# Does the expansion of Space require continuous energy?

1. Jul 2, 2008

### Herbascious J

Does the expansion of Space require continuous energy??

I'm curious; does the expansion of Space require energy? I'm assuming that the expansion of space must have some kind of 'momentum' (the big bang must have required an input of inertial energy directly into the geometrical expansion of space itself). I guess my question is; does this expansion require a continuous input of energy to keep it expanding? My intuition tells me 'No' only because GR is supposed to behave similar to any Newtonian equivalent, but this still requires a kind of inertia behind the expansion, right? Do we have a theory of how this came to be? Was there some instantaneous and temporary Lambda-like 'jolt' that got everything moving right at the first instance of time?

My next question is, is this energy positive? If the universe is now being accelerated by a Lambda-like force, is this force also a form of energy, and if so, is this energy positive or negative? Any input is appreciated.

Clarification: I have heard that the Lambda energy is tension energy in theory, and therefore is a negative form of energy, and therefore is repulsive and not attractive like gravity. BUT, I have also heard that gravity is a negative form of energy in the first place. Does this make Lambda a 'double-negative' and therefore it is a positive form of energy?

2. Jul 2, 2008

### Antenna Guy

Re: Does the expansion of Space require continuous energy??

Does an increase in entropy require energy?

Maybe someone else can provide a more authoritative answer...

Regards,

Bill

3. Jul 2, 2008

### jonmtkisco

Re: Does the expansion of Space require continuous energy??

Hi HJ,
Good grief, you're asking too many questions at once. These seem more like cosmology questions than GR questions.

I think the easiest and most sensible view is that galaxies are expanding apart because they previously expanded apart. The faster moving ones were always moving faster, and the slower ones were always moving slower. All aspects of the expansion slowed down for many Gy due to self-gravity, but later the increasing effect of the cosmological constant became the dominant factor and caused a re-acceleration of the expansion.

The most accepted view is that the original momentum of expansion resulted from inflation (although many cosmologists have doubts about inflation). At the start of the inflation, there was no matter or radiation, just quantum particles called inflatons. The universe gained a very high expansion rate very quickly during inflation, and then mass-energy precipitated out of the decay of the inflatons, with the built-in expansionary momentum left over from inflation.

The cosmological constant (dark energy) is thought to be an attribute of every cubic meter of space. Thus, the more space expands, the more cosmological constant there is. Eventually there is so much of it compared to the amount of matter, that the restraining force of gravity becomes overwhelmed by the negative pressure energy of the cosmological constant.

There is no answer to the question about whether the cosmic expansion inherently "creates" the energy of the cosmological constant. In the standard view, that energy sort of "wells up" in the new space between separating mass-energy particles. In a simpler contrarian view, the energy of the cosmological constant (if it really exists) was already there, and the expansion of the universe simply incorporates that pre-existing energy into its domain.

Jon

4. Jul 3, 2008

### Antenna Guy

Re: Does the expansion of Space require continuous energy??

Forgive my ignorance - what are you talking about?

Regards,

Bill

5. Jul 3, 2008

### Fredrik

Staff Emeritus
Re: Does the expansion of Space require continuous energy??

A non-zero cosmological constant is equivalent to a non-zero density of the vacuum. Take a look here for example. Move the $\Lambda$ to the right-hand side and it becomes

$$\frac{\Lambda c^2}{3}+\frac{8\pi G}{3}\rho=\frac{8\pi G}{3}\bigg(\rho-\frac{\Lambda c^2}{8\pi G}\bigg)$$

So the only thing the cosmological constant "does" in the first Friedmann equation is to change the density from $\rho$ (the density of matter/energy) to the quantity in parentheses.

If you move $\Lambda$ to the right-hand side in the second Friedmann equation too, you will see that $\Lambda$ also contributes to the total pressure in a similar way.

Last edited: Jul 3, 2008
6. Jul 3, 2008

### jonmtkisco

Re: Does the expansion of Space require continuous energy??

Antenna Guy, the term "cosmological constant" is the simplest theory for the "dark energy" that is believed to characterize all space in our universe. It is believed to possess "negative pressure" which means that it has mass/energy which contributes to gravitational slowing of the expansion, while it also possesses a sort of anti-gravity attribute, which not only offsets the slowing effect of its extra gravity, but also provides a boost which is thought to explain why the expansion rate is accelerating.

The cosmological constant was originally invented as a mathematical placeholder by Einstein as a theoretical way to offset the tendency of self-gravitation to cause the universe to collapse. Einstein strongly believed that the universe was static, that is neither expanding or contracting. It wasn't until Hubble published data on cosmic redshift that Einstein came to accept that the universe is probably expanding. Good thing, because other physicists had already concluded that the cosmological constant could not be a stable way of keeping the universe static; it would inevitably tip in the direction of expansion or collapse.

Jon

Last edited: Jul 3, 2008
7. Jul 3, 2008

### gamesguru

Re: Does the expansion of Space require continuous energy??

If expansion creates mass, it requires energy. If dark energy creates negative pressure, it does positive work by expanding and uses energy. I would say the answer is yes, but many people will disagree.

8. Jul 3, 2008

### Herbascious J

Re: Does the expansion of Space require continuous energy??

Hi, yes I agree, I did ask to many questions at once, my apologies. The basic idea that of what I'm driving at is this... GR is supposed to 'behave' similar to Newtonian physics in many ways (this is how Einstein knew he was on the right track). If the expansion of the cosmos was Newtonian, there would be an initial burst of kinetic energy, which would drive all matter apart, with true velocities moving through space. It's important to point out that these velocities require no additional energy once in motion. From that point forward, only the gravitational attraction of all of the matter would slow down the system's expansion.

However, within the GR interpretation, matter (galaxies) are not moving through space, but instead space itself is 'stretching' between them. So my question is... In GR, can space grow (with apparent constant recessional velocities between galaxies) without any required energy input, thus behaving like Newtonian expansion? Is this a natural ability of Space-Time, inherent in GR, to grow and spread out infinitely with out continual energy being applied to the expansion?

Then the last question... Is Lambda a positive or negative from of energy? I hope that makes more sense.
-HJ

9. Jul 3, 2008

### jonmtkisco

Re: Does the expansion of Space require continuous energy??

Hi Herb,
You really should go to the Cosmology forum for answers to these questions.

The prevailing view in cosmology (if there is one currently) is that the expansion of space does not cause galaxies to move apart, instead the movement of galaxies away from each other creates the opportunity for more space to exist. So yes, in that sense one can think of galaxies as retaining the expansionary momentum left over from the end of inflation, less the portion of that momentum lost to gravitational deceleration.

Lambda is considered to be a positive form of energy.

10. Jul 24, 2011

### dsobotka123

Re: Does the expansion of Space require continuous energy??

My opinion, is that since the universe is still very young, It is going to keep on expanding until the energy no longer exist from the big bang. Then what happens? Does the center of the universe start pulling it back? Since we can see galaxies close to the time of when the Universe was formed, who is to say, it has not already stopped expanding and is Collapsing at the same rate it expanded since we are looking "back in time". That does not mean what is happening at this moment is what we are currently observing. I am a computer scientist, and have no business posting here, although I just had some thoughts and questions others may expand (or contract) on :-).

Last edited: Jul 24, 2011
11. Jul 24, 2011

### bcrowell

Staff Emeritus
Re: Does the expansion of Space require continuous energy??

GR doesn't have conservation of energy. (That is, it has locally conserved energy-momentum, but it doesn't have a globally conserved scalar mass-energy that can be defined in all spacetimes. In particular, it doesn't have any such thing for cosmological spacetimes.) We have a FAQ entry on this: https://www.physicsforums.com/showthread.php?t=506985

Expansion doesn't create mass.

That's incorrect, since GR doesn't have conservation of energy. Even if we were talking about a spacetime where it was possible to define a conserved global scalar mass-energy, it couldn't "run out," since it would be conserved.