malibueinstein:

*Image represents a finite universe (top) and infinite universe (bottom two)

How Does It All End?

The universe is filled with tons of stuff, and this stuff is “matter” so it has “mass.”

Because this stuff has mass, and also takes up space, it has a certain density. (Mass over volume = density)  Everything from semi-trucks to feathers to air molecules have density. 

Ending of the Universe - Spoiler Alert:

If the density of the universe is higher than a certain level, the shape of the universe is spherical, like a ball.  The density of matter could pull the very fabric of space back in on itself and cause a great contraction, maybe resulting in the universe shrinking down to a small super dense point, like the center of a black hole.  Space will be brought back by the gravitational force of this dense universe, and all time and space will crash in on each other in a “Big Crunch.”

Right now the universe is not in “thermodynamic equilibrium.”  That means that there is heat and energy disproportionately available in certain places, and it can “do work” when it goes to some other place that doesn’t have that same level of heat and energy.  Think of it like this: it’s much hotter below the Earth’s surface than above the Earth’s surface.  When there is a break in the Earth’s surface, steam can rush out to the colder area, and we can capture it and use it to make electricity. Iceland has been working this system for years with aplomb.

Anyway, if the density of this universe is less than the critical density, then space expands infinitely, everything gets pulled further apart, and all the heat and energy in the universe dissipates as space stretches, thus cooling down to absolute zero. No more work can be done.  No work, no power, no “structures” can be made, living or non-living.  Here the universe ends in a vast, maudlin cosmos of nothingness. 

It’s looking like the expansion of the universe theory is most likely, and to further research the how and why of that I’ll direct you to some reading about dark energy.. Trust me, it’s a good time.

Want to know even more?  NASA has you covered.