October 2026: Darkened Skies and Nebulous Space

Summer break and solar eclipse

0:00The Jogcast, October 2026 edition.

0:12With Phoebe Ryder, Louisa Mason and Sahini Dutta.

0:25I'm Phoebe, and I'm here in the studio with Louisa. We hope that you listeners enjoyed the summer break. This month, we talked to experts about searching for transients, multi-probe cosmology, and planetary nebulae. We also discussed what it's like doing a PhD with some of our guests, and talk about what we got up to over the summer. We know some of you got busy watching the eclipse. Yeah, so for listeners in the UK and across Europe, we were treated to a celestial event with the solar eclipse in August. For folks in Manchester, we experienced a 95% coverage.

0:56Not a dimming in the light, but like a slight dip in the temperature and a change in the colours that we saw in the sky and the surroundings. Whether you were armed with a colander, or lucky enough to get hold of a pair of sold-out eclipse glasses, we hope that you took a few minutes out of your busy lives to experience this celestial event. Listener Bob Ashworth sent us a photo of himself enjoying the eclipse on a dock, wearing the Jodcast t-shirt in solidarity with Jodrell Bank. He got the t-shirt from Stuart Lowe and Chris North from buying the very first copy of their book, Cosmos,

1:33which myself and Phoebe have been exploring their interactive infographics online. I mean, it's well worth checking out. Yeah, definitely. Whereas for those lucky enough to find themselves in Spain, France, or on a cruise in the Atlantic, they had the chance to experience totality, the perfect alignment of the passing moon with the sun, causing complete darkness for those in its path. The fact that the distance to the sun is exactly four times the distance to the moon, and the sun is four times bigger than the moon, really makes this a cosmic coincidence,

2:05highlighting our place in this cosmos. Jodcast t-shirt was out in France and captured the moment she and her friends observed totality in this short audio clip. Sous-titrage ST' 501

2:43If you have a great eclipse story, we would love to hear how you experienced it. Feel free to send in the story via email or on our socials.

Multi-probe cosmology with Alex Reeves

2:51Next up, we hear from Alex Reeves, who talks to Sahini about his experience as a PhD student, growing up with the Jodcast, his work in multi-probe cosmology, and the various computational techniques he uses in his work. Hi everyone, today we have Alex here, visiting us from ED in Zurich. Alex, thank you for sitting down with us. Hi, thanks. To start with, can you explain your work and what you are doing, where you are right now? Sure, yeah.

3:21So I guess I should start with the fact that this is a massive full circle moment for me. I used to listen to this podcast when I was 15, 16, first getting into physics, so it's really an honour and a pleasure to be here talking to you today. So my work is what I've been doing for the past four years of my PhD, and I would say there's two main strands to this work. One is all about multi-probe cosmology, so trying to combine several different probes in cosmology together, which has several advantages over just looking at one probe at a time.

3:54And another strand of the work is to try to work out how we can extract the most information from galaxy clustering, which is a particular probe that we use in cosmology. That sounds very interesting. So when you talk about multi-probe cosmology, what probes are you looking at right now, and why do you like to pick specifically those things? Yeah, that's a really good question. So right now, I'm trying to be very broad with my approach. So I'm looking at several large-scale structure probes.

4:24So this includes galaxy clustering, where we just take the positions of galaxies across the sky, and we look at the density of galaxies in different redshifts and in different positions, and we build this map, and from that we can extract lots of cosmological information. Another of the probes that I look at is broadly called gravitational lensing. So this affects the light from galaxies and causes distortions in their shapes, and by looking at correlations in their shapes in different parts of the sky,

4:55we can work out something about the gravitational lenses that cause this deflection, so the dark matter. This also affects the photons from the CMB, and that's CMB lensing, and I also look at this in my work as well. And then the other probe that I'm quite fond of, but is a bit more niche, is called the integrated Sachs-Wolf effect. And this is where CMB photons move in and out of gravitational potentials at late time, that are time-evolving, and this means that they enter when the gravitational potential has one size,

5:28and then when they leave, the gravitational potential has typically decayed and is now less deep. And that means that when they leave, they get a slight temperature shift. And this is something that we can also pick up and measure in this already interesting probe. That's very cool. So it sounds like you're working a lot on the large-scale structure side of things. So I have two questions broadly, but to start with, what redshifts are you looking at when you investigate any of these probes?

6:00Yeah, that's a good question. So several of these probes span, I would say broadly, redshift 0 to 1.752 for weak lensing, galaxy clustering. For galaxy clustering, it depends a little bit what tracer you use. So we have these different classes of objects that we try to trace in the local universe and in the more distant universe. Out at very high redshift, you can look at quasars. These are typically the highest redshift probe you look at, and these can get up to very high redshifts, 3.5, 4.

6:35And then for the CMB lensing, this effect starts almost immediately after the CMB photons are emitted, so at redshift 1,100. But the peak, or where the lensing is most efficient, where it contributes the most, is around redshift 2. Right, the second follow-up question I had is, what quantities of physical importance are you trying to constrain, or like, what are you trying to determine using these probes? Yeah, that's a really, really good question.

7:05I think, actually, there's a lot of different directions that you can take this kind of work, and this is why I quite like it, because it's quite agile. So something that I think is becoming of increasing interest in cosmology in the last few years is this idea of trying to pin down dark energy. So there was some data from DESI that was released in 2024, and this seemed to suggest that dark energy might evolve with time. So we have this very simple phenomenological W0WA model, and when you fit it to data, it seems that it prefers a region of parameter space,

7:42which is different from a cosmological constant, at relatively low significance, but nevertheless. And I think, in this context, using multiprobes is really interesting, because each of these different probes has a slightly different dependence on these dark energy parameters. And so, potentially, by combining lots of things together, we can try to further pin down the nature of dark energy. One thing to say there is maybe that the ISW, so this more niche integrated sex wolf probe that I was talking about earlier,

8:15is actually really, can be very powerful for discriminating between different models of dark energy at late times. Yeah, that's really cool. It's very interesting to me that we seem to be approaching closer towards, considering what these dark matters and energies are. So, interesting, I'll keep my eye out on that. Now, switching gears a little bit, maybe. So, we have talked about the physics side of things, but what methods do you use?

8:51Because I think a lot of people are now interested in developing methods to sort of prove these very difficult and hard to constrain things. So, what methods do you use, and what's something cool that you're using now? Yeah. So, I guess most of the methods I use are computational. So, I do a lot of what's known as Bayesian analysis, where you take some data and you try to fit a model to it, and you try to constrain some parameters of this model.

9:21And this kind of inference problem has been around for a very long time in cosmology. So, this is kind of the bread and butter of what a cosmologist does. But I think what's really nice is, as our technology has got better, as our hardware has got better, the speed and the amount of computation you can do, the amount of analysis that was previously completely unimaginable is now possible to do in a few minutes. For example, there's been a lot of improvement in GPU hardware.

9:54And so, one of the things I've tried to do in my PhD is try to utilize as much as possible GPUs for doing Bayesian analysis, for doing inference. And there you can also leverage some advances on the software side, which allow you to run your code on a GPU and also compile it, and this massively improves the speed. Yeah, that is really interesting because I am also doing Bayesian analysis. So, it's really interesting to hear about other people doing it in different ways,

10:28but you're essentially doing the same things. So, talking about speed, how, like, do you have an estimate of, like, pick any inference that you, like an inference pipeline that you run, you have an estimate of how long it takes you to run one inference and how has the inculcation of, like, GPU processes. You know, when I first started my PhD, taking over a project from a previous student,

10:58there the inference took days to run. One of the metrics that we're always looking for when we do Bayesian analysis is this idea of convergence. And so, you can make these statistical measures of how converged your analysis is, and it took, yeah, maybe a day and a half to reach this. And actually, this is really prohibitive because as a scientist working and trying to change things, trying to modify things, trying to figure out how things work, not having quick feedback on how your system is evolving

11:30when you change some analysis setting, it makes it really difficult. I actually only realized that when I did start thinking, okay, maybe there's some ways to speed this up. Now, you know, in my pipeline, you can run the same kind of analysis that was running before in 10 minutes, depending on the analysis setup. This really changed how quickly progress was then made because, of course, you don't just run this analysis once. You want to run it in lots of different settings.

12:00In my case, I'm combining lots of different probe combinations and see how they measure things. Because the estimate of the speed, I think this is something that I've also worked on a little bit, is, you know, what is actually giving you the speed gain here? And just switching from CPU to GPU can already give you a factor of 10. I mean, this can really be a huge gain if your code is very nicely portable to a GPU. Of course, it's, I think, very encouraging to know that it can be better and improvements can be made.

12:33All right, so we have talked about sort of working on this and this is something I personally also experienced that we don't do a lot of physics on a day-to-day basis because I think a lot of people imagine that you are looking through a telescope still as an astronomer.

A day in the life of a cosmologist

12:52So what does your day in a life look like now? And is this what you imagined it would be like to be a quote-unquote astronomer? Okay, I'm very biased, but I think this little corner of astrophysics, this cosmology, is very nice in that sense because you can do this nice mix of a bit of computational work, a bit of data analysis, but also thinking a little bit about theory and using theory to inform how you should perform your data analysis

13:22and why you should look for a particular thing. So for example, in my case, this would be which probe or which redshift would be interesting to look at for a physical effect that I'm looking for, such as dark energy. So I think it's a really nice mix between those two things. Now the day-to-day is, of course, a lot of computational work, as you mentioned, but also quite a lot of writing notes, trying to think through what the next best steps are. I always try and keep a pad of paper

13:53next to my computer, constantly scribbling down random ideas and equations and stuff. Yeah, I guess, what did I think it would be to be doing this? I don't even really remember. I guess maybe people have an idea in the head of more of a pen-and-paper theorist. So what do you like working on more? Is it the theory side of things and coming up with plans of what's going to be the next thing that needs to be done?

14:23Or is it getting really hands-on and writing the goals, doing computations, running tests, and all of that? Honestly, running a data analysis and getting some results and visualising them and looking at them is an amazing feeling. I mean, it feels great when you can start, you know, showing plots to your collaborators and discussing what they mean. I mean, this is really great. But also just talking about ideas and ideas for the future. Yeah, I completely get that. So what has been...

14:56Two things maybe. What has surprised you most about your PhD journey and what has been, like, your greatest learning? And this doesn't have to be academic per se. But yeah, what has been your greatest learning? This maybe sounds kind of bad, but I think what has surprised me maybe the most is to realise that science is, at the end, done by humans and humans usually make a lot of mistakes. I think I kind of had this idea that science was sort of

15:27this beautiful set in stone thing where everyone was really smart and I could never really understand how they came to their results. And I think doing a PhD or doing any kind of research, you start to realise that actually at the edges of what we know, it's really uncertain that actually people are making mistakes all the time. There are bugs in code. There are, you know, things that get revised later. Just this sort of messiness of it a little bit has surprised me. I think it's also really fun. And in some ways,

15:57I think it's also encouraging because it kind of takes it away from this kind of, what seems like a really big, different place and actually brings it to a more human level. More as being my biggest learning, one of the things you really learn during a PhD is perseverance and just things never really work as you imagine them to work and they always take a lot longer than you think they're going to take and you just have to keep going. And I think that in itself is a really important thing to go through.

16:28So what is something that you know now you wish you had known when you were starting your PhD? I think probably the idea that you can be really agile and what I mean to say is of course listen to your supervisor, try and, you know, work with their ideas and think about them. but I think it's also important to how you feel about your project, how you feel about the direction

16:58that you're taking it in. Speaking to lots of people, I became more confident about speaking to people and actually that was the key for me. This is what really helped me in my PhD was speaking to lots of people in the field and bouncing ideas of them and making collaborations. Right, so last question from me. What's your favourite astrophysical object or phenomena and why? Okay, I'm going to give a really, really cheesy answer.

17:30I first got a little handheld telescope when I was 16. It was around the time that I started listening to this podcast. Yeah, it really inspired me. And one of the first things I looked at was the Pleiades. It's also called the Seven Sisters, this cluster of seven stars. And it's, I mean, you can see it with the naked eye but through the telescope it was just beautiful. I mean, I don't really have a good science reason for why I like it. It's just really pretty.

18:00Yeah, it is. So, since then I've always really liked that. It's been a lot of my, you know, computer backgrounds. I've got a little poster of it on my wall. So, yeah, it's just a bit random but this is... No, I think it's awesome to keep reminding yourself of like where it's all starting. Great, that's it for me. Thank you so much for coming. Thank you very much. It was a pleasure. Thanks, Alex and Sahini.

Planetary nebulae and dust extinction

18:24Next up, Sahini is talking to one of JBCA's most recent graduates, Alec Chukai, about finishing his PhD, working on planetary nebulae and the most dreaded question for every PhD student. What's next? Hi, everyone. Joining me in the studio today to talk about his PhD experience is Dr. Alec Chukai. He recently finished his PhD from JBCA and is, as we say, PhD done. So, welcome to the podcast, Alec. Thank you.

18:54So, how does it feel to finally have your PhD? Has it sunk in yet? I don't know if it has sunk in yet. I didn't have a strong sensation at either kind of the big milestones of your thesis submission deadline and your viva. Those are the two big parts of kind of finishing a PhD. And I think actually the point when I felt the most and I felt a release of relief and like, ah, this is amazing, was the point not so much when I submitted my thesis but the point about two weeks before submitting my thesis when I realized,

19:26okay, everything's going to be all right and I'm going to finish this on time and I'm going to create a thesis that I'm happy with. That was probably the strongest feeling. By the very end of my PhD, I was very happy with the body of work that I produced. So, that's something that's nice, I think. So, speaking of your work, tell us a little bit about what you have worked on in your PhD. Right. So, I've been working on dust, dust extinction and then the way that my PhD has kind of gone is that I specifically then kind of pivoted

19:57to looking at plantar nebulae. So, that's really evolved, not even really stars anymore, kind of dead stars, post-stellar objects. So, what I've been doing is I've been looking at plantar nebulae which are these very beautiful kind of, when a star dies, it releases, or when an intermediate mass star dies, I should be more specific. It releases a lot of the material that was contained within its stellar envelope and then what you're left with is a really, really hot remnant star,

20:27a white dwarf and because of that's really hot, it kind of sweeps out a lot of that material, it ionizes it, it kind of lights it up in big, shiny, pretty colors. And so, what I've been doing is studying the extinctions towards these objects which have, in the past, been really poorly constrained and I used a technique to more strongly constrain those extinctions and so there's a paper that I've put out that's about that and then, I mean, the way that I've been doing that is by fitting

20:58stellar atmosphere models to the data that exists for the central stars, for those objects that are at the middle of the plantar nebulae and so, what I've then done after that, what's my kind of the final chapter of my thesis is looking at some of the other results that we're getting out of fitting those models to those stars and whether there's any other interesting results that we can kind of pick out like looking for whether those stars have binary companions which we think is a really important mechanism for the shaping of the plantar nebulae. You mentioned

21:29that you pivoted from DAS to working on plantar nebulae. Was that sort of a conscious decision or was it just something that kind of happened? I don't think there was ever like a meeting for example between myself and my supervisor when we specifically decided okay this is a pivot that we're going to do. I think it was somewhat driven by well two things. So the first was that that was I think the area that we were getting results that I was really happy with. I spent the first year

21:59of my PhD kind of looking at trying to derive extinctions kind of with based on blind fits to stars without kind of any prior information about the temperature or the extinction and I found that really tough. I think that's quite a common experience to find the project that you do in the first year of your PhD pretty challenging. You're not as good at being a researcher as you later will become and it's really interesting because I kind of look back on that project that I did and I mean I was never happy with the results at the time. I'm still not happy with the results

22:30but I have a lot more of a sense of what was limiting me and where I could actually go in order to get the results that I think would actually be more meaningful and more interesting. I think it was kind of a product of having results in what I'd been trying to do that I wasn't very happy with and then we tried something looking at specifically the very hot stars which are at the centre of Plancha Nebulae and we got really good results for those and results that I was really happy with and I think do shed interesting light on these objects

23:00so I guess that we followed what was most interesting. The other thing is that in the second year of my PhD I did a research visit to Macquarie University in Sydney and the group there is working much more on stars evolved objects they are specifically looking at common envelope interactions nowadays and simulating that so working on Plancha Nebulae within the context of that group felt like a much more natural thing for me to be doing.

23:31Right, I think that's also part of the journey of a PhD that you figured out what you want to do and how to do it. Yes, yes. I have several questions but you mentioned that you work on extinction based things so can you sort of explain that? Yes, sorry I realised when I was discussing that I really should tell you about what that is. So extinction it's an interesting word because it sounds very morbid and kind of scary, right? But it's nothing like that

24:01and really when we talk about extinction I guess a more natural and intuitive way for people to think of it is extinguishing. The word extinction does come from extinguish. When I say that what I mean is the effect by which the light from an object that you're looking at is like attenuated is the kind of the technical word we use the effect by which the light is lost due to intervening material which exists within the interstellar environment within our galaxy. So I think

24:33it's very common to think of space as being empty being a vacuum but there is dust there there's gas that's what the stars are being formed out of and that is having an effect the light of the objects which we see through them and so constraining that being able to understand the effects of that are really important because you want to be able to actually know the intrinsic properties of the object that you're looking at in order to be able to fully understand what's going on there. So measuring

25:03extinction correcting for extinction is a really important kind of baseline part of astronomy and so I guess in a way I've been working on that because we want to correct for extinction when we study specifically planetary nebulae is what I've been doing it for. In terms of planetary nebulae we're also interested in whether the nebula itself provides a significant extinction effect. So whether when you're looking at a central star of a planetary nebula, when you're looking at the central star, is that kind of veil around it, that shroud

25:34of the nebula, to what extent is that extincting your central star that you're looking at. And actually the results of my work suggest that in a lot of cases the effect of that by the time the nebula is particularly blown out and when you're looking at the big objects which is what I've been studying the most clearly there's not a big effect from the nebula itself so that's one of the results of my work. Although it is size dependent there definitely are objects where you've got a very clear dust lane and you have got massive extinction of the central

26:04object so yeah it's very object dependent. Yeah it's interesting we do the same thing in cosmology but we call them foregrounds. Yes. So yeah to close this session and not to put you in a more stressful position but what's next? What do you want to do? It's interesting I don't have a strong sense of what I want to do in life in general. I have some kind of almost romantic ideas I guess you could say about things that might be quite nice to do but realistically I

26:35don't think I'm particularly trained to do. I really really enjoyed kind of some of the research that I was doing in the final months of my PhD and I would welcome an opportunity to continue that but I also have an awareness that I personally find research to be a relatively solitary environment. I find that you know a lot of the time you are working on your own problem and you're on your own with that problem even if you're in a really great workspace which JBCA is you

27:05know your problems are yours. I don't fully like that I would prefer to be working in a team that's kind of tackling things together and so I don't necessarily see myself in research and in the long term. We'll see. I thought that finishing I would have a much stronger sense of where I wanted to go what I wanted to do I still don't know what I want to do and I have to say I think that I might be one of those people that never knows what they want to do and just takes opportunities when they come. There's a beauty to that I guess because

27:36life then turns into an adventure you never know what's gonna happen. Exactly. Yeah I love that. It was really nice to chat with you Sahini thank you for having me. Thanks for talking to us Alec and all the best for your future.

Jodcast twentieth birthday and outreach

27:51What has the JOGcast been up to over the summer? Over the summer the JOGcast team headed down to Birmingham for the National Astronomical Meeting where we held a little birthday party for the JOGcast's 20th birthday. At this event we got to speak to other astronomers from across the UK and chat to them about their thoughts and experiences doing outreach. The party theme continued as we asked participants to play past the parcel, pin the antenna on the interferometer, as well as signing our birthday banner. Overall the session was a great success and huge fun. We thought that we'd share some of the outreach experiences that stuck with

28:22people. So one person said that whilst working as a planetarium presenter, with each year more and more attendees felt like university and the study of astronomy was accessible and possible for them. Someone else said they loved running World Space Week astronomy activities for their school whilst they were at school. Another said that there had been some primary school students visit and the children's excitement to hear about astronomy reminded them why they chose it in the first place. And someone's journey came full

28:53circle when they went back to the Royal Institute to give a presentation, having been there many times as a child to watch talks. We had an amazing time at this event and it was lovely to have a chance to hear about and learn from other outreach activities and programmes. If you're interested to learn more about other ways to get involved with astronomy in the UK, keep an eye out on our social media as we showcase some of the activities that we've learned about. And if you're sad that you missed out on an opportunity to sing Happy Birthday to the Jodcast, do not fear.

29:23We have a live event coming up at Jodrell Bank. For this anniversary celebration, Jodcasters past and present will reunite to share behind-the-scenes stories, memorable moments and fascinating insights from two decades of exploring the universe. Tickets are available on the Jodrell Bank website and we'll be sharing the details on our social medias very soon. Thank you to Alex Reeves and Alex Chukai for the interviews. The audio editors were Louisa Mason and Josh Bishop. The show editor was Jordan Norris. The producer was Lily Correa Magnus. Until next time,

29:53Jodan!