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

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