I propose to build an instrument that cools YbF molecules to microK temperature using laser light, and throws them up as a fountain in free fall. This will be used to detect CP-violating elementary particle interactions that caused our universe to evolve an excess of matter over antimatter These interactions cause the charge distribution of the electron to be slightly non-spherical and it is this property, the permanent electric dipole moment (EDM), that the ultracold molecules will sense.Laser cooling of any molecule is very new, with first results emerging from a few laboratories including mine. Developing a fountain of molecules will be a major advance in the state of the art. As well as being the key to the new EDM instrument, this will be important in its own right because ultracold molecules have major applications in chemistry, quantum information processing and metrology.In the fountain, the electron spin of each molecule will be polarized. On applying a perpendicular electric field, the spins will precess in proportion to the EDM. At present the (warm) YbF molecules in my lab precess for only 1ms. This gives us world-leading sensitivity, but has not been sufficient to detect the CP-violating forces being sought. The fountain however will achieve precession times of almost a second, giving over 1000x more rotation. The increase in sensitivity should reveal a clear EDM, providing information about the fundamental laws of physics, and the important CP-violating physics of the early universe, which is currently not understood.By advancing the preparation of ultracold molecules, this project will address a key question in particle physics and cosmology: the nature of CP-violating physics beyond the standard model. The approach is radically different from standard accelerator physics and complements it. The sensitivity is sufficient to detect some proposed new forces that are beyond the reach of any current collider experiment.