Many of our scientists have a very lamentable tendency to pretend to know things they do not actually know. We see this massively in the world of biology. But we also see it in the world of cosmology, the study of the large-scale physical universe.
For more than 20 years, scientists have presented in their publications and articles a “composition of the universe” pie chart. By presenting such a chart, they are purporting that they understand the makeup or composition of the universe. The typical “composition of the universe” pie chart looks like this:
Source: NASA / WMAP Science Team
Notice the precision in the chart, including the claim that exactly 71.4% of the universe consists of dark energy. You would never suspect from such precise numbers that such numbers are just shots in the dark or wild guesses. But that's exactly what they are. No one has ever observed any particles of dark energy or any particles of dark matter. Scientists have been searching for such particles for many years, without success. Neither dark matter nor dark energy has any place in the Standard Model of particle physics.
Claims that dark matter exists are based mainly on the fact that existing theories of gravity do not explain observed behavior of stars as they revolve around the center of the galaxy. The hypothesis of dark matter is only one of many possible explanations for such a discrepancy. There are alternate explanations such as the idea that gravity may work in ways more complicated than we imagine. We don't actually know whether dark matter exists.
Claims about the existence of dark energy are based on evidence that the expansion of the universe is accelerating. It was in the late 1990's that there first arose the belief that the expansion of the universe is accelerating. Around 1998 two groups announced that after studying supernova explosions in distant stars, the groups had come to the conclusion that the expansion of the universe was accelerating. It was claimed that we must believe in dark energy to explain this acceleration of the universe's expansion. Such a claim was always suspect, as dark energy is only one theoretical way to explain why a universe might have an accelerating expansion. But nonetheless, members of these groups (Perlmutter, Schmidt and Riess) won the 2011 Nobel Prize in physics.
But now Subir Sarkar and his colleagues have written a paper (accepted for publication) calling the evidence for dark energy into question. Sarkar and his colleagues have re-analyzed the supernova data, and have concluded that the expansion of the universe is probably not accelerating. The scientists claim that there is a motion of our local region of space that was not recognized by Perlmutter, Schmidt and Riess, and that such a motion caused the scientists to reach the wrong conclusion. Sarkar and his colleagues say, “Thus the cosmic acceleration deduced from supernovae may be an artefact of our being non-Copernican observers, rather than evidence for a dominant component of ‘dark energy’ in the Universe.” If Sarkar and his colleagues are right, two of the top 10 claims of cosmologists (that dark energy exists and that the expansion of the universe is accelerating) are unfounded.
The issues are highly technical, and it will be some time before the dust settles and we find out which group is right on this matter. For the time being, I may merely note that this seems to be further evidence of the strong tendency of scientists and science writers to start declaring certain things as facts long before any very clear proof has appeared for the things they are calling facts.
In the article here, we have an example of such overconfidence and dogmatism. Fraser Cain (publisher of the Universe Today web site) claims that scientists “know” that dark energy exists. He claims that there are “multiple lines of evidence,” and lists some space probes (Wilkinson and Planck) that studied the cosmic background radiation. He states:
“NASA’s Wilkinson Microwave Anisotropy Probe studied the Cosmic Microwave Background Radiation of the Universe for 7 years, and put the amount of dark energy at 72.8% of the Universe. ESA’s Planck spacecraft performed an even more careful analysis and pegged that number at 68.3% of the Universe. Astronomers know that dark energy exists. There are multiple lines of evidence.”
But Sarkar is quoted in Physics Today as stating that the cosmic background radiation “does not directly measure dark energy” and that the idea that it does is a “widely propagated myth.” On this point he is right. The idea that we can understand the composition of the universe by studying the cosmic background radiation makes no sense at all. Being uniform to about 1 part in 10,000, such radiation teaches us very little.
There are quite a few “widely propagated myths” spread about by scientists, whose teachings are often the unproven dogmas of their cherished belief system. Cain has made the mistake of thinking that we can know something is true merely because “there are multiple lines of evidence.” Each of multiple lines of evidence can be weak, and they may add up to no convincing basis for believing in something. A hundred different dubious beliefs can be supported by "multiple lines of evidence" if there are enough people trying to look everywhere for such evidence. For example, if you believe that extraterrestrials are currently visiting us, you can point to "multiple lines of evidence" such as UFO sightings, alien abduction stories, strange cattle mutilation incidents and crop circles.
Cain has also failed to see that an estimate of something is not a determination of something. The fact that two teams of scientists estimated a particular level of dark energy did not mean that they had done anything to establish that such a level exists. An examination of the paper on dark energy published by the Planck team should give you no confidence that the team has any kind of good understanding of how much dark energy there is. The paper merely claimed to be studying "the implications of Planck data for models of dark energy," not to be providing evidence that dark energy exists.
When asked about the composition of the universe, the only honest answer for a scientist or a science writer to give is to say that he does not understand such a thing. But instead our scientists have again and again spoken as if they knew things they did not know: that dark energy exists and that dark matter exists. Such dogmatic overconfidence takes a particularly absurd form when we see graphs like the one at the top of this post, in which scientists make claims such as the claim that the universe is “71.4% dark energy,” when they are not even sure that dark energy exists. That's kind of like saying that your girlfriend will live as your wife for 40.7 years before you even ask her to marry you.