Is There a Definable Centre of the Universe?

[muccasen]

You Kidding? Current thinking is 3 spatial 1 time and a number of curled up ones totalling 11 or so

:uhh: But I favour 4 macro spatial + Loads o other macros not relevant to us but take no notice of me on that 1 !

 

[yuiop]

Thanks Marcus for the links. A lot to read there. Maybe take me 2008 and 2009. :P

I notice the Rovelli in your sig is referenced in the last paper. Never quite figured out if view Rovelli with derision or admiration.

 

[marcus]

Thanks Marcus for the links. A lot to read there. Maybe take me 2008 and 2009. :P

I notice the Rovelli in your sig is referenced in the last paper. Never quite figured out if view Rovelli with derision or admiration.

I'm not telling you to read those papers. I remarked that some people were modeling the universe with S³ spatial slices and I offered the links as corroboration in case you wanted to check.

It is just evidence to back up what I said. You can just glance at the papers and see, or you can take my word for it and forget the papers! I am happy either way. Or if you want a page reference to where it says.

It's a standing offer, if I say something that seems questionable about current research then ask for a link to some example, I will try. Sometimes I have forgotten where I saw something. And this is NOT to load you down with homework, it is only an offer of optional documentation.

Yeah, I have the greatest respect for Rovelli.

You say you could either admire or deride him which makes me curious. What do you find admirable? (I would naturally prefer to hear what you think are his strong points but, if you want, describe what you see as shortcomings as well.)

 

[yuiop]

...
It's a standing offer, if I say something that seems questionable about current research then ask for a link to some example, I will try. Sometimes I have forgotten where I saw something. And this is NOT to load you down with homework, it is only an offer of optional documentation.

Yeah, I have the greatest respect for Rovelli.

You say you could either admire or deride him which makes me curious. What do you find admirable? (I would naturally prefer to hear what you think are his strong points but, if you want, describe what you see as shortcomings as well.)

First, I appreciate the offer and am grateful to you for taking time to post the links.

As far as Rovelli is concerned I meant to say "Never quite figured out if you view Rovelli with derision or admiration." I have no strong feelings on Rovelli but he seems to do good, ground breaking work in a difficult field. It more about his statement "We have calculated Newton's law starting from a world with no space and no time." Do you agree with that claim? Some might regard it as a bit extravagant or premature.

 

[mathboy]

Please excuse my ignorance, because I'm a math major. But on a surface of a 2-sphere, a person moving along a great arc will eventually return to his starting point. So in the 3-dimensional universe, will a person eventually return to his starting position if he moved in a straight line indefinitely? If not, what is the current theory about where he will end up moving towards?

 

[yuiop]

Please excuse my ignorance, because I'm a math major. But on a surface of a 2-sphere, a person moving along a great arc will eventually return to his starting point. So in the 3-dimensional universe, will a person eventually return to his starting position if he moved in a straight line indefinitely? If not, what is the current theory about where he will end up moving towards?

In post #30 on page 2 of this thread Marcus answered a very similar question. He said travelers going in opposite directions in S4 space eventually meet (aproximately). I think it follows that if they continued, they would eventually arrive at aproximately their starting points, but I'll let Marcus confirm that.

 

[mathboy]

Ok, I checked that post so the answer would be yes, and based on the hypothesis in post #30 it can be proven mathematically by composing the the path functions.

 

[marcus]

In post #30 on page 2 of this thread Marcus answered a very similar question. He said travelers going in opposite directions in S4 space eventually meet (aproximately). I think it follows that if they continued, they would eventually arrive at aproximately their starting points, but I'll let Marcus confirm that.

Wallace can give you a better discussion of this (he has in the past) or one of the mods (SpaceTiger, Pervect...).

If you just freeze space at some instant of time and explore that particular geometry then it is simple to say what happens. Except for bumpiness, if it is a perfect S³, you get back.

But expanding makes it harder to talk about. And with acceleration it is even harder.

Right now we have this cosmological horizon of about 16 billion LY which says that if TODAY a galaxy is 16 GLY away and today we send a flash of light towards that galaxy then it will never get there----because of accelerated expansion.

Now we can see lots of galaxies that today are at that distance, and even farther. So we are looking at galaxies to which (because of accelerated expansion) we could not succeed in sending a signal to.

All that makes it more complicated to talk about traveling even at the speed of light and making a full circuit. Unless you imagine freezing space and not letting it expand until you had finished making the trip.

So instead of confirming I guess I will just mumble apologetically this time.

BTW Ned Wright has a January 2007 paper where he says a "best fit" Omega is 1.011 and with the amount of positive curvature that indicates we'd have S³ with a circumference of 800 billion LY. So even if you could stop expansion in its tracks by some miracle it would still take you 800 billion years to make the circuit.
And that "best fit" picture is still merely conjecture. The data doesn't statistically rule out the case of exactly 1----the flat infinite case.
==========================
A propos Rovelli. I decided to start an informational thread about what he said about "from a world with no space and no time"
https://www.physicsforums.com/showthread.php?t=207309
I give some links to a few papers and discuss the history of their getting Newton's Law. It happened in 2005-2006. It was a pretty good achievement and it didn't come easily. LQG research has been in progress since around 1990.