Skip navigation.

Institute for the Physics and Mathematics of the UniverseWPI

Joint Seminars on Cosmology and Gravitation

  • 24 (Tue), November, 2009, 14:00 - 16:30
    Place: IPMU General Research Building Room 630, Kashiwa Campus of the University of Tokyo.
    14:00 - 15:00

    Alexei A. Starobinsky (Landau Institute / RESCEU)

    Title: To the local beginning of inflation and beyond

    Abstract: Though such observable quantities as the spectrum and statistics of primordial scalar and tensor perturbations are independent on initial conditions at the local beginning of sufficiently long inflation, there exist many other quantities and effects which do depend on them. Observing these effects or their consequences will give some knowledge on the initial conditions. Examples are probabilities (branching ratios) of decay into different vacua and rms fluctuations of light scalar fields after inflation. Analytic expressions for these quantities derived using the original (probability conserving) stochastic approach are presented and their dependence on the initial conditions is discussed. Generically there seems to be no rule fixing these conditions uniquely. In particular, 'eternal' inflation is not eternal enough for this purpose, as well as for reaching equilibrium values for all observables.

    15:00 - 15:30 coffee break
    15:30 - 16:30

    Masato Minamitsuji (Sogang University)

    Title: TBA

    Abstract: TBA

  • 16 (Fri), October, 14:00 - 17:30
    Place : South tower of Building 55, Room number 410
    14:00 - 15:30

    Dr. Hiroyuki Abe (Waseda univ.)

    Title : Phenomenological (cosmological) aspects of nonperturbative

    moduli stabilization

    Abstract : We show some phenomenological and cosmological aspects of higher-dimensional supergravity/string models, where some of moduli fields arising from the compactification of extra dimensions are stabilized by nonperturbative effects such as gaugino condensations (the KKLT-type stabilization).

    Within such a framework, we find a parameter region avoiding the so- called little hierarchy problem in the minimal supersymmetric standard model, and derive typical superparticle spectra which may allow the lightest superparticle to be a dark matter candidate.

    15:30 - 16:00 Break
    16:00 - 17:30

    Dr. Antonino Flachi (YITP)

    Title : Brane world black holes and quantum back reaction

    Abstract : We analyze the effects of the back reaction due to a conformal field theory (CFT) on a black hole spacetime with negative cosmological constant.

    We study the geometry numerically obtained by taking into account the energy momentum tensor of CFT. We find a sequence of configurations without a horizon in thermal equilibrium ({\it CFT stars}), followed by a sequence of configurations with a horizon.

    We discuss the thermodynamic property of the system and how back reaction effects alter the space-time structure.

    We also provide an interpretation of the above sequence of solutions in terms of the AdS/CFT correspondence. The dual five-dimensional description is given by the Karch-Randall model, in which a sequence of five-dimensional floating black holes followed by a sequence of brane localized black holes correspond to the above solutions.

  • 28 (Tue), July, 2009,13:30 - 17:00
    Place: ICRR, University of Tokyo, Room 601 (Kashiwa Campus)
    13:30 - 15:00 Takeo Moroi (Tohoku)
    Title: Decay of scalar condensate in QFT
    15:30 - 17:00 Masayasu Kamimura (RIKEN/Kyushu)
    Title: Big-bang nucleosysnthesis reactions catalyzed by SUSY particle stau
  • 10 (Wed), June, 2009, 14:00 - 17:30
    Place: KEK, 4th Building Room 244 (2nd Floor)
    14:00 - 15:30 Nobuchika Okada (KEK)
    Title: Higgs portal to dark matter in the Nightmare Scenario
    Abstract: Even if new physics beyond the standard model exists at some high energy, it is generally possible that the LHC experiment could find only the Higgs boson but nothing else. This is particle physicists' Nightmare Scenario. On the other hand, the existence of the dark matter has been established from various observations and according to the Weakly Interacting Massive Particle (WIMP) dark matter hypothesis, a stable and charge-neutral WIMP with mass of O(1 GeV)-(1 TeV) is a promising candidate for thermal relic dark matter. We discuss the way how to incorporate such a WIMP dark matter in the nightmare scenario and investigate a possibility that the dark matter can overcome the nightmare through its coupling with the Higgs boson.
    16:00 - 17:30 Hideki Maeda (Centro de Estudios Cientificos)
    Title: Dynamical black holes with symmetry in Einstein-Gauss-Bonnet gravity
  • 22 (Fri), May, 2009
    Place: Multi-purpose hall, 3rd floor, Tachikawa Hall,Rikkyo University
    14:30 - 16:00 Motoyuki Saijo (Rikkyo University)
    Title: Collapse of a rotating supermassive star to a supermassive black hole
    abstract: We all believe that there exists a supermassive black hole in the centre of galaxy, but the actual formation process is still a mystery. Here we stand on the path to form a supermassive black hole from a supermassive star. In order to focus on this dynamical process, three dimensional general relativistic hydrodynamics is desirable. After a brief introduction about the supermassive objects and expected gravitational waves to be detected, I will review the ideas in numerical relativity which we use in our computation. Then, I will demonstrate our computational results about the dynamic behaviour of the black hole formation process. Here, I will especially focus on the post black hole formation stage, that there exists a quasi-periodic waves after the ringdown in gravitational waveform. The typical frequency of the quasi-periodic waves approximately correspond to gravitational waves from a particle at ISCO orbiting around a Kerr black hole. The radiation may be detected in the future space-based detector LISA, which may tells us the environment of the supermassive object.
    16:00 - 16:30 Break
    16:30 - 18:00 Dr. Leor Barack (University of Southampton, UK)
    Title: Gravitational self-force (with application to extreme-mass-ratio binary inspirals)
    abstract: The radiative inspiral of compact stellar-mass objects into massive black holes in galactic nuclei is a key source for LISA -- the proposed space-based gravitational wave observatory. Each of these inspiralling objects is an extremely effective probe of the strong-field geometry of the central black hole, and its gravitational wave signal encodes a wealth of detailed information about this geometry. Decoding this information will require accurate theoretical templates of the gravitational waveforms, which, in turn, requires an accurate model of the orbital evolution. The inspiral dynamics can be described in a perturbative fashion in terms of an effective gravitational self-force. Knowledge of the local self force acting on the inspiralling body is an important prerequisite in the program to model astrophysical inspirals. I will begin this talk by reviewing the general theory of the gravitational self-force in curved spacetime, and proceed to describe how this theory is being applied today in actual calculations of the self force for inspiral orbits. As a particular application, I will present a recent calculation of the shift in the location and frequency of the ISCO (innermost stable circular orbit) of a Schwarzschild black hole due to the conservative piece of the gravitational self force.
  • 22 (Wed), April, 2009
    Place: RESCEU, University of Tokyo
    14:30 - 15:30 Takehiko Asaka (Niigata)
    Title: Neutrino masses and baryon asymmetry of the universe
    16:30 - 17:30 Damien A. Easson (IPMU)
    Title: Searching for the microphysics of Inflation
  • 27 (Fri), February, 2009, 13:30 - 17:00
    Place: IPMU Prefab. B, Kashiwa Campus of the University of Tokyo.
    13:30 - 15:00 Niayesh Afshordi (Perimeter Institute)
    Title: The end of the cosmological constant problem!
    abstract: Cosmological constant problem is arguably the deepest gap in our understanding of modern physics. I first introduce a modification of Einstein gravity, gravitational aether, which decouples the quantum field theory vacuum from gravity, solving the cosmological constant problem. I will then show how the formation of stellar-mass black holes could trigger the onset of cosmic acceleration (and dark energy) in the context of this model. The model can be tested through its effect on big bang nucleosynthesis, cosmological structure formation, and possible correlations between black hole formation and cosmic acceleration at late times.
    15:00 - 15:30 coffee break
    15:30 - 17:00 Ghazal Geshnizjani (Perimeter Institute)
    Title: Observational Evidence for Cosmological-Scale Extra Dimensions
    abstract: I present a case that current observations may already indicate new gravitational physics on cosmological scales. The excess of power seen in the Lyman-alpha forest and small-scale CMB experiments, the anomalously large bulk flows seen both in peculiar velocity surveys and in kinetic SZ, and the higher ISW cross-correlation all indicate that structure may be more evolved than expected from LCDM. I argue that these observations find a natural explanation in models with infinite-volume (or, at least, cosmological-size) extra dimensions, where the graviton is a resonance with a tiny width. The longitudinal mode of the graviton mediates an extra scalar force which speeds up structure formation at late times, thereby accounting for the above anomalies. The required graviton Compton wavelength is relatively small compared to the present Hubble radius, of order 300-600 Mpc. Moreover, with certain assumptions about the behavior of the longitudinal mode on super-Hubble scales, our modified gravity framework can also alleviate the tension with the low quadrupole and the peculiar vanishing of the CMB correlation function on large angular scales, seen both in COBE and WMAP. This relies on a novel mechanism that cancels a late-time ISW contribution against the primordial Sachs-Wolfe amplitude.
  • 28 (Wed), January, 2009, 14:00 - 17:30
    Place: Waseda University
    14:30 - 15:30 Alexei Starobinsky (Landau Institute / RESCEU)
    Title: f(R) models of inflation and dark energy in the Universe
    Abstract : Since 1980, a variant of the f(R) fourth-order theory of gravity (with small one-loop non-local corrections) was known to provide an internally self-consistent scenario of the early Universe with an initial quasi-de Sitter (inflationary) stage followed by the graceful exit to the radiation-dominated FRW stage via reheating in the regime of a narrow parametric resonance, in which all matter in the Universe arises as a result of gravitational particle creation. Its predictions regarding spectra of primordial density perturbations and gravitational waves remain in agreement with the most recent observational data. A few years ago it was proposed to use this class of models for description of dark energy in the present Universe. However, this problem appeared to be more complicated, mainly due to the presence of non-relativistic matter and radiation, so many attempts in this direction failed. Still recently some f(R) models of dark energy have been found which can satisfy laboratory, Solar system and cosmological tests. They represent an interesting alternative to the standard cosmological model with a cosmological constant. Further problems and most critical tests for such models are outlined.
    16:00 - 17:30 Brian Powell ( IPMU )
    Title : What we know (and may ever know) about inflation
    Abstract : I will discuss the current status of inflation in light of recent CMB and LSS measurements, focusing on a novel numerical technique of potential reconstruction based on the flow formalism. This approach reveals that little information is known about the earliest moments of observable inflation, indicating that non-slow-roll behavior is possible. The associated power spectra exhibit strong suppression of power on large scales. In addition to exotic inflationary dynamics, such spectra might also be produced by modifying the initial vacuum state of fluctuations, as might arise from a pre-inflationary expansion phase. Lastly, I will discuss the future prospects of potential reconstruction as might be expected with the upcoming Planck mission. I will focus on potential reconstruction within the larger class of non-canonical inflation models, a class including DBI and k-inflation, and conclude that a measurement of non-Gaussianities will be essential for a successful reconstruction program.
  • 10 (Wed), December, 2008, 14:10 - 18:30
    Place: KEK, Tsukuba campus, Yon-Go-Kan Bldg
    14:10 - 15:40 David Wiltshire (U. Canterbury)
    Title: Cosmological equivalence principle and dark energy without dark energy
    Abstract: The problem of the synchronization of clocks and normalization of gravitational energy in general relativity does not have a unique or obvious solution in the absence of exact symmetries of the background. I return to first principles and extend the strong equivalence principle in application to averaged dynamical fields in cosmology, to include the role of the evolving average background density in the calibration of inertial frames. This leads to a "radically conservative" solution to the problem of dark energy in cosmology, using only general relativity and matter obeying the strong energy condition. The proposal yields a model universe which appears to be quantitatively viable, in terms of its fit to supernovae luminosity distances, the angular scale of the sound horizon in the cosmic microwave background anisotropy spectrum, and the baryon acoustic oscillation scale. I will briefly overview the observational status of the proposal, as well as discussing the foundational issues.
    15:40 - 15:50 discussion
    15:50 - 16:10 Coffee break
    16:10 - 17:40 Toshifumi Futamase (Tohoku U)
    Title: Recent developements of weak lensing
    17:40 - 17:50 discussion
    18:30 - dinner
  • 14 (Tue), October, 2008, 14:00 - 17:30
    Place: RESCEU, University of Tokyo
    14:30 - 15:30 Shuichiro Yokoyama (Nagoya U)
    Title: New delta N formula for multi-component inflation
    16:30 - 17:30 Masahiro Morikawa (Ochanomizu U)
    Title: Cosmic dark turbulence and scaling of gravitating system
  • 30 (Tue), September, 2008, 14:30 - 18:00
    Place: Rikkyo University (Ikebukuro), Building No 4, Room 4232
    14:30 - 16:00 Takashi Tamaki (Waseda U / Rikkyo U)
    Title: Diversity of gravitating Q-ball
    16:30 - 18:00 Atsushi Taruya (U of Tokyo)
    Title: Nonlinear gravitational evolution of large-scale structure and precision cosmology

  • 23 (Wed), July, 2008, 14:00 - 17:00
    Place: IPMU Prefab. B, Kashiwa Campus of the University of Tokyo.
    14:00 - 15:00 Latham Boyle (CITA)
    Title: Binary black hole merger: symmetry and the spin expansion
    Abstract: Two spinning black holes emit gravitational waves as they orbit, and eventually merge to form a single black hole. How do the properties of the final black hole depend on those of the initial black holes? We regard binary black hole (BBH) merger as a map from a simple initial state (two black holes, with dimensionless spins a and b) to a simple final state (a Kerr black hole with mass m, dimensionless spin s, and recoil kick velocity k). By expanding this map around a = b = 0 and applying symmetry constraints, we obtain a simple formalism that is remarkably successful at explaining existing BBH simulations, and makes detailed new predictions about the merger process. We discuss some astrophysical/cosmological applications of this formalism.
    15:30 - 17:00Contributed talks
    • Kazunori Nakayama (ICRR, U of Tokyo)
      "Impacts of gravitational wave background detection on SUSY"
    • Keisuke Izumi (Kyoto U)
      "How to construct the seed metric of many blackrings"
    • Kin-ya Oda (Osaka U)
      "Black hole thermodynamics for quark-gluon hydrodynamics"