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.

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.

Scientists estimate that the radius of the observable universe is about 46.5 billion light-years. As the universe keeps expanding, some stars and galaxies we see today will eventually move out of sight.

Overview

The universe might be very big, but we can only see part of it. This part is called the observable universe. We can only see this part 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 when their light finally arrives. However, some galaxies 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." This is because the universe is expanding. 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 mean the "observable universe." This is because we can only see the parts close enough for their light to reach us. We cannot know about parts 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 at least 1.5×10³⁴ light-years across! Others think the whole universe might be smaller than what we can see. However, it is hard to test this idea.

Size

The observable universe is a giant sphere all 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, or 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 has a great many galaxies — maybe up to around 2 trillion — and perhaps as many as 10²⁴ stars. There are more stars than grains of sand on all of Earth's beaches!

We can guess how much ordinary matter, like the stuff we are made of, exists in the observable universe. If we imagine all this matter as mostly hydrogen atoms, there could be about 10⁸⁰ of them. This big number comes from dividing the total mass of ordinary matter by the very small 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 shows how matter is arranged across the observable universe. Scientists study this by looking at light and energy from space. They find that matter groups together in patterns, forming clusters and long strings of galaxies. These patterns stop at a certain distance, called 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 saw 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. 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 looks at how far the light has traveled. Another way calculates the actual distance to the object. Scientists use these different ways when they talk about how far away objects in space are.

Horizons

Main article: Cosmological horizon

The universe has limits to what we can see, called cosmological horizons. The most famous is the particle horizon. This shows how far we can see because the universe is not infinitely old. Other horizons show how far we might see in the future, because space is expanding. There are also special horizons linked to the surface where light first became visible, and others for neutrinos and gravitational waves.