Galileo's Pendulum

Successful scientific theories are strong structures, comprised of many parts and predicting many measurable things. Like a building, they can have parts rebuilt, additions constructed, or even parts demolished without ruining the essential structure. Yet, it’s a very human habit to think that a challenge to one portion of a theory will bring the whole thing down immediately, or that the way to be successful is to come up with a whole new theory to explain something that’s well understood with an existing model.

A case in point: in the last few months alone, I’ve seen several new theories attempting to overthrow or revise the standard cosmological theory, the Big Bang model. These include several highlighted in popular publications, such as “Cosmologist claims Universe may not be expanding” (Nature‘s News) and “Our universe may contain TARDIS-like regions of spacetime” (io9). However, it strikes me that these theories are formulated to solve problems that, well…aren’t really problems.

The Big Bang has stood for decades as the most widely accepted model of the structure and evolution of our Universe. The basic idea is simple: galaxies are all getting farther apart from each other on average, with the rate of that growth (roughly) proportional to the distance between any two galaxies. That means a galaxy far from the Milky Way appears to be moving away from us at a faster rate than one close by; doubling the distance appears to approximately double that rate. The Big Bang extrapolates from that information to say that, if galaxies are far apart today, they must have been much closer in the past. At some point in the distant past, all matter in the observable Universe must have been very tightly compressed. All of this is what we usually mean by “the Big Bang”.

Of course, there’s more to cosmology and even the Big Bang model than that. The expansion of the Universe is accelerating due to the influence of dark energy, and about 80 percent of all the mass in the cosmos is made up of invisible dark matter — both things discovered observationally rather than intrinsically part of the Big Bang. The Universe is also remarkably similar in every direction (isotropic) and of roughly uniform density on the largest scales (homogeneous). These aspects aren’t inevitably linked: older cosmology books are full of alternative ideas that have mostly fallen away as the data have improved.

While the details have emerged and changed over the decades, the Big Bang itself has survived. Observations of a variety of objects and phenomena have showed that the Universe is expanding, while also verifying that things were much hotter and denser in the past. The discovery and subsequent measurement of the cosmic microwave background (CMB) helped establish the Big Bang model’s superiority over competing ideas, since it showed the Universe was hot enough in the past to ionize every atom and dense enough that the cosmos was opaque. Similarly, the theory known as Big Bang nucleosynthesis (BBN) predicted the measured abundances of the lightest elements fused in the hot dense environment that constituted the entire Universe during its first few minutes. (These elements are completely separate from dark matter and its “controversies”.)

“No one will expect a frontal attack!”

I have pitches to real publications (i.e., ones with editors) about both of the papers I mentioned above, so to keep from scooping myself, I won’t discuss them in detail here. However, they both seem to approach standard cosmology on strong sides rather than places where it could conceivably break down: the expansion of spacetime and the homogeneity of the Universe. That’s not to say that these approaches would always fail, but observations are so well established that it’s odd to think we’d be served well by challenging them with radical new ideas.

Any alternative to standard cosmology has a big list of observations to explain. That’s not just expansion of the Universe and its acceleration, but also BBN, the cosmic microwave background (which includes stuff like dark energy and dark matter in its power spectrum!), the distribution of galaxies, and so forth. All these data may seem like small things individually, but together they’re pretty formidable. Sure, you might be able to explain why galaxies appear to be moving away from us with an alternative theory, but that’s not enough to be a viable cosmology without taking those other things into account. The “Tardis” spacetime mentioned above is a pretty cool sounding alternative to dark energy, but there’s a huge chunk of the cosmic microwave background power spectrum that needs explaining if it’s correct.

Again, that’s not to say that the Big Bang model as it stands is perfect. There are some small nagging problems (e.g., the “axis of evil” in the CMB power spectrum) and some substantial mysteries that may yet yield new theories of the Universe. However, it’s important to distinguish between the question of (say) dark energy’s identity and its existence. The former is a big challenge to us, while saying dark energy doesn’t exist brings up a slew of other difficulties. It’s sexier to try to bring down the whole edifice of modern cosmology (or at least a big chunk of it), but it’s also the most difficult — even dangerous — way to achieve success. There’s a reason the Big Bang theory and its attached theoretical apparatus is widely accepted: it explains what we observe very well. A little caution and examination of the whole cosmological blueprint might be wise before wielding the sledgehammer of demolition.