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Australia Telescope Compact Array (ATCA, www. 10.1126/science.1106444 REVIEW Inflationary Cosmology: Exploring the Universe from the Smallest to the Largest Scales Alan H. Guth* and David I. Kaiser* Understanding the behavior of the universe at large depends critically on insights blend of concepts from particle physics and about the smallest units of matter and their fundamental interactions. Inflationary gravitation. The last few years have been a cosmology is a highly successful framework for exploring these interconnections remarkably exciting time for cosmology, with between particle physics and gravitation. Inflation makes several predictions about new observations of unprecedented accuracy the present state of the universe—such as its overall shape, large-scale smoothness, yielding many surprises. Einstein_s legacy is and smaller scale structure—which are being tested to unprecedented accuracy by a flourishing in the early 21st century. new generation of astronomical measurements. The agreement between these Inflation was invented a quarter of a century predictions and the latest observations is extremely promising. Meanwhile, ago, and has become a central ingredient of physicists are busy trying to understand inflation’s ultimate implications for the current cosmological research. Describing dra- nature of matter, energy, and spacetime. matic events in the earliest history of our universe, inflationary models generically pre- The scientific community is celebrating the In the span of just a few months during 1905 dict that our universe today should have several International Year of Physics in 2005, Einstein introduced key notions that would distinct features—features that are currently honoring the centennial of Albert Einstein_s dramatically change our understanding of being tested by the new generation of high- most important year of scientific innovation. matter and energy as well as the nature of precision astronomical measurements. Even as space and time. The centennial of these inflation passes more and more stringent seminal developments offers an enticing empirical tests, theorists continue to explore Center for Theoretical Physics, Laboratory for Nuclear Science and Department of Physics, Massachusetts opportunity to take stock of how scientists broader features and implications, such as what Institute of Technology, Cambridge, MA 02139, USA. think about these issues today. We focus in might have come before an inflationary epoch, *To whom correspondence should be addressed. particular on recent developments in the field how inflation might have ended within our E-mail: [email protected] (A.H.G.); [email protected] of inflationary cosmology, which draws on a observable universe, and how inflation might 884 11 FEBRUARY 2005 VOL 307 SCIENCE www.sciencemag.org E INSTEIN’S L EGACY S PECIAL arise in the context of our latest understanding to fundamental particle physics continues to tions or directions—Einstein’s gravitational of the structure of space, time, and matter. evolve. equations give a particularly simple result. The Particle theory has been changing rapidly, expansion of the universe may be described by and these theoretical developments have pro- Inflationary Basics introducing a time-dependent ‘‘scale factor,’’ S vided just as important a spur to inflationary According to inflationary cosmology (1–3), a(t), with the separation between any two ob- ECTION cosmology as have the new observations. Dur- the universe expanded exponentially quickly jects in the universe being proportional to a(t). ing the 1960s and 70s, particle physicists dis- for a fraction of a second very early in its Einstein’s equations prescribe how this scale covered that if they neglected gravity, they history—growing from a patch as small as factor will evolve over time, t. The rate of ac- could construct highly successful descriptions 10j26 m, one hundred billion times smaller celeration is proportional to the density of mass- of three out of the four basic forces in the than a proton, to macroscopic scales on the energy in the universe, r, plus three times its universe: electromagnetism and the strong and order of a meter, all within about 10j35 s— pressure, p: d2a/dt2 0 j4pG (r þ 3p)a/3, weak nuclear forces.