
How big is the entire universe? - sajid
http://scienceblogs.com/startswithabang/2012/07/18/how-big-is-the-entire-universe/
======
goodside
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
> [...] 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
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.

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lifeisstillgood
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
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?

~~~
goodside
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>

~~~
arikrak
Was it instantly huge after the big bang or did it expand much faster than the
speed of light?

~~~
nessus42
Both! If the universe is flat, then it was _always_ infinitely big. And,
according to the inflationary universe theory, it expanded faster than the
speed of light too.

In this case, you have to think of "expanding" as being like _stretching_.
I.e., things within the infinitely big universe rapidly got farther apart from
each other.

~~~
arikrak
How could it have always been infinitely big if there was a big bang where
everything was once?

~~~
nessus42
Everything we can see (which is called our "Hubble sphere") started off as a
tiny little (and very massive) speck of stuff at the beginning of time. There
were infinitely many tiny little specks, and they all became different Hubble
spheres. But all the Hubble spheres all overlap each other, forming one
continuous space. The same thing was true about the little massive specks at
the Big Bang.

As I said above, at the time of the Big Bang, the universe was like a very
dense sheet of rubber, and then it started rapidly stretching, getting less
and less dense over time, until it is now the density that we see around us.

Infinities can definitely give you a headache, though. For instance, there are
just as many odd integers as there are integers.

------
xefer
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.

~~~
mootothemax
_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.

~~~
nessus42
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_.

~~~
mattstreet
I think you make a very important point. Even if the universe was infinitely
large, there might be a point where past that the conditions can't exist that
would allow something like our planet to function. For infinity to grant you
duplicates of yourself the possibility must exist within that "zone of
infinity". Gravity for example might be stronger or weaker.

------
boredguy8
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>

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

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

------
thangalin
The article used one of my drawings without my permission.

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

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

------
nessus42
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!

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

How does that follow?

~~~
nessus42
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.

------
tokenadult
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...](http://en.wikipedia.org/wiki/Universe#Size.2C_age.2C_contents.2C_structure.2C_and_laws)

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
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?

~~~
Nogwater
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
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.

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

------
nrmehta
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...](http://www.ted.com/talks/brian_greene_why_is_our_universe_fine_tuned_for_life.html)

------
ars
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.

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

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

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

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hansbo
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
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.

------
kapkapkap
Relevant -- <http://scaleofuniverse.com/>

~~~
thangalin
Also relevant -- <http://www.davidjarvis.ca/dave/gallery/star-sizes/>

------
javert
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.

~~~
laughinghan
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".

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

~~~
fragsworth
Unfortunately, "plain English" is not sufficient to describe all physical
phenomena, particularly with scales we aren't exposed to in day-to-day life.

~~~
javert
If that's truly the case, which I doubt, people should seriously just stop
trying to write physics articles for the layman.

By the way, scale has nothing to do with any of the objections I'm raising.
It's not a problem of scale.

