The universe is defined as everything that physically exists. However, the term universe may be used in slightly different contextual senses, denoting such concepts as the cosmos, the world or Nature. Current interpretations of astronomical observations indicate that the age of the universe is almost 14 billion years, and that its diameter is at least 93 billion light years.

According to the prevailing scientific model of the universe, known as the Big Bang, the universe expanded from a gravitational singularity, a point in spacetime at which all the matter and energy of the observable universe was concentrated. Since the singularity, the universe has been expanding to its present form, possibly with a “brief period” of cosmic inflation. Several independent experimental measurements support this theoretical expansion and, more generally, the Big Bang theory. Recent observations indicate that this expansion is accelerating because of the low density of the universe, and that most of the matter and energy in the universe is fundamentally different from that observed on Earth and not directly observable. The imprecision of current observations has hindered predictions of the ultimate fate of the universe.

Experiments and observations suggest that the universe has been governed by the same physical laws and constants throughout its extent and history. The dominant force at cosmological distances is gravity, and general relativity is currently the most accurate theory of gravitation. The remaining three fundamental forces and all the known particles on which they act are described by the Standard Model. The universe has at least three dimensions of space and one of time, although extremely small additional dimensions cannot be ruled out experimentally. Spacetime appears to be smooth and simply connected, and space has very small mean curvature, so that Euclidean geometry is accurate on the average throughout the universe. Conversely, on a quantum scale spacetime is highly turbulent.

The word universe is usually defined as encompassing everything. However, using an alternate definition, some have speculated that this “universe” is just one of many disconnected “universes”, which are collectively denoted as the multiverse. For example, in bubble universe theory, there are an infinite variety of “universes”, each with different physical constants. Similarly, in the many-worlds hypothesis, new “universes” are spawned with every quantum measurement. Since these universes are, by definition, disconnected from our own, these speculations cannot be tested experimentally.

Throughout recorded history, several cosmologies and cosmogonies have been proposed to account for observations of the universe. The earliest quantitative geocentric models were developed by the ancient Greeks, who proposed that the universe possesses infinite space and has existed eternally, but contains a single set of concentric spheres of finite size, corresponding to the fixed stars, the Sun and various planets, rotating about a spherical but unmoving Earth. Over the centuries, more precise observations and improved theories of gravity led to Copernicus’ heliocentric model and the Newtonian model of the solar system, respectively. Further improvements in astronomy led to the characterization of the Milky Way, and the discovery of other galaxies and the microwave background radiation. Careful studies of the distribution of these galaxies and their spectral lines have led to much of modern cosmology.

Some speculative theories have proposed that this universe is but one of a set of disconnected universes, collectively denoted as the multiverse, altering the concept that the universe encompasses everything. By definition, there is no possible way for anything in one universe to affect another. If two “universes” could affect one another, they would be part of a single universe. Thus, although some fictional characters travel between parallel fictional “universes”, this is, strictly speaking, an incorrect usage of the term universe. The disconnected universes are conceived as being physical, in the sense that each should have its own space and time, its own matter and energy, and its own physical laws. Thus such physical disconnected universes should be distinguished from the metaphysical conception of alternate planes of consciousness, which are not thought to be physical places. The concept of a multiverse of disconnected universes is very old. For example, Bishop Etienne Tempier of Paris ruled in 1277 that God could create as many universes as he saw fit, a question that was being hotly debated by the French theologians.

There are two scientific senses in which multiple universes are discussed. First, disconnected spacetime continua may exist. Presumably, all forms of matter and energy are confined to one universe and cannot “tunnel” between them. An example of such a theory is the chaotic inflation model of the early universe. Second, according to the many worlds hypothesis, a parallel universe is born with every quantum measurement. The universe “forks” into parallel copies, each one corresponding to a different outcome of the quantum measurement. However, both senses of the term “multiverse” are speculative and may be considered unscientific. No experimental test in one universe could reveal the existence or properties of another non-interacting universe.