Scott Dodelson on dark matter and modified gravity (guest post)
Following the recent incident, we and the SciLogs team decided to invite a renown colleague to write a guest blog post. Thinking about possible guest bloggers who are experts in the field of cosmology and approach theories such as MOND with the necessary scientific skepticism, we arrived at Scott Dodelson as one candidate.
Scott is a very well-respected cosmologist. He is a scientist at Fermilab and a professor in the Department of Astronomy and Astrophysics and the Kavli Institute for Cosmological Physics at the University of Chicago. His research focuses on the largest and smallest scales of the universe: the interplay of cosmology and particle physics. He investigates the nature of dark matter and dark energy, works on the cosmic microwave background and is also interested in modified gravity theories. In addition to his many papers, he has written the textbook “Modern Cosmology”.
We are very pleased that Scott Dodelson has accepted to write this guest post. Thank you, Scott!
Is modified gravity a viable alternative to dark matter? Or is dark matter so compelling that pursuits of modified gravity should be abandoned?
There are good reasons to believe in dark matter and to be optimistic about our chances of detecting it in the coming decade. Dark matter explains the flat rotation curves in galaxies; it accounts for the deflection of light far from the centers of galaxies and by galaxy clusters. Many aspects of galaxy clusters make sense only if dark matter is present. Perhaps most importantly, it is the key component in our modern story of how we got here: the standard cosmological model is called CDM or "Cold Dark Matter". The small inhomogeneities captured in maps of the cosmic microwave background (CMB) grew to be the vast structure we see today via gravitational instability, but the story holds together only if dark matter is also present. The story works and it has been tested by observing the spectra of both the CMB and the distribution of matter on large scales. It is true that dark matter does not easily explain some phenomena on small scales, but there is a ready explanation for this: predictions on small scales are hard. Apart from the non-linearity of gravity, baryons play an important role on small scales, and incorporating these effects into numerical simulations is challenging. It is easiest to make predictions on large scales and those easy predictions have been confirmed with exquisite precision. Beyond all this lies the suite of experiments poised to detect dark matter. Thousands of scientists are now hunting for the particles that comprise dark matter by studying collisions at the LHC; by manning underground laboratories designed to detect it; and by launching satellites to observe the debris created when two dark matter particles in space collide and annihilate. We have reason to be optimistic.
Why then pursue modified gravity?
First, the people who study modified gravity (MG) tend to focus on small scale data rather than large scale data. They are serious, smart scientists who make observations and fit MG models to the data. These fits tend to be pretty good, often with very few free parameters and therefore the scientists gain confidence in their models. This focus on different data or different slices through the data presents a challenge to the dark matter model. Eventually, dark matter will have to explain these data sets as well. Slicing and combining things in different ways leads to different challenges than might otherwise arise. Even if you believe in dark matter, you want to confront the data in all forms. The simple (slightly condescending) way of saying this is to say that CDM must ultimately reduce to MONDian phenomenology on small scales.
More importantly, dark matter has not yet been detected. This is not the time to raise the barriers and decree that only those who accept dark matter are serious scientists. We are optimistic, but we have to accept the possibility that dark matter will not be detected in the next decade. Our initial feedback from the LHC shows no hint for the simplest model that contains dark matter, supersymmetry (although these early data are certainly not conclusive). There have been hints in direct and indirect detection experiments, but certainly nothing definitive. It is possible that we will need to think of something completely new. In so doing we are going to have to drop some assumptions, weight evidence differently than we do now. The MG community does this now by downweighting large scale data and focusing more on small scales. This may end up being the correct approach, or we may need to think of something even more radical. I do not know how to do this (How do we encourage a revolution?) but I am pretty sure suppressing alternatives is moving in the wrong direction.
The communities now are quite disparate and find it difficult to engage one another. Is the MG vs. dark matter dispute identical to the disagreements between people from different religions, say, virtually impossible to resolve because the two sides cannot communicate? Certainly not. We are scientists, and facts will change our minds. Some examples of things the vast majority of the MG community accepts or will accept:
- MG is not theoretically favored over dark matter because "dark matter is something new". Both approaches are changing the fundamental lagrangian of nature by adding new terms and new degrees of freedom.
- The fact that Xenon100 or Fermi (or perhaps AMS in a few days) has not seen dark matter does not mean the theory is excluded. There is plenty of room in theories like supersymmetry and even more in other more generic models.
- If dark matter is detected unambiguously via direct and/or indirect detection, then MG would indeed fall outside the realm of reasonable scientific investigation.
On the other hand, our dispute does share similarities with those that divide adherents of religion. We are passionate, we come at things from different directions with different preconceptions, so it is sometimes difficult to speak the same language, to focus on a single question. At the end of the day, just like the devout in different religious traditions, we are all after the same goal, in our case, trying to understand nature. It is premature to state that our way is the only way.