universes

Universes are the totality of all physical existence. A universe comprises space, time, matter, energy, and the laws that govern them. In scientific contexts, the term often refers to the observable universe—the portion that can be measured or inferred from light and other signals. The full extent of the universe beyond the observable region, and what form it might take, remain open questions.

The universe is estimated to be about 13.8 billion years old. The observable universe has a radius of about 46 billion light-years, roughly 93 billion light-years in diameter. The cosmos is expanding, a phenomenon inferred from redshifts of distant galaxies and described by the Big Bang model. The expansion appears to be accelerating, driven by dark energy. The large-scale structure includes galaxies, clusters, filaments, and voids, shaped by gravity and the initial fluctuations from the early universe.

Beyond the observable realm, some theories propose a multiverse—a collection of universes with different properties. A common framework classifies multiverses into levels. Level I posits different initial conditions in regions beyond our horizon but identical physical laws. Level II posits causally disconnected regions formed by eternal inflation with potentially different constants. Level III corresponds to the many-worlds interpretation of quantum mechanics, yielding branching histories. Level IV encompasses the idea that all mathematical structures correspond to real universes.

These ideas are speculative and debated. They are difficult to test directly, and some proposed signatures are controversial. Nevertheless, multiverse concepts arise in cosmology, quantum theory, and the study of fundamental constants, contributing to discussions about what constitutes a universe and how its properties arise.

In science, "universe" usually refers to the sum of what can be observed and inferred within physics, while "multiverse" refers to speculative extensions beyond our own cosmos. The term also appears in culture and philosophy, where it is used more loosely.