SHORT KNOW

 Before Hubble , most astronomers thought that the universe didn’t change. But if the universe is expanding, what does that say about where it was in the past? If the universe is expanding, the next logical thought is that in the past it had to have been smaller. How Did the Universe Form? The Big Bang theory - (the theory that the universe originated sometime between 10 billion and 20 billion years ago from the cataclysmic explosion of a small volume of matter at extremely high density and temperature ) is the most widely accepted cosmological explanation of how the universe formed. If we start at the present and go back into the past, the universe is contracting -- getting smaller and smaller.  What is the end result of a contracting universe? According to the Big Bang theory, the universe began about 13 to 14billion years ago. Everything that is now in the universe was squeezed into a very small volume. Imagine the entire known universe compressed into a single, hot, chaot...

 After discovering that there are galaxies beyond the Milky Way , Edwin Hubble went on to measure the distance to hundreds of other galaxies. His data would eventually show how the universe is changing, and would even yield clues as to how the universe formed. Redshift . If you look at a star through a prism, you will see a spectrum, or a range of colors through the rainbow. The spectrum will have specific dark bands where elements in the star absorb light of certain wavelengths. By examining the arrangement of these dark absorption lines, astronomers can determine the composition of elements that make up a distant star.  In fact, the element helium was first discovered in our Sun—not on Earth—by analyzing the absorption lines in the spectrum of the Sun. While studying the spectrum of light from distant galaxies, astronomers noticed something strange. The dark lines in the spectrum were in the patterns they expected, but they were shifted toward the red end of the spectrum, ...

What did the ancient Greeks recognize as the universe? In their model, the universe contained Earth at the center, the Sun, the Moon, five planets, and a sphere to which all the stars were attached. This idea  held for many centuries until new ideas and better observing instruments allowed people to recognize that Earth is not the center of the universe.  Galileo's telescope revealed four moons orbiting Jupiter (not Earth) and many more stars than are visible to the naked eye. More importantly, Galileo's experiments established the principal of inertia which countered the physical arguments the Greeks used against a rotating and moving  Earth. 4th century BCE — Aristotle, building on the ideas of earlier astronomers, proposes that the Sun, Moon, planets, and stars revolve around a stationary Earth . This is known as the geocentric theory, meaning that the universe revolves around the earth.  2nd century AD — Ptolemy publishes a book that describes a mathematical pr...