Dark Energy

[rynner2]

Which stars would be the oldest then? Would it be the ones that formed nearest the centre of the area where the big bang occured or the ones further out in space?

The big bang had no centre, strange as that may seem. At any point in the universe now you would see the rest of the universe expanding away from you, so any point might seem like 'the centre'.

When the big bang occurred, that point was the entire universe.

So the oldest stars are just the ones that happen to be oldest now. (The lifetime of a star depends on its mass - very massive stars burn out relatively quickly, while dwarf stars go on almost for ever. Google on Hertsprung-Russell diagram for more info.)

 

[QuaziWashboard]

Which stars would be the oldest then? Would it be the ones that formed nearest the centre of the area where the big bang occured or the ones further out in space?

The big bang had no centre, strange as that may seem. At any point in the universe now you would see the rest of the universe expanding away from you, so any point might seem like 'the centre'.

When the big bang occurred, that point was the entire universe.

So the oldest stars are just the ones that happen to be oldest now. (The lifetime of a star depends on its mass - very massive stars burn out relatively quickly, while dwarf stars go on almost for ever. Google on Hertsprung-Russell diagram for more info.)

Ahh right. Y'see I was just wondering if the resulting shockwaves from the oldest stars going supernova could be acting on the spreading universe that has been slowed down by gravitational forces, thereby speeding them up again.

 

[OneWingedBird]

Y'see I was just wondering if the resulting shockwaves from the oldest stars going supernova could be acting on the spreading universe that has been slowed down by gravitational forces, thereby speeding them up again.

iirc the theory that the universe is expanding is based on observing the red-shift of supernova (and red shift is quite an arcane thing in itself), i think a viable alternative theory might be that the universe isn't expanding, but that the constants that these calculations are based on could have been different in the early universe...

 

[Anonymous]

Less gravity pulling things together = more speed as they fly apart = less gravity = more speed = less gravity....

No, no, no!

Moving things continue at the same velocity unless acted upon by a force...
(Newton's First Law)

So if God were to wave his magic wand and abolish all gravity right now, the universe would continue to expand at its present rate....


...the expansion would not suddenly accelerate!

Are you sure?

Because I was told that if you launch a rocket in a vacuum then if nothing slows it down, it will pick up speed gradually over time up to the speed of light.

 

[rynner2]

Because I was told that if you launch a rocket in a vacuum then if nothing slows it down, it will pick up speed gradually over time up to the speed of light.

While the rocket is firing, yes. The rocket exhaust imposes a force on the rocket which does accelerate it.

But as soon as the rocket runs out of fuel, the acceleration ends, and the rocket coasts on at whatever speed it had reached by that time.

Subsequently it may slow down if it encounters friction with interstellar material, or it may speed up if it wanders into a strong gravitational field.

But accelerations or decelerations all require a force to act:
this is Newton's Second Law.

 

[Anonymous]

Because I was told that if you launch a rocket in a vacuum then if nothing slows it down, it will pick up speed gradually over time up to the speed of light.

While the rocket is firing, yes. The rocket exhaust imposes a force on the rocket which does accelerate it.

But as soon as the rocket runs out of fuel, the acceleration ends, and the rocket coasts on at whatever speed it had reached by that time.

Subsequently it may slow down if it encounters friction with interstellar material, or it may speed up if it wanders into a strong gravitational field.

But accelerations or decelerations all require a force to act:
this is Newton's Second Law.

Ok - so there is nothing at all (no force) moving the planets and stars etc?

Because I think there is...and it is not necessarily dark energy.

Though it might be.

 

[Anome]

There is a force moving the planets and stars. It's called gravity. That's why we orbit the sun, and the sun orbits the centre of the galaxy.

The reason the universe is expanding (according to the most commonly accepted theory - this is still under dispute) is the big bang. Under most versions of this theory, the expansion is entirely due to the acceleration during the first fractions of a second of the universe's existence.

Of course, this should mean the expansion is slowing down, as gravity overcomes this initial impulse. There is some evidence, however, that it's actually speeding up which may be due to dark energy, or some other unaccounted for phenomenon.

 

[headnspace]

I like good conversations like this.

Now, if the universe is expanding because of the "big bang" what would the expected universe look like with much less gravity? Would it make a difference if gravity was much greater? Scientists keep saying that the force of gravity is such a value that if it was a fraction greater, then the universe would have already collapsed, and if it was a fraction less, then the universe would be headed toward infinite expansion.

Now, with Dark Energy, you don't need the restrictions on gravity that you had before. Even if gravity was much greater, then a "dark energy" could counteract that to keep the universe expanding. If gravity was much less, then you wouldn't need as much "dark energy" to make the universe keep expanding.

Now, does that make "Dark energy" a function of mass, or a function of space? In other words, I've read that "dark energy" is the energy that space itself carries with it, just by being there. What are some thoughts on "dark energy" being a natural part of space, and not of mass or energy?

 

[rynner2]

Re: I like good conversations like this.

Now, does that make "Dark energy" a function of mass, or a function of space? In other words, I've read that "dark energy" is the energy that space itself carries with it, just by being there. What are some thoughts on "dark energy" being a natural part of space, and not of mass or energy?

Now since cosmologists and others, who make researching these sort of things the major part of their profession, don't yet really understand what is going on, I doubt whether the average poster on a MB like this will have anything of value to add to the debate.

No doubt, in 5 years time (or, 10 or 20...), when the professionals have finshed chewing over it, one of them will write a book on it explaining it all to the general public. This is one of those exciting times in science, when things are in a state of flux - but most of us will have to wait a few years to get the full story of who discovered what, whose theories looked good (until some other theorist blasted them to pieces!), and what final consensus was arrived at.

"The universe is not only stranger than we imagine; it is stranger than we can imagine."

(I've seen this quote attributed to various people, including j. b. s. haldane and A.C.Clark!)

 

[Anome]

There was a theory that Dark Energy was just the Vacuum energy (the energy of empty space), but it doesn't add up.

 

[eburacum]

Some reseachers reckon they have found an axion, which is one of the candidates for Dark Matter...

http://www.iop.org/EJ/abstract/0954-3899/34/1/009

not really much overlap between Dark Matter and Dark Energy really; this post is probably irrelevant to the matter at hand.
Sorry about that...

 

[Van_Eck]

Isn't dark energy just that, a form of energy, and not a fundamental force like electromagnetism and gravity?

 

[Anome]

Yes, Van_Eck. It's just some energy that's floating around somewhere, just as Dark Matter is just some matter floating around somewhere. It's just that we can't see it, so it has caused people to speculate it might be because of particles we haven't found yet, or strange properties of particles we do know about, as well as a number of other possibilities.

 

[Rrose_Selavy]

Re: I like good conversations like this.

Now since cosmologists and others, who make researching these sort of things the major part of their profession, don't yet really understand what is going on, I doubt whether the average poster on a MB like this will have anything of value to add to the debate.

headnspace wrote:

I don't think dark energy exists in the way some of these scientists are thinking of it. (but, I could be totally wrong, I have not done the calculations to prove/disprove my own thoughts; well, because the math is hard)

I'm with Rynner on this one - these are complex subjects that people devote their careers to- - we can speculate but if the "common sense" solution was so obvious we wouldn't need "dark matter theory in the first place.

 

[Anonymous]

There is a force moving the planets and stars. It's called gravity. That's why we orbit the sun, and the sun orbits the centre of the galaxy.

The reason the universe is expanding (according to the most commonly accepted theory - this is still under dispute) is the big bang. Under most versions of this theory, the expansion is entirely due to the acceleration during the first fractions of a second of the universe's existence.

Of course, this should mean the expansion is slowing down, as gravity overcomes this initial impulse. There is some evidence, however, that it's actually speeding up which may be due to dark energy, or some other unaccounted for phenomenon.

The phenomenon IMO is that the matter has broken beyond the gravity skien of the universe - and now the matter, akin to dust, has spilled out of the boundries of the balloon of gravity etc.

Just before it occours, there may have been a ripple that pushes the matter out, to acclererate it in a slingshot effect before the instant the balloons walls are breached by the speeding matter and any force immediately before it and the skiens walls.

IMO.

 

[rynner2]

Er, sorry, could you run that past me one more time?

 

[stu neville]

Just before it occours, there may have been a ripple that pushes the matter out, to acclererate it in a slingshot effect before the instant the balloons walls are breached by the speeding matter and any force immediately before it and the skiens walls...

Sort of like the way that warp drive is supposed to work? It creates a wave, or ripple, that places the vessel, like a surfer, out of the fluid of linear space/time thus preventing the physical and temporal "drag" they exert and simultaneously giving it forward motion and thus enabling speeds and energy that would be impossible otherwise? Or am I barking up the wrong supernova?

 

[Anonymous]

Just before it occours, there may have been a ripple that pushes the matter out, to acclererate it in a slingshot effect before the instant the balloons walls are breached by the speeding matter and any force immediately before it and the skiens walls...

Sort of like the way that warp drive is supposed to work? It creates a wave, or ripple, that places the vessel, like a surfer, out of the fluid of linear space/time thus preventing the physical and temporal "drag" they exert and simultaneously giving it forward motion and thus enabling speeds and energy that would be impossible otherwise? Or am I barking up the wrong supernova?

I was picturing the universe as a thin transparent balloown filled with star dust and bits of rocks and gravity wells and black holes as well.

So I imagined the balloon bursting because too much air had been used to inflate it.

In slow motion.

That ripple effect would happen before the wall is breached.

Then at the instant of the breach, stardust etc is flung out - and disperses into the beyond.

Next - what is beyond?

 

[rynner2]

Good try, Stu! 8)

I think I'll stick with Newton's Laws - you knew where you were with them!

 

[QuaziWashboard]

I reckon it's all down to centrifugal force.
Everything's revolving around everything else right? And as we know, anything that revolves creates centrifugal force which forces objects outwards. So why is everything speeding up? Well as we all know, when something rotates, it's outer edge moves faster that it's middle, and the further the distance from the centre, the faster the outer edge travels. It's as simple as that.
Scientists? Pah!! Who needs 'em :lol:





PS. please don't take this post seriously as I probably haven't a clue what I'm talking about.

 

[OneWingedBird]

centrifugal force got rebadged as centripetal force some time back, though iirc it's not technically a force in itself, so much as an effect of conservation of momentum.

 

[rynner2]

centrifugal force got rebadged as centripetal force some time back, though iirc it's not technically a force in itself, so much as an effect of conservation of momentum.

erm..

cen·trip·e·tal /sɛnˈtrɪpɪtl/
–adjective
1. directed toward the center (opposed to centrifugal).

http://dictionary.reference.com/browse/centripetal


So gravity is a centripetal force, as is the reaction of the wall-of-death upon the motorbike whizzing round inside, or the tension in the string when you whirl a conker....

 

[QuaziWashboard]

centrifugal force got rebadged as centripetal force some time back, though iirc it's not technically a force in itself, so much as an effect of conservation of momentum.

erm..

cen·trip·e·tal /sɛnˈtrɪpɪtl/
–adjective
1. directed toward the center (opposed to centrifugal).

http://dictionary.reference.com/browse/centripetal


So gravity is a centripetal force, as is the reaction of the wall-of-death upon the motorbike whizzing round inside, or the tension in the string when you whirl a conker....

But seeing as how planets and other space objects have no strings attached or walls to keep them in place.....it's centrifugal all the way.

 

[OneWingedBird]

But seeing as how planets and other space objects have no strings attached or walls to keep them in place.....it's centrifugal all the way.

in the case of orbits, i'm not sure it's actually either, though the old centrifugal/centripital thing is starting to look horribly complicated:

Centrifugal force (from Latin centrum "center" and fugere "to flee") is a term which may refer to two different forces which are related to rotation. Both of them are oriented away from the axis of rotation, but the object on which they are exerted differs.

* A pseudo or "fictitious" centrifugal force appears when a rotating reference frame is used for analysis. The (true) frame acceleration is substituted by a (fictitious) centrifugal force that is exerted on all objects, and directed away from the axis of rotation.

* A real or "reactive" centrifugal force occurs in reaction to a centripetal acceleration. This centrifugal force is equal in magnitude to the centripetal force, directed away from the center of rotation, and is exerted by the rotating object upon the object which imposes the centripetal acceleration. Although this sense was used by Isaac Newton,[1] it is only occasionally used in modern discussions.[2][3][4][5]

Both of the above can be easily observed in action for a passenger riding in a car. If a car swerves around a corner, a passenger's body seems to move towards the outer edge of the car and then pushes against the door.

In the reference frame that is rotating together with the car (a model which those inside the car will often find natural), it looks like a force is pushing the passenger away from the center of the bend. This is not an actual force exerted by some other object, but results from ignoring the car's acceleration. A number of physicists treat it much as if it were a real force, as they find that it makes calculations simpler and gives correct results.

However, the force with which the passenger pushes against the door is very real. That force is called a reaction force because it results from passive interaction with the car which actively pushes against the body. As it is directed outward, it is a centrifugal force. Note that this real centrifugal force does not appear until the person touches the body of the car.

iirc orbits are just a case of the object having sufficent momentum that they're constantly falling towards a gravitational centre that they always miss - does centrifugal/centripal come into that at all?

 

[Anonymous]

Good try, Stu! 8)

I think I'll stick with Newton's Laws - you knew where you were with them!

Did I not go with Newton's Laws there then?

I mean balloons do burst in real life if you blow too much air into them...and if there was stuff in a balloon then when it burst it would be forced out by the air rushing to get out - wouldn't it?

Surely that is not breaking any laws?

 

[QuaziWashboard]

But seeing as how planets and other space objects have no strings attached or walls to keep them in place.....it's centrifugal all the way.

in the case of orbits, i'm not sure it's actually either, though the old centrifugal/centripital thing is starting to look horribly complicated:

Centrifugal force (from Latin centrum "center" and fugere "to flee") is a term which may refer to two different forces which are related to rotation. Both of them are oriented away from the axis of rotation, but the object on which they are exerted differs.

* A pseudo or "fictitious" centrifugal force appears when a rotating reference frame is used for analysis. The (true) frame acceleration is substituted by a (fictitious) centrifugal force that is exerted on all objects, and directed away from the axis of rotation.

* A real or "reactive" centrifugal force occurs in reaction to a centripetal acceleration. This centrifugal force is equal in magnitude to the centripetal force, directed away from the center of rotation, and is exerted by the rotating object upon the object which imposes the centripetal acceleration. Although this sense was used by Isaac Newton,[1] it is only occasionally used in modern discussions.[2][3][4][5]

Both of the above can be easily observed in action for a passenger riding in a car. If a car swerves around a corner, a passenger's body seems to move towards the outer edge of the car and then pushes against the door.

In the reference frame that is rotating together with the car (a model which those inside the car will often find natural), it looks like a force is pushing the passenger away from the center of the bend. This is not an actual force exerted by some other object, but results from ignoring the car's acceleration. A number of physicists treat it much as if it were a real force, as they find that it makes calculations simpler and gives correct results.

However, the force with which the passenger pushes against the door is very real. That force is called a reaction force because it results from passive interaction with the car which actively pushes against the body. As it is directed outward, it is a centrifugal force. Note that this real centrifugal force does not appear until the person touches the body of the car.

iirc orbits are just a case of the object having sufficent momentum that they're constantly falling towards a gravitational centre that they always miss - does centrifugal/centripal come into that at all?

Well if it's a decreasing orbit that would eventualy end with one body crashing into another, then that would be centripital but if it's an expanding orbit such as one similar to the slingshot effect used to propel probes further into space, then it would be centrifugal......I think. :?

 

[eburacum]

The example of an object in orbit is a demonstration of why centrifugal force doesn't really exist. The object concerned would move in a straight line through space if it were not acted upon by the force of gravity of (say for instance a planet); the gravity is your centripetal force, moving the object constantly toward the centre of gravity of that planet. But the object cotinues to move ever forward, along a curved line which encompasses the planet in an orbit. There is no need for any centripetal force here at all.

 

[headnspace]

astronomy, orbits, and conservation of angular momentum

Rynner wrote:

Now since cosmologists and others, who make researching these sort of things the major part of their profession, don't yet really understand what is going on, I doubt whether the average poster on a MB like this will have anything of value to add to the debate.

I said earlier:

Let me know if this went way over your Head, and I might try to explain it a little better.

I knew that in introducing this topic, I was on the edge of understanding (also meaning my own).

In my astronomical studies, in days long past, there was discrepancy with theory and observation. One of the discrepancies was with the expansion of the universe. It looked like there should be about 10 times as much mass as could be seen. (thus the term "dark matter" defining stuff we couldn't see). Dark energy was barely mentioned when I left school, so I didn't really study it or the reasons it was developed as a theory to define observed discrepancies with current theory. This is a very speculative subject, and the discussion has been awesome so far. Could an early spin in the universe have created speedy matter that outruns gravity?

Now, less speculative is what happens to an orbit over time. In most cases, to conserve angular momentum in a system, the orbiting object slows down, thus increasing the distance to the host object. (both object spinning, each having a slowing effect on the other) So, I think to conserve angular momentum in the universe, things would slow down, not speed up as observed.

 

[OneWingedBird]

Could an early spin in the universe have created speedy matter that outruns gravity?

but wouldn't that have to travel faster than the speed of light? as gravity itself is subject to relativity?

 

[crunchy5]

Could an early spin in the universe have created speedy matter that outruns gravity?

but wouldn't that have to travel faster than the speed of light? as gravity itself is subject to relativity?

If that was the case it might explain the dark aspect of the matter.