Time, Cosmic

Cosmic time is a measure of time on a scale of billions of years, during which the universe is cooling and expanding from its initial hot, dense state, and stars and galaxies are evolving. The universe had its beginning about 14 billion years ago in the big bang. By contrast, the earliest signs of life on earth only date back to about 3.7 billion years ago, and cosmic time is vast in comparison with the scale of a human lifetime. An even shorter timescale marks the life of quantum particles, the smallest forms of matter and energy that wink in and out of existence so quickly that some cannot be detected directly.

Time itself did not exist until the birth of the universe in the big bang. Albert Einstein showed that it is the fourth dimension of spacetime (the other three being the conventional dimensions of space) and that any directional movement is also a movement through time. Throughout cosmic time, stars and galaxies evolve. This is evident in the birth, growth, and death of stars and in the formation and changes in galaxies. Change is a key factor in the slow-moving cycle of cosmic time, and a more narrow definition states that a single unit of cosmic time is the span of time through which the universe has changed fundamentally.

The fundamental change involves the shifting balance between matter and another, as yet poorly understood, form of energy. Recent observations indicate a change in the Hubble constant, a measure of the rate of expansion of the universe since just after the big bang. This evidence that the rate of expansion is accelerating was an unexpected finding, because calculations indicated that the amount of normal matter and energy present in the universe would lead to a gradual slowing down of expansion. However, scientists have discovered a previously unrecognized form of repulsive substance, a dark energy, that acts like negative gravity, pushing matter apart, and this may account for the acceleration. with the passage of time, matter will become more dilute, carried along, like raisins in a loaf of rising bread. Dark energy appears in minute amounts within this expanding space and, acting as a repulsive factor, will eventually overtake matter as the dominant factor in the universe. The fundamental change wherein dark energy overtakes [Page 1259]matter is calculated to be about 10 billion years—a single unit of cosmic time.