Dark matter is an enigmatic form of matter not expected to emit light, yet it is essential to understanding how the rich tapestry of stars and galaxies we see in the night sky evolved. As a fundamental building block of the universe, a key question for astronomers is whether dark matter is smooth or clumpy, as this could reveal what it is made of. Since dark matter cannot be observed directly, its properties can only be determined by observing the gravitational lensing effect, whereby the light from a more distant object is distorted and deflected by the gravity of the dark object.

“Hunting for dark objects that do not seem to emit any light is clearly challenging,” said Devon Powell at the Max Planck Institute for Astrophysics and lead author of the study. “Since we can’t see them directly, we instead use very distant galaxies as a backlight to look for their gravitational imprints.”

The research is published in the journal Nature Astronomy.

Gravitational waves are energy-carrying waves produced by the acceleration or disturbance of massive objects. These waves, which were first directly observed in 2015, are known to be produced during various cosmological phenomena, including mergers between two black holes that orbit each other (i.e., binary black holes).

A neutron star’s viscosity determines how the star interacts with gravitational waves, a behavior that could be useful to the study of neutron-star interiors.

The detection of gravitational waves from mergers of black holes and neutron stars has opened a window onto the strong-gravitational-field regime, allowing physicists to put constraints on various gravitational theories [1, 2]. These observations also have the power to probe the ways in which such compact objects interact with gravitational waves hitting their boundaries or, in the case of neutron stars, passing through their interiors [3]. Valentin Boyanov at the University of Lisbon in Portugal and his colleagues have now investigated such interactions in detail, analyzing how an object’s response to passing gravitational waves is influenced by its viscosity [4]. Their results could allow researchers to extract information about the internal structure of neutron stars from future gravitational-wave measurements.

Boyanov and colleagues tackle the following questions: Under what conditions do viscous compact objects such as neutron stars reflect or absorb gravitational waves? And to what extent do these interactions mimic those of black holes? At first, it might seem that black holes in particular cannot be reflective―after all, their defining feature is that they absorb everything that falls on them. But in practice, whether a black hole absorbs or reflects gravitational waves depends on the frequency of those waves. High-frequency gravitational waves cross the event horizon and are absorbed, adding to the black hole’s mass and angular momentum. For low-frequency waves, on the other hand, the curved space time around the black hole constitutes a potential barrier to the wave propagation: The waves are “reflected,” meaning that they scatter off this region with their phase or their propagation direction altered.

If recent discoveries that dark energy is evolving hold any water, our Universe will collapse under its own gravity on a finite timeline, new calculations suggest.

Based on several recent dark energy results, a new model finds that the Universe has a lifespan of just 33.3 billion years. Since we are now 13.8 billion years after the Big Bang, this suggests that we have a smidge less than 20 billion years left.

For another 11 billion years, the Universe will continue to expand, before coming to a halt and reversing direction, collapsing down to the hypothetical Big Crunch, say physicists Hoang Nhan Luu of Donostia International Physics Center in Spain, Yu-Cheng Qiu of Shanghai Jiao Tong University in China, and corresponding author Henry Tye of Cornell University in the US.