Category: Dark Matter

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Scientists are trying to understand the invisible and hypothetical ‘dark matter’ – the stuff that we know exists by inference of its gravitational influence on the matter we can see. The most common held notion of dark matter is that it exists in ‘halos’ or clumps that surround galaxies. But a new study predicts that galaxies like our own Milky Way, also contain a disk of dark matter. Using the results of a supercomputer simulation, scientists from the University of Zurich and the University of Central Lancashire say that if dark matter in fact resides as a disk within a galaxy, it could allow physicists to directly detect and identify the nature of dark matter for the first time.

Physicists believe dark matter makes up 22% of the mass of the Universe (compared with the 4% of normal matter and 74% comprising the mysterious ‘dark energy’). But, despite its pervasive influence, no-one is sure what dark matter consists of.

This ‘standard’ theory of dark matter is based on supercomputer simulations that model the gravitational influence of the dark matter alone. The new work includes the gravitational influence of the stars and gas that also make up our Galaxy.

Stars and gas are thought to have settled into disks very early on in the life of the Universe and this affected how smaller dark matter halos formed. The team’s results suggest that most lumps of dark matter in our locality merged to form a halo around the Milky Way. But the largest lumps were preferentially dragged towards the galactic disk and were then torn apart, creating a disk of dark matter within our Galaxy.

“The dark disk only has about half of the density of the dark matter halo, which is why no one has spotted it before,” said lead author Justin Read. “However, despite its low density, if the disk exists it has dramatic implications for the detection of dark matter here on Earth.”

The Earth and Sun move at some 220 kilometres per second along a nearly circular orbit about the center of our Galaxy. Since the dark matter halo does not rotate, from an Earth-based perspective it feels as if we have a ‘wind’ of dark matter flowing towards us at great speed. By contrast, the ‘wind’ from the dark disk is much slower than from the halo because the disk co-rotates with the Earth.

“It’s like sitting in your car on the highway moving at a hundred kilometres an hour”, said team member Dr. Victor Debattista. “It feels like all of the other cars are stationary because they are moving at the same speed.”

This abundance of low-speed dark matter particles, the science team says, could be a real boon for researchers because they are more likely to excite a response in dark matter detectors than fast-moving particles. “Current detectors cannot distinguish these slow moving particles from other background ‘noise’,” said Prof. Laura Baudis, a collaborator at the University of Zurich and one of the lead investigators for the XENON direct detection experiment, which is located at the Gran Sasso Underground Laboratory in Italy. “But the XENON100 detector that we are turning on right now is much more sensitive. For many popular dark matter particle candidates, it will be able to see something if it’s there.”

If so, its possible that the dark disk could be directly detected in the very near future.

Sources: Monthly Notices paper, Royal Astronomical Society

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Rumors are spinning faster than a neutron star about the possibility that a European satellite mission called PAMELA may have made a direct detection of dark matter, the mysterious particles thought to make up as much of 85% of all matter in the Universe. Word got out in August at a conference about dark matter in Stockholm, Sweden where the PAMELA (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics) team presented their preliminary findings to a few selected physicists. What information has leaked out says the satellite has detected more positrons than can be explained by known physics and that this excess exactly matches what dark matter particles would produce if they were annihilating each other at the center of the galaxy. But the PAMELA team is not allowing any more information to be made public, until they re-analyze their data and allow other scientists to evaluate and verify the findings. This is good, if not wonderful, in all respects – making sure their findings are peer reviewed before publishing their work and going public. (Does anyone remember the cold fusion debacle?) But in what seems to cross the line of good science — as well pushing the boundaries of what is just plain polite, two other scientists have published an abstract based on what was revealed to them at the conference.

Ever since cosmologists “concocted” dark matter to explain the matter that was obviously missing from the universe’s equation, scientists have speculated, worked, created models and worked some more to determine exactly what dark matter is. Recent findings (see here and here)seem to be bringing us closer to finding this mysterious substance, providing clues to what this stuff might be. The PAMELA data seems to point towards positrons, or anti-electrons.

Marco Cirelli from the CEA near Paris in France and Alessandro Strumia from the Università di Pisa in Italy presented their own analysis of the PAMELA data in this abstract. They say the data agrees with their own model called Minimal Dark Matter in which the particle responsible is called the “Wino.” They do reference their own work but interestingly, many of their references are from talks given at the conference on August 18-22. At one point they note, “The preliminary data points for positron and antiproton fluxes plotted in our figures have been extracted from a photo of the slides taken during the talk, and can thereby slightly differ from the data that the PAMELA collaboration will officially publish.”

Is this just a desire to “publish” something first, or is this real science?

Sources: ArXiv, ArXiv blog, Nature

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More news on dark matter this week: By analyzing light from dwarf galaxies that orbit the Milky Way, scientists believe they have discovered the minimum mass for galaxies in the universe – 10 million times the mass of the sun. This mass could be the smallest known “building block” of the mysterious, invisible substance called dark matter. Stars that form within these building blocks clump together and turn into galaxies. Scientists know very little about the microscopic properties of dark matter, even though it accounts for approximately five-sixths of all matter in the universe. “By knowing this minimum galaxy mass, we can better understand how dark matter behaves, which is essential to one day learning how our universe and life as we know it came to be,” said Louis Strigari, lead author of this study from the University of California, Irvine.

Dark matter governs the growth of structure in the universe. Without it, galaxies like our own Milky Way would not exist. Scientists know how dark matter’s gravity attracts normal matter and causes galaxies to form. They also suspect that small galaxies merge over time to create larger galaxies such as our Milky Way.

The smallest known galaxies, called dwarf galaxies, vary greatly in brightness, from 1,000 times the luminosity of the sun to 10 million times the luminosity of the sun. At least 22 of these dwarf galaxies are known to orbit the Milky Way. UCI scientists studied 18 of them using data obtained with the Keck telescope in Hawaii and the Magellan telescope in Chile, with the goal of calculating their masses. By analyzing stars’ light in each galaxy, they determined how fast the stars were moving. Using those speeds, they calculated the mass of each galaxy.

The researchers expected the masses to vary, with the brightest galaxy weighing the most and the faintest galaxy weighing the least. But surprisingly all dwarf galaxies had the same mass – 10 million times the mass of the sun.

Manoj Kaplinghat, a study co-author and physics and astronomy assistant professor at UCI, explains this finding using an analogy in which humans play the role of dark matter.

“Suppose you are an alien flying over Earth and identifying urban areas from the concentration of lights in the night. From the brightness of the lights, you may surmise, for example, that more humans live in Los Angeles than in Mumbai, but this is not the case,” Kaplinghat said. “What we have discovered is more extreme and akin to saying that all metro areas, even those that are barely visible at night to the aliens, have a population of about 10 million.”

Since dwarf galaxies are mostly dark matter – the ratio of dark matter to normal matter is as large as 10,000 to one – the minimum-mass discovery reveals a fundamental property of dark matter.

“We are excited because these galaxies are virtually invisible, yet contain a tremendous amount of dark matter,” said James Bullock, a study co-author and director of UCI’s Center for Cosmology. “This helps us better understand the particle that makes up dark matter, and it teaches us something about how galaxies form in the universe.”

The scientists say clumps of dark matter may exist that contain no stars. The only dark matter clumps they can detect right now are those that are lit by stars.

Scientists hope to learn about dark matter’s microscopic properties when the Large Hadron Collider in Switzerland becomes operational later this year. The device will accelerate two beams of nuclei in a ring in opposite directions and then slam them together to recreate conditions just after the Big Bang. By doing this, scientists hope to create the dark matter particle in the lab for the first time.

Source: University of California, Irvine

Dark matter was theorized to exist relatively recently, and we’ve come a long way in understanding what makes up a whopping 23% of our Universe. Our own galaxy is surrounded by a halo of dark matter that adds to its mass. A recent paper on the dark matter closer to home – right here in our own Solar System – reveals that it is denser and more massive than in the galactic halo.

Dark matter is just plain weird stuff. It doesn’t give off light, has mass and reacts gravitationally with “normal” matter – the stuff that we and our planet and the stars are composed of. Just like normal matter, it “clumps” up, or accretes, because of this gravitational attraction; we find more dark matter near galaxies than in the vast expanses between them.

Dark matter isn’t just far off in the Milky Way or somewhere on the other side of the Universe, though: it’s right here at home in our Solar System. In a recent paper submitted to Physical Review D, Ethan Siegel and Xiaoying Xu of the University of Arizona analyzed the distribution of dark matter in our Solar System, and found that the mass of dark matter is 300 times more than that of the galactic halo average, and the density is 16,000 times higher than that of the background dark matter.

Over the history of the Solar System, Xu and Siegel calculate that 1.07 X 10^20 kg of dark matter have been captured, or about 0.0018% the mass of the Earth. To get a handle on this number, the mass of Ceres – the largest object in the asteroid belt between Mars and Jupiter – is about 9 times this amount.

Siegel and Xu calculated how much dark matter the Solar System has swept up over it’s 4.5 billion-year lifespan by modeling the composition of the background dark matter halo in the orbit of the Solar System around the galaxy, and calculating just how much dark matter would be trapped by the Solar System as it moves through this halo. They ran this calculation for the Sun and each one of the eight planets separately, giving the distribution of the matter throughout the Solar System, as well as the total amount captured.

Much like when you drive your car through a light snowfall, dark matter “sticks” to the Solar System when it is gravitationally bound by the Sun and planets. Just as some of the snow melts on your windshield (hopefully), some doesn’t stick to the hood and most just flies right by, dark matter isn’t distributed evenly throughout our Solar System, either. Some planets have more dark matter surrounding them than others, depending on where they are. Shown below is the density distribution of the dark matter in the Solar System

The first spike is Mercury, and the next two spikes are Venus and Earth (Mars doesn’t show up). The next is Jupiter, followed by a small bump from Saturn and finally Uranus and Neptune combined create the last small bump.

How does the local dark matter effect interactions in the Solar System? Well,it doesn’t have a large effect on the orbits of the planets, nor does it slow down the Solar System in its orbit around the galactic center appreciably.

“Planetary orbits, if there were enough dark matter present, would have their perihelia precess faster than if there were no dark matter. The amount of dark matter allowed from these observations is considerably greater than the amount I predict. The errors on the measurements of perihelion precession are in units of hundredths of an arc second per century…Even if you assume the dark matter is at rest with respect to the galaxy that the Solar System moves through (which is the extreme example), the Sun is of order 10^30 kg; capturing a 10^20 kg clump of dark matter will slow you down by about 20 microns/second over the lifetime of the Solar System. So that would be small.” – Ethan Siegel in an email interview.

And, alas, the mystery of the Pioneer anomaly is not going to be solved by this revelation, as the mass of the captured dark matter is not enough to explain the odd motions of that spacecraft.

The discovery of a higher density and mass of dark matter in our neighborhood may aid in the study and detection of dark matter, though. Knowing the mass and density distribution of the local dark matter – and thus knowing how much and where to look for it – will provide astronomers looking into solving exactly what it’s made up of with more information .

“Our determination of the local dark matter density and velocity distribution are of great importance to direct detection experiments. The most recent calculations that have been carried out assume that the properties of dark matter at the Sun’s location are derived directly from the galactic halo. By comparison, we find that terrestrial experiments should also consider a component of dark matter with a density 16,000 times greater than the background halo density,” wrote Xu and Siegel.

Source: Arxiv, email interview with Ethan Siegel

It is thought that primordial or “Population III” stars were born in dense clouds of dark matter, 100 million years after the Big Bang. During the period between birth and dark matter depletion, these first stars were effectively but into a “deep freeze” where normal star development was prevented. After this period when all the dark matter fuel had been consumed, these stars were allowed to commence normal stellar evolution, dying out within a few hundred thousand years. But say if a Population III star was born in an exceptionally dense cloud of dark matter? How long could “normal stellar evolution” be frozen for? According to new research, dark matter could theoretically freeze the star indefinitely, over timescales longer than the age of the Universe…

This amazing theory comes from research carried out by Gianfranco Bertone and his team at the Paris Institute of Astrophysics in France. The thought that the first stars, born over 14 billion years ago, could possibly inhabit the Universe today is a very impressive idea. These primordial stars are thought to have been seeded inside dense clouds of dark matter, where gravity caused dark matter compression. As the matter became concentrated, non-baryonic particles may have begun annihilating, stopping natural hydrogen fusion (the mechanism commonly associated with star creation). “Normal” stellar evolution was therefore paused and the “dark star” phase began as dark matter annihilation heated the stellar cores.

It has long been the assumption that the “dark star” phase occurred for a short period of time in the early Universe where vast halos of dark matter may have dominated. Once the dark matter fuel ebbed away, primordial stars were left to self-destruct in a flurry of accelerated evolution. Now Bertone and his colleagues believe a few primordial specimens might be alive today, hidden inside particularly dense clouds of dark matter, in galactic centres, keeping some of the Universe’s first stars in a state of suspended animation.

“There could be conditions in the early universe where stars form in big enough reservoirs of dark matter to last until the present day.” – Gianfranco Bertone.

One of the most exciting implications to come from this research is the fact that these ancient relics may be observed, what’s more, we may have already seen some. “A frozen star would appear much bigger and colder than a normal star with the same mass and chemical composition,” says Marco Taoso, co-investigator in the French group. If stars matching the characteristics of these frozen stellar bodies are (or already have been) found, the discovery would have huge consequences for the quantum search for supersymmetry, indicating dark matter was indeed made up of massive “superpartners” to ordinary matter.

If dark matter influenced stars a few hundred thousand years after the Big Bang, can it still influence stellar evolution today? Researchers believe this could be the case. Present-day stars evolving in regions of dark matter clouds may be influenced by non-baryonic particles. White dwarfs are formed after the death of Sun-like stars and it is believed that should the dwarf star encounter a cloud of dark matter, it could be resurrected as a dark matter burner, shining like 30 Suns.

It will be interesting to see if there have already been any observations of these primordial stars, possibly providing more indirect evidence of dark matter in our Universe.

Source: New Scientist