The deceleration parameter q in cosmology is a dimensionless measure of the cosmic acceleration of the expansion of the universe. It is defined by: In the physical sciences, a dimensionless number (or more precisely, a number with the dimensions of 1) is a quantity which describes a certain physical system and which is a pure number without any physical units; it does not change if one alters ones system of units of measurement...

where a is the scale factor of the universe and the dots indicate derivatives by proper time. Recent measurements of dark energy imply that the expansion of the universe is actually accelerating; the minus sign and name "deceleration parameter" are historical. The scale factor, parameter of Friedmann-Lemaître-Robertson-Walker model, is a function of time which represents the relative expansion of the universe. ... Proper time is time as measured by the clock for an observer who is traveling through spacetime. ... In physical cosmology, dark energy is a hypothetical form of energy which permeates all of space and has strong negative pressure. ...

The Friedmann acceleration equation can be written as The Friedmann equations relate various cosmological parameters within the context of general relativity. ...

where ρ is the energy density of the universe, p is its pressure, and w is the equation of state of the universe. In cosmology, the equation of state of a perfect fluid is characterized by a dimensionless number w, equal to the ratio of its pressure p to its energy density ρ: . It is closely related to the thermodynamic equation of state and ideal gas law. ...

This can be rewritten as

by using the first Friedmann equation, where H is the Hubble parameter and K = 1, 0 or − 1 depending on whether the universe is hyperspherical, flat or hyperbolic. Hubbles law is the statement in astronomy that the redshift in light coming from distant galaxies is proportional to their distance. ... The shape of the Universe is a subject of investigation within cosmology. ... The shape of the Universe is a subject of investigation within cosmology. ... The shape of the Universe is a subject of investigation within cosmology. ...

The derivative of the Hubble parameter can be written in terms of the deceleration parameter: Hubbles law is the statement in astronomy that the redshift in light coming from distant galaxies is proportional to their distance. ...

Except in the speculative case of phantom energy (which violates all the energy conditions), all postulated forms of matter yield a deceleration parameter . Thus, any expanding universe should have a nonincreasing Hubble parameter and the local expansion of space is always slowing (or, in the case of a cosmological constant, proceeds at a constant rate, as in de Sitter space). Phantom energy is a hypothetical form of dark energy with equation of state . ... In mathematics and physics, n-dimensional de Sitter space, denoted , is the maximally symmetric, simply-connected, Lorentzian manifold with constant positive curvature. ...

Observations of the cosmic microwave background demonstrate that the universe is very nearly flat, so: WMAP image of the CMB anisotropy,Cosmic microwave background radiation(June 2003) The cosmic microwave background radiation (CMB) is a form of electromagnetic radiation that fills the whole of the universe. ...

This implies that the universe is decelerating for any cosmic fluid with equation of state w greater than − 1 / 3 (any fluid satisfying the strong energy condition does so, as does any form of matter present in the Standard Model, but excluding inflation). However, observations of distant type Ia supernovae indicate that q is negative; the expansion of the universe is accelerating. This is an indication that the gravitational attraction of matter, on the cosmological scale, is more than counteracted by negative pressure dark energy, in the form of either quintessence or a positive cosmological constant. The Standard Model of Fundamental Particles and Interactions The Standard Model of particle physics is a theory which describes the strong, weak, and electromagnetic fundamental forces, as well as the fundamental particles that make up all matter. ... Remnant of Keplers Supernova, SN 1604. ... In physical cosmology, dark energy is a hypothetical form of energy which permeates all of space and has strong negative pressure. ... In physics, quintessence is a hypothetical form of dark energy postulated as an explanation of observations of an accelerating universe. ... The cosmological constant (usually denoted by the Greek capital letter lambda: Λ) occurs in Einsteins theory of general relativity. ...

Before the first indications of an accelerating universe, in 1998, it was thought that the universe was dominated by dust with negligible equation of state, . This had suggested that the deceleration parameter was equal to one half; the experimental effort to confirm this prediction led to the discovery of acceleration.