@PdotdQ and the @physicists, what are we to make of this? I thought it was flat, but now it might be closed? How big might it be if it is closed?

The Planck team, whose data this paper reanalyzed had already performed very robust statistical analysis of the data and disagreed with this conclusion. Even this paper’s authors’ do not seem wholly convinced:

“The point here is not that the universe is closed,” Melchiorri said by email. “The problem is the inconsistency between the data. This indicates that there is currently no concordance model and that we are missing something.” In other words, ΛCDM is wrong or incomplete.

That ΛCDM could be wrong or incomplete is already well known, and in fact preferred by a large fraction of cosmologists.

Nevertheless, the fact that this paper, which is very anti-mainstream, is published in Nature Astronomy (impact factor of ~10), continues to prove that there is **no bias against non-mainstream thought in the physics community**.

I’m confused about the consensus though.

When you say the universe is flat, don’t you meant “indistinguishable” from flat?

Moreover, it is most likely closed, right? Just so large due to inflation that it’s indistinguishable from flat, right?

Yes; of course we cannot measure to infinite precision to confirm that it is actually flat.

It could be closed but the curvature is so low that we cannot detect it. However, this paper said that the Universe is *more* closed than that.

Just for kicks, I’ll point out that the article glosses over the fact that the universe can be both flat *and* closed if it isn’t simply connected.

But that means the journalist has this debate wrong. It isn’t about whether the universe is closed, but whether or not we can detect a deviation from flatness. Moreover, saying the universe is “flat” does not mean we think it stretches to an actual infinity in all directions.

Correct.

Correct again, flat does not necessarily mean stretching to infinity in all directions.

So then from a theoretical point of view, does inflation always produce closed universes? That’s what the balloon blowing analogy seems to teach. Does the math allow for universes that are unbounded and open? Or are closed universes inevitable?

If I understand this right, it would seem that the consensus is that we are most likely in a closed universe so large that it looks flat to us.

Not necessarily - inflation can produce universes of all kinds of shapes. The math does allow for unbounded and open Universe. Indeed, the most popular view is that the universe is completely flat and unbounded. Of course, as we discussed before, we cannot tell whether this is actually the case.

The most popular view among physicists is that the universe is completely flat and unbounded. However, the proper scientific consensus is that we have an upper bound for the absolute value of the cosmological curvature. Going beyond that formally requires a leap of faith.

So if the universe is unbounded, does that mean it is infinite in size?

If it is precisely flat and bounded, what would it’s edges look like?

To properly answer this question we need to go to more technicalities than I think is productive. I think probably suffices to explain this intuitively: bounded means that it has an edge, while finite means that it has a finite volume. You can have a finite, unbounded Universe. For example, the surface of the earth has no edge - it is unbounded, but it is clearly finite. Similarly, a *closed* universe is finite, but unbounded.

I don’t think anybody knows - the point is this possibility has not been ruled out.

I suppose I mean perfectly flat and unbounded. In that case it is it necessarily infinite?

If that is the case, does that mean the most common view among physicists is that the universe is infinite?

Yes, these universes are infinite.

Yes, I believe so. Of course I never actually took a poll, but nearly everyone I talked to believes this.

The other possibility is that they think it is open and flat, but never considered carefully if it was infinite and the bizzarity that would entail.

I don’t think that is a fair assessment. The fact that standard FLRW metric gives infinite volume for open and flat cosmologies is taught at the undergraduate level.

Okay. Thanks for clarifying this. Very interesting.

Great point (and leads right into my preface that I am not back here on the forum to discuss evolution in any way, shape, or form.)

From the articles:

“the discordance between the CMB data, which suggests the universe is closed, and other data pointing to flatness represents a cosmological crisis that calls for drastic rethinking.”

“According to the new analysis, the large amount of lensing of the CMB suggests that the universe may be about 5% denser than the critical density…so that gravity wins and the cosmos closes in on itself.”

“A closed Universe can provide a physical explanation for this effect, with the Planck cosmic microwave background spectra now preferring a positive curvature at more than the 99% confidence level.”

If the Universe is closed, unbounded, finite, but time is exotic and duple in construction, then I guess I should have been able to make this prediction, but I missed it. The CMB should obviously return a closed (curvature) system while light from hard-body cosmic sources would indicate flat space, and oddly enough, all within an FLRW spacetime. I think this is intriguing news. Thanks for sharing.