The Cosmology Large Angular Scale Surveyor (CLASS) is an experiment to measure the signature of a gravitational-wave background from inflation in the polarization of the cosmic microwave background (CMB). CLASS is a multi-frequency array of four telescopes operating from a high-altitude site in the Atacama Desert in Chile. CLASS will survey 70% of the sky in four frequency bands centered at 38, 93, 148, and 217 GHz, which are chosen to straddle the Galactic-foreground minimum while avoiding strong atmospheric emission lines. This broad frequency coverage ensures that CLASS can distinguish Galactic emission from the CMB. The sky fraction of the CLASS survey will allow the full shape of the primordial B-mode power spectrum to be characterized, including the signal from reionization at low . Its unique combination of large sky coverage, control of systematic errors, and high sensitivity will allow CLASS to measure or place upper limits on the tensor-to-scalar ratio at a level of r = 0.01 and make a cosmic-variance-limited measurement of the optical depth to the surface of last scattering, τ .Recently, the BICEP2 experiment announced the detection of B-mode polarization at of 40-200, 5 but it is unclear whether this signal is cosmological or Galactic in nature. These results have generated strong interest in complementary experiments and have highlighted the importance of multi-frequency observations for foreground subtraction. A measurement of B-modes in the CMB would constitute important evidence for inflation and a measurement of the energy scale at which inflation occured. The tensor-to-scalar ratios, r ≤ 0.1, being probed correspond to E ∼ 10 16 GeV, near grand-unified-theory (GUT) energy scales. The gravitational waves from inflation are our only probe of the physics at such enormous energies and at such early times, just 10 −35 seconds after the Big Bang. They would also provide the first firm evidence for the existence of quantum-gravitational effects. 6 Detecting primordial gravitational waves requires greater frequency coverage to definitively rule out Galactic foreground contamination, as well as a measurement of the B-mode signal over a wider range of angular scales to verify the full shape of the B-mode power spectrum.A number of experiments are searching for B-mode polarization. Notably, the Planck satellite has mapped the entire sky in nine frequency bands from 30 to 857 GHz, allowing measurement of CMB polarization over a broad range of angular scales with the ability to remove Galactic foreground contamination; however, it is yet to be seen whether Planck will have the ability to constrain this signal. In this paper we present the Cosmology Large Angular Scale Surveyor (CLASS), which is leading the effort to map the CMB polarization at large angular scales from the ground. CLASS will observe in four frequency bands centered on 38, 93, 148, and 217 GHz. CLASS is uniquely poised to measure inflationary gravitational waves through its ability to measure CMB polarization at the largest angular scales, a...
A spherical geometry for a resonant-mass gravitational wave antenna offers significant improvements over traditional cylindrical antennas. However, completing a detector requires breaking the bare antenna's spherical symmetry by attaching multiple mechanical resonators and transducers. To fully assess the merits of such detectors, it is essential to be able to calculate the detector's sensitivity and the accuracy of the extractable signal information without relying on exact mathematical transducer or resonator symmetries to simplify the analysis, as has been done in previous work. Without making such assumptions, this paper generalizes the fundamental sensitivity limits, known for cylindrical detectors, that arise from the back-action noise present in any linear amplifier, and from thermal Brownian motion noise when detection bandwidth is limited. Optimal signal detection and estimation methods are derived by generalizing techniques used for one-dimensional detectors to the case of multiple interacting transducers. Formulas for the optimized signal-to-noise ratio are derived which generalize the connection between bandwidth and sensitivity known for one-dimensional detectors. A demand for isotropic sensitivity then gives requirements on transducer placement and matching. Comparing bandwidth anisotropies, the detector design proposed by Johnson and Merkowitz is found to be superior to an alternative proposal by Lobo and Serrano, and to be reasonably robust against asymmetries. In addition to sensitivity limits and optimal data analysis methods, limits are derived for the accuracy of reconstructed signal parameters such as direction, polarization, phase, and arrival time. ͓S0556-2821͑97͒01614-7͔
We report on results of computer simulations of spherical resonant-mass gravitational wave antennas interacting with high-frequency radiation from astronomical sources. The antennas were simulated with three-mode inductive transducers placed on the faces of a truncated icosahedron. Overall, the spheres were modeled with a sensitivity of about three times the standard quantum limit. The gravitational radiation data used was generated by three-dimensional numerical computer models of inspiraling and coalescing binary neutron stars and of the dynamical bar-mode instability of a rapidly rotating star. We calculated energy signal-to-noise ratios for aluminum spheres of different sizes cooled to 50 mK. We find that by using technology that could be available in the next several years, spherical antennas can detect coalescing binaries out to slightly over 15 Mpc, the lower limit on the distance required for one event per year. For the rapidly rotating star, we find, for a particular choice of the radius at centrifugal hangup, spheres are sensitive out to about 2 Mpc. The event rate is estimated to be about 1 every 10 years at this distance. ͓S0556-2821͑96͒04916-8͔ PACS number͑s͒: 04.80.Nn, 04.30.Db, 95.55.Ym
High-performance, integrated spectrometers operating in the far-infrared and submillimeter ranges promise to be powerful tools for the exploration of the epochs of reionization and initial galaxy formation. These devices, using high-efficiency superconducting transmission lines, can achieve the performance of a meter-scale grating spectrometer in an instrument implemented on a 4 inch silicon wafer. Such a device, when combined with a cryogenic telescope in space, provides an enabling capability for studies of the early universe. Here, the optical design process for Micro-Spec (μ-Spec) is presented, with particular attention given to its two-dimensional diffractive region, where the light of different wavelengths is focused on the different detectors. The method is based on the stigmatization and minimization of the light path function in this bounded region, which results in an optimized geometrical configuration. A point design with an efficiency of ~90% has been developed for initial demonstration and can serve as the basis for future instruments. Design variations on this implementation are also discussed, which can lead to lower efficiencies due to diffractive losses in the multimode region.
We report on the diffusive-ballistic thermal conductance of multi-moded single-crystal silicon beams measured below 1 K. It is shown that the phonon mean-free-path ℓ is a strong function of the surface roughness characteristics of the beams. This effect is enhanced in diffuse beams with lengths much larger than ℓ, even when the surface is fairly smooth, 5-10 nm rms, and the peak thermal wavelength is 0.6 µm. Resonant phonon scattering has been observed in beams with a pitted surface morphology and characteristic pit depth of 30 nm. Hence, if the surface roughness is not adequately controlled, the thermal conductance can vary significantly for diffuse beams fabricated across a wafer. In contrast, when the beam length is of order ℓ, the conductance is dominated by ballistic transport and is effectively set by the beam area. We have demonstrated a uniformity of ±8% in fractional deviation for ballistic beams, and this deviation is largely set by the thermal conductance of diffuse beams that support the micro-electro-mechanical device and electrical leads. In addition, we have found no evidence for excess specific heat in single-crystal silicon membranes. This allows for the precise control of the device heat capacity with normal metal films. We discuss the results in the context of the design and fabrication of large-format arrays of far-infrared and millimeter wavelength cryogenic detectors. * Electronic address: karwan.rostem@nasa.gov
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