Expert: Courtney Seligman - 11/26/2008

Question
QUESTION: Lets just say for instance that;

1).  We have two Astronomers on two planets say 4ooo light years apart from each other.  Both have the same equipment and know the exact laws of physics and cosmology.

2).  Each can communicate instantly with each other.  I know they can't, but for the sake of the argument.

Here is the problem!

In a direct line of sight between each viewer, there is an expansion of the Universe, lets say a value of 4 for each 1000 light year segments of space in that direct line of sight.  I see out of my Telescope in his area that the accumunated expansion reaches a factor of 4 in his region of space and as I look out, I see that the factor reaches even more as I see space further out than his region in a direct line of sight.

I ask my fellow astronomer what he sees out of his telescope, what factor expansion he notices in a direct line of sight.

If he looks out directly towards me, is it a factor of 4 that I see?

Is it a larger factor if he looks out in our direct line of sight further out?

If I look out I would see a factor of, say, 7 past his region.  If he say its only 2 meaning 2000 light years in a direct line of sight going further out, there is a paradox existing between our veiws.  We both can't be right?

Thanks,

Mike


ANSWER: Because the expansion of space is very small unless the volume of space is very large, your example would be better stated in terms of hundreds of millions of light years, instead of thousands; but let's say that there was a significant expansion, so that the conditions you set can be met...

I think you must mean an expansion rate of 1 unit per 1000 light years, or 4 units per 4000 light years. Then, whatever distance you see something from you, you would see it moving away at 1 unit per 1000 light years of distance. If 6000, 10000, or 20000 light years away, you would measure an expansion rate of 6, 10 or 20 units.

The other astronomer, looking back at you, would see exactly the same thing, except that his distance from things would be different than yours. Since your distance from him is the same as his distance from you, you would both see an expansion rate of 4, between you; but for things on the other side of him, he would see an expansion rate 4 units less than you do, because he is 4000 light years closer to them.

This is not, however, a paradox. It simply means that since he is moving away from you at 4 units, the net rate that things further from you than he is will appear to have, relative to him, will be reduced by his 4 units of motion, relative to you.

In your last example, I presume you mean that you are looking at something 6000 light years from you (not 7000, since you say it is only 2000 light years further than he is), so you would see it moving away from you at 6 units, but since he is already moving away from you at 4 units, he would see it only moving away from him at 2 units, which is exactly what he'd expect, given its distance of 2000 light years.

I realize that the above is a bit convoluted, but the numbers you used didn't quite jibe with each other, and I wanted to make sure I answered the question you meant to ask, whether the numbers were right, or not. If I've misunderstood what you meant, or you need further discussion, let me know, and I'll do what I can to make things clearer.

Courtney Seligman
Professor of Astronomy
Long Beach City College

---------- FOLLOW-UP ----------

QUESTION: Dear Professor Seligman;

Thanks for the wonderful reply.  I have been wanting to ask that question for a long time but couldn't find anyone qualified or wanted to answer it.  I wish as a young man I would have gone to college and studied Physics, Astronomy and Cosmology.  Its a faciniating subject and I have books galore on various subjects on it.

Yes, his factor would be a factor of 2 if he were looking out 2000 light years and my factor would be 6.

One last question and I'll leave you alone:)

The latest theory states that due to dark matter/dark energy taking president over gavity and making the expansion of the Universe even more faster, the Universe will expand forever increasing its speed of expansion approuching and may exceed the speed of light.

If the Universe is like a bubble in a bath tub with other Universes on the other side of the bubble, what would happen to our Universe at that time and at that edge?

And, yes, I do believe there is a wall that seperates the universes from each other if in fact there are other universes like that of a bubble in a bath tub.  Due to Quantum Theory, universes can exist in any form or structure so if there were a universe with very different properties on the other side, they would destroy each other on contact.  So, if you follow this theory, there would have to be a solid wall of some type to seperate the universes.

I would think at that edge all matter would discinigrate to nothing and just leave photons that will evaporate over time.  And I think it would only happen at that edge since it would take time for the rest of the universe to catch up, so to speak.  The Universe would become colder and colder as time passes however due to accumulated Entropy.

I also wonder what is on the other side of the "visible Universe" (a place or distance where mankind will never see beyond no matter how much technology we have).  More Universe perhaps made of basic building blocks of matter, just forming galaxies, clounds of matter?

Thanks Again:)

Mike


Answer
The Universe doesn't have to expand any faster, for the speed of expansion in a given region to become faster than the speed of light. You just have to wait until the current expansion increases the size of that region enough, so that the expansion rate increases to the speed of light.

Suppose that we consider a region a few billion light years in radius, with a current expansion rate of 1/3 the speed of light. In about ten billion years, due to the current expansion, it will be about twice its current size. But with twice the size, if the rate of expansion for any unit of empty space stayed the same, it would expand at twice the rate, or 2/3 the speed of light. In another ten billion years it would be twice the size again, and have twice the expansion rate, or 4/3 the speed of light.

In other words, even if the expansion rate is a constant, the expansion of a given region will accelerate, as it gets larger. And you don't need to suppose that anything (dark matter, dark energy, whatever...) will increase the expansion rate, to attain faster-than-light expansion rates, for regions which have grown sufficiently large.

Now, there are theories in which the actual rate of expansion of a given region of empty space would accelerate, causing the expansion to grow even faster; but the one theory of space-time which is known to be reasonably accurate -- Einstein's Theory of General Relativity -- does not suggest that such an acceleration of the basic rate of growth should exist, so in the absence of evidence to the contrary, I tend not to believe such speculation.

=========== (on to your next topic)

In multiverse theories of the Universe (that ours is merely one of an infinite number of Universes), the individual Universes are often imagined as being like soap bubbles floating in some vast space. But that is merely a way of visualizing things, and has nothing to do with reality. The empty 'space' between the individual Universes doesn't exist, in any meaningful sense. Each Universe contains all of the space, time, energy and matter which can ever be accessed or observed, from that Universe. And there is no space, time, energy or matter associated with the nonexistent 'space' between the 'bubbles' which represent individual Universes. Nor, for that matter, do any of the bubbles have an actual 'edge', in the way you'd expect them to, if they really were bubbles. For anyone inside such a 'bubble', even if near the 'edge', no matter which way they looked, they'd be looking INSIDE the bubble, as that is the only space-time which has any connection to that Universe.

Now, let's suppose one of the bubbles gets bigger. What does that mean? It means that the distances between things inside that Universe get larger and larger, over time. But that has nothing to do with how close the bubble is to any other bubble, because with the 'space' between the bubbles being a nonexistent, totally imaginary 'thing', they might as well be infinitely far apart, and getting bigger, when you're infinitely far from something, doesn't change the fact that you're still infinitely far away from it.

As a result, you don't need to suppose that there's some sort of wall separating the Universes. Each Universe is wholly and completely independent of any other Universe, and no observation of any other Universe could ever be made, save from inside that other Universe (in which case, no observation could ever be made, of our Universe).

============= (sort of another topic, but with some connection)

As far as the particles disintegrating, there are theories that this will happen, a very, very, very long time in the future. Right now, the age of the Universe is about 14 billion years, which is 1.4, with ten zeroes added, to make it a really big number. Some theories propose that when the Universe is around 100 'zeroes' old -- that is, a billion billion billion billion billion billion billion billion billion billion times older than now -- the fundamental particles will begin to break down, and all matter will 'evaporate'. There is no experimental evidence to back this up, so that might not happen, anyway; but even if it does, it is so far in the future that it's hard (if not impossible) to comprehend the timescales involved.

Now, in the above paragraph, there is no reference to where you are in the Universe. Since, for any part of the Universe, the only way you can look is 'inside' the Universe, it doesn't make any difference where you are. Either nothing will ever disintegrate, or everything will, no matter where you are (at that incredibly distant time).

As far as the Universe becoming colder, it has been doing that, ever since its beginning. At the end of the Cosmic Fireball, about 14 billion years ago, it had a temperature of hundreds of thousands of degrees; but it has now cooled off to less than 3 Kelvins, and as it expands more and more, its average temperature will cool off, correspondingly. When it is twice its current size, it will be only half as 'warm'; and when a hundred times its current size, it will be only a hundredth as 'warm'.

=============== (final topic)

As far as what's on 'the other side' of the Observable Universe, I presume you mean, what would we see, if we could see those regions which are currently moving away from us at a little over the speed of light, so the light from them can never reach us. The answer would be, basically just what we see around us, right now. Remember that, in the example at the start of this answer, I pointed out that eventually, regions now relatively close to us, and moving away at relatively low velocities, will eventually be farther from us, and moving away at more than the speed of light. Whether they're close enough for us to see, or too far away to see, they would be just the same; the only thing different is that in the one case, the empty space between us and them isn't growing fast enough to overcome the velocity of the light coming from them to us, and in the other case, the greater distance allows the expansion of space to overcome the speed of light, and prevent the light from reaching us. As a result, although we can never observe things beyond the 'edge' of the Observable Universe, we have no reason to suppose that they're any different from the things near us -- and for all practical purposes, we could think of the Universe as going on forever and ever, even though in any given part of it, you can only see the region close enough to you to be 'Observable'.

Again, I apologize for the delay in answering your question. If my Internet connection hadn't died last night, you'd have received my answer Wednesday morning; but hopefully, this answer will reach you soon after I've posted it.

I hope you are surrounded by loving family and friends during the upcoming holiday season, and throughout the years to come. For they're the most important 'Universe' of all.

Courtney Seligman