least for all single-field models, both new and chaotic (Linde 1983). Thanks to the
results from WMAP, that’s exactly what we’ve observed (Bennett et al. 2013).
Inflation also, owing to its stretching of quantum fluctuations across extremely
large scales, predicts the existence of superhorizon fluctuations. These, theoretically,
would be visible in the TE cross-correlation (temperature/polarization) spectrum of
the microwave background, a prediction that has since been verified (Planck Collaboration 2015a). Inflation also predicts that there should be an upper limit to the
maximum reheat temperature achieved at the hot Big Bang that follows the end of
inflation; from the fluctuations in the cosmic microwave background, we find it’s
more than 100 times lower than the Planck scale (Bassett et al. 2005).
Finally, inflation also predicts a generic spectrum of tensor fluctuations—gravitational wave fluctuations—that ought to imprint themselves on the B-mode polarization signal in the cosmic microwave background. Although the spectrum is
model-independent in inflationary spacetimes, the magnitude and amplitude of the
signal is highly model-dependent (de Bernardis et al. 2009). Despite the purported
and incorrect claims of detection by the BICEP2 collaboration a few years ago
(BICEP2 Collaboration 2014), this remains a promising and active area of research
that could further validate or challenge inflation.
9.5 Discussion
The Big Bang remains one of the greatest theoretical frameworks and achievements
of modern cosmology. Its four cornerstone predictions, of the abundances of the
light elements from Big Bang nucleosynthesis, of the existence and spectrum of the
cosmic microwave background, of the Universe’s expansion rate and its evolution
over time, and the formation of large-scale structure, have been validated and
verified in glorious detail. However, the inference that you can extrapolate the
expanding, cooling Universe all the way back in time to t ¼ 0 and the existence of
a singularity is problematic at best, leading to the assumption of a slew of
ill-motivated initial conditions.
The idea of cosmic inflation, however, removes the singular beginning to the Big
Bang and instead replaces the earliest stages with an exponentially expanding phase
to the early Universe. During this phase, the energy in the Universe exists in the form
of the inflaton field, which drives the de Sitter-like behavior of the Universe during
this time. Inflation is not only compatible with the four cornerstones of the Big Bang,
but it dynamically sets up the initial conditions required to make the Big Bang
compatible with our observations. In addition, cosmic inflation makes a number of
generic predictions that are relatively model-independent; of the six presented in
Sect. 9.4, four have been verified and two are consistent with inflation’s predictions
to the limits of our best observations.
Inflation, therefore, occurs before the Big Bang, setting up the initial conditions
for what we see in our observable Universe during its final moments. It is only the
final ~10
À33 s of inflation that imprint itself on what we can observe; although there
94
E. R. Siegel
results from WMAP, that’s exactly what we’ve observed (Bennett et al. 2013).
Inflation also, owing to its stretching of quantum fluctuations across extremely
large scales, predicts the existence of superhorizon fluctuations. These, theoretically,
would be visible in the TE cross-correlation (temperature/polarization) spectrum of
the microwave background, a prediction that has since been verified (Planck Collaboration 2015a). Inflation also predicts that there should be an upper limit to the
maximum reheat temperature achieved at the hot Big Bang that follows the end of
inflation; from the fluctuations in the cosmic microwave background, we find it’s
more than 100 times lower than the Planck scale (Bassett et al. 2005).
Finally, inflation also predicts a generic spectrum of tensor fluctuations—gravitational wave fluctuations—that ought to imprint themselves on the B-mode polarization signal in the cosmic microwave background. Although the spectrum is
model-independent in inflationary spacetimes, the magnitude and amplitude of the
signal is highly model-dependent (de Bernardis et al. 2009). Despite the purported
and incorrect claims of detection by the BICEP2 collaboration a few years ago
(BICEP2 Collaboration 2014), this remains a promising and active area of research
that could further validate or challenge inflation.
9.5 Discussion
The Big Bang remains one of the greatest theoretical frameworks and achievements
of modern cosmology. Its four cornerstone predictions, of the abundances of the
light elements from Big Bang nucleosynthesis, of the existence and spectrum of the
cosmic microwave background, of the Universe’s expansion rate and its evolution
over time, and the formation of large-scale structure, have been validated and
verified in glorious detail. However, the inference that you can extrapolate the
expanding, cooling Universe all the way back in time to t ¼ 0 and the existence of
a singularity is problematic at best, leading to the assumption of a slew of
ill-motivated initial conditions.
The idea of cosmic inflation, however, removes the singular beginning to the Big
Bang and instead replaces the earliest stages with an exponentially expanding phase
to the early Universe. During this phase, the energy in the Universe exists in the form
of the inflaton field, which drives the de Sitter-like behavior of the Universe during
this time. Inflation is not only compatible with the four cornerstones of the Big Bang,
but it dynamically sets up the initial conditions required to make the Big Bang
compatible with our observations. In addition, cosmic inflation makes a number of
generic predictions that are relatively model-independent; of the six presented in
Sect. 9.4, four have been verified and two are consistent with inflation’s predictions
to the limits of our best observations.
Inflation, therefore, occurs before the Big Bang, setting up the initial conditions
for what we see in our observable Universe during its final moments. It is only the
final ~10
À33 s of inflation that imprint itself on what we can observe; although there
94
E. R. Siegel