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Exotic black holes could be a byproduct of dark matter

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Exotic black holes could be a byproduct of dark matter


For every kilogram of matter that we can see — from the computer on your desk to distant stars and galaxies — there are 5 kilograms of invisible matter that suffuse our surroundings. This “dark matter” is a mysterious entity that evades all forms of direct observation yet makes its presence felt through its invisible pull on visible objects.

Fifty years ago, physicist Stephen Hawking offered one idea for what dark matter might be: a population of black holes, which might have formed very soon after the Big Bang. Such “primordial” black holes would not have been the goliaths that we detect today, but rather microscopic regions of ultradense matter that would have formed in the first quintillionth of a second following the Big Bang and then collapsed and scattered across the cosmos, tugging on surrounding space-time in ways that could explain the dark matter that we know today.

Now, MIT physicists have found that this primordial process also would have produced some unexpected companions: even smaller black holes with unprecedented amounts of a nuclear-physics property known as “color charge.”

These smallest, “super-charged” black holes would have been an entirely new state of matter, which likely evaporated a fraction of a second after they spawned. Yet they could still have influenced a key cosmological transition: the time when the first atomic nuclei were forged. The physicists postulate that the color-charged black holes could have affected the balance of fusing nuclei, in a way that astronomers might someday detect with future measurements. Such an observation would point convincingly to primordial black holes as the root of all dark matter today.

“Even though these short-lived, exotic creatures are not around today, they could have affected cosmic history in ways that could show up in subtle signals today,” says David Kaiser, the Germeshausen Professor of the History of Science and professor of physics at MIT. “Within the idea that all dark matter could be accounted for by black holes, this gives us new things to look for.”

Kaiser and his co-author, MIT graduate student Elba Alonso-Monsalve, have published their study today in the journal Physical Review Letters.

A time before stars

The black holes that we know and detect today are the product of stellar collapse, when the center of a massive star caves in on itself to form a region so dense that it can bend space-time such that anything — even light — gets trapped within. Such “astrophysical” black holes can be anywhere from a few times as massive as the sun to many billions of times more massive.

“Primordial” black holes, in contrast, can be much smaller and are thought to have formed in a time before stars. Before the universe had even cooked up the basic elements, let alone stars, scientists believe that pockets of ultradense, primordial matter could have accumulated and collapsed to form microscopic black holes that could have been so dense as to squeeze the mass of an asteroid into a region as small as a single atom. The gravitational pull from these tiny, invisible objects scattered throughout the universe could explain all the dark matter that we can’t see today.

If that were the case, then what would these primordial black holes have been made from? That’s the question Kaiser and Alonso-Monsalve took on with their new study.

“People have studied what the distribution of black hole masses would be during this early-universe production but never tied it to what kinds of stuff would have fallen into those black holes at the time when they were forming,” Kaiser explains.

Super-charged rhinos

The MIT physicists looked first through existing theories for the likely distribution of black hole masses as they were first forming in the early universe.

“Our realization was, there’s a direct correlation between when a primordial black hole forms and what mass it forms with,” Alonso-Monsalve says. “And that window of time is absurdly early.”

She and Kaiser calculated that primordial black holes must have formed within the first quintillionth of a second following the Big Bang. This flash of time would have produced “typical” microscopic black holes that were as massive as an asteroid and as small as an atom. It would have also yielded a small fraction of exponentially smaller black holes, with the mass of a rhino and a size much smaller than a single proton.

What would these primordial black holes have been made from? For that, they looked to studies exploring the composition of the early universe, and specifically, to the theory of quantum chromodynamics (QCD) — the study of how quarks and gluons interact.

Quarks and gluons are the fundamental building blocks of protons and neutrons — elementary particles that combined to forge the basic elements of the periodic table. Immediately following the Big Bang, physicists estimate, based on QCD, that the universe was an immensely hot plasma of quarks and gluons that then quickly cooled and combined to produce protons and neutrons.

The researchers found that, within the first quintillionth of a second, the universe would still have been a soup of free quarks and gluons that had yet to combine. Any black holes that formed in this time would have swallowed up the untethered particles, along with an exotic property known as “color charge” — a state of charge that only uncombined quarks and gluons carry.

“Once we figured out that these black holes form in a quark-gluon plasma, the most important thing we had to figure out was, how much color charge is contained in the blob of matter that will end up in a primordial black hole?” Alonso-Monsalve says.

Using QCD theory, they worked out the distribution of color charge that should have existed throughout the hot, early plasma. Then they compared that to the size of a region that would collapse to form a black hole in the first quintillionth of a second. It turns out there wouldn’t have been much color charge in most typical black holes at the time, as they would have formed by absorbing a huge number of regions that had a mix of charges, which would have ultimately added up to a “neutral” charge.

But the smallest black holes would have been packed with color charge. In fact, they would have contained the maximum amount of any type of charge allowed for a black hole, according to the fundamental laws of physics. Whereas such “extremal” black holes have been hypothesized for decades, until now no one had discovered a realistic process by which such oddities actually could have formed in our universe.

The super-charged black holes would have quickly evaporated, but possibly only after the time when the first atomic nuclei began to form. Scientists estimate that this process started around one second after the Big Bang, which would have given extremal black holes plenty of time to disrupt the equilibrium conditions that would have prevailed when the first nuclei began to form. Such disturbances could potentially affect how those earliest nuclei formed, in ways that might some day be observed.

“These objects might have left some exciting observational imprints,” Alonso-Monsalve muses. “They could have changed the balance of this versus that, and that’s the kind of thing that one can begin to wonder about.”



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Early dark energy could resolve cosmology’s two biggest puzzles

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Exotic black holes could be a byproduct of dark matter


A new study by MIT physicists proposes that a mysterious force known as early dark energy could solve two of the biggest puzzles in cosmology and fill in some major gaps in our understanding of how the early universe evolved.

One puzzle in question is the “Hubble tension,” which refers to a mismatch in measurements of how fast the universe is expanding. The other involves observations of numerous early, bright galaxies that existed at a time when the early universe should have been much less populated.

Now, the MIT team has found that both puzzles could be resolved if the early universe had one extra, fleeting ingredient: early dark energy. Dark energy is an unknown form of energy that physicists suspect is driving the expansion of the universe today. Early dark energy is a similar, hypothetical phenomenon that may have made only a brief appearance, influencing the expansion of the universe in its first moments before disappearing entirely.

Some physicists have suspected that early dark energy could be the key to solving the Hubble tension, as the mysterious force could accelerate the early expansion of the universe by an amount that would resolve the measurement mismatch.

The MIT researchers have now found that early dark energy could also explain the baffling number of bright galaxies that astronomers have observed in the early universe. In their new study, reported in the Monthly Notices of the Royal Astronomical Society, the team modeled the formation of galaxies in the universe’s first few hundred million years. When they incorporated a dark energy component only in that earliest sliver of time, they found the number of galaxies that arose from the primordial environment bloomed to fit astronomers’ observations.

You have these two looming open-ended puzzles,” says study co-author Rohan Naidu, a postdoc in MIT’s Kavli Institute for Astrophysics and Space Research. “We find that in fact, early dark energy is a very elegant and sparse solution to two of the most pressing problems in cosmology.”

The study’s co-authors include lead author and Kavli postdoc Xuejian (Jacob) Shen, and MIT professor of physics Mark Vogelsberger, along with Michael Boylan-Kolchin at the University of Texas at Austin, and Sandro Tacchella at the University of Cambridge.

Big city lights

Based on standard cosmological and galaxy formation models, the universe should have taken its time spinning up the first galaxies. It would have taken billions of years for primordial gas to coalesce into galaxies as large and bright as the Milky Way.

But in 2023, NASA’s James Webb Space Telescope (JWST) made a startling observation. With an ability to peer farther back in time than any observatory to date, the telescope uncovered a surprising number of bright galaxies as large as the modern Milky Way within the first 500 million years, when the universe was just 3 percent of its current age.

“The bright galaxies that JWST saw would be like seeing a clustering of lights around big cities, whereas theory predicts something like the light around more rural settings like Yellowstone National Park,” Shen says. “And we don’t expect that clustering of light so early on.”

For physicists, the observations imply that there is either something fundamentally wrong with the physics underlying the models or a missing ingredient in the early universe that scientists have not accounted for. The MIT team explored the possibility of the latter, and whether the missing ingredient might be early dark energy.

Physicists have proposed that early dark energy is a sort of antigravitational force that is turned on only at very early times. This force would counteract gravity’s inward pull and accelerate the early expansion of the universe, in a way that would resolve the mismatch in measurements. Early dark energy, therefore, is considered the most likely solution to the Hubble tension.

Galaxy skeleton

The MIT team explored whether early dark energy could also be the key to explaining the unexpected population of large, bright galaxies detected by JWST. In their new study, the physicists considered how early dark energy might affect the early structure of the universe that gave rise to the first galaxies. They focused on the formation of dark matter halos — regions of space where gravity happens to be stronger, and where matter begins to accumulate.

“We believe that dark matter halos are the invisible skeleton of the universe,” Shen explains. “Dark matter structures form first, and then galaxies form within these structures. So, we expect the number of bright galaxies should be proportional to the number of big dark matter halos.”

The team developed an empirical framework for early galaxy formation, which predicts the number, luminosity, and size of galaxies that should form in the early universe, given some measures of “cosmological parameters.” Cosmological parameters are the basic ingredients, or mathematical terms, that describe the evolution of the universe.

Physicists have determined that there are at least six main cosmological parameters, one of which is the Hubble constant — a term that describes the universe’s rate of expansion. Other parameters describe density fluctuations in the primordial soup, immediately after the Big Bang, from which dark matter halos eventually form.

The MIT team reasoned that if early dark energy affects the universe’s early expansion rate, in a way that resolves the Hubble tension, then it could affect the balance of the other cosmological parameters, in a way that might increase the number of bright galaxies that appear at early times. To test their theory, they incorporated a model of early dark energy (the same one that happens to resolve the Hubble tension) into an empirical galaxy formation framework to see how the earliest dark matter structures evolve and give rise to the first galaxies.

“What we show is, the skeletal structure of the early universe is altered in a subtle way where the amplitude of fluctuations goes up, and you get bigger halos, and brighter galaxies that are in place at earlier times, more so than in our more vanilla models,” Naidu says. “It means things were more abundant, and more clustered in the early universe.”

“A priori, I would not have expected the abundance of JWST’s early bright galaxies to have anything to do with early dark energy, but their observation that EDE pushes cosmological parameters in a direction that boosts the early-galaxy abundance is interesting,” says Marc Kamionkowski, professor of theoretical physics at Johns Hopkins University, who was not involved with the study. “I think more work will need to be done to establish a link between early galaxies and EDE, but regardless of how things turn out, it’s a clever — and hopefully ultimately fruitful — thing to try.”

We demonstrated the potential of early dark energy as a unified solution to the two major issues faced by cosmology. This might be an evidence for its existence if the observational findings of JWST get further consolidated,” Vogelsberger concludes. “In the future, we can incorporate this into large cosmological simulations to see what detailed predictions we get.”

This research was supported, in part, by NASA and the National Science Foundation.



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Plant-derived secondary organic aerosols can act as mediators of plant-plant interactions

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A new study published in Science reveals that plant-derived secondary organic aerosols (SOAs) can act as mediators of plant-plant interactions. This research was conducted through the cooperation of chemical ecologists, plant ecophysiologists and atmospheric physicists at the University of Eastern Finland.

It is well known that plants release volatile organic compounds (VOCs) into the atmosphere when damaged by herbivores. These VOCs play a crucial role in plant-plant interactions, whereby undamaged plants may detect warning signals from their damaged neighbours and prepare their defences. “Reactive plant VOCs undergo oxidative chemical reactions, resulting in the formation of secondary organic aerosols (SOAs). We wondered whether the ecological functions mediated by VOCs persist after they are oxidated to form SOAs,” said Dr. Hao Yu, formerly a PhD student at UEF, but now at the University of Bern.

The study showed that Scots pine seedlings, when damaged by large pine weevils, release VOCs that activate defences in nearby plants of the same species. Interestingly, the biological activity persisted after VOCs were oxidized to form SOAs. The results indicated that the elemental composition and quantity of SOAs likely determines their biological functions.

“A key novelty of the study is the finding that plants adopt subtly different defence strategies when receiving signals as VOCs or as SOAs, yet they exhibit similar degrees of resistance to herbivore feeding,” said Professor James Blande, head of the Environmental Ecology Research Group. This observation opens up the possibility that plants have sophisticated sensing systems that enable them to tailor their defences to information derived from different types of chemical cue.

“Considering the formation rate of SOAs from their precursor VOCs, their longer lifetime compared to VOCs, and the atmospheric air mass transport, we expect that the ecologically effective distance for interactions mediated by SOAs is longer than that for plant interactions mediated by VOCs,” said Professor Annele Virtanen, head of the Aerosol Physics Research Group. This could be interpreted as plants being able to detect cues representing close versus distant threats from herbivores.

The study is expected to open up a whole new complex research area to environmental ecologists and their collaborators, which could lead to new insights on the chemical cues structuring interactions between plants.



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Folded or cut, this lithium-sulfur battery keeps going

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Most rechargeable batteries that power portable devices, such as toys, handheld vacuums and e-bikes, use lithium-ion technology. But these batteries can have short lifetimes and may catch fire when damaged. To address stability and safety issues, researchers reporting in ACS Energy Letters have designed a lithium-sulfur (Li-S) battery that features an improved iron sulfide cathode. One prototype remains highly stable over 300 charge-discharge cycles, and another provides power even after being folded or cut.

Sulfur has been suggested as a material for lithium-ion batteries because of its low cost and potential to hold more energy than lithium-metal oxides and other materials used in traditional ion-based versions. To make Li-S batteries stable at high temperatures, researchers have previously proposed using a carbonate-based electrolyte to separate the two electrodes (an iron sulfide cathode and a lithium metal-containing anode). However, as the sulfide in the cathode dissolves into the electrolyte, it forms an impenetrable precipitate, causing the cell to quickly lose capacity. Liping Wang and colleagues wondered if they could add a layer between the cathode and electrolyte to reduce this corrosion without reducing functionality and rechargeability.

The team coated iron sulfide cathodes in different polymers and found in initial electrochemical performance tests that polyacrylic acid (PAA) performed best, retaining the electrode’s discharge capacity after 300 charge-discharge cycles. Next, the researchers incorporated a PAA-coated iron sulfide cathode into a prototype battery design, which also included a carbonate-based electrolyte, a lithium metal foil as an ion source, and a graphite-based anode. They produced and then tested both pouch cell and coin cell battery prototypes.

After more than 100 charge-discharge cycles, Wang and colleagues observed no substantial capacity decay in the pouch cell. Additional experiments showed that the pouch cell still worked after being folded and cut in half. The coin cell retained 72% of its capacity after 300 charge-discharge cycles. They next applied the polymer coating to cathodes made from other metals, creating lithium-molybdenum and lithium-vanadium batteries. These cells also had stable capacity over 300 charge-discharge cycles. Overall, the results indicate that coated cathodes could produce not only safer Li-S batteries with long lifespans, but also efficient batteries with other metal sulfides, according to Wang’s team.

The authors acknowledge funding from the National Natural Science Foundation of China; the Natural Science Foundation of Sichuan, China; and the Beijing National Laboratory for Condensed Matter Physics.



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