# B-modes and the nature of inflation

## Abstract

Observations of the cosmic microwave background do not yet determine whether inflation was driven by a slowly-rolling scalar field or involved another physical mechanism. In this paper we discuss the prospects of using the power spectra of scalar and tensor modes to probe the nature of inflation. We focus on the leading modification to the slow-roll dynamics, which entails a sound speed c{sub s} for the scalar fluctuations. We derive analytically a lower bound on c{sub s} in terms of a given tensor-to-scalar ratio r, taking into account the difference in the freeze-out times between the scalar and tensor modes. We find that any detection of primordial B-modes with r > 0.01 implies a lower bound on c{sub s} that is stronger than the bound derived from the absence of non-Gaussianity in the Planck data. For r ∼> 0.1, the bound would be tantalizingly close to a critical value for the sound speed, (c{sub s}){sub *} = 0.47 (corresponding to (f{sub NL}{sup equil}){sub *} = -0.93), which we show serves as a threshold for non-trivial dynamics beyond slow-roll. We also discuss how an order-one level of equilateral non-Gaussianity is a natural observational target for other extensions of the canonical paradigm.

- Authors:

- D.A.M.T.P., Cambridge University, Wilberforce Road, Cambridge, CB3 0WA (United Kingdom)
- Canadian Institute for Theoretical Astrophysics, 60 St. George Street, Toronto, ON M5S 3H8 (Canada)
- Deutsches Elektronen-Synchrotron DESY, Theory Group, Notkestrasse 85, D-22603 Hamburg (Germany)

- Publication Date:

- OSTI Identifier:
- 22382032

- Resource Type:
- Journal Article

- Resource Relation:
- Journal Name: Journal of Cosmology and Astroparticle Physics; Journal Volume: 2015; Journal Issue: 01; Other Information: Country of input: International Atomic Energy Agency (IAEA)

- Country of Publication:
- United States

- Language:
- English

- Subject:
- 79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; COSMOLOGICAL INFLATION; DETECTION; FREEZING OUT; RELICT RADIATION; SCALAR FIELDS; SCALARS; SPECTRA; TENSORS; VELOCITY

### Citation Formats

```
Baumann, Daniel, Green, Daniel, and Porto, Rafael A., E-mail: dbaumann@damtp.cam.ac.uk, E-mail: drgreen@cita.utoronto.ca, E-mail: rporto@ias.edu.
```*B-modes and the nature of inflation*. United States: N. p., 2015.
Web. doi:10.1088/1475-7516/2015/01/016.

```
Baumann, Daniel, Green, Daniel, & Porto, Rafael A., E-mail: dbaumann@damtp.cam.ac.uk, E-mail: drgreen@cita.utoronto.ca, E-mail: rporto@ias.edu.
```*B-modes and the nature of inflation*. United States. doi:10.1088/1475-7516/2015/01/016.

```
Baumann, Daniel, Green, Daniel, and Porto, Rafael A., E-mail: dbaumann@damtp.cam.ac.uk, E-mail: drgreen@cita.utoronto.ca, E-mail: rporto@ias.edu. Thu .
"B-modes and the nature of inflation". United States.
doi:10.1088/1475-7516/2015/01/016.
```

```
@article{osti_22382032,
```

title = {B-modes and the nature of inflation},

author = {Baumann, Daniel and Green, Daniel and Porto, Rafael A., E-mail: dbaumann@damtp.cam.ac.uk, E-mail: drgreen@cita.utoronto.ca, E-mail: rporto@ias.edu},

abstractNote = {Observations of the cosmic microwave background do not yet determine whether inflation was driven by a slowly-rolling scalar field or involved another physical mechanism. In this paper we discuss the prospects of using the power spectra of scalar and tensor modes to probe the nature of inflation. We focus on the leading modification to the slow-roll dynamics, which entails a sound speed c{sub s} for the scalar fluctuations. We derive analytically a lower bound on c{sub s} in terms of a given tensor-to-scalar ratio r, taking into account the difference in the freeze-out times between the scalar and tensor modes. We find that any detection of primordial B-modes with r > 0.01 implies a lower bound on c{sub s} that is stronger than the bound derived from the absence of non-Gaussianity in the Planck data. For r ∼> 0.1, the bound would be tantalizingly close to a critical value for the sound speed, (c{sub s}){sub *} = 0.47 (corresponding to (f{sub NL}{sup equil}){sub *} = -0.93), which we show serves as a threshold for non-trivial dynamics beyond slow-roll. We also discuss how an order-one level of equilateral non-Gaussianity is a natural observational target for other extensions of the canonical paradigm.},

doi = {10.1088/1475-7516/2015/01/016},

journal = {Journal of Cosmology and Astroparticle Physics},

number = 01,

volume = 2015,

place = {United States},

year = {Thu Jan 01 00:00:00 EST 2015},

month = {Thu Jan 01 00:00:00 EST 2015}

}