Physicists are in the dark on dark matter

We know it’s out there, but experts don’t know what makes up the invisible dark matter that accounts for most of our universe.

Physicists are in the dark on dark matter
MIT astrophysicist Peter Fisher explained the bullet cluster (background) to an audience in Brooklyn. In the image behind him, two galaxies (red) collide with each other but the dark matter (blue) is unaffected and pulls ahead, separating from the galaxy cluster. [Image credit: Dan Robitzski]
By | Posted December 14, 2016
Posted in: Physical Science
Tags: , , ,

There’s a lot of stuff in the universe. Billions and billions of galaxies, each containing billions and billions of stars, all sail away from each other, carried by the universe’s constant expansion. And yet, all this visible stuff we’ve found counts for just a small percentage of what’s out there. Luminous matter — everything that we interact with and see — makes up only about 15 percent of all the mass in the universe. Dark matter, named so for its complete invisibility, accounts for all the rest. But for all its abundance, scientists have no idea what it is.

I went back to Pioneer Works, the Brooklyn museum and science outreach center, for the latest in their series of “Science Controversies” panels. Sitting all around me in the large, open-floor building were fellow physics enthusiasts, most of whom had traded in a suggested donation for a plastic cup of wine. Once more, three decorated physicists tried to answer the big questions: What is this stuff? What is the dark matter that makes up the majority of our universe and will we ever be able to “see” it for ourselves? Much like my last visit, there were few definitive answers but lots of good guesses.

“Everything we have ever seen, which has been brought to us by light predominately, all of that makes up a tiny fraction of what’s out there,” said host Janna Levin, the science director at Pioneer Works and an astronomer at New York City’s Barnard College. We can see luminous matter because when electromagnetic radiation — in the form of a beam of light — hits something, it bounces off and reflects an image that our eyes can pick up.

When it comes to dark matter, however, the light passes right through. No reflection, no appearance at all. “Everything we know about the things we interact with are based on their electromagnetic interactions,” added panelist Elena Aprile, an astrophysicist at Columbia University.

We may never have directly observed dark matter, but physicists have strong evidence indicating that it’s out there. When they looked into how galaxies formed during the early moments of the universe, their findings were problematic. With just luminous matter, galaxies simply didn’t have enough mass to form or maintain a structure. “In order for them to have formed, there must be dark matter,” said Levin, explaining how galaxies required the gravitational forces provided by the extra mass, not unlike an invisible foundation, in order to stay together. “Whatever dark matter is, it’s crucial to the formation of all the structures in the universe.”

If there were no dark matter, said panelist Peter H. Fisher, a particle physicist at the Massachusetts Institute of Technology, “[galaxies] would basically fly apart.” He explained that scientists realized something was missing when the critical density of the universe — the amount of matter necessary to keep the universe from expanding uncontrollably or collapsing — was calculated to be ten times the amount of visible matter. “The best way to think of a galaxy is a ball of dark matter with some regular matter spaced throughout,” Fisher said.

We don’t see that big ball of dark matter, but we do see what it does. A fundamental concept of physics is that gravity pulls matter together. Gravitational forces guide planets and stars along their orbits and can even bend light. As light travels from the luminous matter of distant galaxies to us, the dark matter it passes along the way bends and distorts the image.

“All the evidence [for dark matter] is found through the gravitational effects that dark matter has on everything we see and observe,” said Aprile.

If the universe were the empty void it appears to be, the light bouncing off these distant objects would travel in a straight line. But because the images appear in the form of distorted arcs and circles — and because bent light can travel around objects that would normally be in the way — we know that there is quite a bit of mass out there. By reconstructing the image through a complicated process called gravitational lensing, scientists like Aprile can calculate precisely how much mass, in the form of dark matter, the light passed. This process essentially reverse-engineers the universe based on the extent to which light is affected.

But for everything scientists do know about dark matter, there is still no consensus on what dark matter is made of. “This material is fundamentally different from anything [else] in the universe,” said Levin.

The leading explanation for dark matter comes from recent research into the nature of WIMPs, or weakly-interacting massive particles. Ultra-lightweight — the word massive only finding its way into the acronym because WIMPs do in fact have mass — WIMPs are a proposed class of particles that don’t interact with other particles except through gravity, which is pitifully-weak at small scales. “A billion particles of dark matter could be passing through us every second and we would never know it,” said Levin.

When researchers calculated how many WIMPs, which would have been generated during the big bang, ought to remain in the universe, the number matched the quantity of actual dark matter calculated to be out there. At the Large Hadron Collider in Geneva, experiments designed to recreate the conditions of the early universe are currently underway. “The real hope now is to create dark matter,” said Levin, explaining that the experiments are trying to generate neutrinos — tiny particles that we already know exist — to test whether or not they are a good candidate for dark matter. Neutrinos have mass but rarely interact with other particles, so some physicists think they may be the WIMPs responsible for dark matter.

Aprile also thinks that dark matter is likely made of WIMPs, but her experiments take a different approach than those at the collider. After years of planning and construction, her Xenon1T study has begun collecting data on neutrinos from sensitive detectors shielded deep underground. The hope is that by setting up the most sensitive apparatus possible, any neutrino passing by could be detected.

“I always have to explain that I don’t see when the WIMP interacts,” said Aprile. “We have to do lots of analysis with millions and millions of data.”

So far, dark matter remains hidden. That may be because the technology isn’t there yet, or because dark matter isn’t made of neutrinos — which are just one of many types of WIMPs — after all. “We all agree that this may be the wrong way to detect dark matter,” Aprile said, “but if we don’t do it then we won’t know.”

Fisher suspects that the latter might be the case. After his own experiments — like all others before it — failed to detect WIMPs, Fisher moved on to a new search. Now he suspects that the key to understanding dark matter might be a MACHO: Massive Compact Halo Object.

Arguably the most MACHO objects in the universe, black holes could provide the galaxy-stabilizing mass that researchers are looking for. Fisher explained an idea originally proposed, then written off, and then once more supported by Stephen Hawking: that during the big bang, it was possible to get enough energy together in the same place to form small, primordial black holes.

These black holes could be very small — even as small as a proton — but because black holes are so dense, a black hole that size would have as much mass as the moon. MACHOs spread throughout the universe, some in the centers of galaxies and stars, would give off enough gravitational forces to fit the bill as a candidate for dark matter.

MACHOs are no easier to spot that WIMPs, unfortunately. “One of these could go zipping through the earth and we wouldn’t really know it. It might scoop up a few kilograms,” said Fisher.

Both Fisher and Aprile conceded that they didn’t really know who was right. “[Dark matter] may be particles, it may be primordial black holes, something else. It may be anything,” said Aprile. “As curious human beings, we just have to keep searching and find out what this damn thing is.”

Posted in: Physical Science

Related Posts


comments

All comments are moderated, your comment will not appear on the site until it has been approved.

  1. Superfluid dark matter fills ’empty’ space, strongly interacts with and is displaced by baryonic matter.

    ‘The Milky Way’s dark matter halo appears to be lopsided’
    http://arxiv.org/abs/0903.3802

    “the emerging picture of the dark matter halo of the Milky Way is dominantly lopsided in nature.”

    The Milky Way’s halo is not a clump of dark matter traveling along with the Milky Way. The Milky Way’s halo is lopsided due to the baryonic matter in the Milky Way moving through and displacing the superfluid dark matter, analogous to a submarine moving through and displacing the water.

    What ripples when galaxy clusters collide is what waves in a double slit experiment, the superfluid dark matter which fills ’empty’ space.

    Superfluid dark matter displaced by baryonic matter relates general relativity and quantum mechanics.

    mpc755, December 14, 2016 at 12:44 pm
  2. How much money has the world spent on huge expensive experiments to find the totally unmotivated WIMPs and MACHOs yet? How much has the Dutch government blown on Eric Verlinde yet? Now compare that to how much has been spent to detect a well motivated axion at 50 to 250 μeV? Something that is now guaranteed to exist?

    Just sayin.

    Thomas Lee Elifritz, December 14, 2016 at 4:23 pm
  3. Another attempt to post a comment

    Maybe the whole concept of the Dark energy and Dark matter is NOT adequate…

    Here is a quick summary of some observations on a liquid vortex.

    Vortices are all the way around us. Assuming different kinds of vortices possess common generics, at least at some extent, we can get a great insight in understanding the hardly accessible and/or observable vortices, for instance, cosmic black holes, by observing the easily accessible ones, for example, liquid vortices. Observing liquid vortices, also, allows to run the research with reasonable resources, in reasonable time, under large variety of desired conditions and, what is especially important, to observe the developments taking place beneath the funnel bottom, which in a way could be considered as analogous to the black hole bottom.
    The liquid vortex I’ve observed was arranged to confine the vortex developments proceeding in the water thin surface layer thus making it to act as a two-dimensional rather than a tri-dimensional one and to consider the developments as being reasonably generic ones likely related to the black hole developments. The collection of videos of a liquid vortex located in the middle of a reasonably large water pool recorded under a variety of different conditions could be accessed at YouTube under name Yehiel Gotkis.
    Recently, a number of new findings about the black holes was made and published. Surprisingly, or maybe not, many of the effects discovered were also observed occurring with our liquid vortex. Thorough scrutinizing of the liquid vortex observations lead me, among other things, to a shocking grasp questioning even the widely-adopted views on the existence of the dark matter and dark energy.
    The observed liquid vortex naturally sucking the water was pulling-in also the whatever stuff was floating over the surface: foam, dry leaves and all kinds of floating debris. Considering the analogy with the black holes, the upper surface of the water pool could be thought to be associated with the spacetime, and the floating stuff with the regular matter. For this duo, the interaction between the material object and the spacetime causes appearance of an enveloping depression, or a “dent”, over the spacetime at the location of the object. As in the case of the liquid vortex, the spacetime and the matter appear to be inherently adhered to each other by gravity both statically (as stiction) and dynamically (as friction), making them to meticulously follow each other’s movement. This inherent matter/spacetime bonding could be guessed of possibly causing effects analogous to hydrodynamic drag and friction – gravitational drag and gravitational friction in this case.
    If these or similar relations between the matter and the spacetime take place in the case of spinning black holes, then the black holes should be thought of pulling-in, together with the surrounding regular matter also the spacetime, being whirling around the black hole as liquid whirls while flowing into the vortex (the rationale abbreviated furthermore as BHSSR, Black Hole Sucking Spacetime Rationale).
    Intriguingly, the BHSSR allows to explain the well-known legendary observations, namely, the galaxy rotation curves anomaly and the Universe accelerated expansion, with no necessity of introducing the two famous but still challenging to prove hypotheses:
    • The dark matter –
    As per the BHSSR, the pulled-in into the black hole whirling spacetime should add some extra momentum to the rotation of the visible matter in the black hole proximity,
    • The dark energy –
    As per the BHSSR, at the galactical periphery, where the pulling axipetal force
    diminishes, the keeping on centrifugal force induced by the spinning spacetime, accelerates the matter away from the galaxy center.

    Another important occurrence I would like to attract your attention is associated with the development of a spiral galaxy-like shape when a handful of floating shredded dry leaves was spread over the vortex area (video at https://www.youtube.com/watch?v=l8HBO6E8LAg). The deep significance of this development stirred my mind and I would like to share it with you here:
    • The shredded leaves while circling around the vortex were always arranging in a spiral galaxy-like shape. What could drive the initially disarranged flock of shredded intellect-less dry leaves to organize themselves in a spiral shape? How did they know about the spiral geometry to follow?
    With the knowledge about the liquid vortex I got, the answer looks obviously clear to me now… It is the whirling water managing the floating stuff to arrange in the spiral shape. The complex spiral structure could never been established without being governed by the vortex whirling flows.
    Conclusion:
    If in a system under observation, engaged with another, partnering sub-system, a spiral shape is observed it must be assumed that the partnering sub-system contains area(s) comprising whirling shape(s) and an associated vortex, driving the formation of the spiral(s).
    This important deduction leads us to an unambiguous conclusion about what may cause formation of the galaxy spiral geometry:
    There must be a vortex-actuated whirling media in the partnering sub-system the galaxy matter is engaged with, driving it to form its spiral arrangement. And what could be the origin of this sub-system, the vortex and the whirling media belong to? The uniquely possible open option is the spacetime whirlingly flowing into the black hole vortex funnel. So,
    The black hole vortex actuates the spacetime to whirlingly flow into its funnel, and the whirling spacetime in its turn “orders” the galactic regular matter to arrange itself in a spiral shape.
    And once again about the spacetime/matter tandem relations,
    The matter and the spacetime are likely adhered (as a tandem) to each other by the gravity forming an inseparably engaging duo: where the spacetime there the matter. And vice versa.
    This is it. Thank you for your attention.

    Yehiel Gotkis, January 15, 2017 at 12:41 am
  4. Hey, it was already published. Dear moderator, can you please remove one of the versions, better the first one. Thanks

    Yehiel Gotkis, January 15, 2017 at 12:44 am
post your comment