Topics Space and the universe

How big is the universe?

The part of the universe we can see, called the observable universe, is about 93 billion light-years across. That is the size of the region whose light has had time to reach us since the Big Bang. It contains hundreds of billions of galaxies, each with hundreds of billions of stars, give or take. But the observable universe is not necessarily the whole thing.

Here is the interesting part: nobody knows how big the whole universe is. It could be much larger than what we can see, and it could even be infinite. Measurements suggest space is very close to flat, which fits with a universe that is enormous, but that does not prove it goes on forever. Scientists disagree about what lies beyond our horizon, and honest answers include a lot of "we can't tell."

An episode on this question would start with the number and then take it apart: why 93 billion light-years when the universe is only about 14 billion years old, how we measure distance at all, and what "expanding" really means. bre's hosts are AI, so they can get things wrong, and you can press Talk to ask a question as you listen.

What a bre episode would cover

An outline of the episode bre would make for this question. Every episode is written fresh when you ask, so yours will differ.

  1. The number everyone quotesAbout 93 billion light-years across is the size of the observable universe. We define what observable means and why it is a boundary of sight, not an edge of space.
  2. Why it is bigger than its ageThe universe is roughly 14 billion years old, yet the observable part is far wider than 28 billion light-years. Space itself has stretched while light traveled.
  3. How we measure cosmic distanceAstronomers build a ladder of methods, from parallax on nearby stars to exploding stars in distant galaxies. Each rung checks the next.
  4. What lives inside itHundreds of billions of galaxies, mostly separated by vast empty gaps. Most of the matter is not the kind we can see.
  5. Is there an edge?Nobody has found one. Space looks very close to flat, which is consistent with a universe that is huge or infinite, but it is not proof.
  6. What we may never knowLight from beyond our horizon can never reach us, so some questions may stay open. We cover what is solid and what is only a good guess.

How the episode might open

A sample exchange between two of bre’s AI hosts, bre and Arlo. Both are AI; this is written by AI, as every bre episode is.

  1. breAI host

    Okay, let's start with the question everyone actually wants answered. How big is the universe? And I'll warn you, the honest answer has two parts.

  2. ArloAI host

    Two parts. Go on then.

  3. breAI host

    Part one: the observable universe is about 93 billion light-years across. That's the region whose light has had time to get here. Part two: nobody knows how much lies beyond it.

  4. ArloAI host

    So the number's a boundary of what we can see. Not the size of everything.

  5. breAI host

    Right. Picture standing on a ship in open ocean. The horizon is a circle around you, but the sea doesn't stop there.

  6. ArloAI host

    Fair. But the universe is about 14 billion years old. How's the width 93 billion? Who counted that?

  7. breAI host

    Good catch, and that's the best part. Space itself stretched while the light was traveling, so the starting points drifted far away.

  8. ArloAI host

    Stretching space. Right. Explain that slowly.

Questions people also ask

Does the universe have an edge?
No edge has ever been observed. The observable universe has a boundary, but that is the limit of what light has reached us, not a wall. Whether the whole universe is finite or infinite is still unknown, and scientists debate it.
Is the universe infinite?
We don't know. Measurements show space is very close to flat, which allows for an infinite universe, but also for a finite one far larger than we can see. The evidence cannot settle it so far.
How can the universe be bigger than its age in light-years?
Because space itself expands. Light left distant objects long ago, and while it traveled, the space between us and those objects stretched. Their current distance is therefore much greater than the light's travel time would suggest.
How many galaxies are in the universe?
The observable universe holds hundreds of billions of galaxies, by common estimates. The exact count is uncertain, because faint and distant galaxies are hard to detect. Telescopes like Hubble and James Webb keep refining the picture.

Related topics

More: all 300 topics, space and the universe, or the longer reads on /learn.

bre’s hosts are AI, and every episode is generated, so they can be wrong: check anything that matters. This page outlines what an episode would cover. It is for interest and learning, not medical, financial or legal advice.