1. Black Holes and Gravitational Waves
The processed waveform of GW150914. Credit: LIGO Scientific Collaboration Link to high-resolution image.
Black holes and gravitational waves are among the most fascinating predictions of Einstein's theory of gravity.
Black holes are ubiquitous: they are responsible for many X-ray sources in the Milkyway, quasars are believed to be powered by their immense gravity and there is indirect evidence that every galaxy hosts a massive black hole at its core.
The evidence is indirect because astronomical black holes don't emit any electromagnetic radiation and hence the name black. However, Einstein's theory tells us that they must emit gravitational radiation.
The first evidence that gravitational waves exist came from observations of double and single pulsar binary systems. It was found that their orbits were decaying in precise agreement with general relativity. The most famous puslar binary system being the Hulse-Taylor binary.
On at 9:51 GMT on 14th September 2015 everything changed. The two LIGO detectors picked up the first ever direct signature of gravitational waves from the final stages of two black holes merging together.