Wat you are completely right, there is no way to know there is not ever just one of something in nature, but I was using my intuition when I made that statement.  If there is any proof of it in what we see all around us, all I can say is that something that occurs one time, should at least theoretically, be able to happen more than once.  I dont see why our universe or us for that matter should be so special.  So I guess it's also the anthropogenic argument.  In some ways it reminds me of the intelligent life on other worlds (or life on other worlds at all) argument.  Ironic you mentioned agnosticism!  I'm agnostic too, but as far as religion is concerned, I find the idea of polytheism far more likely than monotheism.  "Gods" would just be a substitute for extremely advanced intelligent species that can create universes haha.

Sean Carroll in the statement above seems to come to the conclusion that having multiple universes preserves symmetries like the arrow of time.

I like Poplawski's too, when you think about it, the universe can be defined as the inside of a black hole for the simple reason we can never leave it!  Could you imagine that the reason why singularities exist is for the simple reason is that they each contain a different universe with different dimensions therefore they cannot be described by the physics of our universe?

Nice to see that your thread is finally getting some attention Watsisname .

It also only takes one counterexample to falsify it, and the universe itself could be the counterexample.

I think it's not falsifiable. How do you want to know if there is really only one of a thing? In order to know this you would have really to search the whole universe!
And if there are other universes, then maybe there is this one thing there again!

Exactly; it is not falsifiable in principle.  What I mean is that it doesn't follow that if everything in the universe exists more than once (and if that premise is even true), then the universe must, too.  It's a faulty generalization because the universe itself could be the one thing that doesn't fit the rule.

This topic is soo mind boggling and bewildering for my tiny little brain . I really don't care whether there are/aren't other universes. Our universe is soo amazingly huge and complex that I consider, for all practical purposes, each galaxy to be an island universe on/off its own.

Speaking of brains (mine excluded ) and since my real life profession is health-related, I find it really fascinating that the structure of the brain is very similar to the structure of the universe, but obviously on a completely different size scale: The brain contains billions of neurons connected together in a web that resembles the galactic web that Watsisname,'s OP picture shows.
I just googled "brain neurons and synapses similar with universe structure" and it returned some quite interesting results one of them is a study done by both an astrophysicist and a neuroscientist and I am linking it here.
I will definitely be following this thread and many many thanks to Watsisname, for this open course for my little tiny brain..

I will definitely be following this thread and many many thanks to Watsisname, for this open course for my little tiny brain..

Thanks, and glad you're enjoying it!

I find it really fascinating that the structure of the brain is very similar to the structure of the universe, but obviously on a completely different size scale

Yes!  I think this is a very neat coincidence.  The structure that evolution developed to efficiently store and transfer information in brains happens to look similar to the structure that is formed by gravitational collapse of a fluid of particles.  Sometimes completely different physics leads to very similar effects, like how hurricanes look a lot like grand design spiral galaxies.

This is a pretty good video lecture for showing how the cosmic web structure formed in the early universe.  The whole video is relevant, but you can also skip to 19:38 to see a bunch of cool simulations, and after 33:30 he compares the simulations to what we actually see in the sky.  

[youtube]YcUU_QrTrv0[/youtube]


Shortly after the Big Bang the universe was essentially homogeneous (uniformly filled with matter).  But small variations from that uniformity grow stronger with time.  Denser regions grow denser by collapsing under their own gravity, and then the matter in their surroundings starts falling into them as well.  Combined with the expansion of space, this produces a complex web of sheets and filaments where galaxies form.  The space in between where matter has been emptied out from becomes the "voids".  You can see this structure in Space Engine as well if you enable the procedural galaxy generation and fly off to large distances.  The voids are almost eerie -- huge stretches of space where you encounter almost nothing.  

(There is actually some matter in the voids, mostly tenuous gas that has been thrown off by violent supernovae explosions in the early universe.  This shows up in observations as the Lyman-alpha Forest, where the gas absorbs some of the spectrum of distant galaxies and quasars, but at many different distances (and hence redshifts), resulting in a complicated "forest" of absorption lines.  More recent cosmological simulations include the stellar evolution and death, and how well they reproduce the Lyman-alpha forest serves as another way to test them.)

The role of dark matter also turns out to be incredibly important for understanding how the cosmic web structure formed.  Dark matter outweighs normal matter in the universe by about a factor of 5, so it was the initial variations in the density of dark matter that acted as the "seeds" for the growth of the cosmic web, with regular matter (mostly gas) falling in to them to create the galaxies.

Exactly; it is not falsifiable in principle.  What I mean is that it doesn't follow that if everything in the universe exists more than once (and if that premise is even true), then the universe must, too.  It's a faulty generalization because the universe itself could be the one thing that doesn't fit the rule.

It's a fantastic topic of discussion though.  And being a big fan of mathematics, I've always felt that anything that can be shown to be possible by mathematics should actually exist somewhere.  Your arguments for an omniverse were compelling also (I use the term omniverse over multiverse because omniverse covers a whole ensemble of different types of universes, from those with different physical laws, to those that are identical to ours but have an altered timeline.)  The symmetry that would balance out some of the oddities of our universe with the existence of other universes also seem compelling to me.

Wat this is a pretty fascinating thread and topic of discussion, I can easily say that this is the most thought-provoking thread I have seen anywhere in any forum on the internet.  I belong to an email group called Theoretical Physics on Yahoo and we used to have these kinds of discussions back a few years ago but unfortunately email groups have become much less popular in the last couple of years.  I belonged to several of those groups and they've all become inactive- including one that was run by David Deutsch.

I would love to hear more from Doc in this thread too, you and he always have the most thoughtful posts!

So SE does model the superclusters and supervoids really well?  I'd love to see that structure on a macro scale, seeing chains of thousands of galaxies puts everything in perspective.

So SE does model the superclusters and supervoids really well?

It does, but with a lot of the exposure settings it is difficult to really appreciate the web-structure of super-clusters. I had to max out the exposure and saturation to really see the patterns.

Thanks, how do you max the saturation?  I've been looking for saturation settings to bring out the colors of the stars better.

A good way to see the cosmic web structure in Space Engine is to fly to one of the faces of the cube (10x10x10Gpc) where galaxies are generated, orient the camera so that you're pointing straight out from that face, turn the magnitude limit up to about +18, and then fly straight backward in to the cube by a few hundred million light years.  This way you see a small cross section of the universe, rather than a huge distance, so the walls and filaments of galaxies are much easier to see.

Here's a quick animation to help also give a sense of depth:

[center][/center]

[left]Compare with the 2dF galaxy survey.  I think SE does a pretty good job emulating the real thing. [/left]

[center][/center]

Thanks, how do you max the saturation?  I've been looking for saturation settings to bring out the colors of the stars better.

Technically there is no such thing in the game engine. What I did to imitate a saturation/desaturation effect was to adjust the gamma settings in conjunction with the exposure setting. As for bringing out the colors of the stars as per their spectral type, I tried everything, with little notable effect. Yes, it would be nice if the engine had star spectrum-color exaggeration.

Wow, Wat that shows it rather well!  I've been trying to visualize what a 3D flat space would look like (flat on the scale of the observable universe at least) and the closest shape I can come up with is that of a tesseract (4D cube- if you want to visualize time as a spatial dimension.)  But I think it "should" still be more like a glome (4D sphere) on scales much larger than that of the observable universe.

I was thinking about what you said about not being able to ever visit parallel timeverses if the universe is infinite, and of course while I think thats true, if some of the variants of string theory are correct, and there's a second time dimension (F-theory), than you could have timeverses separated from each other in the second temporal dimension, therefore they would not necessarily be separated by space, but by time instead.  This doesn't make it any easier and there are good reasons for not thinking we have a second temporal dimension in our universe at least, but it's fun to think about and pretty easy to visualize all timelines emerging at the big bang/bounce and separating from each other across the second time dimension as the universe rapidly expands in space also, and then (if) the universe does ever collapse or bounce back all the timelines would converge right back upon themselves again, (perhaps) to rinse and repeat.

Stellator, thanks, where can I adjust the gamma settings? I see gamma settings in my Nvidia Control Panel, is that where I should do it?

Perhaps a saturation slider will be incorporated into the next version, it wouldn't be that hard to do.  There is a digital vibrance thing in the Nvidia Control Panel but that also affects everything else displayed on that monitor, I wish there was a way to apply it for a specific window lol.

3D flat space on cosmological scales requires no new imagination.  It's the same geometry that you are already intimately familiar with.   If you held two laser pointers side by side so they emit parallel beams, then those beams stay parallel forever.  That's a feature of flat geometry.

Observations are consistent with the space being flat, but it could also be slightly positively or negatively curved.  In fact it would be very easy for it to be either of those.  If the density of the universe turns out to be slightly more than the critical density, then space is positively curved.  If slightly less than the critical density, then space is negatively curved.  Positive curvature would make those two laser beams come together after some distance, just like lines of longitude come together on the Earth.  Negative curvature would instead spread them apart.

For space to curve back on itself to make the volume of the universe finite (the entire thing, not just the observable part), and so that going far enough in any direction brings you back to where you started, then the space needs to be positively curved.  It's entirely possible that this is how the universe really is, since with the uncertainties in the measurements it's a roughly 50-50 chance the density is greater than the critical density.



By the way, I've thrown the term "critical density" out there a few times, but I don't think I defined what it is yet.  It is the specific density of matter and energy that would make the universe flat.  Which is just the reverse of what I've already said, so that's not helpful.  What's helpful is that it works out to be about 5 to 6 proton masses per cubic meter (the exact number depends on the value of the Hubble constant today), and as a formula it is given by



where H is the Hubble constant and G is the gravitational constant.  We can also write it as 0.001123 protons per cubic meter times H², if H is in km/s/Mpc.


5 or 6 protons for each cubic meter in space needed to make the universe flat might sound very tiny.  It is tiny, even compared to the density of the interstellar medium, which is a better vacuum than anything we can make on Earth.  But it's greater than the density of a typical "void" in the cosmic web.  So on the average, the density of the universe works out to be pretty darn close to the critical density, which is a pretty surprising coincidence (not just for making the geometry flat), and as you mentioned, ALEX, the leading idea for why it works out that way has to do with the inflationary hypothesis.  Maybe more on that topic, later.

I was trying to think of different shapes and for some reason a fish tank came to my mind and I was really hoping our universe doesn't look like a fish tank (for obvious reasons lol).

If the universe was even slightly positively curved and you drew a triangle on a scale large enough to measure that curve, the angles would add up to slightly more than 180 degrees, just like if you drew such a large triangle on the surface of the earth?

Fascinating thing about protons and critical density, I remember reading awhile back there was some uncertainty with protons (which are really a composite of three quarks of course).  The uncertainty lay with the theory that there's a possibility that protons aren't completely stable and will break down into other particles after a long period of time (like neutrons do, but their lifetime would be much longer than that of a neutron.)  I read about this years ago and wondered if any headway had been made in determining the lifetime of a proton and if we found out that it was finite, even if it was very long, how that would alter our cosmological theories.  I also remember reading that there are 10^87 electrons in the universe so there should be a similar amount of protons (I dont know how they would come to this estimate, especially since there may be large parts of the universe we can't even see.)