If there was a Big Bang 15 billion years ago, shouldn’t all of the matter be distributed as if on an empty sphere centered in the big bang location? You’d think after 15 or so billion years of expanding universe there’d be a heck of a big hole out there. I’m sure this a simplistic explosion concept that can be shot down by E=mc2 some how, but I just dont get it.
This leads me to many more questions about the universe and seeing “back in time” by observing objects 15 billions light years away. - but one question at a time.
You have to think of space (and time) being created at the Big Bang, rather than a big explosion in an ‘empty’ universe waiting for matter to expand into it. Matter is (sort of) uniformly distibuted through all space and is expanding along with space not moving apart though pre-existing space.
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You have to think of space (and time) being created at the Big Bang
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Ugh, I hate that word “created”: it implies a nothing-to-something transition which no cosmologist I know of seriously entertains.
The universe is time and space. There is no such thing as ‘before’ all of time or ‘next to’ all of space. All of space is expanding: the galaxies stay where they are and the space between them expands. “Big Bang” is the IMO utterly misleading phrase (actually a term of insult from the guy who got it wrong) for the infinitely (or near infinitely) dense state at one end of the object called the universe from which the expansion proceeds, rather like ‘North Pole’ is the point from which the object called Earth expands. There need be no before that state (or, at least, the state ‘before’ may be identical, ie. the Big Bang exists for all t<0. Since it is undefined in this region you can pretty much describe it either way). One might say that the universe has always existed.
Well, the sort of stuff that astrophysicists will come out with when you ask a question about the big bang often makes my head hurt, but let’s try this as a possible explanation.
You seem to be thinking of the big bang as if it were a bomb exploding, with all of the mater being propelled at roughly the same speed in every possible direction. I suspect that, if we ignore the bit about the space in the universe being limited in the first few milliseconds after the big bang, that the reality would look a little bit more like equal distribution. Some matter is propelled at high speed in all directions, some is propelled at medium speed in all directions, some is propelled at low speed in all directions, and some has no ‘speed’ at all but just kind of sits where the big bang happened. (Except that relativity says that all of these are equivalent, but we can hopefully ignore that right now.)
Wherever you ‘sat’ in the first few moments after the big bang, you would see matter rushing away from you in all dimensions, just like we can now see distant galaxies moving away from us in all directions, because every galaxy had a slightly different momentum from the big bang, and those slight differences now define their location in the universe.
Sorry old chap but this is much more wrong than my use of the word ‘created’. Let me have another go without using the ‘c’ word, although Sentient M will still find fault.
You could look at it this way. The universe started***** with no size at all, with all the energy/matter squished in a point. Between then and now the universe has become less cramped but it is not expanding out into anything. There is no ‘outside’.
*****How else to put this? Maybe just say ‘15 Billion years ago’.
The question most people ask here is, “Where is the center of the universe?” i.e., where was the “center” of the big bang explosion? As if we’re a good distance off, watching some fireworks display.
The answer is this: the big bang happened EVERYWHERE in the universe all at once.
Over large scales i.e magaparsecs (Mpc) the Universe can be thought of as being homogeneous.
That is, the average mass density per unit volume over a sufficiently large volume is constant or at leaste tends towards a constant. Admittedly over small scales there is localised clumping i.e. galaxies, clusters and superclusters, but on even larger scales (many Mpc) the mass density will tend towards a constant value. Whether the actual magnitude of this is known I am not sure, but the magnitude of the average density decreases as the size of the volume increases.
For example:
The milky way has an average density of about 2x10-21 Kg/m3 (how do I superscript BTW?)
The local group has a mean density of 0.5 x 10-25 kg/m3
A typical supercluster has an avarage density of about 2 x 10-26kg/m3.
Sorry for the dodgy script.
So in answer to your question on avarage the matter is evenly distributed throughout the universe, although whether the universe is spherical is a matter of debate.
The big bang ‘happened’ everywhere. No matter is or was propelled: the space between matter expanded and continues to expand (with gravity counteracting that expansion locally, thich is why galaxies and indeed humans don’t themselves disperse).
Again, they are not and never were moving. Their location is a function of precisely how space ‘condensed’ (think of the sonic waves from the big “bang” - again the analogy should be treated with caution!)
Mea culpa, my friend. I hate sounding so boorish and adversarial, but I believe that the key to understanding cosmology (and especially the initial state of spacetime) is the exact language one uses. I happen to think that spatial language is far more useful in this regard than aything having temporal connotations. The Big Bang is a place more than an explosion.
Much better! Again, sincere apologies for being such a insufferable pedant.
I prefer “13.7 billion years away, at one end of this region of the universe”, but yours is just as good really.
I read your other post, SentientMeat, but didn’t quite understand it (not your fault – I’m stupid).
If space and time is the universe, why was there ever a big bang? I mean, if everything just is, why did time and space suddenly decide to pop up from their infinitely dense and tiny status? Why isn’t everything just one infinitely unchanging blob?
In your very last post in that thread, you said:
Are you saying that our universe – which began with the big bang – is part of a timeless, spaceless universe which, for some reasonless reason, enabled time and space to begin to at a certain timeless time? So basically, there was a bang of some sort and we are incapable of understanding anything beyond that?
If we consider time as another dimension of space, everything is ‘unchanging’: the past and future exist and there is no universal “now” - we humans simply move from one “initial now” (birth) through subsequent “nows” to a “final now” (death). The same could be said for the entire universe.
As for the “decision to expand”, again I’d suggest we divest ourselves of the cause-and-effect clothing which our time-dwelling brains are so used to and try to start thinking in spatial terms. If the universe is an object just like Earth, we do not ask “Why does Earth decide to expand from the North Pole out to the equator?” We can explain the shape of the Earth without such appeal to causation. Cosmology seeks to explain the shape of the universe.
“Our universe” might well be simply a region of the universe. Other regions might not have the spatial or temporal characteristics of ours, being two-dimensional, four-dimensional or having no dimension which could be called ‘time’. So the entire universe can’t be spaceless (zero dimensional) or timeless (unchanging in configuration) since we live in a region which is 3 dimensional and temporal.
It may be that there are regions of the universe ‘attached’ to that initial singularity, which could be said to be “before” it. We might or might not know anything about such regions (although I would call them protoscientific rather than pseudoscientific in this regard). What there wasn’t was a “state of nothingness” from which a universe emerged. There was always “something”, although we don’t yet (and might never) know the nature of that “something”.
I’ve managed (I think) to follow everything else except this. I can get that the big bang happened everywhere in the universe at once, given that all the matter is really standing still and only the space between the matter is growing, BUT…
All the matter that makes up the universe still has to make a shape, right? If not a sphere, then an egg, or oval, or even a blob, right?
So I would think that, just like we can see the distribution of the stars in our galaxy and figure out that we’re on the edge of one of the arms and can tell where the center of the galazy is, how come we can’t do the same to the universe?
… If the answer is that we don’t have telescopes good enough to see the edges, or that there is too much matter in the way to see the edges (too many trees to see the forrest), those answers would make sense to me.
I’m afraid that only makes sense if you posit an extra dimensional ‘outside’ from where you can observe the entire universe. Imagine trying to work out the shape of the Earth: we might get ships to sail right around the surface and return to the same point, and thus work out that the Earth is a slightly flattened sphere. With the three dimensional universe, what if the ships did exactly the same, returning to the depature point? (We’re ignoring stuff like faster-than-light travel or a 26 billion year round trip at this juncture!)
The complication is that our telescopes are observing the galaxies in the past. Since the universe is only about 14 billion years “old” (ie. the distance from us to the Big Bang is 14Bn years), when we look 14 billion light years away, we are literally looking at the Big Bang (again, everywhereis the Big Bang).
So our ship-voyage calculations are rather more complicated. Added to this is the further complication that very very near the initial singularity, the expansion of the universe has a completely different rate, which allowed light from any one region to spread to all other regions and provide a far more uniform view. The conclusion from all of this is that the universe is a hypersphere with a slightly flattened end near the Big Bang, and with an unknown shape at the other end.
For the record, I stand by my attempt to come up with an answer that guys like Gene and I can understand without our brains twisting up. (Though I do enjoy getting into the other stuff.) What I said was a simplification, to demonstrate a fairly simple point. I should probably have marked it more clearly as such.
The best analogy to explain this is to think of a series of dots drawn on the surface of a slightly inflated balloon which we’ll call the Dotiverse. The surface, as far as the DotWorlds, and the DotPeople living on them, is a two dimensional surface even though it has a curvature that can only be defined by a third dimension which is normal to the surface. As the Dotiverse expands more, the DotWorlds become further and further away from each other, as measured along the surface of the balloon, and therefore makes it increasingly more expensive to get ahold of that AlphaDot slave-DotGirl porn you’ve acquired a taste for.
However, relative to the surface of the Dotiverse, there is no center of expansion; all the DotWorlds are travelling away from each other at a rate of expansion proportional to their distance from one another. (Two nearby DotWorlds are travelling less slowly from one another than two distant DotWorlds.) The center of expansion is external (and therefore indiscernable) to the observable Dotiverse, and various DotCosmologists, DotPhilosophers, and DotTheologions make a substantial living arguing over why the Dotiverse is expanding, what kind of DotGod is doing the inflating, and whether or not D’He is going to run out of breath before the fabric breaks, rupturing the fabric of DotSpace and sending the entire Dotiverse careening about the room only to be consumed and then coughed up by the Schrödinger’s Cat whose gotten bored with this business about waiting about in a box while a bunch of nerds with thick glasses and bad oral hygiene argue about collapsing waveforms.
Similarly, our universe can be thought of as existing on the surface of a “hypersphere” (or a hypersurface, anyway, as it isn’t uniform) and expanding from the center as a function of t. Since our “objective” measurement of t is based only on inference from phenomena that are relative to t (like how fast other universes are speeding away from us) we can’t see the center of expansion; every object on our three-surface (analogous to the 2D surface of the Dotiverse balloon) appears to be moving away from every other object at a rate proportional to the dividing space.
It is, of course, more complicated than a simple balloon, and large scale astronomic structures like The Great Attractor and The Great Wall interfere with the normal rates of expansion. Currently one of the great questions in astronomy and cosmology is how heterogeneous structures were seeded and began to form out of the (presumably) uniform matrix of primordial matter that initially expanded from the singularity. Discovering/mapping/modeling the expansion of matter may give us clues to the structure and basic physical principles that dominated the premacrostructured “quark soup” that existed in the early seconds of the universe.