Artist's rendition via NASA

Dark matter: it is one of the most troubling things in the universe. After all, how can something that is so crucial to our existence be something that we can't see?  Well, as a crude analogy, the very air we breathe fits into this same category. We cannot see air, but no one here is disputing the premise that air exists.

Of course, this is because we can prove that air is there by measuring it; however, we were unable to really see or define it for years--for centuries. In fact, we didn't even discover oxygen (the third most abundant element in the universe that makes up nearly 21% of the earth's atmosphere) until the 1770s. In a similar way (but using a very different approach than the one used to define air), we have started to get verification for dark matter through the scientific method.

According to our model of cosmology, dark matter is thought to make up 26.8% of the total energy density of the observable universe.  This is in stark contrast to regular matter, which makes up a mere 4.9% of the universe. Thus, in order to truly understand the cosmos, we need to define and understand the existence of dark matter--this fact is inescapable.

So, let's get to it...

 Overview of Dark Matter

Image credit: LUX

First, dark matter neither emits nor absorbs electromagnetic radiation (in short, light). Hence the name dark matter. Most scientists believe that WIMPs constitute dark matter, or weakly interacting massive particles.  These would kind of be like neutrinos, but with one major exception--instead of being near massless, they would be quite heavy.

The first body of evidence that supports WIMPs is the fact that stars in galaxies appear to be moving the same speed (more or less) regardless of whether they are in a spiral arm or near the central bulge.

This is a tad problematic, because traditional thought asserts that objects should slow down as they get farther away from the galactic center (where the supermassive black hole typically resides). This is because (per Newton) gravitational effects are inversely proportional to the square of the distance.  In other words, given their observed speeds, the stars on the outer spiral arms should have been flung off into the interstellar medium by their kinetic energy.

3D map of dark matter in our universe. Image credit: ESA Planck homepage

This startling conclusion  means one of two possibilities has to be true: Newton's laws have to be flawed (along with our understanding of gravity and rotation), or some new unobservable form of matter was everywhere interacting with things gravitationally.  Here at FQTQ, we have covered articles on the alternative hypothesis (the hypothesis that Newton is wrong). One example of this is the modified theory of gravity, called MOND.  That article can be found here.

But working under the hypothesis that Newton is not wrong, astronomers went looking for dark matter in galaxy clusters.  With x-ray telescopes (like the Chandra X-ray Observatory), they were hoping to find extra gas that may account for this "missing" matter.  They found some, but not nearly enough to do away with the problem.

In time, they calculated that there had to be a whopping 5 to 6 times as much dark matter as there was baryonic matter ("normal" matter, such as stars and gas) holding everything in place.

This number corresponds further with the theory of Big Bang nucleosynthesis, a model which very accurately predicts chemical abundances that come in at about 5% the total matter density of the observable universe.  We know that these numbers are supported from the Planck Sattelite and WMAP missions, and that total matter density is a lot higher than this.  The evidence for that is observed from large-scale structure formation and detailed analysis of anisotropies of the CMB.

The next line of evidence to support this is gravitational lensing.  Here, light is warped around gravitationally strong objects, magnifying objects behind a dense object. The larger the angle of bending, the stronger the gravity.  Using this guide, astronomers have noted that galaxy clusters have much higher mass (due to the sharp angle of light bending) then we would normally assume the had based strictly on the amount of luminous matter that constitutes them.

 The Role of LUX

Bullet cluster: image credit Wikipedia

Short for "Large Underground Xenon" detector, it is hailed as the most accurate and sensitive dark matter detector in the world.  This is important because dark matter is speculated to interact very, very rarely (aside from gravitationally) with regular matter.  Just how rare you ask?

Well, imagine you are able to shoot a single WIMP of dark matter at a block of solid lead.  In order to get a 50% chance of particle interaction, that solid block of lead would have to extend some 200 light-years. For comparison, the closest star to the Sun is Proxima Centauri, and that is 4 light-years away. So (to put it midly) 200 light-years is rather far away.

This rarity of interaction gives any dark matter researcher a large error of margin due to unwanted particle interactions. So one of the main goals of LUX is to ensure isolation.  To shield it from background radiation and cosmic rays, the LUX is located 4,850 feet underground, submerged in 71,600 gallons of pure de-ionized water.  The sensitivity of LUX in its environment makes it the most advanced detector in the world.

Image credit: LUX

After the first 90 day trial run, scientists concluded that LUX was working beautifully.  The sensitivity is twice as good as the next best dark matter detector, and it can look for WIMPS up to 40 times the mass of the proton.  After the preliminary results, LUX has shown a null result, providing strong evidence that the other previous experiments that cited low mass observations were likely to be false signals due to interference.

As the experiment continues, the chances of a successful nucleus recoil go up dramatically, and if it happens LUX will detect it.  And in a short period of time, we may have our first indisputable evidence for these elusive particles that make up our universe (but don't get too, too excited. This is expected to take around a decade before significant evidence can be expected).

To read more about how LUX works from their homepage here.


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