Physicists are reimagining dark matter

Dark matter emits no light and scientists only know it exists because when they look into the night sky, they see galaxies spinning much faster than they expected. The laws of physics dictate that these galaxies should tear themselves into pieces at such a rapid rate. But astronomers don’t see galaxies flying apart, meaning they contain much more stuff than can be seen as stars, and so there is more gravitational force holding everything together.

For decades astronomers have been able to measure how dark matter behaves and pinpoint the parts of the sky where it should exist. But what exactly it is—remains a mystery. The leading hypothesis, known as “cold dark matter” (CDM), suggests that dark matter is a fluid of particles that move slowly relative to the speed of light, and interact with each other and the universe. Mainly interacts with everything else. Gravity.

give up the ghost

But after more than half a century of fruitless searching, the CDM is in trouble, due to a series of recent astronomical anomalies. Physicists are moving toward a different theoretical framework, known as “self-interacting dark matter” (SIDM), which proposes the existence of a hidden universe of dark particles and dark forces that exist beyond normal There exists a parallel to the familiar particles and forces of matter. This dark universe may also have had its own “Dark Big Bang,” which would have occurred sometime after the more familiar Big Bang that began the universe 14 billion years ago.

The CDM is a pillar of the Standard Model of cosmology. The second, aimed at explaining dark energy, is denoted by the Greek letter lambda. Together these details almost perfectly reproduce the evolution of the large-scale structures that astronomers see in the universe today – galaxies, galactic clusters, and giant galactic superclusters.

The theory is agnostic about what particles actually make up dark matter. But the most promising candidates so far are known as Weakly Interacting Massive Particles (WIMPs). These are particles with masses up to 1,000 times that of a proton and models of the early universe predict that – if WIMPs exist – they should be present in just the right amount for dark matter today, a happy coincidence known as “WIMP”. Is known. Miracle”.

Dozens of highly sensitive detectors have been built around the world to detect WIMPs. Many hide hundreds of meters underground to avoid the noise and have the greatest chance of detecting the subtle interactions that WIMPs make with normal matter. There are also hopes that particle accelerators like the Large Hadron Collider (LHC) at CERN in Switzerland might discover a WIMP candidate or two in the sizzle of high-energy particle collisions. Unfortunately, despite more than 40 years and millions of dollars spent on the search, WIMPs remain stubbornly elusive.

This is not the only problem with CDM. This theory works great at reproducing the large-scale structures of the universe, but, as new telescopes allow astronomers to look deeper into distant galaxies and rapidly improving supercomputer simulations at smaller scales, While the implications of the CDM allow us to explore, it is becoming clear that this theory does not do that well at reproducing the more subtle structures of the universe.

the dark side of the Moon

Two discrepancies emerge. The first concerns the structure of galaxies. The CDM implies that, because it moves slowly and feels the effects of gravity, dark matter should accumulate to unfathomably high density in the cores of galaxies. But this is not what astronomers have seen. As you travel from the edge of a real galaxy toward its center, the density of dark matter increases. But, several thousand light-years away from the center, the density reaches a plateau and then remains constant until the core.

The second anomaly concerns satellite galaxies. The CDM implies that larger galaxies should be orbited by thousands of smaller satellite galaxies. But this isn’t even what astronomers have seen. The Milky Way, and galaxies like it, are orbited by a handful of satellite galaxies and the ones astronomers see are also smaller than the CDM predicts.

These discrepancies may be explained by SIDM. Simple versions of the theory propose only a new elementary dark matter particle and a new fundamental “dark force”; More complex versions contain a smorgasbord of new dark particles and forces, which constantly interact with each other. More complex versions are inspired by the well-established Standard Model of particle physics, which is a quantum mechanical description of all the particles (such as quarks and electrons) and forces (strong, weak, and electromagnetic) of ordinary matter.

One version of SIDM introduces a new dark force that is equivalent to electromagnetism, which is felt by a hypothetical particle with a “dark” charge – a dark electron, essentially – which interacts with a “dark photon”. -Negotiates by providing. However, unlike the familiar photon, which is massless and carries the electromagnetic force, dark photons can potentially carry mass.

SIDM addresses the problems affecting CDM while preserving all the features that make CDM attractive in the first place. If dark matter could interact, its particles would be able to scatter from each other, creating a pressure at the center of galaxies that prevents dark matter from reaching unfathomably high density. This is similar to the pressure in a balloon, which is caused by air molecules bouncing off each other. This would explain why the density of dark matter in the galactic core is so much lower than predicted by the CDM.

This idea kills two birds with one stone. In a paper published in the journal Monthly Notices of the Royal Astronomical Society in late 2022, Victor Moreno and colleagues at Durham University showed that galaxies that have less concentrated dark matter in their cores more violently cannibalize satellite dwarf galaxies. are, known as “”. Tidal stripping”, where the gravitational pull of a larger galaxy strips both matter and dark matter from its satellites. This would explain why there are fewer satellites than predicted by the CDM and why those observed are smaller than predicted by the CDM. .They have been reduced to a smaller size or made non-existent.

In addition to resolving issues with CDM, SIDM also makes predictions that allow it to be tested against CDM. In the conditions that defined the early universe, but thankfully no longer exist in the universe today, both the CDM and SIDM allow the possibility of “dark stars”. These are not stars as we know them today, but, rather, solar-system-sized clouds of gas in which dark matter and its antimatter counterpart—dark antimatter—are constantly waging a war of mutual annihilation because of such interactions. Releasing more energy faster than even nuclear fusion, these huge diffuse clouds of gas would glow with light. An entire modern galaxy.

In a paper published in the journal PNAS in July 2023, Katherine Freese, a particle physicist at the University of Texas, and her colleagues identified three ancient objects in the data collected by James: quite old, quite bright and dark. -Stars were compact enough to be candidates. Webb Space Telescope. “If these do become dark stars – and this can be confirmed with more data – their mass, temperature and emission spectra could one day be used to distinguish between dark-matter models, including CDMs and SIDMs ,” she says.

Dr. Freeze also makes a case for a dark Big Bang that could have given rise to dark matter independently of normal matter in the days following the Big Bang. The traditional model of the universe states that matter and dark matter originated at the same time. However, the earliest evidence of dark matter emerged not long after the early evolution of the universe, when cosmic structure began to form.

One explanation for this is that matter and dark matter did not, in fact, appear together, but that dark matter entered the universe in a second cataclysmic release of energy from the vacuum – the Dark Big Bang – which occurred a month after the conventional Was after. big Bang. The model that Dr. Freese and his co-author Martin Winkler discovered would explain why dark matter can be completely separated from conventional matter and also naturally produce SIDM candidates. If there were such a dark Big Bang, it would leave a clear signature – a pattern in the frequencies of gravitational waves humming in the universe – that could be picked up by future gravitational-wave detectors.

Finally, there may also be ways to directly detect self-interacting dark matter. The fact that SIDM candidates are much lighter than WIMPs means that conventional WIMP detectors working over the past few decades are likely to miss them. New experiments could change this.

The FASER detector at the LHC, which began collecting data in 2022, is designed to detect extremely light dark-matter particles, such as dark photons, that can be produced in collisions at the LHC. Similarly, the SuperCDMS experiment in SNOLAB will start in 2024. Located deep underground in a working mine in Canada, SuperCDMS is designed to detect microscopic collisions between light dark matter particles – including SIDM candidates – and atoms in silicon and germanium crystals.

no longer afraid of the dark

However, for now, dark matter remains steadfast in its refusal to reveal its secrets. Fortunately, physicists are not short of ideas. SIDM may not be what reveals the true nature of dark matter, but an idea will eventually do so.

Meanwhile, it offers a romantic view of the universe. There’s comfort in the idea that, somewhere out there, astronomers looking through telescopes made of dark atoms that magnify dark photons may also be scratching their heads, wondering where that tiny amount of matter from their universe might be. Why is it missing?

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© 2024, The Economist Newspaper Limited. All rights reserved. From The Economist, published under license. Original content can be found at www.economist.com

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Published: 06 May 2024, 05:00 PM IST