In past posts, when I’ve written about the expansion of the universe, I’ve generally referred to the rate of that expansion, the Hubble constant, as being around 70 km/s/megaparsec, that is, for every megaparsec a galaxy is distant from us, it’s moving away at 70 kilometers per second faster. So a galaxy 100 megapasecs away is moving away at 7000 km/s, and one 200 megaparsecs away at 14000 km/s.
But I have to admit this was an oversimplification. 70 km/s was actually a rough and rounded averaging of two measurements for the expansion, one taken using the cosmic distance ladder, and the other using observations of the cosmic background radiation. The former currently yields about 74 km/s, and the latter about 67 km/s.
Everyone for a long time thought this difference was just a measuring artefact that would eventually be smoothed out. Everyone is turning out to be wrong. As this news story discusses, the two measurements have been refined extensively. Confidence in these individual measurements are pretty high, and the margins of error don’t overlap.
In other words, either one or both of these methods has assumptions in it that are wrong, or there is something completely unexpected going on in the universe that cosmologists haven’t yet accounted for. For most scientists, this is a reason for excitement. This kind of issue typically leads to new insights.
However, it’s led to a debate that someone has been asking me to comment on. Bjorn Ekeberg, a philosopher of science, has focused in on this problem, along with others, to assert that cosmology has some big problems, calling into question the overall big bang cosmology. This drew a response from cosmologist and science writer Ethan Siegel pushing back against Ekeberg’s claim and accusing him of being anti-science. Ekeberg has responded accusing Siegel of being a “temple guard” for big bang cosmology.
Name calling aside, who’s right here? Not being a professional physicist, or knowledgeable enough to read raw physics papers, my comments are inevitably based on what various science writers have provided.
But in considering Ekeberg’s position, it’s worth reviewing the evidence for the overall big bang model. Physicists in the 1920s figured out that, under general relativity, the universe could not be static. It had to be either expanding or collapsing. If it was expanding, it was smaller yesterday than today, and smaller the day before. Following that back led to a period in the past where everything was all bunched up together, very dense and very hot. The physics of the early universe could be mathematically deduced and predictions made. (This led Einstein to fudge his equations a bit by adding a cosmological constant, making the universe it predicted static.)
Then in the late 1920s, Edwin Hubble discovered that the light from every galaxy beyond a certain distance was red shifted, with the amount of red shift being proportional to the distance. Red shift is a doppler effect that happens when something is moving away from the observer. Hubble had discovered that the universe is indeed expanding. (Einstein concluded that the cosmological constant was his biggest blunder.)
Still, cosmology was slow to just accept the big bang model. (It didn’t help that Hubble’s early estimates of the age of the universe had it younger than geologist estimates of the Earth’s age.) It continued to be debated for decades, until the discovery of the cosmic background radiation in the 1960s, which provided evidence for the calculations of the physics of the early universe. That was enough for most cosmologists. The big bang became settled science.
As a lay person, reading this through the translations of the experts, classic big bang cosmology seems pretty solid. I think Ekeberg, by implying it isn’t, oversells his thesis. But it’s worth noting that this settled version doesn’t get into what caused the big bang in the first place.
Ekeberg also has issues with the ideas of dark matter and dark energy. My understanding of these terms is that they’re essentially place holders, labels for our ignorance. So criticism of them as theories has always struck me as premature.
The most often touted alternative to dark matter is MOND (modified Newtonian dynamics), but no simple modification to the equations seem able to account for all the observations. Whatever is causing the rapid rotation of galaxies and other intergalactic effects seems to require something that is present in varying densities or intensities. Dark matter may eventually be so different from matter as we understand it that the word “matter” might not be appropriate, but until then, the term really just refers to something mysterious causing varying gravitational effects.
This seems even more true for dark energy. The fact that, against all expectations, the expansion of the universe is actually accelerating rather than decelerating, has to be caused by something, some form of unknown energy. (Ironically, dark energy has resurrected Einstein’s cosmological constant.)
Granted, it does seem unnerving that this results in 95% of the matter and energy in the universe being unobservable and unaccounted for. It’s easy to take this number and other measurement issues and accuse cosmologists of not knowing what they’re doing. Easy, but I think facile. The widely accepted theories that we now have are grounded in observation. Anyone is free to propose alternatives, but to be taken seriously, those alternative have to account for at least as much of the data as the current theories.
I do think one area where Siegel is overconfident is cosmic inflation. I’ve written about the concerns on this before. Some version of inflation might turn out to be true, but I think his stance that it’s a settled issue isn’t justified yet. And the fact that a significant portion of physicists are starting to question inflation, including some of its earliest supporters who now say it generates more issues than it solves, should make the rest of us cautious in our stance toward it.
So, does cosmology have issues? Of course, and Siegel admits as much. But is the overall big bang cosmology model in crisis as Ekeberg seems to contend? I think this is vastly overstating the issues. But only time and the data will tell. Of course, this controversy will likely lead to more sales for Ekeberg’s book.
What do you think? Is the overall big bang model in trouble? Or is this just about fine tuning the details, such as the age of the universe? If it is in trouble, what might replace it?