Relativity Question

[Biodizzle]

This is my first post. I had a thought occur to me the other day that seemed intriguing, but did not add up to me. I am fairly new at physics as a recreational hobby, so don't be insulted if this seems silly.

 

It is a known fact that the universe is accelerating in its expansion. This implies a speed of expansion less than that of c, due to Einstein's Relativity Theory. Is it therefore possible for information in the form of electromagnetic/gravitational waves to "leak out" of our universe? If so what implications may this hold?

 

If this is completely out of line, tell me where my flaw in logic is

.

[Saryctos]

If this is completely out of line' date=' tell me where my flaw in logic is

.[/quote']

right here

It is a known fact that the universe is accelerating in its expansion.

[Biodizzle]

Wait,

 

I thought that increasing redshift values of distant galaxies and quasars as their distances from Earth increase, indicates a universe that is not only expanding, but accelerating in its expansion.

[Biodizzle]

I seem to be incorrect as to the accelerating expansion of the universe being a "known fact", but there is VERY strong evidence for it through observation. Here is an example I took from one of many websites that discusses this issue. Look into it further. It is incredibly interesting......[/i]

 

Why do we think that the expansion of the Universe is accelerating?

 

"The universe appears to be expanding at an accelerating rate. This is inferred because distant supernovae are unexpectedly dim. This is interpreted as implying that the expansion of the universe is faster now than it was before. This expansion is in turn explained by some mysterious repulsive force that is pushing the universe apart."

 

This can be a topic of another debate, but for now, and assuming, that the universe is accelerating in its expansion, can someone answer my question?

[Jacques]

It is a known fact that the universe is accelerating in its expansion.

I would say strong evidence from the supernovae survey like you said.

This implies a speed of expansion less than that of c, due to Einstein's Relativity Theory.

Galaxies can recess faster than the speed of light. Recession is not a motion of the galaxies themself, but more an expansion of space between them (taht what I was told, maybe someone can confirm that?)

[timo]

This is my first post. I had a thought occur to me the other day that seemed intriguing' date=' but did not add up to me. I am fairly new at physics as a recreational hobby, so don't be insulted if this seems silly.

 

If this is completely out of line, tell me where my flaw in logic is. [/quote']

Some physics concepts are really easy to grasp verbally. Someone has an idea, builds up a mathematical framework around it and publishes it in a book called something like "principles of mathematics". Statements like "in absence of air resistance, all bodies fall down equally fast" may be unintuitive, but you can easily understand their meaning -even if you don´t know to which mathematical framework they translate- as they stood at the beginning of the developement of some framework.

But some statements about physics are -in my opinion- absolutely impossible to understand without knowing the mathematical framework. I´m talking about statements that are derived from this framework (not nessecarily all of them, though).

 

If you´re only interested in serious replies, you may skip the following; i´ll get serious again after the next quote. But for now:

 

 

Take some time for a brief history: BREAKING AN ACADEMIC NUTSHELL TO SPREAD RESEARCH IN THE UNIVERSE

 

Let´s say you have a great new theory about gravitation which is also consitent with special relativity. It bases on a four-dimensional geometric space whose structure is descibed by an object with ten entries and a complicated differential equation which describes how these ten entries can be calculated from a given distribution of masses.

With this great new theory you try to describe the universe. After making some assumptions you have come up with a reasonable coordinate system to describe the universe and a convincing assumption for the mass distribution in the universe (with some free parameters). You can start plugging these into the very complicated differential equation for the ten entries of the object describing the geometry of your 4D-universe.

Your calculations turn out to be successful. They are consistent with observations and show a coordinate-dependent behaviour for the ten entries which all of your fellow physicists who also work in the field of examining the very complicated differential equation get really excited about - a breakthrough in physics.

Some newspaper gets interested in writing about this breakthrough in physics. But for some reason, the reporter decides that most of his readers wouldn´t understand it when you say "using <whatever assumptions> we have found a solution to <very complicated differential equation for object describing geometry> and it is <equation given in the chosen coordinate system>". He asks you if you can rephrase is so that it´s easier to understand and perhaps a bit more interesting. Hell no, you can´t. How could such an elegant solution be made any easier? But then, you need to get people interested in your work - because where there´s no interest, there´s no money; and you don´t want to risk losing your job and having to do real work to make a living.

- "err ... the metric is constant in the coordinates x,y and z but varies with the time-coordinate", you tell the reporter.

* "That doesn´t sound like news anyone will get excited about", the reporter replies. "Besides, what´s a metric, anyways".

- "That´s the object describing the structure of the universe, it´s the single most important thing in our whole theory."

* "Oh, so then it seems I understand why your research is so popular. If this 'metric' describes the structure of the universe and you calculated it, then there certainly are some physically interesting results you came up with, no?", the reporter asks in a last attempt to get some human-understandable information out of that weird freak (you).

- "Of course. For example, we could show that in our coordinate system, there´s only one nonzero entry in the time-associated entries of the metric which is constant. But even more importantly, all the other entries scale with the same factor which is only dependent on time. And this factor increases with time".

* "What does this mean when those entries increase with time? Can I somehow visualize that?". Obviously, you have met an experienced science reporter. He already managed to understand spacetime curvature and the bending of light on large masses by seeing the rubber-sheet analogon. And luckily, you actually CAN visualize it because you already had to do one of these annoying comedy shows (lecture) for the working drones (PhD students).

- "Well, I can give it a try. Imagine a balloon. Mark two points on the balloon - that would be two stars. Now, blow up the balloon. The two points -the stars- remain on the same position on the balloon. But the distance between them increases as the balloon expands". Finally you found someone interested in that analogon. The article will show that stupid optican who looked at you as if you were an alien when you told him you need new glasses because the universe expanded too much.

* "So one could say the universe expands, too?"

- "Yes, in the sense that distances between points increase over time."

 

The little story comes to an end here. Of course, it´s completely made up and has nothing to do with how things are in reality. It also has no moral.

Have you ever read "The Robertson-Walker metric is [math] ds^2 = dt^2 - R^2(t) \left[ \frac{dr^2}{1-kr^2} - r^2d\theta ^2 + r^2 sin^2 \theta d\phi ^2 \right] [/math]" in a newspaper article about a cosmological topic, btw?

 

 

 

It is a known fact that the universe is accelerating in its expansion.

This implies a speed of expansion less than that of c, due to Einstein's Relativity Theory.

No it doesn´t imply it. It´s a terrible pitfall in relativity: When physicists say that the universe expands, this is not meant in the sense of a shockwave of an explosion expanding out through ... well, there you have your first problem: A shockwave expands in/through air. What does the universe expand into when there´s nothing else than the universe?

The expansion is usually visualized by a balloon that´s blown up (for later use, let´s place an ant on the balloon). If you paint two points on the balloon and start to blow it up, the distance between the two points will increase even though the points stay on the same place on the balloon. This increase in distance is NOT limited by lightspeed. In this analogy, the radius of the balloon is the time and the very surface of the balloon is space, btw. And it´s only an analogy so don´t stretch it too much or the balloon will burst .

Now for the ant mentioned before. The ant can move around on the surface of the balloon freely. But it´s only a small animal so its speed is limited. In fact, it can´t go faster than lightspeed. If you blow up the balloon sufficiently fast, that poor animal will never get from A to B.

Is it therefore possible for information in the form of electromagnetic/gravitational waves to "leak out" of our universe?

No, even if you blew up the balloon very slowly (or even not at all), the ant could walk around the surface of the balloon but never leave it. Unless, of course, Mokele stumbles across this thread and points out that some ants can fly - but we´re talking about an ant that moves at lightspeed and this type can´t fly unless someone manages to prove otherwise.

 

Oh, and welcome to SFN. Don´t be afraid, usually you won´t have to bother with overlength-posts - at least not on scientific topics, that is.

[Biodizzle]

Atheist,

 

Thanks for clearing this up. This makes much more sense to me now. I have heard of this balloon analogy quite a few times, but have never really fully appreciated it because it seems way over-simplified. Can you extend this analogy to our three space dimensional/one time dimensional universe? Essentially, how does space bend back onto itself (if we start in a straight line and travel infinitely far, we will end up where we started)? I am puzzled by this, and how this works in a three-D universe. It is easy to see on the 2D surface of a balloon. Thanks!

[Daecon]

I thought I heard that the Universe can expand faster than light speed, and Galaxies can be dragged apart faster than light speed, because it's space that's expanding, not the Galaxies moving through space.

 

This explains why the boundary of the "visible" Universe is 14 billion light-years, even though the Universe could be f**knows larger.