[wwisconsin (OP)]

Why do physicists always refer to "The" Big Band as though there was only one Big Bang.  Are they not simply referring to the most recent Big Bang?  Don't the known laws of physics, including the concepts of entropy (thermodynamic equilibrium) or gravity suggest that the universe will eventually contract to a single mass (or black hole).  And if so, what or who is to say that this has not already occurred at least once and is really a continuous cycle of Big Bangs and contractions over time? I realize that this is a hypothetical question, but I don't understand why it is rarely if ever mentioned.

[evan_au]

Why do physicists always refer to "The" Big Bang as though there was only one Big Bang.  Are they not simply referring to the most recent Big Bang?

 
It is thought that the (most recent) Big Bang would have produced truly extreme temperatures, pressures, energies and particles that we can only imagine. This would have erased any light or sounds from a previous Big Bang.

Maybe some subtle future physics may allow us to peer beyond the Big Bang, but for now, any such pre-history has been lost in the flash of "the" Big Bang.

Don't the known laws of physics, including the concepts of entropy (thermodynamic equilibrium) or gravity suggest that the universe will eventually contract to a single mass (or black hole).

Not necessarily. According to Einsteins's General Relativity, one of two major outcomes were expected:

• Go on expanding forever (just slowing down a bit or a lot)
  
• Expand, stop, and collapse into a Big Crunch
   

The difference between these outcomes is determined by the average density of the universe. And all measurements showed it finely balanced between these two outcomes (a "Flat" universe).

However, this discovery of the accelerating expansion of the universe fits neither of these models. If the expansion continues, we may end up with a "Big Rip".

And if so, what or who is to say that this has not already occurred at least once and is really a continuous cycle of Big Bangs and contractions over time?

Einstein reportedly considered such a cyclic universe a real possibility.

Entropy does not like perpetual motion machines - but we are not entirely sure that entropy applies on the scale of fundamental particles (the interaction of two subatomic particles) or at cosmological scales.

See: https://en.wikipedia.org/wiki/Ultimate_fate_of_the_universe#Theories_about_the_end_of_the_universe

[JohnDuffield]

Why do physicists always refer to "The" Big Band as though there was only one Big Bang.

Because that's the only one we have evidence for.

Are they not simply referring to the most recent Big Bang?

They're referring to "the" Big Bang because that's the only one we have scientific knowledge of.

Don't the known laws of physics, including the concepts of entropy (thermodynamic equilibrium) or gravity suggest that the universe will eventually contract to a single mass (or black hole).

No they don't. There's no overall gravity in the universe. And if there was, gravity might make matter fall towards the centre, but it doesn't make space fall towards the centre. Instead space expands.   

And if so, what or who is to say that this has not already occurred at least once and is really a continuous cycle of Big Bangs and contractions over time? I realize that this is a hypothetical question, but I don't understand why it is rarely if ever mentioned.

People have speculated about this sort of thing, see the cyclic model. But there's no actual evidence for any such cyclic model. 

[jeffreyH]

What Evan and John say is correct. There are limits to what we can determine about what happened at or before the big bang itself. Science is always striving to learn more and with higher energy colliders things may become clearer over time.

[IAMREALITY]

I'm not sure that's a question that can ever be answered, but I'm also not sure it's one that needs to be.  And the reason I say that is because no matter how many times it's occurred, if it has been cyclic, the only reality (all the things that encompasses) is the one that's left in the here and now; the one created from this singular bang.  All other remnants of past bangs would no longer exist, so their relevance would be questionable.  I also would assume, if such a process was cyclic, that anything that could ever be determined about existence and our universe would be able to be discovered within the confines of this one particular bang as well, making the relevance of past bangs even more questionable.

That's at least the off the top of my head impression when just taking a moment to think about the question.

I had initially assumed it to be geared towards a different angle though; not asking how many bangs our universe has gone through, but rather how many bangs have occurred with our universe being merely the product of one.  In that case, the answer is likely to be either one or infinity.

[jeffreyH]

I'm not sure that's a question that can ever be answered, but I'm also not sure it's one that needs to be.  And the reason I say that is because no matter how many times it's occurred, if it has been cyclic, the only reality (all the things that encompasses) is the one that's left in the here and now; the one created from this singular bang.  All other remnants of past bangs would no longer exist, so their relevance would be questionable.  I also would assume, if such a process was cyclic, that anything that could ever be determined about existence and our universe would be able to be discovered within the confines of this one particular bang as well, making the relevance of past bangs even more questionable.

That's at least the off the top of my head impression when just taking a moment to think about the question.

I had initially assumed it to be geared towards a different angle though; not asking how many bangs our universe has gone through, but rather how many bangs have occurred with our universe being merely the product of one.  In that case, the answer is likely to be either one or infinity.

Well that was a very interesting answer. It summed things up quite well.

[PmbPhy]

Evan did a great job at addressing the question. I'm just going to add to what he posted. My compliments to Evan. Well done my friend!  []

Why do physicists always refer to "The" Big Band as though there was only one Big Bang. Are they not simply referring to the most recent Big Bang? 

They don't. The physicists who specialize in the physics of the universe are called Cosmologists and their field is called cosmology, i.e. the study of the universe. Cosmologists include the concept of multiple Big Bangs as conceivable possibilities regarding the history of the universe. The scenario in which the universe stops expanding and then reverses and collapses in on itself is known as The Big Crunch. There is also a scenario in which this keeps on happening over and over again. That models is referred to as an oscillating universe or cyclic universe.

This is all explained on Wikipedia at:

https://en.wikipedia.org/wiki/Big_Crunch
https://en.wikipedia.org/wiki/Cyclic_model

Note: I'd like to make a suggestion which I hope you will find useful. Try to refrain from grouping all physicists into one group, as if we (i.e. physicists) all think the same way and have the same beliefs and ideas about physics. Sometimes its appropriate and sometimes its not. In this case you appear to be assuming that all of us physicists made the same bad mistake. And indeed it'd be a bad mistake because there's no reason a priori to assume that there was one and only one Big Bang.

Don't the known laws of physics, including the concepts of entropy (thermodynamic equilibrium) or gravity suggest that the universe will eventually contract to a single mass (or black hole).

Re - Don't the known laws of physics,...

If you throw a rock up into the sky, it doesn't mean that it will eventually come to rest, reverse its motion, and then fall back to Earth. I think that may have been what you had in mind when you said - ...or gravity suggest that the universe will eventually contract... . If we give a rock a velocity large enough then it can escape the Earth's gravitational field. That speed is called the escape velocity. For this to happen the total energy, which is the sum of the kinetic energy and potential energy, must be greater than zero. When the total energy is exactly zero then the rock will come to rest at infinity. This corresponds to the case where k = 0. If we give the rock more energy then it corresponds to the case k > 0. If we don't give it enough energy resulting in a total energy which is less than zero, i.e. negative, then the rock will eventually stop and fall back down to earth.

Since I don't know what your background is in math and physics I'll explain it using two sources. The first is simply to point you to Wikipedia:

Oscillating Universe:  To learn about the oscillating universe please see: https://en.wikipedia.org/wiki/Cyclic_model

Big Crunch: To learn about the Big Crunch please see: https://en.wikipedia.org/wiki/Big_Crunch

The second is to explain it myself with the help of a cosmology text just in case you have the math skills to follow along. I recommend downloading and following along from the text An Introduction to Modern Cosmology by Andrew Liddle (2003). You can obtain this text online for free. All you have to do is go to http://book4you.org/ and register. There is no cost and no need to have a credit card. After you've registered, login and go to http://book4you.org/book/451779/8ae8ca  That's the website for the text. Just click on Download and you'll be all set. It's a PDF file so open it with Adobe Acrobat and scroll down to page 40. That page is where the section 5.5 starts which is entitled 5.5  Evolution including curvature. When they speak of curvature here they're referring to spatial curvature not spacetime curvature. There's a difference.

Let's assume that the cosmological constant is zero. Then there are three possibilities:

1) k < 0: Rate of expansion never becomes zero.
2) k = 0: Rate of expansion approaches zero as t -> infinity.
3) k < 0: Rate of expansion goes to zero, stops and then starts to contract. The result is a Big Crunch

If the cosmological constant is greater than zero then it behaves like anti-gravity and we have an accelerating expansion of the universe. That can also happen with negative pressure. That's what we think dark energy is, i.e. either a positive cosmological constant, negative pressure or some combination of both.

And if so, what or who is to say that this has not already occurred at least once and is really a continuous cycle of Big Bangs and contractions over time? I realize that this is a hypothetical question, but I don't understand why it is rarely if ever mentioned.

There's nothing to say that it hasn't happen and cosmologists are indeed well aware of this fact.

[JohnDuffield]

...You didn't actually think you were the first one to think about this, did you? Physicists are very smart people. That's why they have the reputation for being highly intelligent.

And some people have a reputation for being patronising, condescending, and wrong.

[wwisconsin (OP)]

Thanks everyone. I am satisfied with the with the responses and truly feel more enlightened on the topic. Although PmbPhy's answer seemed a bit condescending, the other responses provided some directions for me to explore further.

[Bill S]

Hi wwisconsin; welcome.   So often threads die quietly without the OP saying if her/his question was answered. It's great to have a new poster who sets us all a good example.

[PmbPhy]

Although PmbPhy's answer seemed a bit condescending, the other responses provided some directions for me to explore further.

Then I take it that my response wasn't very useful. Sorry about that. It was my sincere desire to be helpful.

I want to apologize if I appeared to be condescending. It wasn't my intention to come off that way. However, I will admit that when people come here posting comments where they appear to say that physicists have been wrong all along and haven't thought of what they have, being a physicists myself my feathers tend to get ruffled by such comments. However that's no excuse for being condescending. However it wasn't my intention to be that way. However what may have contributed to that is that I misread the following statement that you made. What you actually wrote was

Don't the known laws of physics ....

However, what I thought that you wrote was this

Don't they known laws of physics ...

which to me meant that you was stating that we all didn't know something obvious and elementary when in fact you didn't. I went back and edited that post to make it appear not to be condescending. Again, my apologies. I hope you can forgive me for appearing to be condescending.

[jeffreyH]

Having thought about this I have come to some conclusions. In our universe we see evidence of expansion via the redshift of galaxies. This expansion indicates to us that the universe originated from a dense region. We can therefore say that the number of possible big bangs is >= 0. We cannot say > 0 since we do not have observational evidence to back that up. We can infer that from the observations we make now but we were not witness to the big bang itself and will never be certain that it actually happened.

[garth john greiner]

I'm not much of a believer in all this expansion and contraction buissiness I think the universe is born lives a life span then bigins to fade and then die's I think it is a finite structure and all its resources eventually get consumed.
 I prefer the idea of  there being more than one universe, I struggle with the notion that there is only one. Where's the diversity in that ?, what happens when the universe runs out of what fuels it, everything will eventually
be consumed and what then ??

[dlorde]

... I prefer the idea of  there being more than one universe, I struggle with the notion that there is only one.

That's as may be, but it isn't science. What you prefer and can cope with may bear no relation to reality. People once preferred the idea that the Earth was the centre of the universe and everything revolved around it; people once preferred to think of atoms as little planetary systems, and particles as tiny billiard balls. They were wrong.

... what happens when the universe runs out of what fuels it, everything will eventually be consumed and what then ??

Then, just a heat-death of darkness and evanescent virtual particles - assuming the accelerating expansion doesn't result in a 'big rip' in which spacetime itself is destroyed and the universe undergoes a phase change to something else (no idea what that might be).

I did once see an interesting hypothesis that suggested that, at some point in the expanding heat-death, scale would become meaningless and the vast emptiness would become equivalent (mathematically & physically) to a hot, dense system with minimal entropy (i.e. another big bang)... unfortunately I don't recall the details of how it was supposed to work.