What Shape Is the Universe, Closed or Flat?

https://www.quantamagazine.org/what-shape-is-the-universe-closed-or-flat-20191104/

by theafh • 7 years ago

87 26 7 years ago

tabtab• 7 years ago

Atheists are going to have a hard time if it turns out the Universe is shaped like a giant Pikachu :-)

Some models suggest our universe is just one "foam-bubble" in a big ocean of universes. Hell, the mole on your face may contain its own universes. It really could be "turtles all the way down", or Pikachus all the way down.

https://www.vice.com/en_us/article/j5yngp/the-universe-is-ma...

Because of our Earthly experience, we tend to assume there's an absolute limit to the quantity of "stuff", but perhaps that's not true: there is infinite stuff, we just can't touch most of it.

hombre_fatal• 7 years ago

What's up with the Pikachu references? I don't get the joke.

earthboundkid• 7 years ago

The universe is the dream of an enormous sleeping Pokémon called Slumblord. If it should ever wake, all of our reality will disappear in an instant and be forgotten.

“Worship not God but Regigigas, on whom the world rests.”

— Henry David Thoreau

rafaelvasco• 7 years ago

The bubble shape makes much more sense to me than the linear plane shape. Whatever the truth is, the most accepted model today is most likely wrong;

ars4• 7 years ago

Time for Einstein Gravity Probe C.

Einstein suggested we could directly measure the curvature of the universe by comparing distances between probes located at angles to each other.

They would need to be pretty far apart from each other for this to work, but they can be very small: just an RTG and an antenna.

So hopeful light enough to build up an enormous velocity.

tasty_freeze• 7 years ago

For the satellites to communicate to each other they need some combination of a powerful transmitter and a large receiving antenna, so that puts a limit on just how small they can be.

ars4• 7 years ago

I was thinking a laser rather than radio. In radio terms: very very high gain.

The receiver can be a mylar reflecting dish, which would be lightweight.

Use a gyroscope to steer the entire probe (both for transmitting and receiving).

They only need to communicate very very sporadically, and you only need a single bit of data transmitted (amongst each other).

Make some kind of schedule where the probes face each other, measure distance (i.e. time between send and reply), then steer toward the next probe.

To earth you would basically use Hubble to read the data (and let each probe send duplicate data back to earth).

namirez3• 7 years ago

You are right but this has already been done using the CMB data and the universe seems flat with a 0.4% uncertainty. I don't think there is any need for artificial probes for it.

Source: https://arxiv.org/abs/1502.01589

ars4• 7 years ago

That's exactly the point of the article - the CMB data is far less clear than advertised.

jessriedel• 7 years ago

Not remotely feasible. The overall curvature of the universe is swamped by local inhomogeneities until you get to scales that are much larger than galaxies. So you'd only be measuring the curvature due to nearby matter unless you had proves traveling at relativistic speeds and were willing to wait millions of years.

keymone• 7 years ago

If you know mass distribution between and around probes, could you account for it? or is it a case where we can only know the mass distribution by observing the curvature?

zaarn• 7 years ago

Well, if you have a few billion years to send the probes to the required distances, would likely be possible. But I don't think there is an RTG capable of powering even a laser based system short of feeding matter to a black hole, which kinda will make it difficult to build a probe around it.

Florin_Andrei• 7 years ago

They would need to be pretty far apart from each other for this to work

Yeah, like light-years apart. Many light-years, too.

ianai• 7 years ago

Seems like it would have to be open since the boundaries are permanently expanding. The set can’t contain its accumulation points if new points will always be created. Granted, infinite sets like aleph-0 and -1 are typically considered both open and closed, iirc. Been a while since I took topology.

patrickthebold• 7 years ago

I'm pretty sure when the article says 'closed' the really mean 'compact' in the topological sense.

If you have a topological space, the entire space is always both open and closed. https://en.wikipedia.org/wiki/Topological_space#Definition_v...

foxes• 7 years ago

"Closed manifold" means compact with boundary. I think physics might also just mean something with positive "Ricci curvature". Certainly closed 3-manifolds are all equivalent to 3-spheres [0]. I think 4-manifolds are more complicated?

In relativity, the idea is all time-like (basically path of ordinary matter) curves will all converge, while the other case, time-like curves always diverge (so open).

[0] https://en.wikipedia.org/wiki/Poincaré_conjecture

Sniffnoy• 7 years ago

Granted, infinite sets like aleph-0 and -1 are typically considered both open and closed

This statement is meaningless. It makes no sense to speak of whether a set is open or closed outside the context of a topology.

Also, like... in any topological space of the sort we usually deal with (specifically: T_1 spaces), any open set must be infinite, as any finite set is closed? So I'm really not sure what you're thinking of here. (And in something like Euclidean space -- or a manifold -- any open set must be uncountably infinite...)

(Also, as others have already mentioned, "closed" here doesn't mean "closed" in the sense of point-set topology; "closed manifold" means a compact manifold [without boundary, if manifolds with boundary are being considered]. Yes, this is a bit confusing at first.)

unzadunza• 7 years ago

I read a book a long time ago (Poetry of the Universe: A Mathematical Exploration of the Cosmos, by Robert Osserman) about the shape of the universe. In it he talks about quasars. If you observe 2 of them in opposite directions, each 13 billions light years from earth, then they must be 26 billion light years from each other. But the universe is only a little less than 14 billion years old so how can this be? He argues that the universe must be a hypersphere.

I read this more than 20 years ago, and I'm no cosmologist, so forgive me if I butchered that summary.

YayamiOmate• 7 years ago

But that's not the only explanation. If you strech space in each direction you get the same effect.

If you have a baloon with a band attached to two opposite points on it's surface and blow the ballon, the distance between points (the band length) on the surface will change 2 times as the distance from the middle. Moreover, if you place some markers on the band the distance between them will also strech. You get that effect in plain Euclidean 3d space. It's not that surprising even for nice orthogonal geometry.

kevin_bloch• 7 years ago

Maybe I was way off the first time I read it, but I could have sworn that Stephen Hawking's "Black Holes and Baby Universes" already claimed, decades ago, that the universe looped like that.

If I remember right, there was some analogy likening us to ants crawling on the surface of the Earth, but I could be conflating it with something else.

What Shape Is the Universe, Closed or Flat?

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