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Dark matter search narrows

Fornax dwarf galaxy

This faint smattering of stars is actually a small galaxy. Scientists have been unable to spot evidence of certain kinds of dark matter particles within this galaxy and nine others.

STUDIES DECADES AGO OF THE ROTATION of galaxies, and of the movement of groups of galaxies, led scientists to conclude that the universe contained more matter than could be detected in the normal ways.

Being unseen at visible wavelengths, and with its nature unknown, the putative matter was dubbed “dark matter“, and according to popular models it comprises over 80 per cent of all the matter in the universe.

In the early years of investigation into this strange phenomenon, two broad candidates emerged—MACHOs and WIMPS.

MACHOs were hypothetical “massive compact halo objects”, ie. large bodies such as dim stars, black holes or large free-floating planets that would inhabit the outer or “halo” regions of a galaxy. WIMPs are hypothetical “weakly interacting massive particles”, ie. sub-atomic particles that could pervade space but not interact much with normal forms of matter.

Artist's impression of NASA’s Fermi Gamma-ray Space Telescope

The research used two-years of data collected by NASA’s Fermi Gamma-ray Space Telescope (artist's impression).

Research programmes failed to find evidence of MACHOs, so dark matter investigations now focus on WIMPs.

WIMPs could take many forms—perhaps as one or more of the familiar particles, such as neutrinos, or maybe as-yet-unknown particles.

In new research using two-years of data from NASA’s Fermi Gamma-ray Space Telescope, a team that includes astrophysicist Jennifer Siegal-Gaskins (Caltech) has been able to rule out certain kinds of WIMPs.

According to some models, when two WIMPs collide, they can annihilate each other and produce a burst of gamma rays with specific wavelengths. Such energy bursts would be detectable with Fermi.

The scientists studied 10 small galaxies that circle our Milky Way galaxy, looking for telltale gamma ray signs of WIMP collisions within them. They didn’t spot any.

This negative result will help scientists by eliminating particular kinds of WIMPs from the field of candidates, and will enable them to focus on searches for other kinds.

More information: New Insights on Dark Matter

Story by Jonathan Nally. Images courtesy NASA / Sonoma State University / Aurore Simonnet / ESO / Digital Sky Survey 2.

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Hubble bursts dark energy bubble

Galaxy NGC 5584

Galaxy NGC 5584 was one of eight galaxies astronomers studied to measure the universe's expansion rate. Two special kinds of stars—Type Ia supernovae and Cepheid variable stars—were used as "cosmic yardsticks", due to their predictable brightnesses.

  • Our universe seems to be expanding faster and faster with time
  • ‘Dark energy’ proposed as an explanation, but its nature remains a mystery
  • Hubble observations have ruled out one dark energy hypothesis

ASTRONOMERS USING NASA’s Hubble Space Telescope have ruled out one explanation for the nature of dark energy after recalculating the expansion rate of the universe to unprecedented accuracy.

The universe appears to be expanding at an increasing rate. Some think this is because it is filled with a ‘dark energy’ that works in the opposite way to gravity.

An alternative to that hypothesis is that an enormous ‘bubble’ of relatively empty space eight billion light-years wide surrounds our galactic neighbourhood.

If we lived near the centre of this void, observations of galaxies being pushed away from each other at accelerating speeds would be an illusion.

This hypothesis has now been invalidated because astronomers have refined their understanding of the universe’s present expansion rate.

“We are using the new camera on Hubble like a policeman’s radar gun to catch the universe speeding,” said Adam Riess of the Space Telescope Science Institute (STScI) and Johns Hopkins University, and leader of the science team. “It looks more like it’s dark energy that’s pressing the [accelerator] pedal.”

Portion of NGC 5584 with Cepheid locations marked

A portion of galaxy NGC 5584 with the location of Cepheid variable stars marked.

The observations helped determine a figure for the universe’s current expansion rate to an uncertainty of just 3.3 percent. The new measurement reduces the error margin by 30 percent over Hubble’s previous best measurement in 2009.

Cosmic yardsticks

Riess’ team first had to determine accurate distances to galaxies near and far from Earth, and then compare those distances with the speed at which the galaxies are apparently receding because of the expansion of space.

They used those two values to calculate the Hubble constant, the number that relates the speed at which a galaxy appears to recede to its distance from the Milky Way.

Because we cannot physically measure the distances to galaxies, astronomers have to find stars or other objects that serve as reliable cosmic yardsticks. These are objects with known intrinsic brightness—brightness that hasn’t been dimmed by distance, an atmosphere or interstellar dust. Their distances, therefore, can be inferred by comparing their intrinsic brightness with their apparent brightness as seen from Earth.

To calculate long distances, Riess’ team chose a special class of exploding star called Type Ia supernovae. These stellar blasts all have similar luminosity and are brilliant enough to be seen far across the universe.

By cross-correlating the apparent brightness of Type Ia supernovae with pulsating Cepheid stars (another class of stars whose intrinsic brightness is known), the team could accurately gauge the distances to Type Ia supernovae in far-flung galaxies.

WFC3

Hubble's latest camera, the Wide Field Camera 3, was instrumental in the research.

Bubble is burst

By using the sharpness of Hubble’s new Wide Field Camera 3 (WFC3) to study more stars in visible and near-infrared light, the team eliminated systematic errors introduced by comparing measurements from different telescopes.

“WFC3 is the best camera ever flown on Hubble for making these measurements, improving the precision of prior measurements in a small fraction of the time it previously took,” said Lucas Macri, a collaborator on the Supernova Ho for the Equation of State (SHOES) Team from Texas A&M in College Station.

Knowing the precise value of the universe’s expansion rate further restricts the range of dark energy’s strength and helps astronomers tighten up their estimates of other cosmic properties, including the universe’s shape and its roster of neutrinos, or ghostly particles, that filled the early cosmos.

“Thomas Edison once said ‘every wrong attempt discarded is a step forward,’ and this principle still governs how scientists approach the mysteries of the cosmos,” said Jon Morse, astrophysics division director at NASA Headquarters in Washington.

“By falsifying the bubble hypothesis of the accelerating expansion, NASA missions like Hubble bring us closer to the ultimate goal of understanding this remarkable property of our universe.”

Adapted from information issued by STScI. NGC 5584 image credit: NASA / ESA / A. Riess (STScI/JHU), L. Macri (Texas A&M University) / Hubble Heritage Team (STScI/AURA). NGC 5584 illustrations credit: NASA / ESA / L. Frattare (STScI) / Z. Levay (STScI).

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Where is the antimatter?

The Alpha Magnetic Spectrometer is a particle physics experiment module that is to be mounted on the International Space Station. It is designed to search for various types of unusual matter by measuring cosmic rays. Its experiments will help researchers study the formation of the Universe and search for evidence of dark matter and antimatter.

Final testing is being completed at ESA’s European Space Research and Technology Centre (ESTEC) facility in the Netherlands and delivery to the Kennedy Space Center in Florida is expected in early September 2010.

Launch is targeted for February 2011 on space shuttle Endeavour flight STS-134, the last flight in the shuttle programme.

Adapted from information issued by ESA / Wikipedia.

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Supercomputer to boost Australian astronomy

A simulation of dark matter distribution

A simulation of the spread of dark matter in the universe, produced using a current-generation Swinburne University supercomputer. The new supercomputer will be up to 100 times better.

A multi-million dollar upgrade to Swinburne University’s supercomputer will make it a leading research facility for the Australian astronomy community.

The upgrade, which will receive $1 million from the Federal Government’s Education Investment Fund (EIF) and $2 million from Swinburne, will dramatically increase the speed and capacity of the facility—now known as ‘gSTAR’.

The EIF funding will finance the installation of Graphics Processing Units (GPUs), or ‘extra brains’ for the supercomputer. Originally developed by the computer gaming industry, GPUs are a type of processor designed to perform simple tasks in a massively-parallel way that leads to enormous increases in computational power.

The Swinburne contribution will be used to upgrade the existing Central Processing Units (CPUs) and the mass storage system and pay for a new machine room to host the facility.

According to Swinburne astrophysicist Dr Darren Croton, the installation of the GPUs will boost the supercomputer’s speed between two and 100 times, depending on the application.

“This means an astrophysics simulation that would previously have taken three months to complete might only take a single day.

“This huge advance in power gives us the opportunity to tackle problems that are potentially 100 times harder,” he said.

A rack of computer equipment

The new supercomputer will used technology adapted from games computers.

Specially designed for astronomy

While there are other supercomputer facilities in Australia that are also starting to use GPU technology, they cater to a wide range of researchers and interests.

“Because these are general purpose facilities, they have to be set up in a very general way,” Croton said.

“The gSTAR’s power lies in its unique application. It will be optimised for astronomy simulations and data processing, which means it will have the same amount of power as other facilities for about one percent of the cost. That’s bang for your buck.”

Croton said that the university will make the gSTAR a national facility for astronomers across the country.

“We’re making the gSTAR and its predecessor available to astronomers from other universities and research centres.”

“In exchange the National Computational Infrastructure (NCI) National Facility is funding a support person who will provide expertise and guidance to researchers, helping them optimise their code.”

The upgrade, which will see the raw power of the Swinburne supercomputer go from 10 teraflops to 600 teraflops, is expected to be completed early- to mid-next year.

Adapted from information issued by Swinburne University / Image by Dr Gregory Poole, Centre for Astrophysics and Supercomputing, Swinburne University of Technology.

Hubble confirms the universe is expanding faster

A map showing the expected location of dark matter withing a region of deep space

A map showing the expected location of dark matter withing a region of deep space

A new study led by European scientists presents the most comprehensive analysis of data from the most ambitious survey ever undertaken by the NASA/ESA Hubble Space Telescope.

The researchers have, for the first time ever, used Hubble data to probe the effects of the natural gravitational “weak lenses” in space and characterise the expansion of the Universe.

A group of astronomers, led by Tim Schrabback of the Leiden Observatory, conducted an intensive study of over 446,000 galaxies within the COSMOS field, the result of the largest survey ever conducted with Hubble. In making the COSMOS survey, Hubble photographed 575 slightly overlapping views of the same part of the Universe using the Advanced Camera for Surveys (ACS) onboard Hubble. It took nearly 1,000 hours of observations.

In addition to the Hubble data, researchers used redshift data from ground-based telescopes to assign distances to 194,000 of the galaxies surveyed (out to a redshift of 5).

“The sheer number of galaxies included in this type of analysis is unprecedented, but more important is the wealth of information we could obtain about the invisible structures in the Universe from this exceptional dataset,” says Patrick Simon from Edinburgh University.

An illustration showing how Hubble looks back in time to "map" evolving dark matter

Hubble looks back in time to "map" evolving dark matter by splitting the background galaxy population into discrete epochs of time (like cutting through rock strata). By measuring the redshift of the "lensing" galaxies used to map the dark matter distribution, scientists can put them into different time/distance "slices".

In particular, the astronomers could “weigh” the large-scale matter distribution in space over large distances. To do this, they made use of the fact that this information is encoded in the distorted shapes of distant galaxies, a phenomenon referred to as weak gravitational lensing.

Using complex algorithms, the team led by Schrabback has improved the standard method and obtained galaxy shape measurements to an unprecedented precision. The results of the study will be published in an upcoming issue of Astronomy and Astrophysics.

The meticulousness and scale of this study enables an independent confirmation that the expansion of the Universe is accelerated by an additional, mysterious component named dark energy. A handful of other such independent confirmations exist.

Astronomers compared real observations with two predictions – one for a dark matter-dominated universe, the other one dominated by dark energy.

COSMOS Project Astronomers compared real observations with two simulations – one for a dark matter-dominated universe, the other one dominated by dark energy. The dark energy one is the closest match.

Scientists need to know how the formation of clumps of matter evolved in the history of the Universe to determine how the gravitational force, which holds matter together, and dark energy, which pulls it apart by accelerating the expansion of the Universe, have affected them.

“Dark energy affects our measurements for two reasons. First, when it is present, galaxy clusters grow more slowly, and secondly, it changes the way the Universe expands, leading to more distant — and more efficiently lensed — galaxies. Our analysis is sensitive to both effects,” says co-author Benjamin Joachimi from the University of Bonn.

“Our study also provides an additional confirmation for Einstein’s theory of general relativity, which predicts how the lensing signal depends on redshift,” adds co-investigator Martin Kilbinger from the Institut d’Astrophysique de Paris and the Excellence Cluster Universe.

The large number of galaxies included in this study, along with information on their redshifts is leading to a clearer map of how, exactly, part of the Universe is laid out; it helps us see its galactic inhabitants and how they are distributed.

“With more accurate information about the distances to the galaxies, we can measure the distribution of the matter between them and us more accurately,” notes co-investigator Jan Hartlap from the University of Bonn.

“Before, most of the studies were done in 2D, like taking a chest X-ray. Our study is more like a 3D reconstruction of the skeleton from a CT scan. On top of that, we are able to watch the skeleton of dark matter mature from the Universe’s youth to the present,” comments William High from Harvard University, another co-author.

Image credits: NASA, ESA, J. Hartlap (University of Bonn), P. Simon (University of Bonn) and T. Schrabback (Leiden Observatory)