3.2. The cosmological principle has been falsified
The cosmological principle is the longstanding notion that our little piece of space and time shouldn’t be special. We should, taking the big picture, be quite average. Keel 2007 states the principle as follows:
“Viewed on a sufficiently large scale, the properties of the universe are the same for all observers.” This amounts to the strongly philosophical statement that the part of the universe which we can see is a fair sample, and that the same physical laws apply throughout.
This principle suggests that when we look out at the universe it should look generally the same in every direction. And similarly with time: when we look backwards and project forwards we should expect more or less the same universe as we see now. We cannot, of course, look out at the universe without looking backwards in time, but we can project the future, and under the cosmological principle the future should generally look the same as the present and the past. Or so the principle supposes.
The cosmological principle was a reasonable assumption in our first efforts at developing modern empirically-informed cosmological theories because with a sample size of just one—our little planet and our human species constituting the only example of intelligent life that we know of at this point—we should indeed assume that the rest of the universe is essentially like our neighborhood of the universe, until proven wrong . That is, we shouldn’t assume that things are radically different outside of our particular milieu because all we know with any intimacy is our own milieu, until we have good evidence to suggest otherwise. This is really just common sense.
It is becoming increasingly apparent, however, that the cosmological principle is inaccurate, both spatially and temporally. There are a number of very large structures in our universe, including the CMB just discussed as well as many others, that seem to contradict the cosmological principle. The largest baryonic structure, and most recently discovered, is the Hercules-Corona Borealis Great Wall discovered in 2013 by Horvath, et al. (Horvath, 2015; Bagoly et al 2015). This structure is 2-3,000 megaparsecs (Mpc) in size. The Sloan Great Wall was the previously largest structure at about 400 Mpc.
Even larger structures in the background energy structure of the universe, the CMB discussed above, have been found and dubbed playfully “the axis of evil” or AOE, because of the implications of this very large-scale structure for the standard ΛCDM model of cosmology. The AOE, as its name suggests, is a literal axis that extends through the entire universe, showing that there is, if the data and its interpretation are accurate, an identifiable orientation to the universe. Examining the Wilkinson Microwave Anisotropy Probe (WMAP) data in a 2006 paper, Wiaux et al. conclude that “nothing at present allows us to discard the possibility of a global universe anisotropy, simple violation of the cosmological principle hypothesis.”
Land and Magueijo 2008 states in reviewing the evidence for the AOE: “it must be said that while everyone agrees on the presence of the ‘axis of evil’ in the data, its extent is still debated.” Liu et al 2016, in reviewing new data on the AOE, supports the existence of the AOE and states: “If the anomalies are not caused by foreground residuals or systematic effects, we are facing a challenge [in our] understanding of fundamental physics and the nature of the cosmos.”
These matter and energy structures, if new data continue to support their existence, should be considered strong support for refuting the spatial cosmological principle because they show that the universe is not isotropic at the large-scale.
Temporally, matters are not quite so clear since our theories of cosmic evolution are themselves still evolving. However, under the Big Bang cosmology, the universe does not seem to be, in the flat universe that we are thought to be living in, temporally symmetric either. This issue is deep and highly debatable, as Carroll’s excellent book, From Eternity to Here , describes (Carroll 2010). Carroll makes a case for temporal symmetry based on a cyclic universe model in which the past and the future do in fact look essentially the same. Carroll acknowledges that his arguments are new and speculative, and we are assured of much thriving debate on this issue for years to come. However, even if the universe is temporally symmetric (at very large timescales) we already know, based on the arguments above, that it is not spatially symmetric. Based on this reasoning alone, we can conclude that the Cosmological Principle is very likely not accurate, and should probably be considered falsified.
This information supports the notion that there is a preferred reference frame with a definite orientation in space and time – the Cosmic Microwave Background and the large-scale matter structures just discussed – which further weighs against SR’s view that there is no preferred frame.