6.2 Gravitational Waves Review

summary

Along with density perturbations, inflation also predicts gravitational waves.

In 1916, Albert Einstein predicted the existence of gravitational waves—a product of the general theory of relativity.
General Relativity defines gravity as a phenomenon resulting from the curvature of spacetime.
Massive, accelerating objects can potentially generate changes in this curvature, which travel at the speed of light, propagating outward from the source.
These propagations are known as gravitational waves.

Gravitational waves transport quantized energy.

Just as a photon is a carrier of electromagnetic radiation, a gravitational wave carries gravitational radiation.
On the quantum scale, the gravitational field is constantly fluctuating.
Inflation stretches these fluctuations from microscopic to astronomical wavelengths, where they behave as classical gravitational waves.

Direct detection of gravitational waves would provide strong evidence for the inflationary theory

Gravitational waves have been called the “smoking gun” for inflation.
Although this gravitational radiation has not yet been directly detected, there is considerable indirect evidence for its existence.
Gravitational waves perturb the plasma of the early universe, imprinting a swirling pattern in the polarization of the cosmic microwave background—these are called B-modes.
Some of the difficulty in detecting gravitational waves stems from the noise in the low frequencies where antennas currently operate.
Gravitational waves are believed to have frequencies between 10^-16 Hz and 10⁴ Hz.

New experimental evidence may be able to lend support to inflation.

Measurement of the size of gravitational waves would determine the energy scale during inflation.
Findings from the BICEP2 experiment attempted to show that the energy density of the universe during inflation is on par with the energy scale of Grand Unified Theories.
BICEP2 measured that $$r=0.20^{ \ +0.07}_{ \ -0.05}$$ where $r$ is the ratio of the power in gravitational wave perturbations to the power in density perturbations.
If $V$ is the energy density of the universe at the time of inflation, then $$\left[V\left(\hbar c\right)^3\right]^{3/4}=2.2\times 10^{16} \,\textrm{GeV} \,\left(\frac{r}{0.2}\right)^{1/4}$$ and so $V$ is right at the scale of Grand Unified Theories.