The expanding universe was suspected since Einstein’s theory of relativity, but it wasn’t until 2000 that evidence was found. Hubble’s redshift analysis led to the discovery that the universe is expanding at an accelerating rate, likely due to mysterious “dark energy.”
Ever since Einstein’s general theory of relativity was applied to what is known about the universe as a whole, it has been suspected that the universe was expanding. When Edwin Hubble and Milton Humason discovered in the 1920s that virtually all the galaxies in the sky were speeding away from us, this gave rise to the suspicion that the universe was expanding at a rapid rate. Yet it wasn’t until 2000 that decisive evidence for the expansion of the universe was finally found, in the form of extensive redshift detections of very distant objects.
The expanding universe is often cited as the most significant discovery of modern cosmology. In Einstein’s time, the steady state theory was the predominant one, which claimed that the universe remained balanced at the same size. Einstein, whose equations predicted universal expansion or contraction but not stasis, artificially introduced a stabilizing variable into his equations, called the “cosmological constant.” After realizing from Hubble’s observations that the universe was probably expanding, he later called it his “biggest mistake.”
The phenomenon that first led cosmologists and astronomers to predict an expanding universe was the Hubble redshift analysis. Using the Mount Wilson Observatory, which was the best telescope in the world at the time, Hubble observed distant galaxies and saw that they appeared progressively red. He has looked further into space and, by extension, even further back in time, as light takes many billions of years to travel from these galaxies to Earth. Because the redness factor correlated so well with increasing distance, Hubble suspected it was a reliable phenomenon with a physical cause.
The cause has been determined as follows: as the universe expands, the space beneath it increases in volume, but the light passing through it remains the same. Expanding space lengthens the wavelength of light, making it longer and therefore redder. An analogy sometimes used is that of dots on the surface of an expanding balloon. Draw a wave pattern on a balloon, and as you blow it up, notice how the wave stretches and lengthens. This is the same principle that underlies the redshift phenomenon.
In 1998, it was discovered that the universe is not only expanding, but that it is probably expanding at an accelerating rate. The physical reason for this is attributed to a mysterious “dark energy,” so named because we barely know anything about it.
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