Thirty years ago, cosmology was described as a search for two numbers: the current expansion rate (or Hubble constant), H0, and its change over time, the deceleration parameter, q0 (Sandage, 1970). But that was before the discovery of giant walls of galaxies, voids, dark matter, tiny variations in the cosmic microwave background radiation (CMB), dark energy and the acceleration of the Universe. Today, the subject has become vastly richer, and the numbers being sought are more numerous but more closely tied to fundamental theory. An overall picture has emerged that accounts for the origin of structure and geometry of the Universe, as well as describing its evolution from a fraction of a second onward.
In this new and still-evolving picture rooted in elementary particle physics, in a tiny fraction of a second during the early history of the universe, there was an enormous expansion called inflation. This expansion smoothed out wrinkles and curvature in the fabric of spacetime, and stretched quantum fluctuations on subatomic scales to astrophysical scales. Following inflation was a phase when the Universe was a hot thermal mixture of elementary particles, out of which arose all the forms of matter that exist today. Some 10,000 years into its evolution, gravity began to grow the tiny lumpiness in the matter distribution arising from quantum fluctuations into the rich cosmic structures seen today, from individual galaxies to the great clusters of galaxies and superclusters.
Recent observations of the universe have not only strengthened and expanded the big-bang model, but they have also revealed surprises. In particular, most of the universe is made of something fundamentally different from the ordinary matter we are made of. (By ordinary matter, we mean matter made of neutrons and protons; the jargon for this is baryons, the technical term for particles made of quark triplets.) About 30% of the total mass-energy is dark matter, composed of particles most likely formed early in the universe. Two thirds is in a smooth "dark energy" whose gravitational effects began causing the expansion of the universe to speed up just a few billion years ago. Ordinary matter, the bulk of it dark, only accounts for the remaining 4% of the total mass-energy density of the universe. While the remnant (thermal) microwave background from the hot big bang contributes only about 0.01%, it encodes information about the spacetime structure of the Universe, about its early history, and possibly even about its ultimate fate.
We have also learned much about the organization of the universe. In the nearby universe, galaxies are distributed in a "cosmic web" composed of sheets and sinuous filaments interspersed with voids (see Figures 1 and 2). Though inhomogeneous on these apparently vast scales, the Universe becomes more and more homogeneous when viewed on even larger scales from 100 Mpc out to the current horizon of 10,000 Mpc.
Figure 1. The Universe observed: A slice of the Universe constructed from the positions of 60,000 galaxies in the Sloan Digital Sky Survey. Voids and walls can be clearly seen (image courtesy of SDSS).
Figure 2. The Universe simulated: The distribution of dark matter in a large-scale numerical simulation of the Universe. The cosmic web of dark matter - with its sheets, sinuous filaments and voids is apparent (image courtesy of the Virgo Consortium).
In the first part of this review, we describe the universe - its structure, composition and global properties. Then we proceed to discuss our current understanding of its origin and early evolution, emphasizing the deep connections between physics on the smallest and largest scales. We end by discussing some recent and more speculative ideas from theory, as well as posing some of the "big questions" confronting cosmology today.