given the size of our observable Universe, it’s only the last few dozen e-foldings of
inflation that have any observable impact on our Universe today.
Inflation, therefore, can reproduce the successes of the hot Big Bang by giving
you the exact state you’re seeking, beginning well above the earliest temperatures
and energies we’re currently capable of probing. In addition to that, cosmic inflation
solves all five of the Big Bang’s puzzles that we mentioned in Sect. 9.3.
• The horizon problem, that the Universe is the same temperature in causally
disconnected regions, is explained by those different regions having been in
causal contact in the past. Inflation stretched those regions apart to much larger
distant scales, but they originated from the same Planck-scale volume, where they
had time to achieve the same properties, like temperature and density.
• The isotropy problem, that the Universe has the same large-scale structure
properties in all directions, is solved because the seed overdensities and
underdensities that inflation creates, due to the quantum fluctuations that get
stretched across the Universe, were created by the same process everywhere
within our causal horizon.
• The monopole problem, that the Universe should be filled with these relic
particles from a high-energy epoch in the distant past, is solved by a combination
of two factors. First, any monopoles that existed during the inflationary phase are
inflated away by the exponential expansion of space, meaning that there ought to
be, at most, one magnetic monopole in the observable Universe. Second, so long
as the maximum temperature we achieve after the end of inflation is below the
scale that produces new monopoles, we wouldn’t expect our Universe to
contain any.
• The flatness problem, which notes that the Universe appears spatially flat, is now
explained because no matter what conditions the Universe begins with, inflation
will stretch it so that it appears indistinguishable from flat to an observer that can
only see tens of billions of light years in any direction.
• And the small fluctuation problem is resolved, so long as inflation occurs well
below (by a factor of ~10
3 or so) the Planck scale.
With these resolutions in place, we can see where inflation has succeeded in the
realm where the original formulation of the hot Big Bang could not.
In addition, inflationary theories, including models of new inflation and chaotic
inflation (Linde 1983), make six generic predictions that we can look to the Universe
for confirmation, validation, or falsification. Inflation predicts a nearly-perfectly
scale-invariant set of density fluctuations, parametrized by the scalar spectral
index, n s . Inflation predicts that n s should be close to but not quite equal to 1; we
measure it (Planck Collaboration 2015b) to be approximately 0.97. Inflation also
predicts that there should be curvature fluctuations in the Universe, creating a spatial
curvature that ought to be somewhere between 10
À4
! Ω k ! 10
À6 , consistent with
our best constraints that Ω k < 0.003 (Alam et al. 2017).
In principle, there are two types of density fluctuations that could have existed
and imprinted themselves on the Universe: adiabatic and isocurvature fluctuations.
Inflation predicts that the fluctuations in our Universe should be 100% adiabatic, at
9 Before the Big Bang
93
inflation that have any observable impact on our Universe today.
Inflation, therefore, can reproduce the successes of the hot Big Bang by giving
you the exact state you’re seeking, beginning well above the earliest temperatures
and energies we’re currently capable of probing. In addition to that, cosmic inflation
solves all five of the Big Bang’s puzzles that we mentioned in Sect. 9.3.
• The horizon problem, that the Universe is the same temperature in causally
disconnected regions, is explained by those different regions having been in
causal contact in the past. Inflation stretched those regions apart to much larger
distant scales, but they originated from the same Planck-scale volume, where they
had time to achieve the same properties, like temperature and density.
• The isotropy problem, that the Universe has the same large-scale structure
properties in all directions, is solved because the seed overdensities and
underdensities that inflation creates, due to the quantum fluctuations that get
stretched across the Universe, were created by the same process everywhere
within our causal horizon.
• The monopole problem, that the Universe should be filled with these relic
particles from a high-energy epoch in the distant past, is solved by a combination
of two factors. First, any monopoles that existed during the inflationary phase are
inflated away by the exponential expansion of space, meaning that there ought to
be, at most, one magnetic monopole in the observable Universe. Second, so long
as the maximum temperature we achieve after the end of inflation is below the
scale that produces new monopoles, we wouldn’t expect our Universe to
contain any.
• The flatness problem, which notes that the Universe appears spatially flat, is now
explained because no matter what conditions the Universe begins with, inflation
will stretch it so that it appears indistinguishable from flat to an observer that can
only see tens of billions of light years in any direction.
• And the small fluctuation problem is resolved, so long as inflation occurs well
below (by a factor of ~10
3 or so) the Planck scale.
With these resolutions in place, we can see where inflation has succeeded in the
realm where the original formulation of the hot Big Bang could not.
In addition, inflationary theories, including models of new inflation and chaotic
inflation (Linde 1983), make six generic predictions that we can look to the Universe
for confirmation, validation, or falsification. Inflation predicts a nearly-perfectly
scale-invariant set of density fluctuations, parametrized by the scalar spectral
index, n s . Inflation predicts that n s should be close to but not quite equal to 1; we
measure it (Planck Collaboration 2015b) to be approximately 0.97. Inflation also
predicts that there should be curvature fluctuations in the Universe, creating a spatial
curvature that ought to be somewhere between 10
À4
! Ω k ! 10
À6 , consistent with
our best constraints that Ω k < 0.003 (Alam et al. 2017).
In principle, there are two types of density fluctuations that could have existed
and imprinted themselves on the Universe: adiabatic and isocurvature fluctuations.
Inflation predicts that the fluctuations in our Universe should be 100% adiabatic, at
9 Before the Big Bang
93