# From Scattering Amplitudes to Gravitational Waves

The Knut and Alice Wallenberg Foundation granted in 2018 in total 640 million SEK in project grants for 2018 to 22 research projects which where assessed to hold the highest international standards and have the opportunity to lead to future scientific breakthroughs. The Department of Physics and Astronomy was main applicant in two and fellow applicant in one of these projects that was granted in total 78.5 million SEK during five years.

## Project Description

Main applicant: Henrik Johansson, Division of Theoretical Physics

Project title: From Scattering Amplitudes to Gravitational Waves

Grant amount: 19 100 000 SEK during five years

Funder: Project grant from the Knut and Alice Wallenberg Foundation

The aim of this project is to develop new analytical methods for efficient calculations in particle physics, gravity and string theory, relevant to physical processes ranging from subatomic collisions between elementary particles to astronomical collisions between black holes. The strategic importance of this research is emphasized by two Nobel-awarded global experiments: the particle accelerator LHC in Geneva and the LIGO gravitational wave detectors in the United States, which directly rely on precise calculations of these two extreme physical processes.

Although the distance scales relevant to subatomic collisions and to black hole collisions are spectacularly different, the underlying mathematical description is quite uniform. Collisions of elementary particles are described by quantum mechanical scattering amplitudes and black holes are described by the general theory of relativity. In modern theoretical physics, both of these processes can be understood mathematically through so-called Feynman diagrams. The goal of this project is to simplify the computational methods that, directly or indirectly, rely on Feynman diagrams. In achieving this, we also expect to better understand the theories that describe the four fundamental forces of nature: electromagnetism, weak and strong force (all described by gauging theories), as well as gravity.

A significant breakthrough in our understanding of gauge and gravitational theories was made in 2008 by the main applicant and two American colleagues; we observed that the two types of theories have identical underlying mathematical descriptions. The Feynman diagrams of gravity proved to be double copies of the Feynman diagrams for gauge theories, which in turn could be described by a new type of mathematical Lie algebra. The impact of these results gave rise to a new research field in theoretical physics with intense global activity. Since 2014, after the main applicant moved to Uppsala University, Sweden has been a major center in this field. The proposed project will strengthen Sweden's central role in this research.

When the LIGO experiment in September 2015 discovered the first gravitational waves from binary black hole mergers, a new window for observations of the universe opened up. In order to fully utilize this new observational window, both computational methods and experiments needs to be significantly upgraded. The theoretical methods that this project will develop are based on the description developed by the applicant where gravity is viewed as a double copy of strong-force interactions. Recently, research groups in the United States and Britain have demonstrated that the double-copy description has the potential to revolutionize analytical calculations needed to model gravity waves. However, a lot of ground work remains before the double copy can overtake traditional methods. This project will use a variety of approaches, including inspiration from particle physics and string theory, to solve the remaining problems.

This will be a five-year cooperation project between the Department of Theoretical Physics at Uppsala University and the Nordic Institute for Theoretical Physics (Nordita) in Stockholm. Within the project, there will be employed PhD students and postdoctoral researchers in connection with both institutions. This will not only be a strong research environment for achieving the goals of the project, but will also provide the next generation of researchers in Sweden with the opportunity to work on the cutting edge of theoretical physics.