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Gravity

Gravity

Nearby stars could reveal wormhole at the centre of the Milky Way

12 Nov 2019
S2 orbiting Sagittarius A*
Wormhole detector: artist’s impression of path of the star S2 as it passes very close to the supermassive black hole at the centre of the Milky Way. (Courtesy: ESO/M Kornmesser)

Gravity passing through a hypothetical wormhole at the centre of the Milky Way could alter the orbits of nearby stars – according cosmologists in China and the US who are developing new ways to search for wormholes.

A theoretical consequence of Einstein’s general theory of relativity, wormholes are “shortcuts” that link distant points in space. They can, in theory, be “dug” through space–time by the immense mass of a gravitational singularity such as a black hole. To date, wormholes have not been observed and it is not known whether they do exist in nature.

To establish if wormholes exist, astronomers need to know what observational signatures they should look for. De-Chang Dai of the Centre for Gravity and Cosmology at Yangzhou University and Dejan Stojkovic of the University at Buffalo, suggest that evidence of a wormhole could be extracted from the motions of stars around Sagittarius A*, which is the supermassive black hole at the centre of the Milky Way.

If there is a wormhole associated with Sagittarius A*, then the mouth of the wormhole would be larger than the black hole’s event horizon – the radius at which nothing can escape the black hole. This means that particles and forces could pass through the wormhole without being gobbled up by the black hole.

Gravity leaks

De-Chang and Stojkovic point out that a wormhole would be a two-way street for gravity, electromagnetic radiation and electric charge. The gravity of a star, or stars, orbiting the other end of the wormhole should therefore propagate through the wormhole, to pull on stars orbiting Sagittarius A*, and vice versa.

Over the past three decades, astronomers have been monitoring several dozen stars that orbit close to Sagittarius A*. One such star is called S2, which comes to within 130 au (about 20 billion km) of the black hole. De-Chang and Stojkovic considered a scenario in which there is a wormhole around Sagittarius A*, and at the other end of the wormhole there is another star that is in an identical orbit as S2. The gravitational force that they would exert on each other would be the same as if they were 260 au apart in normal space.

Detecting the gravitational effect of this wormhole twin on S2 would require acceleration measurements at a precision of about 10–6 m/s2, which is about 100 times better than is currently possible. As a result, astronomers will have to wait for the next generation of 30–40 m telescopes – such as the Extremely Large Telescope – which De-Chang and Stojkovic say should be good enough to detect the additional gravitational pull of stars at the other end of a wormhole.

Neil Cornish of the eXtreme Gravity Institute at Montana State University, agrees that, in principle, stars at opposite ends of a wormhole could imprint their gravitational influence on one another. “I concur with [De-Chang and Stojkovic’s] analysis that masses on the other side of the wormhole would impact the orbits of objects on our side,” he says.

Dark objects

However, Cornish points out that unseen, dark objects orbiting close to Sagittarius A*, such as stellar-mass black holes and neutron stars, could also perturb S2’s orbit by a similar amount.

“If we were to see a perturbation of a star’s orbit around Sagittarius A*, what would be the most likely explanation?” asks Cornish. “I know what I would bet on!”

There are other problems with a wormhole hypothesis, in particular how the mouth of the wormhole can be kept open and stable. “You need either negative energy or some elaborate set-up that basically does the same thing – provides repulsion to keep the wormhole open,” says Stojkovic.

Bottled dark energy

The concept of negative energy is not hypothetical. Dark energy, which is the mysterious force that is accelerating the expansion of the universe, is a form of negative energy, its repulsive force pushing the universe increasingly far apart, counteracting gravity. However, whether nature could bottle dark energy, or some other exotic matter with the same properties, into a wormhole is not known.

Cornish also flags up quantum feedback effects that would reverberate through the wormhole, “analogous to feedback when a microphone is placed close to a speaker,” rendering the wormhole unstable.

Clearly, it is a long shot to expect to find a wormhole at the centre of our galaxy. However, should one exist, something as simple as the motions of the stars might betray its presence.

The research is described in Physical Review D.

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