An increase in the expansion rate of the universe, attributed to dark energy. In 1998, astronomers presented evidence that the universe's expansion is not slowing down, as would be expected due to the gravity from the abundances of matter strewn throughout the cosmos. Instead, the observations of distant, exploded stars revealed that cosmic expansion is accelerating (Fig. 1). Since this initial discovery, multiple other lines of evidence, including observations of the afterglow of the big bang, known as the cosmic microwave background, and in the clustering of galaxies over cosmic history have made the case for this surprising result more secure. See also: Big bang theory; Cosmic background radiation; Matter (physics); Physics; Universe

The study of the universe and the objects in it through scientific investigation. The goal of astronomical research is to understand celestial objects and the nature and evolution of the cosmos (Fig. 1). Because much of contemporary astronomy uses the laws, mathematics, and methods of physics, the terms "astronomy" and "astrophysics" are usually used interchangeably. However, modern astronomy also uses techniques from many other scientific disciplines, including chemistry, geology, and biology, for which the terms astrochemistry, planetary science, and astrobiology are regularly used. Even astrometry, the measurement of the positions and motions of stars and other astronomical objects, has links to astrophysical investigations. See also: Astrobiology; Astrometry; Astrophysics; Biology; Chemistry; Geology; Physics

The theory that the universe began in a hot, dense state and has since expanded and cooled. The big bang is the generally accepted cosmological theory for the origin, properties, and evolution of the universe (Fig. 1). See also: Cosmology; Universe

The electromagnetic radiation leftover from the origin of the universe in the big bang. Cosmic background radiation is the oldest light in the universe, emitted when the 13.8-billion-year-old universe was about 380,000 years old. The most studied cosmic background radiation peaks in the microwave portion of the spectrum and is thus known as the cosmic microwave background or CMB. This light provides unique cosmological information regarding the age, composition, earliest conditions, and subsequent evolution of the universe (Fig. 1). The cosmic microwave background was emitted before any astronomical objects such as stars or galaxies existed, and is now observed to be like the thermal emission from a black object (blackbody) with a temperature of 2.725 K [Kelvin above absolute zero, or −273.15 °C (−454.8°F)] observed all over the sky. The energy density of this radiation is larger than that of any other cosmic radiation field. See also: Absolute zero; Astronomy; Blackbody; Electromagnetic radiation; Galaxy; Heat radiation; Radiation; Star; Temperature; Universe

Hypothetical thin tubes that carry very large amounts of vacuum energy. They belong to the class of objects known as topological defects. As the universe expands, its temperature decreases and its microscopic properties, such as the strength with which fundamental particles interact, sometime change abruptly in what are called phase transitions. A well-known example of such a transition is when water freezes into ice when the temperature drops below 0°C (32°F). Strings are predicted to form at these transitions. In this context, the lowest energy state of the universe, which is called the “vacuum” although it is not necessarily empty, can have a nontrivial structure in space, and strings appear as defects in this structure. See also: Dark energy; Energy level (quantum mechanics); Phase transitions

The constant gravitational source added to the general theory of relativity by Albert Einstein to balance the gravitational field equations necessary for a static universe. In formulas it is denoted Λ (lambda) and has units of curvature, or (length)⁻². The existence of the cosmological constant would mean that empty space gravitates as if it were filled by a static, uniform energy density ρ = Λc⁴/8πG and pressure p = −ρ, where G is Newton's constant of gravitation and c is the speed of light. It is speculated to be responsible for dark energy, because a sufficiently large cosmological constant would cause the accelerated expansion of spacetime. However, the physical origin of the cosmological constant is not understood. Quantum theory predicts the existence of the cosmological constant, but with an energy density that is approximately 10¹²⁰ times too large to be compatible with observations. This gross mismatch between theory and observation, known as the cosmological-constant problem, is a deep enigma. It is thought that a resolution requires a quantum theory of gravity. Testing for the existence of the cosmological constant is a major goal of current physics and astronomy research. See also: Accelerating universe; Dark energy; Quantum gravitation; Relativity

The study of the origin, development, and fate of the universe, including the origin of galaxies, the chemical elements, and matter itself. As a modern scientific discipline, cosmology incorporates astronomy, physics, and chemistry, among many other fields. Cosmologists construct and refine theoretical models that encompass and describe the observable, physical universe. Fundamental questions cosmology seeks to answer include the size and age of the universe, how the universe developed its evident structure of billions of galaxies arranged in a so-called cosmic web, and what the universe's ultimate fate will be (Fig. 1). See also: Astronomy; Chemistry; Galaxy formation and evolution; Physics; Universe

The cyclic universe theory is a model of cosmic evolution according to which the universe undergoes endless cycles of expansion and cooling, each beginning with a “big bang” and ending in a “big crunch”. The theory is based on three underlying notions: First, the big bang is not the beginning of space or time, but rather a moment when gravitational energy and other forms of energy are transformed into new matter and radiation and a new period of expansion and cooling begins. Second, the bangs have occurred periodically in the past and will continue periodically in the future, repeating perhaps once every 10¹² years. Third, the sequence of events that set the large-scale structure of the universe that we observe today took place during a long period of slow contraction before the bang; and the events that will occur over the next 10¹² years will set the large-scale structure for the cycle to come. Although the cyclic model differs radically from the conventional big bang–inflationary picture in terms of the physical processes that shape the universe and the whole outlook on cosmic history, both theories match all current observations with the same degree of precision. However, the two pictures differ in their predictions of primordial gravitational waves and the fine-scale statistical distribution of matter; experiments over the next decade will test these predictions and determine which picture survives.

An entity that comprises the majority of the mass-energy of the universe and is responsible for its accelerating expansion. The term “dark energy” derives from the inference that it is nonluminous and does not interact with normal matter. According to multiple lines of evidence, dark energy is estimated to make up approximately 70% of the mass-energy of the universe, with “normal” matter comprising around 5% and dark matter about 25%. Dark energy is thought to be the driver behind the accelerating expansion of the universe over time (Fig. 1). See also: Dark matter; Energy; Mass; Matter (physics); Universe

Particles or objects that exert a gravitational force but do not emit any detectable visible light or other electromagnetic radiation of any kind. Dark matter is the dominant form of matter in our Galaxy and in the universe, estimated to comprise approximately 85% of all matter. In terms of the total composition or energy density of the universe, dark matter is estimated to account for about 25%, regular matter is estimated at about 5%, and the remaining 70% is attributed to dark energy. Astronomers have inferred the presence of dark matter through its gravitational effects and have shown that dark matter is not composed of ordinary particles. Physicists have suggested several plausible candidates for dark matter; ongoing and planned experiments are capable of detecting these new particles. There are controversial claims of dark-matter detection. If our understanding of gravity as explained by German-born U.S. theoretical physicist Albert Einstein’s theory of general relativity is correct, as more than a century’s-worth of experiments have verified with ever-increasing precision, then dark matter must exist. However, it is possible that relativity is not correct, and a new physics paradigm will be needed to solve the mystery of dark matter. See also: Dark energy; Elementary particle; Gravity; Matter; Relativity; Universe