COBE-DMR and CMBR Anisotropies

Outline

COBE-DMR and CMBR Anisotropies
Cosmic Seeds and the Face of God
Cosmic Seeds and Galaxies

COBE's Differential Microwave Radiometer (DMR)

measures difference in temperature between spots on the sky separated by 60 degrees
operates at 3 different wavelengths:

= 3.3mm (strong CMBR signal)
= 5.7mm (strong CMBR signal)
= 9.5mm (strong Galactic signal)

helps to subtract Galaxy's contribution

2 independent measurements made at each wavelength

6 horns

Instrument spins as it orbits the Earth

measures temperature differences of all pairs of points separated by 60 degrees
data analysis transforms temperature difference measurements to a map of on the sky

Here are some images of the DMR results:

The anisotropy map

the top panel is the "dipole" anisotropy due to the motion of the Earth and Sun
The middle panel has the "dipole" anisotropy subtracted and is dominated by the Galaxy
The bottom panel has the Galaxy subtracted.

The DMR instrument has an angular resolution of 7 degrees
The maps are smoothed on a scale of 10 degrees
The amplitude of the fluctuations in the bottom panel is = 30 µK (1µK = 10^-6K)

or = 10^-5

The temperature fluctuations in the bottom panel are thought to be intrinsic fluctuations in the temperature of the CMBR itself

How do we know?

Check out the maps in the 3 different wavelengths:
= 9.5mm is 31.5 GHz (note the strong Galactic signal)
= 5.7mm is 53 GHz
= 3.3mm is 90 GHz

Note that the 53 GHz and 90 GHz signals look similar

the 53 and 90 GHz signals are mostly fluctuations in the CMBR

this would have the same pattern in the 2 maps

they also have some instrumental noise

this is different in the two maps
the "signal" in these 4-year averaged maps is 2 times the noise
1 year maps had equal amounts of noise and signal

A pure CMBR signal would look the same in every map
Radiation from electrons in our Galaxy would be brightest in the 31.5 GHz map

Here's another projection of the "Cosmic Signal" in the 53 GHz map

Cosmic Seeds (the Face of God?)

The announcement of the temperature fluctuations seen by COBE-DMR was a big deal
Smoot's "face of God" quote:

Does this look like the face of God to you?
The leading theories have the origin of the temperature fluctuations at t ~ 10^-34sec

so if God started the Big Bang, we are getting pretty close

Why all the fuss?

Fluctuations were originally expected to be much larger than = 10^-5
Upper limits were coming close to the values of favored theories
Stories in the popular press had erroneously claimed that lack of meant trouble for the Big Bang theory

AC says that COBE-FIRAS results were more important scientifically, but your esteemed Professor does not agree!
Consistent with favored inflationary Universe model

the size of the fluctuations is the same on all scales:

add up the different scales:

We expect lots of structure on angular scales too small to be seen by COBE:

Here's another view of the COBE-DMR map:

Here's what the upcoming Microwave Anisotropy Probe (MAP) is supposed to see:

MAP should have an angular resolution of 0.1-0.2 degrees - 50 times better than COBE-DMR
The smallest scripture on the COBE map will by now have about the same size as the "Great Wall" in the Galaxy distribution:

The "Great Wall" is the passing horizontally through these CfA "slice of the Universe" maps

Above is the map of a slice 6 degrees thick, and below is the combination of 6 such slices

How Cosmic Seeds Lead to Galaxies

Seeds are small fluctuations in the density of matter

possibly generated in an "inflationary universe" phase
they have higher density => higher T => positive
they have stronger gravity => negative

due to gravitational redshift

these 2 effects tend to cancel, but the redshift is stronger

high density regions have negative

Here's a space-time diagram that is supposed to show how this works:

This plot uses "co-moving" coordinates - the coordinates move with the expansion

we're at the top vertex and the red lines are our past lightcone
we see the CMBR anisotropy only where the red lines intersect the "last scattering surface" which is the shaded region on the bottom
there is no effect when the light passes through the fluctuations later

the photons are blue shifted when they fall in and red shifted when they climb out - these effects exactly cancel!

Here are the trajectories of individual particles move in the same kind of space-time diagram:

The fluctuations in the density grow with time

dense regions tend to collapse and become denser

with respect to the average of the Universe
only in the final stages does the collapse overcome the overall expansion

sparse regions don't slow their expansion as much as regions of average density so they appear to grow sparser