Talks & Events
KICP Friday Noon Seminars: 2002
MAJORANA: a next generation double-beta decay project
The neutrino mass interest generated by recent atmospheric and solar neutrino oscillation results has led to the idea of a next generation neutrino mass experiment. Double-beta decay can measure the effective neutrino mass, but only with the availability of large quantities of enriched material, new underground facilities, and new technologies for background rejection. The Majorana Project seeks to build such an experiment based on the 20+ years of double-beta decay experience, plus several technological advances including the introduction of commercially available segmented germanium detectors.
Polarization of the Cosmic Fireball
The Cosmic Microwave Background (CMB), believed to be the remnant radiation from the Big Bang, has recently provided a wealth of information about the properties of our Universe. After a (very) brief review of theory and past Cosmic Microwave Background research I will focus on the benefit, prospects, and status of observing the polarization of the CMB. In particular I will discuss the design, observations, and data analysis of CoMPaSS (Cosmic Microwave Polarization at Small Scales), our recent polarization experiment.
The fundamental Planck scale may be of the order TeV in models of extra dimensions. If this is the case, particle collisions with center-of-mass energy larger than a few TeV and sufficiently small impact parameter are expected to generate black holes. For instance, high energy cosmic rays may produce black holes in the Earth's atmosphere. Production of spherically symmetric black holes has been discussed in recent literature. However, in presence of extra dimensions, one should also expect the creation of higher-dimensional objects (p-branes). In this talk we compute the cross section for p-brane creation. In spacetimes with asymmetric compactifications, we find that the cross section for the formation of a brane completely wrapped on small extra dimensions is higher than the cross section for the creation of black holes. Therefore, we predict that branes are more likely to be created in super-Planckian scattering processes than black holes. Future hadron colliders and cosmic rays detectors may be able to detect formation of these branes.
Galaxies and Mass: SDSS Measurements on Halo Scales
The luminous galaxies we observe are embedded in an extensive dark matter environment. Uncertainty in how our luminous tracers relate to the dark matter limits our understanding of large scale structure and galaxy formation. In this talk we describe SDSS weak lensing and dynamical measurements relating the luminous properties of galaxies to the dark matter environments in which they form. We also touch on the growing interplay between galaxy measurements and structure formation simulations, showing a example parallel analysis conducted on real and simulated universes.
First results from the Auger Engineering Array in Argentina
Xavier Bertou will speak about the first results from the large scale Auger prototype in Argentina.
Light from Galactic Dark Matter
The hierarchical clustering observed in cold dark matter simulations result in highly clumped galactic halos. If the dark matter in our Halo is made of weakly interacting massive particles (WIMPs), their annihilation products should be detectable in the higher density and nearby clumps. We consider WIMPs to be neutralinos and calculate the radiation from their annihilation products. In particular we concentrate upon the synchrotron emission of electrons and positrons in the galactic magnetic field and the gamma ray emission from the decay of neutral pions. Several new and promising avenues for the dark matter detection will be discussed.
"Standard Model Parameters, Black Holes, and Cosmology
BJ is a very well known particle theorist who is turning his thoughts to Cosmology. He happens to be in town for personal reasons. Given our cancellation, he has kindly agreed to share some of his more speculative thoughts with us at our lunch. It should be fun for all.
Making Dark Matter and Deriving Constraints with Neutrino Mixing in the Early Universe
Starting with a general description of mixed neutrinos in the early universe, one can explore different regimes of their behavior. In one, small active neutrino mixing with singlet neutrinos can overproduce primordial helium, distort the CMB, overproduce a diffuse photon background, or supply the dark matter. Decay photons from such a dark matter candidate may be detected with modern X-ray telescopes. In another regime, active-active neutrino mixing can constrain cosmological neutrino degeneracies (lepton numbers) given the large mixing angle solution to the solar neutrino problem and the primordial helium abundance.
Precision Studies with Galaxy Clusters?
Clusters of galaxies provide a critical interface between cosmological models of large-scale structure formation and the astrophysics of galaxy formation. Rich data streams emerging from large observational surveys offer the opportunity to impose detailed constraints on cosmology and astrophysics, but realizing this potential requires the ability to accurately compute expectations for cluster observables. This talk will review the current state of affairs in computational modeling of clusters, emphasizing which pieces of the problem appear to be solved to high precision (few per cent level) and which pieces need more attention. Uncertainty in the absolute mass scale of clusters remains the fundamental impediment to precise constraints on cosmology from cluster observations.
The Energy Spectrum and Mass Composition of Cosmic Rays at High Energy
The derivation of the energy spectrum and mass composition of cosmic rays above 10**17 eV presents a considerable challenge. As well as necessitating accurate measurements of the cascades that the cosmic rays produce in the atmosphere, one must use models of particle physics processes in an energy range well above that explored by terrestrial accelerators to draw inferences. I will discuss recentwork done to improve our understanding of these issues and argue that, while there remains much to do, we are making substantial progress.
Extreme Astronomy: Neutrinos from Beyond the Edge
Single cosmic-ray particles of Joule-scale energies--a billion times higher than the strongest earth-based accelerators can produce--are peppering earth at a rate of several tens of thousands per day, apparently in defiance of the so-called Greisen-Zatsepin-Kuzmin (GZK) cutoff. This GZK cutoff is actually a kind of absorption edge in the cosmic ray spectrum: if these particles originate at distances large compared to a few Mpc, then they should be scattered and absorbed by the cosmic microwave background, which appear as gamma-rays in the rest frame of the particles. Such scattering must in turn yield pions and thus neutrinos, and these high energy GZK neutrinos are perhaps the clearest signature of the cosmic-ray scattering process from which they arise. Their detection, however, is a difficult and compelling challenge, requiring targets of tens to hundreds of cubic km of water-equivalent fiducial mass and several years of operation. We discuss a series of new initiatives aimed at discovering the GZK neutrinos, exploiting the Askaryan effect: strong coherent radio emission from high energy cascades in solid dielectric media such as natural ice and rock salt. This process, which dominates the secondary emission from particle showers at PeV energies and above, may provide the most cost-effective and direct approach to instrumenting the huge volumes necessary for GZK neutrino physics.
Imaging the Early Universe with ACBAR
Primary anisotropies of the Cosmic Microwave Background (CMB) encode a wealth of information about the early Universe. Recent degree-scale experiments have begun to exploit the potential of the CMB as a precision probe of cosmology with encouraging results. High resolution images of primary anisotropies can be used to produce improved constraints on cosmological parameters. In addition, the interaction of the CMB with intervening matter can produce secondary anisotropies that exhibit a sensitive dependence on the growth of structure. I will discuss the potential of high resolution observations of the CMB as a cosmological probe and the new generation of experiments designed for this task. In particular, I will focus on the Arcminute Cosmology Bolometer Array Receiver (ACBAR) which is now beginning its second season of observations at the South Pole.
Small Experiments, High Precision and Some Large Questions
Update on the EDGE LDB Project
Angular Power Spectrum of the Sunyaev-Zel'dovich Effect
Measurement of the angular power spectrum of the Sunyaev-Zel'dovich (SZ) effect offers a simple way to determine dark-halo abundance at high redshift. In contrast to a conventional number-count analysis of halo abundance, the SZ angular power spectrum is insensitive to observational selection effects, e.g., flux, surface brightness, or volume limit of the survey. The SZ angular power spectrum is also insensitive to poorly known gas physics in the central region of gas in halos, as it is determined by the outer part of gas-pressure profile rather than the inner part. Theoretical understanding to the SZ angular power spectrum is, however, still in the early stage of its development. In this work, we attempt to make a refined analytic prediction for the SZ angular power spectrum, compare it with hydrodynamic simulations, and argue that our prediction approximates the simulations well, and can be used to fit forthcoming data of the SZ angular power spectrum to extract cosmological information.
Cosmic Inflation and the Arrow of Time
Cosmic inflation claims to make the initial conditions of the standard big bang "generic". But Boltzmann taught us that the arrow of time arises from very non-generic ("low entropy") initial conditions. I discuss how to reconcile these perspectives. The resulting insights give an interesting way to understand and compare inflation and other ideas that purport to offer alternatives to inflation.
Dark Matter, Antimatter, and Local Cosmic Rays
Several recent measurements have been made to search for high-energy antiparticles in the local space environment. Most of the detected antiparticles are produced by high-energy interactions in the interstellar medium, but there are ongoing searches for features that could be related to dark matter particle annihilations. Ambitious investigations are in progress or planned that could provide more definive results.
Reflections on Inflation
Inflationary models are generally credited with explaining the large scale homogeneity, isotropy, and flatness of our universe as well as accounting for the origin of structure. We argue that the explanations provided by inflation for the homogeneity, isotropy, and flatness of our universe are not satisfactory, and that a proper explanation of these features will require a much deeper understanding of the initial state of our universe. On the other hand, inflationary models are spectacularly successful in providing an explanation of the deviations from homogeneity. The main aim of the talk is to point out that the fundamental mechanism responsible for providing deviations from homogeneity--namely, the evolutionary behavior of quantum modes with wavelength larger than the Hubble radius--will operate whether or not inflation itself occurs. The key difference is that if inflation did not occur, one must directly confront the issue of the initial state of modes whose wavelength was larger than the Hubble radius at the time at which they were "born," and one's predictions will depend on these assumptions. Under some simple hypotheses concerning the "birth time" and initial state of these modes--namely, that semiclassical physics can be applied at all times on spatial scales larger than the grand unification scale and that all modes are "born" in their ground state--it is shown that, e.g., non-inflationary fluid models in the extremely early universe would result in a similar density perturbation spectrum and amplitude as inflationary models, without any "fine tuning." Such models should give a larger contribution of tensor modes than inflationary models, since there is no "slow role" enhancement of the scalar modes.
Relativistic Corrections to the Sunyaev-Zeldovich Effects for Clusters of Galaxies
High-temperature plasmas exist inside the clusters of galaxies. The temperature is generally 5-15 keV. These high temperature electrons interact with the 2.728 K cosmic microwave photons and distort the Planck spectrum.This is the well-known Sunyaev-Zeldovich effect. Since some of the clusters have temperature as high as 15 keV, it is extremely important to include the relativistic corrections in the Sunyaev-Zeldovich effect. This has been successfully carried out recently by our group as well as some other groups. In this talk I will discuss the importance of the relativistic corrections. In fact they will be extremely important for the forthcoming short-wavelength observations of the Sunyaev-Zeldovich effect.
Studying the Reionization of the Universe with Recent and Future Observations
The first stars and quasars reionized the hydrogen in the universe by redshift six. Observations are beginning to probe this phase transition in the universe, and the near future should produce a wealth of data on the first sources of light and the reionization era. We model and interpret the spectra of current sources at the highest known redshifts, and study what can be learned in the future from Lyman alpha absorption.
Probing the State and Composition of the Intergalactic Medium
Radiative and mechanical feedback from galaxies and quasars play a crucial role in determining the characteristics of the intergalactic medium (IGM), including its temperature, ionization state, and heavy element content. However, the extent and power of feedback remain uncertain, and observational probes of the IGM at a variety of redshifts are needed. I will describe several techniques to constrain feedback scenarios in the coming years. For example, observations of the 21 cm hyperfine transition of hydrogen in the neutral IGM at high redshifts allow us to probe its thermal state and the early radiation background. Studies of metal absorption lines at high redshift can also constrain the extent of heavy element pollution in the IGM. Finally, I will describe how observations of the environments of radio jets can probe the physics of relativistic outflows.
CBR Anisotropy as a Probe of New Physics: Holographic discreteness of inflationary fluctuations
Arguments based on black hole thermodynamics and quantum unitaritary suggest that the Hilbert space of any physical system is discrete and finite, with a maximum information content equivalent to n=A/4 binary spins, where A is the area of the two-dimensional bounding surface in Planck units. Thus the effect of quantizing gravity considerably reduces the number of degrees of freedom below that of quantum field theory. This "holographic entropy bound" is described and used to estimate the quantum-gravitational discreteness of inflationary perturbations. In the context of scalar inflation perturbations produced during standard slow-roll inflation, and assuming that horizon-scale perturbations ``freeze-out'' in discrete steps separated by one bit of total observable entropy, it is shown that the Hilbert space of a typical horizon-scale inflation perturbation is equivalent to that of about 10^5 binary spins-- approximately the inverse of the final scalar metric perturbation amplitude, independent of other parameters. Holography thus suggests that in a broad class of fundamental theories, inflationary perturbations carry a limited amount of information (about 10^5 bits per mode) and should therefore display discreteness not predicted by the standard field theory. Some manifestations of this discreteness may be observable in cosmic background anisotropy.
Mapping the Dark Matter: Mass Selected Galaxy Clusters
The distortion of images of faint high-redshift background galaxies can be used to probe the intervening mass distribution. This weak gravitational lensing effect can be used to detect dark matter in clusters of galaxies, allowing one to effectively "image" and "weigh" these dark objects. In addition, if photometric redshifts of background source galaxies are available, mass tomography enables one to ascertain the cluster redshift. This opens up the possibility of mapping the 3-d locations of a mass selected sample of galaxy clusters from weak gravitational lensing alone. The efficacy and reliability of these techniques is investigated using a large ensemble of fast cosmological N-body simulations specifically tailored to investigate the statistics of lensing by clusters. Recent purported detections of baryon poor "dark clusters" are reviewed and interpreted. An Adaptive Matched Filtering scheme which combines tomography and matched filtering is introduced and proves superior to filtering techniques used in previous studies. The possibility of using mass selected cluster samples to probe cosmological parameters is discussed. Specifically, the redshift distribution of clusters is a sensitive probe of the equation of state parameter of the dark energy w, and is robust against the uncertain state of baryons in clusters.
TeV Gamma-Rays, Active Galaxies and Cosmology
A new observational window of the extreme cosmos has been opened with the advent of second generation ground based Gamma-ray detectors. The majority of these detectors utilise the Imaging Atmospheric Cerenkov Technique (IACT) which has been pioneered by the Whipple Collabortion. One of the main successes of IACT has been the detection of objectsbelonging to the blazar class of Active Galactic Nuclei, at energies above 300 GeV. These TeV observations are beginning to provide an important tool in constraining the models of particle acceleration in AGN. As TeV gamma-rays interact with the Cosmic Infrared Background, they provide us with a novel way of probing this background. By accurately measuring the energy spectra at TeV energies from extragalactic objects at different distances, preliminary estimates of the CIB flux have been derived. TeV gamma-ray observations may also be used to contrain the energy scale of quantum gravity, and to search for signatures of dark-matter particles. An overview of the IACT, and results of TeV observations on AGN to date, will be presented.
Kaluza-Kein Dark Matter
In a certain class of models with extra dimensions, the Lightest Kaluza-Klein Particle (LKP) is stable and turns out to be a viable dark matter candidate and a typical WIMP. In this talk, I will start by reviewing the particle physics framework for this class of models with TeV^-1 compactification radius and their phenomenological motivations. I will next present the relic density calculation of the LKP (which is likely to be a Kaluza-Klein photon) and discuss (direct and indirect) detection issues.
Is the hunt for the B-mode a waste of time?
Recent results appear to indicate that the detection of primordial gravity waves from inflation may be a hopeless task. First, foregrounds from lensing put a strict lower limit on the detectability of the B-mode polarization signal in the Cosmic Microwave Background, the "smoking gun" for tensor (gravity wave) fluctuations. Meanwhile, widely accepted theoretical arguments indicate that the amplitude of gravity waves produced in inflation will be below this limit. I will argue that failure is not inevitable, and that the effort to detect the primordial signal in the B-mode, whether it succeeds or fails, will yield crucial information about the nature of inflation.
Signatures of primordial helicity in the CMBR
I will discuss ways in which helical primordial magnetic fields could be constrained by measurements of the CMBR. If there were helical flows in the primordial plasma at the time of recombination, they would produce parity violating temperature-polarization correlations. However, the magnitude of helical flows induced by helical magnetic fields is unobservably small. I will describe an alternate scheme for extracting the helicity of a stochastically homogeneous and isotropic primordial magnetic field using Faraday rotation measure maps of the CMBR and the power spectrum of B-type polarization.