PDA

View Full Version : expansion



geeyathink
2010-Apr-17, 08:57 AM
The expansion of space is a standard three + one dimensonal observation? Its space that is expanding not spacetime, correct?

Is the following in the ballpark;

14 billion parsecs to edge of observable universe from earth
14 billion parsecs equals 14,000 megaparsecs
hubble constant says 50 to 100 km per second / per megaparsec (I think this may be outdated)
14,000 mpc times 100 km per second equals 1,400,000 km per second (edge of observable universe expanding away from earth)
1,400,000 km per second divided by the speed of light (approx. 300,000 km per second)
edge of observable universe moving away from earth around 2.3 to 4.6 times the speed of light

I don't understand how the km per sec / per megaparsec works, If we took a one megaparsec chunk that was at the edge of the universe would the rate of expansion of that mpc (in relation to itself) be the same as the rate of expansion for a one mpc chunk (in relation to itself) that started at earth?


Is there a mainstream theory explaining expansion, what is it?

If there were no expansion of space what would the stuff in the universe be doing or predicted to be doing? Would it be collapsing or still expanding but the expanding would be slowing instead of speeding up?

Is it only space and not the objects in it that are expanding? I've heard it thought both ways. (in forums)


I understand that someone at almost the edge of our observable universe would also see themselves as being at the center of thier universe. Is there a reason they would have to see uniformity throughout thier universe like we do, or could they see half thier universe as full but the other half as almost empty? Could they (the energy they were eventually made from) have been ejected during or shortly after the big bang in such a way for them to have a completely different observable universe ?

Is the Dark Flow still on the table or has it been done away with? If its still on the table I would ask a question about it.

Thank you very much for any replies,

Cougar
2010-Apr-17, 10:44 PM
I don't understand how the km per sec / per megaparsec works, If we took a one megaparsec chunk that was at the edge of the universe would the rate of expansion of that mpc (in relation to itself) be the same as the rate of expansion for a one mpc chunk (in relation to itself) that started at earth?

Well, it depends on which "edge" of the universe you're talking about. The CMB has a redshift on the order of 1,000. That's the edge of our perception of any distant electromagnetic radiation, i.e., light. But that light has taken billions of years just to get here. Back then the rate of expansion would have been different. But a region of space "back then" has moved on, just like us, even though we can't yet see where it is "now" due to the lag time in light. Wherever it is "now," I expect its rate of expansion is comparable to ours, even if it's in a very distant region of the universe.


Is there a mainstream theory explaining expansion, what is it?

Big Bang plus inflation as expressed through General Relativity. Of course, inflation is on a much more flimsy base than the Big Bang model, which is pretty well established with some significant predictions and confirmations. Inflation seems to fit, but it's not "as mainstream" as the now volumes of work that is the Big Bang theory.

DrRocket
2010-Apr-18, 12:03 AM
The expansion of space is a standard three + one dimensonal observation? Its space that is expanding not spacetime, correct?

That is correct. Spacetime contains all of time -- past, present and future -- and all of space. It is completely static and describes physics completely deterministically. In fact all of the physics is in it.

Now, just to make sense of the statement that space is expanding takes a bit of work. Spacetime is a single thing. It is not space and time but a single concept -- spacetime. And because spacetime is curved, there is no global way to separate time from space. They are mixed together.

But, if one assumes that spacetime is both homogeneous and isotropic, which it is not, but which does appear to be a good approximation on the very largest scales, then one can show that spacetime can be decomposed as a one-parameter foliation via space-like hypersurfaces and the parameter serves as a surrogate for "time". Thus to say that space is expanding is to say that the distance between corresponding points increases as the surrogate "tiime" increases -- stuff is getting farther apart. This is physically seen as an increasing separation among very distant galaxies that are not gravitationally bound to one another.


Is the following in the ballpark;

14 billion parsecs to edge of observable universe from earth
14 billion parsecs equals 14,000 megaparsecs
hubble constant says 50 to 100 km per second / per megaparsec (I think this may be outdated)
14,000 mpc times 100 km per second equals 1,400,000 km per second (edge of observable universe expanding away from earth)
1,400,000 km per second divided by the speed of light (approx. 300,000 km per second)
edge of observable universe moving away from earth around 2.3 to 4.6 times the speed of light

I don't understand how the km per sec / per megaparsec works, If we took a one megaparsec chunk that was at the edge of the universe would the rate of expansion of that mpc (in relation to itself) be the same as the rate of expansion for a one mpc chunk (in relation to itself) that started at earth?

Be careful with the word "edge". The observable universe may have an edge in the sense that beyond that boundary lies stuff that we will never see. But the entire universe is generally believed to be a 4-manifold without boundary and the space-like slices are also without boundary.

The rate of expansion is uniform, which is part of the assumption of homogeneity. That implies that the farther an object is from the earth, the more rapidly it recedes from the earth, and in fact that the rate of recession is linear with distance. This is exactly what you see with respect to two ink dots on a balloon as the balloon is inflated.



Is there a mainstream theory explaining expansion, what is it?

This is all based on general relativity. You can find an explanation in most books on general relativity or in Principles of Physical Cosmology by P.J.E. Peebles.


If there were no expansion of space what would the stuff in the universe be doing or predicted to be doing? Would it be collapsing or still expanding but the expanding would be slowing instead of speeding up?

Is it only space and not the objects in it that are expanding? I've heard it thought both ways. (in forums)

It is space that is expanding. That expansion would also cause objects in it to expand if it were not for other forces that hold material things together -- the electromagnetic, weak and strong forces and gravity. The net result is that these other forces counteract the tension introduced by expansion of space and overwhelm it, so that the expansion of space on material bodies has a miniscule effect.



I understand that someone at almost the edge of our observable universe would also see themselves as being at the center of thier universe. Is there a reason they would have to see uniformity throughout thier universe like we do, or could they see half thier universe as full but the other half as almost empty? Could they (the energy they were eventually made from) have been ejected during or shortly after the big bang in such a way for them to have a completely different observable universe ?

The cosmological principle asserts that the universe, on the largest scales, is homogeneous and isotropic. That means that it is the same everywhere and looks the same in all directions. This is an assumption, but it seems to be valid based on what we can see.

The big bang is taken to be an expansion of space itself. Imagine spacetime as a cone, and the big bang as the apex. So what you have is space expanding out from the point, and not material expanding out into some pre-existing space. So, the energy resulting from the big bang is everywhere, all the time.

Due to the rate of expansion, and moreso if you accept the theory of inflation, there are parts of the universe that are so far away from us that light has not had sufficient time to travel the expanding distance between its point of origin and us. That limits the "observable" universe to only part of the whole universe, the observable universe being that portion of the universe from which we could, in principle have received a light signal. Every point in the universe has associated with it its own unique "observable universe". If the cosmological principle is correct, those different observable universes have identical properties.


Is the Dark Flow still on the table or has it been done away with? If its still on the table I would ask a question about it.

Thank you very much for any replies,

Here (http://arxiv.org/abs/0809.3734) is the original paper on dark flow. It is, so far as I know, still controversial. Knock yourself out. I am personally pretty skeptical that it is important or even real. Time will tell.

dhd40
2010-Apr-21, 05:16 PM
Spacetime is a single thing. It is not space and time but a single concept -- spacetime. ... They are mixed together.



Yes, that's how I understand Brian Greene's description:

Quoting (not literally) from his excellent book „The Fabric of the Cosmos“:


For an object moving through space and time, these two kinds of motion are always complementary. Some of its light-speed motion is diverted from motion through time into motion through space, keeping their combined total unchanged.
(=worldlines?)

So, when space expands ... time shrinks (because c=const) ... or what? IF time shrinks (or experiences any other change) .. where? In the „place“ where space expands?

Hopefully, these lines of thought are not too ridiculous, and I will get an explanation

speedfreek
2010-Apr-21, 06:38 PM
Greene's description is talking about an object moving through space and time, not the expansion of space over time. In reference to DrRocket's description, the parameter for time marches onwards at a constant rate, whilst the distance increases between corresponding points on the space-like hypersurfaces.

geeyathink
2010-Apr-24, 08:36 AM
Thank you very much for the help,

DrRocket, I checked on the book by Peebles, its out of my price range at the time. If I get the chance I'll check the library. Is it the type of book I could ignore the math but still get some understanding?

I found a site that is easy to understand but its ten years out of date, does anyone know if this http://www4.ncsu.edu/unity/lockers/users/f/felder/public/kenny/papers/cosmo.html is an accurate (as far as it goes) mainstream view, I'm hoping so. Its easy to read

The more I try to digest the more I am forced to digress. To see if I am even comparing apples to apples;

'observible universe' is a well defined, standard term? I chose it hoping to avoid the ambiguity of discussing the universe in total.
If so, how fast are the earth and the edge of the observible universe moving away from each other? ( I only ask this same question again because nobody came right out and said "it can't be known" or "its a secret" or anything like that )


For future use on any physics topics;

Is there even an official description of what is included in mainstream science?

Is there one place where the mainstream view of all the different physics theories are delineated? Not a complete explanation of each, just whats included. .

Is it in general ok for people in my prediciment (dumb as a rock w/ no knowlege or math skills) to refer to wikipedia for discussions here? I have noticed that it doesn't get referred to and I can guess why but regardless I hope that for amatures (only trying to learn, not forge new theories of thier own) its ok? I don't want to get dis-barred or have my avatar tarred and feathered or whatever happens to bad guys around here. :)

It turns out the site I had been using to gauge mainstream (wikipedia) doesn't even take on the concept of what mainstream science is. The sub-headings on the wikipedia site for 'mainstream' are; film, media, literature, music, scociology, gender and education. I had thought mainstream science was well defined, and to a big point I know it is, but I guess some areas get grey at the edges.

The wikipedia page on expansion seems to be 'metric expansion of space' and has warnings at the beginning that it may be misleading and is in need of an expert. :(

Sorry for the ranting. I sound like I need to step back from the edge.

Thanks,

noncryptic
2010-Apr-24, 10:56 AM
The cosmological principle asserts that the universe, on the largest scales, is homogeneous and isotropic. That means that it is the same everywhere and looks the same in all directions. This is an assumption, but it seems to be valid based on what we can see.

It's just that we don't know how large the universe is, so we also don't know how large the largest scales are.
Which doesn't take away from the fact that within the observable universe it appears to be homogeneous and isotropic at the largest scales of the observable universe.


http://www4.ncsu.edu/unity/lockers/users/f/felder/public/kenny/papers/cosmo.html

Others are better equipped than me to judge the information there, but at a quick glance it looks to be ok in the basics, except that it does not mention accelerated expansion, dark matter and dark energy.

DrRocket
2010-Apr-24, 05:59 PM
Thank you very much for the help,

DrRocket, I checked on the book by Peebles, its out of my price range at the time. If I get the chance I'll check the library. Is it the type of book I could ignore the math but still get some understanding?

I found a site that is easy to understand but its ten years out of date, does anyone know if this http://www4.ncsu.edu/unity/lockers/users/f/felder/public/kenny/papers/cosmo.html is an accurate (as far as it goes) mainstream view, I'm hoping so. Its easy to read

The more I try to digest the more I am forced to digress. To see if I am even comparing apples to apples;

'observible universe' is a well defined, standard term? I chose it hoping to avoid the ambiguity of discussing the universe in total.
If so, how fast are the earth and the edge of the observible universe moving away from each other? ( I only ask this same question again because nobody came right out and said "it can't be known" or "its a secret" or anything like that )


For future use on any physics topics;

Is there even an official description of what is included in mainstream science?

Is there one place where the mainstream view of all the different physics theories are delineated? Not a complete explanation of each, just whats included. .

Is it in general ok for people in my prediciment (dumb as a rock w/ no knowlege or math skills) to refer to wikipedia for discussions here? I have noticed that it doesn't get referred to and I can guess why but regardless I hope that for amatures (only trying to learn, not forge new theories of thier own) its ok? I don't want to get dis-barred or have my avatar tarred and feathered or whatever happens to bad guys around here. :)

It turns out the site I had been using to gauge mainstream (wikipedia) doesn't even take on the concept of what mainstream science is. The sub-headings on the wikipedia site for 'mainstream' are; film, media, literature, music, scociology, gender and education. I had thought mainstream science was well defined, and to a big point I know it is, but I guess some areas get grey at the edges.

The wikipedia page on expansion seems to be 'metric expansion of space' and has warnings at the beginning that it may be misleading and is in need of an expert. :(

Sorry for the ranting. I sound like I need to step back from the edge.

Thanks,

There is no "official scorebook" of mainstream science. You have to get that by reading reputable publications and ignoring the junk publications. It takes a bit of experience to know the difference. In general, I find the magazines Science News and Scientific American to be pretty good. I find The American Scientist (the publicatin of the Sigma Xi scientific honorary society) to be extremely good. I personally find many other so-called science magazines (like Discover) to be questionable. Most peer-reviewed journals are very good and it is what is published there that really defines mainstream science. There are a few junk journals that make a pretense of being peer-reviewed that ought to be avoided, and the only way to recognize them is by their content and editorial board.

You can read Peebles book, and skip the math if you like, but you will be skipping the real justification for some important things. You can just read the prose and the conclusions, take them on faith, and gain some understanding -- because Peebles is a very solid physicist.

Wikipedia is a useful source, but it is largely unregulated and sometimes things get into Wikipedia because of the personal agenda of someone who takes the trouble to edit the content. So, while it is usually good, it is not always so and can be unreliable. Another source is Scholarpedia. Scholarpedia has articles that are reviewed and which are written only by real experts and only by invitation. It is not as big as Wiki, and does not cover as much, but what it covers is gold-plated.

http://www.scholarpedia.org/

BTW Princoiples of Physical Cosmology by Peebles is available on the used book market through Alibris and others. I just checked and there copies in reportedly good condition available for $0.99 plus $3.99 shipping (yes a bit under $5 total).

geeyathink
2010-Apr-28, 07:48 AM
Thanks again all,
DrRocket, thanks for the tips. I hope scholarpedia grows it seems great. I checked and my library doesn't have the Peebles book, it turned up in the search and I got all excited, but there were no catalog numbers for it. I asked the librarian what was up, she told me it had either been stolen or lost or something. Bummer. All these years and I never thought about purchasing used books from an on line vendor, I may go that route. I am checking out the magazines you suggested, most excellent.

I have no choice but to take this stuff on faith. It isn't a matter of desiring to skip the math, I don't think I could learn it. Thats why I've been trying to be careful about sticking with whats accepted as mainstream. At this point someone could tell me the moon was made out of cheese and I wouldn't know the difference (a slight exaggeration but you get the idea)

Thanks,