Recent Developments in Cosmology Josh Frieman Quarknet Argonne

Recent Developments in Cosmology Josh Frieman Quarknet, Argonne National Laboratory, July 2002

Cosmology: an ancient endeavor §How did the world around us come into being? §Has it always been like this or has it evolved? §If the Universe is changing, how did it begin and what will it be like in the future? And (how) will it end? Early Cosmology: the Universe evolved from a beginning Babylonian cosmology: Enuma elish Judeo-Christian cosmology: Genesis Greek and Roman myths and philosophers Modern cosmology: expanding Universe established 1929, evolving Universe established in 1965 (discovery of Cosmic Microwave Background Radiation by Penzias & Wilson, Nobel Prize in 1978)

Modern Science: --The Universe is knowable through repeatable observations --The Universe can be described in terms of universal physical laws Modern Cosmology: Archaeology on the Grand Scale --We cannot (yet) create universes in the laboratory and study them --We must observe stars, galaxies, cosmic radiations, etc, and use them as `pottery shards’ to reconstruct what the Universe was like at much earlier times, to weave a coherent story of cosmic evolution based on our understanding of physical laws. Fortunately, there are surprisingly few ways (given the laws of physics) to make a Universe that looks like ours today.

The macroscopic Universe observed: a hierarchy of Structure. . .

Human scale: Size ~ 100 cm Mass ~ 100 kg ~ 1029 atoms Density ~ 0. 6 gm/cm 3 Structures organized by atomic interactions Sarah Frieman b. March 26, 2001

Planets: Size ~ 1010 cm~1010 cm Mass ~ 1026 kg ~ 1054 atoms Density ~ 0. 6 gm/cm 3 Structures determined by atomic interactions & gravity

Brown Dwarf Star (Planet/star transition) Ordinary Stars: Size ~ 1011 cm Mass ~ 1030 kg ~ 1057 atoms Density ~ 0. 5 gm/cm 3 Hot gas bound by gravity

M 87 Nebula in Orion (star forming region in our galaxy) Interstellar gas clouds & star clusters: Size ~ 1 parsec ~ 3 light-yr ~ 3 x 1018 cm Mass ~ 105 Msun

An Infrared view of the Milky Way (our galaxy) Galaxies: Size ~ 1022 cm ~ 10 kiloparsec (kpc) Mass ~ 1011 Msun Self-gravitating systems of stars, gas, and dark matter

A Brief Tour of Galaxies Images from the Sloan Digital Sky Survey (SDSS): An on-going project to map the Universe, the SDSS will catalog roughly 70 million galaxy images and measure 3 D positions for ~700, 000 of them by the time it is completed in 2005

UGC 03214: edge-on spiral galaxy in Orion

NGC 1087 spiral galaxy in Aries

Clusters of Galaxies: Size ~ 1025 cm ~ Megaparsec (Mpc) Mass ~ 1015 Msun Largest gravitationally bound objects: galaxies, gas, dark matter

Cluster of Galaxies `giant arcs’ are galaxies behind the cluster, gravitationally lensed by it

Gravitational Lensing Basically, the same effects that occur in more familiar optical circumstances: magnification and distortion Apparent position 2 True position 2 Apparent Position 1 True Position 1 Objects farther from the line of sight are distorted less. Observer Gravitational “lens” “Looking into” the lens: extended objects are tangentially distorted. . .

Helen Frieman b. 9/20/99

Helen behind a Black Hole Gravitational Lens

Mapping the Mass in a Cluster of Galaxies via Gravitational Lensing: Most of the Mass in the Universe is Dark (it doesn’t shine) Dark Matter

Superclusters and Large-scale Structure: Filaments, Walls, and Voids of Galaxies 100 Million parsecs (Mpc) You Are Here `Pizza Slice’ 6 degrees thick containing 1060 galaxies: position of each galaxy represented by a single dot

Superclusters and Large-scale Structure: Filaments, Walls, and Voids of Galaxies 100 Million parsecs (Mpc) You Are Here

Superclusters and Large-scale Structure: Filaments, Walls, and Voids of Galaxies Coma cluster of galaxies 100 Million parsecs (Mpc) You Are Here

Early SDSS Data ~200, 000 Galaxies Mapped in 3 D so far

The Big Bang Theory: a well-tested framework for understanding the observations and for asking new questions The Universe has been expanding isotropically from a hot, dense `beginning’ (aka the Big Bang) for about 14 billion years The only successful framework we have for explaining several key facts about the Universe: Hubble’s law of galaxy recession: expansion Uniformity (isotropy) of Microwave background Cosmic abundances of the light elements: Hydrogen, Helium, Deuterium, Lithium, cooked in the first 3 minutes

The Big Bang Theory Not `just a theory’, but one of the most firmly established paradigms in science: The Standard Cosmological Model

The Big Bang Theory The Big Bang is an idealization, a simplified description (analogous to the approximation of the Earth as a perfect sphere), and cosmologists are now occupied with mapping out/filling in the details. Even so, certain basic elements of the model remain to be understood: e. g. , the natures of the Dark Matter & Dark Energy which together make up 95% of the mass-energy of the Universe These puzzles do NOT mean that the Big Bang Theory is wrong—rather, it provides the framework for investigating them.

The Big Bang Theory: Are there human implications?

(as seen on public buses and roadside billboards)

Spectrum of Light from Galaxies receding slowly Redshift of Galaxy Emission & Absorption Lines: recession velocity v/c ≈ z = / 0 (approximation for objects moving with v/c << 1) receding quickly

Hubble Space Telescope in Orbit Measured distances to galaxies using Cepheid Variable stars

Hubble (1929) Hubble Space Telescope (2000)

Modern `Hubble Diagram’ Extend to larger distances using objects brighter than Cepheids

The Microwave Sky: The Universe is filled with thermal radiation: Cosmic Microwave Background (CMB) COBE Map of the Temperature of the Universe On large scales, the Universe is (nearly) isotropic around us (the same in all directions): CMB radiation probes as deeply as we can, far beyond optical light from galaxies: snapshot of the young Universe (at 400, 000 years old) T = 2. 7 degrees above absolute zero Scale of the Observable Universe: Size ~ 1028 cm Mass ~ 1023 Msun

CMB Earth (nearly) isotropic not

The Cosmological Principle A working assumption (hypothesis) aka the Copernican Principle: We are not priviledged observers at a special place in the Universe: At any instant of time, the Universe should appear ISOTROPIC (over large scales) to All observers. A Universe that appears isotropic to all observers is HOMOGENEOUS i. e. , the same at every location (averaged over large scales).

The Microwave Sky: COBE Map of the Temperature of the Universe Dipole anisotropy due to our Galaxy’s motion through the Universe T = 2. 728 deg above absolute zero Red: 2. 7+0. 001 Blue: 2. 7 -0. 001 Red: 2. 7+0. 00001 deg Blue: 2. 7 -0. 00001 deg

The Microwave Sky: COBE Map of the Temperature of the Universe T = 2. 7 degrees above absolute zero Red: 2. 7+0. 001 Blue: 2. 7 -0. 001 Map with Dipole anisotropy removed: fluctuations of the density of the Universe (plus Galactic emission) Red: 2. 7+0. 00001 deg Blue: 2. 7 -0. 00001 deg

Cosmology as Metaphor: From The New Yorker, March 5, 2001: `A hiss of chronic corruption suffuses the capital like background radiation from the big bang. ’ --Hendrik Hertzberg `The Talk of the Town’

Physical Implications of Expanding Universe An expanding gas cools and becomes less dense as it expands. Run the expansion backward: going back into the past, the Universe heats up and becomes denser. Expanding Universe plus known laws of physics imply the Universe has finite age and a `singular’ (nearly infinite density and Temperature) beginning about 14 Billion years ago: THE BIG BANG

Big Bang Nucleosynthesis Origin of the Light Elements: Helium, Deuterium, Lithium, … When the Universe was younger than about 1 minute old, with a Temperature above ~ 1 billion degees, atomic nuclei (e. g. , He 4 nucleus = 2 neutrons + 2 protons bound together) could not survive: instead the baryons formed a soup of protons & neutrons. As the Temperature dropped below this value (set by the binding energy of light nuclei), protons and neutrons began to fuse together to form bound nuclei: the light elements were synthesized as the Universe expanded and cooled.

BBN predicted abundances Fraction of baryonic mass in He 4 Deuterium to Hydrogen ratio h = H 0/(100 km/sec/Mpc) Light Element abundances depend mainly on the density of baryons in the Universe Lithium to Hydrogen ratio baryon/photon ratio

BBN Theory vs. Observations: Observational constraints shown as boxes Remarkable agreement over 10 orders of magnitude in abundance variation Concordance region: b = 0. 04 Strongest constraint comes from Deuterium. Excellent agreement w/ more recent CMB measurements b 4 He

Recent CMB experiments: Going to smaller angular scales higher resolution

Boomerang Recent CMB Anisotropy Experiments: South Pole DASI

CMB Angular Power Spectrum Angular power spectrum is a statistical way to characterize the spatial structure in a 2 -dimensional image or map

Oscillation of the Photon. Baryon fluid when the Universe was 400, 000 yrs old Imprint on the Microwave sky

Theoretical dependence of CMB anisotropy on the baryon density Angular frequency Angular separation on the sky

Microwave Background Anisotropy Probes b (Baryon Density) b = 0. 04 Boomerang experiment (2001) DASI experiment (2001)

Einstein’s General Relativity Matter and Energy curve Space-Time All bodies move in this curved Space-time A massive star attracts nearby objects by distorting spacetime

Gravity: Newton vs. Einstein Newton: 1) gravitation is a force exerted by one massive body on another. 2) a body acted on by a force accelerates Einstein: 1) gravitation is the curvature of spacetime due to a nearby massive body (or any form of energy) 2) a body follows the `straightest possible path’ (aka geodesic) in curved spacetime

Einstein: space can also be globally curved What is the geometry of three-dimensional space?

Microwave photons traverse a significant fraction of the Universe, so they can probe its spatial curvature Sizes of hot and cold spots in the CMB give information on curvature of space: In curved space, light bends as it travels: fixed object has larger angular size in a positively curved space: CMB spots appear larger. Opposite occurs for negatively curved space.

Position of first Peak probes the spatial Curvature of the Universe

Microwave Background Anisotropy Probes Spatial Curvature W 0 = 1. 03 0. 06 Boomerang experiment (2001) W 0 = 1. 04 0. 06 DASI experiment (2001)

Einstein: space can also be globally curved What is the geometry of space? Recent observations of the Microwave background anisotropy indicate it is flat

Probes of the Matter Density: m Current evidence: Galaxy kinematics From galaxy clusters and other probes: m ~ 0. 3 X-ray gas Lensing

rotation velocity Observed: flat, M ~ d Keplerian: v ~ d-1/2 blueshift redshift Typical rotation speed ~200 km/sec and visible disk size ~ 10 kpc

Clusters of Galaxies: Size ~ 1025 cm ~ Megaparsec (Mpc) Mass ~ 1015 Msun Largest gravitationally bound objects: galaxies, gas, dark matter

The 2 Dark Matter Problems Observations indicate: visible matter ~ 0. 01 baryons ~ 0. 04 dark matter ~ 0. 3 BBN+CMB Dark Baryonic matter composed of protons, neutrons, (more fundamentally of quarks) Dominant component of Dark Matter is Non-baryonic requires a new component beyond quarks, . . .

Basic Dark Matter Questions How much is there? What is the value of ? Current evidence suggests ~0. 3. Where is it? Is it just clustered with the luminous material? Not precisely, since Dark halos extend beyond luminous galaxies. Are there completely dark galaxies or clusters? What is it? BBN+CMB mostly not made of baryons (i. e. , protons, neutrons, quarks, etc). It could be a new Weakly Interacting Massive Particle (WIMP). Supersymmetry models predict these. Ultimate Copernican principle: We’re not even made of the central stuff of the Universe!

Dark Energy and the Accelerating Universe Brightness of distant Type Ia supernovae indicates the expansion of the Universe is accelerating, not decelerating. If General Relativity is valid, this requires a new form of stuff with negative effective pressure*: Dubya DARK ENERGY Characterize by its equation of state: *more specifically, p < (w < 1/3) pressure w = p/ density

p = (w = 1) Size of the Universe Accelerating SNe Ia + CMB indicate m 0. 3 DE 0. 7 Empty Open Closed Today Cosmic Time

Evidence for Dark Energy I. III. IV. Direct Evidence for Acceleration Brightness of distant Type Ia supernovae: Standard candles measure luminosity distance d. L(z sensitive to the expansion history H(z) Supernova Cosmology Project High-Z Supernova Team II. Evidence for `Missing Energy’ CMB Flat Universe: 0 = 1 Clusters, LSS Low matter density m 0. 3 missing = 1 – 0. 3 = 0. 7 and missing stuff can only dominate recently for structure to form: w < – 0. 5

Discovery of SNe Ia at `high’ redshift z ~ 0. 5 – 1

Intrinsic Brightness vs. Time Physical model: White dwarf star, accreting mass from a companion star, explodes when it exceeds a critical mass (Chandrasekhar) Luminosity Type Ia Supernovae Peak Brightness as a calibrated `Standard’ Candle Time

Apparent Brightness Fainter 42 SNe Ia distance m(z) = M+5 log(H 0 d. L)=(1+z) dz’/H(z’)

Dark Energy density CMB and Supernovae • CMB + SNIa • orthogonal constraints m = 0. 31 L = 0. 71 Dark matter density 0. 13 0. 11

The Early Universe: the key to Large-scale Structure From our vantage point 13 billion years after the Big Bang, we are now trying to unravel what happened in the earliest tiny fraction of a second, when the Universe was 0. 0000000000000000001 seconds old! We can test our ideas about the Very Early Universe by observing the distributions of galaxies and of cosmic radiations in space. This has been a major breakthrough in cosmology over the last decade.

Inflation An epoch of very rapid expansion, during which the size of the Universe grows faster than time This means that comoving observers appear to be accelerating away from each other. As we saw, there is mounting evidence (from Type Ia Supernovae) that the Universe recently (~10 billion years ago) entered such an accelerating phase of expansion. The Universe may now be in the early stages of Inflation.

Inflation in the Early Universe A hypothetical epoch of very rapid (`accelerated’) expansion very early in cosmic history (perhaps around t ~ 10 -33 seconds), during which the size of the Universe grew faster than time. If this period of `Superluminal’ expansion lasts long enough, then it effectively stretches any inhomogeneity & space curvature, `explaining’ why the Universe today appears homogeneous and flat. Theory arose in 1980 (A. Guth) from considerations of symmetry-breaking phase transitions in Grand Unified Theories.

Inflation Models: Scalar Field slowly rolls down a hill Potential energy density High Temperature: Symmetry is restored, = 0. Low Temperature: Symmetry is broken = + or - field Potential energy function must be fairly `flat’ so the field rolls slowly: probably not a Higgs field, must be something else

After rolling down, scalar field oscillates around the bottom REHEATING Potential energy density High Temperature: Symmetry is restored, = 0. High Temp. Low Temperature: Symmetry is broken = + or - At the end of inflation, the Universe is very cold. field Reheating: Oscillating field energy transformed to other particles as it decays: Universe heats back up to high Temperature: `another’ bang that creates all the matter and energy in the Universe.

Who is the `inflaton’ field? Originally it was thought a GUT Higgs field would do the trick. With the death of `old’ inflation, this hope dimmed. Inflation requires a new scalar field with a very flat potential energy function. Currently, there is no consensus among particle physics theorists as to the identity of this hypothesized inflaton field. Inflation has thus been described as a theory in search of a model.

Density Perturbations & Structure Inflation provides a physical mechanism for producing the initial `seed’ perturbations which grew into Large-scale Structure Density Perturbations from Quantum Mechanics: Classically, the scalar field rolls down its potential at the same speed everywhere in the Universe: = (t). According to Quantum Mechanics, the amplitude (or rolling speed) of the field fluctuates: it differs from place to place by a small amount, = (x, t). These field fluctuations imply spatial fluctuations in the energy density of the Universe. During, reheating, these become fluctuations in the density of all matter & radiation particles. This is a crucial but originally unforeseen consequence of theory, now seen to be in excellent agreement with CMB observations.

1 -dimensional cross-section field space field

Evidence for Inflation • Large-scale homogeneity and isotropy (by design) • Spatial flatness (Euclidean): total = 1 • (Power) Spectrum of density perturbations inferred from CMB experiments agrees to high precision with spectrum of quantum fluctuations predicted by inflation Future: -more precise measurements by satellites (MAP, Planck) -measurement of CMB polarization possibly test inflationary prediction for gravity wave spectrum and distinguish between different inflation models

The Structure Formation Cookbook 1. Initial Conditions: Start with a Theory for the Origin of 2. Density Perturbations in the Early Universe 3. Your Favorite Inflation model 4. 2. Cooking with Gravity: Growing Perturbations to Form Structure 5. Set the Oven to Cold, Hot, or Warm Dark Matter 6. Season with a few Baryons and add Dark Energy 7. 3. Let Cool for 14 Billion years (or buy a Really Big Computer) 8. 4. If it looks, smells, and tastes like the real thing, then publish the recipe. If not, publish anyway, and then start over with

Early Evolution of Structure in a Simulated Big Bang Universe Filled with Dark Matter `The Cosmic Web’ Galaxies and Clusters form at the intersections of sheets and filaments, very similar to the Structure seen in galaxy surveys Today

Evolution of Structure in the Universe

SDSS 2. 5 meter Telescope

Galaxy Clustering in the SDSS Redshift Survey ~100, 000 galaxies Voids, sheets, filaments

Probing Neutrino Mass and Baryon Density Wiggles Due to Non-zero Baryon Density SDSS + MAP: will constrain sum of stable neutrino masses as low as ~ 0. 5 e. V

Some Key Questions for 21 st Century Cosmology How did the hierarchy of large-scale structure, from stars to galaxies to clusters and beyond, originate? Did this structure arise from the expansion stretching of microscopic quantum ripples in the fabric of spacetime during the earliest moments of the Big Bang, a theory known as Cosmic Inflation? What is the nature of the Dark Matter that makes up most of the mass of the Universe? Is it in the form of exotic elementary particles as yet undiscovered? (The Ultimate Copernican Principle) What is the nature of the Dark Energy that is causing the expansion of the Universe to Accelerate? Will the Universe continue to accelerate forever? What happened `before’ the Big Bang? Is this question meaningful? Are there more than 3 spatial dimensions? Can we ever detect them?

CMB Sky: 1992 circa Jan. 2003

MAP Satellite launched June 2001 Planck Satellite planned for ~2008

Proposed satellite mission to observe several thousand SNe Ia out to z ~ 1. 7

Despite major recent advances in cosmology, fundamental mysteries remain Unlike the ancient mystics, however, we hope these unexplained phenomena can in principle be understood, by a combination of new theoretical insight and experimental advances: scientists are perpetual optimists. So far, this optimism has been justified by the continued progress of science. What are the ultimate limits to our understanding of the Universe?

References T. Ferris, The Whole Shebang (Touchstone Books 1997) B. Greene, The Elegant Universe (Vintage, 1999) A. Guth and A. Lightman, The Inflationary Universe J. Silk, A Short History of the Universe C. Hogan, The Little Book of the Big Bang More advanced: A. Liddle, An Introduction to Modern Cosmology E. Linder, First Principles of Cosmology