In this Universe, just because you can see it doesn’t mean you can reach it. Conversely, just because you can’t see it today doesn’t necessarily mean that its light won’t arrive tomorrow. Because the fabric of the Universe itself is expanding, and because the maximum speed at which any signal — whether massive or massless — can travel through that fabric of space is finite, there are both: Today, there’s a well-understood limit for how far we can observe in any direction in space: 46.1 billion light-years.
That limit is the size of the presently observable Universe, and it’s set by the combination of the initial expansion rate at the start of the hot Big Bang, the matter-and-energy contents of the Universe, the amount of time that’s elapsed since the hot Big Bang occurred, and the value of the maximum signal speed through space: the speed of light. Inside of the presently observable Universe is a smaller region that, if we sent a signal or a spacecraft today, could someday reach objects within that volume: the reachability or communicability limit. Outside of the presently observable Universe is a larger region whose light will someday arrive at our eyes: the future visibility limit.
And beyond that limit lies the unobservable Universe, whose true extent is unknown. Here’s what defines those four key regions, and what might change our current ways of thinking. This illustration shows galaxies receding mutually away from one another within the expanding Universe: a consequence of both the Big Bang and the late-time onset of dark energy.
Although the expansion has been occurring continuously ever since the Big Bang, the fact that we see the expansion ongoing today doesn’t necessarily mean we can extrapolate everything back to a singular point in space and time in our Universe’s past. There are several keys to understanding how far we can reach and how far we can see at any point in the history of the Universe, but they all require us to begin with two key concepts: the idea of the expanding Universe and the framework of the hot Big Bang. When most of us think of expansion, we think about something stretching and becoming thinner: like the skin of a balloon that’s being blown up.
The Universe isn’t actually like that, however. As space itself expands, nothing about it thins or changes; there’s simply more of it than there was at the beginning. The matter and energy that’s within it can become less dense — keeping the number of particles the same while increasing the volume — but the space itself is no different after expanding than before.
What can change, however, is the rate of expansion itself. The rate of expansion is related to the (square root of the) total energy density of space at any given moment. As the Universe expands, the component of energy inherent to space itself (dark energy) remains constant and doesn’t dilute, but the portions that are made of matter and radiation do dilute, getting less dense.
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