Chapter 17: Cosmology

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
  • The Scope of Astronomy
  • Astronomy as a Science
  • A Scale Model of Space
  • A Scale Model of Time
  • Questions

Chapter 2
Early Astronomy

  • The Night Sky
  • Motions in the Sky
  • Navigation
  • Constellations and Seasons
  • Cause of the Seasons
  • The Magnitude System
  • Angular Size and Linear Size
  • Phases of the Moon
  • Eclipses
  • Auroras
  • Dividing Time
  • Solar and Lunar 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 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
  • 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 4
Matter and Energy in the Universe

  • Matter and Energy
  • Rutherford and Atomic Structure
  • Early Greek Physics
  • Dalton and Atoms
  • The Periodic Table
  • 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 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 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
  • Mars Close-Up
  • 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 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
  • Discovery 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 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 9
Planet Formation and Exoplanets

  • 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 Exoplanets
  • Doppler Detection of Exoplanets
  • Transit Detection of Exoplanets
  • The Kepler Mission
  • Direct Detection of Exoplanets
  • Properties of Exoplanets
  • Implications of Exoplanet Surveys
  • Future Detection of Exoplanets
  • Questions

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 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 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 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 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 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 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
  • Types of 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 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
  • The Future of the Contents of the Universe
  • 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 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 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?

String Theory and Cosmology


How ambitious should scientists be in their hunger for explanations? Is it really possible to explain the universe and everything it contains? In the earliest age of science, Archimedes tried to identify a few axioms or principles from which he could deduce anything, and the Atomists believed that the diversity of all observed phenomena was caused by the collision of atoms. Much later, the power of Newton’s universal law of gravitation led the French mathematician Pierre-Simon Laplace to suggest that a sufficiently powerful intellect (we might say a computer) that knew the positions and motions of all particles at one time, could calculate their motions and positions at any time in the future.

The determinism implied by Newtonian mechanics and gravity was abhorrent to many humanists and philosophers because we humans are collections of particles, so perhaps our choices and free will are illusory. The 20th century reshaped these grand expectations with a dose of uncertainty. Determinism is in practice thwarted by the probabilistic nature of quantum theory, by the unpredictable and emergent properties of complex systems, and by extreme sensitivity to initial conditions that leads to mathematical chaos. Since a "theory of everything" must also be a self-consistent, mathematical theory, notable physicists like Freeman Dyson and Stephen Hawking concluded that the search for an ultimate theory with a small number of principles might be fruitless. Even if we find a set of equations that describes everything in the universe, we’re still left with unanswered questions about origin and meaning. As Stephen Hawking has asked: "What is it that breathes fire into the equations and makes a universe for them to describe "

Most scientists accept that no single theory can be used to understand and predict the behavior of every physical system. Rather, they hope that they can continue the march towards the unification of the four fundamental forces, where those forces that are each distinct in our current low-energy universe are melded into a single superforce at sufficiently high energy. Grand unified theories try to unite the electromagnetic force with the weak and strong nuclear forces. The most promising theories are based on supersymmetry, where known particles have a "shadow" partner and there’s no distinction between the particles that make up matter and the particles that carry forces.

The final step on this road is the unification of grand unified theories of elementary particles with general relativity, our best theory of gravity. Einstein beat his head against this brick wall for thirty years, until he was on his deathbed. The problem’s hard because particles are grainy and discrete while gravity is smooth and supple. They’re as distinct as wood and marble. Particle theory only works when gravity is so weak we can pretend it doesn’t exist and general relativity only works when the graininess and uncertainty of quantum theory are ignored. Quantum gravity has been the "Holy Grail" of physics since Einstein’s death yet there’s no chance of any lab experiment creating the conditions where the four fundamental forces are unified. Instead, all roads point back to the big bang, and something called the Planck time.

Projecting the expanding universe back towards a singularity — a state of infinite temperature and density — the limit of physical understanding is reached at the Planck time. The Planck time is an infinitesimal 10-43 seconds after the big bang. It’s the smallest interval where time has any meaning. At that very early epoch, the universe was a mere 10-35 meters across, or one hundred billion billionth of the size of a proton. That distance is called the Planck scale. At that tiny size, space and distance measurements don’t have any meaning. Space-time might be foamy rather than smooth and continuous as general relativity would predict. The temperature back then was a staggering 1032 Kelvin, hot enough that the puny force of gravity was equal to the other forces that are far stronger in the current, cold universe. These benchmarks define the limits of both measurement and understanding. Here’s another way to think of the Planck time and the Planck scale, independent of whether or not we are describing the early universe. Heisenberg’s uncertainty principle says evanescent or virtual particles come and go all the time, and they can be massive if their lifetimes are very short. Einstein’s general relativity says enough mass in a small enough space can create a black hole: a region with gravity so strong its escape velocity is the speed of light. Combining these ideas, there is a scale small enough for virtual black holes to exist. It’s the Planck scale. In the first instant of time after the big bang, space was curved on the scale of particles, particles had the attributes of black holes, and space-time distortions were governed by quantum uncertainty. Think of a pot of water on a hard boil and that’s a very crude approximation for the seething space-time foam of the very early universe.

Physicists think they have found a way forward by making an audacious leap beyond the Standard Model. Think of a guitar string. It’s under tension, and depending on the amount of tension and how it’s plucked, it gives off different sounds. Different harmonics or excitation modes of the string make different musical notes. Now imagine the string is freed from the guitar but it still has tension so it can oscillate and vibrate. Some of these floating guitar strings remain open, with both ends free, and others are closed, forming a loop. Now imagine that these strings are invisibly small and far smaller than any particle — they are one-dimensional objects on the Planck scale, about 10-35 meters. That, in a very small nutshell, is what physicists came up with for unifying gravity and quantum mechanics. In string theory, each different particle is a mode of vibration or "note" of an invisibly small string. The open and closed strings can interact and combine. As a string moves through time, it traces out a sheet or a tube, depending on whether it’s open or closed. The vibration modes of the string generate the mass, spin, and charge that a conventional particle would have. By adding supersymmetry to the mix, strings can describe both particles and forces, so an electron is a vibrating string, but so too is a graviton, which carries the gravity force. The theory, renamed superstring theory, naturally includes gravity as well as all the interactions of particles in the Standard Model.

The promise of the new theory was so great that many smart young physicists were willing to learn the gruelingly complex and abstract mathematics needed for a quantum theory of interacting strings. But there were two problems and one big surprise. The first problem was that the size and the energy scale of strings are many trillions of times beyond what can be probed by lab experiments or accelerators, so there seemed to be no way to test the theory. Second, detailed work in the 1980s showed there were five different types of string theory, each fiendishly difficult to work on, with no apparent way to decide between them. And the surprise? All the supersymmetric string theories involved ten space-time dimensions!

The next breakthrough was achieved in the 1990s by theoretical physicists at several universities, most notably Edward Witten at Princeton. What they thought were five completely different string theories were actually different ways of looking at the same theory. Imagine each theory was like a large planet where we only knew of a small island somewhere on the planet. It’s so difficult to explore the theory mathematically that we don’t know what else might be found on the planet. As techniques improve, we’re able to travel around the seas on each planet and find new islands. Only then is it realized that those five string theories are islands on the same planet, not different ones! This world has been called M-theory, where the M stands for membrane. Many theorists think that a formulation using membranes will be the most productive way forward. The fundamental object is a membrane or sheet rather than a string. The general object, called a "brane," can range in dimension from zero to nine. A point is a zero-brane, a line is a one-brane, a surface or membrane is a two-brane, and so on up to dimensions that have no name to describe them. M-theory is a beast to work with because the number of different types of membranes in different dimensions increases exponentially. The number of distinct physical states in a theory with ten or eleven dimensions is essentially infinite. However, the number of states that correspond to a universe roughly like ours, with just four dimensions of space and time, is a more tractable number, "only" 10500! Each of these states has hidden dimensions on the Planck scale and a unique and different set of forces and particles on the macroscopic scale. This situation is called the string theory "landscape." The question arises: What if our universe represents one of these states, while the others represent other possible universes, radically different from each other and from ours?

String theory notched up an important success in 1996 when it was used to explain the surprisingly large entropy of black holes. For the first time, a result derived from classical physics had been derived using string theory, demonstrating the explicit connection between strings and gravity. However, string theory has been subject to a substantial backlash as its enormous promise seems unfulfilled. In the past decade, several hundred exceptionally talented theoretical physicists have written thousands of papers on string theory, yet it’s not been tested so it can’t be confirmed or refuted. Some argue that the complexity and non-uniqueness of the theory mean that it can’t be tested and therefore isn’t truly science.

Is string theory a Theory of Everything or a Theory of Nothing? As usual in these heated academic debates, the truth is likely to be somewhere in between. String theory has provided real insights into the unification of quantum mechanics and gravity, and although the conditions where hidden dimensions can’t be created in the lab, the hypothesis makes predictions of effects at lower energies. The Large Hadron Collider will test supersymmetry, for example, which is a key component of string theories. Edward Witten, the string theory guru who was made a full professor at age 29, has said: " String theory is a part of 21st-century physics that fell by chance into the 20th century." He noted that the technical tools required to create the theory were still being invented. We’re exploring a vast undiscovered country and are still building the vehicles that we need for the journey. It will take time and patience to do this difficult work.


Author: Chris Impey