Sky and Telescope - August 2017 - 34

Beyond the Lamppost

the effect on the stars' motions. WIMPs will predominantly
form cloud-like clumps, whereas axions might collect into
super-high-density arcs, called caustics, and these two structures would plow through streams in a different way.
Caustics are a controversial idea, and recent particle
physics work suggests that they might never form. On the
astronomy side, we're starting to amass the observational
data to test the proposal. Europe's Gaia satellite is measuring
the positions and motions of more than a billion stars in the
Milky Way and might uncover holes in stellar streams. In the
mid-2020s, NASA's infrared WFIRST mission will also survey
large areas of the sky, and combined with ground-based measurements of how quickly stars are moving along their line of
sight, we may be able to make another level of breakthrough.

Getting Warmer

0.06
0.04
0.02
0
-0.02
-0.04
3.0

3.2

3.4

3.6

3.8

Energy (keV)

p HINT OF DARK MATTER? An unexpected "bump" in the X-ray emission from the Perseus Cluster (background image, shown in X-rays) and more
than 70 other galaxy clusters might be created when sterile neutrinos transform into active ones. The slight bump is circled in the spectrum. The cluster's X-ray glow in the background image spans roughly 500,000 light-years.

34

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

N ASA / CXC / SAO / E. BULBUL E T A L.

Extra signal
(counts per second per keV)

The second non-WIMP candidate we will discuss is generically called warm dark matter, or WDM. The "warm" basically
means that its particles have a somewhat greater general

speed than CDM, and so some of the smaller clumps that
might form in CDM don't in WDM.
While most of our observations of the universe fit neatly
into expectations from CDM, there are small but persistent
puzzles. Some of these have to do with the existence of the
smaller clumps within our Milky Way's galactic family or
even at the earliest times of the universe. Astronomers have
been having trouble finding as many small satellite galaxies around the Milky Way as CDM predicted should exist,
although recent computer simulations show that the predictions probably were just overzealous because they didn't
include normal matter (S&T: May 2017, p. 34).
Another cold versus warm signature might be revealed by
counting how many small clouds of clumpy hydrogen gas
there are in the vast spaces between galaxies. It's the same
idea as with satellite galaxies. If dark matter is too warm to
clump gravitationally, then the gas - which would ride along
with the dark matter as it coalesced into clouds - will be
spread out, too.



Sky and Telescope - August 2017

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