• fourth "live post" from the Texas Symposium on Relativistic Astrophysic

    From Phillip Helbig (undress to reply)@21:1/5 to All on Fri Dec 20 17:53:07 2019
    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 cosmological constant.

    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
    oldest objects.

    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
    the latter.

    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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