Travel the universe with Dr. Ethan Siegel as he answers the biggest questions of all
Imagine how different our understanding of the Universe would be if we saw nothing beyond the Milky Way.
Our corner of the Universe was gifted with a plethora of bright, nearby galaxies to light our way through the cosmos.
The Large (top right) and Small (lower left) Magellanic Clouds are visible in the southern skies, and helped guide Magellan on his famous voyage some 500 years ago. In reality, the LMC is located some 160–165,000 light-years away, with the SMC slightly farther away at 198,000 light-years distant. Along with Triangulum and Andromeda, these four galaxies beyond our own are visible to the naked human eye. (ESO/S. BRUNIER)
The spirals and ellipticals in our backyard showed us, a century ago, that the Milky Way wasn’t alone.
This sketch from the mid-1840s is the first ever one to reveal the spiral structure of any nebula in the night sky. Now known to be a spiral galaxy, Messier 51, the Whirlpool Galaxy, is one of the most well-studied galaxies beyond our Milky Way. (WILLIAM PARSONS, 3RD EARL OF ROSSE (LORD ROSSE))
Even earlier astronomers still had copious bright galaxies they could observe with their telescopes.
A selection of approximately 2% of the galaxies in the Virgo cluster. There are approximately 1,000 large galaxies in the Virgo cluster, a large fraction of which were discovered way back in the 18th century. The Virgo cluster is located some 50–60 million light-years away from our Milky Way, and is the largest concentration of galaxies in the extremely nearby Universe. (JOHN BOWLES OF FLICKR)
By measuring the speeds and distances of these galaxies, we discovered the expanding Universe.
The ‘raisin bread’ model of the expanding Universe, where relative distances increase as the space (dough) expands. The farther away any two raisin are from one another, the greater the observed redshift will be by time the light is received. The redshift-distance relation predicted by the expanding Universe is borne out in observations, and has been consistent with what’s been known all the way back since the 1920s.(NASA / WMAP SCIENCE TEAM)
Without them, we might never have understood our cosmic origins: the hot Big Bang.
A visual history of the expanding Universe includes the hot, dense state known as the Big Bang and the growth and formation of structure subsequently. The full suite of data, including the observations of the light elements and the cosmic microwave background, leaves only the Big Bang as a valid explanation for all we see. As the Universe expands, it also cools, enabling ions, neutral atoms, and eventually molecules, gas clouds, stars, and finally galaxies to form. (NASA / CXC / M. WEISS)
Unfortunately, not every observer in the Universe gets so lucky.
Streams of dark matter drive the clustering of galaxies and the formation of large-scale structure, as shown in this KIPAC/Stanford simulation. While the locations where stars, galaxies, and clusters of galaxies emerge are most notable, the enormous cosmic voids separating the matter-rich structures are just as important to understanding our Universe. (O. HAHN AND T. ABEL (SIMULATION); RALF KAEHLER (VISUALIZATION))
Most galaxies clump together in groups, clusters, or along filaments, but some reside in underdense regions.
This figure shows the relative attractive and repulsive effects of overdense and underdense regions on the Milky Way. Note that, despite the large number of galaxies clumped and clustered nearby, there are also large regions that have extremely few galaxies: cosmic voids. While we have a few substantial ones nearby, there are even larger and lower-density voids found in the distant Universe. (YEHUDA HOFFMAN, DANIEL POMARÈDE, R. BRENT TULLY, AND HÉLÈNE COURTOIS, NATURE ASTRONOMY 1, 0036 (2017))
The Universe’s large-scale structure contains great cosmic voids as well as overdense clumps.
A region of space devoid of matter in our galaxy reveals the Universe beyond, where every point is a distant galaxy. The cluster/void structure can be seen very clearly. If we were to live in an extremely underdense/void region, we might not have discovered a single galaxy beyond our own until our astronomical tools advanced to near-modern standards. (ESA/HERSCHEL/SPIRE/HERMES)
In these extremely underdense regions, however, galaxies still occasionally form.
Although it’s relatively nearby at just 293 million light-years away, the galaxy MCG+01–02–015 has no other galaxies surrounding it for approximately 100 million light-years in all directions. To the best of our knowledge, it’s the loneliest galaxy in the Universe. (ESA/HUBBLE & NASA AND N. GORIN (STSCI); ACKNOWLEDGEMENT: JUDY SCHMIDT)
Although a long-exposure, deep image of MCG+01–02–015 appears to show many other galaxies in its vicinity, most are far more distant (and a few are closer), but none are within 100 million light-years of the major galaxy itself. (ESA/HUBBLE & NASA AND N. GORIN (STSCI); ACKNOWLEDGEMENT: JUDY SCHMIDT)
In all directions, we find no other galaxies within 100 million light-years of it.
In between the great clusters and filaments of the Universe are great cosmic voids, some of which can span hundreds of millions of light-years in diameter. While some voids are larger in extent than others, the void that houses MCG+01–02–015 is special because it is so low in density that, rather than having only a few galaxies, it only contains this one known galaxy at all. It is possible, however, that small, low surface brightness galaxies exist in this region after all. (ANDREW Z. COLVIN (CROPPED BY ZERYPHEX) / WIKIMEDIA COMMONS)
If we had grown up there, our telescopes would not have observed other galaxies until the 1960s.
Italian astronomer Paolo Maffei’s promising work on infrared astronomy culminated in the discovery of galaxies — like Maffei 1 and 2, shown here — in the plane of the Milky Way itself. Maffei 1, the giant elliptical galaxy at the lower left, is the closest giant elliptical to the Milky Way, yet went undiscovered until 1967. Technology would have needed to advance to approximately these levels to detect a single galaxy beyond MCG+01–02–15. (WISE MISSION; NASA/JPL-CALTECH/UCLA)
Perhaps we are truly fortunate: our serendipitous position in the Universe allowed us to understand it.
The various galaxies of the Virgo Supercluster, grouped and clustered together. On the largest scales, the Universe is uniform, but as you look to galaxy or cluster scales, overdense and underdense regions dominate. The full extent of this illustration, which maps out the nearby galaxies to the Milky Way (with us at the center), would have exactly one galaxy in it, MCG+01–02–015, if it were centered on the loneliest galaxy known in the Universe today. (ANDREW Z. COLVIN, VIA WIKIMEDIA COMMONS)
Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.
