Deep analogies between the broken symmetries of superfluid 3He and those of the Universe mean that quantized vortices mirror cosmic strings. Vortices are created when the superfluid transition is crossed too fast for the liquid to follow and different regions become independently superfluid. The resultant disorder in the superfluid phase cannot be completely annealed, leaving topological defects, in this case vortices. Analogous processes in the early universe should have created cosmic strings.

The analogy is only complete near absolute zero where there is no normal fluid to mask the properties of the condensate. Here the creation of vortices, after local heating by neutron irradiation, shows behaviour suggesting competition between the two superfluid phases as the liquid recools. Such processes shed light on similar but elusive competing quantum vacua in the early Universe. At low temperatures vortices can only decay by radiation. It is thought that kinks left after reconnections propagate rapidly, leading to the decay of the vortex by radiation of quasiparticles from broken pairs. A cosmic string network should decay analogously by the radiation of gravitational waves.

We intend to study the energy processes involved in vortex tangles, both the energy released on setting up the network and that released on the final decay at microkelvin temperatures in the pure condensate. ULANC will attempt the measurement in the high-resolution quasiparticle energy detector by observing the decay of a vortex tangle generated inside the bolometer. TKK will observe the heat released in the inverse process when a previously stationary condensate in a rotating container is suddenly converted to a vortex lattice. Both methods will require high-sensitivity energy detection. CNRS will investigate the effect of pressure on the dynamics associated with the competition between the two superfluid phases as the vortices are created.

The several coherent phases of superfluid 3He provide us with phase boundaries which are absolutely unique in being boundaries between two fully-ordered condensates with different symmetries. The smooth transformation of the order parameter across the boundary yields the most highly ordered 2D structure to which we have experimental access. In Lancaster phase boundaries are studied as analogues of branes in the early Universe. The motivation being that brane interaction and annihilation is thought of as a possible trigger for inflation. Preliminary work has shown that brane-annihilation (mutual annihilation of two phase boundaries) leads to the generation of topological defects, validating those braneworld scenarios where such defects are predicted. Oscillating branes have been studied in all three core institutes in the search for the various aspects of Schwinger pair production.

ULANC will devise methods to identify the topological defects left after boundary ("brane") annihilation. CNRS will investigate the direct interaction of a micromechanical oscillator with the recently observed 2D "cosmological defect" and investigate the conditions of its creation and destruction, and the dissipation mechanism.

There is great current interest in condensed-matter analogues where aspects of Black Holes and their associated horizons can be simulated. The pure superfluid 3He condensate can throw light on several Black-Hole processes.

In the superfluid the Landau critical velocity plays the role of the velocity of light, marking the threshold where excitations can be created with zero energy. Any scattering object moving through the condensate at this velocity freely creates excitations (costing no energy) with the consequent destruction of the condensate. In the absence of scatterers, when the liquid exceeds this critical velocity, some quasiparticles develop negative energies. This is the analogue of the ergoregion around a Black Hole where the negative-energy quasiparticles clearly cannot escape. This property of the excitation gas provides a whole range of Black-Hole analogue behaviours. For example, an excitation injected into the "ergoregion" can pair break, leaving a trapped negative-energy daughter particle and ejecting the other with energy above that of the parent, thus extracting energy from the "Black Hole". This is closely related to the phenomenon of Hawking radiation where the high energy particles can emerge spontaneously.

Other phenomena which can be studied include; the analogue of cosmological particle production during expansion simulated by the rapid change say, of the magnetic field; the analogue of the Unruh effect of particle creation, simulated by a potential gradient moving rapidly in the superfluid; the radiation of fermionic quasiparticles by a moving vortex in turbulent flow of 3He simulating the radiation of gravitational waves by evolving cosmic strings in early Universe, and many more.

Several experimental configurations can provide such scenarios. At TKK instabilities at the interface between the A and B phases of the superfluid, where one phase is in relative motion, mimic several features of Black-Hole behaviour. With a suitable choice of the superfluid layer thickness, the spectrum of excitations on the interface takes the relativistic form with the governing equations mimicking those for the event horizon of a black hole.

The A-B transition can be triggered by neutron absorption where the "new" B phase destroys the existing metastable A phase, in an analogy with various models of inflation. Preliminary work at CNRS suggests a mechanism working through percolation between B-phase seeds created by the absorption, which will give information on the fundamentals of the phase transition dynamics.

Q-balls can be thought of as bubbles trapping the "wrong" phase after phase transitions in the early Universe. In one scenario the Q-ball represents a bunch of supersymmetric particles trapped in the surrounding "normal" matrix. If such a Q-ball were to enter a neutron star, for example, it would convert the neutrons to their boson equivalent and lead to the disintegration of the star.

The Q-ball can modify its surroundings. A powerful analogy is the long-lived domains seen in superfluid 3He where the spin superfluid precesses coherently over a limited region of space back-reacting on the surroundings to perpetuate its own potential well both in the stationary and in the rotating superfluid. At microkelvin temperatures, dissipation processes become very weak and the deflection of the magnetization becomes the conserved quantity in analogy with the conserved Q-ball charge "Q". In 3He we can observe the deflected spin directly by NMR thus probing the inner structure of the ball. Where two such coherent but independent domains can be formed we can also bring them into contact and watch the inner workings of the Josephson effect between them by NMR. The domain lifetime also provides thermometry at the lowest temperatures.

The 3He condensate provides a "scintillator" material for dark-matter detection and other ultrasensitive energy measurements. A conventional scintillator produces optical photons with eV scale energies. The 3He condensate behaves similarly, but the quasiparticles produced (by pair breaking) have energies of order 10^-7 eV, potentially yielding orders of magnitude more sensitivity. This provides a large advantage over current dark-matter detectors based on the nuclear recoil in large (>100 g) semiconductor single crystals cooled typically to 10 mK (e.g. CDMS, Edelweiss, CRESST, etc). However, the use of superfluid 3He as a detector naturally requires state-of-the-art ultralow temperature techniques, currently only accessible in two laboratories worldwide. The possibility of detecting astroparticles with a sensitivity of less than 1 keV using superfluid 3He at 100 μK (two orders of magnitude colder than current experiments) has been demonstrated in Lancaster and CNRS-Grenoble.

A prototype particle detector showing extreme sensitivity and high discrimination has been successfully tested in Grenoble (Projects MacHe3 and ULTIMA). Current results of ULTIMA, achieved with a small-sized detector, have been widely acclaimed by both the low-temperature and the cosmo-particle communities. In a few months the prototype detector will be the first 3He system to run under the stringent astrophysical conditions of very low muon flux in the Canfranc underground laboratory.

Along with several European groups, we are now in a position to undertake the even more ambitious research project, ULTIMA-Plus, taking advantage of the expertise available in the MICROKELVIN Collaboration. The detector cell containing superfluid 3He cooled to 100 μK is arranged in a matrix of bolometers which, by correlation and pulse-shape discrimination, provide background-event rejection. We intend to develop the first operational superfluid 3He neutralino detector with the first measurements at Canfranc. A 100 cell detector (100 grams of 3He) would provide an ideal starting point for non-baryonic Dark-Matter searches. In particular 3He, with one neutron, should provide a clear advantage for detecting that class of WIMPs thought to interact only with unpaired neutrons.

We have the expertise needed to cool large quantities of 3He to the temperatures required for such a unique detector. However, further techniques must be developed to exploit fully the potential of superfluid 3He, including thermometry, mechanical resonators, low noise signal detection, low radioactivity cryogenic materials, etc.

Such microkelvin devices could also provide the basis for more universal bolometric particle detectors applicable to several different types of cosmic particles, especially those with axial interactions. Since many different WIMP candidates have been proposed to account for the Dark Matter excess (neutralinos and axions being currently very popular), we need to develop a range of different types of detector.