Story URL: http://news.medill.northwestern.edu/chicago/news.aspx?id=229790
Story Retrieval Date: 4/17/2015 11:15:51 AM CST
Gamma rays pouring from the center of our galaxy are giving Fermilab scientists an elusive glimpse at the dark side of the universe – dark matter.
Physicists at the Fermilab National Accelerator Laboratory near Batavia, working with researchers around the globe, have discovered significant evidence for the existence of dark matter with the gamma rays.
Dark matter and dark energy, theorized to make up approximately 95 percent of the universe, continue to elude direct observation. That's because dark matter stubbornly interacts with only itself, said physicist Alex Drlica-Wagner, who studies particle physics and cosmology.
“It isn't affected by regular matter like we are,” Drlica-Wagner explained. “Dark matter is constantly going right through the Earth, right through you and me, without any effect.”
However, it is theorized that when dark matter particles do collide, they annihilate on contact, as Drlica-Wagner explained. Rising like a phoenix out of to destruction to balance Einstein's classic equation, gamma rays and other forms of energy or particles are produced and sent out into the void of space.
At the other end of space and time sits NASA's Fermi-LAT satellite, perpetually sweeping back and forth across the sky, feasting on gamma rays emitted tens of thousands, if not hundreds of thousands, of years ago.
Two papers published recently by Fermilab and the American Physical Society, offer evidence for the existence of dark matter amassed in the center of galactic bodies.
One paper looks at gamma rays originating from the Galactic Center and a second reports on gamma rays originating in the Milky Way's dwarf galaxy neighbors.
Researchers found that these gamma rays match models for the emissions from dark matter annihilation.
Both studies are based on the same gamma ray data collected by LAT, an experiment that was launched in 2008, is the size of a small refrigerator and has, as Drlica-Wagner puts it, “the power supply of a toaster.”
LAT, which stands for Large Area Telescope, is “not Galileo’s telescope,” physicist Dan Hooper said. Hooper, who co-authored the paper covering the Galactic Center, explains how it works. “When energetic photons, which we call a gamma ray, hit it,” they produce an electron-positron pair. These particles “create two tracks through the detector and deposit energy as they move through the detector.” Researchers can then use this data to determine the strength and origin of the gamma ray.
Once the data is collected, researchers compare it with dark matter models. This time there was a match. Drlica-Wagner, who co-authored the paper on our galactic neighbors, explained that models are how physicists attempt to describe how matter interacts with other matter around it. The structure of atoms, with protons and neutrons bound together in the nucleus and electrons orbiting outside of it, is an example of a model, albeit a well-proven model.
But no one has directly observed dark matter, so there are many different models attempting to explain how it interacts with itself. Since this interaction, or annihilation, gives off no light, we can't see them. But now the gamma rays offer clues.
By letting the data drive the analysis, Hooper said that his team can explain the existence of the gamma rays with an incredibly simple dark matter model that has been around for 20 years. “It's easy, [after putting the data to the model] you annihilate the quarks, which is what you would expect, you annihilating at the rate you would expect, you distribute the dark matter in the galaxy the way that simulations tells us to expect, everything is vanilla.”
Based on this model, Hooper's team is able to determine that it is “very highly statistically significant” that the gamma rays in question originate from dark matter annihilation. Hooper is reluctant to say they have proven the existence of dark matter because the findings are based on a model that cannot be confirmed until dark matter is directly observed. But he added that “at this point I don't know of any alternative to dark matter that can explain the signal at hand.” That's a claim previous dark matter possibilities could not make.
Drlica-Wagner and his collaborators are less sure of their findings, but for now this has more to do with the amount and strength of data they have to work with. Since neighboring dwarf galaxies are much further away from Earth, the gamma rays coming from our galactic neighbors are not as strong.
Further, there is a limited number of dwarf spheroidal galaxies that can be studied because they are rather hard to find in the sky. A new camera specifically designed to find these galaxies in the southern sky was installed in Chile's European Southern Observatory and started collecting data last year, but for now the research team has only 25 galaxies to study.
As more data is collected in the coming years and more galaxies are discovered, Drlica-Wagner's team will be able to say with more certainly whether or not their data supports Hooper's analysis.
As Hooper, Drlica-Wagner and their many colleagues spend the next few years continuing to collect data and fine-tune their analysis, everyone will be waiting with baited breath for the ultimate goal: direct detection of dark matter and the possible validation of their research.
A note about statistical significance
To fully understand the impact of the evidence, it is important to understand some of the terminology used in particle physics.
In the world of large experiments and data sets, researchers ultimately determine the validity of their argument by using standard deviation, which describes how closely the entirety of the data corresponds to the average of the data.
Standard deviation is represented with the Greek letter sigma - the closer the data stays together, the higher the sigma.
Different research fields have different regulations for what level of sigma is needed to use words like 'evidence' and 'discovery.' Particle physics requires one of the highest levels of sigma for these words to be used.
To say the data lends 'evidence,' there has to be at least a 3 sigma level of certainty, meaning there is a 1 in 370 chance the given data could be represented by a random error or fluctuation in the experiment.
To say a 'discovery' has been made, researchers need 5 sigma, meaning there is only a one in nearly 2 million chance of this same fluctuation or error.
Drlica-Wagner and his co-authors lay claim to only a 2 sigma, which Hooper says doesn't have any official terminology, but could be called a 'hint.' However, since this is in support of Hooper's, it's likely that they would only need to get to a 2.5 or 3 sigma for many researchers to be convinced by the findings.
To put this in perspective, Hooper compared the level of certainty needed for a discovery to dreaming. He noted that a colleague has said “he's not 5 sigma sure that he's not dreaming right now. And he's right - you're fooled often enough that you're only 4 sigma sure that you're not dreaming.”
In the course of their analysis, Hooper and his colleagues reached a 40 sigma level - but he stops short of claiming a 'discovery,' and that is for one simple reason.
Their math relies on the validity of the model they're building it on. If it turns out that 20 years of dark matter theory is entirely flawed, there's a chance the 40 sigma would round down to zilch.