Gizmodo has an interesting article that someone asked my thoughts on. Part of their “Giz asks” series, it asks various physicists what’s at the edge of the universe? The physicists polled include Sean Carroll, Jo Dunkley, Jessie Shelton, Michael Troxel, Abigail Vieregg, and Arthur B. Kosowsky.
They all give similar answers, that space isn’t known to have any edge. It may be infinite, or it may curve back on itself in an extra-dimensional sphere or torus (donut) shape, meaning that if you travel in one direction long enough, you might end up where you started. The best measurements to date imply that space is flat, although we can’t rule out that within the uncertainty of our current measurements that it doesn’t curve in a way that eventually leads one of the other shapes.
Many of the physicists mention the observable universe, and that the actual universe is thought to continue well beyond it, although one pointed out that we can’t rule out major variations just beyond the boundary of our observations.
If you think about the universe as all that we can causally interact with, then the observable universe could be considered our universe, with the “edge” being the edge of what we can observe. Although the edge of observations may change in the future. Currently the furthest thing we can see is the cosmic microwave background radiation. In terms of electromagnetic radiation, it’s hard to imagine we’ll ever see farther than that.
However, if cosmic inflation is correct, and a lot of physicists are convinced it is, then the causal universe might be far larger than the currently observable universe. We can currently detect gravitational waves. If we could ever detect such waves from the period during inflation in the 10-32 seconds after the big bang, when the universe was thought to have expanded to 1030 to 10100 its previous size, then we might be able to infer things about the universe far beyond the limits of electromagnetic observation.
Of course, that range refers to things from the past that could causally affect us. If we only think about what we can causally affect from here on, then due to the ongoing expansion of the universe, the cosmic horizon has a radius of about 14-16 billion light years, which would be the limit of what we could ever conceivably have any causal influence on. As I’ve written about before, this means that most of the universe we can observe is already forever beyond our reach.
But it’s interesting to speculate what might happen if we’re ever able to travel FTL (faster than light). What might we see beyond the observable universe? Would it just be the same kind of stuff we can currently see going on into infinity? Or would we eventually find regions of the universe where things are very different?
I mentioned cosmic inflation. A variation of that idea is eternal inflation, where inflation is the natural state of spacetime, but that due to a random quantum fluctuation, a bubble of low inflation was created, aka our universe. There are different conceptions about what the edge of this bubble might look like. Some see it as a bubble of time as well as space, which we can’t leave because the edges of the bubble are the beginning of our universe in time, the big bang.
Other physicists have speculated that we could travel until we reached regions where the expansion of the universe was faster and faster, until we approached inflationary space. Unless our method of FTL protected us in some manner, we could never enter inflationary space. Aside from it perpetually receding from us, the expansion rate would overcome the nuclear forces holding our atomic nuclei together, not to mention the electromagnetic forces, and we’d be instantly ripped apart.
So travel to another bubble, even with an FTL drive, would probably never be a thing. Even if it was, other bubbles are thought to have different laws of physics. If we ever made it to another bubble, we might find its physics hostile to our form of life.
There have been measurements lately indicating that dark energy, the force driving the current expansion of the universe, may be increasing in strength. If so, then within a few tens of billions of years, the universe as we know it might end in a “big rip.” If we are in a bubble, then that bubble might eventually come to a violent end, perhaps dissolving into inflationary space.
I sometimes wonder if information of any kind, not to mention any form of life, could be preserved in inflationary space. Based on the description, it doesn’t seem so. I don’t know what’s bleaker, the long slow heat death of the universe under constant dark energy, or a big rip in a few tens of billions of year.
So in pondering the edge of the universe, we have edges in observability (currently 13.8 billion years), in inward causality (depends on cosmic inflation), on outward causality (14-16 billion light years), and edges in time including the big bang and one of the possible endings (heat death, big rip, and an increasingly unlikely one we didn’t discuss, the big crunch), as well as possible curving loopbacks and infinite expanses.
Did I miss anything?