Frank Löffler

My main scientific interest has always been the behavior of matter in strongly curved environments, where both the dynamics of nuclear matter as well as general relativity are important. Most of my current research involves simulations of neutron stars, either as single neutron stars that may undergo instabilities, or together with a companion that is either a black hole or another neutron star. Research in this direction is particularly timely and urgent, as gravitational waves from neutron star binaries are expected to be observed in the near future, potentially making a direct connection between such systems and short gamma-ray bursts.

I lead research projects in numerical relativity, relativistic astrophysics and community software engineering. I also consult LSU faculty and staff in questions of technology and software development. This includes efficient use of present IT infrastructure, as well as strategic planning with future trends in mind. To follow and understand the ever-increasing influence of digital technologies on society, future job markets and teaching methods is going to be even more than in the past essential for success. Communicating this to the next generation is one of my motivations to offer lectures at elementary schools, to train and mentor robotics teams at local schools, and to teach regular courses at LSU.


Binary Neutron Star Mergers

Binary neutron star mergers are interesting for several reasons. One of them is the possible connection with some of the most energetic events in the universe, gamma-ray bursts. Currently, the leading hypothesis for the origin of the "short" kind of these gamma-ray bursts is the merger of a neutron star with either a black hole or another neutron star. This hypothesis could be confirmed soon through a concurrent detection of gravitational and electromagnetic waves from such a system. Another reason is that the gravitational wave signal from binary neutron stars contains more information than the recently detected signal from binary black holes. In particular, the merger and post-merger signal carries information about the equation of state of matter in the extreme environment of a neutron star. It is both amazing and intriguing at the same time, how little we still know about these systems. The list of still open questions includes:


Single Neutron Stars

While neutron star binary mergers are currently studied by many groups, research on single neutron stars is still far from being complete. Single and binary neutron star systems are related in two ways: first, a single, hot neutron star can be the intermediate remnant of a binary neutron star merger; and second, both can often be modeled using the same numerical techniques.

However, single neutron stars exhibit interesting behavior also on much longer timescales than binary mergers. Approximations not applicable to a merger are beneficial to study, e.g., instabilities that only occur after a remnant has cooled sufficiently. It is thus vital to find ways to overlap these two regimes, in order to model the same physical system for a much longer time. Preliminary results show that the necessary "relaxation time" depends strongly on the assumed equation of state, and might be longer than previously thought due to neutron star instabilities. These instabilities, if confirmed, would also emit gravitational waves, but whether they could be observed is still an open question.


Software Infrastructure Development

The currently best way to study neutron star dynamics in detail is through computer simulations. These often couple models for different physical processes, e.g., for a general relativistic spacetime, relativistic hydrodynamics, an approximation of the equation of state of nuclear matter, magnetic fields, and potentially neutrino and electromagnetic wave radiation. Combining these to an accurate mathematical description of the physical system that is both numerically stable and reasonably fast requires the development of both new analytic and numerical methods.

The need for dedicated software development in this area has been acknowledged by the USA National Science Foundation, e.g., in a series of my research grants to the software framework "Einstein Toolkit". The same software also has a substantial presence in Europe. One evidence for this is the representation of 9 European countries by researchers attending the Einstein Toolkit workshop in Trento, Italy, in 2016.


Collaborations

The required computational research component makes it necessary to invest in skills that on first glance don't appear to belong into physics, e.g., good software engineering practices and parallel programming. However, they are not only necessary for competitive progress in this area, they also provide a natural basis for collaborations between otherwise isolated fields. In my experience, such collaborations are often very valuable, and I will continue to engage them with interest.

Another crucial component of this type of research is the availability of supercomputer time allocations. Besides my large computing time awards at LSU in the US, I am familiar with allocations from the highly competitive call by PRACE (Partnership for Advanced Computing in Europe), often in collaboration with the gravity group at Parma University in Italy. A gallery with pictures of that work can be found here. In contrast to the majority of the research in this field, we not only publish a general outline of used algorithms, methods and work-flows alongside scientific results. Instead, we ensure all of the used research software is open source, all of our input data is made public in a directly usable form, and all post-processing and analysis steps are also released as free software, making our research as much Open Science as possible.



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