Copernicus Webinar and Colloquium Series

Jul 9, 2020, 10:00 AM → Jan 31, 2025, 4:00 PM Europe/Zurich

Dear colleagues,

 

We would like to invite you to take part in the upcoming 178th Copernicus Webinar (the last one before the summer break).

 

Time:              16th July, Tuesday, at 3pm CEST (9 pm Beijing, 10pm JST, 9 am EDT). 

 

Speaker:        Hassan Firouzjahi (Institute for Research in Fundamental Sciences)

Title: Upper bounds on the mass of fields from vacuum zero point energy fluctuations during inflation 

 

Abstract: We study the fluctuations in the vacuum zero point energy associated to quantum fields and their statistical distributions during inflation. We show that the perturbations in the vacuum zero point energy  have large amplitudes and are highly non-Gaussian. The effects of vacuum zero point fluctuations can be interpreted as the loop corrections in primordial power spectrum and bispectrum. Requiring that the primordial curvature perturbation to remain nearly scale-invariant and  Gaussian imposes strong upper bounds on the mass of fundamental fields during inflation. We show that the fundamental fields can not be much heavier than the Hubble scale during inflation, otherwise their vacuum zero point fluctuations  induce large non-Gaussianities and scale-dependence in primordial perturbations. Imposing the observational upper bound on tensor to scalar ratio, we conclude that all fundamental fields are lighter than $10^{14}$ GeV.

 

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Zoom link:

https://zoom.us/j/4214801692?pwd=TGxVWHJudnI1S25FbXhERlVDMkN0dz09

Meeting ID: 421 480 1692

Passcode: cosmology

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Thu, July 9

3:00 PM → 4:00 PM Relieving the Hubble tension with primordial magnetic fields

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.)

Pogosian_July9_2020.pdf

4:00 PM → 5:00 PM informal discussion/coffee time

Wed, July 15

3:00 PM → 4:00 PM Physical Implications of a Fundamental Period of Time

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

Copernicus.pdf

4:00 PM → 5:00 PM informal discussion/coffee time

Fri, July 17

3:00 PM → 4:00 PM The Cosmic Microwave Background

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)

The Cosmic Microwave Background_2020.pdf

4:00 PM → 5:00 PM informal discussion/coffee time

Thu, July 23

3:00 PM → 4:00 PM Constraining Early Dark Energy with Large Scale Structure

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)

4:00 PM → 5:00 PM informal discussion/coffee time

Wed, July 29

3:00 PM → 4:00 PM High frequency gravitational waves

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

4:00 PM → 5:00 PM informal discussion/coffee break

Thu, August 6

3:00 PM → 4:00 PM Origin of matter and gravitational wave

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

4:00 PM → 5:00 PM informal discussion/Coffee time

Tue, August 11

3:30 PM → 4:30 PM The Noise of Gravitons

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

4:30 PM → 5:00 PM informal discussion/coffee time

Thu, August 20

3:00 PM → 4:00 PM alpha'-Cosmological tale: String Cosmology backgrounds from Classical String Geometry

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

Cosmological_tale_made_with_strings.pdf

4:00 PM → 5:00 PM informal discussion/coffee time

Wed, August 26

3:00 PM → 4:00 PM Primordial black holes as the solution of many cosmological conundra

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

4:00 PM → 5:00 PM informal discussion/coffee time

Tue, September 1

2:00 PM → 3:00 PM Firewalls in General Relativity

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

Fri, September 11

3:00 PM → 4:00 PM Observational Signatures of Multifield Inflation

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)

Observational signatures of multifield inflation with curved field space.pdf

Wed, September 16

6:00 PM → 7:00 PM Signals of a Quantum Universe

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

Green_Copernicus_seminar.pdf

7:00 PM → 8:00 PM informal discussion/coffee time

Mon, September 21

3:00 PM → 4:00 PM Cosmic Bell Tests: Using Quasars to Test Quantum Theory

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)

4:00 PM → 5:00 PM informal discussion/coffee time

Mon, September 28

9:00 AM → 10:00 AM Two-body dynamics and gravitational waves in general relativity

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)

Thu, October 8

10:00 AM → 11:00 AM Probing extremely small-scale primordial perturbations by gravitational waves

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

Thu, October 15

8:00 PM → 9:00 PM Detecting the thermal effect of acceleration in an Analog system.

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

Thu, October 22

7:00 PM → 8:00 PM Pulsar Timing Arrays: The Next Window to Open on the Gravitational-Wave Universe

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

Wed, October 28

3:00 PM → 4:00 PM The status of cosmological tensions after Planck

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)

4:00 PM → 5:00 PM informal discussion

Thu, November 5

4:00 PM → 5:00 PM Einstein-Cartan gravity: Electroweak Symmetry Breaking, Inflation and Dark Matter

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

Einstein-Cartan.pdf

Tue, November 17

3:00 PM → 4:00 PM Exploring the early universe with the stochastic background of induced gravitational waves

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.

Speaker: Dr Guillem Domenech (Padova)

Mon, November 23

4:00 PM → 5:00 PM New Physics and the Black Hole Mass Gap

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)

Copernicus2020.pdf

Thu, December 3

2:00 PM → 3:00 PM Causality with Gravity

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

Fri, December 11

10:00 AM → 11:00 AM A look at axion inflation in string theory

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)

Mon, December 14

3:00 PM → 4:00 PM Black Holes Are Finally in Vogue

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)

Tue, January 12

1:00 PM → 2:00 PM Quantum fluctuations and new instantons

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

Speaker: Prof. Viatcheslav Mukhanov (LMU)

Tue, January 26

2:00 PM → 3:00 PM Hunting for Parity-violating Physics in Polarisation of the Cosmic Microwave Background

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)

Thu, January 28

11:00 AM → 1:00 PM Cosmological Genesis: Approaches and Problems.

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)

Tue, February 2

3:00 PM → 4:00 PM [Colloquium] Are there any fundamental problems with quantum gravity?

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)

Thu, February 4

4:00 PM → 5:00 PM The Topology of Data: from String Theory to Cosmology to Phases of Matter

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)

Tue, February 9

3:00 PM → 4:00 PM Quantum gravity predictions for cosmology: from the beginning to dynamical dark energy.

Speaker: Prof. Christof Wetterich (Universitat Heidelberg)

Thu, February 18

10:00 AM → 11:00 AM Axions, Dark Matter, and the Primordial Density Perturbation

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)

Tue, February 23

2:00 PM → 3:00 PM Gravitational waves from first-order phase transition during inflation

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)

Copernicus_2021_HaipengAN.pdf

Tue, March 2

2:00 PM → 3:00 PM New physics on the horizon? Testing the nature of dark compact objects

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)

Copernicus_Pani_TestsNatureCompactObjects.pdf

Thu, March 4

2:00 PM → 3:00 PM [Colloquium] Emergence of electromagnetic wave and gravitational wave from quantum information (qubit ocean)

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)

210304grav.pdf

Thu, March 11

3:00 PM → 4:00 PM Cosmology from the CMB frequency spectrum

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)

yah_copernicus.pdf

Tue, March 16

2:00 PM → 3:00 PM Gravitational Collider Physics

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)

2021 Copernicus.pdf

Thu, March 25

2:00 PM → 3:00 PM Decoding and bootstrapping cosmological fluctuations

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.)

Tue, March 30

3:00 PM → 4:00 PM An order-unity correction to Hawking radiation

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)

orderunity.key.pdf

Tue, April 6

3:00 PM → 4:00 PM Dark Matter via inverse phase transition

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.)

Sabir_Copernicus.pdf

Thu, April 8

3:00 PM → 4:00 PM [Colloquium] Black Hole Information

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)

Black-Hole-Information-beamer.pdf

Tue, April 13

3:00 PM → 4:00 PM What the Hubble tension really is and how (not) to solve it

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)

Hubble_tension_Sunny_Vagnozzi_Copernicus.pdf

Tue, April 20

3:00 PM → 4:00 PM New Probes of Large-Scale Structure

Progress in cosmology over the past few decades has been quantified by the extent to which we can accurately measure “two-point functions” such as the power spectrum of galaxies; the shear-shear- correlation function; galaxy-galaxy lensing; and most famously the C_l’s of the anisotropies in the cosmic microwave background. New statistics are emerging though that offer potential to infer even more information about the universe. I will open by asking your opinions about what information is most important that is not listed above. Then, I will give my opinion and share some proposals for extending this relatively new class of statistics to learn about the large-scale structure of the universe.

Speaker: Prof. Scott Dodelson (Carnegie Mellon U.)

Scott Dodelson.pdf

Thu, April 29

3:00 PM → 4:00 PM Fundamental Physics from Gravitational Waves

Following the direct discovery of gravitational waves (GWs) by LIGO and Virgo, there are many opportunities to probe fundamental physics using GWs. These include using GWs from astrophysical sources to constrain the graviton mass and search for Lorentz violation, as well as searching for GWs from dark matter in merging neutron stars, from first-order phase transitions in the early Universe, and from loops of cosmic strings, as may be hinted by recent data from the NANOGrav pulsar timing array. The roles that could be played by atom interferometers measuring GWs in the mid-frequency band between LIGO/Virgo and LISA will be highlighted.

Speaker: Prof. John Ellis (King's College London)

Copernicus.pdf

Tue, May 4

3:00 PM → 4:00 PM Black Holes, Unscripted

Black Holes occupy a special place in the fascination of astronomers and physicists. From the most speculative mathematical physicist to the most sensible radio astronomer, everyone has their own narrative of what lies within a black hole, based on their own preconceptions. This is in contrast to the more empirical and agnostic approach that we take in studying (almost) everything else in physics. As an alternative, I will outline what I consider to be a physical roadmap to unravelling what lies within black holes.

Speaker: Prof. Niayesh Afshordi (Perimeter Institute)

Black Holes, Unscripted.pdf

Tue, May 11

3:00 PM → 4:00 PM Symmetries of Black Hole Perturbation Theory

We present novel symmetries of perturbation theory around rotating and non-rotating black holes in general relativity, and discuss their origins and implications for gravitational-wave astronomy. This is motivated by two special aspects of black hole perturbations in four dimensions: isospectrality of quasinormal modes and the vanishing of tidal Love numbers. There turn out to be off-shell symmetries underlying both of these phenomena. One is a duality, which on shell reproduces the famous Chandrasekhar duality and therefore underlies isospectrality, and can be thought of as an extension of electric-magnetic duality to black hole backgrounds. The other is an infinite set of "ladder symmetries" relating modes of different angular momentum or spin, which imply the vanishing of Love numbers. We further discuss the geometric origins of these symmetries.

Speaker: Prof. Adam Solomon (Carnegie Mellon U.)

Copernicus2021.pdf

Thu, May 13

3:00 PM → 4:00 PM Black hole existence and intrinsic angular momentum in general relativity

In this talk , I shall report on how black hole is formed and how classical conserved quantities are defined in general relativity . These are joint works with Schoen , Mu Tao Wang , Poning Chen , and Ye Kai Wang .

Speaker: Prof. Shing-Tung Yau (Harvard University & Tsinghua University)

Tue, May 18

3:00 PM → 4:00 PM Quantum collapse models and cosmic inflation

Attempts to apply quantum collapse theories to Cosmology and cosmic inflation are reviewed. These attempts are motivated by the fact that the theory of cosmological perturbations of quantum-mechanical origin suffers from the single outcome problem, which is a modern incarnation of the quantum measurement problem, and that collapse models can provide a solution to these issues. Since inflationary predictions can be very accurately tested by cosmological data, this also leads to constraints on collapse models. These constraints are derived in the case of Continuous Spontaneous Localization (CSL) and are shown to be of unprecedented efficiency.

Speaker: Prof. Jerome Martin (IAP)

Copernicus21.pdf

Tue, May 25

2:00 PM → 3:00 PM Minimalism in modified gravity

It is generally believed that modification of general relativity inevitably introduces extra physical degree(s) of freedom. In this talk I argue that this is not the case by constructing modified gravity theories with two local physical degrees of freedom. After classifying such theories into two types, I show explicit examples and discuss their cosmology and phenomenology, such as possible amelioration of the H0 tension and stable massive gravity/bigravity cosmology.

Speaker: Prof. Shinji Mukohyama (YITP)

210525Mukohyama.pdf

Tue, June 1

3:00 PM → 4:00 PM Probing high-scale new physics with modulated reheating

Scalar fields with spatially varying background could modulate the reheating process, thereby leaving their imprints in the density perturbations. In this talk we discuss two scenarios using this mechanism to probe physics at very high scales. First, we introduce a “cosmological Higgs collider” where the SM-Higgs-modulated reheating allows us to discover heavy particles and to measure their Higgs couplings in the squeezed primordial bispectrum. Second, we explain that the modulated reheating can act as a “cosmic microscope” that enlarges the small-scale preheating dynamics to CMB scales, providing us a chance to learn non-perturbative dynamics of the preheating.

Speaker: Prof. Zhong-Zhi Xianyu (Tsinghua University)

LightFields_CopernicusWebinar.pdf

Tue, June 8

3:00 PM → 4:00 PM Indirect detection of gravitons through quantum entanglement

We propose an experiment that the entanglement between two macroscopic mirrors suspended at the end of an equal-arm interferometer is destroyed by the noise of gravitons through bremsstrahlung. By calculating the correlation function of the noise, we obtain the decoherence time from the decoherence functional. We estimate that the decoherence time induced by the noise of gravitons in squeezed states stemming from inflation is approximately 20 seconds for 40 km long arms and 40 kg mirrors. Our analysis shows that observation of the decoherence time of quantum entanglement has the potential to detect gravitons indirectly. This indirect detection of gravitons would give strong evidence of quantum gravity.

Speaker: Prof. Sugumi Kanno (Kyushu University)

Tue, June 15

3:00 PM → 4:00 PM A stringy perspective on the coincidence problem

The cosmological coincidence problem is the question of why now? Why do we live at the dawn of dark energy domination, when the energy density of dark matter and dark energy are roughly comparable? In this talk, I will describe how the problem is significantly alleviated, if not entirely solved, in generic string theory models of dark energy.

Speaker: Prof. Antonio Padilla (Nottingham University)

Thu, June 17

3:00 PM → 4:00 PM [Colloquium] Black Hole Thermodynamics, Then and Now

Speaker: Prof. Edward Witten (IAS)

Tue, June 22

3:00 PM → 4:00 PM Self-Organised Localisation

We describe a new phenomenon in quantum cosmology: self-organised localisation. When the fundamental parameters of a theory are functions of a scalar field subject to large fluctuations during inflation, quantum phase transitions can act as dynamical attractors. As a result, the theory parameters are probabilistically localised around the critical value and the Universe finds itself at the edge of a phase transition. We illustrate how self-organised localisation could account for the observed near-criticality of the Higgs self-coupling, the naturalness of the Higgs mass, or the smallness of the cosmological constant.

Speaker: Prof. Tevong You (CERN)

2021_06 - Copernicus - Self-Organised Localisation.pdf

Tue, June 29

3:00 PM → 4:00 PM Probing particle physics and cosmology with cosmic strings and gravitational waves

In this seminar I will present the latest research and results on cosmic strings, which are line-like defects which may be formed in spontaneous symmetry breaking phase transitions in the early universe. Such phase transitions may have occurred at grand unification energy scales, and more generally at lower scales. Through their different observational consequences — which I will discuss here — cosmic strings can therefore probe particle physics, and also cosmology, beyond the standard models, and at energy scales much above those of particle accelerators. Being highly relativistic, cosmic strings are sources of gravitational waves (GWs), and a network of cosmic strings formed in the early universe emits GWs throughout its history generating a stochastic gravitational wave background (SGWB). We discuss the shape of this spectrum, depending on the properties of the string network and also the expansion history of the universe, and the constraints which are (and will be) placed by GW detectors. Additionally, cosmic strings produce short bursts of GWs that can be searched for individually in LIGO-Virgo data, and we present the latest results from the O3 run. Finally, we discuss some other advanced and open topics, including how cosmic strings can be a possible component of dark matter.

Speaker: Prof. Daniele Steer (APC, Paris)

copernicus-strings.pdf

Tue, July 6

3:00 PM → 4:00 PM A higher dimensional view on quantum cosmology

Fundamental questions in cosmology, such as understanding the Big Bang, the cosmic hierarchy problem and boundary conditions for the wave function of the universe, should be possible to address explicitly in (toy?) models that are UV complete. Inspired from string theory I will discuss the role of extra dimensions in all these issues and show that indeed with some varying degree of precision the physics of extra dimensions provides answers. For instance; in a very natural suggestion by Banerjee et al to realise a de Sitter cosmology from string theory, I will explain how Vilenkin's tunneling wave function is selected as the unique boundary condition for the wave function of the universe.

Speaker: Thomas Van Riet

Copernicuswebinar2.pdf

Tue, July 13

3:00 PM → 4:00 PM Clustered PBHs and Stellar Bubbles from the Primordial Universe

We explore the possibility of strongly clustered primordial black holes (PBHs). Such clustered PBHs may be produced by multi-stream inflation. Those PBH bubbles may produce gravitational waves possibly detectable by LISA, or themselves shine with optical signals like stars. Similar clustering mechanisms may be used to study bubbles with other exotic matter such as unstable particles, domain walls, cosmic strings or monopoles.

Speaker: Prof. Yi Wang (HKUST)

stellar bubbles - 20210712.pdf

Thu, July 15

3:00 PM → 4:00 PM [Colloquium] Is Nature Natural? The Electroweak Hierarchy Problem Circa 2021

The discovery of the Higgs boson at the LHC marks the culmination of a decades-long quest for the final piece of the Standard Model. But the discovery of the Higgs also adds new urgency to the hierarchy problem, namely the question of why the Higgs boson is so light despite its unique quantum sensitivity to much higher energy scales. This puzzle is made all the more challenging by the lack of evidence for conventional approaches to the hierarchy problem at the LHC and other experiments. In this talk I'll discuss the essential features of the hierarchy problem and its many possible solutions — ranging from the familiar to the highly speculative — with a particular focus on recently-developed approaches that connect the hierarchy problem to cosmology in diverse ways.

Speaker: Prof. Nathaniel Craig (UC, Santa Barbara)

Copernicus_Colloquium_Naturalness_Craig.pdf

Tue, July 20

3:00 PM → 4:20 PM Primordial features and non-Gaussianities as probes of fundamental physics

During the primordial universe such as the inflationary epoch, all particles with mass up to the Hubble parameter or higher are excited quantum-mechanically or classically. These particles left their imprints in the primordial density perturbations, as primordial features and non-Gaussianities, which may be probed by astrophysical observations of the large-scale structure of the universe today. These informations include the particle mass and spin spectra, and the scale factor evolutionary history a(t) of the primordial universe. The latter would provide a direct evidence for the inflation or an alternative scenario. As an example, we present an inflationary primordial feature model that can explain both the large and small-scale feature anomalies in the currently measured CMB anisotropy spectra, revealing a clip of adventurous history of the Universe during its primordial epoch and realizing some of the properties outlined above. We show how to further test such models in future experiments.

Speaker: Prof. Xingang Chen (Harvard, CfA)

Chen_Copernicus.pdf

Tue, July 27

3:00 PM → 4:00 PM Testing the expansion rate with the cosmological recombination lines and exotic physics

Current measurements of the CMB anisotropies have given us unprecedented precision surrounding the standard ΛCDM model of cosmology and the parameters that make up this model. The data accrued by collaborations like Planck have even allowed us to test additional models of fundamental physics. These models have grown more recently in the context of diluting the tension between low-redshift and high-redshift measurements of the Hubble constant. With the exquisite data, we required a deeper understanding of recombination physics, particularly focused on the relationship between the electrons and the photons. Consequently, we can now calculate the distortions to the CMB black body from such an interaction. In this talk, I am going to demonstrate how we could measure these distortions in future experiments (e.g., Voyage 2050) and discuss the impact on the recombination radiation when we add exotic physics such as early dark energy or fundamental constant variations.

Speaker: Prof. Luke Hart (University of Manchester)

copernicusSlides.pdf

Tue, August 3

3:00 PM → 4:20 PM On the assumptions leading to the information loss paradox

The information loss paradox is usually stated as an incompatibility between general relativity and quantum mechanics. However, the assumptions leading to the problem are often overlooked and, in fact, a careful inspection of the main hypothesises suggests a radical reformulation of the problem. Indeed, I will present a thought experiment that shows the existence of an incompatibility between (i) the validity of the laws of general relativity to describe infalling matter far from the Planckian regime, and (ii) the so-called central dogma which states that as seen from an outside observer a black hole behaves like a quantum system whose number of degrees of freedom is proportional to the horizon area. We critically revise the standard arguments in support of the central dogma, and argue that they cannot hold true unless some new physics is invoked even before reaching Planck scales. This leads to the counterintuitive result that the information loss problem, in its current formulation, is not necessarily related to any loss of information or lack of unitarity. Semiclassical general relativity and quantum mechanics can be perfectly compatible before reaching the final stage of the black hole evaporation where, instead, a consistent theory of quantum gravity is needed to make any prediction.

Speaker: Prof. Francesco Di filippo (YITP, Kyoto)

Francesco Di Filippo.pdf

Tue, August 10

3:00 PM → 4:20 PM Influence Through Mixing: Hotspots as Benchmarks for Black Hole Behaviour

Effective theories are being developed for quantum fields outside black holes, often with an unusual open system feel due to the influence of large number of degrees of freedom lying out of reach beyond the horizon. The absence of comparisons to simpler systems that share these features complicates the interpretation of what is found. This talk describes a simple model aimed to help remedy this, that involves a single external scalar field that mixes in a limited region of space with a large number of hot internal degrees of freedom. Since the model is at heart gaussian it can be solved explicitly, and correlation functions computed for the external field once the others are traced out. The results can be compared with various approximations, such as perturbative and resummed open-system calculations, to test their efficacy, and predictions can be made for the response of external qubit probes and for the evolution and decoherence of the external field itself.

Speaker: Prof. Cliff Burgess (McMaster University, Perimeter Institute)

Hotspots-Copernicus.pdf

Tue, August 31

3:00 PM → 4:20 PM Small scale signatures of non-trivial inflationary and post-inflationary dynamics

The primordial scalar power spectrum is well constrained over large scales, essentially by the observations of the anisotropies in the cosmic microwave background. However, the current bounds on the scalar power spectrum over small scales are considerably weaker. During the last few years, there has been an interest in examining scenarios which generate enhanced scalar power on small scales and lead to significant production of primordial black holes as well as induce secondary gravitational waves (GWs) of possibly detectable amplitudes. In this talk, I shall first outline some of the inflationary scenarios we have examined in this context. Thereafter, I shall describe the scalar bispectrum arising in such scenarios in single field models of inflation and discuss the corresponding imprints on the spectral density of secondary GWs. I shall then illustrate the difficulty in generating enhanced power on small scales from squeezed initial states. Lastly, I shall highlight the manner in which non-trivial post-inflationary dynamics can leave telltale imprints on the spectral density of primary GWs at small scales.

Speaker: Lakshmanan Sriramkumar (Indian Inst. of Tech., Madras)

l.sriramkumar-cw-august-31-2021.pdf

Tue, September 7

3:00 PM → 4:20 PM Understanding Cosmic Acceleration: A Question of Life, the Universe and Everything?

The cosmological constant provides a simple explanation for the observed late-time accelerated expansion of our Universe. Our lack of understanding of it, however, motivates the exploration of alternative explanations such as a modification of General Relativity at cosmological scales. I will first discuss how gravitational wave observations have severely challenged that concept. I will then present a new self-tuning mechanism that provides a simple toy model to simultaneously solve both the old and new aspects of the cosmological constant problem. A possible signature of this mechanism is the variation of fundamental constants across different spacetime regions. I will briefly present new tools that can be used with forthcoming nonlinear cosmological data in the search for New Physics. Finally, I will explore the Emergence of Life across a potential multiverse as an approach to a deeper understanding of the fundamental parameters and laws of our own observable Universe.

Speaker: Lucas Lombriser (University of Geneva)

Copernicus_Webinar_Lombriser.pdf

Tue, September 14

3:00 PM → 5:00 PM Probing dark energy and inflation with gravitational waves

In this talk, I will summarize several recent results about gravitational wave cosmology in the context of dark energy and inflation. In the first part of the talk, I will concentrate on astrophysical gravitational waves and will argue that the spatial clustering of gravitational wave sources provides a wealth of invaluable information concerning the propagation law of gravitational waves. I will demonstrate its importance for constraining deviations from General Relativity on cosmological scales. In the second part of the talk, I will discuss gravitational waves produced during inflation and will revisit the implications of their possible near-future detection for inflationary models. I will particularly present our proposal of resonant gravitational wave production during inflation due to non-linear effects and will discuss the implications for the well-known Lyth bound. I will also present related ideas for producing induced gravitational waves and primordial black holes on small scales.

Speaker: Valeri Vardanyan (Kavli IPMU, University of Tokyo)

Vardanyan_Copernicus_September2021.pdf

Tue, September 21

3:00 PM → 5:00 PM Chemical-Potential-Assisted Particle Production in FRW Spacetimes; Uncalibrated cosmic standards (UCS) and Early-Universe-Physics Insensitive determinations of the Hubble Constant

We analyze gravitational particle production assisted by chemical potential. By utilizing the uniformly smoothed Stokes-line method and Borel summation, we gain insight into the fine-grained history of enhanced particle production. Analytic/semi-analytic formulae describing the production amount, time and width are obtained for both spin-1 and spin-1/2 particles in various FRW spacetimes. Our work also serves as a concrete demonstration of the uniformly smoothed Stokes-line method applied to cosmology.

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To further investigate whether pre-recombination physics alone can resolve the Hubble tension, we explore H0 determinations that are insensitive to early-universe physics. The similarity between the two sound horizons at recombination and at the drag epoch is insensitive to early-universe physics. This allows us to relate the two horizons and treat them as free parameters. Together with Type Ia supernovae, these uncalibrated cosmic standards (UCS) robustly constrain the matter density fraction. Combining with other non-local observations, we obtain several constraints on H0 with significantly reduced sensitivity to early-universe physics, which are consistent with Planck. These results suggest that pre-recombination physics cannot fully resolve the Hubble tension.

Speaker: Xi Tong and Weikang Lin . (Hong Kong University of Science and Technology; Tsung-Dao Lee Institute, Shanghai Jiao Tong University)

UCS_WeikangLin.pdf

XiTong_Chemical-Potential-Assisted Particle Production in FRW Spacetimes_short.pdf

Thu, September 23

3:00 PM → 4:00 PM [Colloquium] GW astrophysics with LIGO/VIRGO data

I will describe our recent work re-analyzing the GW data made public by the LIGO collaboration. More broadly I will discuss some of the outstanding questions related to binary black hole mergers, what the data might be saying and what we might expect in the near future. I will focus on what can be inferred from the spin measurements.

Speaker: Prof. Matias Zaldarriaga (IAS)

Zaldarriaga.pdf

Fri, October 1

3:00 PM → 4:20 PM Emergent Cosmology from the BFSS Matrix Model

The BFSS matrix model is a proposed non-perturbative definition of M-theory in which space is emergent. In this talk, I shall present a new paradigm of early-universe cosmology in the context of the BFSS theory. Specifically, I will show that matrix theory leads to an emergent non-singular cosmology which, at late times, can be described by an expanding phase of Standard Big Bang cosmology. Crucially, the thermal fluctuations in the emergent phase source an approximately scale-invariant spectrum of scalar perturbations and a scale-invariant spectrum of gravitational waves. Hence, this model leads to a successful scenario for the origin of perturbations responsible for the currently observed structure in the universe while providing a consistent UV-complete description.

Speaker: Suddhasattwa Brahma (U. Edinburgh)

Copernicus_Seminar_2021.pdf

Fri, October 8

3:00 PM → 4:40 PM Gauge invariant formulation of the induced gravitational waves

The gauge dependence of the second order induced gravitational waves are widely recognized issue since GWs should be physical observable. While there have been several studies about the gauge dependence, I will talk about a new idea to solve the issue. I revisited the definition of nonlinear tensor modes and their gauge transformation from a covariant perspective and find a way to properly define the gauge invariant energy density of the induced GWs.

Speaker: Atsuhisa Ota (Hong Kong University of Science and Technology)

Tue, October 12

3:00 PM → 4:20 PM Unlocking Dark Matter Physics out of Astrophysical Data Sets

Cosmological observations and galaxy dynamics seem to imply that 84% of all matter in the universe is composed of dark matter, which is not accounted for by the Standard Model of particles. The particle nature of dark matter is one of the most intriguing puzzles of our time.
The wealth of knowledge which is and will soon be available from astrophysical surveys will reveal new information about our universe. I will discuss how we can use new and complementary data sets to improve our understanding of the particle nature of dark matter both at large and small scales.

Speaker: Cora Dvorkin (Harvard University)

Tue, October 19

3:00 PM → 4:20 PM Some Ideas about Cosmic Structure on the Smallest Scales

The present day distribution of dark matter on scales smaller than the mass scale of dwarf galaxies contain a wealth of information on the early history of the early universe as well as the nature of dark matter. This distribution is not reflected in the distribution of gas and stars because the amplitude of dark matter inhomogeneities on these scales are constrained to have little effect on these baryonic tracers. Yet the remnants of the smallest scale dark matter structure surround us, perhaps in the form of dark matter "microhalos’’. This talk will focus on two ideas related to the smallest scale dark matter structures:
1) what are the remnants of the gravitationally bound structures which may have existed during an early phase of matter domination
2) detecting the inevitable small inhomogeneities in ultra-light dark matter required by the uncertainty principle

Speaker: Albert Stebbins (University of Chicago)

20211019_UltraSmallScaleStructure_Copernicus.pdf

Mon, October 25