1. Dark Energy: Why Go Further Steven M. Kahn Stanford U./SLAC

2. Dark Energy: Why Go Further ✻ Many public presentations on dark energy highlight two essential points: – The fact that DE dominates the mass energy of the Universe. – The fact that it is very difficult to account for the value of the energy density in terms of conventional quantum field theories. ✻ The first of these points is meant to capture the interest of astronomers and other natural scientists. We should care about this stuff because it is there, and it plays an important role in cosmic evolution. However, we already know enough about DE for this purpose. For example, the uncertainties in H(z) are no longer very important for our understanding of galaxy evolution. ✻ The second point is more of interest to physicists. It is usually viewed as a question of whether DE is an evolving scalar field, a cosmological constant, or a manifestation of a modification to general relativity. The diagnostics are w(z) and g(z). Absent a specific well-motivated theory, it is indeed difficult to quantity how high in precision we need to go in such measurements before it is no longer worthwhile.

3. Dark Energy: Why Go Further ✻ However, in my view, such emphases lose sight of the most surprising thing about the discovery of dark energy – the fact that it solved so many existing problems in cosmology! In the early 90’s, there were many, many problems with prevailing cosmological models: – The oldest stars appeared to be older than the universe. – Galaxy velocities appeared to be discrepant with the galaxy density. – The universe appeared to be open, which was in conflict with all the arguments that motivated the ideas behind inflation. ✻ Of course, it was known at the time that the introduction of a cosmological constant would have this effect, but without any experimental evidence for it, that was not taken as an especially convincing argument.

4. Dark Energy: Why Go Further ✻ The success of the standard cosmological model is truly astounding, and I personally don’t think we emphasize that enough when we all give talks on this topic. ✻ Yes, the model does involve three forms of deus ex machina: Inflation, dark matter, and dark energy. We don’t really know what any of these things are. ✻ But the model still has a relatively small number of free parameters, and the wealth of diverse astronomical measurements that it appears to describe is rather remarkable (properties of the cmb, the large-scale distribution of galaxies, properties of clusters, kinematical constraints from supernovae, etc.)

5. Dark Energy: Why Go Further ✻ I view the entire picture as a kind of intricate tapestry. It just begs us to pull on various loose threads to see if we can make the whole thing unravel! ✻ Indeed, progress in the history of physics has shown us that when we come up with theories that appear to work better than expected (especially when they have surprising elements to them), it makes sense to try to test them further, with higher and higher precision. (Example: Atomic spectroscopy at the dawn of quantum mechanics.) ✻ I don’t see how we “lose” in this process. Either, the theory continues to work well in the face of increasingly precise measurements, which makes it even more astounding! Or, the theory “breaks” and we learn something new.

6. Dark Energy: Why Go Further ✻ This argument leads to the conclusion that we need to continue to build experiments until we have tested the key elements that hold the tapestry together – i.e. we need to exercise all of the various probes at a comparable level of precision. ✻ That essentially takes us to Stage IV (LSST, DESI, etc.) If the concordance cosmological model remains intact and unmodified after those experiments complete their analyses, I believe it might not make sense to go further, unless we come up with new, qualitatively different approaches for investigating this phenomenon.

7. One Final Point ✻ Others have emphasized that the dark energy facilities we are building have so many other uses for astronomy, that that is another reason why this is a no-lose proposition. ✻ I would take this one step further: These facilities basically provide “maps” of the universe in terms of the key “fields” that characterize its structure: baryons, dark matter, velocities. ✻ I would argue that we should endeavor to make such maps simply because we now have the technology to do so. The maps themselves are very likely to have tremendously long-lasting value, well beyond their utility for dark energy studies. ✻ An analogy might be to the making of detailed topological maps of the Earth to test theories of continental drift. Sure such maps do provide tests of those theories. But one would be hard-pressed to argue that is the only reason, or even the most important reason to make those maps.