A new experiment to understand dark matter

June 15, 2018, Max Planck Society
Schematic image of a pulsar, falling in the gravitational field of the Milky Way. The two arrows indicate the direction of the attractive forces, towards the standard matter—stars, gas, etc. (yellow arrow) and towards the spherical distribution of dark matter (grey arrow). The question is whether dark matter attracts the pulsar only by gravity or, in addition to gravity, by a yet unknown "fifth force." Credit: Norbert Wex, with Milky Way Image by R. Hurt (SSC), JPL-Caltech, NASA and pulsar image by NASA

Is dark matter a source of a yet unknown force in addition to gravity? The mysterious dark matter is little understood and trying to understand its properties is an important challenge in modern physics and astrophysics. Researchers at the Max Planck Institute for Radio Astronomy in Bonn, Germany, have proposed a new experiment that makes use of super-dense stars to learn more about the interaction of dark matter with standard matter. This experiment already provides some improvement in constraining dark matter properties, but even more progress is promised by explorations in the centre of our Milky Way that are underway.

The findings are published in the journal Physical Review Letters.

Around 1600, Galileo Galilei's experiments brought him to the conclusion that in the gravitational field of the Earth all bodies, independent of their mass and composition feel the same acceleration. Isaac Newton performed pendulum experiments with different materials in order to verify the so-called universality of free fall and reached a precision of 1:1000. More recently, the satellite experiment MICROSCOPE managed to confirm the universality of free fall in the gravitational field of the Earth with a precision of 1:100 trillion.

These kind of experiments, however, could only test the universality of free fall towards , like the Earth itself whose composition is dominated by iron (32 percent), oxygen (30 percent), silicon (15 percent) and magnesium (14 percent). On large scales, however, ordinary matter seems to be only a small fraction of matter and energy in the universe.

It is believed that the so-called dark matter accounts for about 80 percent of the matter in our universe. Until today, dark matter has not been observed directly. Its presence is only indirectly inferred from various astronomical observations like the rotation of galaxies, the motion of galaxy clusters, and gravitational lenses. The actual nature of dark matter is one of the most prominent questions in modern science. Many physicists believe that dark matter consists of so far undiscovered sub-atomic particles.

With the unknown nature of dark matter another important question arises: is gravity the only long-range interaction between normal matter and dark matter? In other words, does matter only feel the space-time curvature caused by dark matter, or is there another force that pulls matter towards dark matter, or maybe even pushes it away and thus reduces the overall attraction between normal matter and dark matter. That would imply a violation of the universality of free fall towards dark matter. This hypothetical force is sometimes labeled as "fifth force," besides the well-known four fundamental interactions in nature (gravitation, electromagnetic & weak interaction, strong interaction).

At present, there are various experiments setting tight limits on such a fifth force originating from dark matter. One of the most stringent experiments uses the Earth-Moon orbit and tests for an anomalous acceleration towards the galactic center, i.e. the center of the spherical dark matter halo of our galaxy. The high precision of this experiment comes from Lunar Laser Ranging, where the distance to the Moon is measured with centimeter precision by bouncing laser pulses of the retro reflectors installed on the Moon.

Until today, nobody has conducted such a fifth force test with an exotic object like a neutron star. "There are two reasons that binary pulsars open up a completely new way of testing for such a fifth force between normal matter and dark matter," says Lijing Shao from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany, the first author of the publication in Physical Review Letters. "First, a neutron star consists of matter which cannot be constructed in a laboratory, many times denser than an atomic nucleus and consisting nearly entirely of neutrons. Moreover, the enormous gravitational fields inside a neutron star, billion times stronger than that of the Sun, could in principle greatly enhance the interaction with dark matter."

The orbit of a binary pulsar can be obtained with high precision by measuring the arrival time of the radio signals of the pulsar with radio telescopes. For some pulsars, a precision of better than 100 nanoseconds can be achieved, corresponding to a determination of the pulsar orbit with a precision better than 30 meters.

To test the universality of free fall towards dark matter, the research team identified a particularly suitable binary pulsar, named PSR J1713+0747, which is at a distance of about 3800 light years from the Earth. This is a millisecond pulsar with a rotational period of just 4.6 milliseconds and is one of the most stable rotators amongst the known pulsar population. Moreover, it is in a nearly circular 68-day orbit with a white dwarf companion.

While pulsar astronomers usually are interested in tight binary pulsars with fast orbital motion when testing general relativity, the researchers were now looking for a slowly moving millisecond pulsar in a wide orbit. The wider the orbit, the more sensitive it reacts to a violation of the universality of . If the pulsar feels a different acceleration towards dark matter than the white dwarf companion, one should see a deformation of the binary orbit over time, i.e. a change in its eccentricity.

"More than 20 years of regular high precision timing with Effelsberg and other radio telescopes of the European Pulsar Timing Array and the North American NANOGrav timing projects showed with high precision that there is no change in the eccentricity of the orbit," explains Norbert Wex, also from MPIfR. "This means that to a high degree the neutron star feels the same kind of attraction towards dark matter as towards other forms of standard ."

"To make these tests even better, we are busily searching for suitable pulsars near large amounts of expected ," says Michael Kramer, director at MPIfR and head of its "Fundamental Physics in Radio Astronomy" research group. "The ideal place is the galactic centre where we use Effelsberg and other telescopes in the world to have a look as part of our Black Hole Cam project. Once we will have the Square Kilometre Array, we can make those tests super-precise," he concludes.

Explore further: New study suggests galactic bulge emissions not due to dark matter

More information: Lijing Shao et al. Testing the Universality of Free Fall towards Dark Matter with Radio Pulsars, Physical Review Letters (2018). DOI: 10.1103/PhysRevLett.120.241104

Related Stories

Does dark matter annihilate quicker in the Milky Way?

June 23, 2017

Researchers at the Tata Institute of Fundamental Research in Mumbai have proposed a theory that predicts how dark matter may be annihilating much more rapidly in the Milky Way, than in smaller or larger galaxies and the early ...

Mapping dark matter

July 24, 2017

About eighty-five percent of the matter in the universe is in the form of dark matter, whose nature remains a mystery. The rest of the matter in the universe is of the kind found in atoms. Astronomers studying the evolution ...

NA64 hunts the mysterious dark photon

November 25, 2016

One of the biggest puzzles in physics is that eighty-five percent of the matter in our universe is "dark": it does not interact with the photons of the conventional electromagnetic force and is therefore invisible to our ...

Recommended for you

New study could hold key to hack-proof systems

July 17, 2018

Major data breaches have made worldwide headlines of late but an international consortium of scientists—including a professor from Heriot-Watt—have developed a new technique that could result in hack-proof systems.

Solutions to water challenges reside at the interface

July 17, 2018

In response to rising water scarcity, leading Argonne National Laboratory researcher Seth Darling describes the most advanced research innovations that could address global clean water accessibility. His comprehensive paper ...

Exploding waves from colliding dissipative pulses

July 17, 2018

The interaction of traveling waves in dissipative systems, physical systems driven by energy dissipation, can yield unexpected and sometimes chaotic results. These waves, known as dissipative pulses (DSs), are driving experimental ...

20 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

fthompson495
1 / 5 (11) Jun 15, 2018
There is evidence of dark matter every time a double-slit experiment is performed, as it is the medium that waves.

Dark matter is a supersolid that fills 'empty' space and is displaced by visible matter. What is referred to geometrically as curved spacetime physically exists in nature as the state of displacement of the dark matter. The state of displacement of the dark matter is gravity.

Dark matter ripples when galaxy clusters collide and waves in a double-slit experiment, relating general relativity and quantum mechanics.
Ojorf
3.3 / 5 (14) Jun 15, 2018
No, still absolutely not.

Explained here: https://www.physi....948973/

How many sock puppets do you have?
Mimath224
4.5 / 5 (4) Jun 15, 2018
Imho it's a good idea to ask these questions, then formulate experiments to test. If results imply a fifth force then further experiments can be constructed. If the implication is in the negative, that's okay because we still learn something...then move on.
tallenglish
3 / 5 (2) Jun 16, 2018
I love all these theories of what DM is - but since we can't measure it as it doesn't interact with strong, weak or electromagnetic forces to me suggests it exists in a different part of spacetime to mass and light.

If we think of the universe as a 2D surface, mass is on the top, DM is on the bottom and the only way they can interact is by deforming the 2D plane. Mass being the framework for DM and DM is the framework for mass.

You know everything that has a positive, must have its negative twin, etc. The source of the big bang and as DM evaporates (aka negative entropy) our universe expands to fill the space it leaves behind. Universe always was and always will be, the big bang if it exists at all would be a cyclical event.

If you can believe that, the DM is not special - its just mass, with momentum oscillating between the two halves (in a sine wave like everything else we see).

I don't understand why this is so hard for people to understand?
tallenglish
3 / 5 (2) Jun 16, 2018
DM and mass do not exist in the same space at the same time - it is wrong to say DM goes through mass as that implies they do exist in the same space and time, which if they did they would interact using one of the other forces not just gravity.

DM doesn't slide through mass - it slides past it. My guess is if you go faster than the speed of light, mass and light become DM and go backwards in time relative to us. But most peoples brains break when you try and say time can be both negative and imaginary in nature, most only accept time can be positive and going forward (and thats why they can't ever understand DM as they refuse to understand the conditions it exists under).
Ojorf
3.2 / 5 (9) Jun 17, 2018
since we can't measure it as it doesn't interact with strong, weak or electromagnetic forces to me suggests it exists in a different part of spacetime to mass and light.
Why not look at the evidence? You waste your time making up nonsensical fantasies that contradict experimental evidence and the currant state of knowledge.
How do we know there is such a thing as DM, whatever it may be?
Galaxy rotation curves
Galaxy velocity dispersion
Galaxy cluster masses
Gravitational lensing
CMB anisotropy
Structure formation after BB
Bullet Cluster
Ia supernova measurements
Baryon acoustic oscillations
Redshift-space distortions
Lyman-alpha forest
See? A few of these observations can be explained by modified gravity theories, but not even the majority.
Now have a look at the SM.
There are massless particles, ones without charge or colour charge.
Why is it sooo difficult to imagine a particle only interacting via gravity?
Would explain it ALL, simplest.
ZoeBell
Jun 17, 2018
This comment has been removed by a moderator.
ZoeBell
Jun 17, 2018
This comment has been removed by a moderator.
archytype_net
3 / 5 (8) Jun 17, 2018
The fact that experiment after experiment results in no dark matter ever found would suggest we've been chasing down a red herring!

The assumptions and models are obviously not right. Time for re-evaluation of the fundamentals I'd say.
eachus
2 / 5 (2) Jun 17, 2018
I think it is more likely that DM interacts with the weak force than a (new) fifth force. The proposed sterile neutrinos would interact with the weak force just as other neutrinos do. If such sterile neutrinos exist, there would be a ratio of mass to weak interaction where DM would get blown out of the center of galaxies (and solar systems) by interaction with electrons (positrons, muons, tau, etc.) , and the neutrino background. That would explain why DM hasn't been seen near, on, or in Earth.

Is there a neutrino wind blowing away from the sun? Sure, various solar neutrino experiments have shown it. If this hypothesized weak interaction of dark matter exists, it should be detectable by looking at orbits in the Oort cloud--and perhaps eventually in orbits of spacecraft like Pioneer and New Horizons. The Pioneer Anomaly was explained, but it showed just how effective spacecraft can be in detecting the distribution of mass in the outer solar system.
chuck_in_st_paul
2 / 5 (4) Jun 17, 2018
there is no Dark Matter. There is however Astronomical Plasma.
ZoeBell
Jun 17, 2018
This comment has been removed by a moderator.
milnik
2 / 5 (4) Jun 17, 2018
If science has not yet learned what is matter, what forms it and how it is created, how can it be said about the existence of a dark matter, which has not yet been able to be measured or detected in any way. It means that they are fabrications, because it is a stupid idea to talk about the existence of dark matter. The same is a stupid idea to talk about curvature of space time. In the infinite universe, there is a substance Aether from which matter is formed in two routines. These forms of matter, each for themselves with Aether, cause gravity, that is, the supernovae, and the other is magnetism.
Science does not yet know how gravity and magnetism arises, and what is their role in the behavior of everything in the material energy universe (MEEU).
tgmeloche
2.6 / 5 (5) Jun 17, 2018
Quit wasting public funding chasing dark matter... it is a ghost ... a misunderstanding by many. Learn the principles of atomic gravity. Searching for solutions is the path forward!
Surveillance_Egg_Unit
2.3 / 5 (3) Jun 17, 2018
I love all these theories of what DM is - but since we can't measure it as it doesn't interact with strong, weak or electromagnetic forces to me suggests it exists in a different part of spacetime to mass and light.

If we think of the universe as a 2D surface, mass is on the top, DM is on the bottom and the only way they can interact is by deformbut since we can't measure iting the 2D plane. Mass being the framework for DM and DM is the framework for mass.
...I don't understand why this is so hard for people to understand?

says tallenglish

You had already said, "but since we can't measure it...", and then you go on to exclaim that, "DM is on the bottom..." (in a 2D surface). A bit of double-speak, eh?

--CONTINUED--
Surveillance_Egg_Unit
3.4 / 5 (5) Jun 17, 2018
--CONTINUED--
Dark Matter is an unknown and is being supported by wishful thinking as a placeholder until something better comes along. It is said to possess certain properties for which scientists are attempting to find some relationship to that which they already know, are able to measure and understand.

It will be a long while, if ever, that Science will truly find that special niche for the elusive characteristics of what they have termed, Dark Matter. The same with Dark Energy. Both fall into the same category of WOO until found to be credible AND scientific. Whichever comes first.
ZoeBell
Jun 18, 2018
This comment has been removed by a moderator.
ZoeBell
Jun 18, 2018
This comment has been removed by a moderator.
antialias_physorg
3.7 / 5 (6) Jun 18, 2018
but since we can't measure it as it doesn't interact with strong, weak or electromagnetic forces to me suggests it exists in a different part of spacetime to mass and light.

Neither do neutrinos. No need to go 'extradimensional'.

Dark Matter is an unknown and is being supported by wishful thinking as a placeholder until something better comes along.

Dark matter is the placeholder. It is not supported by 'wishful thinking' because we can measure its effects. We know something is there. Don't get hung up on the word 'matter'. The label 'dark matter' does not mean that exclusively particle/mass based theories fall under this heading.

The same with Dark Energy. Both fall into the same category of WOO

If you say 'dark matter/energy' is woo then you must consider gravity 'woo', as well. We still don't really know how gravity works but we can measure its effects (just as with DM/DE). We have a good model, but no graviton has yet been detected.
eachus
not rated yet Jun 22, 2018
Sterile neutrino have no weak charge, so that they cannot interact with weak force (find link for yourself)


Until and unless sterile neutrinos are found, their properties remain an untested hypothesis. I happen to think that sterile neutrinos, if found will oscillate between three forms same as normal neutrinos. That does require a weak charge. But that, is just guessing without any sterile neutrinos to study.

the scientists would already notify it by balance of normal neutrinos (the neutrino oscillations were revealed in similar way - find link for yourself


Definitely no need, I've been following--and believing in--neutrino oscillations for almost 50 years. To have sterile neutrinos separate from normal neutrinos, you have to have a property which distinguishes them. What I was saying was that if the mass from this and a (hypothesized) interaction with electrons were balanced right, it would match the known distribution of DM.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.