A 6 Feb 2019 article in Scientific American by Corey S. Powell discusses
how various methods of measuring the Hubble Constant are in agreement
within 10 percent or better, but that last 10 percent is proving somewhat
vexing.
The problem is that the errors bars for some of the methods aren't
overlapping with each other and there is an increasing confidence
that discrepancy is not due to experimental error alone.
There is a growing hunch by some researchers that the discrepancies
betray something fundamentally new that we don't understand yet.
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Originally Posted by Corey S. Powell, Scientific American
One possibility is that somebody goofed. As the evidence piles up, however, Knox has come to embrace the other possibility: that the fault lies not with his colleagues but with the universe itself. If so, figuring out why space is not ringing the way they expected could lead cosmologists to previously unknown physics, potentially revealing a whole new aspect of reality.
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Quote:
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Originally Posted by Corey S. Powell, Scientific American
There is one way all of the measurements can be correct, and that is if something is wrong with scientists’ interpretations of those measurements. Knox notes everything we know about the origin of the sound horizon depends on a theoretical model of how the universe behaved during its unseen initial 380,000 years. If the models are wrong and the size of the sound horizon is different than what they predict, that adjustment would change all of the numbers derived from it, including the Hubble constant. “If there is a cosmological solution, it has to be one that results in a smaller sound horizon,” Knox says. Shrink it by just 7 percent, and all of the studies happily agree with one another. The problem is, it is not at all clear what could account for such shrinking. In almost every other way, the model and the observations fit together tightly.
“It’s been really hard to think of an answer that explains everything perfectly. It will have to be something complicated, because we’ve tried all the simple things already,” says Marius Millea, a researcher at the Berkeley Center for Cosmological Physics and one of Knox’s co-authors. He notes it is much easier to tick off the things that do not work: Undiscovered kind of neutrino? Nope. New type of interaction between photons? Uh-uh. They all conflict with the data.
The most convincing explanation, in Knox’s view, is that the very early universe was expanding slightly faster than expected. If so, it would have cooled more quickly and frozen the sound horizon in place a little sooner. Then the sound horizon would be smaller than the one theorists have plugged into their models, and—problem solved! Or rather, then the problem is kicked down the road again, because now you need some explanation for what made the early cosmos take off more quickly.
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Full article here :-
https://www.scientificamerican.com/a...-the-universe/
Paper, "Sounds Discordant: Classical Distance Ladder & ΛCDM -based Determinations of the Cosmological Sound Horizon", by Knox et. al :-
https://arxiv.org/abs/1811.00537v1