# Why can’t we figure out roughly how big the non-observable universe is?

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If we know approximately how long it’s been since the Big Bang, and we know approximately how fast the universe expands/has been expanding, why can’t we get a good estimate on how big the non-observable universe is? Or more specifically, why can’t we figure out the radius on how far matter has spread out since the Big Bang?

In: Planetary Science

To measure something you need to be able to observe it.

By definition, the non-observable universe cannot be observed, so it cannot be measured.

EDIT: Also, this . . .

> Or more specifically, why can’t we figure out the radius on how far matter has spread out since the Big Bang?

. . . is an inacurrate understanding of the Big Bang. Don’t blame yourself; the term “Big Bang” is a terrible description of the event, because it implies an explosion. But that’s wrong. (Fun fact: many folks believe the astronomer who coined the term “Big Bang” intended the term as an insult, because he was a proponent of the idea of a steady-state universe, i.e. an infinitely-existing universe that had no beginning.)

Don’t think of the Big Bang as “matter expanding into an empty universe.” Before the Big Bang *there was no universe* and, similarly, *there was no matter* (at least not as we comprehend it today). So there was nothing for … something to expand “into.” Prior to the Big Bang, the concepts of space and time don’t really have any meaning.

Rather, the better way — albeit more intellectually confusing way — to think of the Big Bang is as the emergence of spacetime itself from a singularity (the latter of which we cannot adequately explain).

The non-observable universe is, by definition, unobservable. We don’t know if it exists. Furthermore, we don’t know how much spacetime there was at the Big Bang. You’re imagining a single dot of matter exploding into the universe, but that’s not accurate. The universe might very well be infinite in both scale and matter, and it was just closer together in the beginning.

The Big Bang didn’t happen in one small place that the universe is expanding away from. It happened everywhere and the universe is expanding by adding space between all the space, not radially out from some center point.

Non-observable is a nice way to say unknowable with our current understanding of physics.

The observable region of the universe is a sphere reaching out to a distance more or less equivalent to the distance light would be able to travel since the Big Bang and reach us.

Anything out of that sphere, if there actually is anything (which we just can’t know), may as well not exist as far as we’re concerned, as it’s unimaginably far away and incapable of exerting any kind of influence on us, and, if cosmic inflation is actually happening (if 3d space is actually being continuously created between everything all the time) those regions will remain unreachable and disconnected basically forever.

Scientists actually don’t know how fast was the universe expanding. There is a possibility the rate of expansion changed for some unknown reason. (This might explain some discrepancies in cosmologists measurements. But maybe not.)

Even assuming the rate was constant, you need to know also the initial size. For example if you know how big the whole universe was when things were closer together (let’s say when universe was half the size), you could just multiply that by 2 and get current size. Problem is the universe might actually have been infinite. That would mean now it’s 2 times infinite, so… it’s infinite.

Maybe somehow an infinite extremely dense universe came into existence which then expands and cools down but it’s still infinite. When you look at current expanding universe and simulate it backwards, you’ll get to conclusion that at some time everything had to be really close together. But no-one lived back then so no-one could check what happened for sure. We can only use current understanding of physics to piece together clues, and these clues need to be observed by telescope first, so they need to be within observable universe.

Because as far as we know (and this is extremely theoretical), the big bang didn’t happen in any one place, but everywhere at once. It’s just that “everywhere” approaches being a single point, because the more you look into the past, the closer everything is (so if you picture two galaxies, no matter the distance, they would be in the same place near the big bang).

Wether all truly comes from a point or not is an open question, but since as far as we know the big bang happened everywhere, we cannot use it to determine the size of the universe, which has no center.

There is a tower somewhere. You can not see it. You can not see pictures of it. You can not observe it in any way, ever.

How tall is it?

You do not know the universe has expanded uniformly during its lifespan. Even a second of error can compound quickly when you are dealing with billions of years. You need to be able to observe something when measuring it. If not, it is just guesswork. You can lower the margin of error of your guess. But it is still a guess.