Astronomers Figure Out How to use Gravitational Lensing to Measure the Mass of White Dwarfs

For the sake of studying the most distant objects in the Universe, astronomers often rely on a technique known as Gravitational Lensing. Based on the principles of Einstein’s Theory of General Relativity, this technique involves relying on a large distribution of matter (such as a galaxy cluster or star) to magnify the light coming from a distant object, thereby making it appear brighter and larger.

However, in recent years, astronomers have found other uses for this technique as well. For instance, a team of scientists from the Harvard-Smithsonian Center for Astrophysics (CfA) recently determined that Gravitational Lensing could also be used to determine the mass of white dwarf stars. This discovery could lead to a new era in astronomy where the mass of fainter objects can be determined.

The study which details their findings, titled “Predicting gravitational lensing by stellar remnants” appeared in the Monthly Noticed of the Royal Astronomical Society. The study was led by Alexander J. Harding of the CfA and included Rosanne Di Stefano, and Claire Baker (also from the CfA), as well as members from the University of Southampton, Georgia State University, the University of Nigeria, and Cornell University.

A Hubble image of the white dwarf star PM I12506+4110E (the bright object, seen in black in this negative print) and its field which includes two distant stars PM12-MLC1&2. Credit: Harding et al./NASA/HST

To put it simply, determining the mass of an astronomical object is one the greatest challenges for astronomers. Until now, the most successful method relied on binary systems because the orbital parameters of these systems depend on the masses of the two objects. Unfortunately, objects that are at the end states of stellar evolution – like black holes, neutron stars or white dwarfs – are often too faint or isolated to be detectable.

This is unfortunate, since these objects are responsible for a lot of dramatic astronomical events. These include the accretion of material, the emission of energetic radiation, gravitational waves, gamma-ray bursts, or supernovae. Many of these events are still a mystery to astronomers or the study of them is still in its infancy – i.e. gravitational waves. As they state in their study:

“Gravitational lensing provides an alternative approach to mass measurement. It has the advantage of only relying on the light from a background source, and can therefore be employed even for dark lenses. In fact, since light from the lens can interfere with the detection of lensing effects, compact objects are ideal lenses.”

As they go on to state, of the 18,000 lensing events that have been detected to date, roughly 10 to 15% are believed to have been caused by compact objects. However, scientists are unable to tell which of the detected events were due to compact lenses. For the sake of their study then, the team sought to circumvent this problem by identifying local compact objects and predicting when they might produce a lensing event so they could be studied.

Animation showing the white dwarf star Stein 2051B as it passes in front of a distant background star. Credit: NASA

“By focusing on pre-selected compact objects in the near vicinity of the Sun, we ensure that the lensing event will be caused by a white dwarf, neutron star, or black hole,” they state. “Furthermore, the distance and proper motion of the lens can be accurately measured prior to the event, or else afterwards. Armed with this information, the lensing light curve allows one to accurately measure the mass of the lens.”

In the end, the team determined that lensing events could be predicted from thousands of local objects. These include 250 neutron stars, 5 black holes, and roughly 35,000 white dwarfs. Neutron stars and black holes present a challenge since the known populations are too small and their proper motions and/or distances are not generally known.

But in the case of white dwarfs, the authors anticipate that they will provide for many lensing opportunities in the future. Based on the general motions of the white dwarfs across the sky, they obtained a statistical estimate that about 30-50 lensing events will take place per decade that could be spotted by the Hubble Space Telescope, the ESA’s Gaia mission, or NASA’s James Webb Space Telescope (JWST). As they state in their conclusions:

“We find that the detection of lensing events due to white dwarfs can certainly be observed during the next decade by both Gaia and HST. Photometric events will occur, but to detect them will require observations of the positions of hundreds to thousands of far-flung white dwarfs. As we learn the positions, distances to, and proper motions of larger numbers of white dwarfs through the completion of surveys such as Gaia and through ongoing and new wide-field surveys, the situation will continue to improve.”

The future of astronomy does indeed seem bright. Between improvements in technology, methodology, and the deployment of next-generation space and ground-based telescopes, there is no shortage of opportunities to see and learn more.

Further Reading: CfA, MNRAS

An Artificial Intelligence Just Found 56 New Gravitational Lenses

Gravitational lenses are an important tool for astronomers seeking to study the most distant objects in the Universe. This technique involves using a massive cluster of matter (usually a galaxy or cluster) between a distant light source and an observer to better see light coming from that source. In an effect that was predicted by Einstein’s Theory of General Relativity, this allows astronomers to see objects that might otherwise be obscured.

Recently, a group of European astronomers developed a method for finding gravitational lenses in enormous piles of data. Using the same artificial intelligence algorithms that Google, Facebook and Tesla have used for their purposes, they were able to find 56 new gravitational lensing candidates from a massive astronomical survey. This method could eliminate the need for astronomers to conduct visual inspections of astronomical images.

The study which describes their research, titled “Finding strong gravitational lenses in the Kilo Degree Survey with Convolutional Neural Networks“, recently appeared in the Monthly Notices of the Royal Astronomical Society. Led by Carlo Enrico Petrillo of the Kapteyn Astronomical Institute, the team also included members of the National Institute for Astrophysics (INAF), the Argelander-Institute for Astronomy (AIfA) and the University of Naples.

The notable gravitational lens known as the Cosmic Horseshoe is found in Leo. Credit: NASA/ESA/Hubble

While useful to astronomers, gravitational lenses are a pain to find. Ordinarily, this would consist of astronomers sorting through thousands of images snapped by telescopes and observatories. While academic institutions are able to rely on amateur astronomers and citizen astronomers like never before, there is imply no way to keep up with millions of images that are being regularly captured by instruments around the world.

To address this, Dr. Petrillo and his colleagues turned to what are known as “Convulutional Neural Networks” (CNN), a type of machine-learning algorithm that mines data for specific patterns. While Google used these same neural networks to win a match of Go against the world champion, Facebook uses them to recognize things in images posted on its site, and Tesla has been using them to develop self-driving cars.

As Petrillo explained in a recent press article from the Netherlands Research School for Astronomy:

“This is the first time a convolutional neural network has been used to find peculiar objects in an astronomical survey. I think it will become the norm since future astronomical surveys will produce an enormous quantity of data which will be necessary to inspect. We don’t have enough astronomers to cope with this.”

The team then applied these neural networks to data derived from the Kilo-Degree Survey (KiDS). This project relies on the VLT Survey Telescope (VST) at the ESO’s Paranal Observatory in Chile to map 1500 square degrees of the southern night sky. This data set consisted of 21,789 color images collected by the VST’s OmegaCAM, a multiband instrument developed by a consortium of European scientist in conjunction with the ESO.

A sample of the handmade photos of gravitational lenses that the astronomers used to train their neural network. Credit: Enrico Petrillo/Rijksuniversiteit Groningen

These images all contained examples of Luminous Red Galaxies (LRGs), three of which wee known to be gravitational lenses. Initially, the neural network found 761 gravitational lens candidates within this sample. After inspecting these candidates visually, the team was able to narrow the list down to 56 lenses. These still need to be confirmed by space telescopes in the future, but the results were quite positive.

As they indicate in their study, such a neural network, when applied to larger data sets, could reveal hundreds or even thousands of new lenses:

“A conservative estimate based on our results shows that with our proposed method it should be possible to find ?100 massive LRG-galaxy lenses at z ~> 0.4 in KiDS when completed. In the most optimistic scenario this number can grow considerably (to maximally ? 2400 lenses), when widening the colour-magnitude selection and training the CNN to recognize smaller image-separation lens systems.”

In addition, the neural network rediscovered two of the known lenses in the data set, but missed the third one. However, this was due to the fact that this lens was particularly small and the neural network was not trained to detect lenses of this size. In the future, the researchers hope to correct for this by training their neural network to notice smaller lenses and rejects false positives.

But of course, the ultimate goal here is to remove the need for visual inspection entirely. In so doing, astronomers would be freed up from having to do grunt work, and could dedicate more time towards the process of discovery. In much the same way, machine learning algorithms could be used to search through astronomical data for signals of gravitational waves and exoplanets.

Much like how other industries are seeking to make sense out of terabytes of consumer or other types of “big data”, the field astrophysics and cosmology could come to rely on artificial intelligence to find the patterns in a Universe of raw data. And the payoff is likely to be nothing less than an accelerated process of discovery.

Further Reading: Netherlands Research School for Astronomy , MNRAS

 

New ‘Einstein Ring’ Discovered By Dark Energy Camera

The "Canarias Einstein Ring." The green-blue ring is the source galaxy, the red one in the middle is the lens galaxy. The lens galaxy has such strong gravity, that it distorts the light from the source galaxy into a ring. Because the two galaxies are aligned, the source galaxy appears almost circular. Image: This composite image is made up from several images taken with the DECam camera on the Blanco 4m telescope at the Cerro Tololo Observatory in Chile.

A rare object called an Einstein Ring has been discovered by a team in the Stellar Populations group at the Instituto de Astrofísica de Canarias (IAC) in Spain. An Einstein Ring is a specific type of gravitational lensing.

Einstein’s Theory of General Relativity predicted the phenomena of gravitational lensing. Gravitational lensing tells us that instead of travelling in a straight line, light from a source can be bent by a massive object, like a black hole or a galaxy, which itself bends space time.

Einstein’s General Relativity was published in 1915, but a few years before that, in 1912, Einstein predicted the bending of light. Russian physicist Orest Chwolson was the first to mention the ring effect in scientific literature in 1924, which is why the rings are also called Einstein-Chwolson rings.

Gravitational lensing is fairly well-known, and many gravitational lenses have been observed. Einstein rings are rarer, because the observer, source, and lens all have to be aligned. Einstein himself thought that one would never be observed at all. “Of course, there is no hope of observing this phenomenon directly,” Einstein wrote in 1936.

The team behind the recent discovery was led by PhD student Margherita Bettinelli at the University of La Laguna, and Antonio Aparicio and Sebastian Hidalgo of the Stellar Populations group at the Instituto de Astrofísica de Canarias (IAC) in Spain. Because of the rarity of these objects, and the strong scientific interest in them, this one was given a name: The Canarias Einstein Ring.

There are three components to an Einstein Ring. The first is the observer, which in this case means telescopes here on Earth. The second is the lens galaxy, a massive galaxy with enormous gravity. This gravity warps space-time so that not only are objects drawn to it, but light itself is forced to travel along a curved path. The lens lies between Earth and the third component, the source galaxy. The light from the source galaxy is bent into a ring form by the power of the lens galaxy.

When all three components are aligned precisely, which is very rare, the light from the source galaxy is formed into a circle with the lens galaxy right in the centre. The circle won’t be perfect; it will have irregularities that reflect irregularities in the gravitational force of the lens galaxy.

Another Einstein Ring. This one is named LRG 3-757. This one was discovered by the Sloan Digital Sky Survey, but this image was captured by Hubble's Wide Field Camera 3. Image: NASA/Hubble/ESA
Another Einstein Ring. This one is named LRG 3-757. This one was discovered by the Sloan Digital Sky Survey, but this image was captured by Hubble’s Wide Field Camera 3. Image: NASA/Hubble/ESA

The objects are more than just pretty artifacts of nature. They can tell scientists things about the nature of the lens galaxy. Antonio Aparicio, one of the IAC astrophysicists involved in the research said, “Studying these phenomena gives us especially relevant information about the composition of the source galaxy, and also about the structure of the gravitational field and of the dark matter in the lens galaxy.”

Looking at these objects is like looking back in time, too. The source galaxy is 10 billion light years from Earth. Expansion of the Universe means that the light has taken 8.5 billion light years to reach us. That’s why the ring is blue; that long ago, the source galaxy was young, full of hot blue stars.

The lens itself is much closer to us, but still very distant. It’s 6 billion light years away. Star formation in that galaxy likely came to a halt, and its stellar population is now old.

The discovery of the Canarias Einstein Ring was a happy accident. Bettinelli was pouring over data from what’s known as the Dark Energy Camera (DECam) of the 4m Blanco Telescope at the Cerro Tololo Observatory, in Chile. She was studying the stellar population of the Sculptor dwarf galaxy for her PhD when the Einstein Ring caught her attention. Other members of the Stellar Population Group then used OSIRIS spectrograph on the Gran Telescopio CANARIAS (GTC) to observe and analyze it further.

Dwarf Dark Matter Galaxy Hides In Einstein Ring

The large blue light is a lensing galaxy in the foreground, called SDP81, and the red arcs are the distorted image of a more distant galaxy. By analyzing small distortions in the red, distant galaxy, astronomers have determined that a dwarf dark galaxy, represented by the white dot in the lower left, is companion to SDP81. The image is a composite from ALMA and the Hubble. Image: Y. Hezaveh, Stanford Univ./ALMA (NRAO/ESO/NAOJ)/NASA/ESA Hubble Space Telescope

Everybody knows that galaxies are enormous collections of stars. A single galaxy can contain hundreds of billions of them. But there is a type of galaxy that has no stars. That’s right: zero stars.

These galaxies are called Dark Galaxies, or Dark Matter Galaxies. And rather than consisting of stars, they consist mostly of Dark Matter. Theory predicts that there should be many of these Dwarf Dark Galaxies in the halo around ‘regular’ galaxies, but finding them has been difficult.

Now, in a new paper to be published in the Astrophysical Journal, Yashar Hezaveh at Stanford University in California, and his team of colleagues, announce the discovery of one such object. The team used enhanced capabilities of the Atacamas Large Millimeter Array to examine an Einstein ring, so named because Einstein’s Theory of General Relativity predicted the phenomenon long before one was observed.

An Einstein Ring is when the massive gravity of a close object distorts the light from a much more distant object. They operate much like the lens in a telescope, or even a pair of eye-glasses. The mass of the glass in the lens directs incoming light in such a way that distant objects are enlarged.

Einstein Rings and gravitational lensing allow astronomers to study extremely distant objects, by looking at them through a lens of gravity. But they also allow astronomers to learn more about the galaxy that is acting as the lens, which is what happened in this case.

If a glass lens had tiny water spots on it, those spots would add a tiny amount of distortion to the image. That’s what happened in this case, except rather than microscopic water drops on a lens, the distortions were caused by tiny Dwarf Galaxies consisting of Dark Matter. “We can find these invisible objects in the same way that you can see rain droplets on a window. You know they are there because they distort the image of the background objects,” explained Hezaveh. The difference is that water distorts light by refraction, whereas matter distorts light by gravity.

As the ALMA facility increased its resolution, astronomers studied different astronomical objects to test its capabilities. One of these objects was SDP81, the gravitational lens in the above image. As they examined the more distant galaxy being lensed by SDP81, they discovered smaller distortions in the ring of the distant galaxy. Hezaveh and his team conclude that these distortions signal the presence of a Dwarf Dark Galaxy.

But why does this all matter? Because there is a problem in the Universe, or at least in our understanding of it; a problem of missing mass.

Our understanding of the formation of the structure of the Universe is pretty solid, at least in the larger scale. Predictions based on this model agree with observations of the Cosmic Microwave Background (CMB) and galaxy clustering. But our understanding breaks down somewhat when it comes to the smaller scale structure of the Universe.

One example of our lack of understanding in this area is what’s known as the Missing Satellite Problem. Theory predicts that there should be a large population of what are called sub-halo objects in the halo of dark matter surrounding galaxies. These objects can range from things as large as the Magellanic Clouds down to much smaller objects. In observations of the Local Group, there is a pronounced deficit of these objects, to the tune of a factor of 10, when compared to theoretical predictions.

Because we haven’t found them, one of two things needs to happen: either we get better at finding them, or we modify our theory. But it seems a little too soon to modify our theories of the structure of the Universe because we haven’t found something that, by its very nature, is hard to find. That’s why this announcement is so important.

The observation and identification of one of these Dwarf Dark Galaxies should open the door to more. Once more are found, we can start to build a model of their population and distribution. So if in the future more of these Dwarf Dark Galaxies are found, it will gradually confirm our over-arching understanding of the formation and structure of the Universe. And it’ll mean we’re on the right track when it comes to understanding Dark Matter’s role in the Universe. If we can’t find them, and the one bound to the halo of SDP81 turns out to be an anomaly, then it’s back to the drawing board, theoretically.

It took a lot of horsepower to detect the Dwarf Dark Galaxy bound to SDP81. Einstein Rings like SDP81 have to have enormous mass in order to exert a gravitational lensing effect, while Dwarf Dark Galaxies are tiny in comparison. It’s a classic ‘needle in a haystack’ problem, and Hezaveh and his team needed massive computing power to analyze the data from ALMA.

ALMA will consist of 66 individual antennae like these when it is complete. The facility is located in the Atacama Desert in Chile, at 5,000 meters above sea level. Credit: ALMA (ESO / NAOJ / NRAO)
ALMA will consist of 66 individual antennae like these when it is complete. The facility is located in the Atacama Desert in Chile, at 5,000 meters above sea level. Credit: ALMA (ESO / NAOJ / NRAO)

ALMA, and the methodology developed by Hezaveh and team will hopefully shed more light on Dwarf Dark Galaxies in the future. The team thinks that ALMA has great potential to discover more of these halo objects, which should in turn improve our understanding of the structure of the Universe. As they say in the conclusion of their paper, “… ALMA observations have the potential to significantly advance our understanding of the abundance of dark matter substructure.”

SpaceX Dragon Launches on Science Supply Run to Station, Booster Hard Lands on Barge

KENNEDY SPACE CENTER, FL – After a 24 hour delay due to threatening clouds, a SpaceX Falcon 9 rocket soared spectacularly to orbit from the Florida Space coast today, April 14, carrying a Dragon on a science supply run bound for the the International Space Station that will help pave the way for deep space human missions to the Moon, Asteroids and Mars.

Meanwhile, SpaceX’s bold attempt to land and recover the 14 story tall first stage of the Falcon 9 rocket successfully reached a tiny ocean floating barge in the Atlantic Ocean, but tilted over somewhat over in the final moments of the approach, and tipped over after landing and broke apart. Here’s a Vine video posted on Twitter by Elon Musk:

See the video of the launch, below.

SpaceX will continue with attempt to soft land and recover the rocket on upcoming launches, which was a secondary goal of the company. SpaceX released some imagery and video with a few hours of the landing attempt.

“Looks like Falcon landed fine, but excess lateral velocity caused it to tip over post landing,” tweeted SpaceX CEO Elon Musk.

Falcon 9 first stage approaches Just Read the Instructions. Image of SpaceX Falcon 9 first start booster in final moments of hard landing on ocean going barge after CRS-6 launch. Credit: SpaceX
Falcon 9 first stage approaches Just Read the Instructions. Image of SpaceX Falcon 9 first start booster in final moments of hard landing on ocean going barge after CRS-6 launch. Credit: SpaceX

The Falcon 9 first stage was outfitted with four landing legs and grid fins to enable the landing attempt, which is a secondary objective of SpaceX.

The top priority was to safely launch the Falcon 9 and deliver critical supplies to the station with the Dragon cargo vessel.

“Five years ago this week, President Obama toured the same SpaceX launch pad used today to send supplies, research and technology development to the ISS,” said NASA Administrator Charles Bolden.

“Back then, SpaceX hadn’t even made its first orbital flight. Today, it’s making regular flights to the space station and is one of two American companies, along with The Boeing Company, that will return the ability to launch NASA astronauts to the ISS from U.S. soil and land then back in the United States. That’s a lot of progress in the last five years, with even more to come in the next five.”

“Looks like Falcon landed fine, but excess lateral velocity caused it to tip over post landing,” tweeted SpaceX CEO Elon Musk.

A chase plane captured dramatic footage of the landing on the ocean going platform known as the ‘autonomous spaceport drone ship’ (ASDS).

It was pre-positioned some 200 to 250 miles offshore of the Carolina coast in the Atlantic Ocean along the rockets flight path flying along the US Northeast coast to match that of the ISS.

The ASDS measures only 300 by 100 feet, with wings that extend its width to 170 feet.

SpaceX Falcon 9 and Dragon blastoff from Space Launch Complex 40 at Cape Canaveral Air Force Station in Florida on April 14, 2015 at 4:10 p.m. EDT  on the CRS-6 mission to the International Space Station. Credit: Ken Kremer/kenkremer.com
SpaceX Falcon 9 and Dragon blastoff from Space Launch Complex 40 at Cape Canaveral Air Force Station in Florida on April 14, 2015 at 4:10 p.m. EDT on the CRS-6 mission. to the International Space Station. Credit: Ken Kremer/kenkremer.com

Overall CRS-6 is the sixth SpaceX commercial resupply services mission and the seventh trip by a Dragon spacecraft to the station since 2012.

CRS-6 marks the company’s sixth operational resupply mission to the ISS under a $1.6 Billion contract with NASA to deliver 20,000 kg (44,000 pounds) of cargo to the station during a dozen Dragon cargo spacecraft flights through 2016 under NASA’s original Commercial Resupply Services (CRS) contract.

The SpaceX Falcon 9 with the Dragon vessel for the CRS-6 launch lifts off for the International Space Station at 4:10 PM eastern time on 4/14/15 from Cape Canaveral.  Credit: Alex Polimeni/AmericaSpace
The SpaceX Falcon 9 with the Dragon vessel for the CRS-6 launch lifts off for the International Space Station at 4:10 PM eastern time on 4/14/15 from Cape Canaveral. Credit: Alex Polimeni/AmericaSpace

Dragon is packed with more than 4,300 pounds (1915 kilograms) of scientific experiments, technology demonstrations, crew supplies, spare parts, food, water, clothing and assorted research gear for the six person Expedition 43 and 44 crews serving aboard the ISS.

After a three day orbital chase, the Dragon spacecraft with rendezvous with the million post Earth orbiting outpost Friday morning April 17.

After SpaceX engineers on the ground maneuver the Dragon close enough to the station, European Space Agency (ESA) astronaut Samantha Cristoforetti will use the station’s 57.7-foot-long (17-meter-long) robotic arm to reach out and capture Dragon at approximately 7 a.m. EDT on April 17.

Cristoforetti will be assisted by fellow Expedition 43 crew member and NASA astronaut
Terry Virts, as they work inside the stations seven windowed domed cupola to berth Dragon at the Earth-facing port of the Harmony module.

The series of images shows the journey the SpaceX Falcon 9 rocket and Dragon spacecraft from its launch at 4:10 p.m. EDT on Tuesday April 14, 2015 from Space Launch Complex 40 at Cape Canaveral Air Force Station in Florida, to solar array deployment. Credit: NASA TV
The series of images shows the journey the SpaceX Falcon 9 rocket and Dragon spacecraft from its launch at 4:10 p.m. EDT on Tuesday April 14, 2015 from Space Launch Complex 40 at Cape Canaveral Air Force Station in Florida, to solar array deployment. Credit: NASA TV

Watch for Ken’s continuing onsite coverage of the CRS-6 launch from the Kennedy Space Center and Cape Canaveral Air Force Station.

Stay tuned here for Ken’s continuing Earth and planetary science and human spaceflight news.

Ken Kremer

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Learn more about SpaceX, Mars rovers, Orion, Antares, MMS, NASA missions and more at Ken’s upcoming outreach events:

Apr 18/19: “Curiosity explores Mars” and “NASA Human Spaceflight programs” – NEAF (NorthEast Astronomy Forum), 9 AM to 5 PM, Suffern, NY, Rockland Community College and Rockland Astronomy Club

Peer Into the Distant Universe: How to See Quasars With Backyard Telescopes

“How far can you see with that thing?”

It’s a common question overhead at many public star parties in reference to telescopes.

In the coming weeks as the Moon passes Full and moves out of the evening sky, we’d like to challenge you to hunt down a bright example of one of the most distant and exotic objects known: a quasar.

To carry out this feat, you’ll need a ‘scope with at least an aperture of 20 centimetres or greater, dark skies, and patience.

Although more than 200,000 of quasars are currently known and they’re some of the most luminous objects in the universe, they’re also tremendously distant. A very few are brighter than magnitude +14, about the brightness of Pluto. Most quasars have an absolute magnitude rivaling our Sun, though if you plopped one down 33 light years away, we’d definitely have other things to worry about.
Continue reading “Peer Into the Distant Universe: How to See Quasars With Backyard Telescopes”

What is Gravitational Lensing?

Gravity’s a funny thing. Not only does it tug away at you, me, planets, moons and stars, but it can even bend light itself. And once you’re bending light, well, you’ve got yourself a telescope.

Everyone here is familiar with the practical applications of gravity. If not just from exposure to Loony Tunes, with an abundance of scenes with an anthropomorphized coyote being hurled at the ground from gravitational acceleration, giant rocks plummeting to a spot inevitably marked with an X, previously occupied by a member of the “accelerati incredibilus” family and soon to be a big squish mark containing the bodily remains of the previously mentioned Wile E. Coyote.

Despite having a very limited understanding of it, Gravity is a pretty amazing force, not just for decimating a infinitely resurrecting coyote, but for keeping our feet on the ground and our planet in just the right spot around our Sun. The force due to gravity has got a whole bag of tricks, and reaches across Universal distances. But one of its best tricks is how it acts like a lens, magnifying distant objects for astronomy.
Thanks to the general theory of relativity, we know that mass curves the space around it. The theory also predicted gravitational lensing, a side effect of light travelling along the curvature of space and time where light passing nearby a massive object is deflected slightly toward the mass.

It was first observed by Arthur Eddington and Frank Watson Dyson in 1919 during a solar eclipse. The stars close to the Sun appeared slightly out of position, showing that the light from the stars was bent, and demonstrated the effect predicted. This means the light from a distant object, such as a quasar, could be deflected around a closer object such as a galaxy. This can focus the quasar’s light in our direction, making it appear brighter and larger. So gravitational lensing acts as a kind of magnifying glass for distant objects making them easier to observe.

We can use the effect to peer deeper into the Universe than would otherwise be possible with our conventional telescopes. In fact, the most distant galaxies ever observed, ones seen just a few hundred million years after the Big Bang, were all discovered using gravitational lensing. Astronomers use gravitational microlensing to detect planets around other stars. The foreground star acts as a lens for a background star. As the star brightens up, you can detect further distortions which indicate there are planets. Even amateur telescopes are sensitive enough to spot them, and amateurs regularly help discover new planets. Unfortunately, these are one time events as this alignment happens only once.

This illustration shows how gravitational lensing works. The gravity of a large galaxy cluster is so strong, it bends, brightens and distorts the light of distant galaxies behind it. The scale has been greatly exaggerated; in reality, the distant galaxy is much further away and much smaller. Credit: NASA, ESA, L. Calcada
This illustration shows how gravitational lensing works. The gravity of a large galaxy cluster is so strong, it bends, brightens and distorts the light of distant galaxies behind it. The scale has been greatly exaggerated; in reality, the distant galaxy is much further away and much smaller. Credit: NASA, ESA, L. Calcada

There’s a special situation known as an Einstein Ring, where a more distant galaxy is warped by a nearby galaxy into a complete circle. To date a few partial rings have been seen, but no perfect Einstein Ring has ever been spotted.

Gravitational lensing also allows us to observe invisible things in our Universe. Dark matter doesn’t emit or absorb light on its own, so we can’t observe it directly. We can’t take a photo and say “Hey look, dark matter!”. However, it does have mass, and that means it can gravitationally lens light originating behind it. So we’ve even used the effect of gravitational lensing to map out dark matter in the Universe.

What about you? Where should we focus our gravitational lensing efforts to get a better look in the Universe? Tell us in the comments below.

Astronomers Discover First Mulitiple-image Gravitationally-lensed Supernova

How about four supernovae for the price of one? Using the Hubble Space Telescope, Dr. Patrick Kelly of the University of California-Berkeley along with the GLASS (Grism Lens Amplified Survey from Space) and Hubble Frontier Fields teams, discovered a remote supernova lensed into four copies of itself by the powerful gravity of a foreground galaxy cluster. Dubbed SN Refsdal, the object was discovered in the rich galaxy cluster MACS J1149.6+2223 five billion light years from Earth in the constellation Leo. It’s the first multiply-lensed supernova every discovered and one of nature’s most exotic mirages.

The rich galaxy cluster MACS J1149+2223 gained notoriety in 2012 when the most distant galaxy when the most distant galaxy found to date was discovered there through gravitational lensing.
The lensed supernova was discovered far behind the rich galaxy cluster MACS J1149.6+2223. The cluster is one of the most massive known and gained notoriety in 2012 when astronomers harnessed its powerful lensing ability to uncover the most distant galaxy known at the time. Credit: NASA/ESA/M. Postman STScI/CLASH team

Gravitational lensing grew out of Einstein’s Theory of Relativity wherein he predicted massive objects would bend and warp the fabric of spacetime. The more massive the object, the more severe the bending. We can picture this by imagining a child standing on a trampoline, her weight pressing a dimple into the fabric. Replace the child with a 200-pound adult and the surface of the trampoline sags even more.

Massive objects like the sun and even the planets warp the fabric of space. Here a planet orbits the sun but does not fall in because of its sideways orbital motion.
Massive objects like the Sun and even the planets warp the fabric of space. Here a planet orbits the Sun but doesn’t fall in because of its sideways orbital motion.

Similarly, the massive Sun creates a deep, but invisible dimple in the fabric of spacetime. The planets feel this ‘curvature of space’ and literally roll toward the Sun. Only their sideways motion or angular momentum keeps them from falling straight into the solar inferno.

Curved space created by massive objects also bends light rays. Einstein predicted that light from a star passing near the Sun or other massive object would follow this invisible curved spacescape and be deflected from an otherwise straight path. In effect, the object acts as a lens, bending and refocusing the light from the distant source into either a brighter image or multiple and distorted images. Also known as the deflection of starlight, nowadays we call it gravitational lensing.

This illustration shows how gravitational lensing works. The gravity of a large galaxy cluster is so strong, it bends, brightens and distorts the light of distant galaxies behind it. The scale has been greatly exaggerated; in reality, the distant galaxy is much further away and much smaller. Credit: NASA, ESA, L. Calcada
This illustration shows how gravitational lensing works. The gravity of a large galaxy cluster is so strong, it bends, brightens and distorts the light of distant galaxies behind it. The scale has been greatly exaggerated; in reality, the distant galaxy is much further away and much smaller. Credit: NASA, ESA, L. Calcada


Simulation of distorted spacetime around a massive galaxy cluster over time

Turns out there are lots of these gravitational lenses out there in the form of massive clusters of galaxies. They contain regular matter as well as vast quantities of the still-mysterious dark matter that makes up 96% of the material stuff in the universe. Rich galaxy clusters act like telescopes – their enormous mass and powerful gravity magnify and intensify the light of galaxies billions of light years beyond, making visible what would otherwise never be seen.

Here we see a central slice of the MACS cluster. A massive elliptical galaxy is responsible for splitting SN Refsdal into four images. It also distorts and lenses the purple-toned spiral galaxy that's host to the supernova. Credit:
This cropped image shows the central slice of the MACS J1149 galaxy cluster. A massive elliptical galaxy lenses the light of SN Refsdal into four separate images. It also distorts the purplish spiral galaxy that’s host to the supernova. Credit: NASA/ESA/M. Postman STScI/CLASH team

Let’s return to SN Refsdal, named for Sjur Refsdal, a Norwegian astrophysicist who did early work in the field of gravitational lensing.  A massive elliptical galaxy in the MACS J1149 cluster “lenses” the  9.4 billion light year distant supernova and its host spiral galaxy from background obscurity into the limelight. The elliptical’s powerful gravity’s having done a fine job of distorting spacetime to bring the supernova into view also distorts the shape of the host galaxy and splits the supernova into four separate, similarly bright images. To create such neat symmetry, SN Refsdal must be precisely aligned behind the galaxy’s center.

What looks like a galaxy with five nuclei really has just one (at center) surrounded by a mirage of four images of a distant quasar. The galaxy lies 400 million light years away; the quasar about 8 billion. Credit: NASA/ESA/Hubble
What looks like a galaxy with five nuclei really has just one (at center) surrounded by a mirage of four images of a distant quasar. The galaxy lies 400 million light years away; the quasar about 8 billion. Credit: NASA/ESA/Hubble

The scenario here bears a striking resemblance to Einstein’s Cross, a gravitationally lensed quasar, where the light of a remote quasar has been broken into four images arranged about the foreground lensing galaxy. The quasar images flicker or change in brightness over time as they’re microlensed by the passage of individual stars within the galaxy. Each star acts as a smaller lens within the main lens.

Color-composite image of lensing elliptical galaxy and distorted background  host spiral (top).The green circles show the locations of images S1–S4, while another quadruply imaged segment of the spiral arm is marked in  red. The bottom panels show two additional lensed images of the spiral host galaxy visible in the galaxy cluster field. Credit: S.L. Kelly et. all
Color-composite image of the lensing elliptical galaxy and distorted background host spiral (top). The green circles, S1-4, show the locations of the supernova images, while another quadruply imaged segment of the spiral arm is marked in red. The bottom panels show two additional lensed images of the spiral host galaxy visible in the galaxy cluster field.  Talk about a funhouse mirror! Credit: P.L. Kelly/GLASS/Hubble Frontier Fields

Detailed color images taken by the GLASS and Hubble Frontier Fields groups show the supernova’s host galaxy is also multiply-imaged by the galaxy cluster’s gravity. According to their recent paper, Kelly and team are still working to obtain spectra of  the supernova to determine if it resulted from the uncontrolled burning and explosion of a white dwarf star (Type Ia) or the cataclysmic collapse and rebound of a supergiant star that ran out of fuel (Type II).

The time light takes to travel to the Earth from each of the lensed images is different because each follows a slightly different path around the center of the lensing galaxy. Some paths are shorter, some longer. By timing the brightness variations between the individual images the team hopes to provide constraints not only on the distribution of bright matter vs. dark matter in the lensing galaxy and in the cluster but use that information to determine the expansion rate of the universe.

You can squeeze a lot from a cosmic mirage!

Searching for Alien Worlds and Gravitational Lenses from the Arctic

The quest for optimal sites to carry out astronomical observations has taken scientists to the frigid Arctic.  Eric Steinbring, who led a team of National Research Council Canada experts, noted that a high Arctic site can, “offer excellent image quality that is maintained during many clear, calm, dark periods that can last 100 hours or more.”  The new article by Steinbring and colleagues conveys recent progress made to obtain precise observations from a 600 m high ridge near the Eureka research base on Ellesmere Island, which is located in northern Canada.

The new telescope that Steinbring and his colleagues tested was located at the Polar Environment Atmospheric Research Laboratory (PEARL).  The observatory can be accessed in winter by 4 x 4 trucks via a 15 km long road from a base facility at sea-level. That base camp is operated by Environment Canada and serviced by an airstrip and resupply ship in summer.  Recently, wide-field cameras developed at the University of Toronto were deployed near Eureka to monitor thousands of stars, with the objective of expanding the exoplanet database.

Earlier work by Steinbring and colleagues indicated that data obtained from PEARL imply that clear weather prevails 68% of the time. After significant testing, the team concluded that the site “can allow reliable, uninterrupted temporal coverage during successive dark periods, in roughly 100 hour blocks with clear skies and good seeing.”

The Polar Environment Atmospheric Research Laboratory (PEARL) is located on Ellesmere Island (image credit: Left,  , right, Tobias Kerzenmacher).
The Polar Environment Atmospheric Research Laboratory (PEARL) is located on Ellesmere Island (image credit: left, wikimedia commons, right, Tobias Kerzenmacher).

However, the optimal conditions can be interrupted by brief but potentially intense storms. In the article the team added that, “the primary issue is wind rather than the cold temperatures.” The PEARL facility is equipped with an important weather probe that conveys on-site conditions at 10 minute intervals, thanks to the Canadian Network for the Detection of Atmospheric Change (CANDAC).

There are numerous challenges that arise when observing from the Arctic, but scientists like Steinbring have worked to overcome them, potentially enabling new studies of gravitational lenses and other pertinent phenomena. Indeed, astronomical observations are likewise being obtained from Antarctica. For example, there is the Antarctic Search for Transiting Exoplanets (ASTEP) 40 cm telescope at Dome C, and three 50 cm Antarctic Survey Telescopes (AST3) at Dome A, Antarctica. Steinbring remarked that floorspace is potentially available for up to 5 more telescopes at PEARL, if the compact design they studied was adopted.

E. Steinbring and his colleagues B. Leckie and R. Murowinski are associated with the National Research Council Canada, Herzberg Astronomy and Astrophysics in Victoria, Canada. An electronic preprint of their article is available on arXiv, and the findings were presented recently at the Adapting to the Atmosphere Conference in Durham, UK.

 

New Simulation Offers Stunning Images of Black Hole Merger

A black hole is an extraordinarily massive, improbably dense knot of spacetime that makes a living swallowing or slinging away any morsel of energy that strays too close to its dark, twisted core. Anyone fortunate (or unfortunate) enough to directly observe one of these beasts in the wild would immediately notice the way its colossal gravitational field warps all of the light from the stars and galaxies behind it, a phenomenon known as gravitational lensing.

Thanks to the power of supercomputers, a curious observer no longer has to venture into outer space to see such a sight. A team of astronomers has released their first simulated images of the lensing effects of not just one, but two black holes, trapped in orbit by each other’s gravity and ultimately doomed to merge as one.

Astronomers have been able to model the gravitational effects of a single black hole since the 1970s, but the imposing mathematics of general relativity made doing so for a double black-hole system a much larger challenge. Over the last ten years, however, scientists have improved the accuracy of computer models that deal with these types of calculations in an effort to match observations from gravitational wave detectors like LIGO and VIRGO.

The research collaboration Simulating Extreme Spacetimes (SXS) has begun using these models to mimic the lensing effects of high-gravity systems involving objects such as neutron stars and black holes. In their most recent paper, the team imagines a camera pointing at a binary black hole system against a backdrop of the stars and dust of the Milky Way. One way to figure out what the camera would see in this situation would be to use general relativity to compute the path of each photon traveling from every light source at all points within the frame. This method, however, involves a nearly impossible number of calculations.  So instead, the researchers worked backwards, mapping only those photons that would reach the camera and result in a bright spot on the final image – that is, photons that would not be swallowed by either of the black holes.

A binary black hole system, viewed from above. Image Credit: Bohn et al. (see http://arxiv.org/abs/1410.7775)
The same binary black hole system, viewed from above. Image Credit: Bohn et al. (see http://arxiv.org/abs/1410.7775)

As you can see in the image above, the team’s simulations testify to the enormous effect that these black holes have on the fabric of spacetime. Ambient photons curl into a ring around the converging binaries in a process known as frame dragging. Background objects appear to multiply on opposite sides of the merger (for instance, the yellow and blue pair of stars in the “northeast” and the “southwest” areas of the ring). Light from behind  the camera is even pulled into the frame by the black holes’ mammoth combined gravitational field. And each black hole distorts the appearance of the other, pinching off curved, comma-shaped regions of shadow called “eyebrows.” If you could zoom in with unlimited precision, you would find that there are, in fact, an infinite number of these eyebrows, each smaller than the last, like a cosmic set of Russian dolls.

In case you thought things couldn’t get any more amazing, SXS has also created two videos of the black hole merger: one simulated from above, and the other edge-on.
 



 



The SXS collaboration will continue to investigate gravitationally ponderous objects like black holes and neutron stars in an effort to better understand their astronomical and physical properties. Their work will also assist observational scientists as they search the skies for evidence of gravitational waves.

Check out the team’s ArXiv paper describing this work and their website for even more fascinating images.