End of the universe

The end of the Universe, known to the Time Lords as "Event Two", is an event that is to take place far off into the future where the universe, or N-Space, will collapse in on itself due to two factors: the fact that the universe is a closed system; and the constant build up of entropy.

The end of the Universe has been projected to occur in varied times, including the years 60,000,000,000 AD (BFA: Zagreus), 100,000,000,000,000 AD (DW: Utopia) and 100,000,000,000,000,000,000,000,000,000,000 (One hundred nonillion) AD. (PDA: The Infinity Doctors)

Before 1981, the universe had already past the point of collapse. Having discovered this, the Logopolitans created Charged Vacuum Emboitments which opened N-Space into another universe, E-Space, allowing entropy into E-Space. Though Logopolis was destroyed in 1981, the CVEs were rebooted and still continue to feed entropy from our universe into E-Space. (DW: Logopolis)

The universe will end sometime around the year 100,000,000,000,000 when the galaxies collapse and all the great civilisations have died out. (DW: Utopia)

It may be that since E-Space is a smaller universe, it will be entirely destroyed by the year 100 trillion, causing entropy to build up in this universe. If the CVEs are redirected to another universe, though, perhaps it can live until 100,000,000,000,000,000,000,000,000,000,000.