|Barred spiral galaxy NGC1672 [Courtesy NASA]|
The big bang cosmology theory received substantial confirmation in 1967 when two radio astronomers in New Jersey detected noise that appeared to be from the cosmos itself, since it was equally intense from all directions, not even varying when they pointed their antenna in the main direction of the Milky Way galaxy. A group of physicists at nearby Princeton University, led by Robert Dicke, immediately recognized that this noise was the primordial echo of the universe itself from 300,000 years after the big bang [Silk1989, pg. 82-85].
However, researchers immediately recognized some puzzles with the notion of a big bang. As mentioned above, observations of the cosmic microwave background found it was equally intense from all directions; other types of astronomical observations, such as "deep field" observations of distant galaxies, also have this property. But such observations raise the question as to how these different regions of the universe could be so well coordinated -- even extrapolating back to the big bang, these different regions should never have been close enough to communicate or coordinate, and communication faster than the speed of light is forbidden. This is known as the "horizon problem" in physics.
A theoretical breakthrough came in 1980 when Alan Guth, who at the time was a junior researcher at the Stanford Linear Accelerator, proposed the theory of inflation [Guth1997]. He proposed that in the first inconceivably brief instant after the big bang, the space-time fabric of the universe expanded by some 30 orders of orders of magnitude (i.e., by a factor of 1030), due to the operation of a "scalar field." In one stroke Guth solved several problems, including the horizon problem -- the reason the universe appears to uniform today is because we are all part of a tiny region shortly after the big bang that was "blown up" like a balloon, thus becoming much more uniform.
These difficulties had led even long-time supporters of inflation to question the theory's viability (although not the viability of the overall big bang cosmology). Paul Steinhardt of Princeton University is one vocal detractor. In 2012 he declared, "We thought that inflation predicted a smooth, flat universe. Instead, it predicts every possibility an infinite number of times. We're back to square one." Similarly, Sean Carroll, a cosmologist at the California Institute of Technology, explains, "Inflation is still the dominant paradigm, but we've become a lot less convinced that it's obviously true. ... If you pick a universe out of a hat, it's not going to be one that starts with inflation." [Gefter2012; Horgan2014].
These controversies were re-ignited with the March 2014 discovery of gravitational wave ripples in the cosmic microwave background radiation, which were heralded as the first solid experimental confirmation of the inflationary theory of the big bang [Overbye2014a]. Proponents of the multiverse were quick to point out the implications. As Alan Guth, the MIT physicist who founded inflation theory, explains, "Most inflationary models, almost all, predict that inflation should become eternal." Frank Wilczek adds, "There doesn't seem to be anything unique about the event we call the big bang. It is a reproducible event that could and would happen again, and again, and again." [Moskowitz2014].
But critics of inflation and the multiverse were also quick to sow skepticism. As Peter Coles of the University of Sussex, UK, writes, "Perhaps there is a part of the multiverse in which the BICEP2 results for inflation provide evidence for a multiverse, but I don't think we live there." [Moskowitz2014]. What's more, the BICEP2 experimental evidence itself, on which the March 2014 discovery was based, has been called into question. A team of researchers at the University of California, Berkeley argues that the "twisting" effect cited in the gravitational wave study could just as easily be accounted for as a result of dust in the Milky Way [Cowen2014]. The latest data points even more strongly to dust as an explanation, so the current consensus is that the BICEP2 data can say nothing about inflation one way or the other [Wolchover2015].
The latest Planck satellite observations are also casting grave doubt on the traditional inflation theory. As described by Ijjas, Steinhardt and Loeb, inflationary theories come in two varieties, which can be compared to two types of ski hills [Ijjas2017]. In one scenario, corresponding to the prevailing theory of inflation dating back several decades, skiers begin at the top of a hill, taken there by a ski lift, and then ski downhill to the valley in a steady, predictable fashion. The problem with this view is that the corresponding inflation theory produces hot and cold spots with larger variation than has been observed in the latest Planck data. What's more, this theory predicts that gravitational waves would have been generated strong enough to be detected today, yet they have not been detected.
In the other scenario, known as a "plateau model," the skiers must be dropped from a helicopter, using a parachute, and land within inches of a particular spot on a ridge, with just the right velocity; otherwise the skier will go off-track to the wrong valley. What's more, the slope is a complicated hill, one with a high risk of avalanche, and a flat ridge with a steep cliff down to the valley. Indeed, the plateau theories strain credibility with how precisely they must be "engineered" to correspond to currently available empirical data [Ijjas2017].
Given these and other difficulties, some researchers in the field (including some who were early pioneers and advocates for the inflation theory) are now wondering if it is time to seriously consider some completely different theoretical frameworks. One leading contender is a "big bounce" scenario, namely a dimly understood transition from a previous cosmological existence to the current universe. One advantage of these theories is that they avoid a stage where quantum effects dominate the physics. However, it should be emphasized that even "big bounce" scenarios do not negate the basic notion of a big bang -- our universe still started approximately 13.8 billion y ears ago as a very small volume of space-time fabric, which grew rapidly in side, proliferating to the universe we see today [Ijjas2017].
An excellent summary of these controversies is given in a 2011 Scientific American article [Steinhardt2011]; in a 2012 New Scientist article [Gefter2012]; and, most recently, in a 2017 Scientific American article [Ijjas2017].
In other words, the overall outline of the big-bang evolution of the universe, as summarized above, is on very solid ground, both theoretically and empirically. In this regard, the status of big bang cosmology is analogous to that of the theory of biological evolution on earth -- while there are many questions as to how the process started and what processes and laws governed very early epochs of the process, the overall history since the origin is hardly in doubt. See Origin.
The big bang and big bang cosmology have led to numerous additional scientific discoveries, some of them rather deep and far-reaching -- see
Big bang, Cosmic coincidences,
Cosmological constant and