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(abridged) The Observatory for Multi-Epoch Gravitational Lens Astrophysics (OMEGA) is a mission concept for a 1.5-m near-UV through near-IR space observatory that will be dedicated to frequent imaging and spectroscopic monitoring of ~100 multiply-imaged active galactic nuclei over the whole sky. Using wavelength-tailored dichroics with extremely high transmittance, efficient imaging in six channels will be done simultaneously during each visit to each target. The separate spectroscopic mode, engaged through a flip-in mirror, uses an image slicer spectrograph. After a period of many visits to all targets, the resulting multidimensional movies can then be analyzed to a) measure the mass function of dark matter substructure; b) measure precise masses of the accreting black holes as well as the structure of their accretion disks and their environments over several decades of physical scale; and c) measure a combination of the local Hubble expansion and cosmological distances to unprecedented precision., Comment: Proceedings paper for the SPIE Astronomical Telescopes and Instrumentation 2008

Low-frequency radio observations of neutral hydrogen during and before the epoch of cosmic reionization will provide hundreds of quasi-independent source planes, each of precisely known redshift, if a resolution of ~ 1 arcminutes or better can be attained. These planes can be used to reconstruct the projected mass distribution of foreground material. A wide-area survey of 21 cm lensing would provide very sensitive constraints on cosmological parameters, in particular on dark energy. These are up to 20 times tighter than the constraints obtainable from comparably sized, very deep surveys of galaxy lensing although the best constraints come from combining data of the two types. Any radio telescope capable of mapping the 21cm brightness temperature with good frequency resolution (~ 0.05 MHz) over a band of width ~> 10 MHz should be able to make mass maps of high quality. If the reionization epoch is at z ~ 9 very large amounts of cosmological information will be accessible. The planned Square Kilometer Array (SKA) should be capable of mapping the mass with a resolution of a few arcminutes depending on the reionization history of the universe and how successfully foreground sources can be subtracted. The Low-Frequency Array (LOFAR) will be able to measure an accurate matter power spectrum if the same conditions are met., Comment: prize wining contribution to proceedings of the Cosmology 2007 conference in Venice, 4 pages, one color figure

We study the constraints which the next generation of radio telescopes could place on the nature of dark energy, dark matter and inflation by studying the gravitational lensing of high redshift 21 cm emission, and we compare with the constraints obtainable from wide-angle surveys of galaxy lensing. If the reionization epoch is effectively at z ~ 8 or later, very large amounts of cosmological information will be accessible to telescopes like SKA and LOFAR. We use simple characterizations of reionization history and of proposed telescope designs to investigate how well the two-dimensional convergence power spectrum, the three-dimensional matter power spectrum, the evolution of the linear growth function, and the standard cosmological parameters can be measured from radio data. The power spectra can be measured accurately over a wide range of wavenumbers at z ~ 2, and the evolution in the cosmic energy density can be probed from z ~ 0.5 to z ~ 7. This results in a characterization of the shape of the power spectra (i.e. of the nature of dark matter and of inflationary structure generation) which is potentially more precise than that obtained from galaxy lensing surveys. On the other hand, the dark energy parameters in their conventional parametrization (Omega_Lambda, w_o, w_a) are somewhat less well constrained by feasible 21 cm lensing surveys than by an all-sky galaxy lensing survey although a 21 cm surveys might be more powerful than galaxy surveys for constraining models with "early" dark energy. Overall, the best constraints come from combining surveys of the two types. This results in extremely tight constraints on dark matter and inflation, and improves constraints on dark energy, as judged by the standard figure of merit, by more than an order of magnitude over either survey alone., Comment: submitted to MNRAS, 12 pages, error in computer code corrected which changed constraints on some cosmological parameters, change to lensing estimator to improve performance

21-cm emission from neutral hydrogen during and before the epoch of cosmic reionisation is gravitationally lensed by material at all lower redshifts. Low-frequency radio observations of this emission can be used to reconstruct the projected mass distribution of foreground material, both light and dark. We compare the potential imaging capabilities of such 21-cm lensing with those of future galaxy lensing surveys. We use the Millennium Simulation to simulate large-area maps of the lensing convergence with the noise, resolution and redshift-weighting achievable with a variety of idealised observation programmes. We find that the signal-to-noise of 21-cm lens maps can far exceed that of any map made using galaxy lensing. If the irreducible noise limit can be reached with a sufficiently large radio telescope, the projected convergence map provides a high-fidelity image of the true matter distribution, allowing the dark matter halos of individual galaxies to be viewed directly, and giving a wealth of statistical and morphological information about the relative distributions of mass and light. For instrumental designs like that planned for the Square Kilometer Array (SKA), high-fidelity mass imaging may be possible near the resolution limit of the core array of the telescope., Comment: version accepted for publication in MNRAS (reduced-resolution figures)

We use the distribution, and particularly the skewness, of high redshift type Ia supernovae brightnesses relative to the low redshift sample to constrain the density of macroscopic compact objects (MCOs) in the universe. The data favors dark matter made of microscopic particles (such as the LSP) at 89% confidence. Future data will greatly improve this limit. This constraint is valid for a range in MCO mass from 10^-2 Msun to 10^10 Msun. Combined with other constraints, MCOs larger than one tenth the mass of Earth (~ 10^-7 Msun) can be eliminated as the sole constituent of dark matter., Comment: 4 pages, 1 figure

Low-frequency radio observations of neutral hydrogen during and before the epoch of cosmic reionization will provide ~ 1000 quasi-independent source planes, each of precisely known redshift, if a resolution of ~1 arcminutes or better can be attained. These planes can be used to reconstruct the projected mass distribution of foreground material. Structure in these source planes is linear and gaussian at high redshift (30~ 10 MHz should be able to make mass maps of high quality. The planned Square Kilometer Array (SKA) may be able to map the mass with moderate signal-to-noise down to arcminute scales, depending on the reionization history of the universe and the ability to subtract foreground sources., Comment: revised and extended, more detailed predictions for observations, 25 pages, 11 figures, submitted to MNRAS

We present a numerical technique to compute the gravitational lensing induced by simulated haloes. It relies on a 2D-Tree domain decomposition in the lens plane combined with a description of N-Body particles as extended clouds with a non-singular density. This technique is made fully adaptive by the use of a density-dependent smoothing which allows one to probe the lensing properties of haloes from the densest regions in the center or in substructures to the low-density regions in the outskirts. 'Smooth Particle Lensing' has some promising features. First, the deflection potential, the deflection angles, the convergence and the shear are direct and separate end-products of the SPL calculation and can be computed at an arbitrary distribution of points on the lens plane. Second, this flexibility avoids the use of interpolation or a finite differentiation procedure on a grid, does not require padding the region with zeros and focuses the computing power on relevant regions. The SPL algorithm is tested by populating isothermal spheres and ellipsoids with particles and then comparing the lensing calculations to the classical FFT-based technique and analytic solutions. We assess issues related to the resolution of the lensing code and the limitations set by the simulations themselves. We conclude by discussing how SPL can be used to predict the impact of substructures on strong lensing and how it can be generalized to weak lensing and cosmic shear simulations., Comment: 11 pages, 11 figures, submitted to MNRAS

Image flux ratio anomalies have been attributed to substructures within the gravitational lens and to small mass halos (M <~ 10^10 Msun) in intergalactic space. In this paper, analytic calculations are presented that help in the understanding of how intergalactic halos affect magnification ratios. It is found that intergalactic halos can produce anomalies at a similar level to those that are observed. Intergalactic halos with masses <10^10 Msun are expected to cause relative deflections between images of order 10 milliarcseconds, which are then magnified by the primary lens. They will also cause fluctuations in the surface density on the several percent level. The importance of intergalactic halos depends strongly on the radial profile of the halos and the primordial power spectrum at small scales. Strongly lensed quasars provide an opportunity to probe these properties. A strong dependence on the QSO redshift is predicted and can be used to distinguish between intergalactic structure and substructure as the cause of magnification anomalies. This analytic approach also explains why some previous semi-analytic estimates disagreed with numerical calculations., Comment: 15 pages, 7 figures, submitted to ApJ

Galactic sized gravitational lenses are simulated by combining a cosmological N-body simulation and models for the baryonic component of the galaxy. The lens caustics, critical curves, image locations and magnification ratios are calculated by ray-shooting on an adaptive grid. When the source is near a cusp in a smooth lens' caustic the sum of the magnifications of the three closest images should be close to zero. It is found that in the observed cases this sum is generally too large to be consistent with the simulations implying that there is not enough substructure in the simulations. This suggests that other factors play an important role. These may include limited numerical resolution, lensing by structure outside the halo, selection bias and the possibility that a randomly selected galaxy halo may be more irregular, for example due to recent mergers, than the isolated halo used in this study. It is also shown that, with the level of substructure computed from the N-body simulations, the image magnifications of the Einstein cross type lenses are very weak functions of source size up to $\sim 1\kpc$. This is also true for the magnification ratios of widely separated images in the fold and cusp caustic lenses. This means that selected magnification ratios for different the emission regions of a lensed quasar should agree with each other, barring microlensing by stars. The source size dependence of the magnification ratio between the closest pair of images is more sensitive to substructure., Comment: 28 pages, 2 tables and 14 figures. Accepted to MNRAS

The gravitational lensing constraints on the small mass end of the LCDM mass function are discussed. Here a conservative approach is taken where only the most difficult to explain image flux anomalies in strong QSO lenses are emphasized. Numerical simulations are performed to compare predictions for the LCDM small scale mass function with the observed flux ratios. It is found that the cusp caustic lens anomalies and the disagreements between monochromatic flux ratios and simple lens models can be explained without any substructure in the primary lenses' dark matter halos. Extragalactic LCDM halos of mass < 10^9 Msun are enough to naturally explain these anomalies. This does not mean that substructure within the host lens is not contributing. In fact, it could dominate the lensing depending on how much of substructure survives in the centers of galactic halos. Spectroscopic gravitational lensing provides more information on the nature of these substructures. Thus far, the one relevant case in which this technique has been used, observations of Q2237+0305, shows evidence of more small mass halos (~ 10^6 Msun$) than is expected in the LCDM model., Comment: to appear in "Impact of Gravitational Lensing on Cosmology", IAU Symposium 225, ed. Mellier & Meylan

This paper primarily addresses the question of whether recent lensing observations probing the small scale structure in the universe are consistent with the LCDM model. A conservative approach is taken where only the most difficult to explain cases of image flux anomalies in strong lenses are considered. Numerical simulations are performed to compare predictions for the LCDM small scale mass function with observed flux ratios. It is found by simulating several represent cases that all the cusp caustic lens anomalies and the disagreements between monochromatic flux ratios and simple lens models might be explained without any substructure in the primary lenses' dark matter halos. Intergalactic LCDM halos are enough to naturally explain these cases. However, thus far, spectroscopic gravitational lensing observations require more small mass halos (~ 10^6 Msun) than is expected in the LCDM model., Comment: submitted to ApJ, 19 pages, 7 figures, replaced with accepted version, more discussion of tests of the computer code

The Cold Dark Matter (CDM) model for galaxy formation predicts that a significant fraction of mass in the dark matter haloes that surround L L* galaxies is bound in substructures of mass 1E4-1E7Msun. The number of observable baryonic substructures (such as dwarf galaxies and globular clusters) falls short of these predictions by at least an order of magnitude. We present a method for searching for substructure in the haloes of gravitational lenses that produce multiple images of QSOs, such as 4-image Einstein Cross lenses. Current methods based on broadband flux ratios cannot cleanly distinguish between substructure, differential extinction, microlensing and, most importantly, ambiguities in the host lens model. These difficulties may be overcome by utilizing the prediction that when substructure is present, the magnification will be a function of source size. QSO broad line and narrow line emission regions are approximately ~1pc and >100pc in size, respectively. When narrow line region (NLR) features are used as a normalisation, the relative intensity and equivalent width of broad line region (BLR) features will respectively reflect substructure-lensing and microlensing effects. Spectroscopic observations of just a few image pairs would probably be able to cleanly extract the desired substructure signature and distinguish it from microlensing. In the rest-optical, the Hbeta/[OIII] region is ideal, since the narrow wavelength range also largely eliminates differential reddening problems. Simulations of Q2237+030 are done as an example to determine the level of substructure that is detectable in this way, and possible systematic difficulties are discussed. This is an ideal experiment to be carried out with near-infrared integral field unit spectrographs on 8-m class telescopes., Comment: 9 pages, 8 figures, submitted to MNRAS, uses LaTeX2e mn2e.cls

When a gravitational lens produces two or more images of a quasar's radio jet the images can be compared to reveal the presence of small structures along one or more of the lines of sight. If mass is distributed smoothly on scales of <~ 10^7 Msun independent bends in the jet images on milli-arcsecond scales will not be produced. All three of the well collimated multiply imaged radio jets that have been mapped on milli-arcsecond scales show some evidence of independent bends in their images. Using existing data we model the lens system B1152+199 and show that it likely contains a substructure of mass ~10^5 - 10^7 h^-1 Msun or a velocity dispersion of ~ 10 km/s. An alternative explanation is that an intrinsic bend in the jet is undetected in one image and magnified in the other. This explanation is disfavored and future observations could remove any ambiguity that remains. The probability of a radio jet being bent by small scale structure both inside and outside of the host lens is then investigate. The known populations of dwarf galaxies and globular clusters are far too small to make this probability acceptable. A previously unknown population of massive dark objects is needed. The standard Cold Dark Matter (CDM) model might be able to account for the observations if small mass halos are sufficiently compact. In other cosmological models where small scale structure is suppressed, such as standard Warm Dark Matter (WDM), the observed bent jets would be very unlikely to occur., Comment: Significantly improved and extended - Better comparison with observations and a more complete treatment of alternative explanations that do not require substructure. Some small errors are corrected. 16 pages, 4 figures, submitted to ApJ

We demonstrate that the flux ratios of 4-image lensed quasars provide a powerful means of probing the small scale structure of Dark Matter (DM) halos. A family of smooth lens models can precisely predict certain combinations of flux ratios using only the positions of the images and lens as inputs. Using 5 observed lens systems we show that real galaxies {\it cannot} be described by smooth singular isothermal ellipsoids, nor by the more general elliptical power-law potentials. Large scale distortions from the elliptical models can not yet be ruled out. Nevertheless we find, by comparing with simulations, that the data can be accounted for by a significant ($\sim 5-10%$) amount of dark substructures within a projected distance of several kpc from the center of lenses. This interpretation favors the Cold Dark Matter (CDM) model other the warm or self-interacting DM models., Comment: 6 pages with 2 PS figs, resubmitted to ApJ Letter, added a figure to show the amount of substruture needed and tables of model parameters

I argue that the cold dark matter (CDM) model requires that even within a few kpc of the center of a galactic halo a significant fraction (greater than a few percent) of the surface density is contained in substructures with masses > 10^3 solar masses. These structures should be light enough to avoid dynamical friction and dense enough to avoid tidal disruption. I then show using the results of numerical simulations that this substructure will significantly alter the flux ratios of multiply imaged quasars (QSOs) without changing the image positions. The degree to which this occurs will depend on the angular size of the QSO and thus the wavelength of the observations., Comment: 5 pages, for proceedings of Marseille 2001 conference "Where's the Matter?", ed. L. Tresse & M. Treyer

We show how observations of multiply-imaged quasars at high redshift can be used as a probe of dark matter clumps (subhalos with masses ~ 10^9 solar masses) within the virialized extent of more massive lensing halos. A large abundance of such satellites is predicted by numerical simulations of galaxy formation in cold dark matter (CDM) cosmogonies. Small-scale structure within galaxy halos affects the flux ratios of the images without appreciably changing their positions. We use numerical simulations to quantify the effect of dark matter substructure on the distribution of magnifications, and find that the magnification ratio of a typical image pair will deviate significantly from the value predicted by a smooth lensing potential if, near the Einstein radius, only a few percent of the lens surface density is contained in subhalos. The angular size of the continuum source dictates the range of subclump masses that can have a detectable effect: to avoid confusion with gravitational microlensing caused by stars in the lens galaxy, the background source must be larger than the optical continuum-emitting region of a QSO. We also find that substructure will cause distortions to images on milli-arcsecond scales and bias the distribution of QSO magnification ratios -- two other possible methods of detection., Comment: accepted for publication in ApJ, 21 pages, 10 figures

A new method for measuring gravitational lensing with high redshift type Ia supernovae is investigated. The method utilizes correlations between foreground galaxies and supernova brightnesses to substantially reduce possible systematic errors and increase the signal to noise ratio. It is shown that this lensing signal can be related to the mass, size and substructure of galaxy halos and galaxy clusters if dark matter consists of microscopic particles. This technique may be particularly useful for measuring the size of dark matter halos, a measurement to which the lensing of galaxies is not well suited, and for measuring the level of substructure in galaxy halos, a problematic prediction of the cold dark matter model. The contributions to the signal from galaxy halos and galaxy clusters are modeled and contributions to the noise from fluctuations in the galaxy number counts, galaxy redshift error, dispersion in SN luminosities and sample variance are estimated. The intrinsic distribution of supernova luminosities and its redshift evolution are removed as major sources of uncertainty. The method is found to be complimentary to galaxy--galaxy lensing. The required observations of $\simgt 100$ supernovae have already been proposed for the purposes of cosmological parameter estimation., Comment: Replaced with final version to be published in MNRAS. 19 pages

Observations of high redshift type Ia supernovae (SNe) will enable us to probe the structure of galaxy halos and the composition of dark matter. The future prospects for this field are briefly discussed here. First the ability of SN observations to differentiate between dark matter made of macroscopic compact objects and dark matter made of microscopic particles is reviewed. Then a new method for probing the structure of galaxy halos and galaxy cluster halos is described. This method utilizes the correlations between foreground galaxy light and supernova brightnesses to substantially decrease possible systematic errors. The technique may be particularly useful for measuring the size of dark matter halos, a measurement to which the galaxy--galaxy lensing is not well suited, and the level of substructure in galaxy halos, a problematic prediction of the cold dark matter model. The required observations of hundreds of SNe at z ~ 1 are already being proposed for the purposes of cosmological parameter estimation., Comment: 5 pages, 3 figures, To appear in "Cosmological Physics with Gravitational Lensing", Kneib J.-P., Mellier Y., Moniez M., Tran Thanh Van J. eds, Recontres de Moriond XX

Dark matter may consist of weakly interacting elementary particles or of macroscopic compact objects. We show that the statistics of the gravitational lensing of high redshift supernovae strongly discriminate between these two classes of dark matter candidates. We develop a method of calculating the magnification distribution of supernovae, which can be interpreted in terms of the properties of the lensing objects. With simulated data we show that >~ 50 well measured type Ia supernovae (\Delta m ~ 0.2 mag) at redshifts ~1 can clearly distinguish macroscopic from microscopic dark matter if $\Omega_o \simgt 0.2$ and all dark matter is in one form or the other., Comment: 8 pages, 2 figures, AASTeX, replaced to conform to the version to be published in ApJL. It is now more clearly written and addresses some possible systematic uncertainties

The weak gravitational lensing of high redshift type Ia supernovae has the potential of probing the structure of matter on galaxy halo scales. This is complementary to the weak lensing of galaxies which probes structure of larger scales. There are already several organized searches for these supernovae being carried out for the purposes of cosmological parameter estimation. A method is proposed for extracting from future supernovae data information on lensing and the structures responsible. This method utilizes the correlations between SN luminosities and foreground galaxies. Simulations of the lensing and uncertainties will be presented. It is found that with a hundred supernovae or more at $z\simgt 1$ or larger, significant measurements of the mass, shape and extent of dark matter halos could be made if they contain a significant proportion of the matter in the universe., Comment: To be published in ``Evolution of Large-Scale Structure: From Recombination to Garching'', Banday T., et al. (ed.). 5 pages, 3 figures

The dispersion in the peak luminosities of high redshift type Ia supernovae will change with redshift due to gravitational lensing. This lensing is investigated with an emphasis on the prospects of measuring it and separating it from other possible sources of redshift dependent dispersion. Measuring the lensing induced dispersion would directly constrain the power spectrum of density fluctuations on smaller length scales than are easily probed in any other way. The skew of the magnification distribution is related to the bispectrum of density fluctuations. Using cold dark matter models it is found that the amount and quality of data needed is attainable in a few years. A parameterization of the signal as a power law of the angular size distance to the supernovae is motivated by these models. This information can be used in detecting lensing, detecting other systematic changes in supernovae and calculating the uncertainties in cosmological parameter estimates., Comment: 12 pages, AASTeX, 4 figures

It has been argued that the power spectrum of the anisotropies in the Cosmic Microwave Background (CMB) may be effectively degenerate, namely that the observable spectrum does not determine a unique set of cosmological parameters. We describe the physical origin of this degeneracy and show that at small angular scales it is broken by gravitational lensing: effectively degenerate spectra become distinguishable at l ~ 3000 because lensing causes their damping tails to fall at different rates with increasing l. This effect also helps in distinguishing nearly degenerate power spectra such as those of mixed dark matter models. Forthcoming interferometer experiments should provide the means of measuringotherwise degenerate parameters at the 5-25% level., Comment: 7 pages, LaTeX, two figures, to be published in ApJ Letters

Some aspects of gravitational lensing by large scale structure (LSS) are investigated. We show that lensing causes the damping tail of the cosmic microwave background (CMB) power spectrum to fall less rapidly with decreasing angular scale than previously expected. This is due to a transfer of power from larger to smaller angular scales which produces a fractional change in power spectrum that increases rapidly beyond $\ell \sim 2000$. We also find that lensing produces a nonzero mean magnification of structures on surfaces of constant redshift if weighted by area on the sky. This is a result of the fact that light-rays that are evenly distributed on the sky oversample overdense regions. However this mean magnification has a negligible affect on the CMB power spectrum. A new expression for the lensed power spectrum is derived and it is found that future precision observations the high-$\ell$ tail of the power spectrum will need to take into account lensing when determining cosmological parameters., Comment: to be published in November 1 Astrophysical Journal, 11 pages, two figures, AASTEX

It has been proposed that the MACHOs in our galaxy could be clumped in globular cluster--like associations or RAMBOs (robust associations of massive baryonic objects) (Moore \& Silk 1995). Here we investigate the effect such clustering has on the microlensing of stars in the Large Magellanic Cloud. We find that the lensing in a one square degree field could be dominated by just a few clusters. As a result the lensing properties vary widely depending on the position and velocity of those clusters which happen to lie between us and the LMC. Moreover we find a large variance in time scale distributions that suggests that the small number statistics could easily be dominated by events in the tails of the unclustered distribution ({\it e.g.} by long periods). We compare our results with the MACHO collaboration's data and find that a ``standard'' halo made entirely of MACHOs is not strongly disfavored if the clusters have masses of $10^6 M_{\odot}$. For less massive clusters such a halo is not as likely. For $10^4 M_{\odot}$ clusters the microlensing statistics are essentially unchanged from the unclustered case. It may be possible to detect very massive clusters from the distribution of events in time-scale and space. We provide some example time- scale distributions., Comment: uuencoded, gzipped PostScript, 14 pages