# The age of the universe vs. the speed of light

1. Nov 30, 2011

### cisco kid

I have recently been watching the series "Into the Universe with Stephen Hawking" and I have a question (commence eye-rolling of anyone reading this). I don't think the series is supposed to be an educational tool necessarily, its more of a horizon expanding view of some high level topics with low level explanations. At any rate, Dr. Hawking stated something that I am having a hard time wrapping my head around. I understand that the laws of physics state that there is a maximum speed limit, which we call the speed of light. This particular episode is explaining the origins of the universe via the big bang. He states "by the time the cosmos was ten minutes old, it was already thousands of light years in diameter." My question is, how could the matter travel thousands of light years in ten minutes? Wouldn't it take thousands of years, if that is the maximum speed?? Obviously I'm missing something here...

2. Nov 30, 2011

### mathman

The general idea is something which goes by the name "inflation", where space expanded at an extremely high rate carrying stuff with it.

http://en.wikipedia.org/wiki/Inflation_(cosmology [Broken])

Last edited by a moderator: May 5, 2017
3. Nov 30, 2011

### Staff: Mentor

The key thing to understand is that nothing can travel THROUGH space at greater than light speed, aka c, however inflation and expansion of the universe happens because space itself is expanding and carrying matter and radiation with it. Thus two things that are a billion light years apart can be moving through space at 200 km/s towards each other yet because of expansion the distance between them is actually increasing over time.

4. Nov 30, 2011

### cisco kid

Crazy. So I guess that is why people use the raisin bread analogy? It's impossible for the raisin to move through the dough at the speed of light, but combined with the expansion of the dough it's speed relative to another raisin would equal a speed faster than light? Something like that?

5. Nov 30, 2011

### Staff: Mentor

That is exactly right!

6. Nov 30, 2011

### cisco kid

Awesome! Thanks for explaining!

7. Nov 30, 2011

### wildwill

but what is it expanding into?

8. Nov 30, 2011

### Staff: Mentor

To the best of our knowledge it is not expanding into anything. Read the FAQ here on the cosmology forum.

9. Nov 30, 2011

### wildwill

then how is it expanding?

10. Nov 30, 2011

### Staff: Mentor

11. Nov 30, 2011

### wildwill

okay then thanks i think i understand it

12. Dec 1, 2011

### Calimero

Hi Drakkith. I fail to understand your first sentence. How do you measure or define object's speed or velocity relative to the space ?!? If you mean that nothing massive can't outrun light, then say so. But nothing massive can't outrun light in expanding space also.

If someone comes here looking for an answer to a basic straightforward question, like cisco kid did, it is much better to link him to the some cosmology tutorial, like ned wright's pages, instead of trying to give him wrong answer.

13. Dec 1, 2011

### D H

Staff Emeritus
Drakkith's answer, while perhaps a bit too terse is essentially correct.

The Davis and Lineweaver paper, http://www.mso.anu.edu.au/~charley/papers/DavisLineweaver04.pdf, describes three common misconceptions regarding the expansion of space:
#1: Recession velocities cannot exceed the speed of light.
#2: Inflation causes superluminal expansion but normal expansion of the universe does not.
#3: Galaxies with recession velocities exceeding the speed of light exist but we cannot see them.​

The paper delves into each of these misconceptions, showing how they are just that (misconceptions, that is).

14. Dec 1, 2011

### Calimero

From the paper you linked to:

"No observer ever overtakes light beam and all observers measure light locally to travel at c"

You completely misunderstood my post. I didn't say anything about superluminal recession velocities between galaxies, not a word.

Again, what does it mean to say: nothing can travel through space at greater then light speed? How do you define speed relative to the space? With aether, or what?

Edit: DH, I feel that we shouldn't have this discussion at all. Bcrowell wrote a nice FAQ on the subject, linking to the same paper you did. It is just painful to see someone with PF mentor badge defending statements like Drakkith's #3 in this thread.

Last edited: Dec 1, 2011
15. Dec 1, 2011

### Staff: Mentor

You don't measure it relative to space, as you cannot as far as I know. However it is believed that the expansion of space is responsible for the increasing distance between objects in the universe. I simply mean that if we could stop expansion of space and just see the velocities of objects due to inertia we would see that many objects billions of light years away are actually traveling towards us. Their current observed recession velocities are due to the expansion of space, not through inertia.

Edit: Actually, if you know the rate of expansion you can calculate the effect that expansion has on objects. Since expansion increases recession velocity by 73.8 ± 2.4 (km/s)/Mpc we can measure the velocity of an object, remove the effect due to expansion, and just see the velocity of that object "moving through space".

What was incorrect about my explanation?

Last edited: Dec 1, 2011
16. Dec 1, 2011

### Calimero

Right.

Debatable, but ok.

Wrong. You can not distinguish between the two. Locally, Hubble flow can be viewed as pure kinematical phenomenon. Objects recede because they have an initial velocity proportional to distance.

Please stop talking about velocity through space.

What was correct? Please read bcrowell's FAQ: At what velocity does the universe expand? Can it be faster than light?
Do you see there any distinction between objects moving through space and objects moving with space as an attempt to explain superluminal recession velocities?

Last edited: Dec 1, 2011
17. Dec 1, 2011

### Cosmo Novice

I fail to see how bcromwells FAQ disagrees with Drakkiths fundamental statements.

I think what Drakkith is saying is clear and essentially correct.

Expansion and increase in the scale factor is responsible for overall expansion of the Universe regardless of individual kinematic motion.

Calimero you are claiming Drakkith post is wron but not stating why? Kind of a difficult debate for him to have.

18. Dec 1, 2011

### Staff: Mentor

There is no need to locally, as there is no expansion locally since we are gravitationally bound to everything within our local galaxy cluster or supercluster I believe.

Umm. No?

The last line of the FAQ:

From the paper linked in the faq:

I'm not seeing where I'm wrong.

19. Dec 2, 2011

### Calimero

Expansion is due to the inertia. It should be clear to you, but it obviously isn't. Galaxies move apart because they did it in the past. Accelerating expansion, like the one we are witnessing, is partly due to inertia, and partly due to the dark energy repulsion. In the early universe it was almost purely inertial. Neither of these are caused with expanding space. The very meaning of the notion of expanding space is not rigorously defined. It is not a law or a cause of anything, rather it is is an intuitive framework for understanding the effects of General Relativity.
It is very wrong to say that universe is expanding because space is expanding and carrying stuff along. And it is at least very misleading to say that it is ok for two objects to recede faster then light when they are moving with space, but not through space.

How come then that there is apparent superluminal motion between distant objects? Again bcrowell's FAQ. If you define distance as proper distance D, and time as cosmological time t, then FRW metric fails to put a limit on distance of validity of Hubble's law, and differential dD/dt can exceed unity. It does not violate special relativity prohibition on light speed in any way.

20. Dec 2, 2011

### D H

Staff Emeritus
No, it is not.

Expansion of space is quite different from inertia. Your view is not consistent with the big bang theory. The inflationary period (the question raised in the opening post) cannot be explained in terms of inertia. Although expansion now is much attenuated compared to that in the inflationary period, even the present-day expansion of space cannot be explained in terms of inertia. The observable universe is estimated to be about 93 billion light years in diameter, yet the universe is estimated to be only about 13.75 billion years. These two figures do not jibe with one another if expansion is inertial only.

You appear to have the mistaken concept that the big bang was an explosion in space. A much better point of view is that the big bang was an explosion of space.