Universe may be curved, not flat

http://www.nature.com/news/universe-may-be-curved-not-flat-1.13776

by ColinWright • 13 years ago

43 33 13 years ago

lutusp3• 13 years ago

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.

pdonis3• 13 years ago

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?

lutusp3• 13 years ago

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?"

pferde• 13 years ago

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• 13 years ago

It's turtles all the way down.

pmiller22• 13 years ago

A monkey, clearly. http://en.wikipedia.org/wiki/Monkey_saddle

p4bl0• 13 years ago

It's clearly a gigantic whale holding an umbrella (well, that reference may be to cryptic for people who don't read French or didn't read the awesome Le Génie des Alpages comic book by F'murrr).

pmiller22• 13 years ago

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?

andrewcooke2• 13 years ago

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 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).

jurjenh• 13 years ago

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• 13 years ago

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• 13 years ago

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°?

andrewcooke2• 13 years ago

i imagine the problem is finding an area of space that's smooth enough to make the measurement accurately. the distribution of mass within would need to be very uniform and "average" (mass curves space; what you're trying to find here is the average curvature). so it would need to be a very large area (since matter in the universe is clumped into galaxies and the like - you want a region so large that those clumps are averaged out).

since the voyagers are only separated by a scale similar to the size of the solar system, and we're talking of sizes much larger than the separation of galaxies (or superclusters), the answer is no (unfortunately) (and of course, at those scales, you have the practical problem of light having a finite speed, so your experiment would take a long time!).

in fact, that's why this comes from cmb (cosmic microwave background) measurements. because those give you, in effect, measurements across the largest chunk of universe observable.

having said all that, measuring changes in curvature is an easier problem. and that's what gravitational wave detectors try to do. and i am pretty sure there have been proposals to do that with satellites (measuring (changes in) distances to each other with lasers).

deletes• 13 years ago

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• 13 years ago

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.

lmm• 13 years ago

The idea that medieval people thought the world was flat is entirely unfair, but still popular.

aninhumer• 13 years ago

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.

OnACoffeeBreak• 13 years ago

You may find this essay by Isaac Asimov interesting: "The Relativity of Wrong" [1]

1: http://chem.tufts.edu/answersinscience/relativityofwrong.htm

znowi• 13 years ago

Don't you laugh. Flat Earth still has a following today.

http://en.wikipedia.org/wiki/Flat_Earth_Society

anakha• 13 years ago

Time to start a Flat Universe society.

jpswade• 13 years ago

https://en.wikipedia.org/wiki/Doughnut_theory_of_the_univers...

3rd3• 13 years ago

How do torus and saddle fit together?

hrkristian• 13 years ago

Putin on a Ritz[1] comes to mind

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

Sharlin• 13 years ago

The geometry of (the surface of) a torus is actually flat, not curved! This is quite unintuitive.

RockofStrength• 13 years ago

http://www.youtube.com/watch?v=wdJyafSBCb0 Howard Bloom's "Big Bagel" theory hand waves an entire scheme for everything. Pretty interesting.

crusso• 13 years ago

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

Universe may be curved, not flat

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