Standing in nothingness before the Big Bang

[Chalnoth]

Texas, your understanding is not correct. We don't know how the universe was created, we only know that the early universe was very dense, very hot, and consisted of a sea of very energetic particles and radiation. Before this point in time we know nothing, as the math in our theories breaks down and starts giving us infinities.

See here: http://en.wikipedia.org/wiki/Chronology_of_the_universe

"Was created" is just bad language. Formed is more neutral word.

And I would say it is just plain false that we know nothing. We don't know specifically what happened yet, and certainly don't know that much. But that doesn't mean we know nothing.

 

[Deleted member 498135]

"Was created" is just bad language. Formed is more neutral word.

And I would say it is just plain false that we know nothing. We don't know specifically what happened yet, and certainly don't know that much. But that doesn't mean we know nothing.

True, a lot of people ask me that if science is right, then explain how the world was created. I say that "you don't have to believe in it, but science is explaining things new everyday, unanswered questions will become answers" (this is after me explaining the Big Bang theory). They don't understand that science is progressing, not to be offensive or rude but don't you think it's more reasonable to believe what science says and explains rather than philosophy and *other beliefs***?

 

[Enigman]

An off-topic and possibly naive question:
Doesn't the big bounce model violate the 2nd law of thermodynamics? Or does it not matter because the 2nd is largely a statistical law?

 

[Chalnoth]

An off-topic and possibly naive question:
Doesn't the big bounce model violate the 2nd law of thermodynamics? Or does it not matter because the 2nd is largely a statistical law?

It certainly seems to violate the second law to me. The proponents of the more recent versions of the loop quantum gravity-based bounce models claim that it doesn't violate the second law of thermodynamics, but I remain incredibly skeptical. The only way in which I've heard it explained clearly sounded more like avoiding the problem rather than solving it.

 

[MathJakob]

Saying that the universe expands faster than light is a nonsensical statement. Expansion cannot possibly be faster (or slower) than the speed of light, because expansion isn't a speed! Expansion is a rate. It's like saying that your car's engine can't revolve at faster than 60 mph: that statement doesn't make sense, because engine revolutions are measured in RPM's, which are not a speed at all.

And just as matter cannot ever outpace a light ray, due to the speed of light limitation, matter also cannot outpace the gravity it produces. So it is fundamentally impossible for matter and gravity to become somehow separated.

Maybe I misunderstood but I thought the current understanding was that the universe is expanding and the rate is increasing more and more and then universe can expand faster than the speed of light withou violating relativity?

So if the rate gets faster and faster, will it not eventually be expanding at a rate greater than the speed of light? I hope I'm making sense.

 

[Chalnoth]

Maybe I misunderstood but I thought the current understanding was that the universe is expanding and the rate is increasing more and more and then universe can expand faster than the speed of light withou violating relativity?

So if the rate gets faster and faster, will it not eventually be expanding at a rate greater than the speed of light? I hope I'm making sense.

A rate is not a speed. Rate is inverse time (think of it as a percent that our universe expands every second...for reference it's 0.0000000000000002% per second, or about 7% per billion years). It is simply not possible to compare a rate to a speed, as the units of speed are distance per time.

Saying, "The universe is expanding faster than the speed of light," is rather like saying, "The length of that road is faster than 15 miles per hour." The sentence doesn't even make sense.

 

[marcus]

An off-topic and possibly naive question:
Doesn't the big bounce model violate the 2nd law of thermodynamics? Or does it not matter because the 2nd is largely a statistical law?

Several explanations have been given, one I like is based partly on the observation that you cannot state the 2nd Law without the distinction of macro and micro degrees of freedom and what defines a macroscopic degree of freedom is what affects a second system. The macro degrees of freedom are what appear in the interaction Lagrangian.
They are what matter to the second system. And BTW the primitive ideas of order and disorder also depend on what matters to the second system, with which the first is interacting.

There was some discussion of that in Rovelli's recent FQXi essay contest entry Relative Information at the Foundation of Physics, which as i recall got second prize. It's a wide audience essay, just 3 pages.

When one focuses on the Loop cosmology big bounce another issue that comes to the fore is the definition of entropy of geometry i.e. entropy of the gravitational field. To date this has not been satisfactorily defined. It would play a huge role in the total entropy, the matter part would be comparatively small. How would one define the entropy of geometry at the bounce when due to quantum effects gravity is repellent?

As long as gravity is universally attractive the gravitational field tends to become CLUMPY. Density and curvature tends to accumulate in knots and clusters. So a clumpy geometry is the HIGH ENTROPY one and smooth even geometry is LOW.

But as soon as gravity becomes repellent then smooth uniform geometry is HIGH entropy, because that is the direction in which things evolve.

1. Can you imagine a partition of the system which endures thru the bounce so that interaction
d.o.f. and hence entropy can be defined at all?

2. If entropy of any sort can be defined consistently thru the bounce, then what about entropy of the grav field, the geometry itself IOW? There is currently no generally accepted definition.

3. If the entropy of the geometry of the universe can be acceptably defined in a regime in which gravity is consistently attractive, then what happens at high density when according to Loop it turns temporarily repellent due to quantum corrections? This would seem to reverse the definition of entropy--turn it around 180 degrees.

So a naive interpretation of 2nd Law does indeed encounter difficulties. It's a good question to be asking Enigman! I think some interesting developments will come out of people taking a closer look at quantum gravity and thermodynamics.

Check out the fourth quarter MIP Poll! there's some new research in the area of QG and thermo.
https://www.physicsforums.com/showthread.php?t=730750

 

[marcus]

Maybe I misunderstood but I thought the current understanding was that the universe is expanding and the rate is increasing more and more and then universe can expand faster than the speed of light withou violating relativity?

So if the rate gets faster and faster, will it not eventually be expanding at a rate greater than the speed of light? I hope I'm making sense.

Basically you are making sense and it's a reasonable question to be asking MathJakob.
Notice that in a uniform pattern of distance expansion NOBODY GETS ANYWHERE, everybody just gets farther apart. Nobody is favored to catch up with anybody else, or to overtake a photon of light. Distance growth is not like ordinary motion where you get somewhere. So the relativity speed limit does not apply to it. There is no reason that distances cannot increase faster than light.

Since the speed of increase is proportional to the size of the distance, it is the very large distances that are increasing faster than light. While shorter distances grow at a barely noticeable rate. (And within a gravitationally bound structure like our galaxy, not at all.)

A good way to think of the current rate of distance growth is 1/144 of a percent per million years

According to the standard cosmic model that nearly all cosmologists use that percentage growth rate is slowly declining towards about 1/173% per million years, where it is slated level off and not decline further.

Since the percentage rate has become nearly constant (slow decline from 1/144 down to 1/173 is not much change) if you watch a particular distance (say between two clusters of galaxies) you would see ALMOST exponential growth of that distance. Like money deposited at a bank at a small but nearly constant percentage rate of interest. that's the "acceleration" they talk about. It is not very dramatic.

 

[timmdeeg]

Since the percentage rate has become nearly constant (slow decline from 1/144 down to 1/173 is not much change) if you watch a particular distance (say between two clusters of galaxies) you would see ALMOST exponential growth of that distance.

I am sceptical regarding this conclusion, as the matter density today is about 25% of the total energy density. The universe will approach an exponential expansion asymptotically once the percentage of matter density goes to zero, hereby assuming that the dark energy acts like a cosmological constant. Perhaps there are estimates, when this will happen.

 

[Chalnoth]

I am sceptical regarding this conclusion, as the matter density today is about 25% of the total energy density. The universe will approach an exponential expansion asymptotically once the percentage of matter density goes to zero, hereby assuming that the dark energy acts like a cosmological constant. Perhaps there are estimates, when this will happen.

Once the matter density goes to zero, the expansion will be exponential, no asymptotic about it.

What is asymptotic is the approach of the matter density to zero. The matter density is already low enough that the current and future expansion is pretty close to exponential.

 

[marcus]

I am sceptical regarding this conclusion, as the matter density today is about 25% of the total energy density. The universe will approach an exponential expansion ...

I'm not sure what conclusion you find dubious, Tim. I'm saying that right now we have almost exponential distance growth at around 1/144% per My
And eventually in future we will have exponential growth at 1/173% per My (according to standard cosmic model).

Maybe the the word "almost" is too vague for you. It is vague! I think you would like to see a TIMETABLE for the decline in percentage growth rate from 1/144 to 1/173 and that is easy to supply. Just google "lightcone cosmological calculator" and look at the column labeled "R" for Hubble radius.
As that increases from 14.4 to 17.3 the percentage rate declines accordingly.

You can compare that with the "T" column which gives the time since start of expansion, i.e. age.

To get better time resolution, first click "set sample chart range" and then "calculate"

that will give a longer table with more gradual increase from today's 14.4 to eventual 17.3.

It also gives a substantial chunk of the past. You can see for instance that the distance growth rate was approximately 1% per My back in the year 67 million. that is what the first row of the table says (if you have pressed "set sample chart range")

If googling does not work, here is the link:
http://www.einsteins-theory-of-relativity-4engineers.com/LightCone7/LightCone.html

I also keep the link in my signature, it's handy for a LOT of purposes. You get to set the parameters of the table.

 

[timmdeeg]

I'm not sure what conclusion you find dubious, Tim. I'm saying that right now we have almost exponential distance growth at around 1/144% per My
And eventually in future we will have exponential growth at 1/173% per My (according to standard cosmic model).

Maybe the the word "almost" is too vague for you. It is vague! I think you would like to see a TIMETABLE for the decline in percentage growth rate from 1/144 to 1/173 and that is easy to supply. Just google "lightcone cosmological calculator" and look at the column labeled "R" for Hubble radius.
As that increases from 14.4 to 17.3 the percentage rate declines accordingly.

Thanks marcus and congratulation to you and Mordred, this table is very helpful. I haven't realized it till now and for sure will be busy with it from now on. And I agree regarding the development of 1/R , we are now already relatively close to exponential expansion, though it will take another 78 Gy to reach R = 17.2999, corresponding to an almost exponential expansion. Hereby I use 'almost' only in the sense to not hide that the approach is asymptotic.

 

[timmdeeg]

Once the matter density goes to zero, the expansion will be exponential, no asymptotic about it.

What is asymptotic is the approach of the matter density to zero.

It seems curious. "Gegen Null gehen" translated into English means "approaching zero", so, seems different from "goes to zero", as I used it wrongly by translating directly. I appreciate your comment.