These 5 divisions define how far we can go in space

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:

places that we can currently see that we could never reach or communicate with,
and places that we cannot currently observe today, but whose light is on the way and will someday reach us in the future.

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.

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. As a result, the expansion rate drops as the energy density drops, causing the Universe to expand more and more slowly as time goes on.

The Big Bang, meanwhile, is normally visualized incorrectly: as a singularity, or the birth of time and space, where all of the matter and energy in the Universe was compressed into a single point. That’s not necessarily true and hasn’t been thought to be the case for over 45 years: since cosmic inflation was first proposed. We now recognize that the Big Bang represents the hot, dense, early state associated with our Universe as-we-know-it in its infancy, but that state was preceded and set up by an earlier state: cosmic inflation, where there was no matter or radiation in the Universe, but only an enormous amount of that relentless energy inherent to space itself.

In the aftermath of inflation, the hot Big Bang commenced. The Universe began expanding extremely rapidly, as the density during those early moments was tremendous. As it expanded, however:

the density dropped,
the kinetic energy of each matter particle and the energy of each radiation particle decreased,
causing the Universe to expand more and more slowly as time went on.

Eventually, the energy density in the form of radiation drops below the total energy density in the form of matter: a transition that occurs a few thousand years after the Big Bang. Neutral atoms form, gravitation leads to black holes, stars, galaxies, and a cosmic web, and eventually, the matter density drops so low that the energy inherent to space itself — today’s dark energy — becomes the dominant form of energy that determines the expansion of the Universe, a transition which occurred about 6 billion years ago.

Throughout all this time, the amount of Universe that an observer can potentially see has been increasing. We can imagine that, at the onset of the hot Big Bang, from every point in the Universe an enormous amount of omnidirectional radiation emerged. At any other point, you’ll see that initially-emitted light arriving from a spherical surface surrounding you based on the speed of light, the expansion of the Universe, and the amount of time that’s elapsed since the hot Big Bang first occurred. Back when a total time of:

1 picosecond had elapsed, that sphere was about the size of the Earth-Sun distance,
1 millisecond had elapsed, that sphere was about the size of one light-year,
1 second had elapsed, that sphere was about the size of a massive globular cluster, around 20-40 light-years in radius,
1 year had elapsed, the Universe was just a little bit smaller than the radius of the Milky Way,
10,000 years had elapsed, the now matter-dominated Universe was about 10 million light-years in radius,
13.8 billion years had elapsed, bringing us up to our dark energy-dominated present, the Universe is 46.1 billion light-years in radius.

It is precisely this line of thinking that leads us to draw conclusions about the size of the observable Universe at any moment in cosmic history: the amount of distance that light generated at the start of the hot Big Bang could have traveled from that event and still reach us.

However, just because that light — emitted 13.8 billion years ago — is reaching us now doesn’t necessarily mean that we, in the here and now, can necessarily reach that same point in space. The Universe is still expanding and has been expanding for all this time. Because of the way that expansion works, we have to remember something subtle. Whenever a signal is emitted, such as a light signal, that signal has to travel through the fabric of space, and that signal travels at a specific speed: for example, the speed of light in a vacuum, or 299,792,458 m/s.

However, when we measure the distance between any two points in the Universe and ask how that distance evolves, that distance evolves owing to cosmic expansion: which is a speed per unit distance, such as 67 km/s/Mpc. This means that for every megaparsec (Mpc, or about 3.26 million light-years) two objects are separated by, those two points are receding from one another by 67 km/s/Mpc. Since the speed of light is 299,792.458 km/s, we can just do a little bit of algebra to figure out how far away an object needs to be for those two points to speed away from one another at speeds that are greater than the speed of light. That value, using the above numbers, works out to about 4.474 megaparsecs, or the equivalent of 14.6 billion light-years.

Now, in reality, even though the Universe is dominated by dark energy, there’s still plenty of matter remaining within it, so the expansion rate will continue to drop. Eventually, it will bottom out somewhere in the range of around 55-60 km/s/Mpc, so objects that are slightly farther away than 14.6 billion light-years will still be reachable: somewhere in the range of 15-18 billion light-years away, depending on a few cosmic parameters that we’re still working to pin down. Objects within that specific distance can be reachable in one of two ways.

They can be reached by a light signal, such as a set of radio waves, that are emitted or transmitted today, alerting any intelligent observers to our presence and announcing “we are here” in a triumphant effort. If we set our sights on a star or galaxy within that critical distance, we could yet be heard and discovered.
Or they can be reached by an advanced enough spacecraft, launched today and accelerating at a constant rate: approaching but never quite reaching the speed of light owing to the laws of special relativity. If we left today, we could in principle reach everything within that 15-18 billion light-year distance of us.

That’s the first limit: the reachability limit of the Universe, which is also the same limit as the (one-way) communicable cosmic limit.

If we wanted to instead ask about a two-way communicable cosmic limit, we’d have to get more restrictive: an object only approximately 10 billion light-years away is the maximum distance we could consider. When our signal (emitted at present) arrived, that object would still be within that 15-18 billion light-year distance of us, and so if they immediately sent us a return message, it would still arrive at our eyes if given enough time. In this sense, it would be smarter, for objects between 10 and 18 billion light-years away, to send a spaceship to make contact, as even if our signals were received, they’d never be returned.

Nevertheless, within about 15-18 billion light-years of us, objects can be reached with a spacecraft and (at least one-way) communicated with if we sent a signal toward them at the speed of light today. Beyond that 15-18 billion light-year limit, we can see those objects as they were in the past, and those objects can potentially see us as we were in the past, but we cannot send signals to them now that will ever reach them as the destination. Beyond the reachability or communicability limit, we can still observe them and they can still observe us, but only as we were in the past.

That goes out to a maximum distance of 46.1 billion light-years. Any observer within that radius of us is presently detectable by us, at least as they were in the distant past. Similarly, given the entirety of the volume of space that we are within 46.1 billion light-years of, observers there can see our location in space — the Milky Way (or its predecessors) — as we were in the distant past. We are within one another’s observable Universe, and so we can, right now, observe light that was emitted from one another’s locations at some point in the distant past.

Beyond the limit of the observable Universe, however, we don’t have to lose hope immediately. There are still objects out there that emitted signals long ago, perhaps even back at the instant where the hot Big Bang can first describe our Universe. Those signals are already on their way, and while we cannot yet see them today, there are plenty of those signals that are already within that critical 15-18 billion light-years radius of us. Just as someone could send a signal that reached us from within that radius today, so, too, will we eventually be able to observe the signals that have already made it this far toward where we are.

This means that, beyond the limits of the presently observable Universe, there’s the “someday observable Universe” beyond that: what we call the future visibility limit.

Conservatively, taking the lower value in the 15-18 billion light-year range and adding that to the 46.1 billion light-years that is our presently observable Universe, that means that any object within 61 billion light-years of us — and perhaps up to a maximum of 64 billion light-years from us — will someday, eventually, be observable to us. It also means that our infant Milky Way, or at least the point in space where the Milky Way now exists, will someday be detectable by an observer at those locations as well. We can never reach or communicate with them, and we can’t even see them today (nor can they see us), but in the future, we’ll be able to collect light that originated from their locations, and vice versa.

It’s only beyond that limit, when we look to distances that surpass the future visibility limit, that we enter the realm of the forever-unobservable Universe. This is the part of the cosmos that not only lies beyond our current cosmic horizon, but that lies beyond our ability to ever see it, hear from it, or contact it, even arbitrarily far into the future. This rounds out the five total regions of space that we have.

The two-way communicable Universe, within about 10 billion light-years of us.
The one-way communicable, or reachable, Universe, beyond 10 billion but within 15-18 billion light-years of us.
The unreachable, but currently observable Universe, beyond 18 billion but within 46.1 billion light-years of us.
The presently invisible, but someday in the future visible Universe, beyond 46.1 billion light-years but within 61-64 billion light-years of us.
And the forever unobservable Universe, beyond 61-64 billion light-years of us.

Because we live in a (spatially) three-dimensional Universe, the differences in volume between these five regions is enormous. As a percentage of the presently observable Universe:

the two-way communicable Universe takes up just 0.1% of the observable Universe’s volume,
the (only) one-way communicable, or reachable, Universe takes up only 0.2-0.5% of the presently observable Universe’s volume,
the non-communicable observable Universe takes up 99.4-99.7% of the full observable Universe’s volume,
the future-visible Universe, but not presently visible Universe, is 133-169% larger than the currently observable volume,
and the forever unobservable Universe is likely millions of times greater in volume, with no upper limit.

Of course, this relies on an assumption that’s consistent with all of our observations, but remains unproven: that dark energy truly is a constant in our Universe, and behaves identically to a true cosmological constant. If instead, dark energy turns out to evolve either in time or across space, these conclusions may yet change dramatically, and these five regions will likely shift, potentially by a tremendous amount. But at the present time, these are the divisions for our Universe to the best of our knowledge. Even though we can see so much of it, there’s only a tiny, nearby sliver of it that we’ll ever be able to reach!

This article is featured on Big Think.

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