Engineering topics

Brandenberger, Robert

Publications and source records attributed to Brandenberger, Robert.

At least 19 records

DOE OSTI2019

Unified superfluid dark sector

We present a novel theory of a unified dark sector, where late-time cosmic acceleration emerges from the dark matter superfluid framework. The system is described by a superfluid mixture consisting of two distinguishable states with a small energy gap, such as the ground state and an excited state of dark matter. Given their contact in the superfluid, interaction between those states can happen, converting one state into the other. This long range interaction within the superfluid couples the two superfluid phonon species through a cosine potential motivated by Josephson/Rabi interactions. As a consequence of this potential, a new dynamics of late-time accelerated expansion emerges in this system, without the need of dark energy, coming from a universe containing only this two-state DM superfluid. Because the superfluid species are non-relativistic, their sound speeds remain suitably small throughout the evolution. We calculate the expansion history and growth of linear perturbations, and compare the results to ΛCDM cosmology. For the fiducial parameters studied here, the predicted expansion and growth function are close to those of ΛCDM, but the difference in the predicted growth rate is significant at late times. The present theory nicely complements the recent proposal of dark matter superfluidity to explain the empirical success of MOdified Newtonian Dynamics (MOND) on galactic scales, thus offering a unified framework for dark matter, dark energy, and MOND phenomenology.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2017

Searching for cosmic strings in CMB anisotropy maps using wavelets and curvelets

We use wavelet and curvelet transforms to extract signals of cosmic strings from simulated cosmic microwave background (CMB) temperature anisotropy maps, and to study the limits on the cosmic string tension which various ongoing CMB temperature anisotropy experiments will be able to achieve. We construct sky maps with size and angular resolution corresponding to various experiments. These maps contain the signals of a scaling solution of long string segments with a given string tension G μ, the contribution of the dominant Gaussian primordial cosmological fluctuations, and pixel by pixel white noise with an amplitude corresponding to the instrumental noise of the various experiments. In the case that we include white noise, we find that using curvelets we obtain lower bounds on the string tension than with wavelets. For maps with Planck specification, we obtain bounds comparable to what was obtained by the Planck collaboration [1]. Experiments with better angular resolution such as the South Pole Telescope third generation (SPT-3G) survey will be able to yield stronger limits. For maps with a specification of SPT-3G we find that string signals will be visible down to a string tension of G μ = 1.4 × 10{sup −7}.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2017

Dynamics of cosmological perturbations and reheating in the anamorphic universe

We discuss scalar-tensor realizations of the Anamorphic cosmological scenario recently proposed by Ijjas and Steinhardt [1]. Through an analysis of the dynamics of cosmological perturbations we obtain constraints on the parameters of the model. We also study gravitational Parker particle production in the contracting Anamorphic phase and we compute the fraction between the energy density of created particles at the end of the phase and the background energy density. We find that, as in the case of inflation, a new mechanism is required to reheat the universe.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2016

Differentiating G-inflation from string gas cosmology using the effective field theory approach

A characteristic signature of String Gas Cosmology is primordial power spectra for scalar and tensor modes which are almost scale-invariant but with a red tilt for scalar modes but a blue tilt for tensor modes. This feature, however, can also be realized in the so-called G-inflation model, in which Horndeski operators are introduced which leads to a blue tensor tilt by softly breaking the Null Energy Condition. In this article we search for potential observational differences between these two cosmologies by performing detailed perturbation analyses based on the Effective Field Theory approach. Our results show that, although both two models produce blue tilted tensor perturbations, they behave differently in three aspects. Firstly, String Gas Cosmology predicts a specific consistency relation between the index of the scalar modes n {sub s} and that of tensor ones n {sub t} , which is hard to be reproduced by G-inflation. Secondly, String Gas Cosmology typically predicts non-Gaussianities which are highly suppressed on observable scales, while G-inflation gives rise to observationally large non-Gaussianities because the kinetic terms in the action become important during inflation. However, after finely tuning the model parameters of G-inflation it is possible to obtain a blue tensor spectrum and negligible non-Gaussianities with a degeneracy between the two models. This degeneracy can be broken by a third observable, namely the scale dependence of the nonlinearity parameter, which vanishes for G-inflation but has a blue tilt in the case of String Gas Cosmology. Therefore, we conclude that String Gas Cosmology is in principle observationally distinguishable from the single field inflationary cosmology, even allowing for modifications such as G-inflation.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2016

Particle production in Ekpyrotic scenarios

We consider Parker particle production in the Ekpyrotic scenario (in particular in the New Ekpyrotic model) and show that the density of particles produced by the end of the phase of Ekpyrotic contraction can be sufficient to lead to a hot state of matter after the bounce. Hence, no separate reheating mechanism is necessary.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2016

Holographic curvature perturbations in a cosmology with a space-like singularity

We study the evolution of cosmological perturbations in an anti-de-Sitter (AdS) bulk through a cosmological singularity by mapping the dynamics onto the boundary conformal fields theory by means of the AdS/CFT correspondence. We consider a deformed AdS space-time obtained by considering a time-dependent dilaton which induces a curvature singularity in the bulk at a time which we call t=0, and which asymptotically approaches AdS both for large positive and negative times. The boundary field theory becomes free when the bulk curvature goes to infinity. Hence, the evolution of the fluctuations is under better controle on the boundary than in the bulk. To avoid unbounded particle production across the bounce it is necessary to smooth out the curvature singularity at very high curvatures. We show how the bulk cosmological perturbations can be mapped onto boundary gauge field fluctuations. We evolve the latter and compare the spectrum of fluctuations on the infrared scales relevant for cosmological observations before and after the bounce point. We find that the index of the power spectrum of fluctuations is the same before and after the bounce.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2016

Cosmic Microwave Background spectral distortions from cosmic string loops

Cosmic string loops contain cusps which decay by emitting bursts of particles. A significant fraction of the released energy is in the form of photons. These photons are injected non-thermally and can hence cause spectral distortions of the Cosmic Microwave Background (CMB). Under the assumption that cusps are robust against gravitational back-reaction, we compute the fractional energy density released as photons in the redshift interval where such non-thermal photon injection causes CMB spectral distortions. Whereas current constraints on such spectral distortions are not strong enough to constrain the string tension, future missions such as the PIXIE experiment will be able to provide limits which rule out a range of string tensions between G μ ∼ 10{sup −15} and G μ ∼ 10{sup −12}, thus ruling out particle physics models yielding these kind of intermediate-scale cosmic strings.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2015

Effects of cosmic string velocities and the origin of globular clusters

With the hypothesis that cosmic string loops act as seeds for globular clusters in mind, we study the role that velocities of these strings will play in determining the mass distribution of globular clusters. Loops with high enough velocities will not form compact and roughly spherical objects and can hence not be the seeds for globular clusters. We compute the expected number density and mass function of globular clusters as a function of both the string tension and the peak loop velocity, and compare the results with the observational data on the mass distribution of globular clusters in our Milky Way. We determine the critical peak string loop velocity above which the agreement between the string loop model for the origin of globular clusters (neglecting loop velocities) and observational data is lost.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2013

Note on structure formation from cosmic string wakes

The search for cosmic strings has been of renewed interest with the advent of precision cosmology. In this note we give a quantitative description of the nonlinear matter density fluctuations that can form from a scaling network of cosmic string wakes. Specifically, we compute the distribution of dark matter halos. These halos would possess strong correlations in position space that should have survived until today. We also discuss the challenges involved in their detection due to their small size and the complex dynamics of their formation.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

Scale-invariant fluctuations from Galilean genesis

We study the spectrum of cosmological fluctuations in scenarios such as Galilean Genesis \cite(Nicolis) in which a spectator scalar field acquires a scale-invariant spectrum of perturbations during an early phase which asymptotes in the far past to Minkowski space-time. In the case of minimal coupling to gravity and standard scalar field Lagrangian, the induced curvature fluctuations depend quadratically on the spectator field and are hence non-scale-invariant and highly non-Gaussian. We show that if higher dimensional operators (the same operators that lead to the η-problem for inflation) are considered, a linear coupling between background and spectator field fluctuations is induced which leads to scale-invariant and Gaussian curvature fluctuations.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

Supersymmetry breaking and dilaton stabilization in string gas cosmology

In this Note we study supersymmetry breaking via gaugino condensation in string gas cosmology. We show that the same gaugino condensate which is introduced to stabilize the dilaton breaks supersymmetry. We study the constraints on the scale of supersymmetry breaking which this mechanism leads to.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

Towards a nonsingular bouncing cosmology

We present a nonsingular bouncing cosmology using single scalar field matter with non-trivial potential and non-standard kinetic term. The potential sources a dynamical attractor solution with Ekpyrotic contraction which washes out small amplitude anisotropies. At high energy densities the field evolves into a ghost condensate, leading to a nonsingular bounce. Following the bounce there is a smooth transition to standard expanding radiation and matter dominated phases. Using linear cosmological perturbation theory we track each Fourier mode of the curvature fluctuation throughout the entire cosmic evolution. Using standard matching conditions for nonsingular bouncing cosmologies we verify that the spectral index does not change during the bounce. We show there is a controlled period of exponential growth of the fluctuation amplitude for the perturbations (but not for gravitational waves) around the bounce point which does not invalidate the perturbative treatment. This growth induces a natural suppression mechanism for the tensor to scalar ratio of fluctuations. Moreover, we study the generation of the primordial power spectrum of curvature fluctuations for various types of initial conditions. For the pure vacuum initial condition, on scales which exit the Hubble radius in the phase of Ekpyrotic contraction, the spectrum is deeply blue. For thermal particle initial condition, one possibility for generating a scale-invariant spectrum makes use of a special value of the background equation of state during the contracting Ekpyrotic phase. If the Ekpyrotic phase is preceded by a period of matter-dominated contraction, the primordial power spectrum is nearly scale-invariant on large scales (scales which exit the Hubble radius in the matter-dominated phase) but acquires a large blue tilt on small scales. Thus, our model provides a realization of the 'matter bounce' scenario which is free of the anisotropy problem.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

IR divergences in inflation and entropy perturbations

We study leading order perturbative corrections to the two point correlation function of the scalar field describing the curvature perturbation in a slow-roll inflationary background, paying particular attention to the contribution of entropy mode loops. We find that the infrared divergences are worse than in pure de Sitter space: they are power law rather than logarithmic. The validity of perturbation theory and thus of the effective field theory of cosmological perturbations leads to stringent constraints on the coupling constants describing the interactions, in our model the quartic self-interaction coupling constant of the entropy field. If the self coupling constant is larger than some critical value which depends in particular on the duration of the inflationary phase, then perturbation theory breaks down. Our analysis may have implications for the stability of de Sitter space: the quantum effects which lead to an instability of de Sitter space will be larger in magnitude in the presence of entropy fluctuations.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2012

The 21 cm signature of a cosmic string loop

Cosmic string loops lead to nonlinear baryon overdensities at early times, even before the time which in the standard LCDM model corresponds to the time of reionization. These overdense structures lead to signals in 21 cm redshift surveys at large redshifts. In this paper, we calculate the amplitude and shape of the string loop-induced 21 cm brightness temperature. We find that a string loop leads to a roughly elliptical region in redshift space with extra 21 cm emission. The excess brightness temperature for strings with a tension close to the current upper bound can be as high as 1deg K for string loops generated at early cosmological times (times comparable to the time of equal matter and radiation) and observed at a redshift of z+1 = 30. The angular extent of these predicted 'bright spots' is x{sup '}. These signals should be detectable in upcoming high redshift 21 cm surveys. We also discuss the application of our results to global monopoles and primordial black holes.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

Multi-stream inflation in a landscape

There are hidden observables for inflation, such as features localized in position space, which do not manifest themselves when only one inflation trajectory is considered. To address this issue, we investigate inflation dynamics in a landscape mimicked by a random potential. We calculate the probability for bifurcation of the inflation trajectory in multi-stream inflation. Depending on the shape of the random bumps and the distance between bumps in the potential, there is a phase transition: on one side of the critical curve in parameter space isocurvature fluctuation are exponentially amplified and bifurcation becomes very probable. On the other side bifurcation is dominated by a random walk where bifurcations are less likely to happen.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2011

Angular 21 cm power spectrum of a scaling distribution of cosmic string wakes

Cosmic string wakes lead to a large signal in 21 cm redshift maps at redshifts larger than that corresponding to reionization. Here, we compute the angular power spectrum of 21 cm radiation as predicted by a scaling distribution of cosmic strings whose wakes have undergone shock heating.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1991

Particle astrophysics

The following scientific areas are reviewed: (1) cosmology and particle physics (particle physics and the early universe, dark matter, and other relics); (2) stellar physics and particles (solar neutrinos, supernovae, and unconventional particle physics); (3) high energy gamma ray and neutrino astronomy; (4) cosmic rays (space and ground observations). Highest scientific priorities for the next decade include implementation of the current program, new initiatives, and longer-term programs. Essential technological developments, such as cryogenic detectors of particles, new solar neutrino techniques, and new extensive air shower detectors, are discussed. Also a certain number of institutional issues (the funding of particle astrophysics, recommended funding mechanisms, recommended facilities, international collaborations, and education and technology) which will become critical in the coming decade are presented.

Sadoulet, Bernard↗

NASA NTRS1988

Cosmic string catalysis of skyrmion decay

The Callan-Witten picture is developed for monopole catalyzed skyrmion decay in order to analyze the corresponding cosmic string scenario. It is discovered that cosmic strings (both ordinary and superconducting) can catalyze proton decay, but that this catalysis only occurs on the scale of the core of the string. In order to do this we have to develop a vortex model for the superconducting string. An argument is also given for the difference in the enhancement factors for monopoles and strings.

Gregory, Ruth↗