Laws of Physics not applicable at the 'moment of creation'?

[JSat92]

Hello!

I'm assuming the model of the Big Bang Theory and I'm asking specifically about the 'moment' of creation; I'm in no way well-versed in this; so please be gentle :

* Are the laws of physics applicable to the beginning on the universe?
* The Law of Conservation will not apply for this moment, would it? Matter and energy cannot be destroyed or created ONLY 'within' the universe, am I correct?
* Does Entropy dictate that the universe couldn't have existed infinitely?

And finally, what are some formidable challenges and alternatives to the Big Bang Theory?

EDIT :
Is the 'moment of creation' termed as the 'singularity?

My apologies if these questions have already been answered, I'm having trouble navigating through the website. It's probably my wireless going haywire. =( If you could provide me with links to the answers, that would be great, thanks!Thank you!

 

[mathman]

The laws of physics always apply. The question is do we know what they are at the beginning of the big bang? Current theories (Quantum and Gen. Rel.) are very good descriptions of what we can observe. However in extreme situations (inside black holes or the beginning of the big bang) they break down.

 

[bcrowell]

* Are the laws of physics applicable to the beginning on the universe?

The only well tested theory of gravity that we have is general relativity. In GR, there is no t=0 in a cosmological model, only t>0. Therefore the issue of the laws of physics at t=0 doesn't arise. Nobody actually believes that GR is correct for times before about 10^-43 seconds (the Planck time), so in that sense the answer to your question is not known.

* The Law of Conservation will not apply for this moment, would it? Matter and energy cannot be destroyed or created ONLY 'within' the universe, am I correct?

There is no law of conservation of energy on cosmological scales. Not now, not ever. We have a FAQ on this: https://www.physicsforums.com/showthread.php?t=506985

* Does Entropy dictate that the universe couldn't have existed infinitely?

Right, if the universe had existed for an infinite time in the past, then we would expect it to have reached thermal equilibrium already, so there could be no stars, life, etc.

Is the 'moment of creation' termed as the 'singularity?

In GR, it's a singularity. In reality, all we know is that it was super-duper dense and hot at times getting back close to the Planck time, which is the earliest time at which we think GR applies. That is, the singularity that's present in a GR model of cosmology is believed not to be a true mathematical singularity in reality.

 

[JSat92]

Thank you very much.

 

[marcus]

...
Right, if the universe had existed for an infinite time in the past, then we would expect it to have reached thermal equilibrium already, so there could be no stars, life, etc.
...

Loop cosmology gets around that, as we've discussed before: a classical universe phase collapses resulting in a bounce and re-expansion: looking like what we see around us.

What makes this work is a quantum correction term in the Friedmann equation that is density dependent. At very high density it dominates, making gravity repellent.

You could say "thermal equilibrium" is different when gravity is repellent, which provides an escape clause to the Second Law---equilibrium is different from what it means under ordinary attractive-gravity conditions. So the bounce, with its episode of repellence, leads to a uniform-density state which is LOW entropy and far from equilibrium when gravity again becomes attractive.

However that be, the loop cosmology people have been studying model universes with a bounce for around 10 years now---and have never seen the Second Law as posing an objection to there being stars life etc after the bounce.

Indeed their model converges rapidly to the classical Friedmann expanding universe within a short time after the bounce.

 

[bcrowell]

Interesting post, Marcus. I hadn't known about these aspects of LQC. Would it be a fair characterization to say that LQG violates the second law of thermodynamics, but this is a feature rather than a bug, because it happens through a well-defined mechanism?

 

[Chalnoth]

Loop cosmology gets around that, as we've discussed before: a classical universe phase collapses resulting in a bounce and re-expansion: looking like what we see around us.

Yeah, I am exceedingly skeptical of this claim. How is it supposed to go from a very high entropy state to an extremely low entropy one?

 

[marcus]

Interesting post, Marcus. I hadn't known about these aspects of LQC. Would it be a fair characterization to say that LQG violates the second law of thermodynamics, but this is a feature rather than a bug, because it happens through a well-defined mechanism?

I just saw your post. It's a crisp cogent view of the situation. My own view is more muddled. I'm still wondering about how the relevant entropy is defined. Whose map of phase-space do we use? Who does the coarse-graining? I will try to sketch how I see it.

Yeah, I am exceedingly skeptical of this claim. How is it supposed to go from a very high entropy state to an extremely low entropy one?

The key element is the entropy of the gravitational field. High gravitational entropy and low gravitational entropy have no intrinsic fixed meaning unless you stipulate something about gravity, like for example the fact that it is attractive.

Anytime gravity changes sign this is going to abruptly redefine the entropy.

You are welcome to be as skeptical as you please. Why not? The definition of the entropy of the grav. field---the entropy of geometry---is still being worked on. However you want to think of it is up to you and all right with me! :-D.

So i'll just give a handwave pictorial intuitive explanation, and you can decide. This is aimed at wide audience so anyone who happens to read might be able to get the picture.

If gravity is attractive then low entropy means uniform field and evenly spread matter and high entropy means clumpy. Geometry&matter evolve towards clumpy.

If you then take a clumpy situation and change the sign of gravity so it repels (which happens starting around 1% of Planck density in these models) then what WAS high entropy is redefined to be low. Low entropy now means clumpy. Clumpy geometry and matter start to spread out.

Gravitational entropy increases as stuff spreads out and becomes more uniform.
Say density goes to 40% right at the bounce and then back down to 1% and at that point gravity starts to be attractive again. But now the gravitational entropy is very low because all matter spread out evenly. Low entropy now means uniform even, again.

BTW loop bounce models generically have a brief period where expansion is much faster than exponential---this has been called "super inflation" because it is faster than ordinary inflation scenarios.

Interesting dynamics---one has equation models and numerical (computer) models. The Hubble parameter H is zero right at the bounce but then quickly rises to something on the order of Planck scale. 1/H ~ 10^-43 second, as I recall.

The main authority on Loop cosmology is Abhay Ashtekar the young people active in the field tend to be his former PhD students and postdocs. I don't know what Ashtekar would say. He has been quoted as saying that Second Law is not violated.

I speculate that it might avoid being violated by ceasing to mean anything in the Planckian regime around the bounce. Maybe entropy itself has no consistent definition. Can you assign a unique number to the entropy in the absence of a unique observer? What if there are two distinct horizons? One going into the bounce and one looking back on the bounce after the fact.

How do we define gravitational entropy in these extreme circumstances?

Lots of interesting problems!

I recently talked with a new loop gravity PhD and he said he and his co-author are right now interested in the problem of defining the entropy of the gravitational field. I gather currently available definitions are apparently not universally applicable or entirely satisfactory. It's something to watch.

Anyway the upshot is that I suspect it simply does not mean anything to invoke the Second Law in a regime where you don't even know how to define and compute a unique number for the entropy. We'll see.

 

[Chalnoth]

So i'll just give a handwave pictorial intuitive explanation, and you can decide. This is aimed at wide audience so anyone who happens to read might be able to get the picture.

If gravity is attractive then low entropy means uniform field and evenly spread matter and high entropy means clumpy. Geometry&matter evolve towards clumpy.

If you then take a clumpy situation and change the sign of gravity so it repels (which happens starting around 1% of Planck density in these models) then what WAS high entropy is redefined to be low. Low entropy now means clumpy. Clumpy geometry and matter start to spread out.

Gravitational entropy increases as stuff spreads out and becomes more uniform.
Say density goes to 40% right at the bounce and then back down to 1% and at that point gravity starts to be attractive again. But now the gravitational entropy is very low because all matter spread out evenly. Low entropy now means uniform even, again.

These statements aren't sensible when we consider the definition of entropy as the number of microstates that replicate the same macrostate. With this definition of entropy, which is generally accepted as being truly fundamental, the stuff that happens in between going from state A to B is pretty much irrelevant: we can learn all we need to learn by considering just state A and state B, independent of the intervening physics.

 

[marcus]

... the definition of entropy as the number of microstates that replicate the same macrostate...

That was the definition I had in mind.

It depends on having an observer's map of phase space showing which microstates are lumped together into the same macro region.

In order for the concept of macrostate to have an operational meaning you need a clear idea of who is doing the measuring and what the variables mean that they are measuring (temperature, pressure,...etc). The observer defines the map.

We differ in our opinions. I do not believe what you are saying is sensible. It would only make sense to say "it doesn't matter what happens in between" if you know that the quantities are WELL-DEFINED in between.

Believing that something exists if you can't define it seems a bit like believing in fairies, does it not?

The def of geometrical entropy involves some non-trivial issues---basically what I was driving at.

 

[Chalnoth]

That was the definition I had in mind.

It depends on having an observer's map of phase space showing which microstates are lumped together into the same macro region.

In order for the concept of macrostate to have an operational meaning you need a clear idea of who is doing the measuring and what the variables mean that they are measuring (temperature, pressure,...etc). The observer defines the map.

We differ in our opinions. I do not believe what you are saying is sensible. It would only make sense to say "it doesn't matter what happens in between" if you know that the quantities are WELL-DEFINED in between.

My argument basically only relies upon the unitarity of the laws of physics. As long as that is the case, it really doesn't matter what happens between two configurations.

 

[marcus]

I think you just aren't getting something. You invoked the 2nd Law (probable nondecrease of entropy) over an interval of time where I don't think you can define the entropy in a unique way.

that is why I compared it to believing in fairies, or god for that matter.

If in some context you cannot operationally define the terms in a law, the law does not exist there. It is meaningless to invoke it.

I have to go pretty soon, we are meeting friends down the coast from here. Don't expect I could convince you of anything however even if I could stay around and, time permitting, respond.

 

[Chalnoth]

I think you just aren't getting something. You invoked the 2nd Law (probable nondecrease of entropy) over an interval of time where I don't think you can define the entropy in a unique way.

But that doesn't matter. The later system is still a different state of the same physical system. And therefore, by unitarity, you can compare their entropies, no matter what happened in between. You can invoke all the complicated dynamics you want, but you won't be able to get around the second law.

You still have a comparison between two states:
1. A collapsing universe with high entropy, getting clumpier and clumpier as it gets denser and denser.
2. An expanding universe with low entropy that is very, very smooth but getting clumpier.

The only conceivable way I can see to reconcile these two states would be if the expanding universe was exponentially larger than the previous collapsing one.

 

[Chronos]

I agree the laws of physics applied even to the very early universe, we just don't have a good understanding of what they were like way back in the day. The four fundamental forces of nature were unified until 10E-43 s and did not finish breaking free until the electoweak force split around 10E-12 seconds after the big event. It reminds me of a large number of participants playing four different sports on the same field at the same time under a unified rule book. The proceedings are very confusing until the rules of the individual sports emerge and the players assume appropriate roles as the game progresses. Try to imagine what the 'unified' rules might look like if the four sports were nascar, boxing, soccer, and polo.

 

[JSat92]

Been away for a couple of days, and I come back to find an interesting read! Cool!