Tuesdays 10:30 - 11:30 | Fridays 11:30 - 12:30
Showing votes from 2017-11-17 12:30 to 2017-11-21 11:30 | Next meeting is Friday Aug 29th, 11:30 am.
A critical challenge to the cold dark matter (CDM) paradigm is that there are fewer satellites observed around the Milky Way than found in simulations of dark matter substructure. We show that there is a match between the observed satellite counts corrected by the detection efficiency of the Sloan Digital Sky Survey (for luminosities $L \gtrsim$ 340 L$_\odot$) and the number of luminous satellites predicted by CDM, assuming an empirical relation between stellar mass and halo mass. The "issing satellites problem", cast in terms of number counts, is thus solved, and imply that luminous satellites inhabit subhalos as small as 10$^7-$10$^8$ M$_\odot$. The total number of Milky Way satellites depends sensitively on the spatial distribution of satellites. We also show that warm dark matter (WDM) models with a thermal relic mass smaller than 4 keV are robustly ruled out, and that limits of $m_\text{WDM} \gtrsim 8$ keV from the Milky Way are probable in the near future. Similarly stringent constraints can be placed on any dark matter model that leads to a suppression of the matter power spectrum on $\sim$10$^7$ M$_\odot$ scales. Measurements of completely dark halos below $10^8$ M$_\odot$, achievable with substructure lensing, are the next frontier for tests of CDM.
Recent observations of the gravitational wave by LIGO motivates investigations for the existence of Primordial Black Holes (PBHs) as a candidate for the dark matter. We propose quasar gravitational microlensing observations in Infrared to the sub-millimeter wavelengths by sub-lunar PBHs as lenses. The advantage of observations in the longer wavelengths is that the Schwarzschild radius of the lens is of the order of the wavelength (i.e. $R_{\rm sch}\simeq \lambda$), so the wave optics features of gravitational lensing can be seen on the cosmological scales. In the wave optics regime, the magnification has a periodic profile rather than monotonic one in the geometric case. This observation can break the degeneracy between the lens parameters and determine uniquely the lens mass as well as its distance from the observer. We estimate the wave optics optical-depth and number of detectable events for sub-lunar lenses and propose a long-term survey of quasars with cadence $\sim$ hour to probe possible fraction of dark matter in form of sub-lunar PBHs.
The Galactic center offers us a unique opportunity to test General Relativity (GR) with the orbits of stars around a supermassive black hole. Observations of these stars have been one of the great successes of adaptive optics on 8-10 m telescopes, driving the need for the highest angular resolution and astrometric precision. New tests of gravitational physics in the strong gravity regime with stellar orbits will be made possible through the leap in angular resolution and sensitivity from the next generation of extremely large ground-based telescopes. We present new simulations of specific science cases such as the detection of the GR precession of stars, the measurement of extended dark mass, and the distance to the Galactic center. We use realistic models of the adaptive optics system for TMT and the IRIS instrument to simulate these science cases. In additions, the simulations include observational issues such as the impact of source confusion on astrometry and radial velocities in the dense environment of the Galactic center. We qualitatively show how improvements in sensitivity, astrometric and spectroscopic precision, and increasing the number of stars affect the science with orbits at the Galactic center. We developed a tool to determine the constraints on physical models using a joint fit of over 100 stars that are expected to be observable with TMT. These science cases require very high astrometric precision and stability, thus they provide some of the most stringent constraints on the planned instruments and adaptive optics systems.
Hawking's area theorem is a fundamental result in black hole theory that is universally associated with the null energy condition. That this condition can be weakened is illustrated by the formulation of a strengthened version of the theorem based on an energy condition that allows for violations of the null energy condition. This result tightens the conventional wisdom that quantum field theoretic violations of the null energy condition account for why the conclusion of the area theorem can be bypassed in the semi-classical context. Shown here is that violations of the null energy condition, though necessary, are not sufficient to violate the conclusion of the area theorem. As an added benefit, the specific form of the energy condition used here suggests that the area non-decrease behavior described by the area theorem is a quasi-local effect that depends, in large measure, on the energetic character of the relevant fields in the vicinity of the event horizon.