Big Bang vs Slightly Smaller Bang vs Slightly Bigger Bang

fatherdougalmag

OK. I'll admit I'm no physicist. My dalliances amount to listening to Feynman and Hawking lectures on audiobook when walking to work

I wasn't sure which forum was best - astronomy or physics - but I felt I'd get a more informed response here.

One thing I here bandied about sometimes is how things would have turned out if the universe had expanded every so slightly slower or ever so slightly faster. For example, if it was slightly slower then we'd have had a crunch by now.

Then I was thinking that that argument assumes that the laws of physics for the slightly slower/faster expanding universe would have been the same as for the universe we're in now. Would it not be a case that, if the universe expanded slightly slower or faster that there would have been a completely different set of laws appropriate to those instances?

And, in a similar vein, when cosmologists are theorising about the demise (or persisted expansion) of the universe, they are basing the behaviour on the laws which operate within the universe. Is it not conceivable that there is another construct outside the realm of our universe thats dictating how the universe will develop.

Or have I just gone too long without food and drink?

Professor_Fink

Hi,

Basically when talking about the speed of expansion, they are refering to the question of how much mass there is in the universe (or at least what the density is). This will determine the end of the universe. There is a critical mass above which the universe will end in a big crunch, and below which the universe will expand for ever. At this mass, the universe would expand slower and slower until it stoped, but never collapse.

The universe, essentially by definition, is a closed system. If it were not, then we would just pick the bigger system to be called our universe. Time and space have no meaning "outside" our universe. Essentially there is no outside.

Hope this helps

Professor_Fink

Also, as regards different expansion rates, these correspond to different initial conditions, not different laws of physics.

fatherdougalmag

Professor_Fink wrote:

Essentially there is no outside.

I guess the nub of my question. Is it an assumption that there is no outside? It smacks a bit of 'the world is flat'.

And when we speak of things like gravity, mass, light, etc. I'm wondering if they only apply to the universe that we grew up with and observe now. If there were different inital conditions, would we end up with these things?

In my mind the Laws of Physics have been derived through observation and experiment. So it follows that the Laws of Physics are defined by the universe as it is and has been. And I wonder that if the universe had a differnt initial condition there would be different behaviours and observations. Consequently there would be different Laws of Physics. Is that a valid proposition?

Thanks for the responses so far. I'm probably wandering off down some philisophical road but I'd rather wander with the right information and facts.

Professor_Fink

My point is that by definition there is no outside. The universe may or may not be infinite, but according to our current understanding of physics, it is meaningless to talk about an outside, in much the same way as saying it is meaningless to talk about the edge of the world (as in falling off the edge). The earths geometry doesn't allow for this, and as far as we can tell, neither does the universes.

If the universe had different intitial conditions, then it is concievable that some of the physical constants would be different (this is essentially the landscape is string theory), but the basic laws of physics would most likely be the same.

Son Goku

I guess the nub of my question. Is it an assumption that there is no outside? It smacks a bit of 'the world is flat'.

The evolution of spacetime (its dynamics), are governed by laws which make no reference to anything external.
These laws are capable of modelling the universe from shortly after its "birth", right up to the present day.
So we can conclude that either there is nothing outside or whatever is outside has no effect on the universe.

In my mind the Laws of Physics have been derived through observation and experiment.

Some laws are and some aren't. Most of the modern theories were derived through attempts synthsize previous concepts and were confirmed at a later date by experiment, but they were not derived from experiment.

And I wonder that if the universe had a differnt initial condition there would be different behaviours and observations. Consequently there would be different Laws of Physics. Is that a valid proposition?

The laws of these different universes (faster or slower expansion) would be the exact same. As Professor_Fink said they are simply different inital conditions. There would be different behaviours, but not completely different theoretical frameworks.

If you throw a ball from your hand at 10 m/s and the trajectory will be different than one thrown at 20m/s, but both are governed by the same laws.

RiderOnTheStorm

there are always laws out there. we know some and can guess others, and some dont apply. But if I understand your question, you are asking if the unknown laws (from outside the universe, or ones that we have not discovered yet) can be seen / measured / have effect. Well.... I guess they are unknow as we dont know about them, so we cant tell what would happen, or would have happend if something basic to our understanding didnt happen. As you say, all our laws / knowledge / understanding is based upon our history / experience and trying to figure out something outside our communal experience is .... well ... outside our knowledge!

i guess its a bit like saying if you throw a ball @ 10m/s it will go x distance, and @ 20m/s it will go 2x and at 1,000,000m/s it will open a wormhole, drop in and come out on 52nd of Julober 5257AD..... in my backyard. If there are unknown forces going to come into play, then their effects will be unknown too!

JoeB-

related questions I have never understood...

Why is it that the laws of physics are the same throughout the entire universe? Maybe they are different in parts that are out of contact with our part?

And is it not possible that these laws change over time? i.e maybe the 'current' laws depend on current conditions and will change over time?

I understand that if the laws change over time or are different in distant regions it would make it very difficult for us humans to understand the universe but that's not a good reason for them to be the same always...

And a final weird question... some people think that it is more likely we are living in a 'simulated' universe like the Matrix... this is on the grounds that there can only be one 'real' universe and many simulated universes, hence it is more likely that an individual is living in a simulated universe... how could this be disproven?