Jurassic worlds' might be easier to spot than modern Earth

 Things may not have ended well for dinosaurs on Earth, but Cornell University astronomers say the "light fingerprint" of the conditions that enabled them to emerge here provide a crucial missing piece in our search for signs of life on planets orbiting alien stars.

Their analysis of the most recent 540 million years of Earth's evolution, known as the Phanerozoic Eon, finds that telescopes could better detect potential chemical signatures of life in the atmosphere of an Earth-like exoplanet more closely resembling the age the dinosaurs inhabited than the one we know today.

Two key biosignature pairs -- oxygen and methane, and ozone and methane -- appeared stronger in models of Earth roughly 100 million to 300 million years ago, when oxygen levels were significantly higher. The models simulated the transmission spectra, or light fingerprint, generated by an atmosphere that absorbs some colors of starlight and lets others filter through, information scientists use to determine the atmosphere's composition.

"Modern Earth's light fingerprint has been our template for identifying potentially habitable planets, but there was a time when this fingerprint was even more pronounced -- better at showing signs of life," said Lisa Kaltenegger, director of the Carl Sagan Institute (CSI) and associate professor of astronomy. "This gives us hope that it might be just a little bit easier to find signs of life -- even large, complex life -- elsewhere in the cosmos."

Kaltenegger is co-author of "Oxygen Bounty for Earth-like Exoplanets: Spectra of Earth Through the Phanerozoic," published in Monthly Notices of the Royal Astronomical Society: Letters. First author, Rebecca Payne, research associate at CSI, led the new models that details a critical epoch including the origins of land plants, animals and dinosaurs.

Using estimates from two established climate models (called GEOCARB and COPSE), the researchers simulated Earth's atmospheric composition and resulting transmission spectra over five 100-million-year increments of the Phanerozoic. Each features significant changes as a complex ocean biosphere diversified, forests proliferated and terrestrial biospheres flourished, influencing the mix of oxygen and other gasses in the atmosphere.

"It's only the most recent 12% or so of Earth's history, but it encompasses pretty much all of the time in which life was more complex than sponges," said Payne. "These light fingerprints are what you'd search for elsewhere, if you were looking for something more advanced than a single-celled organism."

While similar evolutionary processes may or may not unfold on exoplanets, Payne and Kaltenegger said their models fill in a missing puzzle piece of what a Phanerozoic would look like to a telescope, creating new templates for habitable planets with varying atmospheric oxygen levels.

Kaltenegger pioneered modeling of what Earth would look like to faraway observers based on changes over time in its geology, climate and atmosphere -- our "ground truth," she said, for identifying potential evidence of life on other worlds.

To date, about 35 rocky exoplanets have been discovered in habitable zones where liquid water could exist, Kaltenegger said. Analyzing an exoplanet's atmosphere -- if it has one -- is at the edge of technical capability for NASA's James Webb Space Telescope but is now a possibility. But, the researchers said, scientists need to know what to look for. Their models identify planets like Phanerozoic Earth as the most promising targets for finding life in the cosmos.

They also allow scientists to entertain the possibility -- purely theoretical -- that if a habitable exoplanet is discovered to have an atmosphere with 30% oxygen, life there might not be limited to microbes, but could include creatures as large and varied as the megalosauruses or microraptors that once roamed Earth.

"If they're out there," Payne said, "this sort of analysis lets us figure out where they could be living."

Dinosaurs or not, the models confirm that from a great distance, such a planet's light fingerprint would stand out more than a modern Earth's.

Black holes are messy eaters

 New observations down to light-year scale of the gas flows around a supermassive black hole have successfully detected dense gas inflows and shown that only a small portion (about 3 percent) of the gas flowing towards the black hole is eaten by the black hole. The remainder is ejected and recycled back into the host galaxy.

Not all of the matter which falls towards a black hole is absorbed, some of it is ejected as outflows. But the ratio of the matter that the black hole "eats," and the amount "dropped" has been difficult to measure.

An international research team led by Takuma Izumi, an assistant professor at the National Astronomical Observatory of Japan, used the Atacama Large Millimeter/submillimeter Array (ALMA) to observe the supermassive black hole in the Circinus Galaxy, located 14 million light-years away in the direction of the constellation Circinus. This black hole is known to be actively feeding.

Thanks to ALMA's high resolution, the team was the first in the world to measure the amount of inflow and outflow down to a scale of a few light-years around the black hole. By measuring the flows of gasses in different states (molecular, atomic, and plasma) the team was able to determine the overall efficiency of black hole feeding, and found that it was only about 3 precent. The team also confirmed that gravitational instability is driving the inflow. Analysis also showed that the bulk of the expelled outflows are not fast enough to escape the galaxy and be lost. They are recycled back into the circumnuclear regions around the black hole, and start to slowly fall towards the black hole again.

Researchers find gravitational lensing has significant effect on cosmic birefringence

 Future missions will be able to find signatures of violating the parity-symmetry in the cosmic microwave background polarization more accurately after a pair of researchers has managed to take into account the gravitational lensing effect, reports a new study in Physical Review D, selected as an Editors' Suggestion.

How far does the universe extend? When and how did the universe begin? Cosmology has made progress in addressing these questions by providing observational evidence for theoretical models of the universe based on fundamental physics. The Standard Model of Cosmology is widely accepted by researchers today. However, it still cannot explain fundamental questions in cosmology , including dark matter and dark energy.

In 2020, an interesting new phenomenon called cosmic birefringence was reported from the cosmic microwave background (CMB) polarization data. Polarization describes light waves oscillating perpendicularly to the direction it is traveling. In general, the direction of polarization plane remains constant, but can be rotated under special circumstances. A reanalysis of the CMB data showed the polarization plane of the CMB light may have slightly rotated between the time it was emitted in the early universe and today. This phenomenon violates the parity symmetry and is called the cosmic birefringence.

Because cosmic birefringence is challenging to explain with the well-known physical laws, there is a strong possibility that yet to be discovered physics, such as the axionlike particles (ALPs), lies behind it. A discovery of cosmic birefringence could lead the way to revealing the nature of dark matter and dark energy, and so future missions are focused on making more precise observations of the CMB.

To do this, it is important to improve the accuracy of current theoretical calculations, but these calculations so far have not been sufficiently accurate because they do not take gravitational lensing into account.

A new study by a pair of researchers, led by The University of Tokyo Department of Physics and Research Center for Early Universe doctoral student Fumihiro Naokawa, and Center for Data-Driven Discovery and Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) Project Assistant Professor Toshiya Namikawa, established a theoretical calculation of cosmic birefringence that incorporates gravitational lensing effects, and worked on the development of a numerical code for cosmic birefringence that includes gravitational lensing effects, which will be indispensable for future analyses.

First, Naokawa and Namikawa derived an analytical equation describing how the gravitational lensing effect changes the cosmic birefringence signal. Based on the equation, the researchers implemented a new program to an existing code to compute the gravitational lensing correction, and then looked at the difference in signals with and without the gravitational lensing correction.

As a result, the researchers found that if gravitational lensing is ignored, the observed cosmic birefringence signal cannot be fitted well by the theoretical prediction, which would statistically reject the true theory.

In addition, the pair created simulated observational data that will be obtained in future observations to see the effect of gravitational lensing in the search for ALPs. They found that if the gravitational lensing effect is not considered, there would be statistically significant systematic biases in the model parameters of ALPs estimated from the observed data, which would not accurately reflect the ALPs model.

The gravitational lensing correction tool developed in this study is already being used in observational studies today, and Naokawa and Namikawa will continue to use it to analyze data for future missions.

Scientists, philosophers identify nature's missing evolutionary law

 A paper in the journal Proceedings of the National Academy of Sciences today describes "a missing law of nature," recognizing for the first time an important norm within the natural world's workings.

In essence, the new law states that complex natural systems evolve to states of greater patterning, diversity, and complexity. In other words, evolution is not limited to life on Earth, it also occurs in other massively complex systems, from planets and stars to atoms, minerals, and more.

Authored by a nine-member team -- leading scientists from the Carnegie Institution for Science, the California Institute of Technology (Caltech) and Cornell University, and philosophers from the University of Colorado -- the work was funded by the John Templeton Foundation.

"Macroscopic" laws of nature describe and explain phenomena experienced daily in the natural world. Natural laws related to forces and motion, gravity, electromagnetism, and energy, for example, were described more than 150 years ago.

The new work presents a modern addition -- a macroscopic law recognizing evolution as a common feature of the natural world's complex systems, which are characterised as follows:

• They are formed from many different components, such as atoms, molecules, or cells, that can be arranged and rearranged repeatedly
• Are subject to natural processes that cause countless different arrangements to be formed
• Only a small fraction of all these configurations survive in a process called "selection for function."

Regardless of whether the system is living or nonliving, when a novel configuration works well and function improves, evolution occurs.

The authors' "Law of Increasing Functional Information" states that the system will evolve "if many different configurations of the system undergo selection for one or more functions."

"An important component of this proposed natural law is the idea of 'selection for function,'" says Carnegie astrobiologist Dr. Michael L. Wong, first author of the study.

In the case of biology, Darwin equated function primarily with survival -- the ability to live long enough to produce fertile offspring.

The new study expands that perspective, noting that at least three kinds of function occur in nature.

The most basic function is stability -- stable arrangements of atoms or molecules are selected to continue. Also chosen to persist are dynamic systems with ongoing supplies of energy.

The third and most interesting function is "novelty" -- the tendency of evolving systems to explore new configurations that sometimes lead to startling new behaviors or characteristics.

Life's evolutionary history is rich with novelties -- photosynthesis evolved when single cells learned to harness light energy, multicellular life evolved when cells learned to cooperate, and species evolved thanks to advantageous new behaviors such as swimming, walking, flying, and thinking.

The same sort of evolution happens in the mineral kingdom. The earliest minerals represent particularly stable arrangements of atoms. Those primordial minerals provided foundations for the next generations of minerals, which participated in life's origins. The evolution of life and minerals are intertwined, as life uses minerals for shells, teeth, and bones.

Indeed, Earth's minerals, which began with about 20 at the dawn of our Solar System, now number almost 6,000 known today thanks to ever more complex physical, chemical, and ultimately biological processes over 4.5 billion years.

In the case of stars, the paper notes that just two major elements -- hydrogen and helium -- formed the first stars shortly after the big bang. Those earliest stars used hydrogen and helium to make about 20 heavier chemical elements. And the next generation of stars built on that diversity to produce almost 100 more elements.

"Charles Darwin eloquently articulated the way plants and animals evolve by natural selection, with many variations and traits of individuals and many different configurations," says co-author Robert M. Hazen of Carnegie Science, a leader of the research.

"We contend that Darwinian theory is just a very special, very important case within a far larger natural phenomenon. The notion that selection for function drives evolution applies equally to stars, atoms, minerals, and many other conceptually equivalent situations where many configurations are subjected to selective pressure."

The co-authors themselves represent a unique multi-disciplinary configuration: three philosophers of science, two astrobiologists, a data scientist, a mineralogist, and a theoretical physicist.

Says Dr. Wong: "In this new paper, we consider evolution in the broadest sense -- change over time -- which subsumes Darwinian evolution based upon the particulars of 'descent with modification.'"

"The universe generates novel combinations of atoms, molecules, cells, etc. Those combinations that are stable and can go on to engender even more novelty will continue to evolve. This is what makes life the most striking example of evolution, but evolution is everywhere."

Among many implications, the paper offers:

1. Understanding into how differing systems possess varying degrees to which they can continue to evolve. "Potential complexity" or "future complexity" have been proposed as metrics of how much more complex an evolving system might become
2. Insights into how the rate of evolution of some systems can be influenced artificially. The notion of functional information suggests that the rate of evolution in a system might be increased in at least three ways: (1) by increasing the number and/or diversity of interacting agents, (2) by increasing the number of different configurations of the system; and/or 3) by enhancing the selective pressure on the system (for example, in chemical systems by more frequent cycles of heating/cooling or wetting/drying).
3. A deeper understanding of generative forces behind the creation and existence of complex phenomena in the universe, and the role of information in describing them
4. An understanding of life in the context of other complex evolving systems. Life shares certain conceptual equivalencies with other complex evolving systems, but the authors point to a future research direction, asking if there is something distinct about how life processes information on functionality.
5. Aiding the search for life elsewhere: if there is a demarcation between life and non-life that has to do with selection for function, can we identify the "rules of life" that allow us to discriminate that biotic dividing line in astrobiological investigations?
6. At a time when evolving AI systems are an increasing concern, a predictive law of information that characterizes how both natural and symbolic systems evolve is especially welcome

Laws of nature -- motion, gravity, electromagnetism, thermodynamics -- etc. codify the general behavior of various macroscopic natural systems across space and time.

The "law of increasing functional information" published today complements the 2nd law of thermodynamics, which states that the entropy (disorder) of an isolated system increases over time (and heat always flows from hotter to colder objects).

Grasping the three-dimensional morphology of kilonovae

 An advanced new three-dimensional (3D) computer simulation of the light emitted following a merger of two neutron stars has produced a similar sequence of spectroscopic features to an observed kilonova. "The unprecedented agreement between our simulations and the observation of kilonova AT2017gfo indicates that we understand broadly what has taken place in the explosion and aftermath," says Luke Shingles, scientist at GSI/FAIR and the leading author of the publication in The Astrophysical Journal Letters. Recent observations that combine both gravitational waves and visible light have pointed to neutron star mergers as the major site of this element production. The research was performed by scientists at GSI Helmholtzzentrum für Schwerionenforschung and Queen's University Belfast.

The interactions between electrons, ions, and photons within the material ejected from a neutron-star merger determine the light that we can see through telescopes. These processes and the emitted light can be modelled with computer simulations of radiative transfer. Researchers have recently produced, for the first time, a three-dimensional simulation that self-consistently follows the neutron-star merger, neutron-capture nucleosynthesis, energy deposited by radioactive decay, and radiative transfer with tens of millions of atomic transitions of heavy elements.

Being a 3D model, the observed light can be predicted for any viewing direction. When viewed nearly perpendicular to the orbital plane of the two neutron stars (as observational evidence indicates for the kilonova AT2017gfo) the model predicts a sequence of spectral distributions that look remarkably similar to what has been observed for AT2017gfo. "Research in this area will help us to understand the origins of elements heavier than iron (such as platinum and gold) that were mainly produced by the rapid neutron capture process in neutron star mergers," says Shingles.

About half of the elements heavier than iron are produced in an environment of extreme temperatures and neutron densities, as achieved when two neutron stars merge with each other. When they eventually spiral in toward each other and coalesce, the resulting explosion leads to the ejection of matter with the appropriate conditions to produce unstable neutron-rich heavy nuclei by a sequence of neutron captures and beta-decays. These nuclei decay to stability, liberating energy that powers an explosive 'kilonova' transient, a bright emission of light that rapidly fades in about a week.

The 3D simulation combines together several areas of physics, including the behavior of matter at high densities, the properties of unstable heavy nuclei, and atom-light interactions of heavy elements. Further challenges remain, such as accounting for the rate at which the spectral distribution changes, and the description of material ejected at late times. Future progress in this area will increase the precision with which we can predict and understand features in the spectra and will further our understanding of the conditions in which heavy elements were synthesized. A fundamental ingredient for these models is high quality atomic and nuclear experimental data as will be provided by the FAIR facility.

Removal of magnetic spacecraft contamination within extraterrestrial samples easily carried out, researchers say

 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, published in Geophysical Research Letters Oct. 11, 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."

Reproducing contamination

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.

"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.

Dynamo theory

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

Source of electron acceleration and X-ray aurora of Mercury local chorus waves detected

 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*1) 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).

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*2) 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*3). 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."

Future Prospects

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.

Signatures of the Space Age: Spacecraft metals left in the wake of humanity's path to the stars

The Space Age is leaving fingerprints on one of the most remote parts of the planet -- the stratosphere -- which has potential implications for climate, the ozone layer and the continued habitability of Earth.

Using tools hitched to the nose cone of their research planes and sampling more than 11 miles above the planet's surface, researchers have discovered significant amounts of metals in aerosols in the atmosphere, likely from increasingly frequent launches and returns of spacecraft and satellites. That mass of metal is changing atmospheric chemistry in ways that may impact Earth's atmosphere and ozone layer.

"We are finding this human-made material in what we consider a pristine area of the atmosphere," said Dan Cziczo, one of a team of scientists who published a study on these results in the Proceedings of the National Academy of Sciences. "And if something is changing in the stratosphere -- this stable region of the atmosphere -- that deserves a closer look." Cziczo, professor and head of the Department of Earth, Atmospheric, and Planetary Sciences in Purdue's College of Science, is an expert in atmospheric science who has spent decades studying this rarefied region.

Led by Dan Murphy, an adjunct professor in the Department of Earth, Atmospheric, and Planetary Sciences and a researcher at the National Oceanic and Atmospheric Administration, the team detected more than 20 elements in ratios that mirror those used in spacecraft alloys. They found that the mass of lithium, aluminum, copper and lead from spacecraft reentry far exceeded those metals found in natural cosmic dust. Nearly 10% of large sulfuric acid particles -- the particles that help protect and buffer the ozone layer -- contained aluminum and other spacecraft metals.

Scientists estimate that as many as 50,000 more satellites may reach orbit by 2030. The team calculates that means that, in the next few decades, up to half of stratospheric sulfuric acid particles would contain metals from reentry. What effect that could have on the atmosphere, the ozone layer and life on Earth is yet to be understood.

Scientists have long suspected that spacecraft and satellites were changing the upper atmosphere, but studying the stratosphere, where we don't live and even the highest flights enter only briefly, is challenging.

As part of NASA's Airborne Science Program, Murphy and his group fly a WB-57 airplane to sample the atmosphere 11.8 miles (19 km) above the ground in Alaska, where circumpolar clouds tend to form. Similar measurements were made by Cziczo and his group from an ER-2 aircraft over the continental United States. Both groups use instruments hitched to the nose cone to ensure that only the freshest, most undisturbed air is sampled.

The sheltering sky

Like the view of the unruffled surface of the ocean, the stratosphere appears untroubled -- at least to human eyes. Life and civilization take place mostly on the planet's surface and in the troposphere, the atmosphere's very lowest layer. The stratosphere is a surprisingly stable and seemingly serene layer of the atmosphere.

The stratosphere is also the realm of the ozone layer: that gaseous marvel that acts as a global tent to shield the planet and all life on it from the searing, scorching rays of ultraviolet radiation. Without the ozone layer, life would likely never have arisen on Earth. And without it, life is unlikely to be able to continue.

The last decades have been eventful for the stratosphere. The ozone layer came under threat from chlorofluorocarbons in the 1980s, and only coordinated, sustained global efforts of governments and corporations have begun to bear fruit in repairing and replenishing it.

"Shooting stars streak through the atmosphere," Cziczo said. "Often, the meteor burns up in the atmosphere and doesn't even become a meteorite and reach the planet. So the material it was made from stays in the atmosphere in the form of ions. They form very hot gas, which starts to cool and condense as molecules and fall into the stratosphere. The molecules find each other and knit together and form what we call meteorite smoke. Scientists recently started noticing that the chemical fingerprint of these meteoritic particles was starting to change, which made us ask, 'Well, what changed?' because meteorite composition hasn't changed. But the number of spacecraft has."

What goes up

Spacecraft launches, and returns, were once international events. The launches of Sputnik and the Mercury missions were front-page news. Now, a quickening tide of innovation and loosening regulation means that dozens of countries and corporations are able to launch satellites and spacecraft into orbit. All those satellites have to be sent up on rockets -- and most of that material, eventually, comes back down.

Like the wakes of great ships trolling through the ocean, rockets leave behind them a trail of metals that may change the atmosphere in ways scientists don't yet understand.

"Just to get things into orbit, you need all this fuel and a huge body to support the payload," Cziczo said. "There are so many rockets going up and coming back and so many satellites falling back through the atmosphere that it's starting to show up in the stratosphere as these aerosol particles."

Of course, shooting stars were the first space-delivery system. Meteorites fall through the atmosphere every day. The heat and friction of the atmosphere peel material off them, just as they do off human-made artifacts. However, while hundreds of meteors enter the Earth's atmosphere every day, they are increasingly being rivaled by the mass of metals that comprise the tons of Falcon, Ariane and Soyuz rockets that boost spacecraft into space and return again to Earth's surface.

"Changes to the atmosphere can be difficult to study and complex to understand," Cziczo said. "But what this research shows us is that the impact of human occupation and human spaceflight on the planet may be significant -- perhaps more significant than we have yet imagined. Understanding our planet is one of the most urgent research priorities there is." 

New 'Assembly Theory' unifies physics and biology to explain evolution and complexity

 An international team of researchers has developed a new theoretical framework that bridges physics and biology to provide a unified approach for understanding how complexity and evolution emerge in nature. This new work on "Assembly Theory," published today in Nature, represents a major advance in our fundamental comprehension of biological evolution and how it is governed by the physical laws of the universe.

This research builds on the team's previous work developing Assembly Theory as an empirically validated approach to life detection, with implications for the search for alien life and efforts to evolve new life forms in the laboratory. In prior work, the team assigned a complexity score to molecules called the molecular assembly index, based on the minimal number of bond-forming steps required to build a molecule. They showed how this index is experimentally measurable and how high values correlate with life-derived molecules.

The new study introduces mathematical formalism around a physical quantity called "Assembly" that captures how much selection is required to produce a given set of complex objects, based on their abundance and assembly indices.

"Assembly Theory provides a completely new lens for looking at physics, chemistry and biology as different perspectives of the same underlying reality," explained lead author professor Sara Walker, a theoretical physicist and origin of life researcher from Arizona State University. "With this theory, we can start to close the gap between reductionist physics and Darwinian evolution -- it's a major step toward a fundamental theory unifying inert and living matter."

The researchers demonstrated how Assembly Theory can be applied to quantify selection and evolution in systems ranging from simple molecules to complex polymers and cellular structures. It explains both the discovery of new objects and the selection of existing ones, allowing open-ended increases in complexity characteristic of life and technology.

"Assembly Theory provides an entirely new way to look at the matter that makes up our world, as defined not just by immutable particles but by the memory needed to build objects through selection over time," said professor Lee Cronin, a chemist from the University of Glasgow and co-lead author. "With further work, this approach has the potential to transform fields from cosmology to computer science. It represents a new frontier at the intersection of physics, chemistry, biology and information theory."

The researchers aim to further refine Assembly Theory and explore its applications for characterizing known and unknown life, and testing hypotheses about how life emerges from non-living matter. "A key feature of the theory is that it is experimentally testable," said Cronin. "This opens up the exciting possibility of using Assembly Theory to design new experiments that could solve the origin of life by creating living systems from scratch in the laboratory."

The theory opens up many new questions and research directions at the boundary of the physical and life sciences. Overall, Assembly Theory promises to provide profound new insights into the physics underlying biological complexity and evolutionary innovation.

A prehistoric cosmic airburst preceded the advent of agriculture in the Levant

 Agriculture in Syria started with a bang 12,800 years ago as a fragmented comet slammed into the Earth's atmosphere. The explosion and subsequent environmental changes forced hunter-gatherers in the prehistoric settlement of Abu Hureyra to adopt agricultural practices to boost their chances for survival.

That's the assertion made by an international group of scientists in one of four related research papers, all appearing in the journal Science Open: Airbursts and Cratering Impacts. The papers are the latest results in the investigation of the Younger Dryas Impact Hypothesis, the idea that an anomalous cooling of the Earth almost 13 millennia ago was the result of a cosmic impact.

"In this general region, there was a change from more humid conditions that were forested and with diverse sources of food for hunter-gatherers, to drier, cooler conditions when they could no longer subsist only as hunter-gatherers," said Earth scientist James Kennett, a professor emeritus of UC Santa Barbara . The settlement at Abu Hureyra is famous among archaeologists for its evidence of the earliest known transition from foraging to farming. "The villagers started to cultivate barley, wheat and legumes," he noted. "This is what the evidence clearly shows."

These days, Abu Hureyra and its rich archaeological record lie under Lake Assad, a reservoir created by construction of the Taqba Dam on the Euphrates River in the 1970s. But before this flood, archaeologists managed to extract loads of material to study. "The village occupants," the researchers state in the paper, "left an abundant and continuous record of seeds, legumes and other foods." By studying these layers of remains, the scientists were able to discern the types of plants that were being collected in the warmer, humid days before the climate changed and in the cooler, drier days after the onset of what we know now as the Younger Dryas cool period.

Before the impact, the researchers found, the inhabitants' prehistoric diet involved wild legumes and wild-type grains, and "small but significant amounts of wild fruits and berries." In the layers corresponding to the time after cooling, fruits and berries disappeared and their diet shifted toward more domestic-type grains and lentils, as the people experimented with early cultivation methods. By about 1,000 years later, all of the Neolithic "founder crops" -- emmer wheat, einkorn wheat, hulled barley, rye, peas, lentils, bitter vetch, chickpeas and flax -- were being cultivated in what is now called the Fertile Crescent. Drought-resistant plants, both edible and inedible, become more prominent in the record as well, reflecting a drier climate that followed the sudden impact winter at the onset of the Younger Dryas.

The evidence also indicates a significant drop in the area's population, and changes in the settlement's architecture to reflect a more agrarian lifestyle, including the initial penning of livestock and other markers of animal domestication.

To be clear, Kennett said, agriculture eventually arose in several places on Earth in the Neolithic Era, but it arose first in the Levant (present-day Syria, Jordan, Lebanon, Palestine, Israel and parts of Turkey) initiated by the severe climate conditions that followed the impact.

And what an impact it must have been.

In the 12,800-year-old layers corresponding to the shift between hunting and gathering and agriculture, the record at Abu Hureyra shows evidence of massive burning. The evidence includes a carbon-rich "black mat" layer with high concentrations of platinum, nanodiamonds and tiny metallic spherules that could only have been formed under extremely high temperatures -- higher than any that could have been produced by man's technology at the time. The airburst flattened trees and straw huts, splashing meltglass onto cereals and grains, as well as on the early buildings, tools and animal bones found in the mound -- and most likely on people, too.

This event is not the only such evidence of a cosmic airburst on a human settlement. The authors previously reported a smaller but similar event which destroyed the biblical city at Tall el-Hammam in the Jordan Valley about 1600 BCE.

The black mat layer, nanodiamonds and melted minerals have also been found at about 50 other sites across North and South America and Europe, the collection of which has been called the Younger Dryas strewnfield. According to the researchers, it's evidence of a widespread simultaneous destructive event, consistent with a fragmented comet that slammed into the Earth's atmosphere. The explosions, fires and subsequent impact winter, they say, caused the extinction of most large animals, including the mammoths, saber-toothed cats, American horses, and American camels, as well as the collapse of the North American Clovis culture.

Because the impact appears to have produced an aerial explosion there is no evidence of craters in the ground. "But a crater is not required," Kennett said. "Many accepted impacts have no visible crater." The scientists continue to compile evidence of relatively lower-pressure cosmic explosions -- the kind that occur when the shockwave originates in the air and travels downward to the Earth's surface.

"Shocked quartz is well known and is probably the most robust proxy for a cosmic impact," he continued. Only forces on par with cosmic-level explosions could have produced the microscopic deformations within quartz sand grains at the time of the impacts, and these deformations have been found in abundance in the minerals gathered from impact craters.

This "crème de la crème" of cosmic impact evidence has also been identified at Abu Hureyra and at other Younger Dryas Boundary (YDB) sites, despite an absence of craters. However, it has been argued that the kind of shock-fractured quartz found in the YDB sites is not equivalent to that found in the large crater-forming sites, so the researchers worked to link these deformations to lower-pressure cosmic events.

To do so, they turned to humanmade explosions of the magnitude of cosmic airbursts: nuclear tests conducted at the Alamogordo Bombing Range in New Mexico in 1945 and in Kazakhstan, in 1949 and 1953. Similar to cosmic airbursts, the nuclear explosions occurred above ground, sending shockwaves toward Earth.

"In the papers, we characterize what the morphologies are of these shock fractures in these lower-pressure events," Kennett said. "And we did this because we wanted to compare it with what we have in the shock-fractured quartz in the Younger Dryas Boundary, to see if there was any comparison or similarities between what we see at the Trinity atomic test site and other atomic bomb explosions." Between the shocked quartz at the nuclear test sites and the quartz found at Abu Hureyra, the scientists found close associations in their characteristics, namely glass-filled shock fractures, indicative of temperatures greater than 2,000 degrees Celsius, above the melting point of quartz.

"For the first time, we propose that shock metamorphism in quartz grains exposed to an atomic detonation is essentially the same as during a low-altitude, lower-pressure cosmic airburst," Kennett said. However, the so-called "lower pressure" is still very high -- probably greater than 3 GPa or about 400,000 pounds per square inch, equivalent to about five 737 airplanes stacked on a small coin. The novel protocol the researchers developed for identifying shock fractures in quartz grains will be useful in identifying previously unknown airbursts that are estimated to recur every few centuries to millennia.

Taken together, the evidence presented by these papers, according to the scientists, "implies a novel causative link among extraterrestrial impacts, hemispheric environmental and climatic change, and transformative shifts in human societies and culture, including agricultural development."

Large mound structures on Kuiper belt object Arrokoth may have common origin

 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. These results are now also published in the peer-reviewed Planetary Science Journal.

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."

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.

The Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, designed, built and operates the New Horizons spacecraft, and manages the mission for NASA's Science Mission Directorate. Southwest Research Institute, based in San Antonio, directs the mission via Principal Investigator Stern, who leads the science team, payload operations and encounter science planning. New Horizons is part of the New Frontiers Program managed by NASA's Marshall Space Flight Center in Huntsville, Alabama.

Pulsars may make dark matter glow

 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.

Cleaning up two problems

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?

Shining a light on dark matter

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?

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.

Simulating a subtle glow

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.

Putting axion models to a test

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.

Astronomers discover first step toward planet formation

 Astronomers have gotten very good at spotting the signs of planet formation around stars. But for a complete understanding of planet formation, we also need to study examples where planet formation has not yet started. Looking for something and not finding it can be even more difficult than finding it sometimes, but new detailed observations of the young star DG Taurus show that it has a smooth protoplanetary disk without signs of planet formation. This successful non-detection of planet formation may indicate that DG Taurus is on the eve of planet formation.

Planets form in disks of gas and dust, known as protoplanetary disks, around protostars, young stars still in the process of forming. Planet growth is so slow that it's not possible to watch the evolution as it happens, so astronomers observe many protostars at slightly different stages of planet formation to build up a theoretical understanding.

This time an international research team led by Satoshi Ohashi at the National Astronomical Observatory of Japan (NAOJ) used the Atacama Large Millimeter/submillimeter Array (ALMA) to conduct high-resolution observations of a protoplanetary disk around a relatively young protostar, DG Taurus located 410 light-years away in the direction of the constellation Taurus. The team found that DG Taurus has a smooth protoplanetary disk, without any rings which would indicate that planets are forming. This led the team to believe that DG Taurus system will start forming planets in the future.

The team found that in this pre-planet-formation stage, the dust grains within 40 AU (about twice the size of the orbit of Uranus in the Solar System) of the central protostar are still small, while beyond this radius the dust grains have started to grow in size, the first step in planet formation. This is contrary to theoretical expectations that planet formation starts in the inner part of the disk.

These results provide surprising new information about the dust distribution and other conditions at the start of planet formation. Future studies of more examples will further improve our understanding of planet formation.

Colliding neutron stars provide a new way to measure the expansion of the Universe

 In recent years, astronomy has seen itself in a bit of crisis: Although we know that the Universe expands, and although we know approximately how fast, the two primary ways to measure this expansion do not agree. Now astrophysicists from the Niels Bohr Institute suggest a novel method which may help resolve this tension.

The Universe expands

We've known this ever since Edwin Hubble and other astronomers, some 100 years ago, measured the velocities of a number of surrounding galaxies. The galaxies in the Universe are "carried" away from each other by this expansion, and therefore recedes from each other.

The greater the distance between two galaxies, the faster they move apart, and the precise rate of this movement is one of the most fundamental quantities in modern cosmology. The number that describes the expansion goes by the name "the Hubble constant," appearing in multitude of different equations and models of the Universe and its constituents.

Hubble Trouble

To understand the Universe we must therefore know the Hubble constant as precisely as possible. Several methods exist to measure it; methods that are mutually independent but luckily give almost the same result.

That is, almost…

The intuitively easiest method to understand is, in principle, the same that Edwin Hubble and his colleagues used a century ago: Locate a bunch of galaxies, and measure their distances and speeds. In practise this is done by looking for galaxies with exploding stars, so-called supernovae. This method is complemented by another method that analyzes irregularities in the so-called cosmic background radiation; an ancient form of light dating back to shortly after the Big Bang.

The two methods -- the supernova method and the background radiation method -- always gave slightly different results. But any measurement comes with uncertainties, and a few years back the uncertainties were substantial enough that we could blame those for the disparity.

Nevertheless, as measurement techniques have improved, uncertainties have diminished, and we've now reached a point where we can state with a high degree of confidence that both cannot be correct.

The root of this "Hubble trouble" -- whether it is unknown effects systematically biasing one of the results, or if it hints at new physics yet to be discovered -- is currently one of astronomy's hottest topics.

Two methods

The expansion of the Universe is measured in "speed per distance," and is just over 20 km/s per million lightyears. That means that a galaxy located 100 million lightyears away recedes from us at 2,000 km/s, while another galaxy 200 million lightyears away recedes at 4,000 km/s.

But using supernovae to measure distances and velocities of galaxies yields 22.7 ± 0.4 km/s, while analyzing the background radiation of the Universe yields 20.7 ± 0.2 km/s.

It might sound pernickety to care about such a little disagreement, but for instance the number appears in the calculation of the age of the Universe, and the two methods yield an age of 12.8 and 13.8 billion years, respectively.

Crashing neutron stars may help with the answer

One of the greatest challenges lies in accurately determining the distances to galaxies. But in a new study, Albert Sneppen who is a PhD student in astrophysics at the Cosmic Dawn Center at the Niels Bohr Institute in Copenhagen, proposes a novel method for measuring distances, thereby helping to settle the ongoing dispute.

"When two ultra-compact neutron stars -- which in themselves are the remnants of supernovae -- orbit each other and ultimately merge, they go off in a new explosion; a so-called kilonova," Albert Sneppen explains. "We recently demonstrated how this explosion is remarkedly symmetric, and it turns out that this symmetry not only is beautiful, but also incredibly useful."

In a third study that has just been published, the prolific PhD student shows that kilonovae, despite their complexity, can be described by a single temperature. And it turns out that the symmetry and the simplicity of the kilonovae enable the astronomers to deduce exactly how much light they emit.

Comparing this luminosity with how much light reaches Earth, the researchers can calculate how far away the kilonova is. They have thereby obtained a novel, independent method to calculate the distance to galaxies containing kilonovae.

Darach Watson is an associate professor at the Cosmic Dawn Center and a co-author of the study. He explains: "Supernovae, which until now have been used to measure the distances of galaxies, don't always emit the same amount of light. Moreover, they first require us to calibrate the distance using another type of stars, the so-called Cepheids, which in turn also must be calibrated. With kilonovae we can circumvent these complications that introduce uncertainties in the measurements."

Confirms one of the two methods

To demonstrate its potential, the astrophysicists applied the method to a kilonova discovered in 2017. The result is a Hubble constant closer to the background radiation method, but whether the kilonova method can resolve the Hubble trouble, the researchers do not yet dare to state:

"We only have this one case study so far, and need many more examples before we can establish a robust result," Albert Sneppen cautions. "But our method at least bypasses some known sources of uncertainty, and is a very "clean" system to study. It requires no calibration, no correction factor."

New proof for black hole spin

 The supermassive black hole at the heart of galaxy M87, made famous by the first picture of a black hole shadow, has yielded another first: the jet shooting out from the black hole has been confirmed to wobble, providing direct proof that the black hole is spinning.

Super massive black holes, monsters up to billions of times heavier than the Sun that eat everything around them including light, are difficult to study because no information can escape from within. Theoretically, there are very few properties that we can even hope to measure. One property that might possibly be observed is spin, but due to the difficulties involved there have been no direct observations of black hole spin.

Searching for evidence for black hole spin, an international team analyzed over two decades of observational data for the galaxy M87. This galaxy located 55 million light-years away in the direction of the constellation Virgo harbors a black hole 6.5 billion times more massive than the Sun, the same black hole which yielded the first image of a black hole shadow by the Event Horizon Telescope (EHT) in 2019. The supermassive black hole in M87 is known to have an accretion disk, which feeds matter into the black hole, and a jet, in which matter is ejected from near the black hole at close to the speed of light.

The team analyzed data for 170 time frames collected by the East Asian VLBI Network (EAVN), the Very Long Baseline Array (VLBA), the joint array of KVN and VERA (KaVA), and the East Asia to Italy Nearly Global (EATING) VLBI network. In total, more than 20 radio telescopes across the globe contributed to this study.

The results show that gravitational interactions between the accretion disk and the black hole's spin cause the base of the jet to wobble, or precess, much the same way that gravitational interactions within the Solar System cause the Earth to precess. The team successfully linked the dynamics of the jet with the central supermassive black hole, providing direct evidence that the black hole does in fact spin. The jet's direction changes by about 10 degrees with a precession period of 11 years, matching theoretical supercomputer simulations conducted by ATERUI II at the National Astronomical Observatory of Japan (NAOJ).

"We are thrilled by this significant finding," says Yuzhu Cui, lead author on the paper summarizing the research she started as a graduate student at NAOJ before moving to Zhejiang Lab as a postdoctoral researcher. "Since the misalignment between the black hole and the disk is relatively small and the precession period is around 11 years, accumulating high-resolution data tracing M87 structure over two decades and thorough analysis are essential to obtain this achievement."

"After the success of black hole imaging in this galaxy with the EHT, whether this black hole is spinning or not has been a central concern among scientists," explains Dr. Kazuhiro Hada from NAOJ. "Now anticipation has turned into certainty. This monster black hole is indeed spinning."

"This is an exciting scientific milestone that was finally revealed through years of joint observations by the international researchers team from 45 institutions around the world, working together as one," says Dr. Motoki Kino at Kogakuin University, the coordinator of the East Asian VLBI Network Active Galactic Nuclei Science Working Group. "Our observational data beautifully fitted to the simple sinusoidal curve bring us new advances in our understanding of black hole and jet system."