Unruh: Loop Quantum Cosmology paper (new)

In summary, Bill Unruh has found difficulties with closed isotropic loop quantum cosmology. He came up with the Unruh temperature and Unruh radiation about the same time that Hawking came up with Hawking temperature and Hawking radiation. It is nice that Unruh has taken an interest in LQC and something could come of this. It might shake things up and trigger some new growth.
  • #1
marcus
Science Advisor
Gold Member
Dearly Missed
24,775
792
http://arxiv.org/abs/gr-qc/0408074

Bill Unruh has been doing some LQC research

Looking at the positive curvature case
Bojowald and friends mostly looked at the spatially flat case
Unruh found some interesting difficulties with the positive curved universe, especially the kind that eventually collapses

"Difficulties with Closed Isotropic Loop Quantum Cosmology"
Daniel Green and William Unruh


Unruh came up with the Unruh temperature and Unruh radiation about the same time that Hawking came up with Hawking temperature and Hawking radiation. It is nice that Unruh has taken an interest in LQC and something could come of this. It might shake things up and trigger some new growth.

Bojowald is a young guy just barely out of his postdocs and
Unruh is a major figure of the 1970s onwards. Hello.
 
Astronomy news on Phys.org
  • #2
Fascinating paper. Thanks for the link.
 
  • #3
Chronos said:
Fascinating paper. Thanks for the link.

yeah, Bill Unruh is neat. glad someone picked up on this Chronos!

I remember when I saw his original article about the temperature associated with any quantity of acceleration.

it was in Phys. Review Series D and the first thing he did was set
G=c=hbar=k=1
where k is Boltzmann constant
this validated natural units for me. the formulas all became incredibly simple.

then he said that if an observer was accelerating he would see a certain radiation which you and I can't see because we arent----it was like Hawking radiation because one of the virtual particles falls behind and can't catch up (oversimplifying) just like at BH event horizon
so the accelerating observer sees a temperature in the universe which is the temperature of this thermal glow----sheer artifact of his acceleration.

this is a creative mind. this is beautiful. I was impressed all getout.

You Chronos probably know some about Unruh temperature.

did you ever take some sample acceleration and calculate the temperature that goes with it?
 
  • #4
marcus said:
did you ever take some sample acceleration and calculate the temperature that goes with it?
Yes, but,I am reluctant to give any numbers fearing how they might be abused. Oh well.. :devil: It takes some pretty impressive accelerations to generate much of an effect. 1 degree K requires about 2.4E22 cm/s^2. For a temperature of 2.7K, which is what I was curious about, this works out to roughly 7E19 G... [ducking for cover now]. I thought this might somehow relate to the maximum mass of a detectably 'radiating' black hole, but, that is a pretty speculative approach.

Measuring this experimentally is, however, quite the technological challenge. I've heard some proposals involving high energy lasers, but, not about it having yet been tried.
 
  • #5
Chronos said:
Yes, but,I am reluctant to give any numbers fearing how they might be abused. Oh well.. :devil: It takes some pretty impressive accelerations to generate much of an effect. 1 degree K requires about 2.4E22 cm/s^2. For a temperature of 2.7K, which is what I was curious about, this works out to roughly 7E19 G... [ducking for cover now]. I thought this might somehow relate to the maximum mass of a detectably 'radiating' black hole, but, that is a pretty speculative approach.

Measuring this experimentally is, however, quite the technological challenge. I've heard some proposals involving high energy lasers, but, not about it having yet been tried.

Chuckle. Have i been scolding you about numbers? If I did I am sorry. There is no need to duck for cover!

actually I heard of some experiment at Stanford SLAC around year 2000 IIRC or 2001. But I lost the link to it. I don't know if it was a good experiment.

I vaguely remember that if G=hbar=c=k=1 then the formula for the temperature is
T = a/2pi

I will assume your number is right, as a guide to me in trying to remember.

I know that 1.4 kelvin is E-32
(they have Planck temperature listed with the other constants at NIST)

so one would just multipy that by 2pi to get the acceleration that would produce that temp.

2pi E-32

this acceleration will produce 1.4 kelvin.

It looks like I am done but if I want to interpret that acceleration in metric terms then I have to know that the unit of acceleration (G=c=hbar=1) is 5.56E51 meter per second per second (again from the NIST figures for Planck time and length etc.)

so multiplying by 2piE-32 gives 35E19 meter per second per second.

Wow! I get around the same answer you do! I get 3.5E20 meters which is 3.5E22 cm persecondpersecond. It is the right OOM (order of magnitude)

I happened to be calculating 1.4 kelvin while you were doing 1.0 kelvin but we don't worry about trivial details!

So maybe i remembered right and Unruh formula for temp is really

T = a/2pi

but I'm still unsure and need to check this---earlier remembered something different for the denominator
 
Last edited:
  • #6
marcus said:
Chuckle. Have i been scolding you about numbers? If I did I am sorry. There is no need to duck for cover!
Not at all. I was just afraid somebody would use the 'math' to derive some 'fundamental' constant of the universe and push this thread into the 'Theory Development' sub-forum. :surprise:
 
  • #7
4E-23a is approximately correct. The pi factor is unnecessary, it cancels in translation.
 
  • #8
Chronos said:
Not at all. I was just afraid somebody would use the 'math' to derive some 'fundamental' constant of the universe and push this thread into the 'Theory Development' sub-forum. :surprise:

I think you and I can maintain our dignity in whatever pen we are herded :smile:
Be not afraid of eviction Chronos

but I felt a brief elbow in my ribs about my blatantly obvious love for
the fundamental physical constants-----especially when their values are all set to unity.

Trouble is, my memory is pulling a blank and I forget if I have the correct Hawking formula for the temp of a BH of mass M.
and the correct Unruh formula for the temperature of a any give acceleration.
I'm still struggling with this. Want to be able to recapture it. I think it's like this but I or someone should verify:

[tex]T_{BH} = \frac{1}{8 \pi M}[/tex]

[tex]T_{Unruh} = \frac{a}{2 \pi}[/tex]

in any case they are remarkably nice clean formulas, I hope I have the twos and pies right. Maybe with a little practice using them...[edit: I am still at sea. Many people know these---Alejandro for instance. I would welcome a little help. do you have a source for these Chronos?]
 
Last edited:
  • #10
Chronos said:
Looks good from here
http://en.wikipedia.org/wiki/Hawking_radiation
http://relativity.livingreviews.org/Articles/lrr-2001-6/node3.html

great! thanks for confirming. for some reason I didnt want to go
churning up the mud at google
but just for the formulas to emerge in our conversation out of the fog of memory

there is a new paper called "Time before Time" with absolutely no formulas in it!

I found it by this link I use every weeknight

Last twelve months (e.g. 26 August 2003 to 26 August 2004):
http://lanl.arXiv.org/find/nucl-ex,...m+AND+OR+triply+doubly+special/0/1/0/past/0/1

As I was going to fetch the link I saw that Olias had already
noticed Time before Time and posted a link. It looks like a useful paper.
 
Last edited by a moderator:

1. What is Unruh: Loop Quantum Cosmology paper (new)?

The Unruh: Loop Quantum Cosmology paper is a recently published scientific paper that presents a new theory in the field of cosmology. It is authored by renowned physicist, William Unruh, and discusses the application of loop quantum gravity to understand the origin and evolution of the universe.

2. What is loop quantum gravity?

Loop quantum gravity is a theoretical framework that combines the principles of general relativity and quantum mechanics to explain the fundamental structure of space and time. It proposes that space and time are not continuous, but rather made up of discrete, indivisible units called quantum loops.

3. How does Unruh's paper contribute to our understanding of the universe?

Unruh's paper presents a new approach to understanding the universe by applying the principles of loop quantum gravity to cosmology. This theory provides a different perspective on the origin and evolution of the universe, potentially offering new insights and explanations for some of the unanswered questions in cosmology.

4. What are some key findings or implications of Unruh's paper?

One of the key findings of Unruh's paper is that the universe may have originated from a quantum bounce, rather than a singularity as predicted by the Big Bang theory. This theory also suggests that the universe may have existed in a previous cycle before undergoing a quantum bounce and beginning a new cycle.

5. What are some potential criticisms or limitations of Unruh's paper?

Some critics of Unruh's paper argue that there is not enough empirical evidence to support the claims made in the theory. Additionally, some believe that loop quantum cosmology may not be compatible with other established theories, such as inflation theory. Further research and testing will be necessary to fully understand the validity and implications of Unruh's paper.

Similar threads

I What do the authors of the paper mean here exactly by path integral?
    • Quantum Physics
Replies
6
Views
694
I Two recent tests of loop quantum gravity theory
    • Beyond the Standard Models
Replies
6
Views
377
I Trouble for Loop Quantum Gravity?
    • Beyond the Standard Models
Replies
3
Views
2K
A Can the huge energy of quantum vacuum explain the observed value of dark energy?
    • Beyond the Standard Models
Replies
2
Views
1K
I Is loop quantum gravity dual to CFT?
    • Beyond the Standard Models
Replies
13
Views
1K
I Implications of validating loop quantum cosmology
    • Beyond the Standard Models
Replies
28
Views
4K
I How do cosmologists view loop quantum cosmology?
    • Cosmology
Replies
4
Views
1K
Recent papers on self-dual loop quantum black holes
    • Beyond the Standard Models
Replies
28
Views
4K
I A Bold New Take on Quantum Theory
    • Quantum Interpretations and Foundations
Replies
25
Views
1K
Towards self dual Loop Quantum Gravity Jibril Ben Achour
    • Beyond the Standard Models
Replies
4
Views
1K

Hot Threads

Recent Insights

Back
Top