Weird little questions for this here group

[eccoblackfin]

How large is the known universe? Or, put in a different light, do 'we' have any clue as to where the universal center might be, or what it looks like in theory or otherwise? Finally, what's the object, be it galaxy, star, restaurant, etc., furthest away from us?

Comments

[timeprojectile]

The "known universe" is, sort of, about 14 billion light years in radius, which would mean 28 billion in diameter. We can only see the distance the light has been able to travel in the 14 billion years since (this episode of?) the Universe began. But space has expanded since the light set out from these galaxies, so technically I think that would make them farther away today than what their light-travel time would infer. And furthermore, we think the Universe is even larger still, beyond what we can ever see due to the Universe's finite age and increasing expansion, because the best way we have to explain why the cosmic microwave background looks more or less the same even on opposite sides of the sky (which are twice as far away from one another as light has had time to travel) is that it was all once in contact with each other long ago, but an ultra-powerful expansion of space immeasurable fractions of a second after the Big Bang, known as Inflation, blew these similar regions apart.

The farthest thing away that we can see is the cosmic microwave background radiation, which is the afterglow from a hot state of the Universe a few hundred thousand years after the Big Bang. At this point, the universe was opaque, for the same reason the photosphere ("surface") of a star is opaque (because in an ionized gas, the electrons are loosely attracted to and influenced by the positive ions without being bound in an atom, and can emit and absorb any wavelength of energy as they accelerate and decelerate in the electric fields of the ions). The temperature of this star-surface-like Universe was something like 2700 degrees Kelvin (degrees Centigrade above absolute zero). Now the temperature is only about 3 degrees above absolute zero. Because of the opacity of the Universe at this era, we cannot see, with electromagnetic waves of any form, what the Universe before that was like.

And as best we can tell, there is neither a center nor an edge to the Universe. A common analogy is the surface of a sphere, which itself has no center and no edge, but it is finite, and the "center" associated with this surface is in a completely different dimension. I think I was told that thinking of the center of the Universe as being in a different dimension is not quite right, though. So it's really complicated. We also don't know if the Universe even has this "curvature" to it...as best we can tell, it's flat. But, we don't think there is any kind of center to the Universe in any dimension that we can probe. Instead, there are simply regions of higher and lower density of gas, stars, and galaxies.

[eccoblackfin]

Thanks, that answered a lot of questions. It also raised a hell of a lot more though.

Ah, don't you just love astronomy?

[timeprojectile]

That's what science always does. Raise more questions than it answers. Which can be frustrating if you're in one of those moods when you want answers...but wonderful if you're in one of those moods where you want to feel like you're part of something magical and mysterious and beautiful.

[eccoblackfin]

So, with that theory/analogy, we're basicaly just skimming along the surface of a bubble? Does that mean that sometime in the next trillion years or so we'll do a full circle and be smashed together in a new big bang?
Or maybe, we're already being pulled into the next one, the so-called 'dark flow', by the rest of the universe that we don't know about yet o.O

[timeprojectile]

I don't think we know yet, and I'm not sure we CAN know, what the ultimate fate of the Universe will be. Like I said, I think I was told that we can't really think of the curvature of the Universe being in an extra dimension...the sphere thing is not to be taken literally, it's a matter of visualizing how geometry in "curved" space works.

Some say that our universe, though, may be one of many bubbles that blew up in a parent "multiverse" as the result of some quantum fluctuation-type thing. Others say that our universe cycles between big bangs and big crunches, but I don't think it's because our universe is skimming along the surface of a bubble in a circle. In the surface-of-a-sphere analogy, the universe IS the circle, and the "known" or observable universe is a tiny little disk surrounded by fog (the cosmic background and the edge of time) on that circle. So it looks like the Universe ends at the fog and is flat, but it extends far beyond it and is curved. That's the idea.

For several years now I've had an idea, rooted in pure poetry and not in math, about the concept of the "Big Rip" that I heard about when the big buzz in cosmology was that the Universe is accelerating in its expansion and may accelerate out of control to the point of tearing apart galaxy clusters, then galaxies, then stars and people and even atoms. At the time, I'd also heard that there were 11 dimensions of space-time but that 7 of them were curled up, and those curled-up dimensions were the "strings" that matter and energy were made of. (This was M-theory, I think, a version of string theory.) So, given these two ideas, I figured that sometime after atoms were torn apart by the cosmic expansion, the most fundamental particles - the stuff of matter and energy, made of these curled-dimension "strings" - would have to be torn apart too. And tearing apart these strings would mean the unfolding of the dimensions within them. So the Universe would revert back to its original 11 dimensions that it had before the Planck time (the smallest unit of time we can create by multiplying and dividing physical constants involved in the four forces by one another), and this unravelled Universe could have another Big Bang. Possibly a quantum-gravitational fluctuation in 11-dimensional space-time creating a new bubble-universe with its own big bang, space, time, and mass-energy.

But this is all pure speculation. As is the rest of the field. Those who do it as a special area of study simply add some esoteric mathematics to bring some non-poetic logic to their speculations.

[umbran]

Does that mean that sometime in the next trillion years or so we'll do a full circle and be smashed together in a new big bang?

No. The "everything is moving away from everything else" phenomenon is not due to our moving around on the surface of the bubble, but because the bubble is expanding. So long as the bubble continues to expand, things'll continue to move apart.

If there's enough mass in the Universe, eventually the bubble stops expanding, and starts shrinking. Then, things will start oving together, and we head for a Big Crunch or Big Bounce, depending on your favorite theory.

[cultureofdoubt]

Superb answer. And right about "But space has expanded since the light set out from these galaxies, so technically I think that would make them farther away today than what their light-travel time would infer"
The radius is about 46 billion light years "comoving radial distance" - which is basically the distance with the expansion factored out to today's scale (there's other ways of measuring distance you'll sometimes come across which are more practical to measure but give very different values - I think this one is most meaningful here though)