Distant Invisible Galaxy Could be Made Up Entirely of Dark Matter


Astronomers can’t see it but they know it’s out there from the distortions caused by its gravity. That statement describes dark matter, the elusive substance which scientists have estimated makes up about 25% of our universe and doesn’t emit or absorb light. But it also describes a distant, tiny galaxy located about 10 billion light years from Earth. This galaxy can’t be seen in telescopes, but astronomers were able to detect its presence through the small distortions made in light that passes by it. This dark galaxy is the most distant and lowest-mass object ever detected, and astronomers say it could help them find similar objects and confirm or reject current cosmological theories about the structure of the Universe.

“Now we have one dark satellite [galaxy],” said Simona Vegetti, a postdoctoral researcher at the Massachusetts Institute of Technology, who led the discovery. “But suppose that we don’t find enough of them — then we will have to change the properties of dark matter. Or, we might find as many satellites as we see in the simulations, and that will tell us that dark matter has the properties we think it has.”

This dwarf galaxy is a satellite of a distant elliptical galaxy, called JVAS B1938 + 666. The team was looking for faint or dark satellites of distant galaxies using gravitational lensing, and made their observations with the Keck II telescope on Mauna Kea in Hawaii, along with the telescope’s adaptive optics to limit the distortions from our own atmosphere.

They found two galaxies aligned with each other, as viewed from Earth, and the nearer object’s gravitational field deflected the light from the more distant object (JVAS B1938 + 666) as the light passed through the dark galaxy’s gravitational field, creating a distorted image called an “Einstein Ring.”

Using data from this effect, the mass of the dark galaxy was found to be 200 million times the mass of the Sun, which is similar to the masses of the satellite galaxies found around our own Milky Way. The size, shape and brightness of the Einstein ring depends on the distribution of mass throughout the foreground lensing galaxy.

Current models suggest that the Milky Way should have about 10,000 satellite galaxies, but only 30 have been observed. “It could be that many of the satellite galaxies are made of dark matter, making them elusive to detect, or there may be a problem with the way we think galaxies form,” Vegetti said.

The dwarf galaxy is a satellite, meaning that it clings to the edges of a larger galaxy. Because it is small and most of the mass of galaxies is not made up of stars but of dark matter, distant objects such as this galaxy may be very faint or even completely dark.

“For several reasons, it didn’t manage to form many or any stars, and therefore it stayed dark,” said Vegetti.

Vegetti and her team plan to use the same method to look for more satellite galaxies in other regions of the Universe, which they hope will help them discover more information on how dark matter behaves.

Their research was published in this week’s edition of Nature.

The team’s paper can be found here.

Sources: Keck Observatory, UC Davis, MIT

Microlensing Study Says Every Star in the Milky Way has Planets


How common are planets in the Milky Way? A new study using gravitational microlensing suggests that every star in our night sky has at least one planet circling it. “We used to think that the Earth might be unique in our galaxy,” said Daniel Kubas, a co-lead author of a paper that appears this week in the journal Nature. “But now it seems that there are literally billions of planets with masses similar to Earth orbiting stars in the Milky Way.”

Over the past 16 years, astronomers have detected more than 3,035 exoplanets – 2,326 candidates and 709 confirmed planets orbiting other stars. Most of these extrasolar planets have been discovered using the radial velocity method (detecting the effect of the gravitational pull of the planet on its host star) or the transit method (catching the planet as it passes in front of its star, slightly dimming it.) Those two methods usually tend to find large planets that are relatively close to their parent star.

But another method, gravitational microlensing — where the light from the background star is amplified by the gravity of the foreground star, which then acts as a magnifying glass — is able to find planets over a wide range of mass that are further away from their stars.

Gravitational microlensing method requires that you have two stars that lie on a straight line in relation to us here on Earth. Then the light from the background star is amplified by the gravity of the foreground star, which thus acts as a magnifying glass.

An international team of astronomers used the technique of gravitational microlensing in six-year search that surveyed millions of stars. “We conclude that stars are orbited by planets as a rule, rather than the exception,” the team wrote in their paper.

“We have searched for evidence for exoplanets in six years of microlensing observations,” said lead author Arnaud Cassan from the Institut de Astrophysique in Paris. “Remarkably, these data show that planets are more common than stars in our galaxy. We also found that lighter planets, such as super-Earths or cool Neptunes, must be more common than heavier ones.”

The Milky Way above the dome of the Danish 1.54-metre telescope at ESO's La Silla Observatory in Chile. The central part of the Milky Way is visible behind the dome of the ESO 3.6-metre telescope in the distance. On the right the Magellanic Clouds can be seen. This telescope was a major contributor to the PLANET project to search for exoplanets using microlensing. The picture was taken using a normal digital camera with a total exposure time of 15 minutes. Credit: ESO/Z. Bardon

The astronomers surveyed millions of stars looking for microlensing events, and 3,247 such events in 2002-2007 were spotted in data from the European Southern Observatory’s PLANET and OGLE searches. The precise alignment needed for microlensing is very unlikely, and statistical results were inferred from detections and non-detections on a representative subset of 440 light curves.

Three exoplanets were actually detected: a super-Earth and planets with masses comparable to Neptune and Jupiter. The team said that by microlensing standards, this is an impressive haul, and that in detecting three planets, they were either incredibly lucky despite huge odds against them, or planets are so abundant in the Milky Way that it was almost inevitable.

The astronomers then combined information about the three positive exoplanet detections with seven additional detections from earlier work, as well as the huge numbers of non-detections in the six years’ worth of data (non-detections are just as important for the statistical analysis and are much more numerous, the team said.) The conclusion was that one in six of the stars studied hosts a planet of similar mass to Jupiter, half have Neptune-mass planets and two thirds have super-Earths.

This works out to about 100 billion exoplanets in our galaxy.

The survey was sensitive to planets between 75 million kilometers and 1.5 billion kilometers from their stars (in the Solar System this range would include all the planets from Venus to Saturn) and with masses ranging from five times the Earth up to ten times Jupiter.

This also shows that microlensing is a viable way to find exoplanets. Astronomers hope to use other methods in the future to find even more planets.

“I have a list of 17 different ways to find exoplanets and only five have been used so far,” said Virginia Trimble from the University of California, Irvine and the Las Cumbres Observatory, providing commentary at the American Astronomical Scoeity meeting this week, “I expect we’ll be finding many more planets in the future.”

Sources: Nature, ESO, AAS briefing

Quadruply Lensed Dwarf Galaxy 12.8 Billion Light Years Away

Galaxy Cluster MACS J0329.6-0211 lenses several background galaxies including a distant dwarf galaxy. CREDIT: A. Zitrin, et al.


Gravitational lensing is a powerful tool for astronomers that allows them to explore distant galaxies in far more detail than would otherwise be allowed. Without this technique, galaxies at the edge of the visible universe are little more than tiny blobs of light, but when magnified dozens of times by foreground clusters, astronomers are able to explore the internal structural properties more directly.

Recently, astronomers at the University of Heidelberg discovered a gravitational lensed galaxy that ranked among the most distant ever seen. Although there’s a few that beat this one out in distance, this one is remarkable for being a rare quadruple lens.

The images for this remarkable discovery were taken using the Hubble Space Telescope in August and October of this year, using a total of 16 different colored filters as well as additional data from the Spitzer infrared telescope. The foreground cluster, MACS J0329.6-0211, is some 4.6 billion light years distant. In the above image, the background galaxy has been split into four images, labelled by the red ovals and marked as 1.1 – 1.4. They are enlarged in the upper right.

Assuming that the mass of the foreground cluster is concentrated around the galaxies that were visible, the team attempted to reverse the effects the cluster would have on the distant galaxy, which would reverse the distortions. The restored image, also corrected for redshift, is shown in the lower box in the upper right corner.

After correcting for these distortions, the team estimated that the total mass of the distant galaxy is only a few billion times the mass of the Sun. In comparison, the Large Magellanic Cloud, a dwarf satellite to our own galaxy, is roughly ten billion solar masses. The overall size of the galaxy was determined to be small as well. These conclusions fit well with expectations of galaxies in the early universe which predict that the large galaxies in today’s universe were built from the combination of many smaller galaxies like this one in the distant past.

The galaxy also conforms to expectations regarding the amount of heavy elements which is significantly lower than stars like the Sun. This lack of heavy elements means that there should be little in the way of dust grains. Such dust tends to be a strong block of shorter wavelengths of light such as ultraviolet and blue. Its absence helps give the galaxy its blue tint.

Star formation is also high in the galaxy. The rate at which they predict new stars are being born is somewhat higher than in other galaxies discovered around the same distance, but the presence of brighter clumps in the restored image suggest the galaxy may be undergoing some interactions, driving the formation of new stars.

A Magnified Supernova


Supernovae are among astronomers most important tools for exploring the history of the universe. Their frequency allows us to examine how active star formation was, how heavy elements have developed, and the distance to galaxies across vast distances. Yet even these titanic explosions are only so bright, and there’s an effective limit on how far we can detect them with the current generation of telescopes. However, this limit can be extended with a little help from gravity.

One of the consequences of Einstein’s theory of general relativity is that massive objects can distort space, allowing them to act as a lens. While first postulated in 1924, and proposed for galaxies by Fritz Zwicky in 1937, the effect wasn’t observed until 1979 when a distant quasar, an energetic core of a distant galaxy, was split in two by the gravitational disturbances of an intervening cluster of galaxies.

While lensing can distort images, it also provides the possibility that it may magnify a distant object, increasing the amount of light we receive. This would allow astronomers to probe even more distant regions with supernovae as their tool. But in doing so, astronomers must look for these events in a different manner than most supernova searches. These searches are generally limited to the visible portion of the spectrum, the portion we see with our eyes, but due to the expansion of the universe, the light from these objects is stretched into the near-infrared portion of the spectrum where few surveys to search for supernovae exist.

But one team, led by Rahman Amanullah at Stockholm University in Sweden, has conducted a survey using the Very Large Telescope array in Chile, to search for supernovae lensed by the massive galaxy cluster Abell 1689. This cluster is well known as a source of gravitationally lensed objects, making visible some galaxies that formed shortly after the Big Bang.

In 2009, the team discovered one supernova that was magnified by this cluster that originated 5-6 billion lightyears away. In a new paper, the team reveals details about an even more distant supernova, nearly 10 billion lightyears distant. This event was magnified by a factor of 4 from the effects of the foreground cluster. From the distribution of energy in different portions of the spectrum, the team concludes that the supernova was an implosion of a massive star leading to a core-collapse type of supernova. The distance of this event puts it among the most distant supernovae yet observed. Others at this distance have required extensive time using the Hubble telescope or other large telescopes.

Hubble Provides Most Detailed Dark Matter Map Yet

Cosmic Noise


Using Hubble’s Advanced Camera for Surveys, astronomers have been able to chart invisible dark matter in a distant galaxy, which enabled them to create one of the sharpest and most detailed maps of dark matter in the universe. Looking for invisible and indeterminate matter is a difficult job, but one that astronomers have been trying to do for over a decade. This new map also might provide clues on that other mysterious stuff in the universe — dark energy – and what role it played in the universe’s early formative years.

A team led by Dan Coe at JPL used Hubble to look at Abell 1689, located 2.2 billion light-years away. The cluster’s gravity, which mostly comes from dark matter, acts like a cosmic magnifying glass, bending and amplifying the light from distant galaxies behind it. This effect, called gravitational lensing, produces multiple, warped, and greatly magnified images of those galaxies, making the galaxies look distorted and fuzzy. By studying the distorted images, astronomers estimated the amount of dark matter within the cluster. If the cluster’s gravity only came from the visible galaxies, the lensing distortions would be much weaker.

What they found suggests that galaxy clusters may have formed earlier than expected, before the push of dark energy inhibited their growth.

Dark energy pushes galaxies apart from one another by stretching the space between them, thereby suppressing the formation of giant structures called galaxy clusters. One way astronomers can probe this primeval tug-of-war is through mapping the distribution of dark matter in clusters.

“The lensed images are like a big puzzle,” Coe said. “Here we have figured out, for the first time, a way to arrange the mass of Abell 1689 such that it lenses all of these background galaxies to their observed positions.” Coe used this information to produce a higher-resolution map of the cluster’s dark matter distribution than was possible before.

Based on their higher-resolution mass map, Coe and his collaborators confirm previous results showing that the core of Abell 1689 is much denser in dark matter than expected for a cluster of its size, based on computer simulations of structure growth. Abell 1689 joins a handful of other well-studied clusters found to have similarly dense cores. The finding is surprising, because the push of dark energy early in the universe’s history would have stunted the growth of all galaxy clusters.

“Galaxy clusters, therefore, would had to have started forming billions of years earlier in order to build up to the numbers we see today,” Coe said. “At earlier times, the universe was smaller and more densely packed with dark matter. Abell 1689 appears to have been well fed at birth by the dense matter surrounding it in the early universe. The cluster has carried this bulk with it through its adult life to appear as we observe it today.”

Astronomers are planning to study more clusters to confirm the possible influence of dark energy. A major Hubble program that will analyze dark matter in gigantic galaxy clusters is the Cluster Lensing and Supernova survey with Hubble (CLASH). In this survey, the telescope will study 25 clusters for a total of one month over the next three years. The CLASH clusters were selected because of their strong X-ray emission, indicating they contain large quantities of hot gas. This abundance means the clusters are extremely massive. By observing these clusters, astronomers will map the dark matter distributions and look for more conclusive evidence of early cluster formation, and possibly early dark energy.

For more information see the HubbleSite.

Herschel Provides Gravitational Lens Bonanza

The image shows the first area of sky viewed as part of the Herschel-ATLAS survey. The five inset show enlarged views of the five distant galaxies whose images are being gravitationally lensed by foreground galaxies (unseen by Herschel). The distant galaxies are not only very bright, but also very red in colour in this image, showing that they are brighter at the longer wavelengths measured by the SPIRE instrument. Image credits: ESA/SPIRE/Herschel-ATLAS/SJ Maddox (top); ESA/NASA/JPL-Caltech/Keck/SMA (bottom).


One of the predictions of Einstein’s predictions from general relativity was that gravity could distort space itself and potentially, act as a lens. This was spectacularly confirmed in 1919 when, during a solar eclipse, Arthur Eddington observed stars near the Sun were distorted from their predicted positions. In 1979, this effect was discovered at much further distances when astronomers found it to distort the image of a distant quasar, making one appear as two. Several other such cases have been discovered since then, but these instances of gravitational lensing have proven difficult to find. Searches for them have had a low success rate in which less than 10% of candidates are confirmed as gravitational lenses. But a new method using data from Herschel may help astronomers discover many more of these rare occurrences.

The Herschel telescope is one of the many space telescopes currently in use and explores the portion of the spectrum from the far infrared to the submillimeter regime. A portion of its mission is to produce a large survey of the sky resulting in the Herschel ATLAS project which will take deep images of over 550 square degrees of the sky.

While Herschel explores this portion of the electromagnetic spectrum in far greater detail than its predecessors, in many ways, there’s not much to see. Stars emit only very faintly in this range. The most promising targets are warm gas and dust which are better emitters, but also far more diffuse. But it’s this combination of facts that will allow Herschel to potentially discover new lenses with improved efficiency.

The reason is that, although galaxies lack strong emission in this regime in the modern universe, ancient galaxies gave off far more since during the first 4 billion years. During that time, many galaxies were dominated by dust being warmed by star formation. Yet due to their distance, they too should be faint… Unless a gravitational lens gets in the way. Thus, the majority of small, point-like sources in the ALTAS collection are likely to be lensed galaxies. As Dr Mattia Negrello, of the Open University and lead researcher of the study explains, “The big breakthrough is that we have discovered that many of the brightest sources are being magnified by lenses, which means that we no longer have to rely on the rather inefficient methods of finding lenses which are used at visible and radio wavelengths.”

These panels show a zoom of one of the lenses, with high resolution images from Keck (optical light, blue) and the submillimeter Array (sub-millimetre light, red). Image credits: ESA/NASA/JPL-Caltech/Keck/SMA
These panels show a zoom of one of the lenses, with high resolution images from Keck (optical light, blue) and the submillimeter Array (sub-millimetre light, red). Image credits: ESA/NASA/JPL-Caltech/Keck/SMA

Already, this new technique has turned up at least five strong candidates. A paper, to be published in the current issue of Science discusses them. Each of them received followup observations from the Z-Spec spectrometer on the California Institute of Technology Submillimeter Observatory. The furthest of these these objects, labeled as ID81, showed a prominent IR spectral line had a redshift of 3.04, putting it at a distance of 11.5 billion lightyears. Additionally, each system showed the spectral profile of the foreground galaxy, demonstrating that the combined light received was indeed two galaxies and the bright component was a gravitational lens.

This method of using gravitational lenses will allow the Herschel team to probe distant galaxies in detail never before achieved. As with all telescopes, longer wavelengths of observations result in less resolution which means that, even if one of the distant systems were to be broken into distinct portions, Herschel would be unable to resolve them. But the fact that we can see them at all means their spectral signatures of the galaxies as a whole can still be studied. Additionally, as Professor Steve Eales from Cardiff University and the other leader of the survey noted: “We can also use this technique to study the lenses themselves.” This potential to explore the mass of the nearby galaxies may help astronomers to understand and constrain the enigmatic Dark Matter that makes up ~80% of the mass in our universe.

Dr Loretta Dunne of Nottingham University and joint-leader of the Herschel-ATLAS survey adds, “What we’ve seen so far is just the tip of the iceberg. Wide area surveys are essential for finding these rare events and since Herschel has only covered one thirtieth of the entire Herschel-ATLAS area so far, we expect to discover hundreds of lenses once we have all the data. Once found, we can probe the early Universe on the same physical scales as we can in galaxies next door.”

First Quasar Gravitational Lens Discovered (w/video)


Gravitation lensing – a phenomenon that falls out of Einstein’s theory of general relativity – has been observed numerous times, making for some fantastic images of rings, arcs and crosses composed of massive galaxies light years away. As the light from a background object is bent by gravity around a foreground object, multiple, magnified images of the background object are produced from our vantage point.

For the first time, a quasar (quasi-stellar object) has been shown to gravitationally lens a galaxy behind it. About a hundred instances of gravitational lenses that consist of a foreground galaxy and a background quasar have been found, but this is the very first time where the opposite is the case; that is, a quasar bending the light from a background galaxy around it to create a multiple image of that galaxy.

Quasars are thought to be the result of a supermassive black hole at the center of a galaxy attempting to swallow up all of the matter that surrounds it. As the matter bunches up when it gets closer to the black hole, it heats up due to friction and begins to emit light across the electromagnetic spectrum. The light from a quasar can outshine an entire galaxy of stars, making it difficult to separate the light from a background galaxy from the overwhelming glare of the quasar itself.

To make this initial detection (there are surely many to follow), astronomers from the EPFL’s Laboratory of Astrophysics in cooperation with Caltech used data from the Sloan Digital Sky Survey (SDSS). They analyzed 22,298 quasars from the SDSS Data Release 7 catalog, and looked for images that had a strongly redshifted emission spectra. According to the paper announcing the results, “In these spectra, we look for emission lines redshifted beyond the redshift of the [quasar].”

In other words, a quasar that is lensing a galaxy in the background will exhibit a higher redshift than one that is not lensing a background galaxy, since the light from the galaxy and the quasar are combined in the SDSS data. So, quasars that had an expected redshift were thrown out, and a statistical analysis of quasars with emission lines that might mimic a gravitational lens eliminated many more of the objects. This left about 14 objects of the 22,298 analyzed as potential candidates. Of these 14, the team selected one to perform follow-up observations on, named SDSS J0013+1523.

SDSS J0013+1523 lies about 1.6 billion light years away, and is lensing a galaxy that is about 7.5 billion light years away from Earth. Using the Keck II telescope, they were able to confirm that SDSS J0013+1523 was indeed lensing the light from a galaxy located behind it. Hubble images of the discovery are in the works.

Here’s a video produced by the EPFL describing the results.

What is significant about this discovery – besides the novel aspect of a quasar acting as a lens – is that it will allow researchers to better refine their understanding of quasars. When light is bent around an object, it bends because of gravity, and gravity is a result of mass. So, something that is very massive will act as a stronger lens than something that is tiny, and the mass of the object doing all of the lensing work – in this case, the foreground quasar – can be determined.

Their results were published in a letter to Astronomy & Astrophysics on July 16th. The original paper is available for your perusal here.

Source: Eurekalert here and here, Arxiv paper here

Hubble Confirms Cosmic Acceleration with Weak Lensing


Need more evidence that the expansion of the Universe is accelerating? Just look to the Hubble Space Telescope. An international team of astronomers has indeed confirmed that the expansion of the universe is accelerating. The team, led by Tim Schrabback of the Leiden Observatory, conducted an intensive study of over 446,000 galaxies within the COSMOS (Cosmological Evolution Survey) 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 the orbiting telescope. 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,” said co-author Patrick Simon from Edinburgh University.

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. 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.

The astronomers specifically chose the COSMOS survey because it is thought to be a representative sample of the Universe. With thorough studies such as the one led by Schrabback, astronomers will one day be able to apply their technique to wider areas of the sky, forming a clearer picture of what is truly out there.

Source: EurekAlert

Paper: Schrabback et al., ‘Evidence for the accelerated expansion of the Universe from weak lensing tomography with COSMOS’, Astronomy and Astrophysics, March 2010,

Using Gravitational Lensing to Measure Age and Size of Universe


Handy little tool, this gravitational lensing! Astronomers have used it to measure the shape of stars, look for exoplanets, and measure dark matter in distant galaxies. Now its being used to measure the size and age of the Universe. Researchers say this new use of gravitation lensing provides a very precise way to measure how rapidly the universe is expanding. The measurement determines a value for the Hubble constant, which indicates the size of the universe, and confirms the age of Universe as 13.75 billion years old, within 170 million years. The results also confirm the strength of dark energy, responsible for accelerating the expansion of the universe.

Gravitational lensing occurs when two galaxies happen to aligned with one another along our line of sight in the sky. The gravitational field of the nearer galaxy distorts the image of the more distant galaxy into multiple arc-shaped images. Sometimes this effect even creates a complete ring, known as an “Einstein Ring.”
Researchers at the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) used gravitational lensing to measure the distances light traveled from a bright, active galaxy to the earth along different paths. By understanding the time it took to travel along each path and the effective speeds involved, researchers could infer not just how far away the galaxy lies but also the overall scale of the universe and some details of its expansion.

Distinguishing distances in space is difficult. A bright light far away and a dimmer source lying much closer can look like they are at the same distance. A gravitational lens circumvents this problem by providing multiple clues as to the distance light travels. That extra information allows them to determine the size of the universe, often expressed by astrophysicists in terms of a quantity called Hubble’s constant.

“We’ve known for a long time that lensing is capable of making a physical measurement of Hubble’s constant,” KIPAC’s Phil Marshall said. However, gravitational lensing had never before been used in such a precise way. This measurement provides an equally precise measurement of Hubble’s constant as long-established tools such as observation of supernovae and the cosmic microwave background. “Gravitational lensing has come of age as a competitive tool in the astrophysicist’s toolkit,” Marshall said.

When a large nearby object, such as a galaxy, blocks a distant object, such as another galaxy, the light can detour around the blockage. But instead of taking a single path, light can bend around the object in one of two, or four different routes, thus doubling or quadrupling the amount of information scientists receive. As the brightness of the background galaxy nucleus fluctuates, physicists can measure the ebb and flow of light from the four distinct paths, such as in the B1608+656 system that was the subject of this study. Lead author on the study Sherry Suyu, from the University of Bonn, said, “In our case, there were four copies of the source, which appear as a ring of light around the gravitational lens.”

Though researchers do not know when light left its source, they can still compare arrival times. Marshall likens it to four cars taking four different routes between places on opposite sides of a large city, such as Stanford University to Lick Observatory, through or around San Jose. And like automobiles facing traffic snarls, light can encounter delays, too.

“The traffic density in a big city is like the mass density in a lens galaxy,” Marshall said. “If you take a longer route, it need not lead to a longer delay time. Sometimes the shorter distance is actually slower.”

The gravitational lens equations account for all the variables such as distance and density, and provide a better idea of when light left the background galaxy and how far it traveled.

In the past, this method of distance estimation was plagued by errors, but physicists now believe it is comparable with other measurement methods. With this technique, the researchers have come up with a more accurate lensing-based value for Hubble’s constant, and a better estimation of the uncertainty in that constant. By both reducing and understanding the size of error in calculations, they can achieve better estimations on the structure of the lens and the size of the universe.

There are several factors scientists still need to account for in determining distances with lenses. For example, dust in the lens can skew the results. The Hubble Space Telescope has infra-red filters useful for eliminating dust effects. The images also contain information about the number of galaxies lying around the line of vision; these contribute to the lensing effect at a level that needs to be taken into account.

Marshall says several groups are working on extending this research, both by finding new systems and further examining known lenses. Researchers are already aware of more than twenty other astronomical systems suitable for analysis with gravitational lensing.

These results of this study was published in the March 1 issue of The Astrophysical Journal. The researchers used data collected by the NASA/ESA Hubble Space Telescope, and showed the improved precision they provide in combination with the Wilkinson Microwave Anisotropy Probe (WMAP).

Source: SLAC