Hubble's Universe Unfiltered

  • August 2, 2014

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

    by Frank Summers

    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.






    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.]