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Cougar
2008-Mar-30, 01:04 AM
I was looking at the Wikipedia article on the Big Bang and noticed the section on Big bang theory assumptions. I tend to disagree with the way this section is introduced, as follows:



The Big Bang theory depends on two major assumptions: the universality of physical laws, and the cosmological principle. The cosmological principle states that on large scales the universe is homogeneous and isotropic.

Does the theory really depend on the two things listed, and are those really assumptions? The section goes on to describe the observational testing of the cosmological principle and its confirmation to a high degree, so shouldn't that lose its "assumption" label?

Steve Limpus
2008-Mar-30, 01:37 AM
Maybe what they are driving at is since we probably observe only a small portion of the universe, we can only assume the laws of physics and the cosmological principle apply elsewhere.

Even if the fundamental domain is smaller than the observable universe and we see it all, we can only make assumptions about higher (and lower?) dimensions.

Having said that, I like the laws of physics and the cosmological principle. Take them away... and there goes the neighborhood. :neutral:

Delvo
2008-Mar-30, 01:59 AM
That's one of the ways Creationists try to argue against the Big Bang. Whoever the author was apparently managed to hide its Creationist nature pretty well...

George
2008-Mar-30, 02:09 AM
Does the theory really depend on the two things listed, and are those really assumptions? The section goes on to describe the observational testing of the cosmological principle and its confirmation to a high degree, so shouldn't that lose its "assumption" label? That does seem odd. I don't recall Lemaitre basing it on either of these. It emerged from GR.

Perhaps they mean the latest tweaked version only. Had there been greater anisotropies, then inflation would have not been as much a needed addition.

EvilEye
2008-Mar-30, 02:24 AM
We know about mass on Earth, and how much we weigh.

Then before we went to the moon, we did calculations that showed what it SHOULD be like based on the same physics. It turned out to be true.

Same with Mars. Same with our probes trying to reach other planets, and even landing on an asteroid.

The physics of our little place work everywhere. It is not an assumption. It is a scientific theory that keeps being proven true, and until something shows it to be false, it is the accepted truth.

Vanamonde
2008-Mar-30, 08:51 AM
...The physics of our little place work everywhere. It is not an assumption. It is a scientific theory that keeps being proven true, and until something shows it to be false, it is the accepted truth.

When Voyage flew by Saturn, it found things like spokes in the rings and a set of braided rings that defy current explanation. And a hexagon shaped cloud on the north pole! Cassini has confirmed the latter but not the former.

I'm not saying our physics do not apply but we currently don't have an explanation of HOW it applies in all cases. Still mysteries to solve.

Spaceman Spiff
2008-Mar-30, 03:25 PM
When Voyage flew by Saturn, it found things like spokes in the rings and a set of braided rings that defy current explanation. And a hexagon shaped cloud on the north pole! Cassini has confirmed the latter but not the former.

I'm not saying our physics do not apply but we currently don't have an explanation of HOW it applies in all cases. Still mysteries to solve.

The spokes in Saturn's rings are now understood to be associated with electrostatic interactions ('static cling') with lightning in Saturn's upper atmosphere (http://www.agu.org/pubs/crossref/2006/2006GL028146.shtml). The atmospheric hexagon shape over saturn's pole is almost certainly a spectacular example of a Benard Cell (http://en.wikipedia.org/wiki/B%C3%A9nard_cells) (related to convective instablity). See also GoogleImages (http://images.google.com/images?q=Benard+cell&ie=UTF-8&oe=utf-8&rls=org.mozilla:en-US:official&client=firefox-a&um=1&sa=N&tab=wi) (but watch out for the "woo-woo" websites that use science's understanding of these structures promote gobbly-gook) and an excerpt from Koschmieder's book (http://assets.cambridge.org/97805214/02040/excerpt/9780521402040_excerpt.pdf) on the topic.

New findings are often "mysterious". As the data continue to roll in, many of these, however, are quickly explained within our current scientific theories of nature's behavior. Others take longer to understand, and can lead to more general theories of nature, and very occasionally to paradigm shifts.

Also - it isn't just the creationist (ugghh**) who questions the "assumption" of the uniformity of nature's laws over all of observable space-time. While astronomers are now able to conduct tests of this assumption (e.g., setting stringent limits on possible variations in the values of c, G, and other physical constants), one should not simply presume its validity. I would say that it is our assumptions about the nature of gravity (aka curvature in space-time) as well as the associated one, "homogeneous and isotropic", over very large distance scales, that we need to remain vigilant about. We can "assume" with caution and vigilance, without "presuming".


**Their arguments are nearly always ones from some combination of credulity, ignorance, logical fallacy, and deceit so it grates on me to mention them here in this context.

Cougar
2008-Mar-30, 10:24 PM
While astronomers are now able to conduct tests of this assumption (e.g., setting stringent limits on possible variations in the values of c, G, and other physical constants), one should not simply presume its validity. I would say that it is our assumptions about the nature of gravity (aka curvature in space-time) as well as the associated one, "homogeneous and isotropic", over very large distance scales, that we need to remain vigilant about. We can "assume" with caution and vigilance, without "presuming".
I see your point.

I have a bit of a problem with "homogeneous," though. Homogeneous when? Obviously the Big Bang predicts that the very early universe would look very different than the universe in our region today. So the universe is not homogeneous throughout time, although it may be homogeneous at any one particular cosmological time.

Tobin Dax
2008-Mar-30, 10:39 PM
I see your point.

I have a bit of a problem with "homogeneous," though. Homogeneous when? Obviously the Big Bang predicts that the very early universe would look very different than the universe in our region today. So the universe is not homogeneous throughout time, although it may be homogeneous at any one particular cosmological time.
Always. The homogeneity is on large scales. There may be a small clump here and a small clump there, but there are enough clumps around that they're in a pretty uniform distribution through space, and they will stay that way on those scales.

Spaceman Spiff
2008-Mar-30, 10:58 PM
I see your point.

I have a bit of a problem with "homogeneous," though. Homogeneous when? Obviously the Big Bang predicts that the very early universe would look very different than the universe in our region today. So the universe is not homogeneous throughout time, although it may be homogeneous at any one particular cosmological time.

The question isn't "homogeneous when?" it is "homogeneous on what distance scale?". The FRW equations assume homogeneity - and so they may be applied successfully only over spatial regimes where this is true. Gravity has had its impact in the collapsing of matter on locally small scales (which then decoupled from the Hubble flow) with the resulting formation of structure.

Jens
2008-Mar-31, 05:21 AM
Always. The homogeneity is on large scales.

I was under the impression that there is still uncertainty about whether the universe is homogenous at large scales. I think there are those who argue that it is fractal. Has the homogeneity been well demonstrated?

This (http://www.newscientist.com/blog/space/labels/large-scale%20structure.html)is a little dated, and just a blog, but it demonstrates what I'm asking.

mugaliens
2008-Mar-31, 11:39 AM
Maybe what they are driving at is since we probably observe only a small portion of the universe, we can only assume the laws of physics and the cosmological principle apply elsewhere.

Even if the fundamental domain is smaller than the observable universe and we see it all, we can only make assumptions about higher (and lower?) dimensions.

Having said that, I like the laws of physics and the cosmological principle. Take them away... and there goes the neighborhood. :neutral:

Hi, Steve. I don't think I've ever welcomed you to the board, so Welcome!

What you said.

mugaliens
2008-Mar-31, 11:45 AM
I was under the impression that there is still uncertainty about whether the universe is homogenous at large scales. I think there are those who argue that it is fractal. Has the homogeneity been well demonstrated?

This (http://www.newscientist.com/blog/space/labels/large-scale%20structure.html)is a little dated, and just a blog, but it demonstrates what I'm asking.

It's a huge void, Jens. However, on the overall scheme of things, it's very small compared to the known universe.

Statistics, given what they are, predict such things. They also predict black holes, etc. Thus, it's normal.

And life is???

folkhemmet
2008-Mar-31, 12:37 PM
Apparently, the Universe does not have a handedness. This conclusion was reached via an analysis of the spin of ~37,000 SDSS-imaged spiral galaxies. Note: the analysis did not have to turn out this way, as the original less thorough analysis implied that it did! This is an another confirmation, along side the the quasar correlation function, of statistical isotropy. Statistical isotropy should be tested again and again, but I find it a bit tiresome and frustrating how much incredulity certain people express when faced with the simple notion (in accordance with the Copernican Principle) that the geography of the Universe is not incredibly varied.

idav
2008-Mar-31, 07:50 PM
Some of you are taking the word assumption to personally(for lack of a better word). I believe, in context, it's meant to fill the model of a theory. That a theory is built upon other theories and improved with observations and evidence.

dhd40
2008-Mar-31, 09:06 PM
When Voyage flew by Saturn, it found things like spokes in the rings and a set of braided rings that defy current explanation. And a hexagon shaped cloud on the north pole! Cassini has confirmed the latter but not the former.

As Spaceman Spiff said: "The spokes in Saturn's rings are now understood to be associated with electrostatic interactions" ( my addition: with Saturnīs magnetic field).

And the braided rings (quote from the book of "Planetary Ring Systems", E.D.Miner et al ) are apparently a result of complex gravitational interactions with its two shepherding satellites (Prometheus & Pandora)
And if you donīt like the word "apparently", there are even more fascinating phenomena (e.g. small local disturbances called "propellors") which have been predicted theoretically !!


I'm not saying our physics do not apply but we currently don't have an explanation of HOW it applies in all cases. Still mysteries to solve

True, I agree!

dhd40
2008-Mar-31, 09:07 PM
When Voyage flew by Saturn, it found things like spokes in the rings and a set of braided rings that defy current explanation. And a hexagon shaped cloud on the north pole! Cassini has confirmed the latter but not the former.

As Spaceman Spiff said: "The spokes in Saturn's rings are now understood to be associated with electrostatic interactions" ( my addition: with Saturnīs magnetic field).

And the braided rings (quote from the book of "Planetary Ring Systems", E.D.Miner et al ) are apparently a result of complex gravitational interactions with its two shepherding satellites (Prometheus & Pandora)

And if you donīt like the word "apparently", there are even more fascinating phenomena (e.g. small local disturbances called "propellors") which have been predicted theoretically !!


I'm not saying our physics do not apply but we currently don't have an explanation of HOW it applies in all cases. Still mysteries to solve

True, I agree!

Jens
2008-Apr-01, 06:41 AM
It's a huge void, Jens. However, on the overall scheme of things, it's very small compared to the known universe.

Statistics, given what they are, predict such things. They also predict black holes, etc. Thus, it's normal.


My impression from reading the New Scientist blog was that this was bigger than what would be expected statistically. Has it been settled that the void is within the range of expectation?

Ken G
2008-Apr-01, 08:50 AM
Does the theory really depend on the two things listed, and are those really assumptions? The section goes on to describe the observational testing of the cosmological principle and its confirmation to a high degree, so shouldn't that lose its "assumption" label?I think the confusion here is between the difference between a model and the validity of a model. The model does require these assumptions, though one could just as well call them axioms. I'm not sure why they singled out those two and not the others, including general relativity and the presence of expansion as an initial condition. But when you put them all together, you have the Big Bang theory, and can make further modifications to distinguish various versions. When you've done that, you next have the problem of verifying the value of the model. At this stage, we do not ask "is the cosmological principle a true principle of the universe", we ask "does the assumption of the cosmological principle yield useful comparisons with observations". Of course the principle is not "true", it is an idealization, but we can verify that it is an appropriately accurate and useful one. Such verification does not make it any less an assumption of the theory-- it just makes it a good assumption. Spaceman Spiff expressed this distinction in terms of "assumption" vs. "presumption", and how important it is not to confuse the two. We never tell reality how to be, we model it and let it tell us if our models are any good.

astromark
2008-Apr-01, 10:29 AM
I have little to add to Kens excellent explanation...
The nature of assumption over presumption. So it seems like the right time and place for this; ( with apologies for the Trojan horse i am Wheeling in here ). Ignoring this would not offend me.
Robert K Soberman and Maurice Dubin have launched a proposal. Until tested that is all it can be said to be.
Explaining Dark mater and Contradicting the Big Bang.
This could be the beginning of a very interesting chapter.
Presumption, assumption, theory.
Confirmation by observation. Proof of fact. Logical conclusions. Common sense.
Untangling this mess is going to take a while yet. I do like a challenge