Inspired by the Randall-Sundrum brane-world scenario, we investigate the possibility of braneworl... more Inspired by the Randall-Sundrum brane-world scenario, we investigate the possibility of braneworld inflation driven not by an inflaton field on the brane, but by a bulk, dilaton-like gravitational field. As a toy model for the dilaton-like gravitational field, we consider a minimally coupled massive scalar field in the bulk 5-dimensional spacetime, and look for a perturbative solution in the anti-de Sitter (AdS) background. For an adequate range of the scalar field mass, we find a unique solution that has non-trivial dependence on the 5th dimensional coordinate and that induces slow-roll inflation on the brane.
Journal of Cosmology and Astroparticle Physics, Aug 3, 2017
We investigate in details the gravitational wave (GW) from the first-order phase transition (PT) ... more We investigate in details the gravitational wave (GW) from the first-order phase transition (PT) in the extended standard model of particle physics with a dimension-six operator, which is capable of exhibiting the recently discovered slow first-order PT in addition to the usually studied fast first-order PT. To simplify the discussion, it is sufficient to work with an example of a toy model with the sextic term, and we propose an unified description for both slow and fast first-order PTs. We next study the full one-loop effective potential of the model with fixed/running renormalization-group (RG) scales. Compared to the prediction of GW energy density spectrum from the fixed RG scale, we find that the presence of running RG scale could amplify the peak amplitude by amount of one order of magnitude while shift the peak frequency to the lower frequency regime, and the promising regime of detection within the sensitivity ranges of various space-based GW detectors shrinks down to a lower cutoff value of the sextic term rather than the previous expectation.
The inflationary 1-loop tensor power spectrum from an excited spectator scalar field is calculate... more The inflationary 1-loop tensor power spectrum from an excited spectator scalar field is calculated. Recent studies on primordial black holes suggest that the inflationary curvature perturbation may be huge on small scales. An enhanced curvature perturbation may arise from a drastic enhancement of spectator scalar field fluctuations. In this letter, using the in-in formalism, we calculate 1-loop quantum corrections to primordial gravitational waves by such an excited spectator field with a sharp peak in momentum space. We find scale-invariant loop corrections in this full quantum setup, in contrast to the sharply peaked corrections in the previously calculated scalar-induced tensor modes. Especially on super Hubble scales, the primordial gravitational waves are also amplified, which can be understood as a Bogoliubov transformation of the vacuum due to the excited scalar field. This mechanism allows us to probe the scalar field properties on extremely short-distance scales with the current and future cosmic microwave background and gravitational wave experiments, opening a novel window for inflationary cosmology.
Previous studies concerning the interaction of branes and black holes suggested that a small blac... more Previous studies concerning the interaction of branes and black holes suggested that a small black hole intersecting a brane may escape via a mechanism of reconnection. Here we consider this problem by studying the interaction of a small black hole and a domain wall composed of a scalar field and simulate the evolution of this system when the black hole acquires an initial recoil velocity. We test and confirm previous results, however, unlike the cases previously studied, in the more general set-up considered here, we are able to follow the evolution of the system also during the separation, and completely illustrate how the escape of the black hole takes place.
We argue that dark energy with multiple fields is theoretically well-motivated and predicts disti... more We argue that dark energy with multiple fields is theoretically well-motivated and predicts distinct observational signatures, in particular when cosmic acceleration takes place along a trajectory that is highly non-geodesic in field space. Such models provide novel physics compared to ΛCDM and quintessence by allowing cosmic acceleration on steep potentials. From the theoretical point of view, these theories can easily satisfy the conjectured swampland constraints and may in certain cases be technically natural, potential problems which are endemic to standard single-field dark energy. Observationally, we argue that while such multi-field models are likely to be largely indistinguishable from the concordance cosmology at the background level, dark energy perturbations can cluster, leading to an enhanced growth of large-scale structure that may be testable as early as the next generation of cosmological surveys.
There have been various interpretations of Hawking radiation proposed based on the perturbative a... more There have been various interpretations of Hawking radiation proposed based on the perturbative approach, and all have confirmed Hawking's original finding. One major conceptual challenge of Hawking evaporation is the associated black hole information loss paradox, which remains unresolved. A key factor to the issue is the endstage of the black hole evaporation. Unfortunately by then the evaporation process becomes non-perturbative. Aspired to provide a tool for the eventual solution to this problem, here we introduce a new interpretation of Hawking radiation as the tunneling of instantons. We study instantons of a massless scalar field in Einstein gravity. We consider a complex-valued instanton that connects an initial pure black hole state to a black hole with a scalar field that represents the Hawking radiation at future null infinity, where its action depends only on the areal entropy difference. By comparing it with several independent approaches to Hawking radiation in the perturbative limit, we conclude that Hawking radiation may indeed be described by a family of instantons. Since the instanton approach can describe non-perturbative processes, we hope that our new interpretation and holistic method may shed lights on the information loss problem.
Journal of Cosmology and Astroparticle Physics, 2018
According to the equivalence principal, the long wavelength perturbations must not have any dynam... more According to the equivalence principal, the long wavelength perturbations must not have any dynamical effect on the short scale physics up to O(k 2 L /k 2 s). Their effect can be always absorbed to a coordinate transformation locally. So any physical effect of such a perturbation appears only on scales larger than the scale of the perturbation. The bispectrum in the squeezed limit of the curvature perturbation in single-field slow-roll inflation is a good example, where the long wavelength effect is encoded in the spectral index through Maldacena's consistency relation. This implies that one should be able to derive the bispectrum in the squeezed limit without resorting to the in-in formalism in which one computes perturbative corrections field-theoretically. In this short paper, we show that the δN formalism as it is, or more generically the separate universe approach, when applied carefully can indeed lead to the correct result for the bispectrum in the squeezed limit. Hence despite the common belief that the δN formalism is incapable of recovering the consistency relation within itself, it is in fact self-contained and consistent.
International Journal of Modern Physics: Conference Series, 2011
The open inflationary scenario is attracting a renewed interest in the context of string landscap... more The open inflationary scenario is attracting a renewed interest in the context of string landscape. Since there are a large number of metastable de Sitter vacua in string landscape, tunneling transitions to lower metastable vacua through the bubble nucleation occur quite naturally, which leads to a natural realization of open inflation. Recently, it was argued anthropically that string landscape would lead to an estimate of the density parameter in the range 0.998 ~ 0.9996. Although the deviation of Ω0 from unity, the effect of this small deviation on the CMB anisotropies might be significant. If their estimate of the value of Ω0 is correct, although yet there is no consensus about the problem of the probability measure, we might be able to discriminate the string landscape scenario from others. The effect of this small deviation on the large angle CMB anisotropies may be significant for large angle mode in tensor-type perturbation. We found that the wall fluctuation mode gives sign...
In General Relativity, the constraint equation relating metric and density perturbations is inher... more In General Relativity, the constraint equation relating metric and density perturbations is inherently nonlinear, leading to an effective non-Gaussianity in the dark matter density field on large scales-even if the primordial metric perturbation is Gaussian. Intrinsic non-Gaussianity in the large-scale dark matter overdensity in GR is real and physical. However, the variance smoothed on a local physical scale is not correlated with the largescale curvature perturbation, so that there is no relativistic signature in the galaxy bias when using the simplest model of bias. It is an open question whether the observable mass proxies such as luminosity or weak lensing correspond directly to the physical mass in the simple halo bias model. If not, there may be observables that encode this relativistic signature. I. INTRODUCTION In Newtonian gravity, the Poisson equation is a linear relation between the gravitational potential and the matter overdensity. By contrast, in General Relativity (GR) this is replaced by a nonlinear relation, which introduces mode coupling between large and small scales [1-3]. The original result has been confirmed by a number of independent calculations [4-9]. Similar mode coupling can be produced in Newtonian gravity by local-type primordial non-Gaussianity of the gravitational potential [10, 11]. The GR effect has therefore previously been interpreted as an effective local non-Gaussianity on very large scales [6, 12-15]. Recently two papers have argued that a "separate universe" approach can be used to show that no scale-dependent bias arises from the GR corrections on large scales [16, 17]. The argument is that the nonlinear coupling between long-wavelength perturbations on a scale λ L , and the small-scale variance, σ 2 S = δ 2 S , on a scale λ S , vanishes under a local coordinate rescaling and hence is unobservable. The separate universe approach [18, 19] has proved to be a powerful tool to understand the origin of large-scale structure, and primordial non-Gaussianity, from inflation. Accelerated expansion in the very early Universe stretches initial small-scale vacuum fluctuations up to scales much larger than the Hubble scale at the end of inflation. Spatial gradients for such longwavelength modes become small relative to the local Hubble time, and for many scales of interest, the perturbed universe can be treated as a patchwork of "separate universes", each locally obeying the classical Friedmann-Lemaître-Robertson-Walker (FLRW) evolution of an unperturbed universe. The separate universe approach is particularly useful for studying nonlinear perturbations on large scales [18, 20]. For adiabatic perturbations, each separate universe patch follows locally the same evolution as the unperturbed "background" cosmology. The only difference between separate patches is the local expansion, characterised by the comoving metric perturbation ζ. This is defined to be the local perturbation of the integrated expansion rate with respect to a background flat reference cosmology, δN = N −N , where N = dt Θ/3. An important consequence of the uniqueness of the local evolution for adiabatic perturbations is that ζ is conserved on large scales where the separate universe approach is valid [19, 21, 22].
Quintessential inflation explains both inflation and present accelerated expansion. In this model... more Quintessential inflation explains both inflation and present accelerated expansion. In this model these two accelerated expansion is driven by a single scalar field. We construct a model based on an exponential potential. We consider two kinds of reheating process. One is the ordinary reheating in quintessential inflation, using gravitational particle creation. The other is the instant preheating which is much more effective. We calculate the relic gravitational waves produced during inflation in our model. We find that the spectrum of the gravitational waves is quite sensitive to the reheating process. Conversely, the detection or non-detection of primordial gravitational waves at ∼100MHz would have profound implications for the inflationary cosmology.
We study the Gregory–Laflamme instability of a five-dimensional slowly rotating black string in w... more We study the Gregory–Laflamme instability of a five-dimensional slowly rotating black string in which the four-dimensional section is described by the Kerr black hole. We treat the rotation in a perturbative way introducing a small parameter for the rotation. It is found that rotation makes the Gregory–Laflamme instability stronger. Both the critical wavelength at the onset of instability and the growth time-scale are found to decrease as the rotation increases.
Recently, the primordial non-Gaussianity in the curvaton model has been predicted assuming sudden... more Recently, the primordial non-Gaussianity in the curvaton model has been predicted assuming sudden decay of the curvaton. We extend the calculation to non-instantaneous decay by employing δN formalism. The difference between the sudden-decay approximation and our numerical result is larger than 1% only if the non-linearity parameter is small, −1.16 < fNL < 60. Thus it is safe to use the sudden-decay approximation when deriving constraints for the curvaton model from WMAP3 (fNL < 114), but with the Planck forecast |fNL| < 5 one should employ the fully numerical result. Often, the curvaton perturbations δσ have been assumed to be small compared to the background value of the curvaton field σ0. Consequently, the variance ∆ 2 = δσ 2 /σ 2 0 has been assumed to be negligible. However, the measurements of CMB or large-scale structure perturbation amplitude do not constrain the variance if the main contribution to it comes from the ultraviolet (UV) scales, i.e., from smaller than observable scales. We discuss how, even in this case, observational constraints on non-Gaussianity set an upper bound to the small scale variance, ∆ 2 U V < 90.
We propose the Gauss-Bonnet dark energy model inspired by string/M-theory where standard gravity ... more We propose the Gauss-Bonnet dark energy model inspired by string/M-theory where standard gravity with scalar contains additional scalar-dependent coupling with Gauss-Bonnet invariant. It is demonstrated that effective phantom (or quintessence) phase of late universe may occur in the presence of such term when the scalar is phantom or for non-zero potential (for canonical scalar). However, with the increase of the curvature the GB term may become dominant so that phantom phase is transient and w = −1 barrier may be passed. Hence, the current acceleration of the universe may be caused by mixture of scalar phantom and (or) potential/stringy effects. It is remarkable that scalar-Gauss-Bonnet coupling acts against the Big Rip occurence in phantom cosmology.
We consider a brane world in an arbitrary number of dimensions without Z 2 symmetry and derive th... more We consider a brane world in an arbitrary number of dimensions without Z 2 symmetry and derive the effective Einstein equation on the brane, where its right-hand side is given by the matter on the brane and the curvature in the bulk. This is achieved by first deriving the junction conditions for a non-Z 2 symmetric brane and second solving the Gauss equation, which relates the mean extrinsic curvature of the brane to the curvature in the bulk, with respect to the mean extrinsic curvature. The latter corresponds to formulating an explicit junction condition on the mean of the extrinsic curvature, analogous to the Israel junction condition for the jump of the extrinsic curvature. We find that there appears a new type of effective anisotropic fluid on the right-hand side of the effective Einstein equation due to the fact that there is no Z 2 symmetry. The derived equation is a basic equation for the study of Kaluza-Klein brane worlds in which some dimensions on the brane are compactified or for a regularization scheme for a higher codimension brane world, where the Kaluza-Klein compactification on the brane is regarded as a means to regularize the uncontrollable spacetime singularity created by the higher codimension brane.
A black hole has characteristic quasi-normal modes that are excited when it is formed or when the... more A black hole has characteristic quasi-normal modes that are excited when it is formed or when the geometry is perturbed. The state of a black hole when the quasi-normal modes are excited is called 'gravitational ringing', and detections of it constitute a direct confirmation of the existence of black holes. To detect it, a method based on matched filtering needs to be developed. Generically, matched filtering requires a large number of templates, because one has to ensure the proper match of a real gravitational wave with one of template waveforms to keep the detection efficiency as high as possible. On the other hand, the number of templates must be kept as small as possible due to limits in computational capability. In a previous paper, assuming that the gravitational ringing is dominated by the least-damped (fundamental) mode with the smallest imaginary part of the frequency, we constructed an efficient method for tiling the template space. However, the dependence of the template space metric on the initial phase of the wave was not taken into account. This dependence arises because of an unavoidable mismatch between the parameters of the signal waveform and those given discretely in the template space. In this paper, we properly take this dependence into account and present an improved, efficient search method for the gravitational ringing of black holes. In practice, by using the inner product (2. 7) of two functions, we can obtain the root-mean-square errors and correlation coefficients of the parameters.
We investigate the Kaluza-Klein braneworld cosmology from the point of view of observers on the b... more We investigate the Kaluza-Klein braneworld cosmology from the point of view of observers on the brane. We first generalize the Shiromizu-Maeda-Sasaki (SMS) equations to higher dimensions. As an application, we study a (4 + n)-dimensional brane with n dimensions compactified on the brane, in a (5 + n)-dimensional bulk. By assuming that the size of the internal space is static, that the bulk energy-momentum tensor can be ignored, we determine the effect of the bulk geometry on the Kaluza-Klein braneworld. Then we derive the effective Friedmann equation on the brane. It turns out that the Friedmann equation explicitly depends on the equation of state, in contrast to the braneworld in a 5-dimensional bulk spacetime. In particular, in a radiation-dominated era, the effective Newton constant depends logarithmically on the scale factor. If we include a pressureless matter on the brane, this dependence disappears after the radiation-matter equality. This may be interpreted as stabilization of the Newton constant by the matter on the brane. Our findings imply that the Kaluza-Klein braneworld cosmology is quite different from the conventional Kaluza-Klein cosmology even at low energy.
In gravity theories derived from a f (R) Lagrangian, matter is usually supposed to be minimally c... more In gravity theories derived from a f (R) Lagrangian, matter is usually supposed to be minimally coupled to the metric, which hence defines a "Jordan frame." However, since the field equations are fourth order, gravity possesses an extra degree of freedom on top of the standard graviton, as is manifest from its equivalent description in the conformally related, Einstein, frame. We introduce explicitly this extra scalar degree of freedom in the action and couple it to matter, so that the original metric no longer defines a Jordan frame. This "detuning" puts f (R) gravity into a wider class of scalar-tensor theories. We argue that a "chameleon-like" detuning tracing the background matter density may provide purely gravitational models which account for the present acceleration of the universe and evade local gravity constraints.
Previous studies concerning the interaction of branes and black holes suggested that a small blac... more Previous studies concerning the interaction of branes and black holes suggested that a small black hole intersecting a brane may escape via a mechanism of reconnection. Here we consider this problem by studying the interaction of a small black hole and a domain wall composed of a scalar field and simulate the evolution of this system when the black hole acquires an initial recoil velocity. We test and confirm previous results, however, unlike the cases previously studied, in the more general setup considered here, we are able to follow the evolution of the system also during the separation, and completely illustrate how the escape of the black hole takes place.
In recent work we have shown that a black hole stacked on a brane escapes once it acquires a reco... more In recent work we have shown that a black hole stacked on a brane escapes once it acquires a recoil velocity. This result was obtained in the probe-brane approximation, i.e., when the tension of the brane is negligibly small. Therefore, it is not clear whether the effect of the brane tension may prevent the black hole from escaping for small recoil velocities. The question is whether a critical escape velocity exists. Here, we analyze this problem by studying the interaction between a Dirac-Nambu-Goto brane and a black hole assuming adiabatic (quasi-static) evolution. By describing the brane in a fixed black hole spacetime, which restricts our conclusions to lowest order effects in the tension, we find that the critical escape velocity does not exist for co-dimension one branes, while it does for higher co-dimension branes.
Inspired by the Randall-Sundrum brane-world scenario, we investigate the possibility of braneworl... more Inspired by the Randall-Sundrum brane-world scenario, we investigate the possibility of braneworld inflation driven not by an inflaton field on the brane, but by a bulk, dilaton-like gravitational field. As a toy model for the dilaton-like gravitational field, we consider a minimally coupled massive scalar field in the bulk 5-dimensional spacetime, and look for a perturbative solution in the anti-de Sitter (AdS) background. For an adequate range of the scalar field mass, we find a unique solution that has non-trivial dependence on the 5th dimensional coordinate and that induces slow-roll inflation on the brane.
Journal of Cosmology and Astroparticle Physics, Aug 3, 2017
We investigate in details the gravitational wave (GW) from the first-order phase transition (PT) ... more We investigate in details the gravitational wave (GW) from the first-order phase transition (PT) in the extended standard model of particle physics with a dimension-six operator, which is capable of exhibiting the recently discovered slow first-order PT in addition to the usually studied fast first-order PT. To simplify the discussion, it is sufficient to work with an example of a toy model with the sextic term, and we propose an unified description for both slow and fast first-order PTs. We next study the full one-loop effective potential of the model with fixed/running renormalization-group (RG) scales. Compared to the prediction of GW energy density spectrum from the fixed RG scale, we find that the presence of running RG scale could amplify the peak amplitude by amount of one order of magnitude while shift the peak frequency to the lower frequency regime, and the promising regime of detection within the sensitivity ranges of various space-based GW detectors shrinks down to a lower cutoff value of the sextic term rather than the previous expectation.
The inflationary 1-loop tensor power spectrum from an excited spectator scalar field is calculate... more The inflationary 1-loop tensor power spectrum from an excited spectator scalar field is calculated. Recent studies on primordial black holes suggest that the inflationary curvature perturbation may be huge on small scales. An enhanced curvature perturbation may arise from a drastic enhancement of spectator scalar field fluctuations. In this letter, using the in-in formalism, we calculate 1-loop quantum corrections to primordial gravitational waves by such an excited spectator field with a sharp peak in momentum space. We find scale-invariant loop corrections in this full quantum setup, in contrast to the sharply peaked corrections in the previously calculated scalar-induced tensor modes. Especially on super Hubble scales, the primordial gravitational waves are also amplified, which can be understood as a Bogoliubov transformation of the vacuum due to the excited scalar field. This mechanism allows us to probe the scalar field properties on extremely short-distance scales with the current and future cosmic microwave background and gravitational wave experiments, opening a novel window for inflationary cosmology.
Previous studies concerning the interaction of branes and black holes suggested that a small blac... more Previous studies concerning the interaction of branes and black holes suggested that a small black hole intersecting a brane may escape via a mechanism of reconnection. Here we consider this problem by studying the interaction of a small black hole and a domain wall composed of a scalar field and simulate the evolution of this system when the black hole acquires an initial recoil velocity. We test and confirm previous results, however, unlike the cases previously studied, in the more general set-up considered here, we are able to follow the evolution of the system also during the separation, and completely illustrate how the escape of the black hole takes place.
We argue that dark energy with multiple fields is theoretically well-motivated and predicts disti... more We argue that dark energy with multiple fields is theoretically well-motivated and predicts distinct observational signatures, in particular when cosmic acceleration takes place along a trajectory that is highly non-geodesic in field space. Such models provide novel physics compared to ΛCDM and quintessence by allowing cosmic acceleration on steep potentials. From the theoretical point of view, these theories can easily satisfy the conjectured swampland constraints and may in certain cases be technically natural, potential problems which are endemic to standard single-field dark energy. Observationally, we argue that while such multi-field models are likely to be largely indistinguishable from the concordance cosmology at the background level, dark energy perturbations can cluster, leading to an enhanced growth of large-scale structure that may be testable as early as the next generation of cosmological surveys.
There have been various interpretations of Hawking radiation proposed based on the perturbative a... more There have been various interpretations of Hawking radiation proposed based on the perturbative approach, and all have confirmed Hawking's original finding. One major conceptual challenge of Hawking evaporation is the associated black hole information loss paradox, which remains unresolved. A key factor to the issue is the endstage of the black hole evaporation. Unfortunately by then the evaporation process becomes non-perturbative. Aspired to provide a tool for the eventual solution to this problem, here we introduce a new interpretation of Hawking radiation as the tunneling of instantons. We study instantons of a massless scalar field in Einstein gravity. We consider a complex-valued instanton that connects an initial pure black hole state to a black hole with a scalar field that represents the Hawking radiation at future null infinity, where its action depends only on the areal entropy difference. By comparing it with several independent approaches to Hawking radiation in the perturbative limit, we conclude that Hawking radiation may indeed be described by a family of instantons. Since the instanton approach can describe non-perturbative processes, we hope that our new interpretation and holistic method may shed lights on the information loss problem.
Journal of Cosmology and Astroparticle Physics, 2018
According to the equivalence principal, the long wavelength perturbations must not have any dynam... more According to the equivalence principal, the long wavelength perturbations must not have any dynamical effect on the short scale physics up to O(k 2 L /k 2 s). Their effect can be always absorbed to a coordinate transformation locally. So any physical effect of such a perturbation appears only on scales larger than the scale of the perturbation. The bispectrum in the squeezed limit of the curvature perturbation in single-field slow-roll inflation is a good example, where the long wavelength effect is encoded in the spectral index through Maldacena's consistency relation. This implies that one should be able to derive the bispectrum in the squeezed limit without resorting to the in-in formalism in which one computes perturbative corrections field-theoretically. In this short paper, we show that the δN formalism as it is, or more generically the separate universe approach, when applied carefully can indeed lead to the correct result for the bispectrum in the squeezed limit. Hence despite the common belief that the δN formalism is incapable of recovering the consistency relation within itself, it is in fact self-contained and consistent.
International Journal of Modern Physics: Conference Series, 2011
The open inflationary scenario is attracting a renewed interest in the context of string landscap... more The open inflationary scenario is attracting a renewed interest in the context of string landscape. Since there are a large number of metastable de Sitter vacua in string landscape, tunneling transitions to lower metastable vacua through the bubble nucleation occur quite naturally, which leads to a natural realization of open inflation. Recently, it was argued anthropically that string landscape would lead to an estimate of the density parameter in the range 0.998 ~ 0.9996. Although the deviation of Ω0 from unity, the effect of this small deviation on the CMB anisotropies might be significant. If their estimate of the value of Ω0 is correct, although yet there is no consensus about the problem of the probability measure, we might be able to discriminate the string landscape scenario from others. The effect of this small deviation on the large angle CMB anisotropies may be significant for large angle mode in tensor-type perturbation. We found that the wall fluctuation mode gives sign...
In General Relativity, the constraint equation relating metric and density perturbations is inher... more In General Relativity, the constraint equation relating metric and density perturbations is inherently nonlinear, leading to an effective non-Gaussianity in the dark matter density field on large scales-even if the primordial metric perturbation is Gaussian. Intrinsic non-Gaussianity in the large-scale dark matter overdensity in GR is real and physical. However, the variance smoothed on a local physical scale is not correlated with the largescale curvature perturbation, so that there is no relativistic signature in the galaxy bias when using the simplest model of bias. It is an open question whether the observable mass proxies such as luminosity or weak lensing correspond directly to the physical mass in the simple halo bias model. If not, there may be observables that encode this relativistic signature. I. INTRODUCTION In Newtonian gravity, the Poisson equation is a linear relation between the gravitational potential and the matter overdensity. By contrast, in General Relativity (GR) this is replaced by a nonlinear relation, which introduces mode coupling between large and small scales [1-3]. The original result has been confirmed by a number of independent calculations [4-9]. Similar mode coupling can be produced in Newtonian gravity by local-type primordial non-Gaussianity of the gravitational potential [10, 11]. The GR effect has therefore previously been interpreted as an effective local non-Gaussianity on very large scales [6, 12-15]. Recently two papers have argued that a "separate universe" approach can be used to show that no scale-dependent bias arises from the GR corrections on large scales [16, 17]. The argument is that the nonlinear coupling between long-wavelength perturbations on a scale λ L , and the small-scale variance, σ 2 S = δ 2 S , on a scale λ S , vanishes under a local coordinate rescaling and hence is unobservable. The separate universe approach [18, 19] has proved to be a powerful tool to understand the origin of large-scale structure, and primordial non-Gaussianity, from inflation. Accelerated expansion in the very early Universe stretches initial small-scale vacuum fluctuations up to scales much larger than the Hubble scale at the end of inflation. Spatial gradients for such longwavelength modes become small relative to the local Hubble time, and for many scales of interest, the perturbed universe can be treated as a patchwork of "separate universes", each locally obeying the classical Friedmann-Lemaître-Robertson-Walker (FLRW) evolution of an unperturbed universe. The separate universe approach is particularly useful for studying nonlinear perturbations on large scales [18, 20]. For adiabatic perturbations, each separate universe patch follows locally the same evolution as the unperturbed "background" cosmology. The only difference between separate patches is the local expansion, characterised by the comoving metric perturbation ζ. This is defined to be the local perturbation of the integrated expansion rate with respect to a background flat reference cosmology, δN = N −N , where N = dt Θ/3. An important consequence of the uniqueness of the local evolution for adiabatic perturbations is that ζ is conserved on large scales where the separate universe approach is valid [19, 21, 22].
Quintessential inflation explains both inflation and present accelerated expansion. In this model... more Quintessential inflation explains both inflation and present accelerated expansion. In this model these two accelerated expansion is driven by a single scalar field. We construct a model based on an exponential potential. We consider two kinds of reheating process. One is the ordinary reheating in quintessential inflation, using gravitational particle creation. The other is the instant preheating which is much more effective. We calculate the relic gravitational waves produced during inflation in our model. We find that the spectrum of the gravitational waves is quite sensitive to the reheating process. Conversely, the detection or non-detection of primordial gravitational waves at ∼100MHz would have profound implications for the inflationary cosmology.
We study the Gregory–Laflamme instability of a five-dimensional slowly rotating black string in w... more We study the Gregory–Laflamme instability of a five-dimensional slowly rotating black string in which the four-dimensional section is described by the Kerr black hole. We treat the rotation in a perturbative way introducing a small parameter for the rotation. It is found that rotation makes the Gregory–Laflamme instability stronger. Both the critical wavelength at the onset of instability and the growth time-scale are found to decrease as the rotation increases.
Recently, the primordial non-Gaussianity in the curvaton model has been predicted assuming sudden... more Recently, the primordial non-Gaussianity in the curvaton model has been predicted assuming sudden decay of the curvaton. We extend the calculation to non-instantaneous decay by employing δN formalism. The difference between the sudden-decay approximation and our numerical result is larger than 1% only if the non-linearity parameter is small, −1.16 < fNL < 60. Thus it is safe to use the sudden-decay approximation when deriving constraints for the curvaton model from WMAP3 (fNL < 114), but with the Planck forecast |fNL| < 5 one should employ the fully numerical result. Often, the curvaton perturbations δσ have been assumed to be small compared to the background value of the curvaton field σ0. Consequently, the variance ∆ 2 = δσ 2 /σ 2 0 has been assumed to be negligible. However, the measurements of CMB or large-scale structure perturbation amplitude do not constrain the variance if the main contribution to it comes from the ultraviolet (UV) scales, i.e., from smaller than observable scales. We discuss how, even in this case, observational constraints on non-Gaussianity set an upper bound to the small scale variance, ∆ 2 U V < 90.
We propose the Gauss-Bonnet dark energy model inspired by string/M-theory where standard gravity ... more We propose the Gauss-Bonnet dark energy model inspired by string/M-theory where standard gravity with scalar contains additional scalar-dependent coupling with Gauss-Bonnet invariant. It is demonstrated that effective phantom (or quintessence) phase of late universe may occur in the presence of such term when the scalar is phantom or for non-zero potential (for canonical scalar). However, with the increase of the curvature the GB term may become dominant so that phantom phase is transient and w = −1 barrier may be passed. Hence, the current acceleration of the universe may be caused by mixture of scalar phantom and (or) potential/stringy effects. It is remarkable that scalar-Gauss-Bonnet coupling acts against the Big Rip occurence in phantom cosmology.
We consider a brane world in an arbitrary number of dimensions without Z 2 symmetry and derive th... more We consider a brane world in an arbitrary number of dimensions without Z 2 symmetry and derive the effective Einstein equation on the brane, where its right-hand side is given by the matter on the brane and the curvature in the bulk. This is achieved by first deriving the junction conditions for a non-Z 2 symmetric brane and second solving the Gauss equation, which relates the mean extrinsic curvature of the brane to the curvature in the bulk, with respect to the mean extrinsic curvature. The latter corresponds to formulating an explicit junction condition on the mean of the extrinsic curvature, analogous to the Israel junction condition for the jump of the extrinsic curvature. We find that there appears a new type of effective anisotropic fluid on the right-hand side of the effective Einstein equation due to the fact that there is no Z 2 symmetry. The derived equation is a basic equation for the study of Kaluza-Klein brane worlds in which some dimensions on the brane are compactified or for a regularization scheme for a higher codimension brane world, where the Kaluza-Klein compactification on the brane is regarded as a means to regularize the uncontrollable spacetime singularity created by the higher codimension brane.
A black hole has characteristic quasi-normal modes that are excited when it is formed or when the... more A black hole has characteristic quasi-normal modes that are excited when it is formed or when the geometry is perturbed. The state of a black hole when the quasi-normal modes are excited is called 'gravitational ringing', and detections of it constitute a direct confirmation of the existence of black holes. To detect it, a method based on matched filtering needs to be developed. Generically, matched filtering requires a large number of templates, because one has to ensure the proper match of a real gravitational wave with one of template waveforms to keep the detection efficiency as high as possible. On the other hand, the number of templates must be kept as small as possible due to limits in computational capability. In a previous paper, assuming that the gravitational ringing is dominated by the least-damped (fundamental) mode with the smallest imaginary part of the frequency, we constructed an efficient method for tiling the template space. However, the dependence of the template space metric on the initial phase of the wave was not taken into account. This dependence arises because of an unavoidable mismatch between the parameters of the signal waveform and those given discretely in the template space. In this paper, we properly take this dependence into account and present an improved, efficient search method for the gravitational ringing of black holes. In practice, by using the inner product (2. 7) of two functions, we can obtain the root-mean-square errors and correlation coefficients of the parameters.
We investigate the Kaluza-Klein braneworld cosmology from the point of view of observers on the b... more We investigate the Kaluza-Klein braneworld cosmology from the point of view of observers on the brane. We first generalize the Shiromizu-Maeda-Sasaki (SMS) equations to higher dimensions. As an application, we study a (4 + n)-dimensional brane with n dimensions compactified on the brane, in a (5 + n)-dimensional bulk. By assuming that the size of the internal space is static, that the bulk energy-momentum tensor can be ignored, we determine the effect of the bulk geometry on the Kaluza-Klein braneworld. Then we derive the effective Friedmann equation on the brane. It turns out that the Friedmann equation explicitly depends on the equation of state, in contrast to the braneworld in a 5-dimensional bulk spacetime. In particular, in a radiation-dominated era, the effective Newton constant depends logarithmically on the scale factor. If we include a pressureless matter on the brane, this dependence disappears after the radiation-matter equality. This may be interpreted as stabilization of the Newton constant by the matter on the brane. Our findings imply that the Kaluza-Klein braneworld cosmology is quite different from the conventional Kaluza-Klein cosmology even at low energy.
In gravity theories derived from a f (R) Lagrangian, matter is usually supposed to be minimally c... more In gravity theories derived from a f (R) Lagrangian, matter is usually supposed to be minimally coupled to the metric, which hence defines a "Jordan frame." However, since the field equations are fourth order, gravity possesses an extra degree of freedom on top of the standard graviton, as is manifest from its equivalent description in the conformally related, Einstein, frame. We introduce explicitly this extra scalar degree of freedom in the action and couple it to matter, so that the original metric no longer defines a Jordan frame. This "detuning" puts f (R) gravity into a wider class of scalar-tensor theories. We argue that a "chameleon-like" detuning tracing the background matter density may provide purely gravitational models which account for the present acceleration of the universe and evade local gravity constraints.
Previous studies concerning the interaction of branes and black holes suggested that a small blac... more Previous studies concerning the interaction of branes and black holes suggested that a small black hole intersecting a brane may escape via a mechanism of reconnection. Here we consider this problem by studying the interaction of a small black hole and a domain wall composed of a scalar field and simulate the evolution of this system when the black hole acquires an initial recoil velocity. We test and confirm previous results, however, unlike the cases previously studied, in the more general setup considered here, we are able to follow the evolution of the system also during the separation, and completely illustrate how the escape of the black hole takes place.
In recent work we have shown that a black hole stacked on a brane escapes once it acquires a reco... more In recent work we have shown that a black hole stacked on a brane escapes once it acquires a recoil velocity. This result was obtained in the probe-brane approximation, i.e., when the tension of the brane is negligibly small. Therefore, it is not clear whether the effect of the brane tension may prevent the black hole from escaping for small recoil velocities. The question is whether a critical escape velocity exists. Here, we analyze this problem by studying the interaction between a Dirac-Nambu-Goto brane and a black hole assuming adiabatic (quasi-static) evolution. By describing the brane in a fixed black hole spacetime, which restricts our conclusions to lowest order effects in the tension, we find that the critical escape velocity does not exist for co-dimension one branes, while it does for higher co-dimension branes.
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Papers by Misao Sasaki