# A speculative explanation why universe accelerates

1. Jul 10, 2014

### Ookke

Since dark energy and other models for explaining accelerating universe (which is quite hard to explain) are somewhat speculative, I hope that PhysicsForums rules allow some speculation on this subject. I try to keep this short. And this is also a question.

Is there any serious research where accelerating universe is not explained by some internal force speeding up the expansion, but instead, some external force pulling the universe to expand? If we pull the universe from outside (whatever that means) with constant force to every direction, assuming that the force is strong enough to overcome gravitation, isn't accelerating universe just what we can expect? It would be reasonable consequence, without need for dark energy with strange properties or something similar.

Surely we cannot directly observe anything outside our universe, but by studying the expansion rate or mass distribution in universe, we might get some indirect evidence to support this kind of theory.

2. Jul 10, 2014

### Mordred

expansion is homogeneous and isotropic it is the same in every direction and location

homogeneous- one location is the same as another
isotropic no preferred direction

an external pull or expansion radiating outward like an explosion both have a preferred direction and location. So your idea does not work with observational data as your model would have a center and a preferred direction.

Also there is no center of the universe, I suggest reading Phinds Balloon analogy

http://www.phinds.com/balloonanalogy/ : A thorough write up on the balloon analogy used to describe expansion

http://arxiv.org/abs/astro-ph/0310808 :"Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe" Lineweaver and Davies
http://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf: [Broken] "Misconceptions about the Big bang" also Lineweaver and Davies

Last edited by a moderator: May 6, 2017
3. Jul 10, 2014

### George Jones

Staff Emeritus
Physics Forums Rule are quite clear.

Actually, there is a current line of research that is remarkably similar to this, using inhomogeneous Lemaitre-Tolman-Bondi universe models to account for the accelerated expansion of the universe.

Lemaitre-Tolman-Bondi universes are spatially istrotropic about us (look the same no matter which direction we look), but they are not spatially homogeneous. The idea is that we are at the centre of a huge cosmic void. Outside of this void, matter is more dense and the "pull" of this matter causes the acceleration in the expansion of the universe.

By giving up spatial homogeneity, Lemaitre-Tolman Bondi universes can account for the supernovae data, but these models cannot

[QUOTE="Relativistic Cosmology", Ellis, Maartens, and MacCallum (2012)]simultaneously explain SNIa observations, the small-angle CMB, the local Hubble rate and the kinetic Sunyae-Zeldovich effect (Bull, Clifton and Ferreira, (2011).[/QUOTE]

It is still a work in progress, though.

Last edited: Jul 10, 2014
4. Jul 10, 2014

### Ookke

Maybe this is what I had originally in mind, but considering balloon analogy, I don't see why it must be internal "pushing" force that causes the expansion. External "pulling" force should work as well. With rubber sheet analogy, pulling force seems even more natural than pushing. Maybe you are right that this doesn't work in the end, but I'd like to try to fix this.

5. Jul 10, 2014

### Ookke

6. Jul 10, 2014

### Matterwave

I'd like to point out though that the isotropy of the universe about every point (thereby implying homogeneity) is more or less a cosmological assumption, sometimes termed the Copernican principle. It arises from the idea that we are not special (i.e. we are not the center of the universe). It is not an experimentally verifiable fact that expansion is isotropic about points other than the Earth because we have not been to any points other than the Earth.

Last edited by a moderator: May 6, 2017
7. Jul 10, 2014

### Mordred

I'm well aware of the assumption basis however none of our measurements have shown a preferred direction to expansion thus far. So I would have to say its stronger than a mere assumption. Please note the observational data reference in my post. If you have observational data that states otherwise I will gladly look at it. Yes I'm aware of the limitations of those measurements, however there has also been numerous attempts to prove isotropy wrong.

The real question is how much scientific data does it take to consider an assumption as more than a mere assumption. WMAP and Planck data both show a strong agreement with the assumption, so does numerous other data sets. I'm fully confident that any data set that shows otherwise would quickly be all over the internet and news. Confirmed or otherwise lol

all that aside its our responsibility when answering questions to teach what one would learn from the textbooks. To the best of our abilities. AFIAK that is also one of the forum rules, though worded different.

(trust me judging from some of the other science forums where that rule is relaxed enough to allow speculative personal theories, you want none of that here. The posters with personal theories never want to learn the real science just push their own models)(yes I am a member on numerous forums, this one I still consider the best due to its stricter rules)

Until such time that we do have a strong dataset showing isotropy wrong I will continue to answer that the universe is homogeneous and isotropic

Last edited: Jul 10, 2014
8. Jul 11, 2014

### Bandersnatch

In the balloon analogy, the universe is the surface of the balloon. Where is the external force located, if the word "external" doesn't make sense?
In other words, if the universe is all there is, every single thing must, by definition, be internal to it.

I get the feeling you've just misunderstood the analogy, thinking the third dimension of the balloon has got some physical significance, and that dark energy is analoguous to air being blown into it.

Mordred posted links to articles rectifying this. You should read them. After all, you can't fix something you don't understand.

9. Jul 11, 2014

### Jorrie

Not "some external force", but we may perhaps say that dark energy is a negative internal pressure that is "pulling the universe to expand" on the large scale.

10. Jul 11, 2014

### Ookke

Yes it's the surface that represents the universe. More accurately, it represents the universe that we know, our environment that we currently model as 4-dimensional spacetime with quantum effects etc. This is not necessarily all there is. We might live "inside a surface of a balloon" so to speak. Besides, I like rubber sheet analogy better.

Air being blown in a balloon is analogous to external force, which doesn't belong to the surface and doesn't originate from surface, but still has effect on it. Dark energy is something that is thought to be part of our universe, so analogy for that would be some exotic material inside balloon surface material, which causes accelerated expansion.

Maybe this was just misunderstanding in communication, hopefully I clarified my message. If this is still wrong, then I really didn't get it. Anyway, I need to study the subject.

11. Jul 11, 2014

### Ookke

And this is the simplest, most natural and credible theory that scientific community has come up with? In contrast to external force, dark energy is not speculative concept at all

12. Jul 11, 2014

### phinds

Although you say that with sarcasm, it is exactly correct and makes good sense. You should attempt to figure out why instead of trying to overturn solidly established physics.

Last edited: Jul 11, 2014
13. Jul 11, 2014

### Jorrie

The cosmological constant as 'dark energy' is incredibly simple, very natural in spacetime sciences and may perhaps have been speculative in Einstein's time, but not today. An "external force" accelerating expansion cannot claim any of the above.

14. Jul 11, 2014

### Mordred

A good article to read in regards to the cosmological constant is this one

Why all these prejudices against a constant?
http://arxiv.org/pdf/1002.3966v3.pdf

in terms of its pressure influence, as Jorrie pointed out the cosmological constant is very easy to understand

what many posters fail to realize is is that energy-density has a pressure relation determined by its equation of state.

http://en.wikipedia.org/wiki/Equation_of_state_(cosmology)

in essence the FLRW metric and the Einstein field equations both describe the universe in terms of a perfect fluid or ideal gas. The universes thermodynamic history also obeys the rules of an ideal gas. The cosmological constant is an influence upon that gas or fluid. Although it has a positive energy-density it has a negative pressure influence.
$$w=-1$$

see the wiki page for the Freidmann acceleration equation.

Last edited: Jul 11, 2014
15. Jul 12, 2014

### Ookke

16. Jul 12, 2014

### Staff: Mentor

But as you will notice, he also explains how dark energy, properly understood, is a perfectly natural way--in fact, the most natural way--to explain why the expansion of the universe is accelerating.

Also, Carroll doesn't say the negative pressure description is wrong; he just says it probably isn't helpful when trying to explain to a non-scientist why the expansion of the universe is accelerating. That's really a matter of pedagogy, not physics, and it's going to vary with both the explainer and the explainee. For example, here's an article, from Ned Wright's cosmology pages, explaining in pretty simple terms how a vacuum containing quantum fluctuations can have a positive energy density and a negative pressure:

http://www.astro.ucla.edu/~wright/cosmo_constant.html

Then you just look at the second Friedmann equation (which Carroll gives in his article) to see why the net effect of the vacuum's energy plus pressure means accelerating expansion.

17. Jul 16, 2014

### yogi

In 1951 McCrea developed the idea of negative pressure as equivalent to a state of tension. When pressure is everywhere the same, without gradients, it has no perceptible effect, but McCrea went on to show that an expanding negative pressure creates positive energy. Since dE/ds is symbolically force, then it follows that dE/dV (energy per volume) is an isotropic force - so the idea of expanding negative pressure is not inconsistent with the idea of an internally created isotropic expansion force. Moreover, since the amount of energy created by the expanding volume is proportional to the volume, it is not unreasonable to assume the expansion rate will increase. Contrary to the article cited, I would think the interpretation of expanding negative pressure as force is a good way to describe dark energy.