\n<\/p>\n
It\u2019s always amazing, and more than a little humbling, when the universe reminds us that our \u201ccommon sense\u201d is provincial, falling apart on cosmic scales.<\/p>\n
If you\u2019re on the surface of Earth\u2014and I\u2019m betting you are\u2014there are many ways to reliably estimate the distance to some object. One we use almost subconsciously is to compare an object\u2019s apparent size with how big we know it to be. For example, you have a good feel for the size of, say, a typical human. So if you see someone looming large in your vision, you can reckon they\u2019re nearby, whereas if they appear very small, they must be much farther away.<\/p>\n
Of course, some humans are larger or smaller than average, but you can still account for that to get a decent distance estimate. And the overall trend is crystal clear: the farther off an object is, the smaller it appears. The trend is so obvious, in fact, that we can see the rate of change is linear: Double the distance, and the object will appear to be half its previous size. Look at it 10 times farther away, and it will seem to be one tenth as big.<\/p>\n
If you’re enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.<\/p>\n
This all works great for familiar objects up to a few kilometers away, but astronomers are notoriously unsatisfied with these relatively tiny scales. We want to know the distances to objects that are trillions of kilometers away\u2014or even billions of times farther than that!<\/p>\n
In deep images of the sky from giant telescopes, galaxies abound. Some may be relatively close to us\u2014merely some tens of millions of light-years distant\u2014while others may be billions of light-years in the background. Just by looking at the image, how could you tell?<\/p>\n
You might assume the galaxies that appear smaller are farther away, in keeping with our earthly intuition, but this won\u2019t work; like humans, galaxies come in a range of sizes. When you examine an image, you might be looking at a massive galaxy nearly at the edge of the observable universe\u2014or a tiny dwarf galaxy right in our cosmic backyard. Just judging by the image, it\u2019s impossible to tell.<\/p>\n
It\u2019s possible there could be some standard physical scale to galaxies, some way to gauge their distance by linking details of their structure to their overall size, but such scenarios are apparently too simple for the universe\u2019s true complexity.<\/p>\n
It turns out that the standard linear scaling we use on Earth would only apply at cosmic scales if our universe were static\u2014unchanging in size over time\u2014which it isn\u2019t! Instead the universe is expanding, growing larger every day. This phenomenon brings with it a whole slew of bizarre consequences, but a surprising one is that beyond a certain threshold of separation from us, farther-off galaxies appear to get bigger with distance! As so often happens, the cosmos really is a lot odder than you think.<\/p>\n
This seemingly paradoxical effect is a consequence of cosmic expansion coupled with the finite speed of light.<\/p>\n
When we say a galaxy is, for example, 12 billion light-years away, what we usually mean is that the light from that galaxy took 12 billion years to reach us. But during that light\u2019s time in transit, the universe has gotten bigger all the while. That means it was smaller in the past, and the objects in it were much closer together. When the light left that galaxy 12 billion years ago, the galaxy was closer to us, so it appears bigger than expected for that distance once its light arrives here!<\/p>\n
This is certainly counterintuitive and, frankly, weird. Still, it\u2019s borne out by the equations governing how the universe works. The effect holds for all galaxies but is imperceptibly small for those relatively nearby; their light travel time is minuscule compared with the age of the universe, so the universe wasn\u2019t all that much smaller when they emitted the light we now see. Their apparent size isn\u2019t affected enough to detect.<\/p>\n
But the effect ramps up with distance and begins to dominate for objects with a light travel time of about 9.5 billion years. Around that point, we\u2019re looking so far back in time\u2014so far back in the universe\u2019s history of expansion\u2014that the galaxies are effectively magnified, appearing larger than they otherwise would. The exact distance where this effect really kicks in depends on many complicated factors, including how rapidly the universe expands and how much matter it contains. In fact, if we could precisely measure this apparent growth in size, we could then use it to better determine these important cosmological parameters.<\/p>\n
Unfortunately, because galaxies don\u2019t come in standard sizes, that\u2019s quite a difficult task. Worse, this cosmic magnification effect confusingly makes some galaxies look dimmer: if they appear bigger, their light is more spread out, so they become fainter and even more difficult to observe!<\/p>\n
As surprising as this effect is, what may be even more astonishing is that we\u2019ve been able to see far enough to detect it at all from our planetary perch in the backwaters of the Milky Way. Its very existence is one of many reasons astronomers put up a Herculean struggle to determine distances to extremely remote objects. Doing so can reveal information about such objects, of course, but also about the universe around them and the way that it behaved when it was very young. Provided, of course, that we take to heart this hard lesson: once we start talking about distances measured in billions of light-years, our own parochial evolution utterly fails us, and we have to be very careful not to extrapolate willy-nilly from our own experience on Earth.<\/p>\n","protected":false},"excerpt":{"rendered":"
It\u2019s always amazing, and more than a little humbling, when the universe reminds us that our \u201ccommon sense\u201d is provincial, falling apart on cosmic scales. If you\u2019re on the surface of Earth\u2014and I\u2019m betting you are\u2014there are many ways to reliably estimate the distance to some object. One we use almost subconsciously is to compare<\/p>\n","protected":false},"author":1,"featured_media":36337,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[50],"tags":[5414,16177,20314,16098,20315,20316],"class_list":{"0":"post-36336","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-environment","8":"tag-cosmic","9":"tag-illusions","10":"tag-magnification","11":"tag-optical","12":"tag-universes","13":"tag-weirdest"},"_links":{"self":[{"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=\/wp\/v2\/posts\/36336","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=36336"}],"version-history":[{"count":0,"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=\/wp\/v2\/posts\/36336\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=\/wp\/v2\/media\/36337"}],"wp:attachment":[{"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=36336"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=36336"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=36336"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}