Astronomy

Dark Matter Curvature

Dark Matter Curvature


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Does Dark Matter affect Space Time Curvature? If so, then what is the formula to calculate space time curvature due to dark matter?


This answer comes with a small warning. Actual dark matter has not been discovered, so this is theoretical, but a very well understood theory.

All matter, dark or light, affects the curvature of spacetime in the same way. The formula for the curvature is the Einstein Field Equation. The curvature is defined by the value of a 4x4 matrix, called the "stress energy tensor". Which describes the density of energy and momentum in space-time. The formula doesn't care if the energy and momentum comes from regular matter or dark matter.


Yes, the same equation as is used for other matter.

Marc Postman, Ray Villard and Donna Weaver of the Space Telescope Science Institute posted the results of a CLASH study: "News Release 2011-25":

"This image of galaxy cluster MACS J1206.2-0847 (or MACS 1206 for short) is part of a broad survey with NASA's Hubble Space Telescope.

The distorted shapes in the cluster are distant galaxies from which the light is bent by the gravitational pull of an invisible material called dark matter within the cluster of galaxies. This cluster is an early target in a survey that will allow astronomers to construct the most detailed dark matter maps of more galaxy clusters than ever before.

On the NASA website: "A Clash of Clusters Provides New Clue to Dark Matter":

"… this composite image, made using data from the Hubble Space Telescope Chandra X-Ray Observatory, researchers have discovered clues to increase our knowledge of dark matter. This image is a powerful collision of clusters. These clusters show formations of dark matter, where it separated from the ordinary matter. The different color disparities, along with the interesting evidence for dark matter… ".

Another article with the same photo: "heic0818 - Science Release -- Clash of clusters provides new dark matter clue":

"New Hubble and Chandra observations of the cluster known as MACSJ0025.4-1222 indicate that a titanic collision has separated dark from ordinary matter. This provides independent confirmation of a similar effect detected previously in a target dubbed the Bullet Cluster, showing that the Bullet Cluster is not an anomalous case.".

Wikipedia webpage on CLASH - Cluster Lensing and Supernova survey with Hubble.


Dark matter may slow the rotation of the Milky Way’s central bar of stars

The Milky Way (illustrated) contains a central bar of stars (yellow) that rotates along with the galaxy. Researchers reported evidence that the bar is being slowed by the presence of dark matter.

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Dark matter can be a real drag. The pull of that unidentified, invisible matter in the Milky Way may be slowing down the rotating bar of stars at the galaxy’s heart.

Based on a technique that re-creates the history of the slowdown in a manner akin to analyzing a tree’s rings, the bar’s speed has decreased by at least 24 percent since it formed billions of years ago, researchers report in the August Monthly Notices of the Royal Astronomical Society.

That slowdown is “another indirect but important piece of evidence that dark matter is a thing, not just a conjecture, because this can’t happen without it,” says astrophysicist Martin Weinberg of the University of Massachusetts Amherst, who was not involved with the study.

Many spiral galaxies, including the Milky Way, contain a central bar-shaped region densely packed with stars and surrounded by the galaxy’s pinwheeling arms. The bar also has some groupies: a crew of stars trapped by the bar’s gravitational influence. Those stars orbit a gravitationally stable point located alongside the bar and farther from the galaxy’s center, known as a Lagrange point (SN: 2/26/21).

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If the bar’s rotation slows, it will grow in length, and the bar’s tagalongs will also move outward. As that happens, that cohort of hangers-on will gather additional stars. According to computer simulations of the process, those additional stars should arrange themselves in layers on the outside of the group, says astrophysicist Ralph Schönrich of University College London. The layers of stars imprint a record of the group’s growth. “It’s actually like a tree that you can cut up in your own galaxy,” he says.

Schönrich and astrophysicist Rimpei Chiba of the University of Oxford studied how the composition of stars in the group changed from its outer edge to its deeper layers. Data from the European Space Agency’s Gaia spacecraft revealed that stars in the outer layers of the bar tended to be less enriched in elements heavier than helium than were stars in the inner layers. That’s evidence for the group of stars moving outward, as a result of the bar slowing, the researchers say. That’s because stars in the center of the galaxy — which would have glommed on to the group in the more distant past — tend to be more enriched in heavier elements than those farther out.

The bar’s slowdown hints that a gravitational force is acting on it, namely, the pull of dark matter in the galaxy. Normal matter alone wouldn’t be enough to reduce the bar’s speed. “If there is no dark matter, the bar will not slow down,” Chiba says.

But the results have drawn some skepticism. “Unfortunately, this is not yet convincing to me,” says astrophysicist Isaac Shlosman of the University of Kentucky in Lexington. For example, he doubts that the tree ring layering would really occur. It is “hard to believe that this is the case in a realistic system” as opposed to in a simplified computer simulation, he says.

Weinberg, on the other hand, says that although the study relies on a variety of assumptions, he suspects it’s correct. “It’s got the right smell.”

Questions or comments on this article? E-mail us at [email protected]

Citations

R. Chiba and R. Schönrich. Tree-ring structure of Galactic bar resonance. Monthly Notices of the Royal Astronomical Society. Vol. 505, August 2021, p. 2412. doi: 10.1093/mnras/stab1094.


The dark force secretly slowing down our galaxy could be dark matter

When something went wrong in Star Wars, it usually had to do with the Dark Side of the Force, but an unexplained phenomenon in our galaxy is probably connected to dark matter.

Something suspicious is going on in a galaxy not so far, far away. In the core of the Milky Way lies the Hercules stream of stars. Its spin is mysteriously slowing down, and what exactly is going on has been predicted for decades with no luck. Now it has finally been measured and possibly proven. Astrophysicists Rimpei Chiba from the University of Oxford and Ralph Schoenrich of University College London think they have figured out what dark force is doing this — and it isn’t the Sith. Dark matter has seemingly been countering the spin and slowing it down.

More Dark Matter

Billions of stars totaling trillions of solar masses are trapped by a spinning bar at the center of the Milky Way. Chiba and Shoenrich, who recently published a study in Monthly Notices of the Royal Astronomical Society, found that the spin of that bar has slowed down to around 75% of what it was when it first came into existence.

“The finding was not expected,” Chiba, who led the study, and Shoenrich, who was a co-author, told SYFY WIRE via email. “Previous constraints on dark matter have mostly concentrated on mapping the gravitational potential, but the bar's slowdown that we have quantified links to the inertial mass (dynamical response) of dark matter taking up the angular momentum from the galactic bar.”

Dark matter has never before been measured by its inertial mass rather than its gravitational energy. Inertial mass is how much an object resists forces working against it, as opposed to gravitational mass, or the strength of the gravitational force between objects. Dark matter has been slowing the spin of the galactic bar through dynamical friction, or the drag that objects experience as they orbit through dark matter. How much of this drag holds a moving object back depends on where clumps of dark matter are and the amount found in any given region (spatial distribution), as well as how fast any of those particles are moving, anywhere (velocity distribution).

The galactic core (right) of the Milky Way (left). Credit: NASA

Stars in the Hercules stream are gravitationally trapped by the spinning bar, and will move outward as the bar’s spin grows slower and slower. The proof that Hercules stars migrated away from the bar while maintaining their orbits is in their chemistry. Stars that started out in the galactic core are full of heavier elements, while the core is ten times richer in these elements than the halo. These stars are trapped in orbit around the resonance, which occurs when a consistent gravitational influence is exerted by one orbiting body on another, but can still move outward.

“Since our measurement quantifies the loss of the bar's angular momentum, the finding is in tension with alternative gravity theories without dark matter, which has to take up the angular momentum lost by the bar,” Chiba and Shoenrich noted. “We cannot see a different solution for explaining this angular momentum loss.”

The team has run into opposition even though their explanation is the only one that completely makes sense. Though dark matter is still just about as dark and mysterious as it sounds, there are ways to infer it until technology advances enough to be able to detect it otherwise. Some scientists have used models excluding dark matter to show why the spin of the galactic bar is slowing down, but the problem with these is that they end up with little or no slowdown as a result. Others who have proposed models that involve alternative gravity and only some dark matter have not seen the same results that Chiba, Shoenrich, and their research team have.

“You can see it this way — finding this new type of evidence for dark matter is like finding a big island in the ocean,” they said. “Knowing it is there is great, but to explore and use it for further study, you need new tools. At the end of the road, you get new constraints on galactic history and the unique opportunity to differentiate between different dark matter models.”

Sorry Vader, but even the power of the Dark Side probably has nothing on the strength of all the dark matter in the universe.


Dark Matter Density

As advertised the acoustic peaks in the power spectrum are sensitive to the dark matter density in the universe. (Formally, the matter to radiation ratio but the radiation density is fixed in the standard model.)

As we raise the physical density of the dark matter, W m h 2 , the driving effect goes away at a given peak such that its amplitude decreases. Although this effect changes the heights of all the peaks, it is only separable from the baryonic effects with at least three peaks . Note that decreasing the matter density also affects the baryon loading since the dark matter potential wells go away leaving nothing for the baryons to fall into. Having a third peak that is boosted to a height comparable to or exceeding the second peak is an indication that dark matter dominated the matter density in the plasma before recombination. Note that the self-gravity of the photons and baryons still plays a role in the first and second peaks so that the third peak is the cleanest test of this behavior.

Notice also that the location of the peaks, and that of the first peak in particular, changes as we change the dark matter density. The matter to radiation ratio also controls the age of the universe at recombination and hence how far sound can travel relative to how far light travels after recombination. This is the leading order ambiguity in the measurement of the spatial curvature of the universe. We see here that that ambiguity will be resolved when at least three peaks are precisely measured.


Solved: the mystery of how dark matter in galaxies is distributed

Dark matter in two galaxies simulated on a computer. The only difference between them is the nature of dark matter. Without collisions on the left and with collisions on the right. The work suggests that dark matter in real galaxies looks more like the image on the right, less clumpy and more diffuse than the one on the left. The circle marks the end of the galaxy. Credit: Image taken from the article Brinckmann et al. 2018, Monthly Notices of the Royal Astronomical Society, 474, 746.

The gravitational force in the Universe under which it has evolved from a state almost uniform at the Big Bang until now, when matter is concentrated in galaxies, stars and planets, is provided by what is termed 'dark matter." But in spite of the essential role that this extra material plays, we know almost nothing about its nature, behavior and composition, which is one of the basic problems of modern physics. In a recent article in Astronomy & Astrophysics Letters, scientists at the Instituto de Astrofísica de Canarias (IAC)/University of La Laguna (ULL) and of the National University of the North-West of the Province of Buenos Aires (Junín, Argentina) have shown that the dark matter in galaxies follows a 'maximum entropy' distribution, which sheds light on its nature.

Dark matter makes up 85% of the matter of the Universe, but its existence shows up only on astronomical scales. That is to say, due to its weak interaction, the net effect can only be noticed when it is present in huge quantities. As it cools down only with difficulty, the structures it forms are generally much bigger than planets and stars. As the presence of dark matter shows up only on large scales the discovery of its nature probably has to be made by astrophysical studies.

To say that the distribution of dark matter is organized according to maximum entropy (which is equivalent to 'maximum disorder' or 'thermodynamic equilibrium') means that it is found in its most probable state. To reach this 'maximum disorder' the dark matter must have had to collide within itself, just as gas molecules do, so as to reach equilibrium in which its density, pressure, and temperature are related. However, we do not know how the dark matter has reached this type of equilibrium.

"Unlike the molecules in the air, for example, because gravitational action is weak, dark matter particles ought hardly to collide with one another, so that the mechanism by which they reach equilibrium is a mystery," says Jorge Sánchez Almeida, an IAC researcher who is the first author of the article. "However if they did collide with one another this would give them a very special nature, which would partly solve the mystery of their origin," he adds.

The maximum entropy of dark matter has been detected in dwarf galaxies, which have a higher ratio of dark matter to total matter than have more massive galaxies, so it is easier to see the effect in them. However, the researchers expect that it is general behavior in all types of galaxies.

The study implies that the distribution of matter in thermodynamic equilibrium has a much lower central density that astronomers have assumed for many practical applications, such as in the correct interpretation of gravitational lenses, or when designing experiments to detect dark matter by its self-annihilation.

This central density is basic for the correct interpretation of the curvature of the light by gravitational lenses: if it is less dense the effect of the lens is less. To use a gravitational lens to measure the mass of a galaxy one needs a model, if this model is changed, the measurement changes.

The central density also is very important for the experiments which try to detect dark matter using its self-annihilation. Two dark matter particles could interact and disappear in a process which is highly improbable, but which would be characteristic of their nature. For two particles to interact they must collide. The probability of this collision depends on the density of the dark matter the higher the concentration of dark matter, the higher is the probability that the particles will collide.

"For that reason, if the density changes so will the expected rate of production of the self-annihilations, and given that the experiments are designed on the prediction of a given rate, if this rate were very low the experiment is unlikely to yield a positive result," says Sánchez Almeida.

Finally, thermodynamic equilibrium for dark matter could also explain the brightness profile of the galaxies. This brightness falls with distance from the center of a galaxy in a specific way, whose physical origin is unknown, but for which the researchers are working to show that it is the result of an equilibrium with maximum entropy.

Simulation Versus Observation

The density of dark matter in the centers of galaxies has been a mystery for decades. There is a strong discrepancy between the predictions of the simulations (a high density) and that which is observed (a low value). Astronomers have put forward many types of mechanisms to resolve this major disagreement.

In this article, the researchers have shown, using basic physical principles, that the observations can be reproduced on the assumption that the dark matter is in equilibrium, i.e., that it has maximum entropy. The consequences of this result could be very important because they indicate that the dark matter has interchanged energy with itself and/or with the remaining 'normal' (baryonic) matter.

"The fact that equilibrium has been reached in such a short time, compared with the age of the Universe, could be the result of a type of interaction between dark matter and normal matter in addition to gravity," suggests Ignacio Trujillo, an IAC researcher and a co-author of this article. "The exact nature of this mechanism needs to be explored, but the consequences could be fascinating to understand just what is this component which dominates the total amount of matter in the Universe."


New experiment to probe dark matter interactions

This schematic shows the motion of a pulsar falling in the Milky Way’s gravitational field. The yellow arrow indicates motion due to the gravity of normal matter while the white arrow shows motion caused by the dark matter in and around the galaxy. A new experiment is designed to find out if a possible “fifth force” is working with the gravity generated by dark matter. Image: R. Hurt (SSC), JPL-Caltech, NASA and pulsar image by NASA

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 ordinary matter, like the Earth itself whose composition is dominated by iron (32%), oxygen (30%), silicon (15%) and magnesium (14%). 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% 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 centimetre 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 free fall. 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 pulsar 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 matter.”

“To make these tests even better, we are busily searching for suitable pulsars near large amounts of expected dark matter”, 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.


New dark matter map reveals cosmic mystery

The results are a surprise because they show that it is slightly smoother and more spread out than the current best theories predict.

The observation appears to stray from Einstein's theory of general relativity - posing a conundrum for researchers.

The results have been published by the Dark Energy Survey Collaboration.

Dark Matter is an invisible substance that permeates space. It accounts for 80% of the matter in the Universe.

Astronomers were able to work out where it was because it distorts light from distant stars. The greater the distortion, the greater the concentration of dark matter.

Dr Niall Jeffrey, of École Normale Supérieure, in Paris, who pieced the map together, said that the result posed a "real problem" for physics.

"If this disparity is true then maybe Einstein was wrong," he told BBC News. "You might think that this is a bad thing, that maybe physics is broken. But to a physicist, it is extremely exciting. It means that we can find out something new about the way the Universe really is."

Prof Carlos Frenk, of Durham University, who was one of the scientists that built on the work of Albert Einstein and others to develop the current cosmological theory, said he had mixed emotions on hearing the news.

"I spent my life working on this theory and my heart tells me I don't want to see it collapse. But my brain tells me that the measurements were correct, and we have to look at the possibility of new physics," said Prof Frenk.

"Then my stomach cringes, because we have no solid grounds to explore because we have no theory of physics to guide us. It makes me very nervous and fearful, because we are entering a completely unknown domain and who knows what we are going to find."

Using the Victor M Blanco telescope in Chile, the team behind the new work analysed 100 million galaxies.

The map shows how dark matter sprawls across the Universe. The black areas are vast areas of nothingness, called voids, where the laws of physics might be different. The bright areas are where dark matter is concentrated. They are called "halos" because right in the centre is where our reality exists. In their midst are galaxies like our own Milky Way, shining brightly like tiny gems on a vast cosmic web.

According to Dr Jeffrey, who is also part of a department at University College London, the map, clearly shows that galaxies are part of a larger invisible structure.

"No one in the history of humanity has been able to look out into space and see where dark matter is to such an extent. Astronomers have been able to build pictures of small patches, but we have unveiled vast new swathes which show much more of its structure. For the first time we can see the Universe in a different way."

But the new dark matter map is not showing quite what astronomers expected. They have an accurate idea of the distribution of matter 350, 000 years after the Big Bang, from a European Space Agency orbiting observatory called Planck. It measured the radiation still present from that moment, called the cosmic microwave background, or more poetically, the "afterglow of creation".

Drawing on the ideas of Einstein, astronomers, such as Prof Frenk, developed a model to calculate how matter should disperse over the next 13.8bn years to the present day. But the actual observations from the new map are out by a few per cent - it shows that matter is slightly too evenly spread.

As a result, Prof Frenk thinks there may be big changes afoot in our understanding of the cosmos.

"We may have uncovered something really fundamental about the fabric of the Universe. The current theory rests on very sketchy pillars made of sand. And what we may be seeing is the collapse of one of those pillars."

But others, such as Prof Ofer Lahav, of University College London, have a more conservative view.

"The big question is whether Einstein's theory is perfect. It seems to pass every test but with some deviations here and there. Maybe the astrophysics of the galaxies just needs some tweaks. In the history of cosmology there are examples where problems went away, but also examples when the thinking shifted. It will be fascinating to see if the current 'tension' in Cosmology will lead to a new paradigm shift," he said.

The DES collaboration consists of over 400 scientists from 25 institutions in seven countries.


2 Answers 2

There's lots of crazy ways you can go about modifying gravity in the hope of explaining dark matter, but the point is that none of these modifications so far have worked. None of them properly describe our observations (this is what we should care about most), and the best current fit is simple CDM. If you want to propose something else, you really need to have a solid, consistent theory which makes specific predictions and fits with observation too.

Your statement/question, "tell me why it's wrong" isn't a good way of phrasing things. You really need to present a theory and calculate what it predicts, rather than having a vague idea and asking others to falsify it. People work on a huge plethora of ideas - the reason they're not all accepted as correct a priori is because they haven't been shown to be. The burden of proof is on the person presenting new ideas, not the rest of the scientific community to prove them wrong*.

I don't understand why Dark Matter or MOND/"gravity works different on bigger scales" are the only options to explain the observational data.

You're asking why theory X works at fitting the data but theory Y doesn't? This sounds more like a philosophical question.

---Edit---
Lastly, to tie in with your second question, we know the propagating degrees of freedom in GR: there are 2 of them, the two tensor modes (two polarisations of gravitational waves). We understand them quite well, especially since the detection of gravitational waves. They don't behave in the way you seem to be describing here.

In modifications you can have more propagating degrees of freedom, and/or scalar/vector modes, but these are already quite constrained such as by the CMB. In terms of the whole landscape of possible theories and their predictions, I'm not sure anybody can rule out things that behave like dark matter in some way, but it hasn't yet been put forward in a convincing way.

*Of course people do spend a lot of time finding problems & inconsistencies in physical theories, but that's not what we're talking about here.


Dark matter is slowing the spin of the Milky Way's galactic bar

The spin of the Milky Way's galactic bar, which is made up of billions of clustered stars, has slowed by about a quarter since its formation, according to a new study by researchers at University College London and the University of Oxford

University College London

IMAGE: Artist's conception of the Milky Way galaxy. view more

The spin of the Milky Way's galactic bar, which is made up of billions of clustered stars, has slowed by about a quarter since its formation, according to a new study by researchers at University College London (UCL) and the University of Oxford.

For 30 years, astrophysicists have predicted such a slowdown, but this is the first time it has been measured.

The researchers say it gives a new type of insight into the nature of dark matter, which acts like a counterweight slowing the spin.

In the study, published in the Monthly Notices of the Royal Astronomical Society, researchers analysed Gaia space telescope observations of a large group of stars, the Hercules stream, which are in resonance with the bar - that is, they revolve around the galaxy at the same rate as the bar's spin.

These stars are gravitationally trapped by the spinning bar. The same phenomenon occurs with Jupiter's Trojan and Greek asteroids, which orbit Jupiter's Lagrange points (ahead and behind Jupiter). If the bar's spin slows down, these stars would be expected to move further out in the galaxy, keeping their orbital period matched to that of the bar's spin.

The researchers found that the stars in the stream carry a chemical fingerprint - they are richer in heavier elements (called metals in astronomy), proving that they have travelled away from the galactic centre, where stars and star-forming gas are about 10 times as rich in metals compared to the outer galaxy.

Using this data, the team inferred that the bar - made up of billions of stars and trillions of solar masses - had slowed down its spin by at least 24% since it first formed.

Co-author Dr Ralph Schoenrich (UCL Physics & Astronomy) said: "Astrophysicists have long suspected that the spinning bar at the centre of our galaxy is slowing down, but we have found the first evidence of this happening.

"The counterweight slowing this spin must be dark matter. Until now, we have only been able to infer dark matter by mapping the gravitational potential of galaxies and subtracting the contribution from visible matter.

"Our research provides a new type of measurement of dark matter - not of its gravitational energy, but of its inertial mass (the dynamical response), which slows the bar's spin."

Co-author and PhD student Rimpei Chiba, of the University of Oxford, said: "Our finding offers a fascinating perspective for constraining the nature of dark matter, as different models will change this inertial pull on the galactic bar.

"Our finding also poses a major problem for alternative gravity theories - as they lack dark matter in the halo, they predict no, or significantly too little slowing of the bar."

The Milky Way, like other galaxies, is thought to be embedded in a 'halo' of dark matter that extends well beyond its visible edge.

Dark matter is invisible and its nature is unknown, but its existence is inferred from galaxies behaving as if they were shrouded in significantly more mass than we can see. There is thought to be about five times as much dark matter in the Universe as ordinary, visible matter.

Alternative gravity theories such as modified Newtonian dynamics reject the idea of dark matter, instead seeking to explain discrepancies by tweaking Einstein's theory of general relativity.

The Milky Way is a barred spiral galaxy, with a thick bar of stars in the middle and spiral arms extending through the disc outside the bar. The bar rotates in the same direction as the galaxy.

The research received support from the Royal Society, the Takenaka Scholarship Foundation, and the Science and Technology Facilities Council (STFC).

Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.


A New Experiment to Understand Dark Matter

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 (2018 June 15 issue).

Schematic image of a pulsar, falling in the gravitational field of the Milky Way. The two arrows indicate the direction . [more]

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“?

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“?

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 ordinary matter, like the Earth itself whose composition is dominated by iron (32%), oxygen (30%), silicon (15%) and magnesium (14%). 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% 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 free fall. 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 pulsar 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 matter.”

“To make these tests even better, we are busily searching for suitable pulsars near large amounts of expected dark matter”, 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.

BlackHoleCam is an ERC-funded Synergy project to finally image, measure and understand astrophysical black holes. Its principal investigators, Heino Falcke, Michael Kramer and Luciano Rezzolla, test fundamental predictions of Einstein’s theory of General Relativity. The BlackHoleCam team members are active partners of the global Event Horizon Telescope Consortium (EHTC).