Five years ago, San Francisco State researcher Andisheh Mahdavi and his colleagues observed an unexpected dark core at the center of Abell 520, a cosmic "train wreck" of galaxy clusters. With new space-based telescope observations, they have confirmed that the core really does exist. But they are no closer to explaining why it is there.

When galaxy clusters crash into each other, the bright matter of galaxies sticks together with the mysterious substance called dark matter, leaving behind hot gases. Or at least that is what astronomers have observed in similar cosmic wrecks like the Bullet Cluster. But Myungkook James Jee of the University of California, Davis, Mahdavi and their colleagues say Abell 520 has a definite -- but bewildering -- dark matter core that is completely separated from its usual bright partners.

"We tried to come up with models that would explain this, but there were not any good models," said Mahdavi, an assistant professor in the Department of Physics and Astronomy. "There is no way that you could have cold dark matter piling up like this in a region with so few galaxies."

The researchers first identified the dark core in 2007 using a technique called gravitational lensing. Even though the dark core isn't visible, astronomers can get an idea of its location and size by observing how light from galaxies behind it is distorted by the core's gravitational pull.

"We cannot see dark matter because it does not radiate. What we see is the 'effect' of dark matter," Jee explained. "It's similar to how we cannot see wind directly, but we can tell the presence of wind by looking at the vibration of leaves on a tree."

In this case, the galaxies behind the dark core are the tree leaves. But the 2007 observations came in part from ground-based telescopes, which can detect only a few of the galaxies lurking behind Abell 520. The Earth's atmosphere also distorts the view from the ground, "like looking at a tree inside a house through a frosty window," Jee said.

The researchers decided that they needed further observations from the space-based Hubble Telescope to confirm the dark core's presence. "For every ten galaxies that we were able to see from the ground, we can see 100 from space with the Hubble," they noted, "for a total of about 4000 galaxies from space versus 400 from the ground."

The 2007 study was "a result that basically everyone wished would go away," Mahdavi said, but the new observations published in the *Astrophysical Journal* show "without a doubt that there is a dark matter concentration in that piece of the sky."

Their results do not put the mystery to rest, however, since the researchers also note in their study that there are no plausible scenarios yet to explain the existence of the dark core. In all other known collisions, bright galaxy matter and dark matter stay together.

Why is Abell 520 so different? It may be that our understanding of how galaxies grow and collide is incomplete, Mahdavi suggests. Alternatively, a new theory of dark matter interaction could be necessary to explain the mysterious core.

Mahdavi thinks that the first scenario is more likely, and that perhaps there are "some sort of freak initial conditions that would create this amount of dark matter."

"But the only way we understand how galaxies grow up is with supercomputer simulations," he noted. The simulations--which would include recreating galaxy cluster collisions under a variety of conditions -- help to calculate how likely it would be to spot an oddball like Abell 520. "My colleagues tell me the likelihood is nil, but now we have the responsibility to go and do the hard work to check the simulations," he said.

If the simulations don't turn up anything to show that Abell 520 is possible, Mahdavi said the mystery might be best left in the hands of particle physicists to revisit their theories about the nature and interactions of dark matter.

"I'm just as perplexed as I was back in 2007," he said. "It's a pretty disturbing observation to have out there.

**Explore further:**
Hubble provides new evidence for dark matter around small galaxies

## Lurker2358

"My text book and computer says it's impossible, so your observations must be wrong." - physicist.

the true culprit here continues to lie in properly calculating the GRAVITY of the ordinary matter, seeing as how the over-simplification of treating matter as point masses does not work. This only works when almost all of the mass in the system is in the larger object.

To even begin to model 1000 galaxies properly you need 100 trillion to 400 trillion point masses representing stars, and you must do all combinations of vector sums for each star's gravitational acceleration on each other star, which is 400 trillion vector sums, each having 400 trillion terms, and it must be done with partitions of time approaching infinitesmal.

Using the "center of mass" thing does not work, as it severely under-estimates the gravitational acceleration caused by ordinary matter, producing the illusion of Dark Matter.

## Lurker2358

But ignoring the control structures and loads, etc, it's 2E30 operations, PER partition of time, 3/4 of which are multiplications and divisions.

So to model a mere linear approximation of one second's worth of acceleration requires 2E30 calculations, even without control structures loads, and saves.

It would actually take 2 million years to run that calculation for a LINEAR APPROXIMATION of a 1 second partition worth of simulation on an 8 million core computer @ 4ghz per core, and again, ignoring actual "code". that's just idealism.

This is why your stupid model can't figure this out, and your simulations cannot predict the real world.

## HannesAlfven

Perhaps the simulation approach to physics is not all it's cracked up to be, eh?

## Lurker2358

It is standard practice to discard the "constant of integration" after performing an integral, favoring the solution where the constant equals zero.

If you take a non-zero constant of integration, you get solutions that potentially resolve some of the "missing mass", as well as explaining some other freaky effects.

i.e. the constant of integration, K, that should be left over when you integrate for acceleration appears to have units as.

GMK

since you can take out the G and M, treating them as constants when integrating acceleration over distance from CoG.

Since G has units (m^3/kg*s^-2), then GMK has units of (m^3/s^2).

What does that look like?

That is a measure of acceleration of the rate of expansion of a sphere.

(m^3/s) is an expansion rate.

(m^3/s^2) is an acceleration of an expansion.

GMK looks a lot like "Dark Energy".

But we have to ignore the constant of integration...or should we?

## GSwift7

I'm SO glad you figured it all out. Quick! Write a letter to MIT and let them know.

It's calculus and differential equations silly.

## Lurker2358

It does not work, and it's piss easy to prove it doesn't work.

You need vector sums no matter what you do anyway.

If you don't believe me, just take a system of a line of 5 adjacent balls of radius 1, and mass 1, which means each end of the line is 5m from the CoG. For simplicity, ignore G and M and just work with unitary expressions.

Now find gravitational acceleration for a particle at 5m from the center, i.e. at either end of the line.

If you do it individually and add vector sums, you get

A = 1 1/9 1/25 1/49 1/81 = 1.183864

continued...

## Lurker2358

If you use the INCORRECT center of gravity calculation, which is what models have always done, you get:

A = 5/25 = 0.2, under-estimating acceleration by a factor of nearly 600%

Ironically, this is the discrepancy almost identical to Dark Matter, if you imagine that the balls represents stellar systems distributed across the cross-section of a galaxy.

Dark energy supposedly represents 23% of the universe, while ordinary matter supposedly represents just 4%.

23%/4% = ~6.

Which is EXACTLY the error predicted by the above example.

If you increase the number of balls to 7, the error goes to:

A = 1 1/9 1/25 1/49 1/81 1/121 1/169 = 1.9804657

vs

A = 7/49 = 1/7 = .142857

Which is actually an even bigger error.

But it's ok, because Galaxies have a LOT of mass in the center, making up for this, but not enough.

Point is, the ENTIRE damn thing is a bad application of a theory

## Lurker2358

And then you can work on vector sums for balls that are not in a line, to simulate a disk with a big bulge in the center, and using the cosine to find the portion of the acceleration which should be in the direction of the center of the galaxy, and this again will be different value than the over-simplification crap.

I've shown this before in several different ways, and people don't pay it any attention, but it's the truth, and for the same reason the MOON is more strongly attracted to the Earth than to the Sun...

You MUST calculat every vector for every objects relationship to every other object.

Calculus and differentials do not work, because they assume a smooth distribution of mass for a differentiable curve.

## Lurker2358

They have stars and planets, but with huge voids of space between, which is not continuous matter distribution, but rather a STEP distribution, which therefore is not differentiable, because it cannot be represented by a continuous curve.

This produces acceleration vectors which interact both constructively and destructively, which is most certainly NOT being modeled properly by "everything orbits the center of mass at a velocity calculated by the center of masse's gravity" BS.

that's bad math. Very, very bad.

## GSwift7

lol, you really don't even know enough to understand what you are not understanding.

You have to use integrals to get this right. You can't use Newtonian math. You are so far away from being right that I don't know where to begin to explain.

You know we actually do a good approximation of the multibody problem inside every single GPS navigation system on the planet, right? It's just an approximation using a differential equations "trick" but it works. You are right that the math is too complex to work it out exactly, but you obviously don't understand why that is true. Where you are wrong is that it is not unreasonable to do a close approximation and simpley keep in mind that it will be off by some small amount. We do this all the time for many applications.

## bewertow

Your arrogance and stupidity is incredible

## dtyarbrough

## Tuxford

## Tuxford

This is not rocket science, just cosmology. See LaViolette's 'Subquantum Kinectics'.

## hylozoic

## Urgelt

"Dark Matter" is the term used for gravitational phenomena for which no visible matter can be observed.

Buried in the term is an assumption: that gravitational phenomena are always associated with matter, and matter with gravitational phenomena.

How confident are we that we're making a good assumption there?

Stupid question, you say? Eh, I'm sure you're right.

Yeah, they'll find the Higgs, show how it works, everything will be fine. No gravitational fields in nature will ever be found which aren't connected to and a property of matter. Invisible, tricky, half-sticky matter, often enough.

That creaking sound you hear is how my vertebrae always sound when I'm bending over backward to conform to the prevailing wisdom.

If they don't find the Higgs, though, the Standard Model's explanations will be looking a mite ragged. What then? How solid is that assumption?

## vidyunmaya

Three Tier- Three mode Spread needs to be understood with comprehension. My papers ar carnegie-Jan 2003 and STSCI-Astrophysical Lab Symposium-May 2003 clearly explains the concepts.

see Cosmology Review articles- Dec 1999 by me.Read more in my books-COSMOLOGY VEDAS INTERLINKS-BOOKS INFORMATION May 2011 ISBN:978-1-257-96228-0