Inflation physics from the cosmic microwave background and large scale structure

doi:10.1016/j.astropartphys.2014.05.013

Astroparticle Physics

Volume 63, 15 March 2015, Pages 55-65

https://doi.org/10.1016/j.astropartphys.2014.05.013 Get rights and content

Abstract

Fluctuations in the intensity and polarization of the cosmic microwave background (CMB) and the large-scale distribution of matter in the universe each contain clues about the nature of the earliest moments of time. The next generation of CMB and large-scale structure (LSS) experiments are poised to test the leading paradigm for these earliest moments—the theory of cosmic inflation—and to detect the imprints of the inflationary epoch, thereby dramatically increasing our understanding of fundamental physics and the early universe. A future CMB experiment with sufficient angular resolution and frequency coverage that surveys at least 1% of the sky to a depth of 1 uK-arcmin can deliver a constraint on the tensor-to-scalar ratio that will either result in a $5 σ$ measurement of the energy scale of inflation or rule out all large-field inflation models, even in the presence of foregrounds and the gravitational lensing B-mode signal. LSS experiments, particularly spectroscopic surveys such as the Dark Energy Spectroscopic Instrument, will complement the CMB effort by improving current constraints on running of the spectral index by up to a factor of four, improving constraints on curvature by a factor of ten, and providing non-Gaussianity constraints that are competitive with the current CMB bounds.

Introduction

Precision cosmological measurements push the boundaries of our understanding of the fundamental physics that governs our universe. In the coming years, cosmologists will be in a position to make major breakthroughs in our understanding of the physics of the very early universe and be able to probe particle physics and gravity at the highest energy scales yet accessed. A major leap forward in the sensitivity of cosmological experiments is within our technological reach, leveraging past and current experience to tackle some of the most interesting fundamental physics questions.

Cosmic inflation, the theory that the universe underwent a violent, exponential expansion during the first moments of time, is the leading theoretical paradigm for the earliest history of the universe and for the origin of the structure in the universe. Current measurements of the cosmic microwave background (CMB) and observations of the large scale distributions of dark matter and galaxies in the universe are in stunning agreement with the concept of inflation. The next generations of experiments in observational cosmology are poised to decide central questions about the mechanism behind inflation. In this short document, we highlight the importance of experimentally determining the nature of inflation in the early universe and the unique opportunity these experiments provide to explore the physics of space, time, and matter at the highest energies possible: those found at the birth of the universe.

Although the landscape of possible models for inflation is potentially large—and sensitive to quantum gravity corrections to the low-energy quantum field theory—the phenomenology is sufficiently well understood to make concrete distinctions between fundamentally different classes of models that we can test observationally.

One key and generic prediction is the existence of a background of gravitational waves [1] from inflation that produces a distinct signature in the polarization of the CMB, referred to as “B-mode” polarization. The amplitude of primordial gravitational waves, or tensor modes, which can be detected or constrained by observations of the B-mode polarization in the CMB, is fundamentally interesting for several basic reasons. It is proportional to the energy scale of inflation and tied to the range of the inflaton field. In particular, observations promise to reach the level of sensitivity that will enable them to determine whether the field range is larger than the Planck scale in the simplest versions of inflation [2] . This provides a striking ultraviolet-sensitive probe of quantum field theory and quantum gravity, and an observational test of string theoretic large-field inflation. Additionally, in one theoretically developed (though currently speculative) alternative to inflation, the ekpyrotic scenario, the authors of [3], [4] find no mechanism for generation of the tensor perturbations; hence, if these calculations are correct, detection of B-modes would present a convincing refutation of that model. Last but not least, a detection of tensor modes would constitute a stunning measurement of the quantum mechanical fluctuations of the gravitational field.

This motivates a next-generation CMB experiment with the sensitivity and systematics control to detect such a polarized signal at $⩾ 5 σ$ significance, thus ensuring either a detection of inflationary gravitational waves or the ability to rule out large classes of inflationary models. A program to meet these goals by developing a Stage IV CMB experiment, CMB-S4, with O(500,000) detectors by 2020 is described in the companion cosmic frontier planning document (Neutrino Physics from the Cosmic Microwave Background and Large Scale Structure [5] ). Such an experiment would also contribute to inflationary science by strongly constraining the spectrum of primordial density fluctuations, allowing us to distinguish different families of inflationary models.

Possibilities for self-interactions of the inflaton and for additional fields are tested by different limits of the correlation functions of the perturbations. Despite important recent progress, we require substantial improvements before observational constraints on these quantities limit the interactions to be small corrections to slow-roll, or to detect non-Gaussianity if it is present. A concerted theoretical effort combined with observations of large scale structure promises to fill this gap. A detection of primordial non-Gaussianity of the so-called local shape would effectively rule out all models of inflation that involve a single scalar field [6], [7], [8]). The CMB bound on local-model non-Gaussianity is now limited by having only one sky to observe; further improvements will come from measurements of the large scale structure of the universe. The next generation of large scale structure measurements will produce non-Gaussianity constraints that are an important cross-check of the CMB bound and will pave the way for more stringent bounds from future large scale structure measurements.

Section snippets

Inflation science: theoretical motivations

Cosmic inflation, the idea that the universe underwent a period of exponential expansion in the first $10^{- 34}$ seconds of its existence, was proposed in the early 1980s to explain the apparent smoothness and flatness of the universe and the absence of relics such as magnetic monopoles [9]. Quantum fluctuations generated during inflation evolve into the distributions of dark matter and galaxies we observe today [10]. Inflation drives the spatial curvature to nearly zero, and introduces density

Constraining inflation physics with cosmological probes

As detailed in Section 2, the theory of cosmic inflation is the most promising model for the dynamics of the universe at very early times and high energies. Inflation supplies a natural explanation for the smoothness and geometrical flatness of our observable universe, and the predicted consequences of an inflationary beginning to our universe, including a nearly Gaussian distribution of primarily adiabatic density fluctuations with a nearly scale-invariant spectrum, have been spectacularly

Conclusions

The next generation of CMB and LSS experiments are poised to dramatically increase our understanding of fundamental physics and the early universe by probing the inflationary epoch. In particular, constraints on the amplitude of tensor modes will provide unique insight into the physics of inflation only available through CMB observations. A Stage-IV CMB experiment such as CMB-S4 that surveys $> 1 %$ of the sky to a depth of $\sim 1 μ K$ -arcmin will deliver a constraint on the tensor-to-scalar ratio r that

Note added in proofs

After this paper was submitted, the BICEP2 collaboration reported a detection of B-mode polarization at degree angular scales [93]. When interpreted as an inflationary gravitational-wave signal, the BICEP2 measurement implies a value of the tensor-to-scalar ratio of $r = 0.2$ . This result immediately shifts the focus of currently operating and funded CMB experiments to confirming the level of the BICEP2 measurement and its cosmological origin, and it strengthens and clarifies the case for a Stage

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View full text

Inflation physics from the cosmic microwave background and large scale structure

Abstract

Introduction

Section snippets

Inflation science: theoretical motivations

Constraining inflation physics with cosmological probes

Conclusions

Note added in proofs

Phys. Rep.

Phys. Lett.

Phys. Lett.

Relict gravitation radiation spectrum and initial state of the universe (in Russian)

JETP Lett.

What would we learn by detecting a gravitational wave signal in the cosmic microwave background anisotropy?

Phys. Rev. Lett.

Cosmic evolution in a cyclic universe

Phys. Rev. D

Ekpyrotic universe: colliding branes and the origin of the hot big bang

Phys. Rev. D

Non-Gaussian features of primordial fluctuations in single-field inflationary models

JHEP

Single-field consistency relation for the 3-point function

JCAP

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Phys. Rev. D

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JETP Lett.

Signature of gravity waves in polarization of the microwave background

Phys. Rev. Lett.

A probe of primordial gravity waves and vorticity

Phys. Rev. Lett.

Cosmic microwave background observables of small field models of inflation

JCAP

Observable gravitational waves from inflation with small field excursions

JCAP

Natural inflation with pseudo-Nambu–Goldstone bosons

Phys. Rev. Lett.

N-flation

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No-scale supergravity realization of the Starobinsky model of inflation

Phys. Rev. Lett.

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An ignoble approach to large field inflation

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Phys. Rev. D

Spatial curvature falsifies eternal inflation

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What can the observation of nonzero curvature tell us?

Phys. Rev. D

Beyond the simplest inflationary cosmological models

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Inflation as a probe of short distance physics

Phys. Rev. D

Oscillations in the CMB from axion monodromy inflation

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Features of heavy physics in the CMB power spectrum

Jcap

Primordial features as evidence for inflation

JCAP

Primordial non-gaussianities from inflation models

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The shape of non-gaussianities

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Large non-gaussianities in single field inflation

Jcap

Resonant non-gaussianity

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