Universe

The universe comprises all of existence: all forms of matter and energy, and the structures they form, from sub-atomic particles to entire galactic filaments. Since the early 20th century, the field of cosmology establishes that space and time emerged together at the Big Bang 13.787±0.020 billion years ago and that the universe has been expanding since then. The portion of the universe that can be seen by humans is approximately 93 billion light-years in diameter at present, but the total size of the universe is not known.

Some of the earliest cosmological models of the universe were geocentric, placing Earth at the center. During the European Scientific Revolution, astronomical observations led to a heliocentric model. Further observational improvements led to the realization that the Sun is one of a few hundred billion stars in the Milky Way, which is one of a few hundred billion galaxies in the observable universe.

From studying the effects of gravity on both matter and light, it has been discovered that the universe contains much more matter than is accounted for by visible objects; stars, galaxies, nebulae and interstellar gas. This unseen matter is known as dark matter. In the widely accepted ΛCDM cosmological model, dark matter accounts for about 25.8%±1.1% of the mass and energy in the universe while about 69.2%±1.2% is dark energy, a mysterious form of energy responsible for the acceleration of the expansion of the universe. Ordinary ('baryonic') matter therefore composes only 4.84%±0.1% of the universe.

Definition

The physical universe is everything that exists — all space, time, energy, and matter. This includes planets, moons, stars, galaxies, and everything in between, even the empty spaces known as intergalactic space.

Some thinkers also believe that ideas like math and logic are part of the universe. The word "universe" can mean the entire cosmos, the world, or even nature itself.

Etymology

The word universe comes from an Old French term, which itself came from a Latin word meaning 'combined into one'. Ancient Latin writers like Cicero used similar words to describe the whole of everything.

People in ancient Greece had different names for the universe, like 'the all' or 'all things'. These ideas are still found in many languages today, including some German words and English terms like the cosmos, the world, and nature.

Chronology and the Big Bang

Main articles: Big Bang and Chronology of the universe

In this schematic diagram, time passes from left to right, with the universe represented by a disk-shaped "slice" at any given time. Time and size are not to scale. To make the early stages visible, the time to the afterglow stage (really the first 0.003%) is stretched and the subsequent expansion (really by 1,100 times to the present) is largely suppressed.

The Big Bang theory explains how the universe began. It started from an extremely hot and dense state and has been expanding and cooling ever since. We call this beginning the Big Bang, which happened about 13.8 billion years ago.

After the Big Bang, the universe went through several important stages. First, tiny particles came together to form simple atoms like hydrogen and helium. Later, these atoms grouped into stars and galaxies. Today, we can still see evidence of this early time as faint light called the cosmic microwave background. The universe continues to expand, and scientists believe a mysterious force called dark energy is making this expansion speed up.

Physical properties

Main articles: Observable universe, Age of the universe, and Expansion of the universe

Illustration of the observable universe, centered on the Sun. The distance scale is logarithmic. Due to the finite speed of light, we see more distant parts of the universe at earlier times.

Gravity is the main force that affects large parts of the universe. Unlike other forces, gravity’s effects add up over large distances. This makes gravity very important for shaping the universe, even though other forces like electromagnetism are stronger at very small scales.

Because light only travels at a certain speed, we can only see as far as light has had time to reach us. This creates what we call the observable universe, which is about 93 billion light-years wide. The Milky Way, our galaxy, is about 87,000 light-years wide, and our nearest neighbor, the Andromeda Galaxy, is about 2.5 million light-years away. We don’t know if the whole universe is bigger than what we can see or if it goes on forever.

The universe began about 13.8 billion years ago with an event called the Big Bang. Since then, it has been growing bigger all the time. We know this because the light from faraway galaxies looks red, which tells us they are moving away from us. This growing apart of the universe lets stars, planets, and galaxies form over time.

Composition

The universe is made up of different kinds of matter and energy. About 68% of it is something called dark energy, 27% is dark matter, and 5% is ordinary matter—the stuff we can see and touch. Ordinary matter includes atoms, stars, galaxies, and even life. There are also tiny particles called neutrinos and energy in the form of light and other radiation, but these make up less than 1% of the universe.

The formation of clusters and large-scale filaments in the cold dark matter model with dark energy. The frames show the evolution of structures in a 43 million parsecs (or 140 million light-years) box from redshift of 30 to the present epoch (upper left z=30 to lower right z=0).

Matter in the universe isn’t spread out evenly. Some areas are very empty, called voids, while other areas are crowded with stars and galaxies. The universe has huge structures like galaxy clusters and long chains of galaxies called galactic filaments. In between these structures are more empty spaces. The universe looks mostly the same in every direction when you look at very large scales, and it’s filled with a faint glow of microwave radiation left over from the early universe.

Dark energy

Main article: Dark energy

A map of the superclusters and voids nearest to Earth

Dark energy is a mysterious force that makes the universe expand faster and faster. We don’t fully understand what it is, but it makes up most of the universe’s energy. Scientists think it might be a constant energy filling space or something more complex that changes over time.

Dark matter

Main article: Dark matter

Dark matter is invisible material that we can’t see with telescopes, but we know it’s there because of how its gravity affects stars and galaxies. It makes up most of the matter in the universe, even though we haven’t been able to detect it directly.

Ordinary matter

Main article: Matter

Masses and sizes of objects in the Universe

Ordinary matter is what makes up stars, planets, and everything we see. It’s made of atoms, which are tiny pieces of matter with a nucleus and electrons orbiting around it. After the Big Bang, simple atoms formed, and later, stars made heavier elements in their cores.

Particles

Main article: Particle physics

Everything in the universe is made of tiny particles. The most basic ones are called quarks and leptons. Quarks combine to form protons and neutrons, which make up the nucleus of atoms. Leptons include electrons, which orbit the nucleus, and neutrinos, which rarely interact with other matter. Photons are particles of light and carry the force of electricity and magnetism.

Habitability

People have long wondered if life exists somewhere else in the universe. Scientists study this question using tools like telescopes and by thinking about ideas such as the Drake equation. One interesting puzzle is called the Fermi paradox, which asks why we haven't found any signs of extraterrestrial life yet. Some believe life might be a natural part of how the universe works, linking it to the study of astronomy and astrobiology.

Cosmological models

Main article: Solutions of the Einstein field equations

The universe's shape and behavior are described by a theory called general relativity, which was created by Albert Einstein. This theory explains gravity as the way space and time change based on the amount and movement of matter and energy.

One important idea from this theory is the Big Bang, which suggests that the universe began a very long time ago from a very hot, dense point and has been expanding ever since. Scientists use special equations to understand how the universe changes over time. These equations tell us that depending on certain factors, the universe might keep expanding forever or eventually slow down and stop. Right now, we think the universe is flat and will continue to expand.

Some theories also suggest that our universe might be just one of many, called a multiverse, where each universe acts like a separate bubble that cannot interact with the others.

Historical conceptions

Historically, many cultures have had ideas about the origin and structure of the universe. Early theories often described a universe governed by natural laws, while others were based on myths and stories. The modern study of the universe, called cosmology, began with Albert Einstein's general theory of relativity in 1915. This theory helped scientists understand how the universe began and how it changes over time. Today, many cosmologists base their work on Einstein's ideas and the Big Bang theory.

3rd century BCE calculations by Aristarchus on the relative sizes of, from left to right, the Sun, Earth, and Moon, from a 10th-century AD Greek copy

Mythologies

Many cultures have stories about how the world and universe began. These stories often explain the creation through gods or natural forces. For example, some traditions tell of a world egg from which the universe emerged, while others describe a single god creating everything. There are also stories where the universe comes from the union of male and female forces or is crafted from existing materials.

Philosophical models

Flammarion engraving, Paris 1888

Early philosophers in Greece and India tried to understand the universe using reason and observation. They asked questions about what everything is made of and how change happens. Some believed everything was made from a single substance, like water or air. Others proposed that the universe was made of small, indivisible particles called atoms moving in empty space.

Astronomical concepts

Some of the earliest records of astronomy come from ancient Egypt and Mesopotamia. Early astronomers viewed Earth as a flat disk surrounded by water. Later, Greek philosophers developed models placing Earth at the center of the universe, with stars and planets moving in circular paths around it. One notable exception was Aristarchus of Samos, who suggested that Earth orbits the Sun. This idea was later revived by Nicolaus Copernicus in the 16th century.

Model of the Copernican Universe by Thomas Digges in 1576, with the amendment that the stars are no longer confined to a sphere, but spread uniformly throughout the space surrounding the planets

Deep space astronomy

In the 18th century, some thinkers suggested that fuzzy patches in the night sky, called nebulae, might be separate galaxies from our own Milky Way. In the early 20th century, astronomer Edwin Hubble used new telescopes to show that many of these nebulae were indeed distant galaxies. This discovery changed our understanding of the universe, showing that it contained many galaxies beyond our own.

The modern study of the universe began when Albert Einstein applied his theory of relativity to understand the large-scale structure and evolution of the cosmos.