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Darasen
2002-Feb-20, 02:13 PM
I realize this probably a stupid question but it has been bugging me.
The universe starts with a “Big Bang” and super acceleration. The expansion then slows but the universe continues to expand and from what I understand the rate of expansion is increasing.
If my understanding of the sequence of events is correct it leaves me with following question. Doesn’t it take energy to change the acceleration of anything? Where does the energy to expand the universe come from? Isn’t the amount of energy in the universe a constant?

Probably silly questions but I can’t help but wonder.

Darasen

Wiley
2002-Feb-20, 03:29 PM
Actually, I think they are extraordinarily good questions. The answers, I believe, are no one knows.

That the expansion is acclerating has only been known for a few years. So theories like "dark energy" are understandably nascent.

SiriMurthy
2002-Feb-20, 04:11 PM
Boy, Excellent questions. Actually I was wondering myself (after reading Bad Astronomer's pages - somewhere on this site he mentions that the Universe is actually accelerating - I mean expanding with acceleration) how that can be possible. If anything, the universe should be expanding at constant rate, if not decelerating.

If the Universe is accelerating, then the galaxies should be slowly going out of shape, right? Does Newton's laws of motion still hold good in this situation?

/phpBB/images/smiles/icon_confused.gif

Another Phobos
2002-Feb-20, 04:25 PM
In a nutshell (at least as this nut understands it)...

The Big Bang occurred and space rapidly expanded.

Soon after this period of hyperinflation, the rate of expansion slowed. (Not sure why, but it seems to make sense that the energy supporting hyperinflation would dilute out as space got bigger.)

Space continues expanding for billions of years. Local gravitational fields (e.g., within a galaxy) overpower the expansion and there is no expansion at that level...just in the vast empty spaces between galaxies/galaxy clusters). Even in those empty spaces, gravity has some effect to reign-in the expansion of space.

Recent discovery - - the expansion of space may be accelerating. Hypothesis - - there is a 'dark energy' that is VERY weak...far weaker than gravity (which is already a very weak force). So, this dark energy...which pushes space apart...only becomes significant in the vast, vast intergalactic spaces where gravity is so low that this dark energy can start to overcome it.

Much more research is needed regarding this new finding. So, it remains a big question/uncertainty.

SiriMurthy
2002-Feb-20, 04:45 PM
On 2002-02-20 11:25, Another Phobos wrote:
In a nutshell (at least as this nut understands it)...

Recent discovery - - the expansion of space may be accelerating. Hypothesis - - there is a 'dark energy' that is VERY weak...far weaker than gravity (which is already a very weak force). So, this dark energy...which pushes space apart...only becomes significant in the vast, vast intergalactic spaces where gravity is so low that this dark energy can start to overcome it.


I have read something about this "dark energy. You know, I wonder, if we just made up this term to explain something we don't know.

Wiley
2002-Feb-20, 04:53 PM
On 2002-02-20 11:45, SiriMurthy wrote:

I have read something about this "dark energy. You know, I wonder, if we just made up this term to explain something we don't know.


You could be right. It seems like anytime there's something we can't detect, we call it dark: dark energy, dark matter, dark side of the force.

This theory was obtain by waving dark hands.



<font size=-1>[ This Message was edited by: Wiley on 2002-02-20 11:55 ]</font>

Hale_Bopp
2002-Feb-20, 05:33 PM
If you can answer all these questions, you deserve a major prize /phpBB/images/smiles/icon_smile.gif I'll just give you a few things I have picked up along the way.

After the period of inflation, the expansion of the universe was slowing. After several billion years of a slowing expansion, the dark energy increased and led to the currently observed acceleration of the expansion.

Dark energy is thought to exist in all of space. The more space, the more dark energy. Since the expansion of the universe is actually an expansion of space, as the amount of space increases, so does the dark energy. Eventually there is enough dark energy to accelerate the expansion.

What about conservation of energy? Well, I admit to being very sketchy on the details, but I do know that energy is not conserved in general relativity for reasons that I do not understand. I am not sure how dark energy fits in with all this.

To make matters worse, I went to a very technical talk (that I followed only a fraction of) a couple of months ago that talked about the fact that there are actually two separate factors that contribute to the dark energy.

Now, the last point on this confusing topic, Kim Griest at the University of California at SanDiego recently published a paper proposing that the current acceleration of the expansion of the universe if just a phase, much like inflation was. She argues that there may be different types of fields that drive the expansion of the universe in different eras. The current era of inflation may end, and she does not rule out future eras of inflation.

So I guess that sort of sums up what I have been able to make of this puzzle over the last couple of years.

Rob

DStahl
2002-Feb-20, 09:17 PM
I have only a few fragments to add--I'm as puzzled as everyone else.

Minor point--the early inflationary era collapsed rather suddenly in most scenarios, and the force driving the exponential expansion during that era disappeared along with it...the universe was given an immense push, but after the first 10<sup>-10</sup> second or so expansion was no longer driven by the inflationary false vacuum physics.

Mor interesting point--The mass/energy of the universe can, in a sense, be placed on one side of an imaginary balance, and the potential gravitational energy of the universe be placed on the other side. Conventionally, we say the gravitational energy is negative. (I rather naively imagine that two masses, separated by distance x, would yield kinetic energy E<sub>1</sub> in falling together. If they move apart to distance 2x, then they yield kinetic energy E<sub>2</sub> > E<sub>1</sub>.) So I think that as the universe expands and centers of mass become more widely separated the gravitational potential energy of the universe reaches a larger (negative) value. Do the scales balance? Do they come close? I dunno!

Anyone want to straighten me out here? *grin*

--Don Stahl

[Later] Here are some references for the stuff on mass/energy versus gravity, in two languages.

Mathematish: University of New Mexico (http://tesla.phys.unm.edu/phy536/6/node2.html) web page.

English: Dr. Sten Odenwald's (http://itss.raytheon.com/cafe/qadir/q149.html) answer to a question about whether there exists some sort of negative mass/energy to balance the cosmic scales.

<font size=-1>[ This Message was edited by: DStahl on 2002-02-20 16:54 ]</font>

NubiWan
2002-Feb-21, 11:37 PM
On 2002-02-20 11:53, Wiley wrote:


On 2002-02-20 11:45, SiriMurthy wrote:

I have read something about this "dark energy. You know, I wonder, if we just made up this term to explain something we don't know.


You could be right. It seems like anytime there's something we can't detect, we call it dark: dark energy, dark matter, dark side of the force.

<font size=-1></font>


Well, that is exactly right. It's "dark" because the light of science has yet to bring it to understanding, so to speak. We know something is at work by measuring it's influences, but what it is, is unknown, not understood, therefore "dark."

*L* Does that make any sense? *shrug*

DoctorDon
2002-Feb-22, 10:18 PM
On 2002-02-21 18:37, NubiWan wrote:

We know something is at work by measuring it's influences, but what it is, is unknown, not understood, therefore "dark."

*L* Does that make any sense? *shrug*


Actually, it's called dark for the very simple reason that it doesn't interact with light.

We don't know what it is, either, so it makes a nice poetic resonance, but the background of the name is much more prosaic.

Don

DJ
2002-Feb-22, 11:26 PM
I mentioned this in the other thread on dark matter, but it makes more sense based on the last comment. Is it possible that "dark matter" is dark because it is something traveling faster than the speed of light?

If we are not defining dark as the absence of light, but as some_thing, then it seems plausible. I have nothing to back up this hypothesis as I lack the math to even understand it... however I do have some understanding of singularities. And in their case, the gravitational force is strong enough to bend the light cones back on themselves, creating the "black."

Something is at work there, and it's dark, and so is dark matter, and so are bended light cones, so there's my observational logic. /phpBB/images/smiles/icon_frown.gif

DJ

Pi Man
2002-May-14, 04:13 AM
There is a correlation between the acceleration and the cosmological constant that Einstein pounded into his equations to make the universe still static. Whatever causes it, it brings up some interesting consequences. The acceleration makes it possible to have a spherical universe(rather than a flat or saddle shaped universe) that will expand forever, and never approach a stand-still. This was seemingly impossible before this discovery.

P.S. Last I heard, the answer was that nobody knows why it's expanding.

beskeptical
2002-May-14, 10:07 AM
On 2002-02-20 11:25, Another Phobos wrote:
Soon after this period of hyperinflation, the rate of expansion slowed. (Not sure why, but it seems to make sense that the energy supporting hyperinflation would dilute out as space got bigger.)


Wouldn't gravity be the force slowing hyperinflation? And, now if expansion is accelerating, the force for that acceleration is only being theorized?

I heard an interesting talk at the U. of WA re: the above. (I am bad about remembering the professor's name but if someone really wants to know I'll get it for them.)

One aspect of the talk was very interesting to me. Light diminishes in 3 dimensional space at a rate that reflects those 3 dimensions. The force of gravity, on the other hand, diminishes at a rate that suggests 11 dimensions. It is getting very interesting out there.

DoctorDon
2002-May-15, 05:01 AM
On 2002-05-14 06:07, beskeptical wrote:

Wouldn't gravity be the force slowing hyperinflation? And, now if expansion is accelerating, the force for that acceleration is only being theorized?


To answer your first question, the standard explanation I've always heard is that there is some scalar field (somtimes called the inflaton field), the derivative of which shows up in an exponential for the equation of the space-time metric. If this inflaton field were shaped like a sombrero hat, and the initial state of the universe were at the center, the derivative would be zero, and the universe would not inflate, but if the system slid out of that equilibrium (and it would, being unstable), the derivative would become non-zero, and the exponential would cause space to expand really, really quickly, until the inflaton reached the well of the hat brim, where it would be stable and have a zero slope, so inflation would stop. So yes, gravity is driving the inflation, in a general relativistic sense, but it's not gravity like you think about the idea in an everyday sense. At the kind of densities we're talking about, pressure has gravity.

The second question is relevant for all the discussion about dark energy. Yes, the evidence suggests that the universe is accelerating. However, the jury is still way out as far as what's causing that acceleration. The main contending theories I have heard about call themselves names like Dark Energy, Quintessence, The Cosmological Constant, the Ekpyrotic Theory, and Vacuum Energy. There's even a theory called "The Cardassian Universe" which does not require dark energy, but uses a modification of the Friedman-Robertson-Walker metric to get a version of Einstein's equation that will result in an expanding, matter-dominated universe. That is, rather than posit some unknown stuff within the context of known physical models, it suggests we have the right stuff, but our models are only approximations because we usually only measure things on relatively tiny scales. It's like saying we need to modify Newton's second law for rotating coordinate systems rather than suggesting there are "dark forces" like Centrifugal and Coriolis.

I'm not advocating the Cardassian model, I'm just trying to point out the wide range of approaches that people are taking to the subject, to stress how uncertain we all are about what's causing the acceleration. Dark energy is by no means a done deal.

Don