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New Lights on Dark Matter

  • 16th September, 2020

Syllabus: Prelims GS Paper I : Current Events of National and International Importance; General Science.

Mains GS Paper III : Awareness in the fields of IT, Space, Computers, Robotics, Nano-technology, Bio-technology and issues relating to Intellectual Property Rights.

Contextnew-light

Observations by the NASA/ESA Hubble Space Telescope and the European Southern Observatory's Very Large Telescope (VLT) in Chile have found that something may be missing from the theories of how dark matter behaves. This missing ingredient may explain why researchers have uncovered an unexpected discrepancy between observations of the dark matter concentrations in a sample of massive galaxy clusters and theoretical computer simulations of how dark matter should be distributed in clusters. The new findings indicate that some small-scale concentrations of dark matter produce lensing effects that are 10 times stronger than expected.

Dark matter is the invisible glue that keeps stars, dust, and gas together in a galaxy. This mysterious substance makes up the bulk of a galaxy's mass and forms the foundation of our Universe's large-scale structure. Because dark matter does not emit, absorb, or reflect light, its presence is only known through its gravitational pull on visible matter in space. Astronomers and physicists are still trying to pin down what it is.

Galaxy clusters, the most massive and recently assembled structures in the Universe, are also the largest repositories of dark matter. Clusters are composed of individual member galaxies that are held together largely by the gravity of dark matter.

"Galaxy clusters are ideal laboratories in which to study whether the numerical simulations of the Universe that are currently available reproduce well what we can infer fromgravitational lensing ," said Massimo Meneghetti of the INAF-Observatory of Astrophysics and Space Science of Bologna in Italy, the study's lead author.

"We have done a lot of testing of the data in this study, and we are sure that this mismatch indicates that some physical ingredient is missing either from the simulations or from our understanding of the nature of dark matter," added Meneghetti.

The distribution of dark matter in clusters is mapped by measuring the bending of light—the gravitational lensing effect—that they produce. The gravity of dark matter concentrated in clusters magnifies and warps light from distant background objects. This effect produces distortions in the shapes of background galaxies which appear in images of the clusters.

Gravitational lensing can often also produce multiple images of the same distant galaxy.The higher the concentration of dark matter in a cluster, the more dramatic its light-bending effect. The presence of smaller-scale clumps of dark matter associated with individual cluster galaxies enhances the level of distortions. In some sense, the galaxy cluster acts as a large-scale lens that has many smaller lenses embedded within it.

Hubble's crisp images were taken by the telescope's Wide Field Camera 3 and Advanced Camera for Surveys. Coupled with spectra from the European Southern Observatory's Very Large Telescope (VLT), the team produced an accurate, high-fidelity, dark-matter map. By measuring the lensing distortions astronomers could trace out the amount and distribution of dark matter. The three key galaxy clusters, MACS J1206.2-0847, MACS J0416.1-2403, and Abell S1063, were part of two Hubble surveys: The Frontier Fields and the Cluster Lensing And Supernova survey with Hubble (CLASH) programs.

Dark Matter
About eighty-five percent of the matter in the universe is in the form of dark matter, whose nature remains a mystery. The rest of the matter in the universe is of the kind found in atoms. Astronomers studying the evolution of galaxies in the universe find that dark matter exhibits gravity and, because it is so abundant, it dominates the formation of large-scale structures in the universe like clusters of galaxies. Dark matter is hard to observe directly, needless to say, and it shows no evidence of interacting with itself or other matter other than via gravity, but fortunately it can be traced by modeling sensitive observations of the distributions of galaxies across a range of scales.
Galaxies generally reside at the centers of vast clumps of dark matter called haloes because they surround the clusters of galaxies. Gravitational lensing of more distant galaxies by dark matter haloes offers a particularly unique and powerful probe of the detailed distribution of dark matter. So-called strong gravitational lensing creates highly distorted, magnified and occasionally multiple images of a single source; so-called weak lensing results in modestly yet systematically deformed shapes of background galaxies that can also provide robust constraints on the distribution of dark matter within the clusters.
The Center for Astrophysics | Harvard & Smithsonian (CfA) astronomers Annalisa Pillepich and Lars Hernquist and their colleagues compared gravitationally distorted Hubble images of the galaxy cluster Abell 2744 and two other clusters with the results of computer simulations of dark matter haloes. They found, in agreement with key predictions in the conventional dark matter picture, that the detailed galaxy substructures depend on thedark matter halo distribution, and that the total mass and the light trace each other. They also found a few discrepancies: the radial distribution of the dark matter is different from that predicted by the simulations, and the effects of tidal stripping and friction in galaxies are smaller than expected, but they suggest these issues might be resolved with more precise simulations. Overall, however, the standard model of dark matter does an excellent and reassuring job of describing galaxy clustering

To the team's surprise, in addition to the dramatic arcs and elongated features of distant galaxies produced by each cluster's gravitational lensing, the Hubble images also revealed an unexpected number of smaller-scale arcs and distorted images nested near each cluster's core, where the most massive galaxies reside. The researchers believe the nested lenses are produced by the gravity of dense concentrations of matter inside the individual cluster galaxies. Follow-up spectroscopic observations measured the velocity of the stars orbiting inside several of the cluster galaxies to therby pin down their masses.

"The data from Hubble and the VLT provided excellent synergy," s`hared team member Piero Rosati of the Università degli Studi di Ferrara in Italy, who led the spectroscopic campaign. "We were able to associate the galaxies with each cluster and estimate their distances."

"The speed of the stars gave us an estimate of each individual galaxy's mass, including the amount of dark matter," added team member Pietro Bergamini of the INAF-Observatory of Astrophysics and Space Science in Bologna, Italy.

By combining Hubble imaging and VLT spectroscopy, the astronomers were able to identify dozens of multiply imaged, lensed, background galaxies. This allowed them to assemble a well-calibrated, high-resolution map of the mass distribution of dark matter in each cluster.

The team compared the dark-matter maps with samples of simulated galaxy clusters with similar masses, located at roughly the same distances. The clusters in the computer model did not show any of the same level of dark-matter concentration on the smallest scales—the scales associated with individual cluster galaxies.

Astronomers, including those of this team, look forward to continuing to probe dark matter and its mysteries in order to finally pin down its nature.

A Note on Hubble Space Telescope

From the dawn of humankind to a mere 400 years ago, all that we knew about our universe came through observations with the naked eye. Then Galileo turned his telescope towards the heavens in 1610. The world was in for an awakening.

Saturn, we learned, had rings. Jupiter had moons. That nebulous patch across the center of the sky called the Milky Way was not a cloud but a collection of countless stars. Within but a few years, our notion of the natural world would be forever changed. A scientific and societal revolution quickly ensued.

In the centuries that followed, telescopes grew in size and complexity and, of course, power. They were placed far from city lights and as far above the haze of the atmosphere as possible. Edwin Hubble, for whom the Hubble Telescope is named, used the largest telescope of his day in the 1920s at the Mt. Wilson Observatory near Pasadena, California, to discover galaxies beyond our own.

Hubble, the observatory, is the first major optical telescope to be placed in space, the ultimate mountaintop. Above the distortion of the atmosphere, far above rain clouds and light pollution, Hubble has an unobstructed view of the universe. Scientists have used Hubble to observe the most distant stars and galaxies as well as the planets in our solar system.

Hubble's launch and deployment in April 1990 marked the most significant advance in astronomy since Galileo's telescope. Thanks to five servicing missions and more than 25 years of operation, our view of the universe and our place within it has never been the same.

Hubble Space Telescope Facts

NASA named the world's first space-based optical telescope after American astronomer Edwin P. Hubble (1889 -- 1953). Dr. Hubble confirmed an "expanding" universe, which provided the foundation for the big-bang theory.

Mission

  • Launch: April 24, 1990, from space shuttle Discovery (STS-31)
  • Deployment: April 25, 1990
  • First Image: May 20, 1990: Star cluster NGC 3532
  • Servicing Mission 1 (STS-61): December 1993
  • Servicing Mission 2 (STS-82): February 1997
  • Servicing Mission 3A (STS-103): December 1999
  • Servicing Mission 3B (STS-109): February 2002
  • Servicing Mission 4 (STS-125): May 2009

Size

  • Length: 43.5 feet (13.2 m)
  • Weight: At Launch: about 24,000 pounds (10,886 kg)
  • Post SM4: about 27,000 pounds (12,247 kg)
  • Maximum Diameter: 14 feet (4.2 m)
  • Spaceflight Statistics
  • Low Earth Orbit: Altitude of 340 miles (295 nautical miles, or 547 km), inclined 28.5 degrees to the equator
  • Time to Complete One Orbit: about 95 minutes
  • Speed: about 17,000 mph (27,300 kph)

Optical Capabilities

  • Sensitivity to Light: Ultraviolet through Infrared (115–2500 nanometers)

Hubble's Mirrors

  • Primary Mirror Diameter: 94.5 inches (2.4 m)
  • Primary Mirror Weight: 1,825 pounds (828 kg)
  • Secondary Mirror Diameter: 12 inches (0.3 m)
  • Secondary Mirror Weight: 27.4 pounds (12.3 kg)

Pointing Accuracy

  • In order to take images of distant, faint objects, Hubble must be extremely steady and accurate. The telescope is able to lock onto a target without deviating more than 7/1000th of an arcsecond, or about the width of a human hair seen at a distance of 1 mile.

Data Statistics

  • Hubble transmits about 150 gigabits of raw science data every week.

Power Needs

  • Energy Source: The Sun
  • Mechanism: Two 25-foot solar panels
  • Power Generation (in Sunlight): about 5,500 watts
  • Power Usage (Average): about 2,100 watts

Power Storage

  • Batteries: 6 nickel-hydrogen (NiH)
  • Storage Capacity: Equal to about 22 average car batteries

Did you know...

  • Hubble has made more than 1.3 million observations since its mission began in 1990.
  • Astronomers using Hubble data have published more than 15,000 scientific papers, making it one of the most productive scientific instruments ever built. Those papers have been cited in other papers 738,000 times.
  • Hubble does not travel to stars, planets or galaxies. It takes pictures of them as it whirls around Earth at about 17,000 mph.
  • Hubble has circled Earth and gone more than 4 billion miles along a circular low earth orbit currently about 340 miles in altitude.
  • Hubble has no thrusters. To change angles, it uses Newton’s third law by spinning its wheels in the opposite direction. It turns at about the speed of a minute hand on a clock, taking 15 minutes to turn 90 degrees.
  • Hubble has the pointing accuracy of .007 arcseconds, which is like being able to shine a laser beam on President Roosevelt’s head on a dime about 200 miles away.
  • Outside the haze of our atmosphere, it can see astronomical objects with an angular size of 0.05 arcseconds, which is like seeing a pair of fireflies in Tokyo that are less than 10 feet apart from Washington, DC.
  • Due to the combination of optics and sensitive detectors and with no atmosphere to interfere with the light reaching it, Hubble can spot a night light on the surface of the Moon from Earth.
  • Hubble has peered back into the very distant past, to locations more than 13.4 billion light-years from Earth.
  • Hubble generates about 10 terabytes of new data per year. The total archive is currently over 150 TB in size.
  • Hubble weighed about 24,000 pounds at launch but if returned to Earth today would weigh about 27,000 pounds — on the order of two full-grown African elephants.
  • Hubble's primary mirror is 2.4 meters (7 feet, 10.5 inches) across. It was so finely polished that if you scaled it to be the diameter of the Earth, you would not find a bump more than 6 inches tall.
  • Hubble is 13.3 meters (43.5 feet) long — the length of a large school bus.

Miles covered:

Question for Prelims :

Hubble space telescope is installed to observe

(a) Planets
(b) Stars
(c) Sun
(d) Galaxies

Question for Prelims :

"Galaxy clusters are ideal laboratories in which to study whether the numerical simulations of the Universe that are currently available reproduce well what we can infer from gravitational lensing ," Comment.

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