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How Big Is the Universe?

Galaxies as far as the eye can see, at least from Hubble's perspective.
(Image: © NASA/ESA Hubble Space Telescope)

If you've ever dreamed of time traveling, just look out at the night sky; the glimmers you see are really snapshots of the distant past. That's because those stars, planets and galaxies are so far away that the light from even the closest ones can take tens of thousands of years to reach Earth. 

The universe is undoubtedly a big place. But just how big is it?

"That may be something that we actually never know," Sarah Gallagher, an astrophysicist at Western University in Ontario, Canada, told Live Science. The size of the universe is one of the fundamental questions of astrophysics. It also might be impossible to answer. But that doesn't stop scientists from trying. 

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The closer an object is in the universe, the easier its distance is to measure, Gallagher said. The sun? Piece of cake. The moon? Even easier. All scientists have to do is shine a beam of light upward and measure the amount of time it takes for that beam to bounce off the moon's surface and back down to Earth. 

But the most distant objects in our galaxy are trickier, Gallagher said. After all, reaching them would take a very strong beam of light. And even if we had the technological capabilities to shine a light that far, who has thousands of years to wait around for the beam to bounce off the universe's distant exoplanets and return back to us? 

Scientists have a few tricks up their sleeves for dealing with the farthest objects in the universe. Stars change color as they age, and based on that color, scientists can estimate how much energy, and light, those stars give off. Two stars that have the same energy and brightness aren't going to appear the same from Earth if one of those stars is much farther away. The farther one will naturally appear dimmer. Scientists can compare a star's actual brightness with what we see from Earth and use that difference to calculate how far away the star is, Gallagher said. 

But what about the absolute edge of the universe? How do scientists calculate distances to objects that far away? That's where things get really tricky.

Remember: the farther an object is from Earth, the longer the light from that object takes to reach us. Imagine that some of those objects are so far away that their light has taken millions or even billions of years to reach us. Now, imagine that some objects' light takes so long to make that journey that in all the billions of years of the universe, it still hasn't reached Earth. That's exactly the problem that astronomers face, Will Kinney, a physicist at the State University of New York at Buffalo, told Live Science. 

"We can only see a tiny, little bubble of [the universe]. And what's outside of that? We don't really know," Kinney said.

But by calculating the size of that little bubble, scientists can estimate what's outside of it. 

Scientists know that the universe is 13.8 billion years old, give or take a few hundred million years. That means that an object whose light has taken 13.8 billion years to reach us should be the very farthest object we can see. You might be tempted to think that gives us an easy answer for the size of the universe: 13.8 billion light-years. But keep in mind that the universe is also continuously expanding at an increasing rate. In the amount of time that light has taken to reach us, the edge of the bubble has moved. Luckily, scientists know just how far it's moved: 46.5 billion light-years away, based on calculations of universe’s expansion since the big bang. 

Related: If There Were a Time Warp, How Would Physicists Find It?

Some scientists have used that number to try and calculate what lies beyond the limit of what we can see. Based on the assumption that the universe has a curved shape, astronomers can look at the patterns we see in the observable universe and use models to estimate how much farther the rest of the universe extends. One study found that the actual universe could be at least 250 times the size of the 46.5 billion light-years we can actually see.

But Kinney has other ideas: "There's no evidence that the universe is finite," he said, "It might very well go on forever." 

There's no saying for sure whether the universe is finite or infinite, but scientists agree that its "really freaking huge," Gallagher said. Unfortunately, the little part we can see now is the most we'll ever be able to observe. Because the universe is expanding at an increasing rate, the outer edges of our observable universe are actually moving outward faster than the speed of light. That means that our universe's edges are moving away from us faster than their light can reach us. Gradually, these edges (and any restaurants there, as British author Douglas Adams once wrote) are disappearing from view. 

The universe's size, and the sheer amount of it that we can't see — that's humbling, Gallagher said. But that doesn't stop her and other scientists from continuing to probe for answers. 

"Maybe we won't be able to figure it out. It could be seen as frustrating," Gallagher said. "But it also makes it really exciting."

Originally published on Live Science.