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I had recently watched a video and he says big bang could have happen by the slow collision rather than a fast collision. But I know that there was no time before the big bang and fast and slow are measurements of speed which is distance/time. So how can we say fast or slow when there is no time?

Qmechanic
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Avi
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    $\uparrow$ Which video? Which minutes? Related: http://physics.stackexchange.com/q/44538/2451 and links therein. – Qmechanic Apr 25 '16 at 05:00
  • Was the video referring to the ekpyrotic theory i.e. that the Big Bang was the collision between two branes? – John Rennie Apr 25 '16 at 05:09
  • I am sorry that I don't know which video it was and I am not able to get it now. I remember it was about brane theory. – Avi Apr 25 '16 at 05:25
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    How do you know that there was no time before the big bang? Were you there, and when you looked at your watch, it wouldn't work? – CuriousOne Apr 25 '16 at 05:39
  • Big bang is said to be the origin of universe and time is a property of universe.Time originated along with universe. – Avi Apr 25 '16 at 06:20
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    The big bang is a model for the last 13.8 million years. It doesn't say anything about "the origin" of the universe (whatever that is supposed to mean). One can easily make models that go back smoothly to $T_{cosmological}<0$, while still losing basically all information about the pre-big bang state of the universe. Admittedly, the public has been bombarded with a lot of awfully distorted versions of what all of this means, so the confusion is understandable. What's "out there" is simply not what physicists really think about these issues. – CuriousOne Apr 25 '16 at 07:09
  • @CuriousOne I had read that big bang is origin of universe in many places. example – Avi Apr 26 '16 at 04:24
  • I guess that first we need to answer, "what is time?" – CGH May 05 '16 at 14:12

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One of ideas associated with string theory is the ekpyrotic universe. This starts with brane cosmology i.e. the idea that our universe is a four dimensional brane floating around in the ten dimensional string theory spacetime.

There will be many such brane worlds and the ekpyrotic idea is that a collision between two branes would appear just like the Big Bang inside the brane world. So if this idea is correct our Big Bang was just a collision with another brane world and wasn't the beginning of the universe. Time was running smoothly in the ten dimensional string spacetime before, during and after the collision.

For completeness we should note that the ekpyrotic universe is just one of many ideas in string theory for which no compelling evidence exists.

John Rennie
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