The search for 'dark matter' and 'dark energy' just got interesting

The search for 'dark matter' and 'dark energy' just got interesting
We are a big step closer to tracking down what’s hiding in galaxy clusters like Abell 2218. Credit: NASA/ESA via wikipedia

Only about 5% of the universe consists of ordinary matter such as protons and electrons, with the rest being filled with mysterious substances known as dark matter and dark energy. So far, scientists have failed to detect these elusive materials, despite spending decades searching for them. But now, two new studies may be able to turn things around as they have narrowed down the search significantly.

Dark matter was first proposed more than 70 years ago to explain why the force of gravity in galaxy clusters is so much stronger than expected. If the clusters contained only the stars and gas we observe, their gravity should be much weaker, leading scientists to assume there is some sort of matter hidden there that we can't see. Such would provide additional mass to these large structures, increasing their gravitational pull. The main contender for the substance is a type of hypothetical particle known as a "weakly interacting massive particle" (WIMP).

To probe the nature of dark matter, physicists look for evidence of its interactions beyond gravity. If the WIMP hypothesis is correct, dark matter particles could be detected through their scattering off atomic nuclei or electrons on Earth. In such "direct" detection experiments, a WIMP collision would cause these charged particles to recoil, producing light that we can observe.

One of the main direct detection experiments in operation today is XENON100, which has just reported its latest results. The detector is located deep underground to reduce interference from cosmic rays, at the Gran Sasso laboratory in Italy. It consists of a 165kg container of liquid xenon, which is highly purified to minimise contamination. The detector material is surrounded by arrays of photomultiplier tubes (PMTs) to capture the light from potential WIMP interactions.

The search for 'dark matter' and 'dark energy' just got interesting
The vacuum chamber of the atom interferometer. Credit: Holger Muller photo., CC BY

The new XENON100 report has found no evidence of WIMPs scattering off electrons. Although this is a negative result, it rules out many so-called "leptophilic" models that predict frequent interactions between dark matter and electrons.

But the most important consequence of the XENON100 analysis is with regards to the controversial claim of dark matter detection by researchers at the DAMA/LIBRA experiment in Italy, which is in conflict with the results from many other detectors such as the Cryogenic Dark Matter Search. Leptophilic dark matter was proposed as a viable explanation for this discrepancy since exclusions from other experiments would not directly apply. However, the new results from XENON100 firmly rule out this possibility.

Chasing chameleons

Meanwhile, dark energy explains our observation that the universe is expanding at an accelerating rate. Unlike normal matter, dark energy has a negative pressure, which allows gravity to be repulsive, driving the galaxies apart. One of the most promising dark energy candidates is a so-called "chameleon field".

In many dark energy models, we would expect to see significant effects on both laboratory and cosmological scales. However, the attractive feature of a chameleon field is that its impact depends on the environment. At small scales, such as on Earth, the density of matter is high and the field is effectively "screened out", allowing chameleons to evade our detectors. However, in the vacuum of space, the matter density is tiny and the field can drive the cosmic acceleration.

Until now, experiments have only used relatively large detectors, failing to observe chameleons as the density of matter is too high. However, it was recently proposed that an "atom interferometer", operating on microscopic scales, could be used to search for chameleons. This consists of an ultra-high vacuum chamber containing individual atoms and simulates the low-density conditions of empty space so that screening is reduced.

In the second report, researchers implement this idea for the first time. Their experiment works by dropping caesium atoms above an aluminium sphere. Using sensitive lasers, the researchers could then measure the forces on the atoms as they were in free fall. The results were perfectly consistent with only gravity and no chameleon-induced force. This implies that if chameleons exist, they must interact more weakly than we previous thought – narrowing the search for these particles by a thousand times compared to previous studies. The team are hoping that their innovative technique will help them to hunt down chameleons or other particles in a future experiment.

Both of these studies demonstrate how laboratory experiments can answer fundamental questions about the nature of the cosmos. But most importantly, they raise hope that we will one day track down these tantalising substances that make up a whopping 95% of our universe.


Explore further

Snaring a dark energy 'chameleon': Is dark energy hiding from us? Matter may be screening it from our view

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Aug 21, 2015
Dark matter fills 'empty' space and is displaced by the particles of matter which exist in it and move through it.

The Milky Way's halo is lopsided due to the matter in the Milky Way moving through and displacing the dark matter, analogous to a submarine moving through and displacing the water.

The Milky Way's halo is the state of displacement of the dark matter.

The Milky Way's halo is curved spacetime.

The state of displacement of the dark matter is curved spacetime.

The state of displacement of the dark matter *is* gravity.

Aug 21, 2015
I propose removing the current theories of dark energy and dark matter and try replacing them with a theory including a type of matter (probably anti-matter) with a negative mass.
The result of this would be at the big bang an equal amount of matter and anti-matter (with negative mass) was created. The two types would mainly annihilate each other but some would survive repulsing the matter of the other type, resulting in the accelerating expansion of the universe.
As the matter clumped together , it would be surrounded by the anti-matter and this would "squash" the matter clump , explaining the apparent extra gravity galaxies appear to experience.
We should see evidence of this theory as in deep space there would be interaction between the two types of matter as the particles annihilated each other giving of energy.

Aug 22, 2015
Dagnabbit,

Dark energy is well established as cosmological constant, and as the vacuum pressure found in the Casimir Effect which has been repeatedly demonstrated in laboratories.

It's dark matter that we haven't found yet.

Can we please edumacate the physorg writers in this respect? Let me repeat, and sorry for shouting but it seems no one is listening:

DARK ENERGY IS WELL EXPLAINED BY RELATIVITY, AND ITS EFFECTS HAVE BEEN SEEN IN THE LABORATORY

DARK MATTER REMAINS MYSTERIOUS SINCE WE HAVE NOT DETECTED ITS EFFECTS IN THE LABORATORY

Moving right along, the GRAVITATIONAL ASPECTS OF DARK MATTER ARE NOT CONTROVERSIAL, IT IS ONLY THE DETECTION OF DARK MATTER ITSELF IN THE LABORATORY THAT HAS NOT BEEN ACCOMPLISHED AT THIS TIME

Please make note of these facts in future articles. Thanks.

Aug 22, 2015
It's dark matter that we haven't found yet.
So how do we know it's out there?

Aug 22, 2015
I wonder if our regular matter is dark matter to any other creatures/aliens? Our earth would be dark ball. Doesn't really sound feasible, but maybe other creatures can see matter that we cannot.

Aug 22, 2015
"Unlike normal matter, dark energy has a negative pressure, which allows gravity to be repulsive, driving the galaxies apart."

So now we have a new kind of gravity that is repulsive? Sorry, I can't buy this nonsense.

Modern Cosmology is great at creating names for stuff that they can't explain properly in a coherent and self-contained way. Maybe they should switch to stamp collecting.

Aug 23, 2015
"Unlike normal matter, dark energy has a negative pressure, which allows gravity to be repulsive, driving the galaxies apart."

So now we have a new kind of gravity that is repulsive? Sorry, I can't buy this nonsense.

Modern Cosmology is great at creating names for stuff that they can't explain properly in a coherent and self-contained way. Maybe they should switch to stamp collecting.
Good point. Anyway dark energy doesn't shine and it has negative pressure like the vacuum. A bit convoluted so best build your theory of gravity on visible energy.

Aug 23, 2015
Our Universe is a larger version of a galactic polar jet.

Dark energy is dark matter continuously emitted into the Universal jet.

Aug 23, 2015
Dark energy is well established as cosmological constant, and as the vacuum pressure found in the Casimir Effect which has been repeatedly demonstrated in laboratories.


A connection has been postulated between Λ and the negative pressure observed in the Casimir effect,... but such a connection is quite far from being "well established".... especially given it would be a link between QM and GR.

For starters, the Casimir effect comes about by confining degrees of freedom of a quantum field (EM) via physical boundary conditions, .... while there are no such universal boundary conditions for DE to 'be the same thing'.

Also, to observe the Casimir effect requires a vector field, QED, while the hypothesis that DE is modeled by the Λ term as proportional to the metric, is a scalar field.

In the laboratory, the effect of DE as the Λ term would be absolutely infinitesimal.

Aug 25, 2015
Curious, 95% of universe is undetectable. What percent is dark matter and what percent is dark energy??
Still a quantum size experiment would or should yield some of the 95% ¿
Or multiple quantum size experiments. 95% of something does not have to be large!
If the universe is fluid, density of this fluid at quantum level could be different altogether. Is there a dark matter experiment at this level now?

Aug 25, 2015
What percent is dark matter and what percent is dark energy??


Dark-matter is ~84% of the matter in the universe, ....or ~27% of the mass-energy, while dark-energy is ~68% of the mass-energy.

Still a quantum size experiment would or should yield some [....] Is there a dark matter experiment at this level now?


DM is postulated as being a micro-scopic fundamental particle (that does not radiate light), but experiments to detect it can't be 'quantum sized' per se, as we are macro-sized,.... but must be at least sensitive to energy caused by DM particles,.... i.e. colliding with normal matter to cause radiation, etc.

Aug 27, 2015
If the universe is fluid, density of this fluid at quantum level could be different altogether. Is there a dark matter experiment at this level now?
Why not also at the macro level? The gradient of this density would be what I'm thinking of as gravity. Gradient in density at the quantum level would be matter. But these gradients would have to be quantized, meaning they would not normally change at the macro level. So as matter accumulates at the quantum level its region of space will not be affected as much by any changes at the macro level. Specifically any density changes at the macro level will not affect regions of space filled with matter as much, leading to increased differences in density in regions of space with and without matter. But there is no reason density changes can not occur in space in the absence of matter, leading to what we call dark matter at the macro level.

Aug 27, 2015
Wow, somehow every single experiment to find "dark matter" fails.

A negative result is not a failure. Any experiment where you gain information is a success. And being able to rule out large swathes of theories is most definitely a gain in information.

Curious, 95% of universe is undetectable.

It has been detected by its effects. Now it's 'just' a matter of pinning down what's causing these effects.

I wonder if our regular matter is dark matter to any other creatures/aliens?

Probably not, as our type of matter is highly interacting (i.e. very easy to detect with a wide range of methods). E.g. if they know what electromagnetism is (i.e. ifthey are hip to the entire 'speed of light is constant' thing) then they're able to see our type of matter.

Aug 28, 2015
I propose removing the current theories of dark energy and dark matter and try replacing them with a theory including a type of matter (probably anti-matter) with a negative mass.
Note that particles of negative mass would repel each other if particles of positive mass attract each other. Also it may very well be particles of equal negative and positive mass would not interact gravitationally because they have zero total mass. Think of the consequences of particles repelling each other. Leads to some very interesting speculations.

Aug 28, 2015
Wrong. If you set your goal as finding something. You set up an experiment to find it, and then you don't. That is a failure.


In looking for your keys, not finding them in the one or two rooms that you have looked does not imply that your keys don't exist at all,....

THERE WAS NO INFORMATION GAINED.

You ruled out a few rooms to the location of your keys,.... that is information.

Other than proving that an incorrect theory is impossible to prove.

General relativity can be solved in two ways. One, by starting with the stress-energy tensor (starting with the [observed] mass) and solving for the space-time metric (the space-time curvature as expressed in the Einstein tensor),.... the other, by starting with the space-time curvature and solving for the mass-energy present.

Aug 28, 2015
Think of the consequences of particles repelling each other.
This doesn't mean negative mass particles are repelled by positive mass particles. For stronger positive mass gravitational fields, negative mass will still be attracted to the positive mass but with less gravitational pull than if it was positive mass. I was having problems with Hawking radiation believing anti-matter would be attracted into the black hole but the black hole gravitation far outweighs that of the anti-particle (no pun intended) so the attractive force of a matter particle and a similar mass anti-matter particle to the black body is for all practical purposes the same.

Aug 28, 2015
Think of the consequences of particles repelling each other.
So antiparticles can accumulate in regions of denser matter, as in galaxies. But outside galaxies where there are filaments and voids antimatter will not accumulate in the voids but be repulsive. This means in their lowest energy state they will stay as far away from each other as possible. We can never directly detect a single antiparticle in the voids in interstellar space so now we know where all the missing antimatter in the U is hanging out. Similar charges also contribute to this effect since they are repulsive.

Aug 28, 2015
.... continued from above.....

...In the latter theoretical case (solving for T^(μν), given G^(μν)), it would be found that more mass is expected to be present. That that mass is not observed may simply be a consequence of not giving off electromagnetism. By saying that it is rather a consequence of the failure of theory (before ruling out the presence of mass-energy), you're effectively assuming that all forms of mass-energy radiates EM.

I propose removing the current theories of dark energy and dark matter and try replacing them with a theory including a type of matter (probably anti-matter) with a negative mass.


Dark matter and Dark energy are not theories per se,... they are place holder terms for unobserved mass-energy. GR is compatible with any form of mass-energy as the stress-energy tensor is quite generalized,.... only not including gravitational waves, as far as I know.

Aug 28, 2015
Think of the consequences of particles repelling each other.
So now that we know where all the missing antimatter in the U is, we can say the negative gravitational fields of antimatter cancel out the positive fields of matter so the overall curvature of the U is flat and the cosmological constant really is 1.

Aug 28, 2015
Think of the consequences of particles repelling each other.
So now that we know where all the missing antimatter in the U is, we can say the negative gravitational fields of antimatter cancel out the positive fields of matter so the overall curvature of the U is flat and the cosmological constant really is 1.

Aug 28, 2015
I have talked about this many times. "negative mass" is incorrect terminology. The word you are looking for is anti-gravity. Anti-matter = anti-gravity
Funny. Real physicists seem to know exactly what it is.

"In theoretical physics, negative mass is a hypothetical concept of matter whose mass is of opposite sign to the mass of normal matter, e.g. −2 kg. Such matter would violate one or more energy conditions and show some strange properties, stemming from the ambiguity as to whether attraction should refer to force or the oppositely oriented acceleration for negative mass."

Sep 03, 2015
Just because adding dark matter and dark energy makes the models work, does not mean that they exist. There are many other ways of achieving the same result.
A matter galaxy surrounded by particles with repulsive force could appear the same as we observe, with no need to create 95% of the universe that we can not detect, It would probably also explain the missing anti-matter from the big bang, which simply was pushed out of the matter galaxies but would have caused a fast expansion of the universe in the early stages.

Sep 03, 2015
Wow, somehow every single experiment to find "dark matter" fails. Time to drop this made up BS idea yet? Start to look at other explanations which do not involve some magic massive matter that makes up 95% of everything yet is totally undetectable?
Like looking for goblins in the night. However it doesn't answer the question about who moved the furniture around when nobody was home.

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