Hubble proved that the galaxies are all moving away from each other, which implied that the average distance between galaxies in increasing and so the Universe must be changing over time. The horizontal x-axis gives the distance from Earth, in units of Megaparsecs (where 1 Mpc = 3.26 million light years) The vertical y-axis gives the speed in kilometres per second that the galaxy is moving away from us This relationship, which is known as Hubble’s law, is shown in simplified form in the diagram below. The further away a galaxy is from us, the faster it appears to be moving away. Image from NASAĪs discussed in my previous post, it has been known since 1929 that the Universe is expanding, which means that when we look at distant galaxies they appear to be moving away from us. What the Milky Way would look like from a great distance. Our Milky Way is a large galaxy and is believed to contain over 400 billion stars. The Universe is composed of galaxies, each of which contains many billions of stars. How does does the theory support an expanding Universe ? The Sun will last for about 5-6 billion years before it runs out of fuel. So, if we take a large enough region of space, and by large we mean tens of millions of light years across, the average amount of light emitted doesn’t change over time. The Steady State state theory proposes that new stars are continually created all the time at the rate needed to replace the stars which have used up their fuel and have stopped shining. If we take a small region of the Universe, such as the neighbourhood of the Sun, it does change over time as individual stars burn up their fuel and die, eventually becoming objects such as black dwarfs, neutrons stars and black holes. The theory does acknowledge that change takes place on a smaller scale. In other words, the Universe doesn’t evolve or change over time. This states that the Universe is infinite in extent, infinitely old and, taken as a whole, it is the same in all directions and at all times in the past and at all times in the future. At the heart of the Steady State theory is the Perfect Cosmological Principle. This theory was developed in 1948 by Fred Hoyle (1915-2001), Herman Bondi (1919-2005) and Thomas Gold (1920-2004) as an alternative to the Big Bang to explain the origin and expansion of the Universe. However, this hasn’t always been the case and for a while the Steady State theory was very popular. It is now generally accepted by most cosmologists. The Big Bang theory states that the Universe originated from an incredibly hot and dense state 13.7 billion years ago and has been expanding and cooling ever since. This is an elegant alternative theory to the Big Bang, which was very popular among astronomers in the 1950s, but is now obsolete. This post, the latest in my series about cosmology, talks about the Steady State theory.
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