Chapter 16: The Expanding Universe

Chapter 1
How Science Works

  • The Scientific Method
  • Evidence
  • Measurements
  • Units and the Metric System
  • Measurement Errors
  • Estimation
  • Dimensions
  • Mass, Length, and Time
  • Observations and Uncertainty
  • Precision and Significant Figures
  • Errors and Statistics
  • Scientific Notation
  • Ways of Representing Data
  • Logic
  • Mathematics
  • Geometry
  • Algebra
  • Logarithms
  • Testing a Hypothesis
  • Case Study of Life on Mars
  • Theories
  • Systems of Knowledge
  • The Culture of Science
  • Computer Simulations
  • Modern Scientific Research
  • Astronomy
  • Astronomy as a Science
  • A Scale Model of Space
  • A Scale Model of Time
  • Questions
  • Chapter Quiz

Chapter 2
Early Astronomy

  • The Night Sky
  • Motions in the Sky
  • Navigation
  • Constellations and Seasons
  • The Seasons
  • The Magnitude System
  • Angular Size and Linear Size
  • Phases of the Moon
  • Eclipses
  • Aurora
  • Dividing Time
  • Solar Calendars
  • History of Astronomy
  • Stonehenge
  • Ancient Observatories
  • Counting and Measurement
  • Astrology
  • Greek Astronomy
  • Aristotle and Geocentric Cosmology
  • Aristarchus and Heliocentric Cosmology
  • The Dark Ages
  • Arab Astronomy
  • Indian Astronomy
  • Chinese Astronomy
  • Mayan Astronomy
  • Questions
  • Chapter Quiz

Chapter 3
The Copernican Revolution

  • Ptolemy and the Geocentric Model
  • The Renaissance
  • Copernicus and the Heliocentric Model
  • Tycho Brahe
  • Johannes Kepler
  • Elliptical Orbits
  • Kepler's Laws
  • Galileo Galilei
  • The Trial of Galileo
  • Isaac Newton
  • Newton's Law of Gravity
  • The Plurality of Worlds
  • The Birth of Modern Science
  • Layout of the Solar System
  • The Scale of the Solar System
  • The Idea of Space Exploration
  • Orbits
  • History of Space Exploration
  • Moon Landings
  • International Space Station
  • Manned versus Robotic Missions
  • Commercial Space Flight
  • Future of Space Exploration
  • Living in Space
  • Moon, Mars, and Beyond
  • Societies in Space
  • Questions
  • Chapter Quiz

Chapter 4
Matter and Energy in the Universe

  • Matter and Energy
  • Rutherford and Atomic Structure
  • Early Greek Physics
  • Dalton and Atoms
  • The Periodic Table
  • The Structure of the Atom
  • Energy
  • Heat and Temperature
  • Potential and Kinetic Energy
  • Conservation of Energy
  • Velocity of Gas Particles
  • States of Matter
  • Thermodynamics
  • Entropy
  • Laws of Thermodynamics
  • Heat Transfer
  • Thermal Radiation
  • Wien's Law
  • Radiation from Planets and Stars
  • Internal Heat in Planets and Stars
  • Periodic Processes
  • Random Processes
  • Questions
  • Chapter Quiz

Chapter 5
The Earth-Moon System

  • Earth and Moon
  • Early Estimates of Earth's Age
  • How the Earth Cooled
  • Ages Using Radioactivity
  • Radioactive Half-Life
  • Ages of the Earth and Moon
  • Geological Activity
  • Internal Structure of the Earth and Moon
  • Basic Rock Types
  • Layers of the Earth and Moon
  • Origin of Water on Earth
  • The Evolving Earth
  • Plate Tectonics
  • Volcanoes
  • Geological Processes
  • Impact Craters
  • The Geological Timescale
  • Mass Extinctions
  • Evolution and the Cosmic Environment
  • Earth's Atmosphere and Oceans
  • Weather Circulation
  • Environmental Change on Earth
  • The Earth-Moon System
  • Geological History of the Moon
  • Tidal Forces
  • Effects of Tidal Forces
  • Historical Studies of the Moon
  • Lunar Surface
  • Ice on the Moon
  • Origin of the Moon
  • Humans on the Moon
  • Questions
  • Chapter Quiz

Chapter 6
The Terrestrial Planets

  • Studying Other Planets
  • The Planets
  • The Terrestrial Planets
  • Mercury
  • Mercury's Orbit
  • Mercury's Surface
  • Venus
  • Volcanism on Venus
  • Venus and the Greenhouse Effect
  • Tectonics on Venus
  • Exploring Venus
  • Mars in Myth and Legend
  • Early Studies of Mars
  • A Close-Up View of Mars
  • Modern Views of Mars
  • Missions to Mars
  • Geology of Mars
  • Water on Mars
  • Polar Caps of Mars
  • Climate Change on Mars
  • Terraforming Mars
  • Life on Mars
  • The Moons of Mars
  • Martian Meteorites
  • Comparative Planetology
  • Incidence of Craters
  • Counting Craters
  • Counting Statistics
  • Internal Heat and Geological Activity
  • Magnetic Fields of the Terrestrial Planets
  • Mountains and Rifts
  • Radar Studies of Planetary Surfaces
  • Laser Ranging and Altimetry
  • Gravity and Atmospheres
  • Normal Atmospheric Composition
  • The Significance of Oxygen
  • Questions
  • Chapter Quiz

Chapter 7
The Giant Planets and Their Moons

  • The Gas Giant Planets
  • Atmospheres of the Gas Giant Planets
  • Clouds and Weather on Gas Giant Planets
  • Internal Structure of the Gas Giant Planets
  • Thermal Radiation from Gas Giant Planets
  • Life on Gas Giant Planets?
  • Why Giant Planets are Giant
  • Gas Laws
  • Ring Systems of the Giant Planets
  • Structure Within Ring Systems
  • The Origin of Ring Particles
  • The Roche Limit
  • Resonance and Harmonics
  • Tidal Forces in the Solar System
  • Moons of Gas Giant Planets
  • Geology of Large Moons
  • The Voyager Missions
  • Jupiter
  • Jupiter's Galilean Moons
  • Jupiter's Ganymede
  • Jupiter's Europa
  • Jupiter's Callisto
  • Jupiter's Io
  • Volcanoes on Io
  • Saturn
  • Cassini Mission to Saturn
  • Saturn's Titan
  • Saturn's Enceladus
  • The Discoveries of Uranus and Neptune
  • Uranus
  • Uranus' Miranda
  • Neptune
  • Neptune's Triton
  • Pluto
  • The Discovery of Pluto
  • Pluto as a Dwarf Planet
  • Dwarf Planets
  • Questions
  • Chapter Quiz

Chapter 8
Interplanetary Bodies

  • Interplanetary Bodies
  • Comets
  • Early Observations of Comets
  • Structure of the Comet Nucleus
  • Comet Chemistry
  • Oort Cloud and Kuiper Belt
  • Kuiper Belt
  • Comet Orbits
  • Life Story of Comets
  • The Largest Kuiper Belt Objects
  • Meteors and Meteor Showers
  • Gravitational Perturbations
  • Asteroids
  • Surveys for Earth Crossing Asteroids
  • Asteroid Shapes
  • Composition of Asteroids
  • Introduction to Meteorites
  • Origin of Meteorites
  • Types of Meteorites
  • The Tunguska Event
  • The Threat from Space
  • Probability and Impacts
  • Impact on Jupiter
  • Interplanetary Opportunity
  • Questions
  • Chapter Quiz

Chapter 9
How Planetary Systems Form

  • Formation of the Solar System
  • Early History of the Solar System
  • Conservation of Angular Momentum
  • Angular Momentum in a Collapsing Cloud
  • Helmholtz Contraction
  • Safronov and Planet Formation
  • Collapse of the Solar Nebula
  • Why the Solar System Collapsed
  • From Planetesimals to Planets
  • Accretion and Solar System Bodies
  • Differentiation
  • Planetary Magnetic Fields
  • The Origin of Satellites
  • Solar System Debris and Formation
  • Gradual Evolution and a Few Catastrophies
  • Chaos and Determinism
  • Extrasolar Planets
  • Discoveries of Extrasolar Planets
  • Doppler Detection of Extrasolar Planets
  • Transit Detection of Extrasolar Planets
  • The Kepler Mission
  • Direct Detection of Extrasolar Planets
  • Properties of Extrasolar Planets
  • Implications of Extrasolar Planet Surveys
  • Future Detection of Extrasolar Planets
  • Questions
  • Chapter Quiz

Chapter 10
Detecting Radiation from Space

  • Observing the Universe
  • Radiation and the Universe
  • The Nature of Light
  • The Electromagnetic Spectrum
  • Properties of Waves
  • Waves and Particles
  • How Radiation Travels
  • Properties of Electromagnetic Radiation
  • The Doppler Effect
  • Invisible Radiation
  • Thermal Spectra
  • The Quantum Theory
  • The Uncertainty Principle
  • Spectral Lines
  • Emission Lines and Bands
  • Absorption and Emission Spectra
  • Kirchoff's Laws
  • Astronomical Detection of Radiation
  • The Telescope
  • Optical Telescopes
  • Optical Detectors
  • Adaptive Optics
  • Image Processing
  • Digital Information
  • Radio Telescopes
  • Telescopes in Space
  • Hubble Space Telescope
  • Interferometry
  • Collecting Area and Resolution
  • Frontier Observatories
  • Questions
  • Chapter Quiz

Chapter 11
Our Sun: The Nearest Star

  • The Sun
  • The Nearest Star
  • Properties of the Sun
  • Kelvin and the Sun's Age
  • The Sun's Composition
  • Energy From Atomic Nuclei
  • Mass-Energy Conversion
  • Examples of Mass-Energy Conversion
  • Energy From Nuclear Fission
  • Energy From Nuclear Fusion
  • Nuclear Reactions in the Sun
  • The Sun's Interior
  • Energy Flow in the Sun
  • Collisions and Opacity
  • Solar Neutrinos
  • Solar Oscillations
  • The Sun's Atmosphere
  • Solar Chromosphere and Corona
  • Sunspots
  • The Solar Cycle
  • The Solar Wind
  • Effects of the Sun on the Earth
  • Cosmic Energy Sources
  • Questions
  • Chapter Quiz

Chapter 12
Properties of Stars

  • Stars
  • Star Names
  • Star Properties
  • The Distance to Stars
  • Apparent Brightness
  • Absolute Brightness
  • Measuring Star Distances
  • Stellar Parallax
  • Spectra of Stars
  • Spectral Classification
  • Temperature and Spectral Class
  • Stellar Composition
  • Stellar Motion
  • Stellar Luminosity
  • The Size of Stars
  • Stefan-Boltzmann Law
  • Stellar Mass
  • Hydrostatic Equilibrium
  • Stellar Classification
  • The Hertzsprung-Russell Diagram
  • Volume and Brightness Selected Samples
  • Stars of Different Sizes
  • Understanding the Main Sequence
  • Stellar Structure
  • Stellar Evolution
  • Questions
  • Chapter Quiz

Chapter 13
Star Birth and Death

  • Star Birth and Death
  • Understanding Star Birth and Death
  • Cosmic Abundance of Elements
  • Star Formation
  • Molecular Clouds
  • Young Stars
  • T Tauri Stars
  • Mass Limits for Stars
  • Brown Dwarfs
  • Young Star Clusters
  • Cauldron of the Elements
  • Main Sequence Stars
  • Nuclear Reactions in Main Sequence Stars
  • Main Sequence Lifetimes
  • Evolved Stars
  • Cycles of Star Life and Death
  • The Creation of Heavy Elements
  • Red Giants
  • Horizontal Branch and Asymptotic Giant Branch Stars
  • Variable Stars
  • Magnetic Stars
  • Stellar Mass Loss
  • White Dwarfs
  • Supernovae
  • Seeing the Death of a Star
  • Supernova 1987A
  • Neutron Stars and Pulsars
  • Special Theory of Relativity
  • General Theory of Relativity
  • Black Holes
  • Properties of Black Holes
  • Questions
  • Chapter Quiz

Chapter 14
The Milky Way

  • The Distribution of Stars in Space
  • Stellar Companions
  • Binary Star Systems
  • Binary and Multiple Stars
  • Mass Transfer in Binaries
  • Binaries and Stellar Mass
  • Nova and Supernova
  • Exotic Binary Systems
  • Gamma Ray Bursts
  • How Multiple Stars Form
  • Environments of Stars
  • The Interstellar Medium
  • Effects of Interstellar Material on Starlight
  • Structure of the Interstellar Medium
  • Dust Extinction and Reddening
  • Groups of Stars
  • Open Star Clusters
  • Globular Star Clusters
  • Distances to Groups of Stars
  • Ages of Groups of Stars
  • Layout of the Milky Way
  • William Herschel
  • Isotropy and Anisotropy
  • Mapping the Milky Way
  • Questions
  • Chapter Quiz

Chapter 15
Galaxies

  • The Milky Way Galaxy
  • Mapping the Galaxy Disk
  • Spiral Structure in Galaxies
  • Mass of the Milky Way
  • Dark Matter in the Milky Way
  • Galaxy Mass
  • The Galactic Center
  • Black Hole in the Galactic Center
  • Stellar Populations
  • Formation of the Milky Way
  • Galaxies
  • The Shapley-Curtis Debate
  • Edwin Hubble
  • Distances to Galaxies
  • Classifying Galaxies
  • Spiral Galaxies
  • Elliptical Galaxies
  • Lenticular Galaxies
  • Dwarf and Irregular Galaxies
  • Overview of Galaxy Structures
  • The Local Group
  • Light Travel Time
  • Galaxy Size and Luminosity
  • Mass to Light Ratios
  • Dark Matter in Galaxies
  • Gravity of Many Bodies
  • Galaxy Evolution
  • Galaxy Interactions
  • Galaxy Formation
  • Questions
  • Chapter Quiz

Chapter 16
The Expanding Universe

  • Galaxy Redshifts
  • The Expanding Universe
  • Cosmological Redshifts
  • The Hubble Relation
  • Relating Redshift and Distance
  • Galaxy Distance Indicators
  • Size and Age of the Universe
  • The Hubble Constant
  • Large Scale Structure
  • Galaxy Clustering
  • Clusters of Galaxies
  • Overview of Large Scale Structure
  • Dark Matter on the Largest Scales
  • The Most Distant Galaxies
  • Black Holes in Nearby Galaxies
  • Active Galaxies
  • Radio Galaxies
  • The Discovery of Quasars
  • Quasars
  • Gravitational Lensing
  • Properties of Quasars
  • The Quasar Power Source
  • Quasars as Probes of the Universe
  • Star Formation History of the Universe
  • Expansion History of the Universe
  • Questions
  • Chapter Quiz

Chapter 17
Cosmology

  • Cosmology
  • Early Cosmologies
  • Relativity and Cosmology
  • The Big Bang Model
  • The Cosmological Principle
  • Universal Expansion
  • Cosmic Nucleosynthesis
  • Cosmic Microwave Background Radiation
  • Discovery of the Microwave Background Radiation
  • Measuring Space Curvature
  • Cosmic Evolution
  • Evolution of Structure
  • Mean Cosmic Density
  • Critical Density
  • Dark Matter and Dark Energy
  • Age of the Universe
  • Precision Cosmology
  • Future of Astronomical Sources
  • Fate of the Universe
  • Alternatives to the Big Bang Model
  • Space-Time
  • Particles and Radiation
  • The Very Early Universe
  • Mass and Energy in the Early Universe
  • Matter and Antimatter
  • The Forces of Nature
  • Fine-Tuning in Cosmology
  • The Anthropic Principle in Cosmology
  • String Theory and Cosmology
  • The Multiverse
  • The Limits of Knowledge
  • Questions
  • Chapter Quiz

Chapter 18
Life On Earth

  • Nature of Life
  • Chemistry of Life
  • Molecules of Life
  • The Origin of Life on Earth
  • Origin of Complex Molecules
  • Miller-Urey Experiment
  • Pre-RNA World
  • RNA World
  • From Molecules to Cells
  • Metabolism
  • Anaerobes
  • Extremophiles
  • Thermophiles
  • Psychrophiles
  • Xerophiles
  • Halophiles
  • Barophiles
  • Acidophiles
  • Alkaliphiles
  • Radiation Resistant Biology
  • Importance of Water for Life
  • Hydrothermal Systems
  • Silicon versus Carbon
  • DNA and Heredity
  • Life as Digital Information
  • Synthetic Biology
  • Life in a Computer
  • Natural Selection
  • Tree Of Life
  • Evolution and Intelligence
  • Culture and Technology
  • The Gaia Hypothesis
  • Life and the Cosmic Environment
  • Chapter Quiz

Chapter 19
Life in the Universe

  • Life in the Universe
  • Astrobiology
  • Life Beyond Earth
  • Sites for Life
  • Complex Molecules in Space
  • Life in the Solar System
  • Lowell and Canals on Mars
  • Implications of Life on Mars
  • Extreme Environments in the Solar System
  • Rare Earth Hypothesis
  • Are We Alone?
  • Unidentified Flying Objects or UFOs
  • The Search for Extraterrestrial Intelligence
  • The Drake Equation
  • The History of SETI
  • Recent SETI Projects
  • Recognizing a Message
  • The Best Way to Communicate
  • The Fermi Question
  • The Anthropic Principle
  • Where Are They?
  • Chapter Quiz

The Quasar Power Source



Quasar 3C 273 as imaged by the Hubble Space Telescope's Advanced Camera for Surveys

Understanding the quasar power source is one of the most challenging tasks in astronomy. In the most luminous quasars, 1000 times the light from an entire galaxy is contained in a volume not much bigger than the solar system. Normal stellar processes are not efficient enough to produce this fantastic energy density. Can we show this? Yes! Suppose that we use the velocity of the hot gas near the quasar core to get an estimate of the mass in the central parsec. Then assume that normal stellar fusion is causing the energy release (without worrying for a moment about how a sufficient amount of stars could get into such a tiny volume). The Sun releases 0.7% of its mass in the form of radiant energy. Therefore, if M is the total mass of main-sequence stars within the central parsec, E = 0.007 Mc2 is the maximum amount of energy that could come from stars within that small region. The observed amount of energy is 15 to 40 times larger. In other words, the energy source within a quasar is so efficient that it releases 10% to 30% of the mass-energy locked in matter. Stellar fusion cannot do this.

 

The basic ingredient for the model of the quasar power source is a supermassive black hole. This compact object is the gravitational "engine" that powers many of the observed properties of quasars. We have seen that the evidence for black holes in nearby galaxies is quite strong. Quasar black holes would have to be in the range of mass from 106 to 1010 solar masses. There is no observation yet that probes scales close to the Schwarzschild radius. You should recognize that the black hole model for quasars is speculative and does not have the secure status of, for example, the model of stellar evolution.


Centaurus A

According to the standard model, the supermassive black hole at the center of a quasar forms and grows by accreting (gathering) gas and stars from the central parts of a galaxy. Since most galaxies rotate, the black hole is also likely to rotate. The law of conservation of angular momentum indicates that it will be spinning rapidly. No radiation can escape from the event horizon of a black hole. However, material that is falling in will be accelerated and will emit huge amounts of radiation. The region around a supermassive black hole is an enormous particle accelerator, converting the potential energy of material that falls in into radiation and kinetic energy of relativistic particles that flow out. This process is very efficient. In principle, a rotating black hole can convert 20% to 30% of the mass that falls in into pure energy, sufficient to explain the enormous luminosity of quasars. Most of this energy emerges in the form of non-thermal radiation, which can span the entire electromagnetic spectrum. Some quasars have been detected at every wavelength from radio waves the size of a person to ultra-high energy gamma rays the size of an atomic nucleus!

 

Artist's impression of quasar GB1508

A second major ingredient of the quasar model is an accretion disk, the reservoir of hot gas and dust that feeds the black hole. Rotation of the material that falls to the black hole forms a disk rather than a spherical cloud. The proximity of an intense radiation source means that the disk will be hot. Thermal spectra with temperatures of 10,000 to 20,000 K have been observed in a number of quasars, consistent with the expected radiation from an accretion disk. Plasma at this high a temperature puts out most of its energy at ultraviolet and soft X-ray wavelengths and surveys have indeed shown that most quasars are strong X-ray emitters. Such a hot disk would "puff up" into a torus, or doughnut, rather than flatten like a pancake. This geometry would obscure our view of the black hole in the equatorial plane, making it visible only along the poles of the spin axis.

 

Outside the accretion disk are small dense clouds of gas orbiting the supermassive black hole. Clouds at distances of under a parsec are partially ionized by strong ultraviolet radiation from the central engine, and they move at speeds of tens of thousands of kilometers per second in the strong gravity created by the black hole. This region gives rise to the broad lines seen in quasar spectra. Astronomers have developed a clever technique called reverberation mapping to measure the mass of the quasar black hole. The power source varies and with some time delay the clouds that emit the broad emission lines vary too. That time delay gives a size for the broad line region (R) and the width of the emission lines gives the velocity of the gas (ΔV), induced by the gravity of the supermassive black hole. According to Kepler's laws, the black hole mass M = R/G (ΔV)2, where G is the gravitational constant. All quasars show broad emission lines, but many lower-luminosity active galaxies in addition show narrower emission lines, with widths of hundreds of kilometers per second. The gas that gives rise to the narrow lines is much farther from the central engine; the clouds are distributed over kiloparsec scales.


Jet in Centaurus A

Some quasars have the most spectacular signature of nuclear activity: radio jets. Most quasars are weak radio sources; jets are observed only in quasars that are strong radio sources. The region around a supermassive black hole can act as a powerful particle accelerator. The accretion disk blocks most of the outflow, but material can be accelerated in the two directions perpendicular to the disk, along the spin axis. Quasar jets are relativistic. The material that flows out can be moving at 90% to 95% of the speed of light, and the individual high energy particles can be moving at over 99.9% of the speed of light. The region in which the jet forms is extraordinarily complex, with high-energy particles, radiation, shock waves, and magnetic fields. Supercomputers are used to simulate the structure and evolution of a jet. Radio observations reveal a rich variety of astrophysical jets. Some of them emerge from the nuclear regions, leave the host galaxy, and extend for millions of light years into intergalactic space.

 

Can astronomers relate the life story of quasars to our observations of nearby galaxies? Ten billion years ago, black holes grew quickly in the centers of massive galaxies, fueled by the frequent interactions between galaxies in the early, dense universe. The highest redshift quasars place severe constraints on theories of how black holes form and grow. The highest redshift quasar has a redshift of 7.1, which means that a black hole managed to form and grow to several billion solar masses in only 750 million years after the big bang. Since their peak about 5 to 8 billion years ago, quasars have been gradually fading. Supermassive black holes cannot disappear! However, they can lose their ability to shine brightly if they have no fuel. As the universe expands, there are fewer interactions among galaxies, and as stellar populations evolve, the amount of mass loss available to fuel nuclear activity diminishes. By now, most quasars have been starved into a sullen silence, with very occasional episodes of activity.


Quasar 1229+204 in the Core of a Colliding Galaxy

If we add up all the quasar luminosity at the peak epoch of quasar activity and assume the supermassive black hole model, we can work out the mass of quasar remnants that must persist to the present day. The prediction is that most galaxies will have 106 solar mass black holes, 10% will have 107 solar mass black holes, 1% will have 108 solar mass black holes, and so on. Amazingly enough, searches for central black holes in nearby galaxies have found black holes at roughly the predicted rate. The glory of quasars occurred billions of years ago, but we still see their dark shadows, lurking in the hearts of nearby galaxies.

 

Author: Chris Impey
Editor/Contributor: Jessie Antonellis