This X-ray telescope image shows that galaxy NGC455 is surrounded by a cloud of gas that is 10 million degrees Celsius.

NASA/CXC/E. O'Sullivan et al.

X-rays and Light-bending

­­In­ addition to rotation curves, astronomers have used X-ray observations to confirm the large masses of galaxies and galactic clusters. When heated to high temperatures (millions of degrees Celsius), gases emit X-rays. The hotter the matter, the more X-rays emitted. So, when astronomers looked at intra-cluster medium (the spaces between galactic clusters) with X-ray telescopes, they found large gas clouds at tens to hundreds of millions of degrees Celsius. These gas clouds were invisible to optical telescopes. When astronomers estimated the mass from temperature measurements, they confirmed mass-to-light ratios of 100 or more, which provided more evidence for dark matter.

Dark Matter Gravitationally Bends Light

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In his general theory of relativity, Albert Einstein showed that massive objects can distort space-time with their gravity. Let's look at this phenomenon with a massive galactic super-cluster. The super-cluster distorts the space-time around it. Light rays emanating from a distant object behind the super-cluster pass through the distorted space-time. As they do, the light rays bend and converge upon the observer. Therefore, the super-cluster acts as a large gravitational lens, much like an optical lens (see How Light Works).

The distorted image of the distant object can appear in three possible ways depending upon the shape of the lens:

1. Sphere -- image appears as a ring of light known as an Einstein ring

2. Oblong or elliptical -- image gets split into four images and appears as a cross known as an Einstein cross

3. Cluster -- image appears as a series of banana-shaped arc and arclets

By measuring the angle of bending, astronomers can calculate the mass of the gravitational lens (the greater the bend, the more massive the lens). Using this method, astronomers have confirmed that galactic clusters indeed have high masses, as indicated by rotation curves and X-ray images. The high masses exceed the masses measured by luminous matter (i.e. high mass-to-light ratio) and provide evidence of dark matter.

MOND: Modified Newtonian Dynamics

In the 1980s, physicist Mordecai Milgrom suggested that perhaps dark matter did not exist. He thought that Newton's second law of motion (force = mass x acceleration, f = ma), a fundamental law of physics, should be re-examined in the cases of galactic motions. This would be a major change to the way we understand the universe, because Newton's second law forms the basis of many laws of physics.

Milgrom suggested a modification to Newton's second law called MOND, or Modified Newtonian Dynamics. This modification involves adding a new mathematical constant into Newton's second law. MOND has met skepticism from many astronomers and physicists because it was not introduced as a fundamental physics principle, but as a solution to a specific problem.

Also, MOND cannot account for evidence of dark matter discovered by other techniques that do not involve Newton's second law, such as X-ray astronomy and gravitational lenses. Furthermore, physicists recently tested Newton's second law down to accelerations as low as 5x10-14 m/s2­ and reported that f = ma holds true with no necessary modifications (see American Institute of Physics News Update: "Newton's Second Law of Motion," April 11, 2007). The fate of MOND is still questionable and being explored.