Scientists Say: Observable universe

arc: A curve, often mapping out what appears to be part of a circle.

Big Bang: The rapid expansion of dense matter and space-time that, according to current theory, marked the origin of the universe. It is supported by astronomers’ current understanding of the composition and structure of the universe.

constant: Continuous or uninterrupted. (in mathematics) A number that is known and unchanging, usually based on some mathematical definition. For example, π (pi) is a constant equal to 3.14. . . and defined as the circumference of a circle divided by its diameter.

cosmic: An adjective that refers to the cosmos — the universe and everything within it.

light-year: The distance light travels in one year, about 9.46 trillion kilometers (almost 6 trillion miles). To get some idea of this length, imagine a rope long enough to wrap around the Earth. It would be a little over 40,000 kilometers (24,900 miles) long. Lay it out straight. Now lay another 236 million more that are the same length, end-to-end, right after the first. The total distance they now span would equal one light-year.

matter: Something that occupies space and has mass. Anything on Earth with matter will have a property described as “weight.”

radiation: (in physics) One of the three major ways that energy is transferred. (The other two are conduction and convection.) In radiation, electromagnetic waves carry energy from one place to another. Unlike conduction and convection, which need material to help transfer the energy, radiation can transfer energy across empty space.

radio: Referring to radio waves, or the device that receives these transmissions. Radio waves are a part of the electromagnetic spectrum that people often use for long-distance communication. Longer than the waves of visible light, radio waves are used to transmit radio and television signals. They also are used in radar. Many astronomical objects also radiate some of their energy as radio waves.

radio waves: Waves in a part of the electromagnetic spectrum. They are a type that people now use for long-distance communication. Longer than the waves of visible light, radio waves are used to transmit radio and television signals. They also are used in radar.

radius: (plural: radii) A straight line from the center to the circumference of a circle or sphere. (in physiology) The smaller of two bones in the forearm (the other is the ulna) running between the wrist and the elbow.

telescope: Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.

theory: (in science) A description of some aspect of the natural world based on extensive observations, tests and reason. A theory can also be a way of organizing a broad body of knowledge that applies in a broad range of circumstances to explain what will happen. Unlike the common definition of theory, a theory in science is not just a hunch. Ideas or conclusions that are based on a theory — and not yet on firm data or observations — are referred to as theoretical. Scientists who use mathematics and/or existing data to project what might happen in new situations are known as theorists.

trillion: A number representing a million million — or 1,000,000,000,000 — of something.

universe: The entire cosmos: All things that exist throughout space and time. It has been expanding since its formation during an event known as the Big Bang, some 13.8 billion years ago (give or take a few hundred million years).

visible light: A type of electromagnetic radiation with wavelengths that range between 380 nanometers (violet) and 740 nanometers (red). Visible light has wavelengths that are shorter than infrared light, microwaves, and radio waves but longer than ultraviolet light, X-rays and gamma rays.

wave: A disturbance or variation that travels through space and matter in a regular, oscillating fashion.

X-ray: A type of radiation analogous to gamma rays, but having somewhat lower energy.