Tag Archives: dark matter

Vera Rubin: Astronomy Pioneer

Vera Rubin passed away on December 26, 2016, aged 88. She was a pioneer in the male-dominated world of astronomy, notable for her original work on dark matter,  galaxy rotation and galaxy clumping.

From Popular Science:

Vera Rubin, who essentially created a new field of astronomy by discovering dark matter, was a favorite to win the Nobel Prize in physics for years. But she never received her early-morning call from Stockholm. On Sunday, she died at the age of 88.

Rubin’s death would sadden the scientific community under the best of circumstances. Countless scientists were inspired by her work. Countless scientists are researching questions that wouldn’t exist if not for her work. But her passing brings another blow: The Nobel Prize cannot be awarded posthumously. The most prestigious award in physics will never be bestowed upon a woman who was inarguably deserving.

In the 1960s and ’70s, Rubin and her colleague Kent Ford found that the stars within spiral galaxies weren’t behaving as the laws of physics dictated that they should. This strange spinning led her and others to conclude that some unseen mass must be influencing the galactic rotation. This unknown matter—now dubbed dark matter—outnumbers the traditional stuff by at least five to one. This is a big deal.

Read more here.

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Spacetime Without the Time

anti-de-sitter-spaceSince they were first dreamed up explanations of the very small (quantum mechanics) and the very large (general relativity) have both been highly successful at describing their respective spheres of influence. Yet, these two descriptions of our physical universe are not compatible, particularly when it comes to describing gravity. Indeed, physicists and theorists have struggled for decades to unite these two frameworks. Many agree that we need a new theory (of everything).

One new idea, from theorist Erik Verlinde of the University of Amsterdam, proposes that time is an emergent construct (it’s not a fundamental building block) and that dark matter is an illusion.

From Quanta:

Theoretical physicists striving to unify quantum mechanics and general relativity into an all-encompassing theory of quantum gravity face what’s called the “problem of time.”

In quantum mechanics, time is universal and absolute; its steady ticks dictate the evolving entanglements between particles. But in general relativity (Albert Einstein’s theory of gravity), time is relative and dynamical, a dimension that’s inextricably interwoven with directions x, y and z into a four-dimensional “space-time” fabric. The fabric warps under the weight of matter, causing nearby stuff to fall toward it (this is gravity), and slowing the passage of time relative to clocks far away. Or hop in a rocket and use fuel rather than gravity to accelerate through space, and time dilates; you age less than someone who stayed at home.

Unifying quantum mechanics and general relativity requires reconciling their absolute and relative notions of time. Recently, a promising burst of research on quantum gravity has provided an outline of what the reconciliation might look like — as well as insights on the true nature of time.

As I described in an article this week on a new theoretical attempt to explain away dark matter, many leading physicists now consider space-time and gravity to be “emergent” phenomena: Bendy, curvy space-time and the matter within it are a hologram that arises out of a network of entangled qubits (quantum bits of information), much as the three-dimensional environment of a computer game is encoded in the classical bits on a silicon chip. “I think we now understand that space-time really is just a geometrical representation of the entanglement structure of these underlying quantum systems,” said Mark Van Raamsdonk, a theoretical physicist at the University of British Columbia.

Researchers have worked out the math showing how the hologram arises in toy universes that possess a fisheye space-time geometry known as “anti-de Sitter” (AdS) space. In these warped worlds, spatial increments get shorter and shorter as you move out from the center. Eventually, the spatial dimension extending from the center shrinks to nothing, hitting a boundary. The existence of this boundary — which has one fewer spatial dimension than the interior space-time, or “bulk” — aids calculations by providing a rigid stage on which to model the entangled qubits that project the hologram within. “Inside the bulk, time starts bending and curving with the space in dramatic ways,” said Brian Swingle of Harvard and Brandeis universities. “We have an understanding of how to describe that in terms of the ‘sludge’ on the boundary,” he added, referring to the entangled qubits.

The states of the qubits evolve according to universal time as if executing steps in a computer code, giving rise to warped, relativistic time in the bulk of the AdS space. The only thing is, that’s not quite how it works in our universe.

Here, the space-time fabric has a “de Sitter” geometry, stretching as you look into the distance. The fabric stretches until the universe hits a very different sort of boundary from the one in AdS space: the end of time. At that point, in an event known as “heat death,” space-time will have stretched so much that everything in it will become causally disconnected from everything else, such that no signals can ever again travel between them. The familiar notion of time breaks down. From then on, nothing happens.

On the timeless boundary of our space-time bubble, the entanglements linking together qubits (and encoding the universe’s dynamical interior) would presumably remain intact, since these quantum correlations do not require that signals be sent back and forth. But the state of the qubits must be static and timeless. This line of reasoning suggests that somehow, just as the qubits on the boundary of AdS space give rise to an interior with one extra spatial dimension, qubits on the timeless boundary of de Sitter space must give rise to a universe with time — dynamical time, in particular. Researchers haven’t yet figured out how to do these calculations. “In de Sitter space,” Swingle said, “we don’t have a good idea for how to understand the emergence of time.”

Read the entire article here.

Image: Image of (1 + 1)-dimensional anti-de Sitter space embedded in flat (1 + 2)-dimensional space. The t1- and t2-axes lie in the plane of rotational symmetry, and the x1-axis is normal to that plane. The embedded surface contains closed timelike curves circling the x1 axis, though these can be eliminated by “unrolling” the embedding (more precisely, by taking the universal cover). Courtesy: Krishnavedala. Wikipedia. Creative Commons Attribution-Share Alike 3.0.

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Dark Matter May Cause Cancer and Earthquakes

Abell 1689

Leave aside the fact that there is no direct evidence for the existence of dark matter. In fact, theories that indirectly point to its existence seem rather questionable as well. That said, cosmologists are increasingly convinced that dark matter’s gravitational effects can be derived from recent observations of gravitationally lenses galaxy clusters. Some researchers postulate that this eerily murky non-substance — it doesn’t interact with anything in our visible universe except, perhaps, gravity — may be a cause for activities much closer to home. All very interesting.

From NYT:

Earlier this year, Dr. Sabine Hossenfelder, a theoretical physicist in Stockholm, made the jarring suggestion that dark matter might cause cancer. She was not talking about the “dark matter” of the genome (another term for junk DNA) but about the hypothetical, lightless particles that cosmologists believe pervade the universe and hold the galaxies together.

Though it has yet to be directly detected, dark matter is presumed to exist because we can see the effects of its gravity. As its invisible particles pass through our bodies, they could be mutating DNA, the theory goes, adding at an extremely low level to the overall rate of cancer.

It was unsettling to see two such seemingly different realms, cosmology and oncology, suddenly juxtaposed. But that was just the beginning. Shortly after Dr. Hossenfelder broached her idea in an online essay, Michael Rampino, a professor at New York University, added geology and paleontology to the picture.

Dark matter, he proposed in an article for the Royal Astronomical Society, is responsible for the mass extinctions that have periodically swept Earth, including the one that killed the dinosaurs.

His idea is based on speculations by other scientists that the Milky Way is sliced horizontally through its center by a thin disk of dark matter. As the sun, traveling around the galaxy, bobs up and down through this darkling plane, it generates gravitational ripples strong enough to dislodge distant comets from their orbits, sending them hurtling toward Earth.

An earlier version of this hypothesis was put forth last year by the Harvard physicists Lisa Randall and Matthew Reece. But Dr. Rampino has added another twist: During Earth’s galactic voyage, dark matter accumulates in its core. There the particles self-destruct, generating enough heat to cause deadly volcanic eruptions. Struck from above and below, the dinosaurs succumbed.

It is surprising to see something as abstract as dark matter take on so much solidity, at least in the human mind. The idea was invented in the early 1930s as a theoretical contrivance — a means of explaining observations that otherwise didn’t make sense.

Galaxies appear to be rotating so fast that they should have spun apart long ago, throwing off stars like sparks from a Fourth of July pinwheel. There just isn’t enough gravity to hold a galaxy together, unless you assume that it hides a huge amount of unseen matter — particles that neither emit or absorb light.

Some mavericks propose alternatives, attempting to tweak the equations of gravity to account for what seems like missing mass. But for most cosmologists, the idea of unseeable matter has become so deeply ingrained that it has become almost impossible to do without it.

Said to be five times more abundant than the stuff we can see, dark matter is a crucial component of the theory behind gravitational lensing, in which large masses like galaxies can bend light beams and cause stars to appear in unexpected parts of the sky.

That was the explanation for the spectacular observation of an “Einstein Cross” reported last month. Acting like an enormous lens, a cluster of galaxies deflected the light of a supernova into four images — a cosmological mirage. The light for each reflection followed a different path, providing glimpses of four different moments of the explosion.

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But not even a galactic cluster exerts enough gravity to bend light so severely unless you postulate that most of its mass consists of hypothetical dark matter. In fact, astronomers are so sure that dark matter exists that they have embraced gravitational lensing as a tool to map its extent.

Dark matter, in other words, is used to explain gravitational lensing, and gravitational lensing is taken as more evidence for dark matter.

Some skeptics have wondered if this is a modern-day version of what ancient astronomers called “saving the phenomena.” With enough elaborations, a theory can account for what we see without necessarily describing reality. The classic example is the geocentric model of the heavens that Ptolemy laid out in the Almagest, with the planets orbiting Earth along paths of complex curlicues.

Ptolemy apparently didn’t care whether his filigrees were real. What was important to him was that his model worked, predicting planetary movements with great precision.

Modern scientists are not ready to settle for such subterfuge. To show that dark matter resides in the world and not just in their equations, they are trying to detect it directly.

Though its identity remains unknown, most theorists are betting that dark matter consists of WIMPs — weakly interacting massive particles. If they really exist, it might be possible to glimpse them when they interact with ordinary matter.

Read the entire article here.

Image: Abell 1689 galaxy cluster. Courtesy ofNASA, ESA, and D. Coe (NASA JPL/Caltech and STScI).

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95.5 Percent is Made Up and It’s Dark

Petrarch_by_Bargilla

Physicists and astronomers observe the very small and the very big. Although they are focused on very different areas of scientific endeavor and discovery, they tend to agree on one key observation: 95.5 of the cosmos is currently invisible to us. That is, only around 4.5 percent of our physical universe is made up of matter or energy that we can see or sense directly through experimental interaction. The rest, well, it’s all dark — so-called dark matter and dark energy. But nobody really knows what or how or why. Effectively, despite tremendous progress in our understanding of our world, we are still in a global “Dark Age”.

From the New Scientist:

TO OUR eyes, stars define the universe. To cosmologists they are just a dusting of glitter, an insignificant decoration on the true face of space. Far outweighing ordinary stars and gas are two elusive entities: dark matter and dark energy. We don’t know what they are… except that they appear to be almost everything.

These twin apparitions might be enough to give us pause, and make us wonder whether all is right with the model universe we have spent the past century so carefully constructing. And they are not the only thing. Our standard cosmology also says that space was stretched into shape just a split second after the big bang by a third dark and unknown entity called the inflaton field. That might imply the existence of a multiverse of countless other universes hidden from our view, most of them unimaginably alien – just to make models of our own universe work.

Are these weighty phantoms too great a burden for our observations to bear – a wholesale return of conjecture out of a trifling investment of fact, as Mark Twain put it?

The physical foundation of our standard cosmology is Einstein’s general theory of relativity. Einstein began with a simple observation: that any object’s gravitational mass is exactly equal to its resistance to accelerationMovie Camera, or inertial mass. From that he deduced equations that showed how space is warped by mass and motion, and how we see that bending as gravity. Apples fall to Earth because Earth’s mass bends space-time.

In a relatively low-gravity environment such as Earth, general relativity’s effects look very like those predicted by Newton’s earlier theory, which treats gravity as a force that travels instantaneously between objects. With stronger gravitational fields, however, the predictions diverge considerably. One extra prediction of general relativity is that large accelerating masses send out tiny ripples in the weave of space-time called gravitational waves. While these waves have never yet been observed directly, a pair of dense stars called pulsars, discovered in 1974, are spiralling in towards each other just as they should if they are losing energy by emitting gravitational waves.

Gravity is the dominant force of nature on cosmic scales, so general relativity is our best tool for modelling how the universe as a whole moves and behaves. But its equations are fiendishly complicated, with a frightening array of levers to pull. If you then give them a complex input, such as the details of the real universe’s messy distribution of mass and energy, they become effectively impossible to solve. To make a working cosmological model, we make simplifying assumptions.

The main assumption, called the Copernican principle, is that we are not in a special place. The cosmos should look pretty much the same everywhere – as indeed it seems to, with stuff distributed pretty evenly when we look at large enough scales. This means there’s just one number to put into Einstein’s equations: the universal density of matter.

Einstein’s own first pared-down model universe, which he filled with an inert dust of uniform density, turned up a cosmos that contracted under its own gravity. He saw that as a problem, and circumvented it by adding a new term into the equations by which empty space itself gains a constant energy density. Its gravity turns out to be repulsive, so adding the right amount of this “cosmological constant” ensured the universe neither expanded nor contracted. When observations in the 1920s showed it was actually expanding, Einstein described this move as his greatest blunder.

It was left to others to apply the equations of relativity to an expanding universe. They arrived at a model cosmos that grows from an initial point of unimaginable density, and whose expansion is gradually slowed down by matter’s gravity.

This was the birth of big bang cosmology. Back then, the main question was whether the expansion would ever come to a halt. The answer seemed to be no; there was just too little matter for gravity to rein in the fleeing galaxies. The universe would coast outwards forever.

Then the cosmic spectres began to materialise. The first emissary of darkness put a foot in the door as long ago as the 1930s, but was only fully seen in the late 1970s when astronomers found that galaxies are spinning too fast. The gravity of the visible matter would be too weak to hold these galaxies together according to general relativity, or indeed plain old Newtonian physics. Astronomers concluded that there must be a lot of invisible matter to provide extra gravitational glue.

The existence of dark matter is backed up by other lines of evidence, such as how groups of galaxies move, and the way they bend light on its way to us. It is also needed to pull things together to begin galaxy-building in the first place. Overall, there seems to be about five times as much dark matter as visible gas and stars.

Dark matter’s identity is unknown. It seems to be something beyond the standard model of particle physics, and despite our best efforts we have yet to see or create a dark matter particle on Earth (see “Trouble with physics: Smashing into a dead end”). But it changed cosmology’s standard model only slightly: its gravitational effect in general relativity is identical to that of ordinary matter, and even such an abundance of gravitating stuff is too little to halt the universe’s expansion.

The second form of darkness required a more profound change. In the 1990s, astronomers traced the expansion of the universe more precisely than ever before, using measurements of explosions called type 1a supernovae. They showed that the cosmic expansion is accelerating. It seems some repulsive force, acting throughout the universe, is now comprehensively trouncing matter’s attractive gravity.

This could be Einstein’s cosmological constant resurrected, an energy in the vacuum that generates a repulsive force, although particle physics struggles to explain why space should have the rather small implied energy density. So imaginative theorists have devised other ideas, including energy fields created by as-yet-unseen particles, and forces from beyond the visible universe or emanating from other dimensions.

Whatever it might be, dark energy seems real enough. The cosmic microwave background radiation, released when the first atoms formed just 370,000 years after the big bang, bears a faint pattern of hotter and cooler spots that reveals where the young cosmos was a little more or less dense. The typical spot sizes can be used to work out to what extent space as a whole is warped by the matter and motions within it. It appears to be almost exactly flat, meaning all these bending influences must cancel out. This, again, requires some extra, repulsive energy to balance the bending due to expansion and the gravity of matter. A similar story is told by the pattern of galaxies in space.

All of this leaves us with a precise recipe for the universe. The average density of ordinary matter in space is 0.426 yoctograms per cubic metre (a yoctogram is 10-24 grams, and 0.426 of one equates to about 250 protons), making up 4.5 per cent of the total energy density of the universe. Dark matter makes up 22.5 per cent, and dark energy 73 per cent (see diagram). Our model of a big-bang universe based on general relativity fits our observations very nicely – as long as we are happy to make 95.5 per cent of it up.

Arguably, we must invent even more than that. To explain why the universe looks so extraordinarily uniform in all directions, today’s consensus cosmology contains a third exotic element. When the universe was just 10-36 seconds old, an overwhelming force took over. Called the inflaton field, it was repulsive like dark energy, but far more powerful, causing the universe to expand explosively by a factor of more than 1025, flattening space and smoothing out any gross irregularities.

When this period of inflation ended, the inflaton field transformed into matter and radiation. Quantum fluctuations in the field became slight variations in density, which eventually became the spots in the cosmic microwave background, and today’s galaxies. Again, this fantastic story seems to fit the observational facts. And again it comes with conceptual baggage. Inflation is no trouble for general relativity – mathematically it just requires an add-on term identical to the cosmological constant. But at one time this inflaton field must have made up 100 per cent of the contents of the universe, and its origin poses as much of a puzzle as either dark matter or dark energy. What’s more, once inflation has started it proves tricky to stop: it goes on to create a further legion of universes divorced from our own. For some cosmologists, the apparent prediction of this multiverse is an urgent reason to revisit the underlying assumptions of our standard cosmology (see “Trouble with physics: Time to rethink cosmic inflation?”).

The model faces a few observational niggles, too. The big bang makes much more lithium-7 in theory than the universe contains in practice. The model does not explain the possible alignment in some features in the cosmic background radiation, or why galaxies along certain lines of sight seem biased to spin left-handedly. A newly discovered supergalactic structure 4 billion light years long calls into question the assumption that the universe is smooth on large scales.

Read the entire story here.

Image: Petrarch, who first conceived the idea of a European “Dark Age”, by Andrea di Bartolo di Bargilla, c1450. Courtesy of Galleria degli Uffizi, Florence, Italy / Wikipedia.

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Everywhere And Nowhere

Most physicists believe that dark matter exists, but have never seen it, only deduced its existence. This is a rather unsettling state of affairs since by most estimates dark matter (and possibly dark energy) accounts for 95 percent of the universe. The stuff we are made from, interact with and see on a daily basis — atoms, their constituents and their forces — is a mere 5 percent.

From the Atlantic:

Here’s a little experiment.

Hold up your hand.

Now put it back down.

In that window of time, your hand somehow interacted with dark matter — the mysterious stuff that comprises the vast majority of the universe. “Our best guess,” according to Dan Hooper, an astronomy professor at the University of Chicago and a theoretical astrophysicist at the Fermi National Accelerator Laboratory, “is that a million particles of dark matter passed through your hand just now.”

Dark matter, in other words, is not merely the stuff of black holes and deep space. It is all around us. Somehow. We’re pretty sure.

But if you did the experiment — as the audience at Hooper’s talk on dark matter and other cosmic mysteries did at the Aspen Ideas Festival today — you didn’t feel those million particles. We humans have no sense of their existence, Hooper said, in part because they don’t hew to the forces that regulate our movement in the world — gravity, electromagnetism, the forces we can, in some way, feel. Dark matter, instead, is “this ghostly, elusive stuff that dominates our universe,” Hooper said.

It’s everywhere. And it’s also, as far as human knowledge is concerned, nowhere.

And yet, despite its mysteries, we know it’s out there. “All astronomers are in complete conviction that there is dark matter,” said Richard Massey, the lead author of a recent study mapping the dark matter of the universe, and Hooper’s co-panelist. The evidence for its existence, Hooper agreed, is “overwhelming.” And yet it’s evidence based on deduction: through our examinations of the observable universe, we make assumptions about the unobservable version.

Dark matter, in other words, is aptly named. A full 95 percent of the universe — the dark matter, the stuff that both is and is not — is effectively unknown to us. “All the science that we’ve ever done only ever examines five percent of the universe,” Massey said. Which means that there are still mysteries to be unraveled, and dark truths to be brought to light.

And it also means, Massey pointed out, that for scientists, “the job security is great.”

You might be wondering, though: given how little we know about dark matter, how is it that Hooper knew that a million particles of the stuff passed through your hand as you raised and lowered it?

“I cheated a little,” Hooper admitted. He assumed a particular mass for the individual particles. “We know what the density of dark matter is on Earth from watching how the Milky Way rotates. And we know roughly how fast they’re going. So you take those two bits of information, and all you need to know is how much mass each individual particle has, and then I can get the million number. And I assumed a kind of traditional guess. But it could be 10,000 higher; it could be 10,000 lower.”

Read the entire article here.

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Shedding Light on Dark Matter

Scientists are cautiously optimistic that results from a particle experiment circling the Earth onboard the International Space Station (ISS) hint at the existence of dark matter.

From Symmetry:

The space-based Alpha Magnetic Spectrometer experiment could be building toward evidence of dark matter, judging by its first result.

The AMS detector does its work more than 200 miles above Earth, latched to the side of the International Space Station. It detects charged cosmic rays, high-energy particles that for the most part originate outside our solar system.

The experiment’s first result, released today, showed an excess of antimatter particles—over the number expected to come from cosmic-ray collisions—in a certain energy range.

There are two competing explanations for this excess. Extra antimatter particles called positrons could be forming in collisions between unseen dark-matter particles and their antiparticles in space. Or an astronomical object such as a pulsar could be firing them into our solar system.

Luckily, there are a couple of ways to find out which explanation is correct.

If dark-matter particles are the culprits, the excess of positrons should sink suddenly above a certain energy. But if a pulsar is responsible, at higher energies the excess will only gradually disappear.

“The way they drop off tells you everything,” said AMS Spokesperson and Nobel laureate Sam Ting, in today’s presentation at CERN, the European center for particle physics.

The AMS result, to be published in Physical Review Letters on April 5, includes data from the energy range between 0.5 and 350 GeV. A graph of the flux of positrons over the flux of electrons and positrons takes the shape of a valley, dipping in the energy range between 0.5 to 10 GeV and then increasing steadily between 10 and 250 GeV. After that point, it begins to dip again—but the graph cuts off just before one can tell whether this is the great drop-off expected in dark matter models or the gradual fade-out expected in pulsar models. This confirms previous results from the PAMELA experiment, with greater precision.

Ting smiled slightly while presenting this cliffhanger, pointing to the empty edge of the graph. “In here, what happens is of great interest,” he said.

“We, of course, have a feeling what is happening,” he said. “But probably it is too early to discuss that.”

Ting kept mum about any data collected so far above that energy, telling curious audience members to wait until the experiment had enough information to present a statistically significant result.

“I’ve been working at CERN for many years. I’ve never made a mistake on an experiment,” he said. “And this is a very difficult experiment.”

A second way to determine the origin of the excess of positrons is to consider where they’re coming from. If positrons are hitting the detector from all directions at random, they could be coming from something as diffuse as dark matter. But if they are arriving from one preferred direction, they might be coming from a pulsar.

So far, the result leans toward the dark-matter explanation, with positrons coming from all directions. But AMS scientists will need to collect more data to say this for certain.

Read the entire article following the jump.

Image: Alpha Magnetic Spectrometer (AMS) detector latched on to the International Space Station. Courtesy of NASA / AMS-02.

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Shedding Some Light On Dark Matter

Cosmologists theorized the need for dark matter to account for hidden mass in our universe. Yet, as the name implies, it is proving rather hard to find. Now astronomers believe they see hints of it in ancient galactic collisions.

From New Scientist:

Colliding clusters of galaxies may hold clues to a mysterious dark force at work in the universe. This force would act only on invisible dark matter, the enigmatic stuff that makes up 86 per cent of the mass in the universe.

Dark matter famously refuses to interact with ordinary matter except via gravity, so theorists had assumed that its particles would be just as aloof with each other. But new observations suggest that dark matter interacts significantly with itself, while leaving regular matter out of the conversation.

“There could be a whole class of dark particles that don’t interact with normal matter but do interact with themselves,” says James Bullock of the University of California, Irvine. “Dark matter could be doing all sorts of interesting things, and we’d never know.”

Some of the best evidence for dark matter’s existence came from the Bullet clusterMovie Camera, a smash-up in which a small galaxy cluster plunged through a larger one about 100 million years ago. Separated by hundreds of light years, the individual galaxies sailed right past each other, and the two clusters parted ways. But intergalactic gas collided and pooled on the trailing ends of each cluster.

Mass maps of the Bullet cluster showed that dark matter stayed in line with the galaxies instead of pooling with the gas, proving that it can separate from ordinary matter. This also hinted that dark matter wasn’t interacting with itself, and was affected by gravity alone.

Musket shot

Last year William Dawson of the University of California, Davis, and colleagues found an older set of clusters seen about 700 million years after their collision. Nicknamed the Musket Ball cluster, this smash-up told a different tale. When Dawson’s team analysed the concentration of matter in the Musket Ball, they found that galaxies are separated from dark matter by about 19,000 light years.

“The galaxies outrun the dark matter. That’s what creates the offset,” Dawson said. “This is fitting that picture of self-interacting dark matter.” If dark matter particles do interact, perhaps via a dark force, they would slow down like the gas.

This new picture could solve some outstanding mysteries in cosmology, Dawson said this week during a meeting of the American Astronomical Society in Long Beach, California. Non-interacting dark matter should sink to the cores of star clusters and dwarf galaxies, but observations show that it is more evenly distributed. If it interacts with itself, it could puff up and spread outward like a gas.

So why doesn’t the Bullet cluster show the same separation between dark matter and galaxies? Dawson thinks it’s a question of age – dark matter in the younger Bullet simply hasn’t had time to separate.

Read the entire article after the jump.

Image: An overlay of an optical image of a cluster of galaxies with an x-ray image of hot gas lying within the cluster. Courtesy of NASA.

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Dark Matter: An Illusion?

Cosmologists and particle physicists have over the last decade or so proposed the existence of Dark Matter. It’s so called because it cannot be seen or sensed directly. It is inferred from gravitational effects on visible matter. Together with it’s theoretical cousin, Dark Energy, the two were hypothesized to make up most of the universe. In fact, the regular star-stuff — matter and energy — of which we, our planet, solar system and the visible universe are made, consists of only a paltry 4 percent.

Dark Matter and Dark Energy were originally proposed to account for discrepancies in calculations of the mass of large objects such as galaxies and galaxy clusters, and calculations derived from the mass of smaller visible objects such as stars, nebulae and interstellar gas.

The problem with Dark Matter is that it remains elusive and for the most part a theoretical construct. And, now a new group of theories suggest that the dark stuff may in fact be an illusion.

From National Geographic:

The mysterious substance known as dark matter may actually be an illusion created by gravitational interactions between short-lived particles of matter and antimatter, a new study says.

Dark matter is thought to be an invisible substance that makes up almost a quarter of the mass in the universe. The concept was first proposed in 1933 to explain why the outer galaxies in galaxy clusters orbit faster than they should, based on the galaxies’ visible mass.

(Related: “Dark-Matter Galaxy Detected: Hidden Dwarf Lurks Nearby?”)

At the observed speeds, the outer galaxies should be flung out into space, since the clusters don’t appear to have enough mass to keep the galaxies at their edges gravitationally bound.

So physicists proposed that the galaxies are surrounded by halos of invisible matter. This dark matter provides the extra mass, which in turn creates gravitational fields strong enough to hold the clusters together.

In the new study, physicist Dragan Hajdukovic at the European Organization for Nuclear Research (CERN) in Switzerland proposes an alternative explanation, based on something he calls the “gravitational polarization of the quantum vacuum.”

(Also see “Einstein’s Gravity Confirmed on a Cosmic Scale.”)

Empty Space Filled With “Virtual” Particles

The quantum vacuum is the name physicists give to what we see as empty space.

According to quantum physics, empty space is not actually barren but is a boiling sea of so-called virtual particles and antiparticles constantly popping in and out of existence.

Antimatter particles are mirror opposites of normal matter particles. For example, an antiproton is a negatively charged version of the positively charged proton, one of the basic constituents of the atom.

When matter and antimatter collide, they annihilate in a flash of energy. The virtual particles spontaneously created in the quantum vacuum appear and then disappear so quickly that they can’t be directly observed.

In his new mathematical model, Hajdukovic investigates what would happen if virtual matter and virtual antimatter were not only electrical opposites but also gravitational opposites—an idea some physicists previously proposed.

“Mainstream physics assumes that there is only one gravitational charge, while I have assumed that there are two gravitational charges,” Hajdukovic said.

According to his idea, outlined in the current issue of the journal Astrophysics and Space Science, matter has a positive gravitational charge and antimatter a negative one.

That would mean matter and antimatter are gravitationally repulsive, so that an object made of antimatter would “fall up” in the gravitational field of Earth, which is composed of normal matter.

Particles and antiparticles could still collide, however, since gravitational repulsion is much weaker than electrical attraction.

How Galaxies Could Get Gravity Boost

While the idea of particle antigravity might seem exotic, Hajdukovic says his theory is based on well-established tenants in quantum physics.

For example, it’s long been known that particles can team up to create a so-called electric dipole, with positively charge particles at one end and negatively charged particles at the other. (See “Universe’s Existence May Be Explained by New Material.”)

According to theory, there are countless electric dipoles created by virtual particles in any given volume of the quantum vacuum.

All of these electric dipoles are randomly oriented—like countless compass needles pointing every which way. But if the dipoles form in the presence of an existing electric field, they immediately align along the same direction as the field.

According to quantum field theory, this sudden snapping to order of electric dipoles, called polarization, generates a secondary electric field that combines with and strengthens the first field.

Hajdukovic suggests that a similar phenomenon happens with gravity. If virtual matter and antimatter particles have different gravitational charges, then randomly oriented gravitational dipoles would be generated in space.

More from theSource here.

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