Cosmic crash unmasks dark matter

By Paul Rincon Science reporter, BBC News

Dark matter is shown in blue, ordinary matter is coloured pink

Striking evidence has been found for the enigmatic "stuff" called dark matter which makes up 23% of the Universe, yet is invisible to our eyes.

The results come from astronomical observations of a titanic collision between two clusters of galaxies 5.7 billion light-years away.

Astronomers detected the dark matter because it separated from the normal matter during the cosmic smash-up.

The research team are to publish their findings in the Astrophysical Journal.

They used the Hubble and Chandra space telescopes to study the object MACSJ0025.4-1222 - formed after an incredibly energetic collision between two large galaxy clusters.

Each of these large clusters contains about a quadrillion times the mass of our Sun.

It puts to rest all the worries that the Bullet Cluster was an anomalous case. We have gone out and found another one Richard Massey, Royal Observatory Edinburgh

A technique known as gravitational lensing was used to map the dark matter with Hubble.

If an observer looks at a distant galaxy and some dark matter lies in between, the light from that galaxy gets distorted.

It looks as if it is being seen through lots of little lenses. And each of these lenses represents a piece of dark matter.

Astronomers used the Chandra X-ray telescope to map ordinary matter in the merging clusters, mostly in the form of hot gas, which glows brightly in X-rays.

As the two clusters that formed MACSJ0025 merged at speeds of millions of kilometres per hour, hot gas in the two clusters collided and slowed down.

However, the dark matter kept on going, passing right through the smash-up.

Speeding bullet

This phenomenon has been seen before, in a structure called the Bullet Cluster - which also formed after the collision of two large galaxy clusters. The Bullet Cluster lies closer to Earth, at a distance of 3.4 billion light-years.

"It puts to rest all the worries that the Bullet Cluster was an anomalous case. We have gone out and found another one,"

co-author Richard Massey, from the Royal Observatory Edinburgh, told BBC News.

The study sheds light on the properties of dark matter.

The fact that dark matter does not slow down in the collision supports a view that dark matter particles interact with each other only very weakly or not at all (when one excludes their gravitational interaction).

"Dark matter makes up five times more matter in the Universe than ordinary matter," said co-author Marusa Bradac, from the University of California at Santa Barbara (UCSB).

"This study confirms that we are dealing with a very different kind of matter, unlike the matter that we are made of. And we're able to study it in a very powerful collision of two clusters of galaxies."

Larger sample

The latest astronomical observations suggest that dark matter makes up some 23% of the Universe. Ordinary matter - such as the galaxies, gas, stars and planets - makes up just 4%.

The remaining 73% is made up of another mysterious quantity; dark energy, which is responsible for speeding up the expansion of the cosmos.

The Large Hadron Collider may shed further light on dark matter

According to one model, dark matter may be comprised of exotic sub-atomic "stuff" known as Weakly Interacting Massive Particles (WIMPS).

Others hold that the dark substance consists of everyday matter, rather than some elusive sub-atomic particle. However, this ordinary matter, referred to as Massive Astrophysical Compact Halo Objects (MACHOS), happens to radiate little or no light.

A powerful physics experiment, the Large Hadron Collider, which is currently under construction on the French-Swiss border, could shed further light on this question after it begins operating later this year.

Dr Massey said his team had found other candidates for colliding clusters.

"Ideally, we don't want just one or two, we want lots of these things to really study them statistically," he explained.

"Then we either use the whole lot, or pick out one 'golden bullet' which will provide the best constraints on what dark matter is."

The Hubble Space Telescope failed just after the team had taken their image of MACSJ0025, so they have not yet been able to study these other candidates.

Dr Massey said the astronomers hope to do this after the next Hubble servicing mission with the space shuttle, which is due to launch in October 2008.

Paul.Rincon-INTERNET@bbc.co.uk

Physicist Patricia Burchat sheds light on two basic ingredients of our universe: dark matter and dark energy.

Comprising 96% of the universe between them, they can't be directly measured, but their influence is immense.

16 minutes, TED Seminar. Includes a practical experiment (with a wine glass!) to demonstrate Gravitational Lensing (an effect predicted by Einstein).

Divisive idea 'explains galaxies'

By Paul Rincon Science reporter, BBC News, Belfast

Dwarf galaxies are relatively small galaxies containing a few billion stars

A controversial theory of physics may explain some aspects of galaxy behaviour better than rival, but more widely accepted, ideas.

That is the claim of an astronomer who studied eight so-called dwarf galaxies.

Modified Newtonian Dynamics (Mond) is proposed as an alternative to the widely accepted theory of dark matter to explain the dynamics of galaxies. (Wikipedia)

Garry Angus, from St Andrews University, said Mond effects could be very important for small galaxies.

Details of the study have been presented at the UK National Astronomy Meeting in Belfast and a press release.

Essentially,

Mond would add a new constant of nature - called a0 - to physics, besides the speed of light, Planck's constant, and many others.

Above it, accelerations are exactly as predicted by Newton's second law, which says that a force equals an object's mass times its acceleration.

Below it, gravity decays with distance from a mass, rather than distance squared.

This constant would be so small that it would go unnoticed with the large accelerations that we experience in day-to-day life.

For instance, when we drop a ball the gravity is 100 billion times stronger than a0 and the accelerated motion of the Earth round the Sun is 50 million times stronger.

However, when objects are accelerating extremely slowly, as is seen in galaxies or clusters of galaxies, the constant makes a significant difference to the resulting gravitational forces.

Problem cases

Mr Angus took two key parameters of the eight Milky Way dwarf galaxies and tried to fit them to predictions made by the Mond theory.

These two parameters were the ratio of mass to the amount of light emitted by the stars in the dwarf galaxies (the mass-to-light ratio) and the orbital paths of stars in the galaxies.

"Six out of the eight definitely fit the available data very, very well," the University of St Andrews researcher told BBC News.

"There are two problem cases, and we're going to run simulations to check them."

The most troublesome of these problem cases is a dwarf galaxy called Draco.

But Mr Angus said this could be because this galaxy was falling towards the Milky Way at a speed of 300km/s.

Our galaxy's gravity could be acting upon it, such that tidal forces were beginning to tear Draco apart.

The value for Sextans could also be due to tidal effects but also due to using old measurements of the galaxy's luminosity. Mr Angus said the data were improving all the time for these ultra dim objects.

But he explained: "The larger the systems you go to, the worse Mond fits the data. Presumably there is something we're not understanding at the scales of clusters of thousands of galaxies and upwards."

At these scales, dark matter seems to be the only realistic proposal. This invisible form of matter is thought to make up some 22% of the Universe. The matter we can see makes up a paltry 4%.

The origins of the dark matter theory date back to 1933, when the Swiss astronomer Fritz Zwicky found evidence for unseen mass within a galaxy cluster. This became known as the "missing mass problem".

Observational evidence for dark matter has since been obtained from the study of the motions of galaxies. And this continues to be the dominant theory to explain observations of galaxies.

MODIFIED NEWTONIAN DYNAMICS (MOND)

The MOND theory was first put forward in 1983 by Mordehai Milgrom, now at the Weizmann Institute of Science in Rehovot, Israel. The constant, a0 is equal to 1.2 x 10-10 m/s2. Milgrom points out that this value is also the acceleration that you get by dividing the speed of light by the lifetime of the universe.

DWARF GALAXIES

There are fourteen dwarf galaxies known to orbit the Milky Way, including the Large and Small Magellanic Clouds. The eight selected for this study are all spherical in shape and lie around 240-750 million trillion (x10^16) kilometres outside the Milky Way.

"Even without direct detection, the dark matter theory is difficult to prove or refute and although we may not be able to prove whether MOND is correct, by carrying out these kind of tests we can see if it continues to hold up or if it is definitely ruled out,"

Dark matter clues in oldest stars

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Dark matter clues in oldest stars

By Liz Seward Science reporter, York

Simulation: Warm dark matter would drive filamentary structures

A computer model of the early Universe indicates the first stars could have formed in spectacular, long filaments.

These structures, which may have been thousands of light-years across, would have been shaped by "dark matter".

Scientists know very little about this type of matter, even though it accounts for most of the mass in the cosmos.

The researchers told the British Association (BA) Festival of Science that their work could reveal the true nature of dark matter.

Liang Gao and Tom Theuns from Durham University, UK, also reported their findings in the journal Science.

Quick or slow

Astronomers believe that more than three-quarters of the matter in our Universe may be "dark". It does not reflect or emit detectable light, and so cannot be seen directly - but it does gravitationally pull on normal matter (the gas, stars, and planets we see in space).

It is this interaction that allows scientists to predict its existence - even if they cannot say what it is. Various types of exotic particle seem to be the favoured theory, with equally exotic names such as neutralinos, axions and gravitinos.

The new research, though, may give some clues as to dark matter's properties. Computer modelling suggests there is a link between the structures assumed by early stars and the temperature of the dark matter amongst them.

Tom Theuns, from Durham's Institute for Computational Cosmology, told the festival:

"What we found for the first time is that the nature of the dark matter is crucial to the nature of the first stars.

"In cold dark matter the particles move very slowly; in warm dark matter they move very quickly," he explained.

"We found that if the dark matter consists of these fast moving particles, then the first stars form in very long, thin filaments.

"The filaments have a length about a quarter the size of the Milky Way and contain an amount of matter and gas about 10 million times the mass of the Sun, so that provides a lot of fuel for many stars."

Exotic collection

Some of the stars that formed within the filaments would have had a relatively low mass, which is of interest to astronomers as they have a long lifespan and could still survive today.

Simulation: With cold dark matter, structures become clumpy

Dr Theuns added: "In stark contrast, what happens in (the simulation with) cold dark matter is very, very different.

"Here, the first stars formed in little lumps of dark matter, and just one star per dark matter lump. And these stars are probably very massive as well: 100 solar masses.

"Because these stars are so massive, they die very quickly; so you wouldn't find such stars in the Milky Way today," he said.

Scientists believe that the temperature of the dark matter indicates what kind of particles it is made of.

Observational pointers

The research team hopes answers could come from astronomers who are now scouring the skies to find signs of very old stars.

If dark matter is warm, then some of these very first stars may be in the Milky Way today.

However, detecting the massive stars formed in cold dark matter would require very powerful telescopes capable of "peering into the very distant Universe," Dr Theuns added.

"We don't know what the dark matter is, we don't know what the first stars are. If we bring these two problems together, when we know more about one, then we can say something about the other."

"The Elegant Universe" -A Galaxy Insight

reposted from:http://www.dailygalaxy.com

September 12, 2007

"The Elegant Universe" -A Galaxy Insight

“How can a speck of a universe be physically identical to the great expanse we view in the heavens above?”

Brian Greene, The world's leading string theorist, Professor of Physics at Columbia University and author of The Elegant Universe and The Fabric of the Cosmos: Space. Time. And the Texture of Reality.

The stunning complex of coincidences that render the universe fit for life and intelligence

, is captured by British astronomer Sir Martin Rees: "There are deep connections between stars and atoms, between the cosmos and the microworld."

Scientists have been aware of this great puzzle for decades, but

two recent discoveries have given the quest for an answer to why the universe seems life-friendly a new set of urgency.

The first was the discovery of Dark Energy—although the predominant constituent of the universe—its strength is so astonishingly small that were it even slightly stronger, the universe would be void of galaxies, solar systems and life.

The second great discovery, which is yet to be proved, is M-theory, the reincarnation of superstring theory which posits that the subatomic world are just different modes of vibration of tiny one-dimensional strings of energy

of which only a fraction corresponds to the sub-atomic particle world described by the Standard Model.

For string theory to have the kind of acceptance of general relativity, it's got to make a prediction that is borne out by some experiment.

And as yet, we haven't quite gotten to the stage where we can make definitive predictions which, if they're found, the theory was right, and if they're not found, the theory was wrong.

But we have gotten to the stage where we can make some rough predictions for things that might happen at the accelerator built near Geneva, Switzerland, called the Large Hadron Collider.

If some of the predictions that string theory says might happen are borne out through experiment at that accelerator, then it's quite possible that string theory would be as accepted as general relativity.

However, Steve Giddings a theoretical physicist at the University of California states that:

"No longer can we follow the dream of discovering the unique equations that describe everything we see, and writing them on a single page. Predicting the constants of nature becomes a messy environmental problem. It has the complications of biology."

Posted by Casey Kazan.

Story Links:

BBC Super-String Theory -A breathtaking video about the Grand Unification Theory for the universe and its implications on our understanding of the universe's nature
String Theory -"The Elegant Universe"