Dark matter hunters explore the strangest of places

Search for dark matter from Earth to distant galaxies

Dark matter hunters have been busy. In June, a research team said it found evidence for self-interacting dark matter in an enormous galaxy cluster. In early July, another team said dark matter might be causing a glow on Jupiter’s dark side.

Now three more research groups say they’re extending the hunt for dark matter to new extremes. The first is looking 250,000 light-years away at the Draco dwarf galaxy. The second team thinks we can detect dark matter just over our heads in Earth’s atmosphere. And a third says the signature of dark matter is imprinted on the Earth itself.

Results of a peer-reviewed, 18-year-long stellar survey showing dark matter density in the Draco galaxy appeared in the Astrophysics Journal in July 2024. The scheme for detecting dark matter in Earth’s ionosphere appeared as a preprint paper in Cornell University’s open-source archive – arXiv.org – in May 2024. And the paper proposing a search for dark matter signals in rocks published in July 2024 in the journal Symmetry.

What the heck is dark matter?

We know almost nothing about dark matter. We don’t know what it’s made from. Dark matter can’t be felt or heard … at least not yet. And, of course, it’s so dark we can’t see it.

But – thanks to the work of astronomers like Vera Rubin – we do know that its gravitational effect holds galaxies together. If it weren’t for dark matter, there wouldn’t be enough mass in galaxies to keep stars from flinging into space. That means whatever it is, there’s a whole lot of it. Dark matter makes up roughly 27% of the mass of everything there is.

Dark energy accounts for almost everything else. It makes up 68% or so of the cosmos. That leaves about 5% of the universe’s mass in the form of regular matter like us.

So, dark matter is a sort of cosmic glue holding things together. But we only have tantalizing clues about what it is.

Making a motion map of a pint-sized galaxy’s stars

So the only handle we have on dark matter is its pull on everything else. While the stuff doesn’t appear to interact with electromagnetic radiation – like light, gamma radiation and X-rays – it does warp the fabric of spacetime. In other words, it has gravity.

With that in mind, some very clever people designed computer simulations to show where dark matter should accumulate in galaxies. Their models said it should concentrate at the center of galaxies, in areas known as density cusps. However, some observations suggested dark matter might be evenly dispersed in a galaxy.

So researchers with NASA, ESA and the Space Telescope Science Institute (STScI) spent a decade searching through 18 years’ worth of archival data from the Hubble Space Telescope. With it, they constructed highly detailed models of stellar motions in the Draco dwarf galaxy. STScI’s Eduardo Vitral, the lead author of the study, described what the team found in a NASA press release:

Our models tend to agree more with a cusp-like structure, which aligns with cosmological models. While we cannot definitively say all galaxies contain a cusp-like dark matter distribution, it’s exciting to have such well measured data that surpasses anything we’ve had before.

Scientists are already applying the same method to the Sculptor and Ursa Minor dwarf galaxies. The insight into dark matter this method provides will become more detailed as new instruments like the Nancy Grace Roman Space Telescope come online.

Tuning in dark matter radio signals

Meanwhile, another trio of physicists said the model they created of Earth’s ionosphere shows that dark matter can interact with plasma. And if the plasma has the right frequency – the same as the dark matter – then it produces low-frequency radio emissions.

If that hypothesis is right, then we just need to listen in, said the paper’s authors:

An electrically small dipole antenna targeting the generated radio waves can be orders of magnitude more sensitive to dark photon and axion-like particle dark matter in the relevant mass range. The present study opens up a promising way of testing a hitherto unexplored parameter space which could be further improved with a dedicated instrument.

The catch is dark matter must exist as axions for this to work. These are theoretical elementary particles that scientists first proposed in the 1970s. They, too, have never been definitively detected.

Axion quark nuggets

Another paper appearing on arXiv.org in May 2024 wonders if theoretical dark matter particles are behind strange, yet-to-be-explained events on Earth. Author Ariel Zhitnitsky, a physicist at the University of British Columbia, said the enigmas could be explained by axion quark nuggets striking our planet:

It has been recently argued that there are a number of mysterious observations which are very hard to explain by conventional physics. The mysterious anomalies include (but are not limited to) such unexpected correlations as temperature variation in the stratosphere, the total electron content of the Earth’s atmosphere, the earthquake activity from one hand, and positions of the planets from another hand.

Zhitnitsky’s comment refers to work by other theorists. Unfortunately, the brief available online provides no details. However, the physicist said these and other puzzles may be explained by dark matter striking Earth:

It has been hypothesized that the corresponding mysterious correlations are a result of the ‘streaming invisible matter’ which suddenly becomes very strongly interacting material when entering the Earth’s atmosphere. We propose that some of these (and many other) mysteries might be result of rare (but energetic) events when the so-called axion quark nuggets (AQN) hit the Earth.

Evidence right under our feet?

Theoretical particles come in matched pairs of opposites. In this case, it’s the aptly named “axion antiquark nugget” (AQ¯N). If these nuggets exist, they’ll have very little mass compared to their size. That means we’d need a huge detector, something about, say, the size of Earth.

A paper published just this month (July 2024) proposes a way to look for evidence of dark matter interactions in the rocks below our feet:

This paper presents a new idea for the direct detection of the AQ¯Ns using minerals as natural rock deposits acting as paleo-detectors, where the latent signals of luminescence produced by interactions of AQ¯Ns are registered and can be identified as an increased and symmetrical deposited dose.

In other words, the light produced by AQN/AQ¯N interactions ongoing since the universe began should leave a trace on Earth’s geology. The signal should be detectable, according to the theory, in various minerals from natural deposits. Now, like dark matter, they just have to find it.

Bottom line: Recent research suggests evidence for dark matter is – or might be found – in extreme locations. Researchers are looking in distant galaxies and here on Earth.

Source: Resonant Conversion of Wave Dark Matter in the Ionosphere

Source: HSTPROMO Internal Proper-motion Kinematics of Dwarf Spheroidal Galaxies. I. Velocity Anisotropy and Dark Matter Cusp Slope of Draco

Via NASA

Read more: Do dark matter collisions on Jupiter glow in the infrared?

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