Gravitational lensing: a unique probe of dark matter and dark energy

Abstract

I review the development of gravitational lensing as a powerful tool of the observational cosmologist. After the historic eclipse expedition organized by Arthur Eddington and Frank Dyson, the subject lay observationally dormant for 60 years. However, subsequent progress has been astonishingly rapid, especially in the past decade, so that gravitational lensing now holds the key to unravelling the two most profound mysteries of our Universe-the nature and distribution of dark matter, and the origin of the puzzling cosmic acceleration first identified in the late 1990s. In this non-specialist review, I focus on the unusual history and achievements of gravitational lensing and its future observational prospects.

Figure 1.

(a) Unveiling a new commemorative plaque to Eddington and Einstein at the Roça Sundy site in Príncipe (photo courtesy of Richard Massey). (b) The museum commemorating the 1919 eclipse at the relevant site in Sobral, Brazil, with its current Director, Dr F. de Almeida.

Figure 2.

(a) Hubble Space Telescope image of the first gravitationally lensed source, the high-redshift quasar SBS 0957 + 561. The two nearly comparable images of the same background source are produced by the lensing effect of a foreground elliptical galaxy. (b) The remarkable ‘giant arc’ in the rich cluster of galaxies, Abell 370. The image is that of a distant galaxy distorted by the gravitational potential of the foreground cluster.

Figure 3.

Deployment of multiple images for a strong elliptical lens. (a) Source-plane view, with various locations of the source indicated by coloured dots. Lines represent lensing caustics. (b) Image-plane view (what the observer sees); dotted lines represent critical lines for the relevant source distance. The variously coloured multiple images correspond to the changing position of the source. When the source approaches the caustic, the number of multiple images increases. As surface brightness is conserved, the amount of distortion in the image represents a magnification in the total received brightness as well as a spatial enlargement. Both features are useful advantages of lensing in the study of distant galaxies.

Figure 4.

(a) Hubble Space Telescope images of a selection of elliptical lenses from the SLACS survey (Bolton et al. 2008). The blue ring-like features represent the distorted (and magnified) images of background galaxies lensed by the foreground elliptical galaxy (orange). (b) Distribution of the logarithmic slope γ of the mass density distribution derived from a combination of the lensing geometry and stellar dynamics of the lensing elliptical. The remarkable uniformity in the mass profile argues for early formation concurrent with the assembly of a massive dark matter halo.

Figure 5.

(a) Hubble Space Telescope image of the rich cluster Abell 2218 showing a plethora of distorted arcs and multiple images. Along the critical lines of high magnification were found two of the highest-redshift magnified sources known (at the time of discovery). One is a close pair of images representing a source at redshift 5.7 (Ellis et al. 2001); the other is a triply imaged system at redshift 6.8 (Kneib et al. 2004). (b) A highly magnified star-forming galaxy at z=3.1 for which resolved spectroscopy reveals a rotating disc (Stark et al. 2008): (i) actual image with the Hubble Space Telescope, (ii) reconstructed source-plane image with colour-coded velocity field, and (iii) velocity versus major axis position.

Figure 6.

An idealized illustration of weak gravitational lensing. The blue image represents the projected mass distribution in a given area of the sky (white indicates a higher projected density of dark matter). The white tick marks represent the average shapes and orientations of a population of faint galaxies (assumed statistically to be round in shape) viewed through the dark matter. Where the dark matter is concentrated, the background galaxies are tangentially aligned around the structure; where the dark matter density is weak, the galaxies are aligned radially. The pattern of background galaxies can be used to infer the (invisible) distribution of foreground dark matter.

Figure 7.

(a) Projected distribution of dark matter in the COSMOS field from the analysis of Massey et al. (2007a). The blue map reveals the density of dark matter as inferred from the pattern of weak distortions viewed in background galaxies by the Hubble Space Telescope. (b) Equivalent map for the baryonic matter as revealed by a combination of the stellar mass in galaxies imaged with the Hubble Space Telescope and hot gas imaged with the X-ray satellite XMM–Newton.

Figure 8.

(a) Early halo microlensing event from the MACHO project (Alcock et al. 1993). The absence of a chromatic variation in the (i) blue and (ii) red light curves together with the duration of the event indicate lensing by an unseen line-of-sight compact object of approximately 0.1 solar masses. Too few such events have been detected for the dark matter in the Milky Way to be composed of such objects. (b) Detection of a low-mass exoplanet (OGLE 2005-BLG-390LB) via a small perturbation to the microlensing light curve (Beaulieu et al. 2006). Dataset sources: black, OGLE; green, Robonet; light blue, Canopus; red, Danish; dark blue, Perth; brown, MOA.

Figure 9.

An illustration of milli-lensing in the multiply imaged B2045+265 quasar studied by McKean et al. (2007). (a) Near-infrared adaptive optics image taken with the Keck-II telescope. A–D are multiple images of the background quasar, and G1 is the primary lensing galaxy. A smooth lens model for G1 alone predicts image B to be the brightest image, yet it is anomalously fainter than images A and C. This is due to substructure in the lens arising from the low-mass satellite G2. (b) A lens model for the combined G1+G2 system matches the multiple image positions and fluxes. Caustics in the source plane are shown in blue and critical curves in the image plane in red.