The Mystery of the Expanding Universe
Most recent answer: 10/22/2007
Langley, B.C. Canada
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]
(published on 10/22/2007)
Follow-Up #1: Does the 'fabric of space' stretch like Spandex?
- David (age 48)
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 10/22/2007)
Follow-Up #2: Does space expand in atoms?
- david (age 48)
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.
(published on 10/22/2007)
Follow-Up #3: ... more on the expanding universe
- david (age 48 3/4)
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.
(published on 10/22/2007)
Follow-Up #4: rules on different scales
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.
(published on 10/22/2007)
Follow-Up #5: expansion and atoms
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.
(published on 09/15/2009)
Follow-Up #6: cosmic and micro
- Mr. Anonymous
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.
(published on 04/06/2010)
Follow-Up #7: Questions on gravity, acceleration, galaxies, etc.
- mark (age 36)
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 09/08/2010)
Follow-Up #8: is cosmic expansion meaningful?
- john (age 70)
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.
(published on 05/28/2011)
Follow-Up #9: Does matter expand with the universe?
- Sufyan (age 17)
Riyadh, Saudi Arabia
(published on 10/17/2012)
Follow-Up #10: accelerating expansion
- Kenny (age 16)
philadelphia, pa, USA
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.
(published on 05/16/2013)
Follow-Up #11: where did atoms fit after Big Bang
- bob (age 37)
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.
(published on 01/17/2014)