Cosmic Magnification

In the previous post I noted how wide-area surveys of the sky like the
Deep Lens Survey serve the dual purposes of finding rare objects and
surveying a representative sample of the universe (to determine its
average density, for example), and I described one of our successes in
the former category while promising to post an example in the second
category.  Here it is!

First, we have to understand that the path of light is bent by gravity
and therefore, if we can observe some consequence of this bending, we
can learn about how much mass is between us and the source of light.
I'm not going to explain this in any detail here, but if you wish you
can watch my YouTube video on the subject, or just skip to the part
where I do a demo showing that this bending can lead to magnification.
In that demo I don't specifically point out the magnification, but at
one point you can clearly see that the blue ring on the whiteboard has
been magnified.

If we observe this magnification while looking in one very specific
direction as in the video, we can find how much mass is lurking in the
object which provides the magnification (usually a specific galaxy or
cluster of galaxies).  A few galaxies happen to have background
sources of light lined up just right so that we can see the
magnification easily, so we can learn about those specific galaxies.
But are they representative of galaxies in general?  Probably not,
because the most massive galaxies provide the most magnification and
are more likely to get noticed this way.  Also, having the mass more
concentrated toward the center of the galaxy helps, so if we just
study these galaxies, we will be looking only at the more massive,
concentrated galaxies.

In our wide-field survey, a team led by graduate student Chris
Morrison measured the very small amount of magnification around the
locations of hundreds of thousands of typical galaxies.  Their
statistical analysis doesn't measure the magnification caused by each
galaxy (which would be too small to measure), but it measures the
typical magnification caused by the galaxies in aggregate.  For this
reason, this type of analysis is called "cosmic magnification" which
sounds mysterious but can be thought of as "magnification caused by
the general distribution of mass in the cosmos rather than by a
specific identifiable lump of mass."

The amount of cosmic magnification tells us not only about the
distribution of mass in the universe, but also about the distances
between us, the magnifying masses, and the sources of light.  (Imagine
watching the wineglass demo in my video, but having me move the
wineglass much closer to the whiteboard...you can probably predict
that the magnification will be less.)  These are two very fundamental
things about the universe which astronomers are trying to measure,
because they are both affected by the expansion rate of the universe,
and the expansion rate is unexpectedly accelerating.  Three
astronomers won the 2011 Nobel Prize in Physics for their role in
discovering this acceleration, and ever since they discovered it
(1998), many astronomers and physicists have focused on figuring out
why.  Some attack this question from a theoretical point of view (a
theorist coined the term "dark energy" which has become the popular
term, but be warned that it may not be caused by a new form of energy
at all), and others attack it from an observational point of view: if
we can get better and better measurements of how the expansion is
actually behaving, we can rule out some of the theories which have
been proposed to explain it.  Cosmic magnification has a real role to
play in that process, and Morrison's paper is the first one to
measure, even in a crude way, how cosmic magnification increases as we
increase the distance between us and the masses causing the
magnification.