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Gravitational Waves Could Reveal Primordial Black Hole Mergers

What are primordial black holes (PBHs)?

They are a hypothetical type of black holes that could have formed in the very early Universe (less than one second after the Big-Bang), during the so-called radiation dominated era. The essential ingredient for a primordial black hole to form is a fluctuation in the density of the Universe, inducing its gravitational collapse. There are several mechanisms able to produce such inhomogeneities in the context of cosmic inflation (in hybrid inflation models, for example axion inflation, ...), reheating, or cosmological phase transitions.

In summary, some cosmological models suggest that— immediately after the Big Bang, some 13.82 billion years ago— the early quantum density fluctuations may have been dramatic enough to create black holes — known as primordial black holes — and these ancient Big Bang remnants may still exist to this day.

These theoretical models, however, are hard to test as observing the universe immediately after the Big Bang is very difficult. But recent discoveries of gravitational waves from black hole and neutron star mergers have ushered in a new era of astronomy, and astronomers have an observational tool at their disposal.

So, even if the standard theory predicts that black holes are born from supernovae, which implies that they couldn’t have formed any earlier than the first stars, in a new study published in Physical Review Letters, researchers have proposed that if we have the ability to detect gravitational waves produced before the first stars died, we may be able to carry out astronomical archaeological dig of sorts to possibly find evidence of these ancient black holes.

Savvas Koushiappas of Brown University, Rhode Island, and Abraham Loeb of Harvard University came up with a way to test this idea by calculating the earliest epoch in which baryonic black holes—those made of the matter we see in stars and planets—can form.

"The idea is very simple," Koushiappas said. "With future gravitational wave experiments, we'll be able to look back to a time before the formation of the first stars. So if we see black hole merger events before stars existed, then we'll know that those black holes are not of stellar origin."

Cosmologists measure how far back in time an event occurred using redshift -- the stretching of the wavelength of light associated with the expansion of the universe. Events further back in time are associated with larger redshifts. For this study, Koushiappas and Loeb calculated the redshift at which black hole mergers should no longer be detected assuming only stellar origin.

They find that beyond a redshift of about 40 —where observed objects were formed during the first 65 million years following the big bang—the rate of collisions should drop to less than one per year. This epoch should be within reach of the next generation of gravitational-wave observatories.

Finding evidence for primordial black holes could shed light on the nature of dark matter or on the origin of cosmic structure in the early Universe.

► Read the article from Brown University: "Gravitational waves could shed light on the origin of black holes">>
https://news.brown.edu/articles/2017/11/primordial

► The paper "Maximum Redshift of Gravitational Wave Merger Events", published in Physical Review Letters, 2017>>
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.221104

► Image: Black holes collide
Credit: Simulating eXtreme Spacetimes (SXS) Project (http://www.black-holes.org)

Further reading and references

► Gravitational Waves Could Reveal Black Hole Origins>>
https://physics.aps.org/synopsis-for/10.1103/PhysRevLett.119.221104

► Gravitational Waves Might Reveal Primordial Black Hole Mergers Just After the Big Bang>>
https://astroengine.com/2017/11/30/gravitational-waves-might-reveal-primordial-black-hole-mergers-just-after-the-big-bang/

► Primordial black hole>> https://en.wikipedia.org/wiki/Primordial_black_hole

► NASA Scientist Suggests Possible Link Between Primordial Black Holes and Dark Matter>>
https://www.nasa.gov/feature/goddard/2016/nasa-scientist-suggests-possible-link-between-primordial-black-holes-and-dark-matter

#Astronomy, #PrimordialBlackHoles, #Cosmology, #DarkMatter, #GravitationalWaves, #Astrophysics, #Research
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Achernar: the Flattest Star Known

The ten brightest stars in the nighttime sky in terms of apparent magnitude are, from brightest to least brightest: Sirius, Canopus, Alpha Centauri, Arcturus, Vega, Capella, Rigel, Procyon, Achernar and Betelgeuse.
So Achernar- lying at the southern tip of the constellation Eridanus (The River) at a distance of 139 ± 3 ly- is the 9th brightest star in the sky known!
But that star has other intriguing features: it is the flattest star ever seen and also the brightest Be star in the sky!

Be stars are massive dwarf or subgiant stars that present temporary emission lines in their spectrum, and particularly in the Halpha line. The mechanism triggering these Be episodes is currently unknown, but binarity could play an important role. Actually, based to some studies dating back to 2007 and 2008, Achernar is the primary component of the binary system designated Alpha Eridani. The two components are designated Alpha Eridani A and Alpha Eridani B (known informally as Achernar B).
The WGSN (Working Group on Star Names ) approved the name Achernar for Alpha Eridani A on 30 June 2016 and it is now so entered in the IAU Catalog of Star Names.

Be stars have too a fundamental property of rapid rotation. Theoretically, rotation has several consequences on the star structure. The most obvious is the geometrical deformation that results in a larger radius at the equator than
at the poles. This would obviously cause such stars to become flattened.
In the case of Achernar, its equatorial radius is more than 50% larger than the polar one (because of an unusually rapid rotational velocity), so the star appears oblate in shape.
In other words, this star is shaped very much like the well-known spinning-top toy, so popular among young children. The high degree of flattening measured for Achernar - a first in observational astrophysics - has posed an unprecedented challenge for theoretical astrophysics.

The presence of a circumstellar disk of ionized gas is a common feature of Be stars such as this. The disk is not stable and periodically decretes back into the star. The maximum polarization for Achernar's disk was observed in September 2014, and it is now decreasing.

Achernar is a hot blue main sequence star of spectral classification B6 with about seven times the mass of the Sun, but is roughly 3,150 times more luminous (than the Sun). Its companion star appears to be a blue-white main sequence star of spectral type A, with a stellar mass of about double that of the Sun. The separation of the two stars is roughly 12.3 AU and their orbital period is at least 14–15 years.

► Image: The position of Achernar (lower right).
Source>> https://en.wikipedia.org/wiki/Achernar#/media/File:Position_Alpha_Eri.png

Further reading and references

► H-alpha>> https://en.wikipedia.org/wiki/H-alpha

► The close-in companion of the fast rotating Be star Achernar>> https://arxiv.org/abs/0804.3465

► The spinning-top Be star Achernar from VLTI-VINCI>> https://www.aanda.org/articles/aa/pdf/2003/33/aafd053.pdf

► Flattest Star Ever Seen>> https://www.eso.org/public/unitedkingdom/news/eso0316/

► Achernar: Rapid Polarization Variability as Evidence of Photospheric and Circumstellar Activity>>
https://arxiv.org/abs/0710.4163

► On the Determination of the Rotational Oblateness of Achernar>>
https://arxiv.org/abs/0801.4901

► A-type main-sequence star>> https://en.wikipedia.org/wiki/A-type_main-sequence_star

► B-type main-sequence star>> https://en.wikipedia.org/wiki/B-type_main-sequence_star

► Circumstellar disk>> https://en.wikipedia.org/wiki/Circumstellar_disc

#Astrophysics, #Achernar, #BinarySystem, #Research, #MainSequenceStars, #ConstellationEridanus, #Universe

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First Detection of Both Gravitational Waves and Light Produced by Colliding Neutron Stars

For the first time, scientists have directly detected gravitational waves—ripples in space and time—in addition to light from the spectacular collision of two neutron stars. This marks the first time that a cosmic event has been viewed in both gravitational waves and light.

The discovery was made using the U.S.- based Laser Interferometer Gravitational-wave Observatory (LIGO), funded by the National Science Foundation (NSF); the Europe-based Virgo detector; and some 70 ground- and space-based observatories.

In fact, on 17 August 2017 the NSF's Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States, working with the Virgo Interferometer in Italy, detected gravitational waves passing the Earth. This event, the fifth ever detected, was named GW170817. About two seconds later, two space observatories, NASA’s Fermi Gamma-ray Space Telescope and ESA’s INTErnational Gamma Ray Astrophysics Laboratory (INTEGRAL), detected a short gamma-ray burst from the same area of the sky.

The almost simultaneous detections of both gravitational waves and gamma rays from GW170817 raised hopes that this object was indeed a long-sought kilonova and observations with ESO facilities have revealed properties remarkably close to theoretical predictions. Kilonovae were suggested more than 30 years ago but this marks the first confirmed observation.

Following the merger of the two neutron stars, a burst of rapidly expanding radioactive heavy chemical elements left the kilonova, moving as fast as one-fifth of the speed of light. The colour of the kilonova shifted from very blue to very red over the next few days, a faster change than that seen in any other observed stellar explosion.

Read some scientific releases about this event

► GW170817 Press Release
LIGO and Virgo make first detection of gravitational waves produced by colliding neutron stars
Discovery marks first cosmic event observed in both gravitational waves and light.>>
https://www.ligo.caltech.edu/page/press-release-gw170817

► ESO Telescopes Observe First Light from Gravitational Wave Source
Merging neutron stars scatter gold and platinum into space>>
http://www.eso.org/public/unitedkingdom/news/eso1733/

► GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral>>
http://www.ligo.org/detections/GW170817/paper/GW170817-PRLpublished.pdf

► Hubble observes source of gravitational waves for the first time>> https://www.spacetelescope.org/news/heic1717/

Image: This artist’s impression shows two tiny but very dense neutron stars at the point at which they merge and explode as a kilonova. Such a very rare event is expected to produce both gravitational waves and a short gamma-ray burst, both of which were observed on 17 August 2017 by LIGO–Virgo and Fermi/INTEGRAL respectively. Subsequent detailed observations with many ESO telescopes confirmed that this object, seen in the galaxy NGC 4993 about 130 million light-years from the Earth, is indeed a kilonova. Such objects are the main source of very heavy chemical elements, such as gold and platinum, in the Universe.
Credit: University of Warwick/Mark Garlick

#Astrophysics, #CollidingNeutronStars, #GravitationalWaves
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Waiting for the ESO Press Conference on 16 October 2017...

From the announcement:

"ESO will hold a press conference on 16 October 2017 at 16:00 CEST, at its Headquarters in Garching, Germany, to present groundbreaking observations of an astronomical phenomenon that has never been witnessed before.

The event will be introduced from ESO’s Paranal Observatory in Chile by the Director General, Xavier Barcons, and will feature talks by representatives of many research groups around Europe."

Read the full announcement>>
http://www.eso.org/public/unitedkingdom/announcements/ann17071/

So what is it about?
Scientists have their mouth sewn shut! We can only make assumptions and, of course, respect the embargo imposed by research bodies. Perhaps something new in the field of gravitational-wave astronomy?
Bah, we'll see.

I think that ESO's announcement could have something to do with LIGO's announcement.
Take a look at this link>>
http://www.ligo.org/news/index.php#oct16

We can read:

"11 Oct 2017 -- Scientists representing LIGO, Virgo, and some 70 observatories will reveal new details and discoveries made in the ongoing search for gravitational waves. This will take place on Monday, October 16th, at 10:00am EDT at the National Press Club in Washington, D.C. A live-stream of the press conference can be viewed at this link (https://www.youtube.com/c/VideosatNSF/live). An alternate link (https://youtu.be/mtLPKYl4AHs) will also carry the live-stream, followed by a 30-minute YouTube question & answer session with gravitational-wave scientists.

For additional information see the full media advisory here [pdf]." >> http://www.ligo.org/news/oct16_2017_mediaadvisory.pdf

Let's wait for 16 October!

► Image: artist impression of gravitational waves being created.
R. Hurt/Caltech- JPL

#Research, #Astrophysics, #ESO
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The Nobel Prize in Physics 2017

On October 3, 2017, the Royal Swedish Academy of Sciences has decided to award the Nobel Prize in Physics 2017 with one half to:

Rainer Weiss
LIGO/VIRGO Collaboration

and the other half jointly to

Barry C. Barish
LIGO/VIRGO Collaboration

and

Kip S. Thorne
LIGO/VIRGO Collaboration

"for decisive contributions to the LIGO detector and the observation of gravitational waves".


On 14 September 2015, the LIGO detectors in the USA saw space vibrate with gravitational waves for the very first time. Although the signal was extremely weak when it reached Earth, it is already promising a revolution in astrophysics. Gravitational waves are an entirely new way of following the most violent events in space and testing the limits of our knowledge.

The gravitational waves that have now been observed were created in a ferocious collision between two black holes, more than a thousand million years ago. Albert Einstein was right again. A century had passed since gravitational
waves were predicted by his general theory of relativity, but he had always been doubtful whether they could ever be captured.
LIGO, the Laser Interferometer Gravitational-Wave Observatory, is a collaborative project with over one thousand researchers from more than twenty countries. Together, they have realised a vision that is almost fifty years old. The 2017 Nobel Laureates have, with their enthusiasm and determination,
each been invaluable to the success of LIGO. Pioneers Rainer Weiss and Kip S. Thorne, together with Barry C. Barish, the scientist and leader who brought the project to completion, have ensured that more than four decades of effort led to gravitational waves finally being observed.

Read the press release at Nobelprize.org>>
https://www.nobelprize.org/nobel_prizes/physics/laureates/2017/press.html

Image credits: Abigail Malate, Staff Illustrator
Via Inside Science website>>
https://www.insidescience.org/news/update-three-share-nobel-prize-physics-gravitational-waves

#Physics, #Astrophysics, #NobelPrizeinPhysics2017, #GravitationalWaves
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CO–0.40–0.22*, the Second-Largest Black Hole Candidate Near the Center of the Milky Way

In 2016, I shared the post "Signature of an Intermediate-Mass Black Hole in the Milky Way", related to a study published in The Astrophysical Journal Letters issued on January 1, 2016: "Signature of an intermediate-mass black-hole in the central molecular zone of our galaxy">>
http://dx.doi.org/10.3847/2041-8205/816/1/L7
In short, a team of astronomers led by Tomoharu Oka, a professor at Keio University in Japan, found an enigmatic gas cloud, called CO-0.40-0.22, only 200 light years away from the center of the Milky Way. What makes
CO-0.40-0.22 unusual is its surprisingly wide velocity dispersion- that means in layman words the cloud contains gas traveling at multiple speeds.
That finding led researchers to hypothesize the presence of an invisible black hole (second largest black hole) with a mass of 100 thousand times the mass of the Sun around the center of the Milky Way.

Take a look at 2016 post>> https://plus.google.com/+annaritaruberto/posts/YFpQywDJURU

Recently, 04 September 2017, the study "Millimetre-wave emission from an intermediate-mass black hole candidate in the Milky Way" was published in Nature Astronomy. This new study comes from a team of astronomers led by Tomoharu Oka too.

An excerpt from the abstract tells us: "Recently, we discovered a peculiar molecular cloud, CO–0.40–0.22, with an extremely broad velocity width, near the center of our Milky Way galaxy. Based on the careful analysis of gas kinematics, we concluded that a compact object with a mass of about 10^5M⊙ is lurking in this cloud. Here we report the detection of a point-like continuum source as well as a compact gas clump near the center of CO–0.40–0.22. This point-like continuum source (CO–0.40–0.22*) has a wide-band spectrum consistent with 1/500 of the Galactic SMBH (Sgr A*) in luminosity. Numerical simulations around a point-like massive object reproduce the kinematics of dense molecular gas well, which suggests that CO–0.40–0.22* is one of the most promising candidates for an intermediate-mass black hole."

And again: "The compactness and absence of a counterpart at other wavelengths suggest that this massive object is an inactive IMBH (Intermediate-Mass Black Hole), which is not currently accreting matter. This is the second-largest black hole candidate in the Milky Way galaxy after Sgr A*, as well as the second IMBH candidate in the Galaxy after that in the nuclear subcluster IRS13E (MBH ≈ 1,300M⊙)"

Why would this candidate IMBH be important?

The existence of this candidate black hole (currently referred to as CO-0.40-0.22*) could help scientists to understand the mechanisms that lead to formation of supermassive black holes (SMBHs) at the center of galaxies.

In fact, even if it is widely accepted that black holes with masses greater than a million solar masses (SMBHs) lurk at the centres of massive galaxies, their origins remain unknown, although those of stellar-mass black holes are well understood. One possible scenario is that intermediate-mass black holes (IMBHs), which are formed by the runaway coalescence of stars in young compact star clusters, merge at the centre of a galaxy to form a SMBH.

So far, many candidates for IMBHs have been proposed, but none is accepted as definitive. So, the work of Oka and colleagues - realized through the careful analysis of gas kinematics surrounding this intermediate-mass black hole and of the radiation that it emits - could change things.

CO-0.40-0.22* could just be an intermediate-mass black hole that starts to be fagocitated by Sgr A*, the supermassive black hole in the center of our galaxy. Moreover, the authors have also found that radiation- emitted by the cloud surrounding it- is extremely similar, even if in a reduced version, to that from the nearby Sgr A*.

Finally, Oka and colleagues suggest that CO–0.40–0.22* used to be the nucleus of a dwarf galaxy that was cannibalized by our Milky Way, a few hundred million years ago.

In conclusion, further studies will be needed to confirm the presence of an IMBH at the center of CO–0.40–0.22.
Assuming they succeed, we can expect that astrophysicists will be monitoring it for some time to determine how it formed, and what it’s ultimate fate will be. For instance, if truly CO–0.40–0.22* is slowly drifting towards Sagittarius A* and will eventually merge with it, it will be creating an even more massive SMBH at the center of the Milky Way!

► I summarized the previous information from the paper "Millimetre-wave emission from an intermediate-mass black hole candidate in the Milky Way", published in Nature Astronomy>> https://www.nature.com/articles/s41550-017-0224-z.epdf

► Image explanation: (Figure a) Milky Way’s center seen in 115 and 346 GHz emission lines of carbon monoxide. White regions show the condensation of dense, warm gas. (Figure b) Close-up intensity map around CO-0.40-0.22, a possible intermediate mass black hole. Elipses indicate shell structures in the gas near it. (Figure c) Velocity dispersion diagram taken along the dotted line shown above. The wide velocity dispersion of 100 km/s in CO-0.40-0.22 stands out.
Image via NAOJ >> https://www.nao.ac.jp/en/news/science/2016/20160115-nro.html

#Astrophysics, #GalaxiesandClusters, #Research, #MilkyWayGalaxy, #IntermediateMassBlackHole
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The First Direct Evidence for Light-by-Light Scattering in Heavy-Ion Collisions with the ATLAS Detector at the LHC

According to classical electrodynamics, beams of light pass each other without being scattered. But if we take quantum physics into account, light can be scattered by light, even though this phenomenon seems very improbable.

However, given their potential for revealing new physics, photon–photon collisions, have been a topic of some interest for many decades.

Two-photon physics can be studied with high-energy particle accelerators, where the accelerated particles are not the photons themselves but charged particles that will radiate photons.

In short, despite photons being electrically neutral, the Standard Model (SM) allows two photons to interact via the exchange of virtual charged particles. Several final states are possible, including a pair of photons. The latter process (γγ → γγ, or “light-by-light scattering”) is a purely quantum-mechanical process, well known since the development of quantum electrodynamics (QED) and tested indirectly in several experiments, but the first direct evidence has come from ATLAS.

Light-by-light scattering (γγ→γγ) is, as said previously, a quantum-mechanical process that is forbidden in the classical theory of electrodynamics. This reaction is accessible at the Large Hadron Collider thanks to the large electromagnetic field strengths generated by ultra-relativistic colliding lead (Pb) ions.

Studying more than 4 billion events taken in 2015, the ATLAS collaboration found 13 candidates for light-by-light scattering. This result has a significance of 4.4 standard deviations, allowing the ATLAS collaboration to report the first direct evidence of this phenomenon at high energy.

► Source>> http://atlas.cern/updates/press-statement/atlas-sees-first-direct-evidence-light-light-scattering-high-energy

► The Nature Physics paper "Evidence for light-by-light scattering in heavy-ion collisions with the ATLAS detector at the LHC">> https://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys4208.html

► The preprint version of the study in arXiv>> https://arxiv.org/pdf/1702.01625.pdf

► Image explanation: A light-by-light scattering candidate event measured in the ATLAS detector.
Image credit: CERN

Further reading

► Looking forward to photon–photon physics>> http://cerncourier.com/cws/article/cern/68132

► Two-photon physics>> https://en.wikipedia.org/wiki/Two-photon_physics

#Astrophysics, #Physics, #Research, #ATLASdetector, #PhotonPhotonCollisions
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New Theory on the Origin of Dark Matter: an Alternative to the WIMP Paradigm

As well known- according to the standard model of cosmology- only around 4.9% of the Universe is composed of ordinary (visible) matter, while the remainder consists of around 26.8% dark matter and around 68.3% dark energy.

Up to now, it has been widely accepted that dark matter consists of Weakly-Interacting Massive Particles (WIMPs). Theory says WIMPs are likely heavier than protons and interact via the weak force, or at least interactions related to the weak force.

However, all attempts to find WIMPs using colliders experiments have come up empty. The absence of any convincing signals caused scientists at Johannes Gutenberg University Mainz (JGU) in Germany to start looking for an alternative to the WIMP paradigm.

The current assumption is that dark matter is a cosmological relic that has essentially remained stable since its creation.
"We have called this assumption into question, showing that at the beginning of the universe dark matter may have been unstable," explained Dr. Michael Baker from the Theoretical High Energy Physics (THEP) group at the JGU Institute of Physics. This instability also indicates the existence of a new mechanism that explains the observed quantity of dark matter in the cosmos.

The stability of dark matter is usually explained by a symmetry principle. However, in their paper, Dr. Michael Baker and Professor Joachim Kopp demonstrate that the universe may have gone through a phase during which this symmetry was broken. This would mean that it is possible for the hypothetical dark matter particle to decay. During the electroweak phase transition, the symmetry that stabilizes dark matter would have been reestablished, enabling it to continue to exist in the universe to the present day.

The two physicists claim that the new mechanism they propose may be connected with the apparent imbalance between matter and antimatter in the cosmos and could leave an imprint which would be detected in future experiments on gravitational waves.
In their paper published in the scientific journal Physical Review Letters, Baker and Kopp also indicate the prospects of finding proof of their new principle at CERN's LHC particle accelerator and other experimental facilities.

► Source>> http://www.uni-mainz.de/presse/aktuell/2360_ENG_HTML.php

► Paper "Dark Matter Decay between Phase Transitions at the Weak Scale">> https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.061801

Image explanation: image from Dark Universe, showing the distribution of dark matter in the universe.
Credit: AMNH

#Astrophysics, #DarkMatter, #WimpParadign, #Research, #NewTheory
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Berkeley Lab Scientists Developed New Computer Models to Explore Mergers of a Black Hole with a Neutron Star

The simulations, carried out in part at Berkeley Lab’s National Energy Research Scientific Computing Center (NERSC), are intended to help detectors home in on the gravitational-wave signals. Telescopes, too, can search for the brilliant bursts of gamma-rays and the glow of the radioactive matter that these exotic events can spew into surrounding space.

In separate papers published in a special edition (http://iopscience.iop.org/journal/0264-9381/page/Focus-issue-rattle-and-shine;jsessionid=5229B7F4988EDC235B956CF2FAFF6EFA.c2.iopscience.cld.iop.org) of the scientific journal Classical and Quantum Gravity, Berkeley Lab and other researchers present the results of detailed simulations.

One of the studies models the first milliseconds (thousandths of a second) in the merger of a black hole and neutron star, and the other details separate simulations that model the formation of a disk of material formed within seconds of the merger, and of the evolution of matter that is ejected in the merger.

► Learn more>> https://newscenter.lbl.gov/2017/08/02/new-space-simulations-neutron-star-black-hole-mergers/

► Paper "Dynamical ejecta from precessing neutron star-black hole mergers with a hot, nuclear-theory based equation of state">>
http://iopscience.iop.org/article/10.1088/1361-6382/aa573b/meta

► The preprint (open) version in arXiv>> https://arxiv.org/abs/1611.01159

► Animation showing a simulated merger of a black hole and neutron star. (Credit: Bryant Garcia)

#Astrophysics, #BlackHole, #NeutronStar, #Research, #ComputerModels
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Found the First Firm Evidence of Chiral Majorana Fermion Modes

21 Jul 2017, the study "Chiral Majorana fermion modes in a quantum anomalous Hall insulator–superconductor structure" appeared in Science. The experimental team was led by UCLA Professor Kang Wang, and precise theoretical predictions were made by Stanford Professor Shoucheng Zhang’s group, in collaboration with experimental groups led by Associate Professor Jing Xia at UC-Irvine and Professor Kai Liu at UC-Davis.

What is this? Well, let's do a premise.

A Majorana fermion is a fermion (https://en.wikipedia.org/wiki/Fermion) that is its own antiparticle (https://en.wikipedia.org/wiki/Antiparticle). They were hypothesized by Italian theoretical physicist Ettore Majorana in 1937. The term is sometimes used in opposition to a Dirac fermion, which describes fermions that are not their own antiparticles.

The Standard Model distinguishes between the fermions, which are particles of matter, and the bosons, which carry forces.

The matter particles (fermions) include six quarks and six leptons. The six quarks are called the up, down, charm, strange, top and bottom quark. Quarks typically don’t exist as single particles but lump together to form heavier particles such as protons and neutrons. Leptons include electrons and their cousins the muons and tau particles, along with the three types of neutrinos.

All of these matter particles fall into three “generations.”

With the exception of the neutrino, all of the Standard Model fermions are known to behave as Dirac fermions at low energy (after electroweak symmetry breaking). The nature of the neutrino is not settled; it may be either Dirac or Majorana.

In condensed matter physics, bound Majorana fermions can appear as quasiparticle excitations – the collective movement of several individual particles, not a single one. (Learn more about Majoranas>> https://en.wikipedia.org/wiki/Majorana_fermion)

In superconducting materials, Majorana fermions can emerge as (non-fundamental) quasiparticles.This becomes possible because a quasiparticle in a superconductor is its own antiparticle. So, about 10 years ago, scientists realized that Majorana fermions might also be created in experiments that explore the physics of materials – and the race was on to make that happen.

Coming back to the study pointed out in this post, all superconductors are characterized by an energy gap, a range of energies in which excitations are forbidden. However, the recently discovered class of topological superconductors (TSs) has a unique distinguishing feature: The boundary of a TS hosts gapless states called Majorana modes. The term Majorana stems from similarities with the unusual particles proposed by Ettore Majorana in his seminal 1937 paper. However, although Majorana's particles are fermions that are their own antiparticles, Majorana modes are composite quantum mechanical states, with distinct and perhaps even more intriguing properties. (See>> http://science.sciencemag.org/content/357/6348/294)

In the experiments at UCLA, UC-Davis and UC-Irvine, the team stacked thin films of two quantum materials – a superconductor and a magnetic topological insulator – and sent an electrical current through them, all inside a chilled vacuum chamber.

The top film was a superconductor. The bottom one was a topological insulator, which conducts current only along its surface or edges but not through its middle. Putting them together created a superconducting topological insulator, where electrons zip along two edges of the material’s surface without resistance, like cars on a superhighway.

It was Zhang’s idea to tweak the topological insulator by adding a small amount of magnetic material to it. This made the electrons flow one way along one edge of the surface and the opposite way along the opposite edge.

Then the researchers swept a magnet over the stack. This made the flow of electrons slow, stop and switch direction. These changes were not smooth, but took place in abrupt steps, like identical stairs in a staircase.

At certain points in this cycle, Majorana quasiparticles emerged, arising in pairs out of the superconducting layer and traveling along the edges of the topological insulator just as the electrons did. One member of each pair was deflected out of the path, allowing the researchers to easily measure the flow of the individual quasiparticles that kept forging ahead. Like the electrons, they slowed, stopped and changed direction – but in steps exactly half as high as the ones the electrons took.

These half-steps were the smoking gun evidence the researchers had been looking for.

In an article at Stanford|News, we can read: "In a discovery that concludes an 80-year quest, Stanford and University of California researchers found evidence of particles that are their own antiparticles. These Majorana fermions could one day help make quantum computers more robust."

► Read the article at Stanford|News: "An experiment proposed by Stanford theorists finds evidence for the Majorana fermion, a particle that’s its own antiparticle">> http://news.stanford.edu/2017/07/20/evidence-particle-antiparticle/

► The scientific paper in Science: "Chiral Majorana fermion modes in a quantum anomalous Hall insulator–superconductor structure">> http://science.sciencemag.org/content/357/6348/294

► The preprint version in arXiv>> https://arxiv.org/ftp/arxiv/papers/1606/1606.05712.pdf

► Image: Majorana fermions (blue, red, and purple lines) travel through a topological insulator (horizontal bar) with a superconductor layered on top in this illustration of new experiments to detect the fermions. Green lines indicate electrons travelling on the edges of the topological insulator.
BEIJING SONDII TECHNOLOGY CO LTD. Source>> https://www.sciencenews.org/article/majorana-fermion-detected-quantum-layer-cake

#Physics, #MajoranaFermions, #Research
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