#### The Albert Einstein Institute

Since its foundation in 1995, the Max Planck Institute for Gravitational Physics has established itself as a leading international research center. The research program pursued in five divisions and several independent research groups covers the entire spectrum of gravitational physics: from the giant dimensions of the Universe to the tiny scales of strings.

The Max Planck Institute for Gravitational Physics has two sites:

The Quantum Gravity and Unified Theories Division, the Geometric Analysis and Gravitation Division, the Astrophysical and Cosmological Relativity Division, as well as the independent research groups on Theoretical Cosmology, Geometric Measure Theory and Gravitation and Black Hole Theory are based in Potsdam.

The Laser Interferometry and Gravitational Wave Astronomy Division and the Observational Relativity and Cosmology Division, as well as an independent research group on Pulsar Observation and Data Analysis are based in Hannover.

**Quantum Gravity and Unified Theories Division (Potsdam)**

In the seventies, physicists had already managed to unite three of the four fundamental forces of nature into one theory – the standard model of particle physics. Only gravity is still firmly resisting integration. Although the inherent contradictions between quantum theory and general relativity become apparent only in the unbelievably small dimensions of the Planck scale (10-33 cm), they must be resolved if we want to understand what "happens" inside a black hole or at the Big Bang. The sought-after new theory of quantum gravity promises to unite general relativity and quantum field theory, solving the mathematical contradictions in the process.

Despite intense efforts over the last years it is far from clear at this time what a consistent theory of quantum gravity will look like and what its main features will be. In view of these uncertainties, the best strategy appears to be one which is both diversified and interdisciplinary. For this reason, the division aims to represent all the major current approaches to quantum gravity, in particular supergravity and string theory and their modern developments, as well as canonical quantization (e.g. loop quantum gravity) and discrete models of quantum gravity.

**Geometric Analysis and Gravitation Division (Potsdam)**

Research in gravitation requires close collaboration between physicists and mathematicians. In the “Geometric Analysis and Gravitation“ division physical models and mathematical methods relevant to this field are investigated, with emphasis on Einstein’s general relativity theory.

Einstein’s theory is based on differential geometry, its laws are formulated as partial differential equations. In-depth research in these areas is carried out to study Einstein’s gravitational field equations and its implications for phenomena such as black holes, gravitational waves or the Big Bang singularity. Mathematics establishes the consistency of physical models and provides hints for numerical simulations of observable processes, in particular in relation to gravitational wave detector measurements.

**Astrophysical and Cosmological Relativity Division (Potsdam)**

Physicists are on the verge of directly detecting gravitational waves – ripples in the fabric of space and time. They use laser-interferometer detectors on the Earth such as LIGO (Laser Interferometer Gravitational-Wave Observatory), Virgo, and GEO600, and Pulsar Timing Arrays. The direct detection will enrich our understanding of gravitational phenomena and open a revolutionary new window on our Universe.

Binary systems composed of black holes and/or neutron stars are the most promising and exciting sources for gravitational-wave detectors. However, to significantly increase the probability of identifying gravitational waves in the detector data, the search for these sources requires detailed knowledge of the expected signals.

The research carried out in the "Astrophysical and Cosmological Relativity" division aims at improving our ability to detect and extract unique astrophysical and cosmological information from the observed waveforms, and test fundamental equations of general relativity.

Scientists in this division develop sophisticated analytical and numerical methods to solve the Einstein equations and predict highly-accurate template waveforms. Then, they employ these templates to build algorithms and carry out gravitational-wave searches with LIGO, Virgo and GEO600 experiments.

**Laser Interferometry and Gravitational Wave Astronomy Division (Hannover)**

Today's gravitational-wave observatories operate at a level of sensitivity that enables the first direct detection of strong gravitational waves from nearby astrophysical sources. The “Laser Interferometry and Gravitational Wave Astronomy” division plays a worldwide leading role in this venture. Together with UK colleagues the division operates the gravitational-wave detector GEO600 and develops cutting-edge technologies for this experiment: powerful and extremely stable lasers, so-called squeezed laser light with tailored quantum properties, clever suspension techniques, and innovative detector layouts. Many of these methods were initially developed at the AEI and are now being used worldwide in all of the large gravitational-wave observatories of the LIGO Scientific Collaboration.

The AEI is the worldwide leading research institution in the development of eLISA, a planned gravitational-wave observatory in space. The observatory will consist of three satellites spanning million-kilometres long laser arms, enabling eLISA to hear gravitational-wave signals from the entire Universe, probably even from the Big Bang. In preparation for this mission, the division has a major role in the LISA Pathfinder mission, which will be launched in 2015 to test the measurement and control systems designed for eLISA.

**Observational Relativity and Cosmology Division (Hannover)**

The world-wide network of ground-based gravitational-wave detectors collects large volumes of data. The main research area of the “Observational Relativity and Cosmology” division is the efficient analysis of these data streams to filter out signals from many different astronomical sources. The work of this division focuses on the development and implementation of innovative mathematical methods (data analysis algorithms) and the application of these methods using powerful computer clusters. The division operates its own dedicated computer cluster called Atlas with enormous computing capacities. Atlas consists of more than 6,000 CPU cores and 80,000 GPU cores, making it the largest computer cluster worldwide used for gravitational-wave data analysis.

The division also plays a leading role in the distributed volunteer computing project Einstein@Home. Volunteers from all around the world participate in the search for unknown neutron stars by donating idle computing time on their PCs, laptops or smartphones. Einstein@Home searches for neutron stars in data from gravitational-wave detectors, from large radio telescopes and from the Fermi gamma-ray satellite. More than 50 new neutron stars have already been found in the radio and gamma-ray data.

**Theoretical Cosmology Independent Research Group (Potsdam)**

The aim of the Theoretical Cosmology research group is to enhance our understanding of the early universe and its most mysterious aspect, the big bang. There currently exists no complete theory that satisfactorily explains the early stages of the universe. However, there are promising candidate theories, such as inflationary cosmology and the theory of the cyclic universe. We study and develop these cosmological theories while trying to figure out their relationship with fundamental physics, such as string theory. The big questions guiding our research are: was the big bang really the beginning? How did space and time emerge? What was the role of quantum theory in the early universe? Which aspects of the universe are fixed by mathematical requirements, and which are due to historical accidents? And is our universe unique?

**Geometric Measure Theory Max Planck Research Group (Potsdam)**

The mathematical analysis of General Relativity has made use of a mathematical language, called Geometric Measure Theory, which allows to describe very general surfaces in flat as well as curved spaces. Such surfaces occur explicitly, for example as horizons, or implicitly as in the deduction of the positive mass theorem, or both as in the formulation and proof of the Penrose inequality. The goal of the Max Planck research group Geometric Measure Theory is to construct new tools for the mathematical treatment of surfaces with a priori very intricate local structure. This shall provide further fuel to a machinery that has been successfully employed in the description of General Relatively as well as other – at first sight unrelated – models from the Natural Sciences such as crystal growth.

**Pulsar Observation and Data Analysis Independent Research Group (Hannover)**

The group's research comprises observation and computing-intensive astronomical data analysis to discover and study pulsars through gamma-rays and gravitational waves.

Pulsars, rapidly spinning neutron stars, are some of the most extreme objects in our Universe and important key probes for a wide range of fundamental physics. Yet many aspects are still poorly understood after decades of observations, primarily at radio wavelengths. Now gamma-ray observations with NASA’s Fermi Large Area Telescope, and gravitational-wave observations with Advanced LIGO soon, provide a complementary window of extraordinary opportunity.

A particularly exciting research focus is on extending searches to find new stars that in fact have been inaccessible before on computational grounds. Achieving this requires the development of efficient data analysis methods and powerful computing resources, such as the Einstein@Home volunteer computing system whose capacity is on par with the world’s top supercomputers.

**Gravitation and Black Hole Theory Independent Research Group (Potsdam)**

Astronomical observations suggest the existence of extremal black holes whose horizons spin at nearly the speed of light. Recently, also new symmetries close to the extremal black hole’s event horizon geometry have been found.

Our research aims to utilize these symmetries for the resolution of unsolved problems in General Relativity and Astrophysics. The latest studies of our group comprise models of black hole jets. Other topics include the physics of black holes in higher dimensional General Relativity, the internal structure of black holes, different aspect of black hole thermodynamics, stability and scattering. The ultimate goal of our investigations is to improve our understanding of the most fascinating objects in the Universe, black holes.