What is the observable universe?

Answer 1

The observable Universe is the part of the Universe we can see from Earth because the light from all the objects in it has had enough time to reach us. Light from outside the observable Universe has yet to reach Earth.

The reason we can only see part of the Universe is because of the limited speed of light, and the expansion of the Universe, which is faster than that speed. According to Einstein, nothing in the Universe can move faster than light, but nothing stops the expansion of space from moving faster than light. This results in a large part of the Universe being completely invisible to us.

Answer 2

The furthest we can detect is approximated at some 13 billion years. It is measured in time because we have no good way of telling precisely how far sources are away. It is also measured in 'redshift' (z) but both methods use assumptions with respect to inflation and expansion

Answer 3

The observable universe is the only part of the universe we can see, it might be hard to understand why but here goes... We can only see 13.5 billion light years away from our planet in any direction and this is because the universe was believed to be created 13.5 billion years ago, but the universe is currently expanding faster than the speed of light which means the entire universe is greater than 13.5 billion light years across in any direction from Earth. We can't see past 13.5 billion light years because the universe is expanding faster than the speed of light, so the light beyond 13.5 billion light years will never reach us.

Answer 4

That portion of the universe which is visible with the naked eye or a telescope. It may be argued that a radio telescope expands that distance, but it is not literally a direct observation of phenomena in the universe, but rather a radio signal reception of an unseen (and possibly invisible ) body.

Answer 5

Only about 4 percent of the universe is "ordinary" matter. About 1/4 of the Universe is "dark matter", and the rest is "dark energy". The 4 percent that is matter is the part we can see and observe, the rest is invisible. So all the stars there are, tens of billions of galaxies, are really less than 4 percent of the universe.

Another viewpoint: I don't think the question was about "dark matter" and things like that. This is how I would answer this question:

The Universe started about 13.7 billion years ago with the "Big Bang".

There has been a rapid expansion of space. Space is allowed to expand faster than the speed of light. Cosmologists tell us that space could even be infinite.

However, light can only travel at a fast, but finite speed. So, the "observable" Universe, which we see by light, is probably only a small part of the whole Universe.

Answer 6

There are various estimates. The answer is, for the radius, about 45 billion light

years. That's about 90 billion light years "across" (diameter).

The Universe is about 13.7 billion years old, so light can only have travelled about 13.7 billion light years, at most.

However, the Universe has been expanding and a place that emitted light at the beginning of the Universe could be as far as about 45 billion light years away NOW. That's what astronomers usually mean by "the radius of the observable Universe".

Answer 7

A universe may be infinite, but an observable universe is only everything we can see. An observable universe will always be finite, and the same or usually smaller than a universe.

Answer 8

The Universe is huge and there is only so much of it we can see, so they mean the part of the universe that we can see either with the naked eye or through telescope.