If you started a trip from any place on the Earth's surface and went in a straight line for a long enough distance, you would eventually end up back where you started. You would have made one complete tour around the surface of our planet after traversing around 40,000 kilometers (25,000 miles) — crossing mountains, oceans, deserts, and so on. The final destination would be unmistakable: it would be the same as your beginning position.

Is it possible that it would work the same way in space? Would you eventually return to your starting place if you boarded a spaceship, set off in one direction, and traveled as far as you wanted? It is an intriguing question to investigate. Although all evidence point to "probably not," there are two ways in which the answer could turn out to be yes.

Such movements would be possible if our universe were a finite doughnut, and physicists could potentially estimate its size.

According to a group of astrophysicists, our cosmos may be doubly interconnected, meaning that space is closed in on itself in all three dimensions, like a three-dimensional donut. Such a universe would be finite, and their findings suggest that our entire cosmos is only three to four times larger than the observable universe's limits, which are around 45 billion light-years away.

"We could say: Now we know the size of the universe," astrophysicist Thomas Buchert, of the University of Lyon, Astrophysical Research Center in France, told Live Science in an email.

## Einstein's General Relativity Theory

Physicists explain the universe using Albert Einstein's general relativity theory. Science Alert said Einstein's theory ties spacetime's contents to its bending and warping, which then teaches those contents how to interact. The force of gravity is felt in this way. That language relates the contents of the entire universe — dark matter, dark energy, ordinary matter, radiation, and everything else — to its general geometric structure in a cosmological framework.

For decades, astronomers have disputed whether our universe is "flat" (i.e., imagined parallel lines would stay parallel forever), "closed" (parallel lines would ultimately intersect), or "open" (i.e., parallel lines would eventually overlap) (those lines would diverge).

The universe's geometry determines its fate. A closed world would eventually collapse in on itself, whereas a flat and open universe would continue to grow indefinitely.

But shape is more than just geometry. Topology refers to how shapes can vary while adhering to the same geometric laws, another Live Science report explained.

Our measurements of the universe's contents and shape tell us about its geometry — it is flat — but not about its structure. They do not say whether our universe is multiply-connected, suggesting that one or more of our cosmos' dimensions are connected.

## The Light's Connection

A group of astrophysicists studied the cosmic microwave background from Ulm University in Germany and the University of Lyon in France (CMB). Our universe was a million times smaller when the CMB was released than it is now due to Big Bang Theory, Space.com said. So if it is interconnected, it was much more likely to coil in on itself within the observed bounds of the cosmos at the time. Today, due to the expansion of the cosmos, the wrapping is much more likely to occur at a size beyond the observable limits, making it much more difficult to observe. The CMB provides the best opportunity to see the footprints of a multi-connected cosmos.

The researchers focused on the temperature perturbations - fancy physics speak for bumps and wiggles — in the CMB. The perturbations caused by one or more dimensions in our universe connecting back to itself could not be greater than the distance between the loops. They would just be too big.

In an email to Live Science, Buchert added disturbances in the temperature of the CMB radiation exist in an infinite space in all cases. However, if space is finite, there will be wavelengths missing that are larger than the space's size."

In other words, the disturbances would have a maximum size, which might disclose the universe's architecture.

The team discovered that the CMB measurements were best matched by a multiply-connected universe three to four times larger than our observable bubble. While this finding theoretically implies that you may travel in one way and finish up back where you started, this is not possible in practice. We live in an expanding universe, and the cosmos is expanding at a rate faster than the speed of light on a wide scale. Therefore you would never be able to catch up and complete the loop.

The findings, according to Buchert, are still preliminary. Instrument effects could potentially account for the absent large-scale oscillations.

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