| We live on Earth, which is one of eight planets orbiting our sun. The sun is grouped together with about 100 billion other stars in our galaxy the Milky Way. The Milky Way is shaped like a giant disk 50 to 100 thousand light-years across, with us about 25 thousand light-years from the center. If you go out on a clear, moonless night the stars of our galaxy will appear as a splotchy white band running across the sky, hence the name Milky Way. The other stars you normally see are all in our galaxy too. There are lots of other galaxies in the universe—billions of them at least. Some are off by themselves, but most lie in clusters of anywhere from a few to a few thousand galaxies each. On average, galaxies are distributed in space roughly like dimes spaced a meter apart. A striking feature of these other galaxies is that they are moving away from us! This is not because the other galaxies are moving through space. Rather, the galaxies are more-or-less still relative to space, and it's space itself that is expanding, carrying the galaxies along with it. Observations show that space is currently expanding at a rate of about 7% per billion years. In other words, if the universe were to continue expanding at its present rate, after a billion years all cosmic distances would be stretched by 7%. The concept of an expanding universe has an interesting history. When Einstein in 1917 first applied his geometric theory of gravity (his famous theory of general relativity) to the universe as a whole, he found his equations inconsistent with a universe of constant size. Surprised and perplexed, he introduced a "cosmological constant" Λ into his equation as a fudge factor to make his constant-size universe work. A few years later, in 1922, Alexander Friedmann conceived the idea of an expanding universe. To his delight, he found that Einstein's original equations worked fine in an expanding universe, with no need for a fudge factor. Nevertheless the idea of an unchanging universe was so ingrained in western thinking that not even Einstein could accept Friedmann's work: Einstein regarded Friedmann's model of an expanding universe as a mere mathematical curiosity without physical significance. Fortunately experimental support was quick in coming. During the 1920s the work of many astronomers contributed to the discovery that other galaxies are receding from us. The exciting conclusion, that a galaxy's rate of recession is roughly proportional to the distance from us to the galaxy, was exactly what one would expect in an expanding universe. History has assigned the credit for this conclusion to Edwin Hubble and recorded the date as 1929. In reality Georges Lemaitre had come to the same conclusion two years earlier, in 1927, using essentially the same data and computing the same value for the rate of expansion…. [T]o this day we call the expansion rate the Hubble constant and denote it by the letter H, that is, H = 7% per billion years. The fact that space is expanding means that in the past it must have been smaller. If we look far enough back in time, space had zero size. This was the big bang, the birth of the universe. How long ago was the big bang? To get a rough idea, assume space has been expanding at a constant rate of 7% per billion years. To get back to zero size would require (100%)/7% / billion years) ≈ 15 billion years. Observation shows that at least the visible portion of the universe is both homogeneous and isotropic. This means that any two regions of the universe are basically alike… [and] that no matter where you are in the universe, things look basically the same in all directions. | — Jeffrey R. Weeks, The Shape of Space, Chapter 19 | Indexes/10 |
1 comments:
His joy, he found that Einstein's original equations very well, in an expanding universe, there is no need for a fabrication factors.
Post a Comment