How big is the entire universe?

http://scienceblogs.com/startswithabang/2012/07/18/how-big-is-the-entire-universe/

by sajid • 14 years ago

188 79 14 years ago

hansbo• 14 years ago

Interesting. I had no idea that the curvature of the universe was an indication of its size. Does this imply that a flat universe is also infinite?

calinet6• 14 years ago

This article and the science behind it at this point is making some very interesting assumptions, such as that the Universe is closed (like the Earth is in their analogy).

But I believe at this point, the real answer is that we don't yet know. We're looking at a universe that looks flat and basically saying "well, it could be flat and finite, or it could be closed and curved, or it could be flat and infinite... but last time we thought our plane of existence was flat we ended up way off, so we better not make that assumption again."

Point being: take this conclusion about the size of the Universe with a grain of salt.

Sharlin• 14 years ago

As an example of a flat but finite topology, think of the game "Asteroids" where, if the player's spaceship flies beyond the edge of the screen, it reappears at the opposite edge. This topology actually forms a torus! And, even though a torus definitely seems curved when embedded in 3D, its surface is actually flat - the angles of any triangle drawn on the surface always add up to exactly 180 degrees.

nessus429• 14 years ago

Cosmologist do usually take a flat universe to indicate an infinite universe. They also take "negative curvature" to indicate an infinite universe. Only a universe with "positive curvature" is taken to indicate a finite universe.

lifeisstillgood2• 14 years ago

Well, that blew me away. I would happily have gone for 13billion light years across.

This one little blog (that was a few hours work covering decades of hard work by hundreds of people) has reminded me of the vast importance of education, scientific inquiry and just plain old reading.

One recent suggestion here in the UK is to make maths study compulsory till age 18. If I can go till age 40 and not know how we know how big the universe is, and I am supposedly in the top 5% of educated people, then yes yes yes.

Is there any politican I can vote for who will double the science budget ?

tosseraccount• 14 years ago

Shouldn't it be 13 * 2 = 26 across? 13 light years that way, and 13 light years the opposite way? Is there some "relativity trick" I'm missing? Is some space aliean on the edge of the universe going to see 13 light years in all directions?

ed209• 14 years ago

that would assume that there is a center to the universe. I'm not sure there is (speaking as the lay-est of laypeople). There are articles on the topic which I won't embarras myself by summarising in a HN comment...

http://www.quora.com/The-Universe/Does-the-Universe-have-a-c...

lifeisstillgood2• 14 years ago

considering I knew even less than I thought I did 20 minutes ago I am going to wait for someone who actually knows to chime in.

goodside3• 14 years ago

This is a misconception. If you just want a satisfying mental image of the Universe: It's really big, it doesn't have any point defined as its center, and it doesn't have any wall-like borders. Aliens living 14.6 billion LY away from us see a Universe that looks just like what we see in all directions.

If it is limited in size at all, it's because it wraps around on itself ("if you go straight long enough you end up where you were"). Nobody has firm evidence that it actually does wrap around on itself, and there's reason to believe that if it does, it's on a scale that's much larger than we could ever hope to observe. We will likely never have any sign inconsistent with it being literally infinite regardless of whether or not it is.

If you want a better answer than that, you'll need a very firm grasp of special relativity and a basic understanding of general relativity and the metric expansion of spacetime. In which case, read this: http://arxiv.org/pdf/astro-ph/0310808.pdf

nessus429• 14 years ago

> Shouldn't it be 13 2 = 26 across?*

I guess this depends on how you define "across". Our Hubble sphere is all the space around us for which light has arrived. If the universe is infinitely big, then yes, there are things that we can see that are 26 billion light years from each other. However, they can't see each other, and so they are not causally connected to each other, even though we are causally connected to both.

>Is some space aliean on the edge of the universe going to see 13 light years in all directions?

An alien at the edge of our Hubble sphere will live in a different, overlapping Hubble sphere, from which they will see 13 light years in all directions around themselves.

xefer• 14 years ago

Max Tegmark had an interesting article in Scientific American a while back discussing the implications of an infinite universe:

http://space.mit.edu/home/tegmark/PDF/multiverse_sciam.pdf

Perhaps the most disturbing of which would be the fact that, by definition, at some distance, there would have to be a duplicate of yourself.

hunterjrj• 14 years ago

... Or an infinite number of duplicates of yourself?

mootothemax• 14 years ago

Perhaps the most disturbing of which would be the fact that, by definition, at some distance, there would have to be a duplicate of yourself.

Wouldn't it also mean that there are an infinite number of duplicates out there? Including one typing this very comment, but who typos and leaves the "s" off "duplicates"?

I'm very much not a mathematician nor physicist, can this really be true if the universe is infinite in size? Asking as a layman, at what point when dealing with infinite possibilities and probabilities does something become certain? That's what really terrifies me.

andyjohnson0• 14 years ago

Including one typing this very comment, but who typos and leaves the "s" off "duplicates"

There would be an infinite number of those, too.

nessus429• 14 years ago

If the universe is flat, and therefore infinite, yes it means that most probably there are an infinite number of identical duplicates of you. It does not mean, however, that there are any duplicates of you that make a particular typo. I should think that there are an infinite number of such duplicates for any given typo, BUT it may also be the case that such worlds are impossible. E.g., physics is deterministic enough and chaotic enough that there is no slight nudge to the initial conditions of a world that will result in such a minor eventual difference to occur.

I.e., just because there are an infinite number of parallel worlds, doesn't mean that every imaginable thing occurs in them. What occurs in a parallel world, must be possible.

goatforce5• 14 years ago

"And best of all in possible worlds / nothing is impossible" - The Shamen

creamyhorror• 14 years ago

What if the universe is empty beyond a certain distance from the origin/central point, like a tiny explosion in the middle of an unending emptiness? Then there wouldn't have to be duplicates of you.

(This is about the same thing as the question asked below about whether infinite size implies infinite mass.)

sageikosa2• 14 years ago

...and hence the works of William Shakespeare?

SoftwareMaven• 14 years ago

The way I've heard this describes is with an analogy. If you have an infinite number of decks of cards, shuffled randomly, the same shufflings will show up an infinite number of times. There are only so many ways to put together an infinite number of particles, do the shufflings get reused.

pyoung• 14 years ago

But what if you have an infinite number of cards (or close to it) in each deck?

cgmorton• 14 years ago

If the universe is infinite in size, does that automatically imply that it is infinite in mass?

That is, I can imagine a mass distribution that decreases as it moves away from the center, such that the total amount of mass is finite. In which case these problems don't apply.

I'm not sure whether that's possible though. If space-time is flat, that means there's a specific mass density in that universe, in which case infinite mass exists. For an infinite universe to have finite mass would mean a density that tends to zero, in which case we should observe a saddle shape in the experiments described. That is, unless space is just -locally- (in 14.6 billion light years) flat.

I don't claim to be right. But, you can use this possibility to ease your existential dread at being one of infinitely many near-identical copies. Of course you still have to deal with many-world quantum physics...

rokhayakebe• 14 years ago

The same concept has been argued in details some 2100 years ago by Lucretius in "On the nature of things."

SilasX• 14 years ago

Right, as long as you look through an amount of space that scales superexponentially with the complexity of the entity you want to find a copy of. (With stronger assumptions, like attractors in development of matter according to physical laws, you might be able to improve the bound.)

But don't worry, you get to search in three dimensions, so its distance only scales with the cube root of that!

goodside3• 14 years ago

The short answer is nobody has a plausible upper bound on the size of the entire Universe, and there's no firm consensus on whether it's literally infinite. If it is finite, there is no "wall" or "center", since it's not a big sphere -- a finite Universe just means that if you go straight long enough, you end up in the same spot. It's analogous to how if you stood on the ground of featureless planet, the land you could inhabit is finite, but it has no borders and no point where you could stick a flag in the ground and say "this is the center of the planet's land".

Most of the time when people give hard numbers for the size of the Universe, they either mean A) the portion of the Universe we can see, B) the portion of the Universe we could ever see in principle, or C) the portion of the Universe which could, in principle, be causally influenced by early particle interactions that could have causally influenced us. If you suspect someone might be talking out of their ass about cosmology, ask them how big the Universe is -- if they give you a clean multiple of 14.6 billion light years, they're an idiot.

Technically inclined readers will find this more enlightening than the article above: http://arxiv.org/pdf/astro-ph/0310808.pdf

peterwaller• 14 years ago

> [...] there's no firm consensus on whether it's literally infinite. If it is finite, there is no "wall" or "center", since it's not a big sphere -- a finite Universe just means that if you go straight long enough, you end up in the same spot.

My understanding is that for your interpretation here of "finite" to be true, there must be some curvature. If you only consider the geometry of the universe, then what we see with our instruments does allow us to put a bound on the size of the curvature.

If you want to say something along the lines of "outside of the observable universe is probably cheese whiz" then that's your perogative, and no-one can refute that. But (from the article) the geometry of the universe that we have measured is consistent with an infinite universe, and gives a minimum bound on its size which is many multiples of the observable universe.

Strilanc• 14 years ago

It's mathematically simple for a space to loop without being curved locally, although I am under the impression that there are physics reasons to infer we don't exist in such a space.

The pac-man universe acts euclidean until you have universe-sized effects. For example, a circle will satisfy circumference = diameter * pi until its diameter is greater than the span of the universe. Basically the universe could 'hide' the fact that it is a torus from pac-man by simply being really large.

scott_s2• 14 years ago

I think that confusing A, B or C with "the Universe, all of it" is hardly grounds to call someone an idiot.

goodside3• 14 years ago

There isn't anything of importance in cosmology that's a clean multiple of 14.6 billion LY. Anything under discussion at that scale would by influenced by the Hubble expansion.

But, yeah, being misinformed about cosmology doesn't make you an idiot. Hyperbole.

kennon• 14 years ago

Very cool, but I'm still a little confused. The earth's surface more or less just extends out in two dimensions, but doesn't space extend out in three? How does all of this work when you have take into account the z-axis as well as x and y? How would a three-dimensional object "close" in around itself?

jmmcd• 14 years ago

To imagine a 2d surface closing in on itself, you imagine it in 3d. To imagine a 3d object closing in on itself, you unfortunately have to try to imagine it in 4d.

Splines• 14 years ago

I read Flatland a long time ago as a student and found it enlightening when thinking about 4d.

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

InclinedPlane• 14 years ago

There are different kinds of dimensions. There are simple, Euclidian dimensions where each dimension is perfectly orthogonal to the others, where parallel lines never intersect, etc. But there are other possibilities, non-Euclidean spaces and volumes. This is where the example of the sphere comes in. If you were a 2-dimensional being you could be on a plane or you could be on the surface of a sphere. Similarly, if you are a 3-dimensional being, such as us, you could be within a simple R^3 volume, or you could be on a hyper-sphere or another non-euclidian volume. The math is a little more complex, but similar.

jsmcgd• 14 years ago

The visible curve of the Earth is due to atmospheric distortion? What? Surely not. Surely the curve you see is the horizon of your view?

hcarvalhoalves• 14 years ago

I believe the author is stretching it. In the iceberg photo case, that's just barrel distortion from the camera lens.

tokenadult• 14 years ago

There was a talk about this issue at the Midwest Science of Origins Conference on 31 March 2012 by Marco Peloso of the University of Minnesota. Peloso reviewed the evidence available for the condition of the Universe just after the Big Bang. He also mentioned that current observations are consistent with a "flat" geometry of the entire universe, but pointed out other lines of evidence, not mentioned in the blog post submitted here, consistent with a finite (although very, very large) size for the universe.

Finite size is consistent with current observations and theories, and an issue that the submitted article doesn't have a lot of space to address. Per a Wikipedia article,

http://en.wikipedia.org/wiki/Universe#Size.2C_age.2C_content...

consistent with what I heard in the lecture earlier this year, "The universe appears to have no net electric charge, and therefore gravity appears to be the dominant interaction on cosmological length scales. The universe also appears to have neither net momentum nor angular momentum. The absence of net charge and momentum would follow from accepted physical laws (Gauss's law and the non-divergence of the stress-energy-momentum pseudotensor, respectively), if the universe were finite." Wikipedia cites the Landau and Lifshitz physics textbook from the Soviet Union, Landau, Lev, Lifshitz, E.M. (1975). The Classical Theory of Fields (Course of Theoretical Physics, Vol. 2) (revised 4th English ed.). New York: Pergamon Press. pp. 358–397. ISBN 978-0-08-018176-9, for this statement.

Some multiverse theories such as those mentioned in another comment posted before this one can be overlaid on a simpler theory of a single finite "observable" (in principle) universe with the properties we know from human observation. Testing theories like those, or like the "level I multiverse" theory mentioned in the other comment, still needs more work.

The current Udacity course in physics

http://www.udacity.com/overview/Course/ph100/CourseRev/1

started off its first unit with the students reproducing the effort of Eratosthenes of Cyrene to measure the circumference of the earth more than 2,200 years ago. On reasonable assumptions known to the ancient Greeks, it was possible to get a surprisingly accurate estimate of the earth's circumference, with a major source of error being simply measuring distances between one city and another a few days' journey away.

tocomment• 14 years ago

But if the big bang happened 13 billion years ago there shouldn't be any matter or energy outside of a 13 billion light year radius, right?

brandong• 14 years ago

Due to the expansion of space the edge of the observable universe is now about 46 billion light years away: http://en.wikipedia.org/wiki/Metric_expansion_of_space

Nogwater• 14 years ago

No, unfortunately it doesn't work like that. It's not like the big bang took place "within space" -- like a grenade in the center of a stadium, but the bang is the rapid expansion of space(-time) itself. So, the universe could have even been infinite and very dense, and then BOOM!, it's still infinite but less dense.

The observable universe is even bigger than you'd expect: http://en.wikipedia.org/wiki/Observable_universe#Size

equalarrow• 14 years ago

Yes! Mind blown (again). :)

While it's really enjoyable to talk/think about these topics from a purely scientific point of view, there's another side to all this, which is the sheer beauty of such a thing.

There's a great show on Discovery called How the Universe Works. Season 1 had an episode called Galaxies that sorta touches on some of this. The graphics for the show, first off, are awesome. When they show the super zoomed out view of all the galaxy clusters and how everything is connected, immediately I thought, "wow, those look like neurons".

And that's where the beauty of nature comes in. You can start to see how systems arrange and the patterns used from the micro to the macro. The distances on either level are incomprehensible (strings, atoms, molecules, galaxies, universe), but the patterns are visible.

And this is why I love science. While I'm not a physicist, I always appreciated Feynman because you could see he had that same love for the extremely complex and the simple observations - and he had a great way of describing it. I think the same goes for Sagan, although, in a totally different field. But Sagan too just had that love for the beauty of nature, on top of his accomplishments of the Mariner missions and other planetary science.

I have a 6mo son right now and I hope to instill that love of the beauty of nature through science. We're each an insignificant speck on an insignificant blue speck in an unfathomable universe. But, our connections to each other and our appreciation for the beauty of it all are a privilege. And posts like this, shows like How the Universe Works will be my way of conveying to him his place in it all. I hope he'll appreciate it someday like I do.

I hope, some day all people will.

kapkapkap• 14 years ago

Relevant -- http://scaleofuniverse.com/

thangalin2• 14 years ago

Also relevant -- http://www.davidjarvis.ca/dave/gallery/star-sizes/

acdanger• 14 years ago

"The universe is a big place, perhaps the biggest." -- Kilgore Trout

nessus429• 14 years ago

Another factoid that causes the mind to reel is that a flat universe, which is of infinite size, also has zero total energy. This means that it is conceivable that someday we might figure out how to manufacture entire new universes.

Who says that there's no such thing as a free lunch!

Ralith3• 14 years ago

> zero total energy ... means that it is conceivable that someday we might figure out how to manufacture entire new universes.

How does that follow?

nessus429• 14 years ago

It doesn't follow that we will be able to do it, nor even that it is possible, but before the understanding that flat universes contain zero energy, the claim that you might manufacture a universe would seem to completely absurd. Where would you get all that energy? Especially if energy is conserved?

As it turns out, it doesn't matter if energy is conserved, because a flat universe doesn't have any net energy, and consequently they can come into existence without violating known conservation laws.

In any case, you don't have to take my word for it. Look up Alan Guth, the MIT cosmologist who invented the inflationary universe theory. This is his claim.

javert7• 14 years ago

Why presume that the universe is a surface, instead of a giant volume? After all, the normal way of looking at it is that we're in space, not on it...

This is an especially appalling oversight in an article intended for the intelligent layman.

laughinghan3• 14 years ago

It's an analogy, because while it's easy to visualize how a two-dimensional surface can be curved, it's hard to visualize how a three-dimensional surface can be curved, because we think of it as a "volume".

javert7• 14 years ago

There is no such thing as a "three-dimensional surface" in plain English. I don't know why that is even coming up.

bwood• 14 years ago

If you consider the universe to be a well-defined volume, then there must be some boundary to it. And if there's a boundary, then you can ask the question "What's on the other side of the boundary?". Thinking of the universe as a 2D surface in a 3D space is a way to picture how it's possible to have a finite space with no well-defined boundary.

javert7• 14 years ago

Well, it's not clear to me why we would assume there is a boundary. And I don't know what it means to think of a 3D area as a 2D surface anyway. And I don't think it's possible to have a finite space with no well-defined boundary.

thangalin2• 14 years ago

The article used one of my drawings without my permission.

http://davidjarvis.ca/dave/gallery/

scott_s2• 14 years ago

Email the author, it looks like he thinks the image was made by someone else.

ars• 14 years ago

The first half of this is from the excellent book:

Relativity: The Special and General Theory by Albert Einstein

It's written for the interested layman, the article is from part 3 of the book where Einstein explains this.

boredguy8• 14 years ago

This might be a bit of a nit, but it happens enough: the quote that begins that article is misattributed. It's from Daniel J. Boorstin. http://en.wikiquote.org/wiki/Stephen_Hawking#Misattributed

ctchocula• 14 years ago

Fields Medal winner Terence Tao gives a public lecture on this material:

http://www.youtube.com/watch?v=7ne0GArfeMs

nrmehta• 14 years ago

For the layperson (like me), the below is an intriguing and related TED talk from Brian Greene. A big conclusion is that we now believe the universe is expanding at an accelerating rate based upon the edge of the observable universe. But Greene posits that 1000s of years in the future, this accelerating edge will be too far away for us to observe and the universe will look to future mankind to be more static and small. I'm not physicist so I can't criticize this claim but I found it intriguing.

http://www.ted.com/talks/brian_greene_why_is_our_universe_fi...

How big is the entire universe?

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