This TED-Ed lesson dealt with the cosmic clash between Gravity and Pressure, which created Baryonic Acoustic Oscillations (BAOs) in the very early Universe, close to the beginning of time. We discovered that these waves of matter and radiation left their imprint in the distribution of galaxies and even in the Cosmic Microwave Background, or CMB, which contains the farthest photons we will ever be able to see with our electromagnetic detectors. In other words, the CMB is the oldest possible “photo” of the Universe we can obtain.

But how do we discern, exactly, the contribution of the BAOs to the distribution of galaxies in the Universe? How do we know that, in fact, galaxies are slightly more likely to be separated by 500 million light-years between each other, which is the radius of the BAO acoustic wave in the present-day Universe?

The detection of the peak of the Baryonic Acoustic Oscillation was a landmark discovery by Daniel Eisenstein, now at the Center for Astrophysics | Harvard & Smithsonian, and collaborators, presented in a famous paper in 2005.

To make this discovery, researchers used the most detailed three-dimensional map of galaxies ever created, the Sloan Digital Sky Survey, or SDSS. This is among the most successful sky surveys ever performed, and it is named after the Alfred P. Sloan Foundation which funded the effort. A sky survey is, as the name suggests, a systematic imaging of the sky, which covers it fully or partially. The images of the SDSS are made with a 2.5-meter optical telescope in New Mexico (USA) at the Apache Point Observatory. The journey of the SDSS started in the year 2000—since then, it has collected positions for more than 200 million galaxies in the Universe. The SDSS therefore offers a unique database to study the relative positions of galaxies in the local Universe.

In particular, the team led by Daniel Eisenstein used a preliminary catalog of 46,748 luminous red galaxies up to a redshift z=0.47, when the Universe was about 8.8 billion years old. The relative distances between galaxies were studied with a method named the two-point correlation function. Given a galaxy, this function describes the probability that another galaxy will be found within a given distance from the first one.

Using this mathematical method and the beautiful database provided by the SDSS, astronomers detected with high confidence a bump in the correlation function of nearby galaxies. This bump peaks at around 500 million light-years of distance. Although the plot in the hyperlink above can be difficult to understand, it conveys a very simple fact: galaxies in the local Universe are more likely to be separated by 500 million light-years, instead of 400, or 600, or 800.

This detection represented a spectacular confirmation of the Standard Cosmological Model, which lays its foundation on the observational evidence that the Big Bang occurred about 13.7 billion years ago. The detection of the BAO peak also represented an early example of how big data, obtained from telescopes or computer simulations, are becoming more and more fundamental to the development of cutting-edge research. Another, very recent example of huge amounts of data contributing to an outstanding scientific result is the first image of the region close to the event horizon of a black hole, published in 2019 by the Event Horizon Telescope collaboration.

Love cosmology? Check out these lessons to expand your universe:

The beginning of the universe, for beginners - Tom Whyntie
What is the universe expanding into? - Sajan Saini
Three ways the universe could end - Venus Keus

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