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fourth "live post" from the Texas Symposium on Relativistic Astrophysics
The concluding talk on open problems in cosmology was given by Joe Silk.
I'll mention below a few things I found particularly interesting.
Although the standard cosmological model essentially fits all the data,
nevertheless people have looked for departures, in particular a
non-constant cosmological "constant", usually parameterized as a change
of equation of state with time. Observations are closing in on the
regular cosmological constant, so it will probably be difficult to
justify projects to narrow this down even more, in contrast, say, to
gravitational-wave science, where it is clear that interesting stuff
will be found. For the first time, some of the enthusiasm about
gravitational waves reached me as well. Also, there is no compelling
theoretical reason to expect some sort of "dark energy" other than the
No dark matter has been detected. Experiments for direct detection or
production in colliders have turned up nothing. Possibly
self-annihilation is seen in gamma rays at the centre of the Milky Way,
but this is far from clear. There is still a window for WIMPs as dark
matter, probably very low mass (de Broglie wavelengths measured in
kiloparsecs) or high mass (TeV). Joe also thinks, though, that
primordial black holes might be the best bet.
The tension in the Hubble constant is real, but no-one knows what it
means. If a banal explanation isn't forthcoming, perhaps new physics is
around the corner. Joe also favours the low value, in part because it
allows for a slightly older universe, old enough to accommodate the
It is unclear what will resolve the inconsistencies between numerical
simulations of galaxy formation and observations. While the former
continue to improve to match the latter, what would be interesting are
actual predictions from the former, later confirmed (or ruled out) by
One of the basic questions of cosmology is still unanswered: is the
universe finite or infinite?
The official announcement was made for the next Texas Symposium, to be
held in Prague. Some already knew, and a website (where one can
register) has been up for a while. For a while, the symposia alternated
betwen North America and the rest of the world. The last regular one in
North America was in Vancouver in 2008, and since the 50th-anniversary
symposium in Dallas in 2013, two have been in Europe and one in Africa.
Maybe 2023 or 2025 will see it move back to North America.
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