View Full Version : Big Bang question

2003-Jun-20, 03:53 PM
Whenever I read about the Big Bang theory, it is stated that before the Big Bang, every piece of matter in the universe occupied the same point in space.

But how is this possible, and why is this assumed?

The reason I'm having trouble understanding this is because when I think of an explosion, be it a bomb or whatever, the elements within the object that exploded were not occupying the same point is space, they were simply very close together.

Or am I mistaken in the idea that all the elements in the universe once occupied the same point in space?

2003-Jun-20, 07:52 PM
One of the sticking points may be thinking of the big bang as an explosion. From http://map.gsfc.nasa.gov/m_uni/uni_101bb2.html:

The Big Bang did not occur at a single point in space as an "explosion." It is better thought of as the simultaneous appearance of space everywhere in the universe. That region of space that is within our present horizon was indeed no bigger than a point in the past. Nevertheless, if all of space both inside and outside our horizon is infinite now, it was born infinite. If it is closed and finite, then it was born with zero volume and grew from that.

The big bang was actually the event that created space and time. Matter was created a tiny fraction of a second afterward from the vacuum energy that caused inflation. http://www.pbs.org/deepspace/timeline/ contains a clickable overview of the history of the universe.

2003-Jun-20, 08:35 PM
Ah, thank you very much.

But reading through the link provided, I find myself with many more questions.

The universe begins with a cataclysm that generates space and time, as well as all the matter and energy the universe will ever hold. For an incomprehensibly small fraction of a second, the universe is an infinitely dense, hot fireball. The prevailing theory describes a peculiar form of energy that can suddenly push out the fabric of space. On a rare occasion, a runaway process called "Inflation" can cause a vast expansion of space filled with this energy. The inflationary expansion is stopped only when this energy is transformed into matter and energy as we know it.

Now, would it be possible for this strange form of energy to push into space at anytime? In other words is it possible for another Big Bang to occur at any time, anyplace?

Don't take that as me looking for a catastrophy story or anything, I'm just curious.

Also, how did they come up with such precise times for these occurances (ie: 10-6 seconds after the Big Bang, such and such happened...)?

2003-Jun-20, 09:46 PM
I wish someone else would chime in. Someone who actually knows something about cosmology. I.E. not me.

All right, based on what I remember, inflation was supposed to have been caused by vacuum energy which was dominant while the universe was extremely tiny, but became less dominant as the universe expanded. Today it isn't as much of a factor. Though there's some evidence that a lot of the mass of the universe is in the form of dark energy, which is causing the expansion of the universe to accelerate.

So I'd say no, another big bang probably wouldn't happen. Though if you're wondering if other universes are being created, some believe that on a quantum scale, microscopic wormholes are continually popping into existence and disappearing; that sometimes a tiny portion of spacetime pinches off from the rest of the universe, connected only by a wormhole, and that a baby universe such as that could undergo inflation. I remember a speculative article in Astronomy magazine awhile back that talked about the possibility of creating custom universes in the lab, though we're still pretty far short of the energy requirements needed to do so.

As for the times, I suspect that they mean "10^-6 seconds to within an order of magnitude" some of the time. We have various theories that predict how matter behaves at various temperatures. So, if we know, or can guess, the temperature of the universe based on its size and density, then we can predict what the universe was like at that time. Sometimes we can back this up with observations based on the behavior of particles in particle accelerators, or cosmologically, such as observing the CMB.

Now if I'm wrong someone will correct me, I'm sure. ;)

glen chapman
2003-Jun-21, 01:50 PM
I think the first thing to consider is the Big Bang is a theory, and nothing more. At the moment Big Bang seems to explain what we are seeing in our universe better than any other idea that has been evolved. Perhaps in a hundred years students will shake their heads and say "How could they be so silly" or "Gee those guys were pretty smart after all."

Why did the Big Bang occure? Who knows. Pick an idea, any idea. The biggest problem we have for understand the mechanics, is not knowing what was around before, or what the universe is expanding into.

Whatever the phenomena, we are part of it. Science can't draw from an external view. It's probable we never will.

In terms of things happening so many seconds after the event. It must be remembered we are placing our labels and time frame on events. Time was invented when the universe was. As Stephen Hawkins said. "Time is God's way stopping everything happening at once." Has a sort of elegance about it.

Glen Chapman

Kaptain K
2003-Jun-21, 08:06 PM
If you think that "Big Bang" and "Inflation" are strange, try wrapping your mind around "String" or "'Brane" theory. I think we are making progress, but are still far from a "Theory of Everything". There is plenty of job security in the field of theoretical physics for many years to come! :o

Tim Thompson
2003-Jun-23, 06:09 PM
The easiest way to get around the problem of understanding what the "bang" really was is to basically ignore it. The real key to big bang cosmology is the concept that the universe has evolved with time, from an initial observable state that is very small, very dense, and very hot, to a present state that is much larger, much cooler, and much more sparse. The idea that time & space were "created" at the bang is really based on an overly literal reading of general relativity. If one assumes that GR is the only theory of space time, then one concludes that the universe started out in a "singular" state, where "singular" is a mathematical expression synonymous with "undefined". But once you assume that the universe actually started out as a real tiny thing, you are making the mistake of defining the undefined. The only factually correct statement that can be made about the true initial state of the universe is that it is both unknown, and unknowable, in the context of classical theory (such as GR). Just start with the intial observable state, and we're in business.

But once we add quantum mechanics to the list, the true initial state of the universe becomes both knowable and definable, but in both cases the answer depends on the details of the theory. Perhaps the "bang" was a "quantum fluctuation" in some pre-existing analog for space & time. Or, perhaps it was something more complicated, like a brane collision or a Hagedorn transition in string theory. One of the nice things about string theory as a theory of space time is that it allows an extensive "pre big bang scenario" that can allow the construction of a theoretical universe arbitrarily far into the past. This can also be done with inflation in more classical theory (inflation can stretch into the infinite past), but that seems a less popular notion at the moment.

2003-Jun-24, 04:57 PM
This is probably a stupid question, but it's been nagging on my mind for months now, and I don't know of anybody else to ask.

The denser an object is, the more gravity it has, right? That's why, when the mass of a big enough star is compressed into a small enough space, it collapses into a black hole. So if we're supposing that the mass of the entire universe is compressed into an "infinitely dense, hot fireball," we're talking about a singularity with far a stronger gravity than that of a black hole. So how could the universe have exploded outward if it was being held together by such a powerful force?

2003-Jun-24, 07:26 PM
Welcome to the board. There's never a stupid question.

This is probably a stupid question, but it's been nagging on my mind for months now, and I don't know of anybody else to ask.

The denser an object is, the more gravity it has, right? ...

Not quite. Mass and distance are the only two factors in the gravitational force from an object. Take two objects of identical masses, the first eight times the density of the second. This means the second has a diameter twice that of the first. For any distance outside the diameter of the second (larger) mass the gravity will be identical for both. Once inside this boundary the gravitational force from the first mass continues to increase, while that of the second actually begins to decrease as shells of mass "above" you can be neglected.

Increased density can allow you to get closer to the total mass of an object, thus increasing the experienced gravitational force, but does not in and of itself produce a stronger force.

Unfortunately, I can't provide much else on the rest of your question right now. I'll try later if no one else weighs in.

2003-Jun-24, 08:16 PM
Welcome, Gregory.

Quantum gravity sucks. It all has to do with the fact that space-time itself was expanding outwards and other stuff.

2003-Jun-25, 04:37 PM
Thanks for the info, both of you.

I don't normally inflict my sense of humor on others, but it would go against everything I believe in if I signed off without first pointing out that of *course* quantum gravity sucks. All attractive forces do :D