Freethought & Rationalism Archive

Dark Jedi · 09-30-2002, 05:36 PM

Quote:

Originally posted by Doubting Didymus:
<strong>

Ive often wondered about that, as a physics layman. If the universe could perpetually expand and collapse, wouldn't that contradict the second law of thermodynamics? Or does the theory suggest that it will run down sooner or later?</strong>

I think that it would not run afoul of the second law.

energy is neither created nor destroyed, conservation of mass and all.
Energy is neither introduced nor lost, where would it go (Universe=all that is)?
My take: The universe itself is the only perfect perpetual motion machine.

As for it's current state: I suspect before the last Big Bang was the last Gnab Gib (reverse Big Bang).

eh · 10-01-2002, 02:11 PM

The only problem is, we have no way of allowing for a bounce. Under GR, there is nothing to stop the universe from crushing itself out of existence. If a BC does happen, it might be the end of time itself.

Jesse · 10-01-2002, 02:24 PM

Doubting Didymus:
Ive often wondered about that, as a physics layman. If the universe could perpetually expand and collapse, wouldn't that contradict the second law of thermodynamics? Or does the theory suggest that it will run down sooner or later?

Dark Jedi:
I think that it would not run afoul of the second law.

energy is neither created nor destroyed, conservation of mass and all.
Energy is neither introduced nor lost, where would it go (Universe=all that is)?
My take: The universe itself is the only perfect perpetual motion machine.

You're thinking of the first law of thermodynamics, which says energy is conserved. The second law of thermodynamics says entropy must always increase or stay constant. The second law would indicate that entropy must be higher on each successive "bounce", which according to <a href="http://nedwww.ipac.caltech.edu/level5/Glossary/Essay_clun.html" target="_blank">this</a> page would mean that "Eventually the oscillations would peter out, rather like a bouncing ball which gradually comes to rest as its energy dissipates."

Jesse · 10-01-2002, 02:30 PM

eh:
The only problem is, we have no way of allowing for a bounce. Under GR, there is nothing to stop the universe from crushing itself out of existence. If a BC does happen, it might be the end of time itself.

But most physicists think GR breaks down in the neighborhood of a singularity--we'd need a theory of quantum gravity to know what was really going on at the moment of the Big Crunch (or the Big Bang).

Nat · 10-02-2002, 10:45 AM

I'm rather surprised that the idea of the Big Crunch keeps coming up in discussions of cosmology. With the exception of a certain Stanford husband and wife team of physicists, the idea of the Big Crunch has been all but abandoned since Pearlman et alia provided crucial experiemental verification of a positive Cosmological Constant (CC), or at least, the existence of an accelerating "force" on Universal expnasion. The most promising work has been with the idea of Quintessence and the "pushing" or repulsive gravity effect of dark energy due to its negative pressure (although Newtonian gravitation is not dependant upon pressure, General Relativity is, and GR has been shown again and again to be a more accurate model of gravitation).

Further, most physicists had given up on the Big Crunch idea a while before the Quintessence models were even discussed due to lack of mass in the Universe to halt the expansion. Experiemental and theoretical evidence suggests that there simply is not enough dark matter (consisting mostly of WIMPs, or Weakly Interactive Massive Particles) to cause a gravitational collapse.

Current models and experiements clearly indicate that the Universe is "open" not "closed."

Nat · 10-02-2002, 10:53 AM

"The fact that there 'was' a big bang - necessitates input to 'cause' the explosion. "

Quotes like this make me really mad that physicists decided to keep using Hoyle's insulting moniker of "Big Bang" to describe the expanding Universe model.

Just for the record, Big Bang Cosmology does not, I repeat, does not, posit an "explosion" of any kind. It was not a singular event - it is simply the name of a theory that describes the expansion of spacetime.

What started the expansion (or Big Bang) is not really part of Big Bang Cosmology but is certainly an interesting field of study. There are , of course, many ideas, but they unfortunately are as of yet limited by a weak (read: no) understanding of gravity at a quantum level (we in fact don't even understand gravity at levels as large as a cm). Hopefully the Large Hadron Collider will be able to get up the energies necessary to shed some light on this problem when it opens. There is always hope.

Cheers

Jesse · 10-02-2002, 10:58 AM

Nat:
I'm rather surprised that the idea of the Big Crunch keeps coming up in discussions of cosmology. With the exception of a certain Stanford husband and wife team of physicists, the idea of the Big Crunch has been all but abandoned since Pearlman et alia provided crucial experiemental verification of a positive Cosmological Constant (CC), or at least, the existence of an accelerating "force" on Universal expnasion.

According to <a href="http://www.newscientist.com/news/news.jsp?id=ns99992759" target="_blank">this</a> article, both Andrei Linde and Martin Rees think a Big Crunch cannot yet be ruled out:

Quote:

"A few years ago, nobody would even think seriously about the end of the world within the next 10 to 20 billion years, especially since we learned that the Universe's expansion is accelerating," says Andrei Linde of Stanford University. "Now we see it is a real possibility."

...

He and his colleagues have shown that according to some theories of supergravity, which try to describe gravity within the context of quantum theory, the dark energy from a scalar field will do more than simply reach zero - it will become negative and possibly even plunge as far as minus infinity.

This would slow the rate of expansion of the Universe and then put it into reverse, causing space and time to collapse to a point in a big crunch.

Linde and his colleagues calculate that the typical time for the Universe to start this collapse would be in 10 to 20 billion years from now. Its current age is about 14 billion years.

"This was the greatest surprise," says Linde. "We might be in the middle of the life cycle of the Universe, not at the beginning."

...

England's Astronomer Royal, Martin Rees of Cambridge University, is keeping an open mind. He agrees that a future collapse is possible. "Since we have no idea what the dark energy is, such scenarios cannot be ruled out," he says. "But ultra-long-range forecasts are all exceedingly speculative."

Nat · 10-02-2002, 01:03 PM

Jesse,

Andrei Linde is one part of that Stanford husband and wife team that I mentioned. He could be right, but his ideas on supergravity are bucking the current trend (not that that is a problem). I read his recent articles on the subject, and I remain along with most other physicists, skeptical. He could be right, but I think our understanding of quantum gravity is too limited as yet to givce much weight to his hypotheses.

Even so, remember that Big Crunch scenarios are now, even by their most ardent proponents, considered amazingly speculative.

Cheers

atrahasis · 10-02-2002, 10:02 PM

Nothing existed,,Nothing was always there but not any more...

According to Dr john Baez of university of california said..

Quote:

There's a lot we don't know about nothing.

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10-01-2002, 02:11 PM	#32
eh Senior Member Join Date: Sep 2000 Location: Canada Posts: 624	The only problem is, we have no way of allowing for a bounce. Under GR, there is nothing to stop the universe from crushing itself out of existence. If a BC does happen, it might be the end of time itself.

10-01-2002, 02:24 PM	#33
Jesse Moderator - Science Discussions Join Date: Feb 2001 Location: Providence, RI, USA Posts: 9,908	Doubting Didymus: Ive often wondered about that, as a physics layman. If the universe could perpetually expand and collapse, wouldn't that contradict the second law of thermodynamics? Or does the theory suggest that it will run down sooner or later? Dark Jedi: I think that it would not run afoul of the second law. energy is neither created nor destroyed, conservation of mass and all. Energy is neither introduced nor lost, where would it go (Universe=all that is)? My take: The universe itself is the only perfect perpetual motion machine. You're thinking of the first law of thermodynamics, which says energy is conserved. The second law of thermodynamics says entropy must always increase or stay constant. The second law would indicate that entropy must be higher on each successive "bounce", which according to <a href="http://nedwww.ipac.caltech.edu/level5/Glossary/Essay_clun.html" target="_blank">this</a> page would mean that "Eventually the oscillations would peter out, rather like a bouncing ball which gradually comes to rest as its energy dissipates."

10-01-2002, 02:30 PM	#34
Jesse Moderator - Science Discussions Join Date: Feb 2001 Location: Providence, RI, USA Posts: 9,908	eh: The only problem is, we have no way of allowing for a bounce. Under GR, there is nothing to stop the universe from crushing itself out of existence. If a BC does happen, it might be the end of time itself. But most physicists think GR breaks down in the neighborhood of a singularity--we'd need a theory of quantum gravity to know what was really going on at the moment of the Big Crunch (or the Big Bang).

10-02-2002, 10:45 AM	#35
Nat Regular Member Join Date: May 2002 Location: USA Posts: 100	I'm rather surprised that the idea of the Big Crunch keeps coming up in discussions of cosmology. With the exception of a certain Stanford husband and wife team of physicists, the idea of the Big Crunch has been all but abandoned since Pearlman et alia provided crucial experiemental verification of a positive Cosmological Constant (CC), or at least, the existence of an accelerating "force" on Universal expnasion. The most promising work has been with the idea of Quintessence and the "pushing" or repulsive gravity effect of dark energy due to its negative pressure (although Newtonian gravitation is not dependant upon pressure, General Relativity is, and GR has been shown again and again to be a more accurate model of gravitation). Further, most physicists had given up on the Big Crunch idea a while before the Quintessence models were even discussed due to lack of mass in the Universe to halt the expansion. Experiemental and theoretical evidence suggests that there simply is not enough dark matter (consisting mostly of WIMPs, or Weakly Interactive Massive Particles) to cause a gravitational collapse. Current models and experiements clearly indicate that the Universe is "open" not "closed."

10-02-2002, 10:53 AM	#36
Nat Regular Member Join Date: May 2002 Location: USA Posts: 100	"The fact that there 'was' a big bang - necessitates input to 'cause' the explosion. " Quotes like this make me really mad that physicists decided to keep using Hoyle's insulting moniker of "Big Bang" to describe the expanding Universe model. Just for the record, Big Bang Cosmology does not, I repeat, does not, posit an "explosion" of any kind. It was not a singular event - it is simply the name of a theory that describes the expansion of spacetime. What started the expansion (or Big Bang) is not really part of Big Bang Cosmology but is certainly an interesting field of study. There are , of course, many ideas, but they unfortunately are as of yet limited by a weak (read: no) understanding of gravity at a quantum level (we in fact don't even understand gravity at levels as large as a cm). Hopefully the Large Hadron Collider will be able to get up the energies necessary to shed some light on this problem when it opens. There is always hope. Cheers

10-02-2002, 01:03 PM	#38
Nat Regular Member Join Date: May 2002 Location: USA Posts: 100	Jesse, Andrei Linde is one part of that Stanford husband and wife team that I mentioned. He could be right, but his ideas on supergravity are bucking the current trend (not that that is a problem). I read his recent articles on the subject, and I remain along with most other physicists, skeptical. He could be right, but I think our understanding of quantum gravity is too limited as yet to givce much weight to his hypotheses. Even so, remember that Big Crunch scenarios are now, even by their most ardent proponents, considered amazingly speculative. Cheers

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