A desert no more: astronomers have finally learned how to see the hidden galaxies in a murky epoch of ancient cosmic history

Natural History, June, 2004 by Charles Liu

If a galaxy glows, but no astronomer notices, does it give off light? This celestial version of the classic tree-falling-in-the-forest conundrum comes to mind whenever I think about the "redshift desert." As its name implies, the desert is a zone of the cosmos where, after decades of searching, astronomers had found almost no galaxies--even though it seemed that there should have been lots of them. Now, thanks to the work of a research team led by Charles C. Steidel at the California Institute of Technology in Pasadena, there's proof that the zone isn't a desert at all: the galaxies have been there all along, but no one could identify them. Before I get too far ahead of myself, though, here's a brief--okay, very brief--history of cosmic time, and how astronomers measure it.

Geological sampling puts the age of Earth at 4.56 billion years. Stellar data indicate our Sun is slightly older--about 4.7 billion years old. But what about the universe? It is more than 13 billion years old, so it was already fully mature when the Sun and Earth were born. How were astronomers able to determine its age? The answer is, we measured it with the cosmically appropriate timekeeper--redshift.

Here's how redshift works--schematically, anyway. As Edwin Hubble demonstrated more than seventy years ago, the universe has been expanding since the beginning of time. Now imagine a beam of light traveling from one spot in the universe to another. As the light beam travels through an ever expanding space, it gets "stretched" along with space. The farther it travels, the more the beam is stretched, and so the longer its wavelength becomes. An increase in wavelength is equivalent, at least for visible radiation, to a change in color, according to the familiar order of the rainbow: violet has the shortest visible wavelength, followed by indigo, blue, green, yellow, orange, and, finally, the longest visible wavelength, red. So light emitted from a very distant source--it needs to be millions of light-years away for the effect to be noticeable--gets shifted in color toward the red end of the spectrum. Hence the name: redshift.

In spite of its name, however, "redshift" also happens to electromagnetic radiation that is outside the visible part of the electromagnetic spectrum. Ultraviolet light, for instance, which has shorter wavelengths than visible light, can be redshifted into the visible window; visible light, in turn, can be red shifted into the infrared part of the spectrum, which people feel as radiant heat. By measuring the amount of redshift in the light from a distant source (usually a galaxy or a quasar), astronomers can deduce how long that light has been traveling--and thus, how old the source is, compared to the time elapsed since the big bang.

Astronomers quantify redshift with a number, commonly denoted by the letter z. For light that's not redshifted at all, z is zero. Light for which z is one began its journey to Earth when the expanding universe was just half its current size; light for which z is two left its source when the universe was a third its current size; for z equal to three, the universe was a quarter its current size; and so on. For reference, z is infinite for light from the big bang, if we could see it at all. The cosmic microwave background [see "Sharper Focus," by Charles Liu, May 2003] appeared early enough for its value of z to be about 1,100; the light of the first stars departed sometime corresponding to z values between twenty and ten.

Not surprisingly, then, one of the grandest challenges in observational cosmology today is to search for light from those ancient sources, superfaint and super-redshifted, and to decipher the early history of the universe from such cosmological fossils.

Nearer at hand, though, there's another problem. To measure redshift you need markers in the spectrum of a distant light source to calibrate the "starting point" for the redshifted light--its color when it was first emitted. Those markers are recognizable patterns of bright and dark spectral features. The most prominent of those features, which serve as benchmarks, occur only at a few specific unredshifted, or "rest," wavelengths.

Here on Earth, a number of effects-primarily atmospheric obscuration and technological limitations--have historically made those strong spectral lines all but undetectable for values of z between about 1.4 and 2.0. At those redshifts, the strong lines with rest wavelengths in the visible part of the spectrum shift so far redward that they become infrared light, and get blocked by Earth's atmosphere; meanwhile, the strong lines with ultraviolet rest wavelengths aren't shifted far enough toward the visible-light portion of the electromagnetic spectrum, so they're still not detectable with the electronic cameras that astronomers use.

Since the 1980s astronomers have pushed their instruments--and the design of their experiments--to the limits to study the void in the cosmic historical record, with some success but without major breakthroughs. In their multiyear bid to break into that void, Steidel and his collaborators employed new instrumentation on the ten-meter Keck I telescope on Mauna Kea, Hawai'i, to measure the light of hundreds of faint galaxies in the near-ultraviolet. In addition, they compared their observations with the output of detailed computer models of the spectral-line patterns they expected galaxies in the redshift desert to emit. Their painstaking work reveals what many astronomers suspected but, until now, could never prove: the redshift desert is a mirage. Not only is it empty, it's just as dense with galaxies as any other era in the history of the universe.