Category Archives: Cosmology

First Clear View Of A Boiling Cauldron Where Stars Are Born (Cosmology)

UMD-led team used NASA’s SOFIA telescope to capture high-resolution details of a star nursery in the Milky Way

University of Maryland researchers created the first high-resolution image of an expanding bubble of hot plasma and ionized gas where stars are born. Previous low-resolution images did not clearly show the bubble or reveal how it expanded into the surrounding gas.

The researchers used data collected by the Stratospheric Observatory for Infrared Astronomy (SOFIA) telescope to analyze one of the brightest, most massive star-forming regions in the Milky Way galaxy. Their analysis showed that a single, expanding bubble of warm gas surrounds the Westerlund 2 star cluster and disproved earlier studies suggesting there may be two bubbles surrounding Westerlund 2. The researchers also identified the source of the bubble and the energy driving its expansion. Their results were published in The Astrophysical Journal on June 23, 2021.

“When massive stars form, they blow off much stronger ejections of protons, electrons and atoms of heavy metal, compared to our sun,” said Maitraiyee Tiwari, a postdoctoral associate in the UMD Department of Astronomy and lead author of the study. “These ejections are called stellar winds, and extreme stellar winds are capable of blowing and shaping bubbles in the surrounding clouds of cold, dense gas. We observed just such a bubble centered around the brightest cluster of stars in this region of the galaxy, and we were able to measure its radius, mass and the speed at which it is expanding.”

The surfaces of these expanding bubbles are made of a dense gas of ionized carbon, and they form a kind of outer shell around the bubbles. New stars are believed to form within these shells. But like soup in a boiling cauldron, the bubbles enclosing these star clusters overlap and intermingle with clouds of surrounding gas, making it hard to distinguish the surfaces of individual bubbles.

A team led by UMD astronomers created the first clear image of an expanding bubble of stellar gas where stars are born using data from NASA’s SOFIA telescope on board a heavily modified 747 jet as seen here in this artist’s rendering. Artist Rendering by Marc Pound/UMD

Tiwari and her colleagues created a clearer picture of the bubble surrounding Westerlund 2 by measuring the radiation emitted from the cluster across the entire electromagnetic spectrum, from high-energy X-rays to low-energy radio waves. Previous studies, which only radio and submillimeter wavelength data, had produced low-resolution images and did not show the bubble. Among the most important measurements was a far-infrared wavelength emitted by a specific ion of carbon in the shell.

“We can use spectroscopy to actually tell how fast this carbon is moving either towards or away from us,” said Ramsey Karim (M.S. ’19, astronomy), a Ph.D. student in astronomy at UMD and a co-author of the study. “This technique uses the Doppler effect, the same effect that causes a train’s horn to change pitch as it passes you. In our case, the color changes slightly depending on the velocity of the carbon ions.”

By determining whether the carbon ions were moving toward or away from Earth and combining that information with measurements from the rest of the electromagnetic spectrum, Tiwari and Karim were able to create a 3D view of the expanding stellar-wind bubble surrounding Westerlund 2.

In addition to finding a single, stellar wind-driven bubble around Westerlund 2, they found evidence of new stars forming in the shell region of this bubble. Their analysis also suggests that as the bubble expanded, it broke open on one side, releasing hot plasma and slowing expansion of the shell roughly a million years ago. But then, about 200,000 or 300,000 years ago, another bright star in Westerlund 2 evolved, and its energy re-invigorated the expansion of the Westerlund 2 shell.

“We saw that the expansion of the bubble surrounding Westerlund 2 was reaccelerated by winds from another very massive star, and that started the process of expansion and star formation all over again,” Tiwari said. “This suggests stars will continue to be born in this shell for a long time, but as this process goes on, the new stars will become less and less massive.”

Tiwari and her colleagues will now apply their method to other bright star clusters and warm gas bubbles to better understand these star-forming regions of the galaxy. The work is part of a multi-year NASA-supported program called FEEDBACK.

Additional co-authors of the research paper from UMD’s Department of Astronomy include Research Scientists Marc Pound and Mark Wolfire and Adjunct Professor Alexander Tielens. This work was supported by the NASA-funded FEEDBACK project (Award No. SOF070077). The content of this article does not necessarily reflect the views of this organization.

The research paper, “SOFIA FEEDBACK Survey: Exploring the Dynamics of the Stellar-wind-driven Shell of RCW 49” and the author list is: Tiwari, M., Karim, R., Pound, M. W., Wolfire, M., Jacob, A., Buchbender, C., Güsten, R., Guevara, C., Higgins, R. D., Kabanovic, S., Pabst, C., Ricken, O., Schneider, N., Simon, R., Stutzki, J., Tielens, A. G. G. M., was published on June 23, 2021, in The Astrophysical Journal.

Featured image: The RCW 49 galactic nebula pictured above is one of the brightest star-forming regions in the Milky Way. By analyzing the movement of carbon atoms in an expanding bubble of gas surrounding the Westerlund 2 star cluster within RCW 49, a UMD-led team of researchers have created the clearest image to date of a stellar-wind driven bubble where stars are born. © NASA/JPL-Caltec/E.Churchwell (University of Wisconsin).


Provided by University of Maryland

Picture Of The Week: Clash of the Titans (Cosmology)

A cataclysmic cosmic collision takes centre stage in this Picture of the Week. The image features the interacting galaxy pair IC 1623, which lies around 275 million light-years away in the constellation Cetus (The Whale). The two galaxies are in the final stages of merging, and astronomers expect a powerful inflow of gas to ignite a frenzied burst of star formation in the resulting compact starburst galaxy. 

This interacting pair of galaxies is a familiar sight; Hubble captured IC 1623 in 2008 using two filters at optical and infrared wavelengths using the Advanced Camera for Surveys. This new image incorporates new data from Wide Field Camera 3, and combines observations taken in eight filters spanning infrared to ultraviolet wavelengths to reveal the finer details of IC 1623. Future observations of the galaxy pair with the NASA/ESA/CASA James Webb Space Telescope will shed more light on the processes powering extreme star formation in environments such as IC 1623.

Featured image: IC 1623 © ESA/Hubble & NASA, R. Chandar


Provided by ESA/Hubble

Thus A Cluster Emerges From The Cosmic Web (Cosmology)

An international team of astronomers used the National Science Foundation’s Green Bank Telescope and NASA’s Chandra X-ray Observatory to capture a snapshot of a massive cluster around the time it began to emerge from the cosmic web, nearly 10 billion. Years ago. The study is published in Mnras

A sort of reverse dive: so we can imagine the emergence of any cosmic structure from the cosmos itself. However, this representation is only part of the story, the fruit of a limited perspective. Observing, for astronomers, means focusing attention on phenomena that appear luminous, that is, involving baryon matter . In the formation of cosmic structures, however, light comes almost last. To this, then, it must be added that seeing what was bright when the universe was little more than just born is no small feat.

In an article currently being published in Monthly Notices of the Royal Astronomical Society , a group of scientists managed to capture the light of a cluster of galaxies just as it ignited and emerged from the cosmic web. The cluster is called Idcsj1426 + 3508, is located at redshift 1.75 and its light has traveled almost 10 billion years to reach us.

Let’s start from the beginning: the light does not come first, we said. The texture of the cosmos is in fact woven from dark matterwhich, through gravity, we can imagine as a spider’s web with some more or less large densities: these are the attractors of baryon matter. When an imposing structure such as a cluster begins to form, it is therefore the dark matter that first prepares a comfortable lair in which the baryon matter – the gas – can begin to thicken as it is gravitationally attracted. It therefore seems like a dip in the opposite direction, that of light, because the gas that heats up proportionally to the increase in its density begins to ignite slowly, becoming visible. This is how astronomers were able to measure the properties of Idcsj1426 + 3508, and they did so in an era and with unprecedented precision.

A snapshot that, thanks to the comparison of the properties of the gas of Idcsj1426 + 3508 with those of its most plausible descendants of the present day – those observed in the nearby universe – scientists have been able to evolve over time to predict the history of its future and tumultuous evolution.

Stefano Andreon (researcher of INAF from Brera and first author of this study) observing a very high redshift cluster with the Green Bank Telescope from his home studio

«Everywhere, except in the center, the temperature will become warmer despite the entry of new cold gas from the peripheral regions» explains Stefano Andreon , INAF researcher at the Brera site and first author of the study. The center of the cluster, therefore, will retain its properties unchanged over time despite the tumultuous and hostile environment, maintaining a delicate balance between the perturbations generated by the entry of matter and a response mechanism – in astronomical jargon, feedback– not yet identified. Just outside the center, however, the gas in the newly born cluster seems to be colder than that of today’s clusters, while in the outermost regions the situation is reversed. In order to become similar to his great-grandchildren, therefore, Idcsj1426 + 3508 will have to go through a mechanism that warms the innermost regions, while the external regions cool down and the center remains almost unchanged over time.

The possibility of going so far back in time with extreme precision was offered by the joint use of the Chandra telescope – which, observing at X-ray frequencies, “sees” the density of the tenuous gas that permeates the cluster – and the Mustang2 chamber of the Green Bank Telescpe ( GBT ), in West Virginia, which made it possible to measure the thermal agitation of the gas – which astronomers and physicists call pressure – at various points in the cluster. The data showed that the gas gets hotter as the distance from the center increases, going from 50 to 150 million degrees Celsius.

“But a swallow doesn’t make spring,” Andreon continues. “The evolution of galaxy clusters is probably diverse, which is why we are observing, and inviting colleagues to observe, other clusters that begin to emerge from the cosmic network, in the hope of identifying, in addition to individual differences, a common general pattern” .

“Of course we are interested in how clusters evolve over time, not just to understand their astrophysics and associated phenomenology,” adds Charles Romero , researcher at the Green Bank Observatory and the University of Pennsylvania and co-author of the study. “The evolution of clusters, in fact, is a fundamental step in being able to study the evolution of the universe and cosmology using the clusters of galaxies themselves as tracers”.

Featured image: Image of the cluster Idcsj1426 + 3508 in which the hot gas in the periphery is colored red, the colder gas in the center is blue. Credits: Gbt / Chandra / Sdss / Andreon / Cigan


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A New Type of Gravitational Wave Detector to Find Tennis Ballsized Black Holes (Cosmology)

A new type of gravitational wave detector to find tennis ballsized black holes straight out of the Big Bang

“Detecting primordial black holes opens up new perspectives to understand the origin of the Universe, because these still hypothetical black holes are supposed to have formed just a few tiny fractions of a second after the Big Bang. Their study is of great interest for research in theoretical physics and cosmology, because they could notably explain the origin of dark matter in the Universe”. You can see stars in the eyes of the members of the team led by Professor Fuzfa, astrophysicist at UNamur, when talking about the perspectives of their research. This project is the result of an unprecedented collaboration between the UNamur and ULB, to which the ENS added thanks to the involvement of trainee student Léonard Lehoucq.

The idea was to combine the UNamur expertise in the field of gravitational wave antennas, an idea patented by Professor Fuzfa in 2018 and studied by Nicolas Herman as part of his doctorate, with that of ULB in the booming field of primordial black holes, in which Professor Clesse is one of the central players. They have just developed an application of this type of detector in order to observe “small” primordial black holes. Their results have just been published in the journal Physical Review D. “To this day, these primordial black holes are still hypothetical, because it is difficult to make the difference between a black hole resulting from the implosion of a star core and a primordial black hole. Being able to observe smaller black holes, the mass of a planet but a few centimeters in size, would make the difference,” the team of researchers says. They carry on: “We are offering experimenters a device that could detect them, by capturing the gravitational waves they emit when merging and which are of much higher frequencies than those currently available”.

But what is the technique? A gravitational wave “antenna”, composed of a specific metal cavity and suitably immersed in a strong external magnetic field. When the gravitational wave goes through the magnetic field, it generates electromagnetic waves in the cavity. In a way, the gravitational wave makes the cavity “hiss” (resonate), not with sound but with microwaves.

This type of device, just a few meters in size, would be enough to detect fusions of primordial small black holes millions of light years from Earth. It is much more compact than the commonly used detectors (LIGO, Virgo and KAGRA interferometers) which are several kilometers long. The detection method makes it sensitive to very high frequency gravitational waves (in the order of 100 MHz, compared to 10-1000 Hz for LIGO / Virgo / Kagra), which are not produced by ordinary astrophysical sources such as fusions, neutron stars or stellar black holes.

On the other hand, it is ideal for the detection of small black holes, the mass of a planet and its size goes from a small ball to a tennis ball. “Our detector proposal combines well mastered and everyday life technologies such as magnetrons in microwave ovens, MRI magnets and radio antennas. But don’t take your household appliances apart right away to start the adventure: read our article first, then order your equipment, understand the device and the signal that awaits you at the output,” the researchers say laughingly.

This patented technique is currently at the stage of advanced theoretical modeling, but has all the necessary elements to enter a more concrete phase, with the construction of a prototype. In any case, it paves the way for fundamental research into the origins of our Universe. In addition to primordial black holes, this type of detector could also directly observe the gravitational waves emitted at the time of the Big Bang, and thus probe physics at much higher energies than the ones achieved in particle accelerators.


Reference: Nicolas Herman, André Fűzfa, Léonard Lehoucq, and Sebastien Clesse, “Detecting planetary-mass primordial black holes with resonant electromagnetic gravitational-wave detectors”, Phys. Rev. D, 2 June, 2021. Link to paper


Provided by ULB

The Mystery Of The Missing Dark Matter (Cosmology)

New distance measurements of the diffuse spheroid galaxy Ngc 1052-Df2 place this galaxy at a distance of 72 million light years and confirm that the galaxy is practically devoid of dark matter, a very rare case in the galaxy landscape. This absence of dark matter compared to other galaxies suggests that dark matter exists as a real physical entity and not as a result of a different law of gravitation on a galactic scale.

According to the current paradigm, dark matter makes up about 86% of all matter in our Universe. Its peculiarity is that it does not interact electromagnetically like ordinary matter, but only by gravitational way . For this reason it is difficult to study it, in fact it can only be detected on a large scale by observing the gravitational effects it causes on ordinary matter: unfortunately there is no experimental detection.of dark matter particles. The presence of this matter has been deduced thanks to studies on the velocity curves of spiral galaxies: as we move away from the nucleus of a galaxy, the stars do not decrease their speed as one might expect, but continue to move. quickly. If Newton’s law of gravity holds, this excess of speed indicates that most of the mass of galaxies is made up of invisible matter capable of holding the stars of which they are composed bound together with its own force of gravity: unlike galaxies. they would fall apart. Dark matter in the evolution of the Universe is very important because it is thanks to its intense gravitational effects that, within immense haloes of dark mattergalaxies were formed . Otherwise, after the Big Bang, ordinary matter would never have undergone any process of gravitational collapse and galaxies would not have formed. From this theoretical framework it is expected that each galaxy contains a consistent amount of dark matter: for example the value of the average ratio between dark matter and ordinary , measured for galaxies such as our Milky Way, is of the order of 30 times and increases both for more massive galaxies, and for less massive galaxies.

However, things seem more complex than that, at least as far as the galaxy Ngc 1052-Df2 is concerned . It is an ultra-diffuse galaxy with low surface brightness that is prospectively located in the constellation of the Whale, identified thanks to a large-field survey of the group of galaxies of Ngc 1052. The galaxy contains so little ordinary matter that it is practically transparent, so much is it It is true that in the images that portray it you can see the background galaxies much further away. Morphologically, this galaxy has a spheroidal appearance and does not appear to have a core, spiral arms or a disk of stars. The geometric dimensions are similar to those of the Milky Way.

In a March 2018 article published in Nature, the results of the radial velocity measurements of 10 luminous globular clusters belonging to this evanescent galaxy were published for the estimation of the total mass of the system. The result was that the ratio of dark to bright matter in Ngc 1052-Df2 was about 1, a value about 400 times lower than expected and in stark contrast to what is observed in other galaxies. Put simply, the case of NGC1052-DF2 showed that dark matter is not always coupled with baryon matter , at least on a galactic scale. To confirm this incredible result, the discovery team, led by Pieter van Dokkum of Yale University, focused on precise distance measurementby Ngc 1052-Df2, publishing a new paper in The Astrophysical Journal Letters . In the work of 2018, the distance of the galaxy was assumed to be similar to that of the group of galaxies to which it seemed to belong, namely that of Ngc 1052 at about 65 million light years from us. How does distance fit into estimating the relationship between dark and ordinary matter? To understand this, just think of the fact that the estimation of the mass of a star can be done by measuring its intrinsic brightness and this is obtained by measuring both the apparent brightness and the distance at which the star is located. By scaling this reasoning on a galactic scale we understand that if Df2 were closer to Earth than the 65 million light years adopted, thenits stars would be intrinsically weaker and less massive , so the luminous matter would make a minor contribution to the total mass (which is measured with the radial velocity of globular clusters) and the ratio between dark and luminous matter would increase accordingly. Distance measurement thus becomes a crucial parameter for determining the amount of luminous matter in the galaxy.

To measure the distance of a galaxy you need ” standard candles “, ie stars whose intrinsic brightness is known a priori . The team of astronomers, using the “Hubble” space telescope, focused on measuring the apparent brightness of the red giants located on the periphery of Ngc 1052-Df2 and which, during their evolution, all reach the same brightness peak. In this way, the difference between intrinsic and apparent brightness can be used to measure large intergalactic distances. The new distance estimate tells us that Df2 is 72 million light years awaythat is, the galaxy is further away than the original estimate of 65 million light years. From here it follows that Df2 is really devoid of dark matter, it is not an observational bias .

Moreover, Df2 is not the only galaxy without dark matter, another galaxy, Ngc 1052-Df4 , is also devoid of dark matter. In this case, however, some scientists suggest that dark matter may have been removed from the galaxy due to tidal forces exerted by another passing galaxy.

The discovery of these galaxies devoid of dark matter, paradoxically, confirms that dark matter really exists. In fact, if dark matter were only an effect of a gravitational law different from the Newtonian one, all galaxies should show its presence. The fact that there are galaxies without dark matter means that something is really missing in their structure. Understanding why Df2 is devoid of dark matter will require further observation, the mystery continues.

Featured image: The galaxy poor in dark matter Ngc 1052-Df2 taken with the Hubble Advanced Camera for Surveys between December 2020 and March 2021. The galaxy is so poor in matter that, through it, you can see the background galaxies (Credits: Nasa , Esa, STScI, Zili Shen (Yale), Pieter van Dokkum (Yale), Shany Danieli (Ias), Alyssa Pagan (STScI))


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Provided by INAF

Is Dark Matter Real, Or Have We Misunderstood Gravity? (Cosmology)

For many years now, astronomers and physicists have been in a conflict. Is the mysterious dark matter that we observe deep in the Universe real, or is what we see the result of subtle deviations from the laws of gravity as we know them? In 2016, Dutch physicist Erik Verlinde proposed a theory of the second kind: emergent gravity. New research, published in Astronomy & Astrophysics this week, pushes the limits of dark matter observations to the unknown outer regions of galaxies, and in doing so re-evaluates several dark matter models and alternative theories of gravity. Measurements of the gravity of 259,000 isolated galaxies show a very close relation between the contributions of dark matter and those of ordinary matter, as predicted in Verlinde’s theory of emergent gravity and an alternative model called Modified Newtonian Dynamics. However, the results also appear to agree with a computer simulation of the Universe that assumes that dark matter is ‘real stuff’.

The new research was carried out by an international team of astronomers, led by Margot Brouwer (RUG and UvA).  Further important roles were played by Kyle Oman (RUG and Durham University) and Edwin Valentijn (RUG). In 2016, Brouwer also performed a first test of Verlinde’s ideas; this time, Verlinde himself also joined the research team.

Matter or gravity?

So far, dark matter has never been observed directly – hence the name. What astronomers observe in the night sky are the consequences of matter that is potentially present: bending of starlight, stars that move faster than expected, and even effects on the motion of entire galaxies. Without a doubt all of these effects are caused by gravity, but the question is: are we truly observing additional gravity, caused by invisible matter, or are the laws of gravity themselves the thing that we haven’t fully understood yet?

To answer this question, the new research uses a similar method to the one used in the original test in 2016. Brouwer and her colleagues make use of an ongoing series of photographic measurements that started ten years ago: the KiloDegree Survey (KiDS), performed using ESO’s VLT Survey Telescope in Chili. In these observations one measures how starlight from far away galaxies is bent by gravity on its way to our telescopes. Whereas in 2016 the measurements of such ‘lens effects’ only covered an area of about 180 square degrees on the night sky, in the mean time this has been extended to about 1000 square degrees – allowing the researchers to measure the distribution of gravity in around a million different galaxies.

Comparative testing

Brouwer and her colleagues selected over 259,000 isolated galaxies, for which they were able to measure the so-called ‘Radial Acceleration Relation’ (RAR). This RAR compares the amount of gravity expected based on the visible matter in the galaxy, to the amount of gravity that is actually present – in other words: the result shows how much ‘extra’ gravity there is, in addition to that due to normal matter. Until now, the amount of extra gravity had only been determined in the outer regions of galaxies by observing the motions of stars, and in a region about five times larger by measuring the rotational velocity of cold gas. Using the lensing effects of gravity, the researchers were now able to determine the RAR at gravitational strengths which were one hundred times smaller, allowing them to penetrate much deeper into the regions far outside the individual galaxies.

This made it possible to measure the extra gravity extremely precisely – but is this gravity the result of invisible dark matter, or do we need to improve our understanding of gravity itself? Author Kyle Oman indicates that the assumption of ‘real stuff’ at least partially appears to work: “In our research, we compare the measurements to four different theoretical models: two that assume the existence of dark matter and form the base of computer simulations of our universe, and two that modify the laws of gravity – Erik Verlinde’s model of emergent gravity and the so-called ‘Modified Newtonian Dynamics’ or MOND. One of the two dark matter simulations, MICE, makes predictions that match our measurements very nicely. It came as a surprise to us that the other simulation, BAHAMAS, led to very different predictions. That the predictions of the two models differed at all was already surprising, since the models are so similar. But moreover, we would have expected that if a difference would show up, BAHAMAS was going to perform best. BAHAMAS is a much more detailed model than MICE, approaching our current understanding of how galaxies form in a universe with dark matter much closer. Still, MICE performs better if we compare its predictions to our measurements. In the future, based on our findings, we want to further investigate what causes the differences between the simulations.”

Young and old galaxies

Thus it seems that, at least one dark matter model does appear to work. However, the alternative models of gravity also predict the measured RAR. A standoff, it seems – so how do we find out which model is correct? Margot Brouwer, who led the research team, continues: “Based on our tests, our original conclusion was that the two alternative gravity models and MICE matched the observations reasonably well. However, the most exciting part was yet to come: because we had access to over 259,000 galaxies, we could divide them into several types – relatively young, blue spiral galaxies versus relatively old, red elliptical galaxies.” Those two types of galaxies come about in very different ways: red elliptical galaxies form when different galaxies interact, for example when two blue spiral galaxies pass by each other closely, or even collide. As a result, the expectation within the particle theory of dark matter is that the ratio between regular and dark matter in the different types of galaxies can vary. Models such as Verlinde’s theory and MOND on the other hand do not make use of dark matter particles, and therefore predict a fixed ratio between the expected and measured gravity in the two types of galaxies – that is, independent of their type. Brouwer: “We discovered that the RARs for the two types of galaxies differed significantly. That would be a strong hint towards the existence of dark matter as a particle.”

RAR
A plot showing the Radial Acceleration Relation (RAR). The background is an image of the elliptical galaxy M87, showing the distance to the centre of the galaxy. The plot shows how the measurements range from high gravitational acceleration in the centre of the galaxy, to low gravitational acceleration in the far outer regions. Image: Chris Mihos (Case Western Reserve University) / ESO.

However, there is a caveat: gas. Many galaxies are probably surrounded by a diffuse cloud of hot gas, which is very difficult to observe. If it were the case that there is hardly any gas around young blue spiral galaxies, but that old red elliptical galaxies live in a large cloud of gas – of roughly the same mass as the stars themselves – then that could explain the difference in the RAR between the two types. To reach a final judgement on the measured difference, one would therefore also need to measure the amounts of diffuse gas – and this is exactly what is not possible using the KiDS telescopes. Other measurements have been done for a small group of around one hundred galaxies, and these measurements indeed found more gas around elliptical galaxies, but it is still unclear how representative those measurements are for the 259,000 galaxies that were studied in the current research.

Dark matter for the win?

If it turns out that extra gas cannot explain the difference between the two types of galaxies, then the results of the measurements are easier to understand in terms of dark matter particles than in terms of alternative models of gravity. But even then, the matter is not settled yet. While the measured differences are hard to explain using MOND, Erik Verlinde still sees a way out for his own model. Verlinde: “My current model only applies to static, isolated, spherical galaxies, so it cannot be expected to distinguish the different types of galaxies. I view these results as a challenge and inspiration to develop an asymmetric, dynamical version of my theory, in which galaxies with a different shape and history can have a different amount of ‘apparent dark matter’.”

Therefore, even after the new measurements, the dispute between dark matter and alternative gravity theories is not settled yet. Still, the new results are a major step forward: if the measured difference in gravity between the two types of galaxies is correct, then the ultimate model, whichever one that is, will have to be precise enough to explain this difference. This means in particular that many existing models can be discarded, which considerably thins out the landscape of possible explanations. On top of that, the new research shows that systematic measurements of the hot gas around galaxies are necessary. Edwin Valentijn formulates is as follows: “As observational astronomers, we have reached the point where we are able to measure the extra gravity around galaxies more precisely than we can measure the amount of visible matter. The counterintuitive conclusion is that we must first measure the presence of ordinary matter in the form of hot gas around galaxies, before future telescopes such as Euclid can finally solve the mystery of dark matter.”

Featured image: In the centre of the image the elliptical galaxy NGC5982, and to the right the spiral galaxy NGC5985. These two types of galaxies turn out to behave very differently when it comes to the extra gravity – and therefore possibly the dark matter – in their outer regions. Images: Bart Delsaert (www.delsaert.com).


Publication

The weak lensing radial acceleration relation: Constraining modified gravity and cold dark matter theories with KiDS-1000”, M. Brouwer et al., Astronomy & Astrophysics 2021.


Provided by University of Amsterdam

Primordial Black Holes Give Rise To Mu-Distortion Which Can Be Detected By Future Observations (Cosmology)

If you are a daily reader, you may have came across several articles on our website based on the formation of primordial black holes (PBHs). But, the most popular and natural scenario is, production of PBHs by the collapse of the clumps. In this scenario, PBHs production takes place by the collapse of large overdense clump which is surrounded by an underdense region. After the black hole is formed, the underdense region expands as a shell (shown as a ring in fig 1 below). The shell consists of an underdense and an overdense layer, which is a typical feature of a spherical sound wave packet. As the shell passes by, the fluid density between the black hole and the shell goes back to FRW. Dissipation of the shell due to photon diffusion during the so-called µ-era can release energy into the background, generating µ-distortion in the background photons, and may be seen in Cosmic Microwave Background (CMB).

FIG. 1: Illustrations of the formation of sound shell around a PBH. © Heling Deng

Now, Heling Deng carried out study on the possible μ-distortion in the CMB spectrum around supermassive primordial black holes (PBHs) and how future observations might impose constraints on the PBH density within the mass range 106–1015 M.

He showed that, if there are more than one PBH with M ≳ 106 M within a Silk region, which is of an angular scale of 0.2°, we can have an average distortion µ’ in CMB. The possible non-observation of average distortion in future missions beyond the ΛCDM model (which predicts µ’ ∼ 10¯8) would then place constraints on these black holes. He also suggested that, a bound of particular interest would be f < 10¯3 for 1010M ≲ M ≲ 1012 M.

“Although, such supermassive PBHs are rare in our universe, it is possible that we can see point like distortions with magnitude µM ≳ 10¯7 on some Silk patches in CMB, as long as the black holes have initial mass M ≳ 1012 M.”

Considering that the resolution of future missions could reach δθ ∼ 1°, such a signal in a pixel would be µ1° ≳ 10¯8. The non-observation would imply that these stupendously large PBHs can only constitute a tiny part of the dark matter, with a fraction f < 10¯9 for M ∼ 1012M.

FIG. 2: Constraints on the fraction of dark matter in monochromatic PBHs within the mass range
102– 1015M . The gray regions have been ruled out by current observations. Colored curves are possible upper bounds for fPBH if future observations find certain upper bounds for the average µ-distortion in CMB: µ’ ≲ 10¯5 (blue), µ’ ≲ 10¯6 (orange), µ’ ≲ 10¯7 (green) and µ’ ≲ 10¯8 (red). The purple line is imposed by condition that there should at least be one black holes within the smallest patch future missions can measure. The cyan line is a possible bound if future observations find µ ≲ 10¯8 at an angular scale ∼ 1°. © Heling Deng

If future observations do not see µ-distortion with µ ≳ 10¯8, the shaded regions in the figure 2 (given above), including that with (brown) tilted lines and that with (cyan) dots, should all be excluded. An exciting possibility is that we do see local distortions in CMB, which would be a hint of the existence of stupendously large PBHs, and we will be able to estimate their population.

Finally, although he focused totally on the µ-distortion generated by the Silk damping of the sound shell, he mentioned that the shell itself can be a source of temperature perturbations in CMB. At the time of recombination (t ∼ 1013 s), the shell’s radius (∼ sound horizon) is of an angular size ∼ 1° on the CMB sky, and its thickness is the Silk damping scale ∼ 0.2° (S ∼ Λ), which is comparable to the thickness of the last scattering surface. Therefore, the underdensity and overdensity on the shell (fig. 3) can induce a ring-like feature in CMB and this ring’s radius depends on the black hole’s position relative to the last scattering surface. They estimated the temperature perturbation and found that, PBHs with larger initial masses should be rare on the last scattering surface.

FIG. 3: Sketch of the radiation fluid’s density profile along the radius near the shell at two moments t and t+ ∆t. The shell consists of an underdense and an overdense layer, which is a typical feature of a spherical sound wave packet. The shell’s physical radius is ∼ 2cst, and its thickness increases from S(t) to S(t + ∆t) due to cosmic expansion and the viscosity in fluid. The time it takes for the shell to completely pass through a sphere at the wave front is ∆t ∼ S(t)/cs. During this time, the shell’s wave energy gets dissipated, and the resulting heat is dumped behind the shell (shown as a red line). © Heling Deng

Reference: Heling Deng, “µ-distortion around stupendously large primordial black holes”, Arxiv, pp. 1-21, 2021. https://arxiv.org/abs/2106.09817


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Researchers Trace Dust Grain’s Journey Through Newborn Solar System (Cosmology)

Combining atomic-scale sample analysis and models simulating likely conditions in the nascent solar system, a new study reveals clues about the origin of crystals that formed more than 4.5 billion years ago.

A research team led by the University of Arizona has reconstructed in unprecedented detail the history of a dust grain that formed during the birth of the solar system more than 4.5 billion years ago. The findings provide insights into the fundamental processes underlying the formation of planetary systems, many of which are still shrouded in mystery.

For the study, the team developed a new type of framework, which combines quantum mechanics and thermodynamics, to simulate the conditions to which the grain was exposed during its formation, when the solar system was a swirling disk of gas and dust known as a protoplanetary disk or solar nebula. Comparing the predictions from the model to an extremely detailed analysis of the sample’s chemical makeup and crystal structure, along with a model of how matter was transported in the solar nebula, revealed clues about the grain’s journey and the environmental conditions that shaped it along the way.

The grain analyzed in the study is one of several inclusions, known as calcium-aluminum rich inclusions, or CAIs, discovered in a sample from the Allende meteorite, which fell over the Mexican state of Chihuahua in 1969. CAIs are of special interest because they are thought to be among the first solids that formed in the solar system more than 4.5 billion years ago.

Allende Meteorite
This piece of the Allende meteorite shows the typical crust of material that melted during entry into Earth’s atmosphere. The grain studied in this study was taken from a similar piece, and from deep within the specimen, where little, if any, alteration would occur during the meteorite fall.H. Raab/Wikimedia Commons

Similar to how stamps in a passport tell a story about a traveler’s journey and stops along the way, the samples’ micro- and atomic-scale structures unlock a record of their formation histories, which were controlled by the collective environments to which they were exposed.

“As far as we know, our paper is the first to tell an origin story that offers clues about the likely processes that happened at the scale of astronomical distances with what we see in our sample at the scale of atomic distances,” said Tom Zega, a professor in the University of Arizona’s Lunar and Planetary Laboratory and the first author of the paper, published in The Planetary Science Journal.

Zega and his team analyzed the composition of the inclusions embedded in the meteorite using cutting-edge atomic-resolution scanning transmission electron microscopes – one at UArizona’s Kuiper Materials Imaging and Characterization Facility, and its sister microscope located at the Hitachi factory in Hitachinaka, Japan.

The inclusions were found to consist mainly of types of minerals known as spinel and perovskite, which also occur in rocks on Earth and are being studied as candidate materials for applications such as microelectronics and photovoltaics.

Similar kinds of solids occur in other types of meteorites known as carbonaceous chondrites, which are particularly interesting to planetary scientists as they are known to be leftovers from the formation of the solar system and contain organic molecules, including those that may have provided the raw materials for life.

A slice through an Allende meteorite
A slice through an Allende meteorite reveals various spherical particles, known as chondrules. The irregularly shaped “island” left of the center is a calcium-aluminum rich inclusion, or CAI. The grain in this study was isolated from such a CAI.Shiny Things/Wikimedia Commons

Precisely analyzing the spatial arrangement of atoms allowed the team to study the makeup of the underlying crystal structures in great detail. To the team’s surprise, some of the results were at odds with current theories on the physical processes thought to be active inside protoplanetary disks, prompting them to dig deeper.

“Our challenge is that we don’t know what chemical pathways led to the origins of these inclusions,” Zega said. “Nature is our lab beaker, and that experiment took place billions of years before we existed, in a completely alien environment.”

Zega said the team set out to “reverse-engineer” the makeup of the extraterrestrial samples by designing new models that simulated complex chemical processes, which the samples would be subjected to inside a protoplanetary disk.

“Such models require an intimate convergence of expertise spanning the fields of planetary science, materials science, mineral science and microscopy, which was what we set out to do,” added Krishna Muralidharan, a study co-author and an associate professor in the UArizona’s Department of Materials Science and Engineering.

Based on the data the authors were able to tease from their samples, they concluded that the particle formed in a region of the protoplanetary disk not far from where Earth is now, then made a journey closer to the sun, where it was progressively hotter, only to later reverse course and wash up in cooler parts farther from the young sun. Eventually, it was incorporated into an asteroid, which later broke apart into pieces. Some of those pieces were captured by Earth’s gravity and fell as meteorites.

The samples for this study were taken from the inside of a meteorite and are considered primitive – in other words, unaffected by environmental influences. Such primitive material is believed to not have undergone any significant changes since it first formed more than 4.5 billion years ago, which is rare. Whether similar objects occur in asteroid Bennu, samples of which will be returned to Earth by the UArizona-led OSIRIS-REx mission in 2023, remains to be seen. Until then, scientists rely on samples that fall to Earth via meteorites.

Illustration of the dynamic history that the modeled particle could have experienced during the formation of the solar system
Illustration of the dynamic history that the modeled particle could have experienced during the formation of the solar system. Analyzing the particle’s micro- and atomic-scale structures and combining them with new models that simulated complex chemical processes in the disk revealed its possible journey over the course of many orbits around the sun (callout box and diagram on the right). Originating not far from where Earth would form, the grain was transported into the inner, hotter regions, and eventually washed up in cooler regions.Heather Roper/Zega et al.

“This material is our only record of what happened 4.567 billion years ago in the solar nebula,” said Venkat Manga, a co-author of the paper and an assistant research professor in the UArizona Department of Materials Science and Engineering. “Being able to look at the microstructure of our sample at different scales, down to the length of individual atoms, is like opening a book.”

The authors said that studies like this one could bring planetary scientists a step closer to “a grand model of planet formation” – a detailed understanding of the material moving around the disk, what it is composed of, and how it gives rise to the sun and the planets.

Powerful radio telescopes like the Atacama Large Millimeter/submillimeter Array, or ALMA, in Chile now allow astronomers to see stellar systems as they evolve, Zega said.

“Perhaps at some point we can peer into evolving disks, and then we can really compare our data between disciplines and begin answering some of those really big questions,” Zega said. “Are these dust particles forming where we think they did in our own solar system? Are they common to all stellar systems? Should we expect the pattern we see in our solar system – rocky planets close to the central star and gas giants farther out – in all systems?

“It’s a really interesting time to be a scientist when these fields are evolving so rapidly,” he added. “And it’s awesome to be at an institution where researchers can form transdisciplinary collaborations among leading astronomy, planetary and materials science departments at the same university.”

The study was co-authored by Fred Ciesla at the University of Chicago and Keitaro Watanabe and Hiromi Inada, both with the Nano-Technology Solution Business Group at Hitachi High-Technologies Corp. in Japan.

Funding was provided through NASA’s Emerging Worlds Program; NASA’s Origins Program; and NASA’s Nexus for Exoplanet System Science (NExSS) research coordination network, which is sponsored by NASA’s Science Mission Directorate. NASA and the National Science Foundation provided the funding for the instrumentation in LPL’s Kuiper Materials Imaging and Characterization Facility.

Featured image: Artist’s illustration of the early solar system, at a time when no planets had formed yet. A swirling cloud of gas and dust surrounded the young sun. The cutaway through this so-called protoplanetary disk shows its three-dimensional structure. © Heather Roper


Provided by University of Arizona

New Research Adds A Wrinkle to Our Understanding Of the Origins of Matter in the Milky Way (Cosmology)

New information about how different cosmic rays arrive at Earth hints at unique sources or propagation methods for different elements

New findings published this week in Physical Review Letters suggest that carbon, oxygen, and hydrogen cosmic rays travel through the galaxy toward Earth in a similar way, but, surprisingly, that iron arrives at Earth differently. Learning more about how cosmic rays move through the galaxy helps address a fundamental, lingering question in astrophysics: How is matter generated and distributed across the universe?

“So what does this finding mean?” asks John Krizmanic, a senior scientist with UMBC’s Center for Space Science and Technology (CSST). “These are indicators of something interesting happening. And what that something interesting is we’re going to have to see.”

Cosmic rays are atomic nuclei–atoms stripped of their electrons–that are constantly whizzing through space at nearly the speed of light. They enter Earth’s atmosphere at extremely high energies. Information about these cosmic rays can give scientists clues about where they came from in the galaxy and what kind of event generated them.

An instrument on the International Space Station (ISS) called the Calorimetric Electron Telescope (CALET) has been collecting data about cosmic rays since 2015. The data include details such as how many and what kinds of atoms are arriving, and how much energy they’re arriving with. The American, Italian, and Japanese teams that manage CALET, including UMBC’s Krizmanic and postdoc Nick Cannady, collaborated on the new research.

Iron on the move

Cosmic rays arrive at Earth from elsewhere in the galaxy at a huge range of energies–anywhere from 1 billion volts to 100 billion billion volts. The CALET instrument is one of extremely few in space that is able to deliver fine detail about the cosmic rays it detects. A graph called a cosmic ray spectrum shows how many cosmic rays are arriving at the detector at each energy level. The spectra for carbon, oxygen, and hydrogen cosmic rays are very similar, but the key finding from the new paper is that the spectrum for iron is significantly different.

There are several possibilities to explain the differences between iron and the three lighter elements. The cosmic rays could accelerate and travel through the galaxy differently, although scientists generally believe they understand the latter, Krizmanic says.

“Something that needs to be emphasized is that the way the elements get from the sources to us is different, but it may be that the sources are different as well,” adds Michael Cherry, physics professor emeritus at Louisiana State University (LSU) and a co-author on the new paper. Scientists generally believe that cosmic rays originate from exploding stars (supernovae), but neutron stars or very massive stars could be other potential sources.

Next-level precision

An instrument like CALET is important for answering questions about how cosmic rays accelerate and travel, and where they come from. Instruments on the ground or balloons flown high in Earth’s atmosphere were the main source of cosmic ray data in the past. But by the time cosmic rays reach those instruments, they have already interacted with Earth’s atmosphere and broken down into secondary particles. With Earth-based instruments, it is nearly impossible to identify precisely how many primary cosmic rays and which elements are arriving, plus their energies. But CALET, being on the ISS above the atmosphere, can measure the particles directly and distinguish individual elements precisely.

Iron is a particularly useful element to analyze, explains Cannady, a postdoc with CSST and a former Ph.D. student with Cherry at LSU. On their way to Earth, cosmic rays can break down into secondary particles, and it can be hard to distinguish between original particles ejected from a source (like a supernova) and secondary particles. That complicates deductions about where the particles originally came from.

“As things interact on their way to us, then you’ll get essentially conversions from one element to another,” Cannady says. “Iron is unique, in that being one of the heaviest things that can be synthesized in regular stellar evolution, we’re pretty certain that it is pretty much all primary cosmic rays. It’s the only pure primary cosmic ray, where with others you’ll have some secondary components feeding into that as well.”

“Made of stardust”

Measuring cosmic rays gives scientists a unique view into high-energy processes happening far, far away. The cosmic rays arriving at CALET represent “the stuff we’re made of. We are made of stardust,” Cherry says. “And energetic sources, things like supernovas, eject that material from their interiors, out into the galaxy, where it’s distributed, forms new planets, solar systems, and… us.”

“The study of cosmic rays is the study of how the universe generates and distributes matter, and how that affects the evolution of the galaxy,” Krizmanic adds. “So really it’s studying the astrophysics of this engine we call the Milky Way that’s throwing all these elements around.”

A global effort

The Japanese space agency launched CALET and today leads the mission in collaboration with the U.S. and Italian teams. In the U.S., the CALET team includes researchers from LSU; NASA Goddard Space Flight Center; UMBC; University of Maryland, College Park; University of Denver; and Washington University.The new paper is the fifth from this highly successful international collaboration published in PRL, one of the most prestigious physics journals.

CALET was optimized to detect cosmic ray electrons, because their spectrum can contain information about their sources. That’s especially true for sources that are relatively close to Earth in galactic terms: within less than one-thirtieth the distance across the Milky Way. But CALET also detects the atomic nuclei of cosmic rays very precisely. Now those nuclei are offering important insights about the sources of cosmic rays and how they got to Earth.

“We didn’t expect that the nuclei – the carbon, oxygen, protons, iron – would really start showing some of these detailed differences that are clearly pointing at things we don’t know,” Cherry says.

The latest finding creates more questions than it answers, emphasizing that there is still more to learn about how matter is generated and moves around the galaxy. “That’s a fundamental question: How do you make matter?” Krizmanic says. But, he adds, “That’s the whole point of why we went in this business, to try to understand more about how the universe works.”


Reference: O. Adriani et al. (CALET Collaboration), “Measurement of the Iron Spectrum in Cosmic Rays from 10  GeV/n to 2.0  TeV/n with the Calorimetric Electron Telescope on the International Space Station”, Phys. Rev. Lett. 126, 241101 – Published 14 June 2021. DOI: https://doi.org/10.1103/PhysRevLett.126.241101


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