Movement through space is inevitable. Right now, you’re sitting on a planet that is spinning on its
axis, moving you at up to 1,600 km/h, depending on your latitude. The Earth circles the Sun,
the Sun moves through the Milky Way, the Milky Way moves through the Local Group of galaxies,
and the Local Group moves through our local Supercluster. And even that Supercluster of
galaxies is in motion, heading towards the Shapley Supercluster, where scientists believe that there
is a particularly large concentration of galaxies. I hope all this motion isn’t making you too dizzy.
But on the grandest scales of our universe, something strange is happening. Scientists have
begun to realise that there is not enough mass in the Shapley Supercluster to pull us towards it at
the rate that we see; about 50% of the cause of that motion is unaccounted for. In 2017,
researchers discovered a possible source for the rest of that motion.
But it’s not something pulling us. It’s something pushing.
I’m Alex McColgan, and you’re watching Astrum. Join with me today as we explore the evidence
for the region of space known as the Dipole Repeller, and try to understand
how in a universe filled with gravity, something can push us instead of pull.
Let’s start with a little context. It’s taken a long time for astronomers to recognise that
this motion was taking place. To understand that we were moving, scientists first had to
make a map of the regions of space around us, and although we as a species have been mapping
the stars since the beginnings of civilisation, it was only thanks to Mr. Hubble in 1924 that
we proved there are actually stars in other galaxies outside the Milky Way. Before then,
although astronomers had seen fuzzy little clusters of lights in the night sky that we
now know to be galaxies, the common wisdom was that these were gas-cloud “spiral nebula” within
the Milky Way, not something that existed beyond it. Hubble noticed several Cepheid stars - a type
of variable star with a cycle of variation closely linked to their luminosity - within
one such spiral nebula, and used the fact that this luminosity was very predictable based on
distance to calculate very precisely how far away they were. Surprising many, he proved
that they had to exist beyond the Milky Way. Since then, astronomers have been scrambling
to catch up with the new reality of galaxies and superclusters, and set about mapping the
locations and distances to every galaxy they could find. Hubble had been studying Andromeda,
our closest neighbouring major galaxy, but there were others. By the 1970’s, Stephen Gregory,
Laird Thompson, and William Tifft definitively proved that galaxies didn’t just float randomly
in space, but converged into Superclusters – large filamentary or sheet-like structures that
exist between large bubbles of void. They mapped the first Supercluster – the Coma Supercluster,
which covered a region of space 100 million light-years across. A whopping 95% of all galaxies
are found in these superstructures. It wasn’t until 2014 that our own
supercluster – the Laniakea Supercluster – was fully mapped. You might be surprised to hear
it happened in that order. After all, wouldn’t common sense say it’s easier to map regions of
space closer to us, rather than ones further away? Why was it only nearly a century after
the discovery of other galaxies existing, and 40 years after mapping the first supercluster,
that we finally turned to our own? The answer is that mapping the local regions of space around us
is surprisingly challenging. And it’s all because of the Milky Way getting in the (Milky) way.
The Milky Way is full of gas and dust, and these regions are so difficult to penetrate
that we can’t really see beyond them. Towards the centre of the Milky Way, this effect gets
so bad that scientists have dubbed the whole area the Zone of Avoidance – very little light gets
through it. Not being able to see in this area is particularly relevant to our current discussion,
for reasons we’ll get to later. In contrast to our local area,
distant galaxies are easier to study, as all you have to do is point your telescope
at a single point in the night sky, and let it drink in the light. Mapping the area around us
requires looking at every point in the night sky. It’s proved quite difficult, although
astronomers are now getting a better handle on it. Thanks to their efforts in looking at thousands of
galaxies in our galactic neighbourhood, and by studying galaxies' “peculiar
motion” – their motion relative to the cosmic background radiation, that ignores the expansion
of the universe – scientists have been able to group gravitationally bound galaxies into
Superclusters. At the centre of our Laniakea Supercluster lies a point known as the Great
Attractor, an area 150-250 million light years away from us, that we in the Milky Way
are slowly drifting towards. Sadly, the Great Attractor lies within the Zone of Avoidance,
so it’s tricky to see clearly what lies there. To confound things further, the Laniakea
Supercluster was also found to be moving. It travels towards another supercluster, known as the
Shapley Supercluster, taking us along with it. At the centre of this supercluster lies the Shapley
Attractor, which also is believed to be incredibly dense in terms of mass. But frustratingly,
this second attractor also lies in the zone of avoidance, making it difficult
to see exactly what’s going on over there, too. Thankfully, as x-ray and infra-red telescopes
improved, it became possible to take a better look at these two attractors. But this is where
the mystery begins. What mass is there is not quite massive enough to account
for our apparent motion in that direction. Which is why a team of researchers from the
University of Hawaii published an article in Nature to attempt to explain this discrepancy.
The team was made up of Yehuda Hoffman, Daniel Pomarède, R. Brent Tully & Hélène M. Courtois.
Together they created a new kind of map of the local universe around us. This time,
instead of simply noting galaxies’ positions, they highlighted their motions instead.
This allowed them to create a map of flow lines, which after mathematical analysis,
allowed them to make assumptions about the locations of masses in our nearby superclusters.
Their results were surprising. A lot of our local mass is moving towards the Shapley Supercluster,
which is what you would expect. But they also found a lot of mass is moving away from another
specific region in space. They called this mystery patch of space the “Dipole Repeller.”
Together, the Dipole Repeller and the Shapley attractor both contribute about 50% to our motion,
working together in tandem (hence the name “Dipole”). But how? Gravity is a force that,
so far as we have observed, only pulls. What lies in this region of space that allows a push
to happen? Could it be a strange collection of white holes, the theoretical opposite of
black holes that we’ve talked about in videos before? Or perhaps, some source of anti-gravity?
Not quite. The answer is… nothing. The Dipole Repeller is a void – a bubble
about 100 million light years across, that does not contain many if any major galaxies. It’s one
of the bubbles that exists between the filaments of the universe’s structures. But interestingly,
our galaxy’s current motion lines up much more satisfyingly with the push from this region,
than it does from the pull of the Shapley attractor, which is a compelling argument
in this theory’s favour. So, how does nothing push?
The answer is actually that this could be a pseudo-force, more than it is a real one.
Imagine a universe where all the galaxies were spaced equally. While the gravity of
galaxies above you would pull you, the gravity of galaxies below you would do
the same. So would the pull of galaxies to your left and to your right. In this way, the varying
forces of all these gravities would balance each other, leaving you to not really travel in any
direction – you’d be in perfect equilibrium. But what would happen if we removed the
galaxies from one of these directions? What would happen if we added a void?
Well, then the scales would tip. There would be one direction that no longer pulled you, leaving
the opposite direction’s pull to act on you unimpeded. As a result, you would move away from
the void, as if it were pushing you. Everything near a void will thus move away from it,
making it seem to have a repelling gravitational force. It is something of an optical illusion.
There is another explanation for why a void space might push you, which might work in tandem with
this pseudo force. The universe is expanding, and that expansion shows no sign of slowing down – in
fact, it seems to be speeding up. However, this expansion is counteracted by gravity,
and in areas where there is more gravity, less expansion seems to take place - stars aren’t
just being pushed away from each other, they are also being pulled together. Conversely,
in a void, there is nothing to contain this rampant expansion. As a result,
void spaces literally will swell compared to their supercluster counterparts. It might be
done by warping the fabric of reality, but this also achieves a sort of “push”.
Since 2017, not much more has come to light about the existence of the Dipole Repeller. As this is
still an expanding field of research, there’s some debate about whether it’s really there,
or how influential it might be on our galaxy’s current motion. It is difficult to observe,
due to the Milky Way’s obscuring effect, and there is always a challenge inherent in trying to spot
the absence of a thing, rather than its presence. However, Tully and his fellow researchers hope
that with the aid of future ultra-sensitive surveys in multiple spectrums of light,
it will be possible to map out the few galaxies that lie in this void, and generally confirm its
existence in the region they hypothesise. We are constantly moving through space,
but even the science of space is always in motion. Each new galaxy, along with data regarding its
redshift and travel speed, helps us develop a broader understanding of the movement of the
universe. It’s fascinating to think that not all of that motion is caused by the pull of gravity.
It might turn out that, sometimes, the universe just needs a little push to get going.
There have been a lot of awesome images in Astrum videos over the years. Sometimes,
the visuals we show in our videos are so colourful, so grand, and so awe-inspiring,
that the short 3 second clip doesn’t really do them justice. In those cases,
I felt it would be really cool if there was a way to come back to these images again and again,
to relive the magic of those moments, and capture the power and enormity of space. To that end,
I’ve spent the last couple of months working with artists to develop 70x50cm posters that
captures those epic moments. We’ve created a set of premium prints that look great and act as a
daily reminder of the Universe that surrounds us, perfect for bringing grandeur to your wall
or as an inspiring motif in your office space. Follow the link electrify.art/ASTRUM to check
them out and let me know what you think of them actually in the comments below!