180 Chapter 3 The Universe Was Born With a Big Bang
Do Aliens Exist?
In Chapter 4, we will discuss more about what it’s like at the edge of the universe. But first,
let’s consider one common question about the universe: Do aliens exist?
We’ll tell you the conclusion first. Most
scientists who study the universe believe
that other sentient beings like humans exist
somewhere. This conclusion is based on the
The Cosmological Principle is the hypoth-
esis that the universe is homogeneous (the
same kind) and isotropic (the same from any
direction) on large scales. What that means is if
you take a big chunk of universe, and compare
it to another equally big chunk of universe, they
wouldn’t differ that much. This does not refer
specifically to appearance—although they’ll each
have some areas that have a lot of clusters of
galaxies, some areas with just a few galaxies,
and some areas with no galaxies. It means that
the same physical laws apply everywhere and
don’t behave differently in different places.
F = ma always and forever.
Since we humans, at first, were under the impression that Earth was a unique place in
the universe, we believed in the geocentric theory. However, as the results of observations
of the universe began to be explained more logically, a Sun-centered theory and then the
heliocentric model were developed.
Following this pattern, the theory that life was created only on planet Earth began to
be questioned. According to the Cosmological Principle, our Earth is by no means a special
location. Therefore, there must be other planets in the universe that have environments
similar to Earth’s, and life must have originated there and must be evolving. So according
to this argument, aliens must exist!
Calculating the Number of Extraterrestrial
Although the Cosmological Principle is probably correct as a framework, and space aliens
may exist somewhere, there is still a question as to how common alien life is.
In 1961, the American astronomer Frank Drake (born in 1930) published an interest-
ing equation known as the Drake equation, which enables us to estimate the approximate
distribution of extraterrestrial civilizations in our own galaxy and determine whether we can
communicate with them.