Hunting for extra dimensions with gravitational waves

Right or left, back or forth, up or down, earlier or later: the everyday world our senses experience is, at least apparently, 3+1-dimensional. One would then think that extra dimensions beyond the usual 3+1 spacetime ones, an exciting and common ingredient in many science fiction stories, should be indeed left to science fiction novelists. It turns out, however, that extra dimensions are not at all uncommon in actual physics theories. For several decades, scientists have toyed with the idea of introducing extra dimensions to solve some of the long-standing problems of physics. One such puzzle, known as the “hierarchy problem”, is why gravity is so much weaker than the other three fundamental forces of nature (the strong, weak, and electromagnetic forces).

In some of these extra dimensional theories all particles and forces, except gravity, are confined to the usual 3+1-dimensional spacetime. Gravity, on the other hand, would feel these extra dimensions and travel through them. The “leaking” of gravity into extra dimensions might even help explain why it is so much weaker than all the other known forces. Our senses would not be aware of the fifth dimension, much as the two-dimensional inhabitants of Flatland were not aware of the existence of the third dimension. This is the idea behind “brane cosmology” or “brane-world models”, where our four-dimensional spacetime is restricted to a “brane” which lives in a higher-dimensional space known as the “bulk”. Among the most popular brane-worlds we find the Randall-Sundrum models (named after Lisa Randall and Raman Sundrum, see this paper and this other paper for their original work) where our 3+1-dimensional spacetime is surrounded by an infinite 5-dimensional anti-de Sitter spacetime (or AdSfor short). This AdSspacetime is characterized by its curvature radius: the larger the curvature radius, the more AdSlooks flat (think of the Earth: the larger the Earth radius, the more it appears flat).

This all sounds exciting and – perhaps – looks like it belongs more to the science fiction realm than to reality. However, physicists have good reasons to take these types of conjectures seriously. The question is then: can we ever hope to observationally probe these extra dimensions? A lot of smart people have thought about ways of testing the existence of extra dimensions, for example by looking at particular signatures at colliders such as the LHC or searching for modifications to Newton’s law at short distances (if you are curious, have a look at this review for more information). It turns out there is another way to hunt for extra dimensions, based on an idea by Robert Caldwell and David Langlois, that makes use of a source of gravitational waves. You need to be able to detect both the gravitational waves and the light released from the event. This is the basis of what is known as multi-messenger astronomy. Since (in Einstein’s theory of General Relativity) gravitational waves and light both travel at the same speed — you guessed it, the speed of light! — one would expect the two signals to reach us at the same time. However, if brane-world extra dimensions exist, gravitational waves can take a shortcut in the fifth dimension and actually reach us before the photons do.

Sounds crazy? On August 17, 2017 LIGO and Virgo detected the gravitational waves released during the last minutes of a binary neutron star merger (GW170817) . Within two seconds, Fermi and INTEGRAL also detected a short gamma-ray burst released by the same neutron star merger event. The near-simultaneous arrival of the two signals was used to set very stringent constraints on deviations from Einstein’s general relativity (see for instance this review if you are interested).

Thanks to these two amazing detections, it has also been possible to hunt for extra dimensions using gravitational waves for the first time. OKC researchers, Katie Freese, Luca Visinelli, and Sunny Vagnozzi, in collaboration with Nadia Bolis who was visiting OKC from CEICO in Prague, used the measurements of the time-delay between the gravitational and the electromagnetic signal to put constraints on the curvature radius of the fifth dimension, assuming that gravitational waves travelled along a shortcut in the fifth dimension. They did a careful statistical analysis of the time-delay, taking into account uncertainties in the emission of the gamma-ray burst from the neutron star and the impact of large-scale structure between us and the neutron star on the propagation of the gravitational waves. They concluded that no evidence for non-zero curvature radius could be found and set a 95% confidence level upper limit of 1.997 Megaparsecs on the curvature radius of the fifth dimension. In other words, we can be 95% sure that the curvature radius of the fifth dimension is less than about 6.5 million light-years large.

The results, which appeared in November in the arXiv preprint repository were then published in March in the journal Physical Review D. The tools to reproduce the analysis are publicly available on Github.

This is the first time that researchers have been able to hunt for extra dimensions using gravitational waves.  We like to think it’s no accident that this work was done at the OKC, an institute named after the late Oskar Klein who is considered by many to be the father of extra dimensional theories.

Text by OKC graduate student researcher Sunny Vagnozzi.

OKC leads in international publications at Stockholm University

Stockholm University (SU) is currently in the process of developing a strategy to support and encourage its continued internationalization. One way to quantify the internationalization of a group is by analyzing the number of publications with international co-authors that are produced by that group. An internal SU report, released in the spring of 2018, presents an analysis of international publications for various departments and research centers using data from 2012-2017.

With 96% of all its publications having at least one co-author with an affiliation outside of Sweden, the Oskar Klein Center has the highest percentage of international publications of all the SU departments and centers considered in the analysis. The most frequent affiliation of international co-authors for OKC publications is the United States. Three groups at SU have greater than 90% of their publications with international co-authors : the OKC, the Astronomy department, and NORDITA.

The OKC is also one of the top producers of “Highly Cited” papers (papers that are within the top 1% in number of citations for a given subfield in a given year as determined by the Web of Science) at SU. The only group with a higher ratio of “Highly Cited” papers compared to their total output is the Stockholm Environmental Institute.

Interview with Ankit Beniwal

Hej! My name is Ankit Beniwal, and I’m from Adelaide, Australia. I’m a short-term (6 months)
postdoc at the Oskar Klein Centre, Stockholm, Sweden. Before coming to OKC, I finished my PhD in theoretical particle physics at the University of Adelaide. I also did my first class Honours in theoretical and experimental particle physics at the same university.

Unlike many physicists who became passionate about physics at a young age, I wasn’t aware of physics in general when I was young. This completely changed when I took physics in year 11 and 12. I had an excellent (female) physics teacher named Ms Lindy Bartlett. She loves physics and encourages students by saying “Physics is gold!” By the time I finished high school, my love for physics had grown so much that I decided to undertake an undergraduate degree in physics at the University of Adelaide, and later went on to do a PhD.

I love being a scientist. Not only do we get paid to study the world around us, we are also trying to answer some of the fundamental questions in physics. However, in the particle physics community, there is a strong push for more publications; it’s ultimately not a bad thing, but it is hard for a postdoc applicant as he/she is partially judged on this basis. I solely believe in the quality of work rather than quantity. With that said, I also need to improve on the latter aspect of being a scientist.

What is your field of research?
My research interests include dark matter (DM) phenomenology, astroparticle and Higgs physics. In the past, I’ve studied the phenomenology of Higgs portal DM models where DM interacts with the Standard Model (SM) particles via the Higgs boson. This leads to a rich DM phenomenology at colliders, indirect and direct DM detection experiments. In addition, these models can also help in explaining the observed matter-antimatter asymmetry in our Universe.

At the OKC, I’m working under Prof. Joakim Edsjö on secluded DM models. In these models, the DM particle annihilates into metastable mediators which subsequently decays into SM particles. By having a weak coupling to SM particles, the models are difficult to detect directly. On the other hand, they offer much better indirect detection probes via gamma rays, charged particles and/or neutrinos. The main motivation behind these models is that a neutrino signal from DM annihilation in the Sun would generally be enhanced relative to the standard scenario, i.e., one where DM annihilates directly into SM particles via a short-lived mediator.

What are your research plans for your time in Sweden?
Although 6-months isn’t a long time, my plan is to understand the secluded DM models and their phenomenology at neutrino telescopes in more detail. In particular, I’d like to write up a paper with Joakim and others on this work.

Which of your skills are you most proud of? What new skills would you like to learn in the next year?
I’m particularly proud of the skills that I’ve acquired over the course of my PhD. These include computing skills (being proficient in multiple computing languages and high-performance computing), teaching, tutoring, mentoring and being able to learn new concepts in a short period of time.

There’s always room for more improvement. For instance, I’d like to get better at time management, writing more research papers, and initiating new collaborations.

What advances or new results are you excited about or looking forward to?
In the last few years, tremendous progress has been made in all areas of physics ranging from neutrinos, DM and gravitational waves (GW). Thus, it is an opportune moment to be involved in these areas.

Some future prospects that I’m looking forward to are as follows.
1. Many experiments are underway to better understand the neutrino properties (e.g., neutrino oscillations, CP-violating phase, absolute neutrino masses etc). This is exciting news!
2. Future direct DM search experiments will tell us if the particle description of DM is consistent or not. These experiments are very close to reaching the neutrino floor where they’ll also become sensitive to neutrinos. In addition, many planned experiments will try to either confirm or refute the long-standing annual modulation signal seen by the DAMA experiment.
3. Multiple GW signals have been detected. We are now entering a new era of GW astronomy. Future space-based GW experiments such as LISA will be able to observe GW signals from the electroweak phase transition, a simple mechanism that explains the matter-antimatter asymmetry in our Universe.

If I offered you unlimited funding right now, to be spent on something scientifically relevant, what would you use it for?
If I had unlimited funding, I’d spend it on new computing resources, in particular, on supercomputers. In recent years, it has become increasingly difficult to find new resources for performing multi-dimensional parameter space scans.

With no evidence of a DM signal, we must combine all available data from various DM searches and make statistical inferences on as many DM models as possible. This is the primary goal of the Global And Modular Beyond-the-standard-model Inference Tool (GAMBIT). To achieve these goals, we need a large number of computing resources.

What’s your favorite food? Why?
Being originally from India, I’m obviously biased towards Indian food. In particular, I love butter chicken and plain naan. It’s tasty and mouth-watering!

Why did you choose the OKC?
I enjoy working at the OKC. It has a good mixture of cosmologists, astronomers, experimentalists and particle physicists. Its status at the international level is outstanding. The research staff at OKC are also world-renowned scientists in their field of research.

How do you relax after a hard day of work?
From time to time, I try to explore the city. The weather is getting better day-by-day, so I’ll see more of what the city has to offer.

I’m also trying to learn swimming during my stay in Stockholm. This is an activity that has been on my bucket list for a long time. My goal is to become a proficient swimmer, so I can enjoy the beautiful beaches back home.

What do you hope to see accomplished scientifically in the next 50 years?
In the next 50 years, I hope that we can solve some of the biggest problems that are currently faced by mankind, e.g., global warming, fossil fuels, pollution, poverty, etc. Many people are trying to tackle these issues but more support is required from the government and public to solve them.

Scientifically, the most optimistic scenario in my case would be one where we have discovered DM non-gravitationally. Once we have a DM signal, we can hope to understand its properties, e.g., mass, spin, coupling to SM particles, etc.

Ankit is a postdoc in the Stockholm University Physics Department who joined the OKC in the Spring of 2018.
Thanks Ankit! Try the new Saravana Bhavan in Kista for great South Indian food.

A cold dawn for the first stars

This morning, as I was walking from Tekniska Högskolan metro station to AlbaNova through the Siberian cold which has hit Stockholm, I was thinking about even colder temperatures than the -15 C that I felt on my skin. Did you know that the temperature of Universe was only 3 Kelvin (-270 C) when the first stars were born?! At least that’s what the authors of an article published in ’Nature’ this week claim to have measured and until proven differently they might well be right… Having worked on the Cosmic Dawn (a popular name for the era during which the first stars were born) for many years I was baffled by this news because this temperature is much lower than we thought it was. How could it have been so much colder?

The results in the article were obtained by an experiment called EDGES (Experiment to Detect the Global Epoch of Reionization Signature). The American authors of the paper have over the past decade been trying to measure a signal from hydrogen atoms in the young Universe. When the first stars formed they caused the neutral hydrogen in the Universe to produce this signal. Its strength depends on the temperature of the hydrogen gas and it was produced with a wavelength of 21 cm. However, by the time it reaches us the wavelength has increased to between 1.5 and 5 m (corresponding to frequencies 200 – 60 MHz) since the radiation travelled to us through an expanding Universe. The particular measurement reported in the paper is at a frequency of 78 Mhz, which corresponds to a wavelength of almost 4 m. This means the signal was produced 13.6 billion years ago when the Universe was only 200 million years old!

What makes EDGES special is that it consists of a single antenna which, at least in its original form, fitted in a suitcase. That’s useful because the team of people who worked on the experiment (led by Judd Bowman) is based in Arizona but do their measurements in the desert of Western Australia.Why choose such a remote location? Obviously, not many people live there so there are almost no radio signals made by humans. At other locations, human radio signals can cause big problems; Just think of the FM radio band which runs from 87 to 108 MHz, right in the middle of interesting frequencies for seeing the effect of the first stars. Not picking up these transmissions helps a lot. However, this still doesn’t make it easy to see the far away 21-cm signal from the time of the first stars. The entire sky is filled with bright radio radiation from relativistic electrons and hot plasmas in our own galaxy which is 100,000s, if not million times, stronger than the signal EDGES is trying to see. No Australian desert can help you to avoid the Milky Way!

But similar to people being able to see a small tree on the slopes of a huge mountain, the radio emission from the early Universe can be separated from that of the Milky Way. When you look at different frequencies, the Milky Way radiation shows the same distribution; in other words, it varies in a very regular way with frequency. The 21-cm signal on the other hand is expected to vary significantly from one frequency to another. This is the key to separating them. But since the contrast is quite big, you have to be very sure that your telescope doesn’t artificially produce small variations due to the electronics, interference signals, the Earth’s ionosphere and so on. To get an accuracy of 1 in a 100,000 you need to put in a lot of hard work! In the case of EDGES, they worked their socks off for no less than 10 years out by regularly testing the performance of the antenna in the desert and by frequently testing all the electronic components in the lab.

So after all this hard work, they went out and did the measurements and found a signal! However, something was wrong since the signal was about twice as strong as anything they had expected. Clearly there must be a problem with the antenna. So they went back, changed some things and tried again, only to obtain more or less the same result. They moved the antenna to a different location and measured again; still no change! So after trying many different things and not being able to get anything except a strong signal and not being able to explain it with anything else they decided that it might be real and coming from the time of the first stars.

However, if it is real, it would mean that the temperature of the Universe at the time this signal was produced was at most 3 Kelvin. However, the absolute lowest temperature expected is about 7 Kelvin. Now the difference might not seem much but the problem is that we know of many processes which can increase the temperature but not really of any which can lower it. So, there is a really a problem here, the first stars woke up to a very, very chilly Cosmic Dawn.

However, if we don’t know of any processes which can reduce the temperature then we could try to come up with some. This is exactly what the author of a second paper in Nature did. What is needed is something to cool the early Universe, what could it be? Modern cosmology relies on most of the matter in the Universe to be dark matter, unobservable except through its gravitational effects on normal matter. Now, the type of dark matter which works best in detailed models is known as “Cold Dark Matter” because it’s, well, cold. What if this dark matter would actually interact a little with normal matter? Then the normal matter would lose some of its energy, its heat, to the cold dark matter. It does not need to interact much, just enough to make the temperature drop from 7 to 3 Kelvin. Rennan Barkana, the author of this second paper, worked out the numbers and found this could work. But only if the cold dark matter particles are not too heavy and interact sufficiently with normal matter.

Is this a reasonable explanation? It’s not a type of dark matter particle which is often considered but since we do not know what dark matter is, it’s hard to rule out. Still, it’s an odd result and perhaps the simplest solution is just that despite their best efforts, the EDGES team missed something and the signal is not at all from the time of the first stars but caused by a subtle effect in their equipment. That’s why everyone, including the EDGES team, is hoping another team with a similar radio antenna will confirm their result. Luckily there are several other experiments active which could try this, SARAS, LEDA and possibly NenuFAR, a spin-off from the LOFAR project. However, all of these experiments also need to do the hard work of understanding the tiniest details of their antennae and electronics and how these interact with the radio signals arriving from space and Earth… It may take a while before they are ready.

In the meanwhile I expect there will be a flurry of papers as a result of EDGES result and the proposed cooling by cold dark matter particles. Perhaps the necessary properties for the dark matter particles are already ruled out by some other effects such particles would cause either in a laboratory here on Earth or out in the Universe? Perhaps there are other ‘exotic’ explanations for the detection of such a strong 21-cm signal? We trying to understand a period in the history of the Universe of which we know very little so there is a quite some room for new ideas. There is a lot to look forward to!

Interview with Giovanni Camelio

I was born in Milan, Italy. I took my bachelor and master in Physics at the University of Milan “Degli Studi,” and I took my PhD in Physics in the University of Rome “Sapienza.” I have always wanted to be a scientist; I remember that I chose to become a physicist in high school when I was studying optic waves and in particular the redshift effect. What really impressed me was how the description of that phenomenon was easy and straightforward after putting it in a mathematical form.

The thing I like the most about being a scientist is the possibility to devote my time to study and understand reality. What I really dislike is the rush to publish. This attitude causes the problems that afflict modern science and deprives the work of any pleasure and of its real goal, namely the effort to deepen our understanding of nature.

What is your field of research and/or what project are you involved in at the OKC?
My research field is the study of hot neutron stars. My project here is to implement a code that describes the neutrino diffusion in a rotating neutron star, fully accounting for general relativity effects. We will use this code to study super-massive neutron stars that originate from a binary neutron star merger.

Which of your skills are you most proud of? What new skills would you like to learn in the next year?
I think that my strength as a scientist is my multidisciplinarity (I have worked on different topics in my career) and my intuition. In the next year I would like to have more time to study and improve my mathematical skills.

If you had unlimited funding, to be spent on something scientifically relevant, what would you use it for?
If I had unlimited funding, that in my case specifically means unlimited time, I would work on the problem of determining the oscillation modes of a rotating neutron star. These permit us to study the stability of neutron stars and their gravitational wave emission.

What’s your favorite food? Why?
My favorite food is beef liver with onions. I don’t know why.

How do you relax after a hard day of work?
After work I enjoy the company of friends, and alternatively reading or drawing.

What do you hope to see accomplished scientifically in the next 50 years?
Realistically, in the next 50 years I would like to see consensus on the origin of dark matter. Moreover, it would be great to have a quality evaluation paradigm for scientists that wouldn’t actually harm science.

Giovanni is a postdoc in the SU Astronomy Department who joined the OKC in the fall of 2017.
Thanks Giovanni!

Interview with Rakhee Kushwah

Rakhee Kushwah

I am a talkative girl from India. I finished my Ph.D from Indian Space Research organisation, Bangalore. It is situated in South of India. I love to travel. My love for analysing human psychology means that I also like to meet new people.
I was always fascinated by science because it has a lot of practical applications in day to day life. Thus I chose to do science. I wanted to be in the field of research because I get bored if things are the same every day. Research is about finding something new. I like drawing conclusions based on practical experiences and so I like working with instruments in the lab.
Dislikes do always follow likes because life is all about pairs of opposites. I come from a country with a large population, where researchers often don’t get their due respect because they are just one among the many people living there! This is disheartening at times. I also do not like the uncertainty tagged with this career. Hopefully, it all works out fine at the end!

What is your field of research and/or what project are you involved in at the OKC?
I am working in the Particle and Astroparticle Physics group at KTH. My research is related to space instrumentation. I am currently involved in the activities of developing a new instrument to measure polarization from Gamma Ray Bursts (GRBs).

What are your research plans for your time in Sweden?
I am learning more about detectors which I have not worked with in my previous years of research. So it is very interesting. I am attending some lectures on spacecraft engineering from which I hope to learn more about satellite technologies.

Which of your skills are you most proud of? What new skills would you like to learn in the next year?
I am good with my hands when it comes to being in the laboratory. I am more organized and tidier than most researchers when planning and executing experiments.
I am being exposed to new software tools that are needed to understand different aspects of instrument building. I would love to learn these tools and use them for my experiments.

What advances or new results are you excited about or looking forward to?
I really got excited when the latest GW detection was declared and the associated analysis was presented by many scientists. I would love to learn the details of how such a sensitive detection was made. Hopefully our GRB polarimeter can contribute more to this field.

What is the biggest obstacle that is slowing down your research field right now?
I am not good at coding so it is taking time for me to develop instrument simulation skills. As I am new to Stockholm, the weather and dark evenings make me feel tired sooner in the day than in India.

If I offered you unlimited funding right now, to be spent on something scientifically relevant, what would you use it for?
I would love to make a small X-ray detector (with integrated read out) for undergraduate students to play with. The goal would be to make it at a minimal cost so that all the universities can have this in their labs. I think my knowledge of detectors will help me build such a unit.

What’s your favorite food? Why?
I love Indian food especially because it is prepared with a combination of a variety of spices. After coming here, I have learned to prepare pasta with Indian spices and I love it.

Why did you choose the OKC?
I wanted to be in instrumentation and found this position when I was looking for such an opportunity. The people in the group here have expertise in detection of X-ray polarisation from celestial bodies. I worked on a similar topic during my Ph.D so it was nice to join OKC for continuing in the field of my interest.

How do you relax after a hard day of work?
Watching a movie or preparing a different/new food while listening to my favourite music is what relaxes me most.

What do you hope to see accomplished scientifically in the next 50 years?
I would be happy to see a fully functional fusion reactor on Earth, providing abundant energy with a small input. And it would be nice to have tourist visits to our moon.

Rakhee is a postdoc in the SPHiNX group at KTH who joined the OKC in the fall of 2017.
Thanks Rakhee!

Praise for the CASPEN program from first three OKC participants

Stockholm at sunset with the city hall tower prominently in view
Credits: Björn Olin/Folio/imagebank.sweden.se
The first three OKC:ers to use the CASPEN (Cosmology and Astroparticle Student and Postdoc Exchange Network) program have reported back that their visits were a success! CASPEN provides travel funds for collaborative visits to institutions in the network.

Tanja Petrushevska (former OKC PhD student, now a postdoc at the University of Nova Gorica, Slovenia) used CASPEN to visit Stanford, California and collaborate on a project with former OKC member Manuel Meyer. They worked to estimate the explosion time from core-collapse supernovae in order to search Fermi gamma-ray data for evidence of axion-like particles from these events. Tanja says that, “all in all, it was a very nice and beneficial experience.”

Axel Widmark (OKC PhD student) travelled to the Flatiron Institute in New York, New York to work with David Hogg for two weeks. They made a full joint fit of a subset of Gaia data with a population model that includes binary (and higher multiple) stars under a Bayesian Hierarchical Model framework. Their model can now be used to classify Gaia objects as single stars, binaries, trinaries, and potentially higher multiples. Axel enjoyed experiencing the atmosphere in another research institute and hopes to implement some of his favorite things here at the OKC. Axel says, “overall, my visit to the Flatiron Institute has been very productive and, in my mind, a great success.”

Suhail Dhawan (OKC postdoc) worked on a project which aims to use physical properties of Type Ia supernovae to improve the general classification of supernovae. His collaborators were Robert Schuhmann, Hiranya Peiris and Jason Mcewen. Suhail says, “the program was a great collaborative experience. There was a lot of nice administrative support for the program, so the logistics were made easy (e.g. the support letter for my visa application).”

For OKC:ers who are interested in starting a project with someone in the CASPEN network : first identify a potential collaborator from one of the host institutions and agree on a specific work plan and potential dates for a visit. Then propose a visit using this form.

(Note : there may be visa complications and additional fees if you plan to travel to the USA.)

2017 Year in Review by Jesper Sollerman

The year 2017 is approaching the end… I just resigned from the OKC Steering Group – where I have basically served from the very beginning in 2008. I also handed over the Extreme Object working group which I have stewarded the past six years. Evan O’Connor and Stephan Rosswog will continue that job.

At the end of the year, I wanted to look back at what has been an interesting year for transient astronomy. Starting in December last year, we published an article on a new very-bright transient which we interpreted as a tidal disruption event. The paper was led by my former PhD-student Giorgos Leloudas, previously a postdoc at OKC and now at DARK. The paper was published in the very first issue of Nature Astronomy. Here is a cool animation of an artist impression of the star being stripped apart by the supermassive black hole. There was media attention for this both in Sweden (SU press release) and in the rest of the world (ESO press release).

2017 was supposed to be a calm year, since we closed down our old supernova telescope to upgrade to the new ZTF camera. I started in January observing for a month in sunny Chile for PESSTO together with Francesco Taddia. Swedish TV made a piece on this. In February, Cristina Barbarino and Anders Nyholm took over in Chile and Francesco and I did the data reduction from colder Stockholm.

In February we published the fastest catch ever of a young supernova. The paper on iPTF13dqy was published in Nature Physics, and led by Ofer Yaron at Weizmann. It includes a spectrum taken only 6 hours after the estimated time of explosion for this Type II supernova.

In February we also learned that we were approved a new research environment from VR: the GREAT centre for Gravitational Radiation and Electromagnetic Astrophysical Transients, led by Ariel Goobar, Hiranya Peiris and Stephan Rosswog. This gave us a head start in this field.

A more unexpected discovery was the multiple lensed supernova iPTF16geu. Ariel Goobar and Rahman Amanullah led this paper published in Science.

That was just in time to mention this discovery at the KAW Wallenberg 100 year anniversary in Lund, which I helped organize. I gave a talk about supernova hunting and the coming ZTF. In connection to that meeting, KAW had released their movie – an astronomical journey into space, that covers our research. Check it out! My 10 min presentation at the KAW 100 year symposium was filmed by UR and can be seen here. You can also check out the presentations from Jan Conrad, Matthew Hayes, and Josefin Larsson.

In June we helped organize a large workshop at Albanova on the Physics of extreme gravity stars, and then – after the summer break – on August 17 came the first LIGO detection of a neutron star merger. We worked hectically during the fall until October 16 when we were allowed to release the results. Within ePESSTO we published apaper in Nature, led by Stephen Smartt in Queens – and within the GROWTH collaboration we participated in a paper in Science, led by Mansi Kasliwal at Caltech. More locally, we also published a paper on macronova nucleosynthesis, led by Stephan Rosswog. We invited all of Albanova for a short colloqium (Goobar, Sollerman, Finley, Rosswog – film here) and some drinks.

In my more local universe, two of my PhDs students published their first first-author papers: Anders Nyholm on the bumpy Type II supernova iPTF13z, and Emir Karamehmetoglu on a very long lived Type Ibn – OGLE73. Christoffer Fremling defended his PhD-thesis and went off for a postdoc at CalTech.

In November we published a paper on the very long-lived iPTF14hls, led by Iair Arcavi in Santa Barbara and presented an unusual re-brightening supernova, nicknamed the zombie star in some of the following media coverage.

Then, finally, on November 14 2017 we could announce first light for our new ZTF camera. This is a large step forward for transient surveys. This is indeed what we have been working towards for several years now, with help from KAW funding. This interesting animation done by Rahul Biswas shows how the ZTF will work through the night sky and detect numerous active supernovae each night.

So, in the end, I am looking forward to 2018 when ZTF will become fully operational – while at the same time concluding that in terms of transient science 2017 was very good indeed. 205 Swedish birds in 2017 was also not bad.

Cheers, Jesper

Interview with Erin O’Sullivan

Dr. Erin O’Sullivan

I am from Toronto, Canada. I did my undergraduate degree at the University of Guelph and my PhD at Queen’s University. I chose to become a physicist because I liked how collaborative the work was, even at the high school level. I was fortunate to have a strong science department in high school, including a physics teacher that had a talent for piquing our interest in current physics events. During one class, he discussed the Sudbury Neutrino Observatory and I was really interested in the idea of studying this elusive particle deep in a Northern Ontario mine. I carried this idea with me into my undergraduate career and it set the course of the eventual research I would choose to do. I still like how collaborative the work is and that I get to learn from and share ideas with my collaborators. I also like the freedom that comes with being a researcher, especially that I get to pursue the ideas that are interesting to me. The biggest challenge of being a scientist is having to get through the monotony (getting code to run, making small adjustments to figures, etc) in order to extract the science.

What is your field of research and/or what project are you involved in at the OKC?
I am a neutrino astrophysicist. I am a collaborator on IceCube, as well as the proposed Hyper-Kamiokande experiment.

What are your research plans for your time in Sweden?
I am a software convener for Hyper-Kamiokande, so I spend some time developing the Monte Carlo software package, WCSim. I also have plans to get involved with multi messenger astronomy involving IceCube neutrinos. We would like to expand the number of public alert channels that we send out from IceCube, as well as get involved with the search for coincidences between IceCube’s neutrinos and gravitational events from LIGO-Virgo.

Which of your skills are you most proud of? What new skills would you like to learn in the next year?
I have been fortunate to work on a few different neutrino experiments throughout my career. I did my PhD focusing on solar neutrinos in SNO+, which are low energy (MeV-scale). Before coming to the OKC I was a postdoc with the Super-Kamiokande experiment where I looked for astrophysical neutrinos that were in the mid-energy range (GeV-scale). Now at the OKC, I am working with IceCube where neutrinos with energies above PeV can be measured. So, I guess the skill that I am most proud of is my general knowledge of the whole picture of neutrino detection. I am still learning a lot about detecting neutrinos at the highest energies, so I would like to continue to learn more about that in the next year.

What advances or new results are you excited about or looking forward to?
I would love to measure a galactic supernova using neutrinos. The last time there was a nearby supernova was in 1987 and there were only a few neutrino experiments online and they weren’t as powerful as today’s detectors. If there was a supernova today, detectors would measure a detailed picture of the rate and energy evolution of neutrinos coming from a supernova. This would allow us to see inside a supernova and learn about the progenitor properties, and it would allow us to study how neutrinos behave in dense matter environments. Too bad there are only a few galactic supernovae per century!

Why did you choose the OKC?
I was working in the US when we visited Stockholm for vacation, and I was surprised that there was a similarity between Sweden and Canada. I knew that there was a good IceCube group here, and just after our vacation a faculty position became available in the astronomy department. My husband is an astrophysicist and and getting jobs together can be a challenge, so this seemed like it could be a good opportunity, and indeed it worked out for us.

How do you relax after a hard day of work?
On the weekends I like to explore Stockholm and maybe cook something nice for dinner. Weeknights are pretty busy, but if there’s time I like to catch up on my favorite tv shows (we just finished Stranger Things 2) or play board games (currently Pandemic Legacy 2).

What do you hope to see accomplished scientifically in the next 50 years?
Neutrino astronomy is interesting because neutrinos allow us to see into the interior of violent astrophysical events that are difficult to probe with electromagnetic messengers. I would love for neutrino astronomy to become similar to how we do electromagnetic astronomy now where we could see many neutrinos coming from the same astrophysical object. This would allow us to really start using neutrinos to probe the behavior of astrophysical phenomena. I would also like to see neutrinos enter into the multi-messenger picture where they could be detected in coincidence with gravitational waves or EM detections (or both!). I’m actually hopeful that doing multi messenger physics with neutrinos won’t be 50 years away and could soon be a reality. We have had some recent hints that this could already be happening!

Erin is a postdoc in the SU IceCube group. She joined the OKC in July 2017.
Thanks Erin, and while you’re exploring Stockholm check out Millesgården (one of my favorite places)!

Interview with Fei Xie

An image of Fei Xie with a yellow sweater and black glasses.
Fei Xie
My name is Fei Xie. I am from China and I got my PhD at the Chinese Academy of Science in Beijing.

What is your field of research and/or what project are you involved in at the OKC?
I mainly work on instrument simulations, like detector optimization, on-orbit background estimation, instrument performance simulation, etc. I am working in the SPHiNX group, a satellite for GRB polarization measurements. Now my interest is in polarization measurements at high energies. It’s an area we don’t know much about at this moment. It is challenging but interesting.

Which of your skills are you most proud of? What new skills would you like to learn in the next year?
My coding and Geant4 are quite good. I would like to learn more about the electronics for a better understanding of the instrument in the future.

What is the biggest obstacle that is slowing down your research field right now?
My research relies on the project, so it is not easy to find a long term and stable position.

What’s your favorite food? Why?
I like spicy food as I was born in a family that are good at cooking spicy foods.

How do you relax after a hard day of work?
Movies and reading are my favorite ways to relax.

Fei is a postdoc in the SPHiNX group at KTH who joined the OKC in May 2016.
Thanks Fei, maybe we should all eat more spicy food to keep warm this winter!

Cosmology, astrophysics, astroparticle physics