Why would a theory of quantum gravity be needed to answer the question "What was the nature of the universe before the Big Bang?

[msc545]

[ProbablyClueless]

Because quantum gravity theory is a subset of quantum physics, which relies on logical inferences that can be made between from the behavior of particles that appear to be linked.

The Big Bang needed to be caused by a gravitational event to compress everything down to the specific point. That relies on the assumption that some kind of gravitational force was acting. So quantum gravity theory is the inference of how gravity needed to behave before in order to cause gravity to behave how it does now.

[Sevenpointfive]

i don't know shit about physics. but to me it sounds like he's trying to say the big bang couldn't be true because something doesn't gravitate around nothing. i wish i knew more about universes and how they move. we have all of these universes and never told what we're all in our cellular space of space

[msc545]

Not exactly. This is really asking about what the universe was about prior to the Big Bang, and, implicitly, what caused the Big Bang, and why is it that at least the first question cannot be coherently addressed without having a theory of quantum gravity, which nobody has developed. It may be worthwhile to mention that adjuncively, the current physics states, perhaps conveniently, that time did not exist prior to the Big Bang, which is conceptually difficult but also neatly steps around questions like "how long did the Big Bang last" and "how old was the Universe before the Big Bang occurred", and also, in a quasi-coincidental fashion, eliminates all questions regarding causality since without time there is no causality.

If you now have a headache or stoped reading this altogether, those things are expected responses to this. Have a nice day.

[Sevenpointfive]

that is literally what i just said. i am not the type to ponder mathematics although it's really cool. i had hoped you'd link a definition to your idea of the big bang.

[msc545]

I was not saying the Big Bang didn't happen, but only that we have no idea why or even if it happened, or if it did, how long it lasted or why - so there really is no definition or description of it. The concept has long been used to explain the acceleration of planets and suns and other objects away from each other, which for a long time were thought to be gradually slowing as if the acceleration and then the deceleration over time was the result of an initial explosion, or Bang. This worked well conceptually, except that a few years back, someone discovered that those same objects that had gradually slowed were now speeding up - which calls into question the whole concept of the Big Bang.

But that is really not what this question is about. It is a mathematical question that seeks to understand the relationship between a solution to the Quantum Gravity problem and the description of the Universe prior to the Big Bang, if the Big Bang really happened - which it might not have. The question also has to do with causality and it's relation to gravity and time. Without time, there is no causality, as must be obvious. Without causality, Planck epoch physics falls apart, again obviously.

I have many more questions than answers, which are both interesting and vexing.

[msc545]

I was hoping that somebody on here might have better answers to my questions than the ones I have...

[Sevenpointfive]

it's not going to be better. but i swear to god i had your answer up until i started reading

[msc545]

What is the answer?

[Sevenpointfive]

you can't answer a question with a theory.

[msc545]

Well please describe the theory then.

[Sevenpointfive]

in theory, i wouldn't even know. actually, that might be fact!

[msc545]

Ok I understnd.

[dustybiker2]

https://bigthink.com/starts-with-a-bang/this-simple-thought-experiment-shows-why-we-need-quantum-gravity/

Because of the curve and bends that need to be calculated in to find a closer conclusion. Imperfect still but consistently close.

[exitseven]

Thr first law of Thermodynamics says that matter can neither be crested or destroyed. It must be a constant so that would also make gravity be a constant.

[msc545]

I am asking about *quantum* gravity.

[NathanDavis]

to please our limited comprehension... and appease it as well

[Anonymous]

I don’t know but now I’m curious