7.10. The End of Cosmology?
When the WMAP results came out at the end of this school I was asked "So is this the end of cosmology? We know all the cosmological parameters...what is there left to do? To what precision does one really want to know the value of m?" In his talk, Brian Schmidt asked the rhetorical question: "We know Hubble's parameter to about 10%, is that good enough?" Well, now we know it to about 5%. Is that good enough? Obviously the more precision on any one parameter the better, but we are talking about constraining an entire model of the universe defined by a network of parameters. As we determine 5 parameters to less than 10%, it enables us to turn a former upper limit on another parameter into a detection. For example we still have only upper limits on the tensor to scalar ratio r and this limits our ability to test inflation. We only have an upper limit on the density of neutrinos and this limits our ability to go beyond the standard model of particle physics. And we have only a tenuous detection of the running of the scalar spectral index dn / dlnk 0, and this limits our ability to constrain inflaton potential model builders.
We still know next to nothing about ~ 0.7, most of the Universe. CDM is an observational result that has yet to be theoretically confirmed. From a quantum field theoretic point of view ~ 0.7 presents a huge problem. It is a quantum term in a classical equation. But the last time such a quantum term appeared in a classical equation, Hawking radiation was discovered. A similar revelation may be in the offing. The Friedmann equation will eventually be seen as a low energy approximation to a more complete quantum model in much the same way that 1/2 mv2 is a low energy approximation to pc.
Inflation solves the origin of structure problem with quantum fluctuations, and this is just the beginning of quantum contributions to cosmology. Quantum cosmology is opening up many new doors. Varying coupling constants are expected at high energy (Wilczek 1999) and c variation, G variation, (fine structure constant) variation, and variation (quintessence) are being discussed. We may be in an ekpyrotic universe or a cyclic one (Steinhardt & Turok 2002). The topology of the Universe is also alluringly fundamental (Levin 2002). Just as we were getting precise estimates of the parameters of classical cosmology, whole new sets of quantum cosmological parameters are being proposed. The next high profile goal of cosmology may be trying to figure out if we are living in a multiverse. And what, pray tell, is the connection between inflation and dark matter?