Our view of the overall shape and content of the universe has been radically altered in recent years by the results of moderate-redshift supernova cosmology research. The technique takes advantage of the fact that type Ia supernovae have an intrinsic luminosity predictable from their light curves. Therefore, measurements of the light curve and a spectroscopic redshift give the luminosity distance to a given redshift, a mapping which depends on Omega_mass and Omega_lambda. Unfortunately, at the moderate redshifts accessible to the current generation of telescopes, the observations constrain a combination of Omega_mass and Omega_lambda, not their individual values. Observations of SN Ia at z=2.5 can break the cosmological parameter degeneracy, however, and LINC on LBT has the sensitivity to detect and measure these objects. A key program consuming ~25 nights and taking advantage of the improved improved sensitivity and wide field of view, would be able to constrain Omega_mass uniquely to 5% accuracy.

The best way to understand galaxy formation is to use the enormous light-gathering capability of large telescopes to look back in time at the era of galaxy assembly. The current hierarchical paradigm predicts that the earliest galaxy fragments are small and faint. Unfortunately, the limited sensitivity of current instruments force us to bias our investigations toward atypically luminous and massive galaxies, and toward galaxies that are undergoing a star-bursting episode. A deep multi-color, near-IR survey with LINC could sample a volume of 10^5 cubic Mpc within ~20 nights of observing time, detecting galaxy fragments in that volume with only one one-thousandth of the Milky-Way's mass. Until the NGST mission, no other facility will reach this combination of sensitivity, areal coverage, and number of detected objects.

Current technology limits our ability to resolve individual stars in galaxies further than ~5 Mpc, forcing us to assess their stellar content and formation history using integrated spectral energy distributions. In particular, there is not a single giant elliptical galaxy close enough to be fully resolved into individual stars, a situation which has generated a decade-long debate over their star formation history. LINC will be able to resolve stellar populations in galaxies out to 20 Mpc, and can study the age and metallicity of the stars through a combination of narrow and broad-band filters. With LBT, about 100 luminous galaxies will be accessible to this type of study, compared to the current four. At larger distances, the sensitivity and resolution of the LBT can be used to study the surface brightness fluctuations of marginally resolved stellar populations, which provide a powerful distance indicator (Tonry and Schneider, 1988). Because the maximum measurable distance scales as the square-root of the size of the PSF, LINC will allow distance determinations within a volume 1000 times larger than that currently accessible.

Stellar winds, through their interaction with circumstellar disks and the surrounding media, play a central role in catalyzing and regulating the star foramtion process. Unfortunately, the small angular scales and the presence of obscuring dust have severely limited our ability to probe the regions where these important interactions take place. LINC will provide an unprecedented opportunity to study these fundamental processes. The angular scales sampled by an imaging beam combiner correspond to less than 1 AU at the nearest star forming regions. The ability to form true images will give LINC an enormous advantage over other interferometers in disentangling these complex regions. A monitoring program of circumstellar emission, coupled with high resolution spectroscopy, can give the full, three-dimensional motions of the gas in the near stellar environment. Note that a shock front travelling at 25 km/s in Taurus will move noticeably during a single, week-long observing run.

The majority of main sequence stars in the Galaxy are found in binary and multiple systems, and the distribution of field binary star separations peaks at approximately 50 AU, corresponding to 0.3 arcsec at the distance of the nearest star forming regions. Operating in the near-infrared, LINC will be able to penetrate the obscuring dust in molecular clouds and resolve young binaries with approximately one fifteenth this separation. Even the modest spectroscopic resolution provided by grisms permits spectral classification of the individual components, placing them on the Hertzprung-Russel diagram and allowing an assessment of their relative evolutionary state. Perhaps more importantly, the exquisite precision possible with relative astrometry over a wide field on LBT will enable the measurement and extraction of orbits for a large sample of binaries. The derived dynamical masses can then directly calibrate the mass luminosity relation and the pre-main sequence HR diagram.

As mentioned before, circumstellar disks have an important influence on molecular cloud collapse, by either generating or mediating stellar winds. Disks are also of fundamental importance in planet formation, since they act as both a source of raw material and as a shielding envelope to allow the accumulation of the gas and refractory elements which eventually become a planet. Through accretion and resonant scattering, the planet eventually clears a gap in the disk, inexorably altering its structure and dynamics. LINC can search for evidence of these processes in scattered light -- this is another instance where true imagery is an enormous asset. Circumstellar disks are expected to flatten with increasing age during the transition from protostellar to protoplanetary phase, a process that we hope to follow with a large survey.

For solar system targets, the wide field of view, high spatial resolution, and increased sensitivity of the LINC interferometer will allow ground-based monitoring and investigations that rival spacecraft observations. In fact, this sensitivity may prove to be an occasional problem, since atmospheric features can evolve on timescales short compared to that needed for Earth's rotation to sample all parallactic angles. Imaging the atmosphere and surface of Saturn's largest satellite, Titan, will be a high priority program. As the only other body in the solar system with surface oceans and rainfall, Titan is the subject of much ongoing study and interest. Some planetary scientists believe that Titan's atmosphere resembles that of the Earth prior to the appearance of life and the oxygen it produced. Penetrating the thick cloud deck at near-infrared wavelengths will produce surface imagery of unparalleled clarity and resolution. The 20 mas K band resolution that LINC can provide corresponds to 130 km at Saturn, and Titan's disk will be approximately 40 pixels across. For comparison, HST's resolution at K is ten times coarser, producing an ``image'' of Titan four pixels in diameter. Such observations will also be very topical; the arrival of the Cassini spacecraft with its Titan atmospheric probe, Huygens, will take place at approximately the time of LINC's commissioning.

Current search strategies for extrasolar planets concentrate either on the Doppler shifts in stellar spectral features arising from the reflex motion due to unseen planets, or on photometric changes arising from planetary transits. Both these techniques are biased toward detecting more massive (Jupiter-like) objects close to the host star: the majority of the 50 or so ``hot Jupiters'' discovered to date have semi-major axes below 0.5 AU, forcing a re-evaluation of planetary formation theories. We plan to search for planets using LINC and a very ``old-fashioned'' technique: measuring the astrometric wobble imposed on the parent star by the gravitational tug of the planet. The astrometric precision offered by LINC opens the possibility of pushing these searches into the regime of ``real'' Jupiters, namely 1 Mjupiter objects orbiting at 5 AU from a solar-type star. It should be possible to achieve 0.1 mas astrometric precision with LINC, sufficient to detect the reflex motion of Jupiter on the Sun out to a distance of 100 pc. LINC's wide field of view increases the likelihood of multiple reference stars, improving the precision of relative astrometry. Having several references also removes the ambiguity associated with dual-feed (two object) measurements planned for other interferometers - the star hosting the planet will be the one showing periodic reflex motion with respect to {\it multiple} neighbouring stars. Formation theories also predict that there may be large numbers of isolated planets either created alone or expelled from binary and multiple systems. Searches for free-floating planets are ideally suited to the greater sensitivity and wide field of view of the LINC beam combiner.