Neutron stars are stellar remnants that form in the supernova explosions of massive stars. Each of these objects contains a mass comparable to that of our Sun within a radius of about ten kilometres and exhibits high densities, fast rotation and large magnetic fields. Such conditions cannot be recreated on Earth, making neutron stars amazing cosmic laboratories to study matter under extreme conditions.
While I am interested in many aspects of neutron stars, my work can be broadly separated into two parts: The first focuses on the interface between astrophysics and condensed-matter physics. In contrast, my second research area concerns the population synthesis of isolated neutron stars. More details can be found here:
One of the most exciting aspects of neutron stars is that quantum mechanics strongly influences their interiors. To better understand their behaviour, I study so-called superfluid and superconducting components.
Like the Earth, neutron stars are composed of distinct layers. They have a solid crust and a fluid interior that contain neutrons, protons, electrons and possibly exotic particles. In terms of their high densities, neutron stars are very cold, giving the protons and neutrons the special ability to flow without friction: The charged protons form a superconductor, whereas the neutrons are referred to as a superfluid.
These two are exotic versions of quantum states observed in experiments on Earth. From their laboratory counterparts, we know that superconductors and superfluids create vortices that can be envisaged as tiny, rapidly rotating tornadoes. These small structures interact with their surroundings, affecting the large-scale dynamics of the star.
I research different approaches to include these small-scale effects into theoretical models of neutron stars. Using techniques that are well-known from standard magnetohydrodynamics, I have for example studied the evolution of the magnetic field in the interior of superconducting neutron stars. I have also presented novel ways that low-temperature laboratory experiments could be used to make progress in understanding neutron star astrophysics.
Furthermore, I have been analysing how coupling processes in the interior affect the star's response after a so-called glitch. These sudden spin-ups interrupt the regular spin-down of pulsars and are thought to be a macroscopic manifestation of superfluidity. By connecting the physics on different length scales, I developed predictive models of the glitch rise, showing that assumptions about the microphysics of vortices crucially affect the star's rotational behaviour. Comparing my predictions to the first pulse-to-pulse glitch observations, reported by Palfreyman et al. (2018), I derived constraints on the strength of the frictional mechanisms in the star's interior. An improved analysis of the data, revealing novel details about the internal components of the star, was published in Nature Astronomy.
Using techniques well-known from the study of laboratory superconductors, so-called Ginzburg-Landau models, I have also been exploring the microscale characteristics of the superconducting protons in the neutron star core. Their properties are poorly understood but could significantly impact the stellar magnetism and are thus crucial to understanding the macroscopic magnetic field properties of compact objects. By adapting the Ginzburg-Landau description to the neutron-star interior and connecting it with realistic superfluid parameters and equations of state, my collaborators and I have constructed superconducting phase diagrams and found that the outer core of neutron stars exhibits so-called type-1.5 superconductivity, rather than type-II superconductivity as generally assumed.
In addition to focusing on the neutron-star interior, I also investigate the global population of isolated neutron stars in our Milky Way.
Although about a billion neutron stars are expected to exist in our own galaxy, observational constraints limit us to only detecting a small fraction of them; we only know a few thousand of these compact objects to date. To overcome this gap, so-called population-synthesis approaches are used to model the full population theoretically. Based on our current knowledge of input physics, these approaches focus on simulating synthetic neutron-star populations. Once the simulated samples are created, we compare them to real observations to identify discrepancies and subsequently adjust our theoretical models. This kind of global study, thus, allows us to better constrain the input physics and learn more about neutron stars on an individual level.
I am personally interested in using new computational techniques, specifically machine learning, to perform the comparison between the synthetic samples and the observed characteristics. Machine learning is an artificial intelligence implementation that allows systems to automatically improve and learn from previous experiences without being explicitly told how to do so. These techniques have seen a lot of interest in the astronomy and astrophysics communities, where it is often no longer possible to evaluate large amounts of data by hand.
In our first paper, the MAGNESIA population synthesis team explored the possibility of inferring the properties of the Galactic neutron-star population through machine learning. In particular, we focused on their dynamical characteristics and showed that a convolutional neural network is able to estimate with high accuracy the parameters, which control the current positions of a mock population of pulsars. Our analysis highlights the crucial need for increasing the sample of known pulsars and accurately classifying them, which is one of the main science cases for upcoming radio telescopes, such as the Square Kilometer Array.
A full list of papers can be found on the online databases
arXiv or ORCID.
A list of selected publications is given below.
M. Ronchi, N. Rea, V. Graber, and N. Hurley-Walker, Long-period pulsars as evidence of supernova fallback accretion, Astrop. J., 934, 184 (2022)
T. S. Wood and V. Graber, Superconducting phases in neutron star cores, Univ. 8, 228 (2022)
D. Viganò, A. Garcia-Garcia, J. A. Pons, C. Dehman, and V. Graber, Magneto-thermal evolution of neutron stars with coupled Ohmic, Hall and ambipolar effects via accurate finite-volume simulations, Comp. Phys. Comm., 265, 108001 (2021)
M. Ronchi, V. Graber, A. Garcia-Garcia, J. A. Pons, and N. Rea, Analyzing the Galactic pulsar population with machine learning, Astrophys. J., 916, 100 (2021)
G. Ashton, P. D. Lasky, V. Graber, and J. Palfreyman, Rotational evolution of the Vela pulsar during the 2016 glitch, Nature Astron., 3, 1143 (2019)
V. Graber, A. Cumming, and N. Andersson, Glitch rises as a test for rapid superfluid coupling in neutron stars, Astrophys. J., 865, 23 (2018)
V. Graber, Fluxtube dynamics in neutron star cores, Astron. Nachr., 338, 1090 (2017)
V. Graber, N. Andersson, and M. Hogg, Neutron stars in the laboratory, Intern. J. Mod. Phys. D, 26, 1730015 (2017)
V. Graber, N. Andersson, K. Glampedakis, and S. K. Lander, Magnetic field evolution in superconducting neutron stars, Mon. Not. Roy. Astron. Soc., 453, 671 (2015)
A. Markowsky, A. Zare, V. Graber, and T. Dahm, Optimal thickness of rectangular superconducting microtraps for cold atomic gases, Phys. Rev. A, 86, 023412 (2012)
I taught the undergraduate module PHYS 434 Optics during the 2019 Winter term at McGill University. Below you can find general information on the course as well as the lecture notes, I created.
Classes started on Monday, January 7 and took place every Monday and Wednesday from 2:35 pm to 3:55 pm in the Rutherford Physics Building Room RPHYS 114. General information about the course, teaching assistants and an overview of the course content, prerequisites, evaluation and reading materials can be found in the syllabus. Individual lecture topics, assigned reading materials and important dates are given in the course calendar. Note that both were subject to change throughout the term.
PART I – Electromagnetism and Light Propagation
PART II – Geometric Optics
PART III – Superposition, Polarisation and Interference
PART IV – Diffraction, Fourier Optics and Modern Optics
Since 2021, I have been teaching part of the course Neutron Stars, Black Holes and Gravitational Waves, one of the modules of the Postgraduate Program in High Energy Physics, Astrophysics & Cosmology at the Universitat Autònoma de Barcelona. Some general information on the degree is available here. Below you can find a few more details on the course as well as the lecture notes, I created.
The NSs, BHs and GWs course is currently coordinated by Dr Daniele Viganò and taught by several researchers from the Institute of Space Sciences. For the academic year 2022-2023, classes took place from February 13 to March 16 from 12:00 pm to 2:00 pm and were taught face to face. The module introduces a range of topics related to compact objects, and I covered the subjects of black hole theory and gravitational wave theory.
PART I – Towards General Relativity
PART II – Einstein's Theory of Gravity
PART III – Black Holes
PART IV – Gravitational Waves
The Centre for Research in Astrophysics of Quebec (CRAQ) hosted its annual summer school in June 2019 in Montreal. The topic was Stellar Astrophysics, and I covered Neutron Stars during the Stellar Death section.
General information about the summer school can be found here. My presentation slides and a Jupyter notebook to calculate mass-radius relations for two simple neutron-star model equations of state can be downloaded below.
In July 2021, the Institute of Space Sciences hosted its 4th annual summer school. The school was dedicated to Artificial Intelligence for Astronomy, and I joined as one of the host lecturers and co-organisers.
General information about the summer school, which took place remotely from July 12 to July 16, can be found here. Together with my fellow host lecturers Helena Domínguez-Sánchez and Alessandro Patruno, as well as several external lecturers, we covered a wide range of topics related to Machine Learning in general as well as Deep Learning and its numerous applications.
In particular, I gave an introductory theory lecture on the topic of Deep Learning and Neural Networks and ran a hands-on session, where I introduced the scikit-learn Python library for machine learning and looked at a few examples of clustering algorithms (specifically k-means and Gaussian Mixture Models). Lecture slides and a notebook for the coding session can be accessed below.
I love talking about science and sharing what I know about astronomy and astrophysics with an interested audience. In April 2022, I was interviewed for the German language Astronomy and Space Science podcast raumzeit. Our conversation about the fascinating topic of neutron stars, which was published in October 2022, can be found here. Almost two hours long, but there is still a lot more to learn.
While working as a postdoctoral fellow at the McGill Space Institute in Montreal, I was part of the AstroMcGill outreach team. We regularly hosted Astronomy on Tap, a worldwide initiative combining your two favourite things: astronomy and beer. In September 2017, I had fun talking about 'Neutron stars - a space oddity' .
In December 2018, I gave the monthly public lecture jointly organised by AstroMcGill and PhysicsMatters, the McGill Physics Outreach group. My Public AstroPhysicsNight talk was titled 'Neutron Stars: Extraordinary Cosmic Laboratories for Physicists' and provided a non-specialist introduction to my research (no equations, I promise). You can watch a recording of the lecture here.
In July 2020, I contributed to the Faszination Astronomie Online initiative organised by the Haus der Astronomie. The Haus der Astronomie, which translates to 'House of Astronomy', is a Centre for Astronomy Education and Outreach in Heidelberg, Germany, that runs events for the general public, as well as workshops for students, teachers, and science communicators. In response to the Covid-19 pandemic, the centre moved its German public talk series online and has been regularly streaming about fascinating astronomy topics on its own YouTube-channel. My thirty minute-long talk on pulsar glitches, titled 'Wenn Neutronensterne Schluckauf haben' , can be viewed here.
For NASA's Universe of Learning, I presented a general overview of magnetic fields in neutron stars, the strongest fields we know of, in October 2020. These science briefings are professional learning telecons for the informal science education community, run in partnership with NASA’s Museum & Informal Education Alliance. The monthly events highlight current NASA astrophysics explorations and discoveries from across NASA's astrophysics missions. More information about the event can be found here.
In addition to making scientific content more accessible to the general public, I have also participated at events that aim to make scientists themselves more relatable. One great way of achieving this is via storytelling, and in November 2018, I performed in front of an amazing audience at a Science Story Slam hosted by Broad Science and Confabulation.
In October 2018, I visited a secondary school in Tuttlingen, Germany, to tell the students about the wonders of the solar system and answer all their questions about what it means to be a scientist. We also played a game called 'Moon or Frying Pan' (an idea first spotted here). Try it out! It's actually a lot harder than it seems, but the kids loved it.
While working in Montreal, I volunteered for the Inquiry Institute. The project aims to connect physicists with Montreal school teachers to introduce educators to simple experiments that can be repeated in the classroom and specifically highlight the importance of critical and structural thinking. We, for example, worked on a demonstration that combines a hula-hoop with painted table tennis balls to illustrate the concepts of moon phases and solar eclipses; a set-up that has proven useful in explaining how to encourage pupils to ask critical questions.
In general, demonstrations are an excellent way to get people of all ages interested in science. I have been involved in constructing simple hands-on experiments for open days and public events that help to illustrate complex physical concepts. A few examples that have proven particularly successful over the years:
Combining an old trampoline, striped lycra fabric and marbles of different masses provides a fantastic set-up to visualise the concepts of gravity and space-time.
Wave propagation, reflection and interference can be playfully illustrated using a `jelly baby wave machine'. To build your own, you need jelly babies, duct tape and kebab sticks. Idea first spotted here.
Following the first direct detection of gravitational waves, I constructed a table-top Michelson interferometer (my all-time favourite physics experiment) to visualise the concepts employed by interferometric gravitational wave detectors. I followed instructions provided by the LIGO outreach team.
In February 2021, I joined the 100tífiques initiative, organised by the Fundació Catalana per a la Recerca i la Innovació (FCRI) and the Barcelona Institute for Science and Technology (BIST), in collaboration with the Department of Education of the Generalitat de Catalunya. The event connects female researchers from different disciplines with high schools in Catalonia to promote positive female role models and scientific career paths. For the event, held online in 2021, I spoke to 200 students at Col·legi Reial Monestir de Santa Isabel about how I became a scientist and what I work on. The slides for my talk are available here .
In August 2017, the AstroMcGill outreach team organised a public event for several thousand people at the McGill University campus to view a partial solar eclipse. Seeing so many people excited about astronomy was an amazing experience.
Besides participating in outreach events in person, I occasionally write for blogs that focus on different topics related to science and academia. You can, for example, read an interview with me about the importance of women in research here. If you are interested in learning more about my experience at the 69th Nobel Laureate Meeting dedicated to Physics, which I participated at in the summer of 2019, you can read a post I wrote here.
One aspect of the Lindau Meeting is a poster exhibition, where a pre-selected group of thirty young scientists has the chance to present their research to a general audience. I was one of the lucky participants and my poster won the shared first prize by public vote of all the meeting attendees.
In May 2022, I was awarded a three-year Juan de la Cierva Incorporación Postdoctoral Fellowship at the Institute of Space Sciences (ICE-CSIC) in Barcelona, Spain, to work on magnetic fields in superconducting neutron stars. Since 2020, I have also been working as a senior postdoctoral researcher with Nanda Rea and others at the ICE on the ERC project MAGNESIA. In particular, I am leading the pulsar population-synthesis working group.
The first 18 years of my life, I was lucky enough to be living by Lake Constance in the South of Germany. Having seen many beautiful places over the years, I can say that the Swabian Sea (as the lake is often nicknamed) is one of the most magnificent spots I have been to, and I try to go back as often as possible.