|MadSci Network: Astronomy|
First things first: Distant galaxies aren't *accelerating* away from us. However, more distant galaxies recede from us more quickly than nearby galaxies. This happens because the universe is expanding in all directions according to Hubble's famous equation: v = H*r where v is the speed of a galaxy, H is a number, and r is the distance of the galaxy from our galaxy, the Milky way. The number H is called Hubble's constant and its value has produced some of the most heated debates in all of modern astronomy. The most recent measurements tell us that the value of H is about 70 km/s/Mpc (where 1 Mpc or megaparsec is about 3.3 million light years). Hubble's law says that if galaxy B is twice as far away as galaxy A, B will receed from us twice as quickly. But this explanation doesn't really answer your original question: why is the universe expanding at all? Why isn't it slowing down or remaining static? The energy that is driving the universe's expansion comes from the Big Bang, a colossal explosion that happened about fifteen billion years ago. The word "explosion" isn't really correct, but it's a good approximation. Right now the universe is still expanding. Whether it continues to expand forever, comes to a halt, or begins to contract depends on two things: the amount of matter in the universe and the value of the cosmological constant. First, let's start with the amount of matter in the universe. If there are enough stars, galaxies, and cold dark matter for their mutual gravity to stop the expansion of the universe at some finite time, we live in a closed universe. If there isn't enough stuff in our universe to stop the expansion then we live in an open universe. In a closed universe, the expansion will eventually stop and then reverse. All galaxies will then begin to move towards each other -- the reverse of what we see now. Seems pretty straightforward, right? But cosmologists aren't sure that the picture is so simple. The complication has to do with a mistake that Einstein made in 1915. In the early part of the 1900s, astronomers believed that we lived in a static universe where all galaxies stayed the same distance apart. But in 1915, when Einstein formulated his theory of general relativity, he was dismayed to find that his theory predicted a dynamic universe -- one that was either expanding or contracting. This was contrary to the the accepted ideas of the time. No doubt it made him tear at his hair, perhaps giving rise to the wild hairdo that's become one of his trademarks. To make his new theory agree with the leading ideas of the time, Einstein invented something called the cosmological constant -- a pressure that would counteract the force of gravity and allow the universe to exist in a steady state. But later on, astronomical observations by Edwin Hubble and others showed that all galaxies except the ones very close to the Milky Way were moving away from us. When Einstein heard of this, he abandoned his cosmological constant, calling it the greatest mistake he ever made. But recently, astronomers who study the origin and fate of the universe have begun thinking that maybe there is something to Einstein's idea after all. If the cosmological constant is not zero then it could have big consequences about our understanging of the universe. For instance, even if there is enough matter to close the universe, it might still keep expanding forever. Another consequence is the "loitering universe" where the young universe expanded for awhile but then loitered around in a steady state for several billions of years before resuming its expansion. A good resource for futher reading is chapter 28 of _Universe_ (5th ed.) by Kaufmann and Freedman.
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