A recent experiment to better understand the nature of dark matter constrains a possible "fifth force" of nature to almost zero.

Pulsar-white dwarf binary
An artist's illustration shows what pulsar PSR J1713+0747 and its white dwarf companion might look like.
ESO / L. Calçada

Scientists recently studied a pulsar binary system to constrain the existence of a hypothetical fifth fundamental force of nature.

We already know about four fundamental forces: gravity, electromagnetism, and the strong and weak nuclear forces. However, there are some effects in the universe that cannot be explained by these forces alone. For example, a 2016 experiment in Hungary showed unexpected behavior in the decay of nuclei in the isotype beryllium-8. (After shooting protons at lithium foil, observers saw more electron-positron pairs ejected at a 140-degree angle, which is difficult to explain with standard nuclear physics theories.)

One possibility is the existence of a "fifth force" of nature, which governs the behavior of elementary particles alongside the other four forces. Some scientists suggest this force could work on dark matter, the unseen substance that makes up most of the universe's mass. We can see dark matter’s effects on ordinary matter, but direct detection has eluded scientists and what it’s made of remains unknown.

Testing for a Fifth Force

One research group tested for a fifth force using a pulsar and its white dwarf star companion. Pulsars, whose atoms have been compacted into neutrons, are so dense that their extreme gravitational fields could enhance any possible interactions with dark matter. The white dwarf, while still sardine-packing its atoms, isn’t nearly so compact. General relativity predicts that normal matter ought to fall freely toward dark matter, but a fifth force that has the ability to interact with both normal and dark matter could strengthen or diminish dark matter’s pull. If a fifth force does exist, the Milky Way’s dark matter halo, whose density ought to peak in the galactic center, would pull on the neutron star and the white dwarf in different ways, slightly altering their orbit.

The researchers chose binary pulsar PSR J1713+0747, which is 3,800 light-years from Earth, lying in the direction of the galactic center. Dark matter is believed to be more populous towards the heart of the galaxy, so the pulsar binary system provides an ideal test how a fifth force would act on dark matter and standard matter. The researchers wanted to see if the movements of the pulsar and white dwarf would differ as they orbited one another.

"If there is a fifth force that acts between dark matter and standard matter, it would not be universal," says Lijing Shao (Max Planck Institute for Radio Astronomy, Germany). "It would therefore produce an apparent difference for the neutron star and the white dwarf in their free fall towards dark matter. Thus, the orbit of the neutron star would be different than what is predicted by the general relativity."

Using 20 years of radio observations of this system, the researchers concluded that if a fifth force does exist, it must have less than 1% of gravity’s strength . (And gravity is already the weakest of the four known forces.) The results appear in Physical Review Letters.

The researchers also discovered that the limits on the density of dark matter at this pulsar system were similar to other tests closer to Earth. In other words, the team didn't prove or disprove other observations showing that dark matter density increases towards the center of the galaxy.

Beyond Relativity

Aurélien Hees (Observatory of Paris), who was not involved in the study, noted that this work is the first to investigate interactions between a hypothetical fifth force and dark matter in this way. The short rotational period of the pulsar – just 4.6 milliseconds – and its stable rotation made it a good candidate for constraining the effects of the fifth force, he said.

With the fifth force, he explains, "We expect to see something a little bit beyond relativity. We are trying to search for that with all the observations available from Earth."

Shao says his team hopes to study more binary pulsars closer to the center of the galaxy to better understand the effects of dark matter. Unlike most tests of general relativity, in this case the researchers want to find pulsars moving in relatively slow orbits around their companion The challenge, of course, is finding the pulsars in the first place. He suggested a breakthrough will come when the more sensitive Square Kilometer Array is ready in the 2020s. "Bigger radio telescopes and arrays are better because they more precisely measure the time of the [pulsar signal] arrival," Shao said.




Image of Peter Wilson

Peter Wilson

June 23, 2018 at 10:47 am

"...the Milky Way’s dark matter halo, whose density ought to peak in the galactic center..." Here's where I get lost. If DM's density peaks in the galactic center, then the orbits of stars ought to slow as you go away from the center, like in our solar-system. If DM is in a halo around us, then why are they searching for it in the galactic center?!? Is DM thought to be in a halo around us, or a clump in the galactic center?

You must be logged in to post a comment.

Image of Monica Young

Monica Young

June 25, 2018 at 9:41 am

Peter, I believe the answer is "both"! The density of dark matter is supposed to peak in the galactic center, but the dark matter halo extends out far beyond the stars' orbits.

You must be logged in to post a comment.

Image of GerryH


June 25, 2018 at 5:20 am

Hi Elizabeth,
If the DM which we cannot directly detect, was in other hypothetical dimensions, then we could & will never observe it?

You must be logged in to post a comment.

Image of johnnyiannello


April 23, 2019 at 11:56 pm

Title: The fifth force implies no existance of blackholes?

We know by now that Newton's formula of gravitational potential on gravity, w=G*m(r)/r, (with singularity for star radius=r=0, m(r) is the mass of the star),
is modified by the presence of the "Fifth force" (or said anti-gravity effect) given from the formula of Fischbach E. 1986
(see even Rujula A.D. 1986, Cowsik R. 1990, Thomas J. 1989, formula without singularity).
The Fischbach's formula of the gravitational potential corrected is:
where G is the universal gravitational constant, corrected by a=0.01:0.001, which is the intensity of the fifth force, called ipercharge,
that depends on the relative amount of neutrons upon number of protons, in range L=100:1000 meters, of mass m of the star on radius r.
The question of the title if the fifth force implies no existance of blackholes, is because there is no presence of singularity
in the gravitational potential corrected by Fischbach E.:
lim(r-->0)(1-a*exp(-r/L))/r=a/L. (theoreme of De Hospital for limits).

We know that in General Relativity the Einstein's Field Equations derived from the Newtons formula, (see Weinberg S. 1972 chapter 7.1.3 and 7.1.12),
have the presence of singularity for the radius of the star going to zero: r-->0, where the metric tensor A(r)=g(rr)=1/(1-2*G*m(r)/r),
(see Weinberg in chapter 11.1.11) gives the presence of blackholes with the Schwarzschild radius (1=2*G*m(R)/R).
But if we use the corrected gravitational potential of Fischbach E. 1986 without singularity, modifying the Einstein's Field Equations;
probably the new Einstein's Field Equations shall become without the presence of singularity; it is amazing;
giving a curvature that is bounded, with radius metric tensor A(r)=g(rr)<"curvature limit".
Infact, the metric tensor in radius r is g(rr)=1/[1-(1-a*exp(-r/L))2mG/r] for the Schwarzschild solution (see Weinberg 8.1.7 and 8.2.11);
and you can easily verify that it hasn't any singularity, (so g(rr) doesn't approach infinite value for any radius r, neither with Schwarzschild radius).
So blackholes do not exist for the presence of the fifth force?
But another question is the neutron stars with the fifth force: how are they phenomenologically? Do they exist? And how?

The new Einstein's Field Equations depending on the formula of Fischbach 1986, looks as:
R(ij) - 1/2 * g(ij) * R = {g(mn) * A(mn) + T(mn) * B(mn)} * T(ij) where the indices of tensors are i,j,m,n=1,2,3,4;

where T(ij) is the energy momentum tensor, and R(ij) is the Ricci tensor, and g(ij) is the metric tensor. A(mn) and B(mn) are to be found.

Cowsik R. et al. 1990: Phy.Rev. Lett.64:337
Fischbach E. et al. 1986: Phy.Rev.Lett.57:3
Rujula A.D. 1986: Phy.Lett.180:213.
Thomas J. 1989: Phy.Rev.Lett.63:1963
Weinberg S. 1972 "Gravitation and Cosmology" Wiley.

Good Research. Bye.

You must be logged in to post a comment.

You must be logged in to post a comment.