Observable universe

The observable universe is the part of the universe we can see from Earth. It includes all the matter and energy whose light has had enough time to reach us since the beginning of the cosmological expansion. This region stretches about 46.5 billion light-years in radius, even though the universe itself is only about 14 billion years old. This is because the space between objects has been stretching and expanding over time.

The word "observable" does not mean we can see everything with our telescopes. It means that there is a physical limit based on the speed of light. Light cannot travel faster than this speed, so we can only see objects whose light has had time to reach us. Objects too far away for their light to reach Earth yet are beyond what we can observe.

Scientists estimate that the total amount of ordinary matter in the observable universe is about 1.5×10⁵³ kilograms. As the universe continues to expand at an accelerating rate, some objects that we can see today will eventually move out of our view. Their light will become redder and fainter until they disappear from sight.

Overview

The size of the entire universe is unknown, and it might be infinite. Some parts are too far away for their light to have reached Earth since the Big Bang. In the future, light from more distant galaxies may have time to travel to us, making additional regions observable.

However, some distant regions are moving away from us faster than the speed of light, as described by Hubble's law. The expansion of the universe seems to be speeding up, a phenomenon explained by something called dark energy. Because of this accelerating expansion, there is a limit to how far we can eventually see. Some galaxies will become too faint due to redshift to observe, even though they remain at the same distance from us. This creates a kind of boundary, called a cosmic event horizon, beyond which we cannot see events happening today.

"The universe" versus "the observable universe"

When we talk about the "universe," people often mean the "observable universe." This is the part of the universe we can see from Earth. We can't know anything directly about parts of the universe that are too far away for their light to reach us. Many ideas suggest the whole universe is much bigger than what we can see.

The galaxies we can observe might be just a tiny part of all the galaxies in the universe. Some theories, like those suggested by Alan Guth, think the entire universe could be incredibly huge — much bigger than what we can see.

Size

The observable universe is a huge sphere around us. From Earth, we can see about 14.26 gigaparsecs (which is 46.5 billion light-years) in every direction. This means the whole sphere is about 28.5 gigaparsecs across.

The light we see now from faraway objects left them a long time ago. For example, the oldest light we can see, called the cosmic microwave background, was emitted when the universe was very young. The objects that sent us this light are now very far away, but when they sent the light, they were much closer to us.

Matter and mass

The observable universe contains a vast number of galaxies and stars — up to around 2 trillion galaxies and perhaps as many as 10²⁴ stars. This means there could be more stars than all the grains of sand on Earth!

We can estimate how much ordinary matter exists in the observable universe. If we think of all this matter as made mostly of hydrogen atoms, we find there are about 10⁸⁰ of them. Scientists often say the mass of all this ordinary matter is about 10⁵³ kg. This includes everything we can see, like stars and gas between stars, but it does not include mysterious substances called dark matter and 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 use sky surveys and study different types of electromagnetic radiation, like 21-cm emission, to learn about this structure. They find that matter is organized in a hierarchical way, forming things like superclusters and filaments. Beyond a certain size, around 30 to 200 megaparsecs, there doesn’t seem to be any more organized structure. This is called the End of Greatness. The overall shape of this structure is shown in something called the matter power spectrum.

Most distant objects

Main article: List of the most distant astronomical objects

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

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 decides how far we can see because the universe has a finite age. Other horizons tell us about what we might see in the future, thanks to the expansion of space. There are also special horizons linked to the surface of last scattering, where light, neutrinos, and gravitational waves were last scattered.