• CMB
• Transition Edge Sensor
• B-modes
• development
• LSPE
• cosmology
• sensors

Since its first observation in 1964 by Wilson and Penzias, the Cosmic Microwave Background radiation (CMB) has been studied by many experiments, which have measured many of its characteristics. The relevance given to the study of the CMB is justified by the cosmological significance of this radiation: it is the furthest back in time we can explore using light, since it was released about 380,000 years after the Big Bang, when the Universe became, for the first time, transparent to light. Studying the pattern of this radiation is a valuable method to reconstruct the history of the Universe, both before and after the CMB was released.
Currently we have an almost complete knowledge of some features of the CMB radiation: we know its temperature (2.726 K), its temperature anisotropies (at level 1 part in 100,000), its dipole temperature (~ 3 mK) and other minor features. However, new CMB experiments are now under construction in order to measure the polarization anisotropies spectrum of the CMB at an unpreviously seen sensitivity level. We know that the CMB radiation is intrinsically polarized due to photon-electron scattering in the first moments of life of the Universe and there are two kinds of polarization: an irrotational part (E-modes) and a rotational part (B-modes). In particular, anisotropies in the B-modes spectrum could only have been generated by tensor perturbations (i.e. primordial gravitational waves) that rised up during the so-called inflation: a period of accelerated expansion of the Universe, immediately after the Big Bang. Therefore measuring B-modes anisotropies would be a unique signature of tensor perturbations and, thus, a smoking gun for inflation. This means opening a window on what happened 10^{-32} s after the Big Bang. For these reasons, B-modes anisotropies detection is the ultimate scientific goal of current CMB experiments.
The required sensitivity to be able to detect B-modes anisotropies is very high: we must be able to distinguish two points in the sky having a difference in the polarization field temperature of the order of 100 nK. This is the reason why a huge number of very sensitive detectors must be arranged in current cosmology experiments. The mainly used detectors are Transition-Edge Sensors (TESs) and Kinetic Inductance Detectors (KIDs). In this thesis we focus, in particular, on the development of a Transition-Edge Hot-Electron Microbolometer (THM) protoype, a sensor that exploits superconductivity and the so-called hot electron effect to be very sensitive to temperature changes. In particular we highlight the advantages of using THMs instead of traditional TESs and we show how the measured parameters of some THMs protoypes we fabricated are compatible with the requirements for B-modes detection from space or in a balloon-borne experiment.
In the introductory chapters we present the CMB anisotropies theory, a review of the main features of CMB experiments currently under construction and the general theory of THMs. In the following, we present the fabrication and measurements of superconducting bilayers (to be used as THMs) performed at the National Enterprise for NanoScience and NanoTechnology (NEST) in Pisa. Moreover we describe the mounting of a cryogenic facility at the INFN of Pisa, to be used to test the sensors. The facility was also used to test the cold electronics for the Large Scale Polarization Explorer, an italian balloon-borne experiment. In this context we show some preliminar measurements performed on a TES fabricated at the INFN of Genova.