Hubble's Universe Unfiltered

  • August 2, 2014

    How deep can the Hubble Deep Field peer into the past?

    by John Bintz

    Q: How deep can the Hubble deep field peer into the past?

    A: The most distant objects Hubble has seen are just over 13 billion light-years away. Because light takes one year to travel one light-year of distance, the light left these most distant objects over 13 billion years ago. We see them not as they are today, but as they were 13 billion years in the past. Astronomers have measured and calculated that the Big Bang occurred about 13.7 billion years ago, so we are seeing these objects about 500-700 million years after the beginning of the universe. If the universe were a 70-year-old woman, it would be like looking back to pictures of her when she was only 3 years old.

  • June 10, 2014

    Gravitational Lensing in Action

    by Frank Summers

    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.

    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.

    [NOTE: This post is the third in a series of four, and is a slightly modified version of the same post on the Frontier Fields blog.]

  • June 3, 2014

    Visual "Proof" of General Relativity

    by Frank Summers

    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.

    The above image of galaxy cluster Abell 1689 is a prime example of gravitational lensing (click the image to see a larger version). 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) 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.

    [NOTE: This post is the second in a series of four, and is a slightly modified version of the same post on the Frontier Fields blog.]