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/cm3 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/cm3 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/cm3 Hot gas bound by gravity
M87 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 3D positions for ~700,000 of them by the time it is completed in 2005
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
Apparent position 2 True position 2 Apparent Position 1 True Position 1 Observer Gravitational “lens” “Looking into” the lens: extended objects are tangentially distorted... Gravitational Lensing Basically, the same effects that occur in more familiar optical circumstances: magnification and distortion Objects farther from the line of sight are distorted less.
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 3D so far
The Big Bang Theory:a well-tested framework for understanding the observationsand 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.
Spectrum of Light from Galaxies Redshift of Galaxy Emission & Absorption Lines: recession velocity v/c ≈ z = /0 (approximation for objects moving with v/c << 1) receding slowly 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 (nearly) isotropic Earth 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 Map with Dipole anisotropy removed: fluctuations of the density of the Universe (plus Galactic emission) T = 2.7 degrees 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
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 denseas 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., He4 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 h = H0/(100 km/sec/Mpc) Fraction of baryonic mass in He4 Light Element abundances depend mainly on the density of baryons in the Universe Deuterium to Hydrogen ratio 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 4He
Recent CMB experiments: Going to smaller angular scales higher resolution