End of the universe

The end of the universe, known to the Time Lords as "Event Two" and to others as the "Big Crunch", was an event that took place far off into the future in which the universe, or N-Space, collapsed in on itself due to two factors: the universe was a closed system, and there was a constant build-up of entropy.

Before 1981, the universe had already passed 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 continued to feed entropy from N-Space into E-Space. (TV: Logopolis)

The Doctor prevented the universe from prematurely ending on several occasions, such as at the hands of the Daleks (TV: The Stolen Earth/Journey's End) and the Time Lords (TV: The End of Time). The Silence accidentally caused the universe to undergo total event collapse when they destroyed the Doctor's TARDIS, but the Doctor was able to reverse this. (TV: The Pandorica Opens/The Big Bang)

The Panjistri created the God Machine in an attempt to prevent the end of the universe. (PROSE: Timewyrm: Apocalypse)

The inhabitants of Ember attempted to survive the end of the universe by using the Somnus Foundation on 21st century Earth to transform humanity into a gestalt entity. They were defeated by the Fifth Doctor and Turlough. (AUDIO: Singularity)

The Toclafane, with the aid of, attempted to survive the end of the universe by travelling to 21st century Earth and murdering their ancestors. They were defeated by the Tenth Doctor and Martha Jones. (TV: Last of the Time Lords)

The end of the universe was projected to occur in varied times, including the years 60,000,000,000 AD (AUDIO: Zagreus), 100,000,000,000,000 AD (TV: Utopia) and, in an alternate universe, 100,000,000,000,000,000,000,000,000,000,000 (One hundred nonillion) AD. (PROSE: The Infinity Doctors)

The Fourth Doctor and Romana watched the Universe end from some unknown vantage point. As the dimensions collapsed, the Doctor gave Romana the last Rolo in the Universe. (PROSE: Do You Love Anyone Enough?)