

The Big Bang Was an Explosion of Space, Not in Space - jawngee
http://www.everyjoe.com/articles/the-big-bang-was-an-explosion-of-space-not-in-space-191/

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goodside
I'm not convinced this explanation-by-analogy style is really all that
helpful, particularly with regards to "the big bang is an expanding balloon".
What makes it such a confusing analogy is that most people were already
picturing something like an expanding sphere anyway, just one filled uniformly
with matter moving away from a central point as it expands. And it doesn't
reinforce that the Universe has no center, since most people can easily
imagine what the center of a balloon is.

So, screw the analogy. Here's what we know.

Right now, everything is moving away from us in proportion to its distance
from us. Read that again, because that's _the_ fundamental observation. The
usual intuitive view that lay people have is that we're drifting away from the
Big Bang on inertia left over from the explosion. This would _not result in
what we see_. For everything to move away from us in direct proportion to its
distance, everything would have to be speeding up all of the time. When we're
distance d from the Big Bang, we're drifting away from it at half the speed
that we are when we're distance 2d away. Inertia doesn't do that. So much for
inertia.

Maybe it's not inertia, but some kind of Magic Inertia that we don't
understand? A kind that pushes things faster and faster as time goes on, like
little angels perpetually flapping their wings to add momentum to every atom.
That's great for a while, but you hit another problem: the speed of light. If
you're already moving away at (nearly) light-speed, you can't double your
speed. Doubling your momentum will just give bring you infinitesimally closer
to light-speed. D'oh.

So what is the answer? The closest you can get without worrying about math is
probably this: Space is being created, and distance is somehow being inserted
in the gaps between everything, and this happens continuously and everywhere.
The other, more accurate, way to see it is that the _meaning_ of "distance"
and "duration" (that is, the _metric_ we use to determine the distance between
points, both in space and time) is itself changing as the Universe expands.

This results in some observables that cannot be explained otherwise, and which
we do actually observe. In this model, everything (at intergalactic distances)
does indeed increase its distance from everything else, and in direct
proportion to the original distance. Since this is a change in the spacetime
metric, rather than actual movement within spacetime, it does not result in
relativistic effects that you would see if it were literal movement. There are
galaxies in the sky right now whose distance from us is _literally increasing
at a rate that would be impossible if it were due to movement, since it would
require movement faster than the speed of light_. That's the clincher--the
reason no other model can possibly work.

This "metric expansion of spacetime" maybe sounds like a cop-out, like
something's been invented just to explain the Big Bang, but it hasn't. This is
_exactly_ what matter does in the normal scales of planets and stars. The
changing of the spacetime metric is what gravity fundamentally is, as Einstein
explained with General Relativity. It's the reason clocks run slower on Earth
than they do in space, and it's the reason the planets stay in their orbits.
It's all really just quite ordinary.

~~~
boredguy8
With my limited cosmological understanding, it seems like much of what you're
saying is also key to, in part, understanding that the expansion or shift
isn't a movement THROUGH spacetime, but the expansion OF spacetime.

I have a question. Our current cosmological understanding indicates that no
matter where you are in the universe, you'd see this recession, correct? If
so, bizarre thought experiment: If I could somehow magically teleport from
here to the very edge of observable spacetime, wouldn't I see, broadly, the
same stuff as I see from earth? A whole bunch of receding galaxies?

~~~
goodside
Yes. That's what makes metric expansion of spacetime elegant: it doesn't
require you to suppose there's anything special about our location in the
Universe. If the recession of galaxies were intertial, the only explanation
would be that the Big Bang happened exactly where Earth is now, and we're at
the exact center of the Universe and everything is moving away from us. It's
the key absurdity that needed to be explained away.

Further, we know from more recent evidence that the Universe is homogenous on
scales larger than galactic clusters. On the scales of stars and galaxies, the
Universe has large expanses of empty space sprinkled with massive galaxies.
Zooming out, you can see "clusters" of up to about a thousand galaxies
separated by larger areas that have fewer galaxies. But once you get above
that level, on the order of 10^24 to 10^25 meters, there's simply no more
structure to be found. As far as we can tell, if you take a spherical region
with a diameter 10^25 meters from anywhere in the Universe, it will have
roughly the same amount of mass regardless of where you take it from, and the
Universe is of roughly uniform density.

~~~
boredguy8
So then, and this is where the crazy starts to happen: when people talk about
the size of the universe, I start to get really confused. While there's an
observable limit from our reference point, isn't that functionally a
consequence of the null cone? That is: doesn't the homogeneity irrespective of
location mean that if I were on the edge of the spacetime 'bubble' from
Earth's point of view, there would be a completely different spacetime bubble
with different 'stuff' in it? That is, different galaxies alltogether?

Maybe the question is too absurd, since we can't teleport in the way I
described, but it's...I don't know...interesting.

Thanks!

~~~
goodside
You're right: the "Observable Universe" is defined only relative to a point of
observation. For any spot in the Universe, there's a 93-billion-light-year-
wide sphere centered about that point that constitutes the OU for that point.
What exactly this sphere means it somewhat complicated. The simplest way I can
think to say is this: The radius of the OU is the current distance of the
farthest point in space such that if a radio signal were emitted from that
point 13.7 billion years ago, immediately after the Big Bang, it would be
possible _in principle_ (but by no means in practice) for us to receive that
signal. Anything that is outside of the OU is causally disconnected from us as
a matter of fundamental principle, and nothing that has ever happened or ever
will happen at any point outside the OU can ever affect us in any way, ever.

It was previously (up until a decade or so ago) an open question as to whether
the Universe itself might be smaller than the OU. This sounds absurd, but
consider the example of a hypothetical jet that could circle the Earth ten
times without refueling. While normally the "range" of a jet plane is a circle
about some point, the range of this jet exceeds the size of the Earth itself.
Similarly, if the Universe were smaller than the OU, some of the distant
galaxies we see would actually be repetitions of closer galaxies from an
earlier time and a different angle, since the light had been "looping around"
the Universe once or more before reaching us. Or, as Modest Mouse put it in
one of their better songs: "The Universe is shaped exactly like the Earth / If
you go straight long enough, you end up were you were." Recent evidence from
the Cosmic Microwave Background has made this idea very unlikely, but it's a
useful example to clear up misconceptions about the OU.

So, however big the Universe actually is, it's bigger than the OU, which means
that the stuff cosmologists are debating about is space that cannot, even in
principle, ever be observed. It's an important question, though, since whether
the Universe loops back on itself on the large scale or just keeps going
forever has implications for whether gravity will win out over Hubble
Expansion in the long run, determining the eventual fate of the Universe.

~~~
boredguy8
I'm really curious about how CBR observations solved the question related to
universe size.

Also, then, for me this makes me wonder: is then the big bang simply the
origin of the null cone over whose event horizon we can't see? Clearly this is
a Minkowski spacetime paradigm, and I have no idea where that stands in terms
of general favor.

Also: further reading? I'm guessing "The large scale structure of space-time"
is a bit dated ;)

~~~
goodside
The WMAP space probe allowed better measurements of the CMBR, from which the
relative composition of matter at the time of the CMBR's emission could be
determined (using methods I don't fully understand). Once it was known that
the Universe was X% atomic nuclei, Y% photons, Z% dark matter, etc., this
information implied certain bounds on how quickly the Universe was expanding
during the inflationary epoch (10^-36 to 10^-32 sec after the Big Bang), since
high rates of expansion can overpower the nuclear forces and prevent quarks
from bonding to form protons/neutrons. The math for all of this is,
unfortunately, far over my head. Sorry I can't be of more help.

I'm not fully understanding your next question. The origin of our light cone
is here and now. Minkowski spacetime is an approximation that (almost) works
in the absence of gravitating bodies. Once general relativity gets involved,
certain features of Minkowski spaces start failing, like the fact that you can
always reorient light cones so that they're parallel. This fails even in
intergalactic space, where there is still a non-vanishing Weyl tensor effect
due to Hubble Expansion. So, without getting too much into the detail, in the
real world light cones are (forgive me) uncannily un-coney. Our past light
"cone" collapses back in on itself in the distant past and converges on the
Big Bang, but so do light cones everywhere in the Universe, even outside of
our observable universe. Though, whether a light cone even has meaning before
the inflationary epoch is a question of Grand Unification Theory, which is
_very_ much over my head.

As for further reading, Dodelson's "Modern Cosmology" is great if you're not
afraid of learning the math behind General Relativity:
<http://amzn.com/0122191412> The text doesn't assume very much familiarity
with GR, but it does assume enough mathematical sophistication that you can
fill in any gaps in your knowledge on your own.

(And if you don't like the idea of learning about tensors, you're SOL. Sorry.
There's a very low ceiling of how much one can know about cosmology without
tensor calculus.)

------
mojuba
Except the Big Bang theory doesn't say there was a moment of time zero, the
"moment of creation" before which there was nothing. This is a popular
misconception about the BBT which is driven by our religious beliefs: there
was nothing and then something caused _something_. BBT in its modern form
approaches very dense and very hot moments of the Universe, but doesn't say
anything about "creation".

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thmz
"since space and time are interconnected"

Last week I saw a Ted Talk by Prof. dr. Wubbo J. Ockels telling me that space
and time are not connected. [http://www.tedxamsterdam.com/2009/video-wubbo-
ockels-on-time...](http://www.tedxamsterdam.com/2009/video-wubbo-ockels-on-
time-and-gravity/)

The more I read and hear about space the more I get the feeling we don't know
nothing about it...

~~~
zppx
General relativity remember? In which the spacetime is a simple manifold
(okay, not so simple), if Einstein's theory is right or wrong in such a large
scale (the entire universe) we must confirm or reject this with experiments,
but I see Ockels's theory as some form of solipsism that does not mix
comfortably with my worldview.

But I agree with you, we do not understand the nature of space.

------
Detrus
The flying spaghetti monster said "let there be light," and there was light.
Then light instantly reached all the corners of the singularity. And he said
"whoopsiee, gonna have to edit that out." So he moved a slider on his touch
screen to change the properties of dark energy and there was the big bang, and
obviously light intensity was another slider.

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petercooper
Dr. Pamela Gay and Fraser Cain (of Universe Today) covered this topic really
well on their awesome Astronomy Cast:
[http://www.astronomycast.com/astronomy/the-big-bang-and-
cosm...](http://www.astronomycast.com/astronomy/the-big-bang-and-cosmic-
microwave-background/)

Their "What is the universe expanding into" episode went further:
[http://www.astronomycast.com/astronomy/episode-28-what-is-
th...](http://www.astronomycast.com/astronomy/episode-28-what-is-the-universe-
expanding-into/)

I've been listening to an episode of Astronomy Cast every night for the past
two years now - can't get enough of it :-)

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tfh
From what I understood it's like this:

    
    
      class Universe:
        def __init__(self)
            self.time  = []
            self.space = []
    
      our_universe = Universe() # <== Big Bang

~~~
rvanrooy
without a compiler

~~~
sp332
Dilbert: Today we're going to learn what time is. Imagine a donut fired from a
cannon and spinning at the speed of light. Time is just like that, but without
the donut and the cannon.

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erikstarck
The Big bang was a highly unlikely event made possible by the fact that time
didn't exist before it happened - thus the whole concept of probability over
time falls apart. Or maybe of all unlikely events it was the most likely one
to occur.

In any case I think it's fascinating that tiny creatures such as ourselves can
think thoughts like this. The universe is clearly using us to understand
itself.

~~~
mojuba
> time didn't exist before it happened

Do you see what's wrong here? If there was no time, there could be no
"before". Besides, if you are talking about probabilities of something to
occur, that means there was some cause of that event, and that in turn implies
there was time before time started (?) because causes and effects occur in
time.

These things have nothing to do with the Big Bang theory anyway, let alone all
this kind of speculations make little or no sense.

~~~
s_baar
I'm not sure how to explain the Big Bang, but I'm fairly confident we'll now
have to do it without using the word 'is'.

~~~
stcredzero
Something for Bill Clinton to do in his retirement?

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sigzero
That doesn't really change any arguments on either side at all does it? "OF"
space? Where did that space come from? etc. etc. etc.

~~~
zppx
Well, no model of big bang cosmology says something about how energy came to
exist in this space as well, and based on the principle of mass-energy
equivalence we do not know how stuff came to exist. It is important to keep in
mind that the Big Bang cosmology was a theory originally described by a
catholic priest (Georges Lemaître), this theory is closer to theism than its
direct (academic) competitor in the past, the steady state cosmology, that
predicted an eternal universe, basically it says that time and space existed
since forever, in which new matter is created every moment.

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bartl
Oh, man, good article, but the page that the word "win" links to, in the last
paragraph, is just ridiculous.

