Talks & Events
KICP Friday Noon Seminars: 2004
Multipole Vectors and the CMB Sky
I will present "multipole vectors", a new basis to represent the CMB anisotropy. In this representation, each multipole order l is represented by l unit vectors pointing in directions on the sky and an overall magnitude. Like the usual spherical harmonics, multipole vectors form an irreducible representation of the proper rotation group SO(3) however, they are related to the familiar spherical harmonic coefficients, alm, in a highly nonlinear way. I will present evidence that oriented areas of planes defined by these vectors, between multipole pairs with 2 <= l <= 8 and computed from WMAP maps, are inconsistent with the isotropic gaussian hypothesis at about 99% confidence. I will discuss these results, and future work and possible applications of the multipole vectors.
The Chemistry of H3+ in the Diffuse Interstellar Medium
H3+, the simplest polyatomic molecule, plays a key role in dense interstellar clouds as the initiator of ion-molecule chemistry. The detection of H3+ in diffuse interstellar clouds came as a surprise, however, and suggested a serious (factor of ~100) problem in the simple model of diffuse cloud chemistry. In particular, this observation raised questions as to the applicability of laboratory measurements of the H3+ dissociative recombination rate to interstellar conditions. We have recently measured the dissociative recombination rate of rotationally cold H3+ ions in an ion storage ring, and have also detected H3+ in the classical diffuse cloud towards ? Persei, where the electron fraction is known from previous observations. This combination of new laboratory measurements and astronomical observations has eliminated two of the primary uncertainties in the chemical model, and implies a previously unrecognized and significant enhancement in the cosmic-ray ionization rate in the diffuse interstellar medium.
Galaxy Assembly in the Present Day
New detections of coherent streams and merger remnants in the Galaxy have reinforced our picture of the Galaxy under construction even at the present day. This continuous process of gravitational interaction and accretion leaves its signature on the global density and kinematic structure of the Galaxy, allowing us to unravel the recent merger history of the Milky Way. With the right instrumentation, we can apply this close-up view of galaxy interactions to the interpretation of the kinematics, population content, stellar and dark matter density profiles of nearby galaxies and groups, shedding important light on the processes which set the distribution of galaxy properties we observe today and in the earlier universe.
Observational Constraints on Neutrino Isocurvature Modes from the Curvaton Model of Inflation
In the curvaton model of inflation, where a second scalar field, the "curvaton", is responsible for the observed inhomogeneity, a non-zero neutrino degeneracy parameter may lead to a characteristic pattern of isocurvature perturbations in the neutrino, cold dark matter and baryon components. Where the neutrino degeneracy parameter is the neutrino chemical potential divided by the neutrino temperature and isocurvature perturbations are perturbations in the particle number ratio of different particle species. We find the current data (WMAP, 2dF, HST) can only place upper limits on the level of isocurvature perturbations. These can be translated into upper limits on the neutrino degeneracy parameter. In the case that lepton number is created before curvaton decay, we find that the limit on the neutrino degeneracy parameter is comparable with that obtained from Big-bang nucleosynthesis. For the case that lepton number is created by curvaton decay we find that the absolute value of the non-Gaussianity parameter,fnl, must be less than 10 (95% confidence interval).
Structural Evolution of Cold Dark Matter Substructure and the Missing Satellites Problem
Hierarchical Cold Dark Matter (CDM) models constitute the prevailing paradigm for interpreting the formation and evolution of structure in the universe. A generic prediction of these models is that massive dark matter halos are assembled by numerous merger events, leaving many tightly bound entities known as substructure. I present results on the structural evolution of these low-mass dark matter substructures combining ``low-resolution'' satellites from cosmological N-body simulations of parent halos with N=10^7 particles with high-resolution individual subhalos orbiting within a static host potential. In contrast to earlier investigations indicating that the central density cusp of CDM subhalos becomes shallower as a result of tidal interactions, I find that their inner density slope is unaffected even after several pericentric passages. I discuss the implications of these results for vital issues including the recent attempts to alleviate the missing Galactic satellites problem by means of allowing the observed dwarf spheroidal satellites to be embedded within dark halos with maximum circular velocities as large as 60 km/s.
Modeling SDSS Clusters: Galaxies, Masses, and Cosmology
The Sloan Digital Sky Survey, which is still in progress, has already identified the largest sample of galaxy clusters in the Universe. The abundance of massive clusters is a powerful discriminator between cosmological, but its promise can only be realized by using a large cluster sample with observable properties that can be closely connected to halo mass. Advances in cluster finding techniques, combined with large mock galaxy catalogs that match the relation between galaxy luminosity,color, and environment, now make this possible with optical data. We show how these mock catalogs can be used to calibrate cluster finding techniques and make direct predictions in observational space -- and how a combination of mass measurements and clustering statistics can be used to constrain the cluster mass scale and put constraints both on cosmology and on the relation between galaxies and dark matter halos.
Matter-Imbalance in the Universe
Discrete Spectrum of Inflationary Perturbations
Several arguments suggest that quantum-gravity effects may not only slightly alter the slope of inflationary perturbations, but may also lead to a discrete spectrum, instead of the continuous one predicted by the standard calculation using field theory on a quasi-classical spacetime background. This effect will be discussed from the point of view of the foundations of field theory, by adding simple self-gravity operators to the field Hamiltonian, and from the point of view of the holographic principle. Possible observational effects of a discrete spectrum will be briefly discussed.
Dark matter in Galaxies: Satellites and Neutralinos
Velocities of satellites of galaxies together with weak lensing provide the best way to probe the mass distribution of individual field galaxies at large radii. Using the Sloan Digital Sky Survey we find that the velocity dispersion of satellites declines with distance to the host galaxy. This decline agrees remarkably well with the distribution of dark matter predicted by the cosmological models in the peripheral parts of galaxies. A relation between the satellite velocity dispersion with the host luminosity as well as an estimation of the host mass-to-light is given. We use numerical simulations to test prescriptions for correcting the effects of interlopers who play an important role in the analysis. I will discuss our recent results. If dark matter is made of supersymmetric particles, the center of galaxies should emit gamma-rays produced by their self-annihilation. I will present accurate estimates of continuum gamma-ray fluxes due to neutralino annihilation in the central regions of the Milky Way. We use detailed models of our Galaxy, which satisfy available observational data, and include some important physical processes, which were previously neglected. In the next 1-2 years there is a hope that the new generation of Imaging Atmospheric Cherenkov Telescopes would detect the gamma-ray signal coming from the annihilation products of the SUSY dark matter in galaxy halos.
Adventures with Cold Dark Matter Halo Substructure
I will try to use this talk to address several issues. I plan to begin with a brief introduction to an approximate, analytic calculation of the properties of cold dark matter halo substructure by discussing a problem that many of you are likely rather familiar with, the problem of "missing" dwarf satellites of the Milky Way. I will discuss how such analytic models can be used to probe a wide range of parameter space and how such simple calculations can point towards ways in which substructure can be used as a cosmological probe. After this introduction to the model and some of it's virtues and drawbacks, I will discuss a project to use this model to better understand the physics of galaxy clustering. In particular, I will use this model in conjunction with the Halo Model to show how the processes of satellite accretion, mass loss due to tidal forces and dynamical friction all play a role in determining the galaxy correlation function. In the spirit of having the seminars be an informal discussion meeting, I have chosen to present work that is very much still in progress. As such, I have a request from the audience: Please ask questions! Because this work is in a developmental phase, it can still benefit greatly from questions and criticisms (however harsh) and if I can't finish, that will leave material that I can discuss on another Friday.
Revisiting Hawking Radiation in Odd Spacetime Dimensions
On Beyond Lambda: Dark Energy in the Next Generation
The acceleration of the expansion of the universe points toward new physics frontiers in cosmology, high energy physics, and gravitation. I examine various cosmological probes of the nature of dark energy, emphasizing the challenges in systematics control of the observations and interpretation of the data. Next generation experiments such as the Supernova/Acceleration Probe (SNAP) provide powerful, complementary tests I discuss what Joint Dark Energy Measurements can (and can't) tell us about going on beyond Lambda.
Galaxy environments and galaxy formation at low redshift
(1) Galaxy environments are much more closely related to galaxy spectral properties (ie, star formation histories) than they are related to galaxy structural properties (ie, morphological properties). (2) We can see new galaxy bulges being created in the local Universe,and they are being created at a rate that corresponds to about one percent of the galaxy population per Gyr.
Massive galaxies at high redshift in GOODS: observations in the context of LCDM, and challenges for galaxy & star formation theory
String cosmology and other inflated opinions
String theory provides a context for inflationary cosmology in the framework of a comprehensive theory of gravity and particle physics. The basics of string theory and a few of the current scenarios for inflation within it will be described. Some of the obstacles to further progress will also be discussed.
MAGIC: a Framework for Cosmological Data Analysis
I will discuss powerful statistical techniques which are intended to enable the optimal analysis of real cosmic microwave background data. These techniques allow the exact propagation of statistical uncertainty about the power spectrum from the map (or time-ordered data) to the cosmological parameters under very general assumptions. New features of these methods include the ability to account for correlated noise, arbitrarily shaped observed regions on the sky, specify partial knowledge about foreground contributions in a very flexible way and to incorporate physical priors, if desired. I will present a preliminary application of these methods to the issue of the low power at low l in WMAP and COBE. If time permits, I may mention the promise these methods hold for optimal analyses of other astronomical data sets.
Astrophysics of Dark Energy Stars
Certain predictions of general relativity such as event horizons and closed time-like curves cannot be physically correct for the simple reason that they are inconsistent with ordinary quantum mechanics. On the other hand the overwhelming experimental evidence for relativistic kinematics suggests that general relativity ought to be correct. These seemingly conflicting points of view can be reconciled if it is assumed that an infinite red shift surface corresponds to a quantum critical point of the vacuum of space-time. This leads to an entirely new picture of objects that lie within their Schwarzschild radius, and a new perspective on a variety of astrophysical phenomena including supernovae explosions, gamma ray bursts, relativistic jets, dark energy, and dark matter.
From Spitzer to Herschel and Beyond: The Future of Far-Infrared Space Astrophysics
I will present 1) recent results of semi-analytic galaxy formation models and compare them to the latest from SDSS, GOODS, GEMS, and other searches for high-redshift galaxies 2) new results from the galaxy merger simulations, now including stellar SED modelling and radiative transfer through dust. 3) New techniques for the non-parametric morphological classification of galaxies
Precession, Polaris and the Age of the Pyramids or Ancient Egyptian Astrometry
Just last Quarter, I gave the 59th Compton Lecture Series entitled "The Age of Things: Sticks, Stones, and the Universe." This mutli-disciplinary series of talks dealt with various methods of determining when events happened in the past. Topics covered in these talks included: the recent reconstruction of Ancient Mayan History from dated historical texts the current efforts to determine the timescale of mammalian evolution using molecular dating, and the new estimates of the age of the universe from cosmological data. The second lecture in this series, entitled "Precession, Polaris and the Age of the Pyramids" provides a good example of the cross-disciplinary thrust of these lectures. While the Great Pyramids are among the most impressive ancient monuments on earth, historical records do not yet provide a precise date for the construction of these impressive monuments (residual uncertainties are as large as hundreds of years). Recently, a new method of estimating the age of the pyramids has been proposed, which combines astronomical and archaeological data in a very productive way. If this proposed method works, then the age of the pyramids can be measured to within a decade or so, which would place important new constraints on the chronology of the earliest history of Ancient Egypt.
When Was the Universe Reionized?
The re-ionization history of cosmic hydrogen, left over from the big bang, provides crucial fossil evidence for when the first stars and black holes formed in the infant universe. Current observations provide a mixed message. The large-scale polarization anisotropies of the cosmic microwave background measured by WMAP imply a reionization redshift of 10-20. However, the extent of the ionized regions around the highest redshift quasars indicate a significantly neutral universe at a redshift of 6.4. I will summarize the status of current observational and theoretical studies, and address the possibility that the time evolution of the mean ionization fraction might have been non-monotonic. The truth will likely be revealed over the next decade through observations of the Lyman-alpha spectra of galaxies, quasars and gamma-ray bursts, as well as the detection of intergalactic 21 cm emission from redshifts above 6.
Harvard University 'Models of the ICM with Heating and Cooling: Explaining the Global and Structural X-ray Properties of Clusters'
Non-radiative numerical simulations and self-similar scaling arguments fail to reproduce the observed X-ray scaling relations of clusters. As a result, there has recently been increased interest in models in which either radiative cooling or entropy injection (and/or redistribution) play a central role in mediating the thermal and spatial properties of the intracluster medium. Both sets of models produce results which have been shown to be in good agreement with the mean global properties of clusters. Radiative cooling alone, however, results in fractions of cold/cooled baryons in excess of observationally established limits. And, the simplest entropy injection models, by design, do not treat the ``cooling core'' structure present in many clusters and cannot account for declining entropy profiles towards cluster centers revealed by recent high resolution X-ray observations. We consider models that marry radiative cooling with entropy injection, and confront model predictions for the global and structural properties of massive clusters with the latest X-ray data. The models successfully and simultaneously reproduce the observed L-T and L-M relations, yield detailed entropy, surface brightness, and temperature. Profiles in excellent agreement with observations, and predict cooled gas fraction that is consistent with observational constraints. More interestingly, the model provides a possible explanation for the significant intrinsic scatter present in the L-T and L-M relations, which is crucial if clusters are to be used as probes for precision cosmology studies, such as the determination of sigma_8. Our model also offers a natural way of distinguishing between clusters classically identified as "cooling flow" clusters and relaxed "non-cooling flow" clusters. The former correspond to systems that experienced only mild levels (<300 keV cm^2) of entropy injection while the latter are identified as systems that suffered much higher entropy injection. The dividing line in entropy injection between the two categories corresponds roughly to the cooling threshold for massive clusters. This finding suggests that entropy injection may be an important, if not the primary, factor in determining which class a particular cluster will belong to. These results also suggest that the previously identified relationship between inferred cooling flow strength and the dispersion in the L-T relation is a manifestation of the distribution of entropy injections levels experienced by clusters. This is borne out by the entropy profiles derived from Chandra and XMM-Newton. Finally, we demonstrate that there is a tight observational trend between a cluster's central entropy and its core radius size which may also be accounted for by the model.
Alternate Models for Reionization
The evidence for early reionization from WMAP presents a challenge for the standard models for reionization. I will discuss several alternative ideas that might explain early reionization: decaying particles, non-Gaussian initial conditions, and isocurvature perturbations. I will also briefly summarize my recent work on a variety of other topics.
The observed and predicted Milky Way satellite population
CAPMAP and QUIET: Measuring the Polarization of the Cosmic Microwave Background
CAPMAP is an experiment measuring the E-mode CMB Polarization in the l-range from 200 to 2000 with multiple W-band (90GHz) and Q-band (40GHz) correlation receivers on the 7m Crawford Hill telescope in NJ. Data has been taken for the past 2 years with 4 and then 8 receivers and we are about to deploy a total of 16 for this coming season. Experience from the last data taking periods, first results and the potential of CAPMAP will be presented. Plans for a large array, using miniaturized correlation polarimeters (QUIET), will be shown. QUIET intends to increase the sensitivity of current experiments to unprecedented precision, using in its first season 91 W-band receivers with a later upgrade to 1000.
Possible evidence for spatial fluctuations in dark energy.
The WMAP cosmic microwave background (CMB) first year data was anomalously smooth on the largest spatial scales. We have recently shown that spatial fluctuations in the dark energy, that is causing the expansion of the Universe to speed up, may partially cancel the fluctuations in the CMB on the largest scales. This would imply that the residual fluctuations that are observed on large scales would be due to the integrated Sachs Wolfe effect which is caused by the effect of large scale structure on the CMB at a redshift of about 1. We found that the current WMAP data provides a two sigma detection of the dark energy fluctuations. As this effect only operates at and after about redshift of one, we predict that the EE polarization fluctuations will be unsuppressed on large scales. Reference: Christopher Gordon, Wayne Hu, astro-ph/0406496
Understanding Dark Matter: from Accelerators to Galaxies
Many types of astronomical observations indicate that most of the clustering matter in the universe is non-baryonic and unobserved except for gravitational effects. This material seems to behave as a collisionless gas. Understanding the nature of this component is one of the biggest problems in physics. I briefly describe experimental efforts to understand dark matter, then describe a few recent theoretical developments that may help shed light on the issue.
Weak Lensing and Dark Energy
Since the first detection four years ago, weak gravitational lensing by large scale structure has emerged as an increasingly powerful probe of cosmology. I examine the current state of the art from a theorists point of view using numerical simulations, and speculate on the improvements that will be required if we are to constrain dark energy using future weak lensing surveys.
A new model for quasars and the M-sigma relation
A quasar model is presented in which the accretion disk around the black hole is constantl replenished by matter from stars near the nucleus of a galaxy, as the stars are captured by the accretion disk and are destroyed inside it. The model implies a relation between the mass of the black hole and the central velocity dispersion of the stars that matches the observed relation.
Galaxy Clustering: From HOD's to Brane-Induced Gravity
I discuss how we can use galaxy clustering at large scales to constrain galaxy bias and the galaxy halo occupation distribution (HOD), the matter density from redshift-space distortions, primordial non-Gaussianity from inflation, and large-scale modifications of gravity that explain the acceleration of the universe.