Theo Murphy meeting organised by Dr Steffen Gielen and Dr Jean-Luc Lehners.
Theo Murphy meeting organised by Dr Steffen Gielen and Dr Jean-Luc Lehners.
Quantum theory describes the behaviour of matter, but was it in fact also instrumental in explaining the structure of space and time? This meeting will discuss how the quantum properties of spacetime can change our understanding of the foundations of cosmology, and how their impact could be tested observationally in the near future.
This meeting has taken place.
Image credit: NASA/ESA and Jean-Luc Lehners.
Primordial black holes are a fascination means to probe both the contents and the initial conditions of the universe, because something non-standard needs to have happened in the first second after the Big Bang for them to exist. Dr Byrnes will discuss the extreme sensitivity of the primordial black hole number density to changes in the inflationary model which could have generated them, but how this means the detection of any primordial black hole would be extremely informative.
Primordial black holes (PBHs) may have formed in the early universe and provide an important link between microphysics and macrophysics. In the macroscopic context, it has been proposed that PBHs larger than 1015g could explain various cosmological conundra: the existence of dark matter, the cosmic photon-to-baryon ratio, unexplained microlensing events, some of the LIGO/Virgo/KAGRA gravitational wave events, the puzzling early formation of certain cosmic structures, and the existence of supermassive black holes in galactic nuclei. Since PBHs form from quantum fluctuations in the early universe, this provides an indirect probe of quantum spacetime. In the microscopic context, the existence of Hawking radiation provides a direct probe of quantum spacetime and offers a unification of relativity theory, quantum mechanics and thermodynamics. However, only PBHs could be small enough for such radiation to be detectable and only those smaller than 1015g would have evaporated completely by now. Finally, black holes of 10-5g may provide a probe of higher dimensions, a possible link between elementary particles and black holes, and quantum gravity itself.
Warped compactifications provide a rich arena to explore phenomenological and cosmological questions such as the hierarchy problem, cosmological inflation, dark energy and dark matter. In this talk, the author will discuss gravitational signatures arising from compactifying type IIB string theory on a compact space containing a warped throat. He will focus on the gravitational sector of the resulting 4d EFT, with its tower of Kaluza-Klein (KK) gravitons. By assuming that we live on a (3+1)-dimensional brane within the throat, the author will discuss how the Newtonian potential is modified, and how these modifications can be tested by gravitational experiments. The author will then discuss how the extra massive KK gravitons in warped throats affect gravitational wave signals and its possible tests at current and future gravitational wave experiments.
Dr Brahma will discuss inflation as an open quantum system and derive quantum corrections to cosmological observables, such as the power spectrum, showing explicitly why it is important to go beyond standard (Wilsonian) effective field theory. He will highlight deep insights this approach provides regarding quantum entanglement in the early universe and emphasize the features which apply to other gravitational systems.
Professor João Magueijo will review a recent approach to connecting quantum gravity and the real world by deconstantizing the constants of nature, and using their conjugate as a time variable. This is nothing but a generalization of unimodular gravity. The wave functions are then packets of plane waves moving in a space that generalizes the Chern-Simons functional. For appropriate states they link up with classical cosmology in the appropriate limit. There are however deviations, namely during the matter to Lambda transition, raising the possibility that quantum gravity could be in action here and now. At the other extreme Professor Magueijo will show how this approach can be used to resolve the cosmological singularity, and perhaps more.
The scenario that has been selected as the standard cosmological model is Lambda Cold Dark Matter (ΛCDM), which provides a remarkable fit to the bulk of available cosmological data. However, discrepancies among key cosmological parameters of the model have emerged with different statistical significance. While some portion of these discrepancies may be due to systematic errors, their persistence across probes can indicate a failure of the canonical ΛCDM model. Dr Di Valentino will review these tensions, showing some interesting extended cosmological scenarios that can alleviate them.
Consistency with quantum gravity can have significant consequences on low energy physics. Not every effective field can be consistently coupled to quantum gravity unless it satisfies some additional consistency constraints, dubbed swampland constraints. In this talk Dr Irene Valenzuela will review the most important swampland conjectures and their implications for cosmology. The best understood constraints imply that theories with very small gauge couplings or parametrically large scalar field variations are inconsistent with quantum gravity. The author will also describe recent progress regarding whether accelerating cosmologies at parametrically large time can be embedded in string theory. Furthermore, she will describe a scenario motivated by the swampland program in which the smallness of the cosmological constant emerges from living near a boundary of the quantum gravity field space. A universal feature of this scenario is that there is a light infinite tower of states which is correlated to the value of the vacuum energy. Dr Valenzuela will show how experimental constraints force this tower to correspond to a KK tower (of mass of order neutrino scale) of a single extra mesoscopic dimension of order 10^{-6) m, which we denote as the dark dimension.
Inflation is the leading paradigm for the origin of our observable universe. However, persuaded by some objections to it, several alternatives are also proposed that are so far observationally indistinguishable from inflation. While the arguments for and against inflation (some of which are rather philosophical) remain debatable, it would be of significant value if it is possible to let observation make the ultimate decision. In this talk, the author will introduce the so-called standard clock as such a possibility. The standard clock is a massive field that is naturally expected to exist at the early stages of the universe and which may have distinct imprints on the fluctuations. Importantly, since some features of the standard clock signal are sensitive to the evolution of the universe at its early stages, it can be an observational smoking gun for discriminating inflation from alternatives.
Primordial features are small but strongly scale-dependent corrections to the leading order density perturbations seeded by a wide range of physics in the primordial universe. Information encoded in these primordial feature signals could be used to distinguish the inflation scenario from other alternative scenarios model-independently, and/or to narrow down the physical mechanism underlying the inflation model. The authors discuss the future observational prospects of these primordial feature signals, especially with the measurements of the CMB E-mode polarization in the next decade.
Dr Giulia Cusin will review the effects of peculiar velocities on the observed waveform of a binary system of compact objects visible by LISA. She will then focus on the effects of a velocity component transverse to the line of sight, and show that it gives rise to spin-1 polarisation in the observer frame even in the context of General Relativity. These are pure projection effects, proportional to the plus and cross polarisations in the source frame, hence they do not correspond to new degrees of freedom. The author demonstrates that the spin-1 modes can always be rewritten as pure spin-2 modes coming from an aberrated direction. Since gravitational waves are not isotropically emitted around binary systems, this aberration modifies the apparent orientation of the binary system with respect to the observer: the system appears slightly rotated due to the source velocity.
Professor Mairi Sakellariadou will briefly introduce approaches to quantum gravity and summarise some of their phenomenological and observational implications. She will then employ currently available observational data, with emphasis in gravitational waves, in order to test theories aiming at explaining the origin of spacetime.
In the group field theory (GFT) framework for quantum gravity, effective cosmological dynamics emerge as the hydrodynamics of simple condensates of quanta of geometry. The quantum equations of motion for these GFT condensate states are given in relational terms with respect to the matter field (here assumed to be a scalar field), and these imply dynamics for the volume of the universe that correspond to modified Friedmann equations, with quantum gravity modifications, in a specific regime of the theory corresponding to a Gross–Pitaevskii approximation where interactions are subdominant. The classical Friedmann equations of general relativity are recovered in a suitable semi-classical limit, while the quantum geometries associated with these GFT condensate states are non-singular: a bounce generically occurs in the Planck regime. For some choices of condensate states, these modified Friedmann equations are very similar to those of loop quantum cosmology.
The authors of the talk consider the quantum gravity partition function that counts the dimension of the Hilbert space of a spatial region with topology of a ball and fixed proper volume, and evaluate it in the leading order saddle point approximation. The result is the exponential of the Bekenstein-Hawking entropy associated with the area of the saddle ball boundary. This generalizes the classic Gibbons-Hawking computation which corresponds to the special case when the fixed volume is that of the static patch of de Sitter spacetime, and exhibits the holographic nature of nonperturbative quantum gravity in finite volumes of space.
If the structure in our Universe is sourced by quantum fluctuations during inflation, but not by fluctuations in the inflation, a non-Gaussian signature of the conversion of these fluctuations into curvature may be imprinted in data from galaxy surveys. In the next decade, galaxy surveys will reach an important threshold: the ability to detect the non-Gaussian parameter fNL ~1, which would test a variety of scenarios in which structure is sourced by an additional light field. The author will review the signature of fNL in galaxy surveys and present a new study of astrophysical signals that, if not accounted for, could confuse measurements.