Frequently Asked Questions in Cosmology Tutorial : Part 1 | Part 2 | Part 3 | Part 4 | Age | Distances | Bibliography | Relativity What is the currently most accepted model for the Universe? The current best fit model is a flat ΛCDM Big Bang model where the expansion of the Universe is accelerating, and the age of the Universe is 13.7 billion years. Back to top. What is the evidence for the Big Bang? The evidence for the Big Bang comes from many pieces of observational data that are consistent with the Big Bang. The darkness of the night sky - Olbers' paradox. Why do we think that the expansion of the Universe is accelerating? The evidence for an accelerating expansion comes from observations of the brightness of distant supernovae. What is quintessence? Quintessence, or the fifth essence, is a fifth element beyond the standard earth, air, fire and water of ancient chemistry. If the Universe is only 14 billion years old, why isn't the most distant object we can see 7 billion light years away? What is the redshift?
List of particles This article includes a list of the different types of atomic- and sub-atomic particles found or hypothesized to exist in the whole of the universe categorized by type. Properties of the various particles listed are also given, as well as the laws that the particles follow. For individual lists of the different particles, see the list below. Elementary particles Fermions Fermions are one of the two fundamental classes of particles, the other being bosons. Fermions have half-integer spin; for all known elementary fermions this is 1⁄2. Quarks Leptons Jump up ^ The electron mass is known very precisely as 0.5109989461±0.0000000031 MeVJump up ^ The muon mass is known very precisely as 105.6583745±0.0000024 MeV Bosons Bosons are one of the two fundamental classes of particles, the other being fermions. According to the Standard Model the elementary bosons are: The Higgs boson is postulated by the electroweak theory primarily to explain the origin of particle masses.
Ionization Ionization or ionisation, is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons to form ions, often in conjunction with other chemical changes. Ionization can result from the loss of an electron after collisions with subatomic particles, collisions with other atoms, molecules and ions, or through the interaction with electromagnetic radiation. Heterolytic bond cleavage and heterolytic substitution reactions can result in the formation of ion pairs. Ionization can occur through radioactive decay by the internal conversion process, in which an excited nucleus transfers its energy to one of the inner-shell electrons causing it to be ejected. Uses Everyday examples of gas ionization are such as within a fluorescent lamp or other electrical discharge lamps. Production of ions Avalanche effect between two electrodes. Ionization energy of atoms Figure 1. Semi-classical description of ionization is given by and
Ununoctium The radioactive ununoctium atom is very unstable, due to its high mass, and since 2005, only three or possibly four atoms of the isotope 294Uuo have been detected. While this allowed for very little experimental characterization of its properties and possible compounds, theoretical calculations have resulted in many predictions, including some unexpected ones. For example, although ununoctium is a member of Group 18, it may possibly not be a noble gas, unlike all the other Group 18 elements. It was formerly thought to be a gas but is now predicted to be a solid under normal conditions due to relativistic effects. History Unsuccessful synthesis attempts In late 1998, Polish physicist Robert Smolańczuk published calculations on the fusion of atomic nuclei towards the synthesis of superheavy atoms, including ununoctium. His calculations suggested that it might be possible to make ununoctium by fusing lead with krypton under carefully controlled conditions.
Antimatter In modern physics, antimatter is defined as a material composed of the antiparticle (or "partners") to the corresponding particles of ordinary matter. In theory, a particle and its anti-particle (e.g., proton and antiproton) have the same mass as one another, but opposite electric charge and other differences in quantum numbers. For example, a proton has positive charge while an antiproton has negative charge. A collision between any particle and its anti-particle partner is known to lead to their mutual annihilation, giving rise to various proportions of intense photons (gamma rays), neutrinos, and sometimes less-massive particle–antiparticle pairs. Annihilation usually results in a release of energy that becomes available for heat or work. The amount of the released energy is usually proportional to the total mass of the collided matter and antimatter, in accordance with the mass–energy equivalence equation, E = mc2. Formal definition History of the concept Notation
Plasma (physics) Langmuir described the plasma he observed as follows: For plasma to exist, ionisation is necessary. The term "plasma density" by itself usually refers to the "electron density", that is, the number of free electrons per unit volume. The degree of ionisation of a plasma is the proportion of atoms that have lost or gained electrons, and is controlled by the electron and ion temperatures and electron-ion vs electron-neutral collision frequencies. , is defined as , where is the number density of ions and is the number density of neutral atoms. of the ions through is the number density of electrons. In a plasma, the electron-ion collision frequency is much greater than the electron-neutral collision frequency . , the electron-ion collision frequency can equal the electron-neutral collision frequency: is the limit separating a plasma from being partially or fully ionized. Most of "technological" (engineered) plasmas are weakly ionized gases. is the "electron gyrofrequency" and (where is the velocity, and
Electromagnetic radiation The electromagnetic waves that compose electromagnetic radiation can be imagined as a self-propagating transverse oscillating wave of electric and magnetic fields. This diagram shows a plane linearly polarized EMR wave propagating from left to right. The electric field is in a vertical plane and the magnetic field in a horizontal plane. The two types of fields in EMR waves are always in phase with each other with a fixed ratio of electric to magnetic field intensity. Electromagnetic radiation (EM radiation or EMR) is a form of radiant energy, propagating through space via electromagnetic waves and/or particles called photons. In classical physics, EMR is considered to be produced when charged particles are accelerated by forces acting on them. EMR carries energy—sometimes called radiant energy—through space continuously away from the source (this is not true of the near-field part of the EM field). Physics Theory Maxwell’s equations for EM fields far from sources
Lepton A lepton is an elementary, spin-1⁄2 particle that does not undergo strong interactions, but is subject to the Pauli exclusion principle. The best known of all leptons is the electron, which governs nearly all of chemistry as it is found in atoms and is directly tied to all chemical properties. Two main classes of leptons exist: charged leptons (also known as the electron-like leptons), and neutral leptons (better known as neutrinos). Charged leptons can combine with other particles to form various composite particles such as atoms and positronium, while neutrinos rarely interact with anything, and are consequently rarely observed. The first charged lepton, the electron, was theorized in the mid-19th century by several scientists and was discovered in 1897 by J. J. Thomson. The next lepton to be observed was the muon, discovered by Carl D. Leptons are an important part of the Standard Model. Etymology Following a suggestion of Prof. History Properties
Ion Atom or molecule in which the total number of electrons is not equal to the total number of protons, giving the atom or molecule a net positive or negative electrical charge. An ion () is an atom or molecule that has a net electrical charge. Since the charge of the electron (considered negative by convention) is equal and opposite to that of the proton (considered positive by convention), the net charge of an ion is non-zero due to its total number of electrons being unequal to its total number of protons. History of discovery The word ion comes from the Greek word ἰόν, ion, "going", the present participle of ἰέναι, ienai, "to go". Characteristics Ions in their gas-like state are highly reactive and will rapidly interact with ions of opposite charge to give neutral molecules or ionic salts. As charged objects, ions are attracted to opposite electric charges (positive to negative, and vice versa) and repelled by like charges. Anions and cations Natural occurrences
Radiation Illustration of the relative abilities of three different types of ionizing radiation to penetrate solid matter. Typical alpha particles (α) are stopped by a sheet of paper, while beta particles (β) are stopped by an aluminium plate. Gamma radiation (γ) is damped when it penetrates lead. Note caveats in the text about this simplified diagram. In electromagnetic radiation (such as microwaves from an antenna, shown here) the term "radiation" applies only to the parts of the electromagnetic field that radiate into infinite space and decrease in intensity by an inverse-square law of power so that the total radiation energy that crosses through an imaginary spherical surface is the same, no matter how far away from the antenna the spherical surface is drawn. Gamma rays, x-rays and the higher energy range of ultraviolet light constitute the ionizing part of the electromagnetic spectrum. Ionizing radiation Ultraviolet radiation Main article: Ultraviolet X-ray Beta radiation