It was new at the time, anyway. The model being spoken of is the Big Bang, first suggested by Father Georges-Henri Lemaitre in 1927. (The expanding-universe solutions to general relativity had also been derived by Alexander Friedmann in 1922, but he hadn’t emphasized the nature of the intial singularity in such models.) Lemaitre, a Belgian priest who studied at Harvard and MIT, proposed what he called the “Primeval Atom” or “Cosmic Egg” model of the universe, and derived Hubble’s law, two years before Hubble and Humason actually discovered that the universe is expanding. Einstein wasn’t all that fond of Lemaitre’s idea — having been assured by his astronomer friends that the universe was static — but he encouraged Lemaitre in his investigations.
All of which springs to mind because the Modern Mechanix blog has unearthed a Popular Science article from 1932 by Donald Menzel, an astronomer at Harvard, that explains Lemaitre’s ideas. (The time between Hubble and Humason’s discovery and Menzel’s article is somewhat less than the time between the 1998 discovery of dark energy and Richard Panek’s New York Times Magazine article from yesterday.) Menzel’s piece does a great job of explaining the basics of the Big Bang model, long before it was given that name by Fred Hoyle. Indeed, he touches on many of the questions that still arise in a good Cosmology FAQ! For example, he emphasizes that the redshift is due to the expansion of space, not to the Doppler effect.
The case of the universe is analogous, except that the expansion, being of a three-dimensional volume, cannot be visualized. The phenomena are, however, comparable. The nebulae are not running away from us. Their recession is due to expansion of space. This may, perhaps, seem to be quibbling over terms, since it amounts to the same thing in the end. Nevertheless, the distinction is worth keeping. According to the relativity theory, there is a difference between the running away of the nebulae and expansion of the medium in which they are imbedded.
Sadly, he also appeals to the much-hated balloon analogy for the expansion of the universe, although he uses the surface of the Earth rather than the surface of a balloon; in fact, it’s a better choice. And he’s not afraid of diving into the sticky questions, like “What happened before the Bang?”
DR. LEMAITRE’S hypothesis does away with the old query as to the state of affairs before the beginning of things. Going back to the parent atom we may inquire about what happened before the cosmic explosion took place. The answer is: — Nothing. – Computation shows that space would have closed up around the massive atom and, certainly, nothing can happen where there is no room for it to happen. Time has no meaning in a perfectly static world. The age of the universe is to be reckoned from that prehistoric Fourth of July, when space came into existence. Since then, space has been continually expanding before the onrushing stars, sweeping the way for them, forming a sort of motorcycle squadron to make room for the star-procession to follow.
Like many contemporary cosmologists, Menzel is a little more definitive about this than he really should be. When asked “What happened before the Bang?”, the correct answer is really “We don’t know. According to general relativity, space and time do not exist before the Bang, so there is no such thing as ‘before.’ However, we have no right to think that general relativity is correct in that regime, so… we don’t know.” Few people are sufficiently straightforwardly honest to give that answer.
And what about the future?
SO MUCH for the present. What of the future? Einstein and the noted Dutch astronomer, Willem de Sitter, have talked of some future contraction, which might sweep up the stars along with cosmic dust and eventually bring the world back to its original state. Dr. Lemaitre thinks that such a contraction cannot occur. He prefers to believe that the whole universe was born in the flash of a cosmic sky-rocket and that it will keep expanding until the showering sparks which form the stars have burned to cinders and ashes.
We still don’t know the answer to this one, but the smart money is on Lemaitre (and against Einstein, who liked his dice unloaded and his universes compact). Now that we know the universe is not only expanding but accelerating, the simplest hypothesis is that it will keep doing so. To be honest, of course — we don’t know!
Lemaitre passed away in 1966, a year after Penzias and Wilson detected the microwave radiation leftover from the Primeval Atom.
Nice post, but what’s wrong with the balloon analogy?
Hey–thanks for the plug!
😉
I’m confused by the bit in the New York Times Magazine article from yesterday where he quotes someone (unnamed) as saying we don’t even have a hypothesis to test regarding dark energy. Surely there’s some missing context there? The obvious hypothesis to test is that it’s a cosmological constant.
The balloon analogy is fundamentally confusing, because it gives people the idea that the universe has a center it’s expanding away from, and an outside it’s expanding into. Better not to use an analogy at all.
anon, I’m not sure what that quote was meant to say. I agree that the cosmological constant is the obvious hypothesis to test.
the idea that the universe has a center it’s expanding away from, and an outside it’s expanding into. Better not to use an analogy at all.
This is the part where my head explodes. I don’t understand this.
Far be it from me to disagree with the experts, but if you can convince people that the radial direction of the balloon “doesn’t count”, then isn’t it the best possible visualization of what it means to have expansion of space itself without a center? You get them comfortable with the idea of a 2D surface embedded in a 3D space and then continue the analogy to a 3D surface embedded in a 4D space etc.
Seems to me that the analogy is a good one if it comes with the added explanation. … the alternative being no explanation at all.
Not offering a metaphor at all merely invites people to come up with their own, which may well be worse.
All metaphors are imperfect. But they are how humans understand the world. Without them, we’re in serious trouble — especially on a subject such as the expansion of the universe.
George
The balloon kicks the raisin bread’s ass, though.
The balloon kick’s the raisin bread’s ass, no question. But both contribute a net negative amount to humankind’s understanding of cosmology.
Sometimes analogies are helpful; sometimes the excess baggage that comes along with them gives people a wrong impression that will never go away, and it’s just not worth it. In the case of the expanding universe, it’s much better to contemplate what is happening from the inside — as you are situated in the actual universe as it expands, galaxies receding from you in all directions — than to appeal to any analogy that makes you step outside and view some expanding object embedded in ordinary three-dimensional space. That’s the biggest single source of misunderstanding about the Big Bang model.
I’m guessing it was taken out of context and means that we don’t really have a physical model to test. We can measure the energy density currently, an whether it evolves over time (w and w’), but that still doesn’t give us a model derived from first principles which naturally has those properties. If we do eventually constrain w and w’ to be -1 and 0 respectively, then we get the cosmological constant, without any idea of what that means (our best model for producing a cosmological constant is after all wrong by 132 orders of magnitude).
I’ve always much preferred the raisin-bread analogy to the balloon analogy. A baking loaf of raisin bread is a nice 3d expanding medium that people can get their heads around. And it helps that the raisins are reasonably galaxy shaped. Get people comfortable with the idea that velocity should scale with inter-raisin distance, then generalize to an infinitely large loaf, and you’ve got a reasonably good analogy without the whole center/outside problem.
Damn updates while composing…
Explain please? I use this one to describe things to laypeople quite often.
The problem I’ve found with both the raisin bread and the balloon is that in both 2-D (which students are used to thinking in) and 3-D, the raisins and the pennies/marks are actually moving. I’ve usually found a better analogy by holding a rubber band between two fingers in one hand, and pulling it down with the other. Neither finger is moving, but if your motion is restricted to the surface of the rubber band then the distance between them has become larger as you pull down on the rubber band.
The problem with the raisin bread is that it has an edge, just as the problem with the balloon is that it has an inside and an outside. You can tell people “the universe is like the raisin bread, except without the edge,” or “the universe is like the balloon, except three-dimensional and with no inside or outside,” but you’ve already lost the battle. When explaining the expanding universe, the single most difficult concept to get across is how space can expand without expanding into anything. Trying to analogize that with something that is expanding into a larger space, and then tacking “but ignore the outside” onto the end of your explanation, is rarely productive.
I guess the reason I defend the raisin bread is that it was the first analogy that gave me that “aha” moment in an undergrad astro class, where I really felt I understood how to think about an expanding universe. But perhaps it’s better reserved for physics students, who are more likely to be capable of mentally generalizing to an infinite loaf.
I do think it’s essential in using either metaphor to (1) get people to take the point of view of an observer on one of the raisins/pennies, (2) generalize to an infinitely large object at the end of the explanation, and (3) point out that there is no bread/balloon “medium” doing the expanding in the actual universe. This has the advantage, at minimum, of emphasizing that the analogy is quite inexact. Those who can imagine such things will hopefully get roughly the right idea; those who can’t will hopefully at least recognize that they’ve got a bad mental model.
I just think that most people have a really hard time understanding how an apparent velocity-distance relation corresponds to a uniform expansion and that some sort of analogy is essential. But, lowly grad student that I am, I have substantially less experience teaching these things than the bread-bashers, so perhaps I’ll change my tune someday.
For the sake of learning about more potential pitfalls, are there any problems with the expanding-map analogy to explain the distance-redshift relation? In brief: imagine that the United States were to double in size overnight. From my vantage point in Berkeley, Julianne will appear to have moved substantially farther than Sean, in the same amount of time, so she appears to have a much faster velocity than Sean–but all of us are actually stationary with respect to the expanding country–so the distance-redshift relation is just a geometric consequence of uniform expansion. Any serious conceptual problems introduced here?
I guess the reason I defend the raisin bread is that it was the first analogy that gave me that “aha” moment in an undergrad astro class, where I really felt I understood how to think about an expanding universe. But perhaps it’s better reserved for physics students, who are more likely to be capable of mentally generalizing to an infinite loaf.
This is an interesting point, especially in light of present state-of-the-art cosmological models, but one possible problem with such a generalization is that there’s nothing that absolutely necessitates that the universe be infinite in extent. The balloon surface analogy does demonstrate how something can be finite in extent while lacking a center or boundary.
I agree that it’s a useful analogy for this precise concept , but I think I agree with Sean that it’s a lousy analogy for the expanding universe. My point, which I am articulating poorly, is that analogies like the balloon or the bread can be very useful in explaining certain specific concepts when they are used judiciously and when their failings are very explicitly pointed out to the listener. I’m not ready to abandon them altogether just because being half-assed or overly broad in using them can lead to misunderstandings. Of course it can.
I actually think both balloon and raisin bread are useful, and use them myself. They certainly both have shortcomings, but my personal experience is that the net effect is positive.
Sean, you are advocating a pretty ascetic approach to teaching cosmology! When you were first learning about expanding space way back when, did you not have a metaphor of some sort? The misconceptions you describe — of edges, of a spurious center to the expansion — are real but secondary. If you can’t even convey the idea of expanding space, then all those secondary points are moot.
One of the funny things about learning is that you sometimes have to work with approximate or technically incorrect ideas first, before bootstrapping to the real stuff. The same might be said of the development of science itself. Where would Einstein have been without Newtonian mechanics?
George
I am not against metaphors, or analogies, or approximations. I am against this particular metaphor, the balloon, as an attempt to explain this particular concept, the expansion of space. I think that any attempted explanation that puts the person standing outside of the expanding universe is more counter- than productive. Your mileage may vary.
OK, then, what metaphor should we use?
George
I don’t know of any good metaphors for the expansion of space. I just talk about the real thing, and people seem to like it.
I think the balloon analogy is fine. It’s a nice visualization of expansion without a center. The center you refer to can only be the center of the balloon itself. What’s inside the balloon? The past. The center is the singularity. What’s outside the balloon? The future. The analogy becomes much better once you recognize the radial direction as time, the balloon itself being some (closed) spacelike surface in a 3d spacetime.
But that’s the thing (and I’m aware that in this forum, I sound very stupid) that I can’t get my brain to grasp. It’s frustrating. What is it expanding into? If it’s expanding into nothing, then that’s at least something.
The whole thing feels like a Terry Gilliam movie.
One other problem with the much used balloon analogy is that it plants the idea of a closed universe in the minds of people. Many lay people (who correctly understand the analogy) wrongly think that the universe must be finite.