# Understanding expansion

Jeremy87
I'm trying to get some kind of intuitive understanding of the Big Bang and the expansion of space.

Would it be equivalent to think that instead of space expanding, everything in it is getting smaller, including matter, the speed of light etc?
In the beginning matter would be too large to exist so it was purely energy, and by now we would've shrunken so much that large voids have formed between smaller islands held together by gravity.

Does this way of thinking work, or is there something fundamentally wrong with it? What would make it different from an expanding space?
On a large scale, are galaxy groups fixed in space (which is expanding between them), or are they actually moving *through* space?

PS. My previous topic was removed because it was interpreted as suggesting a new theory.
It's not, I'm just trying to find a good way of seeing the "current" theory more clearly, so I now reworded the question.

## Answers and Replies

Gold Member
Dearly Missed
If you really want to understand expansion as you say (not just think about your own theory) then it will probably take some work but I expect you will succeed if you set aside your own theory for a while, focus on grasping the standard theory of geometry and gravity and keep asking questions about IT as well as the expanding model cosmos derived from it.

1915 GR is the standard law of geometry and gravity that everybody uses, or they use simplifications derived from it, and there are good reasons. It has been tested in a great many ways and always passes with high precision and flying colors. It predicts many kinds of effects often curious ones, which have been observed.

Expansion cosmology was derived from GR in 1922 long before Hubble observed expansion--as a possible solution of the GR equation that would hold under specific conditions. So expansion cosmology is in a sense a theoretical "prediction" of GR.

GR is our Law of Gravity, on earth and solar system scale, galaxy scale, cosmology scale so it is interwoven with how we model reality and it's precise out to 6 decimal place and so on. If you want to throw out expansion, then you abandon GR and you need a new Law of Gravity. It has to explain all the effects that GR does and it has to be at least as accurate in its predictions.

This is not so much "wrong" as impractical. It would be a great deal of work to invent a different theory that governs geometry and how it interacts with matter (which is what gravity is.) You would have to get a theory which did NOT predict expansion but which DID predict frame dragging and gravitational time-slowing and black holes and the bending of light rays as they pass by the sun and the spiraling in of binary neutron stars and the orbit of Mercury etc etc. People have TRIED to invent alternate theories now for almost 100 years and they never seem to work out.

In mathematical science theories are not so much "right or wrong" as they are predictive models meant to be tested until they fail and can be replaced. So you wouldn't be "wrong" to try to invent a theoretical framework with a variable speed of light and variable sizes of atoms etc etc. It would just be impractical and a lot of work. And, it might fail some observational test (as has happened with other alternate theories of gravity/geometry.) Or become prohibitively complicated. (Rival theories have tended to be rather more elaborate than GR, to get all the effects right.)

I think the most promising revolution in this direction that is going on is quantum geometry/gravity. This accepts the wellknown predictions of GR such as expansion and all the rest but challenges the idea that there can be a singularity at the beginning of expansion. It differs from ordinary GR only at very very high density, so the early universe is modeled differently, inflation may occur differently, there may have been a contraction that rebounded and started the expansion, subtle differences in the observable ancient light are predicted as compared with GR, and so on.

I think that is a more attractive revolution and apt to interest more people than what you propose (a reformulation of the Law of Gravity in terms of shrinkage). And it seems considerably more practical. Of course one never knows what will succeed and what will not.

Gold Member
The shrinkage atoms and gravitational orbits, with associated increase in atomic and orbital frequencies (to keep c constant) is a different but usable way to think of expansion. Note that if atoms and gravitational orbits were shrinking (at the same rate), we would not see the shrinking because out yardsticks are also shrinking. We would instead see space expanding.
In fact if you take the standard theory (GR, etc.) and express the results in different units, in which the expansion parameter appears to be constant, you get this picture of things. It is not "real" physics, but it is an interesting way to look at the evolution.

Last edited:
Brainiac2
Wouldn't the photons and cosmic rays reaching us from the past be much bigger if that shrinking scenario were true?

Jeremy87
...
So basically, mathematically it's correct that space is indeed expanding?
Would it still be "OK" to think everything in it is shrinking instead as long as I'm aware that it's really not the case and I'm not a professional cosmologist? Because I find this easier to get an intuitive grasp of, and maybe I can later imagine my "shrinking universe" to expand so that c stays constant.

Wouldn't the photons and cosmic rays reaching us from the past be much bigger if that shrinking scenario were true?
Photons are included in "everything".

Gold Member
Dearly Missed
So basically, mathematically it's correct that space is indeed expanding?
Would it still be "OK" to think everything ...

I'd say GR's correct in the provisional sense that nobody has yet come up with a math model that gives a better fit to the observed changing geometry of the world. All human theories eventually get improved or replaced. No final ultimate "correct".

GR says more than mere uniform expansion. You can have expansion at different rates in different directions (which could never by imitated by mere shrinkage of objects).

GR says you can have expansion along one axis and contraction on another, briefly, and then it can reverse. There can be vibrations in geometry, in effect. Not the kind of thing you could make a "shrink" copy of.

If someone hasn't been paying attention they may think that GR says that the whole universe is undergoing absolutely uniform expansion by the same amount at the same time-rate everywhere, so the appearanceof that could be mimicked by uniform shrinkage.
But that is not what is really happening. That is just a nice on-average approximation that you get from GR by making the simplifying assumption that matter is evenly distributed thru space and everything is happening uniformly.
That is a wonderfully useful approximation, because on large enough scale matter does indeed seem to be roughly uniform. The number and sizes of galaxies are more or less on average the same from place to place. But it is not true down at the level of local detail.

So there is a lot going on with the geometry of space and time that we OBSERVE and which would not be mimicked by some "shrink" scenario.

And time doesn't even run at the same rate everywhere. So if your scenario requires shrinking photons (as you say) and everything else except for large-scale distances, you would have a hard time saying on what time-table and at what rate to shrink things. It would differ all over the map and it would change from time to time even in a given location.

So to answer your "is it OK?" question: sure it is OK to think about the cosmos any way you please. We are free to fantasize the world as we like best. And science does not provide absolute truths, only what might be the simplest most reliable model at any given time. We grope our way towards a better understanding of still-mysterious nature.
So if you really enjoy picturing some kind of pattern of shrinkage, it is surely OK!

But I would say it is not advisable. You'd miss a lot of the richness that way and put obstacles to your further understanding. You're more apt to get led down a blind alley, I think, and be poorly equipped to appreciate how dynamic geometry is.

What do you find that's so hard to understand about the basic expansion model of the cosmos?
It is a simplified approximation derived from GR (our welltested law of gravity). It is far from being the whole GR theory! Just a simplified picture derived from the full theory that fits the largescale date remarkably well.

It says basically that around year 400,000 space was filled with hot gas, uniform to within a tiny fraction of a percent, so we approximate by saying uniform. It was just then cooling enough to become transparent---it was about 3000 degrees kelvin then.

What has cooled that gas is the expansion--that is what has allowed it to gather into denser patches and condense into stars. The expansion of distances since then has been roughly 1000 fold, so the expansion of volumes has been roughly a billion fold.

We still see the ancient light that was released by the glowing hot gas (3000 degrees) at that time, because it had just cooled enough to become transparent. Since distances have increased 1000 fold since then, the temperature of the light has gone DOWN by 1000 fold. (short wavelengths = high temperature, longer wavelengths = cooler light) So the temperature is now about 3 degrees kelvin, and we measure that. We are still swimming in that soup of ancient light that comes more or less uniformly from all directions.

Everything since year 400,000 pretty much follows from that. Cooling by expansion, condensing into stars, gathering into larger clouds and clusters. Slowing of motions relative to the background soup of ancient light. Some lesser but interesting details about the evolving RATES of expansion. That's the broad outline.

That's the basic story of expansion cosmology. I'm curious to know what, if anything, you find hard to understand about it---so hard you flip over to wanting to picture stuff shrinking.

Of course there is EARLY universe history, what happened before about year 400,000 of expansion, especially around the very start. That is not so well understood.

But the story from the moment of clearing, when the hot gas cooled enough to turn transparent and the ancient light was released, that story is well understood and comparatively simple.

If you had to say what the hardest thing to understand about it is, what would that be? I'd like very much to know because there may be others who get stuck at the same point.

Last edited:
Gold Member
If the universe is shrinking, it would follow that the distance of the earth from the sun has increaed over time. Evidence suggests earth's orbit has not measurably changed over the past 3+ billion years.

Last edited:
Jeremy87
The thing I find most difficult to understand is how an infinitely large space can expand further. I mean, I can say it does, and I can probably do whatever math it requires, but it just doesn't click in my head.
It's a lot easier to imagine a fixed (infinite) space, with a finite amount of energy/matter per volume, and the size of matter is shrinking.

I haven't taken any courses in school about this or anything, I've just read poorly explained texts that just say "the universe is expanding", which always sounded to me like the distances between our atoms are also expanding at an equal rate, which would make all our measurements about it useless as the measuring instruments would also be expanding.

What happens to atoms inside solid matter while the space they occupy expands?
Why doesn't it seem to affect things on a small scale?
Where goes this limit between small and large scales?

Gold Member
2021 Award
The thing I find most difficult to understand is how an infinitely large space can expand further. I mean, I can say it does, and I can probably do whatever math it requires, but it just doesn't click in my head.
It's a lot easier to imagine a fixed (infinite) space, with a finite amount of energy/matter per volume, and the size of matter is shrinking.

I haven't taken any courses in school about this or anything, I've just read poorly explained texts that just say "the universe is expanding", which always sounded to me like the distances between our atoms are also expanding at an equal rate, which would make all our measurements about it useless as the measuring instruments would also be expanding.

What happens to atoms inside solid matter while the space they occupy expands?
Why doesn't it seem to affect things on a small scale?
Where goes this limit between small and large scales?

The limit is somewhere around the size of galactic clusters. Things inside that are gravitationally bound and are not affected by expansion. Small objects such as atoms are controlled by forces other than, and stronger than, gravity and are even less subject to expansion.

One thing that might help you get your head around it is to consider how staggeringly TINY the affects of expansion are on small scales. My favorite way of exemplifying this is this:

Even though the universe is expanding, it is still going to be hard to find a parking space. Why? Well if you could go out into intergalactic space where expansion IS effective, and magically draw parking place lines, it would take 20 BILLION years for them to move far enough apart to park another car.

On cosmological scales, on the other hand, the effect is so humongous that galaxies at the edge of our observable universe are receding from us at about THREE TIMES the speed of light (no traffic tickets are issued, however, because nothing is actually MOVING at greater than c, it's just that things are getting farther apart).

Gold Member
Dearly Missed
The thing I find most difficult to understand is how an infinitely large space can expand further...

... poorly explained texts that just say "the universe is expanding", which always sounded to me like the distances between our atoms ...

I try never to say "the universe is expanding". One way to be precise about expansion is to talk about the uniformly distributed ancient matter and the uniform soup of ancient light.
This gives us a well-defined notion of being at rest with respect to Background.

An observer is stationary (or at "CMB rest") if the ancient light is the same temperature in all directions. We know the solar system is moving some 380 km/s in a certain direction with respect to the universe, because the uniform soup of ancient light hotter in that direction. So we adjust our observations to compensate for that. The data is corrected so that it represents the universe as seen by a stationary observer.

If you will take a moment to think about it, you will realize how enormously important it is to have a criterion of being at rest with respect to the light from the universe when it was uniform, before things began falling together in clumps and acquiring random individual velocities.

This is the missing piece of information that the popular books leave out. they should not say "unverse is expanding" it is too vague. Nobody gets a clear idea from that. What they could say instead is distances between stationary observers are increasing according to a certain pattern.

You have no right to expect that they wouldn't. We know now that geometry is dynamic. It is not static like Euclid and the other Greeks thought. It is what it is (approximately Euclidean at small scale) BECAUSE of General Relativity, which our well-tested theory of why geometry is what it is and how it is affected by concentrations of matter. The effects of dynamic geometry are normally very gentle and gradual. The current rate that distances between stationary observers are increasing is only 1% every 139 million years.

So there is no question of a rock expanding, you see. It is distances between observers at CMB rest which expand, not rocks.

Rocks and galaxies stay the same size because of their internal forces. The observers at the ends of a long metal measuring rod could not be stationary with respect to background. How could they be? The atomic forces, metallic bonds between metal atoms, keep the rod the same length! So there is no way that both ends of the rod could be at CMB rest at the same time.

Last edited:
urmother
if all galaxies are moving away from each other due to metric expansion of space how come clusters of galaxies formed in the first place?

Jeremy87
Thanks for all the answers. Just to make a few things clear:

- In smaller things, like a solid rock or even small enough galaxy clusters, the atoms (or galaxies) are actually moving *toward* each other relative the space "beneath them", if their distances stay constant?

- The expansion is kind of like a small "drag" outwards that partly cancels the effect of all attractive forces. If the expansion stopped, would all solid objects become *a tiny bit* smaller, and would perfectly stable orbits start to *slowly* (depending on size) decay?

- When distances get too large, the expansion speed is bigger while gravity gets weaker, and so things move away from each other along with the space they sit on?

Gold Member
Dearly Missed
Thanks for all the answers. Just to make a few things clear:

- In smaller things, like a solid rock or even small enough galaxy clusters, the atoms (or galaxies) are actually moving *toward* each other relative the space "beneath them", if their distances stay constant?

- The expansion is kind of like a small "drag" outwards that partly cancels the effect of all attractive forces. If the expansion stopped, would all solid objects become *a tiny bit* smaller, and would perfectly stable orbits start to *slowly* (depending on size) decay?

- When distances get too large, the expansion speed is bigger while gravity gets weaker, and so things move away from each other along with the space they sit on?

I like the way you put things! It is a clear concrete visualization. Carefully qualified by "kind of like" where you use a metaphor or simile. You seem to think like a physicist, wanting detailed grasp of dynamics.

I don't think anything you say here is physically wrong (but I'm an amateur really, no authority!) just that what you imagine goes well beyond, at some points, our ability to measure. A philosopher might quibble about going beyond what is empirical or operationally meaningful, but to heck with him. Your aim is to understand so you put things in artificially sharp focus.

I think what the orbit would do depends on how gradually or abruptly you stopped expansion.

There are some working relativists or cosmologists around here who I hope you connect with, if you keep asking questions here. There is also the Relativity forum which might be better for more technical questions.

One thing to remember is that we can't determine CMB rest with absolute precision. CMB temperature varies by 10-5 ---a thousandth of a percent. You've seen those mottled red and blue blotchy oval maps of the CMB sky. So we cannot determine the Doppler hotspot caused by our own motion with greater precision. And cannot determine a CMB rest frame with any more accuracy than that. So it's an idealization when I talk about distances between observers at opposite ends of a 1 lightyear long ruler. And say that relative CMB one is moving toward the other. Clearly the motion would be too minuscule to detect.

I don't think orbits would continue to decay if you stopped expansion abruptly (an interesting thought experiment!) but just be a slightly different shape. But it would intrigue me to hear what other people (Brian Powell, George Jones...?) might have to say.

I'll try to comment more later as time permits.

Jeremy87
Well, I am a physics student, just not of this particular field.
I like to understand how things work. Even if their effects are purely academic and likely non-measurable (like the drag and orbits scenarios), saying they don't exist would just confuse me more. I hate it when people ignore things that are incredibly small, as if there was some special rule that made them exactly zero.
It's not like I'm going to ever use things like these in calculations, I just need to know they are there so I can ignore them myself =).

Oh and yes, I didn't think the orbit example through. Their attractive forces would effectively get a little bit stronger, but maybe that just means they will converge into a new smaller stable orbit? (still not my field, though it feels like I should know this...).
Again, by a non-measurable amount, so there's nothing wrong with our laws of gravity =).

Gold Member
Dearly Missed
About the seeming drag (as by a viscous medium) I think it's fascinating that if you give something a kick and start it moving relative to CMB rest then (because of expansion) it will eventually slow back down to CMB rest! as seen by stationary observers.

There is a good pedagogical treatment of this in a 2001 article by Charles Lineweaver and Tamara Davis (they are great at explaining things) on "The tethered galaxy problem". Jorrie just pointed it out to me.
That is a good problem to think about as an exercise in learning to understand expansion and the related physical effects. You have a big galaxy at CMB rest and an ideal cable and you tether a small galaxy at some distance. So the small acquires some speed relative CMB (because kept at constant distance by tether). Then you RELEASE, so both are no inertial. You detach and remove cable. the problem is, what happens? Does momentum carry the little one closer? Or does it stay at constant distance? Or does it drift away? This is sensitive to the acceleration of expansion.
They set up some simple (or at least simple-looking) equations and solve.
Looking it over could give you some intuition about the "drag" or "viscosity-like" business.
They have figures showing plots of trajectories. I hope I'm not boring you. I find this stuff fascinating and tend to go on too much. I'll get the link.
http://arxiv.org/abs/astro-ph/0104349
It's not really drag. It's that where its going backs away from it, so to speak

You might also take a look at Jorrie's post #444 in the Balloon Analogy sticky thread.