The era of gravitational wave astronomy began a few months ago with the first direct detection of gravitational waves. Now, about 150 scientists from all over the world followed the invitation of the Max Planck Institute for Gravitational Physics (Albert Einstein Institute/AEI) in Hannover, Germany, and gathered for discussions of the most recent developments and the next steps in gravitational wave research.

"The discovery electrified scientists from several fields because with gravitational waves we will explore the universe in the most comprehensive way ever," summarized Dr. Badri Krishnan, chair of the local conference organizing committee and staff scientist at the AEI.

In-depth analysis of the observed gravitational wave proved that

+ Advanced LIGO (aLIGO), the data analysis and the waveform templates used for identifying gravitational wave signals work very well.

+ The observed gravitational wave signals match with predictions from general relativity.

+ It is very likely that the universe is densely populated with black holes.

+ Soon we may be seeing gravitational wave signals from black hole binaries once a week or more frequently.

+ In the near future also gravitational wave signals from binary systems with neutron stars and/or black holes may be discovered.

+ The joint observation of the universe with electromagnetic counterparts will shed light on the cosmic objects of our universe, their population statistics and where and when they arise.

"The potential of gravitational wave astronomy is unique. I am delighted that we brought together experts from theoretical and experimental research. Their discussions developed a sustainable basis for the future," said Prof. Bruce Allen, director at the AEI in Hannover.

What follows are some highlights from the conference.

GW150914: The Observed Black Holes
Dr. Collin Capano, AEI Hannover, and Dr. Duncan Brown, Syracuse University, described in their talks how the observed gravitational wave signals were being analyzed. Dr. Ilya Mandel, University of Birmingham, Prof. Tomek Bulik, University of Warsaw, Thomas Tauris, MPIfR, Bonn, and Peter Berczyk, ARI Heidelberg, compared their work to an archaeologist examining a newly discovered dinosaur's skeleton: They gather information, e.g., about the age of the skeleton, the natural environment of the dinosaur and its mode of life. Similarly, astronomers unravel when, where, and how the observed cosmic objects developed and how they can be characterized. So step by step they unveil the bigger picture.

Key lessons learned: the observed gravitational wave signals GW150914 originated from two merging black holes. So for the first time the observation proves the existence of black hole binaries. The signals also proves the existence of stellar mass black holes larger than 30 solar masses.

Theoretical Foundations for the Generation of Gravitational Waves
Gravitational waves are ripples in space-time. They leave characteristic fingerprints in gravitational wave detector data. The prediction of the form of these fingerprints in the most accurate way possible is a task for theoretical astrophysics: Prof. Alessandra Buonanno, director at the AEI in Potsdam, Prof. Luc Blanchet, Institut d'Astrophysique de Paris, Prof. Mark Hannam, University of Cardiff, and Dr. Alessandro Nagar, Institut des Hautes Etudes Scientifiques, develop waveform models which allow us to detect gravitational waves. In their talks they discussed methods to compute these models for colliding black holes, neutron stars or pulsars.

Prof. Thibault Damour, Institut des Hautes Etudes Scientifiques, Dr. Enrico Barausse, Institut d'Astrophysique de Paris, and Dr. Chris van den Broeck, Nikhef, Amsterdam, discussed the various tests of Einstein's theory made possible by the first direct detection of gravitational waves. The observation of the binary black hole allows to probe general relativity in more dynamical and non-linear regimes than possible previously. Damour and Barausse also suggested modifications of general relativity that could be tested by future observations.

Data Analysis
How to extract a gravitational wave signal from the huge amount of data produced by a gravitational wave detector was shown by Dr. Andy Lundgren from the AEI Hannover, Collin Capano and Duncan Brown. The data is cleaned systematically step by step. The noise from earthquakes on the other side of the globe is filtered out the same as that from a passing motorcycle or the noise produced by the instruments themselves.

Future challenges are to improve search efficiency and to maintain adequate computing resources for these searches. Scientists at the AEI Hannover play a significant role in gravitational wave data analysis efforts of the LSC by providing knowledge, manpower. Moreover, the institute operates Atlas, the most powerful computer cluster in the LIGO Scientific Collaboration. Atlas contributes roughly half of the entire computer power available within the collaboration. Additionally AEI runs Einstein@Home, one of the largest distributed volunteer computing projects in the world with more than 400,000 participants.

LIGO and Advanced LIGO
Prof. Benno Willke, AEI and Leibniz Universitat Hannover, outlined the evolution from LIGO to advanced LIGO in his talk. Two examples of core technologies may stand for the precision measurement which was achieved: lasers and mirrors. While initial LIGO's input laser power was 10 watts, aLIGO's input laser power is 180 watts. Regarding the mirrors: initial LIGO's mirrors weighed 10 kg, aLIGO's mirrors weigh 40 kg. Additionally their seismic isolation was improved substantially. Willke emphasized that aLIGO's sensitivity during the first observation run in autumn 2015 was at the lower end of the intended sensitivity. Sensitivity will now be improved step by step, and thus gravitational wave signals will be seen more regularly.

Prof. Bernard Schutz, director emeritus at AEI Potsdam and professor at Cardiff University, highlighted the reliability and efficiency of the researchers because aLIGO was delivered on time and within budget. Additionally it has operated better than the target specifications.

He underlined also the contribution of the British-German gravitational wave observatory GEO600 where aLIGO's core technologies were developed and proved.

Gravitational Wave Observatories - Next Steps
Conference participants agreed that it is now time to bring the Italian-French Virgo detector online for more comprehensive observations of the sky. The Japanese observatory KAGRA and LIGO-India will join the network in about 2020.

Next generation gravitational wave observatories like the Einstein Telescope (ET) will be able to observe a larger fraction of the universe and detect more gravitational wave sources. Considering that LIGO's realization from the first paper to the first announcement in 2015 took about 25 years it is now time to get ET going.

Gravitational Wave Observation in Space and on Earth
ESA's LISA Pathfinder (LPF) Mission is currently proving in space key technologies for a future space-based gravitational wave observatory. The first results are spectacular and recommend a LISA-like mission based on these reliable high precision technologies.

Space- and Earth-based observatories will complement each other. While LISA will observe thousands of binary black hole systems, hundreds of them will go on to coalescence in the LIGO band, leaving the LISA band just weeks before coalescence. So LISA will exquisitely predict just where and when to look. LISA could be launched in about 15 years. Now is the perfect time to tackle the remaining challenges.

Gravitational Wave Sources and Electromagnetic Counterparts
While black holes were the main theme of the meeting, there were a number talks also devoted to merging binaries with neutron stars and the possible electromagnetic counterparts of such events.

Dr. Samaya Nissanke from Radboud University Nijmegen stated that the scientific community is excited in following up such events detected in the gravitational wave spectrum with electromagnetic telescopes over the whole spectrum from gamma-rays down to radio waves. The talk by von Kienlin from MPA described the observations by the Fermi satellite of a possible GRB counterpart to the event.

If confirmed, this would be extremely surprising as merging black holes are not expected to produce electromagnetic signatures. The astronomical methods will join forces in order to increase sensitivity and confidence of the observations. This allows researchers to localize and characterize gravitational wave sources in more detail. We will learn about their origin, population, life cycle and environment. There are many mysteries to be solved.

Prof. Tsvi Piran, Hebrew University of Jerusalem, Prof. Masaru Shibata, Yukawa Institute for Theoretical Physics, and Dr. Kenta Kiuchi, Kyoto University, discussed the physics of neutron star mergers and possible signatures in future observation runs.

One example: The observation of a neutron star binary merger in the gravitational wave spectrum will allow us to follow the full merging process from its early stages to the final ring down. We will learn about the sources' dynamic and fundamental properties. Electromagnetic counterparts can join observation during the very last stages of the merging process. From their data we will learn about the sources' environment, energetics and processes in curved spacetime.