Expansion of the Universe: a two-part question

Assuming the Big Bang theory is correct, how fast is the Universe estimated to be expanding?

Also, are all bodies (both celestial and terrestrial) in space expanding in size to scale of the Universe’s expansion? In other words, suppose the Universe expanded 8.9 x 10¯¹ light years in the last 365 days (a very rough, uneducated, unresearched estimate, but bear with me). Have you, me, the apple on my desk, the Earth, and the molecules which make up everything therein grown the proporitional size in meters? Is there even any way to know, given that all measuring devices have also grown? Or do galaxies simply shift away from each other, remaining the same size?

I know this post isn’t the most delicately written, but I’m no scientist. Any insight would be great. Thanks.


Well, this site has the following numbers.

For the second question, no, objects are not expanding in size. The galaxies are just moving away from each other.


The electromagnetic force holds objects together, and gravity holds galaxies and even galactic clusters together, so neither expand. The space between them is all that is expanding.

I left out something. Gravity is pulling the larger entities together too, but they still have so much momentum from the big bang that they’re still spreading apart. Gravity between galaxies is relatively weak and their momentum is huge.

Sorry to beat a dead horse, but again, how do we know? If a line segment grows by one meter, and your ruler grew by one meter, there’s no real way to tell, is there?


Well certain laws of physics would have to change for us not to know it. For instance, the speed of light would have to increase to keep up with our growth, or else it would appear to us to be slowing down. It’s possible, but not probable. Plus, philosophically, how can space be expanding without us being able to measure it? That implies that space has some kind of substance to it, but it doesn’t. It’s kind of an abstract idea, in a way. I think that space can only be expanding in ways that can be measured. I’m not sure this is coming out in words coherently, so hopefully someone can make better sense of it.

Just to add that the most recent (and accurate) number for the Hubble constant (the number which tells us how fast the universe is expanding right now) is 71(+0.04/-0.03)(km/sec)/Mpc.

I seem to recall reading in one of those physics-for-the-masses types of books that we all were indeed increasing in size, too, as the universe expanded. That is, aside from “expansion” being just staking out a larger territory, that we were also increasing in physical size. I don’t pretent to know that basis for this, nor how to measure it, though.

Which book was this?

On a galactic, and even galactic cluster scale (we’re talking 100s of Mpc here), gravity is dominant, and gravitationally bound objects, such as planets, galaxies, galaxy clusters and super-clusters stay gravitationally bound, at least on timescales corresponding to roughly the age of the universe. That is, we are not being stretched along with space. Indeed, gravity is so strong on local scales, that it can cause galaxies to collide with each other, such as Andromeda, which is currently hurtling towards us, and will eventually merge with our own galaxy. And on a larger scale, our own little local group is due to merge with the Virgo group on a relatively short timescale. Gravity overcomes the Hubble expansion on a fairly regular basis, and on rather large scales. It can stop a galaxy expanding.

The way I tend to demonstrate this is to blu-tack some pennies onto a ballon. When I blow the balloon up, the space between the pennies increases, but the size of the pennies doesn’t. The pennies can be likened to a galaxy. or a galaxy cluster, and the balloon to spacetime.

I can’t remember which book and my library is 1700 miles away. Maybe something by Kip Thorn, or maybe “The God Particle” – the last two I can remember having read.

Keeping in mind I know nothing substantial, I think your balloon example is a good one to expropriate: instead of pennies, magic marker dots. The space between them becomes bigger, but then the dots become bigger as well. The space between them grows in a bigger proportion than the dots.

Except of course, we don’t actually see that happen. We can look at distant galaxies and see how big they are, and they do not show any evidence whatsoever of expanding in the way you suggest. Rather, as I’ve stated above, a gravitationally bound object such as a galaxy does not expand with the Hubble flow, because locally, gravity wins out.

The idea of why why galaxies don’t expand with the Hubble flow is something that does confuse people, and many popular science books do try to explain that. I think you may be misremembering such an explanation.

Conversely then, does this mean that if and when the universe starts to contract that things will not get smaller? No shrinking into a primordial dot?

No. Rather, if the universe starts collapsing back on itself, that’s gravity winning out against the primordial expansion. There’s nothing to stop gravity collapsing everything down to a singularity, as any black hole would testify.

A small nitpick. Although it is often stated this way, although the galaxies are becoming farther apart, they are not actually “moving” away from one another. Like the pennies on the balloon, the galaxies stay where they are (on a macro scale) while the space between them expands. A subtle distinction, but it helps to understand what’s actually happening.

The problem is that the explainations offered tend to idealize “space” as being this large elastic blanket in which we are embedded; people visualize the blanket as expanding uniformly, and the matter within it fixed in relation to the blanket. But matter is not space, and it is perhaps a better analogy to conceptualize atoms, planets, solar systems, and even whole galaxies as lilies floating on the surface of a pond; even if the surface of the pond gets larger as more water is added, and the lilies move away from each other; the increasing water flows around them but the flowers and pads are held firmly together by their vegetable structure.

It is a property of matter (which is a form of energy that expresses itself gravitationally) that it distorts space about itself, causing other matter to be attracted and clump together–like the lilies–rather than spread out evenly; sort of like what Reagonomics did for finance moguls and their money. With large, galactic-scale structures gravity retards space from expanding. Even in larger structures, like The Local Group, gravitational attraction plays some effect, though very minor, on the expansion of local space. And even on much larger scales, enormously massive objects like The Great Attractor have a more significant effect, causing galaxies to stream toward them (but still pulling away from each other).

On more local scales, the electromagnetic force acts on atomic particles and holds together molecules, and the weak and strong nuclear forces act on subatomic particles and serve to keep nuclear particles clinging together; each is increasingly stronger, by many orders of magnitude, than its precedant, and all have much more of an effect to keep particles together than the expansion of space has in pulling them apart, in the same way that a raindrop will not “dilute” or rend apart a lilypad.

That’s a complicated answer, but the simple answer is…er, there is no simple answer. Electrons would still exist in their same shells at particular quanta from the nucleus, molecules would continue to form their present structures, et cetera, up to the point that local gravitational energy, which would be increasing, overwhelms the electromagnetic force. In the heart of the Sun, for instance, gravitationally-induced pressure prevents atoms from properly forming; instead, electrons zip back and forth, sliding between nuclei, without ever letting themselves become attached, like Jane Greer in a '40s noir film. This is called degenerate matter; it’s not quite atomic, but all the components are still intact.

Squeeze further, as with a supernova (in which the lighter mass is blown away, leaving only a superdense core) and the electrons and protons cram together to make a bunch of neutrons and some radiation which disappears into the night like an alimony payment and leaves you with neutronium, held apart only by the particles’ unwillingness to share the same quantum state with one another, like two divas attending a party while wearing the same cut and color of dress.

Add some more matter and eventually the whole party degrades into a massive orgy in which the participants become an inseverable mass of arms, legs, heads, and various other appendages; we don’t know exactly what goes on in there, and would probably be horrified if we found out (except for Bob Guccioni, who would be in a bathroom stall doing things we’d rather not think about), but we speculate that the matter would turn into a quagma, a fluid of quarks too dense for light to penetrate.

Further compression gets us to a point past which we can even guess; the best speculation is that the particles arrange themselves in a matrix in which their quantum states “interlock”, allowing them to achieve smaller and smaller gaps, until they disappear faster than a played-out teen starlett, eventually condensing to a single, unified quantum state into one giant particle. It would take an enormous amount of mass to create this level of compression–quite literally, a universe’s worth–although we hope to simulate something like it for a very, very brief instant using larger and larger particle accelerators, and hence gain a understanding of what happens in the remote corners of space and the furthest reaches of time by looking at things that happen in a space too small to imagine. How cool is that?

In any case, from what we understand right now (and while we are like the proverbial blind men feeling the different parts of the elephant, we have at least identified which smells the worst and have therefore concluded that we must be in the vicinity of an arse) the universe will not contract; the Hubble “constant” is actually a varying value that is slowly increasing over time and the energy balance of space is such that it appears that space will continue to expand away in perpetuity, until there is nothing left to look at but the formless nothingness of space accelerating away at c. And you though 2001: A Space Odyssey was boring?


If you’re interested in this stuff, Simon Singh just came out with a new book called Big Bang: The Origin of the Universe. It’s a pretty straightforward history of the whole thing from a layman’s perspective, and very good.

Well…yes and no. The galaxies aren’t “moving”, in the sense that someone is getting out and pushing or otherwise invoking Netwon’s Second Law, but an observer would have every indication that all galaxies are accelerating away from them, including relativistic effects such as the redshift and Lorentz time and length dilations. But the same is true for any observer in any galaxy, and indeed, an observer standing…er, floating, well away from any galaxy would see (from his reference frame) all galaxies streaming away from him, which would either lead him to conclude that either:

[li]Solipsists (paradoxically, all of them) are right,[/li][li]He has a literal case of cosmic halitosis, or[/li][li]The space in-between galaxies is expanding.[/li][/list]Motion is (heh) a relative thing, and if dx/dt is positive, we can conclude that two objects are in motion with respect to each other, even if neither of them is actually moving relative to anything else.

And because I meant to plug it in the previous post but totally forgot, Alan Dressler’s Voyage To The Great Attractor gives not only a good layman’s underpinnings to modern astronomy and Big Bang theory but also provides insight into how scientific knowledge develops, both by hypthesis and disproof and, sometimes, via serendipity. The researchers in his group (colloqually known as “The Seven Samuari”) discovered after many observations and filtering of data that not only are galaxies moving away from one another (or the intervening space is expanding, as you like it) but the local supercluster of galaxies is actually accelerating toward a massive gravitational object known as “The Great Attractor”. It’s a worthwhile read if you’re interested in the topic of the Big Bang, universal expansion, dark matter and energy, and modern astronomy. (Those who cringe in fear after attempting and giving up on A Brief History of Time need not fear; it’s quite accessible.)


Well, yes, but I can’t take a pond and lillies into a school, or any of the residential courses that I teach. :wink:

Which is what I was trying to convey.

How about this; take the example of a balloon, but instead of talking of taping pennies or drawing dots, make knots of the fabric of the balloon. When you blow up the balloon, the latex between the knots expands, but the material that is bound up remains the same size, other than being distorted to make the knot in the first place, so “local space” isn’t affected by the expansion pressure.

I’ll leave it an exercise of your patience to try to tie knots in a balloon. :dubious: