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by Ethan Siegel
August 07, 2025
from
Medium Website
Article also HERE
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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) |
On
the largest scales,
galaxies
don't simply clump together,
but form
superclusters.
Too bad
they don't remain bound together.
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.
Some of them are enormous structures,
spanning more than a billion light-years across. (But,
despite claims to the contrary, not
much larger.)
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...
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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:
-
the expansion of the Universe, which
works to drive all the matter and energy apart
-
gravitation, which works to pull all
forms of energy together, causing massive material to clump
and cluster together
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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 had
elapsed since the start of the hot 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,
shown below, can grow to quadrillions of solar masses.
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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...!
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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:
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...
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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 a couple of hundred 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,
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.
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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.
...represent some of the
densest and largest concentrations of mass 'close' to the Milky Way.
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...
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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...
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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.
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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 have been detected, but these apparent structures are not
even superclusters:
they're mere associations that haven't been
demonstrated to be part of the same underlying structure.
Even for
the individual clusters within a single supercluster, 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...
Video
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