World Science Scholars
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.

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