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.
Monday, 22 July 2013
Thursday, 18 July 2013
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
Thursday, 4 July 2013
"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
Wednesday, 3 July 2013
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.”Tuesday, 2 July 2013
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.
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:
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.
CERN says:
Good point! Thank you for correcting me on this. I will
modify the text.
Cheers, Pauline
SHREEKANT says:
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%
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%
CERN says:
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:
Tnx. for such a quick response.
It means our immediate surrounding [invisible part]
contain only air? Then what about SUPERSYMMETRY & 96%?
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