All of the objects in the universe are in an extremely complex dance,
constantly being pulled in a variety of directions by other massive objects. The entire motion of
the cosmos - how planets revolve around stars, how stars move around galaxies, and how galaxies shift
in space - is explained by the pull of gravity. However, research claims that not only are we
being pulled in a variety of directions, we’re also being “pushed” on by a mysterious force.
This is Unveiled, and today we’re taking a closer look
at the huge force that’s pushing against our galaxy right now.
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To understand how this force is affecting our speed in the universe, we can think of
the universal structure as ground work. A planetary system, or solar system,
comprises a star and objects, such as planets, that orbit around it. A galaxy consists of
millions of stars, many with their own planetary systems, as well as gas, dust, and dark matter.
Our solar system is located in an interstellar cloud known as the Local Fluff, which is part of
the Orion Arm of our galaxy, the Milky Way. And, just as stars come together, bound by gravity,
so too do galaxies. The Milky Way is part of a collection of galaxies known as the Local Group.
The Milky Way, together with the Andromeda and Triangulum Galaxies, as well as dozens of smaller
dwarf galaxies, make up this group. In general, a “group” is formed of 50 or fewer galaxies,
while for larger masses - of fifty, or more - the term is “clusters”.
Altogether, our Local Group runs for about 10 million light years in diameter.
Our neighborhood of galaxies stretches far across space. But the scale of structures in the universe
gets even bigger than that. Superclusters are wider formations of regular galactic groups
and clusters! And the supercluster that our Local Group is part of is called the Virgo.
What’s incredible, though, is that there are an estimated
10 million (or more) superclusters in the observable universe, total! Interconnected,
for the most part. However, there are also vast areas - sprawling cosmic voids - that
are mostly empty of anything. And they’re crucial to what’s apparently happening to us.
All of the galaxies and groups and clusters and superclusters in the universe are being
pulled in any number of directions, but overall they gravitate towards the largest collection
of mass. At the same time, however, the expansion of the universe has to be taken into account.
Expansion is moving objects in space independently of gravity,
and it’s been doing so since the Big Bang. To separate out this vital factor, then,
scientists refer to the speed of galaxies due to gravity as ‘peculiar velocity’.
And so, back to the “mysterious force” that’s pushing against the Milky Way.
As it happens, our entire area of space is currently being pulled towards another
massive concentration of galaxies called the Shapley Supercluster. The problem is, however,
that the force of that pull isn’t massive enough to account for how fast we’re speeding towards it,
even with expansion taken into consideration. The numbers just don’t add up… and this led scientists
in 2017 to propose the existence of another force - our “mysterious” force - on the opposite side
of the Milky Way, pushing it (and us) onwards. They call this force the Dipole Repeller.
However, the Dipole Repeller is actually a particularly large cosmic void - a supervoid!
How did we get to this conclusion? Using redshift and blueshift observations,
the researchers made a map of the gravitational flow in our area of the universe, dubbed the
Cosmic Flows Project. In so doing, lead researcher, Professor Yehuda Hoffman,
discovered the speeds exhibited by the Local Group didn’t match up to what would be expected.
And the only thing that could explain this contradiction was, again, the presence of another
massive force pressing against us, propelling us forward. In fact, this object (or probable
lack of objects) accounts for about 50% of our overall motion towards the Shapley Supercluster.
Hoffman explained it like this: “In addition to being pulled towards the Shapley Concentration,
we’re also being pushed away from the newly discovered Dipole Repeller”. But the name
“Dipole Repeller” might be a bit misleading. It essentially functions to repel us, but actually
it isn’t exerting any force on us at all… We know that gravity is unique
among the cosmic forces because it always attracts, and never repels. Nevertheless,
while it can’t push us, it can appear to. If we picture our Local Group as a metal ball
and the Shapley Supercluster as a massive magnet, we’ll naturally roll in that direction. However,
for our area of space, we have that massive pull coming from one direction and a lack of any pull
coming from the other direction - from the Dipole Repeller supervoid. As astrophysicist Ethan Siegel
explains: “Gravity is never repulsive, but a less attractive force in one direction than
all the others behaves indistinguishably from a repulsion”. And that’s what’s happening here.
The dipole repeller is a cosmic void that holds very little matter. Although
it still isn’t completely empty. It’s only empty relative to the abundance of stars,
galaxies, and planets elsewhere in the universe. Which is why the amount of gravity it exerts
is basically an unnoticeable amount. It’s like it’s a cosmic ghost town, sitting about half a
billion light years away from Earth, and in the opposite direction to the Shapley Supercluster.
But, even so, it’s this which is to blame for our “mysterious force”. There’s nothing much there,
but it’s still having a major impact.
So far, we haven’t actually observed the Dipole Repeller directly. The scientists who
theorize its existence have inferred it’s there from the speed of galaxies relative
to the amount of matter pulling on us. Voids - or dark spots - of any size can’t easily
be seen through a telescope, and so are often measured based on indirect evidence.
For further research, though, there’s an aim to more closely look at the few galaxies that
are contained within the Dipole Repeller. That’s also a challenge
thanks to the cosmic dust cluttering the way between us and them… but if we could gain
a better understanding of these places, we might also know more about why we move through space
as we do. Mapping the gravitational flow in the overall universe is an important next
step for humanity, and knowing in detail about structures like the Dipole Repeller
could be crucial in the future. To grasp exactly how fast galaxies are moving and why.
At the same time, astrophysicists regularly use gravitational pull to their benefit when launching
spacecraft. Space travel isn’t as simple as just pointing your vehicle in one direction
and turning on the thrusters. It’s much more efficient to sail the gravitational winds,
and scientists currently use other planets’ gravity to accelerate or change the direction
of probes in just the solar system - a neat trick known as a gravity assist, or slingshot.
Many years from now, however, humanity might be planning an intergalactic voyage much further
afield… at which point gravity assists will be a whole new ball game. And in order to
take advantage of slingshotting we’ll have to have precise knowledge of the cosmic flow.
For now, we more simply know the universe is filled with galactic groups, clusters,
superclusters, and cosmic voids. And that all of it’s under the spell of gravity as well as
another impetus; expansion. Gravity works in peculiar ways, though, so that even areas of
space that apparently exert no gravity can appear to push us. As per the Dipole Repeller. And
that’s how scientists discovered a huge force that’s pushing against our galaxy right now.
What do you think? Is there anything we missed? Let us know in the comments, check out these other
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