The Big Bang has happened 15-20 milliard years ago,why there are stars and ..15 milliard light years far?

Question

It much more than 15 milliards years for stars and other objects to go so far distance unless they had gone faster than light!,regarding big bang time,it seems impossible.

Answer 1

I think you mean 15 million light years. The expansion of the universe in the inflationary era was very quick and the speed of expansion was much faster than light , indeed. The restriction of massive particles moving slower than light is within the space time. The space-time itself can expand at any speed. The key point to remember is that space-time, in physics is not just mathematical space of our imagination. It has very physical existence and is governed by a set of quite well understood laws of physics.

See the discussion below:

The Big bang Hypothesis:
The Big Bang is a hypothesis based on the fact that the universe is expanding. The conjecture is that at the beginning of time, all the matter and energy of then universe was packed into a tiny space, in fact into a singularity with INFINITE density.

There was nothing (matter or energy) around that singularity, but you can always construct mathematical space of infinite extent around any point as well as around the Big Bang singularity. The difference is that that mathematical space is not space-time i.e. something physical. It would be just a mental construct.

Expansion after Big Bang:
Hubble discovered that each and every galaxy is moving away from each other in every direction. It is like a number of dots on a balloon, which when inflated move away from each other. It is expansion of each dot from the other in all direction on the surface of the balloon.

Now, just after Big Bang, the space-time that we live in just appeared (like a magic), but of course, due to extreme density of mass-energy , it was highly curved (according to the Einstein's general theory of gravitation). then it started expansion era, of different kind, like inflation, normal expansion, etc. and all the matter/energy particles moved away from each other. This is still happening but at a much slower rate.

Inflation:
Cosmic inflation is a process of extreme accelerated expansion of the Universe. According to the standard inflationary model, proposed in 1981 by A. Guth and elaborated on by many others, the energy of the vacuum was not zero but increased to a huge value by the inflation field. The high vacuum energy density drove this accelerated expansion during the very early instants of the Big Bang. After about 10-35 seconds, there began a brief period of exponentially fast expansion, known as inflation, that ironed out any curves or warps in space and made the universe flat (because it becomes so large).The universe expanded from the size of 10-50 to the 103 m in a brief period of 10-35 to 10-32 second. Thus if you calculate the speed you will naively get 10^(3)/10^(-32) = 10^35 m/sec.

Answer 2

Space itself is expanding. Two objects in space that are traveling in opposite directions from the same starting point have not actually traveled the entire distance that is between them; part of it is essentially the product of space being created between them over time.

EDIT: for those of you who don't know, "milliard" is alternate terminology for a billion.

Answer 3

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Answer 4

I have no idea what a milliard is...if I did, I may be able to explain.

Answer 5

15-20 billion years is = earth year = time
far away from 15 billion light years mean a distance.

Answer 6

I watched a show on the NG channel not to long ago that stated the universe is not just expanding, but that it is speeding up as it does so. Hope that helps.