What are Baryon Acoustic Oscillations & Why do they Matter?
On one of my recent telescope showings, an individual asked me to quantify precisely fast our universe was expanding after our discussion about the evidence for the growing distances between our universe’s galaxies.
A simple google could have spared me the trouble:
But what was intriguing is the science behind it: what makes this intellectual topic so complex that it warrants nearly 18 million articles just about it.
To measure the expansion of the Universe, one can use the size of an object. If we know the size, we can use its angular diameter to tell us its distance from us as seen by examining its red shift. By finding its distance from us, we see how great our universe has expanded. This process of using the physical size is considered measuring with a standard ruler. Standard rulers are objects with an intrinsic size that are time invariant.
The Universe began 13.7 billion years ago in the process called “the Big Bang”. If the Universe were a food product, its mass (ingredients) would correspond to the following:
> roughly 5% normal (baryonic) matter
> roughly 25% cold dark matter
> roughly 70% mysterious dark energy
At the initial start to the Universe, the universe was not perfectly isotropic (= uniform): tiny perturbations and fluctuations occurred allowing for small changes in the matter density landscape allowing for both less dense and over dense areas. These included over-densities of cold dark matter (CDM), photons, baryons and neutrinos. Neutrinos decoupled first. Then CDM, leaving behind photons and baryons held together by Thomson scattering. The dynamic duo fastened together — that is Baryons & Photons — were attracted to to the CDM regions called CDM potential wells. In moving closer to the CDM potential wells, the photons heated up and this countered their movements towards the CDM potential well through the pressure generated from radiation. As this dance of gravity and electromagnetic forces happened, the perturbations formed produced sound waves in the baryon-photon plasma until recombination.
Then things changed.
As the Universe expanded and cooled down due to inflation, the baryons got colder and these formed the first atoms. This is termed as the recombination time. Photons gradually stop coupling with baryons and baryons as a result of recombination, gradually stop coupling with photons (although these two events can happen at different times). From here on out, matter and radiation go about their own paths. From having no more radiation pressure given by photons, the sound speed diminishes and baryons are no longer perturbing.
Radiation reached us from this early time point that is almost isotropic in 1964. This is called the Cosmic Microwave Background Radiation. The fluctuations in densities (as seen by the range of colors) is due to two effects, one of which is the baryon acoustic oscillations.
Why is this important?
From analyzing this data, we can understand the values of certain cosmological constants and parameters. By using the baryon acoustic oscillator as a standard ruler, one can measure the growing distances between standardized objects like galaxies and clusters.
For example, by finding the right density parameters for matter and dark energy, one can examine the expansion of our Universe. Choosing these parameters comes from analyzing the data from Cosmic Microwave Background observations and others. For example, one can use a Monte-Carlo approach to find the cosmological parameters ( https://arxiv.org/pdf/astro-ph/0205436.pdf).
The importance of choosing the right cosmological constants allows scientists to find out the space-time geometry of our Universe.
Sources Considered:
https://www.universetoday.com/wp-content/uploads/2011/12/history.bigbang.jpg
https://en.wikipedia.org/wiki/Hubble's_law
