# Quantum Vacuum and Gravitation: Testing General Relativity in Cosmology

Europe/Berlin
02.430 (Mainz Institute for Theoretical Physics, Johannes Gutenberg University)

### 02.430

#### Mainz Institute for Theoretical Physics, Johannes Gutenberg University

Staudingerweg 9 / 2nd floor, 55128 Mainz
Description

General Relativity has successfully passed all experimental tests, including the spectacular recent detection of gravitational waves by LIGO, a century after their prediction. Yet Einstein’s classical theory remains unreconciled to quantum theory.

Quantum theory and gravity come into closest contact in two areas also at the forefront of observational cosmology and astrophysics, namely in the physics of the accelerating universe and of gravitational collapse to a ‘black hole.’ The paradoxes that beset both arise only when quantum vacuum fluctuations in gravitation are considered.

Taken at face value, present observations imply that some 72% of the energy in the universe is attributable to the energy of the vacuum itself, with a negative pressure p = - ρ. Elucidating the nature and dynamics of this vacuum dark energy and the cosmological ‘constant’ is widely recognized as the most pressing problem of cosmology and fundamental physics today.

The final state of gravitational collapse is the second circumstance where quantum vacuum fluctuations in gravity again come into play. As in cosmology, difficulties appear at this interface, for example in the ‘information paradox’ posed by the enormous Bekenstein-Hawking entropy supposedly associated with a black hole. The resolution of these and related paradoxes and the true nature of the interior of a ‘black hole’ remain very much open questions.

In the next few years we are poised to realize an enormous increase in observational information about the Universe’s Large Scale Structure, as well as potentially direct evidence for or against black hole horizons in tests of strong field gravity. The SDSS-BOSS baryon acoustic oscillation survey, the Atacama Cosmology Telescope, numerous weak lensing surveys, and the Dark Energy Survey will greatly expand our knowledge of structure in the Universe. Imaging and X-ray spectroscopic observations of accreting black holes with current and future instruments will lead to the first direct test of the ‘no-hair’ theorem of spinning black holes. LIGO is expected to provide evidence of many more detections of gravitational waves from compact sources that will test GR in the strong-field and near horizon limit. The Event Horizon Telescope is closing in on the first images of the near horizon region of the supermassive object Sgr A* in the center of the Galaxy. As in cosmology, we are entering an era when a fundamental conundrum at the heart of reconciling quantum physics with Einstein’s theory will become illuminated at long last by observational data.

This intensive two week program at the MITP seeks to bring a select group of theoretical physicists with expertise in Quantum Field Theory and General Relativity, together with observational cosmologists and astrophysicists, in order to develop new approaches to these longstanding problems in the Quantum Vacuum and Gravitation Theory, with the goal of confronting viable theoretical ideas to the wealth of observational data that is now becoming available.

Specific Topics to be considered are:

• Role of Scalar and Non-scalar fields in Cosmology
• Trace Anomaly Induced Effective Action and Dynamics

• Non-ideal Cosmological Fluids and Bulk Viscosity

• Spatially Inhomogeneous Cosmology

• Dynamical Vacuum Energy

• Primordial Non-Gaussianity and Large Scale Structure

• Anomalies in the CMB and Hubble expansion

• Quantum Effects at ‘Black Hole’ Horizons and Interiors

• Gravitational Waves

• Strong Field Astrophysical Tests of General Relativity

The Scientific Program will be posted shortly.

The Distinguished Participants in the Program include:

• Akhmedov, E.
• Argullo, I.
• Bartelmann, M.
• Barvinsky, A.
• Bertacca, D.
• Buonanno, A.
• Burgess, C. P.
• Crocce, H. M.
• Dimastrogiovanni, E.
• Durrer, R.
• Fabris, J.
• Fasiello, M.
• Finelli, F.
• Frolov, V.
• Glavan, D.
• Heisenberg, L.
• Kiefer, C.
• Maroto, A. Lopez
• Mazumdar, A.
• Pani, P.
• Prokopec, T.
• Raveri, M.
• Reuter, M.
• Schwarz, D.
• Serreau, J.
• Starkman, G.
• Starobinsky, A. A.
• Tolley, A.
• Trodden, M.
• Wijewardhana, R.
• Yunes, N.
• Zhitnitsky, A.
• Monday, 13 March
• 09:00 10:00
Coffee 1h
• 10:00 10:30
Welcome: Organizational Meeting 30m
• 10:30 11:45
Loop quantum cosmology and the cosmic microwave background 1h 15m
Loop quantum cosmology has become a robust framework to describe the highest curvature regime of the early universe. In this theory, inflation is preceded by a bounce replacing the big bang singularity. I will summarize the theoretical framework, and explore the corrections to the inflationary predictions for the primordial spectrum of cosmological perturbations that this pre-inflationary, quantum gravity phase of the universe introduces. The impact of the bounce on non-Gaussianity and the exciting relation to the observed large scale anomalies in the CMB will be discussed.
Speaker: Dr Ivan Agullo (Louisiana State University)
• 11:45 12:00
Questions/Discussion 15m
• 12:00 14:00
Lunch 2h
• 14:00 15:00
Infrared dynamics of massive scalars from the complementary series in de Sitter space 1h
We consinder loop IR effects in the $D$-dimensional de Sitter space. We study the real scalar $\phi^4$ theory from the complementary series, i.e. with the mass obeying $0 < m \leq \frac{D-1}{2}$ in units of the Hubble scale. We derive an equation which allows to perform the self--consistent resummation of the leading IR contributions from all loops to the two-point correlation function in the expanding Poincar\'{e} patch of de Sitter space. The equation is applicable only for masses higher than certain value, $m > \frac{\sqrt{3}}{4}\, \left(D-1\right)$: the restriction on the masses comes from the fact that for low enough masses there are large IR loop effects in the vertices. The resummation can be done for such density perturbations of the Bunch--Davies state, which violate the de Sitter isometry. We find solutions of the obtained equation. Some of the solutions have an explosive behavior.
Speaker: Dr Emil Ahkmedov
• 15:00 15:15
Questions/Discussion 15m
• 15:15 15:45
Coffee 30m
• 15:45 16:45
Electrodynamic Effects of Inflationary Gravitons 1h
Photons are conformally coupled to gravity and therefore do not get produced due to cosmological expansion, while gravitons are not and do get produced a lot due to the expansion. Since gravitons couple universally to all matter fields they can induce sizeable effects for conformally coupled fields through loop corrections. I will present computations in two different gauges for the one-loop graviton corrections to photon vacuum polarization in de Sitter spacetime. This is used to quantum correct Maxwell equations, and I will present two cases, (i) correction to dynamical photons, and (ii) correction to the Coulomb force between two point charges. The corrections in both cases exhibit secular growth. Gauge dependence will be discussed.
Speaker: Dr Drazen Glavan (University of Warsaw)
• 16:45 17:00
Questions/Discussion 15m
• Tuesday, 14 March
• 09:00 10:15
Cosmic Evolution and Large-Scale Structure 1h 15m
The evolution of cosmic structures, in particular in its late, non-linear phases, provides important clues on the evolution of the Universe as a whole. In view of possible, subtle deviations from general relativity and the cosmological standard model, non-linear cosmic structures as for example traced by the population of galaxy clusters may act as magnifiers to enhance small dynamical effects above observational thresholds. For drawing such conclusions, we need to understand cosmic structure formation in detail in particular on small scales and at late times. In the first half of this overview talk, I will review the cosmological standard model and the standard approach to the formation and evolution of cosmic structures. In the second half, I will discuss a new approach to cosmic structure formation based on a non-equilibrium, statistical field theory for correlated, classical particle ensembles. First applications of this theory show that it allows to calculate statistical properties of cosmic structures at low orders in perturbation theory even at small scales and deeply in the non-linear regime. The theory builds on first principles and is free of adjustable parameters.
Speaker: Prof. Matthias Bartelmann (Heidelberg University)
• 10:15 10:45
Coffee 30m
• 10:45 11:45
Cosmology with relativistic effects 1h
Since the 1970s, the size of galaxy catalogs has constantly increased in terms of solid-angle and redshift coverage as well as in sampling rate. The next generation of surveys will provide us with the possibility to measure galaxy clustering on scales comparable with the Hubble radius (for example: Euclid, DESI, SKAII, etc.). Theoretical analysis suggests that several relativistic effects might be detectable on these scales. In order to fully exploit the potential of the new datasets, it is therefore imperative to develop analysis tools that include these effects. I have developed a fully general relativistic formalism which recovers and generalizes previous results in the plane-parallel (at-sky) and Newtonian wide-angle approximations. These corrections become important on large scales both at low and high redshifts, and lead to new terms in the wide-angle correlations. Now we need to work out how to apply this formalism for real data and understand which cosmological parameters are particularly affected. This will be important for future surveys and to guide the development on the next generation of large astrophysical experiments.
Speaker: Dr Daniele Bertacca
• 11:45 12:15
Questions/Discussion 30m
• 12:15 14:00
Lunch 1h 45m
• 14:00 15:00
Scale hierarchies and string cosmology 1h
I will discuss the problem of scale hierarchies in string phenomenology of particle physics and cosmology and propose ways to address it. In particular I will present a mechanism for supersymmetry breaking in the presence of a tiny (tunable) positive cosmological constant, discuss connections of supersymmetry breaking and inflation, and describe their phenomenological consequences.
Speaker: Prof. Ignatios Antoniadis (LPTHE Paris and AEC Bern)
• 15:00 15:15
Questions/Discussion 15m
• 15:15 15:45
Coffee 30m
• 15:45 16:45
Cosmology with arbitrary spin coherent fields 1h
Coherent scalar fields play a fundamental role in the description, not only of the inflationary epoch, but also as dark matter candidates such as the axion or in dark energy models. In this talk we will explore the potential role that coherent fields of higher spin could play in cosmology. We will present a general result showing that rapidly evolving coherent fields preserve the isotropy of the background expansion. We will also consider models of coherent bosonic dark matter, providing an explicit example of ultralight vector dark matter and showing the distinctive features at the perturbations level with respect to the scalar case and to conventional cold dark matter.
• 16:45 17:00
Questions/Discussion 15m
• Wednesday, 15 March
• 09:00 10:15
CMB Anomalies 25 years after COBE 1h 15m
Several unexpected features have been observed in the temperature of the microwave sky at large angular scales, from COBE to WMAP and Planck. These include lack of both variance and correlation on the largest angular scales; alignment of the lowest multipole moments with one another and with the motion and geometry of the Solar System; lack of variance in the northern hemisphere, or a hemispherical power asymmetry or dipolar power modulation; a preference for odd parity modes; and an unexpectedly large cold spot in the Southern hemisphere. The individual p-values of the significance of several of these features are in the per mille to per cent level, compared to the expectations of the best-fit inflationary ΛCDM model. There are no good physical models for these anomalies. Rather than focus on a debate about the validity of a posteori statistics, or the relative merits of one statistical measure or another, we will discuss how we can make progress by: (a) considering how the existence of measured anomaly alters the predictions of ΛCDM for other observables; (b) making predictions from reasonable phenomenological expectations for the physics contents of measured anomalies.
Speaker: Prof. glenn starkman (Case Western Reserve University)
• 10:15 10:45
Coffee 30m
• 10:45 11:45
Do we understand the cosmic dipole? 1h
An important assumption of modern cosmology is that the CMB dipole is due to the proper motion of the solar system wrt the CMB. This fixes a reference frame for many observations, e.g. the Hubble diagram of SN1a. Planck observations of the largest multipole moments are consistent with the proper motion hypothesis, but also allow for sizeable non-kinetic contributions to the CMB dipole. The proper motion hypothesis can also be tested at any other wavelength, given good sky coverage and statistics are available. This is the case for modern radio point source catalogues. These allow us to measure the dipole distribution of radio sources at mean redshift of z ~ 1. The radio dipole can be measured at different radio frequencies and is compared to the CMB dipole. First results at three different frequency bands (at three different instruments) indicate an excess wrt to the expected signal based on the propor motion hypothesis only. The radio dipole directions are consistent with the CMB dipole direction at all three frequencies. Together with the alignment of low-l CMB multipole moments with the CMB dipole, I argue that this is further evidence that the largest angular scales of the Universe are not properly understood so far.
Speaker: Prof. Dominik Schwarz (Bielefeld University)
• 11:45 12:15
Questions/Discussion 30m
• 12:15 14:00
Lunch 1h 45m
• 14:00 15:00
Recent developments in the inflationary scenario 1h
I consider reconstruction of inflationary models in GR and f(R) gravity using information on the power spectrum of scalar perturbations only, ambiguity in this procedure and how it can be fixed by some aesthetic assumptions on the absence of new physical scales during and after inflation. Also discussed is the problem on the onset of inflation from generic classical curvature singularity preceding it, and which conditions are needed for this.
Speaker: Prof. Alexei Starobinsky (Landau Institute for Theoretical Physics)
• 15:00 15:15
Questions/Discussion 15m
• 15:15 15:45
Coffee 30m
• 15:45 16:45
Testing primordial non-Gaussianity with CMB and LSS data 1h
Primordial non-Gaussianity (NG) is a powerful probe of Inflation, able to test extra-interactions and extra-degrees of freedom in scenarios beyond the simplest, standard single-field slow-roll paradigm.The most important observational signature of primordial NG is a non-vanishing bispectrum (3-point function in Fourier/harmonic space) of the primordial perturbation field. After showing the link between different bispectrum shapes and corresponding inflationary scenarios, I will describe current bispectrum constraints obtained from the analysis of Planck CMB data. I will then discuss future potential improvements over Planck results, coming from the next generation CMB and galaxy surveys, and their implications. I will conclude by showing the limits that can be in principled achieved in more futuristic experimental scenarios, exploiting 21 cm and CMB spectral distortion fluctuations.
Speaker: Dr Michele Liguori (Univ. of Padova)
• 16:45 17:00
Questions/Discussion 15m
• Thursday, 16 March
• 09:00 10:45
What is the Quantum Matter with Black Holes? 1h 45m
Classical General Relativity (GR) together with conventional equations of state suggest that in complete gravitational collapse a singular state of matter with infinite density could be reached finally, to what is popularly called a "black hole." In addition to its interior singularities, the characteristic feature of a black hole is its apparent horizon, the surface of finite area at which outwardly directed light rays are first trapped. The loss of information to the outside world this implies gives rise to additional difficulties with well-established principles of quantum mechanics and statistical physics. I will overview the historical and most recent approaches to these problems, as well as the status of the gravitational vacuum condensate star proposal we have made in 2001, with a p= - rho interior is actually realized in Schwarzschild's second paper over a century ago. The redshifted surface tension of the condensate star surface is given by $\tau_s = \Delta \kappa/8 \pi G$, where $\Delta \kappa= 1/R_s$ is the difference of equal and opposite surface gravities between the exterior and interior Schwarzschild solutions. The First Law, $dM = dE_v + \tau_s\, dA$ is recognized as a purely mechanical classical relation at zero temperature and zero entropy, describing the volume energy and surface energy change respectively. Since there is no event horizon, the Schwarzschild time $t$ of such a non-singular gravitational condensate star is a global time, fully consistent with unitary time evolution in quantum theory. The $p=-\bar\rho$ interior acts as a defocusing lens for light passing through the condensate, leading to imaging characteristics distinguishable from a classical black hole. Further observational test of gravitational condensate stars vs. black holes is the {\it discrete} surface modes of oscillation and echoes which should be detectable by their gravitational wave signatures.
Speaker: Dr Emil Mottola (Los Alamos National Laboratory)
• 10:45 11:45
Gravitational-wave signatures of exotic compact objects and of quantum corrections at the horizon scale 1h
Gravitational wave (GW) astronomy allows us for unprecedented tests of the nature of compact objects. In this context, I will discuss two signatures of new physics at the horizon scale: GW "echoes" in the postmerger ringdown phase of a binary coalescence, and the tidal deformability of exotic compact objects as measured from the inspiral premerger phase. In the first case, the ringdown waveform of exotic ultracompact objects is initially identical to that of a black hole, and putative corrections at the horizon scale appear only later as a modulated and distorted train of echoes of the modes of vibration associated with the photon sphere. As for the second case, I will discuss the tidal Love numbers of different families of boson stars, gravastars, wormholes, and other toy models for quantum corrections at the horizon scale, showing that they display a universal logarithmic dependence on the location of the surface in the black-hole limit. I will discuss the ability of present and future GW detectors to measure these effects. Both LIGO, ET and LISA can impose interesting constraints on boson stars, while LISA is able to probe even extremely compact objects like gravastars. We argue that these effects provide a smoking gun of new physics at the horizon scale, and that future GW measurements of a binary coalescence provide a novel way to test black holes and general relativity in the strong-field regime.
Speaker: Paolo Pani (Sapienza University of Rome &amp; INFN Roma1 - IST, Lisbon)
• 11:45 12:15
Questions/Discussion 30m
• 12:15 14:00
Lunch 1h 45m
• 14:00 15:00
Remarks on non-singular black hole models 1h
We discuss spherically symmetric metrics which represent non-singular black holes in four- and higher-dimensional spacetimes. We first consider static metrics, which obey the following conditions: (i) Regularity at the center and (ii) Validity of the limiting curvature condition. We describe the Hayward model and its generalizations, which obey these properties. After this we discuss quantum radiation of a massless scalar field from an evaporating spherically symmetric non-singular black hole with finite lifetime. We demonstrate that in a general case there exists a huge outburst of the quantum radiation, emitted from the black hole interior from the domains close to the inner horizon. We discuss a relation of this phenomenon with the mass inflation effect. Finally, we shall make remarks on attempts to solve this problem and to provide self-consistency of the non-singular model in the quantum domain.
Speaker: Prof. Valeri Frolov (University of Alberta, Edmonton, Canada)
• 15:00 15:15
Questions/Discussion 15m
• 15:15 15:45
Coffee 30m
• 15:45 17:00
Panel Discussion: Quantum Effects in Black Holes

Panel Discussion

Conveners: Emil Mottola, Paolo Pani, Valery Frolov
• Friday, 17 March
• 09:00 10:15
The Cosmic Microwave Background: what can we learn from the most precious cosmological data ? 1h 15m
Most numbers in cosmology have been measured using the anisotropies and polarisation of the Cosmic Microwave Background (CMB). So far other data has mainly been used for consistency checks and very few inconsistent measurements exist. The reason for this is twofold: first the theory of the CMB is nearly linear and therefore quite simple. Secondly, the CMB spectrum peaks around frequencies which allow relatively precise observations from the ground and especially from space. In the first part of my talk I review the relatively simple physics behind CMB anisotropies and polarisation and show in some examples how it can be used to measure cosmological parameters. In a second part I outline ideas to go beyond present measurements which mainly constrain cosmological parameters and show how we can use the CMB to test General Relativity on cosmological scales. If there is time, I would also like to outline interesting new developments in the theory of Large Scale Structure observations and argue why we hope that future LSS surveys can compete with and complement CMB observations.
Speaker: Prof. Ruth Durrer (Universite de Geneve)
• 10:15 10:45
Coffee 30m
• 10:45 11:45
Testing Infinite derivative gravity: Bounce and Cosmic inflation 1h
I will be discussing infinite derivative theories of gravity as a perturbative effective field theory of gravity, which in the ultraviolet can become asymptotically free. I will discuss their classical and quantum properties around Minkowski and deSitter backgrounds. I will show how to resolve cosmological singularity problem and I will discuss perturbations around deSitter background. Action which resolves cosmological singularity also gives rise to an ultraviolet completion for Starobinsky inflation. Therefore, a non-singular bounce followed by a period of cosmic inflation can lead to a successful classical and quantum prediction for our Universe. There are some smoking gun signatures for tensor to scalar ratio which I will describe.
Speaker: Dr Anupam Mazumdar (Lancaster)
• 11:45 12:15
Questions/Discussion 30m
• 12:15 14:00
Lunch 1h 45m
• 14:00 15:00
Recent progress in higher-derivative gravity theories 1h
Recently there were some advances in the higher-derivative gravity theories and my purpose is to give the brief survey of our results. First of all, it was shown that the theory with six or more derivatives in the action, which has only complex ghost-like poles, has a unitary S-matrix in the Lee-Wick sense. Second, it was shown that in this class of theories the magnitudes of the masses of the ghost-like states can not be reduced by a gravitational version of the see-saw mechanism. Third, even if the masses are huge, there may be a chance to design an indirect texts for higher derivatives from the astrophysical observations.
Speaker: Prof. Ilya Shapiro (Universidade Federal de Juiz de Fora)
• 15:00 15:15
Questions/Discussion 15m
• 15:15 15:45
Coffee 30m
• 15:45 17:00
Panel Discussion: CMB and LSS
Conveners: Dominik Schwarz, Glenn Starkman, Matthias Bartelmann, Ruth Durrer
• Monday, 20 March
• 09:00 10:15
Effective Field Theories and Modifying Gravity: The View from Below 1h 15m
We live at a time of contradictory messages about how successfully we understand gravity. General Relativity seems to work very well in the Earth’s immediate neighbourhood, but arguments abound that it needs modification at very small and/or very large distances. This talk tries to put this discussion into the broader context of similar situations in other areas of physics, and summarizes some of the lessons which our good understanding of gravity in the solar system has for proponents for its modification over very long and very short distances. The main message is that effective theories (in the technical sense of effective) provide the natural (and arguably only known) precise language for framing proposals. Its framework is also useful, inasmuch as it makes some modifications seem more plausible than others, though there are also some surprises.
Speaker: Prof. Cliff Burgess (McMaster University & Perimeter Institute)
• 10:15 10:45
Coffee 30m
• 10:45 11:45
Positivity Constraints on Effective Field Theories of Gravity 1h
I will discuss how the constraints from unitarity and analyticity lead to an infinite number of positivity constraints on low energy effective theories, starting initially with scalars and then generalising to theories with spin, in particular gravitational theories.
Speaker: Dr Andrew Tolley (Imperial College London)
• 11:45 12:15
Questions/Discussion 30m
• 12:15 14:00
Lunch 1h 45m
• 14:00 15:00
CAN QUANTUM-GRAVITATIONAL EFFECTS BE OBSERVED IN THE COSMIC MICROWAVE BACKGROUND? 1h
In any approach to quantum gravity, it is crucial to search for observational effects. In my talk, I discuss how quantum-gravitational contributions to the anisotropy spectrum of the cosmic microwave background arise in the framework of quantum geometrodynamics (Wheeler-DeWitt equation). A scenario of slow-roll inflation is assumed. I derive concrete predictions for the power spectrum and other cosmic quantities and discuss their observability. I also present a brief comparison with other approaches.
Speaker: Prof. Claus Kiefer (University of Cologne)
• 15:00 15:15
Questions/Discussion 15m
• 15:15 15:45
Coffee 30m
• 15:45 16:45
Cosmological implications of massive vector fields 1h
After reviewing the basic properties of a Proca field, I will discuss how one can generalise it. The resulting theory has non-trivial derivative self-interactions, but still propagates the required three degrees of freedom of a massive vector field. The building blocks are similar to those used in constructing consistent theories for massive gravity and scalar theories. I will then discuss the cosmological implications of this generalised Proca theories.
Speaker: Dr Lavinia Heisenberg (ITS-ETH Zurich)
• 16:45 17:00
Questions/Discussion 15m
• Tuesday, 21 March
• 09:00 10:15
Testing gravity on large scales with current and future Galaxy Surveys. 1h 15m
The striking discovery that the expansion of the Universe is not slowing down but in fact accelerating has turned into one of the main mysteries in Physics today, with explanations that range from new repulsive forces, as the dark energy, or the failure of Einstein's General Relativity (GR). Understanding this puzzle, and others such as the dark matter or the physics of Inflation, has set the programme for the next decade of Observational Cosmology. The goal is to undertake large astronomical surveys that, by scanning millions of galaxies across cosmic time, will study the origin and evolution of the Large Scale Structure of the Universe. In this talk I will review which are some of the probes that these surveys use to test GR, such as galaxy clustering and weak lensing, discussing current constrains and their implications. And particularly the perspective from ongoing and future surveys, such as the Dark Energy Survey (DES) or the ESA/Euclid satellite.
Speaker: Dr Martin Crocce (Institute for Space Science (ICE-CSIC/IEEC))
• 10:15 10:45
Coffee 30m
• 10:45 11:45
Planck results on inflation and perspectives for future observations 1h
After a review of the main Planck results, we discuss the perspectives for future observations to improve our understanding of cosmic inflation.
Speaker: Dr Fabio Finelli (INAF - IASF Bologna)
• 11:45 12:15
Questions/Discussion 30m
• 12:15 14:00
Lunch 1h 45m
• 14:00 15:00
Testing early Universe physics with upcoming observations 1h
Cosmology has seen great progress thanks to precision measurements and is bound to greatly benefit from upcoming Large Scale Structure and Cosmic Microwave Background data. I will point out a number of interesting directions. In particular, I discuss how the microphysics of inflation may be tested in galaxy surveys through “fossil” signatures originating from squeezed primordial correlations. I further elaborate on the constraining power of CMB spectral distortions on small-scale cosmological fluctuations and on particle decays in the very early Universe in relation to reheating. I also describe some of the possible constraints on inflation and reheating from future B-mode observations.
Speaker: Prof. Emanuela Dimastrogiovanni (Case Western Reserve University)
• 15:00 15:15
Questions/Discussion 15m
• 15:15 15:45
Coffee 30m
• 15:45 16:45
LSS probes of Cosmic Acceleration 1h
Operational and near-future astronomical surveys of the galaxy distribution will soon put to the test our best ideas as to the nature of the current cosmic acceleration. I will briefly review the status of some of the most interesting proposals and then focus on LSS probes. The perturbative treatment of LSS, especially in its effective theory realization, can well describe the quasi-linear regime of structure formation. This regime is a precious repository of key information on both early and late-time Universe dynamics. I will elaborate on recent progress towards including a dark energy or a modified gravity component in this framework. Quasi-linear scales have special relevance for testing any beyond LCDM model as these can perturbatively access screening dynamics.
Speaker: Dr Matteo Fasiello (Stanford University)
• 16:45 17:00
Questions/Discussion 15m
• Wednesday, 22 March
• 09:00 10:15
Effective action and very early quantum Universe 1h 15m
We give an overview of the effective action method in cosmology with a special emphasis on its application in the theory of quantum initial conditions for the very early Universe and the models of Higgs and $R^2$ inflation. The role of local gradient expansion and conformal anomaly as calculational methods for the effective action will be discussed and applied to the microcanonical state of the Universe and the model of a new type of hill-top inflation.
Speaker: Dr Andrei Barvinsky (Lebedev Physics Institute, Moscow, Russia)
• 10:15 10:45
Coffee 30m
• 10:45 11:45
Weyl symmetry in the standard model and gravity 1h
Weyl symmetry can be naturally realised in Cartan-Einstein gravity both at the kinematical and dynamical level. The symmetry can be naturally extended to matter fields by a suitable generalization of covariant derivative and by supplementing a dilaton field. Fermionic matter sources skew symmetric torsion while scalar matter sources vectorial torsion trace (gauge fields do not trace torsion). This implies that, when matter fields are integrated out, vectorial torsion and skew symmetric torsion become dynamical. In this talk I will present preliminary results on physical implications of torsion trace and skew symmetric torsion. This talk is based on e-Print: e-Print: arXiv:1606.02677 [hep-th] and unpublished work with Stefano Lucat.
Speaker: Prof. Tomislav Prokopec (Utrecht University)
• 11:45 12:15
Questions/Discussion 30m
• 12:15 14:00
Lunch 1h 45m
• 14:00 15:00
The theta-dependent vacuum energy. The application to cosmology and axion search experiments 1h
It is argued that there is a novel type of energy related to the theta parameter in gauge theories. This energy has so-called non-dispersive nature" and can not be expressed in terms of any local propagating degrees of freedom. Instead, this energy is generated due to the presence of the topologically nontrivial sectors in gauge theories, and tunnelling events between them. The talk is based on 3 recent papers: 1. applications to cosmology (PRD 2015, arXiv:1505.05151), 2. proposal to study this novel type of energy in a tabletop experiment in Maxwell system (PRD 2016, arXiv:1512.00470) 3. New ideas for the axion search experiments sensitive to theta_{QED} (arXiv:1702.00012)
Speaker: Prof. Ariel Zhitnitsky (University of British Columbia)
• 15:00 15:15
Questions/Discussion 15m
• 15:15 15:45
Coffee 30m
• 15:45 17:00
Panel Discussion EFT & Modified Gravity
• Thursday, 23 March
• 09:00 10:15
The Next Theoretical Challenges for Gravitational-Wave Observations 1h 15m
One hundred years after Einstein predicted the existence of gravitational waves on the basis of his theory of General Relativity, LIGO announced the first observation of gravitational waves passing through the Earth emitted by the collision of two black holes one billion four-hundred million light years away. In this talk, I will first review the theoretical groundwork that allowed to identify and interpret the gravitational-wave signals, and carry out tests of general relativity in the strong-field, highly dynamical regime. I will then discuss the next theoretical challenges in solving the two-body problem in General Relativity if we want to take full advantage of the discovery potential of upcoming gravitational-wave observations.
Speaker: Alessandra Buonanno (Max Planck Institute for Gravitational Physics)
• 10:15 10:45
Coffee 30m
• 10:45 12:00
Theoretical Physics Implications of the Advanced LIGO Gravitational Wave Observation 1h 15m
The recent gravitational-wave observations by Advanced LIGO provided the first opportunity to learn about theoretical physics mechanisms that may be present in the extreme gravity environment of coalescing binary black holes. The LIGO-Virgo collaboration verified that this observation is consistent with Einstein's theory of General Relativity and the Kerr hypothesis, constraining the presence of parametric anomalies in the signal. In this talk, I will discuss the plethora of additional inferences that can be drawn on theoretical physics mechanisms from the absence of such anomalies in the data. I will classify these inferences in those that inform us about the generation of gravitational waves, the propagation of gravitational waves and the structure of exotic compact object alternatives to black holes. I will then focus on how GW150914 constrains the generation of gravitational waves (e.g. the activation of scalar fields, black hole graviton leakage into extra dimensions, the variability of Newton's constant, the breakage of Lorentz invariance and parity invariance) as well as the propagation of gravitational waves (e.g. the speed of gravity and the existence of large extra dimensions).
Speaker: Prof. Nicolas Yunes (Montana State University)
• 12:00 12:15
Questions/Discussion 15m
• 12:15 14:00
Lunch 1h 45m
• 14:00 15:00
Nonperturbative renormalization group for scalar fields on de Sitter space 1h
I review recent developments concerning the use of nonperturbative renormalization group techniques for describing the nontrivial infrared regime of light scalar fields in de Sitter space. Long wavelength fluctuations (in units of the spacetime curvature) undergo a dramatic gravitational amplification which is responsible for dramatic phenomena, such as the radiative restoration of spontaneously broken symmetries.
Speaker: Dr Julien Serreau (University Paris Diderot)
• 15:00 15:15
Questions/Discussion 15m
• 15:15 15:45
Coffee 30m
• 15:45 17:00
Panel Discussion: Testing General Relativity with Gravitational Wave Astronomy 1h 15m
• Friday, 24 March
• 09:00 10:00
Viscous models for the dark sector of the Universe 1h
The description of the present universe using a cosmological constant and cold dark matter is, in general, very successful. However, many problems appear at non-linear level, pointing possibly to the necessity of an extension of the cold dark matter paradigm. One of the possibilities in this sense is to introduce viscosity in the dark matter component. First we review the general properties of this viscous fluid, and the applications to cosmology. Later, it is analysed the effects of both bulk and shear viscosities on the perturbations, relevant for structure formation in late time cosmology. It is shown that shear viscosity can be as effective as the bulk viscosity on suppressing the growth of perturbations and delaying the non-linear regime. A statistical analysis of the shear and bulk viscous effects is performed and some constraints on these viscous effects are given.
Speaker: Prof. Julio Fabris (Universidade Federal do Espirito Santo)
• 10:00 10:15
Questions/Discussion 15m
• 10:15 10:45
Coffee 30m
• 10:45 11:15
Multi-messenger time delays from lensed gravitational waves 30m
Speaker: Prof. Mark Trodden
• 11:15 12:00
Scalar Gravitational Waves in the Effective Theory of Gravity 45m
As a low energy effective field theory, classical General Relativity receives an infrared relevant modification from the conformal trace anomaly of the energy-momentum tensor of massless, or nearly massless, quantum fields. The local form of the effective action associated with the trace anomaly is expressed in terms of a dynamical scalar field, the conformalon, that couples to the conformal factor of the spacetime metric, allowing it to propagate over macroscopic distances. Linearized around flat spacetime, this semi-classical EFT admits scalar gravitational wave solutions in addition to the transversely polarized tensor waves of the classical Einstein theory. Astrophysical sources for scalar gravitational waves are considered, with the excited gluonic condensates in the interiors of neutron stars in merger events with other compact objects likely to provide the strongest burst signals.
Speaker: Dr Emil Mottola
• 12:00 12:15
Questions/Discussion 15m
• 12:15 14:00
Lunch 1h 45m
• 14:00 15:15
Can Quantum Effects in Gravity be Tested with Forthcoiming Observations: Open Discussion
• 15:15 15:45
Coffee/End of Program 30m
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