Reading:
This theory states that because space is continually expanding it must have started out at some beginning point.
Let me start by what is going on today, and I'll work my way backwards to the big bang.
The universe, if you look at it in very large scales by "zooming-out" a lot, is expanding as a whole. The analogy I like the best is that of a blueberry muffin expanding as it is baked. Dense structures withing the universe, like stars and galaxies, are held together by their gravity, and they are not expanding. They are the "blueberries". But the space between them, which is only filled with very tenuous stuff (generally extremely rarefied gas), behaves like the muffin dough, and it is expanding. For this reason, the distances between the blueberries (the galaxies) is growing with time. This is not because they are moving away from each other. Rather, the space itself is growing - it is "stretching". Mind boggling, but true. Got the picture in your mind? Space expanding and blueberries flying away from each other? OK, now let's play the movie backwards.
Time is now going backwards, and galaxies are coming closer to each other. If you just keep going, inevitably everything comes very close together, in a crushing mega-dense beginning of the cosmos, where all matter and energy in the universe was stepping on the toes of all other matter and energy. But there is an additional twist. Einstein's general theory of relativity says that if you reach very high densities, time itself starts to act peculiar. In less and less time as we understand it, more and more things can happen. And if you crush all space into itself, you are also crushing time onto itself, and time does not exist. In this first mega-dense instant, where there was no "space" for things to exist in, there was also no "time" for them to evolve in. Time and space were born together. This event is the big bang.
This is where problems of depiction arise. If you have no time and no space, how do you represent the big bang? Unfortunately, the answer is, you can't. The representations you describe in your message are representations of the evolution of the universe, of the expansion of the universe, which are true even at late times, they are not representations of the big bang itself.
NASA is of course right that the big bang is an expansion of everything. On the other hand, an expansion of everything does look a bit like an explosion. It's OK to show a picture of an explosion, as long as you put the right words around it.
The axis of the cone is the "timeline". This is another favorite thing for scientists, because scientists love to draw plots with time as the one axis. The idea is: the size of the oval at any point along the time axis represents the size of the universe at that time. In the beginning, in the big bang, time is at "zero", and the universe is tiny. This is the apex of the cone. Then, as time goes by, the size of the universe (the area of the oval) increases. If this is the cartoon I have in mind, it also shows what there is in the univers at each time instant (photons and ions in the beginning, then dark, then galaxies later on). This is correct too, but again it needs the proper explanation.1
This theory states that because space is continually expanding it must have started out at some beginning point.
Let me start by what is going on today, and I'll work my way backwards to the big bang.
The universe, if you look at it in very large scales by "zooming-out" a lot, is expanding as a whole. The analogy I like the best is that of a blueberry muffin expanding as it is baked. Dense structures withing the universe, like stars and galaxies, are held together by their gravity, and they are not expanding. They are the "blueberries". But the space between them, which is only filled with very tenuous stuff (generally extremely rarefied gas), behaves like the muffin dough, and it is expanding. For this reason, the distances between the blueberries (the galaxies) is growing with time. This is not because they are moving away from each other. Rather, the space itself is growing - it is "stretching". Mind boggling, but true. Got the picture in your mind? Space expanding and blueberries flying away from each other? OK, now let's play the movie backwards.
Time is now going backwards, and galaxies are coming closer to each other. If you just keep going, inevitably everything comes very close together, in a crushing mega-dense beginning of the cosmos, where all matter and energy in the universe was stepping on the toes of all other matter and energy. But there is an additional twist. Einstein's general theory of relativity says that if you reach very high densities, time itself starts to act peculiar. In less and less time as we understand it, more and more things can happen. And if you crush all space into itself, you are also crushing time onto itself, and time does not exist. In this first mega-dense instant, where there was no "space" for things to exist in, there was also no "time" for them to evolve in. Time and space were born together. This event is the big bang.
This is where problems of depiction arise. If you have no time and no space, how do you represent the big bang? Unfortunately, the answer is, you can't. The representations you describe in your message are representations of the evolution of the universe, of the expansion of the universe, which are true even at late times, they are not representations of the big bang itself.
NASA is of course right that the big bang is an expansion of everything. On the other hand, an expansion of everything does look a bit like an explosion. It's OK to show a picture of an explosion, as long as you put the right words around it.
Another thing that scientists like to do when they are trying to describe how a three-dimensional structure (like the universe!) evolve with time, is to make sketches showing how a two-dimensional structure (like a flat sheet) evolves in time,
because we can intuitively comprehend the idea of expansion in two dimensions better than the expansion in three dimensions. Don't let this confuse you - this is an oversimplification done for demonstration purposes. The universe is not a flat sheet (it's a muffin :-). But if you want to show the flat sheet picture, this is also OK as long as you say the right words when you present it (e.g., "the universe is not a flat sheet, but this is just to give you the idea of what expanding space looks like").
The cone is another such simplification for demonstration purposes.
The axis of the cone is the "timeline". This is another favorite thing for scientists, because scientists love to draw plots with time as the one axis. The idea is: the size of the oval at any point along the time axis represents the size of the universe at that time. In the beginning, in the big bang, time is at "zero", and the universe is tiny. This is the apex of the cone. Then, as time goes by, the size of the universe (the area of the oval) increases. If this is the cartoon I have in mind, it also shows what there is in the univers at each time instant (photons and ions in the beginning, then dark, then galaxies later on). This is correct too, but again it needs the proper explanation.1
In other words:
Narative Work:
Sketches
Report
Dates
Activity: Make and eat blueberry muffins
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