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Final Report Summary - NOUCI (Non-gaussianities in the observable universe and the origin of cosmic inhomogeneities)

Executive summary

In general terms, the goal of this project was to contribute to our understanding of the past history and future evolution of the universe we inhabit. This branch of physics is called cosmology, and it is one of the most active areas of contemporary research. Our understanding of the cosmos has grown exponentially in the last 30 years, thanks to a synergy between theory and observations. The final outcome is the so-called Lambda-CDM model, a theoretical framework that describes the evolution of the universe from a tiny fraction of second after the “big bang” until the present. Even though there are still important questions to answer, the predictions of this model agree quite spectacularly with observations. The fact that we can describe the entire universe with such precision, is with no doubts a major milestone in human knowledge.

Efforts in the community of cosmologist are now focused on the unresolved questions in the Lambda-CDM. These are perhaps the most difficult ones to answer, but also contain the most fascinating problems. Just to mention a couple: (i) what is the nature of the so-called dark energy and dark matter that fill our universe; (ii) did the universe “begin” in something similar to a “big bang”, or does it have existed forever? What is remarkable is that we have precise observations about the cosmos, and more to come in the near future, that provide us with a unique opportunity to extend the boundaries of our understanding an answer some of these fundamental questions.

The goal of this project was, generally speaking, to address question (ii) formulated above, and develop a precise framework on which the proposed answer can be contrasted with observations.

More precisely, the Lambda-CDM model describes the universe starting from the onset of the inflationary era, a primordial epoch in which the universe expanded exponentially fast with time. But the model does not describe what happened before that epoch. This project was devoted to develop a mathematical framework to describe our universe before the inflationary era, and to understand the way physical processes that took place during that remote epoch can leave an imprint in the cosmos, accessible to present and future observations.

Although this project has been terminated after only half of the initially planned time, the goals and main objective have been achieved. The theoretical framework and physical ideas developed in this project produced a significant impact in the community, and are now widely used by many research groups. The area of primordial quantum cosmology has now become very active, and the future is promising. It is fair to say that this project has substantially contributed to the way this area of research has developed. The concrete results are described in more detail in the following sections.



Summary description of project context and objectives



The point of inflection in cosmology has been provided by observations, capable of measuring cosmological parameters with precision greater than one percent. Among the observational missions, the COBE satellite launched by NASA in 1989 was a remarkable pioneer. COBE observations of the Cosmic Microwave Background (CMB) guaranteed the Nobel Prize for the team leaders in 2006. The two most influential COBE results were: (1) The CMB is very approximately anisotropic, with a frequency distribution extremely similar to that of a blackbody at a temperature of 2,725 K. This result provides observational support for the Standard Model of the “Hot Big Bang”. (2) The CMB contains small anisotropies in its temperature, really tiny, smaller than one part in hundred thousand! This observation reveals the existence of minute variations in the density of matter and radiation across the universe at the time when the CMB was formed, when the universe was 300,000 years young (its present age is of almost 14,000,000,000 years). A few years after COBE, simulations with supercomputers showed that, when these tiny inhomogeneities in the early universe are evolved using Einstein’s gravitational field equations, grow due to the effects of gravity, and eventually become so large that the give rise to the galaxies, galaxy clusters and superclusters that we observe in the present. In summary, the combination of theory, observation and numerical simulations have taught us that cosmic structures originate in the primordial inhomogeneities present in the universe at the time the CMB was formed.

But still, to understand the real origin of cosmic structures it is needed to unravel the origin of the tiny primordial density perturbations. What is the mechanism responsible for their existence? The most promising idea is the theory of inflation: very early in evolution, immediately after the Big Bang, the universe experienced a short period of extremely violent expansion, almost exponential. Although strange or artificial in appearance, the surprising thing is that this assumption solves many important questions, more than those imagined by its proponents: (1) A large expansion explains why the universe seems so flat and homogeneous on large scales at present: inflation reduces the curvature and dilutes large inhomogeneities. (2) The violent expansion is able to amplify quantum fluctuations, and create real particles purely from the vacuum. (This phenomenon is the gravitational analogue of the creation of electron-positron pairs by intense electromagnetic fields). What is surprising is that the created particles induce a spectrum of perturbations in the density of the universe that is in very good agreement with the inhomogeneities that have been observed in the CMB. In my opinion, the idea that cosmic inhomogeneities, that are responsible for the large scale structures, find their origin in fluctuations of the quantum vacuum in the very primitive universe, is one of the most profound and promising ideas of current theoretical cosmology.

The economy in the assumptions and the robustness in the predictions have made of the inflationary paradigm the most accepted idea among cosmologist. However, the limitations of the inflation model are also evident. One of particular relevance is the fact that the model is based on the gravitational theory of Einstein's general relativity. This is a classical theory of gravity, and its validity is restricted to situations where the energy-momentum density is well below the Planck scale. This assumption is well justified during the inflationary period, where the energy density is less than ten orders of magnitude below the Planck density. However, before inflation the quantum effects of gravity become important and dominate. This causes certain limitations of the inflationary model: (1) The model includes the singularity of the big bang singularity, where matter density and space-time curvature becomes infinite. (2) Since we ignore what happened near the big bang, certain initial conditions must be assumed at the beginning of inflation. These conditions influence the predictions of the model. (3) For certain values of the parameters, inflation involves wavelengths that are below the Planck length, for which the techniques used are not justified.

All these limitations were not considered of major importance for inflationary cosmologist, mainly because a general believe was extended in the early times of the inflationary theory that whatever occurred in the universe before the inflationary epoch was inaccessible to observations. The believe was based on the idea that the enormous inflationary expansion would dilute any remnant of the pre-inflationary universe, washing away memories from the pre-inflationary epoch. Therefore, if this is true, then our ignorance about the way inflation begins is unimportant for observations. However, nowadays enough evidence against this diluting mechanism has been accumulated. As articulated by Agullo and Parker, during inflation there is an effect called stimulated particle creation, similar to the mechanisms that makes a LASER to work. It turns out that this mechanism precludes the universe to forget about the earlier times. The message is that whatever happened before inflation is indeed relevant for observational, in contrast to earlier believes. This is quite extraordinary; before inflation the universe was governed by quantum gravity, and therefore we have the possibility of observing features in the CMB that have a quantum gravity origin.

The goal of this project was to make these ideas mathematical precise and useful for the community of cosmologist. First, one needs to characterize in a precise way the way pre-inflationary physics can be imprinted in the CMB and galaxy distributions. Second, one needs to learn how the universe behaved before inflation, and what kind remnants of that phase can survive. As mentioned before, the difficulty here is found in the fact that Einstein theory of relativity is insufficient to describe the universe in such an extreme regime. One needs to go beyond Einstein and incorporate quantum effects to the gravitational field. One needs quantum gravity. This is an area under development, with several ideas on the market, but with no general consensus on which of the existing proposals is the correct one. The possibility of connecting different ideas with observation is therefore quite exciting, since this could help to rule out some, and favor others.

This project has made concrete progress on these questions.

Reported by

THE CHANCELLOR, MASTERS AND SCHOLARS OF THE UNIVERSITY OF CAMBRIDGE
United Kingdom
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