PARALLEL UNIVERSES 247
verse is large for our universe and is nearly zero for most other universes. Thus there is a small but finite probability that other universes
can exist in the multiverse, but ours has the largest probability. Hawking, in fact, tries to derive inflation in this way. In this picture a universe that inflates is simply more likely than a universe that does not,
and hence our universe has inflated.
The theory that our universe came from the "nothingness" of the
space-time foam might seem to be totally untestable, but it is consistent with several simple observations. First, many physicists have
pointed out that it is astonishing that the total amount of positive
charges and negative charges in our universe comes out to exactly
zero, at least to within experimental accuracy. We take it for granted
that in outer space gravity is the dominant force, yet this is only because the positive and negative charges cancel out precisely. If there
was the slightest imbalance between positive and negative charges on
the Earth, it might be sufficient to rip the Earth apart, overcoming the
gravitational force that holds the Earth together. One simple way to explain why there is this balance between positive and negative charges
is to assume that our universe came from "nothing," and "nothing" has
zero charge.
Second, our universe has zero spin. Although for years Kurt Gôdel
tried to show that the universe was spinning by adding up the spins of
the various galaxies, astronomers today believe that the total spin of
the universe is zero. The phenomenon would be easily explained if the
universe came from "nothing," since "nothing" has zero spin.
Third, our universe's coming from nothing would help to explain
why the total matter-energy content of the universe is so small, perhaps even zero. When we add up the positive energy of matter and the
negative energy associated with gravity, the two seem to cancel each
other out. According to general relativity, if the universe is closed and
finite, then the total amount of matter-energy in the universe should be
exactly zero. (If our universe is open and infinite, this does not have to
be true, but inflationary theory does seem to indicate that the total
amount of matter-energy in our universe is remarkably small.)
verse is large for our universe and is nearly zero for most other universes. Thus there is a small but finite probability that other universes
can exist in the multiverse, but ours has the largest probability. Hawking, in fact, tries to derive inflation in this way. In this picture a universe that inflates is simply more likely than a universe that does not,
and hence our universe has inflated.
The theory that our universe came from the "nothingness" of the
space-time foam might seem to be totally untestable, but it is consistent with several simple observations. First, many physicists have
pointed out that it is astonishing that the total amount of positive
charges and negative charges in our universe comes out to exactly
zero, at least to within experimental accuracy. We take it for granted
that in outer space gravity is the dominant force, yet this is only because the positive and negative charges cancel out precisely. If there
was the slightest imbalance between positive and negative charges on
the Earth, it might be sufficient to rip the Earth apart, overcoming the
gravitational force that holds the Earth together. One simple way to explain why there is this balance between positive and negative charges
is to assume that our universe came from "nothing," and "nothing" has
zero charge.
Second, our universe has zero spin. Although for years Kurt Gôdel
tried to show that the universe was spinning by adding up the spins of
the various galaxies, astronomers today believe that the total spin of
the universe is zero. The phenomenon would be easily explained if the
universe came from "nothing," since "nothing" has zero spin.
Third, our universe's coming from nothing would help to explain
why the total matter-energy content of the universe is so small, perhaps even zero. When we add up the positive energy of matter and the
negative energy associated with gravity, the two seem to cancel each
other out. According to general relativity, if the universe is closed and
finite, then the total amount of matter-energy in the universe should be
exactly zero. (If our universe is open and infinite, this does not have to
be true, but inflationary theory does seem to indicate that the total
amount of matter-energy in our universe is remarkably small.)