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Starts With A Bang

Laniakea, our local supercluster, is being destroyed by dark energy

On the largest scales, galaxies don’t simply clump together, but form superclusters. Too bad they don’t remain bound together.
This visualization of the Laniakea supercluster, which represents a collection of more than 100,000 estimated galaxies spanning a volume of over 100 million light-years, shows the distribution of dark matter (shadowy purple) and individual galaxies (bright orange/yellow) together. Despite the relatively recent identification of Laniakea as the supercluster which contains the Milky Way and much more, it's not a gravitationally bound structure and will not hold together as the Universe continues to expand. A large amount of the normal matter in the Universe isn't found between the galaxies: in the intergalactic medium.
Credit: Tsaghkyan/Wikimedia Commons
Key Takeaways
  • If you look at the cosmic web, on the largest scales of all, galaxy groups and clusters gather together in even larger structures: superclusters.
  • The Milky Way is part of the Local Group, on the outskirts of the Virgo Cluster, which is part of an even larger structure known as the local supercluster: Laniakea.
  • Unfortunately, Laniakea, like all superclusters in the Universe, is only a phantasmal, apparent structure. In time, dark energy will tear it apart completely.

On the largest cosmic scales, planet Earth appears to be anything but special. Like hundreds of billions of other planets in our galaxy, we orbit our parent star; like hundreds of billions of solar systems, we revolve around the galaxy; like the majority of galaxies in the Universe, we’re bound together in either a group or cluster of galaxies. And, like most galactic groups and clusters, we’re a small part of a larger structure containing over 100,000 galaxies: a supercluster. Ours is named Laniakea: the Hawaiian word for “immense heaven.”

Superclusters have been found and charted throughout our observable Universe, where they’re more than 10 times as rich as the largest known clusters of galaxies. Unfortunately, owing to the presence of dark energy in the Universe, these superclusters ⁠— including our own ⁠— are only apparent structures. In reality, they’re mere phantasms, in the process of dissolving before our very eyes.

dark matter
The cosmic web is driven by dark matter, which could arise from particles created in the early stage of the Universe that do not decay away, but rather remain stable until the present day. The smallest scales collapse first, while larger scales require longer cosmic times to become overdense enough to form structure. The voids in between the interconnected filaments seen here still contain matter: normal matter, dark matter and neutrinos, all of which gravitate. (Credit: Ralf Kaehler and Tom Abel (KIPAC)/Oliver Hahn)

The Universe as we know it began some 13.8 billion years ago with the Big Bang. It was filled with matter, antimatter, radiation, etc.; all the particles and fields that we know of today, and possibly even more. From the earliest instants of the hot Big Bang, however, it wasn’t simply a uniform sea of these energetic quanta. Instead, there were tiny imperfections ⁠— at about the 0.003% level ⁠— on all scales, where some regions had slightly more or slightly less matter and energy than average.

In each one of these regions, a great cosmic race ensued. The race was between two competing phenomena:

  1. the expansion of the Universe, which works to drive all the matter and energy apart
  2. gravitation, which works to pull all forms of energy together, causing massive material to clump and cluster together
The growth of the cosmic web and the large-scale structure in the Universe, shown here with the expansion itself scaled out, results in the Universe becoming more clustered and clumpier as time goes on. Initially small density fluctuations will grow to form a cosmic web with great voids separating them, but what appear to be the largest wall-like and supercluster-like structures may not be true, bound structures after all. (Credit: Volker Springel/MPE)

With both normal matter and dark matter populating our Universe ⁠— but not in sufficient quantities to cause the entire Universe to recollapse ⁠— our Universe first forms stars and star clusters, with the first ones appearing when less than 200 million years have passed since the Big Bang. Over the next few hundred million years, structure begins to appear on larger scales, with the first galaxies forming, star clusters merging together, and even galaxies growing to attract matter from the lower-density regions nearby.

As time continues to pass, and we cross from hundreds of millions of years to billions of years in our measurement of time since the Big Bang, galaxies gravitate together to form the Universe’s first galaxy clusters. With up to thousands of Milky Way-sized galaxies in them, massive mergers form giant elliptical behemoths at the cores of these clusters. At the modern extremes, galaxies like IC 1101 can grow to quadrillions of solar masses.

The giant galaxy cluster, Abell 2029, houses galaxy IC 1101 at its core. At 5.5 million light years across, over 100 trillion stars and the mass of nearly a quadrillion suns, it’s the largest known galaxy of all. As massive and impressive as this galaxy cluster is, it’s unfortunately difficult for the Universe to make something significantly larger. (Credit: NASA/Digitized Sky Survey 2)

On even larger spatial scales and even longer timescales, the cosmic web begins to take shape, with filaments of dark matter tracing out a series of interconnecting lines. The dark matter drives the gravitational growth of the Universe, while the normal matter interacts through forces other than gravity, leading to the formation of gas clumps, new stars, and even new galaxies on long enough timescales.

Meanwhile, the space between the filaments ⁠— the underdense regions of the Universe ⁠— give up their matter to the surrounding structures, becoming great cosmic voids. Galaxies dot the filaments, and fall into the larger cosmic structures where multiple filaments intersect. On long enough timescales, the most spectacular nexuses of matter begin attracting one another, causing galaxy groups and clusters to begin forming even larger structures: galactic superclusters.

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 of times fainter than the nearest galaxies appear today. (Credit: Andrew Z. Colvin/Wikimedia Commons)

Superclusters are collections of:

  • individual, isolated galaxies
  • galactic groups
  • large galaxy clusters

These are connected by great cosmic filaments that trace out the cosmic web. Their gravitation mutually attracts these components towards a common center-of-mass, where these large structures span hundreds of millions of light-years and contain upwards of 100,000 galaxies apiece.

If all that we had in the Universe were dark matter, normal matter, black holes, neutrinos and radiation ⁠— where the combined gravitational effects of these components fought against the Universe’s expansion ⁠— superclusters would eventually reign supreme. Given enough time, these enormous structures would mutually attract to the point where they all merged together, creating one enormous, bound cosmic structure of unparalleled proportions.

The flows of nearby galaxies and galaxy clusters (as shown by the ‘lines’ of flows) are mapped out with the mass field nearby. The greatest overdensities (in red) and underdensities (in black) came about from very small gravitational differences in the early Universe. (Credit: H.M. Courtois et al., Astronomical Journal, 2013)

In our own corner of the Universe, the Milky Way lies in a small neighborhood we call our local group. Andromeda is our local group’s largest galaxy, followed by the Milky Way at #2, the Triangulum galaxy at #3, and perhaps 60 significantly smaller dwarf galaxies strewn out over a volume spanning a few million light-years in three dimensions. Our local group is one of many smallish groups in our vicinity, along with the M81 group, the Sculptor group, and the Maffei group.

Larger groups ⁠— like the Leo I group or the Canes II group ⁠— are also abundant in our nearby surroundings, with each containing around a dozen large galaxies. But the most dominant nearby structure is the Virgo Cluster of galaxies, containing more than a thousand galaxies comparable in size/mass to the Milky Way, and located just 50-60 million light-years away. The Virgo cluster is the main source of mass in our nearby Universe.

The Laniakea supercluster, containing the Milky Way (red dot), is home to our Local Group and so much more. Our location lies on the outskirts of the Virgo Cluster (large white collection near the Milky Way). Despite the deceptive looks of the image, this isn’t a real structure, as dark energy will drive most of these clumps apart, fragmenting them as time goes on. (Credit: R.B. Tully et al., Nature, 2014)

But the Virgo cluster itself is just one of a large number of galaxy clusters, themselves collections of hundreds to thousands of large galaxies, that have been mapped out in the nearby Universe. The Centaurus cluster, the Perseus-Pisces cluster, the Norma cluster and the Antlia cluster represent some of the densest and largest concentrations of mass close to the Milky Way.

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They conform very well to this idea of the cosmic web, where “strings” of galaxies and groups exist along the filaments connecting these large clusters, and with giant voids in space separating these mass-containing regions from one another. These voids are tremendously underdense, while the nexuses of these filaments are excessively overdense; it’s very clear that on cosmic timescales, the underdense regions have given up the majority of their matter to the denser, galaxy-rich clusters.

The relative attractive and repulsive effects of overdense and underdense regions on the Milky Way are mapped out here on distance scales of hundreds of millions of light-years. Overdense and underdense regions both pull and push on matter, giving it speeds of hundreds or even thousands of kilometers in excess of what we’d expect from redshift measurements and the Hubble flow alone. These giant collections of galaxies can be divided up into superclusters, but the structures themselves are not gravitationally stable. (Credit: Y. Hoffman et al., Nature Astronomy, 2017)

In our larger galactic neighborhood, going out roughly 100 to 200 million light-years, all of these clusters (excepting Perseus-Pisces, which lies on the other side of a nearby void) appear to have filaments with galaxies and galactic groups between them. It appears to make up a much larger structure, and if you sum up every galaxy in it ⁠— large and small ones alike ⁠— we fully anticipate that the total number should exceed 100,000.

This is the collection of matter that we refer to as Laniakea: our local supercluster. It links up our own massive cluster, the Virgo cluster, with the Centaurus cluster, the Great Attractor, the Norma Cluster and many others. It’s a beautiful idea that represents structures on scales larger than a visual inspection would reveal. But there’s a problem with the idea of Laniakea in particular and with superclusters in general: these are not real, bound structures, but only apparent structures that are currently in the process of dissolving away entirely.

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. The long-held idea that the Universe is held together by structures spanning many hundreds of millions of light-years, these ultra-large superclusters, has now been settled, and these enormous web-like features are destined to be torn apart by the Universe’s expansion. (Credit: Andrew Z. Colvin and Zeryphex/Astronom5109; Wikimedia Commons)

Our Universe isn’t just a race between an initial expansion and the counteracting gravitational force caused by matter and radiation. In addition, there’s also a positive form of energy that’s inherent to space itself: dark energy. It causes the recession of distant galaxies to speed up as time goes on. And ⁠— perhaps most importantly ⁠— it gets more important on larger scales and at later times, which is particularly relevant for the existence of superclusters.

If there were no dark energy, Laniakea would most certainly be real. Over time, its galaxies and clusters would all mutually attract, leading to an enormous grouping of 100,000+ galaxies, the likes of which our Universe has never seen. Unfortunately, dark energy became the dominant factor in our Universe’s evolution approximately 6 billion years ago, and the various components of the Laniakea supercluster are already accelerating away from one another. Every component of Laniakea, including every independent group and cluster mentioned in this article, is not gravitationally bound to any other.

The impressively huge galaxy cluster MACS J1149.5+223, whose light took over 5 billion years to reach us, is among the largest bound structures in all the Universe. On larger scales, nearby galaxies, groups, and clusters may appear to be associated with it, but are being driven apart from this cluster due to dark energy; superclusters are only apparent structures. (Credit: NASA, ESA, and S. Rodney (JHU) and the FrontierSN team; T. Treu (UCLA), P. Kelly (UC Berkeley), and the GLASS team; J. Lotz (STScI) and the Frontier Fields team; M. Postman (STScI) and the CLASH team; and Z. Levay (STScI))

All the superclusters that we’ve identified are not only gravitationally unbound from one another, but they themselves are not gravitationally bound structures. The individual groups and clusters within a supercluster are unbound, meaning that as time goes on, each structure presently identified as a supercluster will eventually dissociate. For our own corner of the Universe, the Local Group will never merge with the Virgo cluster, the Leo I group, or any structure larger than our own.

On the largest cosmic scales, enormous collections of galaxies and quasars spanning vast volumes of space appear to be real ⁠— the Universe’s superclusters ⁠— but these apparent structures are ephemeral and transient. They are not bound together, and they will never become so. In fact, if a structure had not already accumulated enough mass 6 billion years ago to become bound, when dark energy first dominated the Universe’s expansion, it never will. Billions of years from now, the individual supercluster components will be torn apart by the Universe’s expansion, forever adrift as lonesome islands in the great cosmic ocean.


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