Published in Science 307 (2005) 884-890.

astro-ph/0502328

For a PDF version of the article, click here.

Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

**Abstract.**
Understanding the behavior of the universe at large depends
critically on insights about the smallest units of matter and
their fundamental interactions. Inflationary cosmology is a
highly successful framework for exploring these interconnections
between particle physics and gravitation. Inflation makes
several predictions about the present state of the universe -
such as its overall shape, large-scale smoothness, and
smaller-scale structure - which are being tested to
unprecedented accuracy by a new generation of astronomical
measurements. The agreement between these predictions and the
latest observations is extremely promising. Meanwhile,
physicists are busy trying to understand inflation's ultimate
implications for the nature of matter, energy, and spacetime.

**Table of Contents**

- INFLATIONARY BASICS
- BEFORE AND AFTER INFLATION
- BRANE COSMOLOGY: STICKING CLOSE TO HOME
- STRING COSMOLOGY
- CONCLUSIONS
- REFERENCES

The scientific community is celebrating the International Year of Physics in 2005, honoring the centennial of Albert Einstein's most important year of scientific innovation. In the span of just a few months during 1905 Einstein introduced key notions that would dramatically change our understanding of matter and energy as well as the nature of space and time. The centennial of these seminal developments offers an enticing opportunity to take stock of how scientists think about these issues today. We focus in particular on recent developments in the field of inflationary cosmology, which draws on a blend of concepts from particle physics and gravitation. The last few years have been a remarkably exciting time for cosmology, with new observations of unprecedented accuracy yielding many surprises. Einstein's legacy is flourishing in the early 21st century.

Inflation was invented a quarter of a century ago, and has become a central ingredient of current cosmological research. Describing dramatic events in the earliest history of our universe, inflationary models generically predict that our universe today should have several distinct features -- features that are currently being tested by the new generation of high-precision astronomical measurements. Even as inflation passes more and more stringent empirical tests, theorists continue to explore broader features and implications, such as what might have come before an inflationary epoch, how inflation might have ended within our observable universe, and how inflation might arise in the context of our latest understanding of the structure of space, time, and matter.

Particle theory has been changing rapidly, and these theoretical developments have provided just as important a spur to inflationary cosmology as have the new observations. During the 1960s and 70s, particle physicists discovered that if they neglected gravity, they could construct highly successful descriptions of three out of the four basic forces in the universe: electromagnetism and the strong and weak nuclear forces. The "standard model of particle physics," describing these three forces, was formulated within the framework of quantum field theory, the physicist's quantum-mechanical description of subatomic matter. Inflationary cosmology was likewise first formulated in terms of quantum field theory. Now, however, despite (or perhaps because of) the spectacular experimental success of the standard model, the major thrust of particle physics research is aimed at moving beyond it.

For all its successes, the standard model says nothing at all about the fourth force: gravity. For more than 50 years physicists have sought ways to incorporate gravity within a quantum-mechanical framework, initially with no success. But for the past 25 or more years, an ever-growing group of theoretical physicists has been pursuing superstring theory as the bright hope for solving this problem. To accomplish this task, however, string theorists have been forced to introduce many novel departures from conventional ideas about fundamental forces and the nature of the universe. For one thing, string theory stipulates that the basic units of matter are not pointlike particles (as treated by quantum field theory), but rather one-dimensional extended objects, or strings. Moreover, in order to be mathematically self-consistent, string theories require the existence of several additional spatial dimensions. Whereas our observable universe seems to contain one timelike dimension and three spatial dimensions - height, width, and depth - string theory postulates that our universe actually contains at least six additional spatial dimensions, each at right angles to the others and yet somehow hidden from view.

For measurements at low energies, string theory should behave effectively like a quantum field theory, reproducing the successes of the standard model of particle physics. Yet the interface between cosmology and string theory has been a lively frontier. For example, some theorists have been constructing inflationary models for our universe that make use of the extra dimensions that string theory introduces. Others have been studying the string theory underpinnings for inflationary models, exploring such topics as the nature of vacuum states and the question of their uniqueness. As we will see, inflation continues to occupy a central place in cosmological research, even as its relation to fundamental particle physics continues to evolve.