The small satellite that aims to reveal what dark matter is made of

Arrakhis: the tiny satellite that aims to reveal what dark matter is made of

Milky Way over sand dunes in Cervantes, Australia. Credit: Nik Coli/Shutterstock

The European Space Agency (ESA) recently announced a new mission of its science program: a small telescope that orbits the Earth called Arrakhis. But although its name is inspired by the science fiction novel DuneYou won’t be looking for sandworms or “spice” on a desert planet.

Instead, this nimble satellite will vastly outweigh its weight and attempt to track one of the most elusive and mysterious substances in the universe: darkness. affair. This is the term given to hypothetical invisible matter that is believed to be more abundant than normal matter and have a similar gravitational effect on its surroundings.

The mission is classified as fast (F), which means it is smaller, more focused, and has a faster turnaround time (less than ten years to launch) than other ESA mission types. The agency’s previous F mission, selected in 2019, is called comet interceptor. Already stationed at a stable point in the Solar System, this probe is waiting for a comet to appear and fly alongside it, something that will happen around the time Arrakhis launches in the early 2030s.

Follow the light

Since dark matter still evades detection, the mission will target light sources that are sensitive to it. We expect normal matter, the matter that actually emits light, like stars in galaxies—to move primarily under the influence of dark matter, which is more abundant.


Arrakhis: el diminuto satélite que pretende revelar de qué está hecha la materia oscuraCC BY SA“/>

Like the Milky Way, the galaxy NGC 5907 harbors faint streams of stars that surround it. Credit: Wikipedia/R Jay Gabany (Blackbird Observatory) – collaboration; D.Martinez-Delgado(IAC, MPIA), J.Penarrubia (U.Victoria) I. Trujillo (IAC) S.Majewski (U.Virginia), M.Pohlen (Cardiff), CC BY SA

We think the underlying dark matter moves entire galaxies back and forth, like scattered lighthouses across an invisible ocean. Their navigation is bumpy, however, as dark matter is thought to be unevenly distributed throughout the universe, forming a “cosmic web” over great distances and having a lumpier appearance on the scales of galaxies. Some of these clusters should be populated by small galaxies called dwarf galaxieswhile others would be composed entirely of dark matter.

There are also debris from those dwarf galaxies that venture too close to the host galaxies they orbit. As the surrounding dark matter tears these galaxies apart via gravitational tides, they begin to unravel into long streams of stars that engulf vast swaths of space. These thin veils of light are another connection to the invisible. By counting and measuring their shapes, we can infer what kind of particles dark matter is made of, and ultimately what cosmological model is the most accurate.

The agglomeration in space is a strong prediction of our cosmological models, since it simply represents the result of the action of gravity on matter. However, our models give conflicting predictions about the number of these groups, which could be higher or lower depending on what kind of particle or particles we assume that dark matter is composed.

In the “standard” model of cosmology, dark matter particles are assumed to be “cold”, meaning they are heavy and slow moving (an example would be “weakly interacting massive particles” or Wimps). This implies that our Milky Way will contain hundreds of dark matter clumps, some of which will contain dwarf galaxies. But the problem is that we only see a few dozen dwarf galaxies around us, which is very puzzling. It could mean that most of these clumps are made of dark matter.

However, cosmologists have other viable ideas. For example, if dark matter is “warm”—which means that the particles are much lighter and faster, like sterile neutrinos“There would be a lot fewer groups to begin with. Observations can give us the final clue as to which model is correct, but to get there, we first need a dwarf galaxy census orbiting the Milky Way.

The tip of the iceberg

There are strong indications that the dwarf galaxies discovered so far near the Milky Way or other large galaxies are just the tip of the iceberg, and that many more remain hidden after the light of their hosts. Arrakhis will be able to discover this missing population even at great distances from us.

Observing this faint starlight has proven challenging even for Earth’s largest telescopes, requiring very deep imaging and surveying of large portions of the sky. In addition, the Earth’s atmosphere is an obstacle. Arrakhis will observe from space, with an innovative camera that delves into both the optical and near-infrared portion of the spectrum, and with a much wider field of view. (By the way, this type of camera can also look back at earth with excellent resolution.)

The hundred or so Milky Way-like systems to be observed are about 100 million light-years away, where only a few dwarf galaxies have been discovered so far, and there are still no stellar streams. When we know the number of dwarf galaxies that will soon be discovered and how they will look distributed in space, we should be able to pin down the correct cosmological model.

Arrakhis will find many of the missing pieces to the puzzle that dark matter provides, complementing what we already know about the nearby universe and what we will learn in the future from other future telescopes, such as Euclid or the Vera Rubin Observatory.

The hope is that these combined and detailed observations will eventually reveal the dark matter mystery, and help us understand what makes up most of the matter in the cosmos.

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