Parallel Session: COSMOLOGY - EARLY UNIVERSE AND THE ORIGIN OF STRUCTURE (F)
Location: Guildhall Auditorium
F1 - Monday 14:00-15:40 (Matteo Fasiello)
The Formation of the First Quasars in the Universe
Daniel Whalen (Institute of Cosmology and Gravitation, University of Portsmouth)
Over 160 quasars have now been discovered at z > 6, including three that have been found at z > 7. How supermassive black holes (SMBHs) this massive formed by such early epochs poses challenges to current paradigms of cosmological structure formation. I will discuss the potential origins of these objects and new radiation hydrodynamical simulations of how direct collapse black holes (DCBHs) at the nexus of cold accretion flows can grow to a billion solar masses by z ~ 7. I will also present NIR luminosities for the first quasars from birth down to z ~ 7 and discuss how Euclid and E-ELT will inaugurate the era of z = 5 – 15 quasar astronomy in the coming decade.
Will Handley (Kavli Institute for Cosmology (Cavendish Laboratory), University of Cambridge)
Introducing a linearly quantized power spectrum with k0 = 3.225e-4/Mpc and spacing deltak = 2.257e-4/Mpc provides a better fit to the Planck 2018 observations than the concordance baseline, with delta chi squared = −8.55. Quantized primordial power spectra have implications for explaining features in the cosmic microwave background power spectrum, including the low-l suppression of power, the l~20-30 dip as well as oscillations at high multipoles. I will discuss these observations and several potential theories that predict quantized primordial power spectra, such as future conformal boundary cosmologies and kinetically dominated initial conditions for inflation.
Dong-Gang Wang (Leiden Observatory & Lorentz Institute, Leiden University)
Cosmic inflation can be seen as a natural physics laboratory at extremely high energy scales. From the perspective of fundamental realizations, the low-energy effective theories of inflation are typically associated with a curved field manifold, which has drawn a lot of attention recently. One generic question is whether there exists any observational signatures of the inflationary curved field space. In this talk, I will first discuss the role of the curved field space in two recently developed models, ‘multi-field alpha-attracor’ (1711.09478) and ‘shift-symmetric orbital inflation’ (1901.03657). After that I will attack the above generic question in the context of “quasi-single field inflation / cosmological collider physics” (1911.04459), where the extra fields have masses around the Hubble scale and lead to unique predictions in primordial non-Gaussianity. The analysis is performed by using both the multi-field approach and the background effective field theory of inflation, and we explicitly bridge the gap between these two equivalent descriptions. Here due to the curved field space, the standard “collider” signals in the squeezed bispectrum are modified, and a resulting running index directly measures the curvature of the internal field space. Therefore, this new phenomenology in non-Gaussianity may serve as a model-independent way to detect the field space geometry of inflation.
Consistent scalar and tensor perturbation power spectra in single fluid matter bounce with dark energy era
Nelson Pinto-Neto (Department of Cosmology, Astrophysics and Fundamental Interactions, Centro Brasileiro de Pesquisas Físicas)
We investigate cosmological scenarios containing one canonical scalar field with an exponential potential in the context of bouncing models, in which the bounce happens due to quantum cosmological effects. The only possible bouncing solutions in this scenario (discarding an infinitely fine-tuned exception) must have one and only one dark energy phase, occurring either in the contracting era or in the expanding era. Hence, these bounce solutions are necessarily asymmetric. Naturally, the more convenient solution is the one in which the dark energy phase happens in the expanding era, in order to be a possible explanation for the current accelerated expansion indicated by cosmological observations. In this case, one has the picture of a Universe undergoing a classical dust contraction from very large scales, the initial repeller of the model, moving to a classical stiff-matter contraction near the singularity, which is avoided due to the quantum bounce. The Universe is then launched to a dark energy era, after passing through radiation- and dust-dominated phases, finally returning to the dust expanding phase, the final attractor of the model. We calculate the spectral indices and amplitudes of scalar and tensor perturbations numerically, considering the whole history of the model, including the bounce phase itself, without making any approximation nor using any matching condition on the perturbations. As the background model is necessarily dust dominated in the far past, the usual adiabatic vacuum initial conditions can be easily imposed in this era. Hence, this is a cosmological model in which the presence of dark energy behavior in the Universe does not turn the usual vacuum initial conditions prescription for cosmological perturbation in bouncing models problematic. Scalar and tensor perturbations end up being almost scale invariant, as expected. The background parameters can be adjusted, without fine-tunings, to yield the observed amplitude for scalar perturbations and also for the ratio between tensor and scalar amplitudes, r = T/S ≲ 0.1. The amplification of scalar perturbations over tensor perturbations takes place only around the bounce, due to quantum effects, and it would not occur if General Relativity has remained valid throughout this phase. Hence, this is a bouncing model in which a single field induces not only an expanding background dark energy phase but also produces all observed features of cosmological perturbations of quantum mechanical origin at linear order.
We introduce two new effective quantities for the study of comoving curvature perturbations ζ: the space dependent effective sound speed (SESS) and the momentum dependent effective sound speed (MESS) . We use the SESS and the MESS to derive a new set of equations, not involving explicitly entropy or anisotropies, which can be applied to any system described by an effective stress-energy-momentum tensor (EST), including any multi-fields systems, supergravity and modified gravity theories. The MESS is the natural quantity to parametrize in a model independent way the effects produced on curvature perturbations by multi-fields systems, particle production and modified gravity theories and could be conveniently used in the analysis of LSS observations, such as the ones from the upcoming EUCLID mission or CMB radiation measurements. It can be also useful to study in a model independent way the production of primordial black holes.
Beside the degeneracy due to different theoretical scenarios producing the same MESS, we show that in absence of entropy or effective anisotropic stress there is an additional degeneracy related to the freedom in the choice of the initial energy scale of inflation, or to the sign of the Hubble parameter. This implies the existence of an infinite family of dual slow-roll parameters histories which can produce the same spectrum of comoving curvature perturbations, implying that in general there is no one-to-one correspondence between the spectrum and higher order correlation functions. Bounce models are examples of the members of this infinite class of dual models. The combined analysis of data from future CMB and gravitational wave experiments could allow to distinguish between dual models because the primordial tensor perturbations spectra of dual models are in general different.
Many-field Inflation: Universality or Prior Dependence?
Perseas Christodoulidis (Physics, University of Groningen)
We investigate the observational signatures of many-field inflation and present analytic expressions for the spectral index as a function of the prior. For a given prior we employ the central limit theorem and the horizon crossing approximation to derive universal predictions, as found previously. However, we also find a specific dependence on the prior choice for initial conditions that has not been seen in previous studies. Our main focus is on quadratic inflation, for which the initial conditions statistics decouple from those of the mass distribution, while other monomials are also briefly discussed. We verify the validity of our calculations by comparing to full numerical simulations with 100 fields using the transport method.
Calibrating string evolution models with 1032 simulations
José Ricardo Correia (Instituto de Astrofísica e Ciências do Espaço / Faculdade de Ciências da Universidade do Porto)
In the very early Universe it is expected that phase transitions have taken place. Depending on the symmetry broken at the phase transition (and therefore on the details of underlying GUT) one can reasonably expect the formation of topological defects, with cosmic strings being a benign example (in the sense that they are not expected to over close the Universe). The need for higher resolution and extra complexity to model realistic defects, can heavily tax the underlying hardware, to the point where it will be unfeasible to simulate these objects and extract observational constraints. In order to overcome this problem (or to alleviate it to some extent) we’ve recently developed an Abelian-Higgs cosmic string simulation that exploits an architecture capable of higher throughput and bandwidth ceilings: graphics processing units (GPU’s).
In this work we demonstrate using this simulation to improve canonical semi-analytical modelling for string evolution (Velocity-dependant One-Scale model – VOS) in a reasonable amount of time, by producing 1032 simulations. The large number of simulations is required in order to account for the velocity dependencies of the curvature parameter (intricately linked with small-scale structure) and of the overall energy losses of the network (adding an explicit scalar/gauge radiation term). This allows us to point out the relative importance of loop production vs massive radiation as energy loss mechanisms for a network of cosmic strings at radiation and matter era, and a few other key insights regarding the small scale structure of the strings (these are relevant for observational consequences of a network of strings).
Typically, inflation can produce a spike in the curvature perturbation spectrum, which leads to the production of primordial black holes (PBHs), but this is at the expense of assuming a special feature along the inflaton potential at just the right place such that this spike features the desired scale and amplitude so that the PBHs may be cosmologically relevant. I will present a mechanism where this tuning is unnecessary and successful PBH generation can take place regardless of the particular inflaton potential. The required spike in the curvature perturbation is due to an outburst of tachyonic fluctuations on top of a potential maximum. The system finds itself there either because of a period of thermal inflation, much later than primordial inflation, or because of a period of locked inflation at the end of primordial inflation. This subsequent period of inflation is followed by fast-roll inflation as the system rolls down the potential hill to its true minimum. A concrete realisation is studied through a running-mass model. We find that the generated PBHs may account for the dark matter in the Universe or explain the supermassive black holes in the centre of galaxies.
Primordial black holes can form in the early Universe from the collapse of cosmological perturbations after the cosmological horizon crossing. They are possible candidates for the dark matter as well as for the seeds of supermassive black holes observed today in the centre of galaxies. If the perturbation is larger than a certain threshold, depending on the equation of state and on the specific shape of the perturbation, a black hole is formed. In this talk I will discuss the dependence of PBH formation from the initial shape of the curvature profile showing the relation between the threshold amplitude and the initial shape of the inflationary power spectrum of cosmological perturbations, taking into account also possible primordial non-Gaussianity. Although the abundance of PBHs could vary by several order of magnitudes depending on the model of inflation, we find that the threshold of PBH formation is rather solid against non linearities.
Kaloian Lozanov (Max Planck Institute for Astrophysics)
I will talk about our works (arXiv:1608.01213, arXiv:1710.06851, arXiv:1902.06736) in which we calculate the equation of state of the inflaton field after inflation and provide an upper bound on the duration before radiation domination by taking the nonlinear dynamics of the fragmented inflaton field into account. For the models considered, I will discuss how our work significantly reduces the uncertainty in inflationary observables. I will also consider various aspects of the linear and nonlinear dynamics of the inflaton such as parametric self-resonance, the generation of scalar metric perturbations and gravitational waves. I will also review the recent progress in the understanding of the equation of state after inflation in models featuring gauge bosons.
Revisiting non-Gaussianity in multifield inflation with curved field space
Lucas Pinol (Institut d’Astrophysique de Paris)
Multi-field models of inflation with curved field space and non-geodesic motion have recently been under scrutiny as realistic realizations of high-energy physics in the Early Universe. Focusing on 2-field models, we present the covariant 3rd order action in terms of the adiabatic comoving curvature perturbation and the entropic field perturbation. We extend Maldacena’s calculation to such models with generic field space, by showing how we can estimate the right size of each interaction after multiple integrations by parts in the action and careful study of the boundary terms. Furthermore, when entropic fluctuations are heavy we integrate them out at the level of the 2nd and 3rd order actions, leading to an effective single-field theory for the observable adiabatic perturbation only. The sizes of the interactions in this EFT, that explicitly depend on the geometry of the underlying field space, are computed analytically. These results open a new window to investigate the geometry of inflationary models via the observation of primordial non-Gaussianities.
I will introduce the regime of ultra-slow-roll inflation and discuss why we are interested in such a regime, especially in the context of primordial black holes. I will also describe the stochastic formalism for inflation and explain the difficulties that must be considered when using stochastic inflation beyond slow roll. Having explained why the requirements for the stochastic formalism are satisfied beyond slow roll, I will explain how to use stochastic inflation within ultra-slow roll.
Planck scale black hole dark matter from Higgs inflation
Eemeli Tomberg (Department of Physics, University of Helsinki)
If the inflaton potential has a feature that produces strong primordial perturbations, these perturbations may seed the production of primordial black holes (PBHs) during the radiation-dominated era. The PBHs can then contribute to the dark matter. I discuss this possibility in Higgs inflation, where the Standard Model Higgs field, coupled non-minimally to gravity, is the inflaton, and the feature in the potential is produced by realistic quantum corrections. A numerical analysis shows that the production of large PBHs is incompatible with CMB observations, but small black holes can be produced abundantly. These small PBHs evaporate early by Hawking radiation, but if they leave behind Planck mass relics, the relics can constitute all of the observed dark matter.
Tensor non-Gaussianities from inflation with non-Bunch-Davies initial states
Shingo Akama (Department of Physics, Rikkyo University)
It has been found that the primordial non-Gaussianities of the scalar modes from non-Bunch-Davies initial states can be enhanced at the flattened configuration compared with those from the Bunch-Davies state. It is therefore natural to anticipate that the non-Gaussianities associated with the tensor modes can be also enhanced. To see whether this is true or not, we investigate the general properties of the non-Gaussianities generated by both the scalar and tensor modes from the non-Bunch-Davies states. In this talk, we first derive the auto-bispectra of the tensor modes and the cross-bispectra of the scalar and tensor modes with the non-Bunch-Davies states in the most general scalar-tensor theory giving second-order field equations. Then we show that only the non-Gaussianities generated by the cross-interactions among the scalar and tensor modes can be enhanced at the flattened configuration, and additionally find that the non-Gaussian amplitude is highly enhanced if the gravitational theories are modified from general relativity. Based on these results, we also discuss whether it is possible or not to detect the non-Gaussian signatures through the CMB observations.