ST/ ‘Starquakes’ could explain mystery signals

Paradigm

Published in

Paradigm

35 min readOct 20, 2023

Space biweekly vol.86, 29th September — 20th October

TL;DR

Fast radio bursts, or FRBs, are an astronomical mystery, with their exact cause and origins still unconfirmed. These intense bursts of radio energy are invisible to the human eye, but show up brightly on radio telescopes. Previous studies have noted broad similarities between the energy distribution of repeat FRBs, and that of earthquakes and solar flares. However, new research has looked at the time and energy of FRBs and found distinct differences between FRBs and solar flares, but several notable similarities between FRBs and earthquakes. This supports the theory that FRBs are caused by ‘starquakes’ on the surface of neutron stars. This discovery could help us better understand earthquakes, the behavior of high-density matter and aspects of nuclear physics.
By demonstrating that spaceflight doesn’t adversely affect the magnetism of moon rocks, researchers underscore the exciting potential of studying the magnetic histories stored in these samples.
The new study reports the sighting of two ice giant exoplanets colliding around a sun-like star, creating a blaze of light and plumes of dust. Its findings show the bright heat afterglow and resulting dust cloud, which moved in front of the parent star dimming it over time.
Observations during two flybys by the Mio spacecraft as part of the BepiColombo International Mercury Exploration Project have revealed that chorus waves occur quite locally in the dawn sector of Mercury. Mercury’s magnetic field is about 1% of that of Earth, and it was unclear whether chorus waves would be generated like on Earth. The present study reveals that the chorus waves are the driving source of Mercury’s X-ray auroras, whose mechanism was not understood.
Though scientists have long known through observational data that the Milky Way is warped and its edges are flared like a skirt, no one could explain why. Now, astronomers have performed the first calculations that fully explain this phenomenon, with compelling evidence pointing to the Milky Way’s envelopment in an off-kilter halo of dark matter.
The central question in the ongoing hunt for dark matter is: what is it made of? One possible answer is that dark matter consists of particles known as axions. A team of astrophysicists has now shown that if dark matter consists of axions, it may reveal itself in the form of a subtle additional glow coming from pulsating stars.
In the James Webb Space Telescope’s (JWST) first images of the universe’s earliest galaxies, the young galaxies appear too bright, too massive and too mature to have formed so soon after the Big Bang. Using new simulations, a team of astrophysicists now has discovered that these galaxies likely are not so massive after all. Although a galaxy’s brightness is typically determined by its mass, the new findings suggest that less massive galaxies can glow just as brightly from irregular, brilliant bursts of star formation.
A new study posits that the large, approximately 5-kilometer-long mounds that dominate the appearance of the larger lobe of the pristine Kuiper Belt object Arrokoth are similar enough to suggest a common origin. The study suggests that these “building blocks” could guide further work on planetesimal formational models.
A pair of theoretical physicists are reporting that the same observations inspiring the hunt for a ninth planet might instead be evidence within the solar system of a modified law of gravity originally developed to understand the rotation of galaxies.
A newly discovered nearby supernova whose star ejected up to a full solar mass of material in the year prior to its explosion is challenging the standard theory of stellar evolution. The new observations are giving astronomers new insight into what happens in the final year prior to a star’s death and explosion.
And more!

Space industry in numbers

The global smart space market size is projected to grow from USD 9.4 billion in 2020 to USD 15.3 billion by 2025, at a Compound Annual Growth Rate (CAGR) of 10.2% during the forecast period. The increasing venture capital funding and growing investments in smart space technology to drive market growth.

Analysts at Morgan Stanley and Goldman Sachs have predicted that economic activity in space will become a multi-trillion-dollar market in the coming decades. Morgan Stanley’s Space Team estimates that the roughly USD 350 billion global space industry could surge to over USD 1 trillion by 2040.

Source: Satellite Industry Association, Morgan Stanley Research, Thomson Reuters. *2040 estimates

Space industry news

Latest research

Fast radio bursts trigger aftershocks resembling earthquakes, but not solar flares

by Tomonori Totani and Yuya Tsuzuki in Monthly Notices of the Royal Astronomical Society

Fast radio bursts, or FRBs, are an astronomical mystery, with their exact cause and origins still unconfirmed. These intense bursts of radio energy are invisible to the human eye, but show up brightly on radio telescopes. Previous studies have noted broad similarities between the energy distribution of repeat FRBs, and that of earthquakes and solar flares. However, new research at the University of Tokyo has looked at the time and energy of FRBs and found distinct differences between FRBs and solar flares, but several notable similarities between FRBs and earthquakes. This supports the theory that FRBs are caused by “starquakes” on the surface of neutron stars. This discovery could help us better understand earthquakes, the behavior of high-density matter and aspects of nuclear physics.

The vastness of space holds many mysteries. While some people dream of boldly going where no one has gone before, there is a lot we can learn from the comfort of Earth. Thanks to technological advances, we can explore the surface of Mars, marvel at Saturn’s rings and pick up mysterious signals from deep space. Fast radio bursts are hugely powerful, bright bursts of energy which are visible on radio waves. First discovered in 2007, these bursts can travel billions of light years but typically last mere thousandths of a second. It has been estimated that as many as 10,000 FRBs may happen every day if we could observe the whole sky. While the sources of most bursts detected so far appear to emit a one-off event, there are about 50 FRB sources which emit bursts repeatedly.

**Earthquake map.** Data on earthquakes was taken from Japan’s Kanto region (including Tokyo and Narita) and Izumo in the Chugoku region (north of Hiroshima). Black dots represent the epicenters of earthquakes recorded between May 6, 2010, and December 31, 2012.

The cause of FRBs is unknown, but some ideas have been put forward, including that they might even be alien in origin. However, the current prevailing theory is that at least some FRBs are emitted by neutron stars. These stars form when a supergiant star collapses, going from eight times the mass of our sun (on average) to a superdense core only 20–40 kilometers across. Magnetars are neutron stars with extremely strong magnetic fields, and these have been observed to emit FRBs.

“It was theoretically considered that the surface of a magnetar could be experiencing a starquake, an energy release similar to earthquakes on Earth,” said Professor Tomonori Totani from the Department of Astronomy at the Graduate School of Science. “Recent observational advances have led to the detection of thousands more FRBs, so we took the opportunity to compare the now large statistical data sets available for FRBs with data from earthquakes and solar flares, to explore possible similarities.”

So far, statistical analysis of FRBs has focused on the distribution of wait times between two successive bursts. However, Totani and co-author Yuya Tsuzuki, a graduate student in the same department, point out that calculating only the wait-time distribution does not take into account correlations that might exist across other bursts. So the team decided to calculate correlation across two-dimensional space, analyzing the time and emission energy of nearly 7,000 bursts from three different repeater FRB sources. They then applied the same method to examine the time-energy correlation of earthquakes (using data from Japan) and of solar flares (using records from the Hinode international mission to study the sun), and compared the results of all three phenomena.

Totani and Tsuzuki were surprised that, in contrast to other studies, their analysis showed a striking similarity between FRBs and earthquake data, but a distinct difference between FRBs and solar flares.

**Comparing FRBs and earthquakes.** The researchers analyzed the time and energy distribution of FRB and earthquake events, and by plotting the aftershock likelihood as a function of time lag, they found that the two are very similar.

Totani explained: “The results show notable similarities between FRBs and earthquakes in the following ways: First, the probability of an aftershock occurring for a single event is 10–50%; second, the aftershock occurrence rate decreases with time, as a power of time; third, the aftershock rate is always constant even if the FRB-earthquake activity (mean rate) changes significantly; and fourth, there is no correlation between the energies of the main shock and its aftershock.”

This strongly suggests the existence of a solid crust on the surface of neutron stars, and that starquakes suddenly occurring on these crusts releases huge amounts of energy which we see as FRBs. The team intends to continue analyzing new data on FRBs, to verify that the similarities they have found are universal.

“By studying starquakes on distant ultradense stars, which are completely different environments from Earth, we may gain new insights into earthquakes,” said Totani. “The interior of a neutron star is the densest place in the universe, comparable to that of the interior of an atomic nucleus. Starquakes in neutron stars have opened up the possibility of gaining new insights into very high-density matter and the fundamental laws of nuclear physics.”

Establishing a Lunar Origin for Paleomagnetic Records in Apollo Samples

by S. M. Tikoo, J. Jung in Geophysical Research Letters

For decades, scientists have pondered the mystery of the moon’s ancient magnetism. Based on analyses of lunar samples, its now-deceased magnetic field may have been active for more than 1.5 billion years — give or take a billion years. Scientists believe it was generated like the Earth’s via a dynamo process, whereby the spinning and churning of conductive liquid metal within a rocky planet’s core generates a magnetic field. However, researchers have grappled with how such a small planetary body could have sustained a long-lived magnetic field. Some have even questioned the legitimacy of return samples that point to the existence of an ancient dynamo, suggesting magnetism may have been acquired via exposure to strong magnetic fields onboard spacecraft during the return mission or from plasmas produced by massive impacts on the moon.

Stanford University scientists have now demonstrated that the magnetism in lunar samples is not adversely altered by the spacecraft journey back to Earth or certain laboratory procedures, disproving one of the two major oppositions to the ancient dynamo theory. The findings bode well for research stemming from other sample-return missions from space, since any magnetic contamination acquired during flight or on Earth can likely be easily removed.

“You want to know that the spacecraft returning your sample is not magnetically frying your rock, essentially,” said lead study author Sonia Tikoo, an assistant professor of geophysics at the Stanford Doerr School of Sustainability. “We simulated a long-term exposure of a sample to a stronger magnetic field than what the Earth has — something that might be realistic for a spacecraft — and found that for nearly all samples, including several we had previously studied in the context of lunar dynamo records, we could remove that contamination quite easily.”

The study authors conducted two sets of lab experiments on eight samples from four different Apollo missions. They used a magnet to expose the samples to a field strength of about 5 millitesla — about 100 times stronger than the Earth’s magnetic field — for two days to approximately replicate the length of a return journey from the moon. Then, they took the samples into a magnetically shielded lab room to measure how quickly the contamination decayed and test how easily it could be removed using standard techniques. The research shows that basalts (rocks formed by the cooling of lava flows) are generally less susceptible to acquiring magnetic contamination than glass-bearing lunar rocks, but in nearly all cases the resulting contamination could be easily removed using standard methods.

AF demagnetization of VRM initially acquired over a period of 48 hr in a 5 mT field, following an initial viscous decay period of duration ∼3,000 s for various Apollo samples. Squares, triangles, and circles indicate previously undemagnetized, demagnetized, and saturated samples, respectively.

“As a global community, we’re starting to send more sample-return missions to other bodies, so it’s good to know that as long as we’re careful to ensure spacecraft fields are not too high — and it doesn’t have to be zero, necessarily — we can still do paleomagnetism studies along with other research,” said Tikoo, who also holds a courtesy appointment in Earth and planetary sciences. “You don’t always have to send up a heavy magnetic shield that’s going to take up a lot of room and a lot of mass at the expense of other science.”

Paleomagnetism is a branch of geophysics that uses remanent magnetization in rocks from the time of their formation to reconstruct the direction and/or strength of the geomagnetic field. The magnetic history of the moon is important for understanding the evolution of interior thermal history over time, in addition to how a global dynamo field may have controlled the delivery and retention of volatile substances, such as water, at the lunar surface. “An ancient lunar field may even have aided atmospheric retention on the early Earth,” the study authors write.

“Paleomagnetism is a very powerful tool for understanding core processes since we cannot go to the core of the planets, and also to learn about the past behavior of the core,” said study co-author Ji-In Jung, a PhD student in geophysics.

Magnetic fields may protect planets’ surfaces from harmful solar radiation and space weather, enabling the long-term preservation of atmospheres. While various other mechanisms for generating a magnetic field have been proposed, the dynamo theory is the widely accepted explanation of this phenomenon on Earth. Scientists think Earth’s magnetic field may have been essential for the development of conditions that support life, so learning about their presence around other planets and moons is part of the search for evidence of extraterrestrial life.

“In order to know about the internal structures of planetary bodies and their interaction with the atmosphere or other systems, we need to know about planetary dynamo processes,” Jung said.

Magnetic fields can also reveal the overall cooling history of a planetary body, which can, in turn, affect its volcanism and its tectonic regime. For asteroids, researchers want to understand how magnetic fields may have helped material come together in the early solar nebula and eventually build up into larger planets.

The moon’s magnetic history is of particular interest because geophysicists do not understand how a small planetary body like the moon could have generated a long-lived magnetic field, given that it has a small core that would likely have cooled quickly. As a next step, Tikoo aims to continue ongoing work to discriminate between the dynamo and impact hypotheses.

“This study proves that we can do extraterrestrial paleomagnetism with mission-returned samples,” Tikoo said. “I don’t think anybody doubts the ability to do Earth paleomagnetism and I’m happy that we can do it for space, too.”

A planetary collision afterglow and transit of the resultant debris cloud

by Matthew Kenworthy, Simon Lock, Grant Kennedy, et al in Nature

The study reports the sighting of two ice giant exoplanets colliding around a sun-like star, creating a blaze of light and plumes of dust. Its findings show the bright heat afterglow and resulting dust cloud, which moved in front of the parent star dimming it over time.

The international team of astronomers was formed after an enthusiast viewed the light curve of the star and noticed something strange. It showed the system doubled in brightness at infrared wavelengths some three years before the star started to fade in visible light.

Co-lead author Dr Matthew Kenworthy, from Leiden University, said: “To be honest, this observation was a complete surprise to me. When we originally shared the visible light curve of this star with other astronomers, we started watching it with a network of other telescopes.
“An astronomer on social media pointed out that the star brightened up in the infrared over a thousand days before the optical fading. I knew then this was an unusual event.”

The network of professional and amateur astronomers studied the star intensively including monitoring changes in the star’s brightness over the next two years. The star was named ASASSN-21qj after the network of telescopes that first detected the fading of the star at visible wavelengths.

The light curve of ASASSN-21qj from TESS and the periodogram of TESS and ASAS-SN photometry.

The researchers concluded the most likely explanation is that two ice giant exoplanets collided, producing the infrared glow detected by NASA’s NEOWISE mission, which uses a space telescope to hunt for asteroids and comets.

Co-lead author Dr Simon Lock, Research Fellow in Earth Sciences at the University of Bristol, said: “Our calculations and computer models indicate the temperature and size of the glowing material, as well as the amount of time the glow has lasted, is consistent with the collision of two ice giant exoplanets.”

The resultant expanding debris cloud from the impact then travelled in front of the star some three years later, causing the star to dim in brightness at visible wavelengths. Over the next few years, the cloud of dust is expected to start smearing out along the orbit of the collision remnant, and a tell-tale scattering of light from this cloud could be detected with both ground-based telescopes and NASA’s largest telescope in space, known as JWST. The astronomers plan on watching closely what happens next in this system.

Whistler-mode waves in Mercury’s magnetosphere observed by BepiColombo/Mio

by Mitsunori Ozaki, Satoshi Yagitani, Yasumasa Kasaba, Yoshiya Kasahara, Shoya Matsuda, Yoshiharu Omura, Mitsuru Hikishima, Fouad Sahraoui, Laurent Mirioni, Gérard Chanteur, Satoshi Kurita, Satoru Nakazawa, Go Murakami in Nature Astronomy

Observations during two flybys by the Mio spacecraft as part of the BepiColombo International Mercury Exploration Project have revealed that chorus waves occur quite locally in the dawn sector of Mercury. Mercury’s magnetic field is about 1% of that of Earth, and it was unclear whether chorus waves would be generated like on Earth. The present study reveals that the chorus waves are the driving source of Mercury’s X-ray auroras, whose mechanism was not understood.

Since Mercury is the closest planet to the Sun among the solar system planets, it is strongly influenced by the solar wind, a high-speed (several hundred km/s) stream of plasma blowing from the Sun. Explorations of Mercury was first carried out by the Mariner 10 spacecraft in 1974 and 1975, which revealed that Mercury has a magnetic field, and thus a magnetosphere, similar to that of Earth. In the 2000s, the MESSENGER spacecraft provided a detailed picture of the Mercury’s magnetic field and magnetosphere, and revealed that Mercury’s magnetic field center is shifted northward from the planet’s center by approximately 0.2 RM (RM is Mercury’s radius of 2,439.7 km). The third exploration of Mercury is currently being made by the BepiColombo International Mercury Exploration Project thanks to the Mio spacecraft (Project Scientist, Dr. Murakami) and the Mercury Planetary Orbiter (MPO). In particular, unlike Mariner 10 and MESSENGER, the Mio spacecraft is equipped with a full suite of plasma wave instrument (PWI, Principal Investigator Prof. Kasaba) designed specifically to investigate for the first time the electromagnetic environment around Mercury. Electromagnetic waves can efficiently accelerate plasma particles (electrons, protons, heavier ions); as such, they play an important role in the Mercury’s magnetospheric dynamics.

The present study was performed by an international joint research team consisting of scientists from Kanazawa University, Tohoku University, Kyoto University, MagneDesign Corporation, Laboratoire de Physique des Plasmas, France with support from CNES (French Space Agency), and the Institute of Space and Astronautical Science, the Japan Aerospace Exploration Agency (JAXA).

Image of chorus wave generation on Mercury(Image of Mercury. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

The Mio spacecraft, launched on October 20, 2018, is currently on its way to Mercury, with a final insertion in orbit around the planet scheduled for December 2025. Although getting Mio into Mercury’s orbit is technically extremely difficult due to the strong gravity of the Sun as compared to that of Mercury, it is scheduled to enter into orbit around Mercury in 2025 after several flybys of Earth, Venus, and Mercury for gravity assist maneuvers. During the Mercury flybys that occurred on October 1, 2021 and June 23, 2022, the Mio spacecraft had approached the planet at an altitude of approximately 200 km.

The stowed configuration of the spacecraft during the journey to Mercury is not optimal for measuring electromagnetic waves because of the interference noise coming from the spacecraft itself. However, the Mio spacecraft was developed to lower as much as possible its electromagnetic noise level, and thus has been certified as an electromagnetically clean spacecraft through EMC tests. Alternating current magnetic field sensors that can cope with the scorching environment of Mercury have been developed jointly by Japan and France and have allowed the first electromagnetic wave observations around Mercury without being contaminated by the noise from the spacecraft itself. This has revealed the local generation of chorus waves, such as those that are frequently detected in the magnetosphere of Earth. The existence of chorus waves in the magnetosphere of Mercury, which is now confirmed, was predicted (frequency range, intensity, etc.) since 2000s when the plasma wave instrument (PWI) of the Mio spacecraft was designed.

What most surprised the international joint research team, including Dr. Ozaki of Kanazawa University, was the ‘’spatial locality’’ of the chorus waves, which were detected only in an extremely limited region in the dawn sector of the Mercury’s magnetosphere during the two flybys. This means that there is a physical mechanism that tends to generate chorus waves only in the dawn sector of the magnetosphere of Mercury. In order to investigate the cause of the generation of chorus waves in the dawn sector, the international joint research team used the nonlinear growth theory of chorus waves established by Prof. Omura, Kyoto University, to evaluate the effect of curvature of the magnetic field of Mercury, which is strongly distorted by the solar wind. The magnetic field lines in the night sector are stretched by the solar wind pressure, while the magnetic field lines in the dawn sector are less affected resulting in a smaller curvature. Based on the characteristics of the magnetic field lines and the nonlinear growth theory, it is revealed that in the dawn sector, energy is efficiently transferred from electrons to electromagnetic waves along magnetic field lines, creating conditions that favor chorus wave generation. The effect is also confirmed in a numerical simulation of the Mercury environment using a high-performance computer. In this study, the team has revealed the importance of the planetary magnetic field lines, which are strongly affected by the solar wind, on the locality of chorus wave generation thanks to a strong synergy between “spacecraft observation,” “theory” and “simulation.”

In the Mercury flyby observations, the team prepared for the comprehensive electromagnetic environment survey using the planned Mio spacecraft probe in orbit around Mercury. Chorus waves, which were expected to be detected at the time of planning, are observed in a quite local manner, i.e. in the dawn sector of Mercury, which was not expected, and the results show various fluctuations in the magnetosphere of Mercury. The data demonstrate the existence of energetic electrons on Mercury that can generate chorus waves, the possibility of generating active electrons efficiently accelerated by chorus waves, and the generation of X-ray auroras by electrons forcibly precipitating from Mercury’s magnetosphere to the surface of Mercury driven by chorus waves. These observations will have a wide impact on the scientific understanding of Mercury’s environment. The Mio spacecraft is on its way to carry out a comprehensive exploration of Mercury. Based on flyby observations we have found that magnetic field distortion is responsible for the local (i.e. dawn sector) generation of the chorus waves.

The comprehensive exploration of the electromagnetic environment by the Mio spacecraft in Mercury’s orbit will contribute not only to understanding the plasma environment of the entire Mercury’s magnetosphere but also to a deep understanding of the magnetospheric dynamics in general. The magnetosphere acts as a barrier preventing life-threatening cosmic radiations on the planets of the solar system. Comparison of data from Mercury and Earth will strengthen our understanding of this important natural shielding of our home planet.

A tilted dark halo origin of the Galactic disk warp and flare

by Jiwon Jesse Han, Charlie Conroy, Lars Hernquist in Nature Astronomy

The Milky Way is often depicted as a flat, spinning disk of dust, gas, and stars. But if you could zoom out and take an edge-on photo, it actually has a distinctive warp — as if you tried to twist and bend a vinyl LP.

Though scientists have long known through observational data that the Milky Way is warped and its edges are flared like a skirt, no one could explain why.

Now, Harvard astronomers at the Center for Astrophysics | Harvard and Smithsonian (CfA) have performed the first calculations that fully explain this phenomenon, with compelling evidence pointing to the Milky Way’s envelopment in an off-kilter halo of dark matter. The work also bolsters current thinking about how the galaxy evolved and may offer clues into some of the mysteries of dark matter. The new calculations were led by Jiwon Jesse Han, a Griffin Graduate School of Arts and Sciences student affiliated with the CfA. The work includes co-authors Charlie Conroy and Lars Hernquist, both faculty members at the CfA and in the Department of Astronomy.

Our galaxy is located inside a diffuse cloud called the stellar halo, which extends much farther out into the universe. In groundbreaking work published last year, the Harvard team deduced that the stellar halo is tilted and elliptical in shape, like a zeppelin or football. Building on that, the team assumed the same shape for the dark matter halo, the larger entity that encompasses everything in and around the Milky Way. Dark matter makes up 80 percent of the galaxy’s mass but is invisible because it doesn’t interact with light, so the shape of that halo must be inferred. Using models to calculate the orbits of stars within a tilted, oblong dark matter halo, the team found a near-perfect match to existing observations of a warped, flared galaxy.

“A tilted dark halo is actually fairly common in simulations, but no one had explored its effect on the Milky Way,” Conroy said. “It turns out that the tilt is an elegant way to explain both the magnitude and direction of our galaxy’s wobbly disk.”

Scientists had long surmised that the Milky Way formed due to a galactic collision; the astronomers’ work further underscores that hypothesis.

“If the galaxy was just evolving on its own, it would have had this nice, spherical halo, this nice, flat disk,” Han said. “So the fact that the halo is tilted and has a football-like shape suggests that our galaxy experienced a merger event, where two galaxies collide.”

Their calculation of the dark matter halo’s probable shape may also provide clues as to the properties and particle nature of dark matter itself, which remain unsolved mysteries in physics. “The fact that the galaxy is not spherical in our data implies that there is some limit to which dark matter can interact with itself,” Han explained.

Confidence in these findings might lead to better ways to cleverly study the unobservable dark matter that makes up most of the universe. This includes new ways to pick up on kinematic signatures of dark sub-halos, which are miniature dark matter halos zipping around the galaxy.

Novel Constraints on Axions Produced in Pulsar Polar-Cap Cascades

by Dion Noordhuis, Anirudh Prabhu, Samuel J. Witte, Alexander Y. Chen, Fábio Cruz, Christoph Weniger in Physical Review Letters

The central question in the ongoing hunt for dark matter is: what is it made of? One possible answer is that dark matter consists of particles known as axions. A team of astrophysicists, led by researchers from the universities of Amsterdam and Princeton, has now shown that if dark matter consists of axions, it may reveal itself in the form of a subtle additional glow coming from pulsating stars.

Dark matter may be the most sought-for constituent of our universe. Surprisingly, this mysterious form of matter, that physicist and astronomers so far have not been able to detect, is assumed to make up an enormous part of what is out there. No less than 85% of matter in the universe is suspected to be ‘dark’, presently only noticeable through the gravitational pull it exerts on other astronomical objects. Understandably, scientists want more. They want to really see dark matter — or at the very least, detect its presence directly, not just infer it from gravitational effects. And, of course: they want to know what it is.

One thing is clear: dark matter cannot be the same type of matter that you and I are made of. If that were to be the case, dark matter would simply behave like ordinary matter — it would form objects like stars, light up, and no longer be ‘dark’. Scientists are therefore looking for something new — a type of particle that nobody has detected yet, and that probably only interacts very weakly with the types of particles that we know, explaining why this constituent of our world so far has remained elusive.

There are plenty of clues for where to look. One popular assumption is that dark matter could be made of axions. This hypothetical type of particle was first introduced in the 1970s to resolve a problem that had nothing to do with dark matter. The separation of positive and negative charges inside the neutron, one of the building blocks of ordinary atoms, turned out to be unexpectedly small. Scientists of course wanted to know why. It turned out that the presence of a hitherto undetected type of particle, interacting very weakly with the neutron’s constituents, could cause exactly such an effect. The later Nobel Prize winner Frank Wilczek came up with a name for the new particle: axion — not just similar to other particle names like proton, neutron, electron and photon, but also inspired by a laundry detergent of the same name. The axion was there to clean up a problem.

In fact, despite never being detected, it might clean up two. Several theories for elementary particles, including string theory, one of the leading candidate theories to unify all forces in nature, appeared to predict that axion-like particles could exist. If axions were indeed out there, could they also constitute part or even all of the missing dark matter? Perhaps, but an additional question that haunted all dark matter research was just as valid for axions: if so, then how can we see them? How does one make something ‘dark’ visible?

Fortunately, it seems that for axions there may be a way out of this conundrum. If the theories that predict axions are correct, they are not only expected to be mass-produced in the universe, but some axions could also be converted into light in the presence of strong electromagnetic fields. Once there is light, we can see. Could this be the key to detect axions — and therefore to detect dark matter?

Schematic figure showing axion production in neutron star vacuum gaps. The vacuum gap is depicted by a truncated cone on the neutron star surface.

To answer that question, scientists first had to ask themselves where in the universe the strongest known electric and magnetic fields occur. The answer is: in regions surrounding rotating neutron stars also known as pulsars. These pulsars — short for ‘pulsating stars’ — are dense objects, with a mass roughly the same as that of our Sun, but a radius that is around 100,000 times smaller, only about 10 km. Being so small, pulsars spin with enormous frequencies, emitting bright narrow beams of radio emission along their axis of rotation. Similar to a lighthouse, the pulsar’s beams can sweep across the Earth, making the pulsating star easily observable.

However, the pulsar’s enormous spin does more. It turns the neutron star into an extremely strong electromagnet. That, in turn, could mean that pulsars are very efficient axion factories. Every single second an average pulsar would be capable of producing a 50-digit number of axions. Because of the strong electromagnetic field around the pulsar, a fraction of these axions could convert into observable light. That is: if axions exist at all — but the mechanism can now be used to answer just that question. Just look at pulsars, see if they emit extra light, and if they do, determine whether this extra light could be coming from axions.

As always in science, actually performing such an observation is of course not that simple. The light emitted by axions — detectable in the form of radio waves — would only be a small fraction of the total light that these bright cosmic lighthouses send our way. One needs to know very precisely what a pulsar without axions would look like, and what a pulsar with axions would look like, to be able to see the difference — let alone to quantify that difference and turn it into a measurement of an amount of dark matter.

This is exactly what a team of physicists and astronomers have now done. In a collaborative effort between the Netherlands, Portugal and the USA, the team has constructed a comprehensive theoretical framework which allows for the detailed understanding of how axions are produced, how axions escape the gravitational pull of the neutron star, and how, during their escape, they convert into low energy radio radiation.

The theoretical results were then put on a computer to model the production of axions around pulsars, using state-of-the-art numerical plasma simulations that were originally developed to understand the physics behind how pulsars emit radio waves. Once virtually produced, the propagation of the axions through the electromagnetic fields of the neutron star was simulated. This allowed the researchers to quantitatively understand the subsequent production of radio waves and model how this process would provide an additional radio signal on top of the intrinsic emission generated from the pulsar itself.

The results from theory and simulation were then put to a first observational test. Using observations from 27 nearby pulsars, the researchers compared the observed radio waves to the models, to see if any measured excess could provide evidence for the existence of axions. Unfortunately, the answer was ‘no’ — or perhaps more optimistically: ‘not yet’. Axions do not immediately jump out to us, but perhaps that was not to be expected. If dark matter were to give up its secrets that easily, it would already have been observed a long time ago.

The hope for a smoking-gun detection of axions, therefore, is now on future observations. Meanwhile, the current non-observation of radio signals from axions is an interesting result in itself. The first comparison between simulations and actual pulsars has placed the strongest limits to date on the interaction that axions can have with light.

Of course, the ultimate goal is to do more than just set limits — it is to either show that axions are out there, or to make sure that it is extremely unlikely that axions are a constituent of dark matter at all. The new results are just a first step in that direction; they are only the beginning of what could become an entirely new and highly cross-disciplinary field that has the potential to dramatically advance the search for axions.

Bursty Star Formation Naturally Explains the Abundance of Bright Galaxies at Cosmic Dawn

by Guochao Sun, Claude-André Faucher-Giguère, Christopher C. Hayward, Xuejian Shen, Andrew Wetzel, Rachel K. Cochrane in The Astrophysical Journal Letters

When scientists viewed the James Webb Space Telescope’s (JWST) first images of the universe’s earliest galaxies, they were shocked. The young galaxies appeared too bright, too massive and too mature to have formed so soon after the Big Bang. It would be like an infant growing into an adult within just a couple years.

The startling discovery even caused some physicists to question the standard model of cosmology, wondering whether or not it should be upended. Using new simulations, a Northwestern University-led team of astrophysicists now has discovered that these galaxies likely are not so massive after all. Although a galaxy’s brightness is typically determined by its mass, the new findings suggest that less massive galaxies can glow just as brightly from irregular, brilliant bursts of star formation. Not only does this finding explain why young galaxies appear deceptively massive, it also fits within the standard model of cosmology.

“The discovery of these galaxies was a big surprise because they were substantially brighter than anticipated,” said Northwestern’s, Claude-André Faucher-Giguère, the study’s senior author. “Typically, a galaxy is bright because it’s big. But because these galaxies formed at cosmic dawn, not enough time has passed since the Big Bang. How could these massive galaxies assemble so quickly? Our simulations show that galaxies have no problem forming this brightness by cosmic dawn.”
“The key is to reproduce a sufficient amount of light in a system within a short amount of time,” added Guochao Sun, who led the study. “That can happen either because the system is really massive or because it has the ability to produce a lot of light quickly. In the latter case, a system doesn’t need to be that massive. If star formation happens in bursts, it will emit flashes of light. That is why we see several very bright galaxies.”

Faucher-Giguère is an associate professor of physics and astronomy at Northwestern’s Weinberg College of Arts and Sciences and a member of the Center for Interdisciplinary Exploration and Research in Astrophysics(CIERA). Sun is a CIERA Postdoctoral Fellow at Northwestern.

A period that lasted from roughly 100 million years to 1 billion years after the Big Bang, cosmic dawn is marked by the formation of the universe’s first stars and galaxies. Before the JWST launched into space, astronomers knew very little about this ancient time period.

“The JWST brought us a lot of knowledge about cosmic dawn,” Sun said. “Prior to JWST, most of our knowledge about the early universe was speculation based on data from very few sources. With the huge increase in observing power, we can see physical details about the galaxies and use that solid observational evidence to study the physics to understand what’s happening.”

Top: UV magnitude–halo mass relations at z = 8–12. Data for individual galaxies are denoted by the gray dots (no smoothing applied to the SFH). The thick solid curves indicate the range of the 5th and 95th percentiles in the “bursty” and “smoothed” cases, from which the suppression of bright galaxy number counts due to smoothing is apparent. Bottom: UVLFs at z = 8–12 derived from the convolution between the UV magnitude–halo mass relation and the HMF.

In the new study, Sun, Faucher-Giguère and their team used advanced computer simulations to model how galaxies formed right after the Big Bang. The simulations produced cosmic dawn galaxies that were just as bright as those observed by the JWST. The simulations are part of the Feedback of Relativistic Environments(FIRE) project, which Faucher-Giguère co-founded with collaborators at the California Institute of Technology, Princeton University and the University of California at San Diego. The new study includes collaborators from the Flatiron Institute’s Center for Computational Astrophysics, Massachusetts Institute of Technology and University of California, Davis.

The FIRE simulations combine astrophysical theory and advanced algorithms to model galaxy formation. The models enable researchers to probe how galaxies form, grow and change shape, while accounting for energy, mass, momentum and chemical elements returned from stars.

When Sun, Faucher-Giguère and their team ran the simulations to model early galaxies formed at cosmic dawn, they discovered that stars formed in bursts — a concept known as “bursty star formation.” In massive galaxies like the Milky Way, stars form at a steady rate, with the numbers of stars gradually increasing over time. But so-called bursty star formation occurs when stars form in an alternating pattern — many stars at once, followed by millions of years of very few new stars and then many stars again.

“Bursty star formation is especially common in low-mass galaxies,” Faucher-Giguère said. “The details of why this happens are still the subject of ongoing research. But what we think happens is that a burst of stars form, then a few million years later, those stars explode as supernovae. The gas gets kicked out and then falls back in to form new stars, driving the cycle of star formation. But when galaxies get massive enough, they have much stronger gravity. When supernovae explode, they are not strong enough to eject gas from the system. The gravity holds the galaxy together and brings it into a steady state.”

The simulations also were able to produce the same abundance of bright galaxies as the JWST revealed. In other words, the number of bright galaxies predicted by simulations matches the number of observed bright galaxies.

Although other astrophysicists have hypothesized that bursty star formation could be responsible for the unusual brightness of galaxies at cosmic dawn, the Northwestern researchers are the first to use detailed computer simulations to prove it is possible. And they were able to do so without adding new factors that are unaligned with our standard model of the universe.

The Properties and Origin of Kuiper Belt Object Arrokoth’s Large Mounds

by S. A. Stern, O. L. White, W. M. Grundy, B. A. Keeney, J. D. Hofgartner, D. Nesvorný, W. B. McKinnon, D. C. Richardson, J. C. Marohnic, A. J. Verbiscer, S. D. Benecchi, P. M. Schenk, J. M. Moore in The Planetary Science Journal

A new study led by Southwest Research Institute (SwRI) Planetary Scientist and Associate Vice President Dr. Alan Stern posits that the large, approximately 5-kilometer-long mounds that dominate the appearance of the larger lobe of the pristine Kuiper Belt object Arrokoth are similar enough to suggest a common origin. The SwRI study suggests that these “building blocks” could guide further work on planetesimal formational models. Stern presented these findings this week at the American Astronomical Society’s 55th Annual Division for Planetary Sciences (DPS) meeting in San Antonio.

NASA’s New Horizons spacecraft made a close flyby of Arrokoth in 2019. From those data, Stern and his coauthors identified 12 mounds on Arrokoth’s larger lobe, Wenu, which are almost the same shape, size, color and reflectivity. They also tentatively identified three more mounds on the object’s smaller lobe, Weeyo.

“It’s amazing to see this object so well preserved that its shape directly reveals these details of its assembly from a set of building blocks all very similar to one another,” said Lowell Observatory’s Dr. Will Grundy, co-investigator of the New Horizons mission. “Arrokoth almost looks like a raspberry, made of little sub-units.”

New Horizons CA06 LORRI image of Arrokoth, depicting its construction from two lobes, Wenu and Weeyo. Wenu, the larger lobe, displays prominent mound units.

Arrokoth’s geology supports the streaming instability model of planetesimal formation where collision speeds of just a few miles per hour allowed objects to gently accumulate to build Arrokoth in a local area of the solar nebula undergoing gravitational collapse.

“Similarities including in sizes and other properties of Arrokoth’s mound structures suggest new insights into its formation,” Stern, the Principal Investigator of the New Horizons mission, said. “If the mounds are indeed representative of the building blocks of ancient planetesimals like Arrokoth, then planetesimal formation models will need to explain the preferred size for these building blocks.”

There is a good chance that some of the flyby targets for NASA’s Lucy mission to Jupiter’s Trojan asteroids and ESA’s comet interceptor will be other pristine planetesimals, which could contribute to the understanding of accretion of planetesimals elsewhere in the ancient solar system and whether they differ from processes New Horizons found in the Kuiper Belt.

“It will be important to search for mound-like structures on the planetesimals these missions observe to see how common this phenomenon is, as a further guide to planetesimal formation theories,” Stern said.

Modified Newtonian Dynamics as an Alternative to the Planet Nine Hypothesis

by Katherine Brown and Harsh Mathur in The Astronomical Journal

A pair of theoretical physicists are reporting that the same observations inspiring the hunt for a ninth planet might instead be evidence within the solar system of a modified law of gravity originally developed to understand the rotation of galaxies.

Researchers Harsh Mathur, a professor of physics at Case Western Reserve University, and Katherine Brown, an associate professor of physics at Hamilton College, made the assertion after studying the effect the Milky Way galaxy would have on objects in the outer solar system — if the laws of gravity were governed by a theory known as Modified Newtonian Dynamics (or MOND).

MOND proposes Isaac Newton’s famous law of gravity is valid up to a point. That is, when the gravitational acceleration predicted by Newton’s law becomes small enough, MOND allows for a different gravitational behavior to take over. The observational success of MOND on galactic scales is why some scientists consider it an alternative to “dark matter,” the term physicists use to describe a hypothesized form of matter that would have gravitational effects but not emit any light.

“MOND is really good at explaining galactic-scale observations,” Mathur said, “but I hadn’t expected that it would have noticeable effects on the outer solar system.”

Orbital dynamics in the quadrupole approximation.

Mathur and Brown had studied MOND’s effect on galactic dynamics before. But they became interested in MOND’s more local effects after astronomers announced in 2016 that a handful of objects in the outer solar system showed orbital anomalies that could be explained by a ninth planet.

Orbital peculiarities have led to historic discoveries before: Neptune was discovered through its gravitational tug on the orbits of nearby object, the minute precession of Mercury provided early evidence in support of Einstein’s theory of general relativity, and astronomers have recently used orbital dynamics to infer the presence of a supermassive black hole at the center of our Galaxy.

Brown realized MOND’s predictions might be at odds with the observations that had motivated the search for a ninth planet. “We wanted to see if the data that support the Planet Nine hypothesis would effectively rule out MOND,” she said. Instead, Mathur and Brown found MOND predicts precisely clustering that astronomers have observed. Over millions of years, they argue, the orbits of some objects in the outer solar system would be dragged into alignment with the galaxy’s own gravitational field. When they plotted the orbits of the objects from the Planet Nine dataset against the galaxy’s own gravitational field, “the alignment was striking,” Mathur said.

The authors caution that the current dataset is small and that that any number of other possibilities might prove to be correct; other astronomers have argued the orbital peculiarities are the result of observational bias, for example.

“Regardless of the outcome,” Brown said, “this work highlights the potential for the outer solar system to serve as a laboratory for testing gravity and studying fundamental problems of physics.”

Millimeter Observations of the Type II SN 2023ixf: Constraints on the Proximate Circumstellar Medium

by Edo Berger, Garrett K. Keating, Raffaella Margutti, Keiichi Maeda, Kate D. Alexander, Yvette Cendes, Tarraneh Eftekhari, Mark Gurwell, Daichi Hiramatsu, Anna Y. Q. Ho, Tanmoy Laskar, Ramprasad Rao, Peter K. G. Williams in The Astrophysical Journal Letters

A newly discovered nearby supernova whose star ejected up to a full solar mass of material in the year prior to its explosion is challenging the standard theory of stellar evolution. The new observations are giving astronomers insight into what happens in the final year prior to a star’s death and explosion.

SN 2023ixf is a new Type II supernova discovered in May 2023 by amateur astronomer K?ichi Itagaki of Yamagata, Japan shortly after its progenitor, or origin star, exploded. Located about 20 million light-years away in the Pinwheel Galaxy, SN 2023ixf’s proximity to Earth, the supernova’s extreme brightness, and its young age make it a treasure trove of observable data for scientists studying the death of massive stars in supernova explosions.

Type II or core-collapse supernovae occur when red supergiant stars at least eight times, and up to about 25 times the mass of the Sun, collapse under their own weight and explode. While SN 2023ixf fit the Type II description, followup multi-wavelength observations led by astronomers at the Center for Astrophysics | Harvard & Smithsonian (CfA), and using a wide range of CfA’s telescopes, have revealed new and unexpected behavior.

Within hours of going supernova, core-collapse supernovae produce a flash of light that occurs when the shock wave from the explosion reaches the outer edge of the star. SN 2023ixf, however, produced a light curve that didn’t seem to fit this expected behavior. To better understand SN 2023ixf’s shock breakout, a team of scientists led by CfA postdoctoral fellow Daichi Hiramatsu analyzed data from the 1.5m Tillinghast Telescope, 1.2m telescope, and MMT at the Fred Lawrence Whipple Observatory, a CfA facility located in Arizona, as well as data from the Global Supernova Project — a key project of the Las Cumbres Observatory, NASA’s Neil Gehrels Swift Observatory, and many others. This multi-wavelength study revealed that, in sharp contradiction to expectations and stellar evolution theory, SN 2023ixf’s shock breakout was delayed by several days.

“The delayed shock breakout is direct evidence for the presence of dense material from recent mass loss,” said Hiramatsu, adding that such extreme mass loss is atypical of Type II supernovae. “Our new observations revealed a significant and unexpected amount of mass loss — close to the mass of the Sun — in the final year prior to explosion.”

Map from the aggregate SMA data from all six epochs of observations, of the region around SN 2023ixf (white cross) overlaid on a false-color Hubble Space Telescope image, with the synthesized beam shown in the lower left (white ellipse).

SN 2023ixf challenges astronomers’ understanding of the evolution of massive stars and the supernovae they become. Although scientists know that core-collapse supernovae are primary origin points for the cosmic formation and evolution of atoms, neutron stars, and black holes, very little is known about the years leading up to stellar explosions. The new observations point to potential instability in the final years of a star’s life, resulting in extreme mass loss. This could be related to the final stages of nuclear burn-off of high-mass elements, like silicon, in the star’s core.

In conjunction with multi-wavelength observations led by Hiramatsu, Edo Berger, professor of astronomy at Harvard and CfA, and Hiramatsu’s advisor, conducted millimeter-wave observations of the supernova using CfA’s Submillimeter Array (SMA) on the summit of Maunakea, Hawai’i. These data directly tracked the collision between the supernova debris and the dense material lost before the explosion.

“SN 2023ixf exploded exactly at the right time,” said Berger. “Only a few days earlier we commenced a new ambitious three-year program to study supernova explosions with the SMA, and this nearby exciting supernova was our first target.”
“The only way to understand how massive stars behave in the final years of their lives up to the point of explosion is to discover supernovae when they are very young, and preferably nearby, and then to study them across multiple wavelengths,” said Berger. “Using both optical and millimeter telescopes we effectively turned SN 2023ixf into a time machine to reconstruct what its progenitor star was doing up to the moment of its death.”

The supernova discovery itself, and the immediate followup, have significant meaning to astronomers around the world, including those doing science in their own backyards. Itagaki discovered the supernova on May 19, 2023, from his private observatory in Okayama, Japan. Combined data from Itagaki and other amateur astronomers determined the time of the explosion to an accuracy of within two hours, giving professional astronomers at CfA and other observatories a head start in their investigations. CfA astronomers have continued to collaborate with Itagaki on on-going optical observations.