11.3 Baryon acoustic oscillations
3 min read•july 22, 2024
(BAO) are cosmic sound waves that left their mark on the universe's structure. These frozen ripples from the early cosmos now serve as a cosmic ruler, helping us measure vast distances and understand the universe's expansion.
BAO measurements provide crucial insights into the universe's composition and evolution. By studying these primordial echoes, scientists can constrain key cosmological parameters and test theories about , shedding light on the mysterious force driving cosmic acceleration.
Baryon Acoustic Oscillations
Origin of baryon acoustic oscillations
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- In the early universe, baryons and photons were tightly coupled forming a due to high temperature and density
- in the universe caused in the baryon-photon fluid seeded by during
- Competition between gravity and radiation pressure in the baryon-photon fluid led to oscillations
- Gravity compressed the fluid into potential wells
- Radiation pressure resisted this compression
- At the time of (), the universe cooled enough for neutral atoms to form
- Decoupling of baryons and photons caused the oscillations to freeze leaving an on the matter distribution (similar to sound waves in air)
BAO imprint on galaxy distribution
- Frozen baryon acoustic oscillations left an imprint on the distribution of matter in the universe manifesting as a slight excess of galaxies separated by a characteristic scale ()
- Characteristic scale of BAO is determined by the at the time of decoupling
- Sound horizon is the maximum distance a sound wave could travel in the baryon-photon fluid before decoupling (∼150 Mpc)
- In the , BAO appears as a peak at a scale of approximately 150
- Correlation function measures the excess probability of finding galaxy pairs at a given separation compared to a random distribution
- In the , BAO appears as a series of oscillations with a
- Power spectrum is the Fourier transform of the correlation function and describes the clustering of matter at different scales (similar to a musical score)
BAO as cosmic standard ruler
- Characteristic scale of BAO serves as a "standard ruler" for measuring cosmic distances because the is determined by well-understood physics in the early universe and is not affected by later cosmic evolution
- By measuring the apparent size of the BAO scale at different redshifts, cosmologists can infer the
- Angular diameter distance relates the apparent angular size of an object to its physical size and depends on the expansion history of the universe
- By measuring the BAO scale along the line of sight (in the redshift direction), cosmologists can infer the
- Hubble parameter describes the expansion rate of the universe as a function of redshift
- Combining measurements of and from BAO at different redshifts provides a powerful probe of the universe's geometry and expansion history (similar to using a ruler to measure distances on a map)
BAO in cosmological constraints
- BAO measurements provide independent constraints on key cosmological parameters, complementing other probes such as (CMB) and Type Ia supernovae
- By measuring the BAO scale at different redshifts, cosmologists can constrain:
-
- These parameters govern the expansion history and geometry of the universe
- BAO measurements have helped to establish the standard CDM cosmological model
- Universe is dominated by cold dark matter (CDM) and a cosmological constant , which drives the accelerated expansion
- Consistency between BAO measurements and other cosmological probes has strengthened the evidence for dark energy and the
- Future galaxy surveys (DESI, Euclid) will measure BAO with unprecedented precision, enabling stringent tests of cosmological models and shedding light on the nature of dark energy
Key Terms to Review (29)
Accelerating universe: The accelerating universe refers to the observation that the expansion of the universe is speeding up over time, driven by a mysterious force known as dark energy. This concept challenges previous notions that gravity would slow down the expansion and has profound implications for cosmology, influencing our understanding of the fate of the universe and the nature of dark energy.
Acoustic waves: Acoustic waves are pressure waves that travel through a medium, typically air, water, or solids, caused by the vibration of particles. In the context of cosmology, these waves play a crucial role in understanding the early universe, particularly in the formation of baryon acoustic oscillations, which provide insights into the distribution of matter and the large-scale structure of the cosmos.
Angular Diameter Distance: Angular diameter distance is a measure used in cosmology to relate the physical size of an object to its angular size as observed from a given point. This concept is crucial in understanding how the universe's expansion affects our perception of distances, particularly when observing distant celestial objects and phenomena, which can include understanding the observable universe and cosmic horizons as well as baryon acoustic oscillations.
BAO Peak: The BAO Peak, or Baryon Acoustic Oscillation Peak, refers to the characteristic peak observed in the distribution of galaxies in the universe, resulting from sound waves that traveled through the hot plasma of the early universe. These oscillations created slight density variations, which later manifested as large-scale structures in the cosmos, providing crucial information about the universe's expansion and composition.
BAO scale: The BAO scale, or Baryon Acoustic Oscillation scale, refers to the characteristic length scale in the universe determined by the density fluctuations of baryonic matter that were imprinted in the cosmic microwave background radiation during the early stages of the universe. This scale is significant because it acts as a 'standard ruler' for measuring cosmic distances and helps astronomers understand the expansion rate of the universe.
Baryon Acoustic Oscillations: Baryon acoustic oscillations refer to the regular, periodic fluctuations in the density of baryonic matter (normal matter) in the early universe, which arose from the interplay between gravity and pressure waves in the primordial plasma. These oscillations left an imprint on the large-scale structure of the universe, influencing galaxy formation and distribution.
Baryon-photon fluid: The baryon-photon fluid is a state of matter that existed in the early universe, comprising baryonic matter (like protons and neutrons) and photons interacting through radiation pressure. This interaction influenced the dynamics of the universe during its early stages, particularly affecting the formation of cosmic structures through mechanisms like baryon acoustic oscillations.
Characteristic wavelength: The characteristic wavelength is a specific wavelength associated with a physical process, such as the emission or absorption of radiation by particles. This concept is crucial for understanding various phenomena in the universe, including how cosmic structures form and evolve, and it plays a significant role in the study of baryon acoustic oscillations, which reflect sound waves propagating through the early universe's plasma.
Comoving mpc: Comoving megaparsec (mpc) is a unit of measurement in cosmology that accounts for the expansion of the universe when measuring distances between objects. It allows astronomers to express distances in a way that remains consistent over time, even as the universe expands, making it easier to understand the relative positions of galaxies and cosmic structures as they evolve.
Cosmic inflation: Cosmic inflation is a theory proposing that the universe underwent an exponential expansion during its first few moments, around 10^{-36} to 10^{-32} seconds after the Big Bang. This rapid expansion helps explain the uniformity and large-scale structure of the universe we observe today, connecting it to various phenomena such as temperature fluctuations and the cosmic microwave background.
Cosmic microwave background: The cosmic microwave background (CMB) is the remnant radiation from the Big Bang, filling the universe and providing a snapshot of the early cosmos when it was just 380,000 years old. This faint glow, almost uniform across the sky, carries crucial information about the universe's formation, composition, and expansion, connecting various areas of cosmological research and theories.
Cosmic standard ruler: A cosmic standard ruler is an astronomical object or feature used to measure the expansion of the universe, serving as a reference for distance measurements on cosmological scales. These rulers are critical for understanding the universe's geometry and structure, particularly in the context of baryon acoustic oscillations, which imprint a distinct scale in the distribution of galaxies and matter in the universe.
Cosmological Constraints: Cosmological constraints refer to the limitations or conditions derived from cosmological observations and theories that help define the parameters of the universe's evolution, structure, and composition. These constraints are crucial in refining models of the cosmos, such as the behavior of dark energy, the nature of cosmic inflation, and the distribution of matter in the universe. By analyzing large-scale structures and phenomena, scientists can derive values for fundamental parameters like the Hubble constant and density ratios of different energy components.
Dark energy: Dark energy is a mysterious form of energy that makes up about 68% of the universe and is responsible for the observed accelerated expansion of the cosmos. This phenomenon challenges our understanding of gravity and cosmological models, as it seems to have a repulsive effect, counteracting the gravitational pull of matter.
Dark energy equation of state parameter: The dark energy equation of state parameter, often denoted as w, describes the relationship between the pressure and density of dark energy in the universe. It plays a crucial role in understanding the expansion of the universe, particularly in relation to how dark energy influences cosmic acceleration.
David H. Weinberg: David H. Weinberg is a prominent astrophysicist known for his extensive work in cosmology, particularly in the fields of galaxy formation, large-scale structure of the universe, and baryon acoustic oscillations. His research has significantly contributed to our understanding of the distribution of galaxies and the role of baryons in cosmic evolution, helping to advance the field by connecting theoretical predictions with observational data.
Galaxy correlation function: The galaxy correlation function is a statistical tool used to describe the distribution of galaxies in the universe by measuring how the density of galaxies varies with distance. This function helps astronomers understand the large-scale structure of the cosmos and the relationships between galaxies, revealing patterns in their clustering and distribution influenced by gravitational forces and cosmic evolution.
Hubble Parameter: The Hubble parameter, often denoted as H, measures the rate of expansion of the universe at a given time and is defined as the ratio of the velocity of a galaxy to its distance from us. This parameter is essential for understanding cosmic expansion and plays a critical role in models of the early universe, the distribution of galaxies, and the large-scale structure of the cosmos. It is connected to various concepts in cosmology, such as the age of the universe, density parameters, and the effects of dark energy.
Imprint: In cosmology, an imprint refers to the specific signatures left in the cosmic microwave background (CMB) radiation as a result of early universe phenomena, such as baryon acoustic oscillations. These imprints provide critical information about the conditions in the early universe, including density fluctuations and the distribution of matter. Understanding these imprints helps scientists study the evolution of the universe and the formation of large-scale structures.
Lambda cold dark matter model: The lambda cold dark matter (ΛCDM) model is the leading cosmological model that describes the large-scale structure and evolution of the universe. It incorporates the effects of dark energy (represented by the cosmological constant lambda, Λ) and cold dark matter, which together account for the observed phenomena in the universe such as galaxy formation, cosmic expansion, and the cosmic web.
Matter Density Parameter: The matter density parameter, usually denoted as $$\Omega_m$$, is a dimensionless value that represents the ratio of the actual density of matter in the universe to the critical density needed for the universe to be flat. This parameter is crucial in understanding the overall composition and evolution of the universe, especially in relation to cosmic structures formed from baryonic and dark matter, which play a key role in baryon acoustic oscillations.
Matter Power Spectrum: The matter power spectrum is a mathematical representation that describes how matter is distributed across different scales in the universe. It shows the amount of matter, or density fluctuations, as a function of spatial scale, indicating how structures like galaxies and galaxy clusters form and evolve over time. This concept is crucial in understanding cosmic structure formation and relates directly to the patterns observed in baryon acoustic oscillations.
Nikhil Padmanabhan: Nikhil Padmanabhan is a prominent astrophysicist known for his contributions to the understanding of baryon acoustic oscillations, which are critical in studying the large-scale structure of the universe. His work focuses on the implications of these oscillations in the cosmic microwave background radiation and their role in cosmology, helping to shed light on the distribution of matter and energy in the early universe.
Photometric redshift measurements: Photometric redshift measurements are techniques used to estimate the distance of astronomical objects by analyzing the light they emit and determining how much that light has been redshifted due to the expansion of the universe. This method relies on observing the colors of light from distant galaxies and using models of galaxy evolution to infer their redshifts without the need for detailed spectroscopy. These measurements are particularly significant when studying large-scale structures, including baryon acoustic oscillations.
Primordial density fluctuations: Primordial density fluctuations refer to small variations in the density of matter in the early universe, which are thought to be the seeds for the large-scale structure we observe today. These fluctuations are crucial for understanding how matter clumped together to form galaxies, stars, and other cosmic structures, and they play a significant role in models explaining cosmic evolution, the cosmic web, and baryon acoustic oscillations.
Quantum fluctuations: Quantum fluctuations refer to temporary changes in energy levels that occur in empty space due to the uncertainty principle of quantum mechanics. These fluctuations can create virtual particles and influence the fabric of spacetime, playing a crucial role in cosmic phenomena such as the early universe's inflationary period and the formation of large-scale structures in the cosmos.
Recombination: Recombination refers to the epoch in the early universe, approximately 380,000 years after the Big Bang, when electrons combined with protons to form neutral hydrogen atoms. This process allowed photons to travel freely through space, leading to the decoupling of matter and radiation, which has profound implications for the cosmic microwave background (CMB), structure formation, and acoustic oscillations in the early universe.
Sound horizon: The sound horizon is the maximum distance that sound waves could have traveled in the hot, dense plasma of the early universe before the formation of neutral atoms. This concept is essential for understanding the scale of baryon acoustic oscillations, which provide a 'standard ruler' for measuring cosmic distances and play a significant role in studying the large-scale structure of the universe and the effects of dark energy.
Spectroscopic surveys: Spectroscopic surveys are systematic observational studies that utilize spectroscopy to analyze the light emitted or absorbed by celestial objects, allowing astronomers to determine their physical and chemical properties. These surveys can provide essential information about the universe, including the composition, temperature, density, and motion of galaxies, stars, and other astronomical phenomena. The data collected from these surveys is crucial for understanding cosmic structures and the evolution of the universe.