rest mass is in the claimed with 10% accuracy based on the pend on the relation between temperature range of 0.1 to 1, which is many orders of precession of the orbital planes of the two and the rate of expansion of the universe. magnitude beyond any current measurement. LAser GEOdynamics Satellites (LAGEOS) This relation is specified by the Friedmann (25). LAGEOS-1 and LAGEOS-2 are 0.6- equations, which in turn are derived from The Lense-Thirring Effect m-diameter aluminum spherical shells in Einstein’s field equations. Thus, the degree General relativity predicts the existence of Earth orbit, each covered with 426 passive to which Big Bang nucleosynthesis matches black holes and compelling evidence has retroreflectors. the measured abundance ratios of the light been found for their existence in chemical elements is also a test many galaxies (23). Stars that of Einstein’s equations. In 1946, exhaust their hydrogen supply George Gamow wrote that it was cannot start burning carbon un- ‘‘generally agreed I that the less they have more than 4 MR. relative abundance of various Stars with less mass will collapse chemical elements were deter- until the contraction is halted by mined by physical conditions ex- electron degeneracy, according to isting in the universe during the the Pauli exclusion principle. early stages of its expansion, These contracted stars are called when the temperature and densi- white dwarfs. If the mass of the ty were sufficiently high to se- collapsing star is greater than cure appreciable reaction-rates 1.44 MR, then electrons and for the light as well as for the protons will combine to form heavy elements.’’ (26). Gamow neutrons, and the star will col- gave the problem to his graduate lapse further until it is supported student, Ralph Alpher, to work by the neutron degeneracy of the out. Alpher’s Ph.D. thesis and a Pauli exclusion principle. These resulting journal paper (27) ap- more massive contracted stars peared in 1948, showing in de- are called neutron stars. If the tail that the lightest elements mass is greater than 2 to 3 MR, (with atomic numbers e 4) would even the neutron degeneracy be formed in the early universe. cannot support the mass. With Gamow was wrong about the nothing to stop it, the star will heavier elements, which we now collapse to become a . Fig. 2. The distribution of dark matter in space can be deduced from know were created later in stel- The strong gravitational fields as- gravitational lensing effects. (A) An HST picture of the cluster lar processes (28). sociated with neutron stars and Abell 2218 shows distorted images of the background light, including black holes provide a laboratory arcs, contorted galaxy shapes, and multiple images of single objects. Our Dynamic Universe for testing general relativity. [Courtesy of NASA by A. Fruchter and the HST Early Release Obser- General relativity is at the heart For a gyroscope near a rotat- vations Team] With sources of variable luminosity, differential propaga- of modern cosmological models. tion time delays become apparent in the multiple images, such as with j ing black hole, distant stars the 3-year change in the Einstein cross (B and C). In a coaligned system Einstein’s field equation is Rmn 0 provide an inertial reference (source to lens, mass to observer), the source appears as an Einstein ring ½Rgmn 8pGTmn, where Rmn is frame, but the black hole defines (D). [Courtesy of G. Lewis (Institute of Astronomy) and M. Irwin (Royal the Ricci tensor, R is the Ricci a different reference frame. The Greenwich Observatory) from the William Hershel Telescope and J. N. scalar, G is Newton’s gravitational closer the gyroscope is to the Hewitt and E. L. Turner from the National Radio Astronomy Observatory/ constant, g isthemetrictensor, Associated Universities, Inc.] mn black hole, the more it will be and Tmn is the stress-energy tensor. affected by the black hole’s The left-hand side of the equa- reference frame. The gyroscope will precess Analogoustoanacceleratingcharge tion describes the curvature of space and the while trying to follow the rotation of the producing a magnetic field, an accelerating right-hand side describes the content of black hole, but it will lag behind because of mass produces certain ‘‘gravitomagnetic’’ space. Thus, the equation reflects the equiv- its fidelity to the distant stars. In 1918, effects. Three gravitomagnetic effects are: alence of matter and curved space. When Joseph Lense and Hans Thirring introduced (i) the precession of a gyroscope in orbit Einstein applied his field equation to cos- this problem, which does not arise in about a rotating mass, (ii) the precession of mology, he found a problem. His equations Newtonian gravity. The Lense-Thirring pre- orbital planes in which a mass orbiting a implied an unstable (evolving) universe. To cession can be thought of as the dragging of large rotating body constitutes a gyroscopic produce a stable universe, he needed to add a inertial frames around rotating masses. The system whose orbital axis will precess, and parameter to his equation in order to over- rotation twists the surrounding space, (iii) the precession of the pericenter of the come the attractive gravity of all of the mass. perturbing the orbits of nearby masses. orbit of a test mass about a massive rotating In 1917, he introduced what he called the Possible x-ray detections of frame drag- object. NASA’s Gravity Probe B (GPB) cosmological constant as a repulsive anti- ging around neutron stars and black holes seeks to detect and measure the spin-spin gravity term into the field equation. The field have been suggested (24). Unfortunately, at- interaction between its own orbiting gyro- equation with Einstein’s cosmological con-

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j 0 stant term, L,becomesRmn ½Rgmn þ Lgmn Penzias and Wilson found the cosmic mi- The opposing forces of gravitational attrac- 8pGTmn. The cosmological constant can be crowave background (CMB) to be uniform tion and radiation pressure induced adiabatic

ECTION thought of as the energy of the vacuum (that across the sky to within the precision of their oscillations in the photon-baryon fluid.

S is, how much ‘‘‘nothing’’ weighs) and is a measurements. Further measurements were This occurred within the general relativis- constant of nature, with no specified value. performed in order to detect the nonuniform- tic expansion and cooling of the universe. As Einstein fine-tuned its value to balance ity that was expected because the gravita- the universe expanded, the radiation pressure exactly the tendency of mass in the universe tional seeds of the clumpy structures we see dropped and the universe became matter-

PECIAL to collapse gravitationally. in the sky today should be associated with dominated. At that point, the baryons also par- S Now Einstein had a static finite universe, small temperature variations across the sky ticipated in the growth of structures. When the but with a second fundamental gravitational in the CMB. Year after year, experimentalists universe cooled sufficiently for electrons to bind constant of nature. Not only was this a fine- improved technology to reach ever fainter up- to protons to form neutral hydrogen (È3000 K), tuned solution, but it was unstable to per- per limits on the amplitude of these temper- the photons no longer scattered. These CMB turbations; the tiniest of fluctuations would ature fluctuations, and the lack of detectable photons we now observe carry with them the cause the universe to shrink or to expand. fluctuations began to be a problem. Without signature of the epoch of last scattering, called This solution was also based on the large enough primordial gravitational fluctu- decoupling. On an angular scale greater than erroneous assumption that the universe is ations, there wouldn’t be sufficient time for 2-, the photons have been gravitationally red- static. In 1929, determined gravity to cause the growth of the current shifted according to the amplitude of the grav- that the recession velocities of galaxies structure of the universe. A solution to the itational potential at decoupling. (determined by Doppler shifts) are propor- structure problem came with the introduction In 1992, the 27-year search for the anisot- tional to their distances. Assuming that we of cold dark matter (CDM) (33). Although ropy in the CMB ended with the COBE dis- do not occupy a special place in the universe, baryons may not always emit much light and covery of the faint fluctuations in a map of the Hubble concluded that the universe is ex- may be thought of as dark matter, CDM is full sky (Fig. 4B). This discovery supported panding. Given an expanding universe and fundamentally different because CDM par- the basic cosmological picture that structure not a static one, Einstein concluded that his ticles do not interact with light. developed gravitationally in the universe, with cosmological constant was unnecessary. In the early universe, when it was too hot the mass dominated by CDM. For Hubble, stars pro- for electrons to bind to hydrogen atoms, The detection of the primordial gravita- vided the key connection to obtain the dis- there was a plasma of photons, electrons, tional anisotropy caused experimentalists to tance to galaxies. The recession velocities of baryons, and CDM. The photons would redirect their efforts to the characterization galaxies were determined by the Doppler shift frequently scatter off of the free electrons. of the statistics of the anisotropy, especially of their light. More recently, Type Ia super- The seeds of the structure of the universe at smaller angular scales. A precise and ac- novae light curves have been used for distance were also present in the form of primordial curate measurement of the statistics of the determinations. In an attempt to measure the gravitational perturbations. The CDM would anisotropy pattern was predicted to reveal slowing of the expansion of the universe by congregate in regions of gravitational poten- the signature of the adiabatic oscillations, the using supernovae, astronomers discovered tial minima. The combined gravitational pull nature of which depends on the values of key that the expansion of the universe cosmological parameters. CMB is accelerating, not decelerating balloon-borne (34) and ground- (29, 30). Einstein’s cosmological based (35) experiments collected constant has reemerged as one of a wealth of data (36) on the nature many candidate explanations for of the adiabatic oscillations, and the accelerated expansion. the space-borne Wilkinson Micro- wave Anisotropy Probe (WMAP) The Cosmic Microwave mission produced full-sky multi- Background frequency maps of the CMB In working through Big Bang nu- (Fig. 4C) that tightly constrain cleosynthesis in 1949, Alpher and the dynamics, content, and shape Herman estimated (31)thatblack- of the universe (37). body radiation with ‘‘a tempera- Observational cosmologists ture on the order of 5 K’’ would have been productive on many remain today, ‘‘interpreted as the other fronts in the past decade. background temperature which The Hubble Space Telescope (HST) would result from the universal ex- determined the expansion rate of pansion alone.’’ Arno Penzias and the universe, known as the Hubble Robert Wilson won the 1978 Nobel constant (38). The 2- Field Gal- Prize for their 1965 detection of Fig. 3. LIGO is searching for gravitational waves of 3 to 100 ms, which axy Redshift Survey and the Sloan this afterglow of the Big Bang. The are expected from the collapsing cores of supernovae and from Digital Sky Survey have con- radiation has now been precisely coalescing neutron stars and stellar-mass black holes. LISA is sensitive strained cosmological parameters and accurately characterized by to gravitational waves with much longer periods and is currently under (39, 40) by determining the three- using the Cosmic Background Ex- development, with the goal of detecting events such as the coalescence dimensional power spectrum based of supermassive black holes. plorer (COBE) satellite as a black on observations of hundreds of body with a radiation temperature thousands of galaxies. of 2.725 T 0.001 K (Fig. 4A) (32). The black- of the primordial fluctuation and the CDM The combination of the cosmological ob- body spectrum provided evidence that the would attract the baryonic matter, but unlike servations of the past 10 years has led to a radiation had been in thermal equilibrium with the CDM, the baryonic matter would expe- highly constrained cosmological model that matter earlier in the history of the universe, rience enormous radiation pressure that is consistent with these wide-ranging obser- as predicted within the Big Bang model. would prevent the baryons from clumping. vations and general relativity: A flat (Euclid-

882 11 FEBRUARY 2005 VOL 307 SCIENCE www.sciencemag.org E INSTEIN’S L EGACY S PECIAL ean) universe is dominated by 73% , 23% CDM, and 4% baryons, and has aHubbleconstantof71kms–1 Mpc–1.De- spite this knowledge, we still do not know the S specific nature of the dark energy or the ECTION CDM, nor do we understand the physics of the origin of the universe.

Inflation There is evidence that the universe under- went a short early burst of extremely rapid expansion, called inflation. Inflation has been used to explain the so-called flatness, horizon, Fig. 4. NASA’s COBE space mission made a and monopole problems. We measure space to precision measurement of the spectrum of be nearly flat because the inflationary expan- the CMB radiation. The measurement sion drove the initial radius of curvature of the uncertainties are smaller than the thickness universe to become large. We find the CMB of the red curve in the plot (A). COBE also made the first detection of the primordial temperature to be highly uniform across the anisotropy of the cosmic radiation, seen in sky because, with inflation, regions more than the full-sky temperature map (B), where 2- apart on the sky were in causal contact yellow is warmer and blue is cooler. (A model of foreground microwave emission and the Doppler and thus had an opportunity to equilibrate. term have been subtracted.) The primary physical effect is the gravitational redshifting of the light We observe no magnetic monopoles because from primordial gravitational potential wells and blueshifting from primordial gravitational the inflationary expansion drove their densi- potential peaks. (C) The higher sensitivity and higher resolution WMAP full-sky map reveals the interactions of the photon-baryon fluid in the early universe, allowing a host of cosmological ty to an infinitesimal level. parameters to be deduced. [Photo courtesy of NASA’s COBE and WMAP science teams] Inflation is a paradigm, and within the inflationary paradigm there are many specif- ic models. Each model is characterized by a in the theory of quantum chromodynamics associate the dark energy with the energy of tensor-to-scalar ratio (that is, the ratio of (QCD). Gross, Wilczek, and Politzer recently the vacuum. Physical scaling suggests that anisotropy power in gravitational waves to shared the 2004 Nobel Prize in physics for their the vacuum energy should be enormous, al- adiabatic density perturbations) and a spec- work in QCD. Because the electromagnetic though the cosmological constant is tiny by tral index of spatial fluctuations, both of force and the gravitational force both follow comparison. Either there is a subtle reason which are measurable quantities from CMB 1/r2 force laws (the force is inversely propor- for a near-zero but finite vacuum energy, or observations. Inflation generically produces tional to the square of the radial distance r), the dark energy is not vacuum energy. gravitational waves. Inflationary gravitational Einstein felt that these were the most prom- The dark energy could be a sign of the waves will be exceedingly difficult to detect ising forces to try to unify, yet he was un- breakdown of general relativity. A more gener- directly; however, they imprint a signature successful. In 1919, Kaluza proposed to unify al gravity theory may solve the problem, and polarization pattern onto the CMB. The mea- electromagnetism with gravity by introduc- research continues into various modified grav- surement of this pattern could determine the ing an extra dimension, and in 1926, Klein ity theories. Alternatively, the dark energy could energy level of the inflationary expansion. proposed that the extra dimension be com- be a universally evolving scalar field (42). The energy level of inflation is likely to be at pact. Some string theories predict that space- Many other explanations for the dark the scale of the grand unified theory (GUT), time has 10 dimensions. If six dimensions are energy have also been posited. Although it where the strong and electroweak forces are tightly curled up (compacted), then it would is often said that we have no idea what the unified. For GUT-scale inflation, the detection be difficult to establish their existence. We dark energy is, it may be more correct to say of the polarization patterns should be possible, have now eclipsed Einstein with ongoing pro- that we have too many ideas of what the dark albeit very difficult. CMB researchers have gress toward a unified quantum field theory energy might be. And, of course, all of our already begun the search for the primordial and with new ideas for the unification of current ideas may be wrong. gravitational waves. A future NASA mission, gravity from superstring and M theory. Progress in dark energy research is a high- the Einstein Inflation Probe, is currently in its priority challenge for theoretical and experimen- early planning stages and is aimed at detecting Dark Energy tal physicists. Currently, two leading approaches the primordial gravitational waves by their We now believe that the universe is domi- for characterizing the dark energy are the use imprint on the CMB. nated by a completely unpredicted dark en- of supernovae light curves as distance indicators ergy. Einstein’s cosmological constant could and the study of weak gravitational lensing. Unification be the dark energy, and all cosmological Weak lensing takes advantage of the contor- James Clerk Maxwell is famous for pre- measurements are consistent with this inter- tions of the shapes of distant objects by inter- senting the unified theory of electricity and pretation. In this scenario, the energy densities vening objects (lenses). Large statistical lensing magnetism called electromagnetism. The of radiation and matter decrease as the uni- samples, as a function of redshift, probe both electromagnetic force was later quantized by verse expands, but the cosmological constant the geometry and dynamics of the universe. Feynman in a theory called quantum electro- does not. Although the cosmological constant Einstein launched a revolution in physics dymanics (QED). The quantized electromag- was negligible in the early universe, it now that continues to this day, and his theories have netic force was then unified with the weak dominates the energy density of the universe undergone increasingly rigorous tests. Effects force by Weinberg, Salam, and Glashow. Anal- (41). However, the cosmological constant as a predicted by Einstein’s theories have been used agous to the interactions between photons and dark energy candidate is unsatisfying because as tools to comprehend the universe. In many charged particles in QED, quarks carry a we cannot explain why its observed density ways, we are now going beyond Einstein (43). special kind of ‘‘strong charge,’’ arbitrarily is such that it just recently dominates the On the theoretical frontier, efforts toward the called ‘‘color,’’ and interact by the strong force dynamics of the universe. It is tempting to unification of the laws of physics continue. On

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the experimental frontier, tireless efforts are 21. J. H. Taylor, R. A. Hulse, L. A. Fowler, G. E. Gullahorn, atnf.csiro.au/research/cmbr/index.html); Cosmic Anisot- underway to detect CDM directly in under- J. M. Rankin, Astrophys. J. 206, L53 (1976). ropy Polarization MAPper (CAPMAP, http://wwwphy. 22. J. H. Taylor, L. A. Fowler, J. M. Weisberg, Nature 277, princeton.edu/cosmology/); Cosmic Anisotropy Tele-

ECTION ground low-background experiments; to pro- 437 (1979). scope (CAT, www.mrao.cam.ac.uk/telescopes/cat/

S duce CDM in particle accelerators; to make 23. M. Begelman, Science 300, 1898 (2003). index.html); Cosmic Background Imager (CBI, www. measurements to determine the nature of the 24. L. Stella, M. Vietri, Astrophys. J. 492, L59 (1998). astro.caltech.edu/Ètjp/CBI/); CosmicMicrowave Polar- 25. I. Ciufolini, E. C. Pavlis, Nature 431, 958 (2004). ization at SmallScales (COMPASS, http://cmb.physics. dark energy (its sound speed, equation of 26. G. Gamow, Phys. Rev. 70, 572 (1946). wisc.edu/compass.html); Degree Angular Scale Inter- state, and evolution); and to test inflation and 27. R. A. Alpher, H. Bethe, G. Gamow, Phys. Rev. 73, 803 ferometer (DASI, http://astro.uchicago.edu/dasi/);

PECIAL determine what happened in the earliest mo- (1948). Princeton I, Q, and U Experiment (PIQUE, http://

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