How Far Is It To The Edge Of The Universe?

Artist's logarithmic scale conception of the observable universe. Galaxies give way to large-scale... [+] structure and the hot, dense plasma of the Big Bang at the outskirts. This 'edge' is a boundary only in time.

If you were to go as far out into space as you can imagine, what would you encounter? Would there be a limit to how far you could go, or could you travel a limitless distance? Would you eventually return to your starting point, or would you continue to traverse space that you had never encountered before? In other words, does the Universe have an edge, and if so, where is it?

Believe it or not, there are actually three different ways to think about this question, and each one has a different answer. If you consider how far you could go if you:

    • left today in an arbitrarily powerful rocket,
    • considered everything that could ever contact us or be contacted by us from the start of the hot Big Bang,
    • or used your imagination alone to access the entire Universe, including beyond what will ever be observable,

You can figure out how far it is to the edge. In each case, the answer is fascinating.

We often visualize space as a 3D grid, even though this is a frame-dependent oversimplification when... [+] we consider the concept of spacetime. In reality, spacetime is curved by the presence of matter-and-energy, and distances are not fixed but rather can evolve as the Universe expands or contracts.

ReunMedia / Storyblocks

The key concept to keep in mind is that space isn’t how we normally conceive of it. Conventionally, we think about space as being like a coordinate system — a three-dimensional grid — where the shortest distance between two points is a straight line, and where distances don’t change over time.

But both of those assumptions, so thoroughly good in our everyday lives, fail spectacularly when we begin looking at the larger-scale Universe beyond our own planet. For starters, the idea that the shortest distance between two points is a straight line falls apart as soon as you start introducing masses and energetic quanta into your Universe. Because spacetime is subject to curvature, which the presence of matter and energy is the cause of, the shortest distance between two points is inherently dependent on the shape of the Universe between those points.

Instead of an empty, blank, three-dimensional grid, putting a mass down causes what would have been... [+] 'straight' lines to instead become curved by a specific amount. In General Relativity, we treat space and time as continuous, but all forms of energy, including but not limited to mass, contribute to spacetime curvature. If we were to replace Earth with a denser version, up to and including a singularity, the spacetime deformation shown here would be identical; only inside the Earth itself would a difference be notable.

Christopher Vitale of Networkologies and the Pratt Institute

In addition to that, the fabric of spacetime itself does not remain static over time. In a Universe filled with matter and energy, a static, unchanging Universe (where distances between points remain the same over time) is inherently unstable; the Universe must evolve by either expanding or contracting. If Einstein’s General theory of Relativity is correct, this is mandatory.

Observationally, the evidence that our Universe is expanding is overwhelming: a spectacular validation for Einstein’s predictions. But this carries with it a series of consequences for objects separated by cosmic distances, including that the distance between them expands over time. Today, the most distant objects we can see are more than 30 billion light-years away, despite the fact that only 13.8 billion years have passed since the Big Bang.

The farther a galaxy is, the faster it expands away from us and the more its light appears... [+] redshifted. A galaxy moving with the expanding Universe will be even a greater number of light years away, today, than the number of years (multiplied by the speed of light) that it took the light emitted from it to reach us. But we can only understand redshifts and blueshifts if we attribute them to a combination of motion (special relativistic) and the expanding fabric of space (general relativistic) contributions both.

Larry McNish of RASC Calgary Center

When we measure how distant a variety of objects are from their physical and luminous properties — along with the amount that their light has been shifted by the Universe’s expansion — we can come to understand what the Universe is made of. Our cosmic cocktail, at present, consists of:

  • 0.01% radiation in the form of photons,
  • 0.1% neutrinos, an elusive, low-mass particle almost as numerous as photons,
  • 4.9% normal matter, made mostly of the same stuff we are: protons, neutrons, and electrons,
  • 27% dark matter, an unknown substance that gravitates but neither emits nor absorbs light,
  • and 68% dark energy, which is the energy inherent to space that causes distant objects to accelerate in their recession from us.

When you combine these effects together, you get a unique and unambiguous prediction for how far it is, at all times past and present, to the edge of the observable Universe.

A graph of the size/scale of the observable Universe vs. the passage of cosmic time. This is... [+] displayed on a log-log scale, with a few major size/time milestones identified. Note the early radiation-dominated era, the recent matter-dominated era, and the current-and-future exponentially-expanding era.

E. Siegel