200 most important Astronomy topics - Sykalo Eugen 2023

The Baryon Oscillation Spectroscopic Survey (BOSS)

The Baryon Oscillation Spectroscopic Survey (BOSS) is a project that aims to map the distribution of matter in the universe on a large scale. This project is part of the larger Sloan Digital Sky Survey (SDSS) which started in 1998. BOSS was launched in 2009 and was completed in 2014, and its main goal was to measure baryonic acoustic oscillations (BAOs) in the distribution of galaxies.

What are Baryonic Acoustic Oscillations (BAOs)?

BAOs are a pattern of sound waves that were created in the early universe as a result of the interaction of matter and radiation. These waves resulted in a characteristic scale, known as the sound horizon, which can be used as a cosmic ruler to measure the distribution of matter in the universe. The sound horizon was imprinted on the cosmic microwave background radiation (CMB) when the universe was only 380,000 years old. At that time, the universe was a hot, dense plasma of particles that were interacting with each other and with light (photons) through the process of Thomson scattering. This interaction caused sound waves to propagate through the plasma, compressing and rarefying the matter as they traveled. When the universe cooled enough for neutral atoms to form, the photons were able to travel freely through space, creating the CMB. The sound waves left imprints on the CMB in the form of temperature fluctuations that are still observable today. These temperature fluctuations correspond to over- and under-densities of matter in the early universe. The characteristic scale of the sound horizon, which is around 490 million light-years, can be used to measure the distribution of matter in the universe, and to study the evolution of large-scale structures such as galaxies and clusters of galaxies.

How does BOSS work?

BOSS works by using a technique called spectroscopy to measure the redshift of galaxies. Redshift is a measure of how much the light from a galaxy has been stretched due to the expansion of the universe. By measuring the redshift of a large number of galaxies, BOSS is able to determine their distances from us and create a 3D map of the distribution of matter in the universe.

To measure the redshift of galaxies, BOSS uses a spectrograph, which is a device that separates the light from a galaxy into its component wavelengths. The spectrograph then records the intensity of light at each wavelength, creating a spectrum. By analyzing the spectrum, BOSS can determine the redshift of the galaxy, which is related to its distance from us.

BOSS targets two main types of galaxies: luminous red galaxies (LRGs) and emission-line galaxies (ELGs). LRGs are massive, old galaxies that are mostly red in color, while ELGs are less massive, younger galaxies that have strong emission lines in their spectra. By targeting both types of galaxies, BOSS is able to measure the large-scale distribution of matter in the universe over a wide range of distances.

Once BOSS has measured the redshift of a large number of galaxies, it uses this information to create a 3D map of the distribution of matter in the universe. This map is created by dividing the sky into small regions, and then counting the number of galaxies in each region. By comparing the number of galaxies in each region to the expected number based on random fluctuations, BOSS is able to measure the statistical properties of the galaxy distribution, including the characteristic scale of BAOs.

The characteristic scale of BAOs can then be used as a cosmic ruler to measure the expansion rate of the universe. This is done by comparing the observed galaxy distribution to theoretical predictions based on different models of the universe. By comparing the predictions to the observations, BOSS is able to measure the Hubble constant with unprecedented precision, as well as other cosmological parameters such as the matter density of the universe and the equation-of-state parameter of dark energy.

What are the results of BOSS?

BOSS has produced many important results in the field of cosmology. In addition to measuring the expansion rate of the universe, BOSS has also provided important constraints on the properties of dark energy. Dark energy is the mysterious force that is thought to be driving the accelerated expansion of the universe. By measuring the distribution of matter in the universe, BOSS has been able to put limits on the equation-of-state parameter of dark energy, which describes how its pressure changes as its density changes. These constraints have helped to rule out certain models of dark energy, and have provided important clues about its nature.

BOSS has also provided important insights into the history of the universe. By measuring the distribution of galaxies at different distances, BOSS has been able to study how the distribution of matter has evolved over time. This has allowed cosmologists to test different models of the universe and to better understand the physical processes that shaped its evolution. For example, BOSS has shown that the rate of structure growth in the universe is consistent with the predictions of the standard cosmological model, which assumes that the universe is filled with dark matter and dark energy.

Finally, BOSS has provided important constraints on the neutrino mass. Neutrinos are tiny particles that are produced in nuclear reactions, and are thought to have a very small but nonzero mass. By measuring the distribution of matter in the universe, BOSS has been able to put limits on the sum of the masses of the three known types of neutrinos. These limits have helped to narrow down the possible values of the neutrino mass, and have provided important clues about the properties of these elusive particles.