# What is the scale of things you can see with gravitational lensing?

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I'm trying to understand the examples of gravity lensing (using the general relativity property of large masses to bend light like a lens).

Most of the examples I see are of some galaxy (presumably a large mass) between us and a star (or are at least vague enough to not specify.

Yet my intuitive understanding of the sky is that all the stars we see are relatively close, all entirely within the Milky Way galaxy (and many of the stars in our galaxy provide a general glow), and that other galaxies are far enough away that it is difficult to image individual stars. The only single objects large enough comparable to a galaxy would be a quasar. Is that right?

So what then is going on usually with examples of gravity lensing? I find it hard to believe the captions that say a galaxy is enabling seeing more distant stars. I would think one could only apply that concept to a star or galaxy to see something much further away and as big or much bigger. Could one really use a galaxy as a lens to see a star? I wouldn't expect a star to be behind a galaxy from us.

Also, whatever the objects themselves, what is the scale? If the lensing is of a galaxy done by a star, I'd expect the star to galaxy distance ratio to be well below 1:1000 (~width of Milky Way to distance to Andromeda). But for galaxy to galaxy or galaxy cluster to galaxy or quasar lensing what are the likely relative distances?

You are right that the stars seen on the sky are within the Milky Way. Only with a large telescope is it possible to resolve individual stars in other galaxies, and only for the nearest ones.

I don't know which sources you refer to, by I think perhaps you are confusing the different types of gravitational lensing. I cannot explain them better than the excellent review by astromax, but briefly, there are three types:

1. Strong lensing, where are foreground galaxy cluster (i.e. a group of $sim$100-1000 galaxies) magnify and severely distort background galaxies,

2. Weak lensing, where clusters or individual galaxies distort the shapes of many background galaxies on the percent scale, which can only be seen statistically, and

3. Microlensing, where a single object inside the Milky Way happen to pass in front of another single object, also inside the Milky Way. These objects are usually stars or planets, and do not distort the images of the background objects, but merely increase the flux for a while. This effect has been used to find exoplanets.

Whereas type 1 and 2 are more or less static in a human lifetime, type 3 is an event that happens once only for a given set of stars (as seen from Earth).

## Scales

Strong and weak lensing happen on very large scales, from a few hundred million lightyears, up to the order of the size of the observable Universe (e.g. Wong et al. 2014). While the lenses themselves are cluster of galaxies, and thus a few to $sim10$ megaparsec across, the lenses object are typically individual galaxies. Gravitational lensing is most efficient when the lens is halfway between us and the background source.

Occurring in the Milky Way, microlensing, on the other hand, happen on the scale of a few kiloparsec, again with us-lens distance being of the same order as the lens-background object distance (see e.g. this Wikipedia list).

## Gravitational lensing

Can gravitational lensing from intervening galaxies cause the observed ripples in the Cosmic Microwave Background? (Advanced)
Could a different theory of gravity explain the dark matter mystery? (Intermediate)
How are planets detected around other stars? (Intermediate) .

Gravitational Lensing - NASA's Imagine the Universe
Gravitational Lensing - detailed descriptions from UBC
Observations of spacetime bending light - good descriptions with some math from Caltech
Gravitational Lensing tutorial
Einstein Rings
Einstein's rings in space
Wikipedia on "Old Faithful" .

Astronomers Use Gravitational Lensing to Measure Hubble Constant

Researchers Find Seven Isolated Clusters of Dwarf Galaxies .

- by Ricky Leon Murphy:
Introduction
A Gravitational Lens
The Gravity Lens in Use
Gravity Lens and Dark Matter - Microlensing
Gravity Lens and Dark Matter - Weak Lensing
Gravity Lens and Dark Matter - Strong Lensing
Summary
References
Web Sites
Image Credits .

The effect induced on the image of a distant object by a massive foreground object. Light from the distant object is bent into two or more separate images.

: Light around a massive object, such as a black hole, is bent, causing it to act as a lens for the things that lie behind it. Astronomers routinely use this method to study stars and galaxies behind massive objects.

by spherically symmetric lenses with angular momentum p. 393
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As we saw in Black Holes and Curved Spacetime, spacetime is more strongly curved in regions where the gravitational field is strong. Light passing very near a concentration of matter appears to follow a curved path.

effect of massive cosmic structures forming B-modes as it travels across the universe. (Credit: ESA) .

. The distortion or amplification of an object's light due to the presence of a massive object in the light path.

-- weak and strong
Ordinary lenses cause light rays to change their direction. If we design them correctly, we can bring different rays to a focus at a desired location.
Gravity can cause light rays to change their direction, too.

is the displacement of light due to the warping of space by a gravitational lens (a massive object in space that bends light that passes by it, due to the gravitational forces).
.

Another thing that is seen occasionally with quasars and other distant galaxies is one of the effects of Generally Relativity, the distortion of space due to massive objects. Let's say you have a bunch of galaxies in a cluster.

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Lenz's Law
The current induced by an electromotive force will appear in such a direction that it opposes the charge that produced it. [H76]
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A galaxy or other massive object standing between Earth and a more distant object. Its gravity bends the light from the distant object and creates distorted or multiple images of it. See the following two images.

Space WeatherClustersLow Frequency SurveysCosmologyCompact objectsActive Galactic NucleiRadio TransientsNearby galaxiesAstronomy Publications
RESEARCH & INNOVATION
Open Science Cloud
Science Data Centre .

of the light from distant galaxies and quasars by closer galaxies or galaxy clusters enables us to calculate the amount of mass in the closer galaxy or galaxy cluster from the amount of bending of the light. The derived mass is greater than the amount of mass in the visible matter.

in the Galaxy Cluster .
These Hubble telescope photographs show several quasars.
This is the Barred Sprial Galaxy, NGC 1365.

refers to the phenomenon of the light of the source being bent (or lensed) as it travels toward the person observing it.

The creation of a distorted image of a distant quasar or galaxy when its light is focused by the gravity of a galaxy between it and us.
gravitational microlensing .

" causes the light to appear to come from four different points instead of just one lone supernova. Norwegian astrophysicist Sjur Refsdal predicted this type of quadruple-lensed supernova 50 years ago.

to Measure the Mass of White Dwarfs
An international team of scientists has found a way to use white dwarf stars as gravitational lenses, which will allow astronomers to measure their mass.

When we observe some clusters, we see another effect predicted by Einstein, called strong

. Since Einstein predicted that massive objects can warp spacetime, he showed that the light from a background object will be bent if it passes by a massive object, like a galaxy cluster.

These are very faint sources which have been magnified by the

of a foreground cluster of galaxies, and even then barely detected with very long exposures on the Keck telescopes.

The team used data from PLANET (Probing Lensing Anomalies NETwork) and OGLE (Optical

It will use a technique called

, which relies on the gravity of stars and planets to bend and magnify the light coming from stars that pass behind them, from the telescope's viewpoint.

effect, which produces the arc-shaped structures.

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Capillary action
Capillary action, capillarity, capillary motion, or wicking refers to two phenomena:# The movement of liquids in thin tubes.

A concentration of matter such as a galaxy or cluster of galaxies that bends light rays from a background object.

results in duplicate images of distant objects.
Gravity
A mutual physical force of nature that causes two bodies to attract each other.

Dark matter also tugs on light as it passes, bending its path, a phenomenon called

. And now, by studying where that lensing appears in the sky, an international team of scientists have released a detailed, 3D map of dark matter.

Observations of the motions of stars and gas in galaxies, cluster galaxy radial velocities, hot gas properties of clusters, and

of distant, background galaxies by foreground galaxy clusters all suggest large amounts of Dark Matter exist.

Ostriker, J. P. and Vietri, M. "Are Some BL Lacs Artifacts of

In 1933 Zwicky inferred the presence of dark matter by observing that outlying members of the coma cluster were moving more rapidly than could be explained by the calculated mass of the cluster, and four years later he suggested that dark matter could be investigated via

Eksoplanet-eksoplanet MOA dan OGLE ditemukan berdasarkan teknik observasi khusus, yaitu pelensaan-mikro gravitasi, dalam proyek Microlensing Observations in Astrophysics (MOA) dan proyek Optical

In the above image, the VLA data (green) is superimposed on Hubble Space Telescope (HST) data of the Cloverleaf. The four images of the Cloverleaf galaxy are a result of

.
Adapted from the press release on .
New Understanding of Solar Fireworks .

Einstein Ring: A ring- or arc-like image created when light emitted from a distant source is bent by the gravitational influence of a massive foreground object. This effect is called

it allows astronomers to observe objects that would otherwise be too distant to study.

And while quasars are bright, astronomers also only see them because they are bright. The mass in foreground galaxies can act like eyeglasses, magnifying the quasar's image in what astronomers term

Microlensing event - The temporary brightening of a distant object that occurs because its light is focused on the Earth by the

Its rapid rotation would suggest that the quasar grew through a merger with another galaxy, rather than by pulling in space material which would likely result in a much slower rate of spin.

caused by an elliptical galaxy located between RX J1131-1231 and the Earth has created four images of the .

Einstein's theory predicts that the direction of light propagation should be changed in a gravitational field, contrary to the Newtonian predictions. Precise observations indicate that Einstein is right, both about the effect and its magnitude. A striking consequence is

Other telescopes detect X-rays, given off when neutron stars pull material into their gravity. A relatively new field of stellar astronomy involves

, in which space telescopes such as Hubble can observe incredibly distant stars through the natural magnifying effect of foreground galaxies.

Zwicky also uses the virial theorem to deduce the existence of unseen matter (what is now called dark matter) in the universe, as well as the effect of

## Seeing the Universe with Einstein’s Glasses

A survey of 13 million galaxies and 200,000 quasars uses Einstein's theory of gravity to confirm a dark side of the universe.

Light traveling billions of light years to reach us is bent by clumps of matter along its path. Albert Einstein, who is being remembered this year for his amazing scientific output from 1905, predicted this so-called gravitational lensing when he characterized gravity as the curvature of space-time.

Earlier attempts to measure the lensing of far-away quasars had failed to see an effect that matched the standard picture from cosmology - raising some doubts about the validity of that model. But an analysis of the Sloan Digital Sky Survey, released last week, puts an end to the controversy.

"This is exactly what is expected from gravitational lensing and the cosmological model," said Brice Menard of the Institute for Advanced Study, where Einstein worked until his death in 1955.

Specifically, these new results confirm that galaxies - which act as lenses for more distant quasars - are more than meets the eye. The light-emitting matter in the galaxies is not enough to account for their magnifying power. There has to be more mass in the form of dark matter.

"If there was no dark matter, [the researchers] wouldn't have measured the signal they saw," said Josh Frieman of the University of Chicago, who was not involved in the analysis.

According to the most recent estimates, there is about ten times more dark matter than the "normal" matter that makes up stars, gas, and planets.

In the future, Menard and his colleagues plan to use the gravitational lensing signal to map out exactly how the dark matter is distributed around galaxies.

One way to understand gravitational lensing is to picture space as a rubber sheet. The sheet is deformed, or curved, when something heavy, like a bowling ball, is placed on it. If something passes near by - like a rolling marble - the trajectory is deflected, or bent.

This deflection works on light as well. If we look towards a massive object, the light coming from behind may be focused or spread out. In this way, galaxies, which weigh billions of times our Sun, distort the background universe.

The most obvious examples of gravitational lensing are when the light from a quasar is broken up into multiple images, or when the shape of a galaxy is stretched out. These cases are deemed strong lensing.

"Strong lensing is very impressive - it's clear that something is going on," said Ryan Scranton of the University of Pittsburgh.

But Scranton, along with Menard and others, have sifted through the Sloan survey looking for a much more subtle distortion called weak lensing.

"Weak lensing is not as dramatic, but you see it everywhere you look," Scranton told Space.com. "Ultimately, it is more important for doing cosmology."

It is not possible to look at one object and say weak lensing is occurring. It has to be done statistically on a large sample of objects.

Weak lensing of galaxies by galaxies - called cosmic shear - was detected five years ago. The recent findings are for cosmic magnification, or the weak lensing of quasars by galaxies. This has been harder to detect, partly because there was not a large uniform sample of quasars until the Sloan survey came out.

For over six years, the Sloan survey has been methodically mapping out large tracts of the cosmos - identifying stars, galaxies, and quasars.

A quasar is a somewhat rare type of galaxy, which is made extremely bright by hot gas falling onto a huge central black hole. Quasars are often identified by their radio or X-ray emissions, but Scranton and his colleagues used a new method for picking them out in visible light using Sloan's five separate filters.

"The key to it is that we are taking full advantage of the information that is there," Scranton said. "We increased the number of quasars by a factor of four or five."

In a map that accounts for 10 percent of the sky, the researchers measured a change - due to weak lensing - in the number of background quasars around foreground galaxies. The quasars are about 10 billion light years away, while the galaxies are about 2 billion light years away.

This detection is made difficult by the fact that weak lensing causes two effects - magnification and shifting - that tend to cancel each other out.

When a galaxy sits near the line of sight to distant quasars, two things happen (see accompanying figure). The quasars are magnified to look brighter, and their apparent positions in the sky are shifted.

The magnification allows astronomers to see quasars that they would not have been able to see otherwise. In general, this increases the number of quasars visible around a foreground galaxy.

However, the shifting has the opposite consequence. It moves the apparent position of objects away from the lens - decreasing the number of quasars around a foreground galaxy.

"The two effects compete," Frieman said. "One wins some of the time, the other wins the rest of the time."

The net change in the number of quasars is less than one percent. With the large data set from Sloan, the research team observed an excess of bright quasars - but a deficit of faint quasars - in close vicinity of galaxy lenses. This matches predictions.

"Nobody before was able to report both the excess and deficit," Menard said. "Apart from gravitational lensing, it is difficult to get both things to happen."

This research is described in a paper accepted for publication in The Astrophysical Journal.

## Ep. 37: Gravitational Lensing

Astronomers are always trying to get their hands on bigger and more powerful telescopes. But the most powerful telescopes in the Universe are completely natural, and the size of a galaxy cluster. When you use the gravity of a galaxy as a lens, you can peer right back to the edges of the observable Universe.

### Shownotes

Relevant Back Episodes
Our archive is full of background information. Donâ€™t forget to check out these shows from the past!

The Search for Dark Matter Across the Electromagnetic Spectrum What We Learned from the American Astronomical Society The Story of Galaxy Evolution The Largest Structures in the Universe

Gravitational Lensing

– NASA’s Imagine the Universe – detailed descriptions from UBC – good descriptions with some math from Caltech

Telescopes and Projects

Optical Gravitational Lensing Experiment (This is the US mirror, see Warsaw page here) optical telescope concept – named after the objects it studies, MAssive Compact Halo Objects (Australian mirror here) – The Cosmological Evolution Survey

Press Releases

Scientific Papers

Extras: Interactive applets, programs, databases

– visualising gravitational lenses (Francisco Frutos Alfaro) (Mark Boughen) CfA-Arizona Space Telescope LEns Survey of gravitational lenses (gravitational lens database)

### Transcript: Gravitational Lensing

Fraser Cain: This is such a cool topic: here we go. Astronomers have always searched for larger and more powerful telescopes, but the most powerful telescopes in the Universe are completely natural, turning the mass of an entire galaxy into a lens that astronomers can look through. We’re talking about gravitational lenses, which let astronomers peer back into the earliest moments of the Universe.

Pamela, what’s a gravitational lens?

Dr. Pamela Gay: It’s basically this really neat way that the gravity of an object (a star, galaxy or cluster of galaxies) can work just like an optical lens to bend light. In this way, they can bend light that would otherwise go off in some other direction toward the Earth and increase the total amount of light from some distant object that we’re able to see.

Fraser: What’s the underlying principle that’s bending light here?

Pamela: There’s gravity! It’s one of those things that, when you start to realise energy and mass are two sides of the exact some coin, and that light is just energy, and gravity can cause that light to be deflected, to move the same way it can cause you and I to move, it’s possible to start using mass to focus light.

Fraser: So with a gravitational lens, you’ve got light from some more distant object passing some mass like a galaxy, and that mass is warping the space around it so the light follows a different trajectory and bends.

Pamela: A good way to think of it is, if you imagine that it goes: your nose, far, far away a galaxy, and even further back than that, a quasar, light from that quasar is going to be heading off in all directions filling a sphere. Some of that light would normally not just miss your nose and go above your head but miss your nose and hit a star somewhere above your head.

That light that would normally have gone up above you, as it grazes over the top of that galaxy between you and the quasar, it can get bent so that its new path brings it straight to the tip of your nose.

This also has the neat affect that if the alignments are just right, we can see two images of the exact same object. One is the straight view, and the other is seen reflected in a mirror, the same way you can take and use a mirror to look around a corner.

Fraser: Let’s see if I understand this: you’ve got a sphere of light coming out of the quasar, and some of that light is going to be passing very close to this galaxy and what would go in a straight line gets turned in a little or turned as it gets attracted toward the galaxy and so we here on Earth, that’s far down the path, see this light converging back on us because of this warping. So that’s why we see a magnified version of what’s behind it.

Pamela: In fact, the gravity can cause a bunch of different effects. It can distort the light from a background object, this is where you get galaxies that appear as strange arcs around Abell clusters. You can also get what’s called a microlensing event, which is where a background object appears to be a great deal brighter due to an intervening mass.

You can also get neat affects such as double quasars, quadruple quasars, Einsteinian rings, where the light from a background object is multiplied into multiple images or twisted into a ring where there once was just a single point-like object.

Fraser: You say these are wonderful things to look at, but wouldn’t a telescope manufacturer be trying to grind the mistakes out of the mirror? If they saw this kind of stuff?

Pamela: In a real telescope, you really don’t want your telescope to produce fun house images. The reality is that looking at galaxies through gravitational lenses is sometimes just as distorting as if you look through the old deformed glass in extremely old houses, or if you are looking at yourself reflected in a carnival glass. But, we’re allowed to see things we can’t otherwise see, and sometimes the stuff that we’re seeing is invisible stuff, like when the gravitational lenses are made out of dark matter.

Fraser: So I guess the astronomers are going to take what they can get. They don’t have a telescope that powerful, so the fact that there’s one naturally out there that does provide a bit of a distorted image but still allows you to look much further backâ€¦ how much further back can they see?

Pamela: The very most distant galaxy that has ever been detected was found using an Abell cluster to gravitationally lens a background galaxy.

Fraser: So this is an Abell cluster, an intervening cluster of galaxies where it was able to focus the light from this more distant galaxy.

Pamela: In this case it was Abell 2218, and back in 2004 they were able to get a redshift to measure the recession rate of a smear of light that they were able to detect because it was being magnified, it was being lensed by the gravity of this giant cluster of galaxies.

Fraser: So theoretically, how far back could astronomers push this technique?

Pamela: It’s all a matter of how good are we at taking a spectra of a smear of light. If the alignments are right, you can perhaps get multiple galaxy clusters that are gravitationally lensing an object multiple times that, with all of this combined lensing, allows us to look back to objects (that we currently can’t see using existing telescopes) that were formed at the very edge of the Universe in the moments right after the Cosmic Microwave Background was formed. We haven’t found those things yet, but the potential is there, as we look at the smears of light.

Fraser: I guess with a more powerful telescope or with a luckier alignment of foreground object and background object, we might find some of those first objects.

Pamela: We’re also finding objects within our own galaxy (that we can’t find any other way) using gravitational lensing. There was actually a planet, it had a truly terrible name: OGLE 2005-BLG390 (that’s the parent star). It has a planet going around it that we found because the star and the planet gravitationally microlensed a background object and we were able to see the mass of both the star and the planet as the background star was lensed.

Fraser: So we’re just talking about a star here, not a galaxy. This was two stars lining up in our Milky Way and we happened to be in the exact right spot to see the line up.

Pamela: What’s neat in this case is as you have the foreground star passing in front of a background object, we can’t see that star. This was a little red dwarf, too far away for us to be able to see with our telescopes because it just doesn’t give off a lot of light. As it orbited in front of a background object, the background object was something bright enough we can see it every day.

That background object suddenly increased in brightness in a away that isn’t characteristic of a nova or a flare event or any other normal brightening. It increased in brightness in a perfectly symmetrical way that indicated that an object was passing in front of it and then moving out from in front of it at a constant velocity.

In the process of doing this, there was a little blip on the side of that increase and decrease in brightness. That blip corresponded to the planet getting in on the act of microlensing the background light. We were able to find what we think was a rock or an icy planet (one of the smaller mass planets that have been discovered), because of this microlensing event.

Fraser: This is a once in a lifetime opportunity, to see this star and its planet, because you need that line-up, so unless it lines up with another star that we know of, we’ll never see it again.

Pamela: This was a roughly 13-Earth mass planet that we have a observation of, and we have a observation of its star, but still it’s cool!

Fraser: Right, but there’s no chance for follow up observations.

Pamela: Not with current technologies. This is where you wait for the OWL telescopes and the other freakishly large telescopes astronomers are planning to build.

Fraser: I recall it was quite far away, it was like tens of thousands of light years away.

Pamela: It was a star out on the edge of our galaxy, but it’s a new way to get at data in places that we otherwise can’t observe.

Fraser: What’s the process for this the, are astronomers watching stars to see them brighten like that?

Pamela: There are two different projects: OGLE and MACHO. These two programs are regularly looking at certain areas of the sky night after night after night waiting for microlensing events. What they do is take picture after picture of the same region, and as they take these pictures they subtract them off of a previous night’s images and look to see what is different.

In the process of finding these differences, sometimes they’re actually discovering variable stars like the Cepheids and RR Lyraes that I like to study. Sometimes they’re finding nova, sometimes they’re actually finding things like supernova light echoes that are moving through these regions of space. Really cool science.

They’re also (unfortunately at lower events) finding microlensing events. They’re finding lots and lots of RR Lyrae stars, lots and lots of other variable stars. Occasionally, out of the noise, they find these microlensing events that indicate there’s a dark object (a white dwarf, a brown or red dwarf, a neutron star, something that we otherwise can’t see) out in the outskirts of our galaxy, plugging along occasionally lensing light from perhaps the Large Magellenic Cloud stars, perhaps background objects. It’s these lensing events that are allowing us to get a sense of how much of the dark matter in the galaxy is made up of perfectly normal stuff that we just otherwise can’t see.

All the dark matter in the galaxy could be accounted for if there was roughly one ACME brick per solar system volume of space. If that were true, we wouldn’t be able to see out of the galaxy really well. It’s important to find the actual ACME bricks that are out there (which tend to be shaped more like brown dwarfs) and this is one way of doing that.

Fraser: Now, earlier you talked about how the larger gravitational lensing is helping astronomers map dark matter distribution. Can you go into that in some more detail?

Pamela: There was actually a really, really neat Hubble result that just came out. If you havenâ€™t read about it yet, go over and look at our friend the Bad Astronomer’s website. Hubble basically found a smoke ring of dark matter around a galaxy cluster, CL0024+1652.

What they do is, they look at background objects. They assume in this little tiny region on the sky, I have 100 background galaxies. These galaxies are going to have random orientations, random shapes. If I average together all these galaxies’ shapes, they should average out to perfect little circles on the sky. But, if there’s matter between me and those background galaxies, that matter is going to cause all of them to be systematically twisted a little bit, the same way as if all of them were reflected in the same carnival house mirror.

So we look for those slight twists, those slight ellipticities, the slight teardrop shapes that crop up in the background galaxies. When we find these irregularities in their shape, the deviations from being little tiny circles, then we know there’s dark matter and we can map the distribution of the dark matter by reverse engineering what was necessary to make these galaxies not average out to a little disk.

Fraser: So in this case, we don’t have a galaxy in front of another galaxy, we have this invisible dark matter that’s acting as this gravitational lens, distorting the image from the background galaxy.

Pamela: What’s really cool is this dark matter is forming a donut (one of my favourite shapes apparently) around the cluster of visible galaxies. This is the type of thing that can happen when there’s a collision between two systems. You shock the system, one passes through the other and you end up with a ring of material. We’ve seen this in individual galaxies before and after collisions, but now we’re seeing it in an entire cluster and it’s not just the material of the cluster, it’s the dark matter itself that forms the donut. That’s just really cool. We’re not used to thinking of dark matter as actually forming structures,(at least not forming structures on this type of scale), and it’s a really neat, really hard to do discovery.

Every day we’re learning more about the distribution of dark matter. Back in January, after the AAS, we actually reported here on this show about the COSMOS project, and how they’d mapped out the large scale structure of the Universe to find that the structures of the luminous matter generally fell within the structures of dark matter, but didn’t necessarily have precisely the same centres.

Fraser: So I guess this was the same technique: they looked everywhere, looking for that distortion, and then carefully mapped it back to figure out where all the dark matter was.

Pamela: They mapped a fairly significant area on the sky, and they built a 3-D model of dark matter using gravitational lensing of galaxies at various distances away from us. Again, really hard to do, really good, solid science. We don’t know what dark matter is, but every day we’re getting a better and better map of where it is.

We can also use dark matter to sometimes get to repeat our ability to observe specific events. There are quasars out there that have been gravitationally lensed in such a way that when you’re looking at the sky, you see two identical objects that are separated by a few arc seconds to more than a few arc seconds on the sky. This means that you can go out, look directly at the quasar or you can look at the lensed version of the quasar.

The first one of these was actually given the name, Old Faithful (or scientifically, Q0957+561).

Fraser: I like “Old Faithful”

Pamela: Yeah, I like “Old Faithful” too.

We can’t name things well in astronomy. I admit to this fully.

Fraser: There’s too many objects.

Pamela: Yeah, yeah it’s kind of hopeless. But what’s cool about this object is you have two quasars that are far enough apart that any good telescope can clearly resolve them. The two light paths, the one to look directly at the quasar, and the one to look at the gravitationally lensed quasar (where the light has already started to head off somewhere else and then deviates and comes back to us), it’s a difference of over a year.

So if you catch the tail end of the quasar doing something cool (and quasars actually flicker and do neat things on short time-scales indicating stuff going on with the supermassive hole in the centre), if you only catch the tail end of an event, you just go back a year later and watch it occur in the lensed version of the quasar.

Pamela: You don’t get to repeat observations very often. This is like the only way you can get to get a second try at getting your data.

Fraser: It’s like a TiVo for the Universe

Pamela: Exactly, it just requires the mass to be in just the right place.

Fraser: That’s amazing. Are there any other places where gravitational lenses come into astronomy?

Pamela: The primary neat places for them are: looking at these quasars where you get multiple images mapping dark matter using them to zoom in on objects at high redshifts and using them to zoom in on little objects (well, not zoom inâ€¦ using them to detect little tiny objects) in the outer part of the galaxy. These are the main directions, but then there’s also some nifty science that comes out of this just in terms of using theory to do funny things.

There was a scientist down the hallway from me at the University of Texas. Hugo, Hugo Martel. Great Canadian from Quebec. He figured out what distribution of matter would be required to create a lensed image that looked like a smiley face. It’s just a great abuse of science, but that’s the neat thing: you can take a perfectly normal quasar, with a perfectly normal, nice, happy, “I’m a disk” light and twist its light with intervening matter in ways that you can create arcs, in ways you can create smiley faces and all sorts of other neat patterns.

In the process of figuring out what distribution of mass is necessary to make a smiley face, he was able to also figure out what is needed to reverse engineer the distribution of mass between here and there so that when we do find these things that look like waves on the ocean, when we find these things that look like a three-year-old’s version of drawing a seagull, we know what mass is required to get to that observed image.

Fraser: I guess that was my question, as you said earlier on, when astronomers look through telescopes they see these distortions, these fun house mirror images, which in some cases is great because you get a chance to see something and not nothing, but are there techniques to try and reverse engineer the light to try and get a better sense of what the object is? Could there be a day either now or in the future when astronomers can use these lenses and actually rebuild a spiral galaxy image as opposed to a smear around the outside of a galaxy?

Pamela: We’re already there, at a certain level. Just as we had to figure out how to build a corrector for the Hubble Space Telescope based on the observed distortions in the early images, we have also figured out how to mathematically figure out how to get back at the original shape of these distorted galaxies.

What they do is say, “we have these 100+ galaxies that should average out to a nice polite circle on the sky. They don’t.” and then they do the trials. They do the simulations, to figure out where do I need to stick mass in the volume of space between me and these galaxies, to get the perhaps teardrop shape. Once you’ve figured that out, you can reverse engineer the path of the light to get at the original shape. It’s really cool to look at some of these simulations.

With the COSMOS team, they can actually trace the pathway for a beam of light that gets lensed multiple times as it passes from high redshift galaxies to the modern epic. You can see it get bent over and over as it zigs and zags, getting bent by multiple intervening blobs of mass. It’s a maze out there, and the light is forced to run this gauntlet of material because gravity bends light.

Fraser: Now does this technique work across the entire spectrum, does it work from radio waves all the way to gamma rays?

Pamela: Gravity bends everything. There are people, in fact, out there looking to see how gravitational lensing affects our views of the Cosmic Microwave Background. So we’re looking at this in microwaves, we’re looking at this in optical light and infra red light. We’re looking across all the spectrum, trying to understand what is it that we can use this really great artifact of mass and energy being the same thing, to figure out about our Universe.

Fraser: Now there’s one piece of terminology I wanted to talk about. I’ve done a couple stories on this, which are called Einstein rings.

Fraser: I know they have to do with gravitational lenses. Can you explain what those are?

Pamela: This is the neat situation where you get a perfect alignment between us and a distant galaxy or let’s use a quasar (because quasars are neat little point sources).

Fraser: And quasars are the actively feeding supermassive black holes at the hearts of galaxies, right?

Fraser: pouring out tonnes of energy.

Pamela: So you basically have the very centre part of a galaxy pouring out gobs and gobs of light such that an active galaxy that is billions of light years away â€“ so far away that the disc of the galaxy is extremely hard to observe with the largest telescopes â€“ the very centre, the active part, is just the brightness of a normal, faint star. They’re really powerful, fascinating things.

Now, if you take one of these (and they exist in the largest numbers in the early parts of the Universe, when there was just more stuff for central supermassive black holes to be eating). If you look at one of these in the distant Universe and plot a concentration of mass exactly on the line of sight between us and them (so it’s a perfect, straight line: our telescope, the lensing object, the background quasar). The lensing object is going to block the light thatâ€™s trying to get straight at us from what’s being lensed, but the light that’s trying to go above, below, left, right, diagonalsâ€¦ the light that’s trying to go in a perfect ring off in other directions away from us, all that light is going to get bent toward us. If it doesn’t make it all the way into focus, if it doesn’t make it all the way down to a single point before it reaches us, we’ll see that light that’s getting bent as a ring.

Fraser: Is this a temporary situation, will this Einstein ring last for years or could we be anywhere inside the Milky Way and still see it?

Pamela: This type of gravitational lens, made up of quasars at large redshifts and galaxy clusters (or other large-mass objects) at moderate redshift distances, hereâ€¦ human life-scales, not seeing any motion going on. But on cosmic timescales, everything in the Universe is in motion, everything changes, some day those particular Einsteinian rings are going to lose their alignment, but other ones will step forward to take their place.

Fraser: And being an astronomer focussed on this is all about being at the right place at the right time.

Pamela: Well, the whole concept that we’re never really at the exact right place at the right timeâ€¦ this type of thing is always out there, we’re just at the right time for this one particular Einstein ring.

Fraser: Right. Great, I think that covers the concept. I think, astronomers who need to go bigger are just going to have to go out and find themselves a galaxy cluster to look through.
[laughter]

Pamela: Sounds like a plan.

This transcript is not an exact match to the audio file. It has been edited for clarity.

## Gravitational Lensing

We all know what is GRAVITY, but what about gravitational lensing? Is it something related to gravitation? Why is it so much important in Astronomy? How gravitational lensing is observed and what are all can be done with this? Have some patience people this is what we are going to discuss in this section.

The idea of gravitational lensing was first mentioned by our famous scientist Einstein in an unpublished paper in 1912. But the official people with published papers are Orest Khvolsen in 1924 and Frantisek Link in 1936. Still, most of the things are discussed by Einstein with the published paper in 1936. The basic concept of bending of light was mentioned by him in his General Theory of Relativity. Fritz Zwicky said that Gravitational lensing can be done high mass large-scale structures in the Universe. In 1916, Einstein published his General Theory of Relativity with full mathematical calculations. According to General relativity, masses deflect light in a way similar to convex lenses. It gives rise to image distortions.

Then comes Arthur Stanley Eddington, who plans to prove Einstein’s theory of bending of light through the solar eclipse that happened on 29th May 1919. The expeditions aimed to measure the gravitational deflection of starlight passing near the sun. He did his measurements from the West African island Principe and the other place is the Brazilian town of Sobral. Then he published his results in the Royal Society of London which proved the bending of light is possible if a massive object is placed in front of its path. This proved facts brought name and fame for Einsteins General Theory of Relativity.

Well, in some senses we can say that Gravitational lensing is happening because of gravity. The more the gravity or mass in an object (assume galaxy clusters) the more is the gravitational lensing. As Fritz mentioned this effect was proved in 1979 by observing “The Twin Quasar” (SBS0957+561). This paved the way for observational astronomy. Due to gravitational lensing, the light from the distant source gets diverted and looks like the light source is coming from different places, this kind of form a ring called Einstein ring. It is nothing but the “Halo effect” of gravitation when the source, lens, and observer are in near-perfect alignment.

Do you believe me if I say that it is because of gravitational lensing we can observe the universe that is a few billion light-years away from us? Well, you have to. The high mass clusters, distant big galaxies and few other heavily mass objects in the space bends light, as mentioned by Einstein, and act as a lens so that we can observe these distant galaxies and planets. In a sense, you can say this as a natural lens but at a bigger size than our Milkyway. Without gravitational lensing it is impossible to observe and study about these distant galaxies.

Lensing is of three types:

1. Strong lensing: This happens when we happened to notice strong distortions in the lensing such as multiple images, arcs, and Einstein rings. Galaxy clusters form this type of lensing. Strong lensing is observed in radio and x-ray regimes.
2. Weak lensing: This happens when we notice distortions in the background images are smaller and can only be detected by analyzing different kinds of sources to find coherent distortions. This shows us the stretched background image perpendicular to the direction to the center of the lens. This may also provide an important constraint on dark energy in the future.
3. Microlensing: When we observe no distortions in shape and the amount of light received from a background object changes with time. The lensing objects can be stars in the Milkyway and even some distant quasars.

We all know that our universe has many wonders in its sleeves. Observing gravitational lensing is the first step towards exploring its other wonders. Gravitational lensing helped us in many ways such as observing distant galaxies, clusters, and study about them.

Scientists from all around the world are trying to unlock most of the mysteries in the Universe. To understand more about Gravitational lensing check out the book of Einstein’s telescope by Evalyn Gates which I have reviewed before in one of my posts.

## Gravitation Lensing Imagery of the Brightest Infrared Galaxies

Boosted by natural magnifying lenses in space, NASA's Hubble Space Telescope has captured unique close-up views of the universe's brightest infrared galaxies, which are as much as 10,000 times more luminous than our Milky Way.

The galaxy images, magnified through a phenomenon called gravitational lensing, reveal a tangled web of misshapen objects punctuated by exotic patterns such as rings and arcs. The odd shapes are due largely to the foreground lensing galaxies' powerful gravity distorting the images of the background galaxies. The unusual forms also may have been produced by spectacular collisions between distant, massive galaxies in a sort of cosmic demolition derby.

"We have hit the jackpot of gravitational lenses," said lead researcher James Lowenthal of Smith College in Northampton, Massachusetts. "These ultra-luminous, massive, starburst galaxies are very rare. Gravitational lensing magnifies them so that you can see small details that otherwise are unimaginable. We can see features as small as about 100 light-years or less across. We want to understand what's powering these monsters, and gravitational lensing allows us to study them in greater detail."

The galaxies are ablaze with runaway star formation, pumping out more than 10,000 new stars a year. This unusually rapid star birth is occurring at the peak of the universe's star-making boom more than 8 billion years ago. The star-birth frenzy creates lots of dust, which enshrouds the galaxies, making them too faint to detect in visible light. But they glow fiercely in infrared light, shining with the brilliance of 10 trillion to 100 trillion suns.

Gravitational lenses occur when the intense gravity of a massive galaxy or cluster of galaxies magnifies the light of fainter, more distant background sources. Previous observations of the galaxies, discovered in far-infrared light by ground- and space-based observatories, had hinted of gravitational lensing. But Hubble's keen vision confirmed the researchers' suspicion.

Lowenthal is presenting his results at 3:15 p.m. (EDT), June 6, at the American Astronomical Society meeting in Austin, Texas.

According to the research team, only a few dozen of these bright infrared galaxies exist in the universe, scattered across the sky. They reside in unusually dense regions of space that somehow triggered rapid star formation in the early universe.

The galaxies may hold clues to how galaxies formed billions of years ago. "There are so many unknowns about star and galaxy formation," Lowenthal explained. "We need to understand the extreme cases, such as these galaxies, as well as the average cases, like our Milky Way, in order to have a complete story about how galaxy and star formation happen."

In studying these strange galaxies, astronomers first must detangle the foreground lensing galaxies from the background ultra-bright galaxies. Seeing this effect is like looking at objects at the bottom of a swimming pool. The water distorts your view, just as the lensing galaxies' gravity stretches the shapes of the distant galaxies. "We need to understand the nature and scale of those lensing effects to interpret properly what we're seeing in the distant, early universe," Lowenthal said. "This applies not only to these brightest infrared galaxies, but probably to most or maybe even all distant galaxies."

Lowenthal's team is halfway through its Hubble survey of 22 galaxies. An international team of astronomers first discovered the galaxies in far-infrared light using survey data from the European Space Agency's (ESA) Planck space observatory, and some clever sleuthing. The team then compared those sources to galaxies found in ESA's Herschel Space Observatory's catalog of far-infrared objects and to ground-based radio data taken by the Very Large Array in New Mexico. The researchers next used the Large Millimeter Telescope (LMT) in Mexico to measure their exact distances from Earth. The LMT's far-infrared images also revealed multiple objects, hinting that the galaxies were being gravitationally lensed.

These bright objects existed between 8 billion and 11.5 billion years ago, when the universe was making stars more vigorously than it is today. The galaxies' star-birth production is 5,000 to 10,000 times higher than that of our Milky Way. However, the ultra-bright galaxies are pumping out stars using only the same amount of gas contained in the Milky Way.

So, the nagging question is, what is powering the prodigious star birth? "We've known for two decades that some of the most luminous galaxies in the universe are very dusty and massive, and they're undergoing bursts of star formation," Lowenthal said. "But they've been very hard to study because the dust makes them practically impossible to observe in visible light. They're also very rare: they don't appear in any of Hubble's deep-field surveys. They are in random parts of the sky that nobody's looked at before in detail. That's why finding that they are gravitationally lensed is so important."

These galaxies may be the brighter, more distant cousins of the ultra-luminous infrared galaxies (ULIRGS), hefty, dust-cocooned, starburst galaxies, seen in the nearby universe. The ULIRGS' star-making output is stoked by the merger of two spiral galaxies, which is one possibility for the stellar baby boom in their more-distant relatives. However, Lowenthal said that computer simulations of the birth and growth of galaxies show that major mergers occur at a later epoch than the one in which these galaxies are seen.

Another idea for the star-making surge is that lots of gas, the material that makes stars, is flooding into the faraway galaxies. "The early universe was denser, so maybe gas is raining down on the galaxies, or they are fed by some sort of channel or conduit, which we have not figured out yet," Lowenthal said. "This is what theoreticians struggle with: How do you get all the gas into a galaxy fast enough to make it happen?"

The research team plans to use Hubble and the Gemini Observatory in Hawaii to try to distinguish between the foreground and background galaxies so they can begin to analyze the details of the brilliant monster galaxies.

Future telescopes, such as NASA's James Webb Space Telescope, an infrared observatory scheduled to launch in 2018, will measure the speed of the galaxies' stars so that astronomers can calculate the mass of these ultra-luminous objects.

"The sky is covered with all kinds of galaxies, including those that shine in far-infrared light," Lowenthal said. "What we're seeing here is the tip of the iceberg: the very brightest of all."

## Astronomers Use Gravitational Lensing Effect to Detect Intermediate-Mass Black Hole

An artist’s impression of an intermediate-mass black hole. Image credit: Sci-News.com.

Astronomers know that stellar-mass black holes — compact ranging from about 10 times to 100 times the Sun’s mass — are the remnants of dying stars, and that supermassive black holes — those with masses over 100,000 times the mass of the Sun — inhabit the centers of most galaxies.

But scattered across the Universe are a few apparent black holes of a more mysterious type.

Ranging from 100 to 100,000 solar masses, these intermediate-mass black holes have long been posited to reside in the cores of globular clusters. Yet direct observational signatures of their existence are elusive.

“The newly-discovered black hole could be an ancient relic — a primordial black hole — created in the early Universe before the first stars and galaxies formed,” said Professor Eric Thrane, an astronomer in the School of Physics at Monash University and the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav).

“These early black holes may be the seeds of the supermassive black holes that live in the hearts of galaxies today.”

“The discovery sheds new light on how supermassive black holes form,” said James Paynter, a Ph.D. student in the School of Physics at the University of Melbourne.

“While we know that these supermassive black holes lurk in the cores of most, if not all galaxies, we don’t understand how these behemoths are able to grow so large within the age of the Universe.”

The new intermediate-mass black hole was detected thanks to GRB 950830, a gravitationally lensed short burst of gamma-rays.

The gamma-ray burst, emitted by a pair of merging stars, was observed to have a tell-tale ‘echo.’

This echo was caused by the intervening intermediate-mass black hole, which bent the path of the light on its way to Earth, so that astronomers see the same flash twice.

Powerful software developed to detect black holes from gravitational waves was adapted to establish that the two flashes are images of the same object.

“Our findings have the potential to help scientists make even greater strides,” said Professor Rachel Webster, also from the School of Physics at the University of Melbourne.

“Using this new black hole candidate, we can estimate the total number of these objects in the Universe.”

“We predicted that this might be possible 30 years ago, and it is exciting to have discovered a strong example.”

The findings appear in the journal Nature Astronomy.

J. Paynter et al. Evidence for an intermediate-mass black hole from a gravitationally lensed gamma-ray burst. Nat Astron, published online March 29, 2021 doi: 10.1038/s41550-021-01307-1

## Gravitational Lensing in the Canary Islands

The 10.4m Gran Telescopio Canarias (GTC) on La Palma--the largest optical telescope in the world!

Every November the Instituto de Astrofísica de Canarias (IAC) hosts a winter school on a particular astronomical topic in Tenerife, Spain, part of the Canary Islands. I attended this year’s school and got a two week crash course on “The Astrophysical Applications of Gravitational Lensing” from experts in the field while hanging out on a volcanic island with about 50 postdocs and graduate students from all over the world. In addition to daily lectures, the IAC organized two days of excursions to the local observatories–one on Tenerife and the other on a neighboring island, La Palma, home of the Gran Telescopio Canarias. The lectures helped me better understand my own research and opened my eyes to the many uses of gravitational lensing, some of which I didn’t even know existed before the school! Here I’ll outline a few of the ways astronomers use gravitational lensing to study the Universe.

The basics of gravitational lensing

A horseshoe-shaped image behind a single red galaxy. Photo credit ESA/Hubble and NASA.

First off, what exactly is lensing? Einstein’s theory of general relativity naturally predicts gravitational lensing. In general relativity, mass curves spacetime, which means that massive objects will deflect all things that pass by them–including light rays. This means they can focus those rays similarly to a piece of glass. The distortion of spacetime allows a single beam of light from a distant source to reach us from multiple directions thus, background images will be stretched out or split into multiple images if they lie behind a massive object. To visualize the effect, picture looking at a marble through the bottom of a wine glass, as shown in this video (original link here). When the marble (the background object) nears the center of the glass (the lens), the image is stretched all the way around the stem, forming a ring. We see strikingly similar “Einstein rings” in nature: background galaxies which are stretched into giant arcs encircling foreground galaxy clusters.

Neat, right? But why use lensing for astronomy? Recall that the pie chart of mass-energy in the Universe shows 23% dark matter, 72% dark energy, and a mere 5% baryonic (normal) matter. So far only the baryonic matter has been confirmed to emit light detectable by our telescopes (although dark matter may emit gamma rays). Gravitational lensing can probe both dark matter, which only interacts via gravity, and dark energy, which is making the Universe expand at an accelerating rate. What’s more, lensing acts on a remarkable range of scales from extrasolar planets that magnify stars in our own galaxy to galaxy clusters that magnify the most distant quasars. It would be impossible to tell you about all of the applications of lensing in a single post, so I’ll just sample a few of them below.

Finding exoplanets

Closest to home we can use gravitational lensing to search for extrasolar planets. One star can lens another star as the foreground star passes in front of the background star, it will briefly split the background star’s image into several tiny images that will blur together into a single, brighter image. When this happens we say the background star is microlensed. If the foreground star has a planet it will make its own contribution to the microlensing event, detectable as a single sharp spike in the brightness of the background star as the planet passes closest to the star’s line of sight. In contrast to other exoplanet detection methods which preferentially find exoplanets that live close to their host stars, microlensing is sensitive to planets near the frost line, where Jupiter-like planets form, and to free-floating exoplanets, or exoplanets that have been kicked out of their home stellar system. The microlensing events are bright enough to see with a small telescope, but before you run out to your backyard I should caution that they are also extremely rare. Even after monitoring many, many stars for several years, microlensing surveys like OGLE have only found about a dozen exoplanets this way.

The Bullet Cluster: the poster child for dark matter. Red shows the hot, x-ray emitting gas blue shows the mass distribution from weak lensing. (Image credit: NASA/CXC/CfA/STScI)

Weighing the lens: galaxies and galaxy clusters

Farther away, we can model the masses of galaxies and galaxy clusters that lens background galaxies. This is the domain of macrolensing, lensing that remains constant in time and acts on scales large enough to be resolvable with current telescopes. Strong lensing occurs when a background and foreground galaxy lie close to the same line of sight, causing the background galaxy to appear hugely elongated and split into multiple images. Strong lensing puts excellent constraints on the masses of the foreground lensing galaxies – which are, like all galaxies, mostly dark matter. In special cases irregularities in an image can reveal dark matter substructure or a satellite galaxy: if the lens contains dense clumps, part of a lensed image may be more or less magnified than expected. Weak lensing, by contrast, occurs for background galaxies that lie farther from the line of sight of the lens instead of a dramatic effect, many small distant galaxies are slightly distorted. This technique can yield the total masses of galaxy clusters. Note that for weak lensing, we must average over many background galaxies since individual galaxies come with their own intrinsic shapes. Weak lensing can map out the distribution of mass inside a galaxy cluster, and the weak lensing maps of individual clusters like the Bullet Cluster provide the best evidence for dark matter to date: when two clusters collide, the majority of the baryonic matter–the hot gas–feels the impact, while the rest of the material–dark matter and individual galaxies–travels straight through. This results in the baryonic and dark matter being temporarily separated in space, and we can use weak lensing to show that the majority of the mass in the cluster is not in the same place as the baryonic matter. Astronomers also use the results of weak lensing surveys to compare the distribution of cluster masses to cosmological simulations in order to test our theories of structure formation.

Four images of a quasar surround a lens galaxy. (Image credit: NASA/ESA)

A look at the sources: galaxies and quasars

In addition to analyzing the lens itself, we can study the objects behind the lens. Just as a telescope lets us study a star too dim for the naked eye to see, lensing galaxies and galaxy clusters act as giant telescopes that can magnify background galaxies by tens to hundreds of times their original brightness, providing a window into the early Universe. (This is my research!) Multiply-imaged quasars are also rewarding to study, if a bit more complicated. A quasar is the central engine of an active galaxy, an incredibly bright source of energy powered by accretion of material onto a supermassive black hole. This means that, although it is very powerful, a quasar has a very small physical size compared to its host galaxy. The behavior of a microlensing event depends on the size of the background source: the larger the physical extent of the source, the more the signal of a microlensing event is smeared out in time, as the foreground lens passes across the object and magnifies pieces of it bit-by-bit, instead of all at once as it would a point source. A quasar behind a single galaxy will not only be strongly macrolensed by the lens galaxy as a whole, but since the quasar has a small angular size, the individual images will also be microlensed by stars within the lens galaxy as it slowly drifts in and out of alignment with them. Thus, when astronomers model the changes in brightness due to microlensing stars within the lens, they can deduce the size of the lensed quasar accretion disk: a bigger disk will show a smaller microlensing signal. Trying to measure this size without lensing would be like trying to resolve individual grains of regolith on the Moon: basically impossible with current technology.

A snapshot of the "cosmic web," the large-scale structure of filaments and voids that fills the Universe, from the Millennium Simulation (Springel et al. 2005).

Dark energy from cosmic shear

As light rays travel through the Universe they will be deflected by any mass they encounter along the way. The weak lensing distortions felt by the images of galaxies as they travel through the large-scale structure of the Universe is known as cosmic shear. We can quantify the distribution of mass in the universe by mapping out cosmic shear and counting up the strength of density fluctuations of different sizes. This is analogous to the study of the cosmic microwave background, except that the cosmic microwave background shows us the initial density fluctuations at recombination (soon after the Big Bang), while cosmic shear shows us how the original fluctuations developed up through the present day. Cosmic shear is thought to be the most promising way to measure dark energy and get a handle on how quickly the expansion of the Universe is accelerating.

Phew! I hope I’ve convinced you that gravitational lensing can do quite a lot. Watch out for more astrobites on these and other applications of gravitational lensing as we go forward into the bright, lensed future.

If gravitational waves weren't affected by static gravitational fields then you'd be able to tell the difference between "being stationary in a gravitational field" and "accelerating in a rocket in the absence of gravity". That would violate the equivalence principle, which is a founding principle of GR (at least, when formalised somewhat), so it'd be self-contradictory.

To explain - imagine that I'm in a box and feel proper acceleration. I place a source of electromagnetic waves on the floor and a detector on the ceiling. The waves are redshifted when detected. If I'm sitting on a planet, this is due to gravitational redshift. If I'm in a rocket in free space, this is because the waves take time to reach the ceiling and in that time the ceiling has accelerated a little and the waves are redshifted from velocity related Doppler. And if you follow through the maths you find that the amount of redshift is exactly the same, given equal "acceleration due to gravity" in the two situations. Thus I can't use this experiment to break the equivalence principle.

Now replace the EM source and sensor with a gravitational wave source and sensor (I'll leave the implementation details as an exercise for the reader, but this is possible in principle). In the rocket case the waves will be Doppler shifted for the exact same reason as the EM waves - the ceiling has accelerated by the time they are received. So in the sitting-on-a-planet case either the gravitational waves are gravitationally redshifted or I can tell the difference between the two situations in a closed-box experiment and GR is inconsistent. Also, if they aren't redshifted, I could get free energy by shining light down, gaining energy as it falls, and using that energy to power my gravitational wave source, which emits waves that don't lose energy as they climb back up, and using that energy to power my light source and skimming the extra.

Edit: if you find a gravitational wave source in a small box far-fetched, imagine a cosmological source that just happens to emit waves that pass through my box in an upwards direction, and have floor and ceiling mounted detectors.

If gravitational waves weren't affected by static gravitational fields then you'd be able to tell the difference between "being stationary in a gravitational field" and "accelerating in a rocket in the absence of gravity". That would violate the equivalence principle, which is a founding principle of GR (at least, when formalised somewhat), so it'd be self-contradictory.

To explain - imagine that I'm in a box and feel proper acceleration. I place a source of electromagnetic waves on the floor and a detector on the ceiling. The waves are redshifted when detected. If I'm sitting on a planet, this is due to gravitational redshift. If I'm in a rocket in free space, this is because the waves take time to reach the ceiling and in that time the ceiling has accelerated a little and the waves are redshifted from velocity related Doppler. And if you follow through the maths you find that the amount of redshift is exactly the same, given equal "acceleration due to gravity" in the two situations. Thus I can't use this experiment to break the equivalence principle.

Now replace the EM source and sensor with a gravitational wave source and sensor (I'll leave the implementation details as an exercise for the reader, but this is possible in principle). In the rocket case the waves will be Doppler shifted for the exact same reason as the EM waves - the ceiling has accelerated by the time they are received. So in the sitting-on-a-planet case either the gravitational waves are gravitationally redshifted or I can tell the difference between the two situations in a closed-box experiment and GR is inconsistent. Also, if they aren't redshifted, I could get free energy by shining light down, gaining energy as it falls, and using that energy to power my gravitational wave source, which emits waves that don't lose energy as they climb back up, and using that energy to power my light source and skimming the extra.

Edit: if you find a gravitational wave source in a small box far-fetched, imagine a cosmological source that just happens to emit waves that pass through my box in an upwards direction, and have floor and ceiling mounted detectors.

Oh, I hadn't noticed that you decided to try me with a more in-depth explanation after all. That actually makes perfect sense. Thank you. Not so farfetched after all then. The null-path following I can wrap my head around.

## Acknowledgements

It is a pleasure to thank Wes Colley, Frederic Courbin, Emilio Falco, Henk Hoekstra, Neal Jackson, Tomislav Kundić, Geraint Lewis, and Andrzej Udalski for permission to use their figures. I would also like to thank Matthias Bartelmann, Emilio Falco, Jean-Paul Kneib, Bohdan Paczyński, Sjur Refsdal, Robert Schmidt, Liliya Williams, and David Woods for their careful reading of (parts of) the manuscript at various stages and their useful comments. Of particular help were the comments of Jürgen Ehlers and an unknown referee which improved the paper considerably.

## 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.

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

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.

Hubble image of SDSSJ0146-0929, a galaxy cluster that is massive enough to severely distort the spacetime around it, creating the odd, looping curves that almost encircle the cluster – known as an “Einstein Ring.” Credit: ESA/Hubble & NASA Acknowledgment: Judy Schmidt

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.