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Transcript
The Cosmic Microwave Background: A Love Story Dr. Andrew Rivers Northwestern University Big Bang Test 1: Redshift-Distance Relationship Hubble Law Expansion Image Credit: Ernest Norcia Observed redshifts of galaxies due to an expansion of the universe. Image Credit: Freedman et al. (2001) Expansion rate (Hubble Constant) + distance gives origin time. Big Bang Test 1b: Age of Universe Matches Oldest Stars HR Diagram Turn-off Pt. Image Credit: NASA and H. Richer Oldest Stars in M4 (Milky Way Globular Cluster). Stars in cluster form together. Brightest stars turn off first, fainter later. Location of turn-off point=star cluster age Result: Ages of oldest star clusters from turn-off consistent with Big Bang expansion age from Hubble constant. Looking out in space is equivalent to looking back in time. Is it possible to look far enough out in space to observe the Big Bang itself? The Andromeda galaxy is 2.3 Million Light-years away. We see it as it was 2.3 Myr in the past. Most distant galaxy in the universe? 13.3 Billion light years away. Light from 420 million years after Big Bang. A Mysterious Observation Penzias & Wilson were studying Milky Way Radio emission. Calibrated signal in microwave spectrum. Could not account for “noise” observed. Image Credit: Ted Thai/Time Life Pictures/Getty Images) Robert Wilson and Arno Penzias and the Bell Antenna they used to discover the microwave background. Observed Characteristics of Noise • Independent of the direction they pointed the telescope. • Corresponded to an “antenna temperature” of 3.5 K • Not due to atmosphere (would be greater if pointed toward horizon) • Did not vary with time of day or year! – Not from Milky Way or Solar System – Cosmic? About 1% of the fuzz between analog TV channels was the noise observed by Penzias & Wilson Foundation 1: Big Bang Theory: A Hot Past Big Bang universe was more dense and hotter in the past. The Universe has cooled from this hot dense state. Foundation 2: Photon-Matter Interactions - + - - - + + + - + + - Opaque (a fog) Photons of light interact strongly with free (ionized)electrons and protons + + + - - + - + Transparent - Photons of light rarely interact with stable atoms (paired electrons & protons) Prediction of the Big Bang: Formation of Stable Atoms Early Universe: At high enough temperatures, photons would ionize atoms. Early Universe: soup of free protons, electrons and photons (and a few Helium nuclei). Look-Back Time Later Universe: Universe cools enough for stable atoms to form (de-ionization). • As the universe expands it cools • Eventually the temperature drops enough so that protons can bind with electrons electron capture Stable, neutral Hydrogen and Helium atoms Observing the Opaque to Transparent Boundary Looking out in space=looking back in time. We see through the universe until the last-scattering-surface boundary. De-ionization Look-Back Time Plasma Opaque Transparent! Last Scattering Surface Analogy: the sun becomes transparent at the photosphere What does the universe look like 380,000 years after the Big Bang? What would we see? Radiation released at ~3000K (all released at same Temp) Predict: Blackbody Radiation Doppler shift Remember red shift: the radiation is travelling through the universe to us, while the universe is expanding. Image Credit: David Koerner, Northern Arizona University Predict: Light extremely redshifted compared to release. Observed Spectrum of CMB: A near perfect Blackbody Blackbody spectra are produced by one-temperature, opaque and non-reflecting objects. Peak of blackbody indicates the temperature of the universe today. Near perfect Blackbody observed by COBE What will happen to this Blackbody curve as the universe continues to expand? CMB Peak Cosmic Microwave Background Distribution in Space What does the universe look like 380,000 years after the Big Bang? Cosmic Background Explorer (COBE) A baby picture of the universe, as seen by COBE Everything that will ever be is built on this. Is something hiding? Big Bang Test 2: Cosmic Microwave Background Big Bang Theory: expanding, cooling universe goes from opaque to transparent when atoms form Observation Near perfect Blackbody Looking out into the universe and back in time, we can see the last scattering surface (opaque boundary). Doppler shift: observed radiation will be highly redshifted because of universe expansion. Smooth distribution= isotropic & homogeneous universe Subtract the average to see if there is hidden structure (small variations from the mean). The Dipole Anisotropy due to the motion of the Earth with respect to the CMB What happens when we subtract out the Dipole? Is anything else hiding? Subtract the Dipole The Milky Way has some emission in microwaves. This emission is small compared to the CMB, but shows up here. Spots on the Microwave Background: the universe is not entirely smooth! Recall: Average, dipole and MW subtracted COBE reveals small variations in the background: seeds of structure The self-construction of the universe Small temperature variations in CMB imply slight variations in density: seeds of structure formation. Early universe CMB Today’s universe Galaxies /clusters Gravity acts on initial perturbations building structures. Dark matter (HDM, CDM varieties) drives structure formation A (nearly) Perfect Universe: Observations of WMAP & Planck Artist Rendition of WMAP Satellite moving to L2 point http://map.gsfc.nasa.gov/ WMAP & Planck: Precisely Measure Cosmological Parameters By actually measuring features of the CMB, we can determine history & fate of the universe Parameter 1: Hubble Constant Comparing results from COBE and higher res. WMAP Spots seen by three telescopes Where do the spots come from? • Quantum mechanical energy fluctuations during inflationary period. • These fluctuations pulse through the universe as sound waves. • When the universe become transparent sound waves are frozen in (light and matter are decoupled) The Observed Universe of fluctuations Is composed of these peices Image Credit: Clem Pryke, University of Chicago Image Credit: Max Tegmark, MIT The relative strength (power) of these fluctuations is determined by the cosmic parameters of our universe. Largest spots on the CMB are from larger pulsations that only oscillated once before the universe became transparent. WMAP Satellite Full-sky map of CMB fluctuations (http://map.gsfc.nasa.gov/) Sound wave resonances in the early universe WMAP Power spectrum distribution of fluctuations in the CMB. http://map.gsfc.nasa.gov/ Observation: not all spots are equally common Prediction: Oscillations of right frequency so that they reach a maximum compression at time of transparency “resonate” Universe as musical instrument: fundamental and higher harmonics. Relative height of peaks reveals cosmic parameters. Image Credit: ESA and Planck collaboration Planck Power spectrum distribution of fluctuations in the CMB. The recipe of the universe The Answer: From the Cosmic Microwave Background (WMAP), we can intuit the percentages of normal matter, dark matter and dark energy. The New Recipe The Planck universe indicates slightly more normal and dark matter than previous measurements and a slightly older universe (13.8 Billion years).