Chasing light speed

[sjastro]

Originally Posted by skwinty

What I find extremely difficult to understand is the concept of space not existing prior to the big bang. If space was created at the time of the big bang, what was space expanding into, if not more space.

I agree but cosmology is built around a mathematical model, where space is defined mathematically instead of our perceived notion of space.

Apart form assigning coordinates to space, mathematicans define space by it's metric tensor which is a function that describes the shortest pathway between 2 points in space. For Euclidean space this is simply a straight line, for spherical space it's a length of an arc etc.

Cosmology uses a special metric http://en.wikipedia.org/wiki/Friedma...-Walker_metric.

The cosmological model requires galaxies to be fixed in space and expansion to occur of the coordinate scale. Galaxies are therefore fully constrained in the coordinate system. It's therefore not possible to describe the Universe as expanding in space.

Another way of looking at it is to assume that if the Universe does expand in existing space what are the consequences.
Existing space would be an absolute frame of reference for the Universe. Galaxies would be able to exceed the speed of light in this frame of reference.
Another consequence is that since galaxies are now travelling through space the inflation event would not have occurred shortly after the BB. The Universe would not have time to become large enough to prevent matter from collapsing onto itself due to gravity.

Regards

Steven

[Karls48 (Karl)]

"Existing space would be an absolute frame of reference for the Universe. Galaxies would be able to exceed the speed of light in this frame of reference."

That’s what I have been saying in this and other treads.

"Another consequence is that since galaxies are now travelling through space the inflation event would not have occurred shortly after the BB. The Universe would not have time to become large enough to prevent matter from collapsing onto itself due to gravity."

No one knows how big was the original object that caused Big Bang, or what was the velocity of the matter expelled by it. If the velocity has high enough the inertia should overcome gravitational collapse.
This scenario makes Big Bang theory more acceptable to me. Consequences of this would make General Relativity wrong.

[sjastro]

Originally Posted by Karls48

"Existing space would be an absolute frame of reference for the Universe. Galaxies would be able to exceed the speed of light in this frame of reference."

That’s what I have been saying in this and other treads.

"Another consequence is that since galaxies are now travelling through space the inflation event would not have occurred shortly after the BB. The Universe would not have time to become large enough to prevent matter from collapsing onto itself due to gravity."

No one knows how big was the original object that caused Big Bang, or what was the velocity of the matter expelled by it. If the velocity has high enough the inertia should overcome gravitational collapse.
This scenario makes Big Bang theory more acceptable to me. Consequences of this would make General Relativity wrong.

The speed of light has been measured since the 17th century, is found not to exceed c, and be independant of the motion of the observer. All of which means there is no absolute frame of reference or absolute space.

There was no "object" at the BB. The universe was simply too hot. The first hydrogen atoms began to form around 400000 years after the event, the first galaxies around 500 million years later. Any inertial effect from the BB would come from the expansion of space time. You can't refer to inertial effects of expanding matter at the BB because there was no ejection matter in the first place.

I suggest you have a look at this.
http://en.wikipedia.org/wiki/Timeline_of_the_Big_Bang

Regards

Steven

[AGarvin]

Existing space would be an absolute frame of reference for the Universe.

Fom the viewpoint of General Relativity and BB theory, this statement is correct.

GR is a theory of geometry, and within that geometry you choose a set of coordinates (generally comoving coordinates). As the universe expands with the Hubble flow, these coodinates, including "Cosmic" time, move with it. So even though the distance between two points increases with cosmic expansion, a point in space remains the same with respect to another, providing an absolute frame of reference. Of course ALL objects also have local peculiar motion through space, and Special Relativity comes into play.

No one knows how big was the original object that caused Big Bang, or what was the velocity of the matter expelled by it.

Steven has commented on this but just to clarify. What he is saying is that matter didn't expand at all in the BB, the universe did. Matter is inside the expanding universe. Matter only has local peculiar motion. This is why objects can receed from us greater than c....they are moving with the Hubble flow which has no speed of light restriction. It's the local peculiar motion through space, governed by Special Relativity, that has the c restriction.

Anrdew.

[Chippy (Nick)]

Great thread. Thanks to the posters who have explained some of the concepts in detail (and with references). I thought I had a pretty good handle on it - but there's always someone who knows more! ;-)