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Does this universe have a boundary? This question truly perplexes astronomers in their quest for an answer. (Image: Wikipedia)
Our observed universe appeared immensely vast at that time. Centuries later, the boundary of the observable universe expanded once again when astronomers crafted telescopes large enough to reveal that the Milky Way was just one of many galaxies in the universe. Soon after, they also realized that the universe is expanding, with galaxies moving away from each other at an increasingly faster pace.
Since then, ever larger telescopes have allowed us to observe the universe stretching out to a distance of 92 billion light-years and likely containing 2 trillion galaxies. However, astronomers are still pondering how many more universes lie beyond their observational reach.
Virginia Trimble, an astronomer and expert in the history of this field from the University of California, Irvine, says, “The universe is always a bit bigger than what we can see.”
Building larger telescopes will not help us look further into the universe anymore.
Nobel laureate and astrophysicist John Mather, associated with NASA’s Goddard Space Flight Center and also the chief scientist of the James Webb Space Telescope, explains, “Telescopes can only observe what’s possible. You cannot look back in time beyond the age of the universe. Therefore, we are entirely limited.”
At the edge, we see the remaining light from the Big Bang – known as the cosmic microwave background radiation (CMB). But this is not the ultimate edge of the universe. Our universe continues to expand, and we may never know how large it truly is.
In recent decades, cosmologists have tried to solve the mystery of the universe’s size by determining its shape, just as ancient Greek mathematician Eratosthenes calculated Earth’s size using simple trigonometry. In theory, our universe can have one of three possible shapes, depending on the curvature of space: saddle (negative curvature), sphere (positive curvature), or flat (zero curvature).
Very few advocate for a saddle-shaped universe, but a spherical universe might seem familiar to us. Earth is round, as are the Sun and planets. A spherical universe would allow you to travel through space in any direction and return to your starting point, similar to Ferdinand Magellan’s crew circumnavigating the Earth. Einstein referred to this model as a “finite but unbounded universe.”
However, starting from the late 1980s, a series of orbiting observatories studying the CMB showed increasingly accurate measurements indicating that space has no curvature. It is flat to the extent of what astronomers can measure. If it were a sphere, it would be an enormous sphere to the point that not even our entire observable universe shows any curvature.
Mather explains, “The universe is flat like a sheet of paper. According to this, you can keep going on forever in any direction, and the universe will still look the same, whether little or large.” You will never reach an edge of this flat universe; you will only find more and more galaxies.
This is the best understanding with most astronomers. A flat universe fits both observations and theory, making it the focus of modern cosmology.
The issue is, unlike a spherical universe, a flat universe can be infinite – or not. And there is no way to truly distinguish the difference.
Trimble says, “You can look for what would tell you if it’s an infinite universe? Nobody knows.”
So, astronomers are hoping an answer may come from theory – a model that can offer indirect evidence one way or another. For instance, the Standard Model of physics predicted the existence of many particles, such as the Higgs Boson, years before they were actually discovered.
Trimble notes, “If you have a good description of everything you’ve seen so far, and it predicts something that is right, then you will expect that. That’s how most scientists think about how science works.”