Abraham (Avi) Loeb
From Wikipedia, the free encyclopedia
Abraham (Avi) Loeb is an American/Israeli theoretical physicist who works on astrophysics and cosmology. He is currently a professor of Astronomy at Harvard University. Loeb was born in Israel in 1962 and took part in the elite national Talpiot program before receiving a graduate degree in Plasma Physics at age 24 from the Hebrew University in Jerusalem. Between 1988-1993, Loeb was long-term member at the Institute for Advanced Study in Princeton, where he started to work in theoretical astrophysics. In 1993 he moved to Harvard University as an assistant professor in the department of Astronomy, where he was tenured three years later. Loeb was given a number of awards including the Guggenheim Fellowship in 2002. He also holds a visiting professorship at the Weizmann Institute of Science.
Over the past decade, Loeb worked on broad range of research areas in astrophysics and cosmology, including the first stars, the epoch of reionization, the formation and evolution of massive black holes, gravitational lensing, gamma-ray bursts, and 21cm cosmology. Some of his papers pioneered areas that have become by now the focus of established communities of astrophysicists. In particular, Loeb was among the dominant theorists to trigger the intense current research on the first stars and quasars. In a series of papers with his students and postdocs, he addressed how and when the first stars and black holes formed and what effects they had on the young universe. Recently, studies of the first sources of light had become one of the most active frontiers in observational and theoretical cosmology (for Loeb's review of the subject, see http://arxiv.org/abs/astro-ph/0603360). Loeb participated in the first Science Working Group for the James Webb Space Telescope (the successor to the Hubble Space Telescope) whose primary goal is to image the first galaxies in the infant universe. Examples for original ideas named after Loeb include the "Kamionkowski-Loeb Method", the "Loeb-Rybicki Halos", the "Sandage-Loeb Test", and the "Loeb Cafeteria".
[edit] Research Highlights
Below are some highlights from the several hundred papers that Loeb wrote over the past two decades:
• In a paper he wrote with Andy Gould in 1992, Loeb showed how planets can be detected around a star as it gravitationally lenses a background star at Galactic distances. Currently there are several observational groups searching for planets based on this method.
• In 1997 Marc Kamionkowski and Loeb suggested a method to overcome the limited precision achievable when observing the cosmic microwave background (CMB) at the edge of the observable universe from our particular viewing point. Their so-called "Kamionkowski-Loeb Method" uses the scattering of the CMB by X-ray clusters as a probe of the CMB from other viewing points.
• In 1998 Loeb suggested an original method to measure directly the unsteady expansion of the Universe by comparing redshift measurements of the Lyman-alpha forest in quasar spectra at two times separated by a decade or more. The proposed CODEX spectrograph for the future Extremely Large Telescope of ESO is being designed to measure this so-called "Sandage-Loeb (SL) Test".
• In a series of papers in 1997-2001 with Rasalba Perna, Jonathan Granot, and Scott Gaudi, Loeb used the coincidence between the angular size of a Gamma-Ray Burst (GRB) afterglow (micro-arcsecond) and the Einstein radius of a star at cosmological distances, to calculate the expected effect of gravitational micro-lensing on GRB afterglows. A signature similar to the predicted lightcurve was report by Garnavich et al. in 2000.
• In 1999 Loeb and George Rybicki obtained an analytic solution for the halo of scattered radiation around a source of Lyman-alpha photons embedded in a neutral intergalactic medium (before reionization). Future detection of these so-called "Loeb-Rybicki Halos" around early galaxies would provide a direct probe of the density and velocity structure of the primordial cosmic hydrogen in the infant Universe.
• In a NATURE paper he wrote with Eli Waxman in 2000, Loeb predicted that large-scale shocks during structure formation in the universe would accelerate relativistic electrons that would upscatter the microwave background photons to gamma-ray energies and also emit synchrotron radio waves as they spiral around the intergalactic magnetic field. Observations claiming the detection of both signatures were reported subsequently; these studies will continue with upcoming observatories (such as GLAST and LOFAR & MWA).
• The origin the intergalactic magnetic field was linked to outflows emanating from active galactic nuclei in papers that Loeb wrote in 1990 with Ruth Daly and in 2001 with Steve Furlanetto.
• In 2000, Loeb suggested to two observational groups to plot the correlation between the mass of nuclear black holes in galaxies and the velocity dispersion of stars in the central spheroid. Prior to this suggestion, all observers in the field were correlating black hole masses with the spheroid luminosity. The two groups published two papers at the same time (by Gebhardt et al. and Ferrarese & Merritt), including an acknowledgement of Loeb's suggestion. Subsequently, the proposed mass-velocity correlation became the hottest result in this field. Attempts to find better correlations failed.
• In 2002, Loeb showed that extragalactic astronomy will have a limited future in the popular cosmology with a time-independent cosmological constant; once the Universe will age by another factor of ten, our sky will only show the merger product of the Milky-Way and the Andromeda galaxy surrounded by a large vacuum. Loeb showed that, in fact, all sources above a redshift of 1.8 have already left our event horizon by the present time and we will never be able to communicate with them.
• In 2003 Volker Bromm and Loeb used a numerical hydrodynamic simulation to demonstrate that the seeds of supermassive black holes could have been a million times the mass the Sun and formed in dwarf galaxies with a virial temperature just above the cooling threshold of hydrogen. Several independent studies confirmed these results in the recent literature.
• In papers written with Benedetta Ciardi in 2000 and Volker Bromm in 2002 & 2006, Loeb suggested that Gamma-Ray Bursts (GRBs), the brightest explosions in the Universe, can be detected out to the exceedingly high redshifts. Now, the high-redshift frontier is one of the highlights of the SWIFT mission that already detected a GRB at a redshift of 6.3.
• Loeb also explained the origin of the magnetic fields in the collisionless shocks of GRBs through the Weibel instability in a paper with Mikhail Medvedev in 1999. This idea is currently being explored by several groups using plasma-physics simulations.
• In 2004, Loeb and Matias Zaldarriaga published a paper in Physical Review Letters, where they showed that observing the brightness fluctuations of 21cm radiation at redshifts 30-200 could provide the largest data set in cosmology, orders of magnitude larger than the cosmic microwave background or galaxy surveys can provide.
• In a 2004 paper published in NATURE magazine, Loeb showed with Stuart Wyithe that reionization must have ended only ~0.5-1 billion years after the big-bang. This late reionization argument stood in contrast to the popular view at that time, but was later confirmed by the 3-year data from WMAP.
• In another NATURE paper published the same year, Wyithe and Loeb showed based on simple reasoning (involving cosmic variance and light travel time) that the size of the ionized bubbles at the end of reionization is ~30 million light years. Upcoming low-frequency radio observatories (LOFAR, MWA, and PAST) are now tuned to the corresponding angular scale of tens of arcminutes for an optimal detection of 21-cm brightness fluctuations near the end of reionization.
• Rennan Barkana and Loeb showed in 2004 that numerical simulations of reionization must incorporate box sizes bigger than several hundred million light years or else they would be biased by the fact that large-scale density fluctuations modulate the abundance of rare galaxies substantially in the infant Universe. Current state-of-the-art simulations of reionization are designed to satisfy this constraint.
• In three papers written in 2005-2006, Avery Broderick and Loeb showed that the flux and polarization lightcurves as well as the image of a hot spot orbiting the supermassive black hole at the center of the Milky-Way, SgrA*, can be used to infer the spin and accretion physics of the black hole. The predicted signatures provide a very good fit to observational data (involving polarization and flux lightcurves at infrared and sub-mm wavelengths) that were obtained several months later. These papers helped motivate the design of the GRAVITY instrument for the VLT that will monitor centroid shifts in the infrared image of SgrA*.
• In 2006, Loeb and his student Ryan O'Leary explained the recently discovered hyper-velocity stars (moving at a speed of nearly a thousand km/s in the halo of the Milky Way) as ejections from the Galactic center caused by a cluster of stellar-mass black hole near SgrA*. Forthcoming searches for more hyper-velocity stars will be able to test the generic predictions of this model.
• In 2006, Loeb and Zaldarriaga showed that the upcoming generation of low-frequency observatories designed to detect 21cm emission by neutral hydrogen at redshifts 6-15 could also be used to eavesdrop on possible radio broadcasts from Galactic civilizations similar to ours at distances of tens to hundreds of light years (see movie).
Other papers by Loeb shaped the work of astronomers over an encyclopedic range of topics. In 2006 Loeb was featured in a cover story of TIME magazine on the first stars and in a Scientific American article on the dark ages of the Universe.
