Astronomy	The branch of science that deals with the Universe and the various objects, like stars, planets, and galaxies, that we find within it. Cosmology and astrophysics are closely related to astronomy, and the words are sometimes used interchangeably. Cosmology focuses on the Universe’s largest scales in space and time, and astrophysics focuses on the properties and interactions of astronomical objects.
Atom	A small unit of matter composed of protons, electrons, and usually neutrons. Atoms are basic building blocks of the matter we see in the Universe and on Earth. The number of protons in the nucleus of an atom determines which chemical element it is.
Authority	A respectable or credible source; an expert.
Big Bang	A theory, first articulated in the 1920s, proposing that the Universe started out extremely hot and dense and gradually cooled off as it expanded.
Cepheid	A star that fluctuates in brightness and provides astronomers with a reference they can use to measure great distances in the Universe. It was the identification of Cepheids in nearby galaxies that first proved that the Universe consists of more than one galaxy.
Claim	An assertion that something is true.
Claim testing	The use of strategies to decide whether a story or concept should or should not be trusted. The four strategies for claim testing that we use in Big History are intuition, authority, logic, and evidence.
Collective Learning	The ability to share, preserve, and build upon ideas over time.
Cosmic Microwave Background (CMB)	Low-energy radiation pervading the entire Universe, released about 380,000 years after the Big Bang. At this point, the Universe had cooled sufficiently for atoms to form and allow radiation and matter to separate.
Cosmology	The study of the Universe on its largest scales, including its origin.
Doppler Effect	The apparent stretching out or contraction of waves because of the relative movement of two bodies. The Doppler effect explains why an ambulance siren seems higher when the ambulance is traveling toward you than when it is moving away. It also helps astronomers identify whether objects such as stars or galaxies are moving toward us or away from us.
Electromagnetism	One of the four fundamental forces or interactions, along with gravity, the weak nuclear force, and the strong nuclear force. Among other things, electromagnetism is responsible for the interaction between electrically charged particles, including holding electrons and protons together to form atoms. Electromagnetism is also responsible for essentially all molecular interactions.
Electron	A negatively charged subatomic particle that orbits the nucleus of an atom.
Energy	The capacity to do work, associated with matter and radiation. Includes kinetic energy, potential energy, and chemical energy, among others.
Evidence	Concrete, verifiable information that either supports or disproves a claim.
Gravity	The fundamental force of attraction between any two objects that have mass.
Helium	The second simplest of all chemical elements, helium has two protons and (almost always) two neutrons. Helium was produced soon after the Big Bang.
Hydrogen	The simplest of all chemical elements, hydrogen has one proton. Hydrogen was the first element produced after the Big Bang and is the most common element in the Universe.
Inflation	The idea that space and time (space-time) underwent an expansion at a rate much faster than the speed of light during the first 10-36 seconds after the Big Bang.
Intuition	A “gut feeling” that is not necessarily based on logic or evidence.
Light-Year	A measure of distance in space; the distance that light travels in a vacuum in one year. It is equal to roughly 9.5 trillion kilometers, or 5.9 trillion miles.
Logic	The application of systematic reasoning to arrive at a conclusion.
Matter	The physical material of the Universe, including subatomic particles, atoms, and the substances that are built out of them.
Neutron	An electrically neutral subatomic particle present in the nuclei of most atoms. Unlike protons, the number of neutrons in a given element can vary, giving rise to different isotopes of an element.
Nucleus (atomic)	The extremely dense and positively-charged region at the center of an atom that consists of protons and neutrons.
Parallax	The change in the apparent position of an object caused by movement of the observer.
Proton	A subatomic particle with a positive electric charge. The number of protons in an atom (the atomic number) determines which element it is: For example, carbon atoms always have 6 protons, while iron atoms always have 26 protons.
Redshift	The phenomenon in which light waves from distant galaxies are “stretched out,” which for visible light means a shift toward the red side of the spectrum. Redshift provides scientists with strong evidence that the Universe is expanding, since the expansion of space explains the stretching of the light waves.
Scientific Method	The process of gathering evidence to test and refine scientific theories.
Space-time	The unification of space and time into a single four-dimensional continuum or “fabric.” Space makes up three of the dimensions, while time makes up the fourth, and cannot be fully separated from space. Albert Einstein’s General Theory of Relativity holds that all objects with mass interact with space-time by bending it much like a person standing on a trampoline bends the trampoline.
Telescope	An instrument used for viewing distant objects, including planets, stars, and galaxies.
Thermodynamics (first law of)	One form of the law of conservation of energy, which states that energy may change forms but cannot be created or destroyed.