1. Message from ancient universe

2. Old photos of young galaxies

3. Hark! Dark energy!

A hyper-distant galaxy (arrow) made this light several hundred million years after the Big Bang. We are looking through a massive cluster of galaxies called Abell 370 that focuses the light of the incredibly distant galaxy. This image combines infrared light from the 8.3 meter Subaru telescope with visible light from the 10-meter Keck I telescope, both on Mauna Kea, Hawaii. Photo: Courtesy Esther Hu, University of Hawaii.

But theory can be handy to an astronomer, as the early galaxies now creeping into view illustrate. Early in the last century, Albert Einstein proposed that gravity could bend light. Now, "gravitational lenses," AKA "cosmic lenses," built of clusters of galaxies, are focusing light from objects that would otherwise be too faint to see. Although the idea of using a gravitational lens in astronomy was first aired in 1936 or 1937, they were not used for imaging until 1988.

Gravitational lenses quickly became critical to hunters of early galaxies. Richard Ellis, a professor of astronomy and director of Caltech's Optical Observatories, used one (mass: about 100 trillion suns) to see an early galaxy behind the galaxy cluster Abell 2218. The group of galaxies and dark matter, he understates, "is quite massive and acts as a very powerful cosmic lens." It turns out that while cosmic lenses are somewhat bulkier than glass ones, they both bend light according to similar principles.

Gravitational lenses like this, seen by the Hubble Space Telescope in 1990, helped open the deep universe to exploration. Photo: Space Telescope Science Institute

While getting light from early galaxies is a technically sweet operation, what do those photos tell us about the universe during the first billion years after the Big Bang? (Background blip: During that "dark age," any starlight that was around would have been absorbed by clouds of hydrogen gas. When stars and galaxies started condensing from the hydrogen, their nuclear furnaces created radiation that ionized the hydrogen and made it transparent. At that point, electromagnetic radiation started moving across the universe, and the dark age ended.)

Ancient data uncovered
So what does radiation from the early universe tell us about that ancient epoch?

Stars had already formed as early as Big Bang + 470 million years (exactly when they first began burning is not yet known).

The early galaxies now seen contained first-generation stars. "Far more of the energy is in the ultraviolet compared to nearby stellar systems today," says Ellis. Handily, that meets expectations, he adds. "Theorists have long speculated that the first generation of stars would be pristine, not polluted by carbon, magnesium, silicon." (Background blip: Because the Big Bang apparently made only hydrogen and helium, the first stars could not contain other elements, which formed later in stars.)

The early universe was apparently chock-a-block with galaxies. Cosmic lenses "magnify only a very small area of sky," Ellis says, "so the fact that we are finding these objects in these tiny areas means either we are extraordinarily lucky, or they have a very high density in the sky." After examining 10 cosmic lenses, he says, each magnified an early galaxy. "We can use this to estimate that there are probably many thousands in one diameter of the full moon."

Early galaxies were also midgets. Esther Hu, an astronomer at the University of Hawaii who specializes in early galaxies, says they had only a few percent of the mass of big-time, modern galaxies like the Milky Way.

Indeed, the expectation that past galaxies would resemble modern models temporarily derailed the search for the first galaxies, Hu adds, noting that astronomers first started searching for hydrogen emissions stimulated by star formation. "But they weren't successful. What was underpinning the search was the idea that the galaxies you were looking for were just like galaxies now." But it turned out that the nearby, recent universe was different from the early, distant universe, and the early, distant galaxies were also rather faint, making them all the more elusive.

Left: The Abell 370 galaxy shows intense emissions from hydrogen atoms that mark star formation. Right: The galaxy is invisible in visible light. Photo: Courtesy Esther Hu, University of Hawaii.

Looking for the dim bulb of midget galaxies requires longer exposures on bigger and better telescopes, which is how the monsters in Hawaii, and the Hubble Space Telescope, have come to play such key roles.

And how did small galaxies become big ones? Most likely, Ellis says, by merging with their neighbors and slowly assembling into the wondrous spiral and elliptical galaxies we see today.

Dark energy. Can we finally get to the really mysterious stuff?

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