Round One: Big Bang vs. Little Bang

[Steven Taylor]

A quick epistemological issue I'd like to discuss... Can anyone out there give me a good, non-technical explanation for why Cosmologists generally take for granted that evidence of spatial expansion is evidence of some global event (i.e. "the Big Bang"), rather than some local, spatially isolated event (let's call it "a little bang")?

 

[marcus]

Originally posted by Steven Taylor
A quick epistemological issue I'd like to discuss... Can anyone out there give me a good, non-technical explanation for why Cosmologists generally take for granted that evidence of spatial expansion is evidence of some global event (i.e. "the Big Bang"), rather than some local, spatially isolated event (let's call it "a little bang")?

Here is one aspect: expansion and a failure of the model at the beginning of expansion was predicted by the equations BEFORE the actual observation of a redshift-distance relation by Hubble.

Several people very early on had taken Einstein's 1916 GR equation and DERIVED solutions where there was expansion (spatial distances between stationary points were increasing) and where the equations failed to compute at time zero---a kind of 'global' time-boundary to the applicability of the equations.

this limit to the applicability of the GR equation is called a singularity----it is not a property of nature but a limitation on the model---it is also not confined to some finite region of space or to a point: if space is infinite in extent (as is often now assumed as the simplest way to match recent observational data) then the singularity is infinite in spatial extent.

I have not heard any speculation that things behaved like an explosion around time zero----that is an unfortunate image that sticks in people's heads because of the misleading name "Big Bang". What was predicted (already by 1922) and later confirmed by observations is completely unlike an explosion----it is not a bunch of stuff propelled outwards by an initial shockwave and coasting outwards in space.

it has not behaved at all like that

the expansion has been tracked and it does not look like an explosion but like a dynamic change in geometry which is capable
of accelerating and then decelerating and then accelerating as various factors affect it.
right now no contraction is forseen but something we don't know about could turn it around and cause contraction
anyway the words Big Bang convey a incomprehension because the dynamics are unlike those of an explosion.

AND YES I BELIEVE YOU ARE RIGHT there could be a local expansion going on. The events we see could be just a bubble of expansion in a much larger and more varied context. The question for a cosmologist would be, however, does he or she gain anything by complicating the model to include extraneous stuff we can't detect and for which we have no evidence.

Maybe over "there" they have a different kind of dark energy that makes their space contract, while we have a kind that makes our space expand at an accelerating rate. But the prevailing model covers all we can see and its adjusted to fit all the data we can get. Including some speculative "over there" wouldn't help it make testable predictions---and so would be inefficient.

maybe this is the epistemological reason they "take for granted" (as you say) that they are dealing with the whole she-bang.

 

[wolram]

think about it, how could we "know" the difference
BIG BANG is only terminology
in the grand scale of existence "we" could exist for
a micro second or an eternity.
my view of cosmology is that "size", is meaningless .
i am sure that literatate members of this forum
will give you more informed information

 

[marcus]

Originally posted by wolram

i am sure that literatate members of this forum
will give you more informed information

dont let wolram fool you
he is thoroughly literate and canny besides
(which a good many literates arent)
there is a kind of intellectual street-smarts
which I guess can be called canniness

 

[quartodeciman]

answer 1: The Friedmann models are the easiest possible global solutions to Einstein's GR field equations that allow variable scale factors. That makes it easiest for arriving at conclusions, such as what time epoch we might be in now and when the blessed event might have happened. Lemaitre, Robertson, Walker and others cemented the fondness for F models and they sort-of became established. They are homogeneous and isotropic at at time > 0.
Attempts at non-homogeneous GR models of cosmology didn't get very far (got messy!) back in the 1930s and 1940s.

answer 2: Gamow, Alpher and Hermann promised to explain away the cosmic abundances of elements through a nucleosynthesis process of an early dense, ultrahot universe. It didn't quite work out. Most astro people now think elements are created inside stars, either current stars or previous stars that blew up. But the lightest elements (Hydrogen, Helium, Lithium) evidently need another source, so cosmic nucleosynthesis came back. There was controversy about it during the 1950s, but the Cosmic Microwave Radiation discovery by Penzias and Wilson changed all that. Opposition was effectively silenced for a while. True, the effective temperature of this radiation, 2.7K, is lower than the estimates. The early big bang predictions were more like 5K, 10K and up. Oh, well!

answer 3: The redshifts of radiation from remote galaxies, quasars and other stuff have become an indispensable anchor for estimating cosmic distances. So cosmologists all insist that the red shifts come from expansion and nothing else. Find some blue shifts out there. Nobody has found any.

answer 4: Current cosmologists were trained by earlier cosmologists, who were in turn trained by yet early cosmologists that this is the global picture of the universe. In other words, it's a tradition.

answer 5: Believe it or not, it is easier to get big money grants in cosmology if you can promise to see "the beginning of creation" and not just some smaller explosions.