Particle Physics, Cosmology and Quantum Gravity consist of the most fundamental aspects of modern physics and address several important open questions, which include accelerated expansion of the Universe, dark matter, cosmic singularity, matter production in the early Universe and quantum gravity. These objects have been main research topics of the research group of the faculty of physics and they made the following scientific contributions in collaboration with physicists from University of Munich, Stanford University, Technical University of Munich, Stockholm University, etc.
Accelerated expansion of the Universe by dark energy
Observed accelerated expansion of the Universe is one of the most challenging questions in the theoretical cosmology.
Since the astonishing discovery that the expansion of the current universe is speeding up, a number of models has been proposed assuming dark energy such as cosmological constant and quintessence, but it is still a difficult task to find compelling mechanisms.
The most important problem is the so-called coincidence problem, which addresses the question why the transition from deceleration to acceleration of the cosmic expansion happened to coincide with that from matter dominated to radiation dominated era.
The research group of the Physics Faculty studied k-essence theory, which avoids the coincidence problem because of the attractor-like dynamics of the k-scalar field with non-canonical kinetic term in the Lagrangian.
Based on k-field Lagrangian generalizing previous models, they carried out a comprehensive study of classes of k-essence theory with positive energy and no ghost in order to classify all possible cases of models that admit asymptotically stable tracker solutions, whose equation of state and relative energy density approach constant.
Using the classification, they selected the class of models that describe the late-time acceleration and avoid the coincidence problem through the tracking mechanism. In particular, among classes of models with tracker solution, they obtained cosmologically viable models that admit tracker solutions, which have small fraction of energy density during radiation dominated era and large one during matter dominated era.
Cosmic Singularity and Bouncing Universe
Since the Universe is expanding with some temperature, it follows that the temperature and energy density of the early universe were very high. Furthermore, the standard cosmology predicts that there exists in far past a point of time where physical quantities such as the cosmic temperature and energy density blow up, which is called cosmic (big bang) singularity problem.
As the energy scale of the early universe is supposed to be extremely high, the resolution of the cosmic singularity problem mostly invokes theory of quantum gravity such as string theory.
Typical scenario that attempt to solve the singularity problem adopts bouncing involving transition from contraction to expansion. So far many bouncing models have been suggested, but most of them rely on the violation of null energy condition, which may lead to pathology such as vacuum instability.
The research group of the Physics Faculty critically analyzed previous bouncing models and constructed a new bouncing model that consistently resolves the big-bang singularity by means of unstable non-BPS brane in type IIA string theory. In this scenario, non-BPS brane created as the universe bounces through string size regime before expanding as the brane decays. The nice feature in their scenario is that the curvature as well as the derivative of the dilaton remain small (in string units) through the bounce, which justifies using perturbative string theory and the simplest low energy effective action for these fields. In addition, no fine tuning is required and no vacuum instability occurs in their model.
Matter particles production in the early universe
According to the inflationary cosmology, particles of standard model were produced after the accelerated expansion of the early universe (inflation) by some mechanism called cosmic reheating and the maximal temperature of the radiation dominated era (reheating temperature) is a very crucial parameter in the study of beyond standard model.
One important claim of previous studies is that the reheating temperature is upper bounded by the thermal mass of the decay products of inflation and this can significantly affect many results of cosmology.
However, to clarify the role of thermal masses in determining the reheating temperature one should rigorously investigate kinematics of cosmic reheating using non-equilibrium quantum field theory.
To this end, the research group of the Physics Faculty calculated the relaxation rate of a scalar field in plasma of other scalars and fermions with gauge interactions using thermal quantum field theory. It yields the rate of cosmic reheating and thereby determines the temperature of the "hot big bang" in inflationary cosmology.
The total rate originates from various processes, including decays and inverse decays as well as Landau damping by scatterings. At high temperatures, they found that the universe can even be heated through couplings to fermions, which are often assumed to be negligible due to Pauli-blocking.
On the other hand, they also studied dark matter and big-bang nucleosynthesis (BBN) in certain classes of modified gravity to constrain relevant parameters.
In particular they considered dark matter in super symmetric models with axino LSP in Randall-Sundrum II brane model and also investigated f(R)∝Rn gravity in the context of BBN and WIMP (weakly interacting massive particles) dark matter. One important result of this study is that f(R)∝Rn gravity can not differ from Einstein's gravity as much as claimed by other researchers.
D-brane and string quantum effective action
String theory is regarded as a leading contender of quantum gravity in consistency.
Furthermore, since the beginning, D-branes have been very important in the formal development of the string theory as well as in the attempts to apply the theory to particle physics and cosmology.
In all three endeavors (formal, phenomenogical, cosmological), a central role is played by the D-brane moduli. Moreover, its low energy effective action is very important to link string theory to cosmology, for instance, in order to find ways of embedding cosmological models such as inflation in more fundamental physics.
In particular quantum corrections to the metric of the moduli become relevant because they could contribute to interesting dependence on the D-brane moduli that may be used for moduli stabilization. However, due to many difficulties in string loop calculation, quantum corrections to Kahler metric in minimal (N=1) super symmetric sectors were little understood.
In collaboration with German and Swedish colleagues, the research group of the Physics Faculty succeeded in evaluating some quantum corrections to the Kahler metric in poorly understood N=1 sectors.
First, we calculated string one-loop contributions to the Kahler metric of D-brane moduli (positions and Wilson lines) in toroidal orientifolds with branes at angles. We showed that these quantum corrections vanish. This is a stringy effect, which does not follow the usual nonrenormalization theorems of super symmetric field theory. It represents an interesting statement about the underlying string theory rather than a nonrenormalization theorem of the effective field theory. Such statements are somewhat rare in string effective actions and drew attention from the community.
And then, we also evaluated string one-loop contributions to the Kahler metric of closed string moduli in toroidal minimally super symmetric (Calabi-Yau) orientifolds with D-branes. All these works involved developing a large range of new techniques and methods for string amplitude calculations in the presence of D-branes and O-planes.
