Center of the universe

nutgeb

Kronos, the balloon analogy fails entirely once you depart from the surface of the balloon. Only the surface is included in the analogy. There is no analogy in the real universe for any point not located on the surface, such as the center of the balloon.

The same problem occurs whether the bug is inside or outside the balloon.

People have tripped over this conceptual problems many, many times.

ManyNames

If you want a center to the universe, you can call every point on the spacetime map as the center of the universe, since big bang happened everywhere - but not at any single point.

Chronos

Unless we happen to reside very near the 'center' of the universe, which I consider highly improbale, the CMB would be decidedly non-uniform.

DaveC426913

What?? Why are you suggesting there's a centre??


What?? So, time did not exist prior to the fifteenth century or so?


Look, I know this is not literally what you meant but you can't go around posting these pithy platitudes without putting them in some context where they are heavily conditioned. Someone, somewhere is going to say "Time did not exist before clocks, and we might be near he centre of the universe. Yes I have a reference, see this Science Advisor on Physics Forums? No more venerable reference than that..."

BigFairy

As far as i understand, there is no centre.

Cuetek

You know, this is what bugs me about the current model. People cite all of it's tenets as fact without regard to relative probability. It is highly likely that everything we see around us was unimaginably dense and compact at one point. Fine. We have evidence that is very hard to interpret any other way. Something went Bang. You bet.

BUT. To state as fact that the Big Bang happened everywhere is a philosophical statement at best. The eviedence is only that it happened everywhere we are likely to be able to see any time soon, which is NOT to say that we even have any idea what "everywhere" amounts to. That the universe has proven to hold more diverse features beyond the contemporary human ability to resolve them at any given time is as likely now as it was a thousand years ago.

A little perspective and a generous helping of phrases like we think, or presumably, or according to the current model would go a long way in helping people establish a little theoretical context around some of the more tenuous aspects of the standard model.

-Mike

DaveC426913

No, they cite it as the theory it is. The theory part is implicit in all discussions.

If people are coming on to a physics board to discuss physics and do not know this, then they are woefully unprepared.

Chronos

I thought I was suggesting IF that were true, the CMB would not be uniform UNLESS we are at the putative center - a possible, but, unlikely explanation.

I believe you are confusing clocks with mechanical timepieces. Ancient civilizations used the motions of the sun, moon and stars. Motion is the essence of any clock, be it subatomic particles, pendulums, or the stars. Time without motion cannot be quantified, hence is meaningless. I believe that is context Einstein intended with regard to clocks.

I hope this clarification satisfies any perceived contextual needs.

Chronos

Well, Mike, how else can you interpret it? No scientist I know of claims the BB as fact. It is merely our best approximation based on observation and physics. No one disputes our observations are incomplete and theories have error margins. To couch every assertion with 'our best guess is' unnecessarily diverts attention from efforts to propose new ideas. This is the only way to shore up our more fundamental assumptions. If something wrong with the pyramid of current theory, new observations will eventually cause it to collapse under its own weight. Furthermore, no scientist in his right mind would hesitate to attack the slightest inconsistency in any existing theory, however precious it may seem. This is how Nobel's are won.

Cuetek

The theoretical part is not implicit particularly in Cosmology. All theory includes a great deal of subject matter that is widely corroborated. Such factual components of theory may be factually stated with no ill-effect. However, the parts of theory that are either very low on data, long on inductive reasoning or presumptive of issues extending beyond the available evidence are highly speculative. Presenting such formal conjecture as factual by casual ommision is to perpetuate the weakest part of conventional wisdom as truth. Such casual ommision leads to casual acceptance of exactly the parts of theory most likely to be false.


Words have meaning and to present pesumption as fact behind some ill conceived notion that everyone who's anyone "knows" where the line is drawn is pure arrogance and ultimately an impediment to science.

DaveC426913

Um, you do know that implicit is synonymous with unspoken, right?

i.e. the theoretical part of all physics is there as a given, it does not need to be spoken every time.

Anyone who is interested in cosmology will have to already understand the theoretical nature of physics (otherwise, as previously stated, they are in over their heads). Would you ask that every discussion of physics should be qualified so that school children or laypeople don't misunderstand the difference between fact and theory?


Please show one example where cosmological theory is presented as fact.

And for whom are you speaking? Are you having trouble with the difference between fact and theory? Do you treach a class where all your students have trouble? Is this a genuine problem of which you are aware? Or is this a complaint without substance?

inhahe

So we know that galaxies are moving away from us in accordance to their distance and the Hubble constant... but since all motion is relative, we can't very well say which galaxies are moving away from which; to every other galaxy, their neighboring galaxies are moving away from them similarly. BUT...

Why don't we just take the cosmic background radiation, take its red/blue shift, use that to calculate its speed relative to us (which is hence our speed relative to the greater universe), and then multiply that speed backwards by 18 billion years (or however old the universe is estimated to be currently), and then you have the position we'd be in 18 billion years ago, which would be the center of the universe?

xxChrisxx

CMBR doesn't work like that as you are still thinking that the universe is 3D. If you did that you'd find we are the centre of the universe (which we know isn't the case). As it's pretty much uniform and ALL redshifting.

Cuetek

It is not implicit in a forum where people have widely varying comprehension of the theories.

I say, in this forum, it should be. It's not a lot of trouble to add "the math would indicate" or "theory suggests" to passages that are purely speculative like dark energy/dark matter.

It occurs to me that this forum is designed to be as much a resource for younger students as it is an exchange for seasoned enthusiasts. Such indignation over my suggestion that clarifying the degree of presumption in modern cosmology might be useful in this forum is hard for me to take very seriously.

Well, the guy I was responding to with my original complait said the following:

"... since big bang happened everywhere - but not at any single point."

Even if true this is unknowable, much less factual.

Myself, thanks.


No, I'm the one suggesting we formalize the difference between fact and theory a little more in our conversations.

Have you quit beating your wife yet?

-Mike

DaveC426913

OK, so you alone are having trouble with the implicity that BB is a theory. For a moment, I thought you were suggesting anyone else was confused by it.

OK well that's easily rectified.

The BB is our best theory. There are no seriously competing theories. It is not fact.

You now know this. Problem solved.

Cuetek

It's true that nothing will stop the progress of science and sooner or later the data tells the tale. But there is a problem with new data. The CMB and the Super nova recession data are just about the only major new discoveries that have been made recently. And they are both corroborative of there universe having been very compact at some point in the past. So the BB is very attractive and has been for a century or so.

But the weakest part of the BB is not that things were compact some 15 billion years ago, but that everthing in the universe was compact and that there was nothing "outside" or "before" the BB. That is, the cosmological principle (CP) has been take as fact and is considered fact almost reflexively in most exminations of the new data and certainly in all conversations among the experts about the conventional models. But the CP is not only theoretical, it is unlikely.

Coversations about dark matter and dark energy are entirely predicated on the universe being homogeneous. The calculations of the total matter in the universe necessarily require uniformity throughout. And the CP is really an idealization of the locally visible universe and not a careful examination of the most probable disposition of the large scale universe.

All scales of the universe that we have ever examined shows a hierarchical structure, yet we humans always terminate that hierarchy with every new cosmology we devise. Currently the CP terminates the hierarchy by extending the largest visible scale out to whatever extent necessary. The CP is a very handy idealization that allows us to work backwards with all the local material to devise what I imagine to be a very accurate history, but it does not serve us to imagine so rigidly that it is universal. It is unlikely that the CP holds at, say, a million or a billion times the particle horizon.

Every physical phenomenon ever examined has proven to be finite in extent and multiply manifest (that is, for any given physical phenomenon we can find other examples in the universe). Why must the BB be unique. The CP is just as likely to be only a local idealization and if so, discussions about dark matter and dark energy are discussion about what an idealized homogenous universe would look like rather than a universe where larger structures and phenomonon dictated local matter/energy dispositions like they do at all the other scales of the universe we have examined so far.

I don't see what you mean here. To me, the keeping everyone more apprised of the most presumptive aspects would be the better strategy in that it is probably the more presumptive aspects that will need modifying or replacing. I think that it wouldn't be too imposing for cosmologists, if only when talking to the public or the less cognizant groups, to add phrases like "the math suggests" or "the evidence suggests" or even "according to the model" before passages that deal with subject that are short on data, long on presumption or are projections beyond the existing data (eg dark energy, dark matter, curvature of the universe, etc). It would keep us more mindful of our own presumption.

-Mike

Kronos5253

That's a bold statment, and horribly incorrect.