When we look up into the night sky, especially with the aid of a telescope, we see a vast number of stars and galaxies. But scientists have recently discovered that this "visible universe" is perhaps only a small portion (less than 5%) of all the matter and energy in the universe. Just what the rest of the universe is made of is unknown, and scientists have come to refer to it as "dark matter" and "dark energy." So what might be the role of dark energy in the universe?
We know that the universe is expanding. In 1929, astronomer Edwin Hubble found that distant galaxies were not only moving away from Earth, but that the greater the distance away, the faster they moved away. Hubble was able to determine this relationship by measuring the light waves emitted by distant stars. The wavelength of light emitted by celestial bodies shifts depending upon whether the bodies are moving toward Earth or away from it. A change in the wavelength of light is perceived as a change in color. For galaxies moving toward Earth, the wavelength decreases and the shift toward the blue end of the spectrum is known as blueshift; for galaxies moving away, the light they emit shifts toward the red end of the spectrum, known as redshift. A similar phenomenon can be observed with soundwaves: you can detect the contracting or stretching of soundwaves as changes in pitch as the source of the sound approaches or moves away. By observing redshifts in the light emitted by galaxies, Hubble saw that galaxies were moving away from each other at a rate constant to the distance between them.
In 1998, observational data revealed that the universe was not only expanding, but that its expansion was actually accelerating. Scientists had long thought that there were three possible shapes for the universe—closed, flat, and open—all of which included a slowing of the expansion rate. But recent research suggests that none of these descriptions was entirely right. Observations indicate that the shape of the universe is indeed flat, but that its expansion rate has increased within the past 5 or 6 billion years.
But what’s driving this accelerating expansion? Perhaps it's the "dark energy" detected by the researchers discussed in this video.
To learn more about the size and shape of the universe, check out How Big Is the Universe?.
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