I have some somewhat detailed questions about inflation, curvature, and entropy

1. Apr 8, 2012

ocsis2

I posted these on Reddit but some questions weren't answered so I was wondering if people here could help:

First Part

I was informed that the universe did actually exponentially gain energy during inflation and perhaps other periods of its development.

So how does this affect the entropy and energy relation above?

I assumed since the change in energy is equal to the temperature times the change in entropy that the universe started off as having very low volume, very high temperature/pressure, and low entropy. The high pressure drove the expansion in volume which caused temperatures to decrease and entropy to increase.

So that's wrong.

But if the inflationary period came with an exponential growth of energy, how did the universe get colder? Was this compensated by a massively exponential increase in entropy?

Why do we need something with negative pressure to be responsible for inflation. Wouldn't the extremely high positive pressure with low volume of the initial Big Bang conditions be enough to drive inflation?

Second Part

I thank you all in advance for any help you can provide in clearing up my confusion!

Third Part

Why is the idea that the total energy in the universe "zero" so popular (re: Laurence Krauss) and why is the flatness of the universe used to back this up when, according to that post, an open universe would not conserve energy so the total energy of the universe cannot be "zero", can it? What's the energy being defined as zero and why is that energy used to predict things about the universe when "the" energy (as the term is used in that post) is not zero?

Last edited: Apr 8, 2012
2. Apr 8, 2012

Mark M

Hi ocsis, glad you came here.

Well, this is mostly correct. The universe initially gained an enormous amount of energy as inflation began, but it cooled dramatically during inflation. After it ended, the field that drove inflation, the inflaton field, decayed into a hot bath of particles, reheating the universe.

As the universe gains volume, entropy increases, and temperatures decrease. Energy is unaffected, density just decreases. But remember, inflation had a large effect on the energy in the universe, I addressed that above. Normal expansion does not.

After inflation ended, the universe began to cool.

Remember, positive energy/mass/pressure cause attractive gravity, which is the positive curvature of space. Negative pressure, on the other hand, causes repulsive gravity, the expansion of space.

An increase in curvature applies only is the universe is closed or open. If it is exactly flat, it will remain so forever. If it began closed, it will become more and more positively curved. If it is open, it will become more and more negatively curved.

If the universe is closed, it would have a net positive amount of energy. If it is open, it would have a net negative energy. If it is flat, the total energy of everything in the universe would be offset by the negative gravitational energy, resulting in the universe having zero total energy, the ultimate 'free lunch', as Alan Guth calls it.

3. Apr 9, 2012

ocsis2

Will the negative pressure of dark energy which is driving current and future expansion also "flatten" the universe? Why?

4. Apr 9, 2012

Mark M

The expansion due to dark energy is far weaker than that of inflation. As far as I know, it does not flatten the universe.

5. Apr 9, 2012

TheTechNoir

Also I'd like to point out that the big bang theory does not require that the universe was small at the time of the big bang, in fact many if not most cosmologists don't believe this view of an infinitesimally small/dense state of the universe.

6. Apr 9, 2012

ocsis2

Well it had to be pretty dense and small if:

Also, from Wiki,

7. Apr 9, 2012

ocsis2

Thanks for your help here! One more question, how did inflation add energy to the universe? What mechanism did this occur by? In other words, where did that energy come from? Was it due to the quantum fluctuation (or that scalar field rolling down a potential energy hill) to a lower, more stable energy state? This energy then manifested as exponential expansion, and then switched from expansion into heating (by filling up the universe with hot quark-gluon plasma)? Do we know the mechanism for this expansion->reheating switch?

8. Apr 9, 2012

Mark M

Well it was very, very dense, but defining distance of a global scale is a difficult matter. Simply because, if the universe is finite, it wraps back on itself like the surface of a balloon, there is no boundary. If you define the distance as being in between points where the universe began to repeat (in the case of the balloon analogy, the circumference), then it could have been small, but not necessarily. Just small relative to today's universe.

Also, the quote you posted is of the observable universe, what we can see.

9. Apr 9, 2012

TheTechNoir

Not so. It was very dense yes, but we have no strong evidence that says it had to be small. This quotation applies to the observable universe, considering we do not know for certain the size of the universe or if it is even finite or not, we simply don't know it's size in the first 1E-35 seconds. Anywhere from small to infinitely large.

EDIT: Mark got it - thanks.

10. Apr 9, 2012

ocsis2

This brings up a related thing I've often wondered about. Do we have any evidence for the existence of an unobservable universe at the Big Bang?

Or even now? I figure if stuff disappears beyond the cosmological horizon it would qualify as unobservable universe which developed after the Big Bang. Have we observed that yet?

11. Apr 9, 2012

TheTechNoir

Prior to 300,000 years after the big bang the entire universe was unobservable.

Your question was worded a bit wrong/funny but I understand what you're asking so no: in principal it is impossible to know anything with certainty that has and will forever remain outside of the observable universe so the answer is no we don't have evidence of that, but none on the contrary either and multiple reputable or leading models make predictions that suggest a large or infinite size at those earliest moments.

12. Apr 10, 2012

ocsis2

Which models are those?

13. Apr 10, 2012

Mark M

Sorry, for missing these questions, I'll answer them now.

Inflation added energy in two ways, I'll explain them both.

First, was the enormous deposit of energy just as inflation began. Imagine that two spaceships are held together by a rubber band. If the spaceships continue to pull away, the rubber band will get tighter and tighter. As it does so, the kinetic energy from the spaceships is transferred to the rubber band, which will build up a large store of energy.

Similarly, a gravitational field can do the same thing. It can 'store' energy when large objects curve spacetime. (these usually come into effect as gravitational waves)

So, remember, inflation was an enormous burst of repulsive gravity. So, instead of storing energy, it dumped massive amounts of energy into the universe. And I mean massive. In chaotic inflation, this is what creates the 1032 degree temperatures during the planck time.

Once inflation got underway, however, the universe began to cool as it expanded. It cooled down to approximately 1028 degrees. After inflation ended, the inflaton field decayed into an extraordinarily hot bath of radiation, reheating the universe. This radiation, along with the heat from inflation, soon gave birth to quark-gluon plasma through particle pair production. It still remains a mystery how matter managed to outnumber anti-matter. Supersymmetry provides an explanation in which leptons could be converted to Baryons, which would have thrown off the balance.

Eternal inflation actually allows for finite 'bubble universes' that form when inflation ends to become infinite in size. This is because of how dramatically inflation warps time, opposite of the effect on objects orbiting a black hole.