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The Nobel Prize in Physics of 2011 was given for the discovery that the current expansion of the Universe is accelerating, and not decelerating as one would assume from the simple observation that gravity is an attractive force. Indeed, gravity is attractive when applied on the form of matter that we have a most direct experience of: particles, tables, and planets. In order to give rise to acceleration the energy must be either produce negative pressure or modify the laws of gravity altogether. Both proposals now fall under the name of "dark energy," one of the primary research topics at PCCP. Recently, a very general description of dark energy has been proposed which is able to describe both the "accelerating power" of dark energy on the largest cosmological scales and its impact on the formation and evolution of structures (galaxies and galaxy clusters) on smaller scales.

 

What is the cosmic microwave background?

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.

 

What is inflationary theory?

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.
 

How do we learn details about inflation?

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.
 

How do we measure the CMB?

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.

 

 

 

There is a large amount of arbitrariness in the way physical systems can behave. Barring the crucial constraints of being compatible with the laws of special relativity and of quantum mechanics, the explicit form of the equations of motions in particle physics and of condensed matter systems are basically inferred by observation. There is however an important exception whenever we are in the presence of a continuous symmetry and such a symmetry is "broken" by the choice of the system of putting itself in some configuration (in this case the symmetry is "spontaneously broken"). Then the particle content of the theory at low energy — and the interactions among such particles — are entirely dictated by the symmetry breaking pattern. Such particles are the "Nambu Goldstone bosons," for which the 2008 Nobel Prize was awarded, and established the basic idea behind the Higgs Boson, for which the 2013 Nobel Prize was awarded.  Recent research at PCCP has included the study of such particles when the symmetry of relativity is also broken.