{"id":43130,"date":"2023-03-21T09:30:50","date_gmt":"2023-03-21T09:30:50","guid":{"rendered":"https:\/\/www.skyatnightmagazine.com\/?p=117247"},"modified":"2023-03-21T10:32:31","modified_gmt":"2023-03-21T10:32:31","slug":"does-the-universe-have-an-edge","status":"publish","type":"rss_feed","link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/rss_feed\/does-the-universe-have-an-edge\/","title":{"rendered":"Does the Universe have an edge?"},"content":{"rendered":"

What is outside the Universe? Does such a question even make sense? <\/p>

By Govert Schilling\n \t\t<\/p>

Published: Tuesday, 21 March 2023 at 12:00 am<\/p>

\n\n

I<\/span>t\u2019s one of the most perplexing questions in cosmology: does our Universe have an edge?<\/p>\n

If you keep travelling through space in an imaginary faster-than-light spaceship, would you ever arrive at some boundary, unable to go any further?<\/p>\n

And if the Universe does have an edge, what lies beyond it?<\/p>\n

It\u2019s all very hard to imagine.<\/p>\n

Then again, an infinite Universe is just as difficult to wrap your head around.<\/p>\n

After all, there must be something that space is expanding into, right?<\/p>\n

\"""\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
Much of the Universe\u2019s expansion occurred a fraction of a second after the Big Bang, during inflation, and the initial positions of all the matter in the Universe are imprinted on its afterglow. Credit: NASA<\/span><\/figcaption><\/div>\n

Let\u2019s start with a related but easier-to-grasp concept: our Universe has an apparent<\/i> edge, called the cosmological horizon.<\/p>\n

The light emitted right after the Big Bang<\/a> has been travelling for 13.8 billion years through space.<\/p>\n

This means we can only see the Universe up to a current distance corresponding to a light-travel time of 13.8 billion years.<\/p>\n

Thanks to the expansion of space<\/a>, this so-called co-moving distance is approximately 45 billion lightyears, and anything beyond this limit is unobservable to us because not enough time has elapsed since the birth of the Universe for light from these remote regions to reach our telescopes.<\/p>\n

But, just like the familiar horizon seen by sailors on the ocean, this cosmological horizon is not some real, physical boundary.<\/p>\n

And as the ocean stretches beyond the sailor\u2019s horizon, so too does space stretch beyond our observable<\/i> Universe.<\/p>\n

\"""\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
Like a sailor on the ocean, our grasp of the extent of the Universe is limited to the distance we can see.<\/span><\/figcaption><\/div>\n

There\u2019s no reason why there can\u2019t be galaxies<\/a> at these extremely large distances; they\u2019re just invisible to us, no matter how powerful our telescopes are.<\/p>\n

But knowing the Universe goes on beyond 45 billion lightyears still doesn\u2019t tell us whether it\u2019s finite or infinite. <\/span><\/p>\n

One thing\u2019s for sure: the Universe does not have an edge. There\u2019s no physical boundary \u2013 no wall, no border, no fence around the edges of the cosmos. <\/span><\/p>\n

This doesn\u2019t necessarily mean that the Universe is infinitely large though.<\/p>\n

In principle, we could live in a finite Universe, provided that three-dimensional empty space is geometrically curved in a particular way \u2013 a distinct possibility according to Albert Einstein\u2019s theory of general relativity.<\/p>\n

If the Universe has what\u2019s known as positive curvature, it would be like the curved surface of a beach ball, but rather than a 2D surface, it\u2019s 3D space.<\/p>\n

\"""\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
What shape is the Universe? Could it be spherical, flat or shaped like a saddle? Credit: NASA \/ WMAP Science Team<\/span><\/figcaption><\/div>\n

It is finite \u2013 if you were living in this flattened version of the cosmos, you wouldn\u2019t need an infinite amount of paint to cover your 2D Universe \u2013 yet there is no boundary or edge to the surface itself.<\/p>\n

In contrast, a negatively curved Universe would be a higher-dimensional version of a Pringle \u2013 curving upwards along one axis and downwards along the other \u2013 while a flat Universe would resemble a piece of paper.<\/p>\n

Both of these versions would stretch out infinitely.<\/p>\n

Cosmologists have tried to measure the large-scale curvature of space over the past few decades, and the most recent results combined with theoretical arguments seem to indicate that we live in a geometrically flat Universe.<\/p>\n

On the one hand, that\u2019s convenient as our brains aren\u2019t very good at imagining large-scale space curvature \u2013 even here we\u2019ve had to describe our 3D Universe in 2D terms.<\/p>\n

\"""\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
Webb Deep Field, James Webb Space Telescope, July 2022Credit: NASA, ESA, CSA, STScI<\/span><\/figcaption><\/div>\n

On the other hand, this means that our Universe is infinitely large, and that our observable Universe \u2013 the part within our cosmological horizon \u2013 is only an infinitesimally small fraction of an unimaginably large whole.<\/p>\n

In case you were wondering how our infinite, boundless Universe is able to expand, return once again to our 2D analogy.<\/p>\n

If you saw the grid size on a piece of graph paper growing, you would justifiably conclude that the paper is expanding.<\/p>\n

If the paper was so large that you couldn\u2019t see the edge, you\u2019d still draw the same conclusion, even though the piece of paper could also be infinitely large.<\/p>\n

The same is true for an infinite Universe. After all, infinity times two is still infinity!<\/p>\n

This guide originally appeared in the April 2023 issue of <\/strong><\/em>BBC Sky at Night Magazine<\/strong>.<\/strong><\/em><\/p> <\/body><\/html>\n

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