Cosmology Question...

[NeoGreen]

...that my 16-year old asked me after watching the "Dark Matter" presentation at the American Museum of Natural History in NYC last weekend.

A question of cosmology that, I am sorry to say, this old geologist was unable to answer, and which caused me to lose points in the "dad-who-knows-everything" department of parenting. I was never as good at the quick-response made-up answers like Calvin's dad as I have always wanted to be:





So, anywho...the question:

"Cosmologists say that the Universe started as a singularity and immediately after the Big Bang (within the first milliseconds) expanded to a size bigger than the Milky Way. If so, doesn't that rate of expansion exceed the speed of light? How is it possible, I thought nothing could go faster than light?"

So, to all my Cosmologist friends out there in DU, do you have a reasonable (i.e. non-Calvin-dad) explanation?

Thanks ahead of time...
NG

[AuntPatsy]

1. Too cute

[TreasonousBastard]

2. I am not a cosmologist, and so...

may be full of it here, but it is my understanding that the present rules did not exist back then. Light didn't immediately exist, and therefore had no speed limit.

In the picoseconds after the big bang, it wasn't matter and energy spreading out to fill space, which would be limited by the speed of light, but space itself expanding, sucking up light and everything else along with it at incredible speeds. If "speed" would even apply.

Now, the real question is-- just what was space expanding into? What is the non-space that "existed" before the big bang?

[4139]

3. The explanation, which I don't quite grasp:

You ask a good question, one whose answer lies in the subtle difference between expansion that is faster than the speed of light and the propagation of information that is faster than the speed of light. The latter is forbidden by fundamental physical laws, but the former is allowed; that is, as long as you are not transmitting any information (like a light pulse), you can make something happen at a speed that is faster than that of light. The expansion of the Universe is a "growth" of the spacetime itself; this spacetime may move faster than the speed of light relative to some other location, as long as the two locations can't communicate with each other (or, in terms of light rays, these two parts of the Universe can't see each other). According to the theory of inflation, the Universe grew by a factor of 10 to the sixtieth power in less than 10 to the negative thirty seconds, so the "edges" of the Universe were expanding away from each other faster than the speed of light; however, as long as those edges can't see each other (which is what we always assume), there is no physical law that forbids it.
http://curious.astro.cornell.edu/physics/109-the-universe/cosmology-and-the-big-bang/inflation/664-how-can-the-universe-expand-faster-than-the-speed-of-light-during-inflation-advanced

[Igel]

8. This makes sense.

If information can travel no faster than the speed of light, that means there's going to be a limit based on distance and time.

Take an observer in space. I'm also in space and turn on a very bright light source. It will take time for that information to reach them; if you don't like having "light = information," assume I'm sending Morse code. They see the light after it's travelled. Now, that light spreads out in a sphere, but let's simplify it to a circle by saying that all potential observers are on a plane. Each second, the circle of potential observers increases. If you're too far away, you see nothing until the light reaches you. You can't see light that's traveled faster than light speed, so there's a time limit on how quickly you can get the information.

Now let's assume that before the light reaches you I stretch space twofold (or a million-fold) along that plane in a way that's faster than light. You still can't see the light any faster; the light-speed limit on information holds. The best that'll happen is it'll take longer for the information to reach you. In fact, the information is just spread out, so my dots and dashes each become longer.

When my students fail to get this, and since it's not an easy thing to get who can blame them, one of two things happen. Either they insist on a giant time suck in order to understand it--but we don't have that much time and for some there's not enough explanation time left before most of the protons in the universe decay. Or they give up. If they want the details, I start talking about Minkowski space and within a minute their eyes glaze over and they're done. I'm not an expert, but "expert" isn't required.

Or I reason by means of pun. "What goes faster than the speed of light?" They answer, "Nothing." My response is, "Precisely, so you get it now." This confuses them even more since they've gotten precisely nothing. "Space time by itself contains nothing. The one thing that can go faster than the speed of light is nothing. Nothing can go faster than the speed of light."

The check for "understanding" is the groans at the pun. No, it's not quite right but at least they've gotten sort of a glimpse at the core of a hint of a suggestion as to what's discussed. Which for high school seniors can be an accomplishment. For those not pissed off at me, I throw in, "Since information is measured relative to space time, from the information's point of view it hasn't gone faster than the speed of light. It hasn't moved, everything else has but it doesn't notice. And there can be no outside observer that would observe anything violating the light-speed limit on information."

[DetlefK]

4. Imagine an ant crawling on a ballon.

The ant is running at its maximum speed.
Then you blow up the balloon and it expands. The ground the ant is running on moves.
The speed of the ant is in no way related to how fast you are blowing up the balloon.
And if you now measure the velocity of the ant, let's say relative to the nozzle, you will see it is now running faster than it actually can run.

[RussBLib]

5. it's very simple...

It is an unanswerable question considering our current, modern tools.

So, I don't bother with it.

Simple.

(sorry)

[rock]

6. The answer to your question is

In the first few millisecs or less the laws of physics were not stable, i.e. miracles (that is a once in a universe occurrence never to be repeated) could happen . Although no other theory in physics allows/uses such a concoction and would get laughed right out existence if it did, that's the way it is in Cosmology. I'm not making this up.

[NeoGreen]

7. So...what you're saying is...

Last edited Thu Jul 9, 2015, 11:34 AM - Edit history (1)

...step two is as refined as we can get it?



Oiy...I think I'm going to be roasted by a 16-year old tonight...

[rock]

9. LOL!

Yes. It's as simplified as possible.

[AlbertCat]

10. Expansion vs Velocity

Nothing can have a velocity greater than the speed of light. (unless there's some exotic math that says otherwise)

But space expanding is not the same as velocity.

Yes, it is counterintuitive. We don't experience the expansion of space... even tho' the space between you and your computer is expanding even as you read and type. We just cannot perceive it.


Anyway, that's what I got from reading books by Lawrence Krauss.



Another quandary is.... so if the space is expanding...what is it expanding into. We think that way because when we experience expansion, it's like a balloon blowing up and that's how we envision the universe expanding. But we can only experience something expanding into the space around us. That very space expanding doesn't have to have something to expand into.

Counterintuitive again! But it still makes sense. And the math used to prove these things is correct, and math is real.

I also hate the notion of the Big Bang being an explosion. You see it on science programs all the time with a big BOOM to go with the Big Bang. But that's not right. It just started expanding.... it didn't explode.

Anyway.... that is what I have come away with by reading and watching science-y things. I am really just a costume designer from NC. But if I can sorta "get" these notions...and why they can never really be thoroughly "gotten" (all of our experiences do not help us here) then you and your son can get it!

And I hope anyone who knows better will correct my layman knowledge.

[PJMcK]

11. A Universe From Nothing

Actually, the rate of expansion of the universe is increasing and one day in the far distant future, the galaxies will be moving away from one another at velocities exceeding the speed of light. The General Theory of Relativity supports this idea.

There is an excellent lecture viewable on YouTube titled "A Universe From Nothing" by Dr. Lawrence Krauss, a world-renown cosmologist and particle physicist. It's a fascinating presentation of how our universe probably formed, how it works and what will happen in the future. He explains how multi-verses could exist and how dark matter and dark energy affect the our existence. In addition, he illustrates the history of science and explains how science works while making many observations about the ongoing discoveries by scientists around the world and throughout human history.

Although the mathematics and more complex scientific concepts he discusses are difficult for laypeople, Dr. Krauss explains them in an easy to follow manner using PowerPoint projections, a wry and sly sense of humor and remarkable intelligence. I've watched this video several times, as well as some of his other lectures, and I think you'll find your answers there.

[FiveGoodMen]

12. I think I heard Krauss in one of his talks (on Youtube) say...

Nothing can travel through space faster than light, but space can do whatever the hell it wants.

(or something close to that)

[NeoGreen]

14. Ok, so does that mean...

...that, when the expansion (relative to your position) exceeds the speed of light, the sky will go dark and be without (visible) stars?

[3c273a]

13. m=0

We know the ratio e=mc2.
In the early universe there was no matter, until it precipitated out of the fog of energy and locked in the ratio.
As long as m was 0, c was infinite.

[cpwm17]

15. Right now, space is expanding faster than the speed of light.

We very likely only see a very small fraction of our entire Universe (and there are likely many universes beyond.) Most of our Universe is likely beyond our visible horizon and most of it is moving away from us at faster than the sped of light.

Nothing can travel through space at faster than the speed of light, but space can expand faster than the speed of light.