are many fascinating predictions that arise from inflation, we have no way of
knowing how long inflation lasted, how it arose, or whether it was eternal to the
past or had, at some point, a singular beginning of its own. Although there is a
theorem that demonstrates inflating spacetimes are past-timelike-incomplete (Borde
et al. 2001), this theorem only demonstrates that particle trajectories separated by a
finite distance in an inflating spacetime will have a common point-of-origin in the
past. This does not necessitate the existence of a singularity, however, as numerous
scenarios that avoid a singularity in inflating spacetimes have been found.
One remarkable consequence of inflation, even though it does not lead to any
observables, is its prediction of the existence of a multiverse. The inflaton field,
while slowly rolling towards the most stable, equilibrium field value, will spread out,
being quantum in nature. If the rate of slow-roll is slower than the rate of fieldspreading, which it is in practically all models that provide a sufficient amount of
inflation, then there will inevitably exist regions of space where inflation continues
indefinitely. No matter how many regions see inflation come to an end and give rise
to a hot Big Bang, there will be inflating space between these regions to drive them
apart from one another, continuing to generate new, inflating space, and—over
time—to give rise to new, hot Big Bangs that will forever remain causally separated
from the one that created our observable Universe. The generic phenomenon of
eternal inflation (Vilenkin 1983) demands that once inflation begins, there are
regions of it that continue inflating forever into the future. These regions of
spacetime will continuously grow, exit inflation and reheat, creating an everincreasing number of Universes similar to our own. This is why we expect our
Universe to exist as part of a multiverse.
Back when it was first formulated, it was seen as an inevitability that the Big Bang
was something you could extrapolate backwards to arbitrary times, energies, and
temperatures, eventually reaching a singularity. Although the Big Bang can be
accurately described as a hot, dense, expanding initial state, the Big Bang itself
can no longer be said to imply a singularity. There may be a singularity at some early
stage, but it is not a certainty, and if it does exist, it occurs before cosmic inflation
does, which itself precedes and sets up the conditions for the Big Bang. Inflation
makes many other predictions that are unique to it, and a great many of those
predictions have been observationally confirmed. In addition, it predicts the existence of a multiverse, but sadly, the multiverse, as well as any properties of inflation
that existed prior to the final 10
À33 s (or so) of inflation, have no observable impact
on our Universe. The Big Bang still represents an important moment in our cosmic
history, but it is no longer the beginning of it all. Our matter-and-radiation-filled
Universe started with a bang, but cosmic inflation takes us farther back, and does so
more accurately, than the Big Bang on its own ever could.
Acknowledgement E. R. Siegel thanks B. G. Sidharth and FFP15 for support and direction; J. N.
Fry for the education, training, and skills that will stay with me forever; my Patreon supporters for
helping make this trip possible; my wife for all that she is; Will Kinney for helpful advice on the
measurement technique for superhorizon fluctuations; and all the attendees of this program for
enlightening discussions.
9 Before the Big Bang
95
knowing how long inflation lasted, how it arose, or whether it was eternal to the
past or had, at some point, a singular beginning of its own. Although there is a
theorem that demonstrates inflating spacetimes are past-timelike-incomplete (Borde
et al. 2001), this theorem only demonstrates that particle trajectories separated by a
finite distance in an inflating spacetime will have a common point-of-origin in the
past. This does not necessitate the existence of a singularity, however, as numerous
scenarios that avoid a singularity in inflating spacetimes have been found.
One remarkable consequence of inflation, even though it does not lead to any
observables, is its prediction of the existence of a multiverse. The inflaton field,
while slowly rolling towards the most stable, equilibrium field value, will spread out,
being quantum in nature. If the rate of slow-roll is slower than the rate of fieldspreading, which it is in practically all models that provide a sufficient amount of
inflation, then there will inevitably exist regions of space where inflation continues
indefinitely. No matter how many regions see inflation come to an end and give rise
to a hot Big Bang, there will be inflating space between these regions to drive them
apart from one another, continuing to generate new, inflating space, and—over
time—to give rise to new, hot Big Bangs that will forever remain causally separated
from the one that created our observable Universe. The generic phenomenon of
eternal inflation (Vilenkin 1983) demands that once inflation begins, there are
regions of it that continue inflating forever into the future. These regions of
spacetime will continuously grow, exit inflation and reheat, creating an everincreasing number of Universes similar to our own. This is why we expect our
Universe to exist as part of a multiverse.
Back when it was first formulated, it was seen as an inevitability that the Big Bang
was something you could extrapolate backwards to arbitrary times, energies, and
temperatures, eventually reaching a singularity. Although the Big Bang can be
accurately described as a hot, dense, expanding initial state, the Big Bang itself
can no longer be said to imply a singularity. There may be a singularity at some early
stage, but it is not a certainty, and if it does exist, it occurs before cosmic inflation
does, which itself precedes and sets up the conditions for the Big Bang. Inflation
makes many other predictions that are unique to it, and a great many of those
predictions have been observationally confirmed. In addition, it predicts the existence of a multiverse, but sadly, the multiverse, as well as any properties of inflation
that existed prior to the final 10
À33 s (or so) of inflation, have no observable impact
on our Universe. The Big Bang still represents an important moment in our cosmic
history, but it is no longer the beginning of it all. Our matter-and-radiation-filled
Universe started with a bang, but cosmic inflation takes us farther back, and does so
more accurately, than the Big Bang on its own ever could.
Acknowledgement E. R. Siegel thanks B. G. Sidharth and FFP15 for support and direction; J. N.
Fry for the education, training, and skills that will stay with me forever; my Patreon supporters for
helping make this trip possible; my wife for all that she is; Will Kinney for helpful advice on the
measurement technique for superhorizon fluctuations; and all the attendees of this program for
enlightening discussions.
9 Before the Big Bang
95