The Mystery of the Expanding Universe

Q:If the universe is expanding (space is stretching),it would make sense that the spaces between particles are getting bigger. If this is so, then the particles which make up atoms are also affected. Does that mean eventually the spaces between the components of an atom will become to large for the subatomic forces to hold? Are atoms getting weaker!?

-Carl
Langley, B.C. Canada
A: Carl -

Great question! We actually had the opportunity to pass your question on to Nobel prize-winner Leon Lederman recently, and here’s what he told us: The expansion of the universe doesn’t actually affect the spaces between particles. The universe’s expansion is not a force that will rip particles, molecules or even objects apart. The ’fabric of space’ is not stretching - just the distances between really large things like galaxies. So while the distance between the milky way and its nearest neighbor may increase over the next billion years, the distance between the proton and neutron in a deuterium atom’s nucleus will not.

-Tamara

(republished on 07/19/06)

Follow-Up #1

Q:You say "The ’fabric of space’ is not stretching" but surely it is otherwise what is the space that these galaxies are moving into? If it is not existing space stretched then it must be either new space created somehow or pre-exisiting empty space. I know its not the latter so how it it that this new space is created around atoms but not within them. Ditto for molecules. And surely for free atoms in say a gas the avergae space between them does expand as the universe expands?

-David (age 48)
UK
A: You pose a tough fundamental question.  I’ll try to answer as best I can.

First of all is the issue of  ’stretching the fabric of space’; why do galaxies tend to expand relative to each other and not constituents of atoms. Well, it’s an observational fact.  We know by measuring the light of atomic transitions arriving from distant galaxies that the atoms ’there’ are the same size as the atoms ’here’ on earth. These atomic transitions depend on the length scale of space, as well as other parameters.  You might argue that the red shift confuses the issue but by looking at ratios of several transitions any red shift effect cancels out.   The agreement between theoretical calculations and observed ratios of abundance of light elements at the beginnings of the ’big bang’ is additional evidence.

The Hubble expansion rate, on the other hand, is driven by gravity and the equations of general relativity. It’s like after the ’Big Bang’ , or as Calvin said ’The Big Kablooie’, things just started flying apart and have continued, in the past slowing down but more recently speeding up.  Given the average density of matter and energy in the universe, the expansion rate, as well as its acceleration can be calculated.  The latest wrinkle is an additional repulsive term called ’dark energy’ that seems to be needed in order to explain recent observational evidence of expansion acceleration.  But that is another story.

LeeH

Your argument is sound. I’m not sure why we said that ’the fabric of space isn’t stretching’. That metaphor is actually a pretty good, conventional way to describe the equations Lee wrote about. As for atoms, etc, they don’t expand any more than sequins embedded in spandex would expand when the fabric was stretched.  Mike W






(published on 07/26/07)

Follow-Up #2

Q:Thanks we’re getting there now. Just to clarify, the space within atoms is not expanding. The space between separate objects (be they galaxies, stars, planets, or the two pencils on my desk, or free atoms in a gas), is expanding? Or is it only between big objects like galaxies, if so what it the size cut-off and why? One last tricky question (which I did ask before) what about the space between atoms that are locked into molecules? Thanks

-david (age 48)
UK
A: First, I should caution that there are different equally good coordinate systems which assign different time-dependences to various distances. There’s a fairly standard choice, however, in this context, and that’s one where the distances between remote objects grow in  time. In that standard type of coordinate system, the distances between nearby objetcs (even on an atomic scale) would also grow if there were no forces between the objects. So you can take that background growth as a marker of what ’space itself’ is doing, i.e. expanding. Since atoms and molecules are held together by forces, however, they don’t expand. That would hold for pencils and desks too, since each one consists of a fixed collection of atomic spaces.
If you pick some particular part as stationary, then the other parts are moving a little with respect to the coordinate system, due to the forces.
So there’s no exact size cutoff. The question instead is whether the forces are important on the time-distance scale involved. The solar system, and also I believe the galaxy and I think maybe even galactic clusters are in this regard more like atoms or pencils than like the cosmos as a whole. You can basically calculate their sizes using ordinary mechanics (or quantum mechanics for the smaller things) ignoring what space-time is doing on a large scale.

Mike W.

(published on 07/30/07)

Follow-Up #3

Q:Thanks I have it now - the space between my pencils isn’t expanding because there are local forces present (presumably including weak force of gravity at a local scale). If one of my pencils was on another planet or even solar system within the Milky Way, the same would be true. However if one pencil was in another galaxy (excluding Andromeda) then the space between them would be expanding, very rapidly (could be over 2,000km/second) and at an accelerating rate. What if my pencil was sitting in space 1/2 way to another galavy? 1/4 of the way? etc... It has a lot of slowing down to do somewhere to get back to zero.

-david (age 48 3/4)
A: Again, it’s not even possible to make a simple statement about whether the distance between two objects is increasing, without making some standard choice of coordinate system. As for whether ’space’ is really expanding, that’s kind of a semantic question. But for your question, which concerns whether the conventional distance increases, the answer depends on a simple classical issue. Is the pencil bound to the galaxy gravitationally? If so, the size of its orbit, to a good approximation, won’t depend on the universal expansion. If it’s not bound it will be swept farther away by that expansion. Whether it’s bound just depends on whether it does or doesn’t have escape velocity, given its current distance. The farther it is, the lower that escape velocity is, but there’s no sharp cutoff.

Mike W.

(published on 07/31/07)

Follow-Up #4

Q:The one thing that this question has stretched is the distance between my logic.. not to say it’s a concept with difficulty, rather a concept difficult to question. Here’s a question: If we can say stretching fabric is possible but stretching the atoms that make the fabric is impossible, then we know that the mirco and macro realm have separate and distinctive rules. But then what would be the reasoning behing the search for gravity in something as blunt as a graviton. It’s almost like sayin that a green leaf is green because there’s a green particle in it. Gravity is the social factor for atoms, and sure enough sociability within an economy of exchange does not occur because of one thing such as commodification, that is, to give ’nothingness’ a name. The more I think of the topic, the more i reason to look at the creation of the big bang. The only reciprocated value to the universe’s expansion is implosion. And what better term to coin this concept with, on such a macro scale, than Gravity? It seems like gravity’s answer is where it’s anti-force derives from.

-Steve
Ontario, Canada
A: I’m not able to follow most of the points in this question, but will try to answer the ones whose meaning I can track.

It’s not impossible for spatial expansion to pull apart atoms. It’s just that the current expansion of space is far too weak, in comparison with the forces holding atoms together.

The existence of various competing effects, some more important on a small scale and some more important on a large scale, does not mean that there are "separate and distinctive" micro and macro rules. Inside your body, electromagnetic forces are usually more important than gravity. On the scale of the solar system, gravity is usually more important. They both obey the same rules, just with a different quantitative balance between the terms.

The graviton, if it exists, would not be a mere verbal renaming of things, but would show distinct effects (not easy to measure!) such as shot noise, just like the quantum version of electromagnetism.

Finally, I want to make a general point. Scientific statements may have a poetical, evocative side but there’s no point arguing about them unless there’s some sort of observable implication. That requires some precision in formulation.

Mike W.

(published on 08/23/07)

Follow-Up #5

Q:Well, ok maybe the universe isn't expanding fast enough to break the bond holding sub-atomic particles, or what ever else, today, but considering the fact that expasion is accelerating could it be fast enough to rip the universe apart at the sub-atomic level in the future. With dark energy making up ,I think 75% of the universe today, there really seems like there is nothing to slow it down, so inevitabley wouldnt the universe be destined to rip apart.

-Kenny (age 16)
philadelphia, pa, USA
A:Interesting question. I am not 100% sure of this answer, but we can try it for starters. I don't think the current accelerating expansion will ever tear apart small objects directly. Let's say you have two particles bound into a molecule, e.g, H2. The rate of separation of the two atoms is zero in the molecule, even though space is expanding. The acceleration of the expansion does in fact cause a pseudo-force which just slightly stretches the molecule. Now look a billion years later. The expansion rate will have increased, but that still has no effect on the molecule's size. The acceleration will also have increased, because the density of ordinary matter is lower, but even when that density gets close to zero the acceleration will only be about 1.4 times as large as it is now. So the molecular stretching will still be negligible.

However, in the long run as galaxies fly apart, the density of matter will go down. When smaller things (say solar systems) accidentally get knocked apart, the chance of new ones forming will be lower. Ultimately that should mean that all bound  systems (including molecules) will fall apart and not be replaced. That's not going to happen soon.
Mike W.

(published on 01/28/09)

Follow-Up #6

Q:But the expansion is having a small effect on the particle and eventually it would tear it apart in say, billions of years? Have any famous physicists answered this question? Also, I understand that the Big Rip is a theory that states that the accelerating expansion of the universe will cause all the particles to "rip apart" and then begin "floating" faster than light speed away from one another.

-chris
florida
A:The small effect isn't strong enough to tear the atom apart, and unless something drastic changes, it never will be. However, atoms occasionally pick up some energy and fall apart anyway. Once the expansion has gone on long enough, the chance of the atom recombining becomes small.

Mike W.

(published on 09/17/09)

 

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