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Kavli Institute for Cosmology, Cambridge

 
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Wed 03 Mar 16:00: Extracting the Universe from the Wave Function

2 hours 26 min ago
Extracting the Universe from the Wave Function

Quantum mechanics is a theory of wave functions in Hilbert space. Many features that we generally take for granted when we use quantum mechanics—classical spacetime, locality, the system/environment split, collapse/branching, preferred observables, the Born rule for probabilities—should in principle be derivable from the basic ingredients of the quantum state and the Hamiltonian. I will discuss recent progress on these problems, including consequences for emergent spacetime and quantum gravity.

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Tue 18 May 13:00: TBA

Thu, 25/02/2021 - 19:21
TBA

TBA

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Fri 05 Mar 13:00: Cauchy Evolution of Asymptotically Global AdS Spacetimes with No Symmetries

Thu, 25/02/2021 - 11:26
Cauchy Evolution of Asymptotically Global AdS Spacetimes with No Symmetries

I will present the first proof-of-principle Cauchy evolutions of asymptotically global AdS spacetimes with no imposed symmetries, employing a numerical scheme based on the generalized harmonic form of the Einstein equations. In this scheme, the main difficulty in removing all symmetry assumptions can be phrased in terms of finding a set of generalized harmonic source functions that are consistent with AdS boundary conditions. In four spacetime dimensions, we detail an explicit set of source functions that achieves evolution in full generality. A similar prescription should also lead to stable evolution in higher spacetime dimensions, various couplings with matter fields, and on the Poincaré patch. We apply this scheme to obtain the first long-time stable 3+1 simulations of four dimensional spacetimes with a negative cosmological constant, using initial data sourced by a massless scalar field. I will show preliminary results of gravitational collapse with no symmetry assumptions, and the subsequent quasi-normal mode ringdown to a static black hole in the bulk, which corresponds to evolution towards a homogeneous state on the boundary.

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Fri 05 Mar 13:00: Title to be confirmed

Wed, 24/02/2021 - 10:14
Title to be confirmed

Abstract not available

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Mon 01 Mar 13:00: Gravitational waves from inflation

Wed, 24/02/2021 - 08:26
Gravitational waves from inflation

Primordial gravitational waves have the potential to shed new light on the very early universe. In this talk I discuss the prospects for constraining a compelling class of early universe scenarios known as axion-gauge field inflation. The primordial gravitational waves spectrum originating from these set-ups has strikingly different features with respect to those from the minimal inflationary scenario. I then introduce tensor fossils: these are intriguing gravitational waves imprints on the large scale structure and are potentially detectable by upcoming surveys. I will also highlight their implications for testing inflation at interferometer scales.

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Fri 14 May 13:00: Title to be confirmed

Tue, 23/02/2021 - 15:22
Title to be confirmed

Abstract not available

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Wed 24 Feb 14:00: Probing Cosmic Dawn with the first galaxies

Mon, 22/02/2021 - 10:30
Probing Cosmic Dawn with the first galaxies

Abstract not available

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Wed 24 Feb 13:30: How planets grow by pebble accretion

Mon, 22/02/2021 - 10:30
How planets grow by pebble accretion

Abstract not available

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Tue 23 Feb 13:00: Big Steps Toward Small Worlds: Exoplanet Atmosphere Characterization in the 2020s and Beyond

Fri, 19/02/2021 - 09:13
Big Steps Toward Small Worlds: Exoplanet Atmosphere Characterization in the 2020s and Beyond

We now know that exoplanets abound in the Galaxy, with most stars hosting at least one planet. These recently discovered worlds are much more diverse than the planets in the Solar System, and raise many questions about their formation, evolution, and habitability. To address these questions, we turn to atmosphere characterization, which provides a wealth of additional information about the planets. I will discuss the state of the art in atmosphere studies, focusing on recent high-precision, space-based observations of planets ranging in size from Jupiter to Earth. These studies have already revealed planetary atmospheric chemistry, climate, and cloud coverage in unprecedented detail, and they are poised for a revolutionary advance thanks to a spate of new ground-based and space-based observatories.

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Mon 01 Mar 14:00: Some instabilities resulting from the existence of the negative energy modes in a gas-dust disc Subscribe to receive Zoom link

Thu, 18/02/2021 - 23:12
Some instabilities resulting from the existence of the negative energy modes in a gas-dust disc

The Lagrangian approach can be employed to the dynamics of the linear perturbations of gas-dust mixture in protoplanetary disc with the dust streaming through the gas. It shows that gas-dust perturbations may have negative energy provided that there is a bulk settling of dust to the disc midplane. The most simple variant of the negative energy gas-dust mode is constructed of the dust density perturbations transported by the bulk stream of dust though the unperturbed gas environment. This mode is referred to as the streaming dust wave (SDW). SDW coupled with the positive energy inertial wave (IW) in the gas gives rise to the recently discovered dust settling instability (DSI). Further, SDW by itself exhibits exponential growth as gas-dust dynamics becomes dissipative in the presence of turbulence and provided that the effective turbulent viscosity exceeds the effective turbulent diffusion. This new instability can be referred to as a viscous settling instability (VSI). As compared to DSI , VSI is produced by modes with larger wavelengths, while it vanishes in laminar disc and survives up to the largest value of the effective viscosity in turbulent disc.

Additionally, it can be shown that the three-wave resonance is possible between one negative energy SDW and two positive energy IW’s. A particular case of such a resonance is considered in detail in order to show that it gives rise to the non-linear explosive settling instability (ESI) with a physically reasonable time of explosion. DSI , VSI and ESI may serve as early mechanisms for generation of the dust overdensities in the long-standing problem of planetesimal formation.

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  • Speaker: Viacheslav Zhuravlev - Sternberg Astronomical Institute
  • Monday 01 March 2021, 14:00-15:00
  • Venue: Online.
  • Series: DAMTP Astro Mondays; organiser: Cleo Loi.

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Mon 22 Feb 14:00: In search for red giants' internal magnetic field Subscribe to receive Zoom link

Thu, 18/02/2021 - 23:12
In search for red giants' internal magnetic field

One of the “legacy” results of the Kepler mission is the interestingly low rotation rate of the core of subgiant (SG) and red giant (RG) stars, which is about 10 times lower than predicted with the current theory for the transport of angular momentum by purely hydrodynamical mechanisms. This discrepancy points out an order of magnitude issue concerning the understanding of the evolution of the stellar angular momentum in evolved Solar-like stars, a very ubiquitous problem shared by stars of all types and ages. The recent discovery of very low-amplitude dipolar oscillations in a significant fraction of the observed SGs and RGs also points out our misunderstanding of physical processes inside the radiative interiors of evolved Solar-like stars. We thus seek for a missing process taking place inside the core of evolved Solar-like stars, efficient to extract angular momentum from the core to the surface and to perturbate stellar oscillations.

Internal magnetic fields are one amongst the most serious candidates that are currently studied to solve both problems. Stars more massive than ~1.1 Ms are known to develop a convective core during the main-sequence: the dynamo process due to this convection could be the origin of a strong magnetic field, trapped inside the core of the star for the rest of its evolution. Such magnetic fields should impact mixed modes inside the core of RG stars, and their signature should be visible in asteroseismic data. To unravel which constraints can be obtained from these observations, we theoretically investigate the effects of a plausible mixed axisymmetric magnetic field with various amplitudes on the mixed-mode frequencies of red giants. Applying a perturbative method, we estimate the magnetic splitting of the frequencies of simulated mixed dipolar modes that depends on the magnetic field strength and its configuration. A complete asymptotic analysis is derived, showing the potential of asteroseismology to probe the magnetism at each depth as this is done for stellar rotation. The effects of the mass and the metallicity of the stars are also explored. Finally, we infer an upper limit for the strength of the field and the associated lower limit for the timescale of its action to redistribute angular momentum in stellar interiors.

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Fri 19 Feb 11:30: Dance of the Milky Way and the LMC

Wed, 17/02/2021 - 11:05
Dance of the Milky Way and the LMC

The LMC is the largest satellite of our Galaxy and has a total mass roughly 1/6th that of the Milky Way. In this talk, I will review the multiple lines of evidence for the LMC being so massive and then focus on the substantial effect it has on our Galaxy. In particular, I will discuss its effect on many of the stellar streams around the Milky Way which have been perturbed by the LMC . I will then discuss the stellar halo and show that the outskirts of the Milky Way, beyond ~30 kpc, are significantly out of dynamical equilibrium due to the acceleration of the Milky Way by the LMC . I will end with some preliminary predictions of how the dark matter haloes of the Milky Way and LMC are tidally deforming in the presence of each other and how we can hope to measure this effect in the coming years using tidal streams.

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Mon 22 Feb 13:00: Reconstruction of the neutrino mass as a function of redshift

Wed, 17/02/2021 - 10:30
Reconstruction of the neutrino mass as a function of redshift

In this talk, I will present recent work about reconstructing the neutrino mass as a function of redshift z. I will demonstrate that current cosmological data are consistent with a constant neutrino mass in time, with a larger bound on the neutrino mass at low redshifts coinciding with the onset of dark energy domination, of 1.38 eV (95% CL). This result can be explained either by the well-known degeneracy between ∑m_\nu and the dark energy density at low redshifts, or by models in which neutrino masses are generated very late in the Universe. Futhermore, I will explain how the results of the reconstruction can be converted into cosmological limits for models with post-recombination neutrino decay. In this case, we find an upper bound of 0.18 eV (95% CL), which is below the sensitivity of the KATRIN experiment. Thus, a neutrino mass discovery by KATRIN would hint towards models predicting both post-recombination neutrino mass generation and subsequent relic neutrino annihilation. This is joint work with Lena Funcke, Matthias Löffler and Erminia Calabrese.

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Mon 01 Mar 14:00: Some instabilities resulting from the existence of the negative energy modes in a gas-dust disc

Tue, 16/02/2021 - 21:43
Some instabilities resulting from the existence of the negative energy modes in a gas-dust disc

The Lagrangian approach can be employed to the dynamics of the linear perturbations of gas-dust mixture in protoplanetary disc with the dust streaming through the gas. It shows that gas-dust perturbations may have negative energy provided that there is a bulk settling of dust to the disc midplane. The most simple variant of the negative energy gas-dust mode is constructed of the dust density perturbations transported by the bulk stream of dust though the unperturbed gas environment. This mode is referred to as the streaming dust wave (SDW). SDW coupled with the positive energy inertial wave (IW) in the gas gives rise to the recently discovered dust settling instability (DSI). Further, SDW by itself exhibits exponential growth as gas-dust dynamics becomes dissipative in the presence of turbulence and provided that the effective turbulent viscosity exceeds the effective turbulent diffusion. This new instability can be referred to as a viscous settling instability (VSI). As compared to DSI , VSI is produced by modes with larger wavelengths, while it vanishes in laminar disc and survives up to the largest value of the effective viscosity in turbulent disc.

Additionally, it can be shown that the three-wave resonance is possible between one negative energy SDW and two positive energy IW’s. A particular case of such a resonance is considered in detail in order to show that it gives rise to the non-linear explosive settling instability (ESI) with a physically reasonable time of explosion. DSI , VSI and ESI may serve as early mechanisms for generation of the dust overdensities in the long-standing problem of planetesimal formation.

  • Speaker: Viacheslav Zhuravlev - Sternberg Astronomical Institute
  • Monday 01 March 2021, 14:00-15:00
  • Venue: Online.
  • Series: DAMTP Astro Mondays; organiser: Cleo Loi.

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Wed 24 Feb 14:15: The Black Hole Information Paradox: A Resolution on the Horizon?

Tue, 16/02/2021 - 16:55
The Black Hole Information Paradox: A Resolution on the Horizon?

The black hole information paradox — whether information escapes an evaporating black hole or not — remains one of the greatest unsolved mysteries of theoretical physics. The apparent conflict between validity of semiclassical gravity at low curvatures and unitarity of quantum mechanics has long been expected to find its resolution in the deep quantum gravity regime. Recent developments in the holographic dictionary and in particular its application to entanglement, however, have shown that a semiclassical analysis of gravitational physics has a hallmark feature of unitary evolution. I will describe this recent progress and discuss some potential new avenues for working towards a resolution of the information paradox.

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Mon 08 Mar 16:00: Cosmology and Unification

Tue, 16/02/2021 - 15:44
Cosmology and Unification

I will review how future measurements of primordial Non-Gaussianities in the CMB , Large Scale Structure and 21-cm cosmology can probe very heavy particle physics at inflationary energy scales. Intuitively, the production of heavy particles is due to the “Hawking temperature” of inflationary spacetime, and their measurement is via their decay into inflatons. Even greater mass reach is shown to be possible via “chemical potential” enhancement. These mechanisms are applied to plausible targets, namely our most ambitious gauge-Higgs and extra-dimensional theories, such as Orbifold Grand Unification, “dark” sectors, and the Standard Model itself, with particles as exotic as spin-2 Kaluza Klein gravitons. I show how the Hierarchy “Problem” of the Standard Model becomes an asset in this context, readily allowing its particles to have the requisite Hubble-scale masses during inflation, via the mechanism of “Heavy-Lifting”.

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Wed 17 Feb 14:15: The Black Hole Information Paradox: A Resolution on the Horizon?

Tue, 16/02/2021 - 07:41
The Black Hole Information Paradox: A Resolution on the Horizon?

The black hole information paradox — whether information escapes an evaporating black hole or not — remains one of the greatest unsolved mysteries of theoretical physics. The apparent conflict between validity of semiclassical gravity at low curvatures and unitarity of quantum mechanics has long been expected to find its resolution in the deep quantum gravity regime. Recent developments in the holographic dictionary and in particular its application to entanglement, however, have shown that a semiclassical analysis of gravitational physics has a hallmark feature of unitary evolution. I will describe this recent progress and discuss some potential new avenues for working towards a resolution of the information paradox.

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Mon 22 Feb 14:00: In search for red giants' internal magnetic field

Mon, 15/02/2021 - 20:46
In search for red giants' internal magnetic field

One of the “legacy” results of the Kepler mission is the interestingly low rotation rate of the core of subgiant (SG) and red giant (RG) stars, which is about 10 times lower than predicted with the current theory for the transport of angular momentum by purely hydrodynamical mechanisms. This discrepancy points out an order of magnitude issue concerning the understanding of the evolution of the stellar angular momentum in evolved Solar-like stars, a very ubiquitous problem shared by stars of all types and ages. The recent discovery of very low-amplitude dipolar oscillations in a significant fraction of the observed SGs and RGs also points out our misunderstanding of physical processes inside the radiative interiors of evolved Solar-like stars. We thus seek for a missing process taking place inside the core of evolved Solar-like stars, efficient to extract angular momentum from the core to the surface and to perturbate stellar oscillations.

Internal magnetic fields are one amongst the most serious candidates that are currently studied to solve both problems. Stars more massive than ~1.1 Ms are known to develop a convective core during the main-sequence: the dynamo process due to this convection could be the origin of a strong magnetic field, trapped inside the core of the star for the rest of its evolution. Such magnetic fields should impact mixed modes inside the core of RG stars, and their signature should be visible in asteroseismic data. To unravel which constraints can be obtained from these observations, we theoretically investigate the effects of a plausible mixed axisymmetric magnetic field with various amplitudes on the mixed-mode frequencies of red giants. Applying a perturbative method, we estimate the magnetic splitting of the frequencies of simulated mixed dipolar modes that depends on the magnetic field strength and its configuration. A complete asymptotic analysis is derived, showing the potential of asteroseismology to probe the magnetism at each depth as this is done for stellar rotation. The effects of the mass and the metallicity of the stars are also explored. Finally, we infer an upper limit for the strength of the field and the associated lower limit for the timescale of its action to redistribute angular momentum in stellar interiors.

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Fri 26 Feb 13:00: Anomalies in the gravitational recoil of eccentric black-hole mergers with unequal mass ratios

Mon, 15/02/2021 - 15:59
Anomalies in the gravitational recoil of eccentric black-hole mergers with unequal mass ratios

The radiation of linear momentum imparts a recoil (or “kick”) to the center of mass of a merging black hole binary system. In 2019, numerical relativity calculations showed that the addition of orbital eccentricity can lead to an approximate 25% increase in recoil velocities for equal-mass, spinning binaries with spins lying in the orbital plane (“superkick” configurations) arXiv:1910.01598]. I will present recent results [arXiv:2101.11015] obtained from numerical simulations investigating the impact of nonzero eccentricity on the kick magnitude and gravitational-wave emission of nonspinning, unequal-mass black hole binaries. Like the spinning case, nonzero eccentricities at merger can lead to kicks which are larger by up to ~25% relative to the quasicircular case. However, we also find that the kick velocity has an oscillatory dependence on the eccentricity. I will discuss our interpretation of this phenomenon and our observations.

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