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  • richardmitnick 2:13 pm on November 21, 2019 Permalink | Reply
    Tags: , , , Cosmic Rays, , , H.E.S.S. Čerenkov Telescope Array located on the Cranz family farm Göllschau in Namibia near the Gamsberg, MAGIC Čerenkov telescopes at the Observatorio del Roque de los Muchachos (Garfia La Palma Spain))   

    From DESY: “Gamma-Ray Bursts with record energy” 

    DESY
    From DESY

    2019/11/20

    First detection of the cosmic monster explosions with ground-based gamma-ray telescopes.

    The strongest explosions in the universe produce even more energetic radiation than previously known: Using specialised telescopes, two international teams have registered the highest energy gamma rays ever measured from so-called gamma-ray bursts, reaching about 100 billion times as much energy as visible light. The scientists of the H.E.S.S. and MAGIC telescopes present their observations in independent publications in the journal Nature.

    A very-high-energy component deep in the γ-ray burst afterglow; The H.E.S.S. collaboration Nature

    Teraelectronvolt emission from the γ-ray burst GRB 190114C; The MAGIC collaboration Nature

    These are the first detections of gamma-ray bursts with ground-based gamma-ray telescopes. DESY plays a major role in both observatories, which are operated under the leadership of the Max Planck Society.

    H.E.S.S. Čerenkov Telescope Array, located on the Cranz family farm, Göllschau, in Namibia, near the Gamsberg searches for cosmic rays, altitude, 1,800 m (5,900 ft)

    MAGIC Čerenkov telescopes at the Observatorio del Roque de los Muchachos (Garfia, La Palma, Spain)), Altitude 2,396 m (7,861 ft)

    Gamma-ray bursts (GRB) are sudden, short bursts of gamma radiation happening about once a day somewhere in the visible universe. According to current knowledge, they originate from colliding neutron stars or from supernova explosions of giant suns collapsing into a black hole. “Gamma-ray bursts are the most powerful explosions known in the universe and typically release more energy in just a few seconds than our Sun during its entire lifetime – they can shine through almost the entire visible universe,” explains David Berge, head of gamma-ray astronomy at DESY. The cosmic phenomenon was discovered by chance at the end of the 1960s by satellites used to monitor compliance with the nuclear test ban on Earth.

    Since then, astronomers have been studying gamma-ray bursts with satellites, as Earth’s atmosphere very effectively absorbs gamma rays. Astronomers have developed specialised telescopes that can observe a faint blue glow called Čerenkov light that cosmic gamma rays induce in the atmosphere, but these instruments are only sensitive to gamma rays with very high energies. Unfortunately, the brightness of gamma-ray bursts falls steeply with increasing energy. Čerenkov telescopes have identified many sources of cosmic gamma rays at very high energies, but no gamma-ray bursts. Satellites, on the other hand, have much too small detectors to be sensitive to the low brightness of gamma-ray bursts at very high energies. So, it was effectively unknown, if the monster explosions emit gamma rays also in the very-high-energy regime.

    Scientists have tried for many years, to catch a gamma-ray burst with Čerenkov telescopes. Then suddenly, between summer 2018 and January 2019, two international teams of astronomers, both involving DESY scientists, detected gamma rays from two GRB events for the first time from the ground. On 20 July 2018, faint afterglow emission of GRB 180720B in the gamma-ray regime was observed with the 28-metre telescope of the High-Energy Stereoscopic System H.E.S.S. in Namibia. On 14 January 2019, bright early emission from GRB 190114C was detected by the Major Atmospheric Gamma Imaging Čerenkov (MAGIC) telescopes on La Palma, and immediately announced to the astronomical community.

    Both observations were triggered by gamma-ray satellites of the US space agency NASA that monitor the sky for gamma-ray bursts and send automatic alerts to other gamma-ray observatories upon detection. “We were able to point to the region of origin so quickly that we could start observing only 57 seconds after the initial detection of the explosion,” reports Cosimo Nigro from the MAGIC group at DESY, who was in charge of the observation shift at that time. “In the first 20 minutes of observation, we detected about thousand photons from GRB 190114C.”

    MAGIC registered gamma-rays with energies between 200 and 1000 billion electron volts (0.2 to 1 teraelectronvolts). “These are by far the highest energy photons ever discovered from a gamma-ray burst,” says Elisa Bernardini, leader of the MAGIC group at DESY. For comparison: visible light is in the range of about 1 to 3 electron volts.

    The rapid discovery allowed to quickly alert the entire observational community. As a result, more than twenty different telescopes had a deeper look at the target. This allowed to pinpoint the details of the physical mechanism responsible for the highest-energy emission, as described in the second paper led by the MAGIC collaboration. Follow-up observations placed GRB 190114C at a distance of more than four billion light years. This means, its light travelled more than four billion years to us, or about a third of the current age of the universe.

    GRB 180720B, at a distance of six billion light years even further away, could still be detected in gamma rays at energies between 100 and 440 billion electron volts long after the initial blast. “Surprisingly, the H.E.S.S. telescope observed a surplus of 119 gamma quanta from the direction of the burst more than ten hours after the explosion event was first seen by satellites,” says Stefan Ohm, head of the H.E.S.S. group at DESY.

    “The detection came quite unexpected, as gamma-ray bursts are fading fast, leaving behind an afterglow which can be seen for hours to days across many wavelengths from radio to X-rays, but had never been detected in very-high-energy gamma rays before,” adds DESY theorist Andrew Taylor, who contributed to the H.E.S.S. analysis. “This success is also due to an improved follow-up strategy in which we also concentrate on observations at later times after the actual star collapse.”

    The detection of gamma-ray bursts at very high energies provides important new insights into the gigantic explosions. “Having established that GRBs produce photons of energies hundreds of billion times higher than visible light, we now know that GRBs are able to efficiently accelerate particles within the explosion ejecta,” says DESY researcher Konstancja Satalecka, one of the scientists coordinating GRB searches in the MAGIC collaboration. “What’s more, it turns out we were missing approximately half of their energy budget until now. Our measurements show that the energy released in very-high-energy gamma-rays is comparable to the amount radiated at all lower energies taken together. That is remarkable!”

    To explain how the observed very-high-energy gamma rays are generated is challenging. Both groups assume a two-stage process: First, fast electrically charged particles from the explosion cloud are deflected in the strong magnetic fields and emit so-called synchrotron radiation, which is of the same nature as the radiation that can be produced in synchrotrons or other particle accelerators on Earth, for example at DESY. However, only under fairly extreme conditions would the synchrotron photons from the explosion be able to reach the very high energies observed. Instead, the scientists consider a second step, where the synchrotron photons collide with the fast particles that generated them, which boosts them to the very high gamma-ray energies recorded. The scientists call the latter step inverse Compton scattering.

    Observation of inverse Compton emission from a long γ-ray burst; The MAGIC CollaborationNature

    “For the first time, the two instruments have measured gamma radiation from gamma-ray bursts from the ground,” concludes Berge. “These two groundbreaking observations have established gamma-ray bursts as sources for terrestrial gamma-ray telescopes. This has the potential to significantly advance our understanding of these violent phenomena.” The scientists estimate that up to ten such events per year can be observed with the planned Čerenkov Telescope Array (CTA), the next generation gamma-ray observatory. The CTA will consist of more than 100 individual telescopes of three types that will be built at two locations in the northern and southern hemispheres. DESY is responsible for the construction of the medium-sized telescopes and will host CTA’s Science Data Management Centre on its campus in Zeuthen. CTA observations are expected to start in 2023 at the earliest.

    ________________________________________
    Background information

    The detection of the very high-energy gamma rays on Earth was achieved with specialised telescopes that do not observe the cosmic gamma rays directly, but rather their effect on Earth’s atmosphere: When an energetic cosmic gamma ray hits Earth’s atmosphere, it shatters molecules and atoms.

    This process creates an avalanche of particles called an air shower.

    Cosmic rays produced by high-energy astrophysics sources (ASPERA collaboration – AStroParticle ERAnet)

    The shower particles are so energetic that they move faster through the air than light – although not faster than light in a vacuum, which according to Albert Einstein’s theory of relativity is the absolute upper speed limit. The result is a bluish glow, a kind of optical counterpart to the supersonic bang. This Čerenkov light, named after its discoverer, can be observed by Čerenkov telescopes such as those of the H.E.S.S. and MAGIC observatories or the planned CTA.

    The H.E.S.S. observations were first announced at the CTA science symposium in May 2019. The MAGIC observations were distributed in an Astronomers’ Telegram (ATel) on 14 January 2019.

    The H.E.S.S. consortium consists of more than 250 researchers from 41 institutes in 12 countries. The MAGIC consortium brings together 280 members from 37 institutes in 12 countries. The MAGIC group at DESY is partially funded by a grant from the Helmholtz Association for excellent women researchers.

    ________________________________________

    See the full article here .


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    desi

    DESY is one of the world’s leading accelerator centres. Researchers use the large-scale facilities at DESY to explore the microcosm in all its variety – from the interactions of tiny elementary particles and the behaviour of new types of nanomaterials to biomolecular processes that are essential to life. The accelerators and detectors that DESY develops and builds are unique research tools. The facilities generate the world’s most intense X-ray light, accelerate particles to record energies and open completely new windows onto the universe. 
That makes DESY not only a magnet for more than 3000 guest researchers from over 40 countries every year, but also a coveted partner for national and international cooperations. Committed young researchers find an exciting interdisciplinary setting at DESY. The research centre offers specialized training for a large number of professions. DESY cooperates with industry and business to promote new technologies that will benefit society and encourage innovations. This also benefits the metropolitan regions of the two DESY locations, Hamburg and Zeuthen near Berlin.

    DESY Petra III interior


    DESY Petra III

    DESY/FLASH

    H1 detector at DESY HERA ring

    DESY DORIS III

     
  • richardmitnick 8:41 am on October 22, 2019 Permalink | Reply
    Tags: "New all-sky search reveals potential neutrino sources", , , , Cosmic Rays, , ,   

    From U Wisconsin IceCube Collaboration: “New all-sky search reveals potential neutrino sources” 

    U Wisconsin ICECUBE neutrino detector at the South Pole

    From From U Wisconsin IceCube Collaboration

    21 Oct 2019
    Madeleine O’Keefe

    For over a century, scientists have been observing very high energy charged particles called cosmic rays arriving from outside Earth’s atmosphere. The origins of these particles are very difficult to pinpoint because the particles themselves do not travel on a straight path to Earth. Even gamma rays, a type of high-energy photon that offers a little more insight, are absorbed when traversing long distances.

    The IceCube Neutrino Observatory, an array of optical modules buried in a cubic kilometer of ice at the South Pole, hunts for cosmic-ray sources inside and outside our galaxy—extending to galaxies more than billions of light years away—using hints from elusive particles called neutrinos. These neutrinos are expected to be produced by cosmic-ray collisions with gas or radiation near the sources.

    Unlike cosmic rays, neutrinos are not absorbed or diverted on their way to Earth, making them a practical tool for locating and understanding cosmic accelerators. If scientists can find a source of high-energy astrophysical neutrinos, this would be a smoking gun for a cosmic-ray source.

    After 10 years of searching for origins of astrophysical neutrinos, a new all-sky search provides the most sensitive probe of time-integrated neutrino emission of point-like sources. The IceCube Collaboration presents the results of this scan in a paper submitted recently to Physical Review Letters.

    1
    The pre-trial probability of the observed signal being due to background in a 5×5 degree window around the most significant point in the Northern Hemisphere (the hottest spot); the black cross marks the Fermi-3FGL coordinates of the galaxy NGC 1068. Credit: IceCube Collaboration

    Tessa Carver led this analysis under the supervision of Teresa Montaruli in the Département de Physique Nucléaire et Corpusculaire at the University of Geneva in Switzerland. “IceCube has already observed an astrophysical flux of neutrinos, so we know they exist and are detectable—we just don’t know exactly where they come from,” says Carver, now a postdoc at Cardiff University. “It is only a matter of time and precision until we can identify the sources behind this neutrino flux.”

    The principle challenge in searching for astrophysical neutrino sources with IceCube is the overwhelming background of events induced by cosmic-ray interactions in our atmosphere. The signal of faint neutrino sources needs to be extracted via sophisticated statistical analysis techniques.

    Using these methods, Carver and her collaborators “scanned” across the entire sky to look for point-like neutrino sources at arbitrary locations. This scanning method is able to identify very bright neutrino sources that could be invisible in gamma rays, which are also produced in cosmic-ray collisions.

    In order to be sensitive to dimmer sources, they also analyzed 110 galactic and extragalactic source candidates, which have been observed via gamma rays. They then combined the results obtained for individual sources in this list in a “population analysis,” which looks for a higher-than-expected rate of significant results from the individual source list search. This allows researchers to find significant neutrino emission, even if sources in the list are too weak to be observed individually.

    Researchers also employed a “stacking search” for three catalogs of gamma-ray sources within our galaxy. This search layers together all the emission from groups of known objects of the same type under the assumption that they have well-known emission properties. While it can significantly reduce the per-source emission required to observe a large excess of signal over the background, this search is limited in that it requires more knowledge of the sources in the catalog.

    2
    Skymap of -log10(plocal), where plocal is the local pre-trial p-value, for the area between ±82 degrees declination in equatorial coordinates. The Northern and Southern Hemisphere hotspots, defined as the most significant plocal in the given hemisphere, are indicated with black circles. Credit: IceCube Collaboration

    While the different analyses did not discover steady neutrino sources, the results are nevertheless exciting: some of the objects in the catalog of known sources showed a higher neutrino flux than expected, with excesses at the 3σ-level. In particular, the all-sky scan revealed that the “hottest” location in the sky is just 0.35 degrees away from the starburst galaxy NGC 1068, which has a 2.9σ excess over background. NGC 1068 is one of the closest black holes to us; it is embedded in a star-forming region with lots of matter for neutrinos to interact with while the high-energy gamma rays are attenuated, as shown by Fermi and MAGIC measurements.

    NASA/Fermi LAT


    NASA/Fermi Gamma Ray Space Telescope

    MAGIC Čerenkov telescopes at the Observatorio del Roque de los Muchachos (Garfia, La Palma, Spain)), Altitude 2,396 m (7,861 ft)

    This is the most significant excess seen besides TXS 0506-056, the 2017 source that IceCube found to be coincident with a gamma ray flare. Still, these potential neutrino sources require more data with a more-sensitive detector, like IceCube-Gen2, to be confirmed.

    The researchers also found that the Northern Hemisphere source catalog as a whole differed from background expectations with a significance of 3.3σ. Carver says these results demonstrate a strong motivation to continue to analyze the objects in the catalog. Time-dependent analyses, which search for flares of peaked emission, and the possibility of correlating neutrino emission with electromagnetic or gravitational wave observations for these and other sources may provide additional evidence of neutrino emission and insights into the neutrinos’ origin. With continued data-taking, more refined direction reconstruction, and the upcoming IceCube Upgrade, further improvements in sensitivity are on the horizon.

    “We are lucky to have the unique opportunity to be the first people to map the universe with neutrinos, which provides a brand-new perspective,” says Carver. “Also, this progress in neutrino astronomy is accompanied by great strides in gravitational wave physics and cosmic-ray physics.”

    Montaruli adds, “While we are at the dawn of a new era in astronomy that observes the universe not just with light, this is the first time we have begun to see potentially significant excesses of candidate neutrino events around interesting extragalactic objects in time-independent searches.”

    See the full article here .

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    IceCube is a particle detector at the South Pole that records the interactions of a nearly massless sub-atomic particle called the neutrino. IceCube searches for neutrinos from the most violent astrophysical sources: events like exploding stars, gamma ray bursts, and cataclysmic phenomena involving black holes and neutron stars. The IceCube telescope is a powerful tool to search for dark matter, and could reveal the new physical processes associated with the enigmatic origin of the highest energy particles in nature. In addition, exploring the background of neutrinos produced in the atmosphere, IceCube studies the neutrinos themselves; their energies far exceed those produced by accelerator beams. IceCube is the world’s largest neutrino detector, encompassing a cubic kilometer of ice.

    IceCube employs more than 5000 detectors lowered on 86 strings into almost 100 holes in the Antarctic ice NSF B. Gudbjartsson, IceCube Collaboration

    Lunar Icecube

    IceCube DeepCore annotated

    IceCube PINGU annotated


    DM-Ice II at IceCube annotated

     
  • richardmitnick 12:31 pm on October 10, 2019 Permalink | Reply
    Tags: , , , , , Cosmic Rays, , ,   

    From AAS NOVA: “Should We Blame Pulsars for Too Much Antimatter?” 

    AASNOVA

    From AAS NOVA

    9 October 2019
    Susanna Kohler

    1
    Artist’s illustration of Geminga, a nearby pulsar that has been proposed to be the source of excess positrons measured at Earth. [Nahks Tr’Ehnl]

    The Earth is constantly being bombarded by cosmic rays — high energy protons and atomic nuclei that speed through space at nearly the speed of light. Where do these energetic particles come from? A new study examines whether pulsars are the source of one particular cosmic-ray conundrum.

    Cosmic rays produced by high-energy astrophysics sources (ASPERA collaboration – AStroParticle ERAnet)

    An Excess of Positrons

    In 2008, our efforts to understand the origin of cosmic rays hit a snag: data from a detector called PAMELA showed that more high-energy positrons were reaching Earth in cosmic rays than theory predicted.

    INFN PAMELA spacecraft


    INFN PAMELA Schematic

    Positrons — the antimatter counterpart to electrons — are thought to be primarily produced by high-energy protons scattering off of particles within our galaxy. These interactions should produce decreasing numbers of positrons at higher energies — yet the data from PAMELA and other experiments show that positron numbers instead go up with increasing energy.

    Something must be producing these extra high-energy positrons — but what?

    Clues from Gamma-rays

    One of the leading theories is that the excess positrons are produced by nearby pulsars — rapidly rotating, magnetized neutron stars. We know that pulsars gradually spin slower and slower over time, losing power as they spew a stream of high-energy electrons and positrons into the surrounding interstellar medium. If the pulsar is close enough to us, positrons produced in and around pulsars might make it to Earth before losing energy to interactions as they travel.

    Women in STEM – Dame Susan Jocelyn Bell Burnell

    Dame Susan Jocelyn Bell Burnell, discovered pulsars with radio astronomy. Jocelyn Bell at the Mullard Radio Astronomy Observatory, Cambridge University, taken for the Daily Herald newspaper in 1968. Denied the Nobel.

    Dame Susan Jocelyn Bell Burnell at work on first plusar chart 1967 pictured working at the Four Acre Array in 1967. Image courtesy of Mullard Radio Astronomy Observatory.

    Dame Susan Jocelyn Bell Burnell 2009

    Dame Susan Jocelyn Bell Burnell (1943 – ), still working from http://www. famousirishscientists.weebly.com

    3
    Observations from the High-Altitude Water Čerenkov (HAWC) Gamma-Ray Observatory show TeV nebulae around pulsars Geminga and PSR B0656+14. But do these sources also have extended GeV nebulae that would provide more direct constraints on positron density? [John Pretz]

    HAWC High Altitude Čerenkov Experiment, located on the flanks of the Sierra Negra volcano in the Mexican state of Puebla at an altitude of 4100 meters(13,500ft), at WikiMiniAtlas 18°59′41″N 97°18′30.6″W. searches for cosmic rays

    Could nearby pulsars produce enough positrons — and could they diffuse out from the pulsars efficiently enough — to account for the high-energy excess we observe here at Earth? A team of scientists now addresses these questions in a new publication led by Shao-Qiang Xi (Nanjing University and Chinese Academy of Sciences).

    To test whether pulsars are responsible for the positrons we see, Xi and collaborators argue that we should look for GeV emission around candidate sources. As the pulsar-produced positrons diffuse outward, they should scatter off of infrared and optical background photons in the surrounding region. This would create a nebula of high-energy emission around the pulsars that glows at 10–500 GeV — detectable by observatories like the Fermi Gamma-ray Space Telescope.

    NASA/Fermi LAT


    NASA/Fermi Gamma Ray Space Telescope

    6
    Fermi LAT gamma-ray count map (top) and residuals after the background is subtracted (bottom) for the region containing Geminga and PSR B0656+14. [Adapted from Xi et al. 2019]

    Two Pulsars Get an Alibi

    Xi and collaborators carefully analyze 10 years of Fermi LAT observations for two nearby pulsars that have been identified as likely candidates for the positron excess: Geminga and PSR B0656+14, located roughly 800 and 900 light-years away from us.

    The result? They find no evidence of extended GeV emission around these sources. The authors’ upper limits on emission from Geminga and PSR B0656+14 give these objects an alibi, suggesting that pulsars can likely account for only a small fraction of the positron excess we observe.

    So where does this leave us? If pulsars are cleared, we will need to look to other candidate sources of high-energy positrons: either other nearby cosmic accelerators like supernova remnants, or more exotic explanations, like the annihilation or decay of high-energy dark matter.

    Citation

    “GeV Observations of the Extended Pulsar Wind Nebulae Constrain the Pulsar Interpretations of the Cosmic-Ray Positron Excess,” Shao-Qiang Xi et al 2019 ApJ 878 104.
    https://iopscience.iop.org/article/10.3847/1538-4357/ab20c9

    See the full article here .


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    1

    AAS Mission and Vision Statement

    The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

    The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
    The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
    The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
    The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
    The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

    Adopted June 7, 2009

     
  • richardmitnick 5:29 pm on September 14, 2019 Permalink | Reply
    Tags: , , , Cosmic Rays, , ,   

    From Symmetry: “A new way to study high-energy gamma rays” 

    From Symmetry

    09/03/19
    Jim Daley

    The Čerenkov Telescope Array will combine experimental and observatory-style approaches to investigate the universe’s highest energies.

    1

    They permeate the cosmos, whizzing through galaxies and solar systems at energies far higher than what even our most powerful particle accelerators can achieve. Emitted by sources such as far-distant quasars, or, closer to Earth, occasionally ejected from the remnants of supernovae, high-energy cosmic rays are believed to play a role in the evolution of galaxies and the growth of black holes.

    Exactly how cosmic rays originate remains a mystery. Now, an ambitious project—part observatory, part experiment—is preparing to investigate them by studying the gamma rays they produce at sensitivities never achieved before.

    The Čerenkov Telescope Array being built in Chile and Spain’s Canary Islands is the newest generation of ground-based gamma-ray detectors. CTA involves collaborators from 31 countries and comprises more than 100 telescopes of varying sizes. Its detectors will be 10 times more sensitive to gamma rays than existing instruments, which will allow scientists to investigate their properties at a breathtaking range of energy levels—from about 20 billion electronvolts up to 300 trillion electronvolts. This is far above current capabilities: Existing gamma-ray observatories’ energy ranges top out at about 50 trillion electronvolts.

    Rene Ong, an astrophysicist at UCLA and the co-spokesperson for the project, says that CTA is unique in that it will function as both an experiment—zeroing in to investigate specific points and topics of interest—and an observatory—creating an overall record of a portion of the night sky over time.

    It will be the first ground-based gamma-ray observatory, and users will be granted observatory access time for their own projects in a proposal-driven program. “CTA will operate like an astronomical facility with a mix of guest-observer time, dedicated time for major observation projects, and time reserved for the CTA observatory director,” he says.

    Part of what makes CTA an astronomical observatory is that it will make its data freely available, explains Ulisses Barres, an astrophysicist at the Brazilian Center for Physical Research who is leading part of that country’s contribution to CTA’s design and construction.

    Until now, very-high-energy gamma-ray band astronomy research has been conducted by “closed” research groups, which have reserved most or all of their data for their own use. CTA will not only make its data public; just like a typical observatory, it will also structure its data to make it accessible even to nonspecialists and people in other scientific fields.

    “That’s because CTA wants to kind of kick-start astronomy in the [high-energy gamma-ray] band in a new way,” Barres says. “People from other fields can request data from CTA in a competitive way and analyze it, pretty much like what an experimental telescope does.”

    Elisabete de Gouveia Dal Pino, an astrophysicist at the University of São Paulo and also one of the leaders of the CTA Consortium in Brazil, says the project’s design will allow scientists to investigate some of the most energetic events that occur anywhere in the universe. These events are theorized to come mostly from compact sources like supermassive black holes and supernovae explosions.

    “There is a whole slew of processes and particles that we can decipher [by] observing the universe in gamma rays,” Dal Pino says. Other wavelengths have already been probed and are well-developed fields of study, she explains. “This is the last energy band window that we are currently able to open on the universe right now.”

    CTA may also test physics beyond the Standard Model, Ong says. In particular, it will search for dark matter, which scientists think makes up 85% of the known matter in the universe but has yet to be detected, let alone fully understood. It’s possible that gamma rays are produced when dark matter particles bump into one another and self-annihilate.

    CTA’s dark matter program will attempt to discover the nature of this phenomenon by observing the galactic halo, a roughly spherical, thinly populated area that surrounds the visible galaxy and is believed to be home to these particles.

    For now, the project is still in its design and construction phase. Barres says he expects a “critical mass” of telescopes—enough to begin taking useable data—in the northern hemisphere by 2022. “We expect that by the middle of the next decade, CTA may already be fully operational,” he says. “For now, there is a lot of coordination to be done among the partner institutions.”

    See the full article here .


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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 12:17 pm on August 31, 2019 Permalink | Reply
    Tags: , , , Cosmic Rays, , H.E.S.S. Čerenkov Telescope Array located on the Cranz family farm Göllschau in Namibia, MAGIC Čerenkov telescopes at the Observatorio del Roque de los Muchachos (Garfia La Palma Spain)) Altitude 2396 m (7861 ft), , Čerenkov Telescope Array composed of hundreds of similar telescopes to be situated in the Canary Islands and Chile.   

    From Ethan Siegel: “Cosmic Rays Are More Energetic Than LHC Particles, And This Faster-Than-Light Trick Reveals Them” 

    From Ethan Siegel
    Aug 30, 2019

    1
    Cosmic rays, which are ultra-high energy particles originating from all over the Universe, strike protons in the upper atmosphere and produce showers of new particles. The fast-moving charged particles also emit light due to Cherenkov radiation as they move faster than the speed of light in Earth’s atmosphere, and produce secondary particles that can be detected here on Earth. (SIMON SWORDY (U. CHICAGO), NASA)

    Stronger than the LHC and faster than anything except light, the world’s cleverest particle detector sees the particles we could never create on Earth.

    It might be true that there’s an ultimate speed limit to everything in the Universe — the speed of light in a vacuum — but that doesn’t mean there’s a limit to how energetic a single particle can be. As you pump progressively more energy into a massive particle, you can make it move ever faster, asymptotically approaching that ultimate cosmic speed limit. But paradoxically, the more energetic that particle is, the more difficult it is to accurately detect and measure it.

    The reason is straightforward: in order to measure how energetic an initial particle is, you need the energy of its decay and debris products to get deposited in your detector, enabling you to reconstruct its original energy, mass, charge, and so on. Building a larger, more massive detector simply won’t work at cosmic ray energies, which can be millions of times that reached at the LHC. But by slowing down the speed of light, physicists can leverage an incredible trick to measure these cosmic energies after all.

    Here’s how.

    2
    The CMS Collaboration, whose detector is shown prior to final assembly here, is one of the largest, most dense detectors ever constructed. The particles that collide in the center will make tracks and leave debris that deposits energy into the detector, enabling scientists to reconstruct the properties and energies of any particles that were created during the process. This method is woefully inadequate for measuring the energies of cosmic rays. (CERN/MAXIMILIEN BRICE)

    When you increase the energy of a particle, it becomes easier and easier for that particle to interact with another one. Any interaction has a chance to either spontaneously create new particles and antiparticles — via Einstein E = mc² — or to emit a quantum of radiation: a photon. The faster a particle goes, the more likely it is to interact in such a way that it will emit additional particles, losing energy in the process of doing so.

    When you think of ways to make the most energetic particles, the electromagnetic force reigns supreme. Whenever you place a charged particle in an electric field, it accelerates in the direction of the field; whenever you place one in a magnetic field, accelerates perpendicular to both the field direction and the particle’s current motion. The strongest natural accelerators in the Universe aren’t located on Earth, but rather in extreme astrophysical environments: around neutron stars and black holes.

    3
    This artist’s impression depicts the surroundings of a black hole, showing an accretion disc of superheated plasma and a relativistic jet. We have not yet determined whether black holes have their own magnetic field, independent of the matter outside of it. Many of the highest energy cosmic rays have been associated with black hole or neutron star sources. (NICOLLE R. FULLER/NSF)

    Here on Earth, we’ve used particle accelerators to bring objects like protons and electrons as close to the speed of light as laboratory conditions allow, and have gotten awfully close to that ultimate cosmic speed limit set forth by Einstein way back in 1905: c, or 299,792,458 m/s. But as fast and as energetic as we’ve gotten them, they simply don’t compare to the energies of the cosmic rays we’ve seen.

    Fastest Fermilab proton: 980 GeV; 99.999954% the speed of light; 299,792,320 m/s.
    Fastest LHC proton: 7 TeV; 99.999990% the speed of light; 299,792,455 m/s.
    Fastest LEP electron (fastest terrestrial accelerator particle): 105 GeV; 99.9999999988% the speed of light; 299,792,457.9964 m/s.
    Fastest cosmic ray proton: 5 × 10¹⁰ GeV; 99.999999999999999999973% the speed of light; 299,792,457.99999999999992 m/s.

    Earth-based accelerators don’t stand a chance when compared to the absolute fastest particles of all; they aren’t in the same league.

    4
    The galaxy NGC 1275, as imaged by Hubble, shows incredible signs of an active, feeding black hole at its center. The high-energy radiation and particles emitted from this active galaxy are only one of many examples of astrophysical phenomena whose energies far exceed anything we’ve ever generated on Earth. (NASA, ESA, HUBBLE HERITAGE (STScI/AURA))

    We might be able to control electric and magnetic fields incredibly well under laboratory conditions, but our terrestrial energies are limited by the physical constraints of the electromagnets and accelerator facilities we build here on Earth. They’re certainly impressive, but they’re no match for the laboratory of the Universe.

    Black holes, neutron stars, merging stellar systems, supernovae and other astrophysical cataclysms can accelerate particles to energies we’d never be able to equal on Earth. The highest energy cosmic rays travel so close to the ultimate cosmic speed limit, c, that if you were to race an ultra-high energy, cosmic ray proton against a photon to the nearest star and back, do you know what would happen? Over a round-trip journey of nearly 8.5 light years, the photon would arrive first, but just barely. The proton would be a mere 22 microns behind, arriving just 0.7 picoseconds later.

    5
    A portion of the digitized sky survey with the nearest star to our Sun, Proxima Centauri, shown in red in the center. While sun-like stars like our own are considered common, we’re actually more massive than 95% of stars in the Universe, with a full 3-out-of-4 stars in Proxima Centauri’s ‘red dwarf’ class. Barnard’s star, the second-nearest star system after the Alpha Centauri system, is an M-class star as well. (DAVID MALIN, UK SCHMIDT TELESCOPE, DSS, AAO)

    These ultra-high-energy cosmic rays are generated by numerous sources throughout the Universe, and they travel in all directions. Occasionally, one of these particles will have just the right trajectory to strike the Earth. When that serendipitous event occurs, that’s our big chance. That’s our opportunity to measure the energy of the particles that make it down to the ground, and to reconstruct the properties of the original cosmic rays.

    The reason we can do it at all, though, is because we have an atmosphere surrounding the Earth. At hundreds of kilometers thick, this atmosphere acts like a medium, rather than a perfectly pure vacuum. While the speed of light in a vacuum may be fixed and immutable — 299,792,458 m/s — the speed of light in a medium is always slower. Even air, which is pretty close to a vacuum, slows light down to “only” 99.97% of its vacuum speed.

    6
    The Advanced Test Reactor core at Idaho National Laboratory isn’t glowing blue because there are any blue lights involved, but rather because this is a nuclear reactor producing relativistic, charged particles that are surrounded by water. When the particles pass through that water, they exceed the speed of light in that medium, causing them to emit Čerenkov radiation, which appears as this glowing blue light. (ARGONNE NATIONAL LABORATORY)

    A slowdown of 0.03% isn’t that much, but it does enable something remarkable: the high-energy particles that come into contact with our atmosphere will find themselves moving faster than the speed of light in this medium. When that occurs, they emit a special type of radiation: blue light that gets emitted at a specific angle in a cone-like shape, known as Čerenkov radiation.

    Nuclear reactors, which emit fast-moving particles that could potentially irradiate a human being, are surrounded by water for this exact purpose. They shield people from the particles the reactor emits, as these particles are slowed down by the water, emitting a harmless blue light instead. Energy is energy, and by taking it away from the particles themselves and converting it into light, it’s a great way to ensure the safety of those nearby.

    When a cosmic ray strikes our atmosphere, it’s moving much faster than any particle a nuclear reactor will generate, but the physics is very much the same. The Čerenkov radiation emitted will occur at a specific frequency, calculable dependent on the cosmic ray’s energy range. This radiation will be composed of gamma rays, and because it’s created at such a high altitude (hundreds of kilometers up), it will require an enormous array of ground-based telescopes sensitive to gamma rays to detect.

    The idea, then, would be to build a Čerenkov Telescope Array, capable of detecting this light from all over the Earth. When you see even a fraction of the appropriate cone and can trace it back to an individual particle, you can reconstruct its properties in a completely new fashion. Although this is just a proposed project, construction is expected to begin before this year is out.

    H.E.S.S. Čerenkov Telescope Array, located on the Cranz family farm, Göllschau, in Namibia, near the Gamsberg searches for cosmic rays, altitude, 1,800 m (5,900 ft)

    A novel gamma ray telescope under construction on Mount Hopkins, Arizona. A large project known as the Čerenkov Telescope Array, composed of hundreds of similar telescopes to be situated in the Canary Islands and Chile. The telescope on Mount Hopkins will be fitted with a prototype high-speed camera, assembled at the University of Wisconsin–Madison, and capable of taking pictures at a billion frames per second. Credit: Vladimir Vassiliev

    At present, there are many gamma-ray observatories that also work as telescopes, providing what you might call “atmospheric imaging” of these high-energy particles that strike our planet. Observatories such as H.E.S.S. [above], MAGIC and VERITAS have all provided locations and energies for the sources of these high-energy cosmic rays like never before.

    MAGIC Čerenkov telescopes at the Observatorio del Roque de los Muchachos (Garfia, La Palma, Spain)), Altitude 2,396 m (7,861 ft)

    CfA/VERITAS, a major ground-based gamma-ray observatory with an array of four 12m optical reflectors for gamma-ray astronomy in the GeV – TeV energy range. Located at Fred Lawrence Whipple Observatory,Mount Hopkins, Arizona, US in AZ, USA, Altitude 2,606 m (8,550 ft)

    Moving to the Čerenkov Telescope Array will be a tremendous advance. All told, the array is anticipated to consist of 118 dishes: 19 in the northern hemisphere (focusing on lower energies and extragalactic sources), and 99 in the southern hemisphere, focusing on the full spectrum of energies and sources within our own galaxy. At present, 32 countries are involved in this consortium, which is a $300 million endeavor. ESO’s Paranal–Armazones site in the Atacama Desert of Chile will host the greatest number of dishes.

    This isn’t the only mechanism by which we can measure cosmic rays, as when they strike the particles in Earth’s atmosphere, they’ll also produce new particles. These “particle showers” can produce relics that make it down to Earth, and particle-based observatories can be complementary to light-based observatories that observe the associated Čerenkov radiation.

    But the Čerenkov telescopes offer something that the particle-based methods don’t: by only measuring a fraction of what reaches Earth, the energy and trajectory of the incoming particles can be accurately reconstructed. If you wanted to do that with particle-based detectors, you’d need to ensure you were receiving and accurately measuring the energy and momentum from 100% of the particles created in a shower. Even world-class cosmic ray detectors, like the Pierre Auger Observatory, can’t live up to that ambition.

    8
    Cosmic rays produced by high-energy astrophysics sources can reach Earth’s surface. When a cosmic ray collides with a particle in Earth’s atmosphere, it produces a shower of particles that we can detect with arrays on the ground, but even in the absence of particle showers, Čerenkov radiation will be emitted as well. (ASPERA COLLABORATION / ASTROPARTICLE ERANET)

    The other option would be to catch these cosmic ray particles before they ever reached the Earth; you’d need to go to space to see them. But even if you did that, you’d be limited by the sensitivity of your detector and the amount of energy that could be directly deposited within it. Going to space also comes with a tremendous launch cost; the Fermi gamma ray telescope, which detects individual high-energy photons rather than cosmic rays directly, cost approximately $690 million, more than twice the projected cost of the entire Čerenkov Telescope Array.

    Instead, by catching the particles and photons that result from a cosmic ray striking the atmosphere in over 100 locations across the globe, we can come to understand the origin and properties of these ultra-relativistic particles, as well as the astrophysical sources that create them. All of this is possible because we understand the physics of particles moving faster-than-light in one special medium: Earth’s atmosphere. Einstein’s laws might be unbreakable, but the trick of slowing light down enables us to detect something very cleverly that we wouldn’t be able to measure otherwise!

    See the full article here .

    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

    Stem Education Coalition

    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 1:19 pm on July 31, 2019 Permalink | Reply
    Tags: , , , , Cosmic Rays, ,   

    From Ethan Siegel: “How To Prove Einstein’s Relativity In The Palm Of Your Hand” 

    From Ethan Siegel
    July 31, 2019

    1
    Cosmic rays, which are ultra-high energy particles originating from all over the Universe, strike protons in the upper atmosphere and produce showers of new particles. The fast-moving charged particles also emit light due to Cherenkov radiation as they move faster than the speed of light in Earth’s atmosphere, and produce secondary particles that can be detected here on Earth. (SIMON SWORDY (U. CHICAGO), NASA)

    Supernova remnant Crab nebula. NASA/ESA Hubble


    Supernova remnant Crab nebula. NASA/ESA Hubble

    3
    The Tibet ABγ experiment is located at an altitude of 4300 m in Yangbajing, China. With an array of scintillation detectors and underwater water-Cherenkov detectors covering 65,700 m2, the experiment measures the secondary particles that are produced when a high-energy photon strikes the upper atmosphere. An artist’s depiction of such an air shower is shown in the image. Tibet ASγ Collaboration.

    Particle physics is everywhere, even in the palm of your hand.

    When you hold out your palm and point it towards the sky, what is it that’s interacting with your hand? You might correctly surmise that there are ions, electrons and molecules all colliding with your hand, as the atmosphere is simply unavoidable here on Earth. You might also remember that photons, or particles of light, must be striking you, too.

    But there’s something more striking your hand that, without relativity, simply wouldn’t be possible. Every second, approximately one muon — the unstable, heavy cousin of the electron — passes through your outstretched palm. These muons are made in the upper atmosphere, created by cosmic rays. With a mean lifetime of 2.2 microseconds, you might think the ~100+ km journey to your hand would be impossible. Yet relativity makes it so, and the palm of your hand can prove it. Here’s how.

    3
    While cosmic ray showers are common from high-energy particles, it’s mostly the muons which make it down to Earth’s surface, where they are detectable with the right setup. (ALBERTO IZQUIERDO; COURTESY OF FRANCISCO BARRADAS SOLAS)

    Individual, subatomic particles are almost always invisible to human eyes, as the wavelengths of light we can see are unaffected by particles passing through our bodies. But if you create a pure vapor made out of 100% alcohol, a charged particle passing through it will leave a trail that can be visually detected by even as primitive an instrument as the human eye.

    As a charged particle moves through the alcohol vapor, it ionizes a path of alcohol particles, which act as centers for the condensation of alcohol droplets. The trail that results is both long enough and long-lasting enough that human eyes can see it, and the speed and curvature of the trail (if you apply a magnetic field) can even tell you what type of particle it was.

    This principle was first applied in particle physics in the form of a cloud chamber.

    4
    A completed cloud chamber can be built in a day out of readily-available materials and for less than $100. You can use it to prove the validity of Einstein’s relativity, if you know what you’re doing! (INSTRUCTABLES USER EXPERIENCINGPHYSICS)

    Today, a cloud chamber can be built, by anyone with commonly available parts, for a day’s worth of labor and less than $100 in parts. (I’ve published a guide here.) If you put the mantle from a smoke detector inside the cloud chamber, you’ll see particles emanate from it in all directions and leave tracks in your cloud chamber.

    That’s because a smoke detector’s mantle contains radioactive elements such as Americium, which decays by emitting α-particles. In physics, α-particles are made up of two protons and two neutrons: they’re the same as a helium nucleus. With the low energies of the decay and the high mass of the α-particles, these particles make slow, curved tracks and can even be occasionally seen bouncing off of the cloud chamber’s bottom. It’s an easy test to see if your cloud chamber is working properly.

    5
    For an extra bonus of radioactive tracks, add the mantle of a smoke detector to the bottom of your cloud chamber, and watch the slow-moving particles emanating outward from it. Some will even bounce off the bottom! (NASA/GRC/BILL BOWLES)

    If you build a cloud chamber like this, however, those α-particle tracks aren’t the only things you’ll see. In fact, even if you leave the chamber completely evacuated (i.e., you don’t put a source of any type inside or nearby), you’ll still see tracks: they’ll be mostly vertical and appear to be perfectly straight.

    This is because of cosmic rays: high-energy particles that strike the top of Earth’s atmosphere, producing cascading particle showers. Most of the cosmic rays are made up of protons, but move with a wide variety of speeds and energies. The higher-energy particles will collide with particles in the upper atmosphere, producing particles like protons, electrons, and photons, but also unstable, short-lived particles like pions. These particle showers are a hallmark of fixed-target particle physics experiments, and they occur naturally from cosmic rays, too.

    6
    Although there are four major types of particles that can be detected in a cloud chamber, the long and straight tracks are the cosmic ray muons, which can be used to prove that special relativity is correct. (WIKIMEDIA COMMONS USER CLOUDYLABS)

    The thing about pions is that they come in three varieties: positively charged, neutral, and negatively charged. When you make a neutral pion, it just decays into two photons on very short (~10–16 s) timescales. But charged pions live longer (for around 10–8 s) and when they decay, they primarily decay into muons, which are point particles like electrons but have 206 times the mass.

    Muons also are unstable, but they’re the longest-lived unstable fundamental particle as far as we know. Owing to their relatively small mass, they live for an astoundingly long 2.2 microseconds, on average. If you were to ask how far a muon could travel once created, you might think to multiply its lifetime (2.2 microseconds) by the speed of light (300,000 km/s), getting an answer of 660 meters. But that leads to a puzzle.

    7
    Cosmic ray shower and some of the possible interactions. Note that if a charged pion (left) strikes a nucleus before it decays, it produces a shower, but if it decays first (right), it produces a muon that will reach the surface. (KONRAD BERNLÖHR OF THE MAX-PLANCK-INSTITUTE AT HEIDELBERG)

    I told you earlier that if you hold out the palm of your hand, roughly one muon per second passes through it. But if they can only live for 2.2 microseconds, they’re limited by the speed of light, and they’re created in the upper atmosphere (around 100 km up), how is it possible for those muons to reach us?

    You might start to think of excuses. You might imagine that some of the cosmic rays have enough energy to continue cascading and producing particle showers during their entire journey to the ground, but that’s not the story the muons tell when we measure their energies: the lowest ones are still created some 30 km up. You might imagine that the 2.2 microseconds is just an average, and maybe the rare muons that live for 3 or 4 times that long will make it down. But when you do the math, only 1-in-1050 muons would survive down to Earth; in reality, nearly 100% of the created muons arrive.

    8
    A light-clock, formed by a photon bouncing between two mirrors, will define time for any observer. Although the two observers may not agree with one another on how much time is passing, they will agree on the laws of physics and on the constants of the Universe, such as the speed of light. When relativity is applied correctly, their measurements will be found to be equivalent to one another, as the correct relativistic transformation will allow one observer to understand the observations of the other. (JOHN D. NORTON)

    How can we explain such a discrepancy? Sure, the muons are moving close to the speed of light, but we’re observing them from a reference frame where we’re stationary. We can measure the distance the muons travel, we can measure the time they live for, and even if we give them the benefit of the doubt and say that they’re moving at (rather than near) the speed of light, they shouldn’t even make it for 1 kilometer before decaying.

    But this misses one of the key points of relativity! Unstable particles don’t experience time as you, an external observer, measures it. They experience time according to their own onboard clocks, which will run slower the closer they move to the speed of light. Time dilates for them, which means that we will observe them living longer than 2.2 microseconds from our reference frame. The faster they move, the farther we’ll see them travel.

    9
    One revolutionary aspect of relativistic motion, put forth by Einstein but previously built up by Lorentz, Fitzgerald, and others, that rapidly moving objects appeared to contract in space and dilate in time. The faster you move relative to someone at rest, the greater your lengths appear to be contracted, while the more time appears to dilate for the outside world. This picture, of relativistic mechanics, replaced the old Newtonian view of classical mechanics, and can explain the lifetime of a cosmic ray muon. (CURT RENSHAW)

    How does this work out for the muon? From its reference frame, time passes normally, so it will only live for 2.2 microseconds according to its own clocks. But it will experience reality as though it hurtles towards Earth’s surface extremely close to the speed of light, causing lengths to contract in its direction of motion.

    If a muon moves at 99.999% the speed of light, every 660 meters outside of its reference frame will appear as though it’s just 3 meters in length. A journey of 100 km down to the surface would appear to be a journey of 450 meters in the muon’s reference frame, taking up just 1.5 microseconds of time according to the muon’s clock.

    10
    At high enough energies and velocities, relativity becomes important, allowing many more muons to survive than would without the effects of time dilation. (FRISCH/SMITH, AM. J. OF PHYS. 31 (5): 342–355 (1963) / WIKIMEDIA COMMONS USER D.H)

    This teaches us how to reconcile things for the muon: from our reference frame here on Earth, we see the muon travel 100 km in a timespan of about 4.5 milliseconds. This is just fine, because time is dilated for the muon and lengths are contracted for it: it sees itself as traveling 450 meters in 1.5 microseconds, and hence it can remain alive all the way down to its destination of Earth’s surface.

    Without the laws of relativity, this cannot be explained! But at high velocities, which correspond to high particle energies, the effects of time dilation and length contraction enable not just a few but mostof the created muons to survive. This is why, even all the way down here at the surface of the Earth, one muon per second still appears to pass through your upturned, outstretched hand.

    If you ever doubted relativity, it’s hard to fault you: the theory itself seems so counterintuitive, and its effects are thoroughly outside the realm of our everyday experience. But there is an experimental test you can perform right at home, cheaply and with just a single day’s efforts, that allow you see the effects for yourself.

    You can build a cloud chamber, and if you do, you will see those muons. If you installed a magnetic field, you’d see those muon tracks curve according to their charge-to-mass ratio: you’d immediately know they weren’t electrons. On rare occasion, you’d even see a muon decaying in mid-air. And, finally, if you measured their energies, you’d find that they were moving ultra-relativistically, at 99.999%+ the speed of light. If not for relativity, you wouldn’t see a single muon at all.

    Time dilation and length contraction are real, and the fact that muons survive, from cosmic ray showers all the way down to Earth, prove it beyond a shadow of a doubt.

    See the full article here .

    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

    Stem Education Coalition

    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 4:01 pm on February 18, 2019 Permalink | Reply
    Tags: A new technique dubbed STeVE for “starting TeV events, A second technique called LESE for low-energy starting events, , , , Both of these techniques introduce a new online event selection filter that selects starting events based on an initial fast reconstruction, Cosmic Rays, , Gamma-ray emission, However gamma rays can also be produced in environments where neutrino emission would be disfavored, , Searches combining both techniques result in an effective area comparable to ANTARES which thanks to its location in the Mediterranean Sea has a priori a better neutrino view of our galaxy, STeVE and LESE where tested with 3 and 4 years of IceCube data respectively, The gamma-ray galactic sky shows a large concentration of sources in the Southern Hemisphere, The highest energy gamma rays could be produced in the same mechanisms that produce the highest energy neutrinos,   

    From U Wisconsin IceCube Collaboration: “Improving searches for galactic sources of high-energy neutrinos” 

    U Wisconsin ICECUBE neutrino detector at the South Pole

    18 Feb 2019
    Sílvia Bravo

    The search for sources of high-energy neutrinos and cosmic rays has revealed neutrinos from distant galaxies and from all over the sky traveling through the Antarctic ice. Closer sources, though, those that could produce neutrino emission in the Milky Way, have been more elusive.

    In IceCube, the signature of sources such as galactic supernova remnants peaks at low energies, well below 100 TeV, where the large background of atmospheric muons is difficult to filter out. The bulk of galactic neutrino emission is expected in the southern sky, where the Earth cannot serve as a natural filter to remove the million-to-one muon-neutrino signal. In a recent paper by the IceCube Collaboration, two new techniques improve searches at energies from 100 TeV down to 100 GeV. When tested with a few years of IceCube data, these new selections improve the sensitivity and discovery potential, allowing for the first time the search for galactic point-like sources using track events created by muon neutrinos that in many cases are indistinguishable from atmospheric muon tracks. These results have just been submitted to the journal Astroparticle Physics.

    1
    The differential discovery potential at −60° declination for LESE (light blue), STeVE (dark blue), the combined selection (LESE +STeVE) (red), a cascade point-source search (gray), a starting tracks search targeting higher energies (MESE) (gray dashed), throughgoing (light gray dashed), all with the IceCube detector, and of the ANTARES point-like source search (black). In this plot, all results are calculated for an equal three-year exposure. Image: IceCube Collaboration

    Scientists have speculated that at high energies neutrino emission should be associated with gamma-ray emission, since the highest energy gamma rays could be produced in the same mechanisms that produce the highest energy neutrinos. However, gamma rays can also be produced in environments where neutrino emission would be disfavored.

    The gamma-ray galactic sky shows a large concentration of sources in the Southern Hemisphere, where both the galactic center and the majority of the galactic plane are seen from Earth. This is, thus, a region worth exploring with IceCube to look for potential neutrino emission from the same sources that produce the gamma rays.

    However, the most successful searches for high-energy neutrinos select particle interactions that start in the detector—both cascade- and track-like events—or track-like events that come from the northern sky. Track-like events are those that provide a good pointing resolution, which on average is well below 1 degree.

    In previous searches for astrophysical neutrinos using events with the interaction vertex within the detector, a fairly high energy cut was also applied to obtain an efficient selection. The concern is that the majority of galactic neutrino emission could happen at lower energies and, thus, might be removed with this cut. To lower this energy threshold and still preserve a good pointing resolution in the southern sky, researchers have looked closer at track events in IceCube.

    In a new technique dubbed STeVE, for “starting TeV events,” the selection focuses on neutrino events between 10 and 100 TeV and uses techniques developed in a previous IceCube analysis (link to MESE news 414) to remove the background of multiple parallel atmospheric muon events, which has proved to be a resistant background at low energies. In addition, this event selection strategy exploits the difference in the observed photon pattern of bundles of low-energy atmospheric muons compared to individual high-energy muons.

    In a second technique, called LESE, for low-energy starting events, the selection was optimized for neutrinos below 10 TeV. At low energies and due to the small granularity of the IceCube detector, with strings of sensors deployed at horizontal distances of 125 meters, it’s easier for muon tracks to enter the detector without significant energy deposition detected by the outer layers of sensors, which mimics a muon neutrino interacting within the detector volume. LESE aims at selecting track-like events with energies as low as 100 GeV, leveraging the experience gained with veto-based selection techniques in searches for dark matter.

    Both of these techniques introduce a new online event selection filter that selects starting events based on an initial fast reconstruction. This new filter is the first to accept starting events from the entire southern sky while maintaining as large as possible active detector volume.

    STeVE and LESE where tested with 3 and 4 years of IceCube data, respectively, in a search for sources of astrophysical neutrinos anywhere in the southern sky and for neutrino emission from the direction of 96 known gamma-ray sources. No significant excess of neutrino emission was found, but the techniques have proven to be sensitive to strong galactic sources of low-energy astrophysical neutrinos.

    “Studying starting events from the southern sky at these energies poses many new challenges,” explains Rickard Ström, a main analyzer who worked on this study as a PhD candidate at Uppsala University. “We leveraged expertise from previous searches for point sources and exotic signatures such as dark matter. This was the first time IceCube was able to study point sources in the southern sky at these energies and using tracks with degree precision,” adds Ström.

    Searches combining both techniques result in an effective area comparable to ANTARES, which thanks to its location in the Mediterranean Sea has a priori a better neutrino view of our galaxy. STeVE and LESE selections reduce the muon background to a few thousand events per year and significantly improve IceCube’s sensitive and discovery potential of point-like sources in the southern sky with neutrinos with energies below 100 TeV.

    From From U Wisconsin IceCube Collaboration

    See the full article here .

    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

    Stem Education Coalition
    IceCube is a particle detector at the South Pole that records the interactions of a nearly massless sub-atomic particle called the neutrino. IceCube searches for neutrinos from the most violent astrophysical sources: events like exploding stars, gamma ray bursts, and cataclysmic phenomena involving black holes and neutron stars. The IceCube telescope is a powerful tool to search for dark matter, and could reveal the new physical processes associated with the enigmatic origin of the highest energy particles in nature. In addition, exploring the background of neutrinos produced in the atmosphere, IceCube studies the neutrinos themselves; their energies far exceed those produced by accelerator beams. IceCube is the world’s largest neutrino detector, encompassing a cubic kilometer of ice.

    IceCube employs more than 5000 detectors lowered on 86 strings into almost 100 holes in the Antarctic ice NSF B. Gudbjartsson, IceCube Collaboration

    Lunar Icecube

    IceCube DeepCore annotated

    IceCube PINGU annotated


    DM-Ice II at IceCube annotated

     
  • richardmitnick 6:03 pm on December 17, 2018 Permalink | Reply
    Tags: Cosmic Rays, , ,   

    From U Wisconsin IceCube Collaboration: “IceCube and HAWC unite efforts to dissect the cosmic-ray anisotropy” 

    U Wisconsin ICECUBE neutrino detector at the South Pole

    IceCube employs more than 5000 detectors lowered on 86 strings into almost 100 holes in the Antarctic ice NSF B. Gudbjartsson, IceCube Collaboration

    Lunar Icecube

    IceCube DeepCore annotated

    IceCube PINGU annotated


    DM-Ice II at IceCube annotated

    From From U Wisconsin IceCube Collaboration

    17 Dec 2018
    Sílvia Bravo

    HAWC High Altitude Cherenkov Experiment, located on the flanks of the Sierra Negra volcano in the Mexican state of Puebla at an altitude of 4100 meters(13,500ft), at WikiMiniAtlas 18°59′41″N 97°18′30.6″W. searches for cosmic rays

    It was only a few years ago that IceCube provided the first view of the arrival direction distribution of cosmic rays in the Southern Hemisphere. Observations in the Northern Hemisphere, including those from the HAWC gamma-ray observatory earlier this year, had already shown that the number of cosmic rays hitting the atmosphere varied depending on their direction and energy. The anisotropy patterns found in the Southern Hemisphere supported models that pointed to the local interstellar magnetic field as the origin of the dominant effects of this observation.

    In an attempt to better understand the anisotropy, the IceCube Neutrino Observatory and HAWC have united their efforts to study cosmic-ray arrival directions in both hemispheres at the same primary energy. The goal of this combined observation was to get a nearly full-sky coverage to study the propagation of cosmic rays with median energy of 10 TeV through our local interstellar medium as well as the interactions between interstellar and heliospheric magnetic fields. Results have just been accepted for publication in The Astrophysical Journal and include measurements on how the anisotropy modulations are distributed over different angular scales.

    1
    he all-sky distribution in relative intensity of 10 TeV cosmic rays (CR) obtained with the HAWC and IceCube observation. Blue means deficit with respect to the mean CR flux and red excess. On the left, the white arrow indicates the direction of motion of the solar system through the local interstellar medium; the black lines indicate the local interstellar magnetic field lines outside of the heliosphere. On the right, the view of the opposite side of the sky.

    Cosmic rays swirling through space constantly bombard Earth from every direction. Out of every 1,000 cosmic rays there is at most one cosmic ray with a preferred (nonrandom) arrival direction. We refer to this as anisotropy, and this tiny 0.1% effect is what scientists would like to decipher.

    The variations are small but significant and show two different amplitude scales, a large-scale anisotropy with variations of one per mille and a small-scale anisotropy with variations of one per ten thousand.

    The cosmic-ray anisotropy is associated with the distribution of the cosmic ray sources and with the properties of the magnetic fields through which the cosmic rays propagate. However, the limited field of view of any ground-based experiment prevents us from capturing the anisotropy features that are wider than the observable sky.

    The angular variations of this anisotropy support the contribution of two different mechanisms: the mean propagation along the turbulent interstellar magnetic field, which is expected to isotropically diffuse cosmic rays, and the deflection in nearby magnetic fields—the local interstellar magnetic field (LIMF) and the heliosphere—whose relative contribution depends on energy.

    Ground-based experiments typically require averaging the number of cosmic rays along each declination band, to estimate its response to a perfectly isotropic flux. This has the effect of washing out the vertical (north-south) component of the anisotropy. On the other hand, the heliospheric deflections induced on the cosmic-ray particle distribution by the long interstellar propagation are partially aligned along the LIMF and not significantly affected by the north-south blindness.

    In this study, IceCube and HAWC joined efforts to get a full-sky coverage that captures for the first time a full, unbiased picture of the cosmic-ray anisotropy. The work used five years of IceCube data, from May 2011 to May 2016, and two years of HAWC data, from May 2015 and May 2017.

    The fit of the IceCube-HAWC observed anisotropy at 10 TeV shows the expected alignment with the LIMF. Researchers then used this deviation to derive the north-south component of the dipole anisotropy.

    Previous studies of the anisotropy have shown that the dominant dipole variation starts to decrease around 10 TeV and then to abruptly increase again at energies around 100 TeV. This had been explained as a possible effect of the heliosphere, which has a much larger impact for lower energy cosmic rays.

    Deviations of the anisotropy from the LIMF could be due to the motion of the observer and/or to the effects of the heliosphere on the LIMF. However, only a full-sky study of the cosmic-ray anisotropy at different energies will make it possible to distinguish between these or other possible effects, thus enabling a deeper understanding of the properties of the LIMF and the heliosphere.

    See the full article here .

    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

    Stem Education Coalition
    IceCube is a particle detector at the South Pole that records the interactions of a nearly massless sub-atomic particle called the neutrino. IceCube searches for neutrinos from the most violent astrophysical sources: events like exploding stars, gamma ray bursts, and cataclysmic phenomena involving black holes and neutron stars. The IceCube telescope is a powerful tool to search for dark matter, and could reveal the new physical processes associated with the enigmatic origin of the highest energy particles in nature. In addition, exploring the background of neutrinos produced in the atmosphere, IceCube studies the neutrinos themselves; their energies far exceed those produced by accelerator beams. IceCube is the world’s largest neutrino detector, encompassing a cubic kilometer of ice.

     
  • richardmitnick 10:29 am on October 25, 2018 Permalink | Reply
    Tags: , , Cosmic Rays, , NASA ANITA balloon, Similar to the light blue glow of Chernekov light emitted in water surrounding nuclear reactors, Something energetic has travelled through the planet – a phenomenon that is just not allowed within the Standard Model of particle physics, The Askaryan effect, UHECRs- ultra-high energy cosmogenic neutrinos   

    From University of Hawaii via COSMOS: ““Upside down” cosmic rays may be new particle” 

    U Hawaii

    From University of Hawaii

    COSMOS

    25 October 2018
    Alan Duffy

    1
    In the vastness of Antarctica, two subatomic particles are challenging the standard model. Credit: Joseph Van Os/Getty Images

    A handful of particles erupting from the ice of Antarctica may well be the first indication of a new particle of nature.

    Each and every second your body is struck by hundreds of cosmic rays, particles that are created from some of the most energetic events in the known universe, such as exploding stars or accreting blackholes.

    Cosmic rays, as the name suggests, are of cosmic origin and tend to strike the ground from space. Yet NASA’s Antarctic Impulsive Transient Antenna (ANITA) – an array of radio antennas dangling from a balloon 37 metres above the ground – has spotted particles emanating from the ground. This suggests that something energetic has travelled through the planet – a phenomenon that is just not allowed within the Standard Model of particle physics.

    NASA ANITA balloon to detect cosmic ray showers and will monitor 32 potential gaseous contaminants, including formaldehyde, ammonia and carbon monoxide,

    NASA ANITA balloon team

    NASA ANITA balloon carrries scientific instruments above will detect cosmic ray showers


    The intriguing findings are reported in the journal Physical Review Letters, by a team led by Peter Gorham from the University of Hawaii, in the US.

    ANITA’s a balloon circumnavigates Antarctica, detecting cosmic ray showers, in particular ultra-high energy cosmogenic neutrinos (UHECRs), as they collide with ice below.

    The system detects these collisions by the emission of radio waves from the burst of secondary particles generated as the neutrinos move in the ice more quickly than light.

    This process is known as the Askaryan effect, and is similar to the light blue glow of Chernekov light emitted in water surrounding nuclear reactors.

    In its first flight ANITA detected 16 such events, showing the power of this technique to detect the otherwise ghost-like neutrinos. The radio waves reflected back towards the balloon became horizontally polarised – in much the same way that light becomes horizontally polarised when it reflects off a puddle.

    Surprisingly, hidden in the first run of data, a single UHECR was detected from below the horizon from the ice, but without any polarisation from reflection. This was noted at the time, but the evidence wasn’t strong enough to rule out the particle being produced in the ice itself.

    In the third flight of ANITA another 20 UHECRs were detected – and again a single event was detected from the ice without the reflected polarisation.

    The two events now suggest that high energy particles have travelled all the way through the earth, creating a burst of Askaryan radio waves in the ice, like upside-down cosmic rays.

    While standard neutrinos are famously non-interacting, capable of flying through light years of solid lead, their high energy counterparts have a much greater cross-section, or chance, of collision. In fact, the UHECRs spotted by ANITA can’t possibly have travelled through the earth without collision, at least according to the Standard Model of particle physics.

    Possible solutions include interactions beyond the Standard Model, or even an entirely new particle, such as a sterile neutrino. More such detections are required to confirm any such claims but for now these tentative signals are exciting hints of a new era of physics.

    See the full article here .

    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

    Stem Education Coalition

    System Overview

    The University of Hawai‘i System includes 10 campuses and dozens of educational, training and research centers across the Hawaiian Islands. As the public system of higher education in Hawai‘i, UH offers opportunities as unique and diverse as our Island home.

    The 10 UH campuses and educational centers on six Hawaiian Islands provide unique opportunities for both learning and recreation.

    UH is the State’s leading engine for economic growth and diversification, stimulating the local economy with jobs, research and skilled workers.

     
  • richardmitnick 8:23 am on October 17, 2018 Permalink | Reply
    Tags: , , , Cosmic Rays, , Speed of Light, The CMB: the cosmic microwave background, The CNB: the cosmic neutrino background, The Universe Has A Speed Limit And It Isn’t The Speed Of Light, The WHIM: the warm-hot intergalactic medium   

    From Ethan Siegel: “The Universe Has A Speed Limit, And It Isn’t The Speed Of Light” 

    From Ethan Siegel

    Oct 16, 2018

    1
    All massless particles travel at the speed of light, including the photon, gluon and gravitational waves, which carry the electromagnetic, strong nuclear and gravitational interactions, respectively. Particles with mass must always travel at speeds below the speed of light, and there’s an even more restrictive cutoff in our Universe. (NASA/Sonoma State University/Aurore Simonnet)

    Nothing can go faster than the speed of light in a vacuum. But particles in our Universe can’t even go that fast.

    When it comes to speed limits, the ultimate one set by the laws of physics themselves is the speed of light. As Albert Einstein first realized, everyone looking at a light ray sees that it appears to move at the same speed, regardless of whether it’s moving towards you or away from you. No matter how fast you travel or in what direction, all light always moves at the same speed, and this is true for all observers at all times. Moreover, anything that’s made of matter can only approach, but never reach, the speed of light. If you don’t have mass, you must move at the speed of light; if you do have mass, you can never reach it.

    But practically, in our Universe, there’s an even more restrictive speed limit for matter, and it’s lower than the speed of light. Here’s the scientific story of the real cosmic speed limit.

    When scientists talk about the speed of light — 299,792,458 m/s — we implicitly mean “the speed of light in a vacuum.” Only in the absence of particles, fields, or a medium to travel through can we achieve this ultimate cosmic speed. Even at that, it’s only the truly massless particles and waves that can achieve this speed. This includes photons, gluons, and gravitational waves, but not anything else we know of.

    Quarks, leptons, neutrinos, and even the hypothesized dark matter all have masses as a property inherent to them. Objects made out of these particles, like protons, atoms, and human beings all have mass, too. As a result, they can approach, but never reach, the speed of light in a vacuum. No matter how much energy you put into them, the speed of light, even in a vacuum, will forever be unattainable.

    But there’s no such thing, practically, as a perfect vacuum. Even in the deepest abyss of intergalactic space, there are three things you absolutely cannot get rid of.

    The WHIM: the warm-hot intergalactic medium. This tenuous, sparse plasma are the leftovers from the cosmic web. While matter clumps into stars, galaxies, and larger groupings, a fraction of that matter remains in the great voids of the Universe. Starlight ionizes it, creating a plasma that may make up about 50% of the total normal matter in the Universe.

    WHIM-Warm-Hot Intergalactic Medium Trevor Ponman U Birmingham


    The CMB: the cosmic microwave background. This leftover bath of photons originates from the Big Bang, where it was at extremely high energies. Even today, at temperatures just 2.7 degrees above absolute zero, there are over 400 CMB photons per cubic centimeter of space.

    CMB per ESA/Planck


    ESA/Planck 2009 to 2013

    The CNB: the cosmic neutrino background. The Big Bang, in addition to photons, creates a bath of neutrinos. Outnumbering protons by perhaps a billion to one, many of these now-slow-moving particles fall into galaxies and clusters, but many remain in intergalactic space as well.

    CNB- the cosmic neutrino background-Amand Faessler U Tuebingen

    3
    A multiwavelength view of the galactic center shows stars, gas, radiation and black holes, among other sources. But the light coming from all of these sources, from gamma rays to visible to radio light, can only indicate what our instruments are sensitive enough to detect from 25,000+ light years away. (NASA/ESA/SSC/CXC/STScI)

    Any particle traveling through the Universe will encounter particles from the WHIM, neutrinos from the CNB, and photons from the CMB. Even though they’re the lowest-energy things, the CMB photons are the most numerous and evenly-distributed particles of all. No matter how you’re generated or how much energy you have, it’s not really possible to avoid interacting with this 13.8 billion year old radiation.

    When we think about the highest-energy particles in the Universe — i.e., the ones that will be moving the fastest — we fully expect they’ll be generated under the most extreme conditions the Universe has to offer. That means we think we’ll find them where energies are highest and fields are strongest: in the vicinity of collapsed objects like neutron stars and black holes.

    4
    In this artistic rendering, a blazar is accelerating protons that produce pions, which produce neutrinos and gamma rays. (IceCube/NASA)

    U Wisconsin IceCube experiment at the South Pole



    Neutron stars and black hole are where you can not only find the strongest gravitational fields in the Universe, but — in theory — the strongest electromagnetic fields, too. The extremely strong fields are generated by charged particles, either on the surface of a neutron star or in the accretion disk around a black hole, that move close to the speed of light. Moving charged particles generate magnetic fields, and as particles move through these fields, they accelerate.

    This acceleration causes not only the emission of light of a myriad of wavelengths, from X-rays down to radio waves, but also the fastest, highest-energy particles ever seen: cosmic rays.

    Cosmic rays produced by high-energy astrophysics sources (ASPERA collaboration – AStroParticle ERAnet)

    Artist’s impression of the active galactic nucleus (DESY, Science Communication Lab)

    Whereas the Large Hadron Collider accelerates particles here on Earth up to a maximum velocity of 299,792,455 m/s, or 99.999999% the speed of light, cosmic rays can smash that barrier. The highest-energy cosmic rays have approximately 36 million times the energy of the fastest protons ever created at the Large Hadron Collider. Assuming that these cosmic rays are also made of protons gives a speed of 299,792,457.99999999999992 m/s, which is extremely close to, but still below, the speed of light in a vacuum.

    There’s a very good reason that, by time we receive them, these cosmic rays aren’t more energetic than this.

    The problem is that space isn’t a vacuum. In particular, the CMB will have its photons collide and interact with these particles as they travel through the Universe. No matter how high the energy is of the particle you made, it has to pass through the radiation bath that’s left over from the Big Bang in order to reach you.

    Even though this radiation is incredibly cold, at an average temperature of some 2.725 Kelvin, the mean energy of each photon in there isn’t negligible; it’s around 0.00023 electron-Volts. Even though that’s a tiny number, the cosmic rays hitting it can be incredibly energetic. Every time a high-energy charged particle interacts with a photon, it has the same possibility that all interacting particles have: if it’s energetically allowed, by E=mc², then there’s a chance it can create a new particle!

    5
    Whenever two particles collide at high enough energies, they have the opportunity to produce additional particle-antiparticle pairs, or new particles as the laws of quantum physics allow. Einstein’s E = mc² is indiscriminate this way. (E. Siegel / Beyond The Galaxy)

    If you ever create a particle with energies in excess of 5 × 10¹⁹ eV, they can only travel a few million light years — max — before one of these photons, left over from the Big Bang, interacts with it. When that interaction occurs, there will be enough energy to produce a neutral pion, which steals energy away from the original cosmic ray.

    The more energetic your particle is, the more likely you are to produce pions, which you’ll continue to do until you fall below this theoretical cosmic energy limit, known as the GZK cutoff. (Named for three physicists: Greisen, Zatsepin, and Kuzmin.) There’s even more braking (Bremsstrahlung) radiation that arises from interactions with any particles in the interstellar/intergalactic medium. Even lower-energy particles are subject to it, and radiate energy away in droves as electron/positron pairs (and other particles) are produced.

    We believe that every charged particle in the cosmos — every cosmic ray, every proton, every atomic nucleus — should limited by this speed. Not just the speed of light, but a little bit lower, thanks to the leftover glow from the Big Bang and the particles in the intergalactic medium. If we see anything that’s at a higher energy, then it either means:

    1.particles at high energies might be playing by different rules than the ones we presently think they do,
    2.they are being produced much closer than we think they are: within our own Local Group or Milky Way, rather than these distant, extragalactic black holes,
    3.or they’re not protons at all, but composite nuclei.

    The few particles we’ve seen that break the GZK barrier are indeed in excess of 5 × 10¹⁹ eV, in terms of energy, but do not exceed 3 × 10²¹ eV, which would be the corresponding energy value for an iron nucleus. Since many of the highest-energy cosmic rays have been confirmed to be heavy nuclei, rather than individual protons, this reigns as the most likely explanation for the extreme ultra-high-energy cosmic rays.

    6
    The spectrum of cosmic rays. As we go to higher and higher energies, we find fewer and fewer cosmic rays. We expected a complete cutoff at 5 x 10¹⁹ eV, but see particles coming in with up to 10 times that energy. (Hillas 2006 / University of Hamburg)

    There is a speed limit to the particles that travel through the Universe, and it isn’t the speed of light. Instead, it’s a value that’s very slightly lower, dictated by the amount of energy in the leftover glow from the Big Bang. As the Universe continues to expand and cool, that speed limit will slowly rise over cosmic timescales, getting ever-closer to the speed of light. But remember, as you travel through the Universe, if you go too fast, even the radiation left over from the Big Bang can fry you. So long as you’re made of matter, there’s a cosmic speed limit that you simply cannot overcome.

    See the full article here .

    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

    Stem Education Coalition

    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
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