Guest Editors' Introduction: Cosmology and Computation
MARCH-APRIL 1999 (Vol. 1, No. 2) pp. 17-20
1521-9615/99/$31.00 © 1999 IEEE

Published by the IEEE Computer Society
Guest Editors' Introduction: Cosmology and Computation
Joel E. Tohline, Louisiana State University

Greg L. Bryan, MIT
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We have constructed this theme issue on the role of computation in modern cosmological investigations principally because professionals across all disciplines of science and engineering are fascinated by questions related to the universe's origin. However, an issue on this topic is also warranted because over the past decade, rapid technological advancements have triggered revolutionary developments in this field.
Two of our theme articles discuss the computational challenges associated with major observational projects and two detail challenges facing modern simulation efforts. The authors hope that through CiSE's interdisciplinary audience, they will receive constructive feedback from researchers in other fields who are facing similar computational challenges.
With apologies to readers who subscribed to Computers in Physics last year, and gratitude to David Lewin, we are also reprinting the interview with Jeremiah Ostriker (see below) that appeared in CIP's 1998 May/June issue because it provides an excellent introduction to the modern field of computational cosmology.
As you read through the four theme articles, remember that astronomers can directly measure spatial structure at extraordinarily high resolution in only two dimensions—the two angular coordinates that are equivalent to projecting the earth's (or, in the field of cosmology, our Milky Way galaxy's) longitude and latitude system onto the sky. You will encounter, for example, references to square-arcsecond resolution—recall that there are approximately 41,000 square degrees on the sky and each square degree contains 1.3 × 10 7 square arcseconds. However, direct measurements of the distance to various structures and, hence, knowledge about the third spatial dimension are much less straightforwardly obtained. Generally speaking, in discussions of cosmology, the distance to an object is directly related to the velocity at which that object moves away from us as it rides along with the overall Hubble expansion of the universe. Hence, spectroscopic measurements that can provide accurate Doppler (recessional) velocities give positional information in the third dimension, where the recessional velocity relative to the speed of light v/c is related to the Doppler redshift z = Δλ/λ via the general expression, 1 + z = [(1 + v/c)/(1 − v/c)] 1/2 and an object's distance d from our own galaxy can be obtained via the linear relationship, d = v/H o, where the Hubble constant $H_{\rm o} \approx 1.6 \times 10^{-18}$ s −1 is a measure of the universe's present expansion rate.
Remember also that because the speed of light is finite, as we examine structures at greater and greater distances from our galaxy, we view them not as they are at the present time but as they were at a well-defined earlier time t = d/c = v/( cH o). So for example, the cosmic microwave background (CMB) radiation that exhibits a redshift $z \approx 1,000$ and therefore is composed of photons that have traveled unimpeded over a distance of 6 × 10 9 parsecs (2 × 10 10 light-years) gives us direct information about the structure of the universe as it existed approximately 2 × 10 10 years ago!
We hope that these introductory remarks, along with David Lewin's interview with Jeremiah Ostriker, will provide you with an appropriate perspective on this issue's theme and that you will enjoy reading them as much as we have enjoyed bringing them together.
Joel E. Tohline is a professor in the Department of Physics and Astronomy at Louisiana State University. He received his BS in physics from the Centenary College of Louisiana, and his PhD in astronomy from the University of California, Santa Cruz. Contact him at tohline@physics.lsu.edu, www.phys.lsu.edu/faculty/tohline.