Universe

The universe is everything that exists: all matter, all energy, and every structure they create, from the smallest subatomic particles to the largest galactic filaments. Cosmology, a field that took shape in the early 20th century, has shown that the universe has been expanding since it began as a dense fireball in an event known as the Big Bang. The part we can observe currently spans about 93 billion light-years across, but the universe’s total size remains unknown.

Early cosmological models placed Earth at the center, but the Scientific Revolution brought a heliocentric view. Better observations later revealed that the Sun is just one of a few hundred billion stars in the Milky Way, which itself is one of a few hundred billion galaxies in the observable universe. On the largest scales, galaxies are spread uniformly and look the same in every direction. On smaller scales, they cluster into groups and superclusters, forming enormous filaments and voids that give space a foam-like structure. Discoveries in the early 20th century, including general relativity, led to the modern picture of an expanding, isotropic, and homogeneous universe. Evidence has piled up for the Big Bang theory: an initial hot fireball cooled and became less dense as it expanded, allowing the first subatomic particles and simple atoms to form. Giant clouds of hydrogen and helium were gradually pulled toward denser regions, creating the first galaxies, stars, and eventually everything else.

By studying how gravity affects both matter and light, scientists have found that the universe contains far more matter than what we can see—stars, galaxies, nebulae, and interstellar gas. This invisible matter is called dark matter. In the widely accepted ΛCDM model, dark matter makes up about 26% of the universe’s mass and energy, while dark energy—a mysterious force driving the expansion to accelerate—accounts for about 69%. Ordinary (baryonic) matter, the stuff of stars, planets, and visible gas, makes up only about 5% of the universe. Of that ordinary matter, stars, planets, and visible gas clouds form just about 6%.

Many competing ideas exist about the universe’s ultimate fate and about what, if anything, came before the Big Bang.

The physical universe is often defined as “the totality of all space and time; all that is, has been, and will be.” It contains all energy and matter, including planets, moons, stars, galaxies, and the contents of intergalactic space. Some philosophers and scientists also include ideas and abstract concepts—like mathematics and logic—in the definition. The word universe can also refer to the cosmos, the world, or nature.

The word “universe” comes from Old French *univers*, which traces back to Latin *universum*, meaning “combined into one.” Cicero and later Latin authors used *universum* in much the same way we use the word today. Ancient Greek philosophers from Pythagoras onward used terms like *to pan* (“the all”), meaning all matter and space, and *ta panta* (“all things”), which didn’t always include the void. Another Greek synonym was *kosmos* (“the world, the cosmos”). Latin authors had *mundus*, *natura*, and *universum*, and modern languages have equivalents like German *Weltall*, *Kosmos*, and *Universum*. In English, synonyms include everything (as in the theory of everything), the cosmos (as in cosmology), the world (as in the many-worlds interpretation), and nature (as in natural laws).

The leading model for the universe’s evolution is the Big Bang theory. In this model, the earliest state was extremely hot and dense, but the universe cooled as it expanded. The model relies on general relativity and assumptions that space is homogeneous and isotropic. A version with a cosmological constant (Lambda) and cold dark matter—the Lambda-CDM model—matches most observations very well. Much of the very earliest time is not understood. An intense period of rapid expansion called cosmic inflation is proposed to explain many observations and set the initial conditions for the Lambda-CDM model.

Within the first fraction of a second, the universe was so dense and energetic that all particles in the Standard Model were in equilibrium. As expansion cooled it, the universe went through phase transitions, like water freezing. Various elementary particles stabilized, producing a plasma of electrons, protons, and neutrons, with very energetic photons preventing them from binding until about one minute after the Big Bang. Over the next few minutes, some protons and neutrons fused into atomic nuclei through Big Bang nucleosynthesis. This process lasted about 15 minutes, producing helium, small amounts of deuterium, and traces of lithium. No other nuclei formed in significant amounts. All neutrons that didn’t fuse decayed into protons and electrons.