What Is Gravitational Lensing? Its Discovery, Mechanics, and Applications
Einstein’s relativity continues to astound us, even though it was formulated over a hundred years ago. The implications have a wide range, from gravity to reference frame dragging and time-space dilations. A particular implication of the gravity component is the focus of this article known as gravitational lensing and it is one of the few things that Einstein got wrong-or at least not 100% right.
Theory or Reality?
For a short time relativity was an untested idea whose implications of time slowing and space compressing were a hard idea to fathom. Science requires some evidence and this was no exception too. So what better to test relativity with than a massive object like the Sun? Scientists realized that if relativity were right then the Sun’s gravity field should cause light to bend around it. If the Sun could be blotted out then perhaps the area around the perimeter could be viewed. And in 1919 a solar eclipse was going to happen, giving scientists a chance to see if some stars which would be known to be behind the Sun would be visible. Indeed, the theory was proven correct as stars were seemingly out of place but in reality just had their light bent by the Sun. Relativity was officially a hit.
But Einstein went further with this idea. After being asked to look more into it by his friend R.W. Mandl, he wondered what would happen if different alignments had been reached with the Sun. He found several interesting configurations which had the benefit of focusing the displaced light, acting like a lens. He showed this was possible in a December 1936 Science article entitled, "Lens-Like Action of a Star by the Deviation of Light in the Gravitational Field" but felt that such an alignment was so rare that it was unlikely for the actual event to ever be viewed. Even if you could, he just couldn't conceptualize a far-away object being possible to focus enough for an image. Just a year later, Fritz Zwicky (famed originator of the dark matter explanation for star motion in galaxies) was able to show in a 1937 Physical Review that if instead of a star the lensing object were a galaxy then the odds are actually really good for a viewing. Zwicky was able to think about the collective power of all the stars (billions!) that a galaxy contains rather than a point mass. He also foresaw the ability for lensing to be able to test relativity, magnify galaxies from the early universe and find the masses of those objects. Sadly, little to no recognition for the work was met at that time (Falco 18, Krauss).
But scientists in the 1960’s grew more curious about the situation as space interest was at an all-time high. They found several possibilities which are shown throughout this article. Much of the rules from normal optics went into these configurations but a few notable differences were found also. According to relativity, the angle of deflection that the light being bent undergoes is directly proportional to the mass of the lens object (which is causing the bending) and is inversely proportional to the distance from the light source to the lens object (Ibid).
Based on this work, Signey Liebes and Sjur Referd figure out the ideal conditions for galaxy and globular star cluster lens objects. Just a year later, Jeno and Madeleine Bartony wonder about the implications this could have for quasars. These mysterious objects had a huge redshift which implied they were far away but they were bright objects, meaning that they had to be very powerful to be seen from so far away. What could they be? The Bartonys wondered if quasars could be the first evidence for galactic gravitational lensings. They postulated that quasars could in fact be lensed Seyfert galaxies from a far distance. But further work showed that the light output did not match that model, and so it was shelved (Ibid).
Over a decade later Dennis Walsh, Robert Carswell, and Ray Weymann uncovered some strange quasars in Ursa Major, near the Big Dipper, in 1979. There they found quasars 0957+561A and 0957+561B (which I will call QA and QB, understandably) at 9 hours, 57 minutes right ascension and +56.1 degrees declination (hence the 09757+561). These two oddballs had nearly identical spectrums and redshift values indicating they were 3 billion light years away. And while QA was brighter than QB, it was a constant ratio across the spectrum and independent of frequency. These two had to be related, somehow (Falco 18-9).
Was it possible for these two objects to have formed at the same time from the same material? Nothing in galactic models shows that this is possible. Could it be an object that split apart? Again, no known mechanism accounts for that. Scientists then began to wonder if they were seeing the same thing but with two images instead of one. If so, then it was a case of gravitational lensing. This would account for QA being brighter than QB because the light was being focused more without changing the wavelength and therefore the frequency (Falco 19, Villard).
But of course, there was a problem. Upon closer examination QA had jets emanating from it and going in a direction of 5 seconds with one north-east and the other west. QB only had one and it was going 2 seconds to the North. Another problem was that the object which should have been acting as the lens was not to be seen. Fortunately, Peter Young and other Caltech researchers figured it out using a CCD camera, which acts like a group of buckets which fill with photons and then store the data as an electronic signal. Using this, they were able to break up the light of QB and determined that the jet from it was actually a separate object just 1 second apart. Scientists were also able to discern that QA was the actual quasar 8.7 billion light years away with its light deflected and that QB was the image formed courtesy of the lens objects which was 3.7 billion light years away. Those jets ended up being part of a large cluster of galaxies which not only acted like a single big lens but were not in a direct alignment of the quasar behind it, resulting in the mixed result of two seemingly different images (Falco 19, 21).
Science Using Gravitational Lensing
The final result of studying QA and QB was proof that galaxies can indeed become lens objects. Now the focus turned to how to make the best use of gravitational lensing for science. One interesting application is of course to see distant objects normally too faint to image. With a gravitational lens you can focus that light so important properties such as distance and composition can be found. The amount that the light bends also tells us about the lens object’s mass.
Another interesting application once again involves quasars. By having multiple images of a distant object such as a quasar, any changes in the object can have a delayed affect between the images because one light path is longer than the other. From this fact we can watch the multiple images of the object in question until we can see how long the delay is between changes in brightness. This can reveal facts about the distance to the object which can then be compared to methods involving the Hubble constant (how fast galaxies are receding from us) and the acceleration parameter (how the acceleration of the Universe is changing). Depending on these comparisons we can see how far off we are and then make refinements or even conclusions about our cosmological model of a closed, open, or flat Universe (Falco 21-2).
One such far away object has actually been found, in fact one of the oldest known. MAC S0647-JD is a 600 light-year long galaxy that formed when the Universe was only 420 million years old. Scientists that were a part of the Cluster Lensing and Supernova Survey With Hubble used cluster MACS J0647+7015 to magnify the galaxy and hope to ream as much information as possible about this important cosmological stepping stone (Farron).
One of the possible images produced by a gravitational lens is an arc shape, produced by very massive objects. So scientists were surprised when they spotted one from 10 billion light years away and at a time in the early Universe when such massive objects should not have existed. It is by far one of the furthest lensing events ever seen. Data from Hubble and Spitzer indicates that the object, a cluster of galaxies known as IDCS J1426.5+3508, is lensing light from even further (and older) galaxies, allowing for a great science opportunity to study these objects. However, it does present a problem of why the cluster is there when it shouldn’t be. It isn’t even a matter of being just slightly more massive either. It is about 500 billion solar masses, almost 5-10 times the mass clusters of that era should be (STSci).
So do we need to rewrite the science books on the early Universe? Maybe, maybe not. One possibility is that the cluster is denser with galaxies near the center and thus giving them better qualities as a lens. But number crunching has revealed that even this would not be enough to account for observations. The other possibility is that early cosmological models are not right and that matter was more dense than expected. Of course, the study points out that this is only a single case of this kind, so no need to draw rash conclusions (Ibid).
Does gravitational lensing work on different wavelengths? You betcha. And using different wavelengths always reveals a better picture. Scientists took this to a new level when they used the Fermi Observatory to look at gamma-rays coming from a blazar, a quasar which has jets of activity pointed towards us because of its supermassive black hole. Blazar B0218+357, located 4.35 billion light years away, was seen by Fermi because of the gamma-rays emanating from it, meaning that something had to be focusing it. Indeed, a spiral galaxy 4 billion light years away was doing just that. The object made two images if the blazar just a third of an arc second apart, making it one of the smallest separations ever seen. And just like the quasar from before, these images have a delayed lapse in changes of brightness (NASA).
Scientists measured delays in gamma-ray flares averaging 11.46 days apart. What makes this finding interesting is that the delay between the gamma-rays was roughly a day longer than the radio wavelengths. Also, the gamma-ray brightness remained about the same between the images while the radio wavelengths saw a 300% increase between the two! The likely answer to this is the location of the emanations. Different regions about the supermassive black hole produce different wavelengths which can affect energy levels as well as distance traveled. Once such light goes through a galaxy, like here, further modifications may occur based off the lens object’s properties. Such results can offer insights into the Hubble constant and galactic activity models (Ibid).
How about infrared? You betcha! James Lowenthal (Smith College) and his team took infrared data from the Planck telescope and got to look at lensing events for infrared galaxies. Looking at 31 of the best imaged objects, they found the population was 8 to 11.5 billion years ago and making stars at a rate of 1000+ times that our Milky Way. With the lensing events, the team was able to get better modeling and imaging of the early Universe (Klesman).
Falco, Emilio and Nathaniel Cohen. “Gravity Lenses.” Astronomy July 1981: 18-9, 21-2. Print.
Ferron, Karri. "Most Distant Galaxy Found with Gravitational Lensing." Astronomy Mar. 2013: 13. Print.
Klesman, Alison. "Gravitational Lenses Reveal the Universe's Brightest Galaxies." Astronomy.com. Kalmbach Publishing Co., 07 Jun. 2017. Web. 13 Nov. 2017.
Krauss, Laerence M. "What Einstein Got Wrong." Scientific American Sept. 2015: 52. Print.
NASA. “Fermi Makes First Gamma-Ray Study of a Gravitational Lens.” Astronomy.com. Kalmbach Publishing Co., 07 Jan. 2014. Web. 30 Oct. 2015.
STSci. “Hubble Spots Rare Gravitational Arc From Distant, Hefty Galaxy Cluster.” Astronomy.com. Kalmbach Publishing Co., 27 Jun. 2012. Web. 30 Oct. 2015.
Villard, Ray. "How Gravity's Grand Illusion Reveals The Universe." Astronomy Nov. 2012: 46. Print.
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© 2015 Leonard Kelley