on the average, the density of the universe works out to be pretty darn close to the critical density, which is a pretty surprising coincidence (not just for making the geometry flat), and as you mentioned, ALEX, the leading idea for why it works out that way has to do with the inflationary hypothesis.  Maybe more on that topic, later.

PBS Space Time beat me to it.   Here's a whole episode dedicated to the physics of Inflation -- a roughly 40 year old hypothesis which seeks to resolve some outstanding problems with the standard Big Bang model, and understand what exactly powered (and also stopped) the extremely rapid exponential expansion of the very early universe.

[youtube]xJCX2NlhdTc[/youtube]

Wat, I heard PBS is going to do something about the different predictions for the acceleration of the universe (the differing values of the Hubble constant), did they mean what we had already talked about or is it something new?

Wat, I heard PBS is going to do something about the different predictions for the acceleration of the universe (the differing values of the Hubble constant), did they mean what we had already talked about or is it something new?

I think you might be mixing two things.  The Hubble constant is just the current expansion rate, and there are discrepancies in measurements of its value.  We talk about this and possible implications here.  But if they said different predictions for the acceleration, then they mean the equation of state for dark energy (the value of 'w'), which we talked about here and in a few places in Astro Q&A.

Hey Wats, I wouldn't mind hearing your opinion on this, as crazy as some of it sounds:

[youtube]IutpODdk4rU[/youtube]

Hey Wats, I wouldn't mind hearing your opinion on this, as crazy as some of it sounds:

Sorry for forgetting to get back to you on it.  It's a pretty good video as usual from him.  I'm not quite sure what you'd like me to talk about with it, though.  It is kind of a mashup of simple explanation of white holes and some crazy theoretical ideas, as you say.  Personally I do not expect white holes to be important for any cosmological model, even if they do exist which appears very unlikely.

[youtube]K1496gv8KCo[/youtube]
Abstract:  

Planck is an ESA satellite aimed at the observation of the Cosmic Microwave Background. The Planck collaboration has recently published its last legacy release. In this talk I will shortly review the main Planck results and their robustness, highlight some of the curious features present in the data and the Planck point of view on tensions with a few other astrophysical probes, notably with the Hubble constant measurements from local distance measurements.

A lot of new science has come out of Planck data in the last two years.  This talk is an excellent review of the latest (2018) results, how reliable they are, what we've learned from them, and some mysteries such as the discrepancy between different measurements of the Hubble constant (as we discussed earlier in the thread.)  It's also just a great review the CMB itself, and how we use it to test our understanding of the universe.

I hope we finally discover evidence of other universes in CMBR data!  Penrose has conjectured about this!

Planck evidence for a closed Universe and a possible crisis for cosmology

hey this isn't a crisis for me, I've always wanted this to be true (closed or oscillating/cyclic.)

The trouble us, I guess, that if it's true, then much else which has already made it into textbooks, is wrong.

The trouble us, I guess, that if it's true, then much else which has already made it into textbooks, is wrong.

Yes, so it makes me wonder if the two can be combined in some way.  I remember we talked about the value of W and if it was even slightly negative we could have a universe that closes/cycles "at the last moment" when it comes back empty.
https://en.wikipedia.org/wiki/Cyclic_model
The Baum-Frampton model is the one I was talking about.
The value of {\displaystyle w}w in the Baum–Frampton model can be made arbitrarily close to, but must be less than, −1.

https://quantamagazine.us1.list-manage. ... e76a2a020f

Hologram Within a Hologram Hints at Fate of Black Holes
By CHARLIE WOOD

Calculations involving a higher dimension are guiding physicists toward a misstep in Stephen Hawking’s legendary black hole analysis.

Dark matter quasiparticle created

https://www.livescience.com/axion-found ... metal.html
https://www.livescience.com/axion-found ... metal.html

Planck evidence for a closed Universe and a possible crisis for cosmology

My take is that the most likely scenario is that the Planck 2018 power spectrum data do not actually indicate a positively curved universe, because if that is the case then discrepancies between other observations become significantly more severe.  In other words, we have more and stronger reasons for concluding that the universe is (essentially) flat than that it is curved to the extent suggested here.  Personally I would get more confidence in this result if it made the bulk of other observational discrepancies smaller.

In fact, measurements of curvature like this are always problematic, because a wide range of values can be consistent with observations if you allow the densities of matter and energy in the universe to vary.  But because we have independent methods of determining those densities, we can pin down the curvature quite well, and they indicate very little curvature if any.

So there are a few possibilities, some more appealing than others depending on your particular philosophy of science, and whatever ends up being correct will still require modifying our understanding of certain measurements, methods, and/or fundamental physics.  The most exciting possibility is always that there is new fundamental physics behind any measurement discrepancy, but that's also always the least likely possibility.

I remember we talked about the value of W and if it was even slightly negative we could have a universe that closes/cycles "at the last moment" when it comes back empty.

The value of w is negative, with very high confidence.  It is approximately -1, which means the dark energy density is constant despite expansion.  w=-1 leads to endless exponential expansion with a very boring endstate (heat death) for the universe.

If it is less than -1, then the expansion becomes super-exponential and the universe ends dramatically in a Big Rip.  There's still no cyclic behavior there, unless we add in a number of extra assumptions to force such a solution.

The most exciting possibility is always that there is new fundamental physics behind any measurement discrepancy, but that's also always the least likely possibility.

Yes, but only if we had some clear leads for what the new fundamental physics would be.  Leads that are clearer than those for what may have been measured incorrectly.

Planck evidence for a closed Universe and a possible crisis for cosmology

My take is that the most likely scenario is that the Planck 2018 power spectrum data do not actually indicate a positively curved universe, because if that is the case then discrepancies between other observations become significantly more severe.  In other words, we have more and stronger reasons for concluding that the universe is (essentially) flat than that it is curved to the extent suggested here.  Personally I would get more confidence in this result if it made the bulk of other observational discrepancies smaller.

In fact, measurements of curvature like this are always problematic, because a wide range of values can be consistent with observations if you allow the densities of matter and energy in the universe to vary.  But because we have independent methods of determining those densities, we can pin down the curvature quite well, and they indicate very little curvature if any.

So there are a few possibilities, some more appealing than others depending on your particular philosophy of science, and whatever ends up being correct will still require modifying our understanding of certain measurements, methods, and/or fundamental physics.  The most exciting possibility is always that there is new fundamental physics behind any measurement discrepancy, but that's also always the least likely possibility.

I remember we talked about the value of W and if it was even slightly negative we could have a universe that closes/cycles "at the last moment" when it comes back empty.

The value of w is negative, with very high confidence.  It is approximately -1, which means the dark energy density is constant despite expansion.  w=-1 leads to endless exponential expansion with a very boring endstate (heat death) for the universe.

If it is less than -1, then the expansion becomes super-exponential and the universe ends dramatically in a Big Rip.  There's still no cyclic behavior there, unless we add in a number of extra assumptions to force such a solution.

Wat, does the Baum-Frampton model add extra assumptions (like phantom energy?) It states w can be close to -1 but must be less than it.  Also, does this new measurement lie outside of the margin of error (what is the margin of error that we have on w as of now?)