Looking Deeply Into the Distant, Dim Universe

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How far is far? And how do you know when you get there?

This is not a Dr. Seuss riddle, but a line of investigation

being pursued by several international teams of astronomers who have gotten

their hands on the deepest photograph of the universe every made in near-infrared

light – courtesy of Hubble Space Telescope’s new Wide Field Camera 3.

The picture released today by NASA reveals several thousand

far-flung galaxies. The most distant are merely dim points of light, and are

about one billionth as bright as can be seen with the naked eye. They offer a

peek at the universe as it looked about 600 million years after

the big bang. No galaxies have ever been seen before at such early times.

The faraway galaxies look like dying red embers in

Hubble’s view. Be these young galaxies actually blazed fiercely in a torrent of

ultraviolet and visible light flooding the newborn universe. So why do we need

an infrared camera to seen them? Starlight streaking across the expanding universe

is stretched to ever-longer wavelengths. From our view looking 13 billion years

into the past, these galaxies look deceptively lackluster because their light

has been so diluted by the relativistic physics of space and time.

Like rewinding through individual frames of a motion picture,

this Hubble deep field and other photos Hubble has taken since 1995 reveal the stages

of galaxy evolution from small star clusters to full-blown majestic spirals like

our Milky Way galaxy.

Before Hubble’s launch in 1990 astronomers spent several

decades of conjecture, and theoretical modeling to consider how galaxies must evolve

if the big bang really happened. 

Ground-based telescopes were not able to establish which of several

competing theories best describe how galaxies grew in the early universe.

That’s because ground-based telescopes of 20 years ago could only see normal

galaxies out to roughly 7 billion light-years – just halfway across the visible

universe.

This was revolutionized in 1995 in a million-second long Hubble

exposure to make what was then the deepest ever-visible light view of the

universe, called the Hubble Deep Field.  It was the astronomical equivalent of taking the deepest ever

core sample of Earth through layers of geologic strata in our planet’s crust.

When Hubble’s Advanced Camera for Surveys was installed in 2002, astronomers

pushed it to go 2.5 times fainter to make the Hubble Ultra Deep Field (HUDF).

This new infrared picture was taken in the same region as

the HUDF but shows objects that glow at near infrared wavelengths. It is not as

sharp as the visible light HUDF because the telescope is recording longer wavelengths of

light in the infrared.

As my science journalist colleague Joel Achenbach recently

pointed out in a recent Washington Post article, these deep sky images are

really “four-dimensional.” The three dimensions of space are encoded in the

flattened 2D distribution of galaxies across the picture. But the fourth

dimension of time is captured in the fact the picture carries us backward

through a 13 billion-light year long “time corridor.” Einstein would have immediately

grasped the historic impact of this image; it is the pictorial embodiment of

his concept of space-time.

Astronomers have now pushed Hubble to its limits of

sensitivity. To see these same galaxies with a comparable wide view from a

ground-based telescope, you would need a whopping 64 meter-diameter mirror

(with adaptive optics) to match the performance of Hubble's modest-sized 2.4-meter

mirror in space.

This is because Hubble’s infrared sky background (at HUDF-IR

wavelengths) is a factor of 1000 (8 magnitudes) fainter than achievable from

the ground. This gives Hubble a big advantage in sensitivity. (An experimental

new technique, called Multi Conjugate Adaptive Optics would shrink the

necessary ground-based telescope size needed for trying to duplicate the HUDF-IR to a

13.5-meter mirror.)

NASA’s 6.5-meter James Webb Space Telescope, scheduled for

launch in 2014, will make the final push to uncover the first 500 million years

of galaxy birth and assembly. Today’s new Hubble view clearly demonstrates that

Webb will have a lot to go hunting for.