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Thread: 13.7 billion years?

  1. #1 13.7 billion years? 
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    One thing that has bothered me for a long time has to do with the speed of light and how scientists have used this 'constant' to determine the age of the Universe. My question is....Since Einstein proved that gravity bends light rays....and since my high school education informs me that the shortest distance between two points is a straight line, how can we be sure when these photons left the Big Bang, since, by definition, every photon has been zig-zagging its way to Earth? Maybe the Universe is much older? And since science has confirmed that all galaxies are moving away from each other, would someone please explain colliding galaxies to me? Thanks.

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    Moderator Moderator Janus's Avatar
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    While gravity does bend light, it takes a very close pass by a very large mass to bend it signicantly. For example, light just grazing the surface of the Sun is only bent by 1/2117 of a degree. The vast majority of space is empty, and most the light passing through it doesn't pass anywhere deep enough into a gravity field for it to be bent by an measureable amount. Even when it does we know it and can compenstae if needed. For example, if we a looking at a galaxy 5 billion light years away, and the light from it passes by a closer galaxy on its way to us and is bent. We see that nearer galaxy as being almost inline with the further galaxy and know that the light was bent passing it. If we don't see anything between us and the galaxy, we know that the light never passed by anything that could have measureably bent the light on its way to us.

    Besides that, the speed of light as a constant is not used to determine the age of the universe.

    As far as galaxies colliding goes. It is actually clusters of galaxies that are receding from each other not individual galaxies. Gravity holds these clusters together against the general expansion of the universe. The galaxies in these clusters orbit around their mutual center of gravity and from time to time these orbits intersect, leading to collisions.

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    Thanks so much, Janus. Your reply makes a lot of sense.
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    One last thing....Knowing that galaxy clusters are speeding away from each other, therefore increasingly less mutual gravitational pull on each other, why wouldn't the expansion be speeding up? And, with this in mind.....and considering the 'Big Crunch' what point would gravity suddenly reverse the expansion properties of dark energy and bring the Universe back to a 'Big Crunch'? That doesn't make sense to me. And (sorry, I can't stop) why is our orbit around the Sun not growing as the Sun losses mass by burning its fuel? It would seem to me that by the time the Sun has lost enough mass to become a red giant, the orbit of the planets would be wildly different and, in fact, we wouldn't expect the Sun to overcome Earth's orbit, because Earth would be long gone...aimlessly wandering the Universe. And, as far as killer asteroids go, we've already proved we can land a spacecraft on an asteroid. What about landing on the asteroid, rotate the landing engine 180 degrees, fire it up, and literally push the asteroid into another trajectory? Whew! Thanks for letting me get that out. I don't expect a long answer. lol
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    Genius Duck Moderator Dywyddyr's Avatar
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    Quote Originally Posted by jmcon007 View Post
    One last thing....Knowing that galaxy clusters are speeding away from each other, therefore increasingly less mutual gravitational pull on each other, why wouldn't the expansion be speeding up?
    They would only speed up if there was a [constant] force causing the expansion.
    If it were simply the initial "thrust" then acceleration wouldn't happen: a thrown ball doesn't get faster as it rises and gravity reduces.
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