Recently, in cosmology various modifications of gravity have been discussed partly motivated by the dark energy problem.
Many of such attempts are to construct effective theories at low energies,
but as we start thiniking about the quantum stability against radiation
correction, the necessary unnnatural tuning of the model parameters
threatens the validity of the model.
In particular, considering modified gravity theories in the infrared regime,
the theory tends to be strongly coupled and the prediction power is lost
at a relatively low energy scale compared to the Planck scale.
It would be extremely important to investigate the existence of an
example in which such models can be embedded in the framework of a
theory applicable to higher energy scales, to answer the question
about whether or not such models are meaningful as an effective theory
at low energies.
In the bi-gravity theories, we focused on the parameter region of the model in which we pointed out that graviton oscillation occurs. If we focus on low energy phenomena, this model resembles the low energy limit of a certain 5-dimensional Blaine world model. Beginning with this research, the interrelationship among various theoretical models might be clarified. It is also interesting to develop a general discussion about what kind of model is permitted as an effective theory at low energies.
The problem of infrared divergence in the inflation universe tends to be neglected because it is not a problem as far as loop corrections are ignored. However, thinking back in the 1980s when the idea of the inflationary universe was proposed, it seems that there was a similar situation. In general, the problems of homogeneity and flatness were pointed out as a crack in the Big Bang cosmology, and it became a trigger for the idea of inflation. However, at that time, many people thought that the initial condition of the universe had been appropriately chosen to solve the problems of homogeneity and flatness at the moment of the creation of the universe by some mechanism, 430/5000
Sekigaihassan no mondai wa, masani kono y? na tsuginaru hiyaku e no hinto o ataete kureru monode wanai ka to kangaete kenky? o susumete imasu. < Br > < br > kore made no wareware no kenky?de wa, t?sho kara yos? shite ita y? ni, ?ku no sekigaihassan no mondai ga jissai no kansoku-ry? wa nanidearu ka o kichinto k?ryo shite inakatta koto ni kiin shite iru koto ga akiraka ni natte kimashita. Sono ipp? de, j?tai no erabikata ni sukunakutomo mikake-j? wa seigen o tsukenai to sekigaihassan o nokenai to itta, yos?-gai no ketsuron mo michibikidasa rete kimashita. Kore made ni mo, kyokutan'na shuch? no rei to shite, j?ryoku-ha setsud? ga ?kina setto hosei o umidashi, uch? k? no shahei ya ketsug? teis? no einen-teki to itta gekitekina moderu no henkei o michibiku to iu shuch? ga arimasu. Ko no y?na shuch? no konkyo o utagau ronri no jakuten wa tata mi raremasuga, meihaku ni sono y?na gekitekina gensh? no kan?sei o hitei suru ni wa itatte imasen. Shikashi, korera no gensh? no umu ni kanshite wa, jissai no kansoku-ry? wa nande aru no ka to iu kanten de kenky? o susumeru koto de, chakujitsu ni meikakuna kotae ni tadori tsukeru mono to kakushin shite imasu.
I think that the problem of infrared divergence is providing
hints for such a next leap.
In our previous studies, as we had anticipated from the beginning, it became clear that many of the problems of infrared divergence are caused by not considering exactly what the actual observable quantities are. On the other hand, unexpected conclusions have also been drawn such that infrared divergence should not be excluded unless at least some constraint is imposed on the choise of the quantum state of the universe. As an example of extreme assertions, there have been arguments that gravitational wave perturbation produces large infrared correction, leading to dramatic modification of the scenario such as shielding of the cosmological constant term and secular change of the coupling constants. There are many weaknesses of logic that doubt the grounds for such assertions, but it has not definitely been denied the possibility of such a dramatic phenomenon. However, with regard to the presence or absence of these phenomena, I am convinced that by steadily carrying out research from the viewpoint of what the actual observable quatities are, we will steadily reach a clear answer.
Theoretical studies towards the understanding of the orbital evolution
of binary systems are essential when trying to verify gravity theory
from gravitational wave observations, and research is progressing from
both post-Newton approximation and black hole perturbation.
Orbital evolution using black hole perturbation is an efficient
approximation method especially for a sattelite motion in a massive
black hole spacetime with a large mass ratio, and this system is
thought to be the most suitable system to measure black hole spacetime
by gravitational waves.
It can be said that research on the orbital evolution by means of black hole
perturbation started with our paper in 1996, but the subsequent progress
has never been linear. The difficulty is
in the technical aspect of regularizing and evaluating the
Many studies so far have been performed based on the idea of directly evaluating the force by the self-field, that is, self-force. Because if one can directly evaluate the self-force, one can always calculate the necessary physical quantities related to the orbital evolution. However, the self-force is actually a quantity that depends on the gauge chose, and only the gauge invariant derived from an appropriate long-term average is the physical quantity that we intend to evaluate. With this idea, you can see that, at the lowest order in the mass ratio, the self-force can be evaluated by completely avoiding the problem of regularization. The expression for the secular change of the Carter constant is obtained by the adiabatic approximation, also based on this idea. The concise expression we got makes it possible to trace the long-term evolution of orbits much more conveniently with higher accuracy than directly evaluating the self-force. This way of thinking has also succeessflly extended to the resonance orbits, which becomes necessary to make the orbital evolution be more precise.
Furthermore, by further developing this idea, the orbital evolution can be described just by evaluating simple gauge invariant quantities even for the next order in the mass ratio. In fact, this order of accuracy is considered to be sufficent, because higher order effects are indistinguishable from the observation of the binary system with large mass ratio. As clarified in the lowest order calculation, we expect that building a calculation scheme by considering what the observable quantity will significantly reduce the amount of required computation. If so, I believe that it will be a standard way to calculate theoretical templates for gravitational waves in the future.
Braneworld is a new scenario of compactification of extra-dimensions. In this scenario the extension of extra-dimensions is assumed to be relatively large compared with Planck scale. In the usual scheme of Kalza-Klein compactification large extra-dimensions mean presence of many light particles, which contradicts with experiments. However, recently people realized the possibility that ordinary matter fields can be localized on a low dimensional object called brane. In that case, directions orthogonal to the brane are not sensed by ordinary matter fields. Then even if we consider higher dimensional models with relatively large extra-dimensions, the models can be consistent with observations. On the other hand, since spacetime is extended, gravity propagates in a higher dimensional spacetime, which we call bulk. Fortunately or unfortunately, gravity is so weak. Hence, the deviation from the standard Newton's law has not been severely tested below sub-mm scale.
My collaborators and I studied
gravity in braneworld mainly in
RS II model,
and clarified how the prediction of braneworld
deviates from the standard general relativity.
Another aspect on which I'm working is quantum correction
to the brane dynamics. It is well known that
Casimir force arises between two parallel plates.
Analogous phenomena happens in the context of braneworld.
This force may play a role to stabilize the distance between branes.
Gravitational radiation reaction