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The Cosmic Microwave Background: recent observations and perspectives. Marco Bersanelli - Istituto di Fisica Cosmica-CNR, Milano, Italy. 11 de Marzo de 1997

11 de Marzo de 1997. Facultad de C.C. Físicas de la Universidad Complutense de Madrid

Algunos de los miembros de Universitas que venimos desarrollando nuestra investigación en el campo científico invitamos al profesor Marco Bersanelli, amigo y maestro, a dar una conferencia en Madrid. El profesor Bersanelli es miembro del Instituto de Física Cósmica (CNR) en Milán, donde trabaja en la medición de la radiación de fondo. Ésta fue la primera conferencia pública que organizó Universitas, y tuvo lugar el día 11 de Marzo de 1997 con la colaboración del Departamento de Física Atómica, Molecular y Nuclear de la Facultad de C.C. Físicas de la Universidad Complutense de Madrid.
The discovery of the Cosmic Microwave Background (CMB) by Penzias and Wilson (1965) has opened up a new window into the early universe. The serendipitous detection of this diffuse radiation and its cosmological interpretation have provided strong support to the Hot Big bang cosmological paradigm, which has now become the standard scenario of modern cosmology.

What is the CMB? Today we can state with remarkable confidence that the CMB radiation emerges from the "last scattering surface", the region of space-time that corresponds to the epoch of recombination. The CMB photons that we observe have traveled essentially undisturbed for about 99.99 percent of the age of the universe, and bring to us a direct image of those early times. By studying the features of the CMB we can learn about the physics of the first stages of cosmic expansion. 

Soon after the discovery of the CMB it became clear that this radiation needed to exhibit two distinctive characteristics in order for its cosmological origin to be confirmed. First, the spectrum is expected to follow a Planckian distribution, as a result of the high degree of thermodynamic equilibrium which characterized the universe before recombination, i.e. at redshifts z > 1000. Second, an isotropic angular distribution is expected to first order, and no correlation must be found with local structures. Successive experiments have confirmed accurately both predictions. 

The Cosmic Microwave Background spectrum.
During the 70’s a number of measurements were performed, confirming a thermodynamic temperature of the radiation of about 3 K; in the early 80’s the first evidence of the Planckian shape of the spectrum was obtained with high frequency observations. 

Accurate ground based measurements of the CMB spectrum have been carried out by an Italian-American collaboration from 1984 to 1992, with a number of observing campaigns at the White Mountain High Altitude Station, California, and at the Geographic South Pole, Admundsen-Scott Station, Antarctica. These experiments have provided accurate absolute measurements in the Rayleigh-Jeans portion of the spectrum up to a frequency of 90 Ghz. 

At higher frequencies the FIRAS experiment, on board of the COBE (Cosmic Background Explorer) NASA satellite, has achieved outstanding precision throughout the peak of the Planckian curve at frequencies above 60 Ghz. 

The overall experimental effort shows an excellent agreement with the blackbody spectral shape on over three decades in frequency, with a thermodynamic temperature of 2.73 ± 0.01 K. The high accuracy reached in the measurement of the CMB spectrum is indicative of the special status of CMB observations in cosmology. These results can be used to constrain possible energy releases in the early universe up to redshifts z < 107. 
The angular distribution of the CMB.
Immediately after the discovery of the CMB, pioneer experiments were designed and carried out, which confirmed the first-order isotropy of the radiation as expected from its cosmic origin. Already in 1967 upper limits to possible angular structures were set to less than one part in 1000.

Theory, however, predicts the existence of small perturbations (anisotropies) in CMB angular distribution, since at the recombination epoch density perturbations must be present to evolve into the large scale structures that we observe in the present universe. Several experiments were planned in search of CMB anisotropies.

A second major achievement of the COBE satellite was the first detection, announced in 1992, of structure in the CMB, at a level of » 10-5. Since this discovery there has been enthusiastic new interest in the field, fueled by the realization that the CMB can set very significant constraints on our current cosmological paradigm. In particular it has been shown that an accurate reconstruction of the radiation angular power spectrum can provide few-percent estimates of all main cosmological parameters, including the curvature parameter, the baryon density, and the Hubble constant.

The COBE maps, though of extreme value, were obtained with a relatively poor sensitivity (signal to noise ratio of about 1) and with an angular resolution of 7 degrees, which is far from the fundamental limits which can be achieved in such observations. True imaging of the CMB fluctuations over large portions of the sky, and with a sensitivity of 10-6 per resolution element can provide an outstanding breakthrough for cosmology and astrophysics.

This is the goal of the space mission COBRAS/SAMBA (COsmic Background Radiation Anisotropy SAtellite for Microwave Baclground Anisotropies), which has been recently selected by the European Space Agency as the next Medium Size (M3) scientific mission.


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