Expansion of space is an idea that space can actually expand.

If there is a number of objects in this space a uniform expansion is such in which all the distances between those objects are increasing with constant velocity and for each pair of objects this velocity is proportional to the distance between those objects at some "common moment" in the time of expansion. If such expansion existed in nature the volume of space would be constantly increasing and it would be a never ending process.

In a decelerating expansion the velocities between pairs of objects (proportional to the distances at some "common moment") are decelerating with the volume of space increasing until "some common" moment when the expansion stops with maximum volume of space, and then it chages into contraction of space with all the distances between pairs of objects becoming smaller with increasing velocity. The volume of space becomes progressively smaller until all objects end up at one point with volume zero called singularity.

In an accelerating expansion the velocities are accelerating, volume increasing, and so the same as in uniform expansion, it is a never ending process.

Except such three pure cases one can imagine mixed cses in which parts of the space are expanding with various characteristics (uniformly or not) while other parts are contracting. Out of those cases one may select a case in which the total volume of space is not changing and so the space might be expanding and contracting on local scale only and not on global scale.

The space of our universe is believed by most astronomers to be expanding on global scale and this expansion is believed to accelerate.

The hypothesis of expanding space is based on an assumption that the observed redshift of galaxies (discovered by Edwin Hubble in 1929 and called "Hubble redshift" or "cosmological redshift") is caused by the Doppler effect while the galaxies are moving away from us. If the reason for the Hubble redshift were different then the hypothesis of expansion of space would need to be modified or abandoned.

Assumption of global conservation of energy leads to a conclusion that the expansion of space is apparent and Hubble redshift is due to general time dilation rather than Doppler effect, however, as mentioned above, based on astronomical observations the global expansion of space is considered by majority of astronomers to be real, and consequently the assumption of global conservation of energy is abandoned.

There are established parameters that describe the expansion of space. One of those parameters is the ratio of velocity to the distance between objects called Hubble's constant of this expansion denoted by H₀, usually expressed in km / s / Mpc (kilometers per second per megaparsec) and acceleration of this expansion denoted by dH / dt expressed in relative change of Hubble's constant per unit of time. Since km and Mpc are both units of distance km / s / Mpc reduce to 1 / s (which are other suitable units for expressing H₀) then dH / dt may be expressed simply in s ^{- 2}.