Record Breaking “Dark Matter Web” Structures Observed Spanning 270 Million Light Years Across


It is well documented that dark matter makes up the majority of the mass in our universe. The big problem comes when trying to prove dark matter really is out there. It is dark, and therefore cannot be seen. Dark matter may come in many shapes and sizes (from the massive black hole, to the tiny neutrino), but regardless of size, no light is emitted and therefore it cannot be observed directly. Astronomers have many tricks up their sleeves and are now able to indirectly observe massive black holes (by observing the gravitational, or lensing, effect on light passing by). Now, large-scale structures have been observed by analyzing how light from distant galaxies changes as it passes through the cosmic web of dark matter hundreds of millions of light years across…

Dark matter is believed to hold over 80% of the Universe’s total mass, leaving the remaining 20% for “normal” matter we know, understand and observe. Although we can observe billions of stars throughout space, this is only the tip of the iceberg for the total cosmic mass.

Using the influence of gravity on space-time as a tool, astronomers have observed halos of distant stars and galaxies, as their light is bent around invisible, but massive objects (such as black holes) between us and the distant light sources. Gravitational lensing has most famously been observed in the Hubble Space Telescope (HST) images where arcs of light from young and distant galaxies are warped around older galaxies in the foreground. This technique now has a use when indirectly observing the large-scale structure of dark matter intertwining its way between galaxies and clusters.

Astronomers from the University of British Columbia (UBC) in Canada have observed the largest structures ever seen of a web of dark matter stretching 270 million light years across, or 2000 times the size of the Milky Way. If we could see the web in the night sky, it would be eight times the area of the Moons disk.

This impressive observation was made possible by using dark matter gravity to signal its presence. Like the HST gravitational lensing, a similar method is employed. Called “weak gravitational lensing”, the method takes a portion of the sky and plots the distortion of the observed light from each distant galaxy. The results are then mapped to build a picture of the dark matter structure between us and the galaxies.

The team uses the Canada-France-Hawaii-Telescope (CFHT) for the observations and their technique has been developed over the last few years. The CFHT is a non-profit project that runs a 3.6 meter telescope on top of Mauna Kia in Hawaii.

Understanding the structure of dark matter as it stretches across the cosmos is essential for us to understand how the Universe was formed, how dark matter influences stars and galaxies, and will help us determine how the Universe will develop in the future.

This new knowledge is crucial for us to understand the history and evolution of the cosmos […] Such a tool will also enable us to glimpse a little more of the nature of dark matter.” – Ludovic Van Waerbeke, Assistant Professor, Department of Physics and Astronomy, UBC

Source: UBC Press Release

Bubble Experiment Fails to Find Dark Matter


Astronomers have no idea what dark matter is, but they have a few guesses. Since they can’t see the stuff directly, they’re trying to chip away at what it can’t be, peeling away theory after theory. Eventually, there should be a few theories that have withstood the most experiments, and best model what astronomers see out in the Universe. Physicists at Fermilab have made one of those steps forward, constraining the characteristics of dark matter, and overturning a recent discovery… by not seeing anything unusual.

We can’t see dark matter, but we know it’s out there. Galaxies should spin themselves apart but they don’t thanks to being inside a halo of dark matter. Amazing images from the Hubble Space Telescope show dark matter’s gravitational distortion on the light from distant galaxies. Oh, it’s out there all right.

So what is it?

Astronomers have two theories. One is that their ideas about gravity are wrong. By modifying our understanding of how gravity works over large distances, you can remove the need for dark matter entirely.

The other possibility are “weakly interacting massive particles”. These are actual particles, made of “something”, but we can’t see them or detect them in any way except through their pull of gravity.

Particle physicists have been searching for dark matter particles using powerful atom smashers, just like they discovered all the sub-atomic particles they’ve found so far.

A new experiment at the US Department of Energy’s Fermi National Accelerator Laboratory announced this week that they’ve made some headway in this search. According to theories, when dark matter particles interact with regular matter, it’s different from the way regular matter interacts. The Fermilab experiment has ruled out one of the last possible ways that the theories have predicted this should happen.

Their experiment, called COUPP, uses a glass jar filled with a litre of iodotrifluoromethane (a fire-extinguishing liquid known as CF3I. As particles strike the CF3I, it causes tiny bubbles to form in the liquid. The scientists can detect these bubbles as they reach a millimetre in size. By watching the interactions, researchers should be able to know if they’re coming from regular matter or dark matter.

So far, their results contradict another search called the Dark Matter experiment (DAMA) in Italy, who claimed to see dark matter interactions. The results for the DAMA experiment predicted that COUPP should have found hundreds of dark matter interactions, but they didn’t see any.

This research appears in the February 15th issue of the journal Science.

Original Source: Fermilab News Release

Could the First Stars Have Been Powered by Dark Matter?


Early stars that began to form about 200 million years after the Big Bang were strange creatures. From observation, the earliest stars (formed from coalescing primordial gas clouds) were not dense enough to support fusion reactions in their cores. Something within the young suns was counteracting the collapsing gas clouds, preventing the core reactions from taking place. Yet, they still produced light, even in absence of nuclear processes. Could dark matter have had a part to play, fueling the stellar bodies and sparking early stars to life?

New research indicates that the energy generated by annihilating dark matter in the early universe may have powered the first stars. How? Well, the violent early universe will have had high concentrations of dark matter. Dark matter has the ability to annihilate when it comes into contact with other dark matter matter, it does not require anti-dark matter to annihilate. When “normal” matter collides with its anti-component (i.e. electron colliding with positron), annihilation occurs. Annihilation is a term often used to describe the energetic destruction of something. While this is true, the annihilation products from dark matter include huge amounts of energy to create neutrinos and “ordinary matter” such as protons, electrons and positrons. Dark matter annihilation energy therefore has the ability to condense and create the matter we see in the Universe today.

Dark matter particles are their own anti. When they meet, one-third of the energy goes into neutrinos, which escape, one-third goes into photons and the last third goes into electrons and positrons.” – Katherine Freese, Theoretical Physicist, University of Michigan.

Katherine Freese (University of Michigan), Douglas Spolyar (University of California, Santa Cruz) and Paolo Gondolo (University of Utah in Salt Lake City) believe the strange physics of the early “dark stars” may be attributed to dark matter. For a star to form from stellar gas cloud to a viable, burning star, it must cool first. This cooling allows the star to collapse so the gas is dense enough to kick-start nuclear reactions in the core. However, early stars appear to have some form of energy acting against the cooling and collapse of early stars, fusion shouldn’t be possible, and yet the stars still shine.

The group believe that early stars may have passed through two stages of development. As the gas clouds collapse, the stars go through a “dark matter phase”, generating energy and producing normal matter. As the phase progresses, dark matter will slowly be used up and converted into matter. As the star becomes sufficiently dense with matter, fusion processes take over, starting the “fusion phase”. Fusion in turn generates heavier elements (such as metals, oxygen, carbon and nitrogen) during the lifetime of the star. When the early stars’ fuel is used up, it will go supernova, exploding and distributing these heavy elements throughout space to form other stars. The “dark matter phase” appears only to have existed in the very first stars (a.k.a. “population three stars”); later stars are supported by fusion processes only.

However, this exciting new theory will have to wait until the James Webb Telescope goes into operation in 2013 before population three stars can be observed with any great accuracy. Light may then be shone on the processes powering the first “dark stars” of our early Universe.


Dark Matter and Dark Energy… the Same Thing?


I’ve said it many times, but it bears repeating: regular matter only accounts for 4% of the Universe. The other 96% – dark matter and dark energy – is a total mystery. Wouldn’t it be convenient if we could find a single explanation for both? Astronomers from the University of St. Andrews are ready to decrease the mysteries down to one.

Dr. HongSheng Zhao at the University of St. Andrews School of Physics and Astronomy has developed a model that shows how dark energy and dark matter are more closely linked than previously thought.

Dr Zhao points out, “Both dark matter and dark energy could be two faces of the same coin. “As astronomers gain understanding of the subtle effects of dark energy in galaxies in the future, we will solve the mystery of astronomical dark matter at the same time.”

Just a quick explainer. Dark energy was discovered in the late 1990s during a survey of distant supernova. Instead of finding evidence that the mutual gravity of all the objects in the Universe is slowing down its expansion, researchers discovered that its expansion is actually accellerating.

Dark matter was first theorized back in 1933 by Swiss astronomer Fritz Zwicky. He noted that galaxies shouldn’t be able to hold themselves together with just the regular matter we can see. There must be some additional, invisible matter surrounding the regular matter that provides the additional gravitational force to hold everything together.

And since their discoveries plenty of additional evidence for both dark energy and dark matter have been seen across the Universe.

In Dr. Zhao’s model, dark energy and dark matter the same thing that he calls a “dark fluid”. On the scale of galaxies, this fluid behaves like matter, providing a gravitational force. And in the large scales, the fluid helps drive the expansion of the Universe.

Dr. Zhao’s model is detailed enough to produce the same 3:1 ratio of dark energy to dark matter measured by cosmologists.

Of course, any theory like this only gains ground when it starts making predictions that can be tested through observation. Dr. Zhao expects the work at the Large Hadron Collider to be fruitless. If he’s right, dark matter particles will have such low energy that the collider won’t be able to generate them.

The paper was recently published in the Astrophysical Journal Letters in December 2007, and Physics Review D. 2007.

Original Source: University of St. Andrews News Release

Large Hadron Collider Could Detect “Unparticles”


Understanding the mysterious dark matter in our universe is paramount to cosmologists. Dark matter and dark energy makes up the vast majority of mass in the observable universe. It influences galaxy rotation, galactic clusters and even holds the answer to our universe’s fate. So, it is unsurprising to hear about some outlandish physics behind the possible structure of this concealed mass. A Harvard scientist has now stepped up the plate, publishing his understanding about dark matter, believing the answer may lie in a type of material that has a mass, but doesn’t behave like a particle. “Unparticles” may also be detected by the high energy particle accelerator, the Large Hadron Detector (LHD) at CERN going online in a few weeks time. High energy physics is about to get stranger than it already is…

Dark matter is theorized to take on many forms, including: neutron stars, weakly interacting massive particles (WIMPs), neutrinos, black holes and massive compact halo objects (MACHOs). It is hard, however, to understand where the majority of mass comes from if you can’t observe it, so much of what we “know” about this dark source of matter and energy will remain theory until we can actually find a way of observing it. Now, we have a chance, not only to observe a form of dark matter, but also to generate it.
A simulation of a LHC collision (credit:CERN)
Professor Howard Georgi, a Harvard University physicist, wants to share his idea that the “missing mass” of the universe may be held in a type of matter that cannot be explained by the current understanding of physics. The revelation came to him when he was researching what can be expected from the future results of LHC experiments. Beginning with quantum mechanics (as one would expect), he focused on the interactions between sub-atomic particles. Using the “Standard Model”, which describes everything we know and understand about matter in our universe (interactions, symmetry, leptons, bosons etc.), Georgi soon came to a dead end. He then side stepped a basic premise of the standard model: the forces that govern particle interactions act differently at different length scales.

I did think I was crazy,” Prof. Georgi on the moment he stumbled on the “unparticle theory”.

This is one of the major failings of the standard model – the unification of the four universal forces: weak force, strong force, electromagnetic force and gravitational force. The standard model unites the first three, but neglects gravity. Gravity simply does not fit. So Georgi took the bold step and calculated what could be generated by the LHC without the help of standard sub-atomic thinking and scale length restrictions.

The unparticle would therefore be “scale invariant”, a property of fractals. Unparticles can interact over any scale lengths without restriction. When viewed, the unparticle will act as a fractal and will look similar over any scale (a characteristic known as self-similarity). Unparticles will also take on any mass, some or all the mass, depending on the scale you are viewing at. Now the implication of mass suddenly becomes interesting to the dark matter hunters out there. Unparticles could be a huge source of dark matter.

As they have mass, unparticles would also possess an “ungravity”. Ungravity should have a strong, short-distance effect on matter in the observable world, and so, may be observed by sufficiently sensitive gravity probes.

Whether unparticles exist or not, exploring the possibility that standard thinking may need to be slightly re-jigged for the extreme world of high energy particle collisions will surely lead to some healthy scientific debate. For now, we wait in anticipation for when the LHC goes online in May this year.


Supercluster Ruled By the Pull of Dark Matter


Enough of this small stuff, let’s look at the big picture. The really really big picture. In this case, one of the largest patches of the sky ever observed by the Hubble Space Telescope. A new detailed survey was released today that combines 80 separate Hubble images together. In addition to a framework of galaxy clusters, the images show the distribution of dark matter that holds the clusters together – and tears it all apart.

The dark matter survey is part of the Space Telescope Abell 901/902 Galaxy Evolution Survey (STAGES), which looks at one of the larger structures in the Universe: the Abell 901/902 superclusters. This is a region of tremendous violence and chaos. Galaxies are being pulled into the core of the superclusters, and getting distorted and stripped of their gas and dust.

And one of the primary forces of this violence comes from the completely invisible dark matter that makes up the bulk of the matter in the region. Instead of being equally distributed, though, this dark matter has pooled into enormous clumps.

An international team of astronomers used Hubble to measure how individual galaxies are distorted by clouds of dark matter. The dark matter is invisible, but it does have mass, which can pull at the light as it moves past. The astronomers know what different galaxies should probably look like, and then can figure out how much dark matter is in between, distorting the view. It’s actually pretty incredible to see the dark matter map imposed over top of the visible light image.

“Thanks to Hubble’s Advanced Camera for Surveys, we are detcting for the first time the irregular clumps of dark matter in this supercluster,” said Catherine Heymans of the University of British Columbia. “We can even see an extension of the dark matter toward a very hot group of galaxies that are emitting X-rays as they fall into the densest cluster core.”

The Hubble study identified 4 separate regions where the dark matter has pooled into dense clumps, adding up to 100 trillion times the mass of the Sun. They can even make out irregular clumps of dark matter in the supercluster. These areas match the locations where hundreds of old galaxies have already experienced the violent passage from the outskirts of the supercluster into the denser regions.

Original Source: Hubble News Release

Galaxy Cluster Collision Creates a Dark Matter Core


This strange photograph is a composite image of Abell 520, a massive cluster of galaxies in the process of colliding with one another – it’s one of the most massive structures in the Universe. Several different instruments and observatories came together to produce the image, and the final result gave astronomers a big mystery: its dark matter is behaving strangely.

When galaxies collide, three ingredients come into play: individual galaxies and their billions of stars, hot gas in between the galaxies, and the mysterious dark matter that actually makes up the bulk of the mass. Optical telescopes can see the light from the stars in the galaxies, and X-ray observatories, like Chandra can see the radiation pouring out of the superheated gas. But the presence of dark matter has to be calculated by the way it warps light from more distant objects.

During gigantic collisions like this, astronomers believed that the dark matter and galaxies should stay together, even during the most violent collisions. And this was seen in another galaxy collision: the so-called Bullet Cluster. But in the collision of Abell 520, something surprising was seen.

They found a dark matter core, containing hot gas, but no galaxies. For some reason, the galaxies were stripped away from the densest part of the dark matter. Here’s what Dr. Hendrik Hoekstra, from the University of Victoria had to say:

“It blew us away that it looks like the galaxies are removed from the densest core of dark matter. This would be the first time we’ve seen such a thing and could be a huge test of our knowledge of how dark matter behaves.”

In addition to this core, they also found a corresponding “light region”, which had galaxies, but little or no dark matter. Somehow this collision separated the dark matter from the regular matter.

So what could have stripped these two apart? One possibility is that the galaxies and dark matter were torn apart by a series of gravitational slingshots. Unfortunately, the researchers weren’t able to come up with a realistic computer simulation that had gravitational interactions powerful enough to do this.

Here’s the stranger possibility: we know that dark matter is affected by gravity, but maybe there’s also some kind of unknown interaction between particles of dark matter. This would be extremely difficult to detect since we can’t even see the stuff.

The astronomers have secured time with the Hubble Space Telescope, and will come back and take another look with its powerful gaze. This should help answer some of the mysteries they’ve unearthed.

Original Source: Chandra News Release

No Stars Shine in This Dark Galaxy


An international team of astronomers have conclusive new evidence that a recently discovered “dark galaxy” is, in fact, an object the size of a galaxy, made entirely of dark matter. Although the object, named VIRGOHI21, has been observed since 2000, astronomers have been slowly ruling out every alternative explanation.

In a new research paper, entitled 21-cm synthesis observations of VIRGOHI 21 – a possible dark galaxy in the Virgo Cluster, researchers provide updated evidence about this mysterious galaxy.

They have now performed a high resolution observations of VIRGOHI21 using the Westerbork Synthesis Radio Telescope (WSRT), to better pin down the quantities of neutral hydrogen gas. They also did followup observations with the Hubble Space Telescope, looking for any evidence of stars.

Astronomers first suspected there was an invisible galaxy out there when they spied galaxy NGC 4254. This unusual-looking galaxy appears to be one partner in a cosmic collision. All the normal evidence is there: gas is being siphoned away into a tenuous stream, and one of its spiral arms is being stretched out.

But the other partner in this collision is nowhere to be seen.

The researchers’ calculated that an object with 100 billion solar masses must have careened past NGC 4254 within the last 100 million years, creating the gas stream, and tearing at one of its arms. This was the clue that an invisible dark matter galaxy might be lurking nearby.

A detailed search turned up a mysterious object called VIRGOHI21, located about 50 million light-years from Earth. Were it a normal galaxy, you would be able to see it in a powerful amateur telescope. But there’s nothing there. Even in the Hubble Space Telescope, not a single star is shining from this massive region of space.

It was only visible in radio telescopes, which could detect the radio emissions from neutral hydrogen gas located in the cloud.

When they first published their research a few years ago, the astronomy community was understandably skeptical, and proposed several alternative theories to explain the mysterious object.

For example, there could be additional mass associated with VIRGOHI21, and not just dark matter. The discovery of red giant stars in the region would give some indication that this was a more normal interaction. But Hubble turned up nothing.

Dr. Robert Minchin, lead researcher from the Arecibo Observatory, said, “not even the power of Hubble has been able to see any stars in it.”

It’s possible that VIRGOHI21 has always been this way, formed from primordial dark matter and neutral hydrogen after the Big Bang. It’s been cruising the Universe ever since, disrupting galaxies as it goes.

However, there do seem to be ways that galaxies and their dark matter can be separated. Only a few months ago, a ring of dark matter was found surrounding a group of colliding galaxy clusters by the Hubble Space Telescope. Perhaps VIRGOHI21 is the wreckage from one of these cluster collisions; a shred of dark matter hurled out into space.

It could be that there are many of these dark galaxies out there. A new sky survey, carried out with the 305-metre (1000-foot) Aricebo radio telescope in Puerto Rico should tease out more of these objects in the future. The survey is called the Arecibo Galaxy Environment Survey (AGES).

This most recent paper has been accepted for publication in the Astrophysical Journal.

Dark Matter Annihilation at the Centre of the Milky Way


Most of the Universe is a complete and total mystery. And one of these mysteries is dark matter. It’s out there, and astronomers are slowly teasing out its characteristics, but it’s not giving up its secrets easily. The problem is, dark matter only interacts with regular matter through gravity (and maybe through the weak nuclear force). It doesn’t shine, it doesn’t give off heat or radio waves, and it passes through regular matter like it isn’t there. But when dark matter is destroyed, it might give astronomers the clues they’re looking for.

Researchers have theorized that one productive way to search for dark matter might not be to search for it directly, but to look for the resulting particles and energy which are emitted when it’s destroyed. In the environment around the centre of our galaxy, dark matter might be dense enough that particles regularly collide, releasing a cascade of energy and additional particles; which could be detected.

And this theory could help account for a strange result gathered by the Wilkinson Microwave Anisotropy Probe (WMAP), a NASA spacecraft which is mapping the temperature of the Cosmic Microwave Background Radiation (CMBR). This background radiation was supposed to be roughly even across the entire sky. But for some reason, the satellite turned up an excess of microwave emission around the centre of our galaxy.

Perhaps this microwave radiation is the glow of all that dark matter getting annihilated.

This conclusion was reached by a team of US astronomers: Dan Hooper, Douglas P. Finkbeiner and Gregory Dobler. Their work is published in a new research paper called Evidence Of Dark Matter Annihilations In The WMAP Haze.

The excess microwave radiation around our galactic centre is known as the WMAP Haze, and was originally thought to be the emissions from hot gas. Astronomers set about trying to confirm this theory, but observations in other wavelengths failed to turn up any evidence.

According to the researchers, the microwave haze could be explained by annihilating particles of dark matter, like the interaction between matter and antimatter. As dark matter particles collide they could give off any number of detectable particles and radiation, including gamma-rays, electrons, positrons, protons, antiprotons and neutrinos.

The size, shape and distribution of the haze matches the central region of our galaxy which should also have a high concentration of dark matter. And if the dark matter particles are within a certain range of mass – 100 to 1000s of times the mass of a proton – they could release a torrent of electrons and positrons that nicely match the microwave haze.

In fact, their calculations precisely match one of the most attractive dark matter particle candidates: the hypothetical neutralino which is predicted in supersymmetry models. When annihilated, these would produce heavy quarks, gauge bosons or the Higgs boson, and would have the right mass and particle size to produce the microwave haze observed by WMAP.

One of the predictions made in this paper is for the upcoming Gamma Ray large Area Space Telescope (GLAST), due to launch in December, 2007. If they’re correct, GLAST will be able to detect a glow of gamma rays coming from the Galactic Centre, matching the microwave haze, and even put an upper limit of the mass of dark matter particles. The upcoming ESA Planck mission will give an even more precise look at the microwave haze, providing better data.

It might still be mysterious, but dark matter is revealing its secrets slowly but surely.

Original Source: Arxiv (PDF)

Ring of Dark Matter Discovered Around a Galaxy Cluster


Astronomers using the Hubble Space Telescope have turned up a ghostly ring of dark matter, surrounding the aftermath of a collision between two galaxy clusters. This is one of the strongest pieces of evidence ever found for the existence of dark matter; a shadowy substance that only interacts with regular matter through gravity.

Researchers discovered the ring while they were mapping the distribution of dark matter inside the galaxy cluster Cl 0024+17, which is located about 5 billion light-years from Earth. The ring itself is 2.6 million light-years across.

Since dark matter is invisible, the researchers discovered the ring by its gravitational influence on background galaxies. The more dark matter concentrated into an area, the more the light from background objects is distorted, like ripples on a pond of water. We’re fortunate that the head-on collision between the galaxy clusters provided us with a perfect view from our perspective here on Earth.

So how did this ring form? Simulations have shown that when galaxy clusters collide, the dark matter falls into the centre of the combined cluster, and then sloshes back out. As it heads back out, mutual gravity slows it back down, and the dark matter piles up into a ring.

Original Source: Hubble News Release