February 26, 2015 Dr. Frank Summers

The Marvel of Gravitational Lensing

A Giant Lensed Galaxy Arc
The view of a distant galaxy (nearly 10 billion light-years away) has been warped into a nearly 90-degree arc of light by the gravity of the galaxy cluster known as RCS2 032727-132623 (about 5 billion light-years away).
Credit: NASA, ESA, J. Rigby (NASA GSFC), K. Sharon (KICP, U Chicago), and M. Gladders and E. Wuyts (U Chicago)

One of the coolest marvels in the universe is a phenomenon known as “gravitational lensing.” Unlike many topics in astronomy, the images are not what makes it appealing. Gravitational lensing produces streaks, arcs, and other distorted views that are intriguing, but don’t qualify for cosmic beauty pageants. What makes these images special is the intellectual understanding of how they are created, and the fact that they are even possible at all. The back story takes an ordinary, everyday process, and transforms it into cosmic proportions.

Most of us are familiar with the workings of a glass lens. If you have ever used a magnifying glass, you have seen how it changes the view of an object seen through it.

The glass lens collects light across its surface, which is generally much larger than the pupil of a human eye. Hence, a lens can amplify brightness. In addition, the path of a light ray is bent when it passes through the glass lens. [To be specific, the path bends when the light crosses from air to glass, and again when it crosses back from glass to air.] This bending is called refraction, and the common lens shape will focus the light to a point. When we view that collected light, our view of the object can be bigger or smaller depending on the distances involved, both from the object to the lens and from the lens to our eyes. In summary, a glass lens can amplify and magnify the light from an object.

Glass lenses, however, are not the only way that the path of light can be changed. Another way to redirect light comes from Einstein’s theory of general relativity.

My three-word summary of general relativity is “mass warps space.” The presence of a massive object, like a star, warps the space around it. When light crosses through warped space, it will change its direction. The result is that light that passes close enough to a massive object will be deflected. This deflection by mass is similar to refraction by glass.

Clusters of galaxies are huge concentrations of mass, including both the normal matter we see in the visible light from galaxies and the unseen dark matter spread throughout. Many galaxy clusters are massive enough to produce noticeable deflections of the light passing through or near them. The combined gravity in the cluster can warp space to act like a lens that gathers, amplifies, and magnifies light. Such a gravitational lens will be lumpy, not smooth, and will generally create distorted images of background galaxies seen through them. Also, this lensing often produces multiple images of the same background galaxy, as light from that galaxy is re-directed toward us along multiple paths through the cluster.

The simple idea of a glass lens becomes both cosmic and complex in gravitational lensing. Imagine a lens stretching millions of light-years across (many million million millions of miles). We don’t need to construct such a lens, as nature has provided a good number of them through the warping of the fabric of space. These lenses allow us to see very distant galaxies in the universe, some of which could not otherwise be observed. That’s the marvelous reality of galaxy clusters acting as gravitational lenses.

June 12, 2014 Dr. Frank Summers

Gravitational Lensing in Action

In my previous blog post, Visual “Proof” of General Relativity, I discussed how gravitational lensing demonstrates the effects of Einstein’s theory of general relativity in a direct, visual manner. Images created by gravitational lenses show features that are not possible in Newton’s version of gravity.

Although seeing general relativity with your own eyes is kinda awesome, there’s one unsatisfying aspect: you only see the result, not the process. Since you don’t know exactly what those galaxies looked like before the gravitational lensing, it is hard to fully appreciate the magnitude of the distortions. We have no on/off switch for the mass of the galaxy cluster to be able to examine the un-lensed image and compare against the lensed one.

A simulation of gravitational lensing by a cluster of galaxies (click on image for larger version). The galaxies of cluster Abell 2744 (left) are inserted into the Hubble Ultra Deep Field (right) to produce the combined image with gravitational lensing (center).

But we can demonstrate the process of gravitational lensing through scientific visualization. The images above show a simulation of gravitational lensing by a galaxy cluster. On the left is an image of only the galaxies that belong to galaxy cluster Abell 2744; all of the foreground and background objects have been removed. On the right is a deep field image of galaxies. In the center is a simulation of how the galaxies of Abell 2744 would distort the galaxy images in the deep field.

By carefully comparing galaxy images between the right and center panels, one can see how the un-lensed galaxies transform to their distorted lensed versions. The elongated streaks and arcs in the center image generally come from compact, ellipse-shaped galaxies in the right image. But not all galaxies are changed, a fact easily seen by examining the larger, yellow galaxy in the lower right.

The explanation comes from the details of the simulated lensing. The deep field used above is a portion of the Hubble Ultra Deep Field (HUDF), and includes only galaxies for which we have a good measure of their distance. Using those distances and the distance to Abell 2744, we were able to place the galaxies of Abell 2744 at their correct positions within the deep field. HUDF galaxies which are closer than the galaxy cluster would not be lensed, and appear the same in the right and center images. Only those galaxies behind the cluster were transformed by the simulated lensing. Thus, the central image provides a proper simulation of what would be seen if Abell 2744 suddenly wandered across the sky and ended up in the middle of the HUDF.

I note that all of the background galaxies were combined into a single image at a set distance behind the cluster for simplicity. The full, and rather tedious, 3D calculation could have been performed, but was deemed unlikely to provide a significant visual difference for a public-level illustration. I further note that it is an occupational hazard of being a scientist that one feels compelled to provide such full-disclosure details.

The really difficult challenge is to do the reverse of this simulation. Start with an image of gravitational lensing and then work out the mass distribution of the galaxy cluster from the distribution of streaks and arcs. But, hey, no one said being an astrophysicist was easy.

In the final part of this series of blog posts, I’ll provide a more down-to-earth example of gravitational lensing.

May 29, 2014 Dr. Frank Summers

Visual “Proof” of General Relativity

In a previous blog post, “Einstein’s Crazy Idea“, I discussed how Einstein’ s theory of general relativity is a reinterpretation of gravity. Newton’s original idea of gravity visualized it as a force between massive objects. Einstein instead surmised that the presence of mass warps space, and that curved space-time produces the motions we attribute to gravity. Earth’s orbit around the Sun is either a curved path through flat space (Newton) or a straight path through curved space (Einstein).

Both ideas of gravity produce the same observed motions for most cases. But there are a number of situations, generally involving very strong gravitational effects, where general relativity explains phenomena that gravitational forces get slightly wrong. The differences are often subtle and take quite a lot of explanation to appreciate. However, one example is visually obvious: gravitational lensing.

Hubble image of galaxy cluster Abell 1689, showing a large number of lensed arcs (click on the image for larger version). These arcs are distorted images of background galaxies, gravitationally lensed by the mass of the cluster.

The above image of galaxy cluster Abell 1689 is a prime example of gravitational lensing. Throughout the image are numerous small arcs, streaks, and strange-looking objects. Most of these are relatively normal galaxies (a few really are just strange-looking objects), whose images have been stretched and twisted by the galaxy cluster and general relativity.

The combined mass of the thousands of galaxies in the cluster (and their associated dark matter – a topic discussed in the What is Dark Matter? blog post) heavily distorts the space-time around the cluster. Light from more distant galaxies passes through that warped space. The images of those distant galaxies become distorted as if they were being seen through an odd-shaped glass lens. In fact, the physics of light redirection using gravity is entirely analogous to that using lenses. It is the optics of complex lenses, but using mass instead of glass.

Newton’s gravity can not produce such gravitational lensing. Well, to be complete, a gravitational force could produce half of the lensing effect of general relativity, but only if one assumes that photons (i.e., particles of light) have mass. Modern physics considers photons to be massless particles, and hence gravitational lensing does not exist in Newton’s version of gravity, only in Einstein’s general relativity.

For that reason, I like to say that pictures of gravitational lensing are visual “proof” of general relativity. You don’t have to delve into the astronomy, physics, or complex mathematics — just examine the image. Such distortions arise from general relativity.

Now, the visual distortions may be easy to spot, but that’s not to say that these images are easy to interpret. Just the opposite is true. I’ll provide some examples of the complexities of understanding gravitational lensing in my next blog post.

May 23, 2014 Dr. Frank Summers

Einstein’s Crazy Idea

One of the original plates from the 1919 solar eclipse used to measure the effects of general relativity. Click the image for a larger version, and note the horizontal lines that mark stars that were used for the measurements.

General relativity is just plain weird.

The basic idea of gravity we are taught in school comes from Isaac Newton’s “Principia” in 1687. Gravity is a force exerted by objects with mass. The greater the mass, the greater the gravitational force. The larger the distance between objects, the lesser the force ( it decreases with the square of the distance). The gravity of the Sun pulls on Earth and holds it, along with the other planets, asteroids, comets, etc., in orbit.

Not so, according to Albert Einstein in 1915. He came up with a completely new, and quite radical, alternative explanation.

Einstein’s crazy idea is that the presence of mass warps the fabric of space around it. Then, that warped space controls the motion of other masses nearby. Newton’s idea of a gravitational force is thus replaced with four-dimensional space-time geometry. Planets orbiting around stars, and stars traveling through galaxies — these are space-time distortions moving within other space-time distortions. As one famous description puts it: mass tells space how to warp, while warped space tells mass how to move. Yeah, weird.

On the face of it, Isaac and Albert are just describing the same phenomenon from two different points of view: the former sees a force, while the latter sees geometric distortions. And, since the algebraic equations of the gravitational force are so, so, so, so, so very much simpler than the tensor calculus of general relativity, why go to all the relativistic trouble?

The answer is that there are certain situations, generally involving very large masses, where Newton’s gravity is demonstrably wrong. The most famous of these is the precession of the perihelion of Mercury.

The orbit of Mercury is not fixed in space. Each time Mercury orbits the Sun, its orbit rotates by a minuscule amount. The position when Mercury is closest to the Sun, called perihelion, is used to measure this orbit rotation, called precession. While Newton’s gravity predicts a precession of the perihelion of Mercury, the measured value is significantly higher. This mismatch between prediction and observation is resolved by Einstein’s general relativity in that the warping of space at such a close distance to the Sun produces a slightly stronger precession than gravitational force.

The other famous demonstration of general relativity is the bending of light as it passes a massive object. Light rays also have their paths changed by passing through warped space. A total solar eclipse on May 29, 1919, served to test this effect. During the eclipse, astronomers could see stars whose light had passed close to the Sun. Their apparent position on the sky would be shifted from their normal position due to passage through the warped space around the Sun. By observing the precise positions of such stars both before and during the eclipse, astronomers measured the effects of general relativity. (See the image accompanying this post.)

Those 1919 observations did much to confirm that this crazy idea of general relativity reflected the reality of the universe. We now have many tests of general relativity. Most are subtle and require significant explanation. However, there is one that is visually striking, and which is critical to the scientific underpinnings of the Frontier Fields project. I’ll address that in my next blog post ( Visual “Proof” of General Relativity ).