This video is sponsored by Magellan TV.
Dark matter is everywhere in the Universe, acting like an invisible glue that binds galaxies
together.
Without it, galaxies wouldn’t have formed, and we wouldn’t be here to wonder what dark
matter is.
Clusters of galaxies dominated by dark matter act as strong gravitational lenses, bending
the light of the background galaxies into multiple images and arcs of light.
Astronomers can use these galaxy clusters as natural laboratories to test theoretical
models of dark matter.
But new observations by the Hubble Space Telescope show that some concentrations of dark matter
produce lensing effects that are 10 times more frequent and even stronger than predicted.
This new discovery is more than just a little surprising; it could very well signal a gap
in astronomers' current understanding of the very nature of dark matter.
Welcome back to Launch Pad, I’m Christian Ready, your friendly neighborhood astronomer.
For over a century, the Universe has presented us with a particularly nagging problem: it
has more mass than we can see.
Something like 85% of the matter in the Universe is in the form of dark matter, yet we have
no way of directly imaging it.
Instead, we can only infer its existence by the gravity it exerts on normal matter, such
as galaxies.
In fact, our own Milky Way Galaxy has a mass of 1.5 trillion Suns, even though it consists
of "only" about 100 billion stars.
The rest is mostly dark matter.
We’ll talk about how we came to understand that dark matter is a thing, and why these
new results pose a challenge in our understanding of it.
But first, I’d like to thank Magellan TV, who are very kindly sponsoring today’s video.
Magellan is a new documentary streaming service that dives deep into topics such as History,
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In fact, while researching this video I watched "The Mystery of Dark Matter” directed by
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This documentary describes the efforts between astrophysics, cosmologists, and particle physicists
to understand and directly detect dark matter in particle accelerators.
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The idea of unseen matter in galaxy clusters was first raised in 1933, when Fritz Zwicky
measured the velocities of the Coma cluster of galaxies.
He found that the galaxies, especially those at the edges, were moving so fast, the cluster
needed about 400 times the mass of its own galaxies to hold itself together.
Zwicky’s estimates were off by a factor of 10, because he derived his estimate from
Edwin Hubble’s measurement of the expansion rate of the Universe, which itself wasn't
very accurate.
But still, Zwicky was ultimately on the right track.
In the 1960’s and 70’s, Vera Rubin measured the rotation rates of several edge-on spiral
galaxies, including the Andromeda galaxy, our closest spiral neighbor.
Spiral galaxies are essentially giant disks of stars and gas, most of which are concentrated
at the center, with fewer out along the edges.
So, if we were to plot the rotational velocity of the disk with increasing distance from
the center, we'd expect the velocity to fall off with distance, just like planets do in
the solar system.
But if anything, Rubin confirmed what others had noticed before; that the rotation curves
of spiral galaxies were generally flat!
In other words, their outer disks rotate at least as fast, and in many cases, even faster,
than their central regions!
It soon became widely accepted that galaxies must be suspended in halos of dark matter
that give galaxies and clusters the necessary mass to hold themselves together.
As its name implies, dark matter doesn't reveal itself at any wavelength, whether its radio,
infrared, ultraviolet or x-ray.
There's simply far too little "luminous" matter to account for the mass that's out there.
Whatever dark matter is, it's a very different from the stuff that makes up everything from
you and me to stars and galaxies.
We can only detect it via its gravity.
Fortunately, galaxy clusters give us an ideal laboratory to probe and study dark matter.
Their gravity bends and magnifies the light from distant background galaxies in an effect
called "gravitational lensing."
If the background source and the foreground lensing object are both compact enough, the
lens can generate up to four images of the background object in what’s called an Einstein
Cross.
And if the alignment between the background object and lens is almost exactly perfect,
light will pass around all sides of the lensing object and produce an Einstein Ring.
But clusters of galaxies are extended and clumpy by their nature, so instead of crosses
or rings, they produce arcs and multiple smeared images of background galaxies.
That’s actually a good thing because astronomers can use these arcs to map out the distribution
of dark matter within the cluster.
Over the last decade, astronomers have been using the Hubble Space Telescope to look at
a lot of galaxy clusters in two key surveys.
The Cluster Lensing And Supernova survey with Hubble, or CLASH, devoted 525 Hubble orbits
to studying 25 different galaxy clusters from 2010 to 2013.
This was followed by an even deeper survey called the Hubble Frontier Fields.
HFF devoted even more time - 560 Hubble orbits to examine just six galaxy clusters from 2013
to 2017.
These long observing campaigns produced the deepest, most precise images of gravitational
lenses ever attempted.
This data allows astronomers to test their models of dark matter.
To that end, a team of astronomers led by Massimo Meneghetti used 11 different galaxy
clusters from the two surveys to map out the dark matter from their gravitational lenses.
By their nature, galaxy clusters are a combination of smaller, concentrated objects - in this
case, the individual galaxies - and larger-scale structures that make up the overall cluster.
If the individual galaxies are small and compact enough, they'll produce some of the strongest
lensing effects.
For example, in the cluster MACSJ1206, a background galaxy is lensed by a compact cluster galaxy
into an Einstein ring.
Another galaxy is lensed into an Einstein Cross, and a third galaxy is lensed into multiple
images of itself.
To figure out how the dark matter needs to behave, Meneghetti and his team built computer
models of clusters with similar masses, distances, shapes, and member galaxies.
Then they let light from background galaxies pass through them to see how often they made
lenses.
To their surprise, the computer models only made about a tenth of the number of lenses
observed.
Put another way, real clusters made ten times the number of lenses predicted.
A factor of 10 difference is huge, and not something that's easily explained.
So, the team tried to resolve the discrepancy by testing different properties of their model.
For example, galaxies typically have supermassive black holes at their centers.
These black holes accrete matter from the surrounding galaxy, and produces energy in
the process.
But that same energy pushes outward on the surrounding galaxy.
So, the galaxy is puffier and less centrally concentrated.
By turning off the black hole accretion in their model, the team found that the galaxies
in their virtual clusters did become more compact and turn into stronger gravitational
lenses.
But it wasn't nearly enough to bridge the gap between the observations and the simulations.
Besides, not having central black holes accreting doesn’t really make sense in the first place
because the clusters are being seen at a time when central black hole accretion was a fairly
common thing in the Universe.
The team also tested whether dark matter halos from galaxies in front of the cluster could
somehow amplify the small lensing effects.
They didn't.
But none of these tricks worked; real clusters produced more frequent and in many cases,
stronger lenses than our current understanding of dark matter predicts.
This implies that the galaxies in these clusters have dark matter halos that are more compact
than theoretically expected.
Now, I’ll note that in their paper the researchers don’t go so far to say there’s a missing
ingredient in dark matter, or that there has to be some new weird physics going on.
Rather, they’re pointing out the discrepancy between the frequency of gravitational lenses
in the cluster galaxies vs. the simulations.
It remains to be seen what’s actually missing from the simulations, but an order of magnitude
difference is a really interesting result that demands attention.
So what could be going on?
Well, clearly, these dark matter halos appear to be denser than previously thought.
Maybe that's because dark matter interacts with normal matter differently than we think
and compresses the halos in the process.
Or, maybe - yeah, maybe - the very physics of dark matter is fundamentally different
than normal matter?
Or, maybe the observations and even the models are simply being misinterpreted somehow and
there's a straightforward explanation for the discrepancy.
Obviously, it's going to take some additional work to find out, but this is a really interesting
result.
Dark matter is clearly a thing even though we don’t yet fully know what it is.
One thing we don’t expect it to do is coalesce into really small things like planets and
stars because it doesn't seem to otherwise interact with anything, including itself.
Our Sun, for example, is not likely to have huge concentrations of dark matter clumped
inside.
But it does has enough mass to act as a gravitational lens on the scale of the solar system.
That’s why there’s a plan to use the Sun’s gravitational lens to image the surface of
exoplanets.
I made a video about that so feel free to check that out when we’re done here.
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