200912 dark matter

SpaceWatch: A matter about dark matter

Dark matter theories may lack something

Astronomers suspect something is missing in current theories of how dark matter behaves.

An international team has uncovered an unexpected discrepancy between observations of the dark matter concentrations in a sample of massive galaxy clusters and theoretical computer simulations of how dark matter should be distributed in clusters.

The findings, published in the journal Science, indicate that some small-scale concentrations of dark matter produce lensing effects that are 10 times stronger than expected.

“There’s a feature of the real Universe that we are simply not capturing in our current theoretical models,” says senior author Priyamvada Natarajan, from Yale University, US.

“This could signal a gap in our current understanding of the nature of dark matter and its properties, as these exquisite data have permitted us to probe the detailed distribution of dark matter on the smallest scales.”

Astronomers “map” the distribution of dark matter within galaxy clusters via the bending of light that the galaxies produce, a concept called gravitational lensing.

By combining imaging from the Hubble Space Telescope and spectroscopy from ESO’s Very Large Telescope (VLT) in Chile, the astronomers were able to assemble a well-calibrated, high-resolution map of the mass distribution of dark matter in each cluster.

They then compared the maps with samples of simulated galaxy clusters with similar masses located at roughly the same distances. The clusters in the computer model did not show any of the same level of dark-matter concentration on the smallest scales – the scales associated with individual cluster galaxies.

“We have done a lot of careful testing in comparing the simulations and data in this study, and our finding of the mismatch persists,” says lead author Massimo Meneghetti of the INAF-Observatory of Astrophysics and Space Science in Italy. Investigations will continue.

Bennu is getting its rocks off

The asteroid Bennu. Credit: NASA, Goddard, University of Arizona, Lockheed Martin

Detailed observations of the asteroid Bennu reveal that it is ejecting material on a regular basis, adding to an emerging picture of asteroids as quite dynamic worlds.

It is particularly active during two-hour afternoon and evening timeframes, scientists with the OSIRIS-REx NASA mission write in the Journal of Geophysical Research: Planets.

“We thought that Bennu’s boulder-covered surface was the wildcard discovery at the asteroid, but these particle events definitely surprised us,” says principal investigator Dante Lauretta, from the University of Arizona.

The authors considered various mechanisms that could cause the phenomena, including released water vapour, impacts by small space rocks known as meteoroids and rocks cracking from thermal stress.

They say the latter two were found to be the most likely, confirming predictions about Bennu’s environment based on ground observations preceding the space mission.

The video animation below shows the trajectories of particles after their ejection. It emphasises the four largest events detected from December 2018 to September 2019. Additional particles, some with lifetimes of several days, that are not related to the ejections are also visible.

Using software algorithms, the scientists determined that the largest of the particles is about six centimetres in diameter. And as they are ejected at low velocities – like a shower of tiny pebbles in super slow motion – it is considered they are not a threat to the spacecraft.

OSIRIS-REx will get close enough to grab a sample from the surface of Bennu in October and return it to Earth on in September 2023.

Credit: M. Brozovic/JPL/Caltech/NASA/University of Arizona

Are Jupiter’s moons warming each other?

Jupiter’s moons are hotter than they should be, given how far they are from the Sun. Some have warm enough interiors to host oceans of liquid water.

It’s because of a process called tidal heating: gravitational tugs from the moons and Jupiter itself stretch and squish the moons enough to warm them. But the process might not work quite how we thought.

It’s been assumed the planet was responsible for most of the tidal heating, but a study led by Hamish Hay from the Jet Propulsion Laboratory in California, US, and published in Geophysical Research Letters suggests that moon-moon interactions may be the key players.

According to the researchers’ model, Jupiter’s influence alone can’t create tides with the right frequency to resonate with the moons because the moons’ oceans are thought to be too thick. It’s only when they added in the gravitational influence of the other moons that they started to see tidal forces approaching the natural frequencies of the moons.

Jupiter’s largest moons in order of distance from the planet: Io, Europa, Ganymede and Callisto. Credit: NASA

When the tides generated by other objects in Jupiter’s moon system match each moon’s own resonant frequency, the moon begins to experience more heating than that due to tides raised by Jupiter alone, and in the most extreme cases, this could result in the melting of ice or rock internally.

There are some caveats to the initial findings, however: the model assumes that tidal resonances never get too extreme. Hay and his team want to return to this variable and see what happens when they lift that constraint.

Big search but no alien signs

Astronomers using the Murchison Widefield Array (MWA) telescope in outback Western Australia have completed the deepest and broadest search at low frequencies for alien technologies, but come away empty handed.

The MWA has an extraordinarily wide field-of-view that allows millions of stars to be observed simultaneously.

However, despite scanning a patch of sky around the Vela constellation known to include at least 10 million stars, and looking “more than 100 times broader and deeper than ever before”, according to Chenoa Tremblay from Australia’s CSIRO, they detected no powerful radio emissions (technosignatures) that could indicate the presence of an intelligent source.

This is reported officially in a paper in the journal Publications of the Astronomical Society of Australia.

Dipole antennas of the MWA radio telescope in Western Australia. Credit: Dragonfly Media.

While no doubt disappointed, Tremblay and co-author Steven Tingay, from the Curtin University node of the International Centre for Radio Astronomy Research, are not completely surprised.

“[E]ven though this was a really big study, the amount of space we looked at was the equivalent of trying to find something in the Earth’s oceans but only searching a volume of water equivalent to a large backyard swimming pool,” says Tingay.

“Since we can’t really assume how possible alien civilisations might utilise technology, we need to search in many different ways. Using radio telescopes, we can explore an eight-dimensional search space.”

And the search will continue, with even more tech behind it.

The new Square Kilometre Array (SKA), which will have telescopes alongside the MWA in Western Australia and in South Africa, will be 50 times more sensitive. Early science observations with SKA are expected to start in the mid-2020s with a partial array.

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