CARPE DE (dark energy)
CARPE is designed to observe cosmological acceleration and dark energy
from a redshift 1<z<4 using the 21 cm intensity mapping
technique. In the next few years a large optical program such as JDEM
is likely to
tackle the problem of dark energy via spectroscopic galaxy surveys,
supernova surveys, and/or weak lensing measurements. If dark
energy behaves like a cosmological constant, then its effect on the
Hubble expansion is dominant only at z<1 and becomes negligible at z
> 2. In this case, studies of the expansion history at low redshifts
by JDEM would provide the most powerful measurement.
In terms of sensitivity CARPE has comparable area to the GBT and its
sensitivity is significantly higher than the Parkes Multibeam Survey.
The frequency range of 300–1400 MHz places it well to find both local
sources at low luminosities at lower frequencies (where scattering is
worse but pulsars are stronger) and more distant, brighter objects at
higher frequencies (Smits et al., 2008). The extremely large FOV
acts as a multiplier, making the system work like many thousands of
individual telescopes (compare to 13 for the Parkes multibeam), rapidly
surveying the whole sky and aiding in RFI rejection. The number of
CARPE pulsar beams is limited by the processing capability of the
pulsar backend (see MOFF correlator). We will be able to repeatedly
survey the sky, aiding in detection of sporadic sources like the
Rotating Radio Transients (RRATs; McLaughlin et al. 2006) and
intermittent sources (Kramer et al., 2006), and also rapidly precessing
systems like the double pulsar. We will also be able to do
repeated, deep searches of the Magellanic clouds in single pointings
(assuming a Southern Hemisphere site). Given the good bandwidth
(especially at lower frequencies), wide frequency range, and enormous
FOV, pulsar searches with CARPE will directly complement recent and
ongoing surveys: it will repeat the Parkes Multibeam Survey with better
time and frequency resolution, and deeper pointings, and can similarly
probe deeply at low frequencies for a larger part of the Galactic plane
than the GBT350 survey. The exact survey details — frequencies,
coverage strategy, processing parameters — remain to be determined with
detailed simulations, but we are confident that CARPE will prove very
powerful. Concurrent advances in pulsar processing (using
dedicated digital electronics like the GUPPI system; GPUs; Ransom et
al. 2003) will hopefully keep pace with the enormous increases in
data-volume that CARPE can provide.
CARPE GW (gravity waves)Once the pulsars are found, many detailed studies require dedicated pulsar timing observations to probe emission physics, establish orbital parameters, and test gravity. Here, the large area and FOV will allow multiple pulsars to be timed simultaneously. This will help refine pulsar ephemerides and remove systematic effects, with likely dramatic improvements to our sensitivity and ability to detect gravity waves. The sensitivity of a pulsar timing array to gravitational wave detection increases with the number of pulsars in the array and decreases with increased timing precision. CARPE will provide improvements on both of these fronts by providing more pulsars and by increasing precision through longer pulsar observations across a large bandwidth, essential for correcting for propagation effects.
Enabling technologiesTwo of the key CARPE technologies are the broadband feed and the MOFF correlator.
It is difficult to construct an antenna with a very broad feed, a stable focus point, and good impedance match (choose 2 of 3). Rich Bradley is leading the design of the CARPE antenna, and is looking at broadband fractal feeds that look directly at the sky. The lack of a reflector removes the constraint needing a stable focal location, and reduces concern of beam spill over the dish edge. The dual polarization sinuous cone (figure at right) has a very good impedance match and stable beam pattern over the 350–1500 MHz frequency range of CARPE. To increase the collecting area at higher frequencies and avoid grating lobes, a second set of sinuous cones sensitive to only the higher 750–1500 MHz frequency range are inserted at the interstices in a Sierpinski fractal pattern (below right).
The MOFF correlator (Morales 2009) combines the calibration and gridding typically done post-correlation as a part of the correlation operation. The result is an imaging correlator that scales as NlogN with the number of antennas (as opposed to N2) for compact interferometers and produces a fully calibrated optimal map output. This greatly reduces the correlation and post processing requirements, and enables instruments like CARPE with many thousands of antennas. In addition, a fully calibrated electric field image is formed as an intermediate product within the correlator. The electric field pixels can be read out as calibrated full Stokes pulsar beams, and the number of pulsar beams is limited only by the output bandwidth (how much data you can process). In addition we exploring ideas for performing de-disperison within the correlator and modularizing the correlator so CARPE can be built in subunits, with the correlator naturally growing as additional ‘tiles’ of the CARPE instrument are completed.
CARPE TalksA technical talk by Miguel Morales at the University of Washington (keynote, pdf, both files are quite large).