slew of relic particles may be created at this time, including heavy Dirac neutrinos,
potential dark matter candidates, and t’Hooft-Polyakov monopoles (t’Hooft 1974;
Polakov 1974) may all exist under these extraordinary conditions. At the limit of our
extrapolations, we exceed the Planck energy, achieve infinite (or arbitrarily high)
densities, and run into the limits of our fundamental physics theories. We achieve a
singularity, from where space and time themselves are believed to emerge.
9.3 Consequences
A singular beginning to the Universe, and its steady evolution from a set of hot,
dense, uniform, expanding initial conditions, has tremendous implications for what
we’d observe today. If the Universe began with arbitrarily high temperatures and
energies, it would immediately create all the particles, antiparticles, and quanta of
radiation that the quantum laws of nature—and energy constraints given by
Einstein’s E ¼ mc
2
—admitted. Beginning from the birth of the Universe, at a time
t ¼ 0, we would only be able to evolve the Universe forward, even in our imaginations. The first day in the Universe after the Big Bang occurred, as cosmologists
sometimes put it, would be “a day without a yesterday.”
After the first major verification of the predictions of the Big Bang came in, with
the discovery of the cosmic microwave background radiation, the extrapolation back
to a singularity seemed an inevitability. The singular beginning of the Universe
would lead to a hot, dense state full of matter and energy, which would then evolve
through the early stages in the Universe as outlined in Sect. 9.2. It’s true that many of
the stages we’ve passed through in the aftermath of the Big Bang have extraordinary
amounts of evidence supporting them. But extrapolating back to arbitrarily high
temperatures and energies not only lacks that evidence and remained a speculative
extrapolation, but brought with it a significant number of unexplained puzzles
(Siegel 2015) that the assumption of an initial singularity provided no answers for.
The first of these is the horizon problem. Even in an expanding Universe, there’s a
physical limit to how far, in a given amount of time, information (and temperature)
can be exchanged between two disparate locations. Since the discovery of the
cosmic microwave background, we’ve seen that the Universe has the same temperature in all directions in space, yet there’s no physical mechanism that would have
caused this to be the case. In the absence of such a cause, this simply needs to be an
initial condition that the Universe was born with in order for it to be the case in the
standard Big Bang, if it arises from a singular beginning. Without such conditions,
we would expect the temperature (and density) to vary in different directions by
roughly the order of the temperature itself.
The second is the related isotropy problem. When we look out at the Universe in
all directions, today, we see the same types of large-scale structure everywhere. The
types, sizes, populations, and clustering patterns of galaxies are directionindependent, illustrating the tremendous isotropy of the Universe. This also implies
that the seed fluctuations from which today’s large-scale structure arose were the
9 Before the Big Bang
89
potential dark matter candidates, and t’Hooft-Polyakov monopoles (t’Hooft 1974;
Polakov 1974) may all exist under these extraordinary conditions. At the limit of our
extrapolations, we exceed the Planck energy, achieve infinite (or arbitrarily high)
densities, and run into the limits of our fundamental physics theories. We achieve a
singularity, from where space and time themselves are believed to emerge.
9.3 Consequences
A singular beginning to the Universe, and its steady evolution from a set of hot,
dense, uniform, expanding initial conditions, has tremendous implications for what
we’d observe today. If the Universe began with arbitrarily high temperatures and
energies, it would immediately create all the particles, antiparticles, and quanta of
radiation that the quantum laws of nature—and energy constraints given by
Einstein’s E ¼ mc
2
—admitted. Beginning from the birth of the Universe, at a time
t ¼ 0, we would only be able to evolve the Universe forward, even in our imaginations. The first day in the Universe after the Big Bang occurred, as cosmologists
sometimes put it, would be “a day without a yesterday.”
After the first major verification of the predictions of the Big Bang came in, with
the discovery of the cosmic microwave background radiation, the extrapolation back
to a singularity seemed an inevitability. The singular beginning of the Universe
would lead to a hot, dense state full of matter and energy, which would then evolve
through the early stages in the Universe as outlined in Sect. 9.2. It’s true that many of
the stages we’ve passed through in the aftermath of the Big Bang have extraordinary
amounts of evidence supporting them. But extrapolating back to arbitrarily high
temperatures and energies not only lacks that evidence and remained a speculative
extrapolation, but brought with it a significant number of unexplained puzzles
(Siegel 2015) that the assumption of an initial singularity provided no answers for.
The first of these is the horizon problem. Even in an expanding Universe, there’s a
physical limit to how far, in a given amount of time, information (and temperature)
can be exchanged between two disparate locations. Since the discovery of the
cosmic microwave background, we’ve seen that the Universe has the same temperature in all directions in space, yet there’s no physical mechanism that would have
caused this to be the case. In the absence of such a cause, this simply needs to be an
initial condition that the Universe was born with in order for it to be the case in the
standard Big Bang, if it arises from a singular beginning. Without such conditions,
we would expect the temperature (and density) to vary in different directions by
roughly the order of the temperature itself.
The second is the related isotropy problem. When we look out at the Universe in
all directions, today, we see the same types of large-scale structure everywhere. The
types, sizes, populations, and clustering patterns of galaxies are directionindependent, illustrating the tremendous isotropy of the Universe. This also implies
that the seed fluctuations from which today’s large-scale structure arose were the
9 Before the Big Bang
89