
Universe may be curved, not flat - ColinWright
http://www.nature.com/news/universe-may-be-curved-not-flat-1.13776
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deletes
Someday in the future children will be taught in history class how in the 21st
century people though the universe was flat. And they will laugh.

~~~
SEMW
That's not really a fair way to present the situation. It isn't 'In the 21st
century people couldn't imagine the universe could be curved, how terribly
quaint!'.

Rather, it's more like 'In the 21st century the best data available from CMB
anisotropies was that Ω was approximately 1 (or a tiny bit more), with Ω=1
being inside the confidence interval, so most cosmologists used a model where
Ω=1. Later, better data showed that the true value of Ω was actually very
slightly less than 1, so cosmologists started using the new, slightly more
precise value.'

The fact that there's a change in the words you can use to describe the
universe at Ω=1 ('open' at Ω<1, 'flat' at Ω=1, 'closed' at Ω>1), and the
latest update happens to have crossed that line (so makes for good headlines),
doesn't change that this is just another case of science slowly homing in on a
true value.

~~~
aninhumer
This reminds me of a letter from Einstein (EDIT: It was Isaac Asimov) where he
points out that saying the earth is flat is a pretty good approximation of the
curvature of the earth.

It's also worth pointing out that educated people have known the earth is
round since the classical period.

~~~
lsd5you
It was Isaac Asimov I believe.

[http://chem.tufts.edu/answersinscience/relativityofwrong.htm](http://chem.tufts.edu/answersinscience/relativityofwrong.htm)

~~~
talmand
TIL that Asimov supported the notion of "nearly right", which is good enough
as long as you tried really hard and could explain how you got to your
incorrect answer.

~~~
summerdown2
"Nearly right" is good enough, providing it's within the accuracy you need for
any particular job.

~~~
talmand
I suppose that depends on how the question is asked. Taking a question that
has a yes or no answer with one of them being correct and changing the
parameters so that nearly right is good enough doesn't seem proper to me.

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pferde
So, the next question is, what animal is saddled with our universe, and who is
riding it. Perhaps it's The Foe from Pohl's Gateway series? :)

~~~
michielvoo
It's turtles all the way down.

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lutusp
This result is consistent with what one would expect because of the geometric
effect of the recently discovered Dark Energy. A saddle-shaped overall
curvature is consistent with a universe accelerated by Dark Energy.

~~~
pdonis
_A saddle-shaped overall curvature is consistent with a universe accelerated
by Dark Energy._

I wonder about this. As I understand it, the de Sitter spacetime (which would
describe a universe containing dark energy and nothing else) has positive
spacetime curvature, but admits all three types of spatial slicings: closed
(positive spatial curvature, like a sphere), flat, and open (negative spatial
curvature, like a saddle). Wouldn't that mean that _any_ spatial curvature we
observe could be consistent with the presence of dark energy?

~~~
lutusp
We need to distinguish between the curvature of a specific location versus the
curvature of the universe as a whole.

As I understand it, for a universe that's being uniformly accelerated, by for
example dark energy, its overall curvature should be negative. In the absence
of dark energy, and for a universe whose masses possess escape velocity (no
more and no less), the classic zero-energy assumption of modern Big Bang
models, the overall curvature is zero (i.e. flat). For a universe that has
less than escape velocity and will recollapse, an idea that has now been
pretty much abandoned, the curvature is positive.

The escape-velocity zero-curvature universe has an interesting property -- it
has zero net energy, i.e. gravitational potential energy is exactly balanced
by kinetic energy. This means the universe can appear spontaneously, like
quantum virtual particle do, but on a larger scale.

"Because there is a law such as gravity, the universe can and will create
itself from nothing ... Spontaneous creation is the reason there is something
rather than nothing, why the universe exists, why we exist." — Stephen Hawking
in "The Grand Design".

I'm not qualified to say what a negative curvature might do to that pretty
spontaneous-creation picture. It's a nice model, and it offers an answer to
the question "where did all this come from?"

~~~
pdonis
_As I understand it, for a universe that 's being uniformly accelerated, by
for example dark energy, its overall curvature should be negative._

By "overall curvature", do you mean spacetime curvature, or the curvature of a
spatial slice?

As I said in my previous post, the de Sitter spacetime, which is the model for
a universe that is being uniformly accelerated, has uniform positive spacetime
curvature. These Wikipedia links give a good overview (of course Wikipedia is
not always reliable, but these pages appear to be consistent with what I read
in other more knowledgeable sources such as GR textbooks):

[http://en.wikipedia.org/wiki/De_Sitter_universe](http://en.wikipedia.org/wiki/De_Sitter_universe)

[http://en.wikipedia.org/wiki/De_Sitter_space](http://en.wikipedia.org/wiki/De_Sitter_space)

The spacetime curvature scalar R is 4 Lambda, where Lambda is the (positive)
cosmological constant.

The second Wikipedia link also describes the different possible spatial
slicings of de Sitter spacetime; as I noted previously, slicings exist with
all three types of spatial curvature (positive, zero, and negative).

However, it is true that the slicing with negative curvature is the one that's
used in the standard cosmological models of a universe that is dark energy
dominated; that's the FLRW model with exponential expansion that's discussed
in the first Wikipedia link. So if that specifically is what you were
referring to by negative "overall curvature", then I agree; but I would
specify that it's overall negative _spatial_ curvature, and that that model
assumes a particular spatial slicing which is not the only possible one.

~~~
lutusp
> By "overall curvature", do you mean spacetime curvature, or the curvature of
> a spatial slice?

I'm referring to the overall spacetime curvature shared by the universe as a
whole.

> ... and that that model assumes a particular spatial slicing which is not
> the only possible one.

Sorry, out of my depth.

~~~
pdonis
_I 'm referring to the overall spacetime curvature shared by the universe as a
whole._

Then that's positive, not negative, for a spacetime dominated by dark energy.
The curvature that is negative is the curvature of the spatial slices in the
model used in cosmology for a dark energy dominated universe--more precisely,
in the model referred to in the article as being suggested by the new data.

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jpswade
[https://en.wikipedia.org/wiki/Doughnut_theory_of_the_univers...](https://en.wikipedia.org/wiki/Doughnut_theory_of_the_universe)

~~~
3rd3
How do torus and saddle fit together?

~~~
hrkristian
Putin on a Ritz[1] comes to mind

[1][http://putintheritzon.ytmnd.com/](http://putintheritzon.ytmnd.com/)

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jurjenh
> This means that if large triangles could be ‘drawn’ in space, their internal
> angles would add up to less than 180 degrees.

I wonder if this could somehow be done with the voyager 1 and 2, although I
imagine this would involve setting the two voyagers up to look for each other
- something that may well be many orders of magnitude outside what is
possible, though they still seem to be able to contact us.

Also it may not be a large enough distance, but I'd guess its the largest
available to us that we could actually test.

~~~
bdg
It's not the drawing of them in space that's the issue. Remember, you're in
space right now in addition to being on the earth.

The issue is in making the measurement. The 180 degree problem is really
complex.

~~~
rwmj
I'm not saying it's easy to send three probes to the edge of the solar system,
but how come measuring the angles is hard? (Granted you cannot do it with the
Voyager probes).

Also does anyone know what total interior angles you'd have to measure, ie.
would be it ~179° or ~179.99999999°?

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pmiller2
> But this version of inflation cannot account for the Universe’s lopsidedness
> except as a statistical fluke — similar to, for example, a fair coin that
> happens to come up heads many more times than tails over 1,000 flips.

So, does this mean p = .001 for this result? Does the result hold if we choose
a different way to partition the sky into two connected regions?

~~~
andrewcooke
my reading of that section is that they're just trying to illustrate what
statistical error is. i don't think there's any real meaning to the 1000
value.

[edit] but googling for "statistical significance planck anisotropy" turned up
[http://arxiv.org/abs/1303.5083](http://arxiv.org/abs/1303.5083) which says "3
sigma" which is about 0.3% (p=0.003). so it's close.

[edit2] whoa. but you've got the right answer with the wrong maths. if you
toss the same side of the coin 10 times in a row, that's p=0.001 (1/2^10 =
1/1024). getting the same side 1000 times in a row is much, much less likely.

ps since the result is basically (if i understand right) "one side of the
universe is slightly different from the other" then it depends on how you
choose the halves. if you divided things "at right angles" to what gives the
signal, you wouldn't see anything at all. not sure if that is what you meant
by partitioning (mathematically they use something like a 3d fourier transform
- spherical harmonics - and this is a significant deviation from zero for the
lowest frequency component).

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crusso
Lawrence Krauss talks about the physics of the curvature of space and why the
current consensus is that it's flat. I found this Youtube to be really helpful
in understanding something of the subject:

[http://www.youtube.com/watch?v=Z0HqZxXZK7c](http://www.youtube.com/watch?v=Z0HqZxXZK7c)

