The Astrophysical Hotseat

[jadesmar]

[Zag]

This aspect makes the theory seem even more goofy to me. If the expansion is 74.2 kilometers/second/megaparsec, but the region within a galaxy supercluster is exempt from it, then you're going to get some serious warping. An area that big is enough for it to make a real difference. Consider what happens when you heat a piece of glass unevenly -- part of it expands and part doesn't, causing it to crack and break.

Consider the sphere around the supercluster, far enough away that it is outside the supercluster's gravity well. It's expanding. And then consider a sphere which is inside the well (as defined by the escape velocity, we've supposed, but we can move in or out more if the actual definition is something else). What is happening in the space between these two spheres? Is "more space" being created? Effectively, it has to stretch a lot faster to make up the difference.

Consider some light traveling through this section -- does it seem to slow down significantly? Perhaps this is the source of the "gravitational lens" effect that was mentioned in the most recent SciShow. Hmm. I'm starting to think that this makes sense.

[3iff]

[Zag]

Yes, I'm quite sure that light travels at the same speed through a vacuum, regardless of the speed of the observer. That was the foundation of the Theory of Relativity. However, if a localized region of space is expanding faster than the space around it, then light will take more time cross the distance than it does in the part of space that expanded more slowly, because it is like swimming upstream. The net effect is that it moved slower, but the real reason is that it had more space to travel through.

BTW, it is a difference in the speed of light that is why a lens bends the light. Light travels more slowly through the glass than it does through air. (Note that it is only the speed of light in a vacuum that is absolutely constant.) Consider the leading edge of a moving wave of light. It will stay cohesive; but if one part of it is moving more slowly than another, the only way for the edge to stay cohesive is for the light to bend so that the slower part has less distance to travel.

Imagine a line of people holding hands and moving perpendicular to the line. Suddenly, all the ones on the left walk more slowly for a few steps, the whole line will turn left a bit to compensate.

BTW, I've read many times that the current theory of gravitation is not that there is an actual attraction, but that mass causes the space around it to warp. I've never really understood it before. So I'm not really just making this all up. I'm a flea standing on a giant's shoulder, and finally the fog around me has cleared a bit (mostly from the giant explaining it repeatedly as I continue not to get it), but finally I am seeing a little more clearly than I was before. At least, I hope so -- maybe I'm just thickening the fog.

[jadesmar]

[Zag]

The overall expansion of the universe causes the red shift due to (you've remembered correctly) the Doppler Effect. I was wondering specifically about the inconsistency of the expansion. If the whole universe is expanding, but parts of it are not (like the part inside a gravity well of a super-cluster of galaxies), then what happens at the interface between the part that expands away from the super-cluster and the part that is held firm? Space has to expand like crazy there, or else it just tears apart (which doesn't seem, to me, to be an option). Light passing through this section is going to get bent inwards, I think, for the same reason that light is bent when it passes through a lens.

In the latest SciShow, he talks about a "gravitational lens," which seems to coincide rather well with what I'm thinking about.

[extropalopakettle]

[Zag]

They are moving relative to each other however they are going to move within space according to Newtonian mechanics. But my original point which is if space expands uniformly (by 3.24 x 10^-18 percent per second, or whatever), then it doesn't matter. If the ruler expands at the same rate as the thing you're measuring, then there is no net effect.

However, the space within an atom does not undergo expansion (which would cascade to everything else), that's why there is a measurable effect.

[Jedo the Jedi]

I'll just go ahead and throw out the question that began my query, and I'll proceed with more as I go along.

Was there something pre-existent or not? In other words, has something always existed or is there an infinite thing?
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[Jack_Ian]

That really depends on what your definition of nothing is.
If you have $10 in your pocket, but owe $10, do you have any money?
Some scientists believe that the total energy of the universe is 0, in which case it's nothing already.

[jadesmar]

[extropalopakettle]

[Zag]

It's like my office in my last job: in the mornings, it was freezing cold, and in the afternoons it was uncomfortably hot: on average, it was comfortable.

Jedo, no one really knows, but there are a variety of theories that have some support.

For instance, it is a commonly-believed theory that matter and anti-matter do spontaneously appear in empty space, more or less constantly. In 99.99999999999 (more nines) % of the times, they immediately collapse back and become nothing again. However, occasionally they happen to fly apart. This is the theory behind Hawking Radiation, which clears up one apparent violation of the Second law of thermodynamics, and it seems to be held up by experimentation.

Then it is a bit more of a leap to say that if it can happen on a small scale with a measurable frequency, maybe it can just occasionally happen on a really large scale.

The part I wrestle with is that physicists say that the universe is finite in size, that what's beyond it isn't empty space, it just isn't. It isn't there. I don't get that. If the universe is about 14 billion years old, then it shouldn't be any more than 28 billion light-years across times 14 billion years worth of expansion at 74.2 kilometers/second/megaparsec = roughly 60 billion light-years. OK. What if a similar singularity happened 3 billion years ago, 200 billion light-years away. We wouldn't know, of course. But does that have a meaning -- 200 billion light-years away? Why the heck not?

[3iff]

I would presume that any other universe (outside the bounds of our universe) would possibly at some point 'overlap' with ours and cause noticable interactions...albeit a long way away from us so perhaps it's already happened but we won't know for some billions of years.

A favourite question (posed as a joke).
Define the universe. Give three examples.

[Jack_Ian]

Nobody knows the answers to these questions and for the most part, the answer doesn't matter.
Scientists generate a model of our universe and try to match it with observable phenomena. If it matches, then they make predictions and develop experiments to test them. If they match then the model gains some credence and becomes useful for our understanding of the way things work.
Many of the current models have data to back them up, but mostly their differences cannot be tested by experiment as they differ in ways that can not be tested.
Take for example the question of whether the universe has a boundary. If it had a boundary, then that boundary is now receding away from us at faster than the speed of light, so that we will never be able to detect it and we will never be able to reach it. If there's a boundary that can never be reached, then that is, for all practical purposes, the same as having an infinite universe.
I favour the model of an infinite universe in finite space, as it fits better in my head, but there is nothing to suggest that model is better than many other proposed ones.
If you're interested in thinking about what's outside, you can look for info on colliding branes and the Big Bounce.

[Chuck]

[extro...*]

I know we discussed here before that current evidence suggests that the universe is in fact infinite (http://map.gsfc.nasa.gov/universe/uni_shape.html).

And I think it was brought up that a transition from finite to infinite isn't possible, so if it's infinite in size now, it was always infinite in size. But that goes against the common notion that the universe started from something very small. Then I read this, which clarifies that that's a misunderstanding: http://www.cfa.harvard.edu/seuforum/faq.htm#e1

So the picture suggested there is that prior to the "big bang", the universe was infinitely large and densely packed with matter and energy. Our observable universe was compressed to a dense point-like singularity, but that was just one point, no different than any other, in an infinite space filled entirely with such densely packed points. And then, suddenly, it, all of it, the entire densely packed infinite universe, started to expand rapidly.

Just food for thought.

[Coyote]

Pics or it didn't happen.

[Jedo the Jedi]

That actually makes way more conceptual sense to me, though I confess my knowledge on the subject is limited.
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[extro...*]

As the posted link ( http://www.cfa.harvard.edu/seuforum/faq.htm#e1 ) notes, the observation that space is "flat" implies an infinite universe, but it might also be that the universe is finite but so large that its curvature isn't detectable by us. So assume these two possibilities:

1) The universe is large but finite, and in the beginning it was extremely small and densely packed with matter and energy.

2) The universe is infinite, and in the beginning it was infinite and densely packed with matter and energy.

At first I thought, but now doubt, that the first was preferable to Occam's Razor. Thoughts?

[Zag]

I find the infinite, densely-packed universe to be a little ridiculous. The original argument (circa 1920 ish) against an infinite universe is that there would not be a dark sky in an infinite universe, because in every single direction, down to the tiniest point, would ultimately terminate in a star. (I'm not sure why they assumed there can't be anything that is large, opaque, and doesn't produce light, but I think it is that the infinite amount of energy available would light everything up.)

Anyway, I guess the assumption is that the universe started densely packed, but the rapid expansion was too fast for the light from all those objects to reach us. I don't see how that makes any more sense than assuming that there was only a single initial singularity, and I can't see how it could make any difference in any test or experiment we could run (making it a moot point). I suspect that this theory is more a proposal to make more comfortable the people who refuse to grasp a finite universe, because they keep wanting to ask what's beyond the borders.

[extro...*]

Let me take the last sentence first:

[Jedo the Jedi]

Anybody want to tell me the salient points of string-theory, namely what evidence there is for it?
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[Zag]

I got nothin' for ya, Jedo. But I only just noticed extro's reply to my note.

extro, by a "moot point" I mean that learning more couldn't possibly teach us anything useful. This is not true of, say, black holes. By learning more about them, we have learned a lot about how thinks work at the quantum level, which very realistically could lead to clean ways to produce energy. That is, it isn't that specific knowledge about black holes mattered, but that the general understand it leads to could help with something practical. I don't think that this is true for an answer to whether or not the universe is infinite and expanding, or finite and expanding.

[extropalopakettle]

[Lepton*]

String theory: beautifully conceptually but in a way that is hard to explain unless you have grad school quantum field theory. No predictions that are testable in a lab today, or probably ever. There seem to be a very very large number of "string theories", so the idea has lost falsiability.

[Jedo the Jedi]

Let me try something simplistic, and maybe you can say it's more complicated than that but I'm on the right track.

At the subatomic level, it is believed some subatomic particles are somehow connected to other dimensions by these strings (or the subatomic particles are part of these strings). These connections exhibit influence from these other dimensions (or ours to theirs) such that the spin or magnetic pull (or other quality) could be affected. However, there is no empirical evidence yet.
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[Lepton*]

Hey Jedo:

Right track, yes. In string theory, all the "particles" actually are strings. Under that assumption, the ordinary laws of physics couldn't exist unless there are 26 (or 10) dimensions. So the assumption is that the extra dimensions are very small, and the strings can get tangled up around the extra dimensions.

One idea about adding general relativity, black holes, and whatnot to string theory is that the strings are attached to "D-Branes"... but I think that's gone out of fashion.

[Jedo*]

Why are the extra dimensions not parallel universes to ours in which instead of the particle spinning clockwise, it spun counter-clockwise? Is it a mainstay of this (very tenuous) theory that the extra dimensions be small?

Yeah, I pretty much ignored the business about "D-Branes" as they seemed excessive. (Occam's Razor and all that. )

[Lepton*]

If you're interested in this, I'd suggest a pinch of salt and Brian Greene's "Elegant Universe", either the book or the video series.

What is a parallel universe? How does it interact with our universe? Is there a "natural" or elegant way for this to be quantified? If the other 22 or 6 dimensions weren't small, we'd be able to see them.

[Jedo the Jedi]

[Lepton*]

I don't want to come across as a jerk, so please understand that as a teacher I'm dedicating my life to helping people learn, and that I'm happy to try to answer sincere questions. But this is not an honest effort to learn something. There's no shortcut to understanding complex ideas such as these, although the basic ideas are accessible to anyone with the willingness to dedicate a bit of time to it. If you can't sit down for a couple hours and watch the PBS version of Elegant Universe (or read the book, or whatever alternative), then you cannot have the layperson understanding of string theory... sorry!

Rhetorical Analogy:
If that Luke Skywalker guy is some sort of superhero, why does he live in the middle of the desert with his mother?

Answers:
The strings are too small to see, the dimensions are too small to be experienced by familiar-sized objects, and the LHC's energy scale is insufficient to investigate the fundamentals of string theory.

[Zag]

[Jedo the Jedi]

[Jedo the Jedi]

Also, you might misunderstand what I'm doing here. This is only idle curiosity for me because knowledge of this will have almost no impact on my life. As such, the GL is a good first resource. Certainly the information is second-hand (and I always evaluate it as such), but it usually comports with what I have read on wikipedia or with what I remember or have inferred from the schooling I have had. If the answers on here are not satisfactory--whether through inconsistency of logic or insufficiency of information--I will then find another, more direct source. (The horse's mouth, as it were.)

Maybe you just disagree with this approach, but I see nothing wrong (or "not honest") with it. As a teacher, I would encourage my students to use whatever reputable sources they have at hand in order to gain understanding. It's not like I'm writing a paper and need to go read a theory book. *shrug*
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[RubberDuck]

[Jack_Ian]

Good learning comes with good questions.
Sometimes a simple question makes me realise that I didn't know something as well as I thought.

Why is the sky blue?
Shorter wavelengths scattered... etc.

Then why is the sky not violet?
Eh...

Turned out to be quite an interesting learning process for me before I could answer that question.

Another simple question that provoked a much deeper understanding of the way things work was, "Why is the sky dark at night?".
Duh! Because the Sun is on the other side of the Earth.
Not quite. The true answer is far more complex.

So don't feel embarrassed about asking questions. Perhaps even Lepton might learn something from the ensuing discussion (or at least crystallise something for him in a way that might provide more insight).

[Zag]

I just read this article (well, the abstract, that is).

In it, they claim

[bgg1996]

[Zag]