At the Edge of the (theoretically visible) Universe

Factual Questions
21

[QUOTE=Frylock]
(Attention, Exapno Mapcase. Attention Exapno Mapcase. Please note the scare quotes. Please note the scare quotes. Thank you for your attention.)
[/QUOTE]

Your problem is the common one that most people have when trying to get their heads around the fact that the big bang happened everywhere. You really can’t make any progress on it until you can get yourself to stop seeing the universe as the outrushing debris from a central explosion. (Or coming toward us, which is less common.)

From experience, words like boundary or edge or even towards and away are immediate tip-offs of this. It’s from trying to understand the universe using “common sense” experience of our local conditions. None of them apply.

You’re the philosopher. I like asking people why they think that the underlying workings of the fabric of all time and space *should * be understandable at all by human beings, let alone be comparable to our everyday experiences. People almost always immediately respond that the universe should indeed be understandable. But they can’t explain why they think so. It just should.

Maybe that’s why so many people seem to resent modern science.

22

[QUOTE=Exapno Mapcase]
This has to be true from the premise that there are no privileged reference frames. Everything looks the same as everywhere else at the current time.
[/QUOTE]

I think both parts of this statement are true, but I don’t see how the second one follows from the first. ‘No privileged reference frames’ is normally a statement made about inertia, not the physical composition of our local region of space.

23

[QUOTE=Omphaloskeptic]
But keep in mind that this shell is also redshifted, which is to say that it is also receding from us.
[/QUOTE]
Cosmic redshift is worth expanding on … it might help shed some light on the subject. (Heh … .)

Popular science programs often compare red shift to Doppler shift: When a police car is moving toward us the sound waves from its siren are compressed so it has a higher pitch. And when it moves away they are spread out so it has a lower pitch. And red shift is the result of the same process applied to light waves.

This is true. Stars that are moving very fast relative to Earth can have their light red- or blue-shifted. However this mechanism is NOT the source the large red shifts we see with distant galaxies.

The cosmic red shift is not caused by the Doppler effect. Rather it is caused by space stretching around the light on its long, long journey to Earth. It’s a measure of travel time, not of the relative velocity of the source.

24

[QUOTE=Exapno Mapcase]
Your problem is the common one that most people have when trying to get their heads around the fact that the big bang happened everywhere. You really can’t make any progress on it until you can get yourself to stop seeing the universe as the outrushing debris from a central explosion. (Or coming toward us, which is less common.)
[/quote]

I don’t believe the universe is a bunch of stuff rushing out from a central explosion. I think the universe looks that way. The way things look is all I’ve been talking about in this thread. It’s all over every post of mine.

And in this case (and in several other conversations you and I have had) your experience has gotten in the way of your actually reading my posts with any kind of care.

And you’re the writer. Most good writers are good readers as well. Having read my posts in this thread, you know I’m not asking that the universe be understandable–rather, I’m asking how, exactly, it would appear that things were moving relative to us at the boundary of the visible universe.

-FrL-

25

[QUOTE=Omphaloskeptic]
No and yes, respectively.
[/QUOTE]

Just to help me understand what followed that comment, could you clarify what you were saying “no” and “yes” to respectively here? Sorry… :wink:

-FrL-

26

[QUOTE=Frylock]
Whatever the age of the universe is, measured in years, take that number, multiply it by light years, and you have the radius of the in-theory-visible universe.
[/QUOTE]
First of all, even this is wrong. The comoving distance to the cosmic event horizon (the point at which, from our perspective, any light emitted is redshifted to undetectability) is about 46 billion light years, giving the observable universe a diameter of about 92 billion light years. However, when this light was emitted, it was about 40 million light years from where Earth is now. This is due to the continuing and probably accelerating expansion of space which increases with greater distance.

Note that while this is a physical limit–barring any kind of sci-fi superluminal travel, we’ll never be able to observe anything beyond this distance–it isn’t that there is anything special about this boundary except from our own particular inertial reference frame. Anyone in an inertial frame of reference in a different location will observe this horizon to be 46Bly distant from their position, and someone in a reference frame that has been significantly accelerated will perceive a different boundary yet. As Q.E.D. pointed out, we don’t and probably can’t know whether the universe is bounded or not, only that stuff that used to exist to us is now outside of the observable universe any (by currently accepted theory) stuff will continue to pass this distance. (Whether the universe warps back around on itself as mlees suggests is unknown, but so far we’ve seen no indication of repetition in deep field sky surveys, so if it does then it would still seem to be larger than the observable universe.)

panache45 has it right in a sense; the edge of the universe is the future, at least if we assume that the universe is unidirectionally causal and non-teleogical (i.e. you can’t go backwards and it isn’t all planned out ahead of time). This is, of course, an assumption and one that is not strictly necessary for many operations in quantum mechanics, but it is an underlying assumption in relativity and thermodynamics.

We can, of course, see the aftermath of the Big Bang (or, at least the first visible effects as soon as the universe thinned out enough to become transparent to light) in the Cosmic Microwave Background. This is like a picture that has been blown up again and again and again, and while it has become really grainy and dim because of the enlargement, if you look hard enough you can just spot the body poking out from behind the bush. There is no central explosion; the expansion has occurred everywhere, and it is still expanding, and we are still right in the middle of it. Like a raisin embedded in the middle of an expanding loaf of dough, we find all the other raisins accelerating away from us.

Stranger

27

[QUOTE=Stranger On A Train]
First of all, even this is wrong. The comoving distance to the cosmic event horizon (the point at which, from our perspective, any light emitted is redshifted to undetectability) is about 46 billion light years, giving the observable universe a diameter of about 92 billion light years. However, when this light was emitted, it was about 40 million light years from where Earth is now. This is due to the continuing and probably accelerating expansion of space which increases with greater distance.

[/quote]

Thanks for the link. When we detect the CMB, does the CMB look to us as though it is 14 billion years old or 46 billion years old? In other words, here are the two scenarios I am envisioning:

  1. When we do whatever it is we do to measure the distance of the CMB, we get 14 billion–but we know how space has expanded over the past 14 billion years so we know it’s actually 46 billion light years away.

  2. When we do whatever it is we do to measure the distance of the CMB, we get 46 billion.

Is one of these correct, or neither somehow?

-FrL-

28

[QUOTE=Frylock]
Just to help me understand what followed that comment, could you clarify what you were saying “no” and “yes” to respectively here? Sorry… :wink:
[/QUOTE]
Sorry, here it is again:

“No” to this part (the quasars are redshifted; they look like they’re moving away from us)…

… and “yes” to this part (the “shell” is even more redshifted).

The balloon model is really a very good analogy

29

I am utterly unqualified to get tangled up in this discussion, but there’s an article in the March 08 issue of Scientific American that touches on the OP’s question and makes an interesting inference: that given the acceleration of the expansion our observable universe will eventually become devoid of matter save the congealing cluster of our local group of galaxies. The concept of observable horizons is treated very crisply in that article so I recommend it to this discussion.

30

Apparently I drifted off at the end of my last post. I meant to say something like

The balloon model is a very good analogy to cosmological expansion. I think understanding it would answer a lot of your questions.

[QUOTE=Frylock]
When we detect the CMB, does the CMB look to us as though it is 14 billion years old or 46 billion years old? In other words, here are the two scenarios I am envisioning:

  1. When we do whatever it is we do to measure the distance of the CMB, we get 14 billion–but we know how space has expanded over the past 14 billion years so we know it’s actually 46 billion light years away.

  2. When we do whatever it is we do to measure the distance of the CMB, we get 46 billion.

Is one of these correct, or neither somehow?
[/QUOTE]
At this point it probably won’t surprise you too much that the answer is “Neither, really.” At any rate, the CMB is just made up of photons; they have no measurable age (and in their reference frame, they do not age at all). They have a wavelength, a direction, and a polarization, and that’s it. The raw measurements of the CMB are measurements of a blackbody spectrum, which is basically measuring the intensity of the radiation as a function of frequency (like here). To get any measurement of distance or age out of this, we need more information.

For the CMB, one piece of extra information is the initial temperature of the radiation. This radiation is believed to come from a proton-electron plasma just before recombination, and theory gives estimates for how hot that plasma must have been. The ratio of this temperature to the measured CMB temperature then tells us how much the radiation has been redshifted on its journey (specifically, this is the expansion factor 1+z for the CMB).

(Similar but different tricks are used to get redshifts to other sources. In some cases the source has well-known emission lines which can be identified at a different wavelength than they actually emit at. In some cases, clouds somewhere between us and the source can form absorption lines, which at least gives a lower bound on how far away the source is.)

This is still not a distance or an age. To go further than this, a model of the evolution of the universe is needed. In the case of the CMB, some of the extra information is actually in the anisotropy of the radiation, which gives information about how large the surface was at last scattering. To convert this result to a comoving distance as Stranger has done, you need to further know something about how the universe has evolved between then and now.

If all of this seems a terribly oblique and roundabout way of measuring simple quantities… welcome to cosmology, where the definitions of “distance” take a chapter or so. The problem of measuring distances in cosmology (the “distance ladder”) is very old and not at all straightforward.

31

[QUOTE=Omphaloskeptic]
Sorry, here it is again:

“No” to this part (the quasars are redshifted; they look like they’re moving away from us)… and “yes” to this part (the “shell” is even more redshifted).

[/quote]

So if I understand correctly, at distances like this, redshift corresponds to distance corresponds to speed relative to us. Take any object near the boundary, and it will be redshifted by factor X. Now take any object further away, and it will be redshifted by factor Y where Y is greater than X. And this means that Y is moving faster than X. Now both X and Y are moving away from us, but X is moving away from us more slowly than Y, and in this sense, X is moving toward us. (I throw a ball from a moving car at the car behind me. If my own car is going fast enough relative to the car behind me, then the ball will continue to move away from the car behind me, but it still makes an intuitive kind of sense for me to say the ball is moving toward the car–after all, that’s the direction toward which the ball appears to be moving from my perspective.)

So it now appears to me that things near the boundary do appear to me to be moving away from the boundary–though not strictly speaking towards me.

I don’t know if my original reasoning had anything to do with the real reason why this is how appearances turn out, though. As it stands in my head ATM, it seems like it should look like things are moving away from the boundary because the boundary is where I see the big bang. Meanwhile, if I’m understanding correctly, it turns out that things do appear to be moving away from the boundary, because nothing is (appears to be?) moving away from me as fast as the boundary is (appears to be?) moving away from me. But I don’t know if these two lines of reasoning are getting at the same thing, or if my own original reasoning just luckily hits on the “right” conclusion by an invalid means.

Anyway, thanks for all of your help in this thread!

-FrL-