The study of the structure and evolution of the Universe. Observational cosmologists try to measure the properties of the Universe, such as its chemical composition, density, and rate of expansion. Theoretical cosmologists try to explain these properties using the laws of physics. In practice, there are very few observational cosmologists who do not try to understand their observations using theory, although there are many theoretical cosmologists who are not observers. In its broadest sense, cosmology also has philosophical and theological aspects.

Observations show that the Universe is isotropic and homogeneous—that is, it looks roughly the same in every direction, and an observer at any place in the Universe would see roughly the same numbers of galaxies and clusters of galaxies. The assumption that the Universe is homogeneous and isotropic is known as the cosmological principle and is the starting point for most theoretical models. These models are usually based on the general theory of relativity, the theory of gravitation devised by A. Einstein.

Of the four forces of nature, gravity is the only one which operates over large scales. Because gravity is an attractive force, the Universe cannot be at rest—if the Universe were at rest even for an instant, the gravitational attraction between galaxies would immediately make it start to contract. Einstein believed the Universe was static, so he added an extra term, the cosmological constant, to his equations to counter the effect of gravity. When the expansion of the Universe was discovered, he came to regard the introduction of this term as a mistake. Mathematical models of the expansion which are based on the cosmological principle and the general theory of relativity (without the cosmological constant) are called Friedmann universes.

The Friedmann equations lead to the startling conclusion that at some time in the past the Universe was infinitesimally small and infinitely dense. Evidence that it was also infinitely hot—i.e. that the Universe started in a hot Big Bang—came from two lines of observation. The first was the discovery in 1965 of the cosmic microwave background. The second was the measurement of the abundance of helium in the Universe. Although most elements are manufactured by nuclear fusion in stars, there is far too much helium around for it to be made in this way. In the Big Bang theory, the helium and a few other light elements were produced by nuclear fusion in the first three minutes after the Big Bang, and the cosmic background radiation was emitted by the Universe when it was in this early hot phase. There are currently no other plausible explanations of the helium abundance or the cosmic microwave background, and as a result the Big Bang theory is accepted by most cosmologists.

For almost seven decades, the aim of most observational cosmologists was to answer a single question: which one of the three possible Friedmann universes do we live in? The answer appeared to be governed by the average density of the Universe. If the average density were greater than a certain value, called the critical density, the gravitational attraction between galaxies would be strong enough that the Universe would eventually collapse and go through a reverse Big Bang—the Big Crunch. If the average density were less than the critical density, the Universe would expand forever. If the average density were equal to the critical density, the Universe would eventually stop expanding but only after an infinite amount of time.

Various approaches to this question were tried, but all methods had limitations and the answers they gave did not agree. In the late 1990s, however, a new set of observational programmes started to give a consistent set of answers to this and many other questions about the Universe. These programmes included detailed studies of the cosmic background radiation which allowed astronomers to estimate, among other things, the curvature of spacetime; direct measurements of the amount of matter in the Universe; and studies of how the brightness of distant supernovae depend on redshift. All of these gave consistent answers, and the universe they describe—which may well be the one we live in—is consequently known as the concordance universe.

These results, confirmed and refined by NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) and by ESA’s Planck satellite, indicate that the Universe started in a hot Big Bang 13.8 billion years ago. The curvature of spacetime is zero. The Universe is dominated by dark matter rather than by the luminous stuff in stars and galaxies, but only about 20% of this dark matter consists of the protons and neutrons that make up our everyday world. The average density of the Universe is only about 30% of the critical density, and so it will expand forever. Finally, and most surprisingly, the Universe appears to contain a mysterious force which is pushing the galaxies apart and acts rather like Einstein’s cosmological constant. This force has been named dark energy and is causing the Universe’s expansion to accelerate. If these results are correct, they provide answers to some important questions about the Universe. However, there are two new questions for which convincing answers do not yet exist: we do not know with any certainty what the dark matter consists of, and we have even less idea what the dark energy is. In fact, current cosmological research is exploring the possibility that the accelerated expansion of the Universe could be caused by modifications to gravity on cosmological scales, rather than dark energy.