# What is gravitational lensing — and how it helped prove Einstein right

In 1915, a physicist by the name of Albert Einstein published a theory that managed to connect the curvature of space-time with energy. It is called General Relativity, and Einstein focused on it as a way to bring gravity into his previous special relativity work.

The theory of general relativity says that the gravitational attraction between objects’ masses comes from their warping of spacetime. Armed with this insight, general relativity was able to predict many things, including the most famous confirmation that gravitational waves exist. But for around a long time, there was no direct proof. The first direct proof came from gravitational lensing.

As we were discussing, energy and time-space curvature are related. The most massive objects are capable of curving space-time itself. Think of a ball on a stretched sheet: the ball curves the sheet, and if you let a smaller object slide on that sheet, it will move towards the ball.

The surprise is that something absurdly massive can even bend light. Yes, light, the fastest thing to carry information in the universe, can be bent — sort of. That is the ‘innovative’ thing about general relativity, the theory allows even objects which are massless to be affected by gravity. Photons are the particles that constitute light and they have zero mass, thus light can be deformed in the presence of a strong gravitational field.

### How it works — and what do lenses have to do with it

The fact that lenses can distort images does not need relativity at all. A glass filled with water can distort light behind or inside the glass. In photographic lenses, if not corrected, images are curved and don’t look realistic.

Scientists have been aware of lenses and their effects for a long time, but with the advent of telescopes, they also realized that objects with very large masses (some stars, galaxies, black holes) distort light in a similar way to lenses here on Earth. So these celestial objects can be used as a sort of lens — a gravitational lens.

When the lens and the target are close enough (from an astronomical perspective) and closely aligned, multiple images can be formed, appearing in an arc shape — this is called strong lensing. The image multiplication of a light source can be out of sync due to the curvature of space. Some images will take longer to reach the observer because the light is taking a longer path.

When lens and source are in nearly perfect alignment the image deforms to a ring shape, called Einstein–Chwolson ring. The most famous multiple image phenomena are the so-called “Einstein crosses” — where the image of a single source is deformed into a cross shape, four more versions of the target appear due to the gravitational phenomenon.

Meanwhile, weak lensing happens when the image is distorted, but without any copies of the target — just a distortion of it with elongated shapes. Microlensing, on the other hand, has to do with motion, either the source, the lens or us. The motion changes the source’s magnification making objects which are usually hard to observe brighter.

## Proving Einstein right

Gravitational lensing was one of the key techniques used to prove General Relativity. In 1919, a solar eclipse was observable in some countries in the Southern Hemisphere and the Hyades star cluster happened to be in the same view range as the Sun. Sir Frank Watson Dyson sent two expeditions in different locations of the globe to observe the eclipse — coincidentally, two Portuguese-speaking places — one in the Democratic Republic of São Tomé and Príncipe (at the time called ‘the island of Príncipe’, with Arthur Eddington and Edwin Cottingham, and another in the city of Sobral in Brazil with Charles Davidson and Andrew Crommelin.

The team at Sobral found better weather conditions and registered 7 images in contrast to the Príncipe team’s only 2 images. Later, the analysis of the photographic plates was carried out by estimating the deflection angle from the two experiments. With both results, considering the error bar, the observation confirmed the theory. Despite the evidence, the confirmation did not give Einstein immediate prestige. Other eclipses had to help and the scientific community took time to digest the theory.

## What can we find with gravitational lensing?

Lensing effects don’t occur when there is a star or a galaxy in our view range. Dark matter is massive, therefore has a gravitational field. Scientists use gravitational lensing to estimate the amount of dark matter from giant galaxy clusters.

Microlensing can help astronomers/astrophysicists find exoplanets. When a lens is passing in front of a star, its brightness will have a maximum in perfect alignment and as the motion continues it returns to the original magnification. Every time the star is being eclipsed by a planet, anomalies in the brightness evolution will appear and the researchers can confirm the presence of a host planet.

Remember the Cosmic Microwave Background (CMB)? It is the oldest ‘image’ of the universe, when photons could travel freely without interaction with matter. Photons are light, everything on light’s path can bend it. Scientists can know how distorted the CMB is by analyzing dark matter.

The Planck Satellite was the first instrument to give results on the distribution of dark matter in the universe through gravitational lensing. In the image illustrating this distribution, the gray color is to represent the Milky Way and very bright nearby galaxies; they need to be excluded because they mess with the measurements. Dark blue are regions with more dark matter than the bright portions.

If you expect a certain amount of effort, if not struggle just to detect a few galaxies, scientists are already thinking of lensed gravitational waves. How hard could that be, right? They predict a boost in gravitational waves’ signal if they are amplified by strong lensing. The problem is that it also helps increase the noise/errors in the observations. Until then, a lot of work is being done with gravitational lensing, something that came from a very abstract theory, proving theoretical work deserves respect.

# Distant ‘Milky Way Look-Alike’ Challenges Theories of Galaxy Formation

#### Using the phenomenon of gravitational lensing astronomers have examined an extremely distant galaxy that shares many features with the Milky Way. The discovery of a calm galaxy so early in the Universe’s history calls into question our theories of how galaxies form.

Astronomers have discovered that a distant young galaxy that existed in the very early universe shares some surprising similarities with our galaxy. The fact that the young galaxy named SPT-S J041839–4751.9 or SPT0418–47 for short — located 12 billion light-years from Earth — resembles the Milky Way and the galaxies that surround it presents something of a problem. Galaxies that existed 1.4 billion years after the Big Bang, weren’t supposed to be so calm, unchaotic, and well-formed.

Thus the discovery of this throws many of our theories of galactic evolution into question. In fact, this finding fits into a series of recent discoveries that suggest galaxies began forming much earlier in the Universe’s history than previously believed.

“This result represents a breakthrough in the field of galaxy formation, showing that the structures that we observe in nearby spiral galaxies and in our Milky Way were already in place 12 billion years ago,” says Francesca Rizzo, a PhD student from the Max Planck Institute for Astrophysics in Germany, and the lead author of a paper detailing the findings published today in the journal Nature

Whilst the observation of this distant galaxy would not have been possible without the impressive technology of the ESO’s Atacama Large Millimeter/submillimeter Array (ALMA) located in the Chilean Andes, it also hinged on the invention of another galaxy, and an extraordinary feature of Einstein’s theory of general relativity. The team reconstructed the accurate image of SPT0418–47 from the ring-like image received by ALMA as a result of gravitational lensing by an intervening galaxy.

“We have studied in great detail a very distant galaxy. This means going back in time and we see this galaxy when it was very young, 1.4 billion years after the Big Bang,” Rizzo’s co-author, Filippo Fraternali, from the Kapteyn Astronomical Institute, the University of Groningen, Netherlands, tells ZME Science referring to the fact that the light from SPT0418–47 has travelled 12 billion years to reach us.

“Given that galaxies cannot form right after the Big Bang, we can estimate that SPT0418–47 [as we see it] is about 1-billion-years old.”

### A Distant Milky Way Doppelganger With Some Key Differences

SPT0418–47 possesses a central bulge and a rotating disc, two features also displayed by the Milky Way and other local galaxies. However, it lacks the Milky Way’s spiral arms and is also much smaller than our home galaxy. This is the first time that astronomers have spotted a central bulge — stars tightly clustered around the galactic centre — in such a distant, and therefore early, galaxy.

However, Fraternali notes that just because SPT0418–47 resembles the Milky Way at the stage we see it at, that doesn’t mean it evolved just as our galaxy did. “It is important to remark that whilst SPT0418–47 is similar to the Milky Way now, it does not mean that 12 billion years ago the Milky Way looked like SPT0418–47,” the researcher adds.

“In fact, we think that SPT0418–47 will evolve into a galaxy very different from the Milky Way, an elliptical galaxy, much more massive and without gas.”

Another key difference between the two galaxies is that SPT0418–47 is forming stars much more rapidly, unusual for a rotating disk. “SPT-S J041839–4751.9 belongs to a particular population of galaxies that are known as dusty star-forming galaxies,” Simona Vegetti, another co-author on the paper and an expert in gravitational lensing from the Max Planck Institute for Astrophysics, tells ZME Science. “As the name suggests, these are galaxies that are undergoing a significant burst of high-rate star formation.”

Vegetti goes on to explain that astronomers believe that as they age, galaxies like SPT0418-47 will turn into what is referred to as early-type galaxies — a galaxy which has consumed most of its gas and is not forming stars anymore.

“By comparing the properties of SPT-S J041839–4751.9 with those of nearby early-type galaxies, we can learn something about the processes which are responsible for the transformation from one galaxy type to the other,” shes says. “It’s a bit like comparing the properties of young and old people, it gives us some hints on the ageing process.”

But the most puzzling aspect of the distant galaxy is how calm and ordered it appears. Something current cosmological models cannot account for.

### Young and Chaotic?

Our current cosmological models suggest that the Universe that SPT0418–47 as we see it inhabited was a chaotic and turbulent place. And galaxies found during this epoch are expected to reflect these qualities, even if they do possess some structure, this should be washed out by the violent conditions around them.

“The general idea was that galaxies at those distances/times were extremely chaotic and one would barely recognise a disc in formation in amongst massive filaments of infalling gas and powerful explosions due to the extremely intense star formation,” says Fraternali.

Galaxies in the early universe are expected to be the site of powerful phenomena like supernova explosions which release a lot of energy Vegetti explains, adding: “We would expect SPT-S J041839–4751.9 to be very turbulent, or in other words, we expect the motion of gas in this galaxy to move chaotically in winds and outflows.”

But the team’s observations reveal a completely different picture. What they actually found was that the motion of the gas in SPT0418–47 is, in fact, rotating around the centre of the galaxy quietly and in a well-ordered fashion. As Vegetti notes: “It is very hard to explain this behaviour within the context of the latest state-of-the-art numerical simulations of galaxies.”

One of the man questions that remain for Fraternali is how common are these features and the relatively ‘calmness’ of SPT0418–47 in other older galaxies? “Because the galaxy has not been selected by us — it was, by chance, perfectly aligned with the lens along the line of sight — we may argue that it could be representative of a large fraction of massive galaxies at that time.”

Thus, staring back in time to conduct further investigation of these distant stars is of vital importance. But, that, as you may imagine, is no easy task. In fact, the study of SPT0418–47, as Fraternali indicates, was only made possible by the intervention of another galaxy coming between it and us, and the ensuing remarkable phenomena of gravitational lensing.

### Galaxies as Lenses — the Power of Gravitational Lensing

The exact alignment of SPT0418–47 and an intervening galaxy means that it appears as a near-perfect ring to the team at ALMA — a structure referred to by astronomers as an ‘Einstein ring’ by astronomers due to its connection to the theory of general relativity — the geometrical theory of gravity put forward by Einstein in the early years of the 20th Century.

Gravitational lensing hinges on the fact that objects with mass curve the fabric of spacetime around them. The greater the mass, the more extreme the curvature. The most common analogy used to describe this is a stretched rubber-sheet having objects of increasing mass placed on it. A bowling ball creating a greater ‘dent’ on the sheet than a marble or a tennis ball.

This means that an object like a galaxy with tremendous mass curves the path of light travelling past it, often this results in an object behind the lens appearing to be located in a different place. In extreme cases, creating an ‘Einstein Ring’ built up of light that took different curved paths around the intervening galaxy and thus arrived at Earth at slightly different times. But, isn’t just a beautiful and curious phenomenon of gravity, it’s also a powerful observational tool.

“Because these galaxies are very far, it is challenging to study them in great detail using current telescopes, they are not powerful enough,” Vegetti says. “Our team then uses the effect of strong gravitational lensing to overcome this limitation.”

The process used by the team first involves the search for a pair of galaxies that are far away from each other but appear aligned from our vantage point here on Earth. “The galaxy closer to us will then behave like a lens providing us with a magnified view of the more distant galaxy,” Vegetti elaborates. “It’s like observing through a much more powerful telescope.

“When we started studying this object we had no idea of what we were going to find. There are almost no other studies of galaxies so young at such a level of detail.”

Vegetti explains that the next generation of telescopes such as the James Webb Space Telescope and the ESO’s Extremely Large Telescope (ELT) should allow for the study of SPT0418–47 galactic contemporaries in much greater detail. This will allow researchers to discover just how common these features are, and in turn, possibly spark a rethink of how early well-ordered galaxies could form in the Universe’s history. She also reserves special praise for this study’s lead author.

“These new facilities will bring this type of analysis to the next level, allowing us to observe even younger galaxies with an even greater level of detail,” Vegetti concludes. “Francesca Rizzo is leading the way in this line of research. She is a brilliant young scientist with whom I enjoy working, so I am looking forward to our next discovery!”

#### Source

Rizzo. F., Vegetti. S., Powell. D., Fraternali. F., et al, ‘A dynamically cold disk galaxy in the early Universe,’ Nature, [2020].

# Dark matter is colder than we thought — and we know this thanks to Einsteins crosses

Clumps of dark matter can be surprisingly small — and cold.

Astronomers love to give weird names to things, but “dark matter” is pretty self-explanatory. It’s matter, or we think it is, because it exerts a gravitational pull. It’s also dark, cause we can’t see it (although we observe its effect) — and that’s pretty much all we know about it.

Dark matter is estimated to account for approximately 85% of the matter in the universe, and yet we don’t really know what it is. But a new study might help us in that regard.

As weird as it may sound, dark matter seems to “clump together”. Turns out, these clumps can be much smaller than we thought. This confirms a fundamental prediction about dark matter, and can help researchers make an important breakthrough in understanding this enigmatic phenomenon.

### A dark hunt

Dark matter is invisible to all our instruments. It doesn’t emit light or any detectable radiation. We never imaged it in any direct way. So when studying dark matter, astrophysicists look for its effects.

The most prevalent of these effects is its gravitational effect. According to such observations, dark matters appears to be the gravitational “glue” holding galaxies together.

We don’t know what kind of particles dark matter would be made of, but it almost certainly wouldn’t be the electrons, protons, and neutrons we’re familiar with. A popular theory holds that whatever particles it may be made of, these particles wouldn’t move very fast. This would help explain why dark matter tends to clump together, and while the dark matter concentrations across the universe can vary so much.

If this were the case, this would make for “cold” dark matter. A competing theory supports the idea of “hot” dark matter, where particles are moving at relativistic speeds (close to the speed of light).

Clumps of dark matter can help solve this dilemma. “Hot” dark matter wouldn’t allow the formation of small clumps, they simply move too fast to allow small chunks to form. So if we could detect small clumps, this would lend support to the “cold” dark matter hypothesis.

But remember how we said that dark matter can’t be imaged? Yeah, that’s still a problem.

### Gravitational lensing

So instead, researchers took to an old tool: gravitational lensing. But they gave it a new twist.

Gravitational lensing, as the name implies, is the technique of using gravitational attraction as a lens. Everything has a gravitational pull, but objects that are really massive can distort even light itself. While this is often a very subtle distortion, it’s still detectable.

Think of it this way: if we’re looking at a distant, bright galaxy through a telescope, and another massive object is interposed between our telescope and the galaxy, its gravitation can act as a lens, bending the light. This is what was done in this study.

As you might have guessed, this requires a very particular alignment — which means that gravitational lenses must be found — and they may not exist in the directions we want them.

But sometimes, ever so rarely, the objects involved are lined up in such a way that four distorted images are produced around the lensing object. This is called an Einstein cross. This is where things get really interesting.

You might be wondering what any of this has to do with dark matter. Well, the gravitational influence of dark matter clumps should be observable — even that of smaller clumps.

The team used the Hubble Space Telescope to study eight Einstein cross quasars — extremely luminous galactic cores powered by supermassive black holes. These quasars were gravitationally lensed by massive foreground galaxies.

“Imagine that each one of these eight galaxies is a giant magnifying glass,” said UCLA astrophysicist Daniel Gilman, one of the study authors.

“Small dark matter clumps act as small cracks on the magnifying glass, altering the brightness and position of the four quasar images compared to what you would expect to see if the glass were smooth.”

The eight quasars and galaxies were aligned so precisely that the warping effect produced four distorted images of each quasar, almost like looking at a carnival mirror. Such alignments are very rare and were fortunate for this study.

The presence of the dark matter clumps altered the apparent brightness and position of each distorted quasar image. The researchers measured how the light was warped by the lens, and then looked at the brightness and position of each of the images, comparing these against predictions of how the Einstein crosses would look without dark matter. These comparisons allowed them to calculate the mass of the dark matter clumps causing the distortion.

According to the results, small dark matter clumps could exist — and these observations support the existence of colder dark matter.

“Dark matter is colder than we knew at smaller scales,” said Anna Nierenberg of NASA’s Jet Propulsion Laboratory in Pasadena, California, leader of the Hubble survey. “Astronomers have carried out other observational tests of dark matter theories before, but ours provides the strongest evidence yet for the presence of small clumps of cold dark matter. By combining the latest theoretical predictions, statistical tools and new Hubble observations, we now have a much more robust result than was previously possible.”

This does not rule out the possibility of hotter dark matter, but lends more weight to the colder theory. To make matters even more complex, there is also a mixed dark matter model that includes both types. However, this is almost certainly not the last study of this type.

Astronomers will be able to conduct follow-up studies of dark matter using future NASA space telescopes such as the James Webb Space Telescope and the Wide Field Infrared Survey Telescope, both infrared observatories.

It’s remarkable that after decades of service, the Hubble telescope still provides extremely useful information, allowing us to understand aspects of the surrounding universe.

As for dark matter, we won’t unravel its secrets today or tomorrow. We’re still taking baby steps, but one at a time, we’re getting closer to understanding what it really is — and maybe then, it won’t be dark matter anymore.

The team will present its results at the 235th meeting of the American Astronomical Society in Honolulu.

# If we want to find the “missing” dark matter, we have to look beyongblack holes

A new study has ruled out black holes as the universe’s missing dark matter — shutting down an intriguing hypothesis that linked the two.

The likely culprit: black holes. Image credits: NASA / JPL.

### Dark matter

Dark matter is a strange thing, and it already has a historied tale. It was hinted at even before the 1900s by Lord Kelvin, but the first truly convincing evidence didn’t come until the 1970s when Vera Rubin and Kent Ford observed some discrepancies between how galaxies should have rotated, and how they were observed to do.

As years passed, it became clearer and clearer that something was out there — something which we can’t see directly, but which has a noticeable gravitational effect on existing matter. Today, astronomers calculate that dark matter constitutes 84.5% of the total mass in the universe, though no one really knows what dark matter really is. It’s funny to say it, but although it makes up most of our universe, we can’t really find it.

Dark matter does not seem to interact with anything else other than gravity, which doesn’t make things easier.

Several theorists have proposed scenarios in which there are multiple types of dark matter. While this seems plausible (though unproven), actually investigating them is daunting, as each of these types would require a different explanation for its origin, which brings even more complexity to an already puzzling situation. But this approach has its supporters.

For instance, dark matter could consist of different types of black holes, which are also notoriously difficult to study directly, but have a massive gravitational field.

“I can imagine it being two types of black holes, very heavy and very light ones, or black holes and new particles. But in that case one of the components is orders of magnitude heavier than the other, and they need to be produced in comparable abundance. We would be going from something astrophysical to something that is truly microscopic, perhaps even the lightest thing in the universe, and that would be very difficult to explain,” said lead author Miguel Zumalacárregui, a Marie Curie Global Fellow at the Berkeley Center for Cosmological Physics.

In the new study, Zumalacárregui and colleagues set out to see whether this idea holds any truth — after the 2015 detection of gravitational waves from colliding black holes, many astronomers hoped to find that dark matter was simply a swarm of black holes, sprinkled through the universe. But this turned out to not be the case.

### Galaxy lenses

A supernova (bright spot at lower left) and its host galaxy (upper center), as they would appear if gravitationally lensed by an intervening black hole (center). Image credits: Miguel Zumalacárregui / UC Berkeley.

In order to figure things out, Zumalacárregui and colleagues carried out a statistical analysis of 740 of the brightest supernovas discovered as of 2014, looking for a phenomenon called gravitational lensing.

[panel style=”panel-default” title=”Gravity as a lens” footer=””]A regular lens focuses light by means of refraction. A simple lens consists of a single piece of transparent material, which, through its geometry, focuses (or disperses) light a specific focal point.

But astronomers learned that gravity can also be used as a lens — and since black holes are the most massive known objects in the universe, they are often involved in gravitational lensing. A gravitational lens doesn’t have a focal point, but rather a focal line.

Animated simulation of gravitational lensing caused by a black hole on a background galaxy. Image credits: Wiki Commons.

[/panel]

The researchers found that none of the supernovas they studied were lensed by black holes, which strongly suggests that the dark matter is represented by black holes — if this were the case, then you would almost certainly observe the phenomenon at such a sample size.

Other researchers have performed similar but simpler analyses that yielded inconclusive results, but Zumalacárregui and colleagues incorporated the precise probability of seeing all magnifications, from small to huge, as well as uncertainties in brightness and distance of each supernova. Even for the smallest black holes, there would have certainly been some level of lensing.

“You cannot see this effect on one supernova, but when you put them all together and do a full Bayesian analysis you start putting very strong constraints on the dark matter, because each supernova counts and you have so many of them,” Zumalacárregui said. The more supernovas included in the analysis, and the farther away they are, the tighter the constraints.

So where does this leave us when it comes to dark matter? Probably back at the drawing board. Scientists disproved one of the theories, but we’re still not much closer to identifying the source and nature of dark matter.

“We are back to the standard discussions. What is dark matter? Indeed, we are running out of good options,” said Uroš Seljak, a UC Berkeley professor of physics and astronomy and BCCP co-director. “This is a challenge for future generations.”

Miguel Zumalacárregui, Uroš Seljak. Limits on Stellar-Mass Compact Objects as Dark Matter from Gravitational Lensing of Type Ia Supernovae. Physical Review Letters, 2018; 121 (14) DOI: 10.1103/PhysRevLett.121.141101

# Researchers discover a planet so big it might not be a planet after all

Astronomers have spotted a planet 13 times larger than Jupiter, raising questions as to whether it is a planet, or rather something else.

The newly discovered object lies at the border between gas giants and brown dwarfs. Image via Wiki Commons.

NASA’s Spitzer telescope is an infrared space telescope launched in 2003. Initially meant to survive for only 2.5 years, the telescope is still running, allowing astronomers to gather useful data, especially through a technique called microlensing. In gravitational lensing, astronomers study the bending of the light caused by massive objects or clusters. Stars from the Milky Way usually serve as the lensing object. Microlensing is a type of gravitational lensing in which no distortion in shape can be seen, but the amount of light received from a background object still changes in time. The effect is small, such that even a galaxy with a mass more than 100 billion times that of the Sun will produce barely noticeable effects. However, these effects are strong enough to be noticed.

A light source passes behind a gravitational lens (point mass placed in the center of the image). The aqua circle is a source as it would be seen if there was no lens; white spots are the multiple images of the source.

The good thing about microlensing is that it does not rely on the light from the host stars; thus, it can detect planets, even when the host stars cannot be detected. This was the case with an object called OGLE-2016-BLG-1190.

The object might not sound very exciting, but it is. Astronomers estimate it to be at about 13.4 Jupiter masses. This is almost too big to be a planet; it puts it right at the limit between gas giants and a brown dwarf. In other words, we don’t know if it is a humongous planet or a failed star.

“The planet’s mass places it right at the deuterium burning limit, i.e., the conventional boundary between “planets” and “brown dwarfs”. Its existence raises the question of whether such objects are really “planets” (formed within the disks of their hosts) or “failed stars” (low mass objects formed by gas fragmentation),” the paper reads.

The deuterium burning researchers refer to is a nuclear fusion reaction that occurs in stars and some substellar objects, in which a deuterium nucleus and a proton combine to form a helium nucleus.

Deuterium fusion is what makes a star a star. Brown stars occupy the mass range between the heaviest gas giant planets and the lightest stars. They are generally regarded as sub-stellar objects not massive enough to sustain nuclear fusion of ordinary hydrogen, but still massive enough to support the fusion of deuterium.

OGLE-2016-BLG-1190Lb orbits its parent star approximately every three years, two times further away than the Earth is from the Sun. It’s the first planet discovered through microlensing from Spitzer.

Journal Reference: Y.-H. Ryu et al. OGLE-2016-BLG-1190Lb: First Spitzer Bulge Planet Lies Near the Planet/Brown-Dwarf Boundary. arXiv:1710.09974

# Another Einstein predication is confirmed by scientists: gravitational lensing measures star’s mass for the 1st time

In an unprecedented new study, astronomers working with the Hubble Space Telescope have measured the mass of a white dwarf star using a cosmic phenomenon first predicted by Albert Einstein. The technique is essentially centered around an optical illusion called gravitational lensing. For more than 100 years, scientists have proposed that it’s possible to precisely measure a star’s mass using this quirk of General Relativity, though Einstein himself doubted it. Finally, the debate is put to rest with this breakthrough study.

### Zooming in

Famed physicist Albert Einstein predicted, as a result of his Theory of General Relativity, that whenever light from a distant star passes by a closer object, gravity acts like a magnifying lens bending the distant starlight but also brightening it. This effect has been documented extensively around very massive structures such as galaxies. Captioned below is the great galaxy cluster Abell Cluster 2218. Notice the giant, stretched arcs? Those are actually background galaxies that get distorted and magnified by the giant cluster which bends the light. That’s analogous to how normal lenses such as the ones in a magnifying glass or a pair of spectacles work by bending light rays that pass through them in a process known as refraction, in order to focus the light somewhere (such as in your eye). In fact, astronomers often use gravitational lensing as a natural telescope, to great effect.

Image credit: ESA, NASA, J.-P. Kneib and Richard Ellis.

What we’re looking at is called strong gravitational lensing, which is very rare because it implies a fortuitous alignment between a foreground mass and a background galaxy. To understand gravitational lensing we need to go back to Einstein’s theory of general relativity which posits that space is not fixed. Instead, it’s merged with time in a four-dimensional continuum called space-time which is morphed by gravity. Massive objects like a black hole or star create curves in space-time much like a bowling ball causes a dent if you place it on a mattress. It follows that if a ray of light passes near such a massive object, it will follow the distorted curve in space-time and veer away from its straight path.

The deflection directs more light to the observer causing background objects to become brighter. Sometimes the effects create a ring of bright light around the foreground object which scientists refer to as an Einstein right.

The schematic below explains in graphic detail how it all works.

Credit: Starts with a bang!

In 1919, measurements of starlight curving around a total eclipse of the Sun provided one of the first convincing proofs of Einstein’s general theory of relativity. But in a 1936 article in the journal Science, Einstein added that because stars are so far apart “there is no hope of observing this phenomenon directly.” Even Einstein can be wrong though.

An international research team directed by Kailash C. Sahu found it is possible to measure the extremely small displacement caused relative light objects like stars (light compared to a black hole or galaxy) using sensitive instruments mounted on the Hubble Space Telescope. They did so for a white dwarf star called Stein 2051 B located only 18 light-years away from Earth. This star caused a displacement of only 2 milliarcseconds on the plane of the sky, or about equal to the width of a quarter seen from 1,500 miles (2,400 kilometers) away. At such a resolution, it’s no wonder Einstein deemed the feat almost impossible given the technology available during his time.

“Einstein would be proud. One of his key predictions has passed a very rigorous observational test,” said Terry Oswalt of Embry-Riddle Aeronautical University, who was not involved in the research.

The gravity of the white dwarf star warps space and bends the path of light from a more distant object.
Credit: ESA/Hubble & NASA

.

This is the first report of  “gravitational microlensing” by a star other than the sun. The discovery required eight measurements between October 2013 and October 2015 of the shifts in the apparent position of a distant star as its light was deflected around the white dwarf star Stein 2051 B.

“The ring and its brightening were too small to be measured, but its asymmetry caused the distant star to appear off-center from its true position,” Oswalt says. “This part of Einstein’s prediction is called ‘astrometric lensing’ and Sahu’s team was the first to observe it in a star other than the Sun.”

During the close alignment, the distant starlight appeared offset by about 2 milliarcseconds from its actual position. This deviation is so small that it is equivalent to observing an ant crawl across the surface of a quarter from 1,500 miles away. From this measurement, astronomers calculated that the white dwarf’s mass is roughly 68 percent of the sun’s mass. Credit: NASA, ESA, and K. Sahu (STScI) .

The observation revealed Stein 2051 B has a mass that is about two-thirds that of the sun, indicating it formed from a star about 2.3 times the mass of the sun. Previously, another method measured the white dwarfțs mass to 0.5 times the mass of the sun, which didn’t sit well with what we know about white dwarf formation.

“This new tool for determining masses will be very valuable as huge new surveys uncover many other chance alignments over the next few years,” Oswalt said.

“At least 97 percent of all the stars that have ever formed in the Galaxy, including the Sun, will become or already are white dwarfs – they tell us about our future, as well as our history,” the physicist concluded.

Journal reference: K.C. Sahu el al., “Relativistic deflection of background starlight measures the mass of a nearby white dwarf star,” Science (2017). science.sciencemag.org/cgi/doi/10.1126/science.aal2879

# An old looking galaxy found in a young Universe

Many people change a lot after their youth… and so to did our Universe. Nowadays, galaxies contain both dust and gas, but back in the early Big-Bang days, the earliest galaxies had no dust, only gas. Now, a team of astronomers has discovered a very young galaxy with lots of dust – the equivalent of a white-bearded young man.

This spectacular view from the NASA/ESA Hubble Space Telescope shows the rich galaxy cluster Abell 1689, which acted like a gravitational lens. Image credits: NASA.

Dust plays an extremely important, both in planetary and in star formation – it basically acts like a seed, “fertilizing” galaxies for celestial body formation. Cosmic dust are smoke-like particles made up of either carbon (you can consider it a fine soot) or silicates (a fine sand). The elements in dust are synthesised by the nuclear combustion process in stars and driven out into space when the star explodes. These remains then gather into dust clouds, which form new stars and new elements, thus perpetuating the cycle. It’s a very slow process, and it seems that the older a galaxy gets, the more dust it tends to have.

But dust was not around from the beginning; in the early stages of the Universe, gas filled the galaxies, and dust was nowhere to be found… or at least so we thought. Now, a team has found a very distant, young galaxy with a large amount of dust, challenging our previous understanding of early galaxies.

“It is the first time dust has been discovered in one of the most distant galaxies ever observed — only 700 million years after the Big Bang. It is a galaxy of modest size and yet it is already full of dust. This is very surprising and it tells us that ordinary galaxies were enriched with heavier elements far faster than expected,” explains Darach Watson, an astrophysicist with the Dark Cosmology Centre at the Niels Bohr Institute at the University of Copenhagen.

Under normal circumstances, astronomers wouldn’t even have been able to detect this galaxy – it’s so far away and faint – but a fortunate circumstance allowed them to visualize it. In between the Earth and this galaxy, there is a large cluster of galaxies called Abell 1689. The light is refracted by the gravity of the galaxy cluster, thus amplifying the distant galaxy – basically acting like a magnifying glass. The technique is called gravitational lensing. Astronomers explain:

“We looked for the most distant galaxies in the universe. Based on the colours of the light observed with the Hubble Space Telescope we can see which galaxies could be very distant. Using observations from the very sensitive instrument, the X-shooter spectrograph on the Large Telescope, VLT in Chile, we measured the galaxy’s spectrum and from that calculated its redshift, i.e. the change in the light’s wavelength as the object recedes from us. From the redshift we can calculate the galaxy’s distance from us and it turned out to be, as we suspected, one of the most distant galaxies we know of to date,” explains Lise Christensen, an astrophysicist at the Dark Cosmology Centre at the Niels Bohr Institute.

Astronomers are capable of telling which galaxies are younger by observing the wavelengths at which they emit light. Younger galaxies emit much more hot ultraviolet light. The hot ultraviolet radiation heats the surrounding ice-cold dust, which then emits light in the far-infrared.

“It is this far-infrared light, which tells us that there is dust in the galaxy. It is very surprising and it is the first time that dust has been found in such an early galaxy. The process of star formation must therefore have started very early in the history of the universe and be associated with the formation of dust. The detection of large amounts of solid material shows that the galaxy was enriched very early with solids which are a prerequisite for the formation of complex molecules and planets,” explains Darach Watson.

Now, the next step is figuring out how dust “colonized” the early galaxies, something which might potentially reveal how this process is shaping up the Universe now.

Story: the above story is based on materials provided by University of Copenhagen – Niels Bohr Institute

# NASA’s great observatories combine to probe deeper in the Universe

These are NASA Hubble Space Telescope natural-color images of four target galaxy clusters that are part of an ambitious new observing program called The Frontier Fields. NASA’s Great Observatories are teaming up to look deeper into the universe than ever before. The foreground clusters range in distance from 3 billion to 5 billion light-years from Earth. (c) NASA/ESA

Each of NASA’s Great Observatories – Hubble, Spitzer and Chandra – have been designed to peer through the Universe in a characteristic manner. The telescopes have provided along the years massive amount of astronomical data and have helped scientists make important discoveries. What if you combine each of the telescopes’ strong points to assemble one massive probe capable of seeing farther in the Universe than ever before? That’s exactly what  The Frontier Fields ambitious space program will undertake in the following three years, combining the observational power of all three major NASA telescopes along with natural gravitational lenses to study six massive clusters of galaxies.

“The Frontier Fields program is exactly what NASA’s Great Observatories were designed to do; working together to unravel the mysteries of the universe,” said John Grunsfeld, associate administrator for NASA’s Science Mission Directorate in Washington. “Each observatory collects images using different wavelengths of light with the result that we get a much deeper understanding of the underlying physics of these celestial objects.”

The program will tackle galaxy clusters that are among the most massive assemblages of matter known. Because of their humongous mass, these galaxy clusters (hundreds to thousands of galaxies bound together by gravity), exert powerful gravitational fields which can be used to brighten and magnify more distant galaxies so they can be observed. This is called gravitational lensing  and because of it  light rays that would have otherwise not reached the observer are bent from their paths and towards the observer.

Pandora’s Cluster. (c) NASA

The first object the astronomers will be directing their view towards is  Abell 2744 or  Pandora’s Cluster. This giant cluster is actually thought to be the result of four distinct galaxy clusters that piled-up over the span of 350 million years.  Studying this cluster, astronomers hope they can discover galaxies that were formed just a few hundred millions years after the Big Bang.

“The idea is to use nature’s natural telescopes in combination with the great observatories to look much deeper than before and find the most distant and faint galaxies we can possibly see,” said Jennifer Lotz, a principal investigator with the Space Telescope Science Institute in Baltimore, Md.

Each Great Observatory will have its role to play. Hubble tells astronomers in which way to direct their view and how many galaxies or stars are born in a system. Spitzer can relay how old these cosmic bodies are. Chanda, using its  X-ray wavelengths instruments, will image the clusters and tell astronomers what their  mass and gravitational lensing power is.

“We want to understand when and how the first stars and galaxies formed in the universe, and each great observatory gives us a different piece of the puzzle,” said Peter Capak, the Spitzer principal investigator for the Frontier Fields program at NASA’s Spitzer Science Center at the California Institute of Technology, Pasadena.

# Massive dark energy survey launched set to probe its secrets

Dark energy is the mysterious force that drives the Universe’s expansion at an ever increased pace. Probing and understanding this force is thus imperative for astronomers’ and cosmologists’  efforts of peering through the Universe’s secrets. Recently, a new massive project set on probing the nature of dark energy was launched, called the  Dark Energy Survey (DES), and its future findings are already regarded in great promise.

The Dark Energy Camera photographs galaxies from its perch on the Blanco telescope in Chile. (c) Reidar Hahn/Fermilab

DES’ centerpiece is its 570-megapixel digital camera (pictured), capable of imaging 300 million galaxies over one-eighth of the clear night sky from up atop the  the 4-metre Blanco telescope at the Cerro Tololo Inter-American Observatory in the Chilean Andes. Galaxies and galaxy clusters are its main target since these are the brightest objects in the night’s sky and the camera’s high resolution capability can be most efficient. You see, to probe dark energy, the astronomers will measure weak gravitational lensing  – the phenomenon in which incoming light from distant galaxies is skewed and distorted by matter between the galaxies and the point of observation (Earth).

Although measuring weak lensing is difficult, the researchers are confident this can be achieved. How is this important? Well, by measuring the degree of lensing, astronomers can infer and map the matter in between galaxies and Earth. This allow them in turn to create a three-dimensional web that can reveal the fingerprints of dark energy through time. A similar effort to DES has already been operation for quite a while – the Japanese Hyper Suprime-Cam in Hawaii, which relies on an even more detailed, 870-megapixel camera. Although Hyper Suprime can image fainter galaxies, DES can cover a wider patch of the sky. Together, the researchers hope these two projects will help paint a more accurate picture.

Besides weak lensing, DES will also be used to count galaxy clusters and measure their distance away from Earth, and spot distant supernovae, whose otherwise reference light is dimmed as the universe expands. The latter are of great importance since this is how the theory of an accelerating Universe was formed, netting the authors of the paper a Nobel prize in 2011.

# Extremely rare double Einstein ring imaged by Hubble

(c) NASA/ESA

Hubble just never ceases to surprise. The latest astronomical find discovered using the ever resourceful space telescope is a never before encountered double ring pattern known as an Einstein ring. This very rare pattern is the result of a peculiar optical alignment in which three galaxies are perfectly aligned with each other, like beads on a string. The occurrence isn’t just a silly optical trick in space – studying it, astronomers can learn more about dark matter and dark energy, and even the curvature of the Universe.

The phenomenon that gave rise to this peculiar observation is known as gravitational lensing, in which the light emitted by a galaxy in the background gets bent by the gravitational pull of a massive galaxy in the foreground. In our case, one could say we have a double gravitational lens on our hands since a third massive galaxy lies in the foreground. When two galaxies are exactly lined up, the light gets twisted in such a fashion that it forms a shape that resembles a circle, called Einstein’s ring. When three of them are perfectly lined up, such as the case, two concentric rings form.

“Such stunning cosmic coincidences reveal so much about nature. Dark matter is not hidden to lensing,” added Leonidas Moustakas of the Jet Propulsion Laboratory in Pasadena, California, USA. “The elegance of this lens is trumped only by the secrets of nature that it reveals.

The odds of such a phenomenon being observable from Earth’s vantage point are so dim, that the discovery can be considered nothing short of jackpot! In fact, the team of astronomers led by Raphael Gavazzi and Tommaso Treu of the University of California, Santa Barbara were extremely lucky to spot it in the first place. SLACS team member Adam Bolton of the University of Hawaii’s Institute for Astronomy in Honolulu first identified the lens in the Sloan Digital Sky Survey (SDSS). “The original signature that led us to this discovery was a mere 500 photons (particles of light) hidden among 500,000 other photons in the SDSS spectrum of the foreground galaxy,” commented Bolton.

The geometry of the two rings allowed the researchers to establish the mass of the middle galaxy precisely to be a value of 1 billion solar masses – a dwarf galaxy. This is actually the first time a dwarf galaxy’s mass was measured at cosmological distance. The comparative radius of the rings could also be used to provide an independent measure of the curvature of space by gravity.

The results were reported at the 211th meeting of the American Astronomical Society in Austin, Texas, USA. A paper has been submitted to The Astrophysical Journal.

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# Farthest known object in the Universe is 13.3 billion years old

The inset at left shows a close-up of the young dwarf galaxy. This image is a composite taken with Hubble’s WFC 3 and ACS. Credit: NASA, ESA, and M. Postman and D. Coe (STScI) and CLASH Team.

NASA scientists have announced they have discovered the farthest object discovered so far in the Universe, a 13.3 billion old galaxy or a mere 420 million years after the Big Bang.

That’s not to say that its 13.3 billion light years away from Earth, since the Universe has expanded greatly since then and the actual distance might be much greater than this figure. It means that light took 13.3 billion years to reach us.

The galaxy has been dubbed  MACS0647-JD and was discovered using a combination of NASA’s Hubble and Spitzer space telescopes, along with gravitational lensing – an interstellar technique that uses distant galaxies to create a zooming effect for the light that passes through them. Without gravitational lensing, this discovery would have been impossible with the current technology employed in telescopes.

“This [magnification galaxy] does what no manmade telescope can do,” Marc Postman, of Baltimore’s Space Telescope Institute, said in a release. “Without the magnification, it would require a Herculean effort to observe this galaxy.”

Essentially, the scientists have looked into the past – 13.3 billion years into the past. What they saw was a galaxy that was only a tiny fraction of the Milky Way. More exactly, it’s been estimated as being only 600 light years wide. For compassion, the Large Magellanic Cloud, a dwarf galaxy companion to the Milky Way, is 14,000 light-years wide. Our Milky Way is 150,000 light-years across.

Since then it has most likely grown, and even collided already with other galaxies. The previous record holder was a gamma ray burst just 600 million years after the Big Bang.

“Over the next 13 billion years, it may have dozens, hundreds, or even thousands of merging events with other galaxies and galaxy fragments,” Dan Coe, lead author of the study announcing the discovery, said in a release. “This object may be one of many building blocks of a galaxy.”

source: NASA

# Faint galaxy sheds light on the dawn of the Universe – many more to be found

The first galaxies formed very fast after the Big Bang – in cosmic time, that is. It’s estimated that the earliest ones appeared some 500 million years after the Big Bang, a period about which researchers know very little.

### How they observed it

Source: The CLASH team/Space Telescope Science Institute

Even though they are typically very bright, such galaxies are quite hard to observe because they are very far away and only a small fraction of their light can make its way towards Earth, a fraction so small it’s almost impossible to pick up. However, the Hubble telescope managed to detect light from a small galaxy emitted just 500 million years post-Big Bang, a period when the Universe was still in its infancy.

The telescope was able to do this thanks to a phenomena called gravitational lensing: basically, when you have an observe (Hubble), a distant source (the galaxy) and a certain distribution of matter (a galaxy cluster for example), the light emitted by the source can be bent and the observer can observe it easier; gravitational lensing is one of the predictions involved by Einstein‘s general theory of relativity. Basically, the massive gravity of the galaxy cluster acts just like a lens.

In this case, astronomer Wei Zheng and colleagues using the Hubble and Spitzer Space Telescopes reported light was magnified 15 times, making it just strong enough to be observed. Even so, the galaxy MACS 1149-JD appeared as a mere blob, and only after repeated measurements were they able to conclude that it is most likely a galaxy.

### How they know its age

The Universe is expanding; galaxies produce light with specific spectral properties, based on the stars and gas they contain. Combine these two facts, and you can understand that light emitted by early galaxies was stretched shifting the entire spectrum into a different wavelength range, a phenomenon called cosmic redshift. All electromagnetic types of radiation (light included) have an electromagnetic spectrum – the range of all possible frequencies of electromagnetic radiation. Cosmic redshift means light seen coming from an object that is moving away is proportionally increased in wavelength, or shifted to the red end of the spectrum.

So, using multiple measurements from the Spitzer and Hubble telescope, they estimated that the light was emitted 490-505 million years after the Big Bang. But their conclusions are perhaps even more interesting. Instead of suggesting MACS 1149-JD is a special snowflake, astronomers believe there are many more such galaxies, formed in the same era, the ‘first’ era, just waiting to be discovered.

Scientific article was published in Nature

# Nomad planets may litter Milky Way

According to a recent study published by researchers from the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), our galaxy may be ‘infested’ with nomad planets, which wander aimlessly instead of orbiting a star. Furthermore, the study concluded there may actually be 100,000 times more “nomad planets” in the Milky Way than stars.

If this theory is proven correct, it will severely affect our current understanding about planetary formation and will even affect what we think about the origin and abundance of life in our galaxy.

“If any of these nomad planets are big enough to have a thick atmosphere, they could have trapped enough heat for bacterial life to exist,” said Louis Strigari, leader of the team that reported the result in a paper submitted to the Monthly Notices of the Royal Astronomical Society.

The thing is, even though these planets don’t have the luxury of a star to offer them warmth, they might generate enough heat to support life through radioactive decay and tectonic activity. Past studies have shown more than 500 planets outside our solar system, almost all of which orbit stars, and last year alone, astronomers identified about a dozen nomad planets, through a technique called gravitational lensing, which analyzes stars whose light is momentarily refocused by the gravity of other passing planets.

The traditional belief was that roughly two nomads exist for every typical, so-called main-sequence star in our galaxy, but this new research showed that nomad planets may actually be 50.000 times more frequent than that.

“To paraphrase Dorothy from The Wizard of Oz, if correct, this extrapolation implies that we are not in Kansas anymore, and in fact we never were in Kansas,” said Alan Boss of the Carnegie Institution for Science, author of The Crowded Universe: The Search for Living Planets, who was not involved in the research. “The universe is riddled with unseen planetary-mass objects that we are just now able to detect.”

“Few areas of science have excited as much popular and professional interest in recent times as the prevalence of life in the universe,” said co-author and KIPAC Director Roger Blandford. “What is wonderful is that we can now start to address this question quantitatively by seeking more of these erstwhile planets and asteroids wandering through interstellar space, and then speculate about hitchhiking bugs.”

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# Astronomers use massive objects in space as huge telescopes, find brightest galaxy via gravity lens

Whenever a massive object, with an equally massive gravitational pull, like black holes or galaxy clusters, falls between an observer, say a telescope, and a distant target in the background to be observed, than a gravitational lens is formed. Light emitted from the distant object gets twisted by the massive object, and ends up distorted at the telescope – this can be magnified, like if the light passed through a huge telescope. Gravity lenses are critical to astronomical observations of distant objects, which aid scientists learn more about how early galaxies formed, and how the Universe came to be.

Illustration showing how a foreground galaxy cluster that stands between Hubble and the background galaxy to be imaged acts like a lens in space, warping space like a funhouse mirror due to massive gravity. The resulting image is stretched into an arc, which scientists need to correct for an accurate view. (c) NASA

Recently, the Hubble Space Telescope harnessed such a gravitational lens, created by a cluster of closer galaxies located about 5 billion light-years away, and captured a distant galaxy 10 billion light-years away. The researchers found it was  three times brighter than any other seen through a gravity lens, and like many great scientific discoveries, it was all discovered by accident.

“This observation provides a unique opportunity to study the physical properties of a galaxy vigorously forming stars when the universe was only one-third its present age,” NASA officials explained

The problem with gravitational lenses is the distortion itself, which makes zooming possible in the first place. Astronomers aimed the space telescope at the galaxy cluster RCS2 032727-132623, which is surrounded by a nearly 90-degree arc of bright light from an even more distant galaxy. Because of the distortions, the image of the background galaxy is repeated several times. Using Hubble data, astronomers carefully removed the distortions and instead left an clear and enchanting sight of the distant galaxy filled with star-forming areas that shine brighter than similar spots in our own Milky Way.

The zoom in rectangle shows the brightest galaxy so far found through a gravity lens. It's 20 times larger and over three times brighter than typically lensed galaxies.The rounded outlines that form an arc are actually the remnant distortions discussed in the article. (c) NASA

“Hubble’s view of the distant background galaxy is significantly more detailed than could ever be achieved without the help of the gravitational lens,” NASA officials wrote.

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# Hubble captures disk of matter around black hole – birth of a quasar

This picture shows a quasar that has been gravitationally lensed by a galaxy in the foreground, which can be seen as a faint shape around the two bright images of the quasar.The level of detail involved is equivalent to being able to study individual grains of sand on the surface of the Moon while standing on Earth. (c) NASA

The Hubble telescope has captured through an innovative technique, which harnessed light bent from a distant galaxy in a optical lens-like manner, a  direct image of a disk of matter surrounding a black hole.

The disk, made out of gas and dust, slowly swirls around a giant black hole’s center gradually getting consumed. Powered by the disk of matter, huge energy bursts of energy are triggered from within the black hole’s center – these phenoma is typically known as a quasar, the most energetic objects we’re currently aware of in the Universe.

These disks of matter are very well obscured, buried away in distant galaxies from the early Universe, making them impossible to image directly. However, scientists were able to picture the forming quasar with the help of a ancient galaxy which happened to be between Earth and the quasar. The mass of the enormous galaxy bent light from the quasar and directed it toward Hubble, allowing for the light’s redshift to be studied through a process called gravitational lensing.

Researchers had to overcome a number of difficulties, like the fact that dust and gas from the galaxy were making imaging impossible, which forced them to look for subtle changes in the color of the light being output by the quasar. In the published paper, in the latest issue of Astrophysical Journal, the authors showed that the quasar was 18.5 billion light-years away and measured in size  between 60 and 180 billion miles across.

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