Chapter 10: Gravity Sheds Light on the Invisible

When we say we see something, we are referring to how our eyes can detect and respond to light. This is called, “visible light.” When light enters our eyes, we can decompose any incoming light beams into electromagnetic waves with different wavelengths or frequencies. As we know, the human eye can only view light between the narrow range of wavelengths two fifths to seven fifths of a thousandth of a millimeter. However, we must remember that light waves have wavelengths that range from zero to infinity, the human just can’t detect given frequencies.

When we look into the night sky, we can see visible light emitted by stars. I have always wondered why planets appear so much brighter than stars. The truth is that planets reflect light from the sun, which makes then appear brighter than stars. When we observe the Universe in this way, we tend to believe that this is the Universe- this is what it is, but that’s simply not the case. When we look at the sky, we think we can perceive everything, but really, most of the Universe cannot be seen by the naked eye.

The 20th century revolutionized astronomy because astronomers began to view the Universe in wavelengths- wavelengths that were not detected by the human eye. Today, they still continue to do this, which is made possible by the conversion of one type of wavelength into another viewable form, like radio waves. Radios pick up electromagnetic waves with wavelengths greater than a few centimeters, emitted by a transmitter, and then converted into sound energy, which can be processed by the human ear.

In the 1930s, Karl Jansky, an engineer at the Bell Laboratories, showed that the Milky Way emitted radio waves. In the 20th century, after Jansky’s discovery, radio observations were used to view very distant objects. The development of radio telescopes has allowed for the detection of radio waves emitted by galaxies such as the Milky Way. Unlike conventional telescopes, radio telescopes can pick radio waves out of darkness.

Obviously the sky looks different when viewed through an optical telescope and a radio telescope, but the question is, how different? It’s common to think that we’ll observe the same picture that we perceive in the visible sky, but we won’t. Different physical processes emit different light frequencies, especially when there are temperature variations. With a normal optical telescope, we would believe that there is no activity in the invisible light regions, but when we view these areas with a radio telescope, we can interpret that they are actually full of radio waves. The fact of the matter is, the sky looks different, depending on how you look at it.

When we define the visible Universe, we have to be clear in our definition. Technically, the visible Universe is not only what we can see optically, but also what we can detect, like electromagnetic waves of varying wavelength. Using radio telescopes, scientists can detect waves like X-rays, and wavelengths that are a thousandth of a millionth of the wavelength of visible light.

Galaxies are not what they appear to be, they are actually much heavier and larger than they appear, but we are unable to identify what makes them so heavy. This unidentifiable mass is called dark matter. How much of the Universe is dark matter? Today it is believed that the Universe contains almost 100 times more dark matter than visible matter.