# The speed of light

1. Jul 2, 2013

### Alpharup

I have learnt that the universe came from big bang and it's expansion is accelerating....Now let us assume that there is only one universe and that there is no multiverse....
According to postulates of relativity, in any frame of reference, no body can travel faster than the speed of light....Let the cosmic speed limit be 'c'.....
Now since we are present in universe, space can only exist in the universe...Outside the universe, space doesn't exist......
Let the magnitude of acceleration of universe be 'a'.....
My question is "what is the speed of expansion of the universe just after the big bang?"....My arguement is that the speed of expansion of the universe just after big bang is greater than or equal to 'c'....
Let us consider that the speed of expansion of the universe just after big bang is less than 'c'...Let the time after the big bang at which the speed of expansion of the universe becomes 'c' be 't'....
Let us consider that we have have a torch between the times '0 'and 't' at the edge of the universe....When I light up the torch, the light travels a speed of 'c'(According to the special relativity postulate) which is greater than the speed of expansion of the universe..ie...light travels out of the universe...ie...it travels in the absence of space...This is in contradiction to the fact than matter can exist without space....Einstein's postulate is not in contradiction to the fact that space expands faster than light...But here the speed of light is greater than the rate of expansion of space...This is a contradiction that matter(light) can exist without space.....So the big gang theory itself is wrong is if we assume that the speed of expansion of the universe just after big bang is less than 'c'....
Now let us come to a point....If the big bang theory were true if the speed of expansion of the universe just after big bang is less than 'c', we have a possibility;

Either the cosmic speed limit(c) varies with time and is always less than the speed of expansion of universe at any point of time...Say, If the speed of expansion of the universe after time 0.2 seconds is 1 m/s, then the cosmic speed limit at that time,ie...the speed of light(Special Relativity postulate) at that time will always be less than 1 m/s(say 0.99 m/s)...This is not against the special theory of relativity(Because all other objects must travel at a speed less than 0.99 m/s at that instant)

Don't get me wrong..We can modify a postulate of special relativity like "Nothing can travel faster than the speed of light which is the maximum cosmic speed limit at that time in any frame of reference"

But it is highly probable that you won't accept my idea...So we can mostly discard it...So, we have another option of modifying big bang theory itself...But it is speculated that there is high evidence for support of this theory by analyzing red shifts from the movement of galaxies in space...So, the possibility of modifying the big bang theory is highly ruled out....

So, the idea of speed of expansion of the universe just after the big bang being lesser than the speed of light(c) can be ruled out..Hence I suggest that the speed of expansion of the universe just after the big bang is greater than the speed of light(C)..

Now they speculate that the universe has accelerated expansion..If so, what is the magnitude of that acceleration? What is the present speed of expansion of the universe?

2. Jul 2, 2013

### Staff: Mentor

No, that's not what the postulates of relativity say. They say that no body can travel outside the local light cones. In a local inertial frame of reference, that statement is equivalent to saying that no body can travel faster than the speed of light; but that's a much weaker statement than the one you made, that no body can travel faster than the speed of light in *any* frame of reference.

For example, in the usual coordinates that are used in cosmology to describe the universe, it is perfectly possible for objects to travel faster than the speed of light. In fact, it's perfectly possible for light to travel faster than the speed of light (where by "the speed of light" we mean 299,792,458 meters per second)! This is because those coordinates are not inertial coordinates, and the "frame of reference" they describe is not a local inertial frame of reference.

The fact that non-inertial coordinates and non-local frames of reference behave differently than local inertial frames is why we prefer to state the postulates of relativity in a more general form that doesn't depend on local inertial frames, as I did above. The light cone structure of spacetime is invariant; it doesn't depend on which coordinates we choose.

In other words, we don't need to modify special relativity or the big bang theory. You need to modify your understanding of how those theories work.

3. Jul 2, 2013

### Staff: Mentor

In addition to the previous comments, there is no edge to the universe.

Also, what is with the excessive use of . marks? If your keyboard has a sticky key the best thing to do is to pry it up and clean out the sticky substance. Otherwise, it just makes you look ignorant of basic grammar.

4. Jul 3, 2013

### nitsuj

its fun to muse over simple physics like this, but don't mistake that as a "scientific", like I sometimes do :tongue2:

in SR spacetime seems to be the stuff in between events, and not something "physical" itself, there is no "aether" or whatever in SR.

Last edited: Jul 3, 2013
5. Jul 3, 2013

### phinds

No, "they" do not SPECULATE, "we" observe and (sometimes, as in this case) draw firm conclusions based on the data, one of which is that the universe IS expanding at an accelerating rate. I forget what the rate per unit distance is (you can find it posted in many threads here) but at the edge of the observable universe (47billion light years from Earth), the expansion is something like 3c.

EDIT: there's some decent discussion of this in the link in my signature

6. Jul 7, 2013

### TheBC

Spacetime 'in between' events'?
There is no 'in between' events. There are only events, and they are all making up 4D spacetime.

7. Jul 7, 2013

### The_Duck

Hi sharan swarup,

1. In the current theory of cosmology, the universe has no "edge" and no "outside," so it doesn't make sense to talk about a region outside the universe where there is no space. The universe seems to be infinite. See also Wikipedia on the question "what is the universe expanding into?"

2. There was in fact a period of "inflation" near the beginning of the universe when space was expanding so fast that nearby objects were carried away from each other faster than light. Note that this does not require any modification of existing theory of relativity. Space can expand as fast as it likes in general relativity.

3. The current rate of expansion of the universe is given by the Hubble constant.

8. Jul 7, 2013

### nitsuj

Ya sorry, I sometimes forget how strict physics jargon needs to be and use "layman" definitions, but the use should be clear by what I said, in any case the measurements of length / time between happenings. Weird point/

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