====== Expansion of the universe ======

{{tag>Unknowable}}

Current cosmological theory, backed up by many diverse observations, suggests that the universe is not only expanding, but that the speed of expansion is //accelerating// over time.  Thus the limit of the [[https://en.wikipedia.org/wiki/Observable_universe|observable universe]] is not only receding away from us (and from every other location in the universe) but the speed at which it recedes is increasing.

If current theory (and the observations which lead to it) are correct, the implication is that any parts of the universe which are beyond the visibility limit will always remain unknowable.

>Assuming dark energy remains constant (an unchanging cosmological constant), so that the expansion rate of the Universe continues to accelerate, there is a 'future visibility limit' beyond which objects will never enter our observable universe at any time in the infinite future, because light emitted by objects outside that limit would never reach us."  \\ \\ Source : [[https://en.wikipedia.org/wiki/Observable_universe|Wikipedia]]

//Note: //Figures for the rate of expansion - a.k.a. the Hubble Constant - are derived either from measurements of the Cosmic Background Radiation, or from data on supernovae. The figures currently vary by about 9% - which, for a physical constant*, would normally be considered spectacularly inaccurate. (Source : //New Scientist//, Volume 247, Issue 3292, 25 July 2020, Page 15) This discrepancy is known as 'The Hubble Tension'.

As of 2019, a new, third, method of measuring the constant has been developed using the LIGO gravity-wave detector. The results are preliminary, and are still being compiled. See: [[https://arxiv.org/abs/1908.06060v1|arXiv org]]

For a 2023 technical roundup, including a detailed description of Hubble Tension, see [[https://arxiv.org/pdf/2308.10954.pdf|The Local Value of H0]] {{:oa_padlock_grn.png?16|a}}//Proceedings of the International Astronomical Union//, Volume 18, Symposium S376:

//*Note //: Although H<sub>0</sub> is known as the Hubble Constant, that's a misnomer, as the 'constant' is in reality a time-dependent variable.

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Also see : [[content:physics:cosmology:dark_energy]] and [[content:physics:cosmology:universe_shape]]

 ~~stars>3/5~~