Observations reveal carbon monoxide “snow line” in exosolar system Posted on July 22, 2013 by Physics Today

Ars Technica: To understand how our solar system formed over billions of years, researchers have been studying snapshots of nearby systems in various stages of formation. New observations of one such system have revealed the first evidence of a “snow line” for carbon monoxide around another star. Snow lines are the distances from the star at which various substances, such as water and ammonia, freeze. They take their name from a feature of mountains. A team led by Chunhua Qi of Harvard University examined images of the protoplanetary disk around a relatively nearby star similar to the Sun 5 billion years ago. They found a ring of ice showing the reflected emissions of frozen carbon monoxide at a distance from the star roughly the same as the distance of Neptune from the Sun. That observation matched with theoretical predictions. In our solar system, that snow line marks the end of the large planets and the beginning of the region filled with frozen Pluto-like planets and comets. 

Hidden mantle material may help explain Earth’s origins Posted on July 17, 2013 by Physics Today

Science Daily: Scientists have been puzzled by the fact that Earth’s mantle appears to have less lead than predicted by standard theories of planetary evolution. It has long been assumed that the planet formed from meteoritic material ejected from asteroids that smashed into each other, and thus the amount of lead in Earth’s mantle should be comparable to that found in meteorites. Yet until now, such a reservoir has gone undetected. To look for that hidden cache, researchers at MIT have been collecting rock samples from a region of northern Pakistan called the Kohistan arc; a collision of two massive tectonic plates there some 40 million years ago exposed some of Earth’s mantle. An analysis of those rocks revealed that some were much denser than the mantle and contained more lead. Based on that finding, the researchers calculated that roughly 70% of the magma that rises from the mantle during subduction events is so heavy with lead that it crystallizes into dense rock and drops back down into the mantle, where it collects and remains undisturbed. Their results could help further the study of how Earth has evolved.

The Earth's Gold --"A Neutron Star Collision Was the Source" -The Daily Galaxy via CfA, July 18, 2013

A odd glow from a short gamma-ray burst (GRB) in a galaxy 3.9 billion light years away in theconstellation Leo on June 3 by NASA’s Swift space telescope hints that all of Earth's gold is the product of collisions between dead neutron stars. The gamma-ray explosion resulting from dead stars crashing together 24 sextillion miles away created an initial burst that lasted only only two-tenths of a second. But the infrared glow that lingered around the area afterward suggests that gold may have been among the elements thrown out in the collision. After comparing their observations using the powerful ESO telescope in Chile and the Hubble Space Telescope with theoretical models, the astronomers concluded that they were seeing the afterglow from a huge quantity of heavy metals formed in the collision.

The image above is a first direct look, in visible light, at a lone neutron star (RX J185635-3754). Produced with the Wide-Field Planetary Camera 2, Hubble Space Telescope.Current observation and theories say that the heavy elements of the periodic table, such as gold, platinum, lead and  uranium, had their origin in supernova explosions, but failed to explain the volume of gold in our solar system. About a decade ago, researchers in Europe used supercomputers to test their theory that heavier metals like gold and platinum could be formed from the massive explosion that occurs when two ultra-dense dead neutron stars collide.

It has long been understood that Earth’s elements are of cosmic origin. Carbon and oxygen atoms in our bodies, for  example, come from the interior of stars, where they were formed under high pressure and heat. They were later spewed into  the universe in supernova explosions.

A single neutron star might be roughly the size of Manhattan, but contain as much mass as our sun, or more, with all of it crammed together by the force of gravity until even the atoms have collapsed, leaving the object with the density of an atomic nucleus. A teaspoon of neutron-star mass would weigh, on Earth, about 5 billion tons.

“We are all star stuff, and our jewelry is colliding-star stuff,” said Edo Berger, an astronomer who led the research at  the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. In the Milky Way a neutron-star collision is likely to happen about once every 100,000 years, Berger said. But the universe is big, containing many billions of galaxies, and so astronomers doing an all-sky survey will occasionally see one of these rare GRBs.

The Daily Galaxy via CfA

"An Unknown Force of the Universe is Acting on Dark Matter" (4th of July Feature)-July 04, 2013

Today, on the 4th of July, a European team of astronomers led by Hongsheng Zhao of the SUPA Centre of Gravity at the University of St Andrews are prsenting a radical new theory at the RAS National Astronomy Meeting in St Andrews. Their theory suggests that the Milky Way and Anromeda galaxies collided some 10 billion years ago and that our understanding of gravity is fundamentally wrong. Remarkably, this would neatly explain the observed structure of the two galaxies and their satellites.

Dr. Zhao is not unused to controversial theories. In 2009, he led An international team of astronomers that found an unexpected link between mysterious 'dark matter' and the visible stars and gas in galaxies that could revolutionise our current understanding of gravity. Zhao suggested that an unknown force is acting on dark matter.Only 4% of the universe is made of known material. Stars and gas in galaxies move so fast that astronomers have speculated that the gravity from a hypothetical invisible halo of dark matter is needed to keep galaxies together. However, a solid understanding of dark matter as well as direct evidence of its existence has remained elusive.

The team believes that the interactions between dark and ordinary matter could be more important and more complex than previously thought, and even speculate that dark matter might not exist and that the anomalous motions of stars in galaxies are due to a modification of gravity on extragalactic scales.

"The dark matter seems to 'know' how the visible matter is distributed. They seem to conspire with each other such that the gravity of the visible matter at the characteristic radius of the dark halois always the same," said Dr. Benoit Famaey (Universities of Bonn and Strasbourg). "This is extremely surprising since one would rather expect the balance between visible and dark matter to strongly depend on the individual history of each galaxy.

"The pattern that the data reveal is extremely odd. It's like finding a zoo of animals of all ages and sizes miraculously having identical, say, weight in their backbones or something. It is possible that a non-gravitational fifth force is ruling the dark matter with an invisible hand, leaving the same fingerprints on all galaxies, irrespective of their ages, shapes and sizes."

Such a force might solve an even bigger mystery, known as 'dark energy', which is ruling the accelerated expansion of the Universe. A more radical solution is a revision of the laws of gravityfirst developed by Isaac Newton in 1687 and refined by Albert Einstein's theory of General Relativity in 1916. Einstein never fully decided whether his equation should add an omnipresent constant source, now called dark energy.

Dr Famaey added, "If we account for our observations with a modified law of gravity, it makes perfect sense to replace the effective action of hypothetical dark matter with a force closely related to the distribution of visible matter."

The implications of the new research could change some of the most widely held scientific theories about the history and expansion of the universe.

Lead researcher Dr. Gianfranco Gentile at the University of Ghent concluded, "Understanding this puzzling conspiracy is probably the key to unlock the formation of galaxies and their structures."

Journal Reference: Gianfranco Gentile, Benoit Famaey, HongSheng Zhao, Paolo Salucci. Universality of galactic surface densities within one dark halo scale-length. Nature, 2009; 461 (7264): 627 DOI: 10.1038/nature08437

The Daily Galaxy via University of St. Andrews and Physorg.com

White dwarf star throws light on possible variability of a constant of Nature [2 hours ago-in Phy.org]

An international team led by the University of New South Wales has studied a distant star where gravity is more than 30,000 times greater than on Earth to test its controversial theory that one of the constants of Nature is not a constant.

Dr Julian Berengut and his colleagues used the Hubble Space Telescope to measure the strength of the electromagnetic force – known as alpha – on a white dwarf star.

Their results, which do not contradict the variable constant theory, are to be published in the journal Physical Review Letters. Dr Berengut, of the UNSW School of Physics, said the team's previous research on light from distant quasars suggests that alpha – known as the fine-structure constant – may vary across the universe.

"This idea that the laws of physics are different in different places in the cosmos is a huge claim, and needs to be backed up with solid evidence," he says.

"A white dwarf star was chosen for our study because it has been predicted that exotic, scalar energy fields could significant alter alpha in places where gravity is very strong."

"Scalar fields are forms of energy that often appear in theories of physics that seek to combine the Standard Model of particle physics with Einstein's general theory of relativity."

"By measuring the value of alpha near the white dwarf and comparing it with its value here and now in the laboratory we can indirectly probe whether these alpha-changing scalar fields actually exist."

White dwarfs are very dense stars near the ends of their lives. The researchers studied the light absorbed by nickel and iron ions in the atmosphere of a white dwarf called G191-B2B. The ions are kept above the surface by the star's strong radiation, despite the pull of its extremely strong gravitational field.

"This absorption spectrum allows us to determine the value of alpha with high accuracy. We found that any difference between the value of alpha in the strong gravitational field of the white dwarf and its value on Earth must be smaller than one part in ten thousand," Dr Berengut says.

"This means any scalar fields present in the star's atmosphere must only weakly affect the electromagnetic force." Dr Berengut said that more precise measurements of the iron and nickel ions on earth are needed to complement the high-precision astronomical data.

"Then we should be able to measure any change in alpha down to one part per million. That would help determine whether alpha is a true constant of Nature, or not.”

Getting our hands on dark matter-[http://www.quantumdiaries.org/2013/07/01/getting-our-hands-on-dark-matter]

In a previous blog, I reviewed the many ways dark matter manifests itself through gravitational effects. But to this day, nobody has managed an unambiguous direct observation of dark matter.

This is not surprising given we are talking about a completely different and totally unknown type of matter, something not made of quarks and leptons like all visible matter (humans, planets, stars and galaxies).

Nevertheless, just as the quarks and leptons are the building blocks of visible matter, physicists expect dark matter is also made of fundamental particles, albeit dark particles. So we need to catch dark matter particles interacting in some way with particles of regular matter.

So far, all we know is that dark matter reacts to gravitation but not to electromagnetism since it does not emit any light. Maybe it interacts with ordinary matter through the weak nuclear force, the one responsible for radioactive decays. Dark matter would then be made of weakly interacting particles.

Weakly Interacting Massive Particles

One popular hypothesis is that dark matter particles would be WIMPs, which stands for Weakly Interacting Massive Particles. How often can WIMPs interact with matter? It should be less than 0.1 times per year per kilogram of sensitive material in the detector, depending on the WIMP mass.

The detection principle is simple: once in a while, a WIMP will strike a nucleus in one of the detector’s atoms, which will recoil and induce a small recordable vibration.

From Lauren Hsu’s review talk at ICHEP 2012.

The vertical axis shows the number of times a dark particle transfer a given amount of energy to a nucleus. The more massive the detector and the longer you operate it, the higher are the chances of recording a collision.

The detector material also matters as seen on the plot above: collisions are more energetic, hence easier to detect,  with Germanium (Ge) than with heavier nuclei like Xenon (Xe), but the total number of collisions is higher with the latter material.

These detectors are placed deep in mines or tunnels to block cosmic rays that would induce false signals in the detector. Eliminating all sources of background is the biggest challenge facing these experiments.

Dark matter wind

If the Universe is full of dark matter, we on Earth should feel a wind of dark particles as we travel around the Sun. This rate is evaluated to be of the order of a million particles per square centimetre per second for a WIMP ten times heavier than a proton.

And just like a cyclist riding on a circular track on a windy day, we should feel a head wind of dark matter particles in June and a tail wind in December since there is a greater concentration of dark matter in the centre of the galaxy.

  

Imagine now a detector operating on Earth and sensitive to WIMPs. The variations in the wind intensity would be detected as an annual modulation in the number of dark matter particles hitting the detector throughout the year.

This is exactly what the DAMA/LIBRA experiment claims to observe for more than a decade now as shown on the plot below. Their signal is loud and clear (8.9 sigma) but unfortunately, refuted by several experiments.

 

Three other experiments have also reported signals: CoGent sees a faint modulation while both CRESST and CDMS observed a few events in excess of the expected background.

All would be great if these four experiments would all agree on the characteristics of the dark matter particle but that is unfortunately not the case.

Many theorists have deployed heroic efforts to devise new models to explain why some experiments see a signal while others do not, but no model is widely accepted yet. The situation remains terribly confusing as can be appreciated from the plot below.

 

The vertical axis represents the possible rate at which a dark matter particle could interact with regular matter while the horizontal axis gives the mass of the hypothetical dark particle. The areas in solid colours delimit the possible values obtained by the four experiments having a signal. Only CoGent and CDMS agree.

The lines show the limits placed on the allowed dark matter interaction rate and mass by some of the experiments that reported no signal. All values above those lines are excluded, meaning the null experiments are in direct contradiction with the four groups that reported a signal.

As frustrating as this might seen, it is in fact not surprising given the complexity of these experiments. It could be due to experimental flaws or there might be a theoretical explanation.

Many experiments are collecting more data and new ones are being built. With theorists and experimentalists being hard at work, hopefully there will soon be a breakthrough.

Stay tune for the next blog for a review of astronomical experiments.

4 Responses to “Getting our hands on dark matter”

Matthew says:

July 1, 2013 at 5:47 am

Correction: “we should feel a head wind of dark matter particles during the summer months and a tail wind during the winter” … if we live in the northern hemisphere.

Reply

CERN says:

July 1, 2013 at 5:54 am

Good point! Thank you for correcting me on this. I will modify the text.

Cheers, Pauline

SHREEKANT says:

July 2, 2013 at 6:18 am

The location[condition] where we are searching ‘the high impact of DM’ is correct? Our atmosphere is now almost STABLE.
At present condition, it is true that little more impact seen in Xe than Ge/Si [here interaction with Nucleus taken, not Atom].
It is also true that DM is more near the GALAXY & …., but what about other places?
DA,DM are not far away to feel & their interactions with Baryon is not too complicated to understand.Is only air comes out when we fill water in empty bottle?
The role of DE energy is interesting. The role of 94-96% is very important in this universe. Our present theory is only based on the knowledge of 4-6%

Reply

CERN says:

July 2, 2013 at 6:40 am

Hello,

of course, it is hard to say we know how to look for dark matter. Nobody know yet if and how it may interact with matter. So we test many hypotheses.

Dark matter is also seen to be concentrated in galaxy centres so indeed, it is a good place to look.

Outside a galaxy centre, the density of dark matter is much reduced. For example, in our Solar system, far away from the galaxy center, the quantity of dark matter within the Solar system amounts to 0.0000000000001 times the mass of the sun. So yes, when you fill up a bottle, it is essentially just air coming out. There is very little dark matter here to start plus it permeates regular matter anyway.

I hope this helps, Pauline

SHREEKANT says:

Your comment is awaiting moderation.
July 2, 2013 at 7:49 am

Tnx. for such a quick response.

It means our immediate surrounding [invisible part] contain only air? Then what about SUPERSYMMETRY & 96%?

Reply