Glossary - Cosmic Dark to Cosmic Dawn

Annihilation (of particles): Particle annihilation occurs when a subatomic particle and its respective antiparticle collide, causing both to disappear and release energy.
Pages: Big Bang, Exotic Physics

Image: A diagram of particle annihilation.
Accretion: The process by which a large astronomical body or mass, such as a galaxy or black hole, accumulates more matter through its gravitational attraction.
Pages: First Black Holes, First Galaxies, Galaxies at Cosmic Noon

Image: This artist's conception shows a supermassive black hole at the center of a remote galaxy digesting the remnants of a star through accretion.
Credit: JPL
Accretion Disk: The matter infalling on a dense, rotating object, often a black hole, typically forms a thin disk. Inside the disk, material spirals into the central object.
Pages: First Black Holes

Image: Artist's conception of a bright black hole accretion disk.
Credit: JPL
Atmospheric Distortion: As light travels through our atmosphere, it encounters the molecules and other particles. Sometimes, the light bounces off these particles, heading in a different direction. This is a problem for astronomers, because we would like to trace each light wave back to its source. Adaptive optics is one way to correct these distortions, but of course another solution is to avoid the atmosphere entirely by placing the telescope in space!
Pages: Space Telescopes

Image: Transmission and absorption in Earth atmosphere. Credit: NASA
Adaptive Optics: A telescope system that compensates for blurring by Earth's atmosphere by bending or flexing the mirror in ways that cancel out the atmosphere's effect.
Pages: Near-Infrared Telescopes

Image: Keck's Laser Guide Star System to measure changes in the atmosphere for the adaptive optics system.
Credit: W. M. Keck Observatory
ALMA (Atacama Large Millimeter Array): A large observatory in the Atacama Desert of Chile, which observes radio waves with millimeter wavelengths. It has a huge array of science objectives, including observations of gas and dust in galaxies during the Cosmic Dawn.
Pages: Other Telescopes

Image: ALMA Observatory
Credit: © EFE/Ariel Marinkovic
ATHENA: A planned space telescope that hopes to measure the X-rays produced as black holes swallow nearby gas in distant galaxies (amongst other things). X-ray telescopes must be placed in space, because the Earth's atmosphere absorbs X-rays.
Pages: Other Telescopes

Image: Athena space observatory. Mirror of the telescope located upfront, science payload located opposite.
Credit: X-IFU Consortium © DB/X-IFU
Angular Resolution: The ability to distinguish features in an image, or the smallest angle between two objects for which those objects can be seen as separate.
Pages: Radio Telescopes

Image: A diagram showing that the further an object, the smaller the angular size/extent, highlighting the need for good angular resolution in astronomy.
Credit:NASA Fermi Classroom
Big Bang: The Big Bang model describes the initial phase of the Universe's expansion, from which it began in a hot, dense state and then expanded rapidly and cooled.
Pages: Big Bang

Image: Snapshot of an artist's animation of the big bang.
Credit: NASA's Goddard Space Flight Center /CI Lab
Black Hole: A collection of matter so dense that nothing, not even light, can escape its pull. Some massive stars end their lives as black holes, once their fuel supply is exhausted.
Pages: Exotic Physics, First Stars, Light Fills The Universe, Galaxies at Cosmic Noon

Image: An artist's conception of the most primitive supermassive black holes known, at the core of a young, star-rich galaxy.
Credit: JPL
Cosmic Dawn: The first appearance of stars, galaxies, and other luminous objects and structures in the universe. It has not yet been observed but likely occurred a few dozen million years after the Big Bang.
Pages: First Stars, Light Fills The Universe, First Galaxies, Epoch of Reionization

Image: A timeline running from the Big Bang (right), tothe present (left). In the middle is the Reionization Period where the initial bubbles caused the cosmic dawn.
Credit: NASA/STScI
Cosmic Microwave Background (CMB): After the Big Bang, the Universe was full of high-energy photons. Until about 400,000 years after the Big Bang, these photons interacted strongly with electrons throughout the Universe. But after that time, those electrons combined with protons to form hydrogen atoms. After that, the photons could not interact with matter, and they traveled in straight lines until the present day. During that period, they redshifted so much that they appear as microwaves. The CMB was emitted when the universe had cooled enough to form neutral hydrogen; before this time, the universe was opaque.
Pages: Big Bang, Epoch of Reionization, Other Telescopes

Image: 13.7 billion year old temperature fluctuations (+/- 200 microKelvin - shown as color differences) that correspond to the seeds that grew to become the galaxies.
Credit: NASA / WMAP Science Team
Cosmic Inflation: A period of extraordinarily fast expansion very early in the Universe's history which generated tiny fluctuations in the matter density, creating the seeds of galaxies.
Pages: Big Bang

Image: A timeline of our universe, with cosmic inflation as the first big event after the big bang, and when the universe made it's fastest increase in size.
Credit: NASA
Cosmic Noon: As dark matter clumps grow, their galaxies do as well, forming stars rapidly. This process peaks about three billion years after the Big Bang, a period known as the Cosmic Noon of galaxy formation.
Pages: Galaxies at Cosmic Noon

Andromeda galaxy, our closest neighbor.
Image Credit: NASA Goddard
Cosmic Rays: High energy particles, such as protons and electrons, that are accelerated near the speed of light by unknown processes. They are common throughout the galaxy, and astronomers suspect many are created following supernova explosions
Pages: Radio Telescopes

Image: Illistration of Cosmic Rays from Galactic Centers.
Credit: Pierre Auger Observatory Team
Cosmic Strings: Vast filament-like structures of very high densities that may have occurred during the Dark Ages early in the Universe's history. Cosmic strings would have formed as relics of high-energy particle physics transitions in the early Universe.
Pages: Exotic Physics

Image: Snapshot of a cosmic string network during the transition from the radiation epoch to the matter epoch.
Credit: B. Allen & E.P. Shellard cosmic string simulations
Cosmic Web: The network of sheets, filaments, and voids into which matter in the Universe is organized. Gravitational growth creates asymmetric matter clumps that take the form of a network of intersecting sheets and filaments, separated by large regions of empty space, making the “skeleton” of the universe.
Pages: Cosmic Web

Image: A snapshot of the cosmic web, from the same simulation as above. This snapshot is at the present day, 13.7 billion years after the Big Bang.
Credit: Springel et al. (2005)
DAPPER (Dark Ages Polarimeter Pathfinder): A small lunar-orbiting telescope that would use the Moon as a shield from terrestrial interference to observe the average spin-flip background at several points during the Cosmic Dawn.
Pages: Lunar Telescopes

Image: concept of DAPPER on the moon.
Credit: University of Colorado and Tend Studios
DARE (Dark Ages Radio Explorer): A lunar-orbiting telescope that would use the Moon as a shield from terrestrial interference to observe the spin-flip background during the Cosmic Dawn, over a larger set of times than DAPPER.
Pages: Lunar Telescopes

Image: DARE mission concept
Credit: Credit: NASA NESS, University of Colorado, Boulder
Dark Ages: A time in the universe after the CMB was emitted but before stars or any other objects began producing light, spanning approximately 400,000 years to 50 million years after the Big Bang.
Pages: Cosmic Web, Exotic Physics, First Stars, Epoch of Reionization
Dark Matter: A form of matter that only interacts through gravity – its identity remains a mystery, but its gravity drove the formation of all structure in the Universe. There are many independent lines of evidence for dark matter in astronomy, but it has never been identified in laboratories.
Pages: Cosmic Web, Exotic Physics, First Stars, First Black Holes, First Galaxies, Larger and Larger Galaxies, Epoch of Reionization

Image: Small Magellanic Cloud superimposed with one half of a model of its dark matter (right of center), with lighter colors indicating greater density.
Credit: Dark matter, R. Caputo et al. 2016; background, Axel Mellinger, Central Michigan University"

Electron: A subatomic (smaller than an atom) particle that holds negative charge.
Pages: Big Bang, Cosmic Web, Spin Flip Background, Epoch of Reionization, Galaxies at Cosmic Noon

Image: Hydrogen, the simpliest atom.
Elliptical Galaxy: A common type of massive galaxy in the nearby Universe. Elliptical galaxies are spherical or (American) football-shaped collections of stars that lack any of the gas that can act as the fuel for future star formation. The origin of this lack of gas is a mystery but it may be connected to their central supermassive black holes.
Pages: Epoch of Reionization, Galaxies at Cosmic Noon
Emission Lines: Specific wavelengths of radiation emitted by a source with unique energy levels, such as an atom or molecule. In these systems, quantum mechanics demands that the electrons only exist at very specific energies. When an electron drops from one energy level to a lower one, it emits a photon (or particle of light) with a very specific energy and wavelength. When many of these events occur together, astronomers detect many photons with that same wavelength - which appears as an excess of photons at a specific wavelength.
Pages: Space Telescopes
Exoplanet: A planet outside of our solar system, orbiting another star.
Pages: Lunar Telescopes
FARSIDE (Farside Array for Radio Science Investigations of the Dark Ages and Exoplanets): A planned mission that would involve the construction of an array of radio antennae on the lunar surface, deployed using robotic rovers. The array would observe the Sun, exoplanets, and eventually the Cosmic Dawn. By placing the array on the far side of the Moon, the Moon itself would block radio waves from Earth - one of the major challenges to observing low-frequency radio waves.
Pages: Lunar Telescopes

Image: Concept of FARSIDE: A Low Radio Frequency Interferometric Array on the Lunar Farside
Credit: Courtesy of Blue Origin
Feedback: A key feature of star formation is that the stars themselves are powerful energy sources. They produce an extraordinary amount of light, and some end their lives in massive explosions called supernovae. This energy can disrupt nearby gas clouds, preventing those clouds from forming stars - and sometimes even shoving the gas entirely outside the galaxy!
Pages: Light Fills the Universe, First Galaxies, Larger and Larger Galaxies, Galaxies at Cosmic Noon
Galactic Buldge: A spherical or (American) football-shaped central region of a spiral galaxy. It contains gas and stars at a variety of ages around a supermassive black hole.
Pages: Our Galaxy: The Milky Way
Galactic Disk: The thin, pancake-like shape of a spiral galaxy. It contains most of the stars and star-forming gas clouds, as well as the spiral structure.
Pages: Galaxies at Cosmic Noon, Our Galaxy: The Milky Way

Image: The barred spiral galaxy M83, also known as the Southern Pinwheel, 15 million light-years away in the constellation Hydra.
Credit: NASA, ESA, and the Hubble Heritage Team
Galactic Halo: A large spherical region around a galaxy containing a sparse population of very old stars but very little gas. Many of these stars are organized into globular clusters.
Pages: Our Galaxy: The Milky Way

Image: The barred spiral galaxy M83, also known as the Southern Pinwheel, 15 million light-years away in the constellation Hydra.
Credit: NASA, ESA, and the Hubble Heritage Team
Galaxy: A discrete group of stars bound together by gravity that is able to form stars over long time periods. Galaxies form inside large clumps of dark matter.
Pages: First Galaxies

Image: The Andromeda galaxy, the most massive in the Local Group of galaxies (which encompasses our own Milky Way). This galaxy is much bigger than the first structures!
Credit: JPL, Caltech, NASA Galaxy Evolution Explorer
Galaxy Cluster: A very massive dark matter clump, more than a million billion times the mass of our Sun, that contains thousands of individual galaxies. Like galaxies, galaxy clusters are held together by dark matter. They are the most massive gravitationally-bound objects in the Universe today.
Pages: Light Fills The Universe, Galaxies at Cosmic Noon

Image: An assortment of majestic star cities that vary in age, shape, and stellar populations in galaxy cluster Abell 370.
Credit: NASA, ESA, J. Lotz and the HFF Team (STScI)
Gamma Ray: High-energy electromagnetic radiation, with the shortest wavelength on the electromagnetic spectrum, less than 10^-12 meters.
Pages: Exotic Physics

Image: Shows the types of wavelengths going from longest to shortest.
Credit: NASA
Globular Clusters: Clumps of about a million old stars packed closely together. Globular clusters contain some of the oldest stars in our Galaxy, but their origin is mysterious.
Pages: Galaxies at Cosmic Noon
Gravitational Wave: Ripples in space-time generated by massive gravitational events, such as the collisions of black holes. Gravitational waves are predicted by Einstein’s General Theory of Relativity.
Pages: Other Telescopes

Image: An artist's impression of gravitational waves generated by binary neutron stars.
Credit: R. Hurt/Caltech-JPL
Gravity: A force by which a star or other body draws matter or other objects toward its center. The strength of gravity is proportional to the mass of the objects and decreases as their separation increases.
Pages: Cosmic Web, Exotic Physics, First Stars, First Black Holes, First Galaxies
Heavy Elements: In astronomy, chemical elements with more than two protons are considered heavy - in other words, every element except hydrogen and helium! This definition is useful because hydrogen and helium were created in abundance by the Big Bang, but almost every other element is created much later, inside of stars. Elements with more protons than iron must be created in a supernova, and are considered heavy to astronomers.
Pages: First Galaxies, Larger and Larger Galaxies, Our Galaxy: The Milky Way
HERA (Hydrogen Epoch of Reionization Array): A large radio telescope in the Karoo Desert of South Africa, and one of the premier radio telescopes studying the Cosmic Dawn. HERA is an interferometer, so it combines signals from an array of individual radio dishes.
Pages: Space Telescopes

Image: Hydrogen Epoch of Reionization Array (HERA) 2016
Credit: HERA Construction Photo log
Hubble Space Telescope: Launched in 1990, this NASA telescope has detected hundreds of galaxies from the first billion years of the Universe’s history and obtained many of the images on this website. The telescope is named after Edwin Hubble, the astronomer who determined that the Universe is bigger than our Galaxy and discovered that the Universe is expanding.
Pages: Larger and Larger Galaxies, Space Telescopes, Infrared Telescopes

Image: Hubble Space Telescope in Earth Orbit.
Credit: JSC
Hydrogen Gas: Gas containing hydrogen, the simplest chemical element. Hydrogen consists of a single proton and electron bound together by their electrical attraction.
Pages: Light Fills The Universe, Spin Flip Background, Epoch of Reionization
Infrared Light: Light that is slightly less energetic than visible light, or with a slightly longer wavelength. Because light redshifts as it travels through the Universe, the visible and ultraviolet light produced by galaxies during the Cosmic Dawn is in the infrared range when it reaches us.
Pages: Our Galaxy: The Milky Way, Space Telescopes

Image: Shows the types of wavelengths going from longest to shortest.
Credit: NASA
Infrared Telescope: Infrared Telescopes measure near-infrared light, rather than the optical light our eyes see, and operate in an analogous way to digital cameras.
Pages: Infrared Telescopes

Image: The summit of Mauna Kea, Hawaii, with W. M. Keck Observatory.
Credit: NASA/JPL
Intensity Mapping: A method in which a telescope observes the sky with such blurry vision that it cannot distinguish individual galaxies. Even though the telescope cannot make out individual galaxies in a particular patch of the sky, it can measure all the light coming from them - including the faint ones invisible to larger telescopes.
Pages: Other Telescopes

Image: A simulated field with galaxy positions (Left) and the corresponding (CO) intensity map.
Credit: Patrick Breysse from Kovetz et al 2017
Interference: In physics, interference occurs when two or more waves combine. If the crests and troughs of the waves line up, they combine into a stronger wave, but if the crests of one wave overlap the troughs of another, the wave weakens. With radio telescopes, this interference can be unintentional from a nearby transmitter (like a radio station), which makes astronomical observations extraordinarily difficult!
Pages: Spin-Flip Background, Radio Telescopes, Lunar Radio Telescopes
Interferometer: A special kind of observatory - most commonly observing in the radio range - that combines signals from many different individual telescopes into a more powerful observation.
Pages: Radio Telescopes, Lunar Telescopes
Intergalactic: Pertaining to or located in the areas between galaxies.
Pages: Light Fills The Universe, Spin Flip Background, Epoch of Reionization, Galaxies at Cosmic Noon, Radio Telescopes, Other Telescopes
Interstellar Medium: Gas and dust that fills the space between the stars inside of a galaxy. This is the material from which the stars form.
Pages: Light Fills The Universe, Larger and Larger Galaxies
Ionize: To change the electric charge of an atom by adding or removing electrons, yielding a net electric (non-neutral) charge. In ionized hydrogen, the proton and electron have been separated, so the spin-flip background cannot be produced.
Pages: Spin Flip Background, Epoch of Reionization

Image: Energy, such as in the form of a photon, exciting the electron enough so that it breaks it's bond with the hydrogen proton. This leaves a free electron and a positively charged hydrogen ion.
Ionosphere: The layer of the earth's atmosphere ionized by solar radiation that absorbs and reflects radio waves. The layer is about 50 to 600 miles (80 to 1,000 km) above the surface.
Pages: Radio Telescopes

Image: A sunset on the Indian Ocean, blue layers are the upper atmosphere including the ionosphere.
Credit: Expedition 23 crew member on the International Space Station (ISS)
Irregular Galaxy: A “messy” galaxy with a disturbed configuration, rather than a disk or elliptical galaxy. Usually, such galaxies are either rather small or in the midst of colliding with another nearby galaxy.
Pages: Larger and Larger Galaxies

Dwarf Galaxy NGC 1140, a modern galaxy similar to those in the early Universe.
Image Credit: NASA Goddard
James Webb Space Telescope: To be launched in late 2021, this infrared NASA telescope will succeed the Hubble Space Telescope and be able to see further into the Universe’s history than any telescope before. It is named after the NASA Administrator James Webb, who ran the program when it was only a ‘fledgling’ space agency in the 1960s, and who many believe led the program to have the high value on science that now motivates missions such as these today.
Pages: Larger and Larger Galaxies, Space Telescopes

Image: James Webb Space Telescope concept
Credit: Northrop Grumman
Jets (of a Black Hole): While most of the matter near a black hole flows into it, powerful magnetic fields can deflect some of the material away - and even accelerate it into a long linear jet of material flowing far out of the black hole's environment.
Pages: Galaxies at Cosmic Noon

Image: A snapshot of Centaurus A's central black hole's jets.
Credit: ESO/WFI (visible); MPIfR/ESO/APEX/A.Weiss et al. (microwave); NASA/CXC/CfA/R.Kraft et al. (X-ray)
Keck Telescopes: A pair of infrared telescopes on top of Mauna Kea in Hawaii, which are some of the largest telescopes today with primary mirrors about 10 meters across (or over 30 feet).
Pages: Infrared Telescopes

W. M. Keck Observatory in Mauna Kea, Hawaii.gr
Image Credit: Ethan Tweedie Photography/W. M. Keck Observatory
Light-Year: The distance light can travel in one year, about six trillion miles (or nearly ten trillion kilometers).
Pages: Cosmic Noon, Our Galaxy: The Milky Way

Credit: NASA/JPL-Caltech
LIGO (Laser Interferometer Gravitational-Wave Observatory): An experiment that detected the first gravitational waves from nearby black holes in 2015. LIGO consists of two widely-separated interferometers within the United States—one in Hanford, Washington and the other in Livingston, Louisiana—operating in unison to detect gravitational waves.
Pages: Other Telescopes
LISA (Laser Interferometric Gravitational Observatory): Due to launch in the 2030s, this observatory hopes to use gravitational waves to study distant black holes, possibly all the way back to the Cosmic Dawn.
Pages: Other Telescopes

Image: The LISA mission will consist of three spacecraft, using lasers to precisely measure how their separation changes.
Credit: © NASA/Simon Barke
Luminosity: A measure of brightness defined as the total energy emitted by an object per unit time. It’s most commonly measured in joules per second or watts, but astronomers often use other units, especially magnitudes.
Pages: Cosmic Web, First Stars
Lyman-Alpha Line: An emission line from the hydrogen atom, in which the electron shifts from its first excited state to its lowest-energy state. Because nearly all of the atoms in the Universe are hydrogen, this can be an extremely important line for astronomers.
Pages: Infrared Telescopes
Messier: Charles Messier was a French Astronomer in the 18th century who catalogued many of the brightest nebulae in the night sky. His catalog is still used to identify these objects today.
Pages: Our Galaxy

Image: Messier objects as seen by the Hubble Space Telescope.
Credit: NASA, ESA/Hubble and the Hubble Heritage Team
Meteor Trails: A trail of ionized gas behind a meteor passing through the upper atmosphere that is capable of reflecting radio waves.
Pages: Lunar Telescopes

Image: A time-lapse of Perseid meteors, August 2009.
Credit: NASA/JPL
Microwave Light: A type of electromagnetic wave that is more energetic than radio, but less energetic than infrared. Microwaves have a wavelength from one meter to one millimeter. And yes, microwave ovens use this type of light to heat your food!
Pages: Big Bang and CMB, Other Telescopes

Image: Shows the types of wavelengths going from longest to shortest.
Credit: NASA
Milky Way: The galaxy in which our solar system resides. It is a fairly average spiral galaxy today.
Pages: First Stars, Light Fills The Universe, First Galaxies, Larger and Larger Galaxies, Radio Telescopes

Image: This infrared image from NASA Spitzer Space Telescope shows hundreds of thousands of stars crowded into the swirling core of our spiral Milky Way galaxy.
Credit: NASA/JPL-Caltech
Molecular Hydrogen: A pair of two hydrogen atoms latched together. This configuration is often written as H₂.
Pages: Light Fills The Universe

Image: A diagram of two hydrogen bound to form a molecule.
Nancy Grace Roman Telescope: The Roman Telescope is a future space telescope that will have a wide-angle camera enabling it to survey huge parts of the sky. (In contrast, most astronomical telescopes have tiny fields of view.) It will observe some of the brightest galaxies during the Cosmic Dawn. It is named for Nancy Grace Roman, one of the founders of NASA's astronomy program.
Pages: Infrared Telescopes

Image: Nancy Grace Roman Telescope concept
Credit: GSFC/SVS
Nebula: A tenuous cloud of gas between the stars. Often, nebulae are the sites of star formation.
Pages: Space Telescopes

Image: Hubble's most famous image: towers of dust and gas in the Eagle Nebula (M16), named 'Pillars of Creation'.
Credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA)
Neutral Hydrogen: Atomic hydrogen with no net charge, meaning it contains both a proton and electron.
Pages: Big Bang, Cosmic Web, Exotic Physics

Image: Hydrogen, the simpliest atom, in its neutral form.
Neutron: A subatomic particle with the same mass as a proton but without an electric charge. Neutrons are found in the nuclei of nearly all atoms, except for hydrogen.
Pages: Big Bang

Image: Diagram of helium atom, green indicating neutrons.
Nuclear Fusion: This process occurs when the nuclei of light elements collide and stick together as heavier elements, such as hydrogen combining to form helium. During fusion, some of the matter inside the nuclei of the original elements is converted to energy and released, making the reaction exothermic.
Pages: First Stars

Image: A diagram of a particular type of fusion called deuterium–tritium fusion, where deuterium and tritium (forms of hydrogen with neutrons) combine to form helium as well as an extra neutron and release energy. There are many forms of nuclear fusion in a star, combining to make elements up to iron.
Photon: Particles of light. According to quantum mechanics, light has the properties of both a particle and a wave. But it always travels in discrete packets called photons.
Pages: Big Bang

Image: common representaion for a photon in diagrams.
Polarization (of Light): Light oscillates as a wave with crests and troughs. Polarization describes the direction in which those oscillations occur.
Pages: Epoch of Reionization, Other Telescopes
Population III: See Primordial Stars.
Primary Mirror: The largest mirror on a telescope, which determines how much light it collects and how sharply it can image objects. Telescopes have other mirrors (secondaries and tertiaries) to redirect the light reflected by the primary mirror to the cameras or other measuring devices.
Pages: Infrared Telescopes

Image: James Webb Space Telescope's primary mirror after the telescope was assembled into its final form.
Credit: NASA/Chris Gunn
Primordial Black Holes: A small black hole that formed in the dark ages through direct gravitational collapse or other exotic means, before any stars had first formed.
Pages: Exotic Physics
Primordial Gas: Gas composed entirely of hydrogen and helium and lacking any heavier elements. This mixture was created just a few minutes after the Big Bang, while heavier elements could not be created until the first stars formed.
Pages: Light Fills The Universe, Spin Flip Background
Primordial Stars: The first stars in the universe, also known as Population III stars. These stars formed from gas clouds that lacked any elements heavier than hydrogen or helium, so they formed through different mechanisms than stars like our Sun.
Pages: First Stars, Larger and Larger Galaxies
Proton: A positively charged subatomic particle found in the nucleus of an atom. The number of protons in the nucleus determines the atomic number of the element.
Pages: Big Bang, Cosmic Web, Spin Flip Background, Epoch of Reionization

Image: Hydrogen, the simpliest atom.
Quasar: An extremely bright astronomical object whose exceptional luminosity is generated by gas falling onto a supermassive black hole.
Pages: First Black Holes, Epoch of Reionization, Galaxies at Cosmic Noon

Image: Artist concept illustrates a quasar, or feeding black hole
Credit: JPL
Quenching: Generally meaning to stifle or supress, but in astronomy referred to as the halting of star formation in a galaxy due to a lack of cold gas. Its cause is not entirely clear, but it may be related to powerful quasars inside of massive galaxies.
Pages: First Black Holes, Epoch of Reionization, Galaxies at Cosmic Noon
Radio Waves: Low-energy electromagnetic radiation (or light) with the longest wavelength on the electromagnetic spectrum, as short as 1 millimeter or as long as hundreds of kilometers.
Pages: Exotic Physics, Radio Telescopes

Image: Shows the types of wavelengths going from longest to shortest.
Credit: NASA
Radio Telescope: Telescopes that observe light with long wavelengths - or radio waves. Radio telescopes usually act like very powerful satellite dishes.
Pages: Radio Telescopes

Image: Parkes telescope in Australia, part of the Commonwealth Scientific and Industrial Research Organization.
Credit: CSIRO/Shaun Amy
Redshift: As they travel through the expanding Universe, the wavelengths of photons are stretched out. This makes them lose energy and at least for visible light, become more red. The redshift is often represented by the letter z, which corresponds to the fractional change in the wavelength relative to the original.
Pages: Big Bang, First Galaxies

Image: The universe is expanding, and that expansion stretches light traveling through space in a phenomenon known as cosmological redshift.
Credit: NASA, ESA, and L. Hustak (STScI)
Reionization: When galaxies began to grow, the hot stars within them sent out such energetic photons that the protons and electrons in the intergalactic hydrogen gas were torn apart from each other - meaning the hydrogen was ionized across the Universe!
Pages: Big Bang, First Galaxies, Spin Flip Background, Epoch of Reionization

Star cluster NGC 3603, a modern example of a young cluster surrounded by gas and dust, helping us understand massive star formation in the early universe.
Image Credit: NASA, ESA, R. O'Connell, F. Paresce, E. Young, WFC3 Science Oversight Committee, and Hubble Heritage Team
SPHEREx: A NASA mission that will launch in 2024 and aims to track emission lines across the Universe. Some of these will come from early galaxies.
Pages: Other Telescopes

Image: SPHEREx concept
Credit: NASA JPL
Spin: In quantum mechanics, many subatomic particles have a quantity called spin, which describes their angular momentum (or their rotational inertia). Spin has both a strength (how fast the object rotates) and a direction.
Pages: Spin Flip Background
Spin-Flip Background, Spin-Flip Radiation or Spin-Flip Signal: see spin-flip transition.
Spin-Flip Transition: There is a tiny energy difference between atomic states in which the spins of protons and electrons are aligned with each other and in which they are opposite each other: when an electron “flips” its spin between these states in a hydrogen atom, it produces a photon (or particle of light) with a wavelength of about 21 centimeters (or about 8 inches).
Pages: Spin Flip Background

Image: Ground state hyperfine levels of hydrogen (parallel and antiparallel) with the spin-flip transition, emitting radiation at 1420 MHz. The corresponding wavelength is 21 cm.
Credit: By Tiltec - Own work, Public Domain, Wikimedia Commons
Spiral Galaxy: At the present day, disk galaxies usually show clear spiral patterns. These patterns are generated by spiral waves (also known as density waves): these are small compressions traveling outward through the galaxy, triggering star formation that appears as spiral patterns in blue light.
Pages: Our Galaxy
Spiral Waves: See Spiral Galaxy.
Star: A massive ball of gas bound by gravity so tightly that it heats up to temperatures high enough to sustain nuclear fusion, which in turn creates pressure to balance the enormous gravity of the star's material.
Pages: First Stars

An artist's conception of a blue giant, a star similar to those in the Cosmic Dawn.
Image Credit: NASA's Goddard Space Flight Center/S. Wiessinger
Supermassive Black Hole: In contrast to a stellar mass black hole, which is similar in size to our Sun, a supermassive black hole is millions to billions times the mass of our Sun, and almost always found at the center of a galaxy.
Pages: First Black Holes, Larger and Larger Galaxies, Galaxies at Cosmic Noon
Supernova: A powerful explosion in which nearly the entire outer layers of a massive star are expelled with enormous energies. Supernovae occur at the end of a massive star’s life, and they are so bright that they can be seen from far across the Universe.
Pages: First Stars, First Galaxies, Larger and Larger Galaxies, Galaxies at Cosmic Noon

Image: The Crab Nebula is a modern example of a supernova remnant, a type of dead star.
Credit: NASA, ESA, J. Hester, A. Loll (ASU)
Synchrotron Radiation: When electrons traveling near the speed of light encounter magnetic fields, they travel in spirals and emit light. Synchrotron radiation is responsible for aurorae on Earth (when particles from the Sun encounter the Earth's magnetic field) and also for strong radio emission from the Milky Way.
Pages: Lunar Telescopes

Image: This all sky background image is mostly due to the synchrotron background.
Credit: NASA Goddard
Thirty-Meter Telescope (TMT): A new giant telescope that hopes to begin observing in the next decade. The mirrors will be about one hundred feet across – larger than three tennis courts laid side to side! This extraordinary size will enable it to better study the faint sources of the Cosmic Dawn.
Pages: Lunar Telescopes

Image: Artist's conception of the future 30-meter telescope.
Credit: TMT International Observatory
Ultraviolet Light: Also known as UV light, this is electromagnetic radiation with wavelengths shorter than visible light ( less than 400 nanometers) but longer than X-rays ( more than 10 nanometers). UV light is created by very massive, hot stars and quasars.
Pages: First Black Holes, Light Fills The Universe, Spin Flip Background, Epoch of Reionization, Other Telescopes

Image: Shows the types of wavelengths going from longest to shortest.
Credit: NASA
X-Ray: High-energy electromagnetic radiation with wavelengths shorter than ultraviolet light ( less than 10 nanometers) but longer than gamma rays ( more than 10 picometers) on the electromagnetic spectrum. In astronomy, X-rays are usually created by matter near black holes.
Pages: First Black Holes, Light Fills The Universe, Spin Flip Background, Other Telescopes

Image: Shows the types of wavelengths going from longest to shortest.
Credit: NASA
z: See Redshift.