the observed universe. If the dark matter is an elementary particle, it cannot be one of those in the Standard Model. The Standard Model is incomplete. Astronomy was beginning to pose challenges to physics.
By the 1980s, the expanding universe had gotten its own theoretical model, which is by now so well verified that it deserves also to be called standard: the so-called Inflationary Big Bang Model. At its earliest moments, our entire universe was unimaginably small and unimaginably hot. It suddenly “inflated” in a “big bang.” The Inflationary Big Bang Model naturally explains why the universe appears to us so nearly flat and so nearly uniform in all directions. The universe continued to expand after the brief episode of rapid inflation ended; this evolved into the expansion that Edwin Hubble first observed. As the universe expanded it cooled, and its composition changed from particles only found by accelerators today to the hydrogen and helium atoms currently observed. This process was completed after about 300,000 years, when electrically charged electrons and ions first combined into these atoms.
In the 1990s, astronomers showed with exquisite precision that microwaves almost uniformly filling the universe in all directions today are the light radiation created when atoms first formed. This cosmic background radiation was subsequently shifted to longer wavelengths by the ongoing expansion of the universe. Precision observations of the microwave background not only confirmed many aspects of the Inflationary Big Bang Model but also became an important tool for learning more about both physics and astronomy.
By 1995, astronomers and physicists could be reasonably satisfied with their century’s work. Einstein’s theory of relativity, the Standard Model of particle physics, and the Inflationary Big Bang Model had passed the experimental tests that science had devised in the past two generations. With the exception of the dark matter puzzle, the foundations seemed reasonably solid, as far as they went. However, Einstein’s relativity and basic particle physics were now irretrievably entangled with one another, and understanding the relationship between the two had become one of the central unanswered questions of contemporary science.
In 1998, astronomy again shook the foundations of physics. Astronomers, by relating the apparent brightness of Type Ia supernovas in distant galaxies to their speed of recession, found that the expansion of the universe—of space itself—is speeding up. The speedup implies the existence of a new kind of energy, “dark energy,” that comprises 70 percent of the total mass-energy density in the universe. Einstein’s equations for an expanding universe allow a so-called cosmological constant that acts the same everywhere and, within today’s observational limits, could account for the speedup. However, basic physics theories have no natural explanation for the size of the observed acceleration rate.
The discovery of dark energy has caused huge excitement. The questions fly thick and heavy. Can an understanding of dark energy (and dark matter) teach us ways in which particle physics models should be extended? Should we not test the degree to which dark energy is exactly constant, as Einstein’s cosmological constant predicts? Or, going beyond Einstein, will we find that it varies? Can the distant universe be observed with the exquisite precision needed to detect its variation? If dark energy does vary, what would be learned about the physics of particles? Either answer—constant or varying—has profound implications for both physics and astronomy. There is renewed interest in testing Einstein’s theory. Should we not now investigate general relativity experimentally where it has never been tested before—in the so-called strong-field regime? Can we do this by observing the gravitational waves generated when two black holes merge. Will this be how we first detect gravitational waves directly? What will be learned when we do detect them directly? Will it be found that there are deviations from Einstein’s general relativity? Can we detect the gravitational waves generated at the moment of inflation? If so, will we learn about particle energy scales vastly higher than those attainable in accelerators? Do atoms behave in unexpected ways when they are at the high temperatures and pressures associated with the strong gravitational field near a black hole? Shouldn’t a more complete census of black holes be made?
Amidst all the new questions awaiting answers, two points emerge with clarity. The first is that the 21st century in astronomy and physics will be very different from the 20th. We are going to have to go beyond Einstein. Second, the understanding of the inflationary big bang universe is sufficiently secure that scientists can use the universe