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! [

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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_Crunchhttps://en.wikipedia.org/wiki/Cyclic_modelNote: 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_CrunchThe 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.