# Expanding Universe here?

#### Ikon Rahu

I am curious about the expanding universe - that is, that the space itself is expanding. Why can't we notice the effects of an expanding universe here in our own solar system? Why doesn't space expand here around us? Wouldn't this result in more space between our molecules and so on until we would see more space between the objects in our local environement?

#### PaulS1950

We are gravitationally bound to other objects in our galaxie which is bound to the local cluster. The expansion of space only occurs between bound clusters.

Here is a question:
Is the "density" of space lower where it is expanding?

#### Drakkith

Staff Emeritus
2018 Award
Here is a question:
Is the "density" of space lower where it is expanding?
I don't believe one could say that space has a density, only that matter and energy within it have a density.

#### mikeph

The Hubble constant describing expansion is about 10^-17 s^-1, so you might expect that after one second of expansion a metre rule will in fact be 1+10^-17 m long. There are already a load of vibrations going on between the atoms at much greater amplitudes than 10^-17 m and with much faster periods than 1 second, so the expansion is effectively an extremely weak perturbation. Since the oscillations are stable over these greater amplitudes, the weak perturbations decrease over time, and the ruler remains, in its equilibrium position, at 1m long.

Essentially it's such a small effect over human scales that most objects with binding energy have sufficient restoring force to return to equilibrium straight away. The main "victims" of the expansion are cosmological-scale objects which are so large that their internal binding cannot overcome the expansion, and other objects such as photons who have no binding energy and are powerless to resist the expansion.

#### Mark M

I am curious about the expanding universe - that is, that the space itself is expanding. Why can't we notice the effects of an expanding universe here in our own solar system? Why doesn't space expand here around us? Wouldn't this result in more space between our molecules and so on until we would see more space between the objects in our local environement?
Because of gravity. Even gravity, the weakest force, is able to negate the effect of expansion inside of the galaxies. Dark energy does, however, have very tiny effect - but it's like an ant trying to move a bulldozer.

#### phinds

Gold Member
Because of gravity. Even gravity, the weakest force, is able to negate the effect of expansion inside of the galaxies. Dark energy does, however, have very tiny effect - but it's like an ant trying to move a bulldozer.
I agree w/ your analogy but not its conclusion. If an ant pushes on a bulldozer, it's not the case that the ant has a little tiny effect, it is the case that the ant has absolutely no effect at all because it cannot to any extent overcome the forces holding the bulldozer in place.

#### Mark M

I agree w/ your analogy but not its conclusion. If an ant pushes on a bulldozer, it's not the case that the ant has a little tiny effect, it is the case that the ant has absolutely no effect at all because it cannot to any extent overcome the forces holding the bulldozer in place.
Oops, thanks for pointing that out. My point is that the effect of dark energy is there, but is irrelevant.

#### Drakkith

Staff Emeritus
2018 Award
I agree w/ your analogy but not its conclusion. If an ant pushes on a bulldozer, it's not the case that the ant has a little tiny effect, it is the case that the ant has absolutely no effect at all because it cannot to any extent overcome the forces holding the bulldozer in place.
I remember a thread about this. Did we ever come to a conclusion? I thought the expansion increased the distance between objects ever so slightly but doesn't keep increasing it. You can think of it as a very slight reduction in the force holding objects together. IE the distance between the Earth and the Sun is increased by like 0.1 mm due to expansion, but it doesn't keep increasing. (Ignoring acceleration of the expansion)

#### Mark M

The normal expansion has no effect at all on gravitationally bound systems. The cosmological constant, however, does have a slight effect.

#### phinds

Gold Member
I remember a thread about this. Did we ever come to a conclusion? I thought the expansion increased the distance between objects ever so slightly but doesn't keep increasing it. You can think of it as a very slight reduction in the force holding objects together. IE the distance between the Earth and the Sun is increased by like 0.1 mm due to expansion, but it doesn't keep increasing. (Ignoring acceleration of the expansion)
Yeah, I'm getting confused here myself and I think I'm going to have to do a page on dark energy / expansion / acceleration the way I did on the balloon analogy, and try to get everyone here (well, the most active players anyway) to agree.

For example, I've been saying that objects inside gravitationally bound systems (galactic clusters and smaller, but particularly solar systems and smaller) are not affected AT ALL by dark energy, but it has been pointed out to me that this is not correct and that although the affect is infinitesimal, it is not zero in gravitationally bound systems. Atoms, on the other hand are governed by forces which are NOT affected by dark energy, so they really do see zero affect from dark energy.

As to your very good question of whether or not the tiny effect on, say, a solar system, reaches an asymptotic limit, or continues, I have not a clue, having heard both points of view.

It is this kind of confusion on my part that drove me to do the ballon analogy page. I do that kind of thing primarily to teach myself, and hope for a beneficial side effect of helping others (and saving us all a LOT of keystrokes).

Paul

#### Chronos

Gold Member
The best guess I heard on solar system effects is around 40 meters on earth orbit over the last 4.5 billion years, but, that is somewhat less than the effect of radiative loss of solar mass loss over the same period of time. Cosmological affects due to dark energy are totally insignificant over such short distances.

#### phinds

Gold Member
The best guess I heard on solar system effects is around 40 meters on earth orbit over the last 4.5 billion years, but, that is somewhat less than the effect of radiative loss of solar mass loss over the same period of time. Cosmological affects due to dark energy are totally insignificant over such short distances.
Yes, but insignificant is still NOT the same as zero since the two would imply completely different operations, so I need to stop telling people that it's zero (except for atoms/molecules)

#### Mark M

Expansion has no effect on the orbit of the earth. What can is dark energy. One way to see why is the fact that dark energy is a uniform negative pressure, so it functions as a kind of 'anti-gravity' (of course, it may be just a constant curvature, which amounts to the same thing.) So, you must take that force into account for, say, the orbit of a planet. But it is negligible. However, nothing affects the size of atoms. The strong force is far to powerful.

#### phinds

Gold Member
Expansion has no effect on the orbit of the earth. What can is dark energy. One way to see why is the fact that dark energy is a uniform negative pressure, so it functions as a kind of 'anti-gravity' (of course, it may be just a constant curvature, which amounts to the same thing.) So, you must take that force into account for, say, the orbit of a planet. But it is negligible. However, nothing affects the size of atoms. The strong force is far to powerful.
Yes, Chronos has alerted me to this. One thing I'm still not clear on is that I have heard both of the following regarding the effect of dark energy / the cosmological constant and I'd like to hear what you guys have to say (also see post #10)

1) the effect on the earth's orbit is tiny and has gotten as big as it's going to get [I have no idea why]
2) the effect on the earth's orbit is tiny and will contrinue to grow (but still be negligible)

#### Naty1

I'd like to know the basis for Chronos quoted estimate in post #11.

Correct me, somebody, if I misinterpreted another thread discussion, but I concluded that the FLRW cosmological [large scale] model [for homogeneous, isotropic conditions] did NOT apply at galactic distances....too much lumpiness within galaxies. In addition my understanding is that nobody knows how to solve the EFE for representative galactic conditions outside the FLRW model....how to include the lumpiness in other words.

So it still seems to me we instead say something like 'gravitationally bound systems and things inside them are not thought to expand [or are generally not considered to expand] but we have no exact solution, no good model, for such conditions.

#### Mark M

Yes, Chronos has alerted me to this. One thing I'm still not clear on is that I have heard both of the following regarding the effect of dark energy / the cosmological constant and I'd like to hear what you guys have to say (also see post #10)

1) the effect on the earth's orbit is tiny and has gotten as big as it's going to get [I have no idea why]
2) the effect on the earth's orbit is tiny and will contrinue to grow (but still be negligible)
It depends on the nature of dark energy.

Firstly, keep in mind that even without dark energy, the orbit of the earth will grow. That's because the Sun and the earth emit gravitational waves over time, and exhibit gravitational recession, a consequence of general relativity. But let's ignore that.

Next, let's assume dark energy has a constant strength, so that it doesn't vary with time. With that in mind, I would have to say two is correct. Let's say we had two objects moving through a region in which dark energy was extremely strong (just a thought experiment). Since the force from DE is constant, the two objects will accelerate away, diverging to infinity.

So, we should be able to conclude that two is correct.

Naty1, regular expansion does not affect gravitationally bound objects. Dark energy does.

#### Naty1

I just stumbled across this paper:

In an expanding universe, what doesn’t expand?
Richard H. Price
http://arxiv.org/pdf/gr-qc/0508052v2.pdf

HAven't read it yet....paper also references other research....

Mark M:
regular expansion does not affect gravitationally bound objects. Dark energy does.
Why do you think so...any source??

#### Mark M

Naty1, take a look at Newton's law for the gravitational force, while taking into account the effect of a cosmological constant: $$F = {GMm \over r^2} - {\Lambda m c^2 \over 3} r$$ You can see that the cosmological constant reduces that gravitational force between two objects, expanding orbits.

That's because dark energy is a constant repulsive gravitational force (whether it be from a negative pressure, or a constant curvature.).

Take a look at the paper you posted - exponential expansion affects bound objects, however slightly.

#### phinds

Gold Member
It depends on the nature of dark energy.

Firstly, keep in mind that even without dark energy, the orbit of the earth will grow. That's because the Sun and the earth emit gravitational waves over time, and exhibit gravitational recession, a consequence of general relativity. But let's ignore that.
Yes, I agree that it exists and I agree that it should be ignored because it's not what we are discussing, which is the effects of dark energy / the cosmological constant.

Next, let's assume dark energy has a constant strength, so that it doesn't vary with time. With that in mind, I would have to say two is correct. Let's say we had two objects moving through a region in which dark energy was extremely strong (just a thought experiment). Since the force from DE is constant, the two objects will accelerate away, diverging to infinity.

So, we should be able to conclude that two is correct.

Naty1, regular expansion does not affect gravitationally bound objects. Dark energy does.
Yes, all of this is now what I believe to be true

#### phinds

Gold Member
So it still seems to me we instead say something like 'gravitationally bound systems and things inside them are not thought to expand [or are generally not considered to expand] but we have no exact solution, no good model, for such conditions.
I LIKE that, with a trailer caveat saying "so, they MAY expand, but if they do so, the result is so small as to be totally negligible"

#### Naty1

"In an expanding universe, what doesn't expand?" by Price and Romano,

http://arxiv.org/abs/gr-qc/0508052,

I skimmed the article and the conclusion seems to be :

We have presented a simple definitive question about the influence of the expansion of the universe on a very particular system: a classical “atom.” ...... atoms are in no danger of being disrupted by cosmological expansion.

#### Neuroglider

That Price & Romano paper ("What doesn't expand") uses an awfully simple model of an atom, specifically, of the electron's orbit. We know that an electron doesn't have a planetary-type orbit around the nucleus (in fact, it passes through the nucleus). I assume that this still fits the Price & Romano argument, however, because the momentum of the electron increases as r decreases and counteracts any anticentric forces, including expansion.

Still, the authors use Newtonian and relativistic analyses but ignore quantum mechanics. According to quantum mechanics, there is a non-zero probability of finding any given electron anywhere in space. So... is it possible that expansion gets lucky now and again, capturing an electron that has strayed so far from its atom's nucleus that the electromagnetic force is too weak? Is it possible, in fact, that expansion is what is responsible for the inconsistency of electron orbits in the first place? Or is that just stringy chaos?

Incidentally, Price & Romano cite Bonnor (1999, Class. Quantum Grav. 16 1313) and claim that their analysis is consistent with his. But Bonnor additionally considered an Einstein-de Sitter model and concluded that under that system "the atom expands, but at a rate which is negligible compared with the general cosmic expansion."

#### Naty1

Mark M: Is that equation in your post within the quoted article? I am not familiar with that equation.....is that available in say Wikipedia?.... I have no idea about the assumptions from which it is built.

Take a look at the paper you posted - exponential expansion affects bound objects, however slightly.
Perhaps, but that is not how I read the article.

from the conclusions:
....And we have found a simple definitive answer: Expansion forces increase with increasing atomic radius, while atomic forces decrease. This amounts to an instability with respect to the disruption of an atom. If the atomic accelerations are initially stronger than the cosmological, then the subsequent expansion will become less and less important. The atom will not “partially” take part in the expansion. If, on the other hand, the cosmological effect is initially stronger, the atomic radius will increase and the atomic forces will become less and less important. The atom will fully take part in the expansion.......atoms are in no danger of being disrupted by cosmological expansion
Even if my interpretation is accurate, I would not necessarily take this as definitive, either, as

" We will put this classical atom in a homogeneous universe
in which expansion is described by an expansion factor a(t), where t is time..."
How realistic IS that? I do not know. And the author points to a different paper [#6] for cosmological expansion effects on galaxy clusters...I don't know what that one sez.

#### Mark M

Naty1,

I've seen the equation from a post by Chalnoth, see post #27 here:

The reason behind the equation is simple - the cosmological constant has an effect that is opposite to gravity. It accelerates objects away from each other.

I was citing the caption under Figure 2 in the paper you posted, namely the line saying:

Due to the exponential increase in a(t), the physical radius grows without bound.
Also, I never said that expansion affects bound systems. I said that accelerated expansion does, the cosmological constant. It's because it takes a constant value everywhere. So, it has a small effect on all systems, however negligible.

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#### Mark M

See this article by John Baez about how normal metric expansion affects objects within a gravitationally bound system:

http://math.ucr.edu/home/baez/physics/Relativity/GR/expanding_universe.html

Obviously, as he explains, objects in bound systems are NOT affected by metric expansion.

Dark energy is what I've been speaking about - since it's a uniform negative pressure (or a constant negative curvature), it affects everything. However small these effects are (small enough that they won't even affect atoms), they can increase orbits, by an extremely small margin.

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