Observations of General Relativity at strong and weak limits

TL;DR

This paper reviews tests of Einstein's General Relativity in both weak and strong gravitational fields, focusing on the zero gravity surface concept and the no-hair theorem, with recent observational evidence and future prospects.

Contribution

It provides a comprehensive overview of recent observational tests of GR in extreme gravitational regimes, highlighting new empirical evidence for the zero gravity surface and black hole properties.

Findings

Dwarf galaxy motions support the zero gravity surface concept.

Binary black hole system OJ287 tests the no-hair theorem.

Current observations constrain alternative gravity models.

Abstract

Einstein's General Relativity theory has been tested in many ways during the last hundred years as reviewed in this chapter. Two tests are discussed in detail in this article: the concept of a zero gravity surface, the roots of which go back to J\"arnefelt, Einstein and Straus, and the no-hair theorem of black holes, first proposed by Israel, Carter and Hawking. The former tests the necessity of the cosmological constant Lambda, the latter the concept of a spinning black hole. The zero gravity surface is manifested most prominently in the motions of dwarf galaxies around the Local Group of galaxies. The no-hair theorem is testable for the first time in the binary black hole system OJ287. These represent stringent tests at the limit of weak and strong gravitational fields, respectively. In this article we discuss the current observational situation and future possibilities.