What is the universe, and is it infinite?

What is the universe?

The universe is everything that exists: all of space and time, and all the matter and energy in it. That includes every planet, star and galaxy, the gas and dust between them, light, and the dark matter and dark energy that we can't see directly. It has been expanding since the Big Bang, about 13.8 billion years ago, according to NASA.

Distances in the universe are so large that we measure them in light-years: the distance light travels in a year. Light moves at 299,792 km every second, so in a year it covers about 9.46 × 10¹² km, or 9.46 trillion km. Even sunlight takes about 8.3 minutes to reach Earth. You can practise numbers this size with our Scientific Notation Calculator.

Is the universe infinite?

There are two different questions hiding here: how big is the part we can see, and how big is the whole thing?

The observable universe is finite

We can only see light that has had time to reach us since the universe began. That defines a sphere around Earth called the observable universe. NASA estimates it is about 92 billion light-years across. That is about 8.7 × 10²³ km.

Why is it bigger than 13.8 billion light-years, if the universe is only 13.8 billion years old? Because space has kept expanding while the light was travelling. The most distant galaxies we see sent their light out over 13 billion years ago, and since then they have been carried much further away. So the edge of what we can see is now about 46 billion light-years from us in every direction.

The whole universe: finite or infinite?

Beyond the observable universe, we can't look. NASA's answer is plain: science "has no reliable estimate of the actual size of the entire universe", though scientists are pretty sure it is bigger than what we can see. The best clue is the shape of space:

• Flat: parallel lines stay parallel forever. A flat universe could go on forever, an infinite universe.
• Curved like a ball's surface (closed): travel far enough in one direction and you'd come back to where you started. This universe would be finite, but with no edge.
• Curved like a saddle (open): this would also go on forever.

Measurements of the cosmic microwave background, the oldest light in the universe, show that space is very close to flat. But "very close" leaves room for a slight curve too small to measure, which could make the universe huge but finite. Space could also be flat but wrap around on itself, like a video game screen where leaving one side brings you back on the other. So the question is still open, and it may never be fully answered, because we can never see beyond our observable patch.

What is the universe made of?

Everything we can see, all the stars, planets, gas and people, makes up only a small part of it. NASA says about 68 to 70% of the universe is dark energy, the unknown cause of the expansion speeding up. Most of the rest is matter, and about 85% of all matter is dark matter, which has gravity but doesn't give off or absorb light. That leaves ordinary matter, everything made of atoms, at only about 5% of the total.

The ordinary matter is gathered into galaxies. A 2016 study using Hubble data concluded that the observable universe holds at least 10 times more galaxies than the earlier estimate of about 200 billion, which would mean roughly 2 trillion. Galaxies cluster into groups and clusters, which line up along huge threads and sheets with empty voids between, a pattern called the cosmic web.

Where is the center of the universe?

As far as we can tell, there isn't one. The universe isn't expanding outwards from a point into empty space; space itself is stretching everywhere. Picture raisins in a loaf of bread rising in the oven: every raisin sees all the others moving away, and the farther ones moving faster, but no raisin is the centre. From any galaxy, the universe looks the same as it does from ours.

The observable universe does have a centre: us. But that is only because it is defined by how far light can reach any observer. An astronomer in a galaxy billions of light-years away would sit at the centre of their own observable universe. Try it: this question gives you a statement about the Big Bang and the expansion and asks if it is a myth or a fact.

How was the universe created?

The best explanation of how the universe developed is the Big Bang theory: about 13.8 billion years ago, the universe was extremely hot and dense, and it has been expanding and cooling ever since. Particles, then atoms, then stars and galaxies formed as it cooled. The evidence is strong: galaxies moving apart, the leftover glow of the cosmic microwave background, and the amounts of hydrogen and helium. What the theory doesn't explain is what came before or what caused it. Our guide to the Big Bang goes through the evidence and the leading ideas.

Are we alone in the universe?

We don't know yet. No life beyond Earth has been found. But the odds of planets being out there have never looked better: NASA lists more than 6,000 confirmed planets orbiting other stars, and those are only the ones near enough and easy enough to detect. Many stars have planets, and some planets orbit in the zone where liquid water could exist on their surface.

Scientists are looking in several ways: robots searching Mars for signs of past life, missions to the icy moons of Jupiter and Saturn, which have oceans under their surfaces, telescopes studying the atmospheres of distant planets for gases that life might produce, and radio searches for signals. Finding even simple microbes elsewhere would be one of the biggest discoveries in history.

Common misconceptions

• "The universe is expanding into something." There is no outside for it to expand into. Distances within it are growing.
• "The edge of the observable universe is the edge of the universe." It is just the limit of how far we can see.
• "The universe is 13.8 billion light-years across." That is its age in years. The observable part is about 92 billion light-years across, because space has expanded.
• "Galaxies move away from us because we are at the centre." Every galaxy sees the same thing.
• "Space is empty." Even between galaxies there is thin gas, light, dark matter and dark energy.

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