Baryon Acoustic Oscillations: 2D Sound Horizon in the Early Universe

What Baryon Acoustic Oscillations Are

Baryon acoustic oscillations (BAOs) are large-scale structures in the distribution of galaxies that trace back to sound waves propagating through the early universe's plasma. These waves, driven by pressure and density fluctuations, left imprints on the cosmic microwave background radiation and continue to influence galaxy clustering today.

The 2D representation of BAOs visualizes these oscillations as a circular wavefront expanding from the photon-baryon plasma during the epoch of recombination, providing a tangible way to understand their dynamics.

How They Work

During the early universe's radiation-dominated era, photons and baryons (protons and neutrons) were tightly coupled in a plasma. As density fluctuations grew due to gravitational instability, these fluctuations created pressure waves that propagated through the plasma at the speed of sound, c_s(t).

These waves traveled outward from regions of higher density, creating ripples in the distribution of baryons that became visible as BAOs when the universe cooled enough for photons and baryons to decouple.

Why They Matter

BAOs are significant because they provide a standard ruler for measuring distances in the universe. By observing the clustering of galaxies, cosmologists can map out the large-scale structure of the cosmos and test theories about dark energy and modified gravity.

Furthermore, BAOs offer insights into the early universe's conditions, such as the baryon density (Ω_b) and the presence of dark matter, which affects the propagation speed and final size of these oscillations.

Real-World Applications

BAOs have been used in precision cosmology to constrain parameters like the Hubble constant (H_0), the density of dark matter, and the equation of state for dark energy. They serve as a powerful tool for validating models of cosmic evolution.

In addition, BAOs are crucial for understanding the growth of structure in the universe and testing whether our current cosmological model accurately describes the observed large-scale distribution of galaxies.

Frequently asked questions

Try it live

Everything above runs in your browser — open Baryon Acoustic Oscillations — 2D Sound Horizon and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.