Before a certain point, you start dealing with things like (near?) infinite temperatures and densities that science as we know it can't deal with. If the pre-big bang universe was a singularity as we suspect, it would hide it's past pretty effectively. The 'why' is probably better asked to a philosopher or a theologian.
Welcome to Physics Forums, afton! As Grogs suggests, the cause of the Big Bang is unknown. Big Bang Theory explains what happened to the universe after the first split second following the beginning, but does not explain the beginning itself (time=0). There are a few science-based ideas about it, but there's no well-supported/strong theory to explain it. Perhaps some of the other members would like to mention a few of those ideas here.
The following is a science-based speculation. I'm reserve the right to be irresponsible if I got this all wrong. (But at least I give references!) Alex Filippenko speaks about this issue very briefly in lecture # 39 of his Introduction to Astronomy course. That lecture is called The Ultimate Free Lunch?. You can purchase his video course on Astronomy here (although I certainly wouldn't recommend buying it just for that one speculative lecture! He certainly didn't clear up any details.) Introduction to Astronomy Video The following is my own personal interpretation of what Filippenko said, combined with some other stuff that I know (which isn't much!) about QFT and the Higgs field Basically all Filippenko states in his lecture is that some scientists have speculated that the entire universe began as a quantum fluctuation that lasted long enough for the process of inflation to take hold (he mentions the idea of inflation in a prior lecture). Once the process of inflation begins there is no longer any way for the quantum fluctuation to annihilate itself and all hell breaks loose from that point onward. He suggests that energy conservation has somehow been maintain during the whole thing. To be quite honest I came away from that lecture with more questions than answers. I still have the videos though and intend to watch this again a few more times. He doesn't go into any mathematics at all. This is strictly a course designed for laymen! The idea of inflation itself is not well understood. My understanding is that it has something to do with a Higgs field that got hung up in some kind of energy well. I've read about this in Brian Greene's book "The Fabric of the Cosmos", and other sources including Steven Pollock's lecture on Particle Physics also from The Teaching Company,… Particle Physics Video Lectures Pollock talks about the Higgs particle (or Higgs Field) in lecture 19 of his videos. Trying to understand how all this works mathematically is what has sparked me to start learning the mathematics of QFT. I have a long journey ahead of me in that regard, by the way. In any case, it seems to me that for this speculative conclusion to have any merit we must assume that a quantum field has pervaded all of spacetime prior to the creation of our universe. In other words, there must have already been a quantum field to fluctuate in the first place! The way that Filippenko describes it, (if I remember correctly) quantum fluctuations where happening everywhere all the time. It was only when one lasted for an abnormally long period of time that inflation was able to take hold and the whole Higgs-field effect took over. I suppose that all of this has some justification within the formal mathematics of QFT. Although, it isn't sold as a solid scientific theory either. Even Filippenko addresses it as nothing more than interesting speculation. Although, he must feel pretty confident that it has some mathematical validity or I don't believe that he would have mentioned it at all. I'll have to watch Filippenko's video again and see just how bad I screwed all of this up. :yuck:
The best explanation I know is the one according to string theory. Where 2 5-dimensional branes collide with each other and produce a huge amount of energy which makes our universe. Otherwise I think we don't have squat to explain it. A series about string theory produced by NOVA is online in it's entirety. Go to the part that talks about the big bang. Elegant Universe
This is a great question. As it is one of the great unknowns I can talk a load of twaddle, and no one can prove me wrong. If only our universe was closed it would be easy to answer the question, we just cycle between big bang and big crunch. Much to my annoyance the universe is open. Is current knowledge indicating that the total energy of our universe is zero? If the sum of all energy and mass is equalled by the sum of exotic “negative” energy and anti matter it would tie in quite well with a theory based on the random fluctuation at a quantum level which essentially says that the universe just appeared from nothing. The universe came from nothing, and is still nothing. Needless to say, there are a few loose ends here. For example, if we started from a random fluctuation wouldn’t there have been a need for space/time to contain the random fluctuation? What’s to stop another random fluctuation happening now within our universe? Questions, questions, I’m getting a headache. Are there any geniuses out there with some suggestions.
Perhaps the universe did not have a beginning. It depends on the clock you are using. The CMB is blueshifted if you go (in imagination) backwards in time towards the big bang. It tends towards infinite blue shift as you approach t = 0. Therefore if you use as the 'tick' of your clock the 'beat' of a typical photon sampled from the CMB (say at peak black body intensity) there are an inifinite number of 'beats' between now and that 'singularity'. Measured by such a clock the universe becomes infinitely old and there is no beginning. Just a thought. Garth
I don't find brane collisions very satisfactory. Still leaves the question begging 'where did branes come from, mommy?" An infinitely old universe also carries a lot of baggage. Paradoxes aside, why do high redshift [i.e., ancient and distant] objects look so much different than nearby objects? The observational evidence suggests the recent [nearby] has evolved over time.
Without a doubt! I'm not even sold on the Big Bang Theory myself. At least not in the sense that it started at an infinitesimal point. Seems to me if that were the case then there would be one special direction that we could point to and say that it started from there. I more inclined to believe that the universe kind of started everywhere at once. Not at some particular infinitesimal point which a theory concerning a quantum fluctuation would certainly suggest. Well the theory is that spacetime is in a constant state of fluctuation. There are virtual particles appearing and disappearing all around us all the time in numbers that could easily be thought of as infinite. This is the nature of spacetime. Although, the idea that the universe began as a quantum fluctuation does seem to suggest that spacetime fluctuations existed prior to the actual creation of the universe and therefore spacetime itself must have existed. Maybe just not in any permanent form of matter like we see today. I actually think about this a lot. If quantum fluctuations are so plentiful, even if there is a very small probability that any one of them will last long enough for inflation to occur, it still seems that they would occur at least often enough that we'd see evidence of other universes inflating within our own universe! However, there are mathematical explanations of why this might not be able to occur inside of a universe that has already undergone inflation. For example, the Higgs field in our universe has already caused inflation and everything that goes with it. The presence of this Higgs field could then prevent any quantum fluctuations within our current universe from lasting long enough to errupt into yet another inflation. The fluctuation has to last a highly improbable amount of time before it can inflate. (at least that's my understanding) The problem with many of these theories is that you need to be more than just a genius. You also need to be highly educated in mathematics, as well as being highly creative in your thinking. Even then you don't get anywhere. But instead of getting headaches you go into some sort of blissful state of euphoria where you don't really care what it all means anymore. You just get high on the mystical elegance of the mathematical symmetries and end up becoming an abstract-junkie.
I feel the same way about the brane theory. It's just all too speculative. We don't even have any evidence that branes even exist. We do, however, have experimental evidence that quantum fluctuations occur all the time on a regular basis. I'm not sure how solid the idea of inflation is though (I mean in terms of what might cause it). All I know, (and I could be terribly wrong about this), is that inflation has something to do with the mathematics of QFT and the Higgs field. It's my understanding that QFT predicts inflation somehow. I don't even pretend to know the mathematics behind it. I'm working toward learning QFT, but I have a very long way to go even in basic fields like differential geometry, and group theory, before I can even begin to study QFT. But I'm plugging away ever so slowly. I would love to actually understand the mathematical framework of all of this. I wish there were more video lectures available on these higher-level mathematics and physics courses. I learn best by going over the stuff many times. I just can't learn from attenting fast-paced classes one time. I need to have a pause, rewind, and replay button to control the lecturer!
There's a theory that I really don't like, but has attracted some attention, perhaps because its spectacularity, and goes by the names of Cosmological Natural Selection, or Cosmic Natural Selection, or Darwinian Cosmology; was proposed by Lee Smolin. He presented this theory in 1997 in his book "The Life of the Cosmos". Basically the theory says: - Every time that a black hole is created, a new Universe is formed springing from the BH singularity - Therefore, the Big Bang of our universe is interpretated as one of such black hole creations - All the material that falls into the Black hole, ends up in the Baby Universe Some questions that I have: -Is possible to travel from our Universe to our mother Universe? -What created the space in which the original, the first Black Hole lived? (Or perhaps this hierarchy of ancestors Black Holes extends infinitely towards the past? That is, there wasn't an original Black Hole) Some obscure points (no pun intended ) that I see: -Why should a Black Hole create a new Universe? -Where's all the material that is supposed to be coming from our mother universe?
That may be the case (could you supply a reference regarding QFT and inflation?) but the flat fields of quantum physics and the curved space-time of GR do not play together well. This is a huge problem for the people trying to extend gravity to the quantum level. My (admittedly amateur and math-challenged) readings in this field lead me to believe that when curved space-time is interpreted as "practically" flat on Planck scales, some kinematic models of gravitation might be tenable, but right now, dynamical models of quantum gravitation in a GR framework still seem hopelessly out of reach. My as-yet poorly-developed ZPE model of gravitation does not rely on curved space-time to mediate gravity. It offers the advantage of flat reference frames and a real quantum-level mechanism for gravitational attraction. If the Athena project demonstrates a differential in the gravitational infall rates for electrically neutral matter vs antimatter, this model will get a chance.