Observable universe

The observable universe is a spherical region of the universe that includes all the matter we can see from Earth. This means that the electromagnetic radiation from these distant astronomical objects has had enough time to travel to our Solar System and Earth since the beginning of the cosmological expansion. The edge of this observable area is about 14.26 gigaparsecs, which is roughly 46.5 billion light-years away.

The word observable does not mean we can see everything with our telescopes. Instead, it describes a limit set by the speed of light. Because nothing can go faster than light and because the universe is only about 14 billion years old, we can only see objects whose light has had time to reach us. Objects too far away are beyond our view, hidden past what is called the particle horizon.

Scientists estimate that the radius of the observable universe is about 46.5 billion light-years. They also believe the total amount of ordinary matter in the universe might be around 1.5×10⁵³ kg. As the universe keeps expanding, some stars and galaxies we see today will eventually move out of sight, appearing to freeze in time and becoming dimmer and redder over billions of years.

Overview

The universe might be infinite, but we can only see a part of it called the observable universe. This is because light from very far away has not had enough time to reach us since the Big Bang. In the future, we might see more galaxies as their light finally arrives, but some will move away so fast that their light will become too faint to see.

There is a limit to what we can ever see, called the "future visibility limit," because the universe is expanding and some galaxies will always stay too far away for their light to ever reach us. Right now, the farthest we can see is about 46 billion light-years away, but we will never see beyond 62 billion light-years, even in the far future.

"The universe" versus "the observable universe"

When people talk about "the universe," they often really mean the "observable universe." This is because we can only see and know about the parts of the universe close enough for their light to reach us. We cannot know anything about parts of the universe that are too far away for their light to reach Earth.

Scientists think the whole universe is much bigger than what we can see. Some ideas suggest it could be so big that it is at least 1.5×10³⁴ light-years across! Others even think the whole universe might be smaller than what we can see, meaning some far-away galaxies might actually be copies of closer ones. However, it is hard to test this idea because we would see these galaxies at different ages.

Size

The observable universe is a huge sphere around us. We can see light and other signals from everything inside this sphere. The distance from Earth to the edge of this sphere is about 14.26 gigaparsecs, which is the same as 46.5 billion light-years.

This means the whole sphere across has a diameter of about 28.5 gigaparsecs or 93 billion light-years. Even though the universe is very old, about 13.8 billion years, we can see objects that are now much farther away because space itself has been stretching since the beginning.

Matter and mass

The observable universe contains a huge number of galaxies — up to around 2 trillion — and possibly as many as 10²⁴ stars. That's more stars than there are grains of sand on all the beaches of Earth!

We can estimate how much ordinary matter, like the stuff we're made of, exists in the observable universe. If we think of all this matter as being made mostly of hydrogen atoms, there might be about 10⁸⁰ of them. This huge number comes from dividing the total mass of ordinary matter by the tiny mass of one hydrogen atom. The mass of all the ordinary matter we can see is about 10⁵³ kg. This includes everything from the gas between stars to the gas between galaxies, but not mysterious things called dark matter or dark energy.

Large-scale structure

Main article: Large-scale structure of the universe

The large-scale structure of the universe describes how matter is arranged across the entire observable universe. Scientists study this by looking at light and other types of energy coming from space. They find that matter groups together in a pattern, forming clusters and long strings of galaxies. Beyond a certain distance, these patterns stop, a point known as the End of Greatness.

Most distant objects

Main article: List of the most distant astronomical objects

The farthest known object in space is a galaxy called MoM-z14. In 2009, scientists found a bright flash of light called a gamma ray burst, named GRB 090423. This flash came from a star that exploded when the universe was very young, only 630 million years old. The light from this event has traveled about 13 billion light-years to reach us.

When we talk about distances in space, there are different ways to measure them. One way is to look at how far the light has traveled, while another way calculates the actual distance the object would be if we could measure it right now. Scientists sometimes use these different ways when they discuss how far away objects in space are.

Horizons

Main article: Cosmological horizon

The universe has limits to what we can observe, called cosmological horizons. The most well-known is the particle horizon, which determines how far we can see because the universe has a finite age. Other horizons relate to how far we might see in the future, thanks to the expansion of space. There are also special horizons linked to the surface where light first became visible, and others for neutrinos and gravitational waves.