WMAP
WMAP
• The Wilkinson Microwave Anisotropy Probe was designed to measure the CMB.
– Launched in 2001
– Ended 2010
• Microwave antenna includes five frequency bands.
– 22 to 90 GHz
– High angular and frequency resolution
L2 Orbit
• WMAP uses the stable L2 point in the sun-earth system.
– About 4 times lunar orbit
– Always faces away from the sun
– Complete sweep every 6 months
• WMAP orbits L2 in an elliptical orbit.
– Onboard propulsion to correct orbit
Anisotropy
• Spatial anisotropy is very small.
– 0.0002 K fluctuations
• Absolute temperature measurement includes statistical and systematic error.
• Coherent errors are the same for all points.
– Remove error by measuring differences not absolutes
Precision Instrument
• WMAP combines precise frequency and spatial resolution.
– 20 uK statistical
– 5 uK systematics
– 0.3° spatial pixel
•Back-to-back telescopes provide differential measurements.
– 140° separation
Differential Signal Processing
Shape of Space
• Space can be flat or curved.
– 3D not just 2D
• The Riemann curvature parameter determines the direction and extent of curvature.
– Positive curvature closed
– Negative curvature open
Collapsing Universe
• Matter contributes to gravitation.
– Pulls the universe together
• The orange line represents a universe dominated by high density matter.
– Few billion years old
– Eventual collapse
Critical Density
• At critical density the matter is sufficient for a flat universe.
– Asymptotically reaches maximum size
– Green curve
• At lower density gravity cannot stop expansion.
– Expansion slows but continues
– Blue curve
Dark Energy
• Dark energy represents energy tied in space itself.
• This energy drives expansion.
– Effectively opposes gravity
– Fraction sets the rate
• The red curve has a large fraction of the matter as dark energy.
Initial Appearance
• The initial appearance of matter is called the Big Bang.
– Not really an explosion
– High density matter and energy
• Since space is expanding there is no central point.
• Cooling radiation become the cosmic microwave background.
One Second
• The temperature decreases as the universe expands.
– One second = 1010 K
• At this point protons and neutrons can fuse into light nuclei.
– Proton + neutron = deuterium
– Deuterium + deuterium = helium 4
– Also helium 3 and lithium
Nucleosynthesis
• The density of ordinary matter determines the likely mix of light nuclei.
• WMAP measures ordinary matter at 4.6% of the total.
– Predicted mix matches other observations
Hot Radiation
• For 300,000 years it was too hot for atoms to form.
• The plasma and free electrons were opaque.
• When the temperature fell to 3000 K, atoms formed.
• The universe was transparent, with a burst of light everywhere.
Blackbody Radiation
• The CMB spectrum is consistent with radiation at 2.725 K.
• The peak is in the microwave region.
– Fluctuations are an order of magnitude smaller
Solar Motion
• Solar motion can be seen in low resolution anisotropy.
– Blue 2.721 K
– Red 2.729 K
• Central line is the galactic plane.
CMB Anisotropy
• After subtraction the residual anisotropy represents the CMB.
– Blue 2.7249 K
– Red 2.7251 K
• Central line is the galactic plane.
Matter Fractions
• The anisotropy points at the fraction of matter of different types.
– Ordinary matter 4.6%
– Cold dark matter 23%
– Dark energy 72%
Shape and Age
• The mix of matter is consistent with a flat universe (2%).
– Determine H to within 5%
– 73.5 (km/s)/Mpc
• The Hubble constant along with matter mixture sets the age.
– Low matter density
– 1/H0 = 13.7 billion years (1%)
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