Arrakhis: a tiny satellite designed to show what dark matter is made of

This article was originally published in The Conversation. (will open in a new tab) The publication published an article in Expert Voices: Op-Ed & Insights on

Andrea Font (will open in a new tab)Lecturer in Theoretical Astrophysics, Liverpool John Moores University

The European Space Agency (ESA) recently announced (will open in a new tab) new mission of its scientific program (will open in a new tab): A small telescope in Earth orbit, dubbed Arrakhis. But while its name is inspired by the science fiction novel Dune (will open in a new tab)“, he will not look for sandworms or “spice” on a desert planet.

Instead, this nimble satellite will jump above its weight and try to track down one of the most elusive and mysterious substances in the universe: dark matter. It is a term given to hypothetical invisible matter thought to be more common than ordinary matter and to have a similar gravitational effect on its surroundings.

On the subject: What part of the Universe is made up of dark matter?

The mission is classified as fast (F), which means it is smaller, more focused, and has a faster turnaround (less than ten years before launch) than other ESA mission types. The agency’s previous F-mission, selected in 2019, is called Comet Interceptor. (will open in a new tab). This probe, already parked at a stable point in the solar system, is waiting for a comet to appear and fly past it, which should happen around the time Arrakhis launches in the early 2030s.

follow the light

As dark matter still eludes detection (will open in a new tab), the mission will target light sources sensitive to it. We expect that normal matter – stuff that actually emits light, like stars in galaxies – will move mostly under the influence of dark matter, which is more abundant.

We believe that entire galaxies are moving from side to side under the influence of the dark matter underlying them, like lighthouses spread over an invisible ocean. However, their voyage is bumpy, as dark matter is thought to be unevenly distributed throughout the universe, forming a “cosmic web.” (will open in a new tab)‘at vast distances, and on a galactic scale, they look more lumpy. Some of these clumps must be inhabited by small galaxies called dwarf galaxies, while others are made entirely of dark matter.

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

Like the Milky Way, NGC 5907 contains faint streams of stars that surround it. (Image credit: Wikipedia/R. Jay Gabani (Blackbird Observatory) Collaboration; D. Martinez-Delgado (IAC, MPIA), J. Penarrubia (W. Victoria) I. Trujillo (IAC) S. Majewski (W. Virginia) , Mr Paulin (Cardiff), CC BY-SA) (will open in a new tab)

Lumpiness in space is a reliable prediction of our cosmological models because it simply represents the effect of gravity on matter. However, our models give conflicting predictions about the number of these clumps, which can be more or less depending on the type of particle or particles. (will open in a new tab) we assume that dark matter is composed of.

In the “standard” model of cosmology, dark matter particles are considered “cold”. (will open in a new tab)which means they are heavy and slow moving (an example would be “weakly interacting massive particles” or WIMPs). (will open in a new tab)). This means that our Milky Way will contain hundreds of clumps of dark matter, 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. This may mean that most of these clumps are composed of dark matter.

However, cosmologists have other viable ideas. For example, if dark matter is “warm” (will open in a new tab) – this means that particles are much lighter and faster, such as sterile neutrinos (will open in a new tab) – for a start there would be much less lumps.

Observations can give us a definitive clue as to which model is correct, but to do that, we first need an accurate census of dwarf galaxies. (will open in a new tab) revolves around the Milky Way.

tip of the iceberg

There are strong indications that dwarf galaxies found near the Milky Way or other large galaxies are just the tip of the iceberg, and that many more remain hidden. (will open in a new tab)behind the light of their masters. Arrakhis will be able to locate this missing population even at a great distance from us.

A galaxy modeled with cold dark matter (left) and warm dark matter (right). There are far more clumps of cold dark matter, which can contain dwarf galaxies, than clusters of warm dark matter. (Image credit: Aquarius/Virgo/ICC Durham University., CC BY-NC-ND) (will open in a new tab)

Observing this faint starlight has proven challenging even for the largest telescopes on Earth, as it requires very deep imaging and large areas of the sky to be captured. In addition, the earth’s atmosphere is a hindrance.

Arrakhis will be watching from space with an innovative camera that looks deeper into both the optical and near-infrared spectrum, and has a much wider field of view. (By the way, this type of camera can also look at the Earth (will open in a new tab) with excellent resolution.)

The hundred or so systems like the Milky Way that will be observed are about 100 million light-years away, where only a few dwarf galaxies (will open in a new tab) have not yet been discovered, and there are no stellar streams yet. When we know the number of dwarf galaxies that will soon be discovered and how they will be distributed in space (will open in a new tab)we must be able to determine the correct cosmological model.

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

It is hoped that these detailed combined observations will finally solve the mystery of dark matter and help us understand what most of the matter in space is made of.

This article is republished from The Conversation (will open in a new tab) under a Creative Commons license. Read original article (will open in a new tab).

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