What are uses for dark matter

New tests offer conclusive signatures for dark matter and modified gravity

Gravitational waves enable astronomy to search for the possible origin of phenomena that can only be explained by the presence of dark matter or by changes in the laws of gravitation. EU research has provided insightful signatures that could lead to the discovery of undiscovered particles, forces and fields.

The discovery of waves in spacetime, which were generated by the collision of two black holes, confirms Einstein's theory. But it does a lot more: it offers astronomy a tool for testing new physics. The gravitational waves could shed light on puzzles such as dark energy and dark matter, which do not fall under the theory of the Standard Model of elementary particle physics. In addition, they could be helpful to astronomy researchers in describing gravity according to the principles of quantum mechanics.

Refine Einstein's theory of gravity

One idea that the StronGrHEP project has investigated is the expansion of Einstein's general theory of relativity with a new theory, the so-called scalar tensor gravitation. This indicates that the universe is filled with an additional field that has yet to be discovered. This research was funded under the Marie Skłodowska-Curie Actions. This means that the supernova explosion of a dying star would not only be visible as an outbreak of gravitational waves, but that there would also be an afterglow of gravitational waves to be detected. “We could aim the Laser Interferometer Gravitational Wave Observatory (LIGO) at regions of the sky where stars have exploded - for example Kepler's Supernova - and try to detect such an afterglow from the scalar field, which may persist for centuries after the actual explosion “, Notes project coordinator Ulrich Sperhake.

Black holes as messengers of modified theories of gravity

“When two black holes collide, they merge into a single black hole that emits gravitational waves - similar to a bell that starts to ring when struck by a hammer. Certain frequencies [tones] are embedded in these gravitational waves, which are similar to the individual tones in a musical chord. The frequency spectrum of these tones is like a fingerprint, with which not only a black hole can be identified, but also the theory of gravity, which determines its vibrations, ”explains Sperhake. Astronomers can recognize the signals if they remain above the LIGO / VIRGO sensitivity threshold. The project team calculated such a tone from the event horizon of a black hole in binary - it would be of less strength than the one discovered in 2015. This fingerprint will make it possible to seek or rule out evidence of gravitational wave echoes. The latter come from imitators of black holes - exotic compact objects like wormholes that have no event horizon. These data form the basis for testing the theory of gravity in depth and possibly finding evidence of its partial collapse. The very first image of a black hole captured by the Event Horizon Telescope in 2019 could also indicate modified gravity. The picture corresponded to the theoretical predictions: Black holes are dark shadows surrounded by a crown of light that is emitted from different regions of the accretion disk. The extreme attraction of the black hole distorts the light, making the disk resemble a helmet. The team examined these shadows of black holes in detail, under the prisms of modified gravity. The resulting images showed a remarkably complex structure that is reminiscent of fractal images.

Prominent Dark Matter Candidates

"Black hole shadows could also be influenced by the presence of dark matter," added Sperhake. The team modeled what shadows might look like when the theoretical subatomic axion particles are in place. Very bright boson fields are another class of excellent dark matter candidates. If they do exist, they should condense around rotating black holes, forming clouds in the process. “Just like the oceans on Earth, these clouds are hit by the currents of other massive bodies. We have shown that the clouds are torn away from their host, the black hole, when the currents are sufficiently large, in a violent flow-related disturbance event, ”concluded Sperhake.

key terms

StronGrHEP, black hole, gravitational waves, dark matter, modified gravity, standard model, scalar tensor gravitation