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).
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