# Copernicus Webinar and Colloquium Series

Europe/Zurich
Description

To promote scientific discussions during this pandemic, we are organizing an online seminar series, i.e. Copernicus Webinar Series, seeking the most outstanding speakers to introduce innovative ideas and important progress in the field of gravity and cosmology. This series is named after the famous Polish Astronomer, Nicolaus Copernicus, whose discovery eventually marked the dawn of modern science.

We would like to invite you to take part in the upcoming 40th Webinar talk,

speaker:         Sunny Vagnozzi (Cambridge)

Time:         13th April, Tuesday, 3pm, Krakow time (9pm Beijing time, 10pm JST, 9am EDT)

Title:          What the Hubble tension really is and how (not) to solve it

Abstract:

Despite being arguably one of the hottest topics in the recent literature, there are several widely spread misconceptions concerning what the Hubble tension really is. Moreover, leaving these misconceptions aside, no compelling model to solve the Hubble tension has been found so far, despite a huge number of attempts (and false alarms). I will begin by explaining what the Hubble tension really is, presenting three different interpretations in order of increasing "correctness". I will then discuss why so many proposed models have failed so far, and what in my opinion is the way forward towards constructing a realistic solution to the tension. I will close by presenting the "10 commandments" which I believe every "Hubble hunter" should abide by.

• Thursday, 9 July
• 15:00 16:00
Relieving the Hubble tension with primordial magnetic fields 1h

The standard cosmological model determined from the accurate cosmic microwave background measurements made by the Planck satellite implies a value of the Hubble constant H0 that is 4.2 standard deviations lower than the one determined from Type Ia supernovae. The Planck best fit model also predicts lower values of the matter density fraction Om and clustering amplitude S8 compared to those obtained from the Dark Energy Survey Year 1 data. We show that accounting for the
additional inhomogeneities in the baryon density induced by primordial magnetic fields present in the plasma prior to recombination can help to solve both the H0 and the S8-Om tensions. The required field strength is just what is needed to explain the existence of galactic, cluster, and extragalactic magnetic fields without relying on dynamo amplification. Our results show clear evidence for this effect and motivate further detailed studies of primordial magnetic fields, setting several well-defined targets for future observations.

Speaker: Levon Pogosian (Simon Fraser U.)
• 16:00 17:00
informal discussion/coffee time 1h
• Wednesday, 15 July
• 15:00 16:00
Physical Implications of a Fundamental Period of Time 1h

If time is described by a fundamental process rather than a coordinate, it interacts with any physical system that evolves in time. The resulting dynamics has recently been shown to be consistent provided the fundamental period of the time system is sufficiently small. A strong upper bound T_C < 10^{-33}s of the fundamental period of time, several orders of magnitude below any direct time measurement, can be obtained from bounds on dynamical variations of the period of a lab system evolving in time. Possible cosmological implications will be discussed.

Speaker: Martin Bojowald
• 16:00 17:00
informal discussion/coffee time 1h
• Friday, 17 July
• 15:00 16:00
The Cosmic Microwave Background 1h

Precision measurements of the Cosmic Microwave Background (CMB) have the potential to provide information about the birth and evolution of our universe. I will review how we extract cosmological parameters from the CMB from both temperature and polarization maps. The Atacama Cosmology Telescope (ACT) has been making measurements of the CMB since 2006. I will discuss the recent results from ACT and their implications. I will also provide a glimpse of the instrumentation being developed for the upcoming Simons Observatory.

Speaker: Mark Devlin (University of Pennsylvania)
• 16:00 17:00
informal discussion/coffee time 1h
• Thursday, 23 July
• 15:00 16:00
Constraining Early Dark Energy with Large Scale Structure 1h

The Hubble tension is conventionally viewed as that between the cosmic microwave background (CMB) and the SH0ES measurement. A prominent proposal for a resolution of this discrepancy is to introduce a new component in the early universe, which initially acts as "early dark energy" (EDE), thus decreasing the physical size of the sound horizon imprinted in the CMB and increasing the inferred H_0, bringing it into near agreement with SH0ES. However, this impacts cosmological observables beyond the CMB -- in particular, the large scale structure (LSS) of the universe across a range of redshift. The H_0 tension resolving EDE cosmologies produce scale-dependent changes to the matter power spectrum, including 10% more power at k=1 h/Mpc. Motivated by this, I will present the results of two analyses of LSS constraints on the EDE scenario. Weak lensing and galaxy clustering data (from, e.g., the Dark Energy Survey) significantly constrain the EDE model, and the resulting H_0 is in significant tension with SH0ES. Complementary to this, including data from the Baryon Oscillation Spectroscopic Survey (BOSS), analyzed using the effective field theory (EFT) of LSS, yields an EDE H_0 value that is in significant (3.6\sigma) tension with SH0ES. These results indicate that current LSS data preclude the EDE model as a resolution of the Hubble tension, and, more generally, that the EDE model fails to restore cosmological concordance. A sensitivity forecast for EUCLID suggests that future LSS surveys can close the remaining parameter space of the model.

Speaker: Dr Evan McDonough (MIT)
• 16:00 17:00
informal discussion/coffee time 1h
• Wednesday, 29 July
• 15:00 16:00
High frequency gravitational waves 1h

Gravitational waves in the MHz to GHz range are window to the very early Universe, and thus provide a unique way to probe physics around the energy scale of grand unification. The detection of such relic gravitational waves is however extremely challenging. In this talk, I will highlight the fundamental processes generating such high frequency gravitational waves and discuss recent progress in searching for these primordial messengers. I will focus in particular on a natural cosmological gravitational wave detector, based on the conversion of gravitational waves into photons in the presence of cosmic magnetic fields. I demonstrate that this conversion leads to a distortion of the cosmic microwave background (CMB). The measurements of the radio telescopes EDGES and ARCADE can be cast as a bound on the gravitational wave amplitude. For the strongest magnetic fields allowed by astrophysical constraints, these constraints exceed current laboratory constraints by about seven orders of magnitude. Future advances in 21cm astronomy may conceivably push these bounds below the sensitivity of cosmological constraints on the total energy density of gravitational waves.

Speaker: Valerie Domcke
• 16:00 17:00
informal discussion/coffee break 1h
• Thursday, 6 August
• 15:00 16:00
Origin of matter and gravitational wave 1h

We exist today thanks to the asymmetry between matter and anti-matter. Its origin, however, has been one of the major mysteries in cosmology and particle physics. Arguably the leading theory called leptogenesis is that the asymmetry is generated by the decay of heavy neutrinos at the temperature above 10^9 GeV. I review this theory and point out that the gravitational wave will be an important test. If time permits, I will also discuss a different origin of the asymmetry which is also tied to the origin of dark matter, again with a gravitational wave signature.

Speaker: Prof. Hitoshi Murayama
• 16:00 17:00
informal discussion/Coffee time 1h
• Tuesday, 11 August
• 15:30 16:30
The Noise of Gravitons 1h

For the purpose of describing observed phenomena, it has thus far been sufficient to regard gravity as a classical field obeying Einstein’s equations. Here we treat the gravitational field as a quantum field and consider the implications for gravitational wave detectors. We present a formalism to obtain the quantum effects of gravity based on the Feynman-Vernon influence functional. We find that the separation of free-falling particles is subject to random fluctuations (“noise”), with the classical geodesic deviation equation being replaced by a stochastic equation. The statistical characteristics of the noise depend on the quantum state of the gravitational field; for certain classes of quantum states, the noise can be greatly enhanced. Detection of this fundamental noise would constitute direct evidence for the quantization of gravity and the existence of gravitons.

Speaker: Maulik Parikh
• 16:30 17:00
informal discussion/coffee time 30m
• Thursday, 20 August
• 15:00 16:00
alpha'-Cosmological tale: String Cosmology backgrounds from Classical String Geometry 1h

In this talk, I intend to give a pedagogical, nonetheless biased, introduction to String Cosmology. After briefly reviewing the Lambda-CDM model and motivating inflation, we will remind ourselves that early universe cosmology remains singular and waiting for alternatives. That will be our cue to consider string theory to define our gravity sector and string thermodynamics to define our matter sector. We will learn how that doesn't work, unless we consider more string corrections (in fact, infinity of them!). Once we are stringy enough, we will be able to build a full cosmological model that is non-singular and already poses itself as an alternative to inflation at the background level.

Speaker: Guilherme Franzmann
• 16:00 17:00
informal discussion/coffee time 1h
• Wednesday, 26 August
• 15:00 16:00
Primordial black holes as the solution of many cosmological conundra 1h

Studies of primordial black holes (PBHs) usually focus on constraints on their abundance, since this has interesting implications for cosmology even if they never formed. However, recently attention has turned to the possibility that they may actually exist and solve various cosmological conundra. The most exciting possibility is that they provide the dark matter, although this is only feasible in a few mass windows. In particular, if they form at the QCD phase transition, the tiny collapse fraction required might naturally explain the cosmic photon-to-baryon ratio and the comparability of the PBH and baryon densities. Even if PBHs provide only a small fraction of the dark matter, they might still explain some of the OGLE and quasar microlensing events, the LIGO/Virgo gravitational wave events, the spatial coherence in the fluctuations of the source-subtracted cosmic infrared and soft X-ray backgrounds, some anomalies associated with Ultra Faint Dwarf galaxies, and the supermassive black holes in galactic nuclei. With a suitable extended mass spectrum, they might even explain all these anomalies. So an exciting new era in PBH research has began, with various forthcoming observations able to probe this proposal.

Speaker: Prof. Bernard Carr
• 16:00 17:00
informal discussion/coffee time 1h
• Tuesday, 1 September
• 14:00 15:00
Firewalls in General Relativity 1h

We present spherically symmetric solutions to Einstein’s equations, which are equivalent to canonical Schwarzschild and Reissner-Nordstrom black holes on the exterior, but with singular (Planck-density) shells at their respective event and inner horizons. The locally measured mass of the shell and the singularity are much larger than the asymptotic Arnowitt-Deser-Misner mass. The area of the shell is equal to that of the corresponding canonical black hole, but the physical distance from the shell to the singularity is a Planck length, suggesting a natural explanation for the scaling of the black hole entropy with area. The existence of such singular shells enables solutions to the black hole information problem of Schwarzschild black holes and the Cauchy horizon problem of Reissner-Nordstrom black holes. While we cannot rigorously address the formation of these solutions, we suggest plausibility arguments for how “normal” black hole solutions may evolve into such states. We also discuss the stability of these structures.

Speakers: Ryan McManus, Surjeet Rajendran
• Friday, 11 September
• 15:00 16:00
Observational Signatures of Multifield Inflation 1h

Slow-roll single-field inflation constitutes the main paradigm of the Early Universe. But this model suffers from a number of conceptual issues that naturally lead to the consideration of multifield models of inflation with curved field space, which have recently been under scrutiny as realistic realisations of high-energy physics in the Early Universe. I will show that the non-trivial internal geometry reshuffles observational predictions from inflation, at the level of the background (geometrical destabilisation of inflation), of linear fluctuations (spectral index, tensor-to-scalar ratio) and can lead to exotic type of non-Gaussianities (bispectrum, higher-order correlation functions). This last fact in particular motivates the thorough analysis of non-Gaussianities in this large class of models. For that, we revisit Maldacena's calculation of the bispectrum in a 2-field context. As a byproduct, we also derive the effective single-field theory including interactions, when the fluctuation perpendicular to the trajectory (isocurvature mode) can be integrated out, and explicitly show the effect of the curvature of the field space on the bispectrum. Time permitting I will also mention other projects that might interest the audience, such as multifield stochastic inflation and multifield/multi-fluid reheating after inflation.

Speaker: Dr Lucas Pinol (IAP)
• Wednesday, 16 September
• 18:00 19:00
Signals of a Quantum Universe 1h

The idea that structure in the universe was created from quantum mechanical vacuum fluctuations during inflation is very compelling, but unproven. Testing this proposal is challenging because the universe we observe is effectively classical. I will explain the origin of this challenge and how it can be circumvented if we observe equilateral primordial non-Gaussianity. In particular, we will see that the absence of an accompanying folded non-Gaussian signal is only possible (assuming locality) with quantum vacuum fluctuations.

Speaker: Prof. Daniel Green
• 19:00 20:00
informal discussion/coffee time 1h
• Monday, 21 September
• 15:00 16:00
Cosmic Bell Tests: Using Quasars to Test Quantum Theory 1h

For decades, physicists have conducted experimental tests of quantum entanglement, a phenomenon that Albert Einstein once dismissed as "spooky action at a distance." Despite Einstein's misgivings, the experiments have consistently found results compatible with quantum theory. Yet every experiment has been subject to one or more "loopholes," which would still allow for an explanation without the need for quantum mechanics. Arguably the most subtle and stubborn loophole is known as "freedom of choice," and concerns whether any unknown mechanism could have affected both the selection of measurements to be performed on the entangled particles and the outcomes of those measurements. To address this loophole, one should obtain random numbers that can be expected to be uncorrelated with any aspect of the entanglement experiment. In recent “Cosmic Bell” experiments, we used real-time astronomical observations of distant quasars as a classical source of binary numbers, to determine which measurements to perform on pairs of entangled particles. We found clear violations of Bell’s inequality, providing even more compelling evidence that quantum entanglement is a robust feature of our world. These experiments push back to at least 7.8 Gyr ago the most recent time by which any (non-quantum-mechanical) local-realist mechanism could have exploited the freedom-of-choice loophole to engineer the observed Bell violation, excluding any such mechanism from 96% of the spacetime volume of the past light cone of our experiment, extending from the big bang to today.

Speaker: Prof. David Kaiser (MIT)
• 16:00 17:00
informal discussion/coffee time 1h
• Monday, 28 September
• 09:00 10:00
Two-body dynamics and gravitational waves in general relativity 1h

The monumental discovery of gravitational waves by the LIGO-Virgo detectors confirms the sophisticated predictions from general relativity and emphasizes the importance of theoretical works (both analytical and numerical) on the compact binary dynamics: two black holes or neutron stars initially detected in close inward spiralling orbits will merge to form a single massive black hole, producing a burst of gravitational waves. In this talk, after motivating remarks on the properties of gravitational waves and the new gravitational astronomy, we shall describe the analytic two-body problem in general relativity, and the post-Newtonian (PN) approximation which allows an accurate description of the inspiralling phase of compact binaries, and plays a crucial role in the definition of the gravitational wave templates used in the data analysis of the detectors. We shall also mention some recent developments where the PN approximation is computed using the effective field theory and a diagrammatic expansion in the classical limit of general relativity.

Speaker: Luc Blanchet (IAP)
• Thursday, 8 October
• 10:00 11:00
Probing extremely small-scale primordial perturbations by gravitational waves 1h

According to the theory of inflation, the primordial perturbations existed over a wide range of length scales from meter size at the smallest scale up to at least the Hubble horizon on the largest scale. Stochastic gravitational waves have been attracting a lot of interest recently as a new powerful probe of the primordial perturbations on very small scales. In this talk, I will give a brief overview in this field and present a recent search for such gravitational waves by using LIGO O2 data and its implications.

Speaker: Prof. Suyama Teruaki
• Thursday, 15 October
• 20:00 21:00
Detecting the thermal effect of acceleration in an Analog system. 1h

Given the intimate connection of gravity and its dependence on the changing flow of time from place to place, it is surprizing that General Relativistic effects can be modeled in other systems. In 1981 I showed that even the Hawking effect has analogies in other systems, which has spawned an active experimental effort in the past few decades. A harder case has turned out to to model the thermal effect that an accelerated detector in the vacuum. Following Bell and Leinaas in 1983, we showed that a circularly accelerated detector can also show a themal effect. This talks will present a way of implimenting a broad band detector of the quantum fluctuations in a BEC which may also be just realisable with an interferometric detector by borrowing techniques from LIGO, with the interferometer in frequency space rather than in real space.

Speaker: Prof. Bill Unruh
• Thursday, 22 October
• 19:00 20:00
Pulsar Timing Arrays: The Next Window to Open on the Gravitational-Wave Universe 1h

Galaxy mergers are a standard aspect of galaxy formation and evolution, and most (likely all) large galaxies contain supermassive black holes. As part of the merging process, the supermassive black holes should in-spiral together and eventually merge, generating a background of gravitational radiation in the nanohertz to microhertz regime. An array of precisely timed pulsars spread across the sky can form a galactic-scale gravitational wave detector in the nanohertz band. I describe the current efforts to develop and extend the pulsar timing array concept, together with recent limits which have emerged from international efforts to constrain astrophysical phenomena at the heart of supermassive black hole mergers.

Speaker: Chiara Mingarelli
• Wednesday, 28 October
• 15:00 16:00
The status of cosmological tensions after Planck 1h

The results of the ESA Planck satellite have enabled extrordinary progress in our understanding of the universe in the past few years. Furthermore, its (sub)-percent measurement of cosmological parameters allowed us to discover a few inconsistencies with other astrophysical probes, which might point towards a crisis of the current standard model of cosmology. In this talk, I will review some of these inconsistencies and highlight the prospects for the future.

Speaker: Prof. Silvia Galli (IAP)
• 16:00 17:00
informal discussion 1h
• Thursday, 5 November
• 16:00 17:00
Einstein-Cartan gravity: Electroweak Symmetry Breaking, Inflation and Dark Matter 1h

It is well-known since the works of Utiyama and Kibble that the gravitational force can be obtained by gauging the Lorentz group, which puts gravity on the same footing as the Standard Model fields. The resulting theory - Einstein-Cartan gravity - happens to be very interesting. First, it may generate the electroweak symmetry breaking by a non-perturbative gravitational effect. In this way, it does not only address the hierarchy problem but opens up the possibility to calculate the Higgs mass. Second, the model incorporates inflation at energies below the onset of the strong-coupling of the theory. Finally, it inevitably contains a four-fermion interaction that originates from torsion associated with spin degrees of freedom. This interaction leads to a novel universal mechanism for producing singlet fermions in the Early Universe. These fermions can play the role of dark matter particles. The mechanism is operative in a large range of dark matter particle masses: from a few keV up to ∼10^8 GeV.

Speaker: Prof. Mikhail Shaposhnikov
• Tuesday, 17 November
• 15:00 16:00
Exploring the early universe with the stochastic background of induced gravitational waves 1h

I will discuss the current status of the secondary gravitational waves induced by the curvature perturbation and why they might be an important source of the cosmological stochastic gravitational wave background. As a practical example, I will use the latest NANOGrav results on the stochastic background of nanohertz gravitational waves to constrain the equation of state parameter of the early universe and the mass of the associated primordial black holes.

• Monday, 23 November
• 16:00 17:00
New Physics and the Black Hole Mass Gap 1h

In this talk I will demonstrate the potential of the black hole mass gap to probe new physics. The mass gap, in which no black holes can be formed, is a standard prediction of stellar structure theory. I will show that new physics that couples to the Standard Model can act as an additional source of energy loss in the cores of heavy stars, dramatically altering their evolution, resulting in large shifts of the gap. I will also discuss how new contributions may modify the stellar equation of state. The gravitational wave observations by the LIGO/Virgo collaboration will bring the edges of the black hole mass gap in sight in the coming years, making this a promising novel probe of new physics.

Speaker: Djuna Croon (TRIUMF)
• Thursday, 3 December
• 14:00 15:00
Causality with Gravity 1h

In standard effective field theories, the notion of causality is intrinsically linked with that of subluminality and with a set of positivity constraints to be imposed on the low-energy scattering amplitudes. I will highlight how the presence of gravity leads to a more subtle relation between causality, (sub)luminality and positivity bounds. I will clarify why a mild level of superluminality is not in contradiction with causality, analyticity or Lorentz invariance and show how consistent gravitational low energy effective theories can self-protect by ensuring that any time advance and superluminality calculated within the regime of validity of the effective theory is necessarily unresolvable for such theories. These considerations are particularly relevant for putting constraints on cosmological and gravitational effective field theories and I will provide explicit criteria to be satisfied so as to ensure causality and a standard high energy completion in gravitational effective field theories.

Speaker: Prof. de Rham Claudia
• Friday, 11 December
• 10:00 11:00
A look at axion inflation in string theory 1h

We will take a look at axion inflation in string theory, taking a somewhat eclectic approach guided by some mechanism classes and (semi-)explicit examples. Looking at models with either 1 or 2 axions, we will argue that (up to manifestly tuning for small-field models) inflation can arise from 2 different mechanisms - either monodromy, or hybrid inflation. Cautiously incorporating both known limits of top-down theory knowledge as well as bottom-up ‘effective quantum gravity’ conjectural constraints, should lead to a ‘theory error blob’ of CMB observable predictions describing the ‘mechanism equivalence class’. We outline this using harmonic hybrid inflation as a representative of 2-axion hybrid inflation, and then describe a systematic method of propagating the ‘theory error’ to observable predictions using machine learning and information geometry

Speaker: Prof. Alexander Westphal (DESY)
• Monday, 14 December
• 15:00 16:00
Black Holes Are Finally in Vogue 1h

With black holes, what you see is not what you get. They are extreme structures of spacetime that represent the ultimate prison, from where even light cannot escape. After decades of being a subject of mathematical interest, recently black holes became a topic of direct observational studies, for which two Nobel prizes were awarded over the past three years. I will describe some of the most exciting frontiers in current multi-messenger studies of astrophysical black holes.

Speaker: Prof. Abraham (Avi) Loeb (Harvard University)
• Tuesday, 12 January
• 13:00 14:00
Quantum fluctuations and new instantons 1h

I will discuss how quantum fluctuations modify the Coleman theory of the decay of false vacuum

Speaker: Prof. Viatcheslav Mukhanov (LMU)
• Tuesday, 26 January
• 14:00 15:00
Hunting for Parity-violating Physics in Polarisation of the Cosmic Microwave Background 1h

Polarised light of the cosmic microwave background, the remnant light of the Big Bang, is sensitive to parity-violating physics. In this presentation we report on a new measurement of parity violation from polarisation data of the European Space Agency (ESA)’s Planck satellite. The statistical significance of the measured signal is 2.4 sigma. If confirmed with higher statistical significance in future, it would have important implications for the elusive nature of dark matter and dark energy.

Speaker: Prof. Eiichiro Komatsu (Max-Planck-Institute for Astrophysics)
• Thursday, 28 January
• 11:00 13:00
Cosmological Genesis: Approaches and Problems. 2h

Cosmological Genesis is a scenario without initial singularity,
in which the Universe starts off from nearly Minkowski state with nearly vanishing energy density, then the energy density increases, the expansion rate grows; at some later epoch the energy density is converted into heat, and the conventional hot epoch begins (variant: at some later epoch energy density stops increasing and inflationary epoch begins).
Clearly, this scenario requires exotic form of matter, which violates energy conditions, and/or exotic modification of gravity. This talk will concentrate on scalar-tensor theories of Horndeski type, and generalizations thereof. Despie initial high expectations, there are problems with stable and subluminal Genesis in these theories. These problems, and attempts to solve them, will be the main focus.

Speaker: Prof. Valery Rubakov (INR Moscow)
• Tuesday, 2 February
• 15:00 16:00
Are there any fundamental problems with quantum gravity? 1h

I plan to informally discuss several issues that have traditionally been raised in various approaches to quantizing gravity. They are invariably related to the concepts that are thought to be fundamental in one of the two theories (quantum and GR) but are (allegedly) at odds with the other one. I will discuss some of the key issues in my talk, such as Bell non-locality and the equivalence principle, only to conclude that they are, in my view, not fundamentally an obstacle. Lack of experiments, on the other hand, is a real obstacle, but, even here, we are closer than ever to being able to test the quantum nature of gravity in the lab. I will describe how.

Speaker: Prof. Vlatko Vedral (Oxford)
• Thursday, 4 February
• 16:00 17:00
The Topology of Data: from String Theory to Cosmology to Phases of Matter 1h

We are faced with an explosion of data in many areas of physics, but very so often, it is not the size but the complexity of the data that makes extracting physics from big datasets challenging. As I will discuss in this talk, data has shape and the shape of data encodes the underlying physics. Persistent homology is a tool in computational topology developed for quantifying the shape of data. I will discuss three applications of topological data analysis: 1) identifying structure of the string landscape, 2) constraining cosmological parameters from CMB measurements and large scale structures data, and 3) detecting and classifying phases of matter. Persistent homology condenses these datasets into their most relevant (and interpretable) features, so that simple statistical pipelines are sufficient in these contexts. This suggests that TDA can be used in conjunction with machine learning algorithms and improves their architecture.

Speaker: Prof. Gary Shiu (University of Wisconsin-Madison)
• Tuesday, 9 February
• 15:00 16:00
Quantum gravity predictions for cosmology: from the beginning to dynamical dark energy. 1h
Speaker: Prof. Christof Wetterich (Universitat Heidelberg)
• Thursday, 18 February
• 10:00 11:00
Axions, Dark Matter, and the Primordial Density Perturbation 1h

I will present new ideas about how the QCD axion or axion-like particles can make up the dark matter of our universe, and/or explain the origin of the primordial density perturbation. For axion dark matter, I will introduce a novel production mechanism that invokes a kinetic mixing between the axion and the inflaton. I will show that this mechanism opens up new windows in the axion parameter space, where conventional scenarios such as vacuum misalignment cannot work. The impact of primordial electromagnetic fields on the axion window will also be discussed. For the density perturbation, I will demonstrate that an axion-like particle coupled to a new confining gauge group is a perfect candidate of a curvaton, and that the resulting density perturbation has distinct signatures that are testable in upcoming experiments.

Speaker: Prof. Takeshi Kobayashi (Nagoya University)
• Tuesday, 23 February
• 14:00 15:00
Gravitational waves from first-order phase transition during inflation 1h

I will talk about the properties of the gravitational wave (GW) signals produced by first-order phase transitions during the inflation era. I will show that the power spectrum of the GW oscillates with its wave number. This oscillatory feature corresponds to the instantaneous nature of the first-order phase transition. I will also show that we can get information about how the universe evolves during and after inflation from the slopes of different parts of the spectrum. I will also present simple models that first-order phase transition can happen and finish during inflation. I will also show that this signal can be observed directly by future terrestrial and spatial GW detectors and through the B-mode spectrum in CMB.

Speaker: Prof. Haipeng An (Tsinghua University)
• Tuesday, 2 March
• 14:00 15:00
New physics on the horizon? Testing the nature of dark compact objects 1h

Gravitational-wave astronomy and new electromagnetic facilities allows us for unprecedented tests of the nature of dark compact objects and provide a novel way to search for new physics. I will give an overview of the many recent result in this area including, shadows, constraints on the multipolar structure, ringdown tests, gravitational-wave echoes, and tidal effects in binaries.

Speaker: Prof. Paolo Pani (Sapienza University of Rome)
• Thursday, 4 March
• 14:00 15:00
Emergence of electromagnetic wave and gravitational wave from quantum information (qubit ocean) 1h

From quantum theory, we know that all elementary particles are waves. For example, photons are waves that satisfy Maxwell equation. Here we discuss the possibility that our space is a qubit ocean. We show that, if the qubits that form the space are properly entangled, the deformation of the qubit ocean corresponds to wave that satisfy Maxwell equation. This is an emergence of electromagnetic wave from quantum information. We then discuss attempts to have an emergence of gravitational wave from qubit ocean, that is only half successful.

Speaker: Prof. Xiao-Gang Wen (MIT)
• Thursday, 11 March
• 15:00 16:00
Cosmology from the CMB frequency spectrum 1h

The frequency spectrum of the CMB was last measured in the nineties by the FIRAS instrument onboard COBE. It was found to be consistent with a perfect blackbody spectrum, up to <1e-4 relative deviations. Today, there is growing interest in re-exploring in more depth this aspect of the CMB, which is complementary to the well-studied CMB anisotropies. In this talk I will briefly review the physics of CMB spectral distortions, and what signals are guaranteed in the standard cosmological model. Beyond this, I will show how CMB spectral distortions can probe dark matter interactions with standard particles, and could thus help shed light on its nature. Lastly, I will discuss the interplay between the CMB monopole temperature T0 and cosmological parameters (in particular the Hubble constant H0) inferred from CMB anisotropies.

Speaker: Prof. Yacine Ali-Haimoud (NYU)
• Tuesday, 16 March
• 14:00 15:00
Gravitational Collider Physics 1h

Gravitational wave astronomy will transform astrophysics in many ways; can it do the same for particle physics? In this talk, I will describe how the gravitational waves emitted by binary black holes offer a new window onto physics beyond the Standard Model. I will focus on probes of ultralight bosons such as axion-like particles and dark photons, which can spontaneously form bound states around rotating black holes. Remarkably, these bound states resemble the proton-electron structure of the hydrogen atom and are therefore often called the "gravitational atoms". In addition, the dynamics of these atoms in binary systems can be formulated as scattering events which are quantified by a S-matrix. These dynamics would significantly backreact on the orbit, thereby affecting the gravitational waves emitted by the binary system. These gravitational wave signatures would also carry imprints of the masses and intrinsic spins of the ultralight bosons, making binary black holes novel detectors of these putative new fields and effectively "gravitational colliders".

Speaker: Prof. Horng Sheng Chia (IAS)
• Thursday, 25 March
• 14:00 15:00
Decoding and bootstrapping cosmological fluctuations 1h

I will review our current understanding of the initial conditions of the universe, and describe what information is available from current and future measurements of cosmological correlation functions. Then I will describe a new method to compute and constrain the possible shapes of those correlation functions, assuming they were generated during inflation. This cosmological bootstrap” draws inspiration from the modern scattering amplitudes program in flat space, as well as the conformal bootstrap of phase transitions. After discussing primordial scalar fluctuations, I will also explain how the consistent propagation of gravitational waves imposes further constraints on the structure of spinning primordial correlators.

Speaker: Prof. Guilherme L. Pimentel (Leiden U.)
• Tuesday, 30 March
• 15:00 16:00
An order-unity correction to Hawking radiation 1h

When a black hole first forms, the properties of the emitted radiation as measured by observers near future null infinity are very close to the 1974 prediction of Hawking. However, deviations grow with time, and become of order unity after a time $t \sim M_i^{7/3}$, where $M_i$ is the initial mass in Planck units. After an evaporation time the corrections are large: the angular distribution of the emitted radiation is no longer dominated by low multipoles, with an exponential falloff at high multipoles. Instead, the radiation is redistributed as a power law spectrum over a broad range of angular scales, all the way down to the scale $\Delta \theta \sim 1/M_i$, beyond which there is exponential falloff. This effect is is a quantum gravitational effect, whose origin is the spreading of the wavefunction of the black hole's center of mass location caused by the kicks of the individual outgoing quanta, discovered by Page in 1980. The modified angular distribution of the Hawking radiation has an important consequence: the number of soft hair modes that can effectively interact with outgoing Hawking quanta increases from the handful of modes at low multipoles $l$, to a large number of modes, of order $\sim M_i^2$. We argue that this change removes on of the primary objections to the Hawking-Perry-Strominger proposed mechanism for purifying the Hawking radiation.

Speaker: Prof. Eanna Flanagan (Cornell University)
• Tuesday, 6 April
• 15:00 16:00
Dark Matter via inverse phase transition 1h

I will discuss a novel mechanism of Dark Matter production through an inverse phase transition. I will focus on a simple Z_2-symmetric model of Dark Matter composed of a scalar singlet. Due to couplings to other matter fields, Z_2-symmetry is spontaneously broken at very early times, and the Dark Matter field is offset from zero. As the Universe expands, Z_2-symmetry is restored, and the Dark Matter field starts oscillating around zero contributing to the pressureless fluid of the Universe. This simple picture of Dark Matter production admits multiple realisations depending on the nature of symmetry breaking couplings: purely gravitational, magnetic, or due to the interaction with the thermal bath. I will discuss phenomenological consequences in each of these cases.

Speaker: Prof. Sabir Ramazanov (Prague, Inst. Phys.)
• Thursday, 8 April
• 15:00 16:00
Black Hole Information 1h

Black hole information is one of the greatest puzzles of theoretical physics from the 20th century that has persisted into the 21st century. After Stephen Hawking discovered black hole evaporation in 1974, in 1976 he predicted that black hole formation and evaporation would cause a pure quantum state to change into a mixed state, effectively losing information from the universe. In 1979 I questioned this conclusion, as many years later did many others, and in 2004 Hawking conceded that black hole evaporation does not lose information. However, a minority of gravitational theorists have not accepted Hawking's concession. There do remain many puzzles about black hole information, such as how it gets out (if it indeed does), and whether there are firewalls at the surfaces of old black holes that would immediately destroy anything falling in.

Speaker: Prof. Don Page (University of Alberta)
• Tuesday, 13 April
• 15:00 16:00
What the Hubble tension really is and how (not) to solve it 1h

Despite being arguably one of the hottest topics in the recent literature, there are several widely spread misconceptions concerning what the Hubble tension really is. Moreover, leaving these misconceptions aside, no compelling model to solve the Hubble tension has been found so far, despite a huge number of attempts (and false alarms). I will begin by explaining what the Hubble tension really is, presenting three different interpretations in order of increasing "correctness". I will then discuss why so many proposed models have failed so far, and what in my opinion is the way forward towards constructing a realistic solution to the tension. I will close by presenting the "10 commandments" which I believe every "Hubble hunter" should abide by.

Speaker: Dr Sunny Vagnozzi (Cambridge University)
• Tuesday, 20 April
• 15:00 16:00
TBA 1h
Speaker: Prof. Scott Dodelson
• Thursday, 29 April
• 15:00 16:00
TBA 1h
Speaker: John Ellis
• Tuesday, 4 May
• 15:00 16:00
TBA 1h
• Tuesday, 11 May
• 15:00 16:00
TBA 1h
• Thursday, 13 May
• 15:00 16:00
TBA 1h
Speaker: Prof. Shing-Tung Yau (Harvard University & Tsinghua University)
• Tuesday, 18 May