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Weak Lensing: Probing the Universe on All Scales
This E-News article is based on Chapter 14 of the LSST Science Book: Weak Gravitational Lensing. The Authors of Chapter 14 are:
Mass bends light. As light from distant galaxies and stars travels to us, mass in its path acts as a lens, bending the light, so we see a distorted image of the object. Strong lensing creates arcs or multiple images, but it is very rare. Weak lensing reveals the background mass by a systematic alignment of the background sources around the lensing mass. Weak lensing studies with LSST will provide a detailed map of half of the sky – a map that will detail the structure of the Universe for scientists and the public, a map that will reveal not only what we can observe but the dark matter and energy that underlie the structure and evolution of the Universe. Bhuvnesh Jain, co-chair of LSST’s Weak Lensing Science Collaboration enthuses, “LSST weak lensing will produce a gorgeous map of the Universe as well as exquisite statistical descriptions of its deepest mysteries.”
Figure 1: A spectacular example of strong gravitational lensing is the nearby galaxy cluster Abell 2218, in which the visible distortion of individual background galaxies can be used to measure the mass of the lensing structure. Image credit: NASA/ESA.
A Brief Review of Lensing
Massive structures lying between the most distant galaxies and Earth form gravitational lenses, which deflect the light of the ancient galaxies as it travels to the observer. Three types of lensing provide information about distant sources: strong, weak, and micro. The type of lensing depends on the distance to the light source and the mass of the lens.
Strong lensing occurs when massive lenses produce multiple images or arcs as seen by the observer (see Strong Gravitational Lensing – LSST Investigates Cosmology, Distant Galaxies and Dark Matter, LSST E-News July 2011, Volume 4, Number 2). Strong lensing occurs only along the densest lines of sight in the Universe, and it can tell us a lot about those particular systems.
To study typical lines of sight which are only slightly distorted, we need to average over many sources near that line of sight and statistically calculate the distortion. This is weak lensing [Figure 2], which we can use to study the Universe in general because all lines of sight are at least weakly affected by overdensities and underdensities near the line of sight. Weak lensing can also complement strong lensing, by pinning down information about the mass distribution outside strongly-lensed areas. Measurements of weak lensing on different scales, from galaxy halos to large-scale structure, allow scientists to constrain models of dark matter, dark energy, and cosmology to reveal the details of these fundamental aspects of the Universe.
Figure 2: This simulation of strong lensing by a massive cluster of galaxies shows a distortion pattern seen when the mass is smoothly distributed over the cluster. Clicking on the image will bring up a MPEG movie (800 kB) showing the evolution of the distortion as the clusters move against the background over half billion years. A full description of the simulation process is available. Courtesy J. A. Tyson, UC Davis.
“Because lensing probes the evolution of structure in the Universe in many complementary ways, it has unmatched statistical power in probing dark energy, which causes the Universe to expand faster, thereby slowing the growth of structure. LSST will offer a variety of opportunities for inquisitive scientists to understand the earliest times and evolution of the Universe in which we live,” says Jain.
Weak lensing around galaxies, termed galaxy-galaxy lensing, provides a direct probe of dark matter surrounding galaxies. Although individual galaxies induce a small stretching distortion called shear, scientists can average all foreground galaxies within a given subsample to obtain a high signal-to-noise ratio. If they know the redshifts, they can relate the shear signal to the projected mass density as a function of the proper distance from the galaxy and observe the averaged dark matter distribution around any given galaxy sample.
Galaxy-galaxy lensing is useful to explore many other properties of galaxies and relate them to the underlying host dark matter halo. For example, after estimating the mass of stars in galaxies, scientists can use the lensing properties to estimate the total mass and thus provide important information about the connection between the visible stellar component and the dark matter halo. This relationship constrains theories about galaxy formation and evolution.
Weak lensing can reveal dark matter halo ellipticity, which is predicted by the current standard model of cosmology (and can invalidate some alternate theories of gravity). The depth of LSST observations will allow exploration of the ellipticity of dark matter halos as a function of galaxy type and redshift to unprecedented precision.
Figure 3: Strong gravitatonal lensing bends light from a distant source to create distorted or multiple images. Credit: Tony Tyson, Greg Kochanski and Ian Dell'Antonio; Frank O'Connell and Jim McManus, adopted from The New York Times
Galaxy Cluster Lensing
Galaxy clusters are the largest structures in dynamic equilibrium in the Universe. Scientists use them as cosmological probes and as astrophysical laboratories. Because they represent the greatest overdensities (areas greater than normal density), scientists can use them to probe the growth of structure. Weak lensing can measure total cluster mass without regard to gas content, star formation history or dynamical state.
LSST will generate the largest and most uniform sample to date of galaxy clusters with gravitational lensing measurements. The cluster weak lensing measurements of shapes, magnitudes, and colors for about 40 galaxies/arcmin2 behind the clusters can be combined to constrain mass profiles of clusters and mass distribution in clusters.
Scientists will study the variation in the shear signal from clusters as a function of redshift to measure the angular diameter distance to clusters. Because the exact shape of the shear versus redshift profile is a function of cosmological parameters, it can be used to study the geometry of the Universe.
Galaxy clusters are sensitive to dark energy and so the LSST sample of weak lensing clusters will be superb for studying dark energy. Because clusters mark the locations of the highest density fluctuations in the early Universe, studies of clusters are complementary to studies of average parts of the Universe (see Large-scale Structure Lensing, below). The combination of cluster and large-scale structure information will allow LSST to constrain cosmological models even more tightly. Furthermore, the number and mass of clusters in the LSST survey will constrain the non-Gaussianity of the primordial fluctuations (that is, the difference from the expected normal distribution), thus probing the physics of inflation. Jain points out, “The depth of LSST will enable vast numbers of clusters to be detected at higher redshift (earlier cosmic epochs) than any other dataset. Clusters are the largest collapsed objects and are sensitive to some of the more intriguing properties of the early Universe, such as whether the fluctuations were Gaussian as expected in simple models of inflation.”
In addition to the chapter authors, the following are members of the Weak Lensing Science Collaboration chaired by Bhuvnesh Jain and David Wittman:
Large-scale Structure Lensing
Lensing by large-scale structure is termed “cosmic shear.” This causes subtle distortions everywhere on the sky, producing images more akin to shower glass than to a magnifying glass. Scientists quantify this by measuring the correlation between galaxy shapes: nearby neighbors (as seen in projection) tend to have similar shapes because of the lensing distortion, but more distant neighbors tend to be less similar. LSST will measure the correlations at different redshifts and across redshifts. The correlations are sensitive to the growth of structure and the expansion history of the Universe and so this work will be a powerful cosmological probe. Combined with baryon acoustic oscillations and type Ia supernovae, which explore expansion history, large scale structure lensing will provide rigorous tests of dark energy and modified gravity models.
Article written by Anna H. Spitz and David Wittman
LSST is a public-private partnership. Funding for design and development activity comes from the National Science Foundation, private donations, grants to universities, and in-kind support at Department of Energy laboratories and other LSSTC Institutional Members:
Adler Planetarium; Brookhaven National Laboratory (BNL); California Institute of Technology; Carnegie Mellon University; Chile; Cornell University; Drexel University; Fermi National Accelerator Laboratory; George Mason University; Google, Inc.; Harvard-Smithsonian Center for Astrophysics; Institut de Physique Nucléaire et de Physique des Particules (IN2P3); Johns Hopkins University; Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) – Stanford University; Las Cumbres Observatory Global Telescope Network, Inc.; Lawrence Livermore National Laboratory (LLNL); Los Alamos National Laboratory (LANL); National Optical Astronomy Observatory; National Radio Astronomy Observatory; Princeton University; Purdue University; Research Corporation for Science Advancement; Rutgers University; SLAC National Accelerator Laboratory; Space Telescope Science Institute; Texas A & M University; The Pennsylvania State University; The University of Arizona; University of California at Davis; University of California at Irvine; University of Illinois at Urbana-Champaign; University of Michigan; University of Pennsylvania; University of Pittsburgh; University of Washington; Vanderbilt University
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