a model of the universe in which the universe expands forever because there is not enough mass to counteract the expansion by means of gravitational attraction.
Model of the universe in which the curvature of space is flat or curved away from itself, entailing that the size of the universe is infinite. According to this model, gravity between objects is not able to stop or reverse the expansion of the universe, thus objects continue to move farther and farther apart as space moves outward. An object moving in a straight line in an open universe would never return to its starting point. According to current cosmological theories, the universe is open if it is insufficiently dense. Such a universe will never end, but will eventually become very cold and dark because stars gradually lose all of their energy. Compare closed universe. See Note at big bang.
If there is not enough matter in the universe to exert a strong enough gravitational force to stop the universal expansion associated with the big bang, the universe is said to be open. (Compareclosed universeandflat universe.)
