100 7. THE FUTURE
Billions of years from now
-13.7 -10 -5 0 5 10 15
Now
Average distance between galaxies
ΩM = 0.3, Ω
Λ
= 0.7
ΩM = 0
ΩM = 0.3
ΩM = 1
ΩM = 6
Figure 7.1: Possible futures of the expansion of the universe, as calculated by different cosmo-
logical models. (Graphic by BenRG, Public Domain.)
e model that best fits the observed data, and also agrees with other important assump-
tions that we will explore later, is the strangest of all. It is marked
M
D 0:3,
ƒ
D 0:7, and it
represents a universe with both dark matter and dark energy. Notice that for all of the models—
this one included—the expansion of the universe is initially slowing down. But this dark energy
model eventually begins to expand faster and faster, accelerating rather than decelerating.
is more complex model that includes dark energy agrees better with our observations of
distant galaxies; the observed look-back time really does seem to have this shape. If this slightly
more complex—but still rather simple—model is correct, then it implies the universe will expand
forever at an ever-accelerating rate.
7.2 THE FUTURE AT SMALL SCALES
While the universe as a whole is expanding, there are other processes at work on the smaller
scales of individual stars and galaxies. e laws of statistics and thermodynamics naturally move
matter and energy from a more ordered state to a more randomized arrangement. e molecules
in a gas injected into one side of a container randomly collide, quickly diffusing until the entire
container is filled, while the opposite process is simply too unlikely to happen. e glass that fell
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