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%)