THE COSMIC MICROWAVE BACKGROUNDSzydagis

03.02.2015

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This special heat is the logo for all of physical cosmology!

The Birth Pangs of the Universe

Activity• How would you explain the nature and meaning of the

CMB to young, high-school or junior-high-level students?• Explain the anisotropies and the polarization as well• Tougher version: what about elementary age youth? (for a

greater challenge, especially if physics major in group)

• Often times being forced to explain something to someone else who has absolutely zero knowledge initially ensures that one learns it more deeply for oneself, as well as more accurately and precisely (think teaching)

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When and What• Somewhere between about 300,000 and 400,000 years

after Big Bang. Pretty early, given multi-billion-year history• We know the temperature range required for nucleosynthesis of the

light elements, so we can extrapolate the cooling of the universe as it expands, given a known rate of expansion, and expansion history

• Many names: recombination, surface of last scattering, primordial echo / faint after-glow of Big Bang, snapshot of early universe, cosmic “baby picture.” What happened?

• High-energy photons (particles of light) bathing the whole universe got low enough in energy (wavelength stretched out by expanding universe) that is in temperature, with the cooling universe, such that they decoupled from the hot plasma (ionized gas) constituting “rest” of the cosmos…

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• “Rest of universe” = mostly protons and electrons, also some neutrons (hydrogen and helium nuclei, and loose electrons)

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W.H. Kinney, Caltech• Photons no

longer hot (or energetic) enough to consistently ionize atoms such as basic hydrogen attempting to form often

• Electrons (re-)combine with nuclei to finally form stable atoms, electrically bound to the (+) protons

Universe became transparent! Remember, microwaves count as light even though invisible to human eyes (and space didn’t need stars before having light, which has existed since the Big Bang). NOT end of its “Dark Ages” of existing as opaque “soup.”

E = O(10 eV) ionizes most atoms, including hydrogen (13.6 eV) but getting the *average* below that is not good enough: tail end will still efficiently knock out electrons. Factor 10 buffer

History of Discovery• These photons (radiation) thus became

free-streaming, coming to us today across time and space as the CMB• Redshifted via expansion from their original

infrared “color” into the microwave band• Temperature falling from ~4,000 degrees

Kelvin (K) to ~2.72548 K now (VERY cold)

• Gamow first (closely) predicted in 40s• Peebles set out to discover, but was

beaten by mistake by Penzias and Wilson, who were working on global satellite communication in 1964 in New Jersey. Found omnidirectional signal• First evidence, also great, of the Big Bang

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Penzias and Wilson (1963)

George Gamow

Jim Peebles (1970)

• Blackbody radiation refers to the photons emitted from an object uniform in temperature (examples include stars, lamps)• Something at ~3 K has peak in its emission spectrum of light at O(1 mm)

wavelength (in microwave regime, order of 100 GHz. Wi-Fi at order 1!)• Recall lecture slide 4 of January 26th presentation on the nature of light

• Near-perfection of the CMB blackbody implies homogenous universe (so we know galaxy clusters vs. voids not big deal)

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• Wait a second, why isn’t my microwave in kitchen as cold as outer space then?

• The story: universe’s CMB is a near-perfect blackbody

(Not to scale)

Recall = inverse of the frequency

Wrinkles in Time• We knew that there had to be

SOME imperfections because then otherwise how would stars, then clusters of them• Whole galaxies formed, then

dense clusters (gravity wells)

• First, must subtract effects of earth’s rotation and revolution, sun motion through galaxy, then bright center of galaxy

• You are looking at < ~0.01% effects (red hotter, blue colder) first observed with COBE. Low resolution, but we got better

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COBE | Berkeley Lab / Smoot Group

Hot spot = cluster,& cold spot = void

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Scientific Progress: Late 20th Century on

Department of Energy Office of Science

NASA/JPL-Caltech/ESA

19922003

2014

Increasing Precision• Continual improvement from original discovery through COBE,

WMAP, Planck, and in between. Can’t do much better though.• Earth-based, earth satellite, now craft at Lagrange point (gravity balance)

• Studying the telltale patterns in the light accurately tells us almost everything we could ever possibly hope to know regarding our primeval universe, as well as the way it is today• Complete energy content of universe; its age, expansion, size, shape• Expansion rate (Hubble’s constant) and history, etc. (fuller list last time)

• WMAP and Planck led to dawn of the precision cosmology era. No more wild speculation. Can even test multiverse theories!

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Image credit: A. E. Lange and P. de Bernardis et al. for the Boomerang collaboration.

Based on our very latest data, we can say that the universe (if just barely closed i.e. positively-curved – Big Crunch closure not necessarily in store) is AT LEAST O(10) *trillion* light-years across (recall 47 billion light-years for visible universe’s radius). With inflation, could be countless orders of magnitude worse (larger). Just think about how much of the universe we’re missing and will likely NEVER be able to observe!

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Integrated Sachs-Wolfe (ISW) Effect

http://cosmictimes.gsfc.nasa.gov/teachers/guide/2006/guide/faster_walk.html (Animation: University of Hawai’i)

Inflation, S-Z, and Other Effects• Inflation flattens universe, smoothing out wrinkles (collapsed

balloon demo), but not entirely so. Rich information encoded!• Dark matter clumps still collect galaxies within them (using gravity alone)

• High-energy electrons hanging out in galactic clusters will give CMB photons a kick, distorting intricate patterns. Life’s lemon

• We study similar phenomena like CIB (star, galaxy formation)

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Wayne Hu, The University of Chicago

high and low density regions

areas of deformation in space-time

Polarization • Recall Jan. 26 light lecture• Property of light or of any

other wave capable of oscillating in any direction other than that of travel (so, not sound). Direction of oscillation basically

• Planck most recently, but also QUIET, BICEP & SPT (South Pole Telescope)

• Looking for E-modes and B-modes (gradient and curl, analogy from E & M)• E density perturbations, stars• B (swirly kind) expected from

lensing of E mode OR from gravitational waves (inflation)

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© 1996-2015 The Physics Classroom, All rights reserved.

Sean Carroll

Another All-Sky Map• Gravity, just like light and everything else, should be able to

act like a wave (ripple on pond of space-time). Tricky to see

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Courtesy: ESA and the Planck Collaboration

Quotes and Homework• (The intricate and subtle deformations in) the CMB is/are…• “…like seeing the face of God.” –George Smoot• “…the handwriting of God.” –Newsweek magazine• “…the Holy Grail of cosmology.” –Michael Turner• “…the scientific discovery of the century, if not all time.” –

Stephen Hawking, on the back cover of Smoot’s famous book

• To read FOR NEXT TIME (Wednesday March 4th) all of these• Cosmic rays: Has Dark Matter Finally Been Found; General Info

http://www.space.com/19845-dark-matter-found-nasa-experiment.html, http://abyss.uoregon.edu/~js/glossary/cosmic_rays.html

• Gamma rays: Possible Hints of Dark Matter in Fermi Data http

://www.symmetrymagazine.org/article/april-2014/possible-hints-of-dark-matter-in-fermi-data

• X-rays: Physics Viewpoint | X-Ray Line May Have Dark Matter Origin http://

physics.aps.org/articles/v7/128 (special thanks to Corwin)• Neutrinos: Ghosts of the Universe http://discovermagazine.com/2014/sept/9-ghosts-of-the-universe

• Gravity waves (astrophysical): Physics - Wave of the Future http://www.nature.com/news/physics-wave-of-the-future-1.15561

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KIPAC