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Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: yor_on on 07/04/2017 08:49:01

Title: Does the universe lose energy as it expands?
Post by: yor_on on 07/04/2017 08:49:01
Geometrically I mean :)

Per inch, cm, whatever you prefer.
Title: Re: Does the universe lose energy as it expands?
Post by: evan_au on 07/04/2017 12:18:09
The Big Bang Theory (http://www.azlyrics.com/lyrics/barenakedladies/bigbangtheorytheme.html) suggests
Quote from: Barenaked Ladies
Our whole universe was in a hot dense state,
Then nearly fourteen billion years ago expansion started...

It is thought to start as a plasma at a temperature of billions of degrees, but as it expanded, the density dropped and it cooled; after around 400,000 years, it cooled enough to form neutral atoms. The glow of that plasma at a temperature of around 3000K was then able to travel freely through now-transparent space. As space expanded further, this radiation was red-shifted by a factor of  over 1000 until now it has an effective temperature around 2.7K. This is what we see as the Cosmic Microwave Background Radiation.
See: https://en.wikipedia.org/wiki/Cosmic_microwave_background

Did the universe "lose" energy? The energy went into the separation of atoms, so that the lower density of atoms had less random motion, and carry less mechanical energy, and this shows as a reduced temperature of space.
Title: Re: Does the universe lose energy as it expands?
Post by: jeffreyH on 07/04/2017 13:01:42
The solidification of matter, along with the tendency of gravity to draw particles together has altered the kinetic energy of the universe as a whole. However this is likely balanced by the forces driving the expansion of spacetime.
Title: Re: Does the universe lose energy as it expands?
Post by: yor_on on 07/04/2017 14:16:48
Hmm, I'm not sure of that one Evan?

The 'forces' keeping matter together has lost 'energy' to a expansion?
How would one prove it?

Temperature is one thing, and the universe as a whole has 'cooled of', from a main stream description.
Then again, does this answer a accelerating expansion?
Title: Re: Does the universe lose energy as it expands?
Post by: PhysBang on 07/04/2017 15:59:17
Yes and no.

If you are talking about the energy density of volumes of given sizes, taken at different times, then, yes, the energy density goes down, because the mass density goes down for volumes of a given size (goes down by the cube of expansion). The energy density of radiation goes down even more, since there is also an effect of redshifting relative the source of the radiation (goes down by the fourth power of expansion). If there is dark energy that is a vacuum energy, then this stays constant, because there is always the same amount of vacuum in a given volume.

If you pick out a volume of space and then follow it over time, then you are tracking an expanding volume, so at least the energy density of matter stays the same. However, the energy of radiation still drops due to redshifting (at the power of expansion). And there is more volume, so there is more vacuum energy, if that is what dark energy is, but the density stays the same, because of the definition of vacuum energy.
Title: Re: Does the universe lose energy as it expands?
Post by: nilak on 07/04/2017 22:11:39
I don't think it is possible for the universe to loose energy. Where would the energy go?
Cosmological research doesn't have a good reputation on being very accurate, therefore if some prediction showed the energy of the universe is decreasing I wouldn't trust it.
What seems clear to me is, if the universe is expanding, its energy density is decreasing.
Title: Re: Does the universe lose energy as it expands?
Post by: evan_au on 08/04/2017 02:15:27
Quote from: yor_on
Hmm, I'm not sure of that one Evan?
The 'forces' keeping matter together has lost 'energy' to a expansion?
How would one prove it?
Gravity is one of those forces keeping matter together. As objects expand apart, their kinetic energy gets turned into gravitational potential energy, and they slow down. This was the dominant interaction up to about 5 billion years after the big bang.

On much smaller scales - closer to the size of an atom (important from 300,000 years after the Big Bang), Van der Waals-type attraction between Hydrogen atoms and Hydrogen molecules is one of the forces holding matter together. As matter becomes less dense, kinetic energy is turned into potential energy, and the gas cools.

Another factor operates when the density of gas is low enough so that atoms are unlikely to bump into each other. The atoms that started heading in our direction all happen to be heading in our direction (a truism), and those that had radically different velocity (speed+direction) are now in a different part of space. So the matter we see around us has relatively low relative velocities, and this makes it relatively cool.   

Energy itself isn't lost, but it is converted from one form to another.

Quote
Then again, does this answer a accelerating expansion?
The "Cosmic Jerk (https://en.wikipedia.org/wiki/Metric_expansion_of_space#Measurement_of_expansion_and_change_of_rate_of_expansion)" is thought to have occurred about 5 billion years after the Big Bang, when the density of matter got low enough that gravity was no longer the dominant force in the universe. Beyond this point, Gravity was weaker than Dark Energy, and so the universe changed from a gravity-induced deceleration to a Dark-Energy induced acceleration.

In Physics, "Jerk" is the change in acceleration, just like acceleration is the change in velocity, and velocity is the change in position).

Where does Dark Energy come from? Physicists think of it as some potential field, but they really don't agree on what kind, yet.
Title: Re: Does the universe lose energy as it expands?
Post by: PmbPhy on 08/04/2017 04:55:26
No. The energy in the universe is a constant of motion.
Title: Re: Does the universe lose energy as it expands?
Post by: nilak on 08/04/2017 08:01:17
No. The energy in the universe is a constant of motion.
If you are you thinking of the energy as kinetic energy of objects from newtonian mechanics, that is an old concept and it is likely it is not correct.
Particles behave more like waves. The energy of a wave can be thought as potential energy of the field.
However, in the case of light, the field values propagate at a constant velocity c, hence there is a motion involved, but I'm not sure what to say.
If you freeze time, you can still have the field excitations. Freezing time is like stopping the momentum. But if you let it go, the values of the field would instantly propagate at c.
For light you don't need the momentum to tell where it will go next. For a ball in newtonian mechanics, if you stop time, unless you remember the velocity of the ball, you will not be able to tell how it is going to move.

Title: Re: Does the universe lose energy as it expands?
Post by: PmbPhy on 08/04/2017 09:05:02
Quote from: Nilak
If you are you thinking of the energy as kinetic energy of objects from newtonian mechanics, that is an old concept and it is likely it is not correct.
I really can't fathom why you'd respond to my comment with such a remark. In the first place kinetic energy has never been a quantity that is conserved. It's only the total energy of a closed system that has ever been conserved. The same is true in quantum mechanics as well. I.e. the total energy of a system is constant - so states quantum mechanics.

Quote from: Nilak
Particles behave more like waves.
That is an incorrect state in general. Its a well-known fact that what an object such a photon or an electron behaves like is dependent on how its observed.

Note: To be frank I can't imagine how you ever came to believe this unless you've never really studied physics in a formal sense or have your own beliefs about quantum mechanics which you hold to be contrary to orthodox quantum mechanics. If its the later then you're in the wrong posting such responses in a subforum dedicated to orthodox physics and should instead be placed in the new theories section. If its the former and you wish to correctly understand quantum mechanics then pick up any text on quantum physics or a good text on general physics. A superb example of the later is The Feynman Lectures on Physics - Volume III[ /b] which you can download from http://bookzz.org/book/2456761/bc273a (To download that PDF file you must join but membership is free. You only need to sign up. No credit card required). In particular see the beginning of chapter I in section 1-1, which reads
Quote
Newton thought that light was made up of particles, but then it was discovered that it behaves like a wave. Later, however (in the beginning of the twentieth century), it was found that light did indeed sometimes behave like a particle. Historically, the electron, for example, was thought to behave like a particle, and then it was found that in many respects it behaved like a wave. So it really behaves like neither. Now we have given up. We say: “It is like neither.”
Also see page 1-6, which reads
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We conclude the following: The electrons arrive in lumps, like particles, and the probability of arrival of these lumps is distributed like the distribution of intensity of a wave. It is in this sense that an electron behaves “sometimes like a particle and sometimes like a wave.”

Quote from: Nilak
The energy of a wave can be thought as potential energy of the field. (etc)
That too is incorrect. In the case of an uncharged particle there is no field to think of but when measured the particle has a definite energy with nothing to do with any field whatsoever to speak of.

Quote from: Nilak
However, in the case of light, the field values propagate at a constant velocity c, hence there is a motion involved, but I'm not sure what to say.
A photon doesn't have a field.

Anyway this has nothing to do with the conservation of energy of the universe. The question was a question concerning classical mechanics. In any case whether a system's energy is conserved or  not has nothing to do with whether its a classical system or a quantum mechanical one.

If you really want to know how and why the energy of the universe is conserved the pick up a copy of The Inflationary Universe by Alan Guth and read why. I forgot where in the book the author explains why but he's a very good friend of mine so I'll ask him where in the book he explains this and post his response. And the author is a first rate physicists and as such knows what he's talking about so I suggest that you pay close attention to the book.

Regarding quantum mechanics there are plenty of great texts out there which explain why your beliefs as you expressed them above are all wrong. Frankly I'd love to hear the source of your beliefs if you claim that its standard, i.e. orthodox, quantum mechanics. Pray tell? :)

BTW - Nothing I said here was intended as an insult but as a genuine concern about what you believe to be true and your attempts to pass them off as actual physics. I.e. I'm only trying to help you, not hurt or insult you. Please keep that in mind. Okay? :)
Title: Re: Does the universe lose energy as it expands?
Post by: nilak on 08/04/2017 11:32:35
That is because you said the energy is a contant of motion, which is associated with kinetic energy. But I said "if".
I can understand kinetic energy of an object that moves, but I don't have a clear view on kinetic energy of EM waves for example.
Photons can have associated fields. In QFT fileds are more fundamental than particles. My opinion is photons are propagating EM field values as packets of EM waves . It is just the classical EM waves don't fully describe their behaviour but it is very close to describing them.
In a world of EM waves only, no gravity, how should we describe the energy? For EM filed I've only seen examples of the energy as potential energy of the E and B fileds.


Particles behave more like waves. Initially light was understood  as an EM wave but after the photoelectric effect was discovered, some properties of particles were found. My opinion is that the way light and matter propagates makes them look as they are objects and in the end that if what we see around us, objects. But I think the matter waves have such a behaviour that make them act like objects. All particles have a deBroglie wavelength therefore the waves are real. But you are right, these hypotheses are supposed to be discussed in the new theories section.

Quote
Particles behave more like waves.

That is an incorrect state in general. Its a well-known fact that what an object such a photon or an electron behaves like is dependent on how its observed
Waves behave like they are dependent on how they are observed . The Doppler effect is one example.

Quote
If you really want to know how and why the energy of the universe is conserved
The energy of a system doesn't fall if the system doesn't transfer energy to  some other system, provided that the energy conservation works. All energy within our world remains within our universe, it doesn't have where to go, unless there is a hidden dimension, and it doesn't seem to be. Even if there was such a parallel world, it would've been part of our universe. Conservation of energy is probably the most important assumption. A theory that predicts the energy of the universe is decreasing would be most probably wrong, and it would be in big trouble.
Title: Re: Does the universe lose energy as it expands?
Post by: yor_on on 10/04/2017 09:48:37
Well, the problem for me is to see what 'energy' is here. It's not the 'forces' we define, unless we mean that there is no way to measure a 'loss'. It can't be the vacuum, if you by it expect it to have a 'energy' of its own, virtual or not. Because if the universe constantly is in a equilibrium, then by 'growing' I would expect more of the 'energy' transferred to those new 'geometrical patches' of a vacuum. The point I'm making is that the universe contain mass, and a geometry. Let's define mass as belonging to forces here.

So it is confusing.
Title: Re: Does the universe lose energy as it expands?
Post by: puppypower on 10/04/2017 11:36:26
The energy is not lost, but is conserved, as an entropy increase.

For example, as an experiment, begin with a cylinder of compressed gas, at temperature T. Open the valve and allow the gas to lower pressure and expand out of the cylinder. What we would see is, would be what appears to be a  red shift, in the energy signal coming from both the stationary cylinder and the expanding gas. Both will get colder and the recorded IR  wavelength profile will appear to red shift. Nothing is even moving close to C.

What appeared to be the lost energy, associated with the temperature drop and/or the recorded red shift, is conserved and is going into entropy; entropy needs energy to increase. The second law implies a universal red shift affect unless energy can be generated faster than entropy. Conceptually, an increase in entropy, first, can result in an apparent red shift as demonstrated by the gas experiment.

If we start the BB from a singularity, the entropy of the universe is at a minimum, since it is a singularity. As the singularity expands and differentiates there are more degree of freedoms which implies the entropy has gone up. This entropy increase will absorb energy and cause what appears to be cooling and red shift.

If the universe expanded from the BB singularity as a type of atomization into lots of smaller particles, this will contain a lot of entropy. An alternative scenario the singularity quantum divides into two, like a mother cell in biology. This second scenario would show a much slower temperature drop, and much less apparent red shift, over time, because the entropy increase is much lower.

This second scenario keeps more energy in play, while also allowing the superstructures of the universe to form easier; after more subsequent quantum divisions of smaller and smaller quantum cells. The reason being is the formation of super structure from smaller quantum cells, would represent an entropy increase and be driven by the second law, which still has energy to play with.

In the more traditional early BB total atomization scenario, superstructure formation would be an entropy decrease, and wold need another source of potential to reverse the second law. It would also need to get hotter to account for the loss of entropy. The second scenario allows a galaxy to puff into place, from a smaller singularity, and then be cool enough for gravity to keep it contained for further development.
Title: Re: Does the universe lose energy as it expands?
Post by: PhysBang on 10/04/2017 13:25:19
The conservation of energy on a universe-wide scale is not something that works the same as conservation of energy in a closed system. Conservation of energy is something that has to change in relativistic physics in order to make any sense, and that means that it doesn't apply to every case in the same way that people thought it does in the classical case.

Here is a nice overview of conservation of energy in cosmology: http://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/
Title: Re: Does the universe lose energy as it expands?
Post by: yor_on on 10/04/2017 16:28:23
Heh Physbang. Found it without you.
Although it tells mountains about good taste to choose the same, eh, blog :)
Well, arguably at least.

It's interesting, especially when he points to cosmological redshifts.
=

And yes, thinking of of it, the way he discuss gravity mirrors the way I wonder about treating it as a force, a 'potential energy' of sorts. and that connects to 'uniform motions' being both provably different, as well as equivalently  'relative'.
Title: Re: Does the universe lose energy as it expands?
Post by: yor_on on 10/04/2017 16:46:57
But does it answer it?
The universe grows, does the energy grow or is it 'loaned'?

It's about what a universe is. A defined 'sphere' as having a boundary, or 'infinite', as in without a boundary. You can't use going out to the left to then come in to the right for that, because doing so imply a boundary to me, even though we won't notice it. A energy coming to be as needed in a 'infinite universe', or one with a boundary in where 'energy' is a loan from something else.
=

Maybe we need some new definitions of 'infinity' here :)
something more than just greater and smaller.
=

Or expressed otherwise. Either 'energy' is a result of interactions within a boundary, or there is no boundary although the interactions still are there. And that too knits to the idea of conservation of energy I think? seems as if you can define a 'open system' as any interactions from where you don't know being such. But a inflationary/expansionary universe doesn't use a boundary in any simple mean, to me at least? The closest I get to a 'whole universe' thinking of it is our ideas of physics holding true anywhere and at all times. And in that universe it's not a loan anymore, it's more like a demand.
Title: Re: Does the universe lose energy as it expands?
Post by: nilak on 10/04/2017 21:04:47
The conservation of energy on a universe-wide scale is not something that works the same as conservation of energy in a closed system. Conservation of energy is something that has to change in relativistic physics in order to make any sense, and that means that it doesn't apply to every case in the same way that people thought it does in the classical case.

Here is a nice overview of conservation of energy in cosmology: http://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/
If the energy is not conserved at large scale, then the OP question makes sense.
I 'm confused about the article. It says that the dark energy increases as the universe expands but the energy of light decreases because of a redshift. In this case, the dark matter increase of energy might compensate the decrease of the energy of light and matter, but they say the total energy is increasing.
Anyway, if energy of the universe is not conserved, the theories that give this result, could be wrong. These studies rely on the cosmological constant that has been massively  underestimated, but its value is still debatable. This is a sign that the theories that describe gravity and dark energy may not be very reliable for answerig the question of energy conservation.