The Strange Case Of The Sombrero Galaxy

Myriad sparkling stars light up the billions of galaxies that dwell in the observable Universe. The observable, or visible, Universe is that relatively small domain of the unimaginably vast Cosmos that we are able to observe. The light traveling to us from more distant regions has not had enough time to reach us since the Big Bang. This is because of the expansion of Space, and the universal speed limit set by light. No known signal can travel faster than light in a vacuum, although Space itself can, and so the very secret of our existence may reside in regions of Spacetime that are far beyond the horizon of our visibility. The galaxies of the Cosmos are far away and mysterious, and the Sombrero Galaxy (Messier 104) stands out in the crowd as one of the most bewitching and bewildering of its starlit kind. In February 2020, a team of astronomers announced that evidence derived from the Hubble Space Telescope (HST) indicates that the Sombrero’s many weird and unexplained attributes are the result of major galaxy mergers–even though its smooth disk displays no signs of a recent catastrophic disruption. However, the Sombrero’s disk may hide the secret of a turbulent past.

The Sombrero has long been a seductively tantalizing object because it seems to travel to the beat of a different drum than other known galaxies. It displays a mystifying mix of shapes found in disk-shaped spiral galaxies (like our own Milky Way), as well as football-shaped elliptical galaxies. The tantalizing mystery of how it acquired its unusual structure becomes ever more bewitching and bewildering with the new evidence from the HST.

The galaxy’s faint halo provides some tattle-tale clues. It is splattered with innumerable stars that are well-endowed with heavier atomic elements–called metals by astronomers. This is because they are later-generation stars. In the terminology astronomers use, a metal is any atomic element heavier than hydrogen and helium, and so the same term has a different meaning for astronomers than it does for chemists. The Big Bang produced only hydrogen, helium, and traces of lithium–but the stars created all the rest. The first generation of stars to dance in the Cosmos were the first to cook up the heavier atomic elements in their nuclear-fusing hearts–and then they sent them screaming into Space when they went supernova. The newly-forged metals were eventually incorporated into later generations of stars. The first stars (Population III) were born depleted of heavy metals, because no stars existed before them to cook them up. The second generation of stars (Population II) were almost, but not quite, depleted of metals, because they were “polluted” with the batch produced in the searing-hot hearts of the first stars. The youngest generation of stars (Population I)–of which our Sun is a member–contain the largest quantity of metals, having received these elements from previous generations of stars.

For this reason, stars with an abundance of heavy metals are usually seen only in a galaxy’s disk. The Sombrero’s metal-rich stars must have been hurled into its halo, as the result of ancient mergers with mature galaxies, that were heavily endowed with metals. The Sombrero galaxy, in its current “adulthood”, is more settled than it was in its “youth”. It is also isolated. This means that there is nothing else dwelling nearby for it to “eat”. This discovery provides a new twist on the way galaxies form in our Cosmos.

“The Sombrero has always been a bit of a weird galaxy, which is what makes it so interesting. Hubble’s metallicity measurements (i.e.: the abundance of heavy elements in the stars) are another indication that the Sombrero has a lot to teach us about galaxy assembly and evolution,” commented Dr. Paul Goudfrooij in a February 20, 2020 Hubblesite Press Release. Dr. Goudfrooij is of the Space Telescope Science Institute (STScI) in Baltimore, Maryland.

“Hubble’s observations of the Sombrero’s halo are turning our accepted understanding of galaxy makeup and metallicity on its head,” added co-investigator Dr. Roger Cohen, who is also of the STScI.

HST’s sensitivity was able to resolve tens of thousands of individual stars inhabiting the Sombrero’s vast, extended halo. The halo is the region situated beyond a galaxy’s central portion, that is typically composed of older stars. These recent observations of the Sombrero are intriguing because they show only a small percentage of older, metal-poor stars in the halo, plus the abundance of metal-rich stars in its disk and central bulge. Therefore, ancient, turbulent galaxy collisions and major mergers provide a possible explanation.

Strange And Beautiful

The Sombrero has captivated skywatchers for years because of its strange structure and great beauty. However, thanks to HSTs recent observations, astronomers are now seeing the Sombrero in a new light. The galaxy displays an extended halo brimming with metal-rich stars with barely any evidence of the predicted metal-poor stars observed in the halos of other galaxies. Astronomers, pouring over the data from the HST, have turned to sophisticated computer simulations to find a solution to this perplexing puzzle that poses a challenge to conventional galaxy-formation theory. Those results indicate the surprising possibility that major mergers occurred in this weird galaxy’s past, even though the Sombrero’s elegant and lovely structure shows no evidence of recent disruption. The results of these new findings are published in the Astrophysical Journal.

“The absence of metal-poor stars was a big surprise, and the abundance of metal-rich stars only added to the mystery,” Dr. Goudfrooij noted in the February 20, 2020 Hubblesite Press Release.

The Sombrero got its name because it resembles the broad rim and high-topped Mexican hat with this name–and, observed from Earth, it is seen almost edge-on. This galaxy is also very bright, and it is easily observed with small telescopes. Because of its brightness, it is just a little beyond the limit of the unaided human eye. The strange and beautiful galaxy resides at the southern edge of the heavily populated Virgo Cluster, and it is one of the most massive objects in that group of galaxies. Indeed, its mass is equivalent to 800 billion Suns. The Sombrero is 50,000 light-years across and is situated 28 million light-years from Earth.

The Sombrero also hosts a heavy population of globular clusters, which are tightly bound spherical collections of stars. It is estimated that this galaxy is orbited by nearly 2,000 globulars–10 times as many as orbit our own barred-spiral Milky Way Galaxy. However, the ages of the clusters are similar to the ages of those circling our Galaxy, ranging from 10 to 13 billion years old.

A smaller disk is embedded within the Sombrero’s bright core, and it is tilted relative to the large disk. X-ray emission indicates that material is swirling down into the galaxy’s compact core, where a supermassive black hole resides in sinister secret, waiting for its dinner to come tumbling down into its voracious maw. This supermassive heart of darkness weighs in at 1 billion times our Sun’s mass. In contrast, our Milky Way’s resident supermassive black hole weighs “only” 4 million times solar mass.

In the 19th century, some astronomers suggested that the Sombrero was just an edge-on disk of brightly shining luminous gas encircling a youthful star–which is the way our own Solar System evolved. However, in 1912, the American astronomer V.M. Slipher (1875-1969) found that the strange object appeared to be flying away from Earth at 700 miles per second. That great velocity provided some of the earliest clues that the Sombrero was really another galaxy, and that the Universe was expanding in all directions.

How Bizarre!

Astronomers expect to find earlier generations of stars, with very small quantities of metals, in a galaxy’s halo–as compared to the more densely populated regions within a galaxy’s main disk. Heavier atomic elements are forged by way of the process of stellar nucleosynthesis, whereby increasingly heavier and heavier atomic elements are created out of lighter ones in a star’s searing-hot core. The longer a galaxy has hosted stars that create heavy metals, the more metal-rich its gas becomes–and the higher the metallictity of the stars that are born “polluted” from that gas. These youthful, high-metallicity stars are usually seen in the main disk of a galaxy, because this is where there is a heavier stellar population.

However, in the mysterious case of the Sombrero, things get complicated because of the presence of a large number of globular clusters containing elderly, metal-poor stars. Such metal-poor, elderly stars are normally expected to travel out of their host clusters and become part of the stellar halo. However, that process failed to work in the Sombrero galaxy. The team of HST astronomers compared their results with recent computer simulations in order to see what could be the origin of such a numerous population of metal-rich stars in this bizarre galaxy’s halo.

Their results were surprising, because they showed that the currently well-ordered, peaceful Sombrero had suffered violent accretion, or major merger events, billions of years ago. In contrast, our Milky Way is thought to have eaten many small satellite galaxies in “minor” accretions over the passage of billions of years. A major accretion event, however, is very different because it involves the merger of two or more similarly massive galaxies that are both well-endowed with higher-metallicity, later-generation stars.

The satellite galaxies only contained low-metallicity stars that were composed primarily of hydrogen and helium–the two lightest atomic elements–born in the Big Bang birth of the Universe almost 14 billion years ago (Big Bang nucleosynthesis). The heavier elements had to be manufactured in the searing-hot interiors of the stars through stellar nucleosynthesis and ultimately incorporated into later stellar generations. This process was somewhat inefficient in the case of small dwarf galaxies such as those circling our own Milky Way–while being considerably more effective in larger, more well-endowed galaxies, such as our own.

The new results for the Sombrero galaxy are surprising because its smooth, unperturbed disk displays no signs of disruption. By comparison, many interacting galaxies, such as the iconic Antennae galaxies, get their name from the distorted appearance of their spiral arms that result from the tidal forces of past violent interactions. Mergers of similarly massive galaxies usually coalesce into single large, smooth elliptical galaxies with extended halos–a process that takes billions of years. However, the Sombrero has always traveled to the beat of a different drum, and it never quite fit into the traditional definition of either a spiral or an elliptical galaxy. Like human nonconformists, it does what it will. It is a galactic mixture of the two.

A Duo Of Dancing Stars

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Small stars like our Sun die with great beauty, encircled by beautiful shrouds of multicolored gases that were once their outer layers–leaving only their relic cores behind as silent testimony to the Universe that they once existed. Our Sun, like other small stars, will first become a bloated red giant that will swell in size to the ghastly point that its flames will engulf the inner planets Mercury, Venus, and possibly Earth. It will then wither into a tiny, dense white dwarf–its former core. In March 2020, an international team, led by University of Warwick (U.K.) astronomers, reported that they had discovered a strange phenomena involving a closely dancing duo of these dead stars. The scientists detected a massive white dwarf star with a weird carbon-rich atmosphere that could really be two white dwarfs that merged together as they performed their bizarre danse macabre in the space between stars–narrowly escaping an explosive destruction.

The astronomers spotted an unusual ultra-massive white dwarf located about 450 light-years from Earth with an atmospheric composition that had never been seen before. This important observation marked the first time that a merged dancing duo of white dwarfs had been discovered by astronomers using its atmospheric composition as a clue to solving the mystery of its true identity.

The discovery, published in the March 2, 2020, issue of the journal Nature Astronomy, could shed new light on the enduring question of how massive white dwarf stars evolve, as well as on the number of supernovae dwelling in our barred-spiral Milky Way Galaxy.

The ultra-massive white dwarf, named WD J0551+4135, was detected in a survey of data derived from the European Space Agency’s (ESA’s) Gaia telescope. The astronomers followed up their discovery with a spectroscopy obtained using the William Herschel Telescope. The scientists focused on those white dwarfs identified as especially massive–an accomplishment that was made possible by the Gaia mission. By breaking apart the light emitted by the strange star, the astronomers were able to determine the chemical composition of its atmosphere, and found that it contained an unusually high level of carbon.

Lead author Dr. Mark Hollands, from the University of Warwick’s Department of Physics, explained in a March 2, 2020 University of Warwick Press Release that “This star stood out as something we had never seen before. You might expect to see an outer layer of hydrogen, sometimes mixed with helium, or just a mix of helium and carbon. You don’t expect to see this combination of hydrogen and carbon at the same time as there should be a thick layer of helium in between that prohibits that. When we looked at it, it didn’t make sense.”

Most white dwarfs are relatively light, weighing-in at about 0.6 times the mass of our Sun. However, WD J0551+4135 weighs in at an impressive 1.14 times solar masses, which makes it almost double the average mass of other white dwarfs. Despite being more massive than our Sun, it is squeezed into a tiny dense ball that is only two-thirds the diameter of Earth.

In order to solve the intriguing mystery, the astronomer-detectives decided to uncover the star’s true origins. The age of WD J0551+4135 also provided them with an important clue. Older stars orbit our Milky Way Galaxy much more swiftly than younger ones, and this strange white dwarf zipped around our Galaxy faster than 99% of the other nearby white dwarfs with the same cooling age. This means that this dead star is much older than it looks.

Dr. Hollands continued to explain in the March 2, 2020 University of Warwick Press Release that “We have a composition that we can’t explain through normal stellar evolution, a mass twice the average for a white dwarf, and a kinematic age older than that inferred from cooling. We’re pretty sure of how one star forms one white dwarf and it shouldn’t do this. The only way you can explain it is if it was formed through a merger of two white dwarfs.”

The Death Of A Small Sun-Like Star

White dwarfs are all that is left of stars, like our own Sun, after they have finished burning their entire necessary supply of nuclear-fusing fuel. At this fatal point, the dying small star has shed its outer gaseous layers into space. A small star’s grand finale contrasts with the noisy and explosive demise of more massive stars, that die in violent and catastrophic supernova blasts. Small stars like our Sun “go gentle into that good night”, and perish with great beauty and relative peace. Indeed, their lovely multicolored gaseous shrouds have inspired astronomers to refer to them as the “butterflies of the Universe”, as homage to their celestial loveliness.

Solitary small stars like our Sun perish gently. However, if there is another stellar actor in the drama, ghastly complications develop. If a small star resides in a binary system with another star, a wild party will inevitably occur. When the first of the duo “dies”, leaving its dense white dwarf core behind, the stellar corpse will gravitationally sip up material from its still-living companion star–and victim. As the vampire-like dwarf continues to steal more and more material from its unlucky companion, it will at last sip up enough material to attain sufficient mass to “go critical.” At this point, the white dwarf pays for its crime and explodes–just like the big guys. This explosion is termed a Type Ia supernova, and it differs from the core-collapse Type II supernovae experienced by more massive stars.

The proposal that WD J0551+4135 is really an object that formed as the result of the merger of a duo of white dwarfs is based on a related, but not identical, theory of its formation. In this case, as one of the two stars expanded at the end of its life into a swollen red giant, it engulfed its companion star, drawing its orbit in ever closer and closer as the first star withered into its white dwarf stage. An encore performance then occurred when the other star became a bloated red giant. Over the passage of billions of years, gravitational wave emission shrunk the orbit further, to the point that the waltzing stellar duo merged together to form a single object.

The Dancers And Their Dance

Even though white dwarf mergers have been predicted to occur, the one involving the unusual WD J0551+4135 is stranger than expected. This is beause most of the mergers in our Milky Way occur between stars sporting different masses, whereas this odd merger likely occurred between a duo of similaly-sized stars. There is also a limit in respect to how big the resulting single white dwarf cam be. This is because, if the resulting stellar corpse weighs-in at over 1.4 times solar masses, it will “go critical” and blow itself to smithereens in a Type Ia supernova explosion. However, it is possible that such fatal stellar explosions can be triggered at slightly lower masses, and so this odd white dwarf is especially useful because it demonstrates how massive a white dwarf can get and still “live” to tell the story.

Because the merger restarts the process of the star’s cooling, astronomers find it difficult to calculate the star’s true age. The stellar corpse probably merged about 1.3 billion years ago–but the duo of original dead stars may have existed for many billions of years before that event.

WD J0551+4135 is important because it is one of only a handful of merged white dwarfs to be identified–and it is the only one to be identified so far on the basis of its composition.

Dr. Hollands explained in the March 2, 2020 University of Warwick Press Release that “There aren’t that many white dwarfs this massive, although there are more than you would expect to see which implies that some of them were probably formed by mergers.”

“In the future we may be able to use a technique called astroseismology to learn about the white dwarf’s core composition from its stellar pulsations which would be an independent method confirming this star formed from a merger. Maybe the most exciting aspect of this star is that it must have just about failed to explode as a suprnova–these gargantuan explosions are really important in mapping the structure of the Universe as they can be detected out to very large distances. However, there remains much uncertainty about what kind of stellar systems make it to the supernova stage,” he added.

“Strange as it may sound measuring the properties of this ‘failed’ supernova and future look alikes is telling us a lot about the pathways to thermonuclear self-annihilation,” Dr. Hollands continued to comment.

Shedding New Light On The Universe’s Shadowland

We live in a mysterious Universe–most of which we are unable to see. What is it made of, and has its composition changed over time? The starlit galaxies, galaxy clusters and superclusters are all embedded within invisible halos composed of transparent material that scientists refer to as the “dark matter.” This mysterious substance creates an enormous, invisible structure throughout Space and Time–a fabulous, fantastic tapestry woven of heavy filaments composed of this “dark” stuff, that is thought to be formed from unidentified and exotic non-atomic particles. In March 2020, a team of scientists announced that they have identified a sub-atomic particle that could have formed the dark matter in the Universe during its Big Bang birth.

Scientists think that up to 80% of the Universe could be dark matter, but despite years of investigation, its origin has remained a puzzle. Even though it cannot be observed directly, most astronomers think that this ghostly form of matter is really there because it does dance gravitationally with forms of matter that can be observed–such as stars and planets. This invisible material is made up of exotic particles that do not emit, absorb, or reflect light.

A team of nuclear physicists at the University of York (U.K.) are now proposing a new particle candidate for this ghostly material–a particle that they recently detected called the d-star hexaquark.

The d-star hexaquark is made up of six quarks–the fundamental particles that normally combine in trios to form the protons and neutrons of the atomic nucleus.

Raise A Quark for Muster Mark

The Irish novelist James Joyce (1882-1941) had a drunken character in Finnegan’s Wake raise a quart of dark beer to toast a man named Finnegan who had just died. He mistakenly said “raise a quark for muster Mark”. The American physicist, Nobel laureate Murray Gell-Mann (1929-2019), who was one of the scientists who proposed the existence of the quark in 1964, thought it was so funny that he named this sub-particle after the drunken host. The Russian-American physicist, George Zweig, also independently proposed the existence of the quark that same year.

A quark is a type of elementary particle that is a fundamental constituent of matter. Quarks combine to create composite particles called hadrons. Hadrons are subatomic particles of a type that includes protons and neutrons, which can take part in the strong interaction that holds atomic nuclei together. Indeed, the most stable hadrons are protons and neutrons–the components that form the nuclei of atoms. Because of a phenomenon termed color confinement, quarks have not been directly observed or found in isolation. For this reason, they have been found only within hadrons. Because of this, a great deal of what scientists have learned about quarks has been derived from studying hadrons.

Quarks also show certain intrinsic properties, including mass, color, electric charge, and spin. They are the only known elementary particle in the Standard Model of Particle Physics to display all four fundamental interactions–also termed fundamental forces–the strong interaction, the weak interaction, gravitation, and electromagnetism. Quarks are also the only known elementary particles whose electric charges are not integer multiples of the elementary charge.

The types of quarks are referred to as flavors: up, down, strange, charm, bottom, and top. The heavier quarks quickly experience a metamorphosis into up and down quarks as the result of a process called particle decay. Particle decay refers to the transformation from a higher mass state to lower mass states. For this reason, up and down quarks are stable, as well as the most abundant in the Universe. In contrast, strange, charm, bottom, and top quarks can only be churned out in high energy collisions–such as those involving cosmic rays or particle accelerators. For every quark flavor there is a corresponding antiquark. The antiquark antiparticle differs from the quark only in certain properties, such as electric charge. The antiquark antiparticles have equal magnitude but an opposite sign.

There was little evidence for the physical existence of quarks until deep inelastic scattering experiments were conducted at the Stanford Linear Accelerator Center in 1968. Accelerator experiments have provided evidence for the existence of all six flavors. The top quark, first observed at Fermilab in 1995, was the last to be discovered.

The Universe’s Shadowland

It is often said that most of our Universe is “missing”, primarily composed as it is of an unidentified substance that is referred to as dark energy. The mysterious dark energy is causing the Universe to accelerate in its expansion, and it is thought to be a property of Space itself.

The most recent measurements indicate that the Universe is composed of approximately 70% dark energy and 25% dark matter. Currently, both the origin and nature of the mysterious dark matter and dark energy are unknown. A considerably smaller fraction of our Universe is composed of so-called “ordinary” atomic matter. “Ordinary” atomic matter–which is really extraordinary–is comparatively scarce. Nevertheless, it is the material that accounts for all of the elements listed in the familiar Periodic Table. Despite being the tiny “runt” of the cosmic litter of three, “ordinary” atomic matter is what makes up stars, planets, moons, and people–everything that human beings on Earth are most familiar with. It is also the precious form of matter that caused life to form and evolve in the Universe.

On the largest scales, the Universe looks the same wherever it is observed. It displays a bubbly, foamy appearance, with extremely massive and enormous filaments composed of dark matter intertwining around one another, creating a web-like structure that is referred to as the Cosmic Web. The ghostly, transparent filaments of the great Cosmic Web are traced out by myriad galaxies blazing with the fires of brilliant starlight, thus outlining the immense, intertwining braids of dark matter that contain the galaxies of the visible Universe. Enormous, cavernous, dark, and almost empty Voids interrupt this web-like pattern. The Voids host few galaxies, and this is the reason why they appear to be entirely empty. In dramatic contrast, the massive starlit filaments of the Cosmic Web weave themselves around these almost-empty Voids, creating a fabulous, complicated, braided knot.

Some cosmologists have proposed that the entire large scale structure of the Universe is really composed of only one filament and a single Void twisted together in an intricate and complex tangle.

Enter The d-Star Hexaquark

The d-star hexaquark is made up of six quarks. These fundamental particles normally combine in trios to form the protons and neutrons of the atomic nucleus. Most importantly, the six quarks in a d-star hexaquark create a boson particle. This indicates that when a large number of d-star hexaquarks are present that can dance together and combine in very different ways to the protons and neutrons. A boson is a particle that carries energy. For example, photons are bosons.

The team of scientists at the University of York propose that in the conditions that existed shortly after the Big Bang, a multitude of d-star hexaquarks could have met up and then combined as the Universe cooled down from its original extremely hot state and then expanded to give rise to a fifth state of matter–what is termed a Bose-Einstein Condensate.

A Bose-Einstein Condensate is a state of matter in which separate atoms or subatomic particles, cooled to near absolute zero, coalesce into a single quantum entity–that is, one that can be described by a wave function–on a near-macroscopic scale.

Dr. Mikhail Bashkanov and Dr. Daniel Watts from the Department of Physics at the University of York published the first assessment of the viability of this new dark matter candidate.

Dr. Watts noted in a March 3, 2020 University of York Press Release that “The origin of dark matter in the Universe is one of the biggest questions in science and one that, until now, has drawn a blank.”

“Our first calculations indicate that condensation of d-stars are a feasible new candidate for dark matter and this new possibility seems worthy of further, more detailed investigation,” he added.

“The result is particularly exciting since it doesn’t require any concepts that are new to physics,” Dr. Watts continued to comment.

Co-author, Dr. Bashkanov, explained in the same University of York Press Release that “The next step to establish this new dark matter candidate will be to obtain a better understanding of how the d-stars interact–when do they attract and when do they repel each other. We are teaching new measurements to create d-stars inside an atomic nucleus and see if their properties are different to when they are in free spae.”

The scientists are planning now to collaborate with researchers in Germany and the United States to test their new theory of.dark matter and hunt for d-star hexaquarks in the Universe.

Sizing Up Neutron Stars

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A neutron star is the lingering leftovers of a massive star that has ended its nuclear-fusing “life” in the brilliant and fatal fireworks of a supernova explosion. These extremely dense city-sized objects are actually the collapsed cores of dead stars which, before their violent “deaths”, weighed-in at between 10 to 29 times the mass of our Sun. These bizarre, lingering relics of heavy stars are so extremely dense that a teaspoon full of neutron star material can weigh as much as a herd of elephants. In March 2020, an international research team of astronomers announced that they have obtained new measurements of how big these oddball stars are. They also found that neutron stars unlucky enough to merge with voracious black holes are likely to be swallowed whole–unless the black hole is both small and/or rapidly spinning.

The international research team, led by members of the Max Planck Institute for Gravitational Physics (Einstein Institute: AEI) in Germany, obtained their new measurements by combining a general first principles description of the mysterious behavior of neutron star material with multi-messenger observations of the binary merger of a duo of neutron stars dubbed GW170817. Their findings, published in the March 10, 2020 issue of the journal Nature Astronomy, are more stringent by a factor of two than earlier limits and demonstrate that a typical neutron star has a radius close to 11 kilometers. In addition, they found that because such unlucky stars are swallowed whole during a catastrophic merger with a black hole, these mergers might not be observable as gravitational wave sources, and would also be invisible in the electromagnetic spectrum. Theoretical work in physics and other sciences is said to be from first principles (ab initio) if it originates directly at the level of established science and does not make assumptions such as empirical model and parameter fitting.

Gravitational waves are ripples in the fabric of Spacetime. Imagine the ripples that propagate on the surface of a pond after a pebble is thrown into the water. Gravitational waves are disturbances in the curvature of Spacetime. They are generated by accelerated masses, that propagate as waves outward from their source at the speed of light. Gravitational waves provide a new and important tool for astronomers to use because they reveal phenomena that observations using the electromagnetic spectrum cannot. However, in the case of neutron star/black hole mergers, neither gravitational wave observations nor observations using the electromagnetic spectrum can be used. This is why such mergers may not be observable.

“Binary neutron star mergers are a gold mine of information. Neutron stars contain the densest matter in the observable Universe. In fact, they are so dense and compact, that you can think of the entire star as a single atomic nucleus, scaled up to the size of a city. By measuring these objects’ properties, we learn about the fundamental physics that governs matter at the sub-atomic level,” explained Dr. Collin Capano in a March 10, 2020 Max Planck Institute Press Release. Dr. Capano is a researcher at the AEI in Hannover.

“We find that the typical neutron star, which is about 1.4 times as heavy as our Sun has a radius of about 11 kilometers. Our results limit the radius to likely be somewhere between 10.4 and 11.9 kilometers. This is a factor of two more stringent than previous results,” noted Dr. Badri Krishnan in the same Max Planck Institute Press Release. Dr. Krishnan leads the research team at the AEI.

Strange Beasts In The Stellar Zoo

Neutron stars are born as the result of the fatal supernova explosion of a massive star, combined with gravitational collapse, that compresses the core to the density of an atomic nucleus. How the neutron-rich, extremely dense matter behaves is a scientific mystery. This is because it is impossible to create the necessary conditions in any lab on Earth. Although physicists have proposed various models (equations of state), it remains unknown which (if any) of these models actually describes neutron star matter.

Once the neutron star is born from the wreckage of its progenitor star, that has gone supernova, it can no longer actively churn out heat. As a result, these stellar oddballs cool as time goes by. However, they still have the potential to evolve further by way of collision or accretion. Most of the basic models suggest that neutron stars are made up almost entirely of neutrons. Neutrons, along with protons, compose the nuclei of atoms. Neutrons have no net electrical charge, and have a slightly larger mass than protons. The electrons and protons in normal atomic matter combine to create neutrons at the conditions of a neutron star.

The neutron stars that can be observed are searing-hot and typically have a surface temperature of 600,000 K. They are so extremely dense that a matchbox containing its material would weigh-in at about 2 billion tons. The magnetic fields of these dead stars are about 100 million to 1 quadrillion times more powerful than Earth’s magnetic field. The gravitational field at the bizarre surface of a neutron star is approximately 200 billion times that of our own planet’s gravitational field.

As the core of the doomed massive star collapses, its rotation rate increases. This is a result of the conservation of angular momentum, and for this reason the newborn neutron star–called a pulsar–can rotate up to as much as several hundred times per second. Some pulsars emit regular beams of electromagnetic radiation, as they rapidly rotate, and this is what makes them detectable. The beams of electromagnetic radiation emitted by the pulsar are so regular that they are frequently likened to lighthouse beacons on Earth.

The discovery of pulsars by Dr. Jocelyn Bell Burnell and Dr. Antony Hewish in 1967 was the first observational indication that neutron stars exist. The radiation from pulsars is believed to be primarily emitted from areas near their magnetic poles. If the magnetic poles do not coincide with the rotational axis of the neutron star, the emission beam will sweep the sky. When observed from a distance, if the observer is situated somewhere in the path of the beam, it will appear as regular pulses of radiation emitted from a fixed point in space–hence the “lighthouse effect.” PSR J1748-2446ad is currently the most rapidly spinning pulsar known, and it rotates at the breathtaking rate of 716 times every second, or 43,000 revolutions per minute, giving a linear speed at the surface of almost a quarter of the speed of light.

There are thought to be approximately 100 million neutron stars in our Milky Way. This number was derived by scientists estimating the number of stars that have gone supernova in our Galaxy. The problem is that most neutron stars are not young, wildly spinning pulsars, and neutron stars can only be easily spotted under certain conditions–for example, if they are members of a binary system or if they are youthful pulsars. However, most of the neutron stars dwelling in our Milky Way are elderly–and cold. Non-accreting and slowly-rotating neutron stars are almost undetectable. However, ever since the Hubble Space Telescope discovered RX J185635-3754, a small number of nearby neutron stars that apparently emit only thermal radiation have been spotted. It has been proposed that soft gamma repeaters are a type of neutron star possessing especially powerful magnetic fields, termed magnetars. However, some astronomers think that soft gamma repeaters are really neutron stars with ancient, fossil disks encircling them.

Any main-sequence (hydrogen burning) star, on the Hertzsprung-Russell Diagram of Stellar Evolution, that sports an initial mass exceeding 8 times that of our Sun, has the potential to become the stellar progenitor of a neutron star. As the aging star evolves away from the main-sequence, additional nuclear burning results in an iron-rich core. When all nuclear fuel in the core has been used up, the core must be supported by degeneracy pressure alone. Stars on the hydrogen-burning main-sequence keep themselves bouncy because they experience a very delicate balance between the squeeze of their own gravity and push of radiation pressure. When radiation pressure can no longer be produced by nuclear fuel burning, gravity crushes the dying star.

Additional deposits from shell fuel burning cause the core of the doomed star to exceed what is termed the Chandrasekhar limit. As a result, temperatures of the dying, doomed massive star soar to more than 5X10 to the ninth power K. At these extremely hot temperatures, photodisintegration (the breaking up of iron nuclei into alpha particles by high-energy gamma rays) occurs. As the temperature soars ever higher and higher, electrons and protons merge to create neutrons by way of electron capture. These liberate a flood of neutrinos. When densities reach nuclear density of 4 X 10 to the seventeenth power kg/m cubed, a combination of strong nuclear force repulsion and neutron degeneracy pressure stops further contraction. The infalling outer envelope of the doomed old star is halted and hurled outward by a flux of neutrinos manufactured in the creation of the neutrons. The elderly star has come to the end of that long stellar road, and it goes supernova. If the stellar ghost sports a mass that exceeds about 3 solar masses, it collapses further and becomes a black hole.

As the core of a massive star is squeezed during a Type II (core-collapse) supernova (or a Type Ib or Type Ic supernova), it collapses into a neutron star. The stellar relic retains most of its angular momentum–but because it only possesses a small percentage of its progenitor star’s radius, a neutron star is born with a very high rotation speed. This stellar oddball slows down over a very long span of time.

Sizing Up A Dense Stellar Oddball

Mergers of a duo of binary neutron stars, such as GW 170817, provide a treasure trove of information about how matter behaves under such extreme conditions, as well as the underlying nuclear physics behind it. GW 170817 was first observed in gravitational waves and the entire electromagnetic spectrum in August 2017. From this type of important astrophysical event, scientists can go on to determine the physical properties of these oddball stars, including their radius and mass.

The research team at AEI used a model based on a first-principles description of how subatomic particles dance together at the extremely high densities found inside neutron stars. Remarkably, as the team of scientists discovered, theoretical calculations at length scales less than a trillionth of a millimeter can be compared with observations of an astrophysical object more than a hundred million light-years from Earth.

“It’s a bit mind boggling. GW 170817 was caused by the collision of two city-sized objects 120 million years ago, when dinosaurs were walking around here on Earth. This happened in a galaxy a billion trillion kilometers away. From that, we have gained insight into subatomic physics,” Dr. Capano commented in the March 10, 2020 Max Planck Institute Press Release.

The first-principles descriptions used by the scientists predicts numerous potential equations of state for neutron stars, which are directly derived from nuclear physics. From these possible equations of state, the researchers chose only those that are most likely to explain different astrophysical observations, which agree with gravitational-wave observations of GW 170817. The team used observations derived from public LIGO and Virgo data, which produce a brief hyper-massive neutron star as the result of the merger, and which agree with known constraints on the maximum neutron star mass from electromagnetic counterpart observations of GW 170817. This approach not only enabled the scientists to derive new information on dense-matter physics, but also to obtain the most stringent limits on the size of neutron stars to date.

“These results are exciting, not just because we have been able to vastly improve neutron star radii measurements, but because it gives us a window into the ultimate fate of neutron stars in merging binaries,” noted Stephanie Brown in the March 10, 2020 Max Planck Institute Press Release. Ms. Brown is co-author of the publication and a doctoral student at the AEI Hannover.

The new results suggest that, with an event like GW 170817, the LIGO and Virgo detectors at design sensitivity will be able to distinguish, from gravitational waves alone, whether the duo of neutron stars or duo of black holes have merged. For GW 170817, observations in the electromagnetic spectrum were central in making that important distinction.

The Laser Interferometer for Gravitational Wave Observatory (LIGO) is a large scale physics experiment and observatory to detect cosmic gravitational waves and to develop gravitational wave observatories on an astronomical level. The Virgo interferometer is a large interferometer designed to detect gravitational waves.

The team of scientists also found that for mixed binaries (a neutron star merging with a black hole), gravitational wave merger observations alone will have a difficult time distinguishing these events from binary black holes. Observations in the electromagnetic spectrum or gravitational waves from after the merger will be crucial to distinguish between the two.

However, it turns out that the new results also suggest that multi-messenger observations of mixed binary mergers are unlikely to occur. “We have shown that in almost all cases the neutron star will not be torn apart by the black hole and rather swallowed whole. Only when the black hole is very small or rapidly spinning, can it disrupt the neutron star before swallowing it; and only then can we expecxt to see anything besides gravitational waves,” commented Dr. Capano in the March 10, 2020 Max Planck Institute Press Release.

In the next decade, the existing gravitational wave detectors will become even more sensitive, and additional detectors will begin observing. The research team expects more gravitational wave detections and possible multi-messenger observations from merging binary neutron stars. Each of these mergers would provide wonderful opportunities to learn more about neutron stars and nuclear physics.

Infinite Space As an Alternative to the Singularity

I have often wondered about the Big Bang Theory. I understand why it is the current favorite for the beginning of the universe. When we look out into space, everything is moving away from us. It would stand to reason that our universe began as an explosion. Wind the clock backward and everything ends up in a central mass. Que the explosion, birth of the universe, everything spreads out and the universe as we know it begins to evolve.

Another problem with the theory that had to be resolved was how parts of the universe that are too far apart happen to be the same uniform temperature. Again, this was solved by claiming the universe was at one time small, came into existence and then a period of inflation followed. This allowed for the uniform temperatures we see when we look at the cosmic background radiation. But why can’t this also be a uniform cooling of space from a reaction that tore through an existing medium rather than the expansion of space itself?

I’m thinking the early universe could have been comprised of nothing but dark matter. There was no free energy, no nothing, not even gravity because, since nothing existed there was nothing to experience gravity. And who knows if gravity effects dark matter? We only know that it has an effect on matter and that is only from observational evidence. Since there isn’t enough stuff in the visible universe, we have Dark matter to solve this problem. Dark energy may be nothing more than a leftover from the event that created the universe. That is to say, only this Dark Material existed before the known universe with the known universe and dark energy coming later.

So, this is the hypothesis I imagine. Infinite space in all directions comprised of dark matter. Some instability occurred that caused a small portion of this matter to destabilize leading to free energy that cooled to form quantum particles; which came together to form particles that went on to form Hydrogen. Hydrogen begins to come together due to gravity forming the structure of the universe we see today with galaxy clusters forming along filaments with large voids in between. All of this existing within a medium of left-over dark matter.

The increasing expansion of the universe being driven by the pure dark matter universe that exists beyond this expanding chain reaction. So, we have some of the dark matter reacting to become free energy with some left-over dark energy. The energy forms matter and the matter form the universe that is being pulled outward faster and faster thanks to the gravitational influence of dark matter. In this way we don’t need to have a singularity expanding and we can answer the question of what the universe is growing into. It’s spreading out into the existing dark matter universe. The dark matter universe IS the universe with our matter universe continuously forming within this medium, free energy released from dark matter.