Find out how old the universe is and how its age is calculated.
The universe is believed to be somewhere between 15 and 20 billion-years-old. The figure is derived from the concept that the universe has been expanding at the same rate since its birth at the Big Bang.
The rate of expansion is a ratio known as Hubble's constant. It is calculated by dividing the speed at which the galaxy is moving away from the earth by its distance from the earth. By inverting Hubble's Constant, that is dividing the distance of a galaxy by its recessional speed, the age of the universe can be calculated.
The estimates of both the velocity and distance of galaxies from the earth are subject to uncertainties and not all scientists accept that the universe has always expanded at the same rate. Therefore, many still hold the age of the universe open to question.