scholarly journals Thermalization of large energy release in the early Universe

2020 ◽  
Vol 498 (1) ◽  
pp. 959-980 ◽  
Author(s):  
Jens Chluba ◽  
Andrea Ravenni ◽  
Sandeep Kumar Acharya
Keyword(s):  
Energy Release ◽  
Degrees Of Freedom ◽  
Light Element ◽  
Large Energy ◽  
Small Scale ◽  
Microwave Background ◽  
Element Abundances ◽  
Relativistic Temperature ◽  
Low Mass ◽  
Thermalization Efficiency

ABSTRACT Spectral distortions of the cosmic microwave background (CMB) provide a unique tool for learning about the early phases of cosmic history, reaching deep into the primordial Universe. At redshifts z ≲ 106, thermalization processes become inefficient and existing limits from COBE/FIRAS imply that no more than Δρ/ρ ≲ 6 × 10−5 ($95{{\ \rm per\ cent}}$ c.l.) of energy could have been injected into the CMB. However, at higher redshifts, when thermalization is efficient, the constraint weakens and Δρ/ρ ≃ 0.01−0.1 could in principle have occurred. Existing computations for the evolution of distortions commonly assume Δρ/ρ ≪ 1 and thus become inaccurate in this case. Similarly, relativistic temperature corrections become relevant for large energy release, but have previously not been modelled as carefully. Here, we study the evolution of distortions and the thermalization process after single large energy release at z ≳ 105. We show that for large distortions the thermalization efficiency is significantly reduced and that the distortion visibility is sizeable to much earlier times. This tightens spectral distortions constraints on low-mass primordial black holes with masses $M_{\rm PBH}\lesssim 2 \times 10^{11}\, {\rm g}$. Similarly, distortion limits on the amplitude of the small-scale curvature power spectrum at wavenumbers $k\gtrsim 10^4\, {\rm Mpc}^{-1}$ and short-lived decaying particles with lifetimes $t_X\lesssim 10^7\, {\rm s}$ are tightened, however, these still require a more detailed time-dependent treatment. We also briefly discuss the constraints from measurements of the effective number of relativistic degrees of freedom and light element abundances and how these complement spectral distortion limits.

scholarly journals Dark matter annihilation in the universe

2014 ◽  
Vol 30 ◽  
pp. 1460256 ◽  
Author(s):  
Pierre Salati
Keyword(s):  
Dark Matter ◽  
Galactic Halo ◽  
Light Element ◽  
Big Bang ◽  
Microwave Background ◽  
Dark Matter Annihilation ◽  
Element Abundances ◽  
Primordial Plasma ◽  
Relic Abundance ◽  
The Universe

The astronomical dark matter is an essential component of the Universe and yet its nature is still unresolved. It could be made of neutral and massive elementary particles which are their own antimatter partners. These dark matter species undergo mutual annihilations whose effects are briefly reviewed in this article. Dark matter annihilation plays a key role at early times as it sets the relic abundance of the particles once they have decoupled from the primordial plasma. A weak annihilation cross section naturally leads to a cosmological abundance in agreement with observations. Dark matter species subsequently annihilate — or decay — during Big Bang nucleosynthesis and could play havoc with the light element abundances unless they offer a possible solution to the 7 Li problem. They could also reionize the intergalactic medium after recombination and leave visible imprints in the cosmic microwave background. But one of the most exciting aspects of the question lies in the possibility to indirectly detect the dark matter species through the rare antimatter particles — antiprotons, positrons and antideuterons — which they produce as they currently annihilate inside the galactic halo. Finally, the effects of dark matter annihilation on stars is discussed.

scholarly journals Abundance Ratios in Metal-Poor Globular Clusters: Deep Mixing and its Effect on Stellar Populations of the Galactic Halo

1998 ◽  
Vol 11 (1) ◽  
pp. 53-57
Author(s):  
Robert P. Kraft
Keyword(s):  
Globular Clusters ◽  
Galactic Halo ◽  
Light Element ◽  
Asymptotic Giant Branch ◽  
Proton Capture ◽  
Low Mass Stars ◽  
Element Abundances ◽  
Deep Mixing ◽  
Low Mass ◽  
Red Giant

Only a bit more than 25 years ago, it seemed possible to assume that all Galactic globular clusters were chemically homogeneous. There were indications that star-to-star Fe abundance variations existed in ω Cen, but this massive cluster appeared to be unique. Following Osborn’s (1971) initial discovery, Zinn’s (1973) observation that M92 asymptotic giant branch (AGB) stars had weaker G-bands than subgiants with equivalent temperatures provided the first extensive evidence that there might be variations in the abundances of the light elements in an otherwise “normal” cluster. Since then star-to-star variations in the abundances of C, N, O, Na, Mg and Al have been observed in all cases in which sample sizes have exceeded 5-10 stars, e.g., in clusters such as M92, M15, M13, M3, ω Cen, MIO and M5. Among giants in these clusters one finds large surface O abundance differences, and these are intimately related to differences of other light element abundances, not only of C and N, but also of Na, Mg and Al (cf. reviews by Suntzeff 1993, Briley et al 1994, and Kraft 1994). The abundances of Na and O, as well as Al and Mg, are anticorrelated. Prime examples are found among giants in M15 (Sneden et al 1997), M13 (Pilachowski et al 1996; Shetrone 1996a,b; and Kraft et al 1997) and ω Cen (Norris & Da Costa 1995a,b). These observed anticorrelations almost certainly result from proton- capture chains that convert C to N, 0 to N, Ne to Na and Mg to Al in or near the hydrogen fusion layers of evolved cluster stars. But which stars? An appealing idea is that during the giant branch lifetimes of the low-mass stars that we now observe, substantial portions of the stellar envelopes have been cycled through regions near the H-burning shell where proton-capture nucleosynthesis can occur. This so-called “evolutionary” scenario involving deep envelope mixing in first ascent red giant branch (RGB) stars has been studied by Denissenkov & Denissenkova (1990), Langer & Hoffman (1995), Cavallo et al (1996, 1997) and Langer et al (1997). The mixing mechanism that brings proton-capture products to the surface is poorly understood (Denissenkov & Weiss 1996, Denissenkov et al 1997, Langer et al 1997), but deep mixing driven by angular momentum has been suggested (Sweigart & Mengel 1979, Kraft 1994, Langer & Hoffman 1995, Sweigart 1997).

scholarly journals Constraints from the cosmic microwave background experiments

2009 ◽  
Vol 5 (S268) ◽  
pp. 17-17
Author(s):  
Joanna Dunkley
Keyword(s):  
Cosmic Microwave Background ◽  
Light Element ◽  
Baryon Density ◽  
Light Elements ◽  
Microwave Background ◽  
Element Abundances

AbstractI will give a review of the current constrains on light element abundances from cosmic microwave background experiments, focusing on results from WMAP and discussing prospects from upcoming data from Planck and ground-based experiments. I will describe how the production of light elements affects the CMB anisotropies, and how we use the data to extract cosmological information that includes constraints on the baryon density, and primordial abundances.

scholarly journals Background Radiation: Probes and Future Tests

1996 ◽  
Vol 168 ◽  
pp. 389-398
Author(s):  
Martin J. Rees
Keyword(s):  
Background Radiation ◽  
Light Element ◽  
Mass Range ◽  
Black Body ◽  
Big Bang ◽  
List Type ◽  
Microwave Background ◽  
Background Spectrum ◽  
Element Abundances ◽  
Microwave Background Radiation

The clearest evidence for the ‘hot big bang’ is of course the microwave background radiation. Its spectrum is now known, from the FIRAS experiment on COBE, to be a very precise black body – indeed, the deviations due to high-z activity, hot intergalactic gas, etc are smaller than many people might have expected. Also the light element abundances have remained concordant with the predictions of big bang nucleosynthesis, thereby giving us confidence in extrapolating back to when the universe was a few seconds old (see Copi, Schramm and Turner 1994 for a recent review). These developments give us grounds for greater confidence in this model than would have been warranted ten years ago. Several things could have happened which would have refuted the picture, but they haven't happened. For instance:(i)Objects could have been found where the helium abundance was far below 23 per cent.(ii)The background spectrum at millimetre wavelengths could have been weaker than a black body with temperature chosen to fit the Rayleigh-Jeans part of the spectrum.(iii)A stable neutrino might have been discovered in the mass range 100eV-1MeV.

scholarly journals Primordial Nucleosynthesis: Constraints on the Birth of the Universe

2018 ◽  
Vol 184 ◽  
pp. 01011
Author(s):  
Grant Mathews ◽  
Motohiko Kusakabe ◽  
Mayukh Gangopadhyay ◽  
Toshitaka Kajino ◽  
Nishanth Sasankan
Keyword(s):  
Cosmic Microwave Background ◽  
Light Element ◽  
Higher Dimensions ◽  
Big Bang ◽  
Energy Scale ◽  
Brane World ◽  
Microwave Background ◽  
Dark Radiation ◽  
Element Abundances ◽  
The Universe

We review the basic elements of big bang nucleosythesis (BBN) and how a comparison of predicted light-element abundances with observations constrains physics of the radiation-dominated epoch. We then summarize some applications of BBN and the cosmic microwave background (CMB) to constrain the first moments of the birth of the universe. In particular, we discuss how the existence of higher dimensions impacts the cosmic expansion through the projection of curvature from the higher dimension in the "dark radiation" term. We summarize current constraints from BBN and the CMB on this brane-world dark radiation term. At the same time, the existence of extra dimensions during the earlier inflation impacts the tensor to scalar ratio and the running spectral index as measured in the CMB. We summarize how the constraints on inflation shift when embedded in higher dimensions. Finally, one expects that the universe was born out of a complicated multiverse landscape near the Planck time. In these moments the energy scale of superstrings was obtainable during the early moments of chaotic inflation. We summarize the quest for cosmological evidence of the birth of space-time out of the string theory landscape. We will explore the possibility that a superstring excitations may have made itself known via a coupling to the field of inflation. This may have left an imprint of "dips" in the power spectrum of temperature fluctuations in the cosmic microwave background. The identification of this particle as a superstring is possible because there may be evidence for different oscillator states of the same superstring that appear on different scales on the sky. It will be shown that from this imprint one can deduce the mass, number of oscillations, and coupling constant for the superstring. Although the evidence is marginal, this may constitute the first observation of a superstring in Nature.

Quantum simulations

2017 ◽  
Author(s):  
Michael P. Allen ◽  
Dominic J. Tildesley
Keyword(s):  
Ab Initio ◽  
Quantum Monte Carlo ◽  
Degrees Of Freedom ◽  
Small Region ◽  
Time Correlation ◽  
Ab Initio Molecular Dynamics ◽  
Classical Dynamics ◽  
Integral Methods ◽  
Quantum Time ◽  
Low Mass

This chapter covers the introduction of quantum mechanics into computer simulation methods. The chapter begins by explaining how electronic degrees of freedom may be handled in an ab initio fashion and how the resulting forces are included in the classical dynamics of the nuclei. The technique for combining the ab initio molecular dynamics of a small region, with classical dynamics or molecular mechanics applied to the surrounding environment, is explained. There is a section on handling quantum degrees of freedom, such as low-mass nuclei, by discretized path integral methods, complete with practical code examples. The problem of calculating quantum time correlation functions is addressed. Ground-state quantum Monte Carlo methods are explained, and the chapter concludes with a forward look to the future development of such techniques particularly to systems that include excited electronic states.

Small‐Scale Cosmic Microwave Background Observations at 8.4 GHz

1997 ◽  
Vol 483 (1) ◽  
pp. 38-50 ◽  
Author(s):  
R. Bruce Partridge ◽  
Eric A. Richards ◽  
Edward B. Fomalont ◽  
K. I. Kellerman ◽  
Rogier A. Windhorst
Keyword(s):  
Cosmic Microwave Background ◽  
Small Scale ◽  
Microwave Background

scholarly journals Task space adaptation via the learning of gait controllers of magnetic soft millirobots

2021 ◽  
pp. 027836492110218
Author(s):  
Sinan O. Demir ◽  
Utku Culha ◽  
Alp C. Karacakol ◽  
Abdon Pena-Francesch ◽  
Sebastian Trimpe ◽  
...  
Keyword(s):  
Human Body ◽  
Degrees Of Freedom ◽  
Bayesian Optimization ◽  
Small Scale ◽  
Soft Robots ◽  
Modeling And Control ◽  
Walking Gait ◽  
Physical Experiments ◽  
Efficient Learning ◽  
Task Environments

Untethered small-scale soft robots have promising applications in minimally invasive surgery, targeted drug delivery, and bioengineering applications as they can directly and non-invasively access confined and hard-to-reach spaces in the human body. For such potential biomedical applications, the adaptivity of the robot control is essential to ensure the continuity of the operations, as task environment conditions show dynamic variations that can alter the robot’s motion and task performance. The applicability of the conventional modeling and control methods is further limited for soft robots at the small-scale owing to their kinematics with virtually infinite degrees of freedom, inherent stochastic variability during fabrication, and changing dynamics during real-world interactions. To address the controller adaptation challenge to dynamically changing task environments, we propose using a probabilistic learning approach for a millimeter-scale magnetic walking soft robot using Bayesian optimization (BO) and Gaussian processes (GPs). Our approach provides a data-efficient learning scheme by finding the gait controller parameters while optimizing the stride length of the walking soft millirobot using a small number of physical experiments. To demonstrate the controller adaptation, we test the walking gait of the robot in task environments with different surface adhesion and roughness, and medium viscosity, which aims to represent the possible conditions for future robotic tasks inside the human body. We further utilize the transfer of the learned GP parameters among different task spaces and robots and compare their efficacy on the improvement of data-efficient controller learning.

scholarly journals QPOs in compact binaries from small scale eruptions in an inner magnetized disk

2020 ◽  
Author(s):  
Nicolas Scepi ◽  
Mitchell C Begelman ◽  
Jason Dexter
Keyword(s):  
Rapid Heating ◽  
Small Scale ◽  
Type B ◽  
Periodic Oscillations ◽  
X Ray ◽  
Compact Binaries ◽  
Heating And Cooling ◽  
Low Mass ◽  
X Ray Binaries ◽  
Standard Disk

Abstract Dwarf novæ (DNe) and low mass X-ray binaries (LMXBs) are compact binaries showing variability on time scales from years to less than seconds. Here, we focus on explaining part of the rapid fluctuations in DNe, following the framework of recent studies on the monthly eruptions of DNe that use a hybrid disk composed of an outer standard disk and an inner magnetized disk. We show that the ionization instability, that is responsible for the monthly eruptions of DNe, is also able to operate in the inner magnetized disk. Given the low density and the fast accretion time scale of the inner magnetized disk, the ionization instability generates small, rapid heating and cooling fronts propagating back and forth in the inner disk. This leads to quasi-periodic oscillations (QPOs) with a period of the order of 1000 s. A strong prediction of our model is that these QPOs can only develop in quiescence or at the beginning/end of an outburst. We propose that these rapid fluctuations might explain a subclass of already observed QPOs in DNe as well as a, still to observe, subclass of QPOs in LMXBs. We also extrapolate to the possibility that the radiation pressure instability might be related to Type B QPOs in LMXBs.

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