👽Galaxies and the Universe Unit 10 – Observing the Cosmos: Redshift Surveys

What's This Unit About?

• Explores the use of redshift surveys to map the large-scale structure and evolution of the universe
• Focuses on how astronomers measure and interpret the redshift of galaxies to determine their distances and velocities
• Covers the scientific principles behind redshift, including the Doppler effect and cosmological redshift
• Examines the various types of redshift surveys conducted by astronomers, such as pencil-beam surveys and wide-field surveys
• Discusses the tools and techniques used to observe and measure redshift, including spectroscopy and photometric redshift estimation
• Delves into the analysis of redshift data to construct 3D maps of the universe and study its properties
• Highlights real-world applications and discoveries made through redshift surveys, such as the detection of large-scale structures (filaments and voids)
• Addresses the challenges faced in conducting redshift surveys and the future directions of this field

Key Concepts and Definitions

• Redshift: The shift of spectral lines towards longer wavelengths (red end of the spectrum) due to the expansion of the universe or the motion of the source away from the observer
  • Cosmological redshift: Caused by the expansion of the universe, proportional to the distance of the galaxy
  • Doppler redshift: Caused by the relative motion of the galaxy away from the observer
• Spectroscopy: The study of the interaction between matter and electromagnetic radiation, used to measure the redshift of galaxies by analyzing their spectra
• Photometric redshift: An estimate of a galaxy's redshift based on its brightness in different wavelength bands, without the need for spectroscopy
• Large-scale structure: The distribution of galaxies and galaxy clusters on scales larger than individual galaxies, forming a cosmic web of filaments, walls, and voids
• Hubble's law: The relationship between a galaxy's distance and its redshift, expressed as $v = H_0 \times d$, where $v$ is the recessional velocity, $H_0$ is the Hubble constant, and $d$ is the distance
• Comoving distance: The distance between two points in the universe that remains constant with the expansion of the universe, used to map the large-scale structure

The Science Behind Redshift

• Redshift occurs when the wavelength of electromagnetic radiation (light) is stretched due to the expansion of the universe or the relative motion of the source
• The amount of redshift is denoted by the letter "z" and is calculated using the formula $z = (\lambda_{observed} - \lambda_{rest}) / \lambda_{rest}$, where $\lambda_{observed}$ is the observed wavelength and $\lambda_{rest}$ is the rest wavelength
• Cosmological redshift is caused by the expansion of the universe, which stretches the wavelength of light as it travels through space
  • The farther a galaxy is from the observer, the more the universe has expanded during the time the light has been traveling, resulting in a higher redshift
• Doppler redshift is caused by the relative motion of the galaxy away from the observer, similar to the change in pitch of an ambulance siren as it moves away from the listener
• The relationship between redshift and distance is described by Hubble's law, which states that the recessional velocity of a galaxy is proportional to its distance from the observer
• The Hubble constant, denoted by $H_0$, represents the current expansion rate of the universe and is a key parameter in cosmology

Types of Redshift Surveys

• Pencil-beam surveys: Narrow, deep surveys that cover a small area of the sky but extend to high redshifts, allowing the study of galaxy evolution over cosmic time
  • Examples: Hubble Deep Field, Hubble Ultra-Deep Field
• Wide-field surveys: Surveys that cover a large area of the sky but may not extend to very high redshifts, providing a more representative sample of the universe at a given epoch
  • Examples: Sloan Digital Sky Survey (SDSS), Dark Energy Survey (DES)
• Targeted surveys: Surveys that focus on specific regions of the sky or types of galaxies, such as clusters or galaxies with active galactic nuclei (AGN)
  • Example: CLASH (Cluster Lensing And Supernova survey with Hubble)
• Multi-wavelength surveys: Surveys that combine observations from different parts of the electromagnetic spectrum (optical, infrared, radio) to provide a more comprehensive understanding of galaxy properties and evolution
  • Example: COSMOS (Cosmic Evolution Survey)
• Time-domain surveys: Surveys that repeatedly observe the same region of the sky to detect changes in galaxy properties over time, such as the emergence of supernovae or the variability of AGN
  • Example: Zwicky Transient Facility (ZTF)

Tools and Techniques for Observation

• Telescopes: Large ground-based and space-based telescopes are used to collect light from distant galaxies, such as the Hubble Space Telescope, the Keck telescopes, and the Very Large Telescope (VLT)
• Spectrographs: Instruments that disperse the light from galaxies into its constituent wavelengths, allowing the measurement of redshift through the identification of spectral lines
  • Examples: DEIMOS (DEep Imaging Multi-Object Spectrograph) on Keck, VIMOS (VIsible Multi-Object Spectrograph) on VLT
• Multi-object spectrographs: Instruments that can simultaneously obtain spectra for multiple galaxies in a single observation, greatly increasing the efficiency of redshift surveys
  • Examples: MOSFIRE (Multi-Object Spectrometer For Infra-Red Exploration) on Keck, MUSE (Multi-Unit Spectroscopic Explorer) on VLT
• Photometric redshift estimation: A technique that estimates redshifts based on the brightness of galaxies in different wavelength bands, using the fact that the spectral energy distribution of galaxies varies with redshift
  • Advantages: Faster and less resource-intensive than spectroscopy, can be applied to fainter galaxies
  • Disadvantages: Less precise than spectroscopic redshifts, can be affected by dust extinction and galaxy evolution effects
• Data reduction pipelines: Software tools that process the raw data from telescopes and spectrographs, removing instrumental effects and calibrating the data to produce science-ready spectra and redshift measurements

Analyzing Redshift Data

• Redshift catalogues: Compilations of redshift measurements for large numbers of galaxies, often including additional information such as galaxy positions, magnitudes, and colors
• Large-scale structure analysis: Using redshift data to map the 3D distribution of galaxies and study the properties of the cosmic web
  • Correlation functions: Statistical measures of the clustering of galaxies as a function of scale, used to quantify the strength and shape of the large-scale structure
  • Power spectrum analysis: Studying the distribution of galaxies in Fourier space to measure the amplitude and scale-dependence of density fluctuations
• Cosmological parameter estimation: Using the observed redshift-distance relation and the clustering of galaxies to constrain key cosmological parameters, such as the matter density, dark energy density, and the Hubble constant
• Galaxy evolution studies: Investigating how the properties of galaxies (star formation rates, masses, morphologies) change with redshift to understand the processes that drive galaxy evolution over cosmic time
• Environmental studies: Examining how the properties of galaxies depend on their environment (clusters, filaments, voids) to shed light on the role of external factors in shaping galaxy evolution

Real-World Applications and Discoveries

• Discovery of the accelerating expansion of the universe: Observations of high-redshift supernovae in the late 1990s revealed that the expansion of the universe is accelerating, leading to the introduction of the concept of dark energy
• Mapping the large-scale structure of the universe: Redshift surveys such as the 2dF Galaxy Redshift Survey and the Sloan Digital Sky Survey have produced detailed 3D maps of the distribution of galaxies, revealing the cosmic web of filaments, walls, and voids
• Detection of baryon acoustic oscillations (BAO): The imprint of sound waves in the early universe on the distribution of galaxies, which serves as a standard ruler to measure the expansion history of the universe and constrain cosmological parameters
• Study of galaxy clusters: Redshift surveys have enabled the identification and characterization of galaxy clusters, the largest gravitationally bound structures in the universe, which serve as laboratories for studying galaxy evolution and cosmology
• Tracing the evolution of galaxy properties: By comparing the properties of galaxies at different redshifts, astronomers have been able to study how star formation rates, masses, and morphologies evolve over billions of years, providing insights into the processes that shape galaxy evolution

Challenges and Future Directions

• Incompleteness and selection effects: Redshift surveys are limited by the sensitivity of telescopes and the ability to obtain spectra for faint or distant galaxies, which can introduce biases in the observed galaxy population
  • Addressing this challenge: Developing more sensitive instruments, using photometric redshift estimation to extend surveys to fainter galaxies, and applying statistical corrections to account for selection effects
• Spectroscopic redshift desert: The difficulty in measuring spectroscopic redshifts for galaxies in the range $1.5 < z < 2.5$ due to the lack of strong emission lines in the optical wavelength range
  • Addressing this challenge: Using near-infrared spectrographs to observe redshifted emission lines, and applying photometric redshift estimation techniques
• Cosmic variance: The uncertainty in cosmological measurements introduced by the fact that we can only observe a finite volume of the universe, which may not be representative of the universe as a whole
  • Addressing this challenge: Conducting larger and deeper surveys to cover more representative volumes, and using multiple independent surveys to cross-check results
• Future redshift surveys: Upcoming projects such as the Dark Energy Spectroscopic Instrument (DESI), the Prime Focus Spectrograph (PFS) on Subaru, and the Wide-Field Infrared Survey Telescope (WFIRST) will provide even larger and more detailed redshift surveys, enabling unprecedented studies of the large-scale structure and galaxy evolution
• Synergy with other observational probes: Combining redshift surveys with other cosmological probes, such as cosmic microwave background measurements, weak lensing surveys, and gravitational wave observations, to provide a more comprehensive understanding of the universe and its evolution