Hidden Cameras Here Article

Geological sampling puts the age of sphere at billion years. Stellar data suggest our Sun is lightly older--about billion years old. But what about the world? It is more than 13 billion years old, so it was previously absolutely mature when the Sun and Earth were born. How were astronomers gifted 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 initiation of time. Now expect a beam of light traveling from one spot in the nature 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 palpable radiation, to a change in color, according to the chummy order of the rainbow: violet has the shortest visible wavelength, followed by indigo, blue, green, yellow, orange, and, someday, the longest discernible wavelength, red. So light emitted from a very formal inception--it needs to be millions of loose-years away for the effect to be striking--gets shifted in color toward the red end of the spectrum. Hence the epithet: redshift.

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

Astronomers quantify redshift with a number, commonly denoted by the missive z. For light that's not redshifted at all, z is nil. Light for which z is one began its journey to sphere when the expanding nature was just half its current stature; light for which z is two liberal its source when the nature was a third its current size; for z equal to three, the nature was a quarter its current size; and so on. For reference, z is eternal 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 flimsy of the first stars departed sometime comparable to z values between twenty and ten.

Not surprisingly, then, one of the grandest challenges in observational cosmology today is to search for effortless from those elderly sources, superfaint and outstanding-redshifted, and to translate the early history of the nature from such cosmological fossils.

Nearer at extremity, though, there's another dilemma. To measure redshift you need markers in the spectrum of a withdrawn easy source to calibrate the "starting point" for the redshifted spongy--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 satisfy as benchmarks, appear only at a few special unredshifted, or "rest," wavelengths.

Here on sphere, a number of effects-chiefly atmospheric obscuration and technological limitations--keep historically made those manly spectral lines all but undetectable for values of z between about and . At those redshifts, the substantial lines with rest wavelengths in the visible part of the spectrum shift so far redward that they become infrared soft, and get blocked by Earth's atmosphere; meanwhile, the resilient lines with ultraviolet rest wavelengths aren't shifted far enough toward the palpable-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 worldwide historical record, with some success but without extensive breakthroughs. In their multiyear bid to break into that void, Steidel and his collaborators busy 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 reckoning, they compared their observations with the turnout of detailed computer models of the spectral-line patterns they expected galaxies in the redshift desert to exhale. Their painstaking performance reveals what many astronomers suspected but, until now, could never prove: the redshift desert is a illusion. Not only is it empty, it's just as dense with galaxies as any other era in the history of the world.

Alas, as cosmologically pure and exquisite as it may be to keep time in units of redshift lone, we astronomers must still (grudgingly!) convert redshift into Earth years, if catholic story is to be relevant to our Earthbound existence. It's a knotty and inexact conversion, but it's accurate enough to say that z values between and reciprocate to cosmic history between about 9 billion and 11 billion years ago. In short, that epoch is, at last, now officially open for study. And that's good news indeed: the Milky Way galaxy formed during the generation, and so now we sustain that, when our galactic home was born, it wasn't an infant alone in a desert, but a baby surrounded by a robust society of galaxies.

It's not that these galaxies were once lost, but now are found. Rather we astronomers were once blind to them--but now, thanks to productive scientists and improved techniques, we see.

CHARLES LIU is a instructor of astrophysics at the City University of New York and an associate at the American Museum of raw version.