H
OT ON THE HEELS of our
story in AG 137 about the
slowdown of Earth’s rotation
comes another discovery about cosmic
spin – but this time on a vastly
different scale. And it sheds light on
the early history of the universe.
Most people understand that when
we look out into space we also look
back in time because the light emitted
by celestial objects takes time to reach
us. We’re always seeing things as they
were in the past. Among other things,
this gives us the possibility of compar-
ing similar objects at different periods
in the universe’s history, by observing
nearby and distant examples. And that’s
exactly what researchers at the Max
Planck Institute for Extraterrestrial
Physics, in Germany, have now done.
Their subjects are spiral galaxies –
gigantic swirling collections of stars,
gas and dust that are often spectacularly
beautiful. Our understanding of them
took a great leap forward in the 1970s,
when it was realised they must have an
invisible component holding them
together – otherwise, their outer
regions would fly off due to their
speed of rotation. This invisible
component reveals itself only by itsgravitational pull, and is known to
astronomers as ‘dark matter’.
Since then, we have learnt a great
deal about dark matter, which we now
know accounts for about five-sixths
of the matter in the universe. We know
it must be made of vast quantities
of some as yet unknown subatomic
particle, and that today’s galaxies sit in
the middle of dense clumps of it. But
the surprise of this new research is that
in the distant past – 10 billion or so
years ago – things were different. By
analysing the rotation of galaxies in
this early phase of the universe, the
researchers have found that they didn’t
have their own individual dark matter
cocoons. It looks as if dark matter,
while not absent in the early universe,
was much more smoothly distributed
than it is today. It’s puzzling that dark
matter didn’t become clumpy until
after the first galaxies had formed –
an unexpected result.FR ED ANSW ERS
YOU R QU E S T IONSGalaxies on the turn
SPACE
buzz
Researchers uncover a new spin on ancient dark matter to reveal a ‘smoother’ universe.
FRED WATSON
is an astronomer at the
Australian Astronomical
Observatory.How do you measure light-years?
John Barbour, Cowra, NSWThis is a great question and conjures
up images of astronomers with
stopwatches timing light beams as
they fly through space. Of course,
that’s not how it works. In fact,
astronomers don’t even use light-
years in their day-to-day work. The
units of distance they do use –
called ‘parsecs’ – are measured by
the slight change in the apparent
direction of a star as the Earth
moves in its orbit around the Sun.
That change is known as parallax,
and one parsec is the distance at
which a star has a parallax of
one second of arc – a tiny angle
amounting to 1/3600 of a degree.
Just for the record, one parsec is
3.262 light years.If you have a space question for Fred,
email it to [email protected]Research shows massive star-forming disc
galaxies in the early universe (at right) were
less influenced by dark matter (shown
in pink) than in the present day (at left)
because it was less concentrated.30 Australian GeographicSCHEMATIC REPRESENTATION: COURTESY ESO / L. CALÇADA