This article is dedicated to the memory of a great cosmologist and a very dear friend, David N. Schramm, who, had he not died tragically in a plane crash, would have been a co-author of this review.
One hundred years ago, we did not know how stars shine and we had only a rudimentary understanding of one galaxy, our own Milky Way. Our knowledge of the Universe - in both space and time - was scant: Most of it was as invisible as the world of the elementary particles.
Today, we know that we live in an evolving Universe filled with billions of galaxies within our sphere of observation, and we have recently identified the epoch when galaxies first appeared. Cosmic structures from galaxies increasing in size to the Universe itself are held together by invisible matter whose presence is only known through its gravitational effects (the so-called dark matter).
The optical light we receive from the most distant galaxies takes us back to within a few billion years of the beginning. The microwave echo of the big bang discovered by Penzias and Wilson in 1964 is a snapshot of the Universe at 300,000 years, long before galaxies formed. Finally, the light elements D, 3He, 4He and 7Li were created by nuclear reactions even earlier and are relics of the first seconds. (The rest of the elements in the periodic table were created in stars and stellar explosions billions of years later.)
Crucial to the development of our understanding of the cosmos, were advances in physics - atomic, quantum, nuclear, gravitational and elementary particle physics. The hot big-bang model, based upon Einstein's theory of General Relativity and supplemented by the aforementioned microphysics, provides our quantitative understanding of the evolution of the Universe from a fraction a second after the beginning to the present, some 13 billion years later. It is so successful that for more than a decade it has been called the standard cosmology (see e.g., Weinberg 1972).
Beyond our current understanding, we are striving to answer fundamental questions and test bold ideas based on the connections between the inner space of the elementary particles and the deep outer space of cosmology. Is the ubiquitous dark matter that holds the Universe together and determines its fate composed of slowly moving elementary particles (called cold dark matter) left over from the earliest fiery moments? Does all the structure seen in the Universe today - from galaxies to superclusters and great walls - originate from quantum mechanical fluctuations occurring during a very early burst of expansion driven by vacuum energy (called ``inflation'')? Is the Universe spatially flat as predicted by inflation? Does the absence of antimatter and the tiny ratio of matter to radiation (around one part in 1010) involve forces operating in the early Universe that violate baryon-number conservation and matter - antimatter symmetry? Is inflation the dynamite of the big bang, and if not, what is? Is the expansion of the Universe today accelerating rather than slowing, due to the presence of vacuum energy or something even more mysterious?
Our ability to study the Universe has improved equally dramatically. One hundred years ago our window on the cosmos consisted of visible images take on photographic plates using telescopes of aperture one meter or smaller. Today, arrays of charge-coupled devices have replaced photographic plates, improving photon collection efficiency a hundredfold, and telescope apertures have grown ten fold. Together, they have increased photon collection by a factor of 104. Wavelength coverage has widened by a larger factor. We now view the Universe with eyes that are sensitive from radio waves of length 100 cm to gamma rays of energy up to 1012 eV, from neutrinos to cosmic-ray particles; and perhaps someday via dark matter particles and gravitational radiation.
At all wavelengths advances in materials and device physics have spawned a new generation of low-noise, high-sensitivity detectors. Our new eyes have opened new windows, allowing us to see the Universe 300,000 years after the beginning, to detect the presence of black holes, neutron stars and extra-solar planets, and to watch the birth of stars and galaxies. One hundred years ago the field of spectroscopy was in its infancy; today, spectra of stars and galaxies far too faint even to be seen then, are revealing the chemical composition and underlying physics of these objects. The advent of computers and their dramatic evolution in power (quadrupling every 3 years since the 1970s) has made it possible to handle the data flow from our new instruments as well as to analyze and to simulate the Universe.
This multitude of observations over the past decades has permitted cross-checks of our basic model of the Universe past as a denser, hotter environment in which structure forms via gravitational instability driven by dark matter. We stand on the firm foundation of the standard big-bang model, with compelling ideas motivated by observations and fundamental physics, as a flood of new observations looms. This is a very exciting time to be a cosmologist. Our late colleague David N. Schramm more than once proclaimed the beginning of a golden age, and we are inclined to agree with him.