The Noise That Gave Birth to Galaxies express
Quantum noise, like the hiss of an untuned radio, filled the infant Universe. Alan Guth showed that superfast expansion (inflation) caused this ripple to freeze, giving rise to clumps of matter — the seeds of galaxies.
A new method extends this picture to any theory of gravity. Previously, calculations only worked for Einstein's general theory of relativity; now they have been generalized to complex variants — with extra dimensions or dark energy, similar to those developed by Andrei Linde. It's like switching bands: each theory yields its own unique 'noise'.
The level of primordial noise decided the fate of the cosmos: had it been slightly different, black holes would have formed instead of stars. Now, by comparing calculations with satellite sky maps, we can for the first time hear exactly what kind of ethereal crackle created our world.
Original abstract
as written by the authors arXiv:2606.07467
During cosmological inflation, inhomogeneities arising from quantum vacuum fluctuations are stretched to become super-Hubble and effectively classical. As many scenarios of the origin involve nonlinearities or a breakdown of perturbativity in the infrared, the limitations of quantum field theory can be addressed using a stochastic description of the dynamics, the so-called stochastic inflation paradigm. However, the stochastic formalism was only recently formulated consistently within full General Relativity and has not yet been extended to more general theories of the early universe, which is the subject of this work. In order to find the stochastic sources for a wide class of fully nonlinear scalar-tensor theories, we apply our gauge-agnostic coarse-graining procedure to the linear equations of the effective field theory of dark energy. Each theory can then be mapped to its own set of stochastic equations of motion by identifying the corresponding coefficients in the EFT. We illustrate this with a few concrete and, in most cases, unprecedented examples, including Gauss-Bonnet, generalized Brans-Dicke, Horndeski, and braiding theories. Finally, we discuss other natural extensions to provide a phenomenologically complete stochastic framework. For example, we showcase the coarse-graining of multifield inflation in full General Relativity and argue for the generality of our procedure and thus its potential applications beyond the realm of inflation.
A slightly different level of quantum noise, and the Universe would consist solely of black holes.