Hubble's Universe Unfiltered

  • March 11, 2016

    A Century Later, General Relativity Is Still Making Waves

    by Frank Summers

    [Note: this article is cross-posted on the Frontier Fields blog]

    In November 1915, Albert Einstein published a series of papers that laid out the ideas, equations, and some astronomical applications of the general theory of relativity. While Isaac Newton described gravity as a force between two massive bodies, Einstein's general relativity re-interprets gravity as a geometric distortion of space and time (see my previous blog post "Einstein's Crazy Idea").

    One example cited in those papers was that general relativity can explain the extra precession of Mercury's orbit that Newton's formulation does not explain. Another prediction, the bending of light as it passes a massive object, was tested and shown accurate less than four years later. This effect, called gravitational lensing, has been shown in tremendous detail by the Hubble Space Telescope (see my previous blog post "Visual 'Proof' of General Relativity"), and is one of the prime motivations behind the Frontier Fields project.

    Last year, scientists celebrated the centennial of general relativity, which has been a resounding success in diverse astronomical situations. However, there was one major prediction that had not yet been tested: gravitational waves.

    General relativity predicts that mass not only can create distortions in space-time, but also can create waves of those distortions propagating across space-time. In cosmology, the global expansion of space over time is a familiar concept. For a gravitational wave, space also stretches/shrinks, but that localized distortion moves across space at the speed of light.

    The Laser Interferometer Gravitational-Wave Observatory (LIGO) is one of the projects designed to observe the minute distortions of gravitational waves. It consists of two detectors, one in Hanford, Washington, and one in Livingston, Lousiana. Each detector has two perpendicular arms, consisting of ultra-high-vacuum chambers four kilometers (two and a half miles) in length.

    For the experiment, a laser light source is split and sent down and back each arm. By measuring how the laser light signals interfere with each other when recombined, extremely precise measurements of any change in distances can be made. The idea is that when a gravitational wave passes by, the minuscule stretch of one arm and shrink of the other will be observable.

    The signal observed in the LIGO event GW150914

    On September 14, 2015, both LIGO detectors observed an event (see the accompanying image). The pattern in the signal indicates that a series of gravitational waves passed through the detectors in about two-tenths of a second. It is extremely important that multiple detectors saw the same event so that local disturbances can be ruled out. Plus, the time delay between the detectors helps measure the speed of the waves.

    To analyze the event, the LIGO team used computer simulations. The shape and duration of the event waveform matched that expected for the merger of two black holes. The amplitude of the detection helped determine how far away the black-hole merger took place. The best fit is a merger of a 36-solar-mass black hole with a 29-solar-mass black hole to form a 62-solar-mass black hole, about 1.3 billion light-years away.

    The energetics of the merger are simply astounding. Recognizing that 36 + 29 = 65, one can see that three solar masses of material did not end up in the resulting black hole. Instead, it was converted in the energy that created the gravitational wave. Released in less than half a second, the peak wattage of the event was greater than the visible-light wattage from all the stars in the observable universe.

    And yet, when detected on Earth, the measured space distortion was smaller than the size of a proton. The reason it took a century to find gravitational waves is because one has to measure subatomic displacements. Gravity is demonstrably the weakest of the four fundamental forces. It takes a tremendous amount of energy to produce a gravitational wave that can be seen at cosmic distances.

    The title and abstract of the paper announcing the discovery of gravitational waves

    There are several major results from this observation. The detection shows, for the first time, that both black-hole mergers and gravitational waves exist. The time delay between detectors, and analysis of the signal at different frequencies, demonstrates that gravitational waves travel at the speed of light. All the results are consistent with the predictions of general relativity.

    This event marks the beginning of gravitational-wave astronomy. With more detectors coming online and planned improvements to current detectors, the field is burgeoning. Dozens to thousands of black-hole or neutron-star mergers, with more detail about each event, should be found in the next decade.

    More than a billion years ago, two black holes merged in a distant galaxy, emitted a tremendous amount of energy, and created a gravitational ripple moving across space. Recently, the LIGO project detected this almost-infinitesimal motion of space, a deviation much smaller than the size of an atom. With that amazing observation, the last major prediction of general relativity was verified. A century later, Einstein still rules.

  • March 4, 2016

    Episode 21: New Views of the Pillars of Creation

    by Frank Summers

    Download this episode


    New Views of the Pillars of Creation

    One of Hubble's most famous images was taken in 1995. The iconic "Pillars of Creation" shows the tall and beautiful gaseous pillar structures that can form inside star-forming regions. Within these dark clouds, stars are being born. Hubble kicked off its 25th anniversary year in 2015 with some images that used its improved cameras to revisit these beautiful pillars. This larger, higher resolution, and expanded wavelength examination uncovered new details, new features, and new perspectives on a classic image.  


    Hubble press release:



    • Most will remember that there was an initial flaw in Hubble's mirror that was corrected in 1993. After that repair mission, it took some time for the public to recognize just how amazing Hubble's views of the universe really were.  The 'Pillars of Creation' image, released in November 1995, was a watershed in that regard. The image was shown on television news and reproduced in newspapers and magazines everywhere. The widespread attention helped cerify Hubble's status to the public as the pre-eminent observatory of our time.

    • The "teapot" in Sagittarius is not the full constellation. It is a star pattern, called an "asterism," within the larger collection of connected stars that makes up the entire constellation. One can search online to see the full Sagittarius constellation as it depicts the archer. A similar asterism is the Big Dipper, which is a star pattern within the constellation of Ursa Major.

    • The visible and infared views of astronomical objects are generally similar enough that one can identify common structures between the views. When using other wavelengths, like X-rays or radio waves, it can be very hard to identify how the two different wavelength views correspond. Astronomers must record the exact sky coordinates of an image in order to be able to precisely compare against views by other telescopes and in other wavelengths.

    • In 2005, for Hubble's 15th anniversary, we released an image of another pillar in the Eagle Nebula. This pillar has a long, thin profile that earned it the nickname of a "stellar spire." As seen in this contextual image, the two pillar regions are near each other in the nebula and both point toward the same group of hot stars.


    Image notes

    Eagle Nebula Pillars (1995)
    Credit: NASA, ESA, STScI, J. Hester and P. Scowen (Arizona State University)

    Sagittarius Region
    Credit: A. Fujii

    Eagle Nebula
    Credit: Timber Rock Observatory,

    Eagle Nebula
    Credit: T.A. Rector (NRAO/AUI/NSF and NOAO/AURA/NSF) and B.A. Wolpa (NOAO/AURA/NSF)

    Eagle Nebula Pillars (2015)
    Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

    Eagle Nebula Pillars in Infrared (2015)
    Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

  • December 29, 2015

    How Hubble 'Sees' Gravity

    by Frank Summers

    [Note: This post is cross-posted on the Frontier Fields blog.]

    Gravity is the familiar force of nature responsible for the diverse motions of a baseball thrown high into the air, a planet orbiting a star, or a star orbiting within a galaxy. Astronomers have long observed such motions and deduced the amount of gravity, and therefore the amount of matter, present in the planet, star, or galaxy. When taken to the extreme, gravity can also create some intriguing visual effects that are well suited to Hubble’s high-resolution observations.

    Einstein’s general theory of relativity expresses how very large mass concentrations distort the space around them. Light passing through that distorted space is redirected, and can produce a variety of interesting imagery. The bending of light by gravity is similar to the bending of light by a glass lens, hence we call this effect “gravitational lensing.”

    The simplest type of gravitational lensing is called “point source” lensing. There is a single concentration of matter at the center, such as the dense core of a galaxy. The light of a distant galaxy is redirected around this core, often producing multiple images of the background galaxy (see the image above for an example). When the lensing approaches perfect symmetry, a complete or almost complete circle of light is produced, called an “Einstein ring.” Hubble observations have helped to greatly increase the number of Einstein rings known to astronomers.

    Galaxy Cluster Abell 2218

    Gravitational lensing in galaxy cluster Abell 2218

    More complex gravitational lensing arises in observations of massive clusters of galaxies. While the distribution of matter in a galaxy cluster generally does have a center, it is never perfectly circularly symmetric and is usually significantly lumpy. Background galaxies are lensed by the cluster with their images often appearing as short, thin “lensed arcs” around the outskirts of the cluster. Hubble’s images of galaxy clusters, such as Abell 2218 (above) and Abell 1689, showed the large number and detailed distribution of these lensed images throughout massive galaxy clusters.

    These lensed images also act as probes of the matter distribution in the galaxy cluster. Astronomers can measure the motions of the galaxies within a cluster to determine the total amount of matter in the cluster. The result indicates that most of the matter in a galaxy cluster is not in the visible galaxies, does not emit light, and is thus called “dark matter.” The distribution of lensed images reflects the distribution of all matter, both visible and dark. Hence, Hubble’s images of gravitational lensing have been used to create maps of dark matter in galaxy clusters.

    In turn, a map of the matter in a galaxy cluster helps provide better understanding and analysis of the gravitationally lensed images. A model of the matter distribution can help identify multiple images of the same galaxy or be used to predict where the most distant galaxies are likely to appear in a galaxy cluster image. Astronomers work back and forth between the gravitational lenses and the cluster matter distribution to improve our understanding of both.

    Three lensed images of a distant galaxy seen through a cluster of galaxies

    On top of it all, gravitational lenses extend Hubble’s view deeper into the universe. Very distant galaxies are very faint. Gravitational lensing not only distorts the image of a background galaxy, it can also amplify its light. Looking through a lensing galaxy cluster, Hubble can see fainter and more distant galaxies than otherwise possible. The Frontier Fields project has examined multiple galaxy clusters, measured their lensing and matter distribution, and identified a collection of these most distant galaxies.

    While the effects of normal gravity are measurable in the motions of objects, the effects of extreme gravity are visible in images of gravitational lensing. The diverse lensed images of crosses, rings, arcs, and more are both intriguing and informative. Gravitational lensing probes the distribution of matter in galaxies and clusters of galaxies, as well as enables observations of the distant universe. Hubble’s data will also provide a basis and guide for the future James Webb Space Telescope, whose infrared observations will push yet farther into the cosmos.

    A "smiley face" gravitational lens in a galaxy cluster

    The distorted imagery of gravitational lensing often is likened to the distorted reflections of funhouse mirrors, but don’t take that comparison too far. Hubble’s images of gravitational lensing provide a wide range of serious science.