Will Cosmology Emerge from the Dark Ages?
To cosmologists, the “dark ages” were not after the fall of Rome, but the time between the release of the microwave background radiation, and the light from the first stars. In a feature article for the November Scientific American, Abraham Loeb discussed how astronomers hope to shed light on this epoch with new telescopes measuring the spin-flip transitions of cold atomic hydrogen, visible as photons with a wavelength of 21 cm. A Harvard astronomer and visiting professor at Weizmann Institute in Rehovot, Israel, Loeb began with a Biblical context:
Today’s news is often forgotten a few days later. But when one opens ancient texts that have appealed to a broad audience over a longer period of time, such as the Bible, what does one often find in the opening chapter? A discussion of how the constituents of the universe–light, stars, life–were created. Although humans are often caught up with mundane problems, they are curious about the big picture. As citizens of the universe we cannot help but wonder how the first sources of light formed, how life came into existence and whether we are alone as intelligent beings in this vast space. Astronomers in the 21st century are uniquely positioned to answer these big questions.
If this begins like a sermon, it only preaches a message that science provides the most satisfying answers to the big questions. Loeb spends most of the time discussing what happened after the big bang, where the microwave background came from, why the “dark ages” are interesting, and how space telescopes tuned to the 21-cm wavelength promise to fill large gaps in astronomers’ understanding. His bio at the end of the article states that he became interested in cosmology because of “ancient philosophical questions.” The Bible, apparently a poor second to science in his view, at least helped people focus on the big picture.
At the end of the article, Loeb listed some of the big mysteries of the dark ages the he hopes the new technologies will solve.
This combined observational and theoretical effort should shed light on various mysteries that now plague the theory of galaxy formation. One set of questions concerns the massive black holes in the centers of galaxies. Over the past decade astronomers have realized that almost every galaxy in the present-day universe, including our own Milky Way, hosts a massive black hole. These holes are believed to be fed with gas in episodic events, triggered by mergers of galaxies. During these growth spurts, the accreting gas shines much more brightly than the entire rest of the galaxy, producing a quasar. The Sloan Digital Sky Survey has revealed that quasars with black holes of more than a billion solar masses already existed at a cosmic age of one billion years. How did such massive black holes come to exist so early? Why did they stop growing?
Another set concerns the size distribution of galaxies. Theorists believe that the ultraviolet radiation produced by dwarf galaxies during the epoch of reionization heated the cosmic gas and suppressed the formation of new low-mass galaxies. How did this suppression unfold over time? Which of the dwarf galaxies we find today were already in existence at the beginning? These are only a few of the many questions whose answers lie in the Dark Ages.
Presumably, this can serve as a microcosm of the state of cosmology. Incidentally, the big bang theory was saved recently. That’s what Science Daily reported a few days ago. A failed prediction about helium-3 in the early universe has now been brought into conformity with theory by a new explanation: stars destroy it before it can be flung into space.
Proud man exalts himself above the word of the Lord and places his own perception, aided albeit by his instruments, as the discoverer of Truth about ultimate things. One only needs to see the history of human speculation about cosmology, with its many upsets, to get a reality check on the likelihood today’s theories have arrived. And one only needs to consider the magnitude of the remaining questions to doubt the propriety of confidence.
Loeb’s tidy picture would require that the universe went from an almost completely homogeneous soup of particles to highly structured arrangement of stars and galaxies and clusters of galaxies in a relatively brief period of time. Not only that, these stars would have had to age, collapse, and form supermassive black holes at the centers of quasars within 7% of the presumed age of the universe, because that is what we now observe at the farthest extent of our current observational capabilities (09/24/2006). Hoping that the next generation of space telescopes will fill in the blanks is only hope. If history is any guide, any answers lurking in the Dark Ages will be outshone by new questions.
Loeb does well to have us ponder the big picture, something often lost in the nightly news. And it is certainly honorable to extend our vision as far as we can, and to seek for understanding. That is a far cry, however, from claiming we already have it.