Much of what we know about the large-scale universe is due to decoding signals from light. The light that reaches the earth from faraway galaxies arrives as a wide spectrum, which is much more than visible light (or white light). Light is actually an electromagnetic wave with a range of wavelengths. White light is but a tiny band in the middle of the spectrum of wavelengths. When white light is refracted, such as passing through a prism, we see the colors of the rainbow.
If we move past the red band the wavelengths get longer, from infrared to microwaves to radio waves. Moving towards the opposite side of the spectrum, past the violet band, the wavelengths get shorter, from ultraviolet to x-rays to gamma rays. Even though the large majority of light is invisible to us, scientists have instruments that can detect information from the spectrum. The following list shows 5 things we know about the universe from decoding light.
1) The Contents of the Universe
Exploring the universe started simple with just visible light; ancient astronomers gazed at the night sky with the naked eye. All the twinkling yellow dots basically looked identical. They could not determine the size and distance of objects. The advent of the telescope added detail to the night sky, such as differentiating stars from planets and discovering individual galaxies.
Space telescopes, like Hubble and Kepler, are now placed in the earth’s orbit. From above the atmosphere, these and other instruments are collecting a tremendous amount of details about the universe. For example, the Hubble Space Telescope focused on a dark spot in the sky for a period of 10 days. In this tiny patch (roughly the opening of a drinking straw) an image of 10,000 galaxies was produced.
2) The Existence of Extrasolar Planets
In recent years, over 1,700 planets outside our solar system have been discovered (some of them are earth-like). Most planets orbit stars, therefore planets can be detected by examining changes in starlight, which are caused by existing planets. Astronomers use a number of methods to find planets. The two most effective methods are:
- The transit method: By observing a star for a period of time a planet will occasionally pass in front of the star. Viewed from earth, a planet will cause the starlight to dim slightly, thus announcing the planet’s presence.
- The Doppler method: As a planet orbits a star it exerts a gravitational effect on the star, which causes the star to wobble slightly. This can be detected by examining variations in the light spectrum as the star moves towards or away from the earth.
3) The Chemical Composition
Light from distant stars and galaxies can be converted into a spectrum of colors. This is achieved with an instrument called a spectroscope, which is attached to a telescope. This is perhaps the most valuable tool for decoding light. As it pertains to the chemical composition of the universe, a particular property of light contains precise information from its source.
When the light spectrum of a star is displayed by a spectroscope, vertical lines (called absorption lines) appear at specific locations, depending on the elements contained in the star. Each element produces a unique pattern of lines, which can be matched with experiments in a laboratory. Even though the information contained in the spectrum will be from a number of elements, the distinct pattern of each element can be sorted out.
By studying the information from light, astronomers have found that all the stars in the universe have more or less the same chemistry (including our sun). Thus, knowing that all the elements originate in stars, the chemical composition of the universe is essentially the same everywhere. The elements found here on earth are plentiful in other galaxies as well, leaving us to speculate that other life-sustaining planets may be out there.
4) The Universe is Expanding
There is another valuable piece of information from the spectroscope that has transformed our view of the universes. It is called a redshift. When the spectrum from distant galaxies is examined, the vertical lines are shifted towards the red end. This is due to the Doppler effect or the Doppler shift, and it has to do with the nature of waves. Light is a wave, and similar to sound waves, incoming waves will be stretched when the source is moving away; thus causing the absorption lines to shift towards the red end of the spectrum.
The conclusion from this information is that galaxies are moving away from us. The universe is expanding. The exception to this rule is that nearby galaxies are not expanding, because they are held together by gravitational forces. But for the universe as a whole, galaxies are moving away from each other. In other words, the earth’s location is not unique; the view from any location in the universe would be similar. Incidentally, it is actually the space that is expanding. The galaxies are rushing away because they are being pulled by the swelling of space.
5) The Big Bang
There are 3 ways we can get to a big bang origin of the universe by studying light:
- The expansion rate: The universe is expanding at a defined rate (based on the redshift), which is simply stated as: distant galaxies that are twice as far away from us are moving twice as fast, and galaxies that are 3 times as far are moving 3 times as fast. This means that if we reverse the timeline, in the distant past all the galaxies would converge at a point of infinite density. This was the moment of creation.
- The cosmic microwave background radiation: The CMBR is the remnant of the intense energy that was created at the big bang. The light from the big bang event has propagated throughout space, and is presently detectable as microwave radiation. Although the radiation is now faint, it is present in all directions of space.
- The agreement between prediction and observation: The amount of lighter elements (hydrogen, helium, deuterium and lithium) that are now present in the universe agrees with the predictions of the big bang theory. The quantity of these elements were detected from light coming from old stars and distant galaxies. The amounts are consistent with what the theory predicts would have been created in the early universe.
From an everyday perspective, light illuminates the world and we see things as a consequence. However, when we examine the large-scale universe our eyes alone are not sufficient. It is remarkable that light from very far away contains information from its source. And if not for ingenious techniques in decoding light, figuratively speaking, we would forever remain in the dark.
References: Richard Dawkins, The Magic of Reality
Lawrence M. Krauss, A Universe from Nothing
Stephen Hawking’s Universe -101- Seeing is Believing (June 14, 2013) https://www.youtube.com/watch?v=5kgPxvJqvEA
The Big Bang: Observational Evidence (June 4, 2012) https://www.youtube.com/watch?v=8WaI-iIlgdI