time dilation

Time dilation is the effect that for some objects, less time as passed than for others, depending their positions and velocities, a consequence of relativity. Special relativity implies time dilation based upon the relative motion of two objects. General relativity also produces time dilation (gravitational time dilation) for objects deeper within a gravitational potential well compared to those not so deep within it. Time dilation challenges intuition based upon daily experience, but is not only proven, but some technology that depends upon high-precision time measurement, such as GPS, requires time dilation to be taken into account: without doing so, it doesn't work. Time dilation is not illusion: two accurate clocks go out of sync if one of them is moved away, then back to the other.

Cosmological time dilation is the apparent time dilation of events viewed from large (high redshift) distances. Supernova light curves at varying distances show the effects, the same type of supernova at a further distance showing the same features, but occurring over a the longer period of time, matching that expected from Hubble expansion. Such a cosmological effect does not require relativity, though relativity affects the amount: it is evident from any radial velocity of a source, and is equivalent to the fact that if a space probe moving away from you sends signals to you at one-second intervals, each successive signal takes longer to reach you and you receive them at intervals longer than a second. The observed dilation is a confirmation that cosmological redshift truly does stem from the expansion of the universe, rather than some kind of attenuation and/or emission in the intervening space.