The Universe Adventure - Glossary: F to J

### Glossary: F to J

Finite- Having limits or bounds, a set amount.

Fission- The process by which a heavy nucleus breaks apart into two, more stable nuclei, resulting in a release of energy.

Flat Universe- A model of the Universe where space is not curved, but is instead, geometrically flat. In a flat Universe, the total energy density equals the critical density. That is, Ω=1.

Flatness Problem- A question in cosmological physics that asks what has caused the Universe to be so flat given that under the influence of gravity, any non-flat area should quickly get larger. For example, a closed Universe should recollapse, while an open Universe should expand so that no two objects are near one another. The Flatness Problem essentially asks why the Universe is balanced between these two extremes.

Fluctuation- A variation in quantity from the average.

Flux- In astronomy, the rate of flow of radiant energy across a given area.

Frame of Reference- The perspective from which events are viewed. For example, a man stands on the side of the road and sees a woman drive past in a car. From the man's frame of reference, he is stationary and the woman speeds past. However, from the woman's frame of reference, the man appears to be speeding past her in the opposite direction but at the same speed. Measurements (like speed or direction) are taken from an "inertial frame of reference", an imaginary static point where events are viewed. If the speed of a spaceship is measured from earth, then the earth is the inertial frame. While the Earth is not actually stationary, we pretend it is for the purpose of measuring.

Fundamental Interactions- In physics, the strong, electromagnetic, weak, and gravitational interactions, or forces. These forces are the mechanisms that govern every observed physical phenomenon, and cannot be explained in terms of other interactions.

Fundamental Particles- The smallest and most basic constituents of matter that transmit the fundamental interactions. In the Standard Model, quarks, leptons, photons, gluons, W+ and W- bosons, and the Z bosons are fundamental. All other known particles are comprised of these fundamental particles.

Fusion- The process by which two or more nuclei of low atomic number or baryons fuse to form a heavier nucleus with a release of energy.

Galaxy- A cluster of billions of stars, gas, and dust held together by gravity. Galaxies come in a variety of shapes (elliptical, spiral, etc.) and sizes. While galaxies are incredibly luminous, physicists speculate that most galactic mass comes from Dark Matter.

General Relativity- A theory that describes gravity as the curvature of space and time due to the presence of mass (rather than a force, as it is treated in classical, Newtonian mechanics) first published by Albert Einstein in 1915.

Geocentric- A model of the Universe, developed by the Greek astronomer Ptolemy, that places the earth at the center of the Universe with the sun and planets located in rotating, concentric, crystalline spheres surrounding Earth.

Geometries- Models of curvature in which the laws of geometry differ depending on the whether the Universe is flat, spherical, or saddle-shaped.

Globular Clusters- A ball-like structure of a few hundred thousand old stars that resides in the halo of a galaxy.

Gluon- An elementary particle that allows for the interaction of quarks, as well the binding of protons and neutrons together in atomic nuclei via the Strong Force.

Gravitational Attraction- The force of attraction between all bodies with mass.

Gravitational Lensing- The distortion of an image due to a very massive object. Light from a distant source can be bent around the object, often allowing us to see behind it.

Hadron- A subatomic particle made up of quarks such as protons and neutrons. All hadrons interact via the Strong Force and are subject to gravity, while charged hadrons are influenced by Electromagnetic Forces.

H0, Hubble Constant- A measure of the present expansion rate of the Universe, named after Edwin Hubble who discovered that the redshifts of galaxies are directly proportional to their distance from the Milky Way. Current techniques measure the Hubble Constant to be approximately between 70 and 80 kilometers per second per megaparsec (A megaparsec is equal to 3.26 x 106 light-years).

Heliocentric- A model first proposed by Nicolaus Copernicus that places the sun, fixed near the center of the Universe, with the Earth and other planets making revolutions once a year.

Homogeneous- The same at every location.

Hubble's Law- The law, formulated by Astronomer Edwin Hubble in 1929, that states the redshift in light coming from distant galaxies is proportional to their distance from the Earth. Hubble's Law is considered to be one of the first pieces of observational evidence for the Big Bang.

Hypothesis- A proposed explanation made on the basis of limited evidence, used as the starting point for further investigation as described by the Scientific Method.

Inflation- A phase of the early Universe marked by rapid, accelerating expansion. Inflation helps explain why the Universe appears flat, homogeneous, and isotropic.

Inhomogeneity- Something not evenly distributed in space; a clump or cluster.

Initial Conditions- The nature of a system's state when time equals zero.

Intensity- The energy moving through a region of area per unit of time. Intensity is proportional to the light's photon density (the number of photons contained in a square meter).

Intrinsic Luminosity- The amount of energy actually emitted into space by an object, as opposed to how bright the object appears from Earth.

Inverse Square- 1 divided by a quantity squared: 1/(r2).

Ion- An atom or molecule with a net electric charge (either positive or negative) resulting from a loss or gain in number of electrons.

Isotopes- Atoms that have the same number of protons but a different number of neutrons.

Isotropic- Having the same value regardless of direction measured.