Embarrassing enough the universe is mostly made by something that we know very little. Indeed, the 68% of the everything is made of “dark energy“, 27% of the everything is made of “dark matter” and the remaining 5% being what we can observe and directly detect! the so called “normal” (baryonic) matter.
The main goal for investigating BAO is to further constrain the properties of the dark energy as a step forward to the knowledge of its nature. BINGO’s goal is to do it via radio emission from atomic hydrogen.
Right after the Big Bang, the Universe was made by a kind of a soup of particles, where matter and light were coupled (i.e. where in thermal equilibrium). The Universe was not 100% homogeneous, and local tiny inhomogeneities may have favour over-dense regions, where surrounding matter is pulled towards those regions via gravity. As matter is pulled, it is also heated, and eventually radiation is emitted, so that the inward gravity related force is balanced by an outward pressure related with the escaping radiation. The pull-in / pull-out caused matter oscillations, and from those oscillations a mechanical wave is created and propagates, like a sound wave. A possible visualization of the process is illustrated below with the slow motion movie of a water drop dropping in to the water:
That sound wave carried baryons, photons and dark matter. Considering that the latter only interacts via gravity, eventually it remained nearby the original over-density location, i.e. at the centre of the sound wave.
From top (1) Universe is an expanding plasma composed by baryonic matter, dark matter and light, tiny inhomogeneities favouring over-densed regions (brighter points) (2) those induced the formation of a propagating wave that drags with it baryons and photons (3) which will travel together until the moment of the decoupling and photons can escape (4) matter remains in place (5) dark matter remained in the centre of the initial over-dense region. Credit: Images from an animation by Martin White, http://newscenter.lbl.gov/2009/10/27/evolving-search-2
Baryons and photons still coupled, progressed with the sound wave until the moment of their decoupling, when the Universe was cool enough and photons did not further interact as much with matter and escaped away (related with the surface of last scattering).
After the decoupling, most of the matter remained at that distance from the centre of sound wave, forming a kind of rings of over-density of matter, seeds for the building up of structures like galaxies and galaxy clusters.
So there is a kind of “primordial” radius that should be imprinted in the matter (note that the central dark matter distribution, also acted as an attractor for the building up of large structure), known as Barionic Acoustic Oscillations or BAO.
2D animations of the evolution of density perturbations: in the top case, the perturbation from a solely origin evolves up to the moment of the decoupling of matter-radiation, in the bottom case, the same perturbation but in this case the origin is at different loci. Credit: http://galaxies-cosmology-2015.wikidot.com/baryon-acoustic-oscillations.
Considering that this was not a solely event, but there were multiples, the resulting “primordial radius” is a statistical quantity. And that is our “cosmological ruler”! A feature that should be imprinted in the whole Universe.
Credit: Chris Blake and Sam Moorfield
Indeed, acoustic oscillations are imprinted on the cosmic background radiation (CMB), the scale set by the distance light travelled in the 380 000 years after Big Bang, at the recombination era, and imprinted in matter distribution since then. Recently, Planck mission determined that scale to be 147.4+/-0.6 Mpc based on CMB measurements.
Rulers & Dark energy
In our expanding Universe, the angular distance between two points changes, and so does the size of the acoustic scale, a rough analogy of an expanding Universe is a filling balloon, the distance between two points on its surface increasing as the balloon expands.
By investigating the variation of angular size of BAO over time, one can infer the rate of expansion of the Universe and thus restrict the properties of dark energy.
In practice, this can be achieved by comparing the BAO radius today, via the distribution of matter, with e.g. the BAO radius at the time of the decoupling of the matter and radiation, via the observational data of the cosmic background radiation.