A galactic test to clarify the existence of dark matter

June 26, 2018, University of Bonn
The distribution of dark matter (above) and stars (below). Credit: © E. Garaldi, C. Porciani, E. Romano-Díaz/University of Bonn for the ZOMG Kollaboration

Researchers at the University of Bonn and the University of California at Irvine used sophisticated computer simulations to devise a test that could answer a burning question in astrophysics: does dark matter actually exist? Or does Newton's gravitational law need to be modified? The new study, now published in the Physical Review Letters, shows that the answer is hidden in the motion of the stars within small satellite galaxies swirling around the Milky Way.

Using one of the fastest supercomputers in the world, the scientists have simulated the distribution of the so-called satellite "dwarf" galaxies. These are that orbit larger galaxies like the Milky Way or Andromeda.

The researchers focused on a relationship called radial acceleration relation (RAR). In disk galaxies, stars move in circular orbits around the galactic center. The acceleration that forces them to change direction is caused by the attraction of matter in the galaxy. The RAR describes the relationship between this acceleration and the one caused by the visible matter only. It provides an insight into the structure of galaxies and their matter distribution.

"We have now simulated, for the first time, the RAR of on the assumption that exists," explains Prof. Dr. Cristiano Porciani of the Argelander Institute for Astronomy at the University of Bonn. "It turned out that they behave as scaled-down versions of larger galaxies." But what if there is no dark matter and instead gravity works differently than Newton thought? "In this case, the RAR of dwarf galaxies depends strongly on the distance to their parent galaxy, while this does not happen if dark matter exists," explains researcher Emilio Romano-Díaz.

This difference makes the satellites a powerful probe for testing whether dark matter really exists. The Gaia spacecraft, which was launched by the European Space Agency (ESA) in 2013, could provide an answer. It was designed to study the stars in the Milky Way and its satellite galaxies in unprecedented detail and has collected a large amount of data.

However, it will probably take years to analyze the data. "Individual measurements are not enough to test the small differences we have found in our simulations," explains doctoral student Enrico Garaldi. "But repeatedly examining the same stars improves the measurements every time. Sooner or later, it should be possible to determine whether the dwarf galaxies behave as in a universe with dark matter—or not."

The cement that holds galaxies together

This question is one of the most pressing issues in cosmology today. The existence of dark matter was suggested more than 80 years ago by the Swiss astronomer Fritz Zwicky. He realized that galaxies move so fast within galaxy clusters that they should actually drift apart. He therefore postulated the presence of invisible matter which, due to its mass, exerts sufficient gravity to keep galaxies on their observed orbits. In the 1970s, his U.S. colleague Vera Rubin discovered a similar phenomenon in spiral galaxies like the Milky Way—they rotate so quickly that the centrifugal force should tear them apart if only visible matter was present.

Today, most physicists are convinced that dark matter makes up about 80 percent of the mass in the universe. Since it does not interact with light, it is invisible to telescopes. Yet, assuming its existence provides an excellent fit to a number of other observations—such as the distribution of background radiation, the afterglow of the Big Bang. Dark matter also provides a good explanation for the arrangement and formation rate of galaxies in the universe. However, despite numerous experimental efforts, there is no direct proof that dark matter exists. This led astronomers to the hypothesis that the gravitational force itself might behave differently than previously thought. According to the theory called modified Newtonian dynamics (MOND), the attraction between two masses obeys Newton's laws only up to a certain point. At very small accelerations, such as those prevailing in galaxies, gravity becomes considerably stronger. Therefore, do not tear apart due to their rotational speed and the MOND theory can dispense with dark matter.

The new study opens up the possibility for astronomers to test these two hypotheses in an unprecedented regime.

Explore further: Dancing with giants: dynamics of dwarf satellite galaxies

More information: Enrico Garaldi et al, Radial Acceleration Relation of ΛCDM Satellite Galaxies, Physical Review Letters (2018). DOI: 10.1103/PhysRevLett.120.261301

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5 / 5 (5) Jun 26, 2018
Hasn't the Bullet Cluster confirmed dark matter exists already?
5 / 5 (6) Jun 26, 2018
Hasn't the Bullet Cluster confirmed dark matter exists already?

I think it true to say that most astrophysicists think that what you say is true. However, there are still a few dissenting voices. For instance, see this article that discusses the ideas of Stacy McGaugh (a highly respected astrophysicist); https://galileosp...-matter/ .
3 / 5 (4) Jun 26, 2018
RK, not they way you and me generally use the word 'confirm'. Here in our neighborhood, we can 'see' that it is overcast but until it rains or the sun burns through? We cannot 'confirm' what the weather will be later in the day.

The data collected to date for the Bullet Cluster, suggests that in this specific situation? The colliding masses of stars, gases and DM are detectable through galactic lensing. We have to have the aggregate mess facing us in the correct position to be visible to us by GL.

It takes tenaciously methodical observations of events umpteen parsecs away and occurring over umpteen aeons of time.

Always with the possibility that we may never have enough information to make an absolute determination. And knowing that someone else, observing some other place? May discover that what you called irreducibly correct? Their analysis of what they saw could be conflicting even contradicting your findings.

Now whatta yah do?
3.7 / 5 (7) Jun 26, 2018
Now whatta yah do?

Well, yah look at all thah options. Then yah look at all thah observations. Then yah make yah bet and put yah money on thah most likely winnah.

80 years ago the most likely explanation was thought to be a particle not interacting via EM. Now with much more data it is still thought to be most likely explanation, just even more so.

It's not quite a done deal yet, but if I had to wager some money I'd put it on Zwicky, not you.
5 / 5 (5) Jun 26, 2018
I thnik most people (and I) would agree with you. The recent gravitational wave/optical detection from the neutron star merger also puts another nail in the coffin of MOND.

However, the problem is that, because there is no physical motivation for MOND, it is possible to construct all kinds of alternatives, including the full range of TeVeS models which can be tweaked in a huge variety of ways.

So, as you say, I'd bet against MOND, but I also note that only a fool would put their shirt on it.
5 / 5 (3) Jun 26, 2018
I apologise for my response to you above being something of a non sequitur. My post began as a continuation of my earlier post to RedKiowa, but I then got myself a little confused.
3.7 / 5 (3) Jun 26, 2018
RK, not they way you and me generally use the word 'confirm'
Well you use it in the sense that, if you agree with it, then its confirmed.

Not quite the way it works willis.
1.8 / 5 (5) Jun 26, 2018
So otto, your astrologer comes to you and tells you that they can 'confirm' that the moon Charon controls the aspects of your life.

Yep, confirmed because if you can't believe your own astrologer, who can you believe?

Who needs to bother with exhaustive observations. Expensive technology. Repetitious experimentation.
Or verified data.

Cause, hey! It's been confirmed! In a pop-science article fudged from a single scientific claim.

Though I gotta admit otto, that your astrologer is a shrewd judge of character. Or perhaps just a shrew?

"Ask not for whom the boatman cometh... He comes for thee!"
1.8 / 5 (5) Jun 27, 2018
Mistaken GR based concepts Dark Matter and Dark Energy were resolved in the 1995 break-through of Suntola's Dynamic Universe (DU) bounce theory based on the motion and gravitational energy balance of the total mass M of 3-D space in closed 4-D Riemannian sphere, Einstein's starting point of GR. Big Bang and standard cosmology theories failed to interconnect the expansion speed C4 of 4-radius (or String) R4 and global Newtonian time T4 to explain true variable speed C of light in space direction. GR confused it with the apparent constant locally observable speed C_obs in EM bound matter (as in interferometers of LIGO). This resulted in blunders of GR/QM foundations showing up in recent global scales of SN1a and GW data with premature 2011 Nobel 'confirmed' beliefs about DM/DE, see Suntola DU web site for literature. C4 explains the discussed motions along the space direction orthogonal to R4, in which direction universe is observed flat as in conformal photogrammetric mapping.
3 / 5 (4) Jun 27, 2018
Sorry annoyingmousie but until your crank woo can blow up a city? It just ain't acceptable as science.

Anyone trying to steer the gullible to these fraudulent sites? Should be required to supply their police record as a career swindler.
1 / 5 (2) Jun 27, 2018
Congratulations nwillsj authority in physics protecting gullible followers from cranks to blow up your world - you are a super fast learner to digest the profound unified DU theory that Einstein attempted to resolve during his past 30 years of life. Did you get as far as Feynman lectures to identify and understand the biggest mystery of science, energy balance at the bouncing era of DU as Big Bang replacement? DU resolved this mystery at every clock cycle by rethinking the physics foundations. DU explains some major mistakes in the foundations of GR/QM theories with practical proofs since ancient times. Latest proofs include the premature Nobel level interpretations of intergalactic SN1a and GW data where cosmic GR based theory starts failing. I agree with your career warning since Galileo and Copernicus times of speaking against religious leaders or their mob with shared beliefs. You may want to google the latest winner of Millennium prize in ALD nano science and PFS web site.
Da Schneib
5 / 5 (4) Jun 28, 2018
Hasn't the Bullet Cluster confirmed dark matter exists already?

@RedKiowa, good question. I see you got a lot of stars for asking.

One of the things about science is that what we call proof, in the mathematical sense, is rarely if ever available. But we can get pretty good evidence, and you're right, the Bullet Cluster is pretty good evidence all by itself. There's a lot more evidence than that, though: galactic rotation curves, galaxy cluster dynamics, gravitational lensing, and very small unevenness (called "anisotropy") in the cosmic microwave background radiation, all point to dark matter. Still, the measurements this article is about will provide a particularly sensitive test, and one that can differentiate between dark matter and MOND, and it's just a matter of collecting enough data and then analyzing it, and the data will already be there anyway, so it's worth doing.
Da Schneib
5 / 5 (4) Jun 28, 2018
Of course, we'll keep looking for dark matter particles too. If we can find and identify a dark matter particle that will account for the effects we see, that will put it "over the top" pretty much. But even then, as @RNP notes, there will almost certainly be dissenters even among professional physicists. There pretty much always are, and their place in science is respected because sometimes they are right and we don't find out until later. But don't confuse these iconoclasts with the trolls who infest science sites. You can tell the difference because the dissenters are generally respectful, whereas the trolls are just looking for attention or running some scam.
4.2 / 5 (5) Jun 29, 2018
And don't forget MACS J0025.4-1222, the Bullet Cluster is not the only one.

1 / 5 (1) Jun 29, 2018
The mystery of modern epicycle blunders (DE,DM etc) in the BB vs. DU world view is the repeat of 400-500 year old Earth vs Sun centered controversy caused by the 'missing dimension' of GRT based spacetime coordinate frame where the metric dimension R4 was fused with the time T4 and the fateful flaw of constant C. The constancy of C was repeatedly 'confirmed' on local energy frames by the practical April joke by Nature. It also fooled some starting points of traditional quantum theories. DU preserved the metric 4th dimension of R4 and kept the absolute or scalar Newtonian time T4 as the 5th dimension using ONE single universal balancing constraint of a closed energy system in 4-sphere. It is INVARIANT to distribution of dark vs visible matter (in terms of MASS concept). GRT based cosmology fails at small values of R4 near CMB where C4 and C were about 10 times C_today explaining 'mysteries' in intergalactic data such as SN1a, GW and galactic rotation curves due to R4 momentum component.

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