Spacetime instability due to quantum gravity

Understand

We show that quantum gravity yields exponentially growing gravitational waves.

• Without a mechanism to stop these modes from growing, the universe would go through a gravitational collapse.
• For Minkowski background, we propose a solution by choosing an integration contour in Fourier space that does not enclose the problematic modes, thus preventing them from showing up in the effective theory.
• It turns out that this is only possible when the modes are removed altogether.