by S. Prasad Ganti

BAO (Baryon Acoustic Oscillation) is a mouthful of an acronym. It interested me because of its relation to our baby Universe. That was a time when our Universe was less than 380,000 years old. With no stars or planets or galaxies. It was just an extremely hot cosmic sea with a soup of baryons (electrons, protons, neutrons) and light trapped within. This article is a summary gathered from the NASA and ESA websites, laced by my 2 cents. 

Further cooling of our universe resulted in the formation of atoms and chemistry took over to create elements like Hydrogen, Helium etc. But the dynamics of the infantile cosmic sea were different and we still have relics from such an ancient past. The first one we know is the CMB (Cosmic Microwave Background) radiation. This noise from the formative years of the Universe can be detected everywhere in the microwave frequencies. 

Starting from the first discovery in the horn antenna of Bell Labs in Holmdel, NJ, to the space based detectors COBE (Cosmic Background Explorer), WMAP (Wilkinson Microwave Anisotropy Probe) and Planck, the picture of the early universe became very sharp. It largely shows a uniform universe with some minor ripples. These ripples got amplified with the expansion of space and became clumps which became stars and galaxies. 

Second relic is the lesser known and esoteric BAO. In the cosmic sea, gravity tried its magic to pull the particles together. Each time this happened, the temperature increased further making it even hotter. The heat caused the particles to repel each other. A tug of war ensued between the two competing forces – attraction caused by gravity and repulsion caused by the heat. This tug of war caused the back and forth motion resulting in oscillations. Gravity won for some of the time and as it weakened, heat started taking over for some time. This oscillation caused the BAO.       

Now we know about Baryons and oscillations. What does acoustic mean in this context? There was no sound or Bose speakers in the early universe ! The matter and air as we know was yet to be formed. I could not find a satisfactory answer to this question. I am surmising that acoustic is taken in its broader sense as it refers to the behavior of plasma. Plasma is a soup of charged particles which have been ripped off from its neutral atoms. 

We know that BAO occurred in the distant past because the relics or imprints of those oscillations are still found in the present day universe. They are the frozen ripples, not a currently flowing one. Something like the finding of dinosaur footprints. These imprints were noticed for the first time in 2005 in the large-scale picture of the universe. Sloan Digital Sky Survey was the first such big picture in which patterns of BAO were found. Similarly such imprints were found in 2dF Galaxy Redshift Survey.

The imprints are in the form of distances between galaxies today. We are likely to find another galaxy within 500 million light years. Not slightly nearer nor farther. This is discounting the local clusters where Andromeda, our neighbor, is only about 2 million light years away. It occurred to me how 500 million light years can be part of a wave. It is too large to be a wavelength. But then the early universe was a lot smaller and the wave got stretched out with the expansion of the universe. What is 500 million light years today could have been a mile or a few inches in those times. Thus, BAO is also used to study the expansion of the universe and the related topic of dark energy. More studies are taking place and more discoveries are in store.