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Gravity

Gravity

Black hole’s shadow boosts Einstein’s general theory of relativity

08 Oct 2020
Black hole shadow
Supermassive test: this simulation of the region around M87 shows the motion of plasma as it swirls around the black hole. The bright thin ring that can be seen in blue is the edge of the shadow. (Courtesy: L Medeiros/C Chan/D. Psaltis/F Özel/University of Arizona/Institute for Advanced Study)

Einstein’s general theory of relativity has triumphed once again after being tested in the strongest gravitational field to date. Dimitrios Psaltis at the University of Arizona and members of the Event Horizon Telescope (EHT) collaboration did their analysis using recent images of M87*, which is a supermassive black hole at the centre of a nearby galaxy. Their results set the stage for even more stringent tests of general relativity in the near future.

For over a century, general relativity has had an excellent track record in explaining observations of the universe. All the same, the theory leaves some big questions unanswered: including how to unify gravity with quantum mechanics and the surprise discovery in 1998 of the universe’s accelerating expansion. As a result, physicists are looking for subtle flaws in general relativity that could lead to the development of a more complete theory.

One way to study the theory’s limitations is to search for discrepancies in how it describes distortions of spacetime by the gravitational fields of massive objects. Initially, these tests used objects in the solar system – famously the motion of Mercury. More recently, gravitational waves created by merging black holes and observed by the LIGO–Virgo collaboration have enabled tests in the gravitational fields of objects as heavy as 150 solar masses. Yet despite the increasingly rigorous constraints imposed by these results, cracks have yet to show in Einstein’s theory.

Billions of Suns

M87* has a mass of about 3.5–6.6 billion Suns and its gravitational field is the largest ever used to test general relativity. In 2019, the EHT released its celebrated image of the shadow of M87* – a dark silhouette, surrounded by bright emission from hot plasma. General relativity provides a precise prediction of the size of the shadow and in the case of M87*, the observed size is within 17% of general relativity’s prediction.

It is possible, therefore that a modified version of general relativity could do a better job at predicting the size of the M87* shadow. To test this, Psaltis and colleagues considered alternative models of gravity that modify the general theory of relativity. They focussed on parameters of these alternative models that affected the models’ predictions of the size of the shadow.

By comparing these predictions to the observed shadow, they we able to constrain modifications to Einstein’s theory by a factor of almost 500 compared with earlier solar system tests. The new constraints are similar to those derived from gravitational wave observations. The EHT collaboration now hopes to impose even stricter limits by imaging the shadow of Sagittarius A* – the supermassive black hole at the centre of our own galaxy, whose mass is far more precisely defined than M87*.

The research is described in Physical Review Letters.

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