Cosmic Microwave Background


Since 1984, there has been several experiments at the Teide Observatory aimed to measure the cosmic microwave background (CMB) radiation and determine its angular power spectrum in scales from a few degrees to several arcminutes.

The primordial matter density fluctuations that originated the present structure of the Universe left imprinted spatial variations in the CMB.

From high sensitivity maps of this radiation we aim to constrain the most relevant cosmological parameters: total energy/matter density, density of cold dark matter, density of baryonic matter, expansion rate of the Universe, density of dark energy, neutrino density, reionization epoch, etc.

There are two experiments operating nowadays in the Observatory: the Very Small Array (VSA), and the COSMOSOMAS experiment.

The VSA is a collaborative project between the Cavendish Astrophysics Group (Cambridge, UK); the Jodrell Bank Observataroy of Univ. of Manchester and the Instituto de Astrofísica de Canarias (IAC). It was installed during 1999 and started operations in October 2000.

The VSA has 14 aerials, each somewhat akin to a satellite TV dish but only 30 cm across. Each antenna is observing at a frequency of 33 GHz. The receivers are based on state of the art High Electron Mobility Transistors (HEMT) which are cryogenically cooled down to 15 K (-258 C).

The performance of the VSA comes from the way the aerials are connected together as an interferometer array - a method pioneered by Cambridge Astronomers. The array measures the extremely faint CMB sky signal common to all the aerials, yet is able to filter out unwanted terrestrial and atmospheric radiation because this arrives at the different aerials with minute time differences of order a billionth of a second. The method allows high precision observations to be made at modest cost.

The Cosmosomas experiment was completely designed and built by the IAC, and started operations in 1998. It consists in two similar instruments, COSMO11 and COSMO15, dedicated to mapping COSMOlogical Structures On Medium Angular Scales, and diffuse emission of our Galaxy. Both instruments are based on a circular scanning sky strategy, consisting of a 60 rpm spinning flat mirror directing the sky radiation into an off-axis paraboloidal antenna, whose size is 1.8-m in the COSMO15 and 2.4-m in the COSMO11.

These antennas focus the radiation on to cryogenically cooled HEMT-based receivers, both operating at a temperature of 20K (-253 C) and in the frequency range of 10-12GHz for COSMO11, and 12-18GHz for COSMO15.

In the COSMO15 instrument, the signal is splitted by a set of three filters, allowing simultaneous observations at 13, 15 and 17GHz.

Thus, four 1-degree resolution sky maps complete in right ascension and covering 20 degrees in declination are obtained every day at these frequencies.

CMB studies begun at Teide Observatory in the mid 80s as the result of a collaboration between the Nuffield Radio Astronomy Laboratories (now JBO) of the University of Manchester and the Instituto de Astrofísica de Canarias. A set of three instruments, collectively designated as "Tenerife CMB Experiment" were installed between 1984 and 1990 to measure CMB anisotropies in the frequency range 10-33 GHz with angular resolution of about 5 degrees. These experiments were first to provide an independent confirmation of the discovery of anisotropies in the CMB by the COBE (NASA) satellite.

Other subsequent CMB experiments conducted at Teide Observatory were: the IAC-Bartol bolometers; the JB-IAC interferometer (which served as a prototype for the VSA); and finally COSMOSOMAS and VSA.

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