All publications

Universality classes for models of inflation

Date of publication: 

2014-07-03 14:30:00

Author: 

P. Binetruy, E. Kiritsis, J. Mabillard, M. Pieroni, C. Rosset

Abstract (arXiv)

We show that the cosmological evolution of a scalar field in a potential can be obtained from a renormalisation group equation. The slow roll regime of inflation models is understood in this context as the slow evolution close to a fixed point, described by the methods of renormalisation group. This explains in part the universality observed in the predictions of a certain number of inflation models. We illustrate this behavior on a certain number of examples and discuss it in the context of the AdS/CFT correspondence.

The Apparent Universe

Date of publication: 

2014-06-06 12:30:00

Author: 

Pierre Binétruy, Alexis Helou

Abstract :

We exploit the parallel between dynamical black holes and cosmological spacetimes to describe the evolution of Friedmann-Lemaître-Robertson-Walker universes from the point of view of an observer in terms of the dynamics of the apparent horizon. Using the Hayward-Kodama formalism of dynamical black holes, we clarify the role of the Clausius relation to derive the Friedmann equations for a universe, in the spirit of Jacobson's work on the thermodynamics of spacetime. We also show how dynamics at the horizon naturally leads to the quantum-mechanical process of Hawking radiation. We comment on the connection of this work with recent ideas to consider our observable Universe as a Bose-Einstein condensate and on the corresponding role of vacuum energy.

See Saw Inflation / Dark Energy

Date of publication: 

2014-05-12 14:00:00

Author: 

George F. Smoot

Abstract:

Motivated by BICEP2 results that imply gravitational waves are produced when the universe has an expansion energy of about Mc 2 ≈10 14   GeV and that a natural extension to the Standard Model of Particle physics is a right-handed neutrino that would or could be at m ν R c 2 ≈10 14   GeV, I propose here a See-Saw Inflation model which fits into the general class of models we have dubbed "Wiggly Whipped Inflation"\cite{Hazra:2014WWI}. The same scalar boson that stabilises the heavy right-handed neutrino mass then becomes the Inflaton whose potential is set by self-coupling and the heavy right-handed neutrino mass coupling. Following this See-Saw Inflation one also finds that the see-saw mechanism provides for an after electroweak symmetry breaking shift in the Inflaton potential at the near GUT scale vev by an amount set by the lightest neutrino and thus as a consequence produces a "Dark Energy" at a scale set by the lightest left-handed neutrino mass m v L 10 −3 eV. The resurrected SO(10) GUT model provides a framework for the development of this scenario and allows a specific prediction for the Inflaton potential and a framework for fitting to neutrino as well as cosmological observables.

Healthy theories beyond Horndeski

Date of publication: 

2014-04-25 09:15:00

Author: 

Jérôme Gleyzes (IPhT, Saclay & U. Paris-Sud 11, Dept. Phys., Orsay), David Langlois, Federico Piazza (APC, Paris & Paris Cent. Cosmol. Phys.), Filippo Vernizzi (IPhT, Saclay).

Abstract (arXiv)

We introduce a new class of scalar-tensor theories that extend Horndeski, or "generalized galileon", models. Despite possessing equations of motion of higher order in derivatives, we show that the true propagating degrees of freedom obey well-behaved second-order equations and are thus free from Ostrogradski instabilities, in contrast to the standard lore. Remarkably, the covariant versions of the original galileon Lagrangians-obtained by direct replacement of derivatives with covariant derivatives-belong to this class of theories. These extensions of Horndeski theories exhibit an uncommon, interesting phenomenology: the scalar degree of freedom affects the speed of sound of matter, even when the latter is minimally coupled to gravity. 

Whipped inflation

Date of publication: 

2014-04-04 13:45:00

Author: 

Dhiraj Kumar Hazra, Arman Shafieloo, George F. Smoot, Alexei A. Starobinsky

Abstract:

Motivated by the idea that inflation occurs at the GUT symmetry breaking scale, in this paper we construct a new class of large field inflaton potentials where the inflaton starts with a power law potential; after initial period of relative fast roll that lasts until after a few e-folds inside the horizon, it transits to the attractor of the slow roll part of the potential with a lower power. Due to the initial fast roll stages of inflation, we find a suppression in scalar primordial power at large scales and at the same time the choice of the potential can provide us a tensor primordial spectrum with high amplitude. This suppression in scalar power with a large tensor-to-scalar ratio helps us to reconcile the Planck and BICEP2 data in a single framework. We find that a transition from a cubic to quadratic form of inflaton potential generates an appropriate suppression in power of scalar primordial spectrum that provides significant improvement in fit compared to power law model when compared with Planck and BICEP2 data together. We calculate the extent of non-Gaussianity, specifically, the bispectrum for the best fit potential and show that it is consistent with Planck bispectrum constraints.

Ruling out the power-law form of the scalar primordial spectrum

Date of publication: 

2014-03-30 13:45:00

Author: 

Dhiraj Kumar Hazra, Arman Shafieloo, George F. Smoot, Alexei A. Starobinsky

Abstract:

Combining Planck CMB temperature [1] and BICEP2 B-mode polarization data [2,3] we show qualitatively that, assuming inflationary consistency relation, the power-law form of the scalar primordial spectrum is ruled out at more than 3σ CL. This is an important finding, since the power-law form of the scalar primordial spectrum is one of the main assumptions of concordance model of cosmology and also a direct prediction of many inflationary scenarios. We show that a break or step in the form of the primordial scalar perturbation spectrum, similar to what we studied recently analyzing Planck data [4] can address both Planck and BICEP2 results simultaneously. Our findings also indicate that the data may require more flexibilities than what running of scalar spectral index can provide. Finally we show that an inflaton potential, originally appeared in [5] can generate both the step and the break model of scalar primordial spectrum in two different limits. The discussed potential is found to be favored by Planck data but marginally disfavored by BICEP2 results as it produces slightly lower amplitude of tensor primordial spectrum. Hence, if the tensor-to-scalar ratio (r ) quoted by BICEP2 persists, it is of importance that we generate inflationary models with large r and at the same time provide suppression in scalar primordial spectrum at large scales.

Superconducting NbN Coplanar Switch Driven by DC Current for CMB Instruments

Date of publication: 

2014-02-28 14:45:00

Author: 

G. Bordier, V. D. Cammilleri, B. Bélier, N. Bleurvacq, A. Ghribi, M. Piat, A. Tartari, M. Zannoni

Abstract: The next generations of cosmic microwave background (CMB) instruments will be dedicated to the detection and characterisation of CMB B-modes. To measure this tiny signal, instruments need to control and minimise systematics. Signal modulation is one way to achieve such a control. New generation of focal planes will include the entire detection chain on chip. In this context, we present a superconducting coplanar switch driven by DC current. It consists of a superconducting micro-bridge which commutes between its on (superconducting) and off (normal metal) states, depending on the amplitude of the current injection. To be effective, we have to use a high normal state resistivity superconducting material with a gap frequency higher than the frequencies of operation (millimeter waves). Several measurements were made at low temperature on NbN and yielded very high resistivities. Preliminary results of components dc behavior is shown. Thanks to its low power consumption, fast modulation and low weight, this component is a perfect candidate for future CMB space missions.

A Small, Rapid Optical-IR Response Gamma-Ray Burst Space Observatory

Date of publication: 

2014-02-20 13:45:00

Author: 

Bruce Grossan, Pawan Kumar, Daniel Perley, George F. Smoot

Abstract:

Here we propose a new gamma-ray burst (GRB) mission, the Next Generation Rapid-Response GRB Observatory (NGRG). As with Swift, GRBs are initially located with a coded-mask X-ray camera. However, the NGRG has two new features: First, a beam-steering system to begin optical observations within ~ 1 s after location; second, a near-IR (NIR) camera viewing the same sky, for sensitivity to extinguished bursts. These features allow measurement of the rise phase of GRB optical-NIR emission. Thus far, the rise time and transition between prompt and afterglow in the optical and NIR are rarely measured. Rapid-response measurements explore many science topics including optical emission mechanisms (synchrotron vs. SSC, photospheric emission) and jet characteristics (reverse vs. forward shock emission, baryon-dominated vs. magnetic dominated). Rapid optical-NIR response can measure dynamic evolution of extinction due to vaporization of dust, and separate star system and galaxy dust extinction. We discuss these measurements, giving reliable detection rate estimates from analysis of Swift data and scaled Swift performance. The NGRG will explore optical/NIR emission measured earlier than ever before, and potentially fainter, more extinguished GRBs than ever before. In the current era, costs are important. Our proposed modest NGRG can still produce new GRB science, while providing rapid GRB alerts for the entire community for post-Swift GRB science. We show that an X-ray instrument barely 1/5 the area of Swift BAT will yield a significant fraction of Swift's detection rate: more than 65 X-ray, and with a 30 cm optical-IR telescope and modern cameras, more than 19 NIR and 14 optical detections each year. In addition, active feedback control of the beam-steering would remove the need for arc sec stabilization of the spacecraft, for a substantial cost saving.

Some Implications of inverse-Compton Scattering of Hot Cocoon Radiation by relativistic jets in Gamma-Ray Bursts

Date of publication: 

2014-02-11 13:00:00

Author: 

Pawan Kumar, George F. Smoot

Abstract: Long Gamma-Ray Bursts (GRB) relativistic jets are surrounded by hot cocoons which confine jets during their punch out from the progenitor star. These cocoons are copious sources of X-ray photons that can be and are inverse-Compton (IC) scattered to MeV--GeV energies by electrons in the relativistic jet. We provide detailed estimates for IC flux resulting from various interactions between X-ray photons and the relativistic jet, and describe what we can learn about GRB jets and progenitor stars from the detection (or an upper limit) of these IC scattered photons.

A mm-Wave Polarisation Analyser Using LEKIDs: Strategy and Preliminary Numerical Results

Date of publication: 

2014-01-29 08:45:00

Author: 

Tartari, A.; Belier, B.; Calvo, M.; Cammilleri, V. D.; Monfardini, A.; Piat, M.; Prêle, D.; Smoot, G. F.

Abstract : The context of this study is the development of polarisation sensitive detectors in view of future Cosmic Microwave Background experiments. Our goal is to demonstrate the possibility to make a mm-wave polarisation analyser at 150 GHz using Lumped Element Kinetic Inductance Detectors (LEKIDs). Although LEKIDs are very attractive for the relative ease of fabrication, they have an intrinsic optical response which is weakly polarisation-senstive, i.e. orthogonal linear polarisations are absorbed with comparable efficiencies (with a separation typically not exceeding few dB). To overcome this difficulty, we achieve a polarised response by means of small (˜ λ × λ) superconducting Nb wire-grids. Each grid is deposited on the rear side of the 300 micron Si substrate, on which 20 nm Al resonators are patterned, so that each pixel may in principle respond as an independent polarisation analyser. Simulations show encouraging results, with a deep (-20 dB) rejection of the unwanted polarisation. Although what we present here is not yet a polarimeter, this pilot study allows us to address some relevant questions that may be crucial in view of a full polarimetric architecture development. In particular, our first prototypes will allow to assess the behaviour of small grids, the interaction between adjacent polarised pixels, and to choose the most suitable resonator geometry. What we present here are preliminary design results about devices which are currently being realised, and soon ready for optical response characterisation

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