The Perseus Galaxy Cluster

You Can't Believe Your Eyes!

  • This corner of the Perseus Galaxy Cluster, located 250 million light years from Earth, is viewed through a snowstorm of stars that reside within our home Galaxy.
    Although you cannot see it, this image is filled with material, called dark matter, that can only be inferred. Photo credit: R. Jay GaBany, Cosmotography.com
  • By: R. Jay GaBany, Cosmotography.com


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The Invisible Universe



Just because you can't see something, doesn't mean it isn't there. For example, anyone who's spent time outside on warm summer days notices that their exposed skin tans (or burns!) over time. The reason isn't due to the sunshine we see, however. It's caused by our skin's reaction to a form of sunlight that's invisible- ultraviolet radiation.
  • Harlow Shapley's observations placed the Sun about 25,000 light years from the center of our home Galaxy.
  • Photo credit: National Academies


Although we human beings have been suntaning and sun burning for thousands of generations, the cause was not identified until 1801 when a Polish pharmacist named Johann Wilhelm Ritter was investigating why silver chloride darkened more quickly under blue light than red. Through a series of experiments, he used a glass prism to produce a rainbow spectrum then carefully exposed small samples of silver chloride to each color. Very little reaction occurred when the chemical was placed under red light but it quickly blackened as he moved it to the violet end of the spectrum. To his surprise, however, the sample had a violent reaction when it was exposed to an area beyond the violet range of color- where he could not see any light. He named this light "Chemical Rays" but it came to be known as ultraviolet.

  • Edwin Hubble, seated behind the the 100-inch Hooker telescope on Mt. Wilson, identified that the Milky Way was not the only galaxy.
  • Photo credit: Exploratorium
Today, we find ourselves in a similar situation- the Universe is being affected by forces we cannot see.

Our understanding of Earth's place in the Universe has dramatically changed over the past 500 years. For example, at the beginning of this period, western knowledge was confident with the centrality of humankind's location in realm of creation- the Sun, Moon, stars and planets were attached to crystalline spheres that revolved around us. Then a Polish cleric, named Nicolas Copernicus, described heavenly motions by advocating that the stars were fixed, the world turned once daily, and all the known planets, including Earth, circled the Sun. Galileo, the famous Italian physicist, mathematician and astronomer, confirmed Copernicus' assertions shortly after pointing one of his hand-made telescopes above the horizon and Johannes Kepler mathematically proved our orbital path was ellipical, not circular. It took another 400 years for the Copernican universe to gain wide acceptance but more surprises awaited because the Sun was still assumed to be the universal focus.
  • In 1931, Father Georges Lemaître proposed the theory now known as the Big Bang.
  • Photo credit: Wikipedia


That idea remained unchallenged until 1920 when Harlow Shapley's observations revealed that the Sun was not at the center of the Milky Way, which was considered the Universe in total, but approximately 28,000 light years from it- about midway to its edge. Shortly following this revelation, humankind experienced another epiphany when, in 1925, Edwin Hubble announced that the thousands of spiral nebulae seen by visual observers and in the astro photographs of that day were not located inside our Galaxy. He was the first to recognize that they were separate island universes located at vast distances from Earth and that each was hurtling from the others at an incredible rate. In short, not only had humankind's concept of the Universe expanded but the Universe, itself, was growing larger every second.

Georges Lemaître, scientist and Roman Catholic priest, was the first to understand the implication of Hubble's discovery. His 1931 paper proposed: if the distance between galaxies is increasing then they must have been closer to each other in the past. At the dawn of time, Lemaître's paper postulated, everything in the Universe must have been compressed into a single, very dense point (that he) called the primeval atom. Today, Lemaître's theory is known as the Big Bang.

Until the end of the last century, it was also widely believed that, over time, gravity would slow this expansion and cause the Universe to eventually contract in something called a Big Crunch.

So, imagine the surprise when independent teams of researchers, who were trying to identify and measure this amount of deceleration, discovered that the Universal expansion was not subsiding- it was increasing!     More »


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