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:
Hu, University of Hawaii.
In astronomy, as in the rest of physics,
there's an in-house rivalry between theoreticians, on the one hand,
and experimenters and observers on the other. The experimenters tend
to think of theoreticians as chalk-pushers, while the theoreticians
mutter about experimenters and observers as glorified electricians.
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
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.
lenses like this, seen by the Hubble Space Telescope in 1990, helped
open the deep universe to exploration. Photo:
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.)
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
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
Dark energy. Can we finally get to
the really mysterious stuff?