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Astrophysics II

🚀Astrophysics II Unit 15 – Observational Cosmology: Methods & Surveys

Observational cosmology uses astronomical data to unravel the universe's mysteries. From Hubble's law to the cosmic microwave background, scientists study the cosmos' large-scale properties, evolution, and origin. Dark matter, dark energy, and the universe's structure are key areas of investigation. Advances in technology have revolutionized our understanding of the cosmos. Surveys like SDSS and missions like Planck have mapped the sky, providing unprecedented data. Future projects like Euclid and the James Webb Space Telescope promise to push the boundaries of our knowledge even further.

Key Concepts

  • Observational cosmology focuses on using astronomical observations to study the large-scale properties, evolution, and origin of the universe
  • Hubble's law describes the relationship between a galaxy's distance and its recessional velocity due to the expansion of the universe
  • The cosmic microwave background (CMB) is the remnant radiation from the early universe, providing a snapshot of the universe ~380,000 years after the Big Bang
    • CMB is nearly uniform in all directions, with small temperature fluctuations on the order of 1 part in 100,000
  • Dark matter, which does not interact with electromagnetic radiation, makes up ~85% of the matter in the universe
    • Its presence is inferred from gravitational effects on visible matter and the rotation curves of galaxies
  • Dark energy is a hypothetical form of energy that permeates all of space and accelerates the expansion of the universe
  • The large-scale structure of the universe consists of galaxies, galaxy clusters, and filaments separated by vast voids
  • Redshift, the shift of spectral lines towards longer wavelengths, is used as a measure of distance in cosmology (cosmological redshift)

Historical Context

  • In 1929, Edwin Hubble discovered a linear relationship between a galaxy's distance and its recessional velocity, known as Hubble's law
  • The discovery of the CMB by Arno Penzias and Robert Wilson in 1965 provided strong evidence for the Big Bang theory
  • The concept of dark matter was introduced by Fritz Zwicky in the 1930s to explain the missing mass in galaxy clusters
  • The accelerating expansion of the universe, attributed to dark energy, was discovered in 1998 through observations of distant supernovae
  • The Cosmic Background Explorer (COBE) satellite, launched in 1989, provided the first precise measurements of the CMB spectrum and anisotropies
  • The Wilkinson Microwave Anisotropy Probe (WMAP), launched in 2001, provided more detailed measurements of the CMB and constrained cosmological parameters
  • The Planck spacecraft, launched in 2009, provided the most precise measurements of the CMB to date, refining our understanding of the universe's composition and evolution

Observational Techniques

  • Photometry measures the brightness of astronomical objects in different wavelength bands, providing information on their luminosity and temperature
  • Spectroscopy disperses light into its constituent wavelengths, allowing the study of an object's composition, temperature, and velocity
    • Spectroscopic redshift surveys map the 3D distribution of galaxies and measure their recessional velocities
  • Interferometry combines signals from multiple telescopes to achieve higher angular resolution than possible with a single telescope
    • Used in radio astronomy to study the CMB and the 21-cm emission from neutral hydrogen
  • Gravitational lensing, the bending of light by massive objects, is used to map the distribution of dark matter and study distant galaxies
    • Strong lensing occurs when a massive object (lens) is aligned with a distant source, creating multiple images or Einstein rings
    • Weak lensing refers to the subtle distortions of background galaxy shapes due to the gravitational influence of intervening matter
  • Supernovae, particularly Type Ia, serve as standard candles for measuring cosmic distances due to their consistent peak luminosity
  • Baryon acoustic oscillations (BAO) are periodic fluctuations in the density of visible matter, serving as a cosmic ruler for measuring distances

Major Surveys and Missions

  • The Sloan Digital Sky Survey (SDSS) has mapped over a third of the sky, providing data on millions of galaxies and quasars
    • SDSS has been instrumental in studying the large-scale structure of the universe and measuring cosmological parameters
  • The Hubble Space Telescope (HST) has made numerous groundbreaking observations, including the Hubble Deep Fields, which revealed thousands of distant galaxies
  • The Chandra X-ray Observatory and XMM-Newton study high-energy phenomena, such as galaxy clusters and the hot gas between galaxies
  • The Atacama Cosmology Telescope (ACT) and the South Pole Telescope (SPT) are ground-based telescopes that study the CMB at high angular resolution
  • The Dark Energy Survey (DES) aims to map the distribution of dark matter and measure the expansion history of the universe using weak lensing and supernovae
  • Euclid, a future ESA mission, will investigate the nature of dark energy and dark matter using weak lensing and galaxy clustering
  • The James Webb Space Telescope (JWST), launched in 2021, will study the early universe, galaxy formation, and exoplanets

Data Analysis Methods

  • Correlation functions measure the clustering of galaxies as a function of scale, providing information on the matter distribution and cosmological parameters
  • Power spectrum analysis decomposes the CMB temperature fluctuations or the galaxy distribution into contributions from different angular or spatial scales
    • The shape and amplitude of the power spectrum depend on cosmological parameters such as the matter density and the Hubble constant
  • Bayesian inference is used to estimate the values of cosmological parameters by comparing theoretical models with observational data
    • Markov Chain Monte Carlo (MCMC) methods are employed to efficiently sample the parameter space and find the most likely values
  • Machine learning techniques, such as neural networks and support vector machines, are increasingly used to analyze large cosmological datasets and identify patterns
  • N-body simulations are used to model the evolution of the large-scale structure of the universe under the influence of gravity
    • These simulations help interpret observational data and test cosmological models
  • Void analysis studies the properties and distribution of cosmic voids, which are large underdense regions in the galaxy distribution
  • Tomographic analysis involves dividing the galaxy sample into redshift slices to study the evolution of cosmic structure over time

Cosmological Models

  • The Lambda Cold Dark Matter (ΛCDM) model is the standard cosmological model, consisting of dark energy (Λ) and cold dark matter (CDM) along with ordinary matter
    • ΛCDM successfully explains a wide range of observations, including the CMB, large-scale structure, and the accelerating expansion of the universe
  • Inflation theory proposes a period of rapid exponential expansion in the early universe, solving several problems in standard Big Bang cosmology
    • Inflation explains the observed flatness, homogeneity, and isotropy of the universe, as well as the origin of primordial density fluctuations
  • Modified gravity theories, such as f(R) gravity and scalar-tensor theories, attempt to explain the accelerating expansion of the universe without invoking dark energy
  • The Friedmann equations describe the evolution of the universe based on its matter and energy content, assuming a homogeneous and isotropic universe
  • The cosmological constant, denoted by Λ, represents the simplest form of dark energy, with a constant energy density throughout space and time
  • Quintessence models propose a dynamic form of dark energy, where the energy density evolves with time due to a scalar field
  • The curvature of the universe, which can be flat, open, or closed, affects the geometry and evolution of the cosmos

Recent Discoveries

  • The Planck mission has provided the most accurate measurements of the CMB temperature and polarization anisotropies, confirming the ΛCDM model with unprecedented precision
  • Gravitational waves from the early universe, predicted by inflation, were potentially detected by the BICEP2 experiment in 2014, but later found to be consistent with dust emission
  • The Hubble tension refers to the discrepancy between measurements of the Hubble constant from the CMB and from local distance indicators, hinting at possible new physics
  • The detection of the BAO feature in the galaxy distribution has provided an independent confirmation of the accelerating expansion of the universe
  • The discovery of ultra-diffuse galaxies, which have low luminosities but sizes comparable to the Milky Way, challenges our understanding of galaxy formation and evolution
  • The Lyman-alpha forest, the absorption lines in the spectra of distant quasars due to intervening neutral hydrogen, has been used to study the distribution of matter at high redshifts
  • The kinetic Sunyaev-Zel'dovich effect, the distortion of the CMB due to the motion of galaxy clusters, has been detected, providing a new probe of the cosmic velocity field

Future Directions

  • The Euclid mission and the Vera C. Rubin Observatory (formerly LSST) will conduct large galaxy surveys to study dark energy and dark matter with unprecedented precision
  • The Square Kilometre Array (SKA) will be the world's largest radio telescope, enabling detailed studies of the early universe, dark energy, and gravity
  • The CMB-S4 experiment will combine data from multiple ground-based telescopes to provide even more sensitive measurements of the CMB polarization, potentially detecting the signature of primordial gravitational waves
  • Intensity mapping of the 21-cm emission from neutral hydrogen will probe the large-scale structure of the universe at high redshifts, complementing galaxy surveys
  • Gravitational wave astronomy, using detectors such as LIGO and LISA, will open new windows into the early universe and the astrophysics of compact objects
  • The search for direct evidence of dark matter particles, through experiments such as XENON and LUX, will continue, aiming to unveil the nature of this mysterious component
  • Advances in machine learning and data analysis techniques will enable more efficient and accurate processing of the vast amounts of data from future surveys and experiments
  • Theoretical and numerical work on the formation and evolution of galaxies, clusters, and the large-scale structure will provide a deeper understanding of the observational results


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© 2024 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.