Hunting for dark energy with the WiggleZ

Glossary

baryon. Refers to all atomic matter, and other heavy subatomic particles that are made from three quarks.

baryonic wiggles. The imprint of sound waves in the early universe on the distribution of matter forming a series of peaks and troughs.

cluster. (astronomy). A group of galaxies held together by gravity. A galaxy cluster can contain from tens to several thousand galaxies.

cosmic microwave background (CMB). Background radiation left over from the Big Bang. The radiation has expanded in wavelength over time so that it is now mostly in the microwave region of the electromagnetic spectrum (but can also be detected at radio and infrared wavelengths). The CMB is at an almost constant intensity throughout the universe and is often used as evidence that the Big Bang occurred.

electromagnetic radiation. Electromagnetic radiation is simply energy which travels through space at about 300,000 kilometres per second – the speed of light. We imagine radiation moving like a wave. The distance between two adjacent wave crests is called a wavelength. The shorter the wavelength, the more energetic the radiation is said to be. Electromagnetic radiation can also be described by its frequency, or how many peaks and troughs per second are reaching an observer. Frequency is inversely related to wavelength. The energy of light, if it is visable to humans, affects the colour it appears. Also, the shorter the wavelength, the greater the frequency of the radiation. Other than wavelength, frequency and energy there is no difference between a radio wave, an X-ray and the colour green. They all possess the same physical nature. For more information see Electromagnetic radiation (Back to Basics, Australian Academy of Science) and Electromagnetic Spectrum (NASA Goddard Space Flight Center, USA).

frequency. A measure of how frequently a wave goes up and down (oscillates) or the number of waves passing by in a second. A hertz is a unit of frequency – 1 oscillation per second; a kilohertz (kHz) is 1000 hertz – 1000 oscillations per second; a megahertz is 1 million hertz – 1 million oscillations per second. For more information see Sound properties and their perception – pitch and frequency (The Physics Classroom, USA).

Hubble constant. By observing distant galaxies Edwin Hubble showed a relationship between a galaxy’s speed away from us (recessional velocity, v) and its distance (d).

Ho=v/d

The Hubble constant, H0, corresponds to the current rate of expansion of the universe.

neutron. A particle having no charge that is a constituent of an atom. It has a mass similar to a proton.

optical fibre. A glass or plastic thread that acts as a guide for lightwaves. Light entering one end of the fibre is trapped and travels down its length as it is reflected internally off the walls. For more information see Communicating with light – fibre optics (Australian Academy of Science).

particle accelerator. A machine, such as the Large Hadron Collider, that accelerates charged particles to high speeds. Electric fields are used to speed up the particles and magnetic fields are used to guide them. Accelerators can be in a ring shape or in a straight line and are used to study subatomic particles.
For more information see LHC the guide (CERN).

plasma. A gas containing free-moving charged particles (ions and electrons). Because the numbers of positive and negative particles are equal, plasma is electrically neutral. Plasma forms when a gas is raised to such a high temperature that the atoms separate into electrons and ions. It is the fourth state of matter because its properties are different from those of solids, liquids, and other gases. Stars such as our Sun are mostly made of plasma.

proton. A particle with positive electric charge of the opposite sign to an electron. Protons are present in the nucleus of all atoms. The proton is the same as a hydrogen ion or the nucleus of a hydrogen atom.

quark. A basic particle that makes up protons, neutrons and other baryons.

redshift. The shift in the spectrum from an object towards longer wavelengths or the red end of the spectrum. Spectral lines from the object are compared to those from sources of light on Earth. Redshift occurs when an object such as a star moves away from the observer or when light travels through the expanding universe.

spectrograph (spectrometer/spectroscope). An instrument used to analyse the spectrum from a source of radiation such as a star. Electromagnetic radiation from the source is spread out into its constituent wavelengths (like a rainbow).

spectroscopy. The technique of detecting and analysing the spectrum of an object to get information on its chemical and physical nature (eg, temperature, motion). Using a spectroscope the radiation or light from an object is dispersed into its different colours or wavelengths (like a rainbow). The position of emission and absorption lines in the spectrum provides information on what chemicals are present. For example, emission at a wavelength of 21 centimetres in the radio range of the spectrum (hydrogen also emits light in the visible region) corresponds to hydrogen. Large telescopes have spectroscopes to measure the properties of astronomical objects.

spectrum. Plural spectra. The distribution of electromagnetic radiation when it is dispersed (eg, the dispersal of visible light into a rainbow). Astronomers gain different information about astronomical objects by examining their spectra from different parts of the electromagnetic spectrum (eg, visible light, radio waves, X-rays).

square degrees. While degrees are used to measure angles in a circle, square degrees are used to measure angles in a sphere. For example, from Earth the Moon has a diameter of approximately 0.5°, but the whole Moon covers around 0.20 deg². For more information see Steradian (Math is fun).

supercluster. A very large grouping of galaxies, or a group of clusters of galaxies. Galaxy superclusters are believed to be the largest structures in the universe with tens of thousands of galaxies covering over one hundred million light-years.

wavelength. The distance between two adjacent wave crests. Visible light and X-rays are both electromagnetic waves and differ from each other only in the length of the wave. The wavelength of visible light ranges from 400 to 700 nanometres while the wavelength of X-rays ranges from about 0.01 to 10 nanometres. For more information see Electromagnetic radiation (Back to Basics, Australian Academy of Science).


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Posted June 2009, edited September 2012.