Big Bang Blackbody Simulator
The Cosmic Microwave Background (CMB)
from the Big Bang, and as such it provides us with a direct view into
the early universe. By mapping out the CMB we are able to view the
universe as it appeared when it was only 400,000 years old.
The finline structure of the Big Bang
The CMB radiates as an almost perfect blackbody at a temperature of 2.7
Kelvin. It is extremely important to be able to simulate the CMB as a
blackbody source to allow us to test instrumentation intended for CMB
The purpose of the Big Bang Blackbody Simulator (BBS) is to construct a
blackbody 'cold load' to measure the microwave response of
Hot-Electron Microbolometers. These
detectors will allow us to measure the faint polarization signals in
the CMB that are expected to be the result of gravitational waves
generated in very first moments of the universe.
The BBS consists primarily of a 100
Ohm chip resistor
on a copper finline structure. The resistor absorbs incident radiation
and radiates as a blackbody when heated. The entire BBS will be mounted
in a copper waveguide. The copper finlines reduce impedance in the
transition from the resistor to waveguide. Once mounted in the copper
waveguide, the BBS will be placed near the detectors in a cryostat and
cooled to below 1 Kelvin. The BBS can then be heated to generate a well
calibrated source of microwave radiation.
Actual size of the BBS
If these polarization signals are detected, they will allow us to study
physics at energy scales far beyond the reach of particle accelerators.
The Transition-Edge Hot-Electron Microbolomoters are also a candidate
technology for the CMBPol satellite, which NASA may launch at the end
of this decade.
Radiation Coupling: Gaussian Optics
A major challenge we face with the BBS
is coupling its radiation to THM detector. Radiation from the BBS must
pass through waveguides, free space, and antennas, encountering
different impedance levels and electromagnetic symmetries. Our beam
coupling patterns are based on a special type of optics called Gaussian
optics. We use Gaussian optics because geometrical optics assumes that
the wavelength is small compared to the size of the structures in the
optical design, which is not the case with our design. Gaussian optics
allows us to model the long wavelengths of the CMB.
Once the microwave radiation leaves the BBS, it travels through a
waveguide into a horn antenna, where it converges to what is called a
Gaussian beam waist. In geometrical optics, we make the approximation
that beams converge to a single point, in Gaussian optics, however, the
beam reaches a minimum size of finite width at its beam waist.
After the microwave radiation leaves
the horn, it briefly spreads out through free space. It then enters a
focusing lens made of a plastic called Rexolite, which focuses the beam
to a new beam waist of about 120% larger than the original waist in the
horn. The new beam waist is located right at the front of the silicon
hyper-hemispherical lens. This final lens then focuses the beam into
the double slot antenna, the first component of the THM.