By Sara Webb - Lecturer, Centre for Astrophysics and Supercomputing, Swinburne University of Technology
Does the universe have an end or an edge? Scientists cannot definitively answer this question, but they have ideas, theories, and measurements to discuss.
This question about the boundaries of the universe is one of those that humans will likely continue to ponder until the end of time.
We are not sure, but we can try to imagine what the edge of the universe might be like, if there is one.
Going back in time
Before we start, we need to go back in time. The night sky has seemed to have always looked the same throughout human history. It has been so stable that people around the world have used the patterns they saw in the stars to navigate and explore.
To our eyes, the sky seems infinite. With the invention of telescopes around 400 years ago, we were able to see farther than our eyes had ever been able to. We continued discovering new things in the sky, found more stars, and then began noticing that there were many strange-looking cosmic clouds.
This gigantic image of the Triangulum Galaxy—also known as Messier 33—was created from 54 different views with Hubble's Advanced Camera for Surveys. With a breathtaking size of 34,372 by 19,345 pixels, it's the second-largest image Hubble has ever produced.
M. Durbin, J. Dalcanton, B.F. Williams/NASA, ESA Astronomers gave these objects the name "nebula," from the Latin word for "mist" or "cloud." Less than 100 years ago, we confirmed for the first time that these cosmic clouds or nebulae were actually galaxies. They resemble the Milky Way, the galaxy our planet resides in, but they are very far away.
What's astonishing is that in every direction we look into the universe, we see more and more galaxies. In this image from the James Webb Space Telescope, which looks at a part of the sky roughly the size of a grain of sand held at arm's length, we can see thousands of galaxies.
It's hard to imagine there could be a limit where all of that stops.
NASA's James Webb Space Telescope has produced the deepest and sharpest image of the distant universe to date. Known as Webb's First Deep Field, this image of the galaxy cluster SMACS 0723 is overflowing with detail.
NASA, ESA, CSA, STScI The edge of the universe
However, there technically is a limit to our universe. We call it the "observable" universe. This is because we don't know if our universe is infinite—that is, whether it goes on forever.
Unfortunately, we may never know because of one inconvenient fact: the speed of light.
We can only see light that has had time to travel to us. Light travels at the exact speed of 299,792,458 meters per second (about 670 million miles per hour). Even at this speed, it takes a long time to cross our universe. Scientists estimate the size of the universe to be at least 96 billion light years across, and likely much larger.
What would we see if there were an edge?
If we traveled to the very edge of the universe as we think it exists, what would actually be there?
Many other scientists (myself included) believe there would simply be... more universe!
As I said, there is a theory that our universe has no edge and might continue infinitely.
But there are also other theories. If our universe has an edge and you crossed it, you could find yourself in a completely different universe. (For now, it's best to leave that for science fiction.)
Even though there is no clear answer to this question, it's precisely questions like these that help us keep exploring and discovering the universe and allow us to better understand our place within it.