A gravitational lens is produced when the gravitational field of a star, galaxy or cluster of galaxies distorts the images of background sources, 'strongly' when multiple images are created and 'weakly' otherwise. Gravitational lenses are now regarded as tools used to: (i) probe the nature of unseen mass, from black holes to dark matter and dark energy; (ii) measure distances and probe the size, shape and expansion of the universe; and (iii) study distant sources in exquisite detail. Future projects using new telescopes, like the Large Synoptic Survey Telescope and the SuperNova Acceleration Probe, will enable detailed, statistical studies of dark matter, and will provide a sensitive and robust measurement of the empirical behavior of dark energy. Past results from Dr. Blandford's group include: (i) demonstrating that dark matter is effectively collisionless and distinct from normal, baryonic matter (ii) a competitively accurate determination of the Hubble constant using a quadruply-imaged variable galaxy nucleus, (iii) measurement of the clustering of the faintest galaxies observed by the Hubble Space Telescope, (iv) understanding the distribution and clustering of distant quasars, and (v) developing new computational tools for cosmological investigations.
The new work will include: (i) more detailed planning for handling the variability data for many thousands of lensed quasars, (ii) analyzing what can be learned from the rare lensed supernovae and gamma ray bursts, (iii) more about galaxy-galaxy weak lensing and what can be learned about the potential wells of galaxies and their halos, (iv) new ideas on systematic errors in gravitational lensing, notably the difficulty of distinguishing a change in the size of the universe from the addition of extra mass, (v) studying the predicted incidence of exceptional lenses that can be used to test the theory of how mass clumps in the expanding universe, (vi) the distribution of sizes of the largest gravitational lenses, and (vii) how to use the gravitational lensing-induced distortion of the clustering of faint galaxies to determine the distribution in space, properties, and histories of these galaxies. It will significantly advance understanding of the size, shape and expansion of the Universe and of the properties of its major constituents, dark matter and dark energy.
Cosmological questions continue to attract much genuine interest among non-scientists and especially among young people who might ultimately become practicing scientists or engineers. Future projects with large publicly-accessible databases are likely to transform the relationship between professional and amateur astronomers.
When a galaxy or cluster of galaxies lies in front of a background galaxy, a gravitational lens is formed and the image if hte background source is distorted or multiplied because the gravitional field of the intervening lens galaxy deflects the rays from the backgrouind galaxy. Gravitational lenses are spendid tools for studing the shape, size, age expansion and contents of the unverse as well as for probing hte dark matter and stellar content of the intervening galaxies and this constitues their principle intellectual merit. Several uses of lensing have been developed with the goal of learning about the universe. In the first, the age and sioze of the univese are measuring the difference in arrivial time along different paths of variations in the source. This has yielded results that are consistent with quite iindependent determinations usiong other methods. It has also been possible toi "weigh" clusters of galaxies which is a crucial step in using them to determine the empirical properties of dark energy. In a very new development, the ALMA millimeter telescope in Chile has been used to find and study gravitational lenses. These are now being used to explore the incidence of predicted substructure in the nuclei of normal galaxies and the masses of the central black holes in very distant galaxies. Another approach to studong the contents and expansion of the universe is to measure the incidence of weak lensing and contributions have been made to the analysis of new data sets and planning the acquisition of future ones. It has also been posssible to show that the stellar content of elliptical galaxies does not change too much over recent cosmic time. However, the more complex spiral glaxies have to hve different proportions of different masses of star and this has now been understood by combining models that explain the distribution of the stars and their speeds as well as the optics of gravitational lenses. Some fascinating secondary results have also been obtained including a new upper limit on the incidence of cosmic strings, a measurement of the correlation function at very small angular separation and a study of microlensing by small "nomadic" bodies that are conjectured to heve been ejected by exoplanetary systems and could be plentiful in the space between the stars of our galaxy. Some of hthis research is directed towards designing observing program optimally with future tleescopes such as LSST and Euclid and the prospects are bright for improving the accuracy of lensing investigations as the number of lenses increases. This research has a broader impact through advancing our cosmological understanding of hte universe at large which is of great interest to non-astronomers, young and old. It also impacts general astrophysical investigations throughg its contribution to an improved understand of the vital parameters that desribe the unverse at large.
