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Observational Cosmology and Instrumentation
The Observational Cosmology and Instrumentation group at IFCA focuses on the study of several topics related to the origin and evolution of the Universe. Our research accounts for the analysis and interpretation of astronomical data, as well as forthe design and development of instrumental devices to perform the measurement of these data. Our major resear topics are:
GAUSSIANITY AND ISOTROPY OF THE COSMIC MICROWAVE BACKGROUND
Current observations tell us that the content, evolution and dynamics of the Universe seem to be well described by a concordance model in which baryonic particles account for only 15% of the matter in the Universe, whereas the remaining 85% appears in the form of non-relativistic, non-baryonic and weakly interactive particles (the Cold Dark Matter). Within the concordance model, matter only represents around 25% of the energy content in the Universe: the dominant 75% is the so-called Dark Energy, which is responsible for the current accelerating expansion of the Universe.
Among the observations that had led to this consensus picture, the Cosmic Microwave Background (CMB) data (a relic electromagnetic radiation coming from an early stage of the Universe evolution) have played a central role. One of the fundamental milestones achieved by the analysis of the CMB observations is the overall agreement of the CMB statistical properties with the predictions made by the Cosmic inflation, within the framework of the Cosmological Principle of homogeneity and isotropy of the Universe.
However, the picture is not yet fixed. One of the most interesting research topics in the cosmology field is, indeed, to check whether the concordance model is sufficient or not, to explain the properties of the CMB temperature fluctuations. In particular, it has been argued that alternative scenarios (like non-standard inflation, topological defects or anisotropic models) might still play a secondary role in the picture of the Universe evolution.
It is precisely in this context where the study of the Gaussianity and isotropy of the CMB signal is crucial. Most of the alternative/complementary scenarios to the standard model predict different levels of Gaussianity and/or isotropy departure. The development and application of sophisticated image processing techniques and statistical approaches to CMB data is one of our main research topics.
COMPONENT SEPARATION OF THE MICROWAVE SKY
As it was said above, the CMB is a very valuable observation to disentangle some of the cosmological properties of the Universe. However, to extract this information from the microwave observations it is not an easy task. The global pipeline that goes from the observations to the final products, like cosmological parameters and the constraints on the compelling for the formation and evolution of the Universe, is a real challenge.
One of the most difficult steps of this chain is the issue of separating the emission due to the CMB from other astrophysical sources that also emitted at microwave frequencies. This activity is the so-called, component separation process. It is also remarkable that this process is not only interesting just because the cleaning of the CMB, but also because provides very useful information about some other physical phenomena that are poorly studied.
The complexity of the source separation problem is very high, and several approaches have been proposed, depending on the kind of solution that one is looking for. To better understand the problem, it is worth to mention that the characteristics and the statistical properties of the different components that form the microwave sky are very heterogeneous. Beside the cosmological emission due to the CMB photons, there are several microwave emissions due to different physical phenomena that take place in our Galaxy and someothers produced by other galaxies and by clusters of galaxies. Among the former, the most important ones are the synchrotron radiation emitted by the charged particles interacting with the magnetic field of the Galaxy, the free-free emission produced by the deceleration of an electron that interacts with an ion, and the thermal and dipolar emission produced by the dust grains of the interstellar medium. These diffuse emissions appear concentrate on the galactic plane, and they present a large scale structure. On the other hand, the contaminant emission due to dusty and radio galaxies and to the Sunyaev-Zeldovich effect (SZ) produced by the CMB photon interaction with the hot and dense electron gas in clusters of galaxies, appear, however, as point-like objects, homogeneously distributed in the sky.
As it was mentioned above, the variety of emissions (CMB, diffuse galactic ones and point-like objects) has addressed to a picture where several tools are used, depending whether one is interested on recovering the CMB, the galactic emissions, the extragalactic point sources and the cluster of galaxies or everything at the same time.
Our group have large experience on the development and application of devoted tools (based on wavelets –like the Mexican Hat Wavelet Family– and optimal filters) to detect the point-like objects. In addition, we have also developed a code (SEVEM, based on the Expectation-Maximization algorithm) focused on the recovery of the CMB emission.
LARGE SCALE STRUCTURE AND SECONDARY ANISOTROPIES
We work in several aspects related to the analysis of the large scale structure (LSS) of the Universe.
One the one hand, we have performed N-body simulations of large volumes (300 Mpc/h) with and without gas particles in order to estimate the Rees-Sciama effect. The derivative of the potential is computed in these boxes and then projected along the line of sight with the observer at the centre of the box at redshift 0. These simulations can be used in studies looking at techniques to detect the ISW and/or Rees-Sciama effects. We have also computed the gravitational lensing effect. We also work on simulations of the high redshift Universe with a focus on the reionization period and the opportunities to study this epoch with future instruments.
In addition to the simulation work, we also investigated on optimal cross-correlation tools to extract the common signal that is present on the CMB maps and on the LSS surveys (due to the gravitational potential evolution). In particular, we were piooners by introducing wavelets in this field. The scientific outcomes produced by these analyses is twofold. First, it probes the common physical origin between the CMB photons emitted at redshift 30000 and the matter distribution up to redshift around 2. Second, it helps to put constraints to the cosmological parameters that define the Dark Energy properties.
SIGNAL PROCESSING AND STATISTICS
A common task that is of crucial importance to all the previous research projects is the statistical analysis of the different astrophysical signals in order to obtain useful information about physical phenomena. Statistical signal processing is the branch of mathematics that treats signals as stochastic processes, dealing with their statistical properties as a means to derive rules from phenomena that apparently evolve in time in an unpredictable manner. Examples of typical statistical signal processing tasks that have already been mentioned here are image processing, model selection, component separation and source detection. In this era of technologically ambitious experiments, we need to process huge loads of data in a fast and robust way. Besides, astrophysical data is usually very complex (multiwavelenght observations, polarization data, non-stationary and non-Gaussian physical processes, complicated systematics of the instrument, etc). For this reason it is important to develop appropriate data processing techniques and to implement them in the supercomputing facilities we have access to in the IFCA. Some of the hot topics we are working with are Bayesian inference, time-frequency analysis, linear and non-linear filtering, image fusion, wavelets, sparse representations, Markov random fields, non-linear sampling, denoising and inverse problems.
Our main goal is to use existing signal processing techniques and to develop new ones to be applied to the study of the Cosmic Microwave Background and the sub-mm Universe, but we are not restricted to thas field. Many of the applications we are working with can be useful in other knowledge areas such as telecommunications, geophysics, bioinformatics and biomedical applications.
INSTRUMENTATION FOR MICROWAVE EXPERIMENTS
Our research on instrumentation for microwave experiments follows two different lines:
PATCH ANTENNA ARRAYS
Nowadays, the new experiments dedicated to study the CMB require multifrequency measurements. As an example, a very important one for our group is the QUIJOTE experiment, which is designed to measure the CMB polarization at 11, 13, 17, 19, 30 and 42 GHz. Most of these experiments use horn antennas in their receivers. These antennas provide the most desirable characteristics such as high beam symmetry, low cross polarization, large bandwidth, low sidelobes level, good return loss, and low attenuation. However, for satellite experiments working at frequencies lower than 100 GHz, and with a large number of receivers, needed to achieve a good instrumental noise, these antennas are not the best option. This is mainly because they are big, and weight. For this reason, planar antenna arrays are an alternative to solve these problems. Our group is making a big effort designing planar antenna arrays to work in the Ka band (26 to 40 GHz). These designs have been fabricated as prototypes and have been measured collaborating with Departamento de Ingeniería de Comunicaciones (DICOM) at Universidad de Cantabria (UC). These arrays could also be included in microwave communications and automotive radar systems.
Some strategies have been developed to get a good matching all over the bandwidth and a uniform radiation pattern. In order to do the design of these prototypes, we have used electric and electromagnetic simulation software. All the designs have been made in alumina substrate. A pair of collimating lenses has also been used to improve the gain, the directivity and reduce the side lobes level. These lenses were also fabricated at UC with low relative permittivity materials.
We work in the implementation of efficient modeling methods with application to the QUIJOTE experiment 30GHz radiometers. Efficient models have been obtained from the different subsystems that form the radiometer (low noise amplifiers, band pass filters, hybrids, cables and detectors). These models allow realistic simulations of the overall radiometer in time and frequency domain, using Gaussian noise as excitation signals. First simulation studies, under nominal conditions, show that the results are close to the expected ones.
Time domain simulations verify the polarimeter behavior in relation to the Stockes parameters (I, Q and U). These simulations also give very important information about the non linear behavior of some radiometer subcircuits. During the designing process of these instruments, it is usual to take into account the non linearity of some circuits by means of the study of some figures of merit, but the use of Gaussian excitation signals makes the circuits to have a more nonlinear behavior than when using typical sinusoidal excitation signals. For this reason, it is necessary to perform realistic linearity tests in order to avoid saturation problems in the detectors.