What happened at the LISA Cosmology Working Group workshop

The LISA Cosmology Working Group Workshop dinner at Särkänlinna restaurant.

Credit: Anna Kormu

Visiting graduate student Daniel Cutting has written a short summary of what happened at last week’s LISA Cosmology Working Group Workshop in Helsinki. Here is what he has to say:

During the last week, the Helsinki Institute of Physics hosted the 5th LISA Cosmology Working group meeting. This group has been formed to develop and inform on the cosmological science that can be conducted with the upcoming space-based gravitational wave detector called LISA.

The LISA mission will be formed of three satellites in an equilateral triangle separated by millions of kilometres following the Earth in an orbit around the Sun. The detector will be sensitive to a large range of sources, ranging from mergers of supermassive black holes in galactic centres to exotic objects such as cosmic strings formed in the early universe.

The meeting began with an update on the current status of the development of the LISA consortium. This is the organising body responsible for developing the technology and science for the LISA mission. In particular, we learnt more about the work packages that the Cosmology Working Group needs to carry out in order for the LISA mission to deliver the science it has promised.

As the day continued, there were a series of talks about the prospects of LISA to participate in multi-messenger detections. An example of multi-messenger astronomy was the recent detection of a neutron star – neutron star merger from both gravitational waves detectors and observatories in various bands of the electromagnetic spectrum. The LISA detector will be able to detect gravitational waves from mergers of compact objects much further from us than ground-based detectors. If we can observe these mergers in the electromagnetic spectrum then we can deduce the distance at which the event occurred, and then trace out the history of the expansion of the universe.

The second day started with several talks on primordial black holes, compact objects that may have formed in the early universe during inflation or a phase transition. As these objects are nearly collision-less and stable, they have been proposed as a dark matter candidate. However, we heard that the range of masses where primordial black holes could constitute all of dark matter is tightly constrained, if not completely ruled out. The primordial black holes would form in clusters, and upon merger, they would form a background of gravitational waves. Depending on the mass range of the primordial black holes, this background could lie within the LISA sensitivity band.

Next were several talks on developing data analysis techniques in order to extract gravitational wave detection events from LISA data. The LISA band is expected to have a large range of sources and backgrounds, and one of the major challenges of the project for cosmologists will be being able to distinguish different signals, some of which may be very weak. The LISA data challenge has been created to investigate this problem. First, a mock catalogue of LISA data with different sources and backgrounds is produced. Then groups can develop techniques to determine what signals are present in the mock data. By building up to more and more realistic data with known sources and signals, we can become prepared for when the real data is available.

The third day was dedicated to the discussion of gravitational waves from cosmic strings and phase transitions. Cosmic strings are thread-like objects of concentrated energy that may have been produced in the early universe. These strings form complex networks permeating the universe, and when strings collide they can form loops. These loops then oscillate, radiating gravitational waves. We heard that for a large range of tensions of the cosmic strings LISA will be able to detect a background of gravitational waves from string loops. We may be able to see rare but strong signals from cosmic string loops called bursts. These events last for a very short duration and look like glitches in the data, and so one of the challenges is how to distinguish them from noise.

Another source of gravitational waves is first-order phase transitions in the early universe. In a first-order phase transition, the universe transitions from one phase to another through the nucleation, expansion and eventual merger of bubbles, similar to water boiling. These highly energetic phenomena can stir up the plasma of particles present in the universe at the time of the transition and create sound waves. The sound waves then source gravitational waves. There was an update on the most recent numerical simulations, and the plans to study stronger, more energetic transitions where the plasma begins to develop turbulence.

In the Standard Model of particle physics, there is no first-order phase transition in the early universe. However, by adding extra undiscovered particles to the theory it is possible to create a first-order phase transition at the scale observable by LISA. There were several talks on different theories of particle physics which do this, and how we might be able to distinguish between them if we did see a signal of a first-order phase transition with LISA.

The penultimate day was split between discussions on inflation and modified theories of gravity. Inflation is a period in the early universe where the universe was expanding at an exponential rate. There is a large range of particle physics models that fit the current understanding of how much the universe expanded during inflation. Some inflationary models can give rise to a background of gravitational waves that LISA could detect. An example of this is where there are extra spectator fields present during inflation. It is also possible that primordial black holes can be created from vacuum fluctuations during inflation. The merger of these compact objects could then form a stochastic background in the LISA band.

The workshop finished with a day focussed on discussions and planning of future projects. New projects were proposed, deadlines set for papers, and technicalities discussed in detail. To see a list of participants, the topics discussed and slides used in talks at the workshop, follow this link.

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