When we look at the night sky,
we see a vast cosmos
filled with stars and galaxies and dust,
a cosmos teeming with luminous phenomena.
So we've all heard some version of that famous Carl Sagan line,
"We are made of star stuff."
And it's true, we are.
And that makes it easy to believe that what matters is what's visible --
us, the trees, the stars --
because it helps us feel connected to everything that we can see.
Today, we know that everything visible in the universe
is composed from a basic set of building blocks
known as elementary particles.
We call this incredibly elegant picture
the standard model of particle physics,
and we understand it in great mathematical detail.
Comprehending the standard model is an enormous achievement.
But we are now certain that it describes very little of what's out there.
It turns out that most of the stuff which fills our universe
is completely invisible to us.
In other words, visible matter,
the kind that we and the stars are made from,
the kind that radiates light,
is not what's normal.
And we, the luminous matter,
we are the cosmic weirdos.
So how do we know?
Well, consider this invisible nonbinary person,
who's hiding in plain sight inside of their suit.
We can't exactly see a person,
but we know that they're there, because the suit is filled out.
So the presence of the invisible enby
is governing how the suit hangs in space-time.
So we can see a similar effect with visible matter.
We can see that stars and galaxies are affected
by the presence of something more,
something completely invisible to us.
So we now know that the universe is more queer and fantastical
than it looks to the naked eye.
(Cheers)
(Laughter)
That's right.
(Laughter and applause)
So how did the universe get this way,
and what exactly is inside?
So I'm a theoretical physicist
with expertise in particle cosmology.
And it's my job to use math
to study the origin and evolution of space-time
and every single thing that's inside of it.
I connect the very small -- elementary particles --
with the extremely large -- galaxies and galaxy clusters,
and I'm a griot of the universe.
I develop creative mathematical narratives
that may just be our cosmic origin story.
Now as a theoretical physicist, I really love doing math
and coming up with different ideas
that may describe our mostly invisible universe.
But it's important to be accountable to data too,
the real stuff.
So after mathematics,
my second favorite tool for addressing these large cosmological questions
is the biggest laboratory we know,
the universe itself.
Observatories with capabilities from visible light
to high-energy X-ray and gamma-ray photons
are still some of the best ways to gain insight
into what's going on in space-time, with the invisible stuff.
So what you're looking at here is the Vera C. Rubin Observatory,
an exciting new facility
that's about to see its first light over the next two years.
It's a leading example of a new generation of telescopes
that are going to change the way we see this mostly invisible universe.
Now it's also the case that swarms of satellites
threaten images from ground-based facilities like this one.
But the Vera C. Rubin Observatory can help us understand
where the invisible stuff is and what it's doing,
which will help us determine what exactly it is.
So when it comes to the cosmic accounting,
here is what we know so far.
We're in the midst of a great cosmic drama,
where space-time is curved and it’s expanding.
And the history and future of that curvature and expansion
is determined by what's inside,
which is mostly not visible stuff like us --
that's only about five percent.
The majority of the energy-matter content in the universe
is something that we call dark energy.
So empty space seems to have an energy associated with it.
And that's increasingly affecting how space-time expands.
After dark energy,
the second-largest ingredient is something that we call dark matter.
So here's the funky thing about dark matter.
Unlike dark energy,
it gravitates exactly like visible matter.
But it's completely unlike us in every other way.
So you might be thinking,
"OK, dark matter.
It clearly has a color associated with it."
It's dark, like my pants.
Right?
But the first thing you should know about dark matter
is that it doesn't have a color,
and at least at first approximation,
light seems to go right through it, so we can't see it.
It's invisible, maybe transparent, maybe clear.
So if you put out your hands
and think about the weight of having a clump of dark matter in your hands --
that's how it would feel,
but your hands would look exactly the same.
Today, we believe
that 80 percent of the normally gravitating matter in the universe
is dark matter.
Dark matter is dominant on the outskirts of galaxies,
and it affects stellar motions on the edges.
This effect is actually how Vera C. Rubin and Kent Ford
found the first substantive evidence for the existence of dark matter.
What you're looking at here
is an artist's rendering of our own galaxy, the Milky Way,
and it's enveloped in a halo of dark matter,
represented here by a blue gas.
We believe that every single galaxy,
or almost every single galaxy,
lives inside of a dark matter halo.
And we think that they're not alone.
The Milky Way itself has around 60 gravitationally bound satellite galaxies
that are in its orbit.
Some of these, you may have seen when observing the night sky,
or you may have heard of,
like the Large Magellanic Cloud and the Small Magellanic Cloud.
Each of these satellites
lives inside of its own dark matter subhalo.
So, like the invisible enby in their suit,
the presence of dark matter
is affecting how galaxies are distributed throughout space-time.
So we can also reverse-engineer where dark matter is --
it's represented here by the bluish purple.
We can look at how images of galaxy clusters
are distorted, which tells us something
about how dark matter is distorting space-time.
So we know something about how much dark matter there is,
and even how the dark matter is distributed,
but what kind of particle is it?
So all that we know
is that it's beyond standard-model physics.
It's not like any of the particles that we have ever seen
or had any kind of contact with.
Alright, so this seems like a potentially terrifying,
intractable problem,
because we're talking about something that we can't see,
something that we can't touch.
You might be thinking,
"OK, they haven't had many ideas about that over the years,
because that just seems really hard."
Right?
So here's the Venn diagram to end all Venn diagrams.
(Laughter)
[Theories of Dark Matter]
So I'll bet y'all a hundred dollars that you can't find a better one.
At least according to me.
(Laughter)
So Tim Tait created this diagram
to help us visualize just some of the hypotheses
that physicists have had over the years to explain the dark matter problem
and how these ideas overlap with each other.
So as you all can see, there's a lot happening here, right?
And hopefully, it's becoming increasingly clear
that this isn't just an astrophysics problem
of galaxies and galaxy clusters,
but this is also a particle physics problem.
In order to understand what's happening on the largest scales,
we need to understand something very small,
like a new particle,
or maybe primordial black holes.
So you've been looking at this for a moment,
and you know what you're all really thinking is,
"What's Chanda's favorite dark matter candidate?"
Right? This is what you're dying to know?
So, I'll end the suspense by telling you ...
that my favorite candidate is something called the axion.
This is the hypothetical particle.
And the first thing that I want to tell you about the axion
is that it was almost called the higglet ...
(Laughter)
And whoever chose "the axion" just completely blew it, OK?
I'm pretty bummed about that.
But the axion is a compelling particle, because it's a twofer:
it addresses a problem that we already had,
a conflict between theory and experiment
in the realm of quark physics.
"OK," you say, "but, like, how can I visualize it?"
At this point in the talk, you should know better, right?
Because to first approximation, dark matter is invisible.
(Laughter)
But I know you really want a visual,
so I'm going to give you one.
Here's what it looks like to me, in my everyday work.
(Laughter)
(Applause)
Which is to say, it's OK if this is unintuitive.
(Laughter)
The universe is a wonderfully strange and fantastical place,
and that's why humans as a species have always wanted to study it.
And this is why we have so much fun, trying to understand it.
So how are we going to go looking for the axion
or any other dark matter particle?
You might think that we have to use traditional particle physics approaches,
like colliders, where we smash particles together
and see what comes out.
But astrophysical signals have something to say.
Telescopes from across the electromagnetic spectrum --
for example, the proposed NASA facility,
the STROBE-X X-ray space telescope --
can help us potentially determine what exactly dark matter is.
But telescopes look at the very large.
How can we use the extremely large to understand something so small?
Well, in the case of the axion,
it helps to pay attention to its quantum classification.
So all particles come in one of two quantum categories,
fermions and bosons.
So fermions, even when things get cold,
like to keep their distance from each other.
They're antisocial.
That's how it is.
Bosons, on the other hand,
when they get below a critical temperature,
they're like five-year-olds on a soccer field,
so they don't have a concept of formation,
they just bunch up together.
So in technical terms,
we call this the formation of a Bose-Einstein condensate,
where all the particles come together and act like one superparticle.
So importantly, axions are bosons,
and so now you have a sense of why I like working with them.
I'm completely enamored with the idea of axion Bose-Einstein condensates.
So usually, we talk about creating these quantum states in the lab,
using atoms,
but now,
we're talking about the possibility of new, maybe galaxy-scale
Bose-Einstein condensates made out of dark matter.
So what you're looking at here
is a simulation developed by a team that I lead.
It's an axion condensate orbiting a central mass.
So it's like a subhalo orbiting its host galaxy.
Maybe the Large Magellanic Cloud orbiting the Milky Way.
As you can see, over the age of the universe,
the subhalo starts to get to get torn apart,
and what my team's work shows
is that the way that this happens with axions
is different than with other dark matter candidates,
because it goes into this special condensate state.
Now imagine the possibility
that there's more than one type of dark matter candidate.
Maybe there's more than one type of dark matter particle?
How much richer this picture can be.
There's no cosmic rule that says there can only be one.
So in the end,
I expect the universe to force us to reevaluate what we thought we knew.
[Making the invisible visible.]
When we honor the land and the sky ...
as our galactic relations ...
and their Indigenous stewards,
it becomes possible for us to imagine
new ways of being in good relations with each other.
That's why I, as a Black queer person,
am so proud to follow in the footsteps of my ancestors,
who studied and dreamed with the night sky,
sometimes of freedom.
Astronomers like Harriet Tubman,
for whom the recently launched James Webb Space Telescope
should be renamed.
(Applause)
I honor the gay NASA employees who were persecuted
under the leadership of JWST's namesake,
even as I share in the tremendous community-wide excitement
for what that facility is going to teach us about dark matter.
And I honor the memory of Vera C. Rubin,
the astronomer who first asked me,
as a young, terrified graduate student,
"How do you think we should solve the dark matter problem?"
We live in an amazing time to be doing dark matter research.
Over the next decade,
we're going to see the universe with incredible accuracy and clarity
thanks to these new telescopes on the ground and in the sky.
We'll probably get some answers,
but we're going to get a host of new questions.
And my team? We're going to be ready.
So the search for dark matter is on.
What's your favorite candidate?
(Laughter)
If it's not an axion, you better fix that.
(Laughter)
Thank you.
(Cheers and applause)