Where is the universe expanding to?
Neuroscientist and science communicator
Daniel is a researcher specialized in neuroglia and brain injuries. His areas of interest are neurosciences, evolution and cosmology. He is the author of Fundamentals of Cosmology, the Science of the Universe (Spanish book)
When people learn about the expansion of the universe, they often ask, “Where is it expanding? What is the universe expanding into? Modern cosmology has a simple and accurate answer to this question.
The evidence we have gathered over the last century denotes that the universe began as an expansion of space and time that we call the Big Bang. Cosmologists say that since then, the universe has been expanding; it is getting bigger every day. For many of us, it is not difficult to get a picture of this. However, there is a recurring question. If the universe is all there is, where does it expand? Or in other words, what is the universe expanding into? We will see that the answer offered by modern cosmology is simple.
These questions are based on everyday experience which tells us that objects expand to where there is more space. Thus, a soap bubble that starts from our cigarette expands into the remaining space in the room in which we are. However, that image is inadequate to represent what cosmologists call the “expansion of the universe”.
Understanding the expasion
To understand this, two elements must be considered. First, the discovery of the expansion of the universe was made in the 1920s by Edwin Hubble, who, by measuring the light spectra of different galaxies, realized that most of them (except those in the local cluster) were moving away from ours. The farther away a galaxy is, the faster it moves away, or technically, the faster it recedes.
In this context, galaxies do not move away from us precisely because they are moving in space, as Earth does around the Sun, but because space is constantly being created between galaxies. Although every galaxy in the universe moves through space, the main cause of their separation is the creation of space between them. Any observer, in any galaxy in the universe, would see exactly the same thing.
So the concept of the expansion of the universe is not well represented by a bubble that we see inflating from the outside, but rather as if we were in a room and we see that the walls and all the objects in it are further and further away from us. That is to say, with the passage of time there are more and more tiles that separate us from the wall, from the bed, from the bedside table.
Fig 1. The expansion of the universe can best be conceptualized if we consider that space is created between two points from our position as observers. Drawing: Julián Restrepo / Published in Foundations of Cosmology, the science of the universe.
The Hubble Horizon
Now, general relativity, which is the theory and equations describing the universe on a large scale, tells us that space-time is a four-dimensional fabric (three spatial and one temporal dimension) that exists only in the universe. In this context, what is the universe? What we, standing from the Earth, in the Milky Way, can see; that is, an observational sphere that comprises 13.7 billion years in the past, equivalent to about 46 billion light-years in every direction we look. That sphere, technically called the Hubble horizon, gets bigger, expands. In 5 billion years, the horizon for people in any galaxy will be 18.7 billion years in the past, and the corresponding distance.
So the answer to the questions we asked at the beginning, first, must be nuanced. When cosmologists talk about the expansion of the universe, they are talking about the creation of space between galaxies and the expansion of Hubble’s horizon. Second, if there is something that is “outside the universe” or something that the universe is “containing,” it is something that we simply do not know, nor can we know with the tools of science. The only thing cosmologists can tell us is that the universe is expanding on itself, because we have no physical or mathematical evidence to tell us that there is anything that exists beyond the universe, beyond space-time. We can’t go out there and see the universe expanding into a larger recipient, if that were to happen.
Some inflationary models suggest that the expansion of the universe can only be interpreted as an effect of galaxies moving away from each other in infinite space-time. However, we have no proof of this fact beyond mathematical models. Also, other inflationary models tell us that our universe is just a bubble in the middle of a multiverse or a hyperspace full of infinite universes; we cannot know that either. On the other hand, string theory suggests that our universe is a brane that can interact with other universes through gravity. We have not collected evidence of this.
In short, when cosmologists talk about the expansion of the universe, they mean the creation of more space between galaxies and the expansion of Hubble’s horizon. On the other hand, if the universe is contained and expanding in hyperspace, it is something outside the bounds of science. It is something that science cannot answer.
The universe is under no obligation to make sense to you
Neil deGrasse Tyson