Topic: Early January Astronomy Bulletin

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A GREEN FLASH ON JUPITER
Spaceweather.com

You've heard of a green flash on the Sun. But a Spanish astrophotographer has captured the rare phenomenon on Jupiter on Dec. 26th. Analysis of the image concludes that it is very likely a mock mirage--the same type of mirage that can create green flashes on the Sun. Mock mirages are caused by atmospheric temperature inversions, in which layers of air are warmer than usual. An extra 1 or 2 degrees Celsius is all it takes. Inversion layers can be quite close to ground. Indeed, Jupiter was only 1/3rd of a degree above the horizon of Arroyo de San Serván, Spain, when the flash was recorded. The low altitude of Jupiter is why the planet looked like a rainbow-coloured smear when the flash occurred. The low atmosphere acts like a prism, spreading the light of stars and planets into their R-G-B components.


NEW ESTIMATES OF NEUTRON STAR SIZE
DOE/Los Alamos National Laboratory

A combination of astrophysical measurements has allowed researchers to put new constraints on the radius of a typical neutron star and provide a novel calculation of the Hubble constant that indicates the rate at which the Universe is expanding. Astronomers studied signals that came from various sources, for example recently observed mergers of neutron stars. They analysed gravitational-wave signals and electromagnetic emissions from the mergers, and combined them with previous mass measurements of pulsars or recent results from NASA's Neutron Star Interior Composition Explorer. It was found that the radius of a typical neutron star is about 11.75 kilometres and the Hubble constant is approximately 66.2 kilometres per second per megaparsec. Combining signals to gain insight into distant astrophysical phenomena is known in the field as multi-messenger astronomy. In this case, the researchers' multi-messenger analysis allowed them to restrict the uncertainty of their estimate of neutron star radii to within 800 metres. Their novel approach to measuring the Hubble constant contributes to a debate that has arisen from other, competing determinations of the Universe's expansion. Measurements based on observations of exploding stars known as supernovae are currently at odds with those that come from looking at the Cosmic Microwave Background (CMB), which is essentially the left-over energy from the Big Bang. The uncertainties in the new multimessenger Hubble calculation are too large to definitively resolve the disagreement, but the measurement is slightly more supportive of the CMB approach.

The primary scientific role in the study was to provide the input from nuclear theory calculations that are the starting point of the analysis. A combination of astrophysical measurements has allowed researchers to put novel constraints on the radius of a typical neutron star and provide a new calculation of the Hubble constant that indicates the rate at which the Universe is expanding. They jointly analysed gravitational-wave signals and electromagnetic emissions from the mergers, and combined them with previous mass measurements of pulsars or recent results from NASA's Neutron Star Interior Composition Explorer. Combining signals to gain insight into distant astrophysical phenomena is known in the field as multi-messenger astronomy. In this case, the researchers' multi-messenger analysis allowed them to restrict the uncertainty of their estimate of neutron star radii to
within 800 metres. Their novel approach to measuring the Hubble constant contributes to a debate that has arisen from other, competing determinations of the Universe's expansion. 


UNDERSTANDING THE DEATH OF STARS
Northwestern University

Any Neapolitan ice cream lover knows three flavours are better than one. New research has found that by studying all three "flavours" involved in a supernova, they've unlocked more clues as to how and why stars die. Scientists look at neutrinos (subatomic particles) for critical information about supernova explosions. While previous research identified three "flavours" of neutrinos, many researchers continued to simplify studies on the topic by studying "vanilla" while ignoring "chocolate" and "strawberry." By including all three flavours in the study, the researchers have developed a deeper knowledge of dying stars and begun to unravel existing hypotheses. In a supernova explosion, 99% of the dead star's energy is emitted through neutrinos. Travelling almost at the speed of light and interacting extremely weakly with matter, neutrinos are the first messengers to reach the Earth and indicate a star has died. Since their initial discovery in the 1950s, particle physicists and astrophysicists have made important strides in understanding, detecting and creating neutrinos. But to limit the complexity of models, many people studying subatomic particles make assumptions to simplify the research -- for example, that non-electron neutrinos behave identically when they are propelled from a supernova. Part of what makes studying neutrinos so complicated is they come from compact objects (the inside of a star) and then interact with one another. That means when one flavour is impacted, much like a melting tub of Neapolitan ice cream, its evolution is affected by all others in the system.

As a result, when an enormous number of neutrinos are sent careening during the massive explosion of a core-collapse supernova, they begin to oscillate. Interactions between neutrinos change the properties and behaviours of the whole system, creating a coupled relationship. Therefore, when neutrino density is high, a fraction of neutrinos interchange flavours. When different flavours are emitted in different directions deep within a star, conversions occur rapidly and are called "fast conversions." Interestingly, the research found that as the number of neutrinos grows, so do their conversion rates, regardless of mass. In the study, the scientist created a non-linear simulation of a "fast conversion" when three neutrino flavours are present, where a fast conversion is marked by neutrinos interacting and changing flavours. The researchers removed the assumption that the three flavours of neutrinos -- muon, electron and tau neutrinos -- have the same angular distribution, giving them each a different distribution. A two-flavour setup of the same concept looks at electron neutrinos and "x" neutrinos, in which x can be either muon or tau neutrinos and where differences between the two are insignificant. While the research could have major implications in both particle and astrophysics, even models used in this research included simplifications. The team hopes to make their results more generic by including spatial dimensions in addition to components of momentum and time.

A BLAZAR IN THE EARLY UNIVERSE
National Radio Astronomy Observatory

The Very Long Baseline Array (VLBA) has revealed previously unseen details in a jet of material ejected at three-quarters the speed of light from the core of a galaxy some 12.8 billion light-years from Earth. The galaxy, dubbed PSO J0309+27, is a blazar, with its jet pointed toward Earth, and is the brightest radio-emitting blazar yet seen at such a distance. It also is the second-brightest X-ray emitting blazar at such a distance. In the image, the brightest radio emission comes from the galaxy's core. The jet is propelled by the gravitational energy of a supermassive black hole at the core, and moves outward. The jet extends some 1,600 light-years, and shows structure within it. At this distance, PSO J0309+27 is seen as it was when the Universe was less than a billion years old, or just over 7 percent of its current age.  An international team of astronomers observed the galaxy in April and May of 2020.  Their analysis of the object's properties provides support for some theoretical models for why blazars are rare in the early Universe.


UNIVERSE IS 13.77 BILLION YEARS OLD
Cornell University

From an observatory high above Chile's Atacama Desert, astronomers have taken a new look at the oldest light in the Universe. Their observations, plus a bit of cosmic geometry, suggest that the Universe is 13.77 billion years old -- give or take 40 million years. The new estimate, using data gathered at the National Science Foundation's Atacama Cosmology Telescope (ACT), matches the one provided by the standard model of the Universe, as well as measurements of the same light made by the European Space Agency's Planck satellite, which measured remnants of the Big Bang from 2009 to '13. In 2019, a research team measuring the movements of galaxies calculated that the Universe is hundreds of millions of years younger than the Planck team predicted. That discrepancy suggested a new model for the Universe might be needed and sparked concerns that one of the sets of
measurements might be incorrect. Now astronomers have come up with an answer where Planck and ACT agree. It speaks to the fact that these difficult measurements are reliable.


SEARCH FOR DARK MATTER FROM THE MULTIVERSE
Kavli Institute for the Physics and Mathematics of the Universe

A research team has made a study of black holes that could have formed in the early Universe, before stars and galaxies were born. Such primordial black holes (PBHs) could account for all or part of dark matter, be responsible for some of the observed gravitational waves signals, and seed supermassive black holes found in the centre of our Galaxy and other galaxies. They could also play a role in the synthesis of heavy elements when they collide with neutron stars and destroy them, releasing neutron-rich material. In particular, there is an exciting possibility that the mysterious dark matter, which accounts for most of the matter in the Universe, is composed of primordial black holes. The 2020 Nobel Prize in physics was awarded to a theorist, Roger Penrose, and two astronomers, Reinhard Genzel and Andrea Ghez, for their discoveries that confirmed the existence of black holes. Since black holes are known to exist in nature, they make a very appealing candidate for dark matter. The recent progress in fundamental theory, astrophysics, and astronomical observations in search of PBHs has been made by an international team of particle physicists, cosmologists and astronomers. To learn more about primordial black holes, the research team looked at the early Universe for clues. The early Universe was so dense that any positive density fluctuation of more than 50 percent would create a black hole. However, cosmological perturbations that seeded galaxies are known to be much smaller. Nevertheless, a number of processes in the early Universe could have created the right conditions for the black holes to form. One exciting possibility is that primordial black holes could form from the "baby universes" created during inflation, a period of rapid expansion that is believed to be responsible for seeding the structures we observe today, such as galaxies and clusters of galaxies. During inflation, baby universes can branch off of our Universe. A small baby (or "daughter") universe would eventually collapse, but the large amount of energy released in the small volume causes a black hole to form.

An even more peculiar fate awaits a bigger baby universe. If it is bigger than some critical size, Einstein's theory of gravity allows the baby universe to exist in a state that appears different to an observer on the inside and the outside. An internal observer sees it as an expanding universe, while an outside observer (such as us) sees it as a black hole. In either case, the big and the small baby universes are seen by us as primordial black holes, which conceal the underlying structure of multiple
universes behind their "event horizons." The event horizon is a boundary below which everything, even light, is trapped and cannot escape the black hole. In their paper, the team described a novel scenario for PBH formation and showed that the black holes from the "multiverse" scenario can be found using the Hyper Suprime-Cam (HSC) of the 8.2m Subaru Telescope, a gigantic digital camera -- the management of which Kavli IPMU has played a crucial role -- near the 4,200 metre summit of Mt. Mauna Kea in Hawaii. Why was the HSC indispensable in this research? The HSC has a unique capability to image the entire Andromeda galaxy every few minutes. If a black hole passes through the line of sight to one of the stars, the black hole's gravity bends the light rays and makes the star appear brighter than before for a short period of time. The duration of the star's brightening tells the astronomers the mass of the black hole. With HSC observations, one can simultaneously observe one hundred million stars, casting a wide net for primordial black holes that may be crossing one of the lines of sight. The first HSC observations have already reported a very intriguing candidate event consistent with a PBH from the "multiverse," with a black hole mass comparable to the mass of the Moon. Encouraged by this first sign, and guided by the new theoretical understanding, the team is conducting a new round of observations to extend the search and to provide a definitive test of whether PBHs from the multiverse scenario can account for all dark matter.