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Category Archives: 16. A Universe of galaxies

From a Berkeley Lab press release (July 21, 2016):

Scientists with the Large Underground Xenon (LUX) dark matter experiment, which operates beneath a mile of rock at the Sanford Underground Research Facility in the Black Hills of South Dakota, have completed their search for the missing matter of the Universe. (See The Cosmos, Section 16.4c, p. 430.)

Although LUX’s sensitivity far exceeded the original expectations of the experiment,  it yielded no trace of a dark matter particle. LUX’s extreme sensitivity makes the team confident that if dark matter particles had interacted with the LUX’s xenon target, the detector would almost certainly have seen them. These new limits on dark matter detection will allow scientists to eliminate many potential models for dark matter particles, offering critical guidance for the next generation of dark matter experiments.

While the LUX experiment successfully eliminated a large swath of mass ranges and interaction-coupling strengths where so-called WIMPs might exist, physicists believe the WIMP model itself remains alive and viable.

Links: full LBL press release, LUX homepage.

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Adapted from a CERN press release, September 18, 2014:

The Alpha Magnetic Spectrometer (AMS) collaboration has recently presented its latest results. These are based on the analysis of 41 billion particles detected with the space-based AMS detector aboard the International Space Station. The results provide new insights into the nature of the mysterious excess of positrons observed in the flux of cosmic rays, which, according to some models, might be evidence of dark matter (see Section 16.4, p. 428). The findings are published in the journal Physical Review Letters.

AMS aboard the International Space Station

Credit: NASA

Cosmic rays are particles commonly present in the Universe, consisting mainly of protons and electrons, but there are also many other kinds of particles, including positrons. Positrons are the antimatter counterparts of electrons, with the same mass but opposite charge. The presence of some positrons in space can be explained from the collisions of cosmic rays, but this phenomenon would only produce a tiny portion of antimatter in the overall cosmic ray spectrum. Since antimatter is extremely rare in the universe, any significant excess of antimatter particles recorded in the flux of energetic cosmic rays indicates the existence of a new source of positrons. Very dense stars, such as pulsars, are potential candidates.

The AMS experiment is able to map the flux of cosmic rays with unprecedented precision and in the results published last week, the collaboration presents new data at energies never before recorded. The AMS collaboration has analyzed 41 billion primary cosmic ray events among which 10 million have been identified as electrons and positrons. The distribution of these events in the energy range of 0.5 to 500 GeV shows a well-measured increase of positrons from 8 GeV with no preferred incoming direction in space.  The energy at which the positron fraction ceases to increase has been measured to be 275±32 GeV.

This rate of decrease after the “cut-off energy” is very important to physicists as it could be an indicator that the excess of positrons is the signature of dark matter particles annihilating into pairs of electrons and positrons. Although the current measurements could be explained by objects such as pulsars, they are also tantalizingly consistent with dark matter particles with mass of the order of 1 TeV. Different models on the nature of dark matter predict different behaviour of the positron excess above the positron fraction expected from ordinary cosmic ray collisions. Therefore, results at higher energies will be of crucial importance in the near future to evaluate if the signal is from dark matter or from a cosmic source.

Links: Full CERN press release.

The Sloan Digital Sky Survey (SDSS) is one of the most ambitious and influential surveys in the history of astronomy. Over eight years of operations it has obtained deep, multi-color images covering more than a quarter of the sky and created 3-dimensional maps containing more than 930,000 galaxies and more than 120,000 quasars.

Credit: Sloan Digital Sky Survey

The education team at SDSS have prepared a variety of astronomical resources, interactive tools, and science projects, for teachers and educators to use. They aim to show us the beauty of the Universe, and share with us their excitement as they build the largest map in the history of the world!

SkyServer‘s tools allow you to access all publicly available data from the Sloan Digital Sky Survey. It offers access to many different types of data, but most users will usually focus on four types: images, spectra, photometric data, and spectroscopic data. See their ‘Getting Started‘ page for more details.

Their projects pages come in both Basic (suitable for high-school and Astronomy 101-level students) and Advanced (for students with a deeper understanding of astronomy) levels.  There are also ideas for extended independent research projects.

Instructor guides are also available.

Mordecai-Mark Mac Low, Curator of Astrophysics at the American Museum of Natural History in New York, presents short, fun features on the history of mysterious dark matter (see Section 16.10) and dark energy (see Section 19.3b).

Credit: AMNH Rose Center for Earth and Space and Hayden Planetarium

The AMNH’s new planetarium show ‘Dark Universe’ celebrates the pivotal discoveries that have led us to greater knowledge of the structure and history of the Universe and our place in it — and to new frontiers for exploration. It is narrated by the planetarium director, Neil deGrasse Tyson. A trailer for the new show may be seen here, along with further information about the show’s creation.

New observations from the Atacama Large Millimeter/submillimeter Array (ALMA) telescope in Chile have given astronomers the best view yet of how vigorous star formation can blast gas out of a galaxy and starve future generations of stars of the fuel they need to form and grow. Dramatic new images show enormous outflows of molecular gas ejected by star-forming regions in the nearby Sculptor Galaxy. These new results help to explain the strange paucity of very massive galaxies in the Universe. The study was published in the journal Nature on 25 July 2013.

The Sculptor Galaxy, also known as NGC 253, is a spiral galaxy located in the southern constellation of Sculptor. Lying at a distance of around 11.5 million light-years from our Solar System it is one of our closer intergalactic neighbors, and one of the closest ‘starburst galaxies,’ those which produce at an exceptionally high rate. Using ALMA, astronomers have discovered billowing columns of cold, dense gas fleeing from the center of the galactic disc. These results may help to explain why astronomers have found surprisingly few high-mass galaxies throughout the cosmos.

Credit: ALMA (ESO/NAOJ/NRAO)/Erik Rosolowsky

See the full ESO press release here, including links to more images and movies.