Sky and Telescope - April 2018 - 4

SPECTRUM

by Peter Tyson

Clear Skies

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IMAGINE IF SPACE WERE OPAQUE. There'd be no such thing as

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N ASA , ESA & G. BACON (STSCI)

optical astronomy, and S&T likely wouldn't exist. (Heaven forfend!)
Fortunately, outer space is crystal clear. It's so transparent that
we can see right across the universe. This is an astonishing fact. We
can't look with our eyes alone into our own oceans because of their
opacity, or into our own forests because of the trees. Yet with the help of our
ﬁnest telescopes, we can perceive objects billions of light-years away.
This equates, of course, to billions of years back in time. Somehow it seems
impossible from our perspective that light, the fastest thing in the cosmos,
takes any time at all to travel somewhere. But because of the unimaginable
distances involved when contemplating the early universe, photons can take a
sizeable fraction of its age to reach us. This, too, is astonishing. Every time we
use a telescope, we're time traveling, and sometimes very, very far into the past.
The ﬁrmament's translucency - in both space and, in a sense, time - works
both ways, naturally. A hypothetical observer on a distant planet could look our
direction and see the Milky Way as it appeared long
ago. With advanced-enough technology, could that
observer even focus in on Earth? Why not? Even as
we're taking in, say, NGC 1275, a galaxy about 225
million light-years away, an observer there could be
admiring the supercontinent Pangaea, which existed
on our planet 225 million years ago.
I digress. Our observers, namely professional
astronomers, are using that remarkable quality of
space, that splendid emptiness, to look back more
Hubble image of GN-z11,
than 13 billion years in the past to a time when
the most distant galaxy
galaxies ﬁrst formed. In images like the one at left,
known to date
those primordial galaxies might be mere splotches of
a few pixels, but there they are.
If that doesn't play enough with your mind, consider this: Those same
astronomers are trying to peer far enough in space and back in time to discern
when galaxies run out. As James Geach writes in his article on page 14, "what
we'd really like to know is when we stop seeing galaxies." Astronomers know
there was a time before galaxies existed. Can they, in essence, wind the clock
back far enough that the ﬁrst galaxies begin to vanish from the view?
Astronomy offers countless opportunities for such head-spinning contemplation. And yet it's not fantasy. This is real. That galaxy-free period is out there,
and we might soon see right up to its very edge.

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Sky and Telescope - April 2018

Table of Contents for the Digital Edition of Sky and Telescope - April 2018