The working group « Tests of fundamental laws » is concerned with the possibility that our knowledge of the correct theory of gravitation is incomplete. There is a possibility that our understanding of gravitation is not correct when the forces in play are very strong.
Gravitational waves were first predicted in the framework of General Relativity by Albert Einstein. The validity of this theory, as well as the existence of the waves themselves, had been well established, but only in the weak-field regime. This means that there is a possibility that our understanding of gravitation is not correct when the forces in play are very strong.
Most of the events that LISA will observe involve huge gravitational forces. By carefully studying the detected signals, LISA will be able to test if the gravitational wave emitted are consistent with General Relativity or if there is any evidence that we require an alternative theory to explain the observations. Many alternative theories have been proposed that include things like a massive graviton, the existence of scalar fields coupled to the tensor gravitational field or additional dimensions in the spacetime.
A related topic concerns black holes themselves. The existence of very compact and massive objects at the center of galaxies is widely accepted but there is a possibility that they are not black holes but more exotic objects, like boson stars. Gravitational waves coming from those objects will probe the structure of the spacetime around them very precisely. This will enable us to decipher their very nature. General Relativity predicts, under certain physical assumptions, that every black hole is described by just two parameters, the mass and the rotation rate. Finding a black hole that had more structure (sometimes called “hair”) would indicate that either General Relativity or one of these physical assumptions was violated in nature.
The task of the working group « Test of fundamentals laws » is to keep track of all the interesting alternatives to General Relativity and to predict what the LISA mission will be able to say about the validity of all those theories. Should a deviation to General Relativity be observed, it would have a huge impact on modern physics.
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