RE: Did the Big Bang happen?
April 27, 2022 at 6:32 pm
(This post was last modified: April 27, 2022 at 6:37 pm by polymath257.)
There are a lot of issues here that overlap and need to be clarified.
1. Standard cosmology is based on General Relativity for the background spacetime geometry (expansion of space).
2. The notion of a singularity is a tricky one: a singularity is NOT an object. it is more a description of something going wrong in our model. For example, the north and south poles are 'coordinate singularities' in the coordinate system using latitude and longitude. The reason is that both the north and south poles don't really have a longitude. That is something going wrong with our mathematical description, but not with the underlying geometry. it is also possible to have singularities in the geometry itself (cusps or 'holes'). The singularity at the beginning of the Big Bang in standard cosmology is closer to a 'hole', an absence, an inability to go further back in time.
3. The term 'Big Bang' can refer to either the singularity or the fact that the universe was hot and dense in the past. Usually, cosmologists mean the latter, not the former.
So, for most cosmologists, there was a Big Bang: the universe was once very hot and dense and expanded to become what we see today. Whether or not there was a singularity, this much happened.
4. As we go back in time, the temperature and pressure of the universe get very large (hot and dense). When the temperatures and pressures are high enough, there can be no molecules, no atoms, etc. So, in a sense, things get very simple at that point. So, during the time of nucleosynthesis, the whole universe was as hot as the core of a star and the pressures were enough for fusion reactions to happen everywhere. This leads to testable predictions about what can be observed now, so there is high confidence that this stage actually existed.
5. Once we get back to a certain point, though, our understanding of high energy particle physics breaks down (this is why we built the LHC). Beyond this point (and that includes the 'inflationary scenario, where the universe expanded exponentially fast) everything is pretty much speculation.
In particular, we don't know why there is more matter than anti-matter in the universe. We have guesses, but nothing concrete has been tested and verified.
6. General Relativity inevitably predicts singularities. At one time, it was hoped that they are just a part of certain symmetrical solutions to the equations, but Hawking showed that thi sis not the case: geometric singularities are inevitable in standard cosmology.
BUT, and this is big, General Relativity does not consider the quantum mechanical aspects of gravity. While we can do quantum theory with general relativity as a background, that doesn't include the quantum aspects of gravity itself. But we also know that quantum gravity effects will become relevant at some point. So GR alone simply won't be enough. The standard model will fail to describe reality at some point.
7. There is no tested quantum theory of gravity. We have a number of proposed ways of dealing with this, but none have been tested: The energies required are simply too large for us to work with at this point.
The theories we do have for quantum gravity suggest that the singularity may be a fault of general relativity, and that it might be 'smoothed out' by quantum effects.if that is the case, time could go back prior to the 'hot dense' stage. There are several different proposals of what happened, including a multiverse which has 'universes' (hot dense pieces) bud off occasionally to a single 'previous universe that contracted to get hot and dense, leading to the current expansion, and a host of other possibilities, depending on the specifics of the theory of quantum gravity being used.
My Opinion: My bet is that bringing in quantum mechanics eliminates the singularity and allows for time to go back infinitely. Whether there is a multiverse or not is very unclear. How to test that possibility is yet to be worked out. In any case, whenever there was time, there was also space, matter, and energy.
1. Standard cosmology is based on General Relativity for the background spacetime geometry (expansion of space).
2. The notion of a singularity is a tricky one: a singularity is NOT an object. it is more a description of something going wrong in our model. For example, the north and south poles are 'coordinate singularities' in the coordinate system using latitude and longitude. The reason is that both the north and south poles don't really have a longitude. That is something going wrong with our mathematical description, but not with the underlying geometry. it is also possible to have singularities in the geometry itself (cusps or 'holes'). The singularity at the beginning of the Big Bang in standard cosmology is closer to a 'hole', an absence, an inability to go further back in time.
3. The term 'Big Bang' can refer to either the singularity or the fact that the universe was hot and dense in the past. Usually, cosmologists mean the latter, not the former.
So, for most cosmologists, there was a Big Bang: the universe was once very hot and dense and expanded to become what we see today. Whether or not there was a singularity, this much happened.
4. As we go back in time, the temperature and pressure of the universe get very large (hot and dense). When the temperatures and pressures are high enough, there can be no molecules, no atoms, etc. So, in a sense, things get very simple at that point. So, during the time of nucleosynthesis, the whole universe was as hot as the core of a star and the pressures were enough for fusion reactions to happen everywhere. This leads to testable predictions about what can be observed now, so there is high confidence that this stage actually existed.
5. Once we get back to a certain point, though, our understanding of high energy particle physics breaks down (this is why we built the LHC). Beyond this point (and that includes the 'inflationary scenario, where the universe expanded exponentially fast) everything is pretty much speculation.
In particular, we don't know why there is more matter than anti-matter in the universe. We have guesses, but nothing concrete has been tested and verified.
6. General Relativity inevitably predicts singularities. At one time, it was hoped that they are just a part of certain symmetrical solutions to the equations, but Hawking showed that thi sis not the case: geometric singularities are inevitable in standard cosmology.
BUT, and this is big, General Relativity does not consider the quantum mechanical aspects of gravity. While we can do quantum theory with general relativity as a background, that doesn't include the quantum aspects of gravity itself. But we also know that quantum gravity effects will become relevant at some point. So GR alone simply won't be enough. The standard model will fail to describe reality at some point.
7. There is no tested quantum theory of gravity. We have a number of proposed ways of dealing with this, but none have been tested: The energies required are simply too large for us to work with at this point.
The theories we do have for quantum gravity suggest that the singularity may be a fault of general relativity, and that it might be 'smoothed out' by quantum effects.if that is the case, time could go back prior to the 'hot dense' stage. There are several different proposals of what happened, including a multiverse which has 'universes' (hot dense pieces) bud off occasionally to a single 'previous universe that contracted to get hot and dense, leading to the current expansion, and a host of other possibilities, depending on the specifics of the theory of quantum gravity being used.
My Opinion: My bet is that bringing in quantum mechanics eliminates the singularity and allows for time to go back infinitely. Whether there is a multiverse or not is very unclear. How to test that possibility is yet to be worked out. In any case, whenever there was time, there was also space, matter, and energy.
