Copernicus Webinar and Colloquium Series

Jul 9, 2020, 10:00 AM → Dec 31, 2024, 4:00 PM Europe/Zurich

Dear Colleagues,

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 126th Copernicus Webinars.

Time:               4th April, Tuesday, at 3:30 pm CEST (9:30 pm Beijing, 10:30pm JST, 9:30 am EDT, 8:30 Alabama)

Speaker:          Sukanya Chakrabarti  (The University of Alabama in Huntsville)

Title: "Towards Precision Measurements of Dark Matter"

Abstract:  For more than a century now, our inference of the mass distributions (including dark matter) in galaxies have been based on modeling the positions and velocities of stars, i.e., using kinematic analyses, which assume equilibrium. These kinematic estimates can be inaccurate for a time-dependent potential, and there are now many lines of observational evidence that show that our Galaxy has had a highly dynamic history.  Technological advances now make it possible for us to carry out extreme-precision time-series measurements of the acceleration of stars that live within the gravitational potential of our Galaxy. I will talk about several different methods of direct acceleration measurements that we have developed, including our recent analysis of compiled pulsar timing data from which we were able to measure the Galactic acceleration for the first time. Given the measured acceleration, we can straightforwardly use the Poisson equation to determine the total density, and the local dark matter density (given an accounting of the stellar density). There are testable differences between popular models of dark matter on small scales, i.e., in their sub-structure. I will discuss the potential for measuring dark matter sub-structure in the Milky Way with pulsar timing and eclipse timing, and for constraining theories of gravity by combining constraints from pulsar timing and extreme precision radial velocity measurements. I will end by discussing our recent work in developing an “acceleration ladder” by calibrating kinematic estimates of the acceleration to direct acceleration measurements.

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Zoom link:
https://zoom.us/j/4214801692?pwd=TGxVWHJudnI1S25FbXhERlVDMkN0dz09
Meeting ID: 421 480 1692
Passcode: cosmology
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Thursday, July 9

1 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

2 informal discussion/coffee time

Wednesday, July 15

3 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 informal discussion/coffee time

Friday, July 17

5 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

6 informal discussion/coffee time

Thursday, July 23

7 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)

8 informal discussion/coffee time

Wednesday, July 29

9 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

10 informal discussion/coffee break

Thursday, August 6

11 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

12 informal discussion/Coffee time

Tuesday, August 11

13 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

14 informal discussion/coffee time

Thursday, August 20

15 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

16 informal discussion/coffee time

Wednesday, August 26

17 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

18 informal discussion/coffee time

Tuesday, September 1

19 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

Friday, September 11

20 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

Wednesday, September 16

21 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

22 informal discussion/coffee time

Monday, September 21

23 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)

24 informal discussion/coffee time

Monday, September 28

25 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)

Thursday, October 8