Universe

The universe is everything that exists: all matter and energy, and the structures they form, from tiny sub-atomic particles to huge galactic filaments. We now know that space and time began with the Big Bang about 13.8 billion years ago and that the universe has been expanding ever since. The part of the universe we can see is about 93 billion light-years wide, but we do not know how big the whole universe is.

Early ideas about the universe placed Earth at the center. Later, during the European Scientific Revolution, we learned that Earth orbits the Sun. The Sun is just one star in the Milky Way, which is one of many galaxies we can see.

By studying gravity, scientists found that there is more matter in the universe than we can see. This hidden matter is called dark matter. Most of the universe’s energy is also hidden and is called dark energy. Only a small part of the universe is made of the matter we can see, like stars and galaxies.

Definition

The 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 people think ideas like math and logic are also part of the universe. The word "universe" can mean the whole cosmos, the world, or even nature itself.

Etymology

The word universe comes from an Old French term. This term came from a Latin word meaning 'combined into one'. Ancient Latin writers like Cicero used similar words to talk about everything.

People in ancient Greece had different names for the universe, like 'the all' or 'all things'. You can still find these ideas in many languages today. This includes some German words and English words 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 tells us how the universe began. It started from a very hot and tight space and has been growing and cooling ever since. We call this start the Big Bang. It happened about 13.8 billion years ago.

After the Big Bang, the universe went through many important steps. First, tiny parts came together to make simple atoms like hydrogen and helium. Later, these atoms formed into stars and galaxies. Today, we can still see proof of this early time as soft light called the cosmic microwave background. The universe keeps growing, and scientists think a hidden force named dark energy is making this growth faster.

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 shapes the universe. Unlike other forces, gravity works over large distances. This makes gravity very important for the universe, even though other forces can be stronger up close.

Because light can only travel so fast, 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 do not 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 helps stars, planets, and galaxies to form over time.

Composition

The universe is made of different kinds of matter and energy. About 68% of it is 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 like light and other radiation, but these are 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. Other areas are full of 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 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.

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.

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 often wondered if life exists somewhere else in the universe. Scientists use tools like telescopes to study this question. They also think 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. This connects to the study of astronomy and astrobiology.

Cosmological models

Main article: Solutions of the Einstein field equations

The universe's shape and behavior are explained by a theory called general relativity. This theory was created by Albert Einstein. It explains gravity as how space and time change with matter and energy.

One key idea is the Big Bang. It suggests the universe started a long time ago from a very hot, dense point and has been expanding since then. Scientists use special equations to learn how the universe changes. These equations show the universe might keep expanding forever or eventually slow down. Right now, we think the universe is flat and will keep expanding.

Some theories also suggest our universe might be just one of many, called a multiverse. Each universe would be like a separate bubble that cannot affect the others.

Historical conceptions

Many cultures have had ideas about how the universe began and what it is made of. Old stories sometimes used myths and tales to explain these ideas. The modern study of the universe, called cosmology, started with Albert Einstein's work in 1915. His ideas helped scientists learn about the beginning of the universe and how it changes. Today, many scientists use Einstein's work 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

Different cultures have tales about the beginning of the world and the universe. These stories often talk about gods or natural forces. For example, some traditions speak of a world egg that the universe came from, while others tell of a god making everything. There are also stories where the universe starts from the joining of different forces or is made from things that already existed.

Philosophical models

Flammarion engraving, Paris 1888

Early thinkers in Greece and India tried to understand the universe by thinking and watching. They asked questions about what everything is made of and how things change. Some thought everything was made from one thing, like water or air. Others believed the universe was made of tiny pieces called atoms moving in empty space.

Astronomical concepts

Some of the first records about stars and planets come from ancient Egypt and Mesopotamia. Early stargazers saw Earth as a flat disk surrounded by water. Later, Greek thinkers made models with Earth at the center of the universe, and stars and planets moving in circles around it. One person, Aristarchus of Samos, thought Earth moved around the Sun. This idea was used again by Nicolaus Copernicus in the 1500s.

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 1700s, some people thought fuzzy spots in the night sky, called nebulae, might be galaxies far from our Milky Way. In the early 1900s, astronomer Edwin Hubble used new telescopes to prove that many of these nebulae were indeed faraway galaxies. This changed our understanding, showing that the universe had many galaxies besides our own.

The modern study of the universe started when Albert Einstein used his theory to learn about the size and changes of the cosmos.