Background Explorer

COBE has produced some of the most dramatic scientific achievements in infrared astronomy. COBE measured the spectral shape of the cosmic background and found it to be a perfect blackbody spectrum at a temperature of 2.37 K. COBE also made the first observations of structure in the distribution of cosmic background radiation that are the probable first step in changes that produced the galaxies, stars, and planets we know today from the primordial smooth distribution of matter produced by the Big Bang. 

The DMR experiment utilized techniques more common to radio astronomy than infrared. The main part of the instrument consisted of sets of opposed antennas looking at different parts of the sky. These antennas looked for differential signals indicating different temperatures at different locations. FIRAS was a version of the FTS discussed earUer. It compared the spectral shape of the cosmic background radiation against the spectral shape of internal blackbody calibrators. The DIRBE instrument was more similar to standard infrared instruments than the other two COBE experiments. DIRBE used a suite of four different detector types to map out the sky at several infrared wavelengths. This instrument has contributed very important information on the contribution of various types of astronomical objects to the total background radiation in the universe. 

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On the last three decades, several space experiments with parts at very low temperatures have been flown. Among these, we mention IRAS (Infrared Astronomical Satellite) launched in 1983 (see Fig. 14.1), COBE (Cosmic Background Explorer) launched in 1989, ISO (Infrared Space Observatory) launched in 1995 and Astro-E (X-ray Observatory), launched in 2000 with instrumentation at 65 mK [35], Some cryogenic space missions are in the preparation or in final phase in Europe, USA and Japan. For example, ESA is going to fly Planck (for the mapping of the cosmic background radiation) and Herschel (called before FIRST Far Infrared and Submillimetre Telescope ) [36], These missions will carry experiments at 0.1 and 0.3 K respectively. 

The development of new low-temperature detection technology and the launch of the Cosmic Background Explorer (COBE) satellite by NASA in 1989 helped to resolve this problem. The results from these observations were amazing - an almost perfect black body curve (Figure 2.3) with a black body temperature of 2.725 0.002 K and a maximum wavelength of the radiation at kmax = 1.05 mm. 

The most accurate measurements of the CMB spectrum to date have come from the Far InfraRed Absolute Spectrophotometer (FIRAS) on the COsmic Background Explorer (COBE) (Boggess et al., 1992). In contradiction to its name, FIRAS was a fully differential spectrograph that only measured the difference between the sky and an internal reference source that was very nearly a blackbody. Figure 9.2 shows the interferograms observed by FIRAS for the sky and for the external calibrator (XC) at three different temperatures, all taken with the internal calibrator (IC) at 2.759 K. Data from the entire FIRAS dataset show that the rms deviation from a blackbody is only 50 parts per million of the peak Iv of the blackbody (Fixsen et al., 1996) and a recalibration of the thermometers on the external calibrator yield a blackbody temperature of... 

Observations with the Cosmic Background Explorer (COBE) have shown that, to a precision of better than 10 , the cosmic microwave background (CMB) is thermal, with a temperature of 2.73 K (Mather et al., 1994). 

For results from CODE (Cosmic Background Explorer), see http //lambda.gsfc.nasa.gov/product/cobe/. 

Taken by NASA s Cosmic Background Explorer, this photograph shows temperature variations in various parts of the sky surveyed by the satellite. (NASA/Photo Researchers, Inc.)... 

These tests provide an answer to the question What happens qualitatively and, whether looking for new phenomena or sorting likely catalysts for special purposes, require the scanning of many contacts as in the typical industrial problem. The risk that an outstanding catalyst may be missed will be minimized if the scouting work is not too closely bounded owing to excessive reliance upon literature or sketchy theory a framework of background exploration is essential. 

COBE Cosmic Background Explorer an orbiting satellite launched in November 1989 for cosmological research. In 1992, statistical studies of measurements on the microwave bacl round radiation indicated the presence of weak temperature fluctuations thought to be imprints of quantum fluctuations in the early universe. See alsoVIMAP. 

The first quantitative evidence for the temperature anisotropy of CMBR was provided by the COBE (Cosmic Background Explorer) satellite in 1992. The angular resolution of its detectors was 7°. This enabled the collaboration to determine the first 20 multipole moments of the fluctuating part of CMBR beyond its isotropic component. It has been established that the degree of anisotropy of CMBR is one part in one hundred thousand (10 ). There are two questions of extreme importance related to this anisotropy ... 

In 1989, the Cosmic Background Explorer (COBE) satellite was developed by NASA s Goddard Space Flight Center to measure the background radiation more precisely. The COBE satellite determined that the backgroimd radiation corresponded to a universe with a temperature of 2.735 K. (Notice the difference in significant figures from the previous measurement.) It went on to measure tiny... 

However, NASA has initiated a study of a dedicated Cosmic Background Explorer Satellite which will include both spectrum and isotropy experiments at several wavelengths. 

J.C. Mather et al., A preliminary measurement of the cosmic microwave background spectrum by the COsmic Background Explorer (COBE) satellite. ApJ 354, L37-L40 (1990)... 

FIGURE 4 The top figure shows the distribution of temperature fluctuations in the cosmic background radiation. The bottom figure shows the exact match between the spectrum of the cosmic background radiation and a blackbody spectrum. CORE, Cosmic Background Explorer DMR, differential microwave radiometer. 

Mather, J. Kelsall, T. (1980). The Cosmic Background Explorer Satellite. Physica Scripta, 21, 671-7. 

In 1989 the Cosmic Background Explorer, COBE, was launched. It made precise measurements of the CBR at wavelengths from a few micrometers out to 1 cm (Fig. 8.1). COBE unambiguously proved the CBR to follow a Planck curve at 2.73 K so there is no doubt that we are seeing the residual radiation left behind from the primordial Big Bang. Tiny variations in the CBR result from the formation of structures during the early evolution of the universe (see also Fig. 8.2). 

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