There are two major assumptions that the scenario of heat death is based upon.
1. Arrow of Time
To physicists, time is a four-dimensional block with three components in space and one in time. Time has one direction that always points from past to present, like an arrow. To see more on this and learn about the philosophical and physical implications involved, you may wish to read Bright Hub’s What is Time?.
2. The Universe is Finite and Isolated
The universe is not infinite. It is closed in space and time, and is an isolated system. This means there will be no energy
exchanges with other systems and there will be no new sources of energy, ever. There will never be any outside intervention from anything or anyone that will share energy with us. Not from aliens, not from wormholes, not from white holes, and certainly not from a Supreme Being.
What Will Happen - And When
The temperature everywhere will become close to absolute zero. Throughout the universe, there will be no heat exchange. All of the stars will die because they will evolve from white dwarfs to black dwarfs. There will be no light at all anywhere, so it will be much darker than shown in the picture on the left.
Since energy will no longer be exchanged, there will be of course will be no energy flow. Energy will exist only as a random motion of atoms and molecules that will be uniformly spread throughout space. No work will be able to be performed for any physical process. It will be impossible for life of any kind to exist because of the cold, and actually there will be no matter left capable of making life.
The caveat: these effects are not expected to happen for trillions and trillions of years, with estimates ranging from 1076 to 10100 years.
How do living beings contribute to this unceasing increase in entropy, and hence to the heat death of the universe? And is there anything you yourself can do to personally stop contributing to the problem? Continue on to part three to find out.
Physics for Scientists and Engineers by Douglas Giancoli
Statistical Mechanics: Entropy, Order Parameters and Complexity by James P. Sethnay
Creation of the Sun and Moon by Michelangelo
Clock by Jorge Barrios
Do Not Enter by Penywise
White Hole from JILA
Sea Ice by Rear Admiral Harley D. Nygren
The Arrow of Time - Professor Paul Davies discusses Heat Death and Cosmology (click link on right under published lectures)