Universe is one big galaxy???

[brianb]

The overall structure of the universe is still not fully understood, but let's suppose that the universe is a simple closed structure like the surface of a higher-dimensional balloon. In that case, we wouldn't be able to point to a center either because that center would exist in a dimension that we don't perceive. To us, the universe would be a continuous, homogeneous structure with that same weird property: if you traveled far enough in one direction, you'd return to where you started.

Actually even that isn't true ... you'd never get "right round" because the universe would have expanded so much that, even if you were travelling at the speed of light, the "right round" point would be staying ahead of you. And, even if you did get "right round", you wouldn't recognise the place you'd started from, because it would have changed so much in the meantime.

[Talitha]

To the ant, the balloon has no center. It lives in a continuous, homogeneous world, which has the weird property that if you walk far enough in any direction, you return to where you started.

Thanks Mick, i understood the expansion/drift part.. i just didn't realize i'd have to think like an ant and only perceive two dimensions.

Still can't wrap my head around something. Suppose i travelled far enough in one direction and returned to the point of beginning... would i be sneaking up on my past, or would i be in the present?

If i'd left a timeclock behind, would it read differently than one i'd carried with me?

And would i be able to make this one-way/round-trip if the universe were open?

[brianb]

Suppose i travelled far enough in one direction and returned to the point of beginning

It's exactly the same as the twins paradox. Doesn't matter how many times you accellerate (provided it's at least once), or what speed you travel at - you'd better travel at the speed of light, else it would take too much of your time to get there = here, but time would have been elapsing whilst you were travelling; and, if the distance travelled is the "circumference of the universe" i.e. 2 * pi * 13.7 billion light years, it's going to take 2 * pi * 13.7 = 86 billion years to get back, even if the universe doesn't expand in the mean time. You won't recognise the galaxy, let alone be able to find the sun, which will have evolved into a white dwarf & been cooling for at least 75 billion of those years ....

But the universe is expanding, there is an "event horizon" beyond which we cannot see because the expansion at that distance is equal to the speed of light, so the bit of the universe we're confined to is always open, meaning that a "straight line trip" will not in general come back to the starting point ... only if the path went close enough to a very massive object to deflect the path through almost 180 degrees would you even get close.

[Talitha]

"there is an "event horizon" beyond which we cannot see because the expansion at that distance is equal to the speed of light, so the bit of the universe we're confined to is always open, meaning that a "straight line trip" will not in general come back to the starting point "

So.. the never-ending event horizon is what keeps our area of the universe open? Sounds like one of those dreams where you're running for all you're worth, but staying in the same spot or worse yet, falling behind.

[brianb]

Sounds like one of those dreams where you're running for all you're worth, but staying in the same spot or worse yet, falling behind.

Indeed. But it's a straightforward consequence of the Second Law of Thermodynamics ... without which the universe would not be habitable to creatures like us.

[vincentnm]

Exlnt reply there Mick.

I understand your ant explanation, as time now,presant.

But what about 13.5 billion years ago when the bang went off.That should be the centre point surely?

Mick.G.

Remember that the big bang created space, and space has been expanding ever since, not just at the outer edges, but from within.

Every point in the current universe was the centre point at the time of the big bang.

So the current universe has no fixed centre, instead every point is the centre

[vhbelvadi]

Many have a problem with the fact that the point of occurance of the big bang explosion is not the centre of the universe. Clicking here and here might help them.

The ants-on-a-balloon explanation at first seems interesting, but is it really valid??

Can anyone give me an example of an object with no centre?

Lastly, can the point with the highest concentration of gravity, in the universe, be considered its centre?

[Ant]

doughnut?

Sorry just kidding...

[vhbelvadi]

doughnut?

Sorry just kidding...

Nice one there ant. My bad, actually.

Let's say, Gimme a non-hollow object without a centre; does that sound better?

[ZOG]

The thing is, and I am in no way trying to contradict anyone here, the big bang theory is just that, a theory, and it's based on our knowledge AT THIS MOMENT IN TIME.

Based on the other theories that have come and gone through the ages (flat earth, sun revolves around earth etc) there is every chance that some time in the future it will be disproved.

Our knowledge of the universe is limited to what our present day instruments can tell us, we think we know how big it is, (the hubble deep field expanded that size not too long ago) and as humans we do have a tendancy to make the facts fit the theory.

A hundred years from now someone could look at this thread and laugh at our lack of knowledge at this moment in time.

[crovax]

Wait, and what about Great Attractor? I am not sure about all Universe but I thought that all observable galaxies are affected by it and rotate around it.

[narrowbandpaul]

big bang happened.

its the simplest theory that explains all the observed evidence (if you add in a bit of inflation at the start). As einstein said, 'a theory should be as simple as possible, but no simpler'

since the big bang explains all the observed evidence (expansion, abundance of light elements) it is almost certainly the correct view, it may be altered though, just as alan guth did with inflation

[room16]

Bare with me on this, im probably miles out....but i have a question regarding the balloon explination.

Im trying to wrap my head around this and I was wondering...

I've always toyed with the idea that we live in some sort of bubble, but not on the outside, on the inside. Not attached to the skin, the skin is the cosmic background radiation and beyond that is as yet unknown.

If this was true, then there would be a central point, where everything, including the cosmic background rays (inner side of the outer skin), speed off into the distance at the same speeds but seemingly faster dependant on where you view it from.

Few more questions, probably easily answered....

In which direction does this cosmic background radiation come from, was it all around the object that spotted it and if so, was it an equal distance away (from all angles) from this object when it was found?

Arghrghr, sooo many questions, just don't have the right words to describe them lol.

[acey]

Hi,

My teacher was unable to answer the question, "are the galaxys in the univese in fact one big galaxy where smaller galaxys (the milkyway) orbit around a central point, as the earth does around the sun?"

Then let me answer it for you.

The model you suggest was considered by Kurt Godel in 1949. He was looking for cosmological models based on Einstein's general relativity, and he found that if the universe as a whole were rotating, then it would be possible to get in a spaceship, go on a very, very long journey, and arrive back not only at the place you left, but also at the time you left. It's a model in which time travel to the past is possible.

This upset Einstein quite a lot, and astronomers were obviously interested to see if there was any evidence that the whole universe is rotating. If it were, then spiral galaxies would all tend to be aligned in the plane of rotation. Godel and other people looked at hundreds of observatory photographs, measuring the position angles of spiral galaxies. But there's no overall alignment - there is no evidence that the universe is rotating. We do not live inside a great big super-galaxy.

Tell your teacher that.

Surveys of the large-scale structure of the universe are based on red-shift, which is taken as the measure of distance (in accordance with the standard big-bang model). Galaxies are found to be bunched in clusters (like our Local Group and the nearby Virgo Cluster); and these clusters are bunched in superclusters.

It was wondered if the superclusters would be assembled in super-dooper clusters, super-dooper-wooper clusters, and so on forever. This would be a fractal universe, with no particular length scale. But observations rule this out.

The superclusters tend to be arranged along "surfaces" with great voids between - rather like foam. This is the largest-scale structure that has been found. It is consistent with the inflationary scenario, in which quantum fluctuations during the first moment of the universe created inhomogeneities that seeded subsequent galaxy formation.

In which direction does this cosmic background radiation come from, was it all around the object that spotted it and if so, was it an equal distance away (from all angles) from this object when it was found?

It comes from every direction and every distance.

Microwaves coming from great distance are more redshifted than those coming from close distance. But microwaves from long distance were emitted at a much earlier time, when the universe was hotter, so they began with a shorter wavelength.

The two effects cancel out, and all the microwaves reaching us have (on average) the same wavelength, corresponding to a very uniform temperature seen all across the sky (once the Earth's own motion is factored out).

Andrew

[acey]

In which direction does this cosmic background radiation come from, was it all around the object that spotted it and if so, was it an equal distance away (from all angles) from this object when it was found?

It comes from every direction and every distance.

Microwaves coming from great distance are more redshifted than those coming from close distance. But microwaves from long distance were emitted at a much earlier time, when the universe was hotter, so they began with a shorter wavelength.

The two effects cancel out, and all the microwaves reaching us have (on average) the same wavelength, corresponding to a very uniform temperature seen all across the sky (once the Earth's own motion is factored out).

[ncjunk]

I like Docs explanation of the balloon.

I was more thinking the balloon as a whole, 3 dimensional, rather than the 2d space occupied by the ants. When you think of the balloon analogy in 3 d you start thinking there must be a centre because the balloon has a centre. So although the balloon analogy is a good one it can cause a bit of confusion.

Neil C

[acey]

It comes from every direction and every distance.

Correction - I typed too fast.

I should have said it comes from every direction and a range of distances.

[room16]

Correction - I typed too fast.

I should have said it comes from every direction and a range of distances.

Ok, so in this thinking, is it not possible that there is a center point where the radiation collides with itself at exactly the same time?

Obviously this isn't possible if it was anything other than spherical universe, kind of like living in a bubble, not on the outside, but the inside.

[Dangerous-Dave]

If our universe has a closed topology, which would be weird, you would be able to see the same object twice!

Only if the universe was small. We can only see so far as we are looking back in time. The observable universe might not be the entire universe!

Please sir my head hurts!

[acey]

For about the first 300,000 years after the big bang, the universe was a plasma so hot that photons couldn't get very far at all in it without hitting something. The universe was effectively opaque.

But at around 300,000 years it cooled enough so that photons could keep going for a long time, possibly forever, without hitting anything.

The cosmic microwave background consists of photons that were emitted at around that time. They were being emitted all over the universe, which is why we see them coming from every direction.

As to "hall of mirror" universes in which you see things twice (or many times), this was looked for quite a long time ago, by trying to find replications of galaxy clusters on observatory photographs. Nothing was found.

More recently, it was proposed to look for replicating patterns of inhomegeneities in the cosmic microwave background. Janna Levin, who was involved in that research, wrote a very nice book about this:

How the Universe Got its Spots: Diary of a Finite Time in a Finite Space: Amazon.co.uk: Janna Levin: Books

But the required patterns haven't been found, so if we're living in a hall of mirrors it must be on a scale larger than the observable universe.

[George]

All other galaxies we can see, are moving away from us.

M31 (Andromeda) is actually moving towards us and will merge with us sometime in the future.

I wonder how that fits in with the balloon theory

[George Jones]

M31 (Andromeda) is actually moving towards us and will merge with us sometime in the future.

I wonder how that fits in with the balloon theory

An expanding universe is a prediction of homogeneous(for a fixed time, the same everywhere in space) and isotropic (for a fixed time, the same in all direction from any given point in space) models of general relativity.

To a good approximation, our universe is homogeneous and isotropic on very large scales, so, on very large scales, our universe expands. On smaller scales, our universe is clumpy (not homogeneous and isotropic) with solar systems, stars, galaxies, clusters of galaxies, etc., so on smaller scales (e.g., a solar system or a cluster of galaxies), expansion is not a necessary feature of our universe.

[room16]

M31 (Andromeda) is actually moving towards us and will merge with us sometime in the future.

I wonder how that fits in with the balloon theory

Simples..... andromeda is in an orbit which crosses the path of the milky way, atleast, it will do and at this point they will collide. They will get relatively further apart, thus stretching the orbit....but still, it will eventually collide...or so we are lead to believe.

[AstroTiger]

The book i am currently reading, the endless universe, explains that objects move away from us according to hubbles law.

For the same period of time of say 2 years

an object 3 light years away would be move 6 light years

another 4 light years would have moved 8.

This is why the more remote an object appears the faster it is (or appears) to be receding from us.

The fact that most objects appear to be expanding would suggest there is no comment point in the universe about which objects orbit?

Please excuse me, i am from a chemical background myself, someone who has always found this facinating.

However i do believe posters will find this book most interesting. Its been gathering dust on my bookshelf for yonks, now hard to put down!

[Cygnus X1]

This idea of everything 'riding' on the skin of a (only possibly) ever expanding balloon, therefore with almost (given that gravity makes objects that are really close gets closer) everything being 'ridden' further away from everything else is quite all right, but surely space has a 'depth'... It's not a 2-D 'skin' being stretched. I am a 3-D object, so is the Earth, so is the Solar System (after all some bodies in our solar system, including some which we have made) are travelling 'up' or 'down'. The Milky Way is light years thick at its center. There are stars and galaxies to be seen all around the Earth, meaning some are left, right, up, down, straight ahead and behind. We are not talking small distances here, so the analogy with the skin of a balloon is far from conclusive.

So how thick is the Universe???