In this image of the Bullet Cluster, the blue area shows what is
thought to be dark matter. Physicists are investigating
connections between a possible fifth force and the direct
detection of dark matter. Image source: NASA / CXC / CIA / STSci
/ Magellan / Univ. of Ariz. / ESO.
(PhysOrg.com) -- No one knows exactly
what a “fifth force” might be, but studies have shown that, if a
long-range fifth force does exist, it could have surprising
effects on the universe’s structure formation. A fifth force
could reduce discrepancies between theory and observation in
several areas of cosmology.
Now, as new research has shown, a fifth
also be connected to dark
matter. In a paper published in Physical
Review Letters, physicists Jo Bovy and Glennys
Farrar were surprised to discover that a fifth force in the dark
sector could place constraints on dark matter that essentially
exclude its direct
spin-independent interactions. Conversely, if future experiments
do detect a spin-independent interaction of dark matter, then
any fifth force in the dark sector must be so weak as to be
“Our study shows that we can strongly constrain some properties
of dark matter, i.e., the combination of its interaction with
the visible sector and the strength of a long-range fifth force
matter particles, through experiments with ordinary
matter,” Bovy, a Ph.D. student at New York
University, told PhysOrg.com.
As for which scenario appears to be more likely - a fifth force
excluding direct detection of dark matter, or direct detection
of dark matter excluding a relevant fifth force - Bovy and
Farrar said that it’s impossible to say in advance. “Both would
be very interesting both theoretically as well as
observationally,” Bovy said.
Previous research has suggested the possibility that a new
long-range, attractive fifth force might exist, which arises in
several extensions of the standard model. Although most dark
matter models predict that the force between dark matter
particles is a short-range force, other models such as supersymmetry and
string theory allow for the existence of a very light boson
which could carry a long-range force in the dark sector.
In the current study, the theorized fifth force - a
nongravitational, long-range attractive force - would couple
directly to dark matter but not to ordinary visible matter.
However, if dark matter particles interacted nongravitationally
with ordinary matter, quantum corrections would make the fifth
force emerge in the visible sector, as well. This quantum
correction occurs when two virtual dark matter particles are
coupled to both the fifth force scalar and to two quarks.
“Our study reveals a hitherto unrecognized connection between
dark matter interactions with ordinary matter, and dark matter
self-interactions,” said Farrar, a physics professor and
Director of the Center for Cosmology and
Particle Physics at New York University.
As Bovy and Farrar explain in their study, in order to be
astrophysically relevant (i.e., on the order of the
gravitational force), the fifth force carrier must have a vacuum
expectation value on the order of the Planck mass. In turn, this
requires that dark matter be very heavy in most scenarios. In
direct detection experiments, scientists look for dark matter by
observing ordinary matter particles recoiling due to being
scattered by dark matter particles. As the mass of dark matter
particles increases, the bounds on the coupling of dark matter
and quarks become more stringent, making the direct detection of
dark matter more difficult.
Besides its implications on the direct detection of dark matter,
a fifth force has also been hypothesized to impact large-scale
structure formation. If a fifth force was attractive and had a
very long range, it would effectively increase the strength of
gravitational interaction, and thus accelerate structure
formation. As previous studies have shown, such a force could
reduce discrepancies between observations and predictions in
several areas, such as by increasing the number of galaxy
clusters and superclusters and reducing voids, which would agree
better with observations.
As the scientists explain, the impacts of a possible fifth force
do not demand its existence, but they do motivate researchers to
explore the implications of a fifth force.
“Jo Bovy and I are looking at what the analogous constraints
are, between possible dark matter annihilation signals (searched
for by Fermi-GLAST and other satellite experiments like PAMELA
and ATIC) and a fifth force,” Farrar said. “Also, in
collaboration with others, I'm studying the effects of a
long-range dark matter force on cosmology and attempting to
improve constraints on it from the Bullet cluster.”
Bovy added that he’s also investigating the implications of
short-range attractive forces between dark matter particles,
which may enable the detection of dark matter annihilation from
satellite galaxies of the Milky Way.
More information: Bovy,
Jo and Farrar, Glennys R. “Connection between a Possible Fifth
Force and the Direct Detection of Dark Matter.” Physical
Review Letters 102,