Joint Seminars on Cosmology and Gravitation

  • Date : 9 (Fri) July, 2010, 14:00-17:30
    Place : Faculty of Science Bldg.1, 233 (Hongo Campus)
    14:00-15:30 Jonathan Ganc (UT Austin)
    Title: A new method for calculating the primordial bispectrum in the squeezed limit.
    Abstract: In 2003, Creminelli and Zaldarriaga proposed a consistency relation for the primordial curvature perturbation of all single-field inflation models; it related the bispectrum in the squeezed limit to the spectral tilt. Notably, their result is produced via classical arguments whereas most bispectrum calculations use quantum field theory (via the in-in formalism, for example). We have applied similar arguments to calculate the squeezed-limit primordial bispectrum using the in-in formalism and have arrived at a generic formula that doesn't rely on a slow-roll approximation. We were not able to verify the consistency relation in all generality, though it should be able to be demonstrated by our technique if it does, in fact, hold in general; we did explicitly verify it for slow-roll inflation (a known result) and for power-law inflation. Our technique could also be useful for calculating the single-field trispectrum in the squeezed limit and could be adapted for certain types of multi-field inflation models.
    15:30-16:00 Break
    16:00-17:30 Norihiro Tanahashi (YITP, Kyoto)
    Title: Extradimension detection by gravitational wave observations
    Abstract: Inspired by the gauge/gravity correspondence, it was conjectured for the RS-II braneworld model, which is composed of five-dimensional AdS bulk spacetime and a four-dimensional brane in it, that the five-dimensional classical gravity in the bulk spacetime is equivalent to some four-dimensional quantum field theory coupled to gravity on the brane. In this scenario, the Hawking radiation from a four-dimensional black hole is largely enhanced due to existence of the new quantum fields, and the enhancement factor is related to the extradimension scale. We would like to discuss observational consequences of this scenario, especially focusing on gravitational wave observations. Black hole binaries are typical targets for the gravitational wave detectors, and those binaries will lose their energy and angular momentum due to the enhanced Hawking radiation. This effect will be detectable if the enhancement factor, i.e., the extradimension scale, is sufficiently large. Observations of this effect may give a new constraints on the extradimension scale. In the presentation, we will introduce constraints on the extradimension scale by observations of LISA which was given by McWilliams (arXiv:0912.4744) recently, and discuss further constraints from BH/BH or NS/BH binary observations by DECIGO.
  • Date : 9 (Wed) June, 2010, 14:00-18:00
    Place : IPMU Seminar room B, Kashiwa Campus of the University of Tokyo
    14:00-15:00 Sanjay Jhingan (Jamia Millia Islamia)
    Title: Gravitational Collapse: Black Holes and Visible Singularities
    Abstract: TBA
    15:00-15:30 Break
    15:30-16:30 Christopher S. Gauthier (Michigan Center for Theoretical Physics)
    Title: K-essence Interactions with Neutrinos: Flavor Oscillations without Mass
    Abstract:In this talk we discuss a novel means of coupling neutrinos to a Lorentz violating background k-essence field. K-essence is a model of dark energy, which uses a non-canonical scalar field to drive the late time accelerated expansion of the universe. We propose that neutrinos couple to the k-essence induced metric G_{\mu\nu}, rather than the space-time metric g_{\mu\nu} The immediate effect that this has will be to modify the energy-momentum relation of the neutrino. This implies that the neutrino velocity will in general be different from the speed of light, even if the neutrino is massless. Later we will see that k-essence can also induce neutrino oscillations even without a neutrino mass term. It will be shown that if k-essence couples non-diagonally to the neutrino flavor eigenstates, then this leads to an oscillation length that goes like \lambda\sim E^{-1}, where E is the neutrino energy. This should be contrasted with the case of purely mass induced neutrino oscillations, which result in a \lambda\sim E type behavior. Thus, k-essence induced neutrino oscillations have a very different observational signature than neutrino oscillations created by mass. However, observations favor a leading order \lambda\sim E behavior. While k-essence induced neutrino oscillations are not favored experimentally, our result places tight constraints on the possible interaction that a k-essence background can have with neutrinos. All nontrivial physical effects discussed here require the sound speed of k-essence fluctuations to be different from the speed of light, and hence are primarily a consequence of Lorentz violation.
    16:30-17:00 Break
    17:00-18:00 Giovanni Marozzi (Institut d'Astrophysique de Paris)
    Title: Covariant and gauge invariant formulation of the cosmological backreaction
    Abstract: I will show, using a gauge invariant prescription to average scalar quantities, a general-covariant formulation of the so-called cosmological "backreaction". These effective covariant equations allow us to describe in explicitly gauge invariant form the way classical or quantum inhomogeneities affect the average evolution of our Universe.
  • Date : 21 (Fri.) May, 2010, 14:00-17:30
    Place : Waseda university, North tower of Building 55, 2F-Conference room
    14:00-15:30 Frans Klinkhamer (Karlsruhe Institute of Technology)
    Title: Two universes from q-theory
    Abstract: The q-theory approach to the main Cosmological Constant Problem gives a general explanation of how the gravitating vacuum energy density can be self-adjusted to zero in an equilibrium state. Perturbations away from the equilibrium state may result in a small positive value of the effective vacuum energy density. Two possible types of perturbations are considered with energy scales set by QCD and electroweak physics. This leads to two possible universes. The first corresponds to a QCD-scale modified-gravity model, which may give a satisfactory description of the present Universe, both qualitatively and quantitatively. The second corresponds to an effective Lambda-CDM model universe with a calculated value for the effective cosmological constant Lambda which is of the correct order of magnitude, provided there is new TeV-scale physics.
    15:30-16:00 Break
    16:00-17:30 Yoshihisa Kitazawa (KEK)
    Title: Boltzmann equation in de Sitter space
    Abstract: In a time dependent background like de Sitter space, Feynman-Dyson perturbation theory breaks down due to infra-red divergences. We investigate an interacting scalar field theory in Schwinger-Keldysh formalism. We derive a Boltzmann equation from a Schwinger-Dyson equation inside the cosmological horizon. Our solution shows that the particle production is compensated by the reduction of the on-shell states due to unitarity. Although the degrees of freedom inside the horizon leads to a small and diminishing screening effect of the cosmological constant, there is a growing screening effect from those outside the horizon.
  • Date: 23 (Fri.) April, 2010, 14:00-17:30
    Place: ICRR, University of Tokyo, Room 601 (Kashiwa Campus)
    14:00-15:00 Masahide Yamaguchi (Tokyo Inst. of Tech.)
    Title: "Dark energy and inflation"
    Abstract: We are going to discuss the relation between dark energy and inflation.
    15:00-15:30 break
    15:30-16:30 Shinji Tsujikawa (Tokyo Univ. of Sci.)
    Title: "Dark energy and modifications of gravity"
    Abstract: The current accelerated expansion of the Universe may originate from some modification to Einstein gravity. In these theories the laws of gravity are modified so that the cosmic acceleration is realized without recourse to a dark energy component, a fact which makes these models attractive. We discuss the cosmological viability of modified gravity dark energy models as well as the consistency with local gravity constraints. Our analysis includes a wide variety of dark energy models such as f(R) gravity and scalar-tensor theories. We also study observational signatures of such modified gravity models and show that these models are in general strongly constrained from observations such as galaxy clustering, cosmic microwave background, and weak lensing.
  • Date: 9 (Tue.), February, 2010, 14:00-17:30
    Place: 4th Bldg Seminar Hall (1st floor) KEK Tsukuba Campus
    14:00-15:30 Yudai Suwa (U Tokyo/KEK)
    Title: "Memory effect of gravitational waves"
    Abstract:The gravitational waves have ``memory effect'', which leads a constantoffset of gravitational signals. In this talk, the formation of memoryby anisotropic energy emission (especially for neutrinos fromastrophysical object) will be discussed.
    15:30-16:00 break
    16:00-17:30 Chiang-Mei Chen (NCU Taiwan/KEK)
    Title: "The RN/CFT correspondence"
    Abstract:We discuss the holographic dual CFT (conformal field theory) of RN(Reissner-Nordstrom) black holes. Firstly we compute the right-movingcentral charge of CFT from the dimensional reduced 2D effectiveaction. We further verify the duality by matching the absorption crosssection of a scalar field propagating in a near extremal RN black holeand the two point function of dual CFT operator.
  • Date: 26 (Tue.), January, 2010
    Place: Room 4151, Ground floor, Rikkyo University (Ikebukuro), Building No 4, Annex
    14:30-16:00 Bernard Carr (Queen Mary University of London/RESCEU)
    Title: NEW COSMOLOGICAL CONSTRAINTS ON PRIMORDIAL BLACK HOLES
    Abstract:Constraints on the formation of primordial black holes - especiallythe ones which are small enough to evaporate - provide a unique probeof the early universe and high energy physics. In this talk I willreview some of the constraints discussed in my recent paper with Kohri,Sendouda and Yokoyama. For evaporating black holes, the dominant onesare associated with big bang nucleosynthesis and the extragalacticphoton background, but there are also other limits associated with thecosmic microwave background, cosmic rays and various types of relicparticles. For larger non-evaporating black holes, important constraintscome from their gravitational and astrophysical effects.
    16:00-16:30 break
    16:30-18:00 Umpei Miyamoto (Rikkyo University)
    Title: Phases of higher-dimensional black holes from fluid/solid mechanics
    Abstract:Recently, importance of black-hole studies in higher-dimensional GRand string/M theory is increasing both from theoretical andexperimental points of view. In this talk, I will talk about how wecan learn about the stability, phase diagrams, and dynamics ofhigher-dimensional black holes from similar fluid/solid phenomena.Then, I will show the background physics behind the similaritiesbetween the black holes and fluid lumps from the view point of arecent progress in the gauge/gravity correspondence.
  • 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: Scalar field in the anisotropic Universe

    Abstract: We discuss the quantization and behavior of a scalar field in the Bianchi-I and Bianchi-IX cosmological models.
    16:30 - 17:30 Antonio Enea Romano

    Title: Apparent cosmological acceleration, dark energy and inhomogeneities.

    Abstract: It well known that inhomogeneous spherically symmetric cosmological models (LTB) can explain the luminosity distance data without invoking dark energy.
    In this context we first clarify the relation between apparent cosmological acceleration $a_{\Lambda CDM}$ and spatially averaged acceleration $a_D$ , and show that in general a positive $a_D$ does not correspond to models which are able to fit observational data, giving some examples.
    We then present some new analytical results in the low red-shift regime for the luminosity distance $D_L(z)$ and the red-shift spherical shell energy density $mn(z)$, and introduce a general approach to the solution of the inversion problem of mapping a LTB model to observations.
  • 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 - 17:00 Contributed 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"

Contact: joint_cosmo@ipmu.jp