"the whole mess will start over again ..."

[DFINITLYDISTRUBD]

Moderation note: This post was moved from the 7177 thread, because it is a question about the future of the universe, not just the motion of the Earth

 

:hihi:

Ack:eek: :eek:

The earth is slowing down. Quick, we better destroy the moon now before it is too late!!:hihi:

 

(sorry, I've got the bad sense of humor back under control:))

:hihi: *BUMP*

 

Tis true though...everything will grind to a halt eventually....at which point the universe which had been expanding will start contracting building to a highly alarming rate until everything smooshes into a singularity similar to the one present right before the first big bang...the tremendous force absorbed will then ignite the next big bang and the whole mess will start all over again.....with life? without life? or do it alternate every few billion cycles or so?:shrug:

[CraigD]

Tis true though...everything will grind to a halt eventually....at which point the universe which had been expanding will start contracting building to a highly alarming rate until everything smooshes into a singularity similar to the one present right before the first big bang...the tremendous force absorbed will then ignite the next big bang and the whole mess will start all over again.....with life? without life? or do it alternate every few billion cycles or so?:shrug:

This is a scenario cosmologists usually call the Big Crunch, big bounce, oscillating universe, and similar phrases.

 

The first cosmologists to grapple with the observation that the visible universe was far from stable, in about the same sense that the solar system is stable – Hubble, Friedmann, etc. around the early 1920s – quickly saw that the universe either would expand forever, or undergo something like the Big Crunch, coming up with a parameter [math]\Omega[/math], omega, Friedmann’s density parameter If [math]\Omega \le 1[/math], the universe is “open”, and will expand forever, if [math]\Omega > 1[/math], it is closed, and will crunch, bounce, oscillate, etc.

 

A decent gauge of scientific consensus is, I think, what’s taught in science classes. When I was in high school (1975-1978), the closed universe was presented as on about equal footing with the open one. When I was in college a few years later, the closed universe was presented as unlikely.

 

The current consensus, based on lots of observation, theory, and exchange of ideas, is that the universe is open, and no Crunch will occur. Knowledgeable proponents of the Crunch (as opposed to ill-informed cranks) tend to postulate either

• that our understanding of gravity, will is still about as empirical as it was 300 years ago, lacking a compelling theoretical explanation of its cause, is somehow unable to describe the universe well enough to rule out a Crunch,

or

• that [math]\Omega[/math] is presently less than 1, but that something about the universe will change in a way that increases it to become effectively greater than 1.

Some of these latter kinds of speculations can be pretty weird, many falling into a line of speculation that future humans, or something like them, will artificially reverse the universe’s expansion. Often, this line speculation takes on a somewhat theological character, as its imagined future super-artificers can reasonable be described as god-like.

 

Serious scientific writing about this is unusual, and in many cases of dubious merit. One that I’ve read is Frank J Tippler’s 19 “The Physics of Immortality”. Since he wrote this and a sequel, the mainstream scientific consensus seems to be that Tippler, once considered a credible, if perhaps sometime overly imaginative, physicist, is now more-or-less insane.

 

The Crunch has and continues to be fertile ground for science fiction. My favorite story featuring it remains Poul Anderson’s 1970 novel “Tau Zero”, though I’ve read many other great stories along similar lines, The idea that a god-like, technological entity engineered out present universe to promote life is hinted at in Arthur C Clarkes 1972-1993 Rama series.

 

In short, though, by the best conventional science, the universe is open, and a Big Crunch isn’t going to happen.

[sanctus]

Actually, to be more precise, there is the special case [math]\Omega ==1[/math] which seems to be the consensus from what I hear/read/see. In this case the universe would be flat, I think this is why Craig wrote for [math]\Omega \le 1 [/math] open between " "...

[DFINITLYDISTRUBD]

Good Grief!!!:hyper: This is quite the unexpected result!

 

Open?

 

Closed?

 

I was merely attempting....actually I have forgotten what the whole intent was:doh:

 

Oh well....now it's gone and got all technicalish....It will likely seem foolish but I do believe that there will come a time when everything in the universe will run out of momentum... I figure in the whole expanding dealy you've got everything's gravity pulling against everything else's gravity so everything should be losing energy in the expanding direction.....

 

Which should mean that at some point the initial energy that sent everything racing outwards will be used up and things will stop...and probably hang there a bit before gravity starts pullin' everything back into the middle of things.

 

Some of these latter kinds of speculations can be pretty weird, many falling into a line of speculation that future humans, or something like them, will artificially reverse the universe’s expansion. Often, this line speculation takes on a somewhat theological character, as its imagined future super-artificers can reasonable be described as god-like.

 

And that be everything I have to offer on the matter ...which is something I merely ponder on occasion.....right along with the highly likely to be a myth "space is a vacuum"....I'd believe that one bout just as much as the alternate lines of thought quoted above.

[CraigD]

Good Grief!!!:hihi: This is quite the unexpected result!

Hah, DD, you’ve wandered into the territory where the greatest minds of early 20th century astrophysics played! We but stand on their dead shoulders and post wikipedia links to them :)

… I do believe that there will come a time when everything in the universe will run out of momentum... I figure in the whole expanding dealy you've got everything's gravity pulling against everything else's gravity so everything should be losing energy in the expanding direction.....

If we stick for the moment with the Friedmann universe model (which, compared to stuff that’s come as a consequences of later observations, is very intuitively sensible), everything does lose momentum in the direction of the Hubble flow, but, based on the best observations available before they started getting really weird, not fast enough to ever halt and reverse it.

 

Like much of astrophysics, this is an area where everyday mechanical intuition can get us into trouble. In the world of our everyday experience, everything does lose momentum, but this is because our everyday world is dominated of frictional forces, while the universe on astronomical scales is not.

 

You might find it helpful to reflect on the physics of spacecraft launched from Earth at greater than Earth escape speed (about 11200 m/s), or better yet, the solar system’s escape speed (about 617500 m/s), and the fact that, despite the saying that “what goes up, must come down”, such spacecraft almost certainly are never coming down. The force of gravity between the Earth and Sun and them continues to slow them down, but not enough, and this acceleration diminishes as the square of the distance the further they travel.

 

Put another way, to say [math]\Omega \le 1[/math], or “the universe is open”, is the same as saying “the universe has achieved escape velocity relative to itself”. If only the Friedmann equations were involved, the expansion of the universe would continuously slow, but at a continuously reduced rate, never reaching zero regardless of how far into the future.

 

As I hinted at above, because observation has not matched theory in an unexpected way, the Friedmann equations are no longer thought to accurately model the universe. Rather than its expansion slowing, the universe’s expansion appears to be increasing. There’s not yet a satisfying explanation of the fundamental underlying mechanism responsible for this, though a name for what’s causing it has been agreed upon: dark energy.

 

If you don’t like an open Friedmann universe, you’re really not going to like dark energy. ;)

[DFINITLYDISTRUBD]

If you don’t like an open Friedmann universe, you’re really not going to like dark energy.

Tis' not so much "like" "don't like" but "can/can't fathom" :)

 

I unfortunately (or fortunately) am bound by "shade tree" mechanics and "country mouse" physics;)

[modest]

As I hinted at above, because observation has not matched theory in an unexpected way, the Friedmann equations are no longer thought to accurately model the universe. Rather than its expansion slowing, the universe’s expansion appears to be increasing. There’s not yet a satisfying explanation of the fundamental underlying mechanism responsible for this, though a name for what’s causing it has been agreed upon: dark energy.

 

If you don’t like an open Friedmann universe, you’re really not going to like dark energy. ;)

 

Before CC steps in and claims victory :hihi:, I'd say the Friedmann equation can accommodate dark energy, quoting it from wiki,

 

There are two independent Friedmann equations for modeling a homogeneous, isotropic universe. They are:

[math]H^2 = \left(\frac{\dot{a}}{a}\right)^2 = \frac{8 \pi G}{3} \rho - \frac{kc^2}{a^2} + \frac{\Lambda c^2}{3}[/math]

which is derived from the 00 component of Einstein field equations Einstein's field equations, and

[math]\dot{H} + H^2 = \frac{\ddot{a}}{a} = -\frac{4 \pi G}{3}\left(\rho+\frac{3p}{c^2}\right) + \frac{\Lambda c^2}{3}[/math]

which is derived from the Trace (linear algebra) trace of Einstein's field equations.

 

or the more common,

 

The first Friedmann equation is often seen in a form with density parameters.

[math]\frac{H^2}{H_0^2} = \Omega_R a^{-4} + \Omega_M a^{-3} + \Omega_k a^{-2} + \Omega_{\Lambda}.[/math]

The last term in each equation can be taken out by setting [math]\Lambda = 0[/math], but that choice most decidedly does not make it a Friedmann equation or not a Friedmann equation. Hence, the Friedmann equation does accurately model the universe. This is no real matter... just avoiding the forest fires with a bit of prevention ;)

 

~modest

[freeztar]

This is no real matter... just avoiding the forest fires with a bit of prevention :evil:

 

"Only you can prevent Friedmann fires!" :lol:

 

In seriousness, it looks like the next decade will be pretty exciting for pinning this issue down further. (not that the last decade wasn't)

[sanctus]

Yep, hope that when we get Planck data eventually (launch postponed again) we will be able to pin down quite a bit

[DFINITLYDISTRUBD]

Originally Posted by CraigD View Post

As I hinted at above, because observation has not matched theory in an unexpected way, the Friedmann equations are no longer thought to accurately model the universe. Rather than its expansion slowing, the universe’s expansion appears to be increasing. There’s not yet a satisfying explanation of the fundamental underlying mechanism responsible for this, though a name for what’s causing it has been agreed upon: dark energy.

I'd call it having seriously outdated data...(but then I really don't know how they figure these things to begin with)

 

It's generally accepted (I think) that the light we recieve from the farthest stars we can see left those start thousands if not millions or billions of years ago (bear with me here) and as best I can figure the expanding universe folk base their (hypothesis's? theories?) on the data they gather by watching what those stars were doing all those years ago as the light (gamma rays radio waves whatever) finally reach observatories satalites here.

 

Now....I'm not sure exactly how long do it take a universe worth of explosion to finish blowin up and the matter thrown by the explosion to finally come to a rest...or at such point for the gravity of all objects within the debris field to reverse the flow (if at all) to create a new "singularity" and reignite as everything hits a critical mass yielding a smaller bang than the preceding one.

 

...

[lawcat]

This is a scenario cosmologists usually call the Big Crunch, big bounce, oscillating universe, and similar phrases.

.

 

CraigD,

 

Is it true that the universe is expanding?

 

If true, is it true that the spacetime between galaxies is expanding?

 

if that is true, is it true that the spacetime within the galaxies is not expanding due to expanding universe, or if it is, it is negligible?

[Qfwfq]

Is it true that the universe is expanding?

The redshift and its proportionality to distance (Hubble's law) indicate that yes, it is.

 

If true, is it true that the spacetime between galaxies is expanding?

If one supposes the whole universe to be correctly described by the Robertson-Walker metric with k = -1 then consequently the volume is finite and increasing. In the k = 0 and k = 1 cases it isn't really a physical question but only a choice of coordinates.

 

if that is true, is it true that the spacetime within the galaxies is not expanding due to expanding universe, or if it is, it is negligible?

The dynamics of a galaxy are not like the dynamics of the overall universe; they are quite independent of each other.

[DFINITLYDISTRUBD]

YouTube - Galaxy Song http://www.youtube.com/watch?v=buqtdpuZxvk&NR=1 related but not enjoy:hihi:

[lawcat]

The redshift and its proportionality to distance (Hubble's law) indicate that yes, it is.

 

If one supposes the whole universe to be correctly described by the Robertson-Walker metric with k = -1 then consequently the volume is finite and increasing. In the k = 0 and k = 1 cases it isn't really a physical question but only a choice of coordinates.

 

The dynamics of a galaxy are not like the dynamics of the overall universe; they are quite independent of each other.

 

I was thinking this:

 

If spacetime is expanding, and galaxies are not, then it means: wherever there is gravity, there is no expansion of spacetime. Wherever there is gravity, there is contraction of space time; at least relative to the constant expansion.

The force of expanding universe is not strong enough to expand even the slightest gravity. Nor is all gravity in the universe enough to slow the expansion of spacetime.

 

So in essence, only matter can contract the spacetime. Otherwise spacetime keeps expanding. All matter exists and none can be created. There isn't enough matter to contract all spacetime.

 

There will never be a big crunch.

 

true or untrue?

[Qfwfq]

If spacetime is expanding, and galaxies are not, then it means: wherever there is gravity, there is no expansion of spacetime. Wherever there is gravity, there is contraction of space time; at least relative to the constant expansion.

Steven Weinberg heads the chapter on the principle of general covariance with a quote of Lewis Carrol, from Alice's Adventures in Wonderland:

 

"Either the well was very deep, or she fell very slowly, for she had plenty of time as she went down to look about her and to wonder what was going to happen next."

 

Since ancient times, philosophers have been aware that we can only check ratios of time and space intervals. We define standard clocks and rulers (currently in terms of electromagnetic radiation from atomic orbital transitions) and express such intervals as ratios to units of measurement. Going much beyind this can quickly transcend epistemology. The matter is very subtle but I find reasons to interpret the dynamics in the sense that the distance between galaxies is increasing and their size is not decreasing.

 

There will never be a big crunch.

 

true or untrue?

IMESHO we currently cannot answer this question.

[lawcat]

 

IMESHO we currently cannot answer this question.

 

Thank you for that wonderful quote from Alice in Wonderland.

 

If this is true:

The force of expanding universe is not strong enough to expand even the slightest gravity. Nor is all gravity in the universe enough to slow the expansion of spacetime.

 

Then, let's begin by saying, spacetime is malleable and expanding in a sense. Gravity only molds it, but does not influence either contraction or expansion of spacetime. If that's true, then gravity and spacetime are completly independent. Spacetime exists. And within it energy exists in various forms.

 

If that is true, then all we perceive is the expanding energy within spacetime--like radiation. But spacetime itself does not expand nor contract. Nor do we have tools of detecting such expansion, if it were to take place, independent of energy fields since we live in and only perceive energy.

 

Since energy changes form, we can perceive it as expanding energy, or moving matter, or contracting density--all with associated magnetic or gravitional or electric fields.

 

Since energy moves inertially, it just goes in one direction unless acted upon by other energy in another. So "theoretically" energy can contract to one point. But the matter, or energy, is so dispersed in various forms, and moving inertially, there is no binding force that would contract it. That is, unless we introduce anti-matters, or other types of boogy men.

 

Only matter has binding force. Radiation has no binding force--it simply propagates through space. All matter lies in the field of radiation. And the field of radiation lies in spacetime. And that radiation is expanding.

 

But since matter can only bind matter, and let's presuppose that it can bind radiation in a highly dense black hole, there is no indication in the observable universe that such enormous matter exists, that can bind all of expanding radiation.

[Qfwfq]

Gosh, that is a rather heterodox treatment of the topic!!!!