(Image: NASA, ESA, the Hubble Heritage Team (STScI/AURA), A. Nota (ESA/STScI), and the Westerlund 2 Science Team)
WE’VE known the size of Earth since the time of the ancient Greeks. The sun, solar system and Milky Way? No problem. But when it comes to the size of the universe, we haven’t got a clue.
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“It’s weird: the size of the observable universe is one of the more precisely known quantities in astronomy, but the size of the whole universe is one of the least well-known,” says , a cosmologist at Fermilab in Batavia, Illinois.
One way to think about the size of the observable universe is to consider how far light emitted at the big bang could have travelled by now. According to our best cosmological models, that distance is about 46 billion light-years. This is the cosmic “horizon”, a sort of three-dimensional equivalent of the 2D-horizon we see on Earth.
“That is as far as we can see and how big, empirically, we can observe the universe to be,” says of Johns Hopkins University in Baltimore, Maryland, who shared the Nobel prize in 2011 for the discovery that the universe’s expansion is accelerating. “Of course we are pretty sure it goes out much farther.”
Why? Because the universe looks very similar no matter which way you look. Take the cosmic microwave background (CMB), the radiation left behind by the big bang. It is largely uniform across the sky, and we have no reason to think that would change beyond the cosmic horizon. There are no signs the universe is tailing off, so it would be a surprise if it abruptly ended.
of the Harvard-Smithsonian Center for Astrophysics says that if the universe is only a little bit larger than what we see, we might expect to find hints of structures much larger than galaxy filaments – formations of superclusters of galaxies. These hints would show up as wild variations in the temperature map of the CMB.
But they don’t, which implies that the universe is much larger than our little observable corner. If that’s the case, Eisenstein says it will be a struggle to figure out how big it is. “It might be a million times larger, or a trillion, or even infinite.”
A truly infinite universe might be tough for us to comprehend. Particularly perplexing is the idea that if the universe is infinite now then it must always have been so, even during the earliest moments in its history when the distances between objects were much smaller. But we would have to go a long way to rule it out.
Read more: “10 mysteries that physics can’t answer… yet”
This article appeared in print under the headline “Is the universe infinite or just very big?”
