Tracking down the force of gravity
17 particles that explain the world: The Standard Model of Physics describes the structure of matter and the fundamental forces interacting between matter particles. The forces are mediated by force carriers (gauge bosons): photons transfer the electromagnetic interaction, gluons the strong, and W and Z bosons the weak interaction. No carrier boson has yet been demonstrated for the fourth fundamental interaction, gravitation. Physicists have coined the term "graviton" for this as yet undemonstrated particle.
In his general theory of relativity, Albert Einstein defined gravitation as a geometrical property of spacetime, curved by mass or energy. His theory describes the graviton as a massless, spin-2 particle – in contrast to the already known, spin-1 gauge bosons.
Massive spin-2 particles
The members of the Max Planck Research Group "Gravitational theories: Massive spin-2 fields" Group study extended theories of gravity. They focus on spin-2 particles which, in contrast to Einstein's theory, possess mass. A massive spin-2 particle would, in a sense, fill a gap.
This is because the standard model includes both massless and massive, low-spin theories. The particles involved have been demonstrated in experiments. Spin-1 examples include the massless photon or the massive W and Z bosons.
The scientists are working to construct the mathematical foundations for a gravitational theory using massive spin-2 particles. They also investigate how such a particle would influence existing particle physics models and cosmology.
The ghost in the theory
In detail, the Group studies what is referred to as the "ghost-free bimetric theory". This describes the interaction of a massive particle with the massless graviton, each being a spin-2 particle. However, the physicists pay a price for this theory: because if general relativity is expanded by a massive particle, an undesirable instability is very likely to be created (a ghost).
For decades, physicists have been attempting to develop a consistent theory without this instability – unsuccessfully. And that, despite the fact that the theoreticians Markus Fierz and Wolfgang Pauli had presented an intrinsically consistent linear theory in flat spacetime as early as 1939.
New approaches for dark energy and dark matter
However, during the 70 years following it has not been possible to transpose the linear theory onto a non-linear scenario. In addition, there were numerous highly convincing arguments against this option and it has therefore gradually fallen into oblivion.
A few years ago, the tide turned: New analyses uncovered the consistent theoretical description of a massive spin-2 particle, and therefore new conceptual models – not only with regard to gravitational theory, but also relating to astrophysical and cosmological phenomena. For example, bimetric theory delivers new approaches to solving the problem of dark energy as the cause of the accelerated expansion of the universe.
A further problem of physics today is dark matter. A series of observations speak in favour of the existence of this mysterious matter, which only interacts weakly, if at all. However, to date, nobody knows what it consists of. The group at the Max Planck Institute for Physics has proposed a massive spin-2 particle – and in future will investigate the effects such a particle would have on phenomenological models.
In addition, the Group is working on a new conceptual framework for spin-2 theories. Here, their studies concentrate on additional symmetries and geometrical interpretations of the new models.
More information on the group "Gravitational theory: massive spin-2 fields"
Heavy spin-2 Dark Matter
Eugeny Babichev, Luca Marzola, Martti Raidal, Angnis Schmidt-May, Federico Urban, Hardi Veermäe, Mikael von Strauss
JCAP 1609 (2016) no.09, 016
Recent developments in bimetric theory
Angnis Schmidt-May (Zurich, ETH) , Mikael von Strauss (Paris, Inst. Astrophys.)
J.Phys. A49 (2016) no.18, 183001
Bimetric gravity is cosmologically viable
Yashar Akrami, S.F. Hassan, Frank Könnig, Angnis Schmidt-Ma, Adam R. Solomon