The world’s most powerful particle collider is waking up from a well-earned rest. After roughly two years of heavy maintenance, scientists have nearly doubled the power of the Large Hadron Collider (LHC) in preparation for its next run. Now, it’s being cooled to just 1.9 degrees above absolute zero.
“We have unfinished business with understanding the universe,” said Tara Shears from the University of Liverpool in a news release. Shears and other LHC physicists will work to better understand the Higgs Boson and hopefully unravel some of the secrets of supersymmetry and dark matter.
On February 11, 2013 the LHC shut down for roughly two years. The break, known as LS1 for “long stop one,” was needed to correct several flaws in the original design of the collider.
The LHC’s first run got off to a rough start in 2008. Shortly after it was fired up, a single electrical connection triggered an explosion, damaging an entire sector (one-eighth) of the accelerator. To protect the accelerator from further disaster, scientists decided to run it at half power until all 10,000 copper connections could be repaired.
So over the last two years, scientists have worked around the clock to rework every single connection in the accelerator.
Now that the step (along with many others) is complete, the collider will operate at almost double its previous power. This was tested early last week, when scientists powered up the magnets of one sector to the level needed to reach the high energy expected in its second run.
With such a powerful new tool, scientists will look for deviations from their initial detection of the Higgs boson, potentially revealing a deeper level of physics that goes well beyond the Standard Model of particle physics.
Many theorists have turned to supersymmetry — the idea that for every known fundamental particle there exists a “supersymmetric” partner particle. If true, the enhanced LHC could be powerful enough to create supersymmetric particles themselves or prove their existence in subtler ways.
“The higher energy and more frequent proton collisions in Run 2 will allow us to investigate the Higgs particle in much more detail,” said Victoria Martin from Edinburgh University. “Higher energy may also allow the mysterious “dark matter” observed in galaxies to be made and studied in the lab for the first time.”
It’s possible that the Higgs could interact with — or even decay into — dark matter particles. If the latter occurs, then the dark matter particles would fly out of the LHC without ever being detected. But their absence would be evident.
So stay turned because these issues might be resolved in the spring of 2015 when the particle accelerator roars back to life.
All galaxies are thought to have a dark matter halo. This image shows the distribution of dark matter surrounding our very own Milky Way. Image credit: J. Diemand, M. Kuhlen and P. Madau (UCSC)
Dark matter is the architect of large-scale cosmic structure and the engine behind proper rotation of galaxies. It’s an indispensable part of the physics of our Universe – and yet scientists still don’t know what it’s made of. The latest data from Planck suggest that the mysterious substance comprises 26.2% of the cosmos, making it nearly five and a half times more prevalent than normal, everyday matter. Now, four European researchers have hinted that they may have a discovery on their hands: a signal in x-ray light that has no known cause, and may be evidence of a long sought-after interaction between particles – namely, the annihilation of dark matter.
When astronomers want to study an object in the night sky, such as a star or galaxy, they begin by analyzing its light across all wavelengths. This allows them to visualize narrow dark lines in the object’s spectrum, called absorption lines. Absorption lines occur because a star’s or galaxy’s component elements soak up light at certain wavelengths, preventing most photons with those energies from reaching Earth. Similarly, interacting particles can also leave emission lines in a star’s or galaxy’s spectrum, bright lines that are created when excess photons are emitted via subatomic processes such as excitement and decay. By looking closely at these emission lines, scientists can usually paint a robust picture of the physics going on elsewhere in the cosmos.
But sometimes, scientists find an emission line that is more puzzling. Earlier this year, researchers at the Laboratory of Particle Physics and Cosmology (LPPC) in Switzerland and Leiden University in the Netherlands identified an excess bump of energy in x-ray light coming from both the Andromeda galaxy and the Perseus star cluster: an emission line with an energy around 3.5keV. No known process can account for this line; however, it is consistent with models of the theoretical sterile neutrino – a particle that many scientists believe is a prime candidate for dark matter.
The researchers believe that this strange emission line could result from the annihilation, or decay, of these dark matter particles, a process that is thought to release x-ray photons. In fact, the signal appeared to be strongest in the most dense regions of Andromeda and Perseus and increasingly more diffuse away from the center, a distribution that is also characteristic of dark matter. Additionally, the signal was absent from the team’s observations of deep, empty space, implying that it is real and not just instrumental artifact.
In a pre-print of their paper, the researchers are careful to stress that the signal itself is weak by scientific standards. That is, they can only be 99.994% sure that it is a true result and not just a rogue statistical fluctuation, a level of confidence that is known as 4σ. (The gold standard for a discovery in science is 5σ: a result that can be declared “true” with 99.9999% confidence) Other scientists are not so sure that dark matter is such a good explanation after all. According to predictions made based on measurements of the Lyman-alpha forest – that is, the spectral pattern of hydrogen absorption and photon emission within very distant, very old gas clouds – any particle purporting to be dark matter should have an energy above 10keV – more than twice the energy of this most recent signal.
As always, the study of cosmology is fraught with mysteries. Whether this particular emission line turns out to be evidence of a sterile neutrino (and thus of dark matter) or not, it does appear to be a signal of some physical process that scientists do not yet understand. If future observations can increase the certainty of this discovery to the 5σ level, astrophysicists will have yet another phenomena to account for – an exciting prospect, regardless of the final result.
The team’s research has been accepted to Physical Review Letters and will be published in an upcoming issue.
The Victor M. Blanco telescope at Cerro Tololo Interamerican Observatory (CTIO) in the Chilean Andes. Credit: Berkeley Lab
Since the early 20th century, scientists and physicists have been burdened with explaining how and why the Universe appears to be expanding at an accelerating rate. For decades, the most widely accepted explanation is that the cosmos is permeated by a mysterious force known as “dark energy”. In addition to being responsible for cosmic acceleration, this energy is also thought to comprise 68.3% of the universe’s non-visible mass.
Much like dark matter, the existence of this invisible force is based on observable phenomena and because it happens to fit with our current models of cosmology, and not direct evidence. Instead, scientists must rely on indirect observations, watching how fast cosmic objects (specifically Type Ia supernovae) recede from us as the universe expands.
This process would be extremely tedious for scientists – like those who work for the Dark Energy Survey (DES) – were it not for the new algorithms developed collaboratively by researchers at Lawrence Berkeley National Laboratory and UC Berkeley.
“Our algorithm can classify a detection of a supernova candidate in about 0.01 seconds, whereas an experienced human scanner can take several seconds,” said Danny Goldstein, a UC Berkeley graduate student who developed the code to automate the process of supernova discovery on DES images.
Currently in its second season, the DES takes nightly pictures of the Southern Sky with DECam – a 570-megapixel camera that is mounted on the Victor M. Blanco telescope at Cerro Tololo Interamerican Observatory (CTIO) in the Chilean Andes. Every night, the camera generates between 100 Gigabytes (GB) and 1 Terabyte (TB) of imaging data, which is sent to the National Center for Supercomputing Applications (NCSA) and DOE’s Fermilab in Illinois for initial processing and archiving.
By studying Type Ia supernova, astronomers can measure dark energy and the expansion of the universe. Credit: NASA/CXC/M. Weiss
Object recognition programs developed at the National Energy Research Scientific Computing Center (NERSC) and implemented at NCSA then comb through the images in search of possible detections of Type Ia supernovae. These powerful explosions occur in binary star systems where one star is a white dwarf, which accretes material from a companion star until it reaches a critical mass and explodes in a Type Ia supernova.
“These explosions are remarkable because they can be used as cosmic distance indicators to within 3-10 percent accuracy,” says Goldstein.
Distance is important because the further away an object is located in space, the further back in time it is. By tracking Type Ia supernovae at different distances, researchers can measure cosmic expansion throughout the universe’s history. This allows them to put constraints on how fast the universe is expanding and maybe even provide other clues about the nature of dark energy.
“Scientifically, it’s a really exciting time because several groups around the world are trying to precisely measure Type Ia supernovae in order to constrain and understand the dark energy that is driving the accelerated expansion of the universe,” says Goldstein, who is also a student researcher in Berkeley Lab’s Computational Cosmology Center (C3).
Goldstein’s new code uses machine learning techniques to vet detections of supernova candidates. Credit: Danny Goldstein, UC Berkeley/Berkeley Lab)
The DES begins its search for Type Ia explosions by uncovering changes in the night sky, which is where the image subtraction pipeline developed and implemented by researchers in the DES supernova working group comes in. The pipeline subtracts images that contain known cosmic objects from new images that are exposed nightly at CTIO.
Each night, the pipeline produces between 10,000 and a few hundred thousand detections of supernova candidates that need to be validated.
“Historically, trained astronomers would sit at the computer for hours, look at these dots, and offer opinions about whether they had the characteristics of a supernova, or whether they were caused by spurious effects that masquerade as supernovae in the data. This process seems straightforward until you realize that the number of candidates that need to be classified each night is prohibitively large and only one in a few hundred is a real supernova of any type,” says Goldstein. “This process is extremely tedious and time-intensive. It also puts a lot of pressure on the supernova working group to process and scan data fast, which is hard work.”
To simplify the task of vetting candidates, Goldstein developed a code that uses the machine learning technique “Random Forest” to vet detections of supernova candidates automatically and in real-time to optimize them for the DES. The technique employs an ensemble of decision trees to automatically ask the types of questions that astronomers would typically consider when classifying supernova candidates.
Evolution of a Type Ia supernova. Credit: NASA/ESA/A. Feild
At the end of the process, each detection of a candidate is given a score based on the fraction of decision trees that considered it to have the characteristics of a detection of a supernova. The closer the classification score is to one, the stronger the candidate. Goldstein notes that in preliminary tests, the classification pipeline achieved 96 percent overall accuracy.
“When you do subtraction alone you get far too many ‘false-positives’ — instrumental or software artifacts that show up as potential supernova candidates — for humans to sift through,” says Rollin Thomas, of Berkeley Lab’s C3, who was Goldstein’s collaborator.
He notes that with the classifier, researchers can quickly and accurately strain out the artifacts from supernova candidates. “This means that instead of having 20 scientists from the supernova working group continually sift through thousands of candidates every night, you can just appoint one person to look at maybe few hundred strong candidates,” says Thomas. “This significantly speeds up our workflow and allows us to identify supernovae in real-time, which is crucial for conducting follow up observations.”
“Using about 60 cores on a supercomputer we can classify 200,000 detections in about 20 minutes, including time for database interaction and feature extraction.” says Goldstein.
Goldstein and Thomas note that the next step in this work is to add a second-level of machine learning to the pipeline to improve the classification accuracy. This extra layer would take into account how the object was classified in previous observations as it determines the probability that the candidate is “real.” The researchers and their colleagues are currently working on different approaches to achieve this capability.
According to a new proposal, GPS satellites may be the key to finding dark matter. Credit: NASA
You know the old saying: “if you want to hide something, put it in plain sight?” Well, according to a new proposal by two professors of physics, this logic may be the reason why scientists have struggled for so long to find the mysterious mass that is believed to comprise 27% of the matter in the universe.
In short, these two physicists believe that dark matter can be found the same way the you can find the fastest route to work: by consulting the Global Positioning System.
Andrei Derevianko, of the University of Nevada, Reno, and Maxim Pospelov, of the University of Victoria and the Perimeter Institute for Theoretical Physics in Canada, proposed this method earlier this year at a series of renowned scientific conferences, where it met with general approval.
Their idea calls for the use of GPS satellites and other atomic clock networks and comparing their times to look for discrepancies. Derevianko and Pospelov suggest that dark matter could have a disruptive affect on atomic clocks, and that by looking at existing networks of atomic clocks it might be possible to spot pockets of dark matter by their distinctive signature.
The two are starting to test this theory by analyzing clock data from the 30 GPS satellites, which use atomic clocks for everyday navigation. Correlated networks of atomic clocks, such as the GPS and some ground networks already in existence, can be used as a powerful tool to search for the topological defect dark matter where initially synchronized clocks will become desynchronized.
The Hubble Space Telescope image of gravitational lensing in the galaxy cluster Abell 2218 indicating the presence of large amount of dark matter. Credit: NASA/Andrew Fruchter/STScI
“Despite solid observational evidence for the existence of dark matter, its nature remains a mystery,” Derevianko, a professor in the College of Science at the University, said. “Some research programs in particle physics assume that dark matter is composed of heavy-particle-like matter. This assumption may not hold true, and significant interest exists for alternatives.”
Their proposal builds on the idea that dark matter could come from cracks in the universe’s quantum fields that could disturb such fundamental properties as the mass of an electron, and have an effect on the way we measure time. This represents a break from the more conventional view that dark matter consists of subatomic particles such as WIMPs and axions.
“Our research pursues the idea that dark matter may be organized as a large gas-like collection of topological defects, or energy cracks,” Derevianko said. “We propose to detect the defects, the dark matter, as they sweep through us with a network of sensitive atomic clocks. The idea is, where the clocks go out of synchronization, we would know that dark matter, the topological defect, has passed by. In fact, we envision using the GPS constellation as the largest human-built dark-matter detector.”
Derevianko is collaborating on analyzing GPS data with Geoff Blewitt, director of the Nevada Geodetic Laboratory, also in the College of Science at the University of Nevada, Reno. The Geodetic Lab developed and maintains the largest GPS data processing center in the world, able to process information from about 12,000 stations around the globe continuously, 24/7.
Artist’s rendering of a vacuum tube, one of the main components of an atomic clock. Credit: NASA
Blewitt, also a physicist, explained how an array of atomic clocks could possibly detect dark matter.
“We know the dark matter must be there, for example, because it is seen to bend light around galaxies, but we have no evidence as to what it might be made of,” he said. “If the dark matter were not there, the normal matter that we know about would not be sufficient to bend the light as much as it does. That’s just one of the ways scientists know there is a massive amount of dark matter somewhere out there in the galaxy. One possibility is that the dark matter in this gas might not be made out of particles like normal matter, but of macroscopic imperfections in the fabric of space-time.
“The Earth sweeps through this gas as it orbits the galaxy. So to us, the gas would appear to be like a galactic wind of dark matter blowing through the Earth system and its satellites. As the dark matter blows by, it would occasionally cause clocks of the GPS system to go out of sync with a tell-tale pattern over a period of about 3 minutes. If the dark matter causes the clocks to go out of sync by more than a billionth of a second we should easily be able to detect such events.”
“This type of work can be transformative in science and could completely change how we think about our universe,” Jeff Thompson, a physicist and dean of the University’s College of Science, said. “Andrei is a world class physicist and he has already made seminal contributions to physics. It’s a wonder to watch the amazing work that comes from him and his group.”
Derevianko teaches quantum physics and related subjects at the University of Nevada, Reno. He has authored more than 100 refereed publications in theoretical physics. He is a fellow of the American Physical Society, a Simons fellow in theoretical physics and a Fulbright scholar. Among a variety of research topics, he has contributed to the development of several novel classes of atomic clocks and precision tests of fundamental symmetries with atoms and molecules.
Their research appeared earlier this week in the online version of the scientific journal Nature Physics, ahead of the print version.
New research suggests that Dark Matter may exist in clumps distributed throughout our universe. Credit: Max-Planck Institute for Astrophysics
We know dark matter exists. We know this because without it and dark energy, our Universe would be missing 95.4% of its mass. What’s more, scientists would be hard pressed to explain what accounts for the gravitational effects they routinely see at work in the cosmos.
For decades, scientists have sought to prove its existence by smashing protons together in the Large Hadron Collider. Unfortunately, these efforts have not provided any concrete evidence.
Hence, it might be time to rethink dark matter. And physicists David M. Jacobs, Glenn D. Starkman, and Bryan Lynn of Case Western Reserve University have a theory that does just that, even if it does sound a bit strange.
In their new study, they argue that instead of dark matter consisting of elementary particles that are invisible and do not emit or absorb light and electromagnetic radiation, it takes the form of chunks of matter that vary widely in terms of mass and size.
As it stands, there are many leading candidates for what dark matter could be, which range from Weakly-Interacting Massive Particles (aka WIMPs) to axions. These candidates are attractive, particularly WIMPs, because the existence of such particles might help confirm supersymmetry theory – which in turn could help lead to a working Theory of Everything (ToE).
According to supersymmetry, dark-matter particles known as neutralinos (aka WIMPs) annihilate each other, creating a cascade of particles and radiation. Credit: Sky & Telescope / Gregg Dinderman.
But so far, no evidence has been obtained that definitively proves the existence of either. Beyond being necessary in order for General Relativity to work, this invisible mass seems content to remain invisible to detection.
According to Jacobs, Starkman, and Lynn, this could indicate that dark matter exists within the realm of normal matter. In particular, they consider the possibility that dark matter consists of macroscopic objects – which they dub “Macros” – that can be characterized in units of grams and square centimeters respectively.
Macros are not only significantly larger than WIMPS and axions, but could potentially be assembled out of particles in the Standard Model of particle physics – such as quarks and leptons from the early universe – instead of requiring new physics to explain their existence. WIMPS and axions remain possible candidates for dark matter, but Jacobs and Starkman argue that there’s a reason to search elsewhere.
“The possibility that dark matter could be macroscopic and even emerge from the Standard Model is an old but exciting one,” Starkman told Universe Today, via email. “It is the most economical possibility, and in the face of our failure so far to find dark matter candidates in our dark matter detectors, or to make them in our accelerators, it is one that deserves our renewed attention.”
After eliminating most ordinary matter – including failed Jupiters, white dwarfs, neutron stars, stellar black holes, the black holes in centers of galaxies, and neutrinos with a lot of mass – as possible candidates, physicists turned their focus on the exotics.
Ongoing experiments at the Large Hadron Collider have so far failed to produce evidence of WIMPs. Credit: CERN/LHC/GridPP
Nevertheless, matter that was somewhere in between ordinary and exotic – relatives of neutron stars or large nuclei – was left on the table, Starkman said. “We say relatives because they probably have a considerable admixture of strange quarks, which are made in accelerators and ordinarily have extremely short lives,” he said.
Although strange quarks are highly unstable, Starkman points out that neutrons are also highly unstable. But in helium, bound with stable protons, neutrons remain stable.
“That opens the possibility that stable strange nuclear matter was made in the early Universe and dark matter is nothing more than chunks of strange nuclear matter or other bound states of quarks, or of baryons, which are themselves made of quarks,” said Starkman.
Such dark matter would fit the Standard Model.
This is perhaps the most appealing aspect of the Macros theory: the notion that dark matter, which our cosmological model of the Universe depends upon, can be proven without the need for additional particles.
Still, the idea that the universe is filled with a chunky, invisible mass rather than countless invisible particles does make the universe seem a bit stranger, doesn’t it?
The galactic core, observed using infrared light and X-ray light.
Credit: NASA, ESA, SSC, CXC, and STScI
The galactic center is a happening place, with lots of gas, dust, stars, and surprising binary stars orbiting a supermassive black hole about three million times the size of our sun. With so many stars, astronomers estimate that there should be hundreds of dead ones. But to date, scientists have found only a single young pulsar at the galactic center where there should be as many as 50.
The question thus arises: where are all those rapidly spinning, dense stellar corpses known as pulsars? Joseph Bramante of Notre Dame University and astrophysicist Tim Linden of the University of Chicago have a possible solution to this missing-pulsar problem, which they describe in a paper accepted for publication in the journal Physical Review Letters.
Maybe those pulsars are absent because dark matter, which is plentiful in the galactic center, gloms onto the pulsars, accumulating until the pulsars become so dense they collapse into a black hole. Basically, they disappeared into the fabric of space and time by becoming so massive that they punched a hole right through it.
Dark matter, as you may know, is the theoretical mass that astrophysicists believe fills roughly a quarter of our universe. Alas, it is invisible and undetectable by conventional means, making its presence known only in how its gravitational pull interacts with other stellar objects.
One of the more popular candidates for dark matter is Weakly Interacting Massive Particles, otherwise known as WIMPs. Underground detectors are currently hunting for WIMPs and debate has raged over whether gamma rays streaming from the galactic center come from WIMPs annihilating one another.
In general, any particle and its antimatter partner will annihilate each other in a flurry of energy. But WIMPs don’t have an antimatter counterpart. Instead, they’re thought to be their own antiparticles, meaning that one WIMP can annihilate another.
But over the last few years, physicists have considered another class of dark matter called asymmetric dark matter. Unlike WIMPs, this type of dark matter does have an antimatter counterpart.
Cosmic Infrared Background ExpeRIment (CIBER) simulation of the density of matter when the universe was one billion years old, as produced by large-scale structures from dark matter. Credit: Caltech/Jamie Bock
Asymmetric dark matter appeals to physicists because it’s intrinsically linked to the imbalance of matter and antimatter. Basically, there’s a lot more matter in the universe than antimatter – which is good considering anything less than an imbalance would lead to our annihilation. Likewise, according to the theory, there’s much more dark matter than anti-dark-matter.
Physicists think that in the beginning, the Big Bang should’ve created as much matter as antimatter, but something altered this balance. No one’s sure what this mechanism was, but it might have triggered an imbalance in dark matter as well – hence it is “asymmetric”.
Dark matter is concentrated at the galactic center, and if it’s asymmetric, then it could collect at the center of pulsars, pulled in by their extremely strong gravity. Eventually, the pulsar would accumulate so much mass from dark matter that it would collapse into a black hole.
The idea that dark matter can cause pulsars to implode isn’t new. But the new research is the first to apply this possibility to the missing-pulsar problem.
If the hypothesis is correct, then pulsars around the galactic center could only get so old before grabbing so much dark matter that they turn into black holes. Because the density of dark matter drops the farther you go from the center, the researchers predict that the maximum age of pulsars will increase with distance from the center. Observing this distinct pattern would be strong evidence that dark matter is not only causing pulsars to implode, but also that it’s asymmetric.
“The most exciting part about this is just from looking at pulsars, you can perhaps say what dark matter is made of,” Bramante said. Measuring this pattern would also help physicists narrow down the mass of the dark matter particle.
Artist’s illustration of a pulsar that was found to be an ultraluminous X-ray source. Credit: NASA, Caltech-JPL
But as Bramante admits, it won’t be easy to detect this signature. Astronomers will need to collect much more data about the galactic center’s pulsars by searching for radio signals, he claims. The hope is that as astronomers explore the galactic center with a wider range of radio frequencies, they will uncover more pulsars.
But of course, the idea that dark matter is behind the missing pulsar problem is still highly speculative, and the likelihood of it is being called into question.
“I think it’s unlikely—or at least it is too early to say anything definitive,” said Zurek, who was one of the first to revive the notion of asymmetric dark matter in 2009. The tricky part is being able to know for sure that any measurable pattern in the pulsar population is due to dark-matter-induced collapse and not something else.
Even if astronomers find this pulsar signature, it’s still far from being definitive evidence for asymmetric dark matter. As Kathryn Zurek of the Lawrence Berkeley National Laboratory explained: “Realistically, when dark matter is detected, we are going to need multiple, complementary probes to begin to be convinced that we have a handle on the theory of dark matter.”
And asymmetric dark matter may not have anything to do with the missing pulsar problem at all. The problem is relatively new, so astronomers may find more plausible, conventional explanations.
“I’d say give them some time and maybe they come up with some competing explanation that’s more fleshed out,” Bramante said.
Nevertheless, the idea is worth pursuing, says Haibo Yu of the University of California, Riverside. If anything, this analysis is a good example of how scientists can understand dark matter by exploring how it may influence astrophysical objects. “This tells us there are ways to explore dark matter that we’ve never thought of before,” he said. “We should have an open mind to see all possible effects that dark matter can have.”
There’s one other way to determine if dark matter can cause pulsars to implode: To catch them in the act. No one knows what a collapsing pulsar might look like. It might even blow up.
“While the idea of an explosion is really fun to think about, what would be even cooler is if it didn’t explode when it collapsed,” Bramante said. A pulsar emits a powerful beam of radiation, and as it spins, it appears to blink like a lighthouse with a frequency as high as several hundred times per second. As it implodes into a black hole, its gravity gets stronger, increasingly warping the surrounding space and time.
Studying this scenario would be a great way to test Einstein’s theory of general relativity, Bramante says. According to theory, the pulse rate would get slower and slower until the time between pulses becomes infinitely long. At that point, the pulses would stop entirely and the pulsar would be no more.
This annotated artist's conception illustrates our current understanding of the structure of the Milky Way galaxy. Image Credit: NASA
Astronomy is notorious for raising more questions than it answers. Take the observation that the vast majority of matter is invisible.
Although astronomers have gathered overwhelming evidence that dark matter makes up roughly 84 percent of the universe’s matter — providing straightforward explanations for the rotation of individual galaxies, the motions of distant galaxy clusters, and the bending of distant starlight — they remain unsure about any specifics.
Now, a group of Australian astronomers thinks there’s only half as much dark matter in the Milky Way as previously thought.
In 1933, Swiss astronomer Fritz Zwicky observed the Coma cluster — a galaxy cluster roughly 320 million light-years away and nearly 2 light-years across — and found that it moved too rapidly. There simply wasn’t enough visible matter to hold the galaxy cluster together.
Zwicky decided there must be a hidden ingredient, known as dunkle Materie, or dark matter, that caused the motions of these galaxies to be so large.
The rotation curve of the Milky Way. Image Credit: Kafle et al.
Then in 1978, American astronomer Vera Rubin looked at individual galaxies. Astronomers largely assumed galaxies rotated much like our Solar System, with the outer planets rotating slower than the inner planets. This argument aligns with Newton’s Laws and the assumption that most of the mass is located in the center.
But Rubin found that galaxies rotated nothing like our own Solar System. The outer stars did not rotate slower than the inner stars, but just as fast. There had to be dark matter on the outskirts of every galaxy.
Now, astronomer Prajwal Kafle, from The University of Western Australia, and his colleagues have once again observed the speed of stars on the outskirts of our own galaxy, the Milky Way. But he did so in much greater detail than previous estimates.
From a star’s speed, it’s relatively simple to calculate any interior mass. The simple equation below shows that the interior mass (M) is equal to the distance the star is from the galactic center (R) times its velocity (V) squared, all divided by the gravitational constant (G):
Kafle and his colleagues used messier physics accounting for the sloppiness of the galaxy. But the point holds, with a star’s velocity, you can calculate any interior mass. And with multiple stars’ velocities you’re bound to be more accurate. The team found the dark matter in our galaxy weighs 800 billion times the mass of the Sun, half of previous estimates.
“The current idea of galaxy formation and evolution … predicts that there should be a handful of big satellite galaxies around the Milky Way that are visible with the naked eye, but we don’t see that,” said Kafle in a news release. This is typically referred to as the missing satellites problem, and it has evaded astronomers for years.
“When you use our measurement of the mass of the dark matter the theory predicts that there should only be three satellite galaxies out there, which is exactly what we see; the Large Magellanic Cloud, the Small Magellanic Cloud and the Sagittarius Dwarf Galaxy,” said Kafle.
These new measurements might prove the Milky Way is not quite the behemoth astronomers previously thought. They also help explain why there are so few satellite galaxies in orbit. But first the results will have to be confirmed as they stand up against numerous other ways to weigh the dark matter in our galaxy.
The results have been published in the Astrophysical Journal and are available online.
On either side of the white line in the picture are two models of how dark matter is distributed in a galaxy similar to the Milky Way. At left, non-interacting cold dark matter creates satellite galaxies. At right, dark matter interacting with other particles makes the number of observed satellite galaxies smaller. Credit: Durham University
Funny how small particle interactions can have such a big effect on the neighbors of the Milky Way. For a while, scientists have been puzzled about the dearth of small satellite galaxies surrounding our home galaxy.
They thought that cold dark matter in our galaxy should encourage small galaxies to form, which created a puzzle. Now, a new set of research suggests the dark matter actually interacted with small bits of normal matter (photons and neutrinos) and the dark matter scattered away, reducing the amount of material available for building galaxies.
“We don’t know how strong these interactions should be, so this is where our simulations come in,” stated Celine Boehm, a particle physicist at Durham University who led the research. “By tuning the strength of the scattering of particles, we change the number of small galaxies, which lets us learn more about the physics of dark matter and how it might interact with other particles in the Universe.”
Artist’s conception of the Milky Way galaxy based on the latest survey data from ESO’s VISTA telescope at the Paranal Observatory. A prominent bar of older, yellower stars lies at galaxy center surrounded by a series of spiral arms. The galaxy spans some 100,000 light years. Credit: NASA/JPL-Caltech, ESO, J. Hurt
Dark matter is a poorly understood part of the Universe, which is frustrating for scientists because it (along with dark energy) is believed to make up the majority of our Cosmos. There are several postulated types of it, but the main thing to understand is dark matter is hard to detect (except, in certain cases, through its interactions with gravity.)
This isn’t the only explanation for why the galaxies are missing, the scientists caution. Perhaps the universe’s first stars were so hot that they affected the gas that other stars formed from, for example.
