How did people come up with the theroy that the singularity was nothing or just a tiny spec? Is there any evidence that supports this theroy?
This is a common question, and though it has been discussed previously, I'll give a brief self-contained answer here.
The singularity is not really a "thing", but a condition in which physics breaks down.
The word "singularity" is a mathematical term. It refers to a condition in which a function diverges or becomes undefined. For example, the function f(x) = 1/x has a singularity when x equals 0.
The theory we use for gravity and spacetime is general relativity. When applied to the universe to track the "scale" and "curvature" of spacetime into the past, the functions diverge to infinitesimal scale and infinite or undefined curvature. This is the singularity; and it marks a point at which classical physics breaks down.
As yet, we do not have a physical theory able to describe these conditions. It will require a combination of relativity and quantum mechanics; which is an unsolved problem in physics.
The theories for relativity, spacetime, and the expanding universe are obtained in the same way as all scientific models. Hypothesize and test.
There is copious evidence for relativity, and for the expanding universe. There is no credible basis for doubt that the universe has expanded from conditions of extreme heat and density some 13 or 14 billion years ago.
It is a mathematical theorem that relativistic physics diverges to a singularity of unbounded density and infinitesimal scale as you extrapolate into the past. This marks a point at which we don't know what is going on, because existing physics can't handle those conditions.
I'm choosing words carefully here. "Scale" is a bit like "size". It does not mean that there is any particular object, like a "speck", which developed into the universe. It means that the entire universe was compressed. Everything we see now was once contained within a small region; but this does not mean that this region had any edge.
Imagine we could look into the distant past, when the whole universe was hot plasma a bit like the middle of the Sun, extending in all directions without edge or bound. Now draw an imaginary sphere within that thick hot dense material. That sphere is the "region" which contained all the matter visible today. "Scale" is just an indication of the factors by which things have expanded.
But what I don't get is why people believe it started from ''nothing''. Wouldn't logic dictate that matter can't come from nothing. Doesn't it make more sense if the ''singularity'' was actully a huge object that got to massive and exploded?
Cosmologists don't know how it started, and they don't usually say things like "it comes from nothing".
Part of the problem is that cosmology is very unintuitive and hard to explain, and when trying to explain it without all the mathematics, we have to use simple word pictures that may help with some of the broad concepts, but which fail to capture the details accurately.
Definite statements that it started from nothing should not appear in any credible exposition of the scientific models. We don't even know that the notion of time is meaningful as you approach the singularity.
However, this is not because of any logical rule about matter not being able to come from nothing. In fact, matter can come from nothing in quantum mechanics; depending on how you think about it. Space is full of virtual particles popping in and out of existence, all the time, from nothing.
The reason we don't say "it comes from nothing" is because we really just don't know about the very earliest beginnings.
Logic is not how to approach this. Logic is a way of carefully figuring out the consequences of your starting assumptions; but logic is useless unless you've got those assumptions right to begin with. In cosmology, that means having to learn a lot of advanced physics. Without that, using "logic" is generally very misleading.
In cosmology, you have to get rid of simple intuitions. You can apply mathematical logic to derive the consequences of a mathematical model, but sweeping intuitive notions are invariably just wrong.
The singuarity is not an object. It is a condition, or state, in which physics breaks down. The notion of something exploding is not a good way to think about the Big Bang, or the singularity.
Intuitive ideas about exploding objects are wrong.
Just to add to what I've just said, when a lay-person asks about creation from "nothing", they are almost certainly implying some form of naive absolute nothing. So when physicists all-to-often reply with the pair-creation argument, they are not addressing the real concern.
Good point: I agree. Thanks for the caution.
The point of pair creation, in my opinion, is simply to shake up the intuitions people bring to the subject; it's not a model for the origin of the universe.
Although it could be close... some folks (Stanford cosmologist Andrei Linde and chaotic inflation?) speculate about a "universe" popping into existence as a quantum fluctuation; though of course as you point out this presumes a quantum foam and a multiverse in which many such events may occur, and so just pushes the "ultimate origin" question to a new domain.
Or it could be totally different... the universe may fit the nice smooth zero-boundary Stephen Hawking notion with no larger meta-universe in which you can speak of our universe as a thing coming into existence at all.
Bottom line (IMO). For a beginner, be ready to drop every intuition or logical assumption you might expect; and stretch your mind as far as it will go. It won't be far enough. For someone who has a good grasp of the subject matter, get ready to revise your understanding. Although much has been discovered about expansion and the history of the universe, there are still enormous gaps in our knowledge.