We show that, by using amplitude-based resummation techniques for Feynman's formulation of Einstein's theory, we get quantum field theoretic 'first principles' predictions for the UV fixed-point values of the dimensionless gravitational and cosmological constants. Connections to the phenomenological asymptotic safety analysis of Planck scale cosmology by Bonanno and Reuter are discussed.
Planck Scale Cosmology and Asymptotic Safety in Resummed Quantum Gravity
在Weinberg的渐近安全方法中，一个UV稳定不动点的有限维临界面产生了一个具有有限个物理参数的量子引力理论。我们认为，在费曼的原始配方的理论的延伸，我们恢复这个不动点UV行为从一个确切的重新安排各自的微扰系列。我们的结果与Alberter { \it et al.}并与Kreimer的最近的Hopf-代数Dyson-Schwinger重整化理论结果进行了比较。我们得到了无量纲引力常数和宇宙学常数的第一性原理预言，我们的结果支持了Bonanno和Schmidter的普朗克尺度宇宙学.最后，我们对目前观测到的宇宙学常数值进行了估计。
We investigate the observational signatures of quantum cosmology in the Cosmic Microwave Background data provided by Planck collaboration. We apply the warm inflationary paradigm with a tachyon scalar field to the loop quantum cosmology. In this context, we first provide the basic cosmological functions in terms of the tachyon field. We then obtain the slow-roll parameters and the power spectrum of scalar and tensor fluctuations respectively. Finally, we study the performance of various warm inflationary scenarios against the latest Planck data and we find a family of models which are in agree…
We present an overview of the phenomenological implications of the theory of resummed quantum gravity. We discuss its prediction for the cosmological constant in the context of the Planck scale cosmology of Bonanno and Reuter, its relationship to Weinberg's asymptotic safety idea, and its relationship to Weinberg's soft graviton resummation theorem. We also discuss constraints and consistency checks of the theory.
Taking into account a fractal structure for the black hole horizon, Barrow argued that the area law of entropy is modified due to quantum-gravitational effects (Barrow in Phys Lett B 808:135643, https://doi.org/10.1016/j.physletb.2020.135643, 2020). Accordingly, the corrected entropy takes the form $S \sim A^{1+\mathrmΔ/2}$, where $0\leq\mathrmΔ\leq1$, indicates the amount of the quantum-gravitational deformation effects. In this paper, based on Barrow entropy, we first derive the modified gravitational field equations through the Clausius relation. We then consider the Friedmann-Lemaître-Robe…
I provide some philosophical groundwork for the recently proposed 'trans-Planckian censorship' conjecture in theoretical physics. In particular, I argue that structure formation in early universe cosmology is, at least as we typically understand it, autonomous with regards to quantum gravity, the high energy physics that governs the Planck regime in our universe. Trans-Planckian censorship is then seen as a means of rendering this autonomy an empirical constraint within ongoing quantum gravity research.
For most initial conditions, cosmologically relevant physical modes were trans-planckian at the bounce time, often by many magnitude orders. We improve the usual loop quantum cosmology calculation of the primordial power spectra -- in the inflationary framework -- by accounting for those trans-planckian effects through modified dispersion relations. This can induce drastic changes in the spectrum, making it either compatible or incompatible with observational data, depending on the details of the choices operated.
A new perspective on the Cosmological Constant Problem (CCP) is proposed and discussed within the multiverse approach of Quantum Cosmology. It is assumed that each member of the ensemble of universes has a characteristic scale $a$ that can be used as integration variable in the partition function. An averaged characteristic scale of the ensemble is estimated by using only members that satisfy the Einstein field equations. The averaged characteristic scale is compatible with the Planck length when considering an ensemble of solutions to the Einstein field equations with an effective cosmologica…
In minisuperspace quantum cosmology, the Lorentzian path integral formulations of the no-boundary and tunneling proposals have recently been analyzed. But it has been pointed out that the wave function of linearized perturbations around a homogeneous and isotropic background is of an inverse Gaussian form and thus that their correlation functions are divergent. In this paper, we revisit this issue and consider the problem of perturbations in Lorentzian quantum cosmology by modifying the dispersion relation based on trans-Planckian physics. We consider two modified dispersion relations, the gen…
We study the implications of the recently proposed Trans-Planckian Censorship Conjecture (TCC) for early universe cosmology and in particular inflationary cosmology. The TCC leads to the conclusion that if we want inflationary cosmology to provide a successful scenario for cosmological structure formation, the energy scale of inflation has to be lower than $10^9$ GeV. Demanding the correct amplitude of the cosmological perturbations then forces the generalized slow-roll parameter $ε$ of the model to be very small ($<10^{-31}$). This leads to the prediction of a negligible amplitude of primordi…
We study quasi-matter bounce cosmology in light of Planck cosmic microwave background (CMB) angular anisotropy measurements along with the BICEP2/Keck Array data. We propose a new primordial scalar power spectrum by considering a linear approximation of the equation of state $w\cong w_0+κ(η-η_0)$ for quasi-matter field in the contracting phase of the universe. Using this new primordial scalar power spectrum, we constrain the zeroth-order approximation of the equation of state $w_0= -\,0.00340\pm 0.00044$ and first-order correction $10^{4} ζ= -1.67^{+1.50}_{-0.83}$ at the $1σ$ confidence level …
We propose that whatever quantity controls the Heisenberg uncertainty relations (for a given complementary pair of observables) it should be identified with an effective Planck parameter. With this definition it is not difficult to find examples where the Planck parameter depends on the region under study, varies in time, and even depends on which pair of observables one focuses on. In quantum cosmology the effective Planck parameter depends on the size of the comoving region under study, and so depends on that chosen region and on time. With this criterion, the classical limit is expected, no…
We search for sterile neutrinos in the holographic dark energy cosmology by using the latest observational data. To perform the analysis, we employ the current cosmological observations, including the cosmic microwave background temperature power spectrum data from the Planck mission, the baryon acoustic oscillation measurements, the type Ia supernova data, the redshift space distortion measurements, the shear data of weak lensing observation, the Planck lensing measurement, and the latest direct measurement of $H_0$ as well. We show that, compared to the $Λ$CDM cosmology, the holographic dark…
We test a class of holographic models for the very early universe against cosmological observations and find that they are competitive to the standard $Λ$CDM model of cosmology. These models are based on three dimensional perturbative super-renormalizable Quantum Field Theory (QFT), and while they predict a different power spectrum from the standard power-law used in $Λ$CDM, they still provide an excellent fit to data (within their regime of validity). By comparing the Bayesian evidence for the models, we find that $Λ$CDM does a better job globally, while the holographic models provide a (marg…
We summarize our investigations of the second-order perturbations in loop quantum cosmology (LQC). We shall discuss, primarily, two aspects. Firstly, whether the second-order contributions arising from the cosmic bounce, occurring at Planck scale, could be large enough to break the validity of perturbation theory. Secondly, the implications of the upper bounds on primordial non-Gaussianity, arrived at by the Planck collaboration, on the LQC phenomenology.
The recent Planck Legacy 2018 release has confirmed the presence of an enhanced lensing amplitude in CMB power spectra compared to that predicted in the standard $Λ$CDM model. A closed universe can provide a physical explanation for this effect, with the Planck CMB spectra now preferring a positive curvature at more than $99 \%$ C.L. Here we further investigate the evidence for a closed universe from Planck, showing that positive curvature naturally explains the anomalous lensing amplitude and demonstrating that it also removes a well-known tension within the Planck data set concerning the val…
Holographic cosmology offers a novel framework for describing the very early Universe in which cosmological predictions are expressed in terms of the observables of a three dimensional quantum field theory (QFT). This framework includes conventional slow-roll inflation, which is described in terms of a strongly coupled QFT, but it also allows for qualitatively new models for the very early Universe, where the dual QFT may be weakly coupled. The new models describe a universe which is non-geometric at early times. While standard slow-roll inflation leads to a (near-)power-law primordial power s…
Cosmological constraints on the scalar-tensor theory of gravity by analyzing the angular power spectrum data of the cosmic microwave background (CMB) obtained from the Planck 2015 results are presented. We consider the harmonic attractor model, in which the scalar field has a harmonic potential with curvature ($β$) in the Einstein frame and the theory relaxes toward the Einstein gravity with time. Analyzing the {\it TT}, {\it EE}, {\it TE} and lensing CMB data from Planck by the Markov chain Monte Carlo method, we find that the present-day deviation from the Einstein gravity (${α_0}^2$) is con…
Loop Quantum Gravity and the The Planck Regime of Cosmology
非常早期的宇宙提供了我们目前测试量子引力理论的最佳舞台。暴胀范式成功地解释了观测到的宇宙微波背景的不均匀性，这在一定程度上已经说明了这一点，因为暴胀范式是基于弯曲宇宙时空的量子场论。然而，这种分析排除了普朗克时代，因为背景时空满足爱因斯坦方程，一直回到大爆炸奇点。利用圈量子引力的技术，这个范式现在已经扩展到从普朗克状态到暴胀开始的自洽理论，涵盖了大约11个数量级的曲率。此外，对于一个狭窄的窗口的初始条件，有偏离标准的范例，与新的效果，如修改的一致性关系，涉及标量和张量功率谱和一个新的来源的非高斯性。因此，宇宙大尺度结构的起源可以追溯到量子引力波动\emph{in the Planck regime}。本报告为广义相对论界提供了这些发展的鸟瞰图。
The spatially closed Friedmann-Lemaître-Robertson-Walker model in loop quantum cosmology admits two inequivalent consistent quantizations: one based on expressing the field strength in terms of the holonomies over closed loops, and, another using a connection operator and open holonomies. Using the effective dynamics, we investigate the phenomenological differences between the two quantizations for the single fluid and the two fluid scenarios with various equations of state, including the phantom matter. We show that a striking difference between the two quantizations is the existence of two d…
In approaches to quantum gravity, where smooth spacetime is an emergent approximation of a discrete Planckian fundamental structure, any standard effective field theoretical description will miss part of the degrees of freedom and thus break unitarity. Here we show that these expectations can be made precise in loop quantum cosmology. Concretely, even when loop quantum cosmology is unitary at the fundamental level, when microscopic degrees of freedom, irrelevant to low-energy cosmological observers, are suitably ignored, pure states in the effective description evolve into mixed states due to …
The goal of this paper is to probe phenomenological implications of large fluctuations of quantum geometry in the Planck era, using cosmology of the early universe. For the background (Friedmann, Lemaître, Robertson, Walker) \emph{quantum} geometry, we allow `widely spread' states in which the \emph{relative} dispersions are as large as $168\%$ in the Planck regime. By introducing suitable methods to overcome the ensuing conceptual and computational issues, we calculate the power spectrum $P_{\mathcal{R}}(k)$ and the spectral index $n_s(k)$ of primordial curvature perturbations. These results …
In this paper, using a qualitative theory of dynamical systems, we study the stability of a unified dark matter-dark energy framework known as quartessence Chaplygin model (QCM) with three different equation-of-states within ultraviolet (UV) deformed Friedmann-Robertson-Walker (FRW) cosmologies without Big-Bang singularity. The UV deformation is inspired by the non-commutative (NC) Snyder spacetime approach in which by keeping the transformation groups and rotational symmetry there is a dimensionless, Planck scale characteristic parameter $μ_0$ with dual implications dependent on its sign that…
We consider five dimensional conformal gravity theory which describes an anisotropic extra dimension. Reducing the theory to four dimensions yields Brans-Dicke theory with a potential and a hidden parameter $z$ which implements the anisotropy between the four dimensional spacetime and the extra dimension. We find that a range of value of the parameter $z$ can address the current dark energy density compared to the Planck energy density. Constraining the parameter $z$ and the other cosmological model parameters using the recent observational data consisting of the Hubble parameters, type Ia sup…
Constraining a de Broglie--Bohm quantum bounce cosmology with Planck data
Scuola Superiore Meridionale（南方高等学院）；INFN Sez. di Napoli（那不勒斯 INFN 分部）；Universidade do Estado do Rio de Janeiro（里约热内卢州立大学）；Faculdade de Tecnologia, Universidade do Estado do Rio de Janeiro（里约热内卢州立大学技术学院）
An alternative to the postulate of dark energy required to explain the accelerated expansion of the universe is to adopt an inhomogeneous cosmological model to explain the supernovae data without dark energy. We adopt a void cosmology model, based on the inhomogeneous Lemaître-Tolman-Bondi solution of Einstein's field equations. The model can resolve observational anomalies in the $ΛCDM$ model, such as the discrepancy between the locally measured value of the Hubble constant, $H_0=73.24\pm 1.74\,{\rm km}\,{\rm s}^{-1}\,{\rm Mpc}^{-1}$, and the $H_0=66.93\pm 0.62\,{\rm km}\,{\rm s}^{-1}\,{\rm M…
We first derive the primordial power spectra, spectral indices and runnings of both scalar and tensor perturbations of a flat inflationary universe to the second-order approximations of the slow-roll parameters, in the framework of loop quantum cosmology with the inverse-volume quantum corrections. This represents an extension of our previous work in which the parameter $σ$ was assumed to be an integer, where $σ$ characterizes the quantum corrections and in general can take any of values from the range $σ\in (0, 6]$. Restricting to the first-order approximations of the slow-roll parameters, we…
We extend the phenomenology of loop quantum cosmology (LQC) to second order in perturbations. Our motivation is twofold. On the one hand, since LQC predicts a cosmic bounce that takes place at the Planck scale, the second order contributions could be large enough to jeopardize the validity of the perturbative expansion on which previous results rest. On the other hand, the upper bounds on primordial non-Gaussianity obtained by the Planck Collaboration are expected to play a significant role on explorations of the LQC phenomenology. We find that the bounce in LQC produces an enhancement of non-…
