Travel the universe with Dr. Ethan Siegel as he answers the biggest questions of all
Billions of years from now, dark energy will push the rest of the Universe out of reach forever. These galaxies will be the last to go.
A look at the night sky, near and far, reveals galaxies everywhere we look.
Hubble’s advanced camera for surveys identified a number of ultra-distant galaxy clusters. If dark energy is a cosmological constant, all of these clusters will remain gravitationally bound themselves, like all galaxy groups and clusters, but will accelerate away from us and one another over time as dark energy continues to dominate the Universe’s expansion. (NASA, ESA, J. BLAKESLEE, M. POSTMAN AND G. MILEY / STSCI)
Throughout the entire observable Universe, an estimated two trillion galaxies exist.
Our Local Group of galaxies is dominated by Andromeda and the Milky Way, but our cosmic neck-of-the-woods contains many dwarf galaxies clustered around each of the large members. Triangulum is the 3rd largest. The Large Magellanic Cloud is the fourth biggest galaxy in the local group, with a slew of others, including M32, between 0.1% and 0.6% the Milky Way’s mass. Approximately 72 galaxies exist in our Local Group in total. (ANDREW Z. COLVIN)
Our Local Group, dominated by Andromeda and the Milky Way, houses about 70 galaxies total.
Different stills from a simulation of the merger of the Milky Way and Andromeda galaxies. When a major merger like this occurs, it may be the case that a large amount of debris is kicked up, creating satellite galaxies dominated by normal matter. The merger will complete in approximately 7 billion years, and the other Local Group galaxies will join on a variety of timescales. (NASA, ESA, Z. LEVAY, R. VAN DER MAREL, T. HALLAS, AND A. MELLINGER)
Over cosmic timescales, gravity will merge us all together into a single, enormous elliptical galaxy.
The figure shows the current stream of galaxies — the flow along in the cosmic super-highway and on the bridge to Virgo, in the region around the Milky Way, Andromeda (M31) and Centaurus A, the latter of which lies definitively beyond the Local Group. (‘PLANES OF SATELLITE GALAXIES AND THE COSMIC WEB,’ NOAM LIBESKIND ET AL., 2015)
But beyond our backyard, all the other galaxies, groups, and clusters are accelerating away from us.
Our local supercluster, Laniakea, contains the Milky Way, our local group, the Virgo cluster, and many smaller groups and clusters on the outskirts. However, each group and cluster is bound only to itself, and will be driven apart from the others due to dark energy and our expanding Universe. After 100 billion years, even the nearest galaxy beyond our own local group will be approximately a billion light years away, making it many thousands, and potentially millions (when you take the different stellar populations that will be inside) of times fainter than the nearest galaxies appear today. (ANDREW Z. COLVIN / WIKIMEDIA COMMONS)
Once you go about 4–5 million light years away, dark energy causes space to expand faster than gravity attracts other objects across space.
The different possible fates of the Universe, with our actual, accelerating fate shown at the right. After enough time goes by, the acceleration will leave every bound galactic or supergalactic structure completely isolated in the Universe, as all the other structures accelerate irrevocably away. We can only look to the past to infer dark energy’s presence, but its implications are larger for the future. (NASA & ESA)
Over time, every other galaxy will see its distance and recession speed increase from our perspective.
Markarian’s chain with the name of the galaxies, located at/near the center of the Virgo Cluster. At its present distance of 60 million light years, it moves away from us at approximately 0.4% the speed of light. After a few tens of billions of years go by, it will be forever unreachable by humanity, even if we traveled towards it at the speed of light. (WIKIMEDIA COMMONS / BILBO-LE-HOBBIT AND PACKBJ)
The Virgo cluster, with thousands of galaxies, is the last large cluster that will remain within view.
The Sculptor galaxy is roughly a Milky Way-sized galaxy that is rapidly forming stars and is surrounded by a small number of satellite galaxies, all bound to it. Beyond it, a looser stream of galaxies, aligned along a filament-like line, may or may not be bound to it. (ESO/J. EMERSON/VISTA. ACKNOWLEDGMENT: CAMBRIDGE ASTRONOMICAL SURVEY UNIT)
Other, small groups are more numerous and closer, but recede with the Universe’s expansion.
Located just outside the Big Dipper, the objects M81 and M82 appear nebulous, but are in fact galaxies located far outside the Milky Way, containing billions of stars apiece. The smaller galaxy, M82, is moving towards us temporarily, but that is only due to the gravitational influence of the group as a whole. Over time, the M81 group will accelerate away as a whole. (MARKUS SCHOPFER / C.C.-BY-2.5)
This includes tightly bound groups, like Sculptor and M81.
The galaxy Centaurus A has a dusty disk component in it, but is dominated by an elliptical shape and a halo of satellites: evidence of a highly evolved galaxy that has experienced many mergers in its past. It is the closest active galaxy to us, but accelerates away from our Local Group. (CHRISTIAN WOLF & SKYMAPPER TEAM/AUSTRALIAN NATIONAL UNIVERSITY)
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. There may yet be additional, closer galaxies on the border between bound and unbound to us located in the plane of the Milky Way that are hitherto undiscovered. (WISE MISSION; NASA/JPL-CALTECH/UCLA)
Still others, like M94, may consist only of free, unbound galaxies.
The galaxy Messier 94 is large, sweeping and beautiful, and is a dominant member of a loosely bound group named for it. It is debated whether the individual members are bound together or will dissociate as time goes on and the Universe expands further. (R JAY GABANY (BLACKBIRD OBS.))
This irregular galaxy, NGC 55, is as small as the Large Magellanic Cloud, but lies only 7 million light years away, making it one of the closest galaxies beyond the Local Group, and placing it in between ourselves and the Sculptor group. (ESO’S LA SILLA OBSERVATORY)
As the Universe expands, they all shall disappear.
The brightest, closest galaxy confirmed to be beyond the local group is NGC 300, at just 6 million light years distant. In around 100 billion years, it will be gone for good: the last reachable galaxy beyond whatever’s left of our Local Group. (ESO / WIDE FIELD IMAGER (WFI))
Mostly Mute Monday tells the astronomical story of an object or phenomenon in images, visuals, and no more than 200 words. Talk less; smile more.
