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Q & A: The Mystery of the Expanding Universe

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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
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.

[see below for further perspectives]

(republished on 07/19/06)

Follow-Up #1: Does the 'fabric of space' stretch like Spandex?

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)
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.

Changes in the Hubble expansion rate, on the other hand, are 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.


That "fabric of space" metaphor is actually a pretty good, conventional way to describe the equations Lee wrote about for how the rate of expansion changes. The average rate of expansion, however, doesn't directly drag anything along with it, unlike a fabric, except for minor effects of friction with the background radiation, etc. A local patch of space is described by special relativity, which doesn't have any preferred state of motion.   Mike W

(published on 07/26/07)

Follow-Up #2: Does space expand in atoms?

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)
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. That fixed expansion doesn't really drag anything along with it, except by actual interactions between real things in space. Since atoms and molecules are held together by forces which set the separation of the parts, they don't expand. That would hold for pencils and desks too, since each one consists of a fixed collection of atomic spaces. Even when the expansion rate changes a bit, that isn't enough to have much of any effect compared to the forces holding those objects together.

If you pick some particular part as stationary, then the other parts are moving a little with respect to the standard coordinate system.
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: ... more on the expanding universe

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)
As for whether 'space' is really expanding, that's kind of a semantic question, because we have the choice of all sorts of different coordinate systems. But your main question concerns whether the conventional distance increases, and we can answer that so long as we're careful about what we mean by "distance" between two objects. So let's pick a standard sort of distance,  measured by how long it takes light to go back and forth between the objects. The time we use to measure "how long" can be any sort of standard local clock.  

Now 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. Things that aren't bound on the average will be swept farther away by that typical 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: rules on different scales

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
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.
The simple spatial expansion doesn't pull apart molecules. On the other hand, the accelerating expansion (usually attributed to dark energy)  acts like a sort of force tending to pull things apart. However, that effect is far too weak, in comparison with the forces holding atoms together, to have a noticeable effect.

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: accelerating expansion

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
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. (Here we're assuming that the acceleration is given by a cosmological constant. You mention the word "rip" which suggests the "big rip", a hypothesis that the acceleration itself will keep increasing. If that were to happen, it could ultimately tear apart even molecules.)

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: expansion and atoms

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

The small effect of the current acceleration isn't strong enough to tear the atom apart, and unless something drastic changes, it never will be. The Big Rip idea suggests the possibility that something drastic like that could happen, but there's no evidence for that currently. 

If I understand correctly, the fate in the usual picture (no Rip) is expected to be something like this: Most things will clump up into black holes. Atoms will lose their identities. Then on an extremely long time scale those will evaporate via Hawking radiation, leaving a very dilute cold gas of photons. There won't be any atoms.

Mike W.

(published on 09/17/09)

Follow-Up #7: cosmic and micro

To make sure we are on the same page: atoms are not expanding enough to rip apart anytime soon, nor molecules, nor solar systems and galaxies due to local gravitational forces (unless collision occurs with another large mass and chance provides variance with this typical occurrence). So, although the universe is expanding, its constituents (protons, neutrons, etc.) are not – according to Lederman. The question initially begged on this topic is of large interest to me. It seems that what may have occurred was confusion in terminology that physicists have designated to certain properties of matter (the fabric). In smaller structures, say between atoms (quantum particles), molecules (relatively bound structures), and larger structures such as solar systems and galaxies (relatively bound), we designate conventional comprehensive strategies, as noted in brackets, to formulate our understanding for certain phenomena. And here is where language brings in the confusion. We see that small molecular structures like pencils or boulders are to be understood on the grounds that larger structures, like galaxies, are to be understood. But, the point at which the expansion question is initally posed was somewhere between small molecular structure and large galactic-molecular structure. To say that expansion does not occur in small subatomic structures can be explained by using terms like gluons or other strong force properties that atoms are constituent of. Expansion does not occur between small scale molecular structures within a shared environment because a shared environment bounds them sufficiently with gravity. To say that expansion does occur in large galactical structures is to commit ourselves to say that structures exceed the spatial limit that mass imposes on surrounding structures. What is illustrated is that both small and large scale molecular bound-ness are due to gravitational forces or lack thereof. However, micro subatomic structural bound-ness is not influenced (directly) by gravity. Here lays a conceptive model that couples subatomic and small-scale molecular structures to be working under the same umbrella in terms of what actually expands (typically), and a model that couples small-scale and large-scale molecular structures of what can expand. In short, atomic and small-scale molecular structures do not expand for different reasons, whereas large-scale molecular structures do for the same reason that small-scale structures do not. In essence, the conceptual groundwork that physics has been construing seems that it could disentangle any sort of semantic confusion in the prior sentence and prevent misconceptions from occurring by not only explaining small and large scale molecular expansion in terms of gravitational force alone, but also by explaining gravity as an emergent property of the subatomic world in a coupled systems model – whereas the subatomic scale world allows for a large scale world, large scale states effect subatomic scale states (eg. collisions resulting in subatomic breakage), and the subatomic scale cycling back, in turn, to once again effect the large scale, etc. By explaining such, we could, in some sense, bound ourselves away from confusing the limits of the large scale with the small scale with the subatomic scale. As an ambition to construing cohesive terminology, we are committed to theoretically investigate the “interface problem” between the micro-quantum and the macro-general sense of the universe by saying what properties in the quantum world lineate with the properties in the macro and how causal properties are connected between the two. This is not to say that the physicist’s concerns are not confronting this nor are of interest to this topic, but that without this issue explained, we will be confronted with questions such as the one initially posted. Here’s the question: Do you believe there is valid connection between subatomic description and macro description? If so, what are your thoughts on the validity of language that separates subatomic structures from the larger molecular (generally relative) structures as being an issue in the physics academia? If not, why not?
- Mr. Anonymous
Canada, eh!
That was quite a question.

First, let me explain a bit further about how things behave in the expansion. If you look at things far away, they are currently flying away from us. Thus even in the absence of any peculiar gravitation effects. they would continue to fly away. There's no need to invoke any 'fabric of space'. If you look at things that are bound (molecules, galaxies) by local forces, the parts are not systematically flying away, so in this basic picture they have no reason to start doing so.

There is, however, a complication. The large-scale expansion is accelerating, indicating that there is a positive cosmological constant. To picture that effect it is useful to think of a 'fabric of space'. Its effect looks like a repulsive gravitational force (although not between the objects but between their spatial locations). A distant object which is not currently moving away from us will gradually begin to, because of this effect. However, on a small scale adding a tiny repulsive force has negligible effect compared to the attractive forces holding together molecules or even galaxies.

One needs to be careful about language in intermediate cases where the accelerating expansion matters, but not enough to overwhelm other forces. Using language that simply ignores it on smaller scales doesn't really make problems.

As for the general question of whether there should be a unified description including gravity and the other forces, almost all of us agree the answer is yes. The string theorists, among others, are attempting to develop such a theory.

Mike W.

(published on 04/07/10)

Follow-Up #8: Questions on gravity, acceleration, galaxies, etc.

OK a multi part question which relates entirely to this discussion. 1) If you apply a force to a mass it moves/accelerates in space but galaxies are not accelerating in space, they are accelerating with space so is there a 'force' acting on them? 2) Is the acceleration of the expanding universe constant or is it accelerating at an accelerating rate? Or to put it anaother way if this force does exist is it increasing with time? 3) I understand that in force bound systems (molecules, solar systems, galaxies) the force holding them together overwhelms any effect of expanding space. However if you were to move two massive objects further and further apart would you eventually reach a point where the effect of gravity has deminished to the extent the the expansion of space takes over? Where the gravitational force of attraction is equal to the Hubble constant (if that makes any sense)? 4) I'm still struggling with the notion that the further away something is the further back in time we see it. This means the further back in time we go the faster things are travelling or, to put it another way, the further forward in time we go the slower things are travelling. So is the Universe slowing down? Sorry for asking so many questions but this stuff fascinates me so thank you in advance for your answers.
- mark (age 36)
 1)  Yes, galaxies do accelerate with respect to each other as well as with respect to our earth reference system.  The force involved is the usual suspect, gravitational attraction.
2)  We don't know yet whether or not the 'acceleration of the universe' is accelerating or not. One can invent theories both for an against this hypotheses but there is no conclusive experimental observations one way or the other. It is an interesting question and many astronomers and astro-physicists are involved in trying to find the answer.  Many of the current and proposed astronomical projects are focused on this problem.  We'll just have to wait and see.

(I think there's pretty much evidence that the cosmological "constant" has truly been nearly constant for billions of years. The acceleration has increased a bit because the positive acceleration is partly canceled by a negative acceleration due to gravity between ordinary masses. As those ordinary masses get more dilute, their effect weakens. So the best guess is that the acceleration will increase, but just slightly. /mw)

3)  Interactions at small distances, nuclear, atomic, molecular, etc are governed by strong and electromagnetic  forces.   At galactic distances, it's gravity, gravity, gravity.  As you know, the force of gravity falls off as the inverse square of the distance between two objects, but the Hubble constant is not a force, it is a measure of the current rate of expansion and has dimensions of  'velocity per distance'.  See .
4)  Light has a finite velocity of propagation.  So if you see something now that you know happened at a distance D from you, then you are actually observing something that took place at a time T = D/c ago.    Consider a friend standing 343 meters away from you and you see him clap his hands.   Well, it will take about 1 second for the sound to reach you but you can see him instantly, (in the limit that the velocity of light is very, very large).  So what you hear actually happened 1 second ago.   Same thing with light.


(published on 09/08/10)

Follow-Up #9: is cosmic expansion meaningful?

This is a very interesting question! For if the universe is expanding (whatever that means) both the space between and inside things is expanding. So our rulers and every means of measure is expanding. So we measure no expansion at all! So when we "perform the observation" that "makes real" (like Schrodinger's Cat)the expansion of the universe, we find it is not expanding at all!
- john (age 70)
Surrey, UK
Hi John- I've marked your question as a follow up to a long old thread on closely related issues.

There's some arbitrariness about what we say is happening to distances, because General Relativity leaves a large collection of different coordinate systems with the same physical effects. In the standard sort of expanding coordinates, however, the expansion is quite measurable. The reason is that for systems bound together by local effects (e.g. atoms) the expansion has essentially no effect on their dimensions. What we are in effect doing is choosing coordinate systems in which the sizes of such bound objects are unchanging. It's in those coordinates that we measure the large-scale expansion.

Mike W.

(published on 05/30/11)

Follow-Up #10: Does matter expand with the universe?

When we say that the Universe is expanding, do we mean that the distance between atoms is increasing? If so, then why don't we expand? Why doesn't matter expand?
- Sufyan (age 17)
Riyadh, Saudi Arabia
Sufyan- I've marked this as a follow-up to several closely related questions. Follow-up some more if those answers aren't quite what you need.

Mike W.

(published on 10/17/12)

Follow-Up #11: where did atoms fit after Big Bang

If atoms dont expand due to cosmological expansion, how did they get to their current size. the early universe wouldnt have had the volume to house all the subatomic particles and atoms at their current size. Sorry if i submitted this twice.
- bob (age 37)
new york

You're right that shortly after the Big Bang the atoms that are around now in the visible universe wouldn't have fit in what became that part of the universe. Instead of atoms there was a soup of different particles at very high energies. Those high energies allow lots of different particle states in a small volume. 

Mike W.

(published on 01/19/14)

Follow-up on this answer.