Tag Archives: dark matter

Vera Rubin R.I.P.

Vera Rubin, whose work helped confirm the existence of dark matter, passed away December 25 at the age of 88.

In the 1960s, Rubin’s interest in how stars orbit their galactic centers led her and colleague Kent Ford to study the Andromeda galaxy, M31, a nearby spiral. The two scientists wanted to determine the distribution of mass in M31 by looking at the orbital speeds of stars and gas at varying distances from the galactic center. They expected the speeds to conform to Newtonian gravitational theory, whereby an object farther from its central mass orbits slower than those closer in. To their surprise, the scientists found that stars far from the center traveled as fast as those near the center.

After observing dozens more galaxies by the 1970s, Rubin and colleagues found that something other than the visible mass was responsible for the stars’ motions. Each spiral galaxy is embedded in a “halo” of dark matter—material that does not emit light and extends beyond the optical galaxy. They found it contains 5 to 10 times as much mass as the luminous galaxy. As a result of Rubin’s groundbreaking work, it has become apparent that more than 90% of the universe is composed of this invisible material. The first inkling that dark matter existed came in 1933 when Swiss astrophysicist Fritz Zwicky of Caltech proposed it. But it was not until Rubin’s work that dark matter was confirmed.

Rubin was a top notch astronomer, which is why she was part of this important discovery. She was also an exception, as at the time relatively few women were interested in becoming astronomers. Be prepared, however, for a slew of articles in the next few days focused not about her work and her contributions to science, but focused instead almost entirely on the sexist oppression she had to overcome in the evil sexist male chauvinist society of mid-twentieth-century America.

All those articles will be wrong. While there were certainly obstacles in Rubin’s way because of her sex, they were hardly as bad as it will be made out to be. Worse, this focus on gender and oppression will distract from honoring the passing of a great astronomer. It will also distract from the significance of her discovery, which continues to baffle astronomers a half century later.

Dark matter unnecessary?

The uncertainty of science: A new analysis of the infrared data from 153 galaxies using the Spitzer Space Telescope suggests that dark matter might not be necessary to explain the rotation of galaxies.

First, this concise and nicely written explanation from the link of why dark matter has been proposed:

Newton’s laws of motion predict that planets that revolve closer to a star move faster than those that are farther away. In principle this should also hold true for stars circling the cores of galaxies, but for nearly a century, astronomers have seen that stars near the outskirts of galaxies orbit at nearly the same velocities as ones near galactic centers.

To explain why these outlying stars travel as quickly as they do without flying out into the void beyond, researchers came up with the idea of dark matter, a substance whose gravitational pull is thought to keep whirling stars in check. Scientists have largely ruled out all known particles as possible explanations for dark matter, and the consensus is that dark matter must be a kind of invisible, intangible material that is only detectable via its gravitational influence.

However, despite decades of trying, researchers have failed to capture a single mote of dark matter, even though it is supposed to make up roughly five-sixths of all matter in the universe. This raises the possibility that dark matter might not be real.

The new research, which I must admit I do not really understand, supposedly suggests that dark matter is unnecessary to explain the motions of stars.

Previous analyses of the orbital velocities of the stars in galaxies often depended on visible wavelengths of light. However, the stars that produce the most visible light are relatively short-lived and prone to fluctuations, and so may not provide the best picture of how matter is scattered overall throughout a galaxy. Instead, McGaugh and his colleagues analyzed near-infrared images collected by NASA’s Spitzer Space Telescope over the past five years. “The stars that generate the most near-infrared light are red giants, that are pretty stable in their output, and so are much better representative of a galaxy’s total mass of stars,” McGaugh said.

The researchers found an extraordinarily close association between the location of normal matter and the way it accelerates around the centers of galaxies. “We were surprised at how tight that relationship was,” McGaugh said. “It looks tantamount to a law of nature.”

Neither the article nor the scientists who did this research however explain clearly how this tight association negates the need for dark matter.

WIMP detector finds nothing

The uncertainty of science: A detector buried a mile underground so that it could only detect the predicted Weak Interacting Massive Particles (WIMP) thought to comprise dark matter has found nothing

Dark matter is thought to account for more than four-fifths of the mass in the universe. Scientists are confident of its existence because the effects of its gravity can be seen in the rotation of galaxies and in the way light bends as it travels through the universe, but experiments have yet to make direct contact with a dark matter particle. The LUX experiment was designed to look for weakly interacting massive particles, or WIMPs, the leading theoretical candidate for a dark matter particle. If the WIMP idea is correct, billions of these particles pass through your hand every second, and also through the Earth and everything on it. But because WIMPs interact so weakly with ordinary matter, this ghostly traverse goes entirely unnoticed.

…“We worked hard and stayed vigilant over more than a year and a half to keep the detector running in optimal conditions and maximize useful data time,” said Simon Fiorucci, a physicist at Berkeley Lab and Science Coordination Manager for the experiment. “The result is unambiguous data we can be proud of and a timely result in this very competitive field—even if it is not the positive detection we were all hoping for.”

This null result, which has its own uncertainties that require confirmation by another experimental test, places significant constraints on the possible nature of the dark matter particle, assuming it exists. And if confirmed, this result makes the hunt to explain the gravitational data of galaxy rotation, something that has been confirmed repeatedly, far more difficult.

Universe’s expansion rate contradicts dark energy data

The uncertainty of science: New measurements of the universe’s expansion rate, dubbed the Hubble constant, contradict theoretical predictions based on previous data.

For their latest paper, Riess’s team studied two types of standard candles in 18 galaxies using hundreds of hours of observing time on the Hubble Space Telescope. “We’ve been going gangbusters with this,” says Riess.

Their paper, which has been submitted to a journal and posted on the arXiv online repository on 6 April, reports that they measured the constant with an uncertainty of 2.4%, down from a previous best result2 of 3.3%. They find the speed of expansion to be about 8% faster than that predicted based on Planck data, says Riess. [emphasis mine]

I highlight the number of galaxies used to get this data because I think these scientists, are being a bit over-confident about the uncertainty of their data. The universe has untold trillions of galaxies. To say they have narrowed their uncertainty down to only 2.4% based on 18 is the height of silliness.

But then, the lead scientist, Adam Riess, recognizes this, as he is also quoted in the article saying “I think that there is something in the standard cosmological model that we don’t understand.”

Astronomers successfully predict appearance of supernova

For the first time ever astronomers have been able to predict and photograph the appearance of a supernova, its light focused by the gravitational lensing caused by a galaxy and the dark matter that surrounds it.

The NASA/ESA Hubble Space Telescope has captured the image of the first-ever predicted supernova explosion. The reappearance of the Refsdal supernova was calculated from different models of the galaxy cluster whose immense gravity is warping the supernova’s light.

What makes this significant is that the prediction models were based on the theory of gravitational lensing and required the presence of dark matter to work. That they worked and were successful in predicting the appearance of this gravitationally bent light (bent by the dark matter it passed through) is a very strong confirmation of both concepts. Up until now I have been somewhat skeptical of gravitational lensing. This confirmation removes some of that skepticism.

Technical problems for cosmic ray detector on ISS

The failure of a second of four cooling pumps on the Alpha Magnetic Spectrometer on ISS threatens the science instrument’s ability to continue its observations.

The AMS continues to gather science data using the three remaining pumps. They are part of a liquid carbon dioxide cooling system that is meant to dissipate heat as the AMS, which is on the outside of the space station, cycles in and out of sunlight during each 90-minute orbit of Earth Only one pump is needed at any given time. One failed in February 2014 and at least one of the other three is showing possible signs of trouble.

Since the 8.5-tonne AMS began operating in 2011, it has tracked more than 69 billion cosmic rays flying through its detectors. Its goal is to search for antimatter and dark matter. In 2013, AMS scientists reported measuring numbers and energies of positrons that hinted at, but did not confirm, the existence of dark matter.

The news article suggests that the instrument is now working with only one reliable pump. It also is possible that repairs might be done by astronauts on ISS during a spacewalk.

Some background: AMS cost $2 billion and about 20 years to build. It only got launched because Congress ordered NASA to launch one more shuttle mission to ISS to get it there.

Is it dark matter, or a previously unrecognized failure of Newton?

Dark matter?

The uncertainty of science: Using new data gathered by the 10-meter Keck telescope in Hawaii, astronomers have found that the outer stars of elliptical galaxies exhibit the same behavior as the outer stars of spirals, suggesting once again the existence of dark matter.

One of the most important scientific discoveries of the 20th century was that the spectacular spiral galaxies, such as our own Milky Way, rotate much faster than expected, powered by [the] extra gravitational force of invisible “dark matter” as it is now called. Since this discovery 40 years ago, we have learned that this mysterious substance, which is probably an exotic elementary particle, makes up about 85 percent of the mass in the Universe, leaving only 15 percent to be the ordinary stuff encountered in our everyday lives. Dark matter is central to our understanding of how galaxies form and evolve – and is ultimately one of the reasons for the existence of life on Earth – yet we know almost nothing about it.

“The surprising finding of our study was that elliptical galaxies maintain a remarkably constant circular speed out to large distances from their centers, in the same way that spiral galaxies are already known to do,” said Cappellari. “This means that in these very different types of galaxies, stars and dark matter conspire to redistribute themselves to produce this effect, with stars dominating in the inner regions of the galaxies, and a gradual shift in the outer regions to dark matter dominance.”

What is most fascinating about this press release, however, is that it also noted that dark matter is only one explanation for the data, and that the failure of Newtonian physics at large distances, instead of dark matter, might also provide an explanation.

However, the [solution] does not come out naturally from models of dark matter, and some disturbing fine-tuning is required to explain the observations. For this reason, the [problem] even led some authors to suggest that, rather than being due to dark matter, it may be due to Newton’s law of gravity becoming progressively less accurate at large distances. Remarkably, decades after it was proposed, this alternative theory (without dark matter) still cannot be conclusively ruled out.

Physicists call this other theory MOND, for modified Newtonian dynamics. It is not a very popular theory, however, and is almost always ignored, even though it appears to work as well as dark matter to explain the motion of stars in galaxies. Instead, most scientists favor dark matter.

For this press release to mention it as suggests the new data favors it over dark matter, which would make this a significant discovery.

Dark matter is even more of a mystery that expected

The uncertainty of science: Using the Hubble and Chandra space telescopes astronomers have discovered that dark matter is not only invisible to direct observation, it is invisible to itself!

In this new research, Harvey and his team realized just how invisible this stuff is, even to itself. As two galactic clusters collide, the stars, gas and dark matter interact in different ways. The clouds of gas suffer drag, slow down and often stop, whereas the stars zip past one another, unless they collide — which is rare. On studying what happens to dark matter during these collisions, the researchers realized that, like stars, the colliding clouds of dark matter have little effect on one another.

Thought to be spread evenly throughout each cluster, it seems logical to assume that the clouds of dark matter would have a strong interaction — much like the colliding clouds of gas as the colliding dark matter particles should come into very close proximity. But rather than creating drag, the dark matter clouds slide through one another seamlessly.

I guarantee that this result is not definitive. The data here is on the very edge of reality, built on too many assumptions. We know that something undetected as yet is influencing the motions of galaxies, but what exactly it is remains completely unknown. These results only make the mystery more mysterious.

Astronomers find an invisible dwarf galaxy

Using dark matter data that suggested the existence of a faint dwarf galaxy 300,000 light years away on the other side of the Milky Way, astronomers have pinpointed its location by finding a tiny cluster of bright Cepheid variable stars, also located at that distance.

“These young stars are likely the signature of this predicted galaxy,” said Chakrabarti, assistant professor in RIT’s School of Physics and Astronomy. “They can’t be part of our galaxy because the disk of the Milky Way terminates at 48,000 light years.” Invisible particles known as dark matter make up 23 percent of the mass of the universe. The mysterious matter represents a fundamental problem in astronomy because it is not understood, Chakrabarti said.

This result is intriguing because it not only found a previously unknown dwarf galaxy orbiting the Milky Way, it also provides further evidence that dark matter, whatever it is, does exist. The dark matter of this unseen dwarf galaxy showed its gravitational effects on Milky Way stars, and when the astronomers looked at the right spot suggested by those effects, they found distant stars that had to belong to the invisible dwarf galaxy, proving it was there. This is comparable to finding Neptune and Pluto by analyzing their gravitational effects and then predicting their location in the sky.

New measurements cut dark matter in Milky Way by half

The uncertainty of science: New more robust measurements by Australian astronomers has shown that the amount of dark matter in the Milky Way galaxy is about half of what previous measurements had estimated.

Without doubt something is causing the outer stars in galaxies to orbit their galaxies at much greater speeds than they should. The answer that astronomers have posited since the late 1950s is that there is additional unidentified mass, dubbed dark matter, lurking as a halo around each galaxy, pulling on those outer stars and making them move faster.

The problem remains that no one has as yet detected this unidentified dark matter. Moreover, there are enormous uncertainties in the measurements of the motions of stars. This result helps narrow those uncertainties.

A new dark matter detector has failed to detect any dark matter after its first three months of operation.

The uncertainty of science: A new dark matter detector has failed to detect any dark matter after its first three months of operation.

Buried about a mile underground in a repurposed South Dakota gold mine, the LUX experiment searches for signs of dark matter particles colliding with the atoms in a vat of liquid xenon. During its first three months of operation, the detector found no such signals whatsoever. “We looked hard for these dark matter particles and we didn’t see anything,” says physicist Rick Gaitskell of Brown University, co-spokesperson for the LUX experiment. The results, presented at a seminar today and submitted to Physical Review Letters for publication, rule out a number of possible masses and characteristics for the particles that make up dark matter. The null result also conflicts with earlier experiments that had reported possible signals of dark matter.

This experiment has not proven that dark matter does not exist. It merely has narrowed significantly the kinds of particles that dark matter could be made of. That the results also contradict evidence from other detectors, however, leaves this specific area of science particularly uncertain.

The Alpha Magnetic Spectrometer on ISS has detected a surplus of positrons, anti-matter electrons, that physicists believe are caused by the existence of dark matter.

The Alpha Magnetic Spectrometer on ISS has detected a surplus of positrons, anti-matter electrons, that physicists believe are caused by the existence of dark matter.

The lead scientist of the experiment also emphasized that dark matter is not the only possible explanation, and that “The detailed interpretation of our data probably will have many theories.”

Astronomers using the Chandra X-Ray Observatory have found that the Milky Way is surrounded by a halo of hot gas.The uncertainty of science: Astronomers using the Chandra X-Ray Observatory have found that the Milky Way is surrounded by a halo of hot gas.

The uncertainty of science: Astronomers using the Chandra X-Ray Observatory have found that the Milky Way is surrounded by a halo of hot gas.

This is the key quote:

The estimated mass of the halo is comparable to the mass of all the stars in the galaxy. If the size and mass of this gas halo is confirmed, it also could be an explanation for what is known as the “missing baryon” problem for the galaxy.

“Missing baryon” is another way to say “dark matter.” In other words, this discovery might prove that it isn’t necessary to invent exotic unknown particles of physics, such as the Weakly Interacting Massive Particles (WIMPs) to explain the missing matter. The missing matter might simply be this hot gas, previously undetected.

Scientists who published a study last month that said they could find no evidence of dark matter in nearby interstellar space, have re-analyzed their data and found that the dark matter is apparently there.

Never mind! Scientists who published a study last month that said they could find no evidence of dark matter in nearby interstellar space have re-analyzed their data and found that the dark matter is apparently there.

More problems for Dark Matter

Vast Polar Structure

A new study by astronomers has found a vast structure of satellite galaxies and star clusters aligned perpendicular to the Milky Way and extending outward above and below the galaxy’s nucleus by as much as a million light years.

In their effort to understand exactly what surrounds our Galaxy, the scientists used a range of sources from twentieth century photographic plates to images from the robotic telescope of the Sloan Deep Sky Survey. Using all these data they assembled a picture that includes bright ‘classical’ satellite galaxies, more recently detected fainter satellites and the younger globular clusters.

“Once we had completed our analysis, a new picture of our cosmic neighbourhood emerged”, says Pawlowski. The astronomers found that all the different objects are distributed in a plane at right angles to the galactic disk. The newly-discovered structure is huge, extending from as close as 33,000 light years to as far away as one million light years from the centre of the Galaxy.

An animation illustrating this galactic distribution is posted below the fold. You can read the actual preprint paper here.

The problem with this polar alignment with the Milky Way’s core is that the theories for explaining the distribution of dark matter do not predict it.
» Read more

Dark matter disappears

The uncertainty of science: A new study has found no evidence of dark matter within 13,000 light years of the Sun, something that had not been expected.

According to widely accepted theories, the solar neighborhood was expected to be filled with dark matter, a mysterious invisible substance that can only be detected indirectly by the gravitational force it exerts. But a new study by a team of astronomers in Chile has found that these theories just do not fit the observational facts. This may mean that attempts to directly detect dark matter particles on Earth are unlikely to be successful.

These findings will be as controversial as the now abandoned faster-than-light neutrino results last fall. Here, however, the new data is likely going to be more robust, which will cause the entire astrophysical community some real conniptions.
» Read more

Higgs announcement from CERN on December 13

CERN will be making an announcement on the status of its search for the Higgs particle on December 13. From this interview of one of its scientists:

The thing I know for sure is that [CERN Director General] Rolf-Dieter Heuer, who must know the results of both experiments, says that on December 13 we will not have a discovery and we will not have an exclusion.

The inteview is fascinating, as he notes how the Higgs research might also have a bearing on the search for dark matter.

Recent results from the Fermi Gamma-ray Space Telescope have found no evidence of dark matter, a result in some conflict with data obtained from several underground research detectors.

The uncertainty of science: Recent results from the Fermi Gamma-ray Space Telescope have found no evidence of dark matter, a result in some conflict with data obtained from several underground research detectors.

The mystery here is that there is no doubt that something causes the outer objects in galaxies to move faster than expected. Scientists have labeled this something as dark matter, guessing that some undetected and unknown mass exists in the outer reaches of galaxies, thereby increasing the gravity potential and hence the velocity in which objects move.

The problem is that they have yet to identify what that dark matter is.

Has dark matter been identified?

From a paper published today on the Los Alamos astro-ph preprint website, scientists suggest that three different physics experiments might have identified dark matter. From the abstract:

Three dark matter direct detection experiments (DAMA/LIBRA, CoGeNT, and CRESST-II) have each reported signals which are not consistent with known backgrounds, but resemble that predicted for a dark matter particle with a mass of roughly ~10 GeV. . . . In this article, we compare the signals of these experiments and discuss whether they can be explained by a single species of dark matter particle, without conflicting with the constraints of other experiments. We find that the spectrum of events reported by CoGeNT and CRESST-II are consistent with each other and with the constraints from CDMS-II, although some tension with xenon-based experiments remains. Similarly, the modulation signals reported by DAMA/LIBRA and CoGeNT appear to be compatible, although the corresponding amplitude of the observed modulations are a factor of at least a few higher than would be naively expected, based on the event spectra reported by CoGeNT and CRESST-II. This apparent discrepancy could potentially be resolved if tidal streams or other non-Maxwellian structures are present in the local distribution of dark matter.

The last sentence above suggests that the differences between the various experiments might be explained by the motion of dark matter itself as it flows through the solar system.

This conclusion is very tentative. The scientists admit that there remain conflicts between the results of the three experiments, and that there also could be explanations other than dark matter for the results. Furthermore, the results of other experiments raise questions about this conclusion.

Nonetheless, it appears that physicists might be closing in on this most ghostlike of all particles in the universe.

Dark matter mysteries

Every year, as part of its educational and research mission, the Space Telescope and Science Institute in Baltimore, Maryland holds a science symposium that focuses one of the big questions of astronomy, inviting over a hundred scientists to come and give their individual perspectives on the state of the field.

This year’s symposium ended yesterday, and the subject was the mysteries of dark matter. Though I wasn’t able to attend the symposium itself, they held a workshop for journalists yesterday, which I did attend. (You can watch the webcast here.)

So, what is dark matter?

First of all, it isn’t dark energy. Dark energy is that mysterious unknown phenomenon that is causing — on vast scales of many billions of light years — the expansion of the universe to accelerate rather than decelerate. It has nothing to do with the question of dark matter.

Second, no one knows. All that scientists do know is that objects in the outer regions of galaxies as well as the galaxies themselves don’t move at the speeds and directions expected if their known mass and gravity were the only forces influencing them. In order to successfully plot their orbits and motions, astronomers have to add a gigantic halo of extra mass, which they have dubbed “dark matter” because it is unseen, undetected, and completely invisible.
» Read more

Experiment fails to find dark matter

The uncertainty of science: An underground experiment in Italy has failed to detect dark matter, as theorized by scientists.

In a paper published online last night, the XENON100 researchers report three events detected during a 100-day run of the experiment last year that might have been due to dark matter1. However, as they expected to see between 1.2 and 2.4 background events — interactions mostly caused by a radioactive contaminant in the xenon — their result is statistically negative and therefore rules out the existence of many of the more strongly interacting and heavier WIMPs.