This spectacular realization would very quickly lead to one of the most important
papers in the history of modern cosmology, which detailed a new scenario for the
Universe’s origin: cosmic inflation (Guth 1981).
Unlike the standard hot Big Bang, where arbitrarily high temperatures, energies,
and densities are achieved at a time t ¼ 0 when you extrapolate backwards from
today, cosmic inflation provided a cutoff to how hot, dense, and energetic the
Universe was in the earliest stages. Rather than continuing back to a singularity,
inflation postulated that there was a time period prior to the hot, dense, expanding
state that the Big Bang describes. In the inflationary scenario, this prior phase
contained no matter, antimatter, or radiation, but rather the Universe existed in a
de Sitter-like state, dominated by energy in the inflaton field, which behaves
similarly to vacuum energy or a cosmological constant.
During this inflationary phase, space expands exponentially, obeying the relation
that a(t) ¼ a 0 e
Ht , where the value of H is determined by the energy scale of inflation.
This exponential expansion doesn’t necessarily need to occur at a rapid rate, but the
exponential property is special because of how relentless it is. For inflation occurring
at the hypothesized GUT scale, for example, it can turn a Planck-scale region
(~10
À35 m) into the size of the observable Universe today (~10
27 m) in a timespan
that’s merely 10
À33 s in duration.
This type of exponential expansion has a number of physical effects that are vital
for both setting up the initial conditions of the Big Bang and ameliorating a number
of cosmic puzzles. The inflationary phase, for example, stretches the Universe flat,
by taking whatever amount and type of spatial curvature that may have pre-existed
and expanding it so thoroughly that, to an observer that can only see the extent of the
Universe that’s accessible to us, it’s indistinguishable from flat. It takes whatever
properties existed in a tiny, causally connected region of space, and stretches that
region across a volume far bigger than that of our observable Universe, giving it the
same properties everywhere. Given that whatever field causes inflation is likely to be
quantum in nature, the field ought to exhibit quantum fluctuations at all times; hence,
these fluctuations should be stretched by the expansion of space to exist, with
practically the same magnitude and spectrum, across all scales and in all locations
in space.
Finally, when inflation ends, the energy that’s inherent to the inflaton field—and
hence, that’s behaving as vacuum energy during the inflationary phase—gets
converted into matter, antimatter, and radiation (Linde 1982; Albrecht and
Steinhardt 1982). Inflation goes on so long as the field that drives it remains out of
the ground state (such as at the top of a potential “hill”), but comes to an end when it
reaches the lowest-energy, most stable state (such as rolling down into a potential
“valley”). The temperature that the Universe reaches, post-inflation, is bounded from
above by the energy scale of inflation, and may be lower depending on the particulars of the reheating process. In the aftermath of this conversion of energy, from
the inflaton field to the matter, antimatter, and radiation permeating all of space, the
conditions at the end of inflation are imprinted on the forms of energy that fill the
Universe as it enters the familiar hot, dense, expanding-and-cooling state. Finally,
92
E. R. Siegel
papers in the history of modern cosmology, which detailed a new scenario for the
Universe’s origin: cosmic inflation (Guth 1981).
Unlike the standard hot Big Bang, where arbitrarily high temperatures, energies,
and densities are achieved at a time t ¼ 0 when you extrapolate backwards from
today, cosmic inflation provided a cutoff to how hot, dense, and energetic the
Universe was in the earliest stages. Rather than continuing back to a singularity,
inflation postulated that there was a time period prior to the hot, dense, expanding
state that the Big Bang describes. In the inflationary scenario, this prior phase
contained no matter, antimatter, or radiation, but rather the Universe existed in a
de Sitter-like state, dominated by energy in the inflaton field, which behaves
similarly to vacuum energy or a cosmological constant.
During this inflationary phase, space expands exponentially, obeying the relation
that a(t) ¼ a 0 e
Ht , where the value of H is determined by the energy scale of inflation.
This exponential expansion doesn’t necessarily need to occur at a rapid rate, but the
exponential property is special because of how relentless it is. For inflation occurring
at the hypothesized GUT scale, for example, it can turn a Planck-scale region
(~10
À35 m) into the size of the observable Universe today (~10
27 m) in a timespan
that’s merely 10
À33 s in duration.
This type of exponential expansion has a number of physical effects that are vital
for both setting up the initial conditions of the Big Bang and ameliorating a number
of cosmic puzzles. The inflationary phase, for example, stretches the Universe flat,
by taking whatever amount and type of spatial curvature that may have pre-existed
and expanding it so thoroughly that, to an observer that can only see the extent of the
Universe that’s accessible to us, it’s indistinguishable from flat. It takes whatever
properties existed in a tiny, causally connected region of space, and stretches that
region across a volume far bigger than that of our observable Universe, giving it the
same properties everywhere. Given that whatever field causes inflation is likely to be
quantum in nature, the field ought to exhibit quantum fluctuations at all times; hence,
these fluctuations should be stretched by the expansion of space to exist, with
practically the same magnitude and spectrum, across all scales and in all locations
in space.
Finally, when inflation ends, the energy that’s inherent to the inflaton field—and
hence, that’s behaving as vacuum energy during the inflationary phase—gets
converted into matter, antimatter, and radiation (Linde 1982; Albrecht and
Steinhardt 1982). Inflation goes on so long as the field that drives it remains out of
the ground state (such as at the top of a potential “hill”), but comes to an end when it
reaches the lowest-energy, most stable state (such as rolling down into a potential
“valley”). The temperature that the Universe reaches, post-inflation, is bounded from
above by the energy scale of inflation, and may be lower depending on the particulars of the reheating process. In the aftermath of this conversion of energy, from
the inflaton field to the matter, antimatter, and radiation permeating all of space, the
conditions at the end of inflation are imprinted on the forms of energy that fill the
Universe as it enters the familiar hot, dense, expanding-and-cooling state. Finally,
92
E. R. Siegel