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For additional details, also see my CV, Recent Activities, and Review Articles. |
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For additional details, also see my CV, Recent Activities, and Review Articles. |
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The second animated frame above shows an example of gravitational lensing, with the effect varied based on the mass of the foreground deflector. The unlensed background sources (here at radio wavelengths as seen at 1.4 GHz using the Very Large Array) are shown in the first image. A real world example of gravitational lensing is the third image where blue arcs surrounding central galaxies are images of a single background source gravitationally lensed to form multiple images. The spatial location and sizes of these arcs can be used to study the foreground mass distribution, which seems to suggest the presence of a large component of matter in an invisible form (thus, "dark-matter"). Similar arcs are commonly observed towards clusters of galaxies, which are some of the largest, therefore most massive, structures in the universe. I am interested in finding similar effects at other wavelengths such as at radio and X-ray bands, which have not been properly studied so far due to instrumental limitations. I have made detailed theoretical predictions on the rate at which lensing events could be found in certain surveys as a function of area, flux (magnitude) etc. These predictions can eventually be tested and used to derive basic properties of the universe, such as the expansion rate and the mass density.
Here is another example of gravitational lensing, illustrated with a moving mass across the famous Hubble Deep Field image from the Space Telescope.
It is now well known that the cosmic microwave background (CMB) provides us with a probe of the earliest times in the universe, more than any other probe we have today, involving galaxies and quasars etc. This background exists in the form of a radio background at microwave wavelengths and has the simple black-body, or Planckian, form for its frequency spectrum. This radiation is generally understood as the leftover radiation from the big-bang era. At the stage of an infant universe growing to a few hundred thousand-years old, this radiation field is tightly coupled to the free electron population. As the universe ages, expands, and cools down, electrons begin to combine with protons to form neutral Hydrogen. During this era, electrons let go of their tight grab on the radiation field and these photons then freely stream in the universe and can be detected today. The universe is filled with these primordial photons and forms a radio noise. In fact, if you extend your arm and grabbed some air with your palm, you managed to collect at least a few hundred of primordial photons from the infant universe. By studying details statistical properties of these photons, we are essentially studying the era when this radiation is let free by electrons. This is similar to the case of a cloudy-day, where when we look at the sky, we see the lower edge of the clouds where the sun light last scatters with particles that form the cloud.
Over the last six years, I have studied various aspects of the cosmic microwave background and its uses to understand the earliest times in the universe. The tiny inhomogeneities in the density distribution, augmented by the presence of dark matter, lead to distinct statistical changes on the photon distribution during the time of last scattering. These properties include the now established acoustic peak structure arising from the sound oscillations in the photon-baryon plasma. At smaller angular scales, which are not currently probed by current experiments due to experimental limitations among other reasons, the CMB is dominated by effects due to large-scale structure, such as lensing and scattering. The scattering of CMB photons via electrons, during late time due to reionization, lead to generation of new anisotropies, which have distinct differences from primary anisotropies, in some cases. For example, the Sunyaev-Zel'dovich effect can be extract from CMB through its unique frequency dependence.
During the last couple of years, the field of cosmic microwave background has seen a substantial increase in research effort due to satellites such as NASA's Wilkinson Microwave Anisotropy Probe. here. While WMAP results are interesting, there are more scientific questions to be addressed with CMB. The European Space Agnecy plans to launch the Planck mission in 2008 t further study CMB while NASA plans for another mission to study detail properties of polarization related to CMB. Currently, I am member of the main working group related to a concept mission proposal related to this satellite and I provide theoretical ideas as well as a detail optimization of the survey procedures so as to maximize the science capabilities from its data.
The Sunyaev-Zel'dovich effect is a spectral distortion of the Cosmic Microwave Background due to inverse-Compton scattering of its photons via relativistic electrons. This effect is expected to occur towards galaxy clusters which contain adequate populations of relativistic electrons, which are also responsible for the observed thermal-Bremsstrahlung X-ray emission. This effect was first predicted by Rashid Sunyaev (now at Max Planck Institute for Astrophysics) and Ya. B. Zel'dovich in 1972 as a way to prove a cosmic origin, instead of say a galactic origin, for the observed microwave background. Between 1977 & 1978, several authors suggested the use of SZ effect as a way to measure distances to galaxy clusters, independent of the so-called distance ladder in astronomy. With distance measurements and observed redshifts for clusters, this effect now allows the determination of the Hubble constant, or the age of the universe, and other cosmological parameters that can be used to determine the geometry of the universe.
I prepared the illustration bellow to explain science aspects of the SZ effect during a general science presentation:
When planets move across the sky, as viewed from Earth, some times they pass in front of random stars in the background sky and dimming them for a few seconds to minutes. When one monitors light coming from that background star, the extent to which light remains diminised during these passings tells us some information on the atmosphere of the foreground planet that blocked the background stellar light. I have obtained and used data from such occultations to study the temperature and pressure structure of the Saturn's north-polar region. The above figure illustrates the path of the star during and after the occultation. It went behind Saturn near north-polar region and reappeared near equator. The Saturn's rings subsequently occulted the star allowing us to study its rings, such as their thickness, as well.
In addition, tiny density inhomogeneities in the atmosphere produce small phase fluctuations which are amplified at Earth due to large distance between Earth and Saturn. During this occultation, we also observed effects related to diffraction for the first time, in the form of fringes, of scintillation spikes known elsewhere in the asttrophysics and physics community as caustics. Our observations were the first to observe them in this context though it had been predicted and expected in other astronomical scenarios, such as the scintillation modulation of pulsar intensity (due to inhomogeneities in the electron distribution of our galaxy) or during microlensing events that involve caustic-crossings.
Recently with a Caltech astronomy graduate student, Alison Farmer, I have calculated details statistics related to an occultation survey that can be used to establish the presence of small (few kilometer size) objects in the outer Solar system. These are expected in the Kuiper Belt, beyond the distance of Neptune, and in the Oort cloud (at far edges of the Solar system). Given the distance and the small size, these objects are rarely found even with largest telecopes given their extremely small brightness. With occultation surveys, however, their presence can be established as they dim background stars randomly for less than a second. Experiments are now underway where many thousands to tens of thousands stars are monitored rapidly (at fractions of a second) and light curves are made to establish if any dimming events exist. In Fall of 2003, I organized a workshop related to this topic with Charles Alcock (formerly at UPenn, now director of Harvard Smithsonian Center for Astrophysics) and Matt Lehner. Talks and summaries from this workshop are available here