As in any good scientific theory there are questions still to be answered, such as the precise nature of the dark matter and dark energy which are prominent actors in the story, or the very interesting question of whether there was a very early phase of inflationary exponential expansion, but these do not suggest the basic picture could be wrong.
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What concerns me is the other meaning of the big bang, which is the further hypothesis that the ultimate origin of our universe was a first moment of time at which our universe was launched from a state of infinite density and temperature. According to this idea, all that exists or has ever existed is It makes no sense to ask what was before that because, before that, there was not even time.
The main problem with this second meaning of big bang is that it is not very successful as a scientific hypothesis because it leaves big questions about the universe unanswered. It turns out that our universe has to have started off in an extraordinarily special state for the universe to evolve to anything like our universe. The hypothesis that there was a first moment of time turns out to be remarkably generic and unconstraining as it is consistent with an infinite number of possible states in which the universe might have started out. This is due to a theorem proved by Hawking and Penrose, that almost any expanding universe described by general relativity has such a first moment of time.
Compared to almost all of these, our own early universe was extraordinarily homogeneous and symmetric.
Big Bang (book) - Wikipedia
If the big bang was the first moment of time there can be no scientific answer because there was no before on which to base an explanation. At this point theologians see their opening and indeed have been lining up at the gates of science to impose their kind of explanation-that god made the universe and made it so. Similarly, if the big bang was the first moment of time there can be no scientific answer to the question of what chose the laws of nature. Rapid cooling allowed for matter as we know it to form in the universe, although physicists are still trying to figure out exactly how this happened.
Georges Lemaître, Father of the Big Bang
About one ten-thousandth of a second after the Big Bang, protons and neutrons formed, and within a few minutes these particles stuck together to form atomic nuclei, mostly hydrogen and helium. Hundreds of thousands of years later, electrons stuck to the nuclei to make complete atoms.
About a billion years after the Big Bang, gravity caused these atoms to gather in huge clouds of gas, forming collections of stars known as galaxies. Gravity is the force that pulls any objects with mass towards one another -- the same force, for example, that causes a ball thrown in the air to fall to the earth. Where do planets like earth come from? Over billions of years, stars "cook" hydrogen and helium atoms in their hot cores to make heavier elements like carbon and oxygen. Large stars explode over time, blasting these elements into space.
This matter then condenses into the stars, planets, and satellites that make up solar systems like our own.
HOW DO WE KNOW THE BIG BANG ACTUALLY HAPPENED?
How do we know the Big Bang happened? Astrophysicists have uncovered a great deal of compelling evidence over the past hundred years to support the Big Bang theory. Among this evidence is the observation that the universe is expanding. By looking at light emitted by distant galaxies, scientists have found that these galaxies are rapidly moving away from our galaxy, the Milky Way.
An explosion like the Big Bang, which sent matter flying outward from a point, explains this observation.
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Another misconception is that we tend to image the singularity as a little fireball appearing somewhere in space. According to the many experts however, space didn't exist prior to the Big Bang. Back in the late '60s and early '70s, when men first walked upon the moon, "three British astrophysicists, Steven Hawking, George Ellis, and Roger Penrose turned their attention to the Theory of Relativity and its implications regarding our notions of time. In and , they published papers in which they extended Einstein's Theory of General Relativity to include measurements of time and space.
Big Bang Theory
Prior to the singularity, nothing existed, not space, time, matter, or energy - nothing. So where and in what did the singularity appear if not in space?
We don't know where it came from, why it's here, or even where it is. All we really know is that we are inside of it and at one time it didn't exist and neither did we. First of all, we are reasonably certain that the universe had a beginning.
Second, galaxies appear to be moving away from us at speeds proportional to their distance. This is called "Hubble's Law," named after Edwin Hubble who discovered this phenomenon in This observation supports the expansion of the universe and suggests that the universe was once compacted. Third, if the universe was initially very, very hot as the Big Bang suggests, we should be able to find some remnant of this heat.
This is thought to be the remnant which scientists were looking for. Penzias and Wilson shared in the Nobel Prize for Physics for their discovery. Finally, the abundance of the "light elements" Hydrogen and Helium found in the observable universe are thought to support the Big Bang model of origins. Is the standard Big Bang theory the only model consistent with these evidences? No, it's just the most popular one. Internationally renown Astrophysicist George F. Ellis explains: "People need to be aware that there is a range of models that could explain the observations….
For instance, I can construct you a spherically symmetrical universe with Earth at its center, and you cannot disprove it based on observations…. You can only exclude it on philosophical grounds.