Sky and Telescope - August 2017 - 6

FROM OUR READERS

Dark Matter Matters

David Britz
Rumson, New Jersey

Monica Young replies: I agree that
gravitational lensing should be a
great way to detect dark matter clumps. So
I checked with Frank van den Bosch, a dark
matter substructure expert at Yale University.
Here's his response (in part):

In Defense of Science
I feel compelled to express my sincere
gratitude for Peter Tyson's timely and
inspirational essay "This Is, and No Mistake" (S&T: May 2017, p. 4). As a longtime reader, I understand and appreciate
that Sky & Telescope is an educational
and scientiﬁc publication that is and
always has been apolitical. Its occasional
forays into the world of politics are never
to promote any one political agenda.
The only goal is to inform the readership as to policies that affect the future
of astronomy and all its allied scientiﬁc
disciplines. Your detailed explanation
of the tenets and processes behind the
scientiﬁc method - and how adhering
to these disciplines has allowed us to
discover not only a useful body of knowledge but also certain "incontrovertible
truths" - touched just the right nerve at
just the right time.
Michael Swanson
Houston, Texas

A U G U S T 2 0 1 7 * SK Y & TELESCOPE

"Detecting substructure via gravitational
lensing occurring in the Milky Way will be
extremely hard. Typically you want your
'lens' (in this case the substructure) to be
roughly halfway between the observer (us)
and the distant 'source' galaxy that is being lensed. Since most galaxies are much
farther away from us than any substructure
in the Milky Way, we wouldn't expect the
latter to cause any measurable lensing of
background galaxies."

That said, we could try to detect substructures around other galaxies. So far
observers have only detected large structures, and as far as I (and van den Bosch)
know, none of these are totally dark matter.
However, a small number of dwarf galaxies
have been found by their image distortions.
But if the technique can be reﬁned, then it's
only a matter of time before we start probing
smaller masses and ﬁnding substructures
that have no stars and only dark matter.

Given your readership, I was kind of
expecting an editorial like yours. Thanks
for that. It means a lot to me.

While I always enjoy reading Peter
Tyson's monthly editorials, the one about
fake news and the scientiﬁc method was
spot on. I was mesmerized by it. You
expressed thoughts that Wendee and I
have been thinking for well over a year
now. And you even managed to include
an appropriate comment by Thoreau, one
of my favorite writers. So keep it going.
S&T has made great leaps forward with
you as its editor.

Marcello Rasparini
Mol, Belgium

I thought I detected a whiff of politics, a
mention of "fake news," in the editorial of May's issue. If Sky & Telescope
goes political, this 30-year subscriber
will drop you like a hot potato. It is that
simple. I used to subscribe to both S&T
and Astronomy magazines. But when
Astronomy got snarky and political back
in the '90s, I canceled my subscription
and haven't looked back. Mr. Tyson
should keep his opinions to himself.
Stay focused on astronomy, okay?
Darrell Mason
Colorado Springs, Colorado

David H. Levy
Vail, Arizona

The truth, the whole truth, and nothing but the truth is a noble aspiration.
But your publication is far from living
up to it, and you could start improving
by looking in the mirror. Your pages
repeatedly assert that astronomers have

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LE AH TISCIONE / SK Y & TELESCOPE

Monica Young's "Spirits of Our Galaxy's
Past" (S&T: April 2017, p. 22) clariﬁed
for me the point that dark matter actually exists as clouds of gravitationally
condensed concentrations rather than
as diffuse clouds of evenly distributed
matter surrounding galaxies. So, assuming that baryonic dark matter interacts
with normal matter through gravity,
wouldn't these concentrations interact
with light the same way that normal
matter does, basically acting as gravitational lenses? If so, what wavelengths
would be ideal for its detection, and
could a survey determine dark matter's
distribution around either our galaxy or
more distant ones?

Sky and Telescope - August 2017

Table of Contents for the Digital Edition of Sky and Telescope - August 2017