Universe began in a hot big bang

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marcus

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Loop Quantum Gravity trying to explain Lambda

Vast, you raised the issue of the cosmological constant
(the assumed constant energy density used to make the model have accelerating expansion so it will fit the data)

The next related conference talk I know of about that is early next month in a Feb 4-14 special seminar where Lee Smolin is supposed to give a talk on

"Cosmological Constant and Quantum Gravity"

here is the webpage that lists the talks:

http://www.ws2004.ift.uni.wroc.pl/html.html [Broken]

You can say it is the current LQG bid in the "Lambda Sweepstakes"

Quantum gravity is the quantum theory of what space consists of, at very small scale, and it seems to me that it is exactly the branch of physical theory you would want to be able to come up with an explanation of Lambda

(if this constant vacuum energy density exists and is 0.6 joules per cubic km, like they say it is)

Dont hold your breath though. No idea what Smolin will say but it is too hard a problem to expect solutions to come quickly. Maybe some incremental progress this year...
 
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Hi Marcus, thank you, there’s a lot of information to take into consideration, but as far as dark energy is concerned, I think the simple model does work better, for the reason that dark energy is divided with conventional matter.
I get the impression that dark energy is simply “explosive energy”, that which occurs in supernova, and at the big bang.
How it gets divided in the early phase of the universe is indeed still a huge question, and as I understand it String theory or M-theory cannot predict how it gets divided.

Quantum Gravity, or Quantum Geometry, is quite hard to understand, but what occurs in that region, does cause energy to be divided. How a quantum structure can divide itself, when conventional matter doesn’t seem to exist yet, may show that different phases have different behaviors, thus the universe divides itself into a cosmological constant and conventional matter some time after inflation.

Well at least they’re focusing on what seems to be the key questions.
 
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Originally posted by marcus
a cubic kilometer of otherwise empty space should have exactly 0.6 joules of vacuum energy in it---constantly and uniformly thru time and space.
How is this energy manifested? Do some of virtual particles survive and carry off this energy?

Thanks.
 

marcus

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Originally posted by Mike2
How is this energy manifested? Do some of virtual particles survive and carry off this energy?

Thanks.
Mike you might want to ask several different people to explain why the cosmologists have come up with a figure of 73 percent. Nereid or selfAdj or various others could.

All I have done is translate the 73 percent they say matches the CMB and supernova observations---the observed flatness and observed increase in expansion rate over time.

You probably need to believe Einstein 1915 GR (as simplified into the Friedmann equation) to understand.
According to GR the curvature and expansion depend on the
average energy density.
So far GR has been passing all tests so it doesnt look like its going to be thrown out so you just have to accept what the equation says
which is that the

observed flatness implies the average density in space
is 0.83 joule per cubic click including all forms (regular matter, transparent matter, visible light, transparent energy, ....everything)

you asked how it manifests itself. the energy content (energy in all forms) manifests itself by space being flat.

if it were more than 0.83 the equation says curvature would be positive, which we dont observe

if it were less than 0.83 the equation says curvature would be negative, which we do not observe

Personally I respect all the work they have put into verifying flatness and also in taking a count or census of different kinds of energy

by counting all the galaxies and other stuff in a given volume they estimate that visible matter only amounts to average 0.033 joule per cubic click

(I tried it one time with a box of space around our galaxy and androm and the rest of the local group out about halfway to the next group and I got an estimate like the official one more or less. It is hard but you can do it.)

Nereid has been giving reasons why the dark matter density should be what they say it is, namely about 0.19 joule per cubic click.
A recent one of her posts gave about 3 reasons. computer sims, obserations of gravitational lensing, stability of clusters etc etc.

Again it is hard but people have been mapping the dark matter and accounting for it and estimating and so on and it manifests itself in various ways by holding galaxies together and lensing etc. So I respect their esimate of 0.19 joule

So you just add it up. What is not accounted for (in the total 0.83 which we have to have because of observed flatness) is about 0.61 joules


0.61 + 0.19 + 0.033 = 0.83

That is the first way it manifests itself. The next is by a fantastic coincidence that blew people away in 1998. This 0.61, or 0.6 not to be too precise, is exactly the right size to explain the accelerating expansion seen in the supernovae!

So that is the second way it manifests itself.

According to the Friedmann equation, again, a constant energy density (a constant number of joules per cubic click) represents a negative pressure of the same amount------minus 0.6 nanonewtons per square meter if you like metric----minus 0.6 nanoPascals----and if you believe GR and the Friedmann this has an expansive effect. Causes just the right amount of acceleration so as to agree with the data.


I expect you might not like this and might prefer, in the last resort, to disbelieve the Einstein equation and the Friedmann derived from it. You are heartily welcome to do so! In that case you may say that the dark energy is not manifested! Deal with it however is best for you.

My point would be that the paradigms in physics are changing and that increasingly GR is calling the shots and saying "this and this is energy and this is how much energy is there, and this is how it is manifesting, and this is its pressure that we observe." It is a changed paradigm and a slightly different scene and different problems and answers, from what it looked like 20 or 30 years ago in the heyday of particle physics where you had different expectations about how energy was going to "manifest" itself.

Like maybe there was a bubble chamber?

And now the bubble chamber is 5 billion light years wide and what you see making tracks are supernovae.
 
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