Chapter summary

• Introduction & Welcome Back — Sophia Gad-Nasr returns to Astrophiz, seven years after Episode 90
• PhD Update: Gravothermal Collapse in Self-Interacting Dark Matter — What Sophia’s 2023 doctorate found, and how dark matter self-interactions could form black holes at galaxy centres
• Cold Dark Matter, WIMPs & Self-Interacting Dark Matter (SIDM) — A refresher on the small-scale structure problem and why SIDM was proposed as a fix
• The LUX-ZEPLIN (LZ) Signal Explained — How the liquid-xenon detector works, what a 248 keV nuclear recoil event means, and why 2.6 sigma isn’t a detection (yet)
• Watching the Nancy Grace Roman Space Telescope Launch — Sophia’s firsthand account of the launch, and why Roman’s 100x collecting area over Hubble matters for dark energy research
• Self-Interacting Dark Matter vs Direct Detection — Why a confirmed LZ signal wouldn’t necessarily prove dark matter self-interacts
• Sophia’s Soapbox: Diversity in Physics & the Big Open Questions — Progress on gender diversity in physics PhD programs, and why dark energy’s fate matters for the universe’s future.

  Here is a list of Sophia’s Socials … look her up:

• Instagram: @Astropartigirl
• Twitter / X: @Astropartigirl
• Facebook Page: Astropartigirl
• Facebook: Sophia Nasr
• Google+: Sophia Nasr
• LinkedIn: Sophia Nasr

Welcome to Astrophiz. We acknowledge Australia’s first astronomers, the Aboriginal and Torres Strait Islander peoples, the traditional owners and custodians of the land we are on. This episode is produced on Yorta Yorta Country.

Join us as we sit down with the world’s leading space scientists to discover exactly how our universe works.
And right now, you’re invited to Zoom over nineteen time zones with me to speak with a truly fabulous astrophysicist, Dr Sophia Gad-Nasr. You’ll love her!

SFX: Morse code snippet

Brendan: Hello, Sophia.

Sophia: Hi, Brendan. How are you doing?

Brendan: I’m very good, thanks, Sophia. Now, listeners, today we’re returning to one of the great open questions in physics: What is dark matter? Long-time listeners will remember today’s fabulous guest from Episode 90, back in September 2019. She’s back — and this time with a doctorate to her name:
Dr Sophia Gad-Nasr.

Welcome Back, Dr Sophia Gad-Nasr

Brendan: In 2019, as a PhD candidate at UC Irvine, Sophia was working on self-interacting dark matter, or SIDM. She’s since been awarded that doctorate and has kept building her reputation as a highly trusted science communicator. She’s known widely — and wildly — online as @Astropartigirl, where “Parti,” spelt P-A-R-T-I, stands for particle.
Look her up, she’s fantastic!
With 350,000 followers, Sophia has long used video and social media to make cosmology understandable and approachable, while continuing to champion diversity in physics.

So welcome, and thanks for speaking with us again today, Sophia.

Sophia: It’s a pleasure to be back. Thank you.

Brendan: Good on you, thank you. Okay, before we talk about dark matter research and a possible breakthrough, can you first fill us in with a summary of what you’ve been up to since we last spoke?

And by the way, congratulations on landing your doctorate — that is awesome.
Can you give us an update, please, Sophia?

PhD Update: Gravothermal Collapse in Self-Interacting Dark Matter

Sophia: Absolutely, thank you so much. Yeah, so I guess the biggest update would be my PhD — that was in September 2023. As you mentioned, my work was focused on self-interacting dark matter. So essentially, you know how if you turn on interactions between dark matter particles, how does that affect the evolution of different-scale dark matter halos, and the galaxies that it surrounds?

I worked on gravothermal collapse.
Essentially, self-interactions can make a halo’s centre less dense, and doubling when it starts  — but as it proceeds, it can actually start to get denser, because what’s happening is that the halo continues to exchange energy between the outside and the inside, and so the core eventually starts to contract and then it becomes extremely dense.

 And so part of what my research studied was whether that can then turn into an avenue for creating black holes at the centres of galaxies.

So yeah, it’s very exciting stuff. Hahahah!

Now I work  in communications, doing my science communication as @AstropartiGirl, as you mentioned.

So I do a lot of that …  I spend a lot of time explaining physics, astronomy, and space exploration, answering questions … helping people and showing people  the awesomeness of physics — because there’s so much that’s great in there, and disentangling it and show how amazing it really is.

I guess the most recent thing I got to do was watch the Nancy Grace Roman Space Telescope launch into space. I saw it live in Florida, and it was really special to me, because I remember during my PhD I thought the Roman could be used for my research — particularly for my thesis work.

So seeing it launch was really special, because I know it’s going to tell us a lot about dark matter in the universe. It was fantastic personally …. And today, here I am. Heheh!  

Brendan: Very good, thanks, Sophia. Do you ever sleep?

Recap: Episode 90 and the Case for Self-Interacting Dark Matter

Brendan: Now, for anyone who missed Sophia’s earlier episode, here’s a quick — and I hope accurate — recap of Episode 90.

Sophia explained the standard cosmological picture of cold dark matter, or CDM: dark matter that’s slow-moving and collisionless, meaning individual particles don’t interact with each other.
It works beautifully at larger scales, correctly shaping galaxies, galaxy clusters, and the cosmic web.

 But … at small scales, many open questions remain — dark-matter-only simulations predict dwarf galaxies with denser centres than we actually observe, though things get more      complicated once you introduce baryons.

We also talked about WIMPs — weakly interacting massive particles, Sophia’s specialty — a long-standing class of dark matter candidates, including particles predicted by supersymmetry.

That small-scale mismatch is where Sophia’s own research comes in. She introduced us to self-interacting dark matter, SIDM, where particles do interact with each other, but with a cross-section that depends on velocity: strong enough to soften slow-moving dwarf galaxies, weak enough to remain consistent with galaxy cluster observations. It’s an elegant way to fix the small scales without breaking the large ones.

Which brings us to this month: the LUX-ZEPLIN experiment — LZ — buried nearly a mile underground in South Dakota, has reported a single unexplained particle interaction, one that some physicists think could be a WIMP.
It’s nowhere near a confirmed discovery, but it’s the most intriguing signal this field has produced.

So, Sophia, over to you — let’s get into what this result might mean for cosmologists and astrophysicists.
Can you tell us two things: first, what was the research that led to the LUX-ZEPLIN announcement, and second, what’s been the reaction across the dark matter community, and your personal response?

The LUX-ZEPLIN (LZ) Signal

Specifically this one … which was the LUX-ZEPLIN …  what they do is use liquid Xenon — they have this a giant thing of liquid Xenon … way, way underground in South Dakota, and it’s very, very far underground so that you can shield it from things like cosmic rays. Because, you know, up here, you have that all over the place, like cosmic rays and other backgrounds that would essentially nullify any sort of detection if we could get them.

And so the way that it works is that when something interacts with the Xenon in the liquid Xenon,  , it produces a flash of light and releases electrons.

 The detector collects those electrons and then generates a second light signal, and so using that, they reconstruct where the interaction happened, and whether it looks more like an electron recoil or a nuclear recoil.

Then this becomes important, because a dark matter particle colliding with a xenon nucleus could make that nucleus recoil — but other particles do that too. So a huge part of the experiment is understanding and rejecting backgrounds.
A lot of that work went into this result as well.

In this analysis, the collaboration extended its search to higher recoil energies, to look at more inelastic-type scattering and similar processes.

And so they found an event consistent with a nuclear recoil of 248 kilo-electron-volts, in a region where very little background is expected — and that was the event that happened …and why this announcement came up … and this was in 2023 … the actual detection was in 2023.

Brendan: Yep

Sophia: Next was the question about the communities reaction … the community’s reaction — for myself, I find this very interesting.

If we can actually directly detect dark matter, that’s the best thing a dark matter physicist can get, because my own work on behaviours of galaxies, infers what’s going on … whereas a direct detection would tell us there’s an actual particle there. So this is really important and very exciting for me.

We still don’t know what caused this . The significance of the event is 2.6 sigma.

To be called a detection, it needs to be three sigma; a discovery is five sigma.

So we’re not at a detection yet, let alone a discovery — but it’s interesting enough that we want to talk about it, and hopefully continue with next steps to see whether an explanation can reproduce the predictions this data set made.

And so I’m really interested to see whether more events occur in these experiments, and what their energies look like, because that tells us a lot about what the dark matter particle actually is.

Brendan: OK … And I haven’t checked up here in Australia, but down in a goldfields town called Stawell in Victoria, Melbourne University has a dark matter detector buried like the South Dakota one — buried two kilometres underground to shield it from stray particles. I haven’t heard what their reaction is, but I’ll chase that up too, Sophia, and let you know if there’s any response from them.

That is so cool — I love how science never sleeps.

Sophia: Heh

Watching the Nancy Grace Roman Space Telescope Launch

Brendan: Now, you mentioned attending launches — I know you were down in Florida for the launch of the Nancy Grace Roman Space Telescope last week, and look, I am very envious, I can tell you.

Tell us about it, please, and what it means for science to have Roman up there.

Sophia: Yeah, absolutely, it was really exciting to get to watch this. We also got to see the boosters landing again, so reusing the boosters too — that was really, really cool.

The Nancy Grace Roman Space Telescope is really special to me, because it’s going to study things like dark matter and dark energy, and give us an idea of how the universe evolved.

The special thing about Roman — if you compare it to Hubble, people often ask what’s the difference, or why do we need another telescope when we already have JWST.

So it has a hundred times more collecting area. So Roman is tuned for larger-scale surveys. Roman can look out into space, find something interesting, and then point JWST or Hubble at it to zoom in and figure out what it is. So it’s a great addition to our flagships.

It’s going to take us so far — one of the things they say is that a survey that would take Roman a month to do would take a n entire century for Hubble to do.

Dark energy is what causes the universe to expand faster and faster over time.

In the standard cosmological picture, we think dark energy is constant, meaning the universe expands at a constant accelerating rate. But when we look at different surveys and probes — the Planck mission, which studied the CMB, versus surveys looking at supernovae, for example — they’re finding different results. So we actually don’t know whether dark energy is constant, or whether it gets stronger or weaker over time. That’s something we’re hoping the Roman telescope will help reveal.

Brendan: Awesome, thank you. Look, can I just move back to dark matter again? Could a dark matter interaction in the LZ experiment tell us whether dark matter is self-interacting?

Self-Interacting Dark Matter vs Direct Detection

Sophia: So it would tell us about a different kind of interaction.
LZ is looking for dark matter that interacts with ordinary matter — specifically Xenon nuclei, in this case.
Self-interacting dark matter is different, in that the collisions happen between dark matter particles themselves, through a force we haven’t discovered yet — a “dark force.”
So a particle could interact very weakly with ordinary matter while interacting much more strongly with other dark matter particles, because those interactions are mediated by a different force.

So a confirmed signal from LZ wouldn’t necessarily tell us whether dark matter is self-interacting or not, because that’s an interaction between dark matter and regular matter. We’d still need to work out the particle model and figure out what it predicts and how that affects the evolution of galaxies. Knowing how it interacts — including the energies at which these collisions happen — is really important for that, so we can understand the particle physics behind the dark matter model.

Brendan: Awesome — one small step at a time. That is fantastic, Sophia, thank you.

Sophia’s Soapbox: Diversity and the Big Questions in Physics

Brendan: So, once again, to finish up, the microphone is all yours, and you’re welcome to give us your favourite rant or rave — on a challenge we face in science, in equity, in diversity, in science denialism, in science career paths, or your own passion for research, or the human quest for new knowledge.
The microphone, Sophia, is all yours.

Sophia: Fantastic, thank you so much. Let’s see — in terms of challenges we face in science, of course diversity is still an issue, but I do see a lot more women in these programs now. Something I really liked about my cohort — in my year doing my PhD at UC Irvine, thirty percent were women. So we had a lot of women, it was really great, and we took note of that. It’s something that still needs work, but I think we’re heading in the right direction.

As for the actual science I’m excited about — of course, dark matter. I did my whole thesis on it, and understanding what kind of particle it is and what kind of interactions it has is incredibly important for us to understand how our universe evolved, and how it could continue to evolve.
Dark energy is important to me too, because the way it behaves — whether it gets stronger or weaker with time — has an effect on how the universe ends. I know that’s billions and billions of years from now, so it’s a lot of time, but it’s very interesting, and it tells us more about how our universe will evolve. That’s something I’m really excited about.

Brendan: Well, thank you so much, Dr Sophia Gad-Nasr. On behalf of all our listeners, and especially from me, it’s been really exciting to speak with you and get that dark matter update.
Keep your eye on @Astropartigirl, everyone — I’ll put links to her social media handles in the show notes. Thank you, Sophia.

Sophia: Thank you so much for having me back. I was thrilled to come back and discuss this stuff with you again, Brendan.

Brendan: Bye.

Sophia: Bye now.

SFX: Morse code snippet

So make sure you subscribe to Astrophiz, tell your Astro Buddies, and join us on the first of each month for Dr Ian Musgrave’s Sky Guide, so you can plan your observing schedule, and on the 15th of each month for an in-depth interview with another one of the world’s leading space scientists.

Have fun. Look up. Clear skies.

SFX: ‘Radio Waaaaaaves’