I’ve written a fair amount about what we do and don’t know about what makes up 95% of the universe. Besides yesterday’s “Ask a Physicist column” in io9, see here, here, or here. Yesterday’s column on dark energy was, on the whole, pretty well received, but there were an enormous number of comments along the lines of (and I’m paraphrasing here):
- “People believed in the Aether once, too.”
- “In a hundred years, scientists are going to think you guys are idiots.”
- “How is this any different than explaining things by invoking gods?”
- “I think they made a mistake somewhere.. Or they have the wrong premises. They need to re-evaluate.” (which is an actual comment)
and so on.
I have a strict “Don’t feed the trolls” policy, since it rarely does any good, and more often simply puts ridiculous arguments on equal footing with well-grounded ones. I’d also like to point out that the title of my column was, “Are physicists just making up dark energy?”, replacing the slightly more antagonistic first draft version, “When it comes to dark energy, are physicists dumb or just lazy?” The point is that I know that introducing dark energy (and dark matter, for that matter) seems like a kludge. I realize that the simplest theoretical models are off by a factor of a googol from the observed densities. In fact, I made those very points from the outset.
But what people (okay, some people) don’t seem to understand is that when physicists introduce concepts like dark matter or dark energy, it’s at a much more precise level than simply saying, “Oh, the universe is accelerating? There must be a mysterious substance making that happen. Next question!”
We instead hypothesize the following, with the specific numbers determined from observation:
- There is a field permeating the universe which makes up approximately 70% of the critical density of the universe
- It has an equation of state (pressure/energy density) of -1.
From this, we make a huge number of very concrete and testable predictions, including:
- The age of the universe (and the expansion factor at any time in the past)
- The brightness of all standard candles at any redshift (distance) from the earth
- The angular size of all standard rulers at any redshift from the earth
- (Along with the dark matter determination) The shape of the universe.
- The linear growth rate of structure at all points in the past.
This means that we’re able to correctly predict all sorts of quantifiable things, from gravitational lensing tomography to the angular scale of the first acoustic peak in the CMB to the supernova redshift-magnitude relation to the number of high redshift clusters to the SZ effect and on and on.
The standard Lambda-CDM cosmology explains all of these things pretty much perfectly. When people make asinine statements like, “Off the top of my head, I can think of 10 different things as dumb as dark energy to explain the perceived acceleration of the universe,” it makes me deeply sad. They clearly don’t get that we’re actually doing predictive science here.
Now, what is dark energy? I don’t know. That isn’t to say that I don’t care. I do, deeply. But the fact remains that we can make all sorts of predictions about our universe without knowing exactly what DE is, and ditto with dark matter. When dark energy is ultimately explained, I personally don’t care whether it turns out to be a cosmological constant, some sort of vacuum energy, or some other sort of field. If all of them make the same predictions, then they are identical from an observational perspective. However to imply that not knowing what, exactly, dark energy is means that it has no more validity than attributing nature to gods or fairies is, I’m afraid, quite ignorant.
It is, of course, the case that something like variable physical “constants” may turn out to up-end our current Lambda-CDM model. But such theories as they are now are completely ad hoc. If you have a problem with an arbitrary (but very simple) field permeating the universe, why, exactly don’t you have a problem with the speed of light arbitrarily knowing how to change throughout the age of the universe?
And what’s more both dark matter and dark energy have strong historical/scientific precedents. Would you have believed in the existence of neutrinos based only on the recoil of the electron from a neutron decay? Would you have believed in anti-matter based on Dirac’s attempts to linearize relativistic quantum mechanics? And dark energy does have laboratory analogs. Vacuum energy has all of the properties (albeit at a much higher energy density) of dark energy; why is it so strange to imagine that the universes is pervaded by a fluid of the same equation of state?
I was chatting with some of the grad students in my department, and it occurred to me that if members of the public were really cognizant of what constitutes a “detection” of a top quark or the Higgs (should it be found) they’d be astonished. We don’t observe these particles directly — we infer them based on recoils and decay detritus. Indirect measures can still tell us a hell of a lot.
I get particularly riled because io9, slashdot, physorg and others report on almost every crazy theory that someone comes up with, and as a result people have become so jaded that they think these theories have the same stature and support as things like dark matter and dark energy. They don’t.
And yes, I normally like to take the high road, but sometimes ignorance about how science really progresses is just too much to take.