Hello, sorry if I am not completely correct with this, as I am just learning about the Big Bang. I read online that at t=0 the universe was much smaller than an atom. But why did time start? And why was the universe even there to begin with?
it's ONLY a theory......don't run away with the concept just yet........
What we know is this: "according to classical General Relativity, all timelike paths within the Big Bang, when followed backward to their origins, flow into a singularity". That's not quite the same as saying that time had a start, and furthermore that statement is probably qualified by quantum gravitational effects that we don't understand.
Also, the big bang may have occurred inside an external manifold that also had time-like dimensions.
As to why the universe came into being is still unknown - at least for now.
Time started when the universe popped into being. Both as a progression of events, and as a way for us humans to comprehend the sequence of what happened when.
[ Physicsoftheuniverse.com Link ]
Totally circular logic. The universe simply did NOT EXIT until the Big Bang happened and CREATED the Universe. TIME did not exist because the Universe did NOT exit.
NO ONE KNOWS WHY OR HOW either time or the Universe popped into existence. For me, random quantum fluctuation in the quantum foam or sub-space, whatever words you want to use to describe whatever existed BEFORE the Universe extied is good explanation. Sometimes the SIMPLEST explanation IS the correct explanation for HOW something happens, but it still does not answer the question WHY..
The Big Bang CREATED the Universe AND time.
Your question starts with the FALSE premise that the Universe ( yes there is a reason for the captial "u") has aways existed, but you are assuming that the Universe did NOT have time. You CANNOT have the Universe WITHOUT including time. Time is an integral component OF the Universe..
YOUR SUBJECTIVE universe has NOT always existed. How far back does your autobiographical memory go. Do you remember your third birthday? I DO remember my third birthday. Many people don't remember STARTING SCHOOL. I DO remember that too.
There was no time before time so it didn't all of a sudden start. Time did begin or it would not exist now. However you seem to be making the mistake of trying to apply probability or likelihood to concepts where time is not a factor. If time does not exist, neither does probability.
Answer any or all of your questions and you will get a noble prize in physics.
There is no explanation for what caused the theoretical Big Bang. The Big Bang Theory doesn't explain creation but merely provides a theory for the universe's history and why the universe is the way it is today. Moreover, your "why" questions operate on the unproven presumptions that, one, there must be a purpose to the universe and, two, the existence of matter, energy, time, etc. had to have had an inception and isn't simply something that has always been.