Did you know that if you stand atop the Shard, the tallest building in London, you can look
southwards and see France on a clear day? When the air is clear, it would appear to be empty and
transparent - but air is far from empty. Right now, the air around you contains approximately
ten-trillion-trillion atoms per cubic meter. That’s a lot of atoms. If you want to get emptier,
you have to go higher. At about 100 kilometres above Earth’s surface, the number of atoms drops
to about a million trillion. About a third of the way to the Moon, there are about 7 million atoms.
And to get much lower, you have to leave our Solar System altogether - interstellar space, which is
the space between star systems, contains about 1 thousand atoms per cubic meter. If you travel even
further, and go to the space between galaxies, that count drops to just ten atoms per cubic
meter. The space between galaxies is massive and sparse. But that space is often part of something
bigger, situated along chains of galaxies carved out galaxy filaments - the long arms
of galaxy walls which can branch for hundreds of millions of lightyears. These filaments group into
concentrated areas called galaxy superclusters, and when viewed on a wide enough scale, these all
sort of connect. But what about the space between filaments? The gaps between large-scale galactic
structures? Well, these spaces are just about as remote and empty as you can get in the universe,
and we have a fitting name for them – voids. We are so fascinated by other galaxies and what
could exist elsewhere, that we often overlook the significance of the voids in between. They
are a component of the universe itself, and they have taught us a great deal about areas
ranging from galaxy superstructure formation to the expansion of the universe itself. And today,
we’re going to revisit some of that knowledge, as we head into the void.
The contents of the macro-universe can be broken down into two structural components - walls,
where matter exists in abundance, and voids, the space in between. A void is a vast,
somewhat spherical region of space with a low cosmic density. We measure cosmic density by
physical galaxy counts as opposed to estimations of mass, so the technical definition for a void
references an area with a low number of galaxies - approximately one tenth of the typical number
inside walls, according to the widely accepted definition. Whereas galaxy walls are composed
of gravitationally-bound structures and filaments, voids are not technically
“structures” - they are the vast spaces between the structures, but are nonetheless
a fundamental property of the universe. Voids can exist both inside galactic walls,
and in the spaces between them. The smallest are about 10 megaparsecs in diameter, with a
megaparsec being equal to roughly 3.2 million lightyears, meaning typical voids are usually
at least 30 million lightyears wide. Larger typical voids can reach up to 10 times that size,
giving them a whopping diameter of 300 million lightyears, or 100 megaparsecs, and at that size,
we have another name for them - supervoids - but they can get even larger still. Voids which form
inside galaxy walls are smaller than voids that form in the low-density spaces of the universe.
These empty spaces can be much bigger, with some of the largest areas we’ve found over a billion
lightyears in diameter. It’s hard to appreciate just how large that scale is. There’s no definite
ratio for the share of galaxy walls and voids in the universe, but what we do know is that the
vast majority of its volume is made up of these massive, near-empty spaces of low cosmic density.
Cosmic Voids became a branch of study in astrophysics in the 1970s,
when redshift surveys became more common. By the end of the decade, voids had been identified in
the foreground of the Coma Cluster and the A1367 Cluster. Redshift surveying technologies have
come a long way since then, and it wasn’t long before we were mapping our universe in exquisite
3D detail for the first time. By the 1990s, 3D redshift surveys, such as the Las Campanas Survey,
were identifying and confirming an abundance of voids throughout the observable universe,
and today we have catalogued hundreds of them. Our nearest is the Local Void, which starts at
the edge of our Local Galactic Group and extends over 200 million lightyears into the darkness.
Voids have existed for as long as the universe. This is because during the first trillionth of
a second of time, our universe was tiny, and its contents was under unimaginable pressure. Within
this ultra-dense primordial soup, quantum fluctuations were causing ripples. And then
the universe suddenly expanded violently in an event known as Cosmic Inflation, and went from
microscopic to immeasurable. Like a snapshot of the tiny universe, the contents of the universe as
it was at that moment was radically elongated, and the tiny acoustic ripples now propagated
across massive cosmic distances, leading to the universe’s early plasma, from which the first
particles formed, being more concentrated in certain areas, and less dense in others. Within
the dense areas, structure formation began to occur. First, dark matter began pulling together
into massive but diffuse filaments over hundreds of millions of years. And then, baryonic matter
traced its path - subatomic particles became atoms, and atoms became gas clouds, and clouds
became stars. The stars collected into groups which formed the first galaxies, and the galaxies
moved into clusters, which became superclusters. The scale really is incredible. These structures
now interlink to form a massive spongelike path over the entire universe - the cosmic web. As
this all took place, the areas of lower density from Cosmic Inflation were emptied out, and now
exist as massive voids between galaxy walls. For this reason, those massive primordial spaces
between walls really are empty - hundreds of millions of lightyears devoid of not just normal
matter, but dark matter as well - it all grouped together in filaments in the early universe and
cleared out incomprehensible gaps in the space in between. We don’t really know how big these voids
can get, and new discoveries continue to push the benchmarks. In contrast, voids which occur inside
galaxy walls form as gravitationally-bound galaxies are pulled towards one another,
which empties out the surrounding space as they converge. The more galaxies merge, the stronger
their overall influence, which attracts more galaxies, and so this process is self-reinforcing.
This leads to denser clusters, and emptier voids. And in fact, when voids form like this,
if two form relatively close to each other; a snowball-like effect lead to something remarkable
While supervoids may be a somewhat less glamorous area of astronomy, one which has continued to
attract particular online attention is the great Bootes Void, which lies on the edge of
the Hercules Supercluster. This void is enormous - estimated to be between 250 million and 330
million lightyears in diameter, and over 200 thousand cubic megaparsecs in volume. Upon its
discovery, scientists disputed whether there had been enough time since galaxy structures
began forming to allow a void of this size to exist. But as it turns out, the Boötes Void
isn’t actually that unusual or unexpected, and the key to understanding why is hidden in the
distribution of a few-dozen galaxies within. 60 isolated galaxies exist within the void where
thousands would usually be expected, and they make up a tube-like formation near the centre.
Scientists now think that this tube may be what’s left of the boundaries of two smaller voids,
which merged as they grew into one another. The near-perfect spherical shape of the void is the
result of a “soap-bubble”-like effect as they merged, and two large voids into one massive void,
leaving the galaxies on the edges of each stranded inside this new supermassive cosmic
abyss. In addition to its size, the Boötes Void is also thought to be one of the sparsest and
most eerily empty of its kind. To put the 60 galaxies into some in sort of perspective,
it’s like driving from coast-to-coast of the United States and passing only a couple of towns
on your way - and this doesn’t even consider that the void is 3-dimensional. As such,
it’s earned itself the nickname “the Great Nothing”, which has been puzzling astronomers
since its discovery in 1981. It’s so enormous that even light takes hundreds of millions of years to
reach the galaxies inside, and for light leaving the galaxies to reach the outside. In fact,
astronomer Greg Aldering at the University of Minnesota said that if humanity had evolved
on a world within one of these few dozen central galaxies, we wouldn’t have known that
other galaxies existed until the 1960s. Imagine scientists in the 60s trying to work that one out.
Another thing that voids have helped us to slightly better understand is the mechanism
of dark energy - the big unknown force which is pushing the universe apart and speeding up its
expansion as it goes. Dark energy expands the universe in all places at once, and not just
at the edges - every part in space grows at the same rate. This means that the swathes of empty
space inside the voids of the universe are also growing, pushing the edges outwards, causing them
to increase in size as the universe ages. Because we now know that the size, shape and evolution of
large voids is partly due to the expanding space within, we can use them to test our
current estimations of dark energy’s abundance. The simultaneous existence of the largest voids
we see in the universe today requires about 70% of the universe's energy density to be dark energy,
a figure which is relatively consistent with our measurements of the Cosmic Microwave
Background - which is a good sign, from a research standpoint at least. Another thing
we can use the Cosmic Microwave Background for is measuring baryonic acoustic oscillation patterns,
a.k.a. the distribution of normal matter and energy across the universe. On this map,
the warmer areas correlate to filaments, and the cooler areas correlate to voids and supervoids.
All of this aligns with and reaffirms our current understanding, except in one place - the WMAP Cold
Spot. In the early 2000s, an area of the CMB which was unusually cold in comparison
to its surrounding areas was identified in the direction of the constellation of Eridanus. It’s
not the temperature of the area itself that’s so unusual, but the difference in temperature
between it and the relatively warm ring around it. The temperature deviates by about 70 microkelvin,
whereas mean temperature variations are usually just under 20 microkelvin or so. And in some
places, the temperature varies by as much as 140 microkelvin. This was a puzzling discovery,
and it quickly became a big mystery in astrophysics in 2013, when the Planck
Satellite also detected the Cold Spot, ruling out any chance of it being a WMAP error. This
thing actually exists somewhere out there - but what is it? Well, a group of scientists
came out and proposed that a supervoid could in fact be responsible. If this hypothetical
Eridanus Supervoid exists, then it is probably at least half a billion lightyears in diameter,
and could be as large as one billion lightyears, totally dwarfing even the Boötes Void. There is
no concrete explanation for the Cold Spot as of yet, and different studies since then
have found conflicting results pertaining to the Supervoid hypothesis, but nothing really
substantial for or against it yet. And so, while it may not be a supervoid, for now at least,
the Cold Spot is yet another perplexing and fascinating mystery of our universe.
Ok, so enough theories and hypotheses, how about a real example of a mind-blowing void which has
significance to us? Well, this is where the elasticity of the term “void” comes into play.
Obviously, when you think of a void, you think of a massive, virtually empty area, like the Boötes
Void, or a completely empty gap between galaxy walls. However, because the term refers to a
lower-than-expected number of galaxies, voids can contain clusters and even superclusters
of galaxies and still have this apparent underdensity. As it so happens, the largest-known
void in the universe contains such galactic structures. The largest void, which you happen to
be sat in the middle of, right now. It might sound crazy, but the KBC Void is the largest-known void,
by definition, in the entire observable universe, with a lower-than-expected concentration of
galaxies across a roughly spherical expanse of nearly - wait for it - 2 billion lightyears. And
yet, it contains you and I, the planet, the galaxy, the local galactic group, and even
a large chunk of the Laniakea Supercluster, which is home to no less than 100 thousand galaxies. So,
it’s not that exciting when you think about it really, but what is interesting is that we are
fairly close to the centre, relatively speaking. While this position doesn’t have any implications
for our existence that we know of, it does have implications for our observations of the universe
around us. Our position in comparison to the rest of the universe is like living in a remote rural
town instead of a city, except the cities are the giant galactic walls. Because these “cities” of
galaxies are denser than us, they are exerting a gravitational influence on us, pulling us through
the void towards the filaments very slowly. This movement may actually be interfering with our
measurements of redshift values, and has probably led to inaccurate estimations in our calculations
of the Hubble Constant - our measure of the rate at which the universe is expanding. This might
explain why the figure we get from analysing the redshift values of galaxies around us to
determine the Hubble Constant is different to the figure we get from analysing the CMB,
which would mean that analysing the Cosmic Microwave Background, or the echo of the Big Bang,
is a more accurate means to study the expansion of the universe - from our perspective at least.
If anything, the KBC Void shows us that not all “voids” really live up to the image of one - we
are simply situated in a less dense region of space, like being out in the intergalactic
countryside of the universe. The only noticeable side-effect this has for us is that our night sky
is a bit darker than it might be on planets within galaxy walls. But it’s certainly a lot less creepy
than it would be if we were in the centre of the Boötes Void - just imagine what it would be like
in one of its isolated galaxies within. In the future, when the universe is so large that all
other galaxies have receded from view, everywhere that isn’t gravitationally bound will expand into
its own massive, isolated pocket of the universe, leaving unfathomably large voids in the space
in between each individual galaxy. And in the distant future, when entropy reaches maximum,
no structures will remain, making the universe one indescribably enormous cosmic void. Huge
swathes of emptiness in space are a natural property of the universe - which always have
been and always will be. But just because it’s empty, doesn’t mean it can’t be interesting.