Scientists like to argue, contra Walt Whitman, that understanding something increases our appreciation of its beauty, rather than detracting from it. The image below, as Evalyn Gates explains, is a perfect example. Evalyn is an astronomer at the University of Chicago, and the author of a great new book on the science of gravitational lensing, Einstein’s Telescope: The Hunt for Dark Matter and Dark Energy in the Universe (Amazon, Barnes & Noble, Powell’s). This post is an introduction to how gravitational lensing gives us some of the most visually arresting and scientifically informative images in all of astronomy.
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I had the pleasure of meeting up with Sean and some other old friends at the World Science Festival in NYC last month, and over champagne at the opening night reception (science has its benefits) Sean graciously invited me to write a guest post on gravitational lensing. It’s a broad topic, mainly because lensing is proving to be such an incredibly useful tool for many areas of cosmology and astronomy, but I have to admit that the visual beauty of the images produced by lensing is part of the appeal for me.
I’m also enamored of the visceral connection between these images and lensing phenomena that all of us encounter in daily life – and the access into a complex theory that this connection affords. The giant arcs, Einstein Rings, and multiple copies of a single distant galaxy or quasar that have now been observed in hundreds of images are concrete visualizations of otherwise abstract concepts of general relativity – they effectively trace out the warps in spacetime created by massive objects, revealing the outline of the cosmos much as the technique of “rubbing” can reveal the writing on an ancient gravestone.
This image, from a recent paper by Adi Zitrin and Tom Broadhurst is both scientifically and visually irresistible:
First, the image itself is really cool. The bright white/yellow galaxies are members of a cluster known as MACS J1149.5+2223, while the blue amoeba-like objects that appear to be invading the cluster are actually five images of a single distant (z ~ 1) spiral galaxy.
This galaxy has been lensed by the warp in spacetime created by the cluster. Light from the galaxy, which lies almost directly behind the center of the cluster but much farther away from us, travels along several curved paths through the cluster lens, producing multiple magnified images of the galaxy. The inset box shows a computer generated model of the unlensed source galaxy, enlarged by a factor of four so that the details, including the spiral arm structure, are visible. Without the lensing power of the cluster, we would see this galaxy as a single small blue smudge.
In general, lensing will both magnify and distort (shear) images of a background source. This lens is fairly unique in that we see large but relatively intact images of the spiral galaxy, which implies that the mass distribution in the central region of the cluster must be nearly uniform. The images in the upper left (#1) and lower right (#2) are especially striking. #1 is magnified but very minimally distorted, while #2, the largest image with a magnification of over 80, seems to be curling its tentacles about one of the galaxies in the cluster.
A close look also reveals the negative parity (mirror symmetry) of the remaining three images – the spiral arms appear to circle in the opposite direction – as expected from lensing. The total magnification of the distant galaxy (the sum of all five images) is about 200, the largest known to date – supporting the authors’s claim that this is “the more powerful lens yet discovered.”
This is not just a pretty picture, however – the image packs a lot of scientific information. The authors extract the mass distribution in the cluster (which has implications for cosmological models), measure the mass-to-light ratio of the bright galaxy in the center of the cluster, and use the magnifying power of the lens to search for even more distant galaxies.
The basic idea is to construct a model of the lens, starting with the cluster galaxies and a dark matter halo; then refine the model to reproduce the multiple images that are seen. Using this refined model it’s possible to predict the location of additional images of a given source, and to identify regions of high magnification that can then be examined for multiple images of other sources. Any additional images that are found can be used to further refine the model and so on.
For example in this system, image #1 is “delensed” to obtain an image of the source galaxy; this model source image is then relensed and the resulting multiple images are compared (size, shape and location) with the observed images. The agreement between observed and modeled images is excellent. Using this lens model nine additional multiply-lensed galaxies (all fainter and at a higher redshift than the spiral galaxy) were found. In total there are 33 images of 10 background source galaxies.
So what does this tell us? The model of the lens outlines the (projected 2D) mass profile of the cluster – which doesn’t seem to agree with numerical simulations for clusters, assuming a standard ΛCDM cosmology. The mass concentration in the center of the cluster is higher than predicted, a result that has also been found for other massive clusters studied with gravitational lensing. This implies that we’re either missing some physics in our simulations, or we may need to modify our cosmological model.
And I suspect that we will hear from this lens again. The most distant galaxy discovered to date, at a redshift of z ~ 7.6, was found courtesy of the cluster lens A1689, and MACS J1149 offers another powerful magnifying glass through which to search for the earliest galaxies in the universe.
Wow! Great post on gravitational lensing. I appreciate your detailed description about what is going on in the image. Nice image to.
I wonder what astronomers would make of these magnifications and shearing if we had no concept of general relativity. I would love to be a fly on the wall in a universe where astronomers see lensing effects but have no concept of GR. 😛
Very nice post, thanks! I found myself continuously scrolling back and forth between the text and the image.
Joseph, I’ve wondered the same thing about relativity and the GPS system. If you didn’t know about special and general relativity, the clocks on your GPS satellites would be out of synch for reasons you would find completely mysterious. In a world where satellite technology had far outstripped fundamental physics, would they have discovered relativity that way?
Joseph,
you can get the entire lensing formalism in a Newtonian framework, except that everything is then off by a factor of 2 compared to GR. We’d probably still understand what’s going on qualitatively but would see constant discrepancies between lensing determined masses and masses measured from X-ray or the virial theorem.
This stuff just makes me happy, but dizzy, but happy. The book is superb by the way.
To what extent is the modelling and matching an ‘art’? I’m curious about just how difficult this is to do in practice.
I guess, I’d like to see pictures of the intermediate steps.
While this image is spectacular, we should remember that recent lifestyle changes in the US allow gravitational lensing to be observed here on Earth:
http://lablemminglounge.blogspot.com/2007/04/relativistic-obesity.html
LL- nice and amusing writeup — as a ‘lenser’, i appreciated this especially, thx for pointing us to it.. ;->
And great little writeup by Evalyn too, i hope to read this book, eventually! -M
Perhaps I’m dense, but I can’t quite match up the features of the image with the description in the text. Any chance this image could be marked up with helpful colored line art indicating what is lensed and what is not? Also, some kind of contour map of the inferred mass density of the lens overlayed on the image would be helpful, if possible of course.
Sili — The modeling procedure (and there are several different techniques) is science rather than art (and apologies if my simplified description gave the wrong impression).
A detailed lens model (including in this case a smooth dark matter component and individual components for the cluster galaxies) is parametrized, and an iterative fitting procedure is performed to minimize the RMS difference between model and observed positions of the images. The method used here also makes use of the photometric redshifts of the lensed galaxies (comparing model vs observations) in order to further constrain certain lens model parameters; they also use model v. observed magnification of the images as a consistency check. The full details can be found in an earlier paper.
Figure 6 in this paper illustrates the variation between the best fitting model and
several others which differ by about 1 sigma from the best fit model.
Of course there is some “art” in science in a sense, as scientists search for creative new methods to improve the modeling, balancing the number of parameters in the model, for example, against the computational challenges and finding ways to robustly extract the most useful information from the available data.
Igor K. — there are additional images in the Zitrin and Broadhurst paper that are helpful, including one of the mass density of the lens.
Sean @3 – I think it’s a given that a society with extremely high-precision stable clocks (that is, frequency standards) and one or both of platforms which can move clocks to regions of lower and higher gravitational potential and platforms which can accelerate a pair of clocks to a high relative velocity will certainly discover gravitational time dilation and Lorentz time dilation even if they have little knowledge of particle physics and have a very poor sky view making them weak on astronomy (e.g., surrounded by dust, only one planetary mass object in local system, no natural satellites).
A planet-based society that has excellent clocks in orbit at all could hardly fail to recover the concept of space-time [starting with (r,frequency_difference) for synchronous orbits at different altitudes, for example], and having such clocks in different orbits and in stationary orbits in satellites of differing masses would almost certainly discover mass energy equivalence through clock-comparison and/or study of doppler redshifting.
Consequently, I don’t think the “frequency anomaly mystery” would last very long.
Now, what does a society that discovers general relativity early (i.e. for whom the gravitational potential gradient is a regular feature of day to day engineering) do when it later develops a quantum hypothesis?
How will a society with poor knowledge of physics construct high precision clocks?
A good question, although a better one would be how a society with poor knowledge of physics put massive objects into stable orbits in the first place. This is wandering into Greg Egan territory.
Sean had wondered about discovering relativity in the presence of GPS; I took that as implying that they already had ballistics, thermodynamics, some chemistry and materials science, and high precision frequency standards, but not necessarily a well defined atomic model or particle physics, astronomy, or cosmology, and with astrophysics focused only on their local star. We had the first four well before the 20th century, I’ll return to clocks, and we were lucky to have had a good sky view with many easily studied periodic events and lots of reasons to look more closely at them (e.g. even a small magnification of the moon or Saturn or Jupiter and its moons reveals interesting extra detail).
Without the “distraction” of lots of long, continuous periodic processes, a society might start with clocks based on short-period oscillation (pendulum, tuning fork, crystal, gyroscope) and refine those. I don’t think it requires QM to get to the point of reading the spin phase of an orbiting gyroscope to sub-ppm accuracies. I don’t think QM is needed to compare clock drift using interferometry either.
Alternatively, we could make this society even luckier by giving them a natural highly-stable high-frequency periodical process clearly visible in their local sky, in contrast to our sky’s several highly-visible low-frequency periodical processes. They might then start developing civil time based on that frequency rather than by dividing up longer processes (rotation, orbit about the local star, etc) like we did. When calibrating against that frequency standard, the gravitational time dilation would become obvious. In effect, this is a society with a natural GPS satellite discovering relativity as they are able to move faster and higher.
On the other other hand, I don’t think that it even necessarily requires atomic-clock-like precision if long term stability can be achieved instead — a society where projects can last thousands of years (due to longevity or meticulous planning, record-keeping and focus) would notice tens of parts per million frequency aberrations due to gravitational time-delay in only a few hundred years even if their clocks are only as precise as those we had on Earth in the 18th century, particularly if they have lots of them in various orbits.
Evalyn,
Gorgeous images, interesting discussion. Thanx.
Of course, now I want a pet amoeba.
Sean,
Occasional guest posts really spice up the site. Good get.
Thank you, dr Gates.
I’m sorry if I seemed to imply any sort of non-rigour. I didn’t mean to suggest that there was untoward fiddling going on. I just did wonder how one would go about the practicalities.
Cheers!
There is another very neat thing about the different images. Assuming that the deflection of the photons caused them to travel different distances until they got to us, what we are actually seeing is not just distorted views of the same galaxy, but also snapshots at different times! If the difference is big enough – can it/has it been estimated? – and the resolution high enough, I could conceivably see a supernova occur in the oldest snapshots and then focus on the same star in a more recent snapshot. Then I could watch the star and wait until it explodes. How cool is that? Even if the difference is not that great, it would still be very neat to have an ‘age’ label on each of the views.
Christopher: you’re right — we’re seeing each snapshot at a different time, and there are actually two effects that contribute to a time delay between images. First is the one you mention — the length of the path the light travels for each image is different. Second is a general relativistic effect (known as the Shapiro delay) — the different light paths travel through different regions of the gravitational field of the cluster, and the stronger the field, the larger the delay.
The time delay hasn’t been estimated for these images, but it has actually been measured for another cluster lens (SDSS J1004) where there are 5 images of a distant quasar. Here the Shapiro delay dominates over the path length delay, and so the light from the quasar images farthest from the center of the cluster arrive first. (Quasars are intrinsically variable, and the delay is measured by finding a pattern in the light intensity of one image, and waiting for this same pattern to appear in the others.) The delays seen in SDSS J1004 are 780 days between the 1st and 2nd images; 40 days between 2nd and 3rd; and over 3 years between 3rd and 4th (the 5th image is difficult to see). So your (definitely cool!) idea could work in principle — just a matter of monitoring one of the images on a regular basis!
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About Sean’s comment (#3) I think one has to consider the general attitude in such a society towards theorizing in physics. In short, its members might be content with hacking solutions to the engineering problems caused by their mysterious GPS timing anomalies; they would rely on epicycles instead of deep understanding.
Of course, this assumes that such hacking would be effective in finding practical solutions:
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Hi Dr. Gates, it was a great pleasure to attend your Hayden Planetarium lecture and I am pretty far along in reading your book that you so graciously signed. Do these lensed images afford us the opportunity to determine the galactic rotation curves of these most distant/ancient galaxies? If so, do these ancient galaxies exhibit the same flat curve or are they more/less Newtonian than more modern galaxies?
Best.
-Allen Everhart
Dr Gates. Has anyone noticed or checked for any scale correlation between the lensing galaxies apparent gravitational mass anomalies and redshift?
I’ve been working on a model which, as a by product, would predict such correlation, and significant discrepancies in the Gmass mismatches, (some masses should be way too high), and explain them. It’s not really an astronomical model and I’m not an astronomer, but it’s a fundamental physics matter.
It could also throw up possible lensing structure anomalies for any blue shifted lensing galaxies compared to redshifted lenses.
Also, is ‘over 3 years’ the highest delay recorded?
Any relevant comments gratefully received.
Peter Jackson