What is cosmology?
Cosmology is the study of the universe as a whole: what it is composed of, how it began, and where it is going. Modern cosmology is based upon the Big Bang Theory, in which the Universe began as a very hot, dense plasma of elementary particles about 13.8 billion years ago and then rapidly expanded. This expansion produced cooling and after about 400,000 years the first atoms formed allowing the Universe to become transparent to light. Eventually some regions condensed to form galaxies, and subsequently suns, planets and finally life on Earth. We now have evidence that the Universe will not only continue to expand, but even accelerate - expand faster and faster - forever.
This first light to emerge after the Universe became transparent is known as the Cosmic Microwave Background, or CMB. This radiation has been traveling through space for billions of years and remains all around us, though very faint. While the radiation was originally at about 3000 K, the expansion of the Universe has caused it to now become quite cold - only about 2.7 K. The CMB was first predicted in 1948 but not detected until 1964, and slight differences across the sky were not detected until 1992. Our ability to measure this first light has now become quite advanced, and we are able to measure the temperature to an accuracy of about a millionth of a degree. PCCP Director George Smoot shared the 2006 Nobel Prize in Physics for his pioneering work in measuring the fluctuations of the CMB.
There is now considerable evidence that the Universe underwent a period of intense expansion, or "inflation," even before the standard Big Bang phase. This inflationary period was likely caused by the "anti-gravity" force resulting from "vacuum energy," or the energy associated with apparently empty space, in a manner very similar to dark energy but much more extreme. This rapid exponential expansion caused the Universe to expand by a factor of at least 10^78 in a mere 10^-32 seconds. Small quantum fluctuations also expanded by this gigantic factor, and so became large regions of slightly more or slightly less repulsive energy. The former become "voids," or regions with no matter, and the latter become galaxies.
There exist a wide variety of inflationary models which are currently consistent with the cosmological data. Although many are superficially similar, they should differ in subtle features which could be compared against experiment. Intense research is currently underway at PCCP to understand the physics responsible for inflation. One approach towards understanding is theoretical: deeper mathematical details can often lead to seeing patterns in models, or the ability to rule out classes of theories. The other approach is experimental: observables often contain a wealth of physics about inflationary physics. The CMB has two properties which are able to provide clues: temperature, as explained above, and polarization, or the direction of the individual photons themselves.
Microwave signals can be detected using experiments either on the ground, in balloons, or satellites in space. Development of the technology for future experiments to the CMB is an active field. Two examples of this are the superconducting bolometers and the superconducting microwave kinetic inductance detectors (MKIDs). Bolometers measure the power of incident electromagnetic radiation by heating up, and the resulting temperature variation is measured through a thermistor. Unlike bolometers, MKIDs are Cooper-pair breaking devices and they are operated well below the criical temperature of the superconductor used to make the sensitive par of the pixel. PCCP and APC, thanks to a LabEx grant, have recently joined a French effort to develop MKIDs in order to measure CMB polarization. In particular, we are developing small arrays with different pixel geometry and arrangement to test different polarization measuring schemes.