caharris Posted February 13, 2011 Share Posted February 13, 2011 I've probably misunderstood something, so if you could help me understand I would appreciate it. If time is relative, how can we actually say how old the universe is? It may be 13.7 billion years old for us, but couldn't it be different somewhere else? These are the questions that haunt me Link to comment Share on other sites More sharing options...
michel123456 Posted February 13, 2011 Share Posted February 13, 2011 This is not speculation. It looks like a fair question. Or do you have a hidden agenda? Link to comment Share on other sites More sharing options...
caharris Posted February 13, 2011 Author Share Posted February 13, 2011 (edited) No agenda, I still state that the universe is 13.7 billion years old and came from natural causes. I just thought it would be wiser to put it in 'Speculations' since it could lead to speculations (For example, if we get 13.7 billion years old from our sense of time (from our gravity and speed) couldn't we say that the universe is only a second old from another perspective, and still be factually correct?) But if you think it should be moved, by all means, let's get it moved Edited February 13, 2011 by caharris Link to comment Share on other sites More sharing options...
ajb Posted February 13, 2011 Share Posted February 13, 2011 When we say the Universe is 13.7 billion years old, this is referring to the cosmological time. The cosmological time for an observer at a fixed spatial point in comoving coordinates is identical to his local measurement of time. That is we consider an observer at rest with respect to a galaxy (probably better is a cluster of gravitational bound galaxies) and their proper time. Remember that on large enough scales the Universe is isotropic and homogeneous, so the clocks will all "tick at the same rate". Using this notion of time you find that all parts of the Universe are the same age, about 13.7 billion years old. There are other ways of defining the time which will give different ages of the Universe. 2 Link to comment Share on other sites More sharing options...
caharris Posted February 13, 2011 Author Share Posted February 13, 2011 So it's measured by an object that moves with the expansion of the universe (so it looks like it isn't moving)? Also, what is a good book/website that explains this in a more in-depth/mathematical detail? Link to comment Share on other sites More sharing options...
ajb Posted February 14, 2011 Share Posted February 14, 2011 So it's measured by an object that moves with the expansion of the universe (so it looks like it isn't moving)? Yes, you can say that. Also, what is a good book/website that explains this in a more in-depth/mathematical detail? I like Andrew Liddle An Introduction to Modern Cosmology, 2nd Edition, Wiley-Blackwell. He explains many of the modern ideas in cosmology without employing general relativity. Thus, you can get acquainted with cosmology before you study general relativity. Steven Weinberg's Cosmology, Oxford University Press seems an up to date introduction to cosmology. It is aimed at graduate students and similar, so I don't know how you would get on with it. Link to comment Share on other sites More sharing options...
caharris Posted February 15, 2011 Author Share Posted February 15, 2011 Thank you for clearing everything up, I do appreciate it. I'm getting ready to start my first year of college this fall (majoring in physics, of course ), so even though your recommended books may be over my head, I can always google it Link to comment Share on other sites More sharing options...
caharris Posted February 16, 2011 Author Share Posted February 16, 2011 By the way, does the figure we get, 13.7 billion, take into account the different accelerations? Link to comment Share on other sites More sharing options...
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