- 3268 - COSMIC RAYS - messengers from the universe? The Earth is being constantly bombarded from space by cosmic rays of an unknown origin! Mysterious cosmic rays traveling at speeds approaching that of light constantly pelt Earth’s upper atmosphere from the depths of space, creating high-energy collisions that dwarf those produced in even the most powerful particle colliders. The atmospheric crashes rain down gigantic showers of secondary particles, not rays, to the surface of our planet.
------------------- 3268 - COSMIC RAYS - messengers from the universe?
- Cosmic Rays were discovered more than a century ago but physicists still don’t know where cosmic rays come from. The short answer to why we can’t trace cosmic rays back to their source due to space being filled with magnetic fields.
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- Charged cosmic-ray particles are redirected by the magnetic fields they pass through on their long journey through space. These magnetic fields in space have local, small, randomly oriented structures, a prediction of the exact path of a cosmic-ray particle is impossible.
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- We know about cosmic rays are comprised of extremely energetic charged particles , like protons, alpha particles, and atomic nuclei like helium and iron, with miniscule proportions of antiparticles thrown into the mix.
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- The average energy of a solar photon is approximately 1.4 electron volts (eV).
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- Imagine a proton that is accelerated so that it has an energy of 100 Joule. This energy corresponds to that of a tennis ball smashed by someone with a velocity of around 124 miles per hour.
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- That means a proton can only reach extreme, macroscopic energy by traveling at almost the speed of light. The universe must be able to accelerate particles to these energies, but we still do not know how it does it.
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- One of the best ways of accelerating particles is a shock front that occurs when a medium with a large velocity runs into a slower one, producing a shock, a sudden change in the properties of the medium. In the case of the universe, the changed properties are velocity and density, and even magnetic fields. Luckily for the cosmic rays, the field becomes highly turbulent in that process. And the combination of a shock front with turbulence is a great particle accelerator.
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- What could produce such a shock front? One likely suspect is supernovae. As a shell of shocked material blasted away from an exploding star, it hits the cool interstellar medium that lies between stars, almost like a cosmic tsunami.
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- A traveling shock front can also be found in active galaxies, where huge plasma jet exist. A better understanding of cosmic rays, as well as their origins, is expected to open an important window into tremendously powerful cosmic events, such as supernovae and collisions between blackholes and neutron stars.
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- It was August 1912 when Austrian-American physicist Victor Hess began a series of flights to the upper atmosphere in a hydrogen-filled balloon equipped with an electroscope to take measurements of ionizing radiation.
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- At the time, it was widely believed that radiation from the Earth itself was responsible for this phenomenon of knocking electrons off atoms. Should this be the case, however ionization should be strongest near the planet’s surface. That’s not what Hess found.
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- Hess discovered something startling. At a nosebleed-inducing altitude of 3.3 miles, ionization rates of the air were three times that measured at sea level. He concluded that the source of this ionization was not coming from below our feet, but instead from above. Further measurements made during a solar eclipse also showed the Sun wasn’t the source of this ionization radiation.
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- During the course of seven balloon trips, Hess discovered cosmic rays — confirmed and named by Robert Millikan in 1925 — coming from beyond our solar system. But, while the detection of cosmic rays has been associated with balloon flights ever since, the upper atmosphere isn’t the most convenient laboratory to investigate the high-energy particle collisions they produce.
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- To study the collisions caused by cosmic rays, particle physicists retreated below ground, employing increasingly monstrous particle accelerators to slam together particles in an attempt to replicate the collisions that cosmic rays spark in the upper atmosphere.
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- With CERN’s Large Hadron Collider (LHC) which is a tunnel 16-mile circumference deep beneath the French/Swiss border. Yet, despite its impressive size, power and utility, the LHC still can’t reach the energies produced by cosmic ray collisions.
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- At roughly the same time that Hess’ balloon ride changed our perspective of the universe forever, a physicist named Einstein was working on a theory that would radically change our understanding of the fabric of space-time. And this theory, many decades later, could provide the next step to decoding cosmic rays.
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- The discovery of gravitational waves , ripples in space-time predicted by Einstein’s theory of general relativity , has made a new form of astronomy possible, allowing us to investigate events and objects that we could never hope to observe in the electromagnetic spectrum alone.
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- This combination of electromagnetic or “traditional” astronomy and gravitational-wave detections along with detecting neutrinos, which are ghost-like particles with virtually no mass or electric charge is known as multi-messenger astronomy.
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- The gravitational-wave signal GW170817 came from the merger of two neutron stars and was observed in 2017. It was significant for both multi-messenger astronomy and identifying potential sources of cosmic rays.
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- Not only did this violent merger become the first such event to be detected in both gravitational waves and electromagnetic radiation, but it also confirmed that the merger of compact stellar remnants can accelerate particles to great speeds, creating cosmic rays.
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- Astronomy enters a new age thanks to multi-messenger signals, cosmic rays, neutrinos, photons, and gravitational waves. Each of these signals carries a message. What can they tell us?
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- The IceCube Laboratory building at the South Pole stands atop an ice field laced with over 5,000 optical sensors. Computers in the lab building perform an initial round of processing before transmitting the data via satellite to researchers at the University of Wisconsin-Madison.
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- For centuries, astronomers studying the stars were limited to information from visible light. Telescopes, photographic plates, and digital detectors were all developed to collect, magnify, and capture that signal. But visible light isn’t the only message the cosmos is sending us.
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- All the big, interesting events, like supernovae, gamma-ray bursts, and mergers, are disruptive. They send out accelerated particles, photons, and waves in space-time.
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- Astronomers call these messengers, and there are four types: photons, neutrinos, cosmic rays, and gravitational waves. Gradually, scientists have unlocked the ability to detect them all.
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- The roots of “multi-messenger astronomy” date to the 1960s, when the U.S. government launched satellites carrying detectors for gamma rays, the most powerful photons, to track Russian nuclear tests. They found plenty of gamma-ray sources, but to their surprise, they weren’t coming from Earth, but from all around the sky.
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- It was obvious that the cosmic rays were coming from very far away, cosmological distances. Around the same time, astronomers detected neutrinos, which are subatomic particles with no charge and very little mass, emerging from the Sun.
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- Cosmic rays are atomic nuclei that have been accelerated to near the speed of light, first seen in balloon experiments about 100 years ago. And although Albert Einstein predicted gravitational waves, the ripples in space-time that occur when massive objects collide, in 1916, they were not observed until 2015.
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- Multi-messenger astronomy is synthesizing these various messengers from violent astronomical events. Astronomers have directly observed gravitational waves and used these observations to understand what happens when neutron stars or blackholes collide.
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- Astronomers have also uncovered new mysteries, including the discovery that rare ultrahigh-energy cosmic rays originate from outside our galaxy, and that high-energy neutrinos form a cosmic background that pervades the universe.
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- Exciting theories abound as to what kinds of exotic objects are sending out these cosmic messengers: superstrings, dark matter, and even “defects” in the structure of the universe.
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- The power of these combined messengers comes from the fact that each one is generated by one of the four forces of nature: photons by the electromagnetic force, gravitational waves by gravity, cosmic rays by the strong nuclear force, and neutrinos by the weak nuclear force.
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- The different messengers are the product of their particular origins, and so their presence (or absence) and their characteristics, such as composition, energy level, and direction, teach us about the object they came from.
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- The detection of these messengers requires vastly different detectors and an unprecedented level of cooperation across disciplines. Instantaneous communication is needed to coordinate observations of fleeting events, and processing the data requires bespoke skills in statistical analysis and data mining.
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- The first object scientists studied with multiple messengers was one close to home: our Sun. In the 1960s, American physicist Raymond Davis Jr. led the first experiment to detect solar neutrinos: a 100,000-gallon (380,000 liters) tank of dry-cleaning fluid placed deep underground in the Homestake Gold Mine in South Dakota.
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- When a neutrino happened to interact with a molecule of that fluid, it transformed into an atom of argon, which Davis could detect. The observations of solar neutrinos have since been confirmed by others, including the series of Kamiokande detectors using pure liquid water in Japan.
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- The Super-Kamioka Neutrino Detection Experiment (or Super-Kamiokande) near Hida, Japan, is an underground stainless steel tank that holds 55,000 tons of water. If a neutrino interacts with a water molecule, it emits light that can be detected by the 13,000 photomultiplier tubes that line the tank’s interior.
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- In 1987, neutrino observatories detected emission from a supernova in the Large Magellanic Cloud, two hours before visible light from the stellar explosion reached Earth. That supernova, SN 1987A, was the first that modern astronomers were able to study with multiple messengers. These observations revolutionized our understanding of core-collapse supernovae, eventually providing direct evidence that these events create neutron stars.
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- Neutrinos are useful messengers because they travel in a straight, traceable line from their origin, passing through nearly every obstacle in their path. This allows scientists to see into locales that radiation cannot penetrate, but , it also makes them difficult to detect.
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- Neutrinos only weakly interact with matter. So you need huge volumes to catch only a few particles, either in man-made water tanks or, on a much bigger scale, by instruments in the ocean or the Antarctic ice sheet.
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- This last option is what has been done with “IceCube“, a neutrino detector at the South Pole. Ice works better than liquid water. when water sloshes around, it makes it harder to trace neutrinos back to their point of origin in the sky, and Antarctic ice is especially transparent and stable.
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- To detect neutrinos, IceCube also needed to be shielded from radiation at Earth’s surface. So scientists drilled with a specially designed hot-water tool into the ice sheet to a depth of about 8,200 feet, and lowered optical modules on long cables into the holes before they refroze. These sensors detect neutrinos by imaging the secondary particles that radiate from points of impact in the ice.
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- When a neutrino strikes a molecule of ice, it produces secondary particles that emit light as they pass through the ice. The most significant multi-messenger detection out of IceCube so far came from a blazar in 2017.
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- Blazars are a type of galaxy with a central supermassive blackhole that shoots out jets of ionized matter at nearly the speed of light. In that event, IceCube’s sensors captured the trail of particles from the impact of a single neutrino from a source in the constellation Orion.
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- Follow-up observations from NASA’s Fermi Gamma-ray Space Telescope showed that in the same direction lies blazar TXS 0506+056, making it the third known individual source of neutrinos (after the Sun and SN 1987A). Furthermore, the gamma-ray observations showed that the blazar was flaring up at the time.
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- The detection of these two messengers, a high-energy neutrino and gamma rays, is strong evidence that blazars are a source of high-energy neutrinos. But it also suggests that blazars play a role in generating another mysterious messenger, cosmic rays, which are critical intermediaries in producing those neutrinos.
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- On the plains of Argentina, the 1,660 water tanks of the Pierre Auger Observatory lie scattered across an area the size of Rhode Island, waiting for cosmic rays to strike the atmosphere overhead. These events create an air shower of secondary particles that the 3,170-gallon tanks are designed to detect as the particles pass through them.
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- Cosmic rays are high-energy protons and atomic nuclei, the leftovers of matter that has been torn apart. These particles represent the only direct source of samples of material from outside the solar system. They move through space at nearly the speed of light, and their exact speed and character are determined by the events that accelerate them.
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- When they strike other material, they interact with it, leaving clues in their wake. For instance, when colliding with interstellar dust, they may form bonds between atoms and create complex organic molecules, the building blocks of life.
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- When cosmic rays are ejected from the core of a blazar and interact with surrounding gas, they may generate neutrinos, like the one detected by IceCube in 2017.
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- That means the next generation of neutrino telescopes will also be useful for investigating cosmic rays. Upcoming facilities like IceCube-Gen2 (an upgrade to the existing array) and the Cubic Kilometre Neutrino Telescope (a planned neutrino detector to be installed at the bottom of the Mediterranean Sea) should be sensitive enough to see the diffuse neutrino flux that high-energy cosmic rays make as they travel through deep space.
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- Astronomers can also use dedicated observatories to study those cosmic rays that survive the trip to Earth from galactic or even extragalactic events. When they hit Earth’s atmosphere, they interact with air molecules, creating showers of secondary particles: X-rays, muons, protons, antiprotons, alpha particles, pions, electrons, positrons, and neutrons.
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- The largest facility designed to capture this subatomic shrapnel is the Pierre Auger Cosmic Ray Observatory, which uses more than 1,600 massive water tanks spread out over 1,160 square miles in western Argentina.
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- Astronomers think that most cosmic rays originating within our galaxy are accelerated by shock waves produced by supernovae. In 2017, researchers reported these cosmic rays appear to originate outside our galaxy. While they suspect actively feeding supermassive black holes produce them, a true multi-messenger detection that includes cosmic rays is needed to verify that theory.
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- The blackholes at the centers of blazars are the most powerful particle accelerators in the cosmos, capable of producing multiple types of messengers, including gamma rays and neutrinos.
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- The process starts with the blazar’s jets of matter, which shoot out into space, forming a shock wave as they plow into surrounding material. The charged particles streaming away from the shock generate magnetic fields that scatter particles back and forth across the shock boundary, gaining energy from the shock every time. This can accelerate protons to nearly the speed of light, turning them into cosmic rays .
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- If these cosmic rays collide with photons, they may produce particles called “pions“. The “neutral pions” decay to gamma rays as well as some electrons and positrons .
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- Some pions may have a charge and they decay to a “muon” and a “muon neutrino” , one of three “flavors” of neutrinos. The muons decay further to an electron or positron and either a muon neutrino or an electron neutrino, another flavor of neutrino. These massagers stream through the Universe.
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- Astronomers have been studying the cosmos with electromagnetic radiation ever since people first looked at stars with the naked eye. But beyond visible light, the electromagnetic spectrum includes all types of electromagnetic radiation: radio waves, microwaves, infrared and ultraviolet light, X-rays, and gamma rays. These last two are blocked by Earth’s atmosphere, so astronomers must rely on spacecraft like Fermi and NASA’s Neil Gehrels Swift Observatory.
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- Gamma rays are produced mainly by nuclear reactions: fusion, fission, and atomic decay. Gamma-ray bursts (GRBs) are born from the explosion of a massive star or the collision of neutron stars. In a matter of seconds, GRBs put out as much energy as the Sun puts out in billions of years.
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- “Swift saellite” acts as an eye in the sky, constantly scanning the cosmos for GRBs. But GRBs are brief and easy to miss. To maximize the chances of spotting one, Swift teams up with gravitational-wave detectors. If the gravitational wave guys tell us they saw something, we have to move quickly, to point in that direction and try to find it.
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- The observations complement each other: Gravitational waves are the fastest messenger, but current gravitational wave instruments aren’t able to accurately localize the exact source of events. On the other hand, X-rays and gamma rays are very useful as direction finders.
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- Cosmic rays are thought to be produced in remnants of supernovae like SN 1987A. The expanding shock waves of these stellar explosions can accelerate protons to near the speed of light.
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On May 22, 2020, astronomers were able to capture five messengers from an astronomical event: X-rays, optical light, infrared light, radio waves, and gamma rays.
It clearly wasn’t a nova or supernova. It was a little too dim, and with faster ejecta. It was a kilonova: a burst created by a neutron star colliding with a blackhole , or , in this case another neutron star.
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- This collision of two neutron stars forms a magnetar, a kind of super neutron star with a powerful magnetic field, which has recently been shown to produce high-energy X-rays and gamma rays. Roughly 30 magnetars have been discovered, all in our galaxy.
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- The resulting kilonova also produces gravitational waves on a grander scale than supernovae. The gravitational-wave facilities are currently offline, so they couldn’t corroborate. That’s because the Laser Interferometer Gravitational-wave Observatory (LIGO) in the U.S. and the Virgo detector in Italy are undergoing upgrades in preparation for their next observing run, set to start next year.
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- Einstein predicted gravitational waves, the most elusive of the messengers, in the early 20th century on the basis of his general theory of relativity. Unlike the messengers of the other three forces, they are not particles, but disturbances in the fabric of space-time that propagate outward at the speed of light and are created when matter is accelerated.
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- The effects of these waves passing through Earth are so small and brief that the LIGO-Virgo detector arms, which range from 1.86 to 2.46 miles long, measure a distortion of only 1/10,000 the width of a proton over a fraction of a second.
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- The first gravitational-wave detectors were built in the 1970s, but it took several decades to improve their sensitivity enough for functional observing.
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- Then from April to October, 2019, an incredible 39 gravitational-wave events were detected.
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- To identify these events and sort them from the noise, the LIGO and Virgo teams use advanced signal processing and analysis methods, including “machine learning“. This new specialty combines skill sets from astronomy, statistics, and computer science.
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- The ability to quickly and accurately identify events in observational data so that alerts can be sent out to the rest of the astronomical community rests upon their software.
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- Since its launch in 2004, NASA’s Neil Gehrels Swift Observatory has served as a sentry, keeping watch for gamma-ray bursts across the sky. Multi-messenger detection capabilities are only set to grow more powerful. All the major facilities involved are either currently undergoing upgrades or have imminent plans to do so.
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- IceCube-Gen2 is due to be deployed during the Antarctic summer of 2022–2023. The Japanese Kamioka Gravitational Wave Detector has recently begun operations and hopes to achieve its target sensitivity by 2024.
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- In addition to the ongoing Advanced LIGO-Virgo upgrade, a new LIGO facility is currently being built in India. And ESA has plans to build a space-borne gravitational-wave observatory, the Laser Interferometer Space Antenna, scheduled for launch in the early 2030s.
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- These new facilities, with their improvements in resolution and sensitivity, will allow for more and better detections, and hopefully even solve some long-standing mysteries.
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- There are many things we hope to learn in the next few decades, such as what is at the center of a neutron star, and the exact conditions which determine whether a merger creates a compact object or a black hole.
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- We also hope to see different kinds of gravitational waves, perhaps ones from continuous sources. And it could be that gravitational waves can teach us about dark matter, the mysterious stuff that makes up most of the matter in the universe and holds together galaxies with its gravitational glue.
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- Multi-messenger astronomy might even hold clues to resolving one of the most fundamental problems in all of science: the fact that the standard model of physics unifies only three of the four known forces (electromagnetic, strong, and weak).
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- General relativity, which describes how gravity arises from the curvature of space-time, also explains the behavior and evolution of the cosmos, but from a different point of view. Yet these two theories become inconsistent with each other at the smallest level, such as the center of a blackhole or the moment of the Big Bang. Can they both be right? Can they be unified into a “Theory of Everything?”
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- Perhaps multi-messenger astronomy has the answer. Perhaps by combining information from all four forces, we can penetrate to the tiniest, earliest core of astronomical-scale phenomena and take a quantum leap in our understanding of how the universe actually works.
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- Like the old parable that teaches us blind men cannot make sense of an elephant by touching just one part of its body, the big picture of massive astronomical objects and events can only be made clear when all possible sources of information are gathered and synthesized. Perhaps then, we will be able to approach the grand unification of the forces and finally make sense of it.
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- September 5, 2021 COSMIC RAYS - messengers from the universe? 3268
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