What is the Universe expanding into?
The universe started with the Big Bang about 13.77 Billion Years ago and has been expanding ever since. We can use telescopes such as Hubble to see the oldest galaxies near the edge of the universe, some 13.25 billion light-years away, and we can calculate that about 170 billion other galaxies occupy the observable universe. All of these Galaxies are moving away from us, which we know because their light appears red (aka redshift), and the faster they move away, the redder they appear. The Andromeda Galaxy (pictured) is the only exception since it is falling towards our galaxy and will collide with us in approximately 4 billion years.
Usually, it is very difficult to work out how far away these galaxies are;; they appear as lights in the sky, so we cannot accurately tell whether a bright light is farther away or a dim light is closer. Occasionally, we can work out how far away a galaxy is if a light of known brightness is switched on. A type 1a supernova is one such light source; it occurs when mass falls onto a white dwarf star. When the star reaches a critical mass, it will violently explode in a supernova.
Since this happens when the star reaches a fixed mass, they always release the same amount of energy and therefore have the same brightness. So, by measuring how bright it appears from Earth, we can calculate how far away it is. Then, by measuring the redshift from that galaxy, we can work out how quickly the galaxy is moving away from us. Hence, we can see that Galaxies that are further away are moving away faster.
Another interesting thing which appears from the graph is that the universe is still accelerating in its expansion. This means a small amount of vacuum energy is pushing galaxies apart and overriding the attraction due to gravity. So the universe is expanding, and that expansion is accelerating.
S. Perlmutter et al. (Supernova Cosmology Project), Astrophys. J. 483, 565 (1997).
So what is it expanding into?
If the universe were finite, the question could be meaningless because space would exist only within the universe, so there would be no outside. In this case, asking where the edge of the universe is is like asking where the edge of the Earth is. If you travel in one direction on Earth, you will go in a circle and end up back where you started; a finite universe would be analogous to this. If you spent enough time going in one direction in a spaceship, you would go in a loop and end up back where you started. This means you could never reach the edge; effectively, there is no edge and, by extension, no outside.
Even if the universe isn't curved in on itself and does have an edge, it could still be permanently beyond our reach or detection. The further away we look in the universe, the faster things move away from us; this means that if you go far enough, there is a point moving away from us at the speed of light (c). Since general relativity states that no information can travel through space faster than c, this means everything beyond that point is permanently beyond our reach; we couldn’t get there or even detect what’s going on there unless we break general relativity and perfect faster-than-light travel. That still means no edge and no outside, at least as far as we can tell, and if there were an outside, we’d have no idea what it is.
Both examples assume the universe is finite, but current evidence suggests the universe may be infinite; this also means there is no outside for the universe to expand into. If you add more to infinity, it cannot get bigger because it's already infinitely big. So the universe isn't expanding into anything, since it already takes up an infinite amount of space, and there is no outside.
But an infinite universe could have an outside if it only appears infinite from inside, while being finite from the outside. At first this appears nonsense but I shall attempt to explain it. The universe is expanding and this means the density of the universe is decreasing. Hence a method for telling the age of the universe is to measure the density of the galaxies. Each row on the diagram below represents the density at one moment in time from the perspective of an outsider, while each column is its position across space. From the perspective of the outside observer the universe looks like an expanding bubble which has densities which are lower towards centre and higher towards the edge.
Now let's consider this from the perspective of an observer inside the universe. Because of relativity, observers' time can run at different rates relative to each other. So a way for different observers to agree on the time since the Big Bang is to measure the density of the universe around them. The same moment in time at different locations in the universe could be agreed upon by the same density of the universe. Since the density is represented by different numbers in the diagram, drawing lines connecting the same numbers represents the same time since the Big Bang. But these lines carry on to infinity, so the same time is spread across infinite space; the universe appears infinite from the inside.
So what about the outside observer? What are they in? This could be explained as Inflationary space; you can imagine this as Swiss cheese (with holes). The multiverse is like Swiss cheese: the cheese is inflationary space, and the holes are different universes. All inflationary space is expanding at an incredible rate; big bangs occur within it, creating holes that also expand, but then their expansion slows, just like our universe did after the Big Bang. This way, we could have an infinite number of universes existing within an infinite inflationary space. This means within this infinite number of other universes, there will be an infinite number of other yous reading this blog. For more information on Infinite universes, I highly recommend Brian Greene’s Book The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos. So, to recap the original question: what is the universe expanding into? This could be meaningless since there isn’t an outside; if there is an outside, we could never know what it’s like. But if we think in terms of universes which are finite from the outside but infinite on the inside, then the outside is inflationary space, which is something like Swiss cheese.
Dr Andy Gibson
BSc PGCE QTS MSc PhD
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