March 18, 2010

Big Bang for beginners-5: Some conceptual challenges

(My latest book God vs. Darwin: The War Between Evolution and Creationism in the Classroom has just been released and is now available through the usual outlets. You can order it from Amazon, Barnes and Noble, the publishers Rowman & Littlefield, and also through your local bookstores. For more on the book, see here. You can also listen to the podcast of the interview on WCPN 90.3 about the book.)

For previous posts in this series, see here.

Although the story of the Big Bang in its essence is quite simple and straightforward, it contains many fascinating subtleties that are worth exploring further. It is good to get some conceptual hurdles and misconceptions out of the way right now.

When we use the words 'Big Bang' it immediately conjure up certain images. We immediately think of familiar explosions, like bombs or firecrackers going off. We envisage a big noise and the exploding pieces hurtling away from the center of the explosion and spreading out into the surrounding space at great speed. This image captures correctly the idea of a hot compressed beginning with a fixed amount of matter spreading out through space and getting cooler and more dilute with time. But there are important ways in which the image is inaccurate.

One simple misconception is to think there was a loud noise at all. The very idea of sound at those huge densities is highly problematic and it is not helpful to think in those terms. But this is a minor misconception. The major misconception that people have is the idea that space always existed and extended all the way to infinity and that the Big Bang occurred in one small region of it and the matter that was created then spread out to fill increasing amounts of that pre-existing space.

What the theory actually says is that the only space that exists is the space occupied by the matter produced in the Big Bang and that, as the matter spread out, it did not fill already existing empty space, but instead it was space itself that was expanding, carrying the matter along with it.

To better understand this difficult idea, a good analogy is raisin bread baking in an oven. The raisins occupy more-or-less fixed positions in the dough. As the bread bakes, the dough expands, carrying the raisins along with it.

The wrong way to interpret this analogy to the Big Bang theory is to think of the dough and raisins as the matter expanding into the pre-existing space of the oven.

The correct way to view the analogy is to think of the bread dough as being space and the raisins as the matter. As the bread bakes, the dough (i.e., space) expands carrying the raisins (i.e., matter) along with it. The hard thing for people to grasp is that there is no space outside of the dough. There is no oven for the dough to expand into. So the 'explosion' we speak of is not of matter expanding into space but of space itself expanding.

In addition to the motion associated with the expansion of space, there is also what we call local motion caused by the forces between objects. So for example, Earth and the planets orbit our Sun under the influence of gravity, and our solar system rotates in the spiral arm of our galaxy the Milky Way, again under the influence of gravitational forces. Protons and neutrons in nuclei move under the influence of nuclear forces and electrons in atoms move under the influence of electromagnetic forces. All these motions are due to forces acting locally and not part of the motion caused by the expansion of space itself. Back to our raisin bread analogy, the raisins are not rigidly embedded in the dough. In addition to the raisins being dragged along by the expanding dough, they may also move around slightly in the dough due to (say) air pockets near them. But when we speak of the motion associated with the Big Bang, we are referring to the motion due to the expansion of space and not these local motions.

(It should be borne in mind that the well-known assertion that nothing can travel faster than the speed of light is popularly interpreted a little too broadly. That limit applies to the speed of particles and information flow. But there are things like the collapse of the wave function and the phase velocity of wave packets that occur at speeds greater than the speed of light. As I described yesterday, in the case of the early universe, space expanded at a much faster rate than the speed of light but that too is allowed by the theory of relativity. In other words, the dough can expand faster than the speed of light but the speed of the raisins relative to the dough has to be less than the speed of light.)

One consequence of the view that the Big Bang consists of space itself expanding is that it did not occur at a point 'in' space (like a blob of dough in an oven) but occurred everywhere in space simultaneously, and that it is space itself that was initially compressed. So it does not make sense to look for a point in the universe where the Big Bang occurred and treat it as the 'center' of the universe. There is also no 'edge' or boundary to the universe, so it does not make sense to ask what exists beyond the edge either.

I will come back to these last points later because they are undoubtedly hard to grasp, especially the idea about the absence of a boundary.

POST SCRIPT: Will Ferrell tries out for a part in West Side Story

Ferrell Sings "West Side Story" on Letterman from Will Ferrell


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>In other words, the dough can expand faster than the speed of light but the speed of the raisins relative to the dough has to be less than the speed of light.

So, what is the speed of a particle relative to "the dough?" Is that even a meaningful concept? Can it be measured, in theory? I thought the constraint was that the particles cannot exceed the speed of light relative to each other. In that case, what would be the apparent speed, relative to an observer on one particle, of the other particles during the initial rapid expansion? Does light get red-shifted by expansion of space?

Posted by Robertallen on March 18, 2010 03:27 PM


I should have been a little more precise in my words. I should have said that the limit is on the speed of the raisins relative to the other raisins. It is this speed that cannot exceed that of light.

I fell prey to the lazy habit of defining the space in which speeds are measured as being framed by the objects in it, the way we use the objects on the Earth to define the "space" in which speeds on Earth are measured.

So instead of speaking of the speed relative to the dough, I should have said the speed relative to the frame of reference defined by the raisins embedded in the dough.

Think of a ruler embedded in the dough and connecting two raisins. As the dough expands, the ruler will also expand. So a raisin that is fixed in the dough will have zero speed because it will occupy the same position on the ruler even though looking from the outside the distance between the raisins will have increased. For a raisin to have a non-zero speed, it will have to move relative to the ruler/dough.

As to the red-shift question, I will quote Wright on this:

"In one view, the spatial positions of galaxies are changing, and this causes the redshift. In another view, the galaxies are at fixed coordinates, but the distance between fixed points increases with time, and this causes the redshift. General relativity explains how to transform from one view to the other, and the observable effects like the redshift are the same in both views."

Posted by Mano on March 18, 2010 04:49 PM

Mano - brilliant analogy! Really helps with my (non)comprehension expressed in the post I just left on Number 4. I look forward to trying to understand non-boundaries, but this explanation has really added a new dimension (pun intended) to my understanding of the universe. Expanding space! Wow! It is such a fundamentally different perspective on what is happening - galaxies aren't moving away from each other, the space they are in is!!! Mate, I really thank you for this insight! So much falls into place when you think about it this way (non-centre of the universe, for example, which I never really got before).

Posted by Bill on March 18, 2010 09:01 PM

Interesting article. Thank you for sharing. Does science have an answer to the question of the source of the forces that caused the big bang?

Posted by Sydney Bookkeeping Services on March 19, 2010 07:07 AM

Great example of analogy! I really like it! Recently I started posting interestnig analogies I found on the web on I thought it could be a good idea to create a place where people can help each other to find useful analogies so I created a simple site ( Check it out!

Posted by Peter on March 21, 2010 08:59 AM

Ok, you just explained something that I've been wondering about for a very long time. Maybe they shouldn't call it the "big Bang" then. Maybe they should have called it the "big expansion" or something like that. I still don't understand where the matter cam from intitially. Something must have put it there to begin with?

Could you pls help me out with this? Also, I liked your bread baking example. Since I write about bread machines that was pretty interesting.



Posted by Bread Machine on July 24, 2010 06:57 PM

I have to admit, I did prescribe to the to the motion that the Big Bang expanded into space. Although it is hard to get your head around the concept that there was nothing, and then in that instant the universe, space and time and the laws of physics, everything, came into being in such a sort space of time.

Posted by Paul on May 25, 2011 05:04 AM

I agree with Paul. Still hurts my head to try and imagine there being nothing at one point.

Posted by Greg on June 13, 2011 08:12 AM

"The big expansion" as opposed to the Big that's interesting. Anyone care to comment on how they would define "the expansion" and how that would be so different from the big bang. I'm just curious...

Posted by Zach on June 21, 2011 05:14 PM

For me, an astronomy not really understand, you talk about some I can not understand.But you give the example of bread in the oven, it is easy to understand, so I can imagine what happened in the universe.From the initial Big Bang to the universe's expansion, it seems that I had there some kind of fallacy.Universe has a boundary, any space has a boundary, I think so.

Posted by amani on August 21, 2011 11:10 PM

Good article. You have a way of explaining things well especially with the analogy.

By the time humans have come even close to fully grasping how the universe started, the human race will probably be almost extinct by then.

Posted by Steve on August 23, 2011 07:22 AM

One thing that I always wondered about was how could the big bang arise out of nothing. Surely some form of matter had to exist in order to create an explosion in the first place, at least that is what I thought until I heard what Stephen Hawking had to say.

Essentially, Hawking states that before the big bang time did not exist, thus without time there cannot be matter, since there is no time for matter to exist in. Therefor it is possible for something to arise out of nothing, and time started from the moment that big bang went off.

Very interesting stuff indeed...please keep posting more!

Posted by Tim on December 13, 2011 02:00 AM