Riccardo Giacconi was an Italian-American Nobel Prize-winning astrophysicist who laid down the foundations of X-ray astronomy. He was a professor at the Johns Hopkins University.
The Greisen–Zatsepin–Kuzmin limit (GZK limit or GZK cutoff) is a theoretical upper limit on the energy of cosmic ray protons traveling from other galaxies through the intergalactic medium to our galaxy. The limit is 5×1019 eV (50 EeV),or about 8 joules (the energy of a proton travelling at ≈ 99.99999999999999999998% the speed of light). The limit is set by the slowing effect of interactions of the protons with the microwave background radiation over long distances (≈ 160 million light-years). The limit is at the same order of magnitude as the upper limit for energy at which cosmic rays have experimentally been detected,although indeed some detections appear to have exceeded the limit,as noted below. For example,one extreme-energy cosmic ray,the Oh-My-God Particle,which has been found to possess a record-breaking 3.12×1020 eV (50 joules) of energy (about the same as the kinetic energy of a 95 km/h baseball).
A blazar is an active galactic nucleus (AGN) with a relativistic jet directed very nearly towards an observer. Relativistic beaming of electromagnetic radiation from the jet makes blazars appear much brighter than they would be if the jet were pointed in a direction away from Earth. Blazars are powerful sources of emission across the electromagnetic spectrum and are observed to be sources of high-energy gamma ray photons. Blazars are highly variable sources,often undergoing rapid and dramatic fluctuations in brightness on short timescales. Some blazar jets appear to exhibit superluminal motion,another consequence of material in the jet traveling toward the observer at nearly the speed of light.
Astrophysics is a science that employs the methods and principles of physics and chemistry in the study of astronomical objects and phenomena. As one of the founders of the discipline,James Keeler,said,Astrophysics "seeks to ascertain the nature of the heavenly bodies,rather than their positions or motions in space–what they are,rather than where they are." Among the subjects studied are the Sun,other stars,galaxies,extrasolar planets,the interstellar medium and the cosmic microwave background. Emissions from these objects are examined across all parts of the electromagnetic spectrum,and the properties examined include luminosity,density,temperature,and chemical composition. Because astrophysics is a very broad subject,astrophysicists apply concepts and methods from many disciplines of physics,including classical mechanics,electromagnetism,statistical mechanics,thermodynamics,quantum mechanics,relativity,nuclear and particle physics,and atomic and molecular physics.
In astroparticle physics,an ultra-high-energy cosmic ray (UHECR) is a cosmic ray with an energy greater than 1 EeV (1018 electronvolts,approximately 0.16 joules),far beyond both the rest mass and energies typical of other cosmic ray particles.
David Norman Schramm was an American astrophysicist and educator,and one of the world's foremost experts on the Big Bang theory. Schramm was a pioneer in establishing particle astrophysics as a vibrant research field. He was particularly well known for the study of Big Bang nucleosynthesis and its use as a probe of dark matter and of neutrinos. He also made important contributions to the study of cosmic rays,supernova explosions,heavy-element nucleosynthesis,and nuclear astrophysics generally.
The Extreme Universe Space Observatory onboard Japanese Experiment Module (JEM-EUSO) is the first space mission concept devoted to the investigation of cosmic rays and neutrinos of extreme energy (E >5×1019 eV). Using the Earth's atmosphere as a giant detector,the detection is performed by looking at the streak of fluorescence produced when such a particle interacts with the Earth's atmosphere.
The IceCube Neutrino Observatory is a neutrino observatory constructed at the Amundsen–Scott South Pole Station in Antarctica. The project is a recognized CERN experiment (RE10). Its thousands of sensors are located under the Antarctic ice,distributed over a cubic kilometre.
VERITAS is a major ground-based gamma-ray observatory with an array of four 12 meter optical reflectors for gamma-ray astronomy in the GeV –TeV photon energy range. VERITAS uses the Imaging Atmospheric Cherenkov Telescope technique to observe gamma rays that cause particle showers in Earth's atmosphere that are known as extensive air showers. The VERITAS array is located at the Fred Lawrence Whipple Observatory,in southern Arizona,United States. The VERITAS reflector design is similar to the earlier Whipple 10-meter gamma-ray telescope,located at the same site,but is larger in size and has a longer focal length for better control of optical aberrations. VERITAS consists of an array of imaging telescopes deployed to view atmospheric Cherenkov showers from multiple locations to give the highest sensitivity in the 100 GeV –10 TeV band. This very high energy observatory,completed in 2007,effectively complements the Large Area Telescope (LAT) of the Fermi Gamma-ray Space Telescope due to its larger collection area as well as coverage in a higher energy band.
Astroparticle physics,also called particle astrophysics,is a branch of particle physics that studies elementary particles of astrophysical origin and their relation to astrophysics and cosmology. It is a relatively new field of research emerging at the intersection of particle physics,astronomy,astrophysics,detector physics,relativity,solid state physics,and cosmology. Partly motivated by the discovery of neutrino oscillation,the field has undergone rapid development,both theoretically and experimentally,since the early 2000s.
Extragalactic cosmic rays are very-high-energy particles that flow into the Solar System from beyond the Milky Way galaxy. While at low energies,the majority of cosmic rays originate within the Galaxy (such as from supernova remnants),at high energies the cosmic ray spectrum is dominated by these extragalactic cosmic rays. The exact energy at which the transition from galactic to extragalactic cosmic rays occurs is not clear,but it is in the range 1017 to 1018 eV.
Katherine Freese is a theoretical astrophysicist. She is currently a professor of physics at the University of Texas at Austin,where she holds the Jeff and Gail Kodosky Endowed Chair in Physics. She is known for her work in theoretical cosmology at the interface of particle physics and astrophysics.
Wendy Laurel Freedman is a Canadian-American astronomer,best known for her measurement of the Hubble constant,and as director of the Carnegie Observatories in Pasadena,California,and Las Campanas,Chile. She is now the John &Marion Sullivan University Professor of Astronomy and Astrophysics at the University of Chicago. Her principal research interests are in observational cosmology,focusing on measuring both the current and past expansion rates of the universe,and on characterizing the nature of dark energy.
HakkıBoran Ögelman was a Turkish physicist and astrophysicist. He was an expert on gamma ray astronomy,the physics of neutron stars,and solar energy and worked on several key topics in modern astrophysics. He made many contributions to high energy astrophysics. In his early professional career he engaged in the SAS-II Small Gamma Ray Astronomy Satellite experiment development,data analysis,and first detection and imaging of our universe in gamma rays with his NASA colleagues,as well as in other fields of physics. His main interests in the field of astrophysics were the study of gamma ray astronomy and compact objects such as neutron stars and pulsars. Ögelman worked at NASA Goddard Space Flight Center,Middle East Technical University (METU) in Ankara,Turkey,Çukurova University in Adana,Turkey,Max Planck Institute (MPI) at Garching,Germany and the University of Wisconsin.
Multi-messenger astronomy is astronomy based on the coordinated observation and interpretation of signals carried by disparate "messengers":electromagnetic radiation,gravitational waves,neutrinos,and cosmic rays. They are created by different astrophysical processes,and thus reveal different information about their sources.
Clement Laurence Pryke is an English-American physicist,focusing in astrophysics and cosmology,particularly on the cosmic microwave background.
Kerstin Perez is an associate professor of Particle Physics at the Massachusetts Institute of Technology. She is interested in physics beyond the standard model. She leads the silicon detector program for the General AntiParticle Spectrometer (GAPS) and the high-energy X-ray analysis community for the NuSTAR telescope array.
Alexey Vikhlinin is a Russian-American astrophysicist notable for achievements in the astrophysics of high energy phenomenon,namely galaxy cluster cosmology and the design of space-based X-ray observatories. He is currently a senior astrophysicist at the Smithsonian Astrophysical Observatory,part of the Center for Astrophysics | Harvard &Smithsonian in Cambridge,Massachusetts. He was recently the Science and Technology Definition Team (STDT) Community Co-chair for the Lynx X-ray Observatory,a NASA-funded Large Mission Concept Study under consideration by the 2020 Decadal Survey on Astronomy and Astrophysics.
Abigail Goodhue Vieregg is a professor of physics at the Enrico Fermi Institute and Kavli Institute of Cosmology,University of Chicago,specializing in neutrino astrophysics and cosmology. Her work focuses on cosmic high-energy neutrinos and mapping the cosmic microwave background.
Joshua A. Frieman is a theoretical astrophysicist who lives and works in the United States. He is a senior scientist at Fermilab and a professor of astronomy and astrophysics at the University of Chicago. Frieman is known for his work studying dark energy and cosmology,and he co-founded the Dark Energy Survey experiment. He was elected a member of the National Academy of Sciences in 2022.
