Glossary of Terms


Anti DeSitter Space: Space-Time with constant negative curvature.

Baryonic Matter: Known matter (as opposed to Dark Matter)

Big Bang: The term is used to refer to the singularity at the beginning of our universe. The Big Bang theory explains how the universe is expanding from its initial state.

Big Crunch: Reversal of the Big Bang, where the whole universe collapses to a singularity.

Big Rip: A scenario where the acceleration of the expansion increases with time, resulting in a ’tearing’ apart of the very fabric of spacetime.

Black Hole: A region of space-time that is bended inward due to the extreme force of gravity. It traps anything even light that passes its event horizon.

Brown Dwarfs: Brown dwarfs are astronomical objects that are too small to sustain hydrogen fusion in their cores.

Chandrasekhar Mass: A mass threshold in stellar structures named after the Indian astrophysicist. When the Chandrasekhar mass limit is breached, the degenerate star is too heavy to support itself, and the object blows up in a supernova.

Concordance Model of Cosmology: Concordance Model of Cosmology is a homogeneous and isotropic solution of Einstein’s theory of gravitation with a cosmological constant.

Cosmic Microwave Background: Remnant E&M radiation from big bang. It is now at a temperature of about 2.7 K (2.7 degrees above absolute zero).

Cosmic Strings: These are different objects from strings in string theories. Cosmic strings are theoretical objects that have been developed at the beginning phase of the universe. Extremely long and narrow but massive, they could be stretched across the universe.

Cosmic Topology: Using Topology to construct cosmological models.

Cosmology: Study of the cosmos that deals with the origin, the evolution, and the fate of the universe.

Cosmological Constant: A term used by Einstein in his general relativity equation for the purpose of counter effecting gravitational attraction. Based on observed acceleration in the expansion of the universe, it has a small value of the order of g.

Cosmological Heat Death: A term that refers to a possible state of the universe in which it expands adiabatically (no new entropy is produced).

Dark Energy: Dark Energy is believed to be the vacuum energy with negative pressure in its simplest form that causes the Universe to accelerate. It accounts for about 75% of the matter/energy of the Universe.

Dark Matter: A kind of matter that cannot be seen directly and its composition is unknown. However, its gravitational effect can be measured. It responds to gravitational force, but it does not respond to strong, weak, and electromagnetic forces. Dark Matter accounts for 22-25% of the total energy of the universe.

Dark Matter Halos: Large structures that extend far beyond the visible portions of galaxies marked by stars, gas, and other forms of ordinary matter.

DeSitter Space: In homogeneous cosmology this term is also used to refer to a flat space with no matter and a cosmological constant.

DeSitter Universe: If the universe ends up in a cosmic heat death, cold and empty of structure, maintaining its entropy and temperature at constant values eternally. Such a universe is known as a DeSitter universe.

Doppler Effect: The shift in frequency of a wave (to a higher frequency, when its source is moving toward a receiver; and to a lower frequency, when the source is moving away from the receiver) for an observer moving relative to the source of the wave.

Entropy: A measure of the disorder or chaos of a closed system.

Event Horizon: An event horizon is a hyper-surface in space-time beyond which events cannot affect an outside observer.

Exotic Material: Hypothetical material inside of a wormhole which has negative average energy density.

Flatness Problem: One of the three problems associated with the standard models of cosmology. The problem has to do with the basic question, why the universe is close to being spatially flat.

Friedmann-Lemaitre-Robertson-Walker Models: Spatially homogeneous and isotropic models of cosmology

General Theory of Relativity: It formulates how gravity bends space-time and it is used to explain and understand the large-scale structure of the universe.

Halos: It is believed that galaxies and galaxy clusters are embedded in giant halos of dark matter.

Hawking Effect: A slow quantum mechanical process that ultimately leads to the decay of  black holes. Through quantum effects, virtual particles are created near the event horizon of a black hole. Although such particles only live for a short time, the tidal stretching force, which is enormous near a black hole, does work on them while they remain in existence. If the work done on the particle — by the tidal force — is large enough, the particle is promoted from virtual existence to “real” existence. The particle can then leave the black hole, and is thus effectively emitted by the hole.

Hawking Radiation: Radiation produced by the Hawking effect.

Hawking Temperature:The Hawking temperature of a black hole is given by the expression

Where  is the mass of the object. Note that this temperature is extremely small: ≈ 6× K for a black hole with one solar mass.

Holographic Principle: The information about the quantum states of a system is contained on the surface area of the system.

Homogeneous: Everywhere the same.

Horizon Problem: One of the three problems associated with the early models of Bing Bang cosmology. The problem has to do with the large-scale uniformity of the universe.

Hubble Constant H: The constant in Hubble’s law which is used to calculate the size and age of the Universe.

Hubble Expansion Law: Simply stated by V= H D equation. Where V is the recession velocity of objects such as galaxies, H is the Hubble constant, and D is distance from Earth.  

Hubble Time: Hubble Time is comparable to the current age of the universe.

Inflationary Cosmology: It states that the universe (the space) went through exponential expansion very early after the big bang.

Isotropic: The same in every direction. In Cosmology, it means the universe looks the same in every direction.

Knot Theory: A branch of mathematics that is used in quantum gravity theories.

LHC: Large Hadron Collider. Is the world’s largest and highest-energy particle accelerator. A circular (27 km in circumference) particle accelerator (proton-proton collider) laboratory at CERN in Geneva.

M-Theory: A theory that unifies five different versions of the string theories and super gravity together, all under one single framework.

MACHOS: Massive Astrophysical Compact Halo Objects

Magnetic Monopole: A single hypothetical magnetic pole. Monopoles have not been detected yet.

Maxwell’s Equations: Four equations developed by J. C. Maxwell that unified electricity and magnetism.

Multiverse: This term is used to refer to Multi-Universes.

Nebulae: A Nebulae is an interstellar cloud of dust and gases.

Neutrino: An elementary particle that is electrically neutral (does not carry electric charge).

Neutron Stars : The star, with roughly the mass of the Sun and a radius of only 10 kilometers, becomes essentially one gigantic nucleus, with most of the electrons and protons combining to make neutrons. Hence the name “neutron stars”.

No Hair Theorem: A constraint which states that only three properties of a black hole can be observed outside its event horizon. (the black hole mass. the spin of the black hole. the electric charge of the black hole)

Planck Length: A quantity associated with quantum gravity. It is about 10-35 cm.

Planck Mass: A quantity associated with quantum gravity. It is about 10-8 kg.

Planck Time: A quantity associated with quantum gravity. It is the time it takes for light to travel a Planck length interval which is about 10-42 seconds.

Quantum Cosmology: A branch of cosmology that uses the laws of quantum mechanics to study the cosmos.

Quantum Gravity: A theory that unifies General Theory of Relativity with quantum mechanics under one single framework. 

Red Dwarfs: Stars belonging to the smallest class of stars which live much longer compared to other stars, are known as “M stars” or “red dwarfs”.

Red Giant: A star of low or intermediate mass will become a Red Giant in its late phase of stellar evolution. The Sun will become a red giant in about 7 billion years from now.

Second Law of Thermodynamics: Entropy of a closed system is always greater than or equal to zero. It cannot decrease.

Smoothness Problem : One of the three problems associated with the standard models of cosmology. The problem has to do with the basic question why the matter is uniformly distributed in the universe.

Singularity: A point in space-time where its curvature becomes infinite. Big Bang is an example of a singularity.

Special Theory of Relativity: It states that laws of nature are the same for all observers regardless of how they move. Also, it describes that space and time are connected and no longer individually absolute.

Speed of Light: One of the constants of the nature. It is 299,792,458 m/s. According to the laws of relativity, nothing can travel faster than speed of light.

Standard Model of Cosmology: Standard model of Cosmology consists of the following theories and models: General Theory of Relativity, Dark Matter, Dark Energy, initial conditions at Big Bang, and Standard model of particle physics.

Stellar Black Holes: These objects have masses in the range of ten to perhaps one hundred Suns.

String Theories: These theories use one dimensional objects (strings) as the basic building block of the universe including matter. M-Theory is the latest version of string theories.

Strong Cosmological Principle: It assumes that all observers in the Universe would see the same in all directions as we do (all what we see up to the horizon is what another inhabitant of the Universe would see up to his/her horizon).

Supermassive Back Holes: Astronomical observations clearly show that almost every large galaxy contains a monster black hole at its core. These black holes come in a range of masses, from about one million to one billion times the mass of our Sun.

Supernova/Supernovae: A supernova is a nuclear stellar explosion at the end of the star’s life.

Thermodynamic Arrow of Time: The thermodynamic arrow of time is based on the Second Law of Thermodynamics, that in a closed/isolated system, entropy increases with time.

Thermodynamical Equilibrium: A system with maximum amount of entropy.

Time Dilation: A consequence of the Special Theory of Relativity. It states that time slows down for an observer in motion or influenced by gravity.

Topology: A branch of mathematics that deals with spatial properties that are preserved under continuous deformations of objects.

Twistor Theory: A background-independent theory that was initially developed by Roger Penrose. It formulates space-time using complex numbers.

Uncertainty Principle: One of the basic principles of quantum mechanics developed by W. Heisenberg. It formulates that one cannot precisely specify the values of two conjugate terms such as position-momentum or time-energy.

Weak Cosmological Principle: It assumes that, on very large scales of billions of light years, the Universe is isotropic and homogeneous, if the distribution of matter and geometry is averaged over.

White Dwarfs: A white dwarf is a very dense small star. Approximately 997 out of every 1000 stars will turn into white dwarfs upon their death. These stellar remnants typically retain somewhat less mass than that of our Sun, but they are much smaller in radius and are one million times denser.

WIMPS: Weakly Interacting Massive Particles.

White Hole: The time reversal of a black hole. Big bang is an example of a white hole. A theoretical region of space-time where matter erupts but cannot enter the region.

Worm Hole: A theoretical object that connects two regions of space-time together. It works as a tunnel and can also connect two regions of different universes together.