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A four channel He-3 cooled balloon-borne bolometer radiometer was constructed. The principal goal of the instrument is to measure the anisotropy of the 3 K cosmic background radiation on angular scales of 4 to 180 deg. The goal is to improve the sensitivity of the measurements to Delta T/T is less than .00001. A secondary goal is to survey the galactic thermal dust emission in the submillimeter range.
NASA's Cosmic Background Explorer (COBE) is designed to investigate the Cosmic Background Radiation (CBR), that permeates the universe as a consequence of the Big Bang. This 3 degree Kelvin radiation is a fossil that contains much information about the early universe. The Far Infrared Absolute Spectrophotometer (FIRAS), will investigate the spectral isotropy of this ancient remnant and look for clues as to the subsequent evolution of the universe. The instrument is a cryogenically cooled, modified Michelson interferometer which operates in the 1 cm to 100 micron wavelength range. FIRAS is designed to provide absolute spectral information, therefore, all possible perturbations to the instrument response must be investigated to minimize distortions of the data. This paper discusses the methodology and resultant variations in the instrument performance noted during room temperature, and liquid nitrogen, (LN2) temperature vibration qualification. Reference alignment shifts in critical components such as the instrument wire-grid beamsplitter are correlated to changes in the instrument spectral response.
Diffuse Infrared Background Experiment (DIRBE) optical alignment and testing methods are discussed. Using strobe videography, vibration and performance testing of a 32 hz tuning-fork chopper was carried out. The Cosmic Background explorer satellite provides improved microwave and IR all-sky maps of the cosmic background radiation from a polar orbit. A liquid helium cryostat houses the DIRBE and the Far IR Absolute Spectrophotometer (FIRAS) instruments at a temperature of 2 K. Differential MicRowave Radiometers (DMRs) provide large scale maps of anisotropy of the 3 K background at wavelengths of 3.3, 5.7, and 9.6 mm. The DIRBE telescope is an IR photometric instrument with 10 wavelength bands between 1 and 300 microns, designed to measure radiation from the epoch of galaxy formation. Stringent stray light requirements mean that the DIRBE flight instrument has to be built and tested in a class 100 environment.
The peculiar velocity correlation tensor in the cosmic background radiation is estimated using observations of large-scale streaming motions. Two models are considered, which assume gravitational instability and random phase fluctuations: the CDM (cold dark matter) model representing the standard Omega = 1 CDM dominated universe with adiabatic scale invariant initial fluctuations and the Peebles' baryon isocurvature (PBI) model, an Omega = 0.4 baryon dominated universe with initial isocurvature (entropy) fluctuations. In both cases, the assumed Hubble constant is H = 50 km/sec Mpc. Unlike the CDM model, low Omega, baryon dominated PBI scenarios containing entropy perturbations can lead to correlated velocities over scales exceeding 4,000 km/sec. Predictions of both the CDM and PBI models are discussed. The velocity field in the local standard of rest frame determined by the galaxies themselves is also considered.
The Cosmic Background Explorer (COBE) is a satellite which is designed to study the 3-K cosmic background radiation. Two cryogenic instruments and a common support structure comprise the Cryogenic Optical Assembly (COA). In order to make definitive measurements of the low-energy FIR radiation, the temperature of the COA must be maintained below 3 K. Temperature control of the COA elements is achieved by the use of a superfluid helium dewar. The thermal design also minimizes power dissipation, transient operational effects, and thermal gradients within the COA. This is accomplished by isolating selected components and heat-sinking others to various locations on the cryogen tank. The thermal design was developed primarily through analysis. Analytical models were created to perform parametric design studies. Ground testing was performed to correlate the thermal models and validate the thermal design. Both test and flight data confirmed that all design requirements were either met or exceeded.
In molecular clouds of modest density and relatively high fractional ionization, the rotational excitation of CN is controlled by a competition among electron impact, neutral impact and the interaction with the cosmic background radiation. The degree of excitation can be measured through optical absorption lines and millimeter-wave emission lines. The available, accurate data on CN in diffuse and translucent molecular clouds are assembled and used to determine electron densities. The derived values, n(e) = roughly 0.02 - 0.5/cu cm, imply modest neutral densities, which generally agree well with determinations by other techniques. The absorption- and emission-line measurements of CN both exclude densities higher than n(H2) = roughly 10 exp 3.5/cu cm on scales varying from 0.001 to 60 arcsec in these clouds.
Goddard celebrates 1990 as a banner year in space based astronomy. From above the Earth's obscuring atmosphere, four major orbiting observatories examined the heavens at wavelengths that spanned the electromagnetic spectrum. In the infrared and microwave, the Cosmic Background Explorer (COBE), measured the spectrum and angular distribution of the cosmic background radiation to extraordinary precision. In the optical and UV, the Hubble Space Telescope has returned spectacular high resolution images and spectra of a wealth of astronomical objects. The Goddard High Resolution Spectrograph has resolved dozens of UV spectral lines which are as yet unidentified because they have never before been seen in any astronomical spectrum. In x rays, the Roentgen Satellite has begun returning equally spectacular images of high energy objects within our own and other galaxies.
Spectral distortions in cosmic background radiation can create instabilities in matter, whose possible role in generating large scale structure is outlined here. A promising physical mechanism is sketched, based on Ly-alpha resonance scattering, with exponentially amplifies perturbations in neutral hydrogen density on large scales after recombination.
We have developed and flown a 1 m diameter Gregorian telescope system for measurements of anisotropy in the Cosmic Background Radiation (CBR). The telescope is incorporated in a balloon-borne stabilized platform with arcminute stabilization capability. To date, the system has flown four times and observed from the ground at the South Pole twice. The telescope has used both coherent and incoherent detectors. We describe the development of the telescope, pointing platform, and one of the receivers employed in making measurements of the CBR. Performance of the system during the first flight and operation on the ground at the South Pole are described, and the quality of the South Pole as a millimeter wave observing site is discussed.
We analyze the effect of a secondary ionization on the evolution of temperature fluctuations in cosmic background radiation. The main results presented in this paper are appropriate analytic expressions of the transfer function relating temperature fluctuations to matter density perturbations at recombination for all possible recombination histories. Furthermore, we particularize our calculation to the standard cold dark matter model, where we study the erasure of primordial temperature fluctuations and calculate the magnitude and angular scale of the damping induced by a late recombination.
This issue on gamma ray astronomy presents many results from observations made with the Compton Gamma Ray Observatory (CGRO) and other spaceborne experiments and includes studies of Seyfert galaxies, X-ray binaries, neutron stars, diffuse cosmic background radiation, pulsars, blazars, gamma ray bursts, the galactic center, and the distribution of Aluminum-26 in the galaxy. The issue begins with a survey of Compton Telescope (COMPTEL) results and concludes with future prospects for the field.
Recent developments in bolometric detector systems for millimeter and submillimeter wave space astronomy are described. Current technologies meet all the requirements for the high frequency instrument onboard the cosmic background radiation anisotropy satellite/satellite for the measurement of background anisotropies (COBRAS/SAMBA) platform. It is considered that the technologies that are currently being developed will significantly reduce the effective time constant and/or the cooling requirements of bolometric detectors. These technologies lend themselves to the fabrication of the large format arrays required for the Far Infrared and Submillimeter Space Telescope (FIRST). The scientific goals and detector requirements of the COBRAS/SAMBA platform that will use infrared bolometers are reviewed and the baseline detector system is described, including the feed optics, the infrared filters, the cold amplifiers and the warm readout electronics.
We investigated the effect of inverse Compton scattering in mildly relativistic static and moving plasmas with low optical depth using Monte Carlo simulations, and calculated the Sunyaev-Zel'dovich effect in the cosmic background radiation. Our semi-analytic method is based on a separation of photon diffusion in frequency and real space. We use Monte Carlo simulation to derive the intensity and frequency of the scattered photons for a monochromatic incoming radiation. The outgoing spectrum is determined by integrating over the spectrum of the incoming radiation using the intensity to determine the correct weight. This method makes it possible to study the emerging radiation as a function of frequency and direction. As a first application we have studied the effects of finite optical depth and gas infall on the Sunyaev-Zel'dovich effect (not possible with the extended Kompaneets equation) and discuss the parameter range in which the Boltzmann equation and its expansions can be used. For high temperature clusters (k(sub B)T(sub e) greater than or approximately equal to 15 keV) relativistic corrections based on a fifth order expansion of the extended Kompaneets equation seriously underestimate the Sunyaev-Zel'dovich effect at high frequencies. The contribution from plasma infall is less important for reasonable velocities. We give a convenient analytical expression for the dependence of the cross-over frequency on temperature, optical depth, and gas infall speed. Optical depth effects are often more important than relativistic corrections, and should be taken into account for high-precision work, but are smaller than the typical kinematic effect from cluster radial velocities.
We present the results of a new calculation of the photodisintegration of ultrahigh energy cosmic-ray (UHCR) nuclei in intergalactic space. The critical interactions for energy loss and photodisintegration of UHCR nuclei occur with photons of the 2.73 K cosmic background radiation (CBR) and with photons of the infrared background radiation (IBR). We have reexamined this problem making use of a new determination of the IBR based on empirical data, primarily from IRAS galaxies, consistent with direct measurements and upper limits from TeV gamma-ray observations. We have also improved the calculation by including the specific threshold energies for the various photodisintegration interactions in our Monte Carlo calculation. With the new smaller IBR flux, the steepness of the Wien side of the now relatively more important CBR makes their inclusion essential for more accurate results. Our results indicate a significant increase in the propagation time of UHCR nuclei of a given energy over previous results. We discuss the possible significance of this for UHCR origin theory.
The neutral hydrogen and the ionized helium absorption in the spectra of high-redshift quasi-stellar objects (QSOs) are unique probes of structure in the universe at epochs intermediate between the earliest density fluctuations seen in the cosmic background radiation and the distribution of galaxies visible today. We present Far-Ultraviolet Spectroscopic Explorer (FUSE) observations of the line of sight to the QSO HE2347-4342 in the 1000-1187 angstrom band at a resolving power of 15,000. Above redshift z = 2.7, the IGM is largely opaque in He II Ly-alpha (304 angstroms). At lower redshifts, the optical depth gradually decreases to a mean value tau = 1 at z = 2.4. We resolve the He II Ly-alpha absorption as a discrete forest of absorption lines in the z = 2.3 - 2.7 redshift range. Approximately 50% of these spectral features have H I counterparts with column densities N(sub HI) > 10(exp 12.3)/sq cm visible in a Keck spectrum. These account for most of the observed opacity in He II Ly-alpha. The remainder have N(sub HI) < 10(exp 12.3)/sq cm, below the threshold for current observations. A short extrapolation of the power-law distribution of H I column densities to lower values can account for these new absorbers. The He II to H I column density ratio eta averages approximately 80, consistent with photoionization of the IGM by a hard ionizing spectrum resulting from the integrated light of quasars at high redshift, but there is considerable scatter. Values of eta > 100 in many locations indicate that there may be localized contributions from starbursts or heavily filtered QSO radiation.
Existing paramagnetic susceptibility thermometers used in fundamental physics experiments near 2.2 K are capable of measuring temperature changes with a precision of about 100 pK in a one-hertz measurement bandwidth, with a demonstrated drift stability of about a nK per day. Commercial electrical heater controllers are only able to control power dissipation to a precision of about ten parts per million (ppm), with an open loop drift of about 50 ppm per day. We have developed an ultra-stable temperature platform with a demonstrated noise of 25 pK in a one-hertz bandwidth, and we have identified the physical source of this residual noise. We used an array of RF-biased Josephson junctions to precisely control the electrical power dissipation in a heater resistor mounted on this thermally isolated cryogenic platform to well beyond our ability to measure, which we estimate is stable to better than a part in 10(exp 12). This Josephson heater controller may be used in a new synchronous demodulation circuit to maintain absolute temperature stability of the stage to about the same level as the demonstrated noise, provided that the He-4 superfluid transition temperature is fundamentally stable at this level. This work may provide a blackbody temperature reference for use in space radiometry applications that is considerably more stable than the temperature of the cosmic background radiation itself. This new technology may enable critical heat capacity measurements in He-4 within a weightless laboratory to a reduced temperature of about 10(exp -11), where the critical fluctuation lengths would be about a cm, and the fluctuation rates would be measurable within the bandwidth of the thermometry.
Reported here are disk-integrated Moon surface microwave brightness temperature (Tb) retrievals covering the frequency range of 23–183 GHz. FullMoon observations obtained from the advanced technology microwave sounder (ATMS) onboard the NOAA-20 satellite during a special spacecraft pitch–maneuver operation forms the basis of the retrievals. Instrument nonlinearity, Earth sidelobe contamination, cosmic background radiation, and reflector thermal emission corrections are applied to the observations to obtain accurate values of the Moon’s Tb at all frequencies. The measured full Moon Tb ranges from ~240 to 293 K with frequency increases from 23 to 183 GHz. A clear frequency trend is detected when the brightness temperature increases.
The contribution of faint Quasi Stellar Objects (QSO's) and other discrete sources to the cosmic X-Ray Background (XRB) is estimated using an optically selected QSO sample in SA 68.2 in conjunction with archived x-ray images from the Einstein Image Proportional Counter (IPC). A new generalization of the 'x-ray image stacking' technique is employed; this new approach tests for a positional 'correlation' between positive x-ray fluctuations in the IPC images, and the locations of the optically selected QSO's. Correlation with thirty SA 68.2 QSO's permits a highly sensitive measurement of the ensemble mean x-ray flux for QSO's with 19 less than B(sub J) less than 22. Although the IPC images are of modest depth, a limiting sensitivity 1.2 times 10 to the minus 14th power erg/sec/sq cm (0.3 to 3.5 keV) is effectively archieved comparable to that of the Rosat medium deep surveys.