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We know that more than a quarter of the universe is dark matter,
an invisible substance whose gravity pulls on the matter we can see.
And because there’s so much of it, we know it plays a huge role in shaping the universe.
Except, in the darkest spaces between galaxies,
it can be hard to know where that dark matter is, and how it’s affecting the space around it.
So, in recent years, scientists have been using an effect called cosmic shear
to indirectly detect dark matter and see how it’s distributed around the universe.
And by mapping it out, they hope to make progress on some of the trickiest problems in cosmology,
like what exactly are the invisible things out there,
and how did they help form the universe as we know it?
Cosmic shear is an effect caused by gravitational lensing, which occurs
when massive objects bend space, so that light passing through gets redirected,
just like if it were passing through a glass lens.
As a result, instead of seeing a crystal-clear galaxy or cluster,
the image gets a little distorted, like you’re seeing it in a funhouse mirror.
Scientists often use the amount of bending
to figure out the masses of individual galaxies or clusters acting as a lens,
because more distortion implies more mass.
But you can also study the average lensing of lots of galaxies at once,
and work out what the overall distortion effect is.
Instead of being like a glass lens,
it’s more like you’re looking at something through a crinkled sheet of plastic wrap.
And the amount by which the shapes of galaxies appear distorted is called the cosmic shear.
Cosmic shear makes circular objects look more elliptical.
It’s a really subtle effect, though, so for any one galaxy,
it can be hard to tell what’s cosmic shear and what’s just the galaxy’s natural shape.
But if you compare lots of galaxies over enormous distance scales,
you can begin to see some trends.
It’s still not the kind of thing you can easily pick out by eye, but for the last two decades,
powerful telescopes and computer simulations have been able to do it.
And these subtle distortions can tell you something about where dark matter is located.
See, usually, galaxies are oriented in random ways.
For example, you generally won’t get a bunch of elliptical galaxies pointing in the same direction.
If you do see something like that, it could be a sign that the ellipses you’re seeing
are actually distortions of more circular galaxies.
These distortions can happen when dark matter in between those galaxies
and us is bending the light that we’re seeing.
So by looking at images of lots of galaxies in different parts of the sky,
and then comparing how distorted the different images look,
scientists can start to map out where the dark matter is in the universe.
In fact, when it comes to mapping dark matter, cosmic shear is the best technique we’ve got.
In 2018, one group of researchers used it to make a map of dark matter in deep space
that was wider and sharper than any map out there.
And it clearly showed that dark matter is part of the scaffolding for the entire universe.
Scientists have thought for a long time that matter in the universe is structured like a web,
with enormous clusters of galaxies connected by long filaments that contain gas and dark matter.
Unfortunately, since it’s not full of brightly shining objects, this cosmic web is nearly undetectable.
But cosmic shear has let us “see” dark matter filaments
stretching between galaxy clusters in large regions of the sky.
And this structure is potentially a really important clue to the past.
Cosmologists think that the way the web looks now
can be traced back to tiny fluctuations in matter right after the Big Bang.
So, the better we can map out its structure now,
the better we can connect it to conditions in the early universe
and find out more about what the universe was like back then.
Cosmologists are also interested in using cosmic shear
to estimate the total amount of dark matter that’s out there,
because our simulations of how the universe has evolved rely on knowing what’s actually in it.
And finally, it could also help us understand
the biggest invisible piece of our universe: dark energy.
Because unlike dark matter, which pulls things together, dark energy pushes space apart.
So, over time, dark matter and dark energy have been in a kind of dance,
pushing and pulling our galaxies over cosmic timescales.
That means that a map of dark matter could tell us something about dark energy, too,
and what it was like at different points in time,
which is something we still don't understand well.
Of course, building up a complete history
of how we got from the chaos of the early universe to the present day
is a huge challenge: one of the biggest in cosmology.
But it’s techniques like cosmic shear that give us new ways
to explore and understand the most spectacular structures in the universe,
and bring the darkest parts of the sky into view.
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