As attractive as the actual methods and results of science are, the language of scientific papers is itself an artform. Lay people such as myself, peripheral to the core engines of research, read such papers as much for their poetry as for their content. A zen-like austerity characterizes the writing style of the best writers.
Professor Scott Tremaine is a Canadian astrophysicist who divides his time between the Canadian Institute for Theoretical Physics at the University of Toronto and the Institute of Astronomy in Cambridge, England. He is the author of a marvelous chapter in Unsolved Problems in Astrophysics, a book that addresses the fundamental open questions of astrophysics. I was reading his chapter today not just for his content, but with sheer admiration for his form of writing. It is joyously laconic, like really good poetry. I want to share with you a short passage from his chapter so that you can see the rhythms of his language, the language of good science writing. Before I do so, I want to describe the rest of the book so you can get some context.
Editors John Bahcall (recently deceased) and Jeremiah Ostriker persuaded top-notch astronomers at the forefronts of research to clearly pose the Big Questions and to show current thinking, a snapshot in time. Present day research in astronomy is developing at an incredible rate. The world of particle physics is slowing down because of the multiple billions of dollars that each particle accelerator costs, but in astronomy, there are so many unanswered questions and -with the Hubble Space Telescope and other spaceborne telescopes - we are finally getting good instruments and good data to answer these big questions. Now we can go down one or two more levels to ask more questions about how our universe developed.
Here are some of the contents of the book.
- Large scale structures of the universe - Do the large scale structures of known luminous matter in the universe provide us evidence of what happened just after the Big Bang? Can we explain the large scale structures, like huge walls of matter, and huge voids?
- Inflation - What caused the huge inflation during the first few hundred thousand years after the big bang?
- Cosmological parameters - Is this a closed space, or an open space? The density parameter Ω. The baryon density parameter ΩB. Einstein's cosmological constant Λ. Is the universe expanding or contracting? The Hubble constant
- Solar neutrinos - Early detectors found shortages in predicted solar neutrinos. But then theory caught up with experimental findings, and the fact that neutrinos have small masses can cause 'flavor-shifting' in flight, and theory and experiment were reconciled. Still, a number of deep mysteries about these elusive particles remain.
- Dark matter and dark energy - Visible matter, light energy, and gravitational energy only constitute about 5% of the existing mass-energy combination of our universe. Dark energy constitutes 70%, and dark matter another 25%. How do dark energy and dark matter square with our existing knowledge of matter and the four known forces and what's known as the Standard Model of physics?
- The highest energy cosmic rays - Some cosmic rays have energies so absurdly high that astrophysicists wonder about their sources, as well as why they seem to be close to certain theoretical limits, such as the GZK limit.
- The morphological evolution of galaxies - Why are some galaxies elliptically shaped, with spiral arms? Why do others look like spherical blobs of light?
- Simulations - Huge numerical simulations show star and galaxy formations and answer questions regarding cosmological constants, like the Hubble constant, and the spatial locations of dark matter around galactic cores
- The Grand Telescopes - Discussions of how huge and complex new spaceborne telescopes like the WMAP (Wilkinson Microwave Anisotropy Probe) will aid astronomers in answering some of the large questions mentioned above
This book was written for specialists and for technically literate lay folk. It's not for the faint of heart, but if you're interested in astronomy you'll want this book on your bookshelf. This is a whodunit mystery, except we don't yet know whodunit -- and we won't know for several hundred years. I have the feeling that such a book is necessary for National Academy of Sciences types who have to write reports recommending for or against the science budgets for the United States federal budget. At this level, they have to have a book on Grand Challenges in Astronomy, Grand Challenges in Particle Physics, Grand Challenges in Genetic Biology, Grand Challenges in Medicine, etc., because they have to weigh the expense of huge multi-year undertakings in one field (astronomy) with those of a disparate field (genetics and biochemistry), because Big Science requires big budgets. This is such a book. It's not the minutae of science, it's a survey, an overview of astronomical research.
With that in mind, read Scott Tremaine's account of the present theory of black holes at galaxy cores in chapter 7, "The Centers of Elliptical Galaxies":
Most astronomers believe that quasars are active galactic nuclei (AGNs), and that the power source for AGNs is accretion onto a massive black hole (BH). The supporting arguments (Rees 1984, Blandford et al. 1991) include the high efficiency of gravitational energy release through disk accretion onto a BH compared to other power sources; the rapid variability of some AGNs, which implies a compact source; and the apparent superluminal1 expansion in some radio sources, implying relativistic outflow which is most naturally produced in a relativistic potential well. Moreover most other plausible power sources eventually evolve into BHs so these objects are likely to be present even if they were not the power source.
The comoving density of quasars is a strong function of redshift, declining by a factor of 102-103 from2 z = 2 to the present (Hartwick and Schade 1990). Thus many local galaxies should contain 'dead quasars' - massive central BHs that show no sign of activity because they are starved of fuel.
"These simple arguments suggest several unsolved problems: Are massive black holes present in the centers of the nearby galaxies? What is the distribution of black hole masses as a function of galaxy luminosity and type? How are the structure and dynamics of galaxies in their central regions related to the central black hole?
Black Holes and Quasars
The local energy density in quasar light is (Chokshi and Turner 1992)
u = 1.3 × 10-15 erg cm-3
If this energy is produced by burning fuel with an assumed efficiency ε ≡ ΔE/(ΔMc2), then the mean mass density of dead quasars must be at least (Soltan 1982, Chokshi and Turner 1992)
ρ• = u/(εc2) = 2.2 × 105⋅(0.1/ε)⋅MSUN⋅Mpc-33
assuming that most of the fuel is accreted onto the BH, and that the universe is homogeneous and transparent.
The mass of a dead quasar may be written
M• = ((LQτ)/(εc2)) =
7 × 108⋅MSUN⋅(LQ/(1012⋅LSUN)⋅(τ/109 y)⋅(0.1/ε)
where LQ is the quasar luminosity4 and τ is its lifetime. An upper limit to the lifetime is the evolution timescale for the quasar population as a whole, ~109 y; however, upper limits to BH masses in nearby galaxies and direct estimates of the BH masses in AGNs both suggest that the typical masses of dead quasars are M• = 107-108 MSUN (Haehnelt and Rees 1993), so that the equation for M• suggests that the lifetime of an individual quasar is only 107-108 y.
To focus the discussion, let us adopt a 'strawman' model in which a fraction f of all galaxies contain a central BH and the BH mass is proportional to the galaxy luminosity. Thus M• = ϒL where ϒ is the (black hole) mass to (galaxy) light ratio. The luminosity density of galaxies is j = 1.5 × 108⋅LSUN Mpc-3 in the blue band (Efstathiou et al. 1988; I assume a Hubble constant H0 = 80 km s-1 Mpc-1); thus
ϒ = (ρ•/(f⋅j)) = (0.0015/f)⋅(0.1/ε)⋅(MSUN/LSUN)
A second estimate of ϒ comes from dividing the typical dead quasar derived above, M• ≈ 107.5MSUN, by the typical luminosity of a bright galaxy, L ≈ 1010LSUN, to get MBH ≈ 10-2.5. If this estimate is to be consistent with the equation for MBH, then f cannot be far from unity; in other words most or all galaxies must contain massive central BHs (Haehnelt and Rees 1993). A possible concern with ubiquitous central BHs is the absence of significant non-stellar radiation from most nearby galaxies with claimed BHs (Fabian and Canizares 1988, Rees 1990, Kormendy and Richstone 1995); however, Narayan et al. (1995) have argued persuasively that the required low accretion efficiency is a natural consequence of advection-dominated accretion flows.
Detection of a significant sample of these exotic objects - or proof that they are not present - would enhance our understanding of both AGNs and the central regions of all galaxies.
The author clearly has a master's command of his subject. Indeed, he's known as a world expert on the subject of theories of galaxy formation. And although he does his own research in theoretical astrophysics he clearly keeps up with the literature of others. (That's not such an easy thing to do.) He quotes others' work and compares their results. This chapter is a fusion of the results of many researchers working independently of one another. The resultant survey of the subject of galaxy formation is just what's needed by those who want a broad overview.
The audience is scientists. Presumably, they have more than a little bit of knowledge of astronomy. They know what black holes are, and galaxies and quasars. He doesn't need to define these terms. He assumes the audience has an understanding of Einstein's famous E=mc2 relation between matter and energy. He references terms they know. He wastes no time, and his is the spare insider argot of specialists, like Trekkies at a Star Trek convention that have every line of every episode memorized.
He gets right to the point. If you can't derive a formula of his, he presumes that if you really care, you'll read the references he cites.
This zenlike nature of good science writing is hugely attractive to scientists who have sat zazen for long and patiently themselves. Years of schooling and research have allowed them to appreciate a fine practioner of the craft. They value another writer's ability to, with a minimum of words and equations, show connections and important new ideas.
Really good researchers, like Albert Einstein, Richard Feynman, and today's Ed Witten, have the ability to surprise even the best of their contemporaries. The Danish Nobel prize winner Niels Bohr had famous intellectual battles with his contemporary, Albert Einstein. Einstein co-authored a famous short paper that kicked off the field of quantum entanglement, where he brought up a paradox of quantum mechanics: how could two particles, born of the same physical process, remain interrelated even though far apart? Bohr's initial reaction was one of disbelief. Then dismissal. Days later, he came back to the paper and attacked it. Then he asked others to look at the paper and attack it too. They simply could not dismiss Einstein's argument of quantum entangled particles. Bohr would pace around his office in Copenhagen, grab his lower lip and mutter, "Einstein... Einstein... Einstein..." The paper, co-authored by Podolsky and Rosen, came to be known in the argot simply as the EPR Paradox. It was short and succinct. It had meaning to only a handful of people in the world, but to those that mattered, it was a deeply upsetting paper. Today, quantum entanglement may become the basis for a new generation of supercomputers and possibly for a new class of encryption systems.
I was surprised that Tremaine's Hubble constant (80 km/sec per megaparsec) is quite a bit larger than the value in my astronomy textbook, Pasachoff & Kutner, who use 55 km/sec/Mpc. Things have improved in the two decades between book publications.
This process is going to accelerate over the next two decades at least. The science of astronomy is becoming well funded since Hubble started sending dramatic photos back to earth. We know so much more about star formation because we have many more instruments both up in space and down on the ground. For once, astronomy is awash in data awaiting theoreticians to sift through and improve our understanding of the large unsolved problems of astrophysics.
In addition to today's current space telescopes, we have many more in the pipeline. Some are exclusively to study the sun. Some will do the grand flyby missions of asteroids, and others will wander through the Solar System to investigate the planets: their gravitational fields, their magnetic fields, their moons, the chemical compositions of the surface and the atmosphere. Finally, others will search the stars. Multispectral instruments ranging from deep ultraviolet to extreme X-ray frequencies will be able to see through the dust of interstellar gasses and the solar winds that surround extremely dense objects. We'll see stars we've never been able to see before with these new night-vision goggles of space.
And the pictures... the pictures will be magnificent.
- superluminal - faster than the speed of light
- z is related to an object's velocity, and is often called the redshift parameter, or simply, the redshift. It's defined as z = Δλ/λ = sqrt((1+(v/c))/(1-(v/c))-1 when an object is moving toward or away from you the observer, and slightly differently when an object is moving transversely to you, where v is the object's velocity and c is the speed of light. z is measured by observing spectral lines at various frequencies. When a known spectral line is redshifted, it is measured to be higher than it is here in an inertial reference frame. So, for example, a spectral line at 3968 Angstroms was measured to be redshifted to 6496 Å. The difference between measured and observed wavelengths is 6496-3968 = 2528 Å and thus z = 2528/3968 = 0.6371. (1 Å = 10-10 meters). You can show that the velocity of the galaxy 3C 123 that produced this redshift is traveling at 0.456 times the speed of light.
The formula for z comes from Pasachoff & Kutner, "University Astronomy", pp. 711-712. The formula for speed versus z is ((1+z)2 - 1) /((1+z)2 + 1) using just a little bit of algebra.
- 1 Megaparsec = 3.262 Megalight-years
- luminosity here is the absolute luminosity, not the apparent luminosity as measured at the earth. One must account for the distance related r-2 factor between apparent and absolute luminosity.
Please note that I'm awaiting Dr. Tremaine's permission to use the extended quote from his book. This may take a while. If you wish to use a quote longer than 250 words, please note that there's a form letter
that makes this easier.
Everything2 Writeups: Articles on galaxies, astronomy, and science writing
- z_evil1 & CapnTrippy, Galaxy
- rk2001 & hopthrisC, Milky Way
- purvis & Oopsz, Cosmology
- E2_Science, Astronomy: astronomy and astrophysics. A great overview and placeholder for E2's collection of articles on these subjects.
- BaronCarlos & Johnny Vector, astrophysics
Galaxies and Quasars
John Bahcall and Jeremiah Ostriker, Eds., Unsolved Problems in Astrophysics, Princeton University Press, (c)1997
- Cosmology and Large Scale Structure
- The Cosmological Parameters, by P.J.E. Peebles
- In the Beginning..., by Paul J. Steinhardt
- Understanding Data Better with Bayesian and Global Statistical Models, by William H. Press
- Large Scale Structure in the Universe, by Neta Bahcall
- The Centers of Elliptical Galaxies, by Scott Tremaine
- The Morphological Evolution of Galaxies, by Richard S. Ellis
- Quasars, by Martin J. Rees
Rita G. Lerner and George L. Trigg, Eds. Encyclopedia of Physics, VCH, (c)1991
Pasachoff and Kuttner, University Astronomy, Saunders, ©1978
David R. Lide, Editor in Chief, CRC Handbook of Chemistry and Physics, 79th Ed, ©1998-1999
- Solar Neutrinos and Unsolved Problems, by John N. Bahcall
- Particle Dark Matter, by David Spergel
- Stars in the Milky Way and Other Galaxies, by Andrew Gould
- Searching for MACHOs with Microlensing, by Charles Alcock
- Globally Asymmetric Supernova, by Peter Goldreich, et al.
- In and Around Neutron Stars, by Malvin Ruderman
- Accretion Flows around Black Holes, by Ramesh Narayan
- The Highest Energy Cosmic Rays, by James W. Cronin
- Toward Understanding Gamma-Ray Bursts, by Tavi Piran
- Wikipedia, "Galaxy"
- HubbieSite, "Gallery of Galaxy Photos"
- Fraser Cain, "Galaxies" Universe Today, April 30, 2009
- Messier, "Galaxies" A nice photo collection of the different morphologies of galaxies: Elliptical, lenticular, and "irregular"
- Sloan Digital Sky Survey, "Galaxies"