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How primordial black holes might explain dark matter

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The Universe began as a strange sort of soup. Large galaxies hadn’t yet formed, and flying particles were hotter than the centres of stars in the Universe today. Of these, tiny particles known as quarks clumped together to form the building blocks of atoms: neutrons, protons and electrons. Later, through a process called cosmic inflation, the Universe expanded like a balloon, smoothing out the clumps.

It could be that the missing matter of the Universe – called dark matter by physicists – formed then, too. Dark matter is one of astronomy’s notorious mysteries. Evidence has grown over the past century that there must be something out there besides the stuff that makes up our tables, our planet, even ourselves. An early hint, in the 1970s, came from the astronomer Vera Rubin, who showed that stars at the edges of galaxies rotate faster than we’d expect from just the mass we can see through telescopes. Something else heavy had to be there, exerting enough gravity to make the galaxies spin. Since the 1980s, astronomers have agreed: dark matter must exist to explain that ‘missing mass’ in our observations, but we can’t see it since it doesn’t interact with light in the typical ways. Our current best estimates show that there should be about five times as much dark matter as ‘regular’ matter in the Universe. In fact, all the things we interact with in our daily lives make up less than 5 per cent of the matter in the Universe.

‘Dark matter makes up 25 per cent of the Universe and we have no idea what it is!’ says Leah Jenks, a theoretical physicist and PhD candidate at Brown University in Rhode Island. ‘So, in that sense, it is a hugely important open question in the field. I think it is also a fun problem to work on because there is a lot of room for creativity in trying to think of new ways we might be able to understand this mysterious missing matter.’

So far, we’ve had many ideas, but little evidence to confirm any of them. Physicists have spent a lot of their time looking for new particles to explain the missing mass. One of the most popular candidates, a collection of big particles called WIMPs, or weakly interacting massive particles, has recently fallen out of favour, since experiments haven’t been able to find evidence for them. Despite looking with particle accelerators, large underground particle-detecting chambers, and other experiments, the Universe has given us no signs that dark matter is one of these elusive particles. This has led scientists to look for other, extremely tiny, particles subject to strange quantum phenomena (known as ‘fuzzy dark matter’) or even changes to the fundamental laws of physics as we know them (an often-shunned theory known as ‘MOND’).

But with a dearth of evidence for any of these ideas, a dark horse theory has emerged. In those first seconds of the Universe, there might have been another ingredient in the primordial soup: black holes. These black holes from the very beginning of time, known as primordial black holes (PBHs), could still be lurking around today – and some scientists believe they could solve the problem of dark matter.

Black holes typically form from the deaths of the largest stars, where all the mass of a star collapses down to one extremely dense point. Their gravitational pull is so great that nothing, not even light, can escape. But in the first second of the Universe, stars didn’t yet exist – so how could black holes have formed back then? Pretty easily, calculations show, from the denser parts of the cosmic soup. The basic recipe, says Alexander Kusenko, professor of physics at the University of California, Los Angeles, is to ‘take a spoonful of the early Universe and squeeze it by 30 per cent.’

Kusenko and some others believe primordial black holes are now the most promising candidate for dark matter that we have. The real appeal of PBHs is their simplicity – unlike other options for dark matter, which require making theories to describe new particles, we already have evidence that black holes are real.

Primordial black holes aren’t a particularly new idea, though. Their story begins back in 1960s Russia, when two physicists – Yakov Zeldovich and Igor Novikov – first considered the possibility of extremely dense objects like black holes forming in the very early Universe. Their calculations showed that these dense regions would grow too fast and heat up the early Universe with their strong radiation, which decidedly conflicted with observations of the real Universe as we know it.

But in the 1970s, the British physicist Stephen Hawking and his graduate student Bernard Carr began looking deeper. Together, they wrote the first paper showing that primordial black holes might be a real possibility after all, finding a way around the problem of runaway growth. ‘That’s what put primordial black holes on the map again, since there was no reason to believe they couldn’t exist,’ says Carr, now emeritus professor of mathematics and astronomy at Queen Mary University of London.

Soon after, Hawking discovered what became known as Hawking radiation, which makes black holes slowly evaporate. For larger, star-sized black holes, the time it would take them to evaporate is far too long to be noticeable – longer than the time the Universe has existed. For small PBHs, however, we might be able to see it. A black hole about the mass of a mountain and the size of a proton, formed billions of years ago when the Universe began, should be showing the explosive end of its life right about now.

Since Carr and Hawking’s original theories in the 1970s, many scenarios have been proposed for how primordial black holes could form in the first fraction of a second of the Universe’s existence. In this long-ago era, the infant Universe was extremely hot, filled with high-energy particles and light, and behaved entirely unlike what we’re familiar with today. The key to making a primordial black hole is somehow making a small region of that hot early Universe slightly denser so that it can collapse into a black hole.

It turns out there are many ways that a part of the early Universe could become extra-dense. The simplest possibility is lumpiness here and there reveals ‘primordial inhomogeneities’. This is not too dissimilar from the fluctuations seen in the cosmic microwave background (CMB), the faint radiation we see from the moments the Universe became transparent a few hundred thousand years after the Big Bang. The CMB shows up in every direction we can point a telescope, but there are some minuscule fluctuations that only our most precise instruments can pick up. These fluctuations are essentially random, a result of the statistical nature of particles moving around in the early Universe. Another possibility is that inflation, the process that quickly expanded the early Universe just after it formed, created more fluctuations in density that could then go on to become black holes.

One model, which Carr is currently working on, proposes that primordial black holes formed at a time known as the QCD transition, an absolutely minuscule 100,000th of a second after the Big Bang. QCD stands for quantum chromodynamics, the branch of theoretical physics that deals with how particles interact via the strong nuclear force, which binds the nuclei of atoms together. The QCD transition, then, is when the Universe went from a soup of quarks and gluons – the components of protons and neutrons, and the glue that binds them together – to a soup of fully formed protons and neutrons. At this unique moment in the Universe’s history, it would be easier to make PBHs. You wouldn’t have to squeeze your spoonful of the Universe as hard to get it to collapse.

Kusenko, on the other hand, is working on a model that adds just one new bit of physics – a new kind of interaction between particles known as ‘Yukawa forces’ – to squeeze together enough matter to form just the right amount of primordial black holes to account for dark matter. Adding a new bit of physics is by no means unprecedented. In fact, that’s practically the job of the theoretical physicists working in this field.

Primordial black holes can wriggle their way into the centres of neutron stars to devour the entire neutron star

A whole host of other models rely on some of the most fantastical ideas in physics. One theory suggests that cosmic strings – cracks in the fabric of the early Universe – might sometimes loop around on each other to create black holes. Another theory proposes that ‘baby universes’ from the multiverse could appear in our own Universe as primordial black holes. If all these phenomena happened, but at slightly different times during the Universe’s first few seconds, the result would be various sizes of primordial black holes.

All the various types of primordial black holes we’ve ruled out: any region coloured in is where we do not see evidence for PBHs. The x-axis shows the mass of the black hole, and the y-axis shows what fraction of dark matter those PBHs can explain (where 10^0 is 100 per cent). Image: Carr and Kuhnel 2020

For decades, a small handful of physicists have considered these theories and others with a focus on how they would affect the early Universe if they panned out to be real. In fact, since primordial black holes would have such outsized influence, physicists have come up with numerous ways to prove or disprove they are real.

The very smallest PBHs are immediately ruled out as a dark matter candidate by Hawking radiation, since they wouldn’t live long enough to still exist now. Black holes about the size of a proton, on the other hand, could survive until the present day, but then explosively evaporate – and we just don’t see anything like that, so they’re ruled out, too. Even if primordial black holes weren’t glowing from Hawking radiation, they would be emitting light as they eat up matter, heating up the area around them. By looking at the history of the Universe’s temperature, scientists can set another limit on primordial black holes.

In our modern Universe, PBHs in a galaxy could be randomly flying about, disrupting the orbits of stars with their gravity and nudging them slightly out of place in ways we can observe. Scientists have also predicted that some PBHs will settle into the centre of the galaxy, like sand falling into a funnel. Measuring the amount of mass at the Milky Way’s centre could tell us the maximum amount of PBHs that could be lurking there.

One of the most dramatic ways primordial black holes can interact with matter, though, is by colliding with other objects in the Universe, like stars and planets. Primordial black holes can wriggle their way into the centres of neutron stars, the dense leftovers from a dead massive star’s core. Once they finally reached the centre, they would quickly devour the entire neutron star from inside, destroying it in a spectacular flash known as a kilonova. This violent event also leaves behind splashes of neutrons, along with heavy elements like platinum and uranium. Two neutron stars colliding can also make a kilonova, although that would stir up gravitational waves – fluctuations in the fabric of space-time itself – so, finding a kilonova without gravitational waves to accompany it could be proof of primordial black holes at work.

The black holes they detected are more massive than you’d expect from a star’s demise

There are a lot of possibilities, and some, like the neutron star scenario, are outlandish. But two methods can help us cut through the uncertainty and find more concrete proof of primordial black holes. One solid method for tracking a primordial black hole is microlensing, which happens when light from a bright object travels past something with a lot of mass. The massive object bends spacetime with its gravity and, since light follows the path of least resistance, the light from the bright object is bent, too. Even if the massive object emits no light, like a black hole, you can still measure its effect on things that do emit light. Three experiments, known as MACHO, EROS and OGLE, have done surveys of the sky looking for this celestial lensing, some of which could be from primordial black holes. These observations have made scientists pretty confident that certain sizes of PBH don’t exist.

The other method for detecting primordial black holes uses gravitational waves generated when massive objects (like black holes) disrupt spacetime. Less than a year after the groundbreaking Laser Interferometer Gravitational-Wave Observatory (LIGO) switched on its detectors in 2015, physicists detected gravitational waves from two black holes – each around 30 times larger than our Sun – spiralling in and merging with each other. Since then, they’ve found many more of these black hole mergers, opening up a plethora of new questions.

The black holes they detected are more massive than you’d expect from a star’s demise, which came as a bit of a surprise to scientists. It’s still a majority view that these larger black holes come from multiple run-of-the-mill star-sized black holes clumping together over time. But, according to Carr, several groups of scientists have argued that these big black holes could instead be primordial black holes, which could naturally form at that size.

Although scientists can’t agree on whether nearby black hole mergers are from stars or primordial black holes, finding evidence of merging black holes sufficiently far away would tip the scales towards primordial black holes. Since light can travel only so fast, when we look far away in the Universe, we’re also looking back in time. A star four light-years away is actually being seen four years in the past, since that’s how long the light we see took to reach us. Gravitational waves also can’t travel faster than light – so, if we see a black hole merger so far away that it’s from before stars formed, that would have to be a merger of primordial black holes.

There is one more observation we could make with gravitational waves that would be surefire evidence of primordial black holes: finding a black hole the size of the Sun. Stars couldn’t produce a black hole that small, so it would have to be from the early Universe.

This leads us back to our original question – if primordial black holes do exist, can they solve the problem of dark matter? Research based on Hawking radiation and microlensing rules out quite a few sizes of primordial black holes, but not all of them. There are three (or four) masses of primordial black holes that could exist and play nice with current observations, and maybe even explain the mystery of dark matter, depending on whom you ask.

Carr thinks that there are four masses of primordial black holes that could exist: asteroid-sized, moon-sized (about ~1/10th the mass of Earth), Sun-sized, and mind-bogglingly huge (more than a billion times the mass of our Sun). Although the huge primordial black holes are interesting, and may explain what we see with gravitational waves, they’re not really a candidate for dark matter. There’s also quite a bit of controversy over whether the Sun-sized ones are possible at all. That leaves asteroid- to moon-sized PBHs as the only well-accepted option to explain dark matter.

Even then, quite a few scientists are sceptical about primordial black holes, which are sometimes dismissed as a bonkers fringe idea. That doesn’t bother Kusenko at all. ‘Healthy scepticism is a welcome and necessary part of scientific discourse,’ he says. ‘So, if these folks’ scepticism is based on some logical arguments, I am happy to hear them.’

However, even he agrees that there are some cons to primordial black holes as dark matter. These small black holes are quite difficult to detect, and it’s looking somewhat unlikely that primordial black holes could make up all of dark matter given the current constraints.

More galaxies could be a sign that primordial black holes were around, helping them get started

What if they don’t have to be all of dark matter, though? A new idea gaining traction is that dark matter may be a collection of different things – an entire ‘dark sector’ that we’re just beginning to investigate. ‘If there is only One True Dark Matter, then any candidate which can’t make up the full dark matter fraction of the Universe is just not satisfactory (hello, PBH!) and can be totally ruled out,’ says Luna Zagorac, a physics PhD candidate at Yale University in Connecticut. ‘But, if it can make up any part of that fraction, then it’s harder to rule out anything [WIMPs, fuzzy dark matter, other candidates, could all be part of the mix].’

So even if primordial black holes can’t explain all of dark matter, they’d certainly have interesting effects on the Universe from the start. Beyond forming dark matter, scientists think that giant primordial black holes could help explain how the biggest black holes in the centres of galaxies – like our own Sagittarius A*, recently imaged by the Event Horizon Telescope – got started.

Constantly improving technology, moreover, should help us investigate primordial black holes. The Square Kilometre Array (SKA) telescope in South Africa, for instance, should be able to make new measurements of quickly spinning dead stars known as pulsars. Pulsars are like lighthouses, emitting huge jets of matter that flash over us as they rotate, and making it look like they blink periodically. Primordial black holes that move between a pulsar and Earth can bend that light and ever so slightly change the period of the pulsar’s blink – an effect we can observe.

New observations with optical telescopes should be able to probe for smaller primordial black holes with microlensing, too. Kusenko and collaborators, such as the physicist Misao Sasaki at the Kavli Institute for the Physics and Mathematics of the Universe in Japan, are searching for small black holes with the Subaru Telescope’s Hyper Suprime-Cam on Hawai‘i. When a small primordial black hole passes in front of a star in our nearest neighbour galaxy, Andromeda, they should be able to spot how the starlight bends around the PBH. The newly launched James Webb Space Telescope should be useful for spotting signs of PBH, too – it’ll peer deeper into the Universe’s past than ever before, finally opening a window to look at the first galaxies. If there are more galaxies or bigger ones than expected, it could be a sign that primordial black holes were around, helping galaxies get started.

Arguably the most exciting prospect for discovering black holes, though, is gravitational waves. Gravitational wave detectors could find ancient black hole mergers, black holes smaller than the Sun, or even signatures of when black holes first formed in the primordial soup of the Universe’s first second of existence. When PBHs collapsed in the very beginning of the Universe, they would have shaken up space-time – contributing to a fuzzy noise of gravitational waves known as the stochastic gravitational wave background. Our current gravitational wave detectors are all on Earth – LIGO in the United States; KAGRA in Japan; and VIRGO in Europe – but there are many more next-generation instruments on the horizon, some of which will be in space. Each of the LISA, TAIJI and DECIGO missions is a set of three spacecraft that will orbit the Sun in triangles, probing different kinds of gravitational waves than we’re able to observe here on Earth. In the coming decades, these new detectors could finally spot a definitive sign of primordial black holes. If scientists detect these exciting objects, then we could finally have a solution to at least part of the mystery of dark matter.

The LISA, TAIJI and DECIGO missions will orbit the Sun in triangles, probing different kinds of gravitational waves. This artist’s rendition visualises the LISA mission orbiting near Earth (not to scale). Image courtesy NASA via Wikimedia

‘PBHs are as good as the other hypothetical candidates, if not better … There are a variety of candidates for dark matter, and a lot of efforts are being made to detect them,’ says Sasaki. ‘So anyone may discover what dark matter is at any moment, maybe tomorrow.’

Carr shares this optimism: ‘I think we’re going to know fairly soon.’

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Primordial Earth Had a Major Difference in Its Skies We Didn’t Realize Until Now

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Standing on Earth almost 4 billion years ago would have been an incredibly hot, desperately lonely, and very short experience – what with there being no oxygen. Now, new research suggests there would have been less lightning around than there is in modern times as well.

 

This could make a difference to any of the hypotheses that suggest lightning may have been involved in sparking the earliest life on our planet. If lightning strikes were actually less common on the early Earth than previously thought, that affects those calculations.

To dig deeper, researchers examined how streamer discharges – the sparks that start lightning – might have formed in an atmosphere dense in carbon dioxide and molecular nitrogen, as the atmosphere of the primordial Earth is now thought to have been.

“Basically, in the nitrogen and carbon-rich atmosphere, you need stronger electric fields for a discharge to initiate,” says physicist Christoph Köhn from the Technical University of Denmark.

Chain reactions of accelerating and colliding electrons known as electron avalanches are crucial to streamer discharges, and how electrons behave changes depending on atmospheric conditions, which is where this newly found discrepancy comes from.

To complicate matters, we’re not exactly certain what the atmosphere of early Earth was like. Here, the scientists used the carbon dioxide and nitrogen hypothesis first put forward in the 1990s by geoscientist James Kasting.

An older proposal from Stanley Miller and Harold Urey, published in the 1950s, suggests that methane and ammonia were actually dominant in the atmosphere during the first billion years of Earth.

 

It was Miller and Urey who first put forward the idea of lightning forming the building blocks of life on Earth, via experiments in gas-filled flasks, but in recent years the thinking over the atmospheric composition at the time has begun to shift.

“Our simulations show that discharges in the Miller-Urey mixture incept at lower fields than in Kasting’s mixture and partly on Modern Earth, which implies that discharges in the atmosphere of Ancient Earth might have been more challenging to incept than previously thought,” write the researchers in the new paper.

What all of this means is that the process of producing and building up the prebiotic molecules key to life, via lightning strikes, would have taken longer if recent ideas about the atmosphere of the early Earth are right.

The researchers don’t specifically quantify how much longer; they only modeled one of the earliest stages in the process of lightning formation, and there remain a lot of unknowns. However, they do say the variations “could potentially make a big difference” in how frequent lightning strikes were.

There’s lots more work to do here, such as expanding the scope of the research to include the entirety of the lightning strike process and add in more models of atmospheric chemistry. Ultimately, we’re still searching for answers to the biggest questions.

“If lightning discharges were responsible for the production of prebiotic molecules, it’s important to get a very good theoretical understanding of what happened,” says Köhn.

“The big question is still, where do all these prebiotic molecules come from?”

The research has been published in Geophysical Research Letters.

 

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Primordial Helium From Billions of Years Ago Seems to Be Leaking Out of Earth’s Core

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Ancient, primordial helium that was forged in the wake of the Big Bang is leaking from Earth’s core, scientists report in a new study.

There’s no cause for alarm. Earth isn’t deflating like a sad balloon. What it does mean is that Earth formed inside a solar nebula – the molecular cloud that gave birth to the Sun, a detail about our planet’s birth that has long been unresolved.

 

It also suggests that other primordial gases may be leaking from Earth’s core into the mantle, which in turn could yield information about the composition of the solar nebula.

Helium on Earth comes in two stable isotopes. By far the most common is helium-4, with a nucleus containing two protons and two neutrons. Helium-4 accounts for arund 99.99986 percent of all the helium on our planet.

The other stable isotope, accounting for just about 0.000137 percent of Earth’s helium, is helium-3, with two protons and one neutron.

Helium-4 is primarily the product of the radioactive decay of uranium and thorium, made right here on Earth. By contrast, Helium-3 is mostly primordial, formed in the moments after the Big Bang, but it can also be produced by the radioactive decay of tritium.

It’s the Helium-3 isotope that has been detected leaking out of Earth’s interior, mostly along the mid-ocean volcanic ridge system, giving us a pretty good indication of the rate at which it escapes the crust.

That rate is about 2,000 grams (4.4 pounds) a year: “about enough to fill a balloon the size of your desk,” explains geophysicist Peter Olson from the University of New Mexico.

 

“It’s a wonder of nature, and a clue for the history of the Earth, that there’s still a significant amount of this isotope in the interior of the Earth.”

What is less clear is the provenance; how much of the helium-3 might be emerging from the core, versus how much is in the mantle.

This would tell us the source of the isotope. When Earth formed, it did so by accumulating material from the dust and gas floating around the newborn Sun.

The only way significant amounts of helium-3 could be inside the planetary core is if it were formed in a thriving nebula. That means, not at its outskirts, and not as it dissipated and blew away.

Olson and his colleague, geochemist Zachary Sharp of the University of New Mexico, investigated by modeling Earth’s inventory of helium as it evolved. First, as it formed, a process during which the protoplanet accumulated and incorporated helium; and then after the Great Impact.

This, astronomers think, is when an object the size of Mars smacked into a very young Earth, sending debris flying into Earth’s orbit, eventually recombining to form the Moon.

 

During this event, which would have re-melted the mantle, much of the helium locked inside the mantle would have been lost. The core, however, is more resistant to impact, suggesting that it could be quite an effective reservoir for holding onto helium-3.

In fact, this is what the researchers found. Using the current rate at which helium-3 is leaking from the interior, as well as models of helium isotope behavior, Olson and Sharp found that there are likely 10 teragrams (1013 grams) to a petagram (1015 grams) of helium-3 in our planet’s core.

This suggests that the planet had to have formed inside a thriving solar nebula. However, several uncertainties remain. The likelihood of all the conditions being met for the sequestering of helium-3 in Earth’s core are moderately low – which means that there may be less of the isotope than the team’s work suggests.

However, it’s possible that there is also abundant primordial hydrogen in our planet’s core, caught up in the same process that may have accumulated helium-3. Looking for evidence of hydrogen leakage could help validate the findings, the researchers say.

The research has been published in Geochemistry, Geophysics, Geosystems.

 

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Physicists Detect Mysterious X Particles in ‘Primordial Soup’ For The First Time

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A mysterious particle thought to have existed briefly just after the Big Bang has now been detected for the first time in the ‘primordial soup’.

Specifically, in a medium called the quark-gluon plasma, generated in the Large Hadron Collider by colliding lead ions. There, amid the trillions of particles produced by these collisions, physicists managed to tease out 100 of the exotic motes known as X particles.

 

“This is just the start of the story,” says physicist Yen-Jie Lee of MIT, and a member of the international CMS Collaboration headquartered at CERN in Switzerland.

“We’ve shown we can find a signal. In the next few years we want to use the quark-gluon plasma to probe the X particle’s internal structure, which could change our view of what kind of material the universe should produce.”

Mere moments after the Big Bang, the very early Universe wasn’t made of the same stuff we see floating around today. Instead, for a few millionths of a second, it was filled with plasma superheated to trillions of degrees, consisting of elementary particles called quarks and gluons. That’s the quark-gluon plasma.

In less time than it takes to blink, the plasma cooled and the particles came together to form the protons and neutrons of which normal matter is constructed today. But in that very brief twitch of time, the particles in the quark-gluon plasma collided, stuck together, and came apart again in different configurations.

One of those configurations is a particle so mysterious, we don’t even know how it’s put together. This is the X particle, and it’s only been seen very rarely and briefly in particle colliders – too briefly to be probed.

 

Theoretically, however, X particles could appear in the very small flashes of quark-gluon plasma that physicists have been creating in particle accelerators for some years now. And this might afford a better opportunity to understand them.

During the Large Hadron Collider’s 2018 run, positively charged atoms of lead were slammed together at high speeds. Each of these roughly 13 billion collisions produced a shower of tens of thousands of particles. That’s a dauntingly colossal amount of data to sift through.

“Theoretically speaking, there are so many quarks and gluons in the plasma that the production of X particles should be enhanced,” Lee says. “But people thought it would be too difficult to search for them because there are so many other particles produced in this quark soup.”

Although X particles are very short-lived, when they decay, they produce a shower of lower-mass particles. To streamline the data analysis process, the team developed an algorithm to recognize the patterns characteristic of X particle decay. Then they fed the 2018 LHC data into their software.

The algorithm identified a signal at a specific mass that indicated the presence of around 100 X particles in the data. This is an excellent start.

 

“It’s almost unthinkable that we can tease out these 100 particles from this huge dataset,” Lee said.

At this point, the data are insufficient to learn more about the X-particle’s structure, but the discovery could bring us closer. Now that we know how to find the X-particle’s signature, teasing it out in future data sets should be a lot easier. In turn, the more data we have available, the easier it will be to make sense of them.

Protons and neutrons are each made up of three quarks. Physicists believe that X particles may be made of four – either an exotic, tightly bound particle known as a tetraquark, or a new kind of loosely bound particle made from two mesons, each of which contain two quarks. If it’s the former, because it’s more tightly bound, it will decay more slowly than the latter.

“Currently our data is consistent with both because we don’t have enough statistics yet. In the next few years we’ll take much more data so we can separate these two scenarios,” Lee says.

“That will broaden our view of the kinds of particles that were produced abundantly in the early Universe.”

The research has been published in Physical Review Letters.

 

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Primordial Black Holes Could Explain Dark Matter and the Growth of Supermassive Black Holes at the Same Time

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It’s that time again. Time to look at a possible model to explain dark matter. In this case, a perennial favorite known as primordial black holes. Black holes have long been proposed as the source of dark matter. In many ways, they are the perfect candidate because they only interact with light and matter gravitationally. But stellar-mass black holes have been ruled out observationally. There simply aren’t enough of them to account for dark matter.

Primordial black holes are a possible solution. Unlike stellar black holes that would have a mass larger than the Sun, primordial black holes could have the mass of a mere planet or less. A planet-mass black hole would be smaller than an apple, and an asteroid-mass black hole could be smaller than a grain of sand. They are known as primordial black holes because they are thought to have formed during the early moments of the universe. The idea hasn’t been tremendously popular, and we have no observational evidence that primordial black holes exist, but a new study has looked at the idea once again.

This study tweaks the original model slightly, proposing that primordial black holes with a range of masses formed almost instantly after the big bang. From their model, they show that some of these black holes could form the seeds of the first stars, and the largest primordial black holes could have rapidly grown into supermassive black holes by gobbling up surrounding hydrogen and helium. This would explain how galaxies and their supermassive black holes seem to have formed so early in the universe. Finally, the smallest primordial black holes would be common enough to explain dark matter.

The James Webb telescope could discover evidence of primordial black holes in the near future. Credit: ESA

Being able to explain black holes, galactic evolution, and dark matter all in one would be a tremendous theoretical boon. But the idea is useless unless the model can be proven. But the authors think the James Webb telescope might be able to do just that. One of the things about primordial black holes is that they likely emit light via [Hawking radiation](/post/great-escape/). According to Hawking’s model, tiny black holes should cause an excess of infrared light in the early universe, which the Webb telescope should be able to pick up.

So if the James Webb Space Telescope does launch this week as planned, and all goes well, we should be able to put this idea to the test. It would be a great holiday gift to finally understand what dark matter truly is.

Reference: Cappelluti, Nico, Günther Hasinger, and Priyamvada Natarajan. “Exploring the high-redshift PBH-LCDM Universe: early black hole seeding, the first stars and cosmic radiation backgrounds.” arXiv preprint arXiv:2109.08701 (2021).

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NASA’s James Webb Space Telescope will spend its first year looking for primordial galaxies, gold-forging explosions, and habitable planets

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An artist’s conception of the James Webb Space Telescope.NASA GSFC/CIL/Adriana Manrique Gutierrez

NASA is about to open a never-before-seen window into the cosmos. Starting next year, astronomers should be able to peer into the atmospheres of planets orbiting distant stars, analyze the aftermath of the universe’s most violent collisions, and look further back in time than ever before.

That’s because the James Webb Space Telescope — JWST, or simply “Webb,” for short — is folded up, full of fuel, and waiting to be loaded onto a rocket in French Guiana.

NASA’s last game-changing space observatory, the Hubble Space Telescope, launched in 1990. It, too, was on a mission to document the 13.8-billion-year history of the universe.

Hubble is still observing the cosmos, and NASA hopes to keep using it for a few more years, possibly into the 2030s. But Hubble could only see so far, and Webb is designed to see even farther.

In collaboration with the European Space Agency (ESA) and the Canadian Space Agency, NASA has spent decades and more than $10 billion building Webb, which is set to launch into space on December 22. While Webb was first conceived of in the 1990s and originally slated to cost $500 million, a redesign and delays both drove up its price tag and pushed back its launch date.

After launch, if all goes according to plan, Webb will spend six months unfolding and adjusting itself, falling into an orbit 1 million miles from Earth. Then it can begin rewriting cosmic history.

The James Webb Space Telescope is unpacked and lifted vertically in a cleanroom at Guiana Space Center in French Guiana, on October 15, 2021NASA/Chris Gunn

The telescope’s main project is to investigate how galaxies formed and grew after the Big Bang — peering into the universe’s depths to capture images of the first galaxies ever formed. Its infrared cameras are so powerful and precise that they could spot a bumblebee 240,000 miles away — the distance between Earth and the moon.

Webb will also help astronomers investigate mysteries they hadn’t considered when NASA first designed the telescope.

“Webb has this broad power to reveal the unexpected. We can plan what we think we’re going to see, but at the end of the day, we know that nature will surprise us more often than not,” Klaus Pontoppidan, a Webb scientist at the Space Telescope Science Institute, said in a press briefing on November 18.

NASA expects the telescope to probe the secrets of the cosmos for at least a decade. Even the telescope’s first year in space is jam-packed, with nearly 400 investigations from thousands of scientists all over the world, Pontoppidan said.

From peering at Mars to investigating ancient galaxies, here are a few of the most exciting projects that Webb — the most powerful space telescope ever built — is expected to tackle in its first year:

Light from the first galaxies is still traveling to Earth, and Webb may spot it

The Hubble Ultra Deep Field is the deepest visible-light image of the cosmos. It includes nearly 10,000 galaxies.NASA, ESA, and S. Beckwith (STScI) and the HUDF Team

As a telescope peers into the distance, it’s also looking back in time. That’s because it takes time for light to travel. When you look at the sun — please, don’t! — you’re seeing light that our star emitted eight minutes ago. When Hubble looks at distant galaxies, it’s seeing light from billions of years ago, as far back as 400 million years after the Big Bang.

“We have this 13.8-billion-year story — the universe — and we’re missing sort of a few key paragraphs in the very first chapter of the story,” Amber Straughn, a scientist on NASA’s Webb team, said in the November 18 briefing. “JWST was designed to help us find those first galaxies.”

Webb is expected to spot galaxies that formed when the universe was just 100 million years old.

It’s 100 times more powerful than Hubble. It’s also using infrared light, which has wavelengths that can cut through dust clouds that may have obscured Hubble’s view, which relied on visible light.

Webb should see deeper into the cosmos and detect galaxies — the first ones formed after the Big Bang — that are too distant and faint for Hubble to pick up.

Looking for gold forged by the universe’s most violent collisions

A supercomputer simulation of a pair of neutron stars colliding, merging, and forming a black hole.NASA Goddard

For the last six years, gravitational-wave detectors on Earth have been sensing ripples in space-time created by the most violent events in the cosmos: black holes and neutron stars crashing into one another.

Scientists think these collisions forged most of the universe’s heavy elements, like silver, gold, and platinum. Webb will try to confirm that by focusing on distant collisions of neutron stars — the dense cores of stars that have collapsed, ejected their outer layers, and died.

Webb will be able to analyze the entire spectrum of infrared light from those collisions. That will allow astronomers to indentify individual elements like gold or platinum in the explosion debris, based on their wavelengths of light.

This method, called spectroscopy, will help astronomers learn about other objects Webb studies, too.

“Spectra will be the bulk of the science,” Antonella Nota, a Webb scientist who leads the ESA office at the Space Telescope Science Institute, said in the briefing. “While an image, we say is worth 1,000 words, spectra for astronomers are just worth 1,000 images.”

Our first glimpse at the atmospheres of planets that could host life

An artist’s impression of the super-Earth exoplanet K2-18b.ESA/Hubble, M. Kornmesser

When it’s not busy studying the most massive objects and ancient galaxies in the universe, Webb will search for less extreme environments, worlds where conditions could be just right to give rise to life.

Exoplanets — worlds orbiting other stars — were barely a field of study when NASA began designing Webb. Two decades later, astronomers have identified dozens of exoplanets that could be temperate enough for alien life. They’re not too cold, not too hot, just right for water — but only if they have hospitable atmospheres.

Webb will watch potentially habitable exoplanets pass in front of their stars, and analyze the spectra of starlight that shines through the planets’ atmospheres. That spectroscopy will indicate to scientists whether the air on other worlds contains compounds that could point to life, like carbon dioxide, methane, or water.

“This telescope is definitely our next big step in our search for potentially habitable planets,” Straughn said.

The James Webb Space Telescope in a Northrop Grumman cleanroom in Redondo Beach, California, on March 4, 2020.NASA/Chris Gunn

These aren’t necessarily Earth-like planets. Stars like the sun are so big and bright that Webb wouldn’t be able to see the tiny Earth-like planets orbiting them. That’s a job for the next great space telescope. Instead, Webb will look at rock planets orbiting stars that are much smaller and dimmer.

Some of its first targets will be planets circling a small star, TRAPPIST-1, just 39 light-years away.

The star has seven rocky planets, three of them in its “Goldilocks zone,” meaning they’re just the right distance to have temperatures that would allow liquid water to exist on their surfaces.

An artistic rendering of what it might look like on the surface of the planet TRAPPIST-1f.NASA/JPL-Caltech

Webb is also set to zoom in on uninhabitable, but fascinatingly extreme, planets. At least one of the planets on its roster is so close to its star that its surface is molten, and it may even rain lava there. Webb should be able to detect that lava rain.

The telescope will also examine every object in our solar system, starting with Mars and working its way outward to the icy objects beyond Pluto.

In those planets, stars, and galaxies, near and far, Webb is sure to uncover major surprises.

“Webb will probably also reveal new questions for future generations of scientists to answer — some of whom may not even be born yet,” Pontoppidan said.

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A Unique Mixture of Salts Could Have Sparked Life on Primordial Earth, Study Hints

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None of us would be around if organisms hadn’t been sparked into life billions of years ago. The question of just how that spark came about continues to fascinate scientists.

New research looking at how the conditions on primordial Earth might have produced life has identified a mixture of salts that, mixed with heat flows from molten rock, could potentially have contributed to the formation of self-replicating biomolecules.

 

This self-replication is a key part of the ‘RNA world’ hypothesis: the idea that ribonucleic acids (RNA) can both store biological information and perform the required structure folding for life to grow and evolve into the state it is today.

In this case, scientists looked at the mixture of magnesium (Mg) and sodium (Na) as it might have been on Earth in its earliest years: for RNA folding to work, a relatively high concentration of doubly charged magnesium ions and a lower concentration of singly charged sodium is required.

“Accordingly, the question arises as to which environments on early Earth might have provided suitable salt conditions for such prebiotic processes. One geologically probable process that produces saline environments is the leaching of salts from basalt,” the international team of researchers writes in their study.

“As a primary partial melt of the Earth’s mantle, basalt is one of the most abundant rock types to be expected in the Earth’s early crust, as well as the crust of other terrestrial planets in our Solar System.”

The team synthesized basaltic glass – which naturally occurs on Earth when melted basalt is rapidly cooled (by contact with ocean water, for instance) – and characterized it in its various forms, including both rock and glass.

 

An analysis of the amount of magnesium and sodium extracted from the glass, under a variety of temperatures and with a variety of grain sizes, always showed significantly more sodium than magnesium.

What’s more, the levels of magnesium were always significantly lower than necessary for prebiotic RNA folding to properly function. The missing part of the process, the researchers discovered, was convective flows of heat.

“This situation changed considerably when heat currents – which are very likely to have been present, owing to the high levels of geological activity expected in prebiotic environments – were added,” says biophysicist Christof Mast, from the Ludwig Maximilians University of Munich in Germany.

“We have shown that a combination of basaltic rocks and simple convection currents can give rise to the optimal relationship between Mg and Na ions under natural conditions.”

The temperature gradients that feature in the narrow cracks and pores of basaltic glass create the convective flows required for salt optimization and also move more ions against the current – creating what’s known as thermophoresis.

Together, convection and thermophoresis increase the number of magnesium ions in the mix, creating conditions where self-replicating RNA can occur, the study shows. The same sort of chemical reactions may have played out on Earth 4 billion years ago.

This leaching of salts from basalt – found in abundance in Earth’s mantle – fits the template for the RNA world hypothesis to work, the research shows. What’s more, it widens out the possibilities in terms of salt mixes that may have helped spark life.

“The principle demonstrated here is applicable to a broad range of salt concentrations and compositions, and, as such, highly relevant to various origin-of-life scenarios,” write the researchers in their published paper.

The research has been published in Nature Chemistry.

 

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Primordial black holes may flood the universe. Could one hit Earth?

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Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of “Ask a Spaceman” and “Space Radio,” and author of “How to Die in Space.” Sutter contributed this article to Space.com’s Expert Voices: Op-Ed & Insights.

Black holes sound pretty scary — dark, powerful, foreboding. And now, astrophysicists have cooked up something else: primordial black holes, forged in the earliest moments of the universe, that flood the present-day cosmos.

So, what are the chances that one of these ancient monsters will come wandering toward Earth? One astrophysicist has run the numbers. 

Related: Swarms of ‘primordial’ black holes might fill our universe

Birthed in the Big Bang

The early universe was a wild, complex time. Far different from the mild-mannered cosmos we inhabit today, the earliest moments of the Big Bang were marked by radical phase transitions, the splitting of fundamental elements and other wild events. While scientists understand the physics of the first few minutes, what happened before that is shrouded in mystery (and a bunch of complicated math).

You need some pretty extreme conditions to form black holes — say, a star collapsing in on itself during the final, catastrophic moments of its life. Stars weren’t around in the first few seconds of the universe’s existence, but there may have been just the right conditions to forge black holes; all you need is a lot of matter or energy crammed into a tiny enough volume.

In the unknown and uncharted reaches of the universe’s distant past, the conditions could have been just right to flood the universe with primordial black holes, which could have any mass, depending on the conditions under which they were made. But interest in primordial black holes waned over the decades as searches for them turned up empty — that is, until we had the Laser Interferometer Gravitational-Wave Observatory (LIGO).

When LIGO detected its first black hole collision, the black holes had rather peculiar masses; each was a few dozen solar masses. That mass range is difficult to achieve with mergers of standard star-based black holes, because mergers would have to be a little too frequent to be plausible). And so, before you knew it, primordial black holes were back in the spotlight.

A dark encounter

The thing about processes in the early universe is that if there is some sort of exotic mechanism that can generate black holes, it’s not going to make a few of them — it’s going to flood the universe with them. In fact, there might be enough primordial black holes roaming the universe to explain at least a portion of dark matter, the mysterious substance that accounts for over 80% of all the matter in the cosmos.

Let’s say a whole bunch of small black holes are swarming the cosmos, as presented in a paper published recently to the preprint database arXiv. What would happen to them?

Thankfully, black holes aren’t 100% black, and they lose mass through Hawking radiation, the complex quantum mechanical process at the black hole event horizon that allows some particles and radiation to escape. The smaller they are, the faster they lose mass. Black holes less than roughly 100 million tons — slightly lighter than a typical asteroid — will lose about half their mass within the current age of the universe. Because of the way Hawking radiation works, black holes that are bigger than that will only lose a small fraction of their mass.

The total number of small black holes in each galaxy depends on how much of the dark matter you want to explain with them, and how big each one is. No matter how you slice it, though, there are a lot.

And each one is fast. Based on computer simulations and observations of galaxy dynamics, dark matter has a velocity of over a hundred miles per second. At that speed, an asteroid-mass black hole could cover the distance between Jupiter and Earth in just a couple weeks. So should we be scared?

Running the numbers

What would happen if an asteroid-mass black hole were to hit Earth? In short, catastrophe. The black hole would puncture our planet’s surface like a hot knife through butter, but it would immediately begin to slow down because of its gravitational interaction with Earth. Any atom or molecule (or person) intersecting the event horizon — the boundary of the black hole beyond which nothing, not even light, can escape — would simply slip away from the known universe, never to be seen again.

In the best-case scenario, the black hole would exit through the other side of our planet, leaving the survivors to clean up the mess. In the worst-case scenario, the black hole would settle into the core of our planet, where its gravity would be enough to allow the black hole to begin feeding. Eventually, it would devour our entire planet.

Thankfully, according to the calculations in the paper, the chances of a black hole settling in Earth’s core are rather minimal. Black holes are just too fast. 

On the other hand, the intersection of our planet with a black hole would lead to another unpleasant reality: heating. During its passage through Earth, the black hole would accrete matter, and that accretion would generate heat (the same heat that powers active galactic nuclei). The impact of an asteroid-mass black hole would end up releasing about the same amount of energy as the impact of a kilometer-wide asteroid.

You know, a dinosaur killer.

Thankfully, black hole collisions are likely rare. In the most “optimistic” scenario — optimistic by the scientists’ standards, that is, so populating the galaxy with the maximum number of black holes — there might be one collision or so every billion years, according to the paper’s calculations.

So, when it comes to black hole collisions, don’t get too scared.

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Ancient Microfossils of Primordial Microbes Found in 3.4 Billion-Year-Old Rocks

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Newly discovered microfossils some 3.42 billion years old are the oldest evidence yet of a particular type of methane-cycling microbe life – and they could help us understand how life gets started in the first place, both on Earth and further out into the Universe.

 

These life forms would have originally existed just below the seafloor in pockets of a rich liquid soup, created from the mixing of cooler seawater from above and the warmer hydrothermal fluids rising up from the depths.

The new findings may answer some of the questions about how and where life first began during the Paleoarchean era (3.2-3.6 billion years ago), or whether indigenous microorganisms like this were around even earlier in Earth’s history.

The outcrop from which a sample was taken. (Cavalazzi et al., Science Advances, 2021)

“We found exceptionally well-preserved evidence of fossilized microbes that appear to have flourished along the walls of cavities created by warm water from hydrothermal systems a few meters below the seafloor,” says paleontologist Barbara Cavalazzi from the University of Bologna.

“Sub-surface habitats, heated by volcanic activity, are likely to have hosted some of Earth’s earliest microbial ecosystems and this is the oldest example that we have found to date.”

The rocks holding the fossils were collected from the Barberton Greenstone Belt in South Africa, near the border with Eswatini and Mozambique – a place where some of the oldest and most well-preserved sedimentary rocks on the entire planet can be found.

 

Analysis of the retrieved sediment showed microfossils with a carbon-rich outer covering around a core that was both chemically and structurally distinct – indicating microorganisms with cellular material wrapped in a wall or membrane.

Further study revealed most of the major chemical elements needed for life, plus other supporting evidence that these microfossils were once microbes: concentrations of nickel similar to those found in modern-day archaea prokaryotes, microbes which use methane rather than oxygen like their distant ancestors did.

“Although we know that archaea prokaryotes can be fossilized, we have extremely limited direct examples,” says Cavalazzi. “Our findings could extend the record of archaea fossils for the first time into the era when life first emerged on Earth.”

Scientists continue to make progress in figuring out how life on Earth got started, and how the inorganic became organic – perhaps through the help of a billion years of lightning strikes, or blasts from hydrothermal vents – but we still don’t know exactly what happened and in what order.

That’s perhaps not surprising, considering how difficult it is to peer back billions of years, but this latest research suggests that subsurface hydrothermal systems could be as important in the creation of life as some scientists have previously hypothesized.

Getting a better understanding about the conditions that life requires to exist and the parameters that it can work inside is going to be useful, not just in tracing the origins of life on Earth, but also in looking for it on other planets.

“As we also find similar environments on Mars, the study also has implications for astrobiology and the chances of finding life beyond Earth,” says Cavalazzi.

The research has been published in Science Advances.

 

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