Hunting Black Holes Through a Gravitational Lens

Need a powerful cosmic magnifying lens to detect that black hole? No problem! All you need is a galaxy and a quirk of Einstein's theory of general relativity.

As we all know, the hunt for black holes requires very powerful telescopes. To spot one of these behemoths you need to detect its X-ray flashes when a tasty asteroid gets eaten, the speed of stars closely orbiting it or another secondary effect.

But there's another way black holes may be spotted.

By using the phenomenon of gravitational lensing, future astronomers may have an extremely powerful means of "seeing" the gravitational power of a supermassive black hole over local space-time.

Cosmic Magnification

Gravitational lensing is a phenomenon that comes straight from Einstein's theory of general relativity. A massive object - like a black hole, or an entire galaxy - will have a powerful gravitational influence over the surrounding space-time, warping it.

Anything, including light, will therefore be influenced by the curvature of space-time surrounding the massive object.

Now, using the awesome power of the Hubble Space Telescope, astronomers have documented many cases of these space-time distortions. The distorted galactic light may appear as "arcs" in Hubble imagery. Occasionally, if the alignment is correct, whole rings may form (pictured top).

These arcs and rings are the light from distant galaxies being distorted by the gravity of a closer galaxy. From Hubble's perspective, the light from the more distant galaxy has been bent around the nearer galaxy, magnifying it.

There are some wonderful applications for this cosmic phenomenon. Most recently, astronomers using Hubble have analyzed the light from lensed arcs of distant galactic light to reconstruct the galactic structure (pictured below). The magnified view has a super-boosting effect for Hubble, allowing the curvature of space-time to act like a gargantuan cosmic magnifying glass.

Now, astronomers of the Harvard-Smithsonian Center for Astrophysics (CfA) think the lensed galactic light may also contain information about the presence of supermassive black holes lurking in the cores of the foreground galaxies.

The amount of gravitational lensing is directly related to the foreground galaxy's mass. The greater its mass, the greater its collective gravity, the greater the lensing effect. If a supermassive black hole is present in the core of that galaxy, this additional compact mass could add some further warping to the structure of the lensed light.

Although the CfA astronomers don't think such a fine structure could be detected by the technology currently available to us, very large arrays of interconnected radio telescopes may detect it.

Warped Communications

If you thought detecting black holes through gravitational lensing was weird, it's about to get even weirder.

In a Discovery News guest article published this week, Pat Galea of Icarus Interstellar Inc. describes a potential mechanism by which gravitational lensing could be used to "amplify" interstellar communications.

If a starship were to ever embark on a journey through the light-years of interstellar space - to Alpha Centauri, say, 4.4 light-years from Earth - a signal transmitted by the starship would take years to be received. Not only would there be the inconvenience of having to wait for very long periods of time, the power of the radio transmission may be too weak to detect after traveling such an extreme distance.

A novel way around this issue could see a relay spacecraft sitting at a distance of around 500 Astronomical Units (AU - where 1 AU is the distance the Earth orbits the sun) from the sun, in the opposite direction to where the starship is traveling.

Using the space-time distortion caused by the sun‘s mass, the solar gravitational lensing effect may be enough to focus the interstellar transmission at the 500 AU mark, allowing interstellar communications to occur.

This may sound like science fiction, but as Hubble is proving, gravitational lensing is potentially a powerful tool with applications from black hole hunting to communicating with interstellar spacecraft.

Image (top): The distant light of a galaxy forms a ring due to the warping of the gravitational field of a galaxy in the foreground. Image (middle): The reconstruction of a magnified galaxy through gravitational lensing. Credits: NASA, ESA, J. Rigby (NASA Goddard Space Flight Center), K. Sharon (Kavli Institute for Cosmological Physics, University of Chicago), and M. Gladders and E. Wuyts (University of Chicago)