- #1
- 24,753
- 794
G. M. Hossain posted a new Loop cosmology paper today.
http://arxiv.org/gr-qc/0411012
Primordial Density Perturbation in Effective Loop Quantum Cosmology
It suggests a way of testing LQG cosmology against standard scenarios by looking at details of the CMB power spectrum. A step in the direction of falsifiability (the criterion of when a theory is scientifically meaningful)
Along with discussing this possible test of the theory, Hossain made a comparison of LQG-induced inflation versus put-in-by-hand or long-shot inflation scenarios. See also
M. Bojowald, “Inflation from quantum geometry,” Phys. Rev. Lett. 89, 261301 (2002) http://arxiv.org/gr-qc/0206054 .
G. Date and G. M. Hossain, “Genericity of inflation in isotropic loop quantum cosmology,” http://arxiv.org/gr-qc/0407069 .
---quote from page 25 of gr-qc/0411012 ---
Before we discuss the implications of possible outcomes of mentioned test let us have a comparative study of standard inflationary scenario and loop quantum cosmology induced inflationary scenario.
In order to have a successful inflation in the standard scenario, generally one requires multi-level of fine tuning of field parameters. In other words one faces several kind of naturalness problems to achieve a successful inflation.
The first one is to start inflation. In standard inflationary scenario it is needed to choose initial field velocity to be sufficiently small so that the equation of state [is approximately -1].
The second one is to sustain inflation. In standard scenario one requires to choose the field potential to be flat enough so that field does not gain momentum quickly.
The third one is to generate sufficient expansion (to solve horizon problem and others). To achieve this in standard scenario, one requires to choose the initial field configuration sufficiently uphill in the potential. In other words, one requires to fine tune initial field configuration.
The fourth one is to end inflation. In many cases this requires sort of potential engineering to have a long flat plateau and then a fast fall-off in the potential profile.
The fifth one is to produce small amplitude for primordial density perturbation. To produce observed small amplitude of density perturbation one needs to fine tune parameters of the field potential. This fine tuning is basically required to compensate the ‘third’ fine tuning.
On the other hand, to achieve the first, second and fourth requirements in loop quantum cosmology induced inflationary scenario one does not require to fine tune the parameters. These requirements are naturally achieved as they simply follow from the spectrum of the inverse scale factor operator.
The fifth requirement i.e. small amplitude, as shown in this
paper, is a natural prediction of effective loop quantum cosmology.
The situation regarding the third problem also gets improved significantly. In the loop quantum cosmology, the generated amount of expansion is controlled by the ambiguity parameter j. Clearly to produce sufficiently large expansion, using loop quantum cosmology alone, one will require to choose the value of j to be large. Thus it is very likely that only the initial part of the inflation was driven by loop quantum cosmology modification. It has been argued in [40, 41, 42] that the loop quantum cosmology induced inflationary phase can lead to a secondary standard inflationary phase. This follows from the fact that the in-built inflationary period of loop quantum cosmology can produce favourable initial conditions for an additional standard inflationary phase. Since the observed part of CMB angular power spectrum generally corresponds to early period of inflation then it may well be the situation where the observed part of the CMB angular power spectrum corresponds to the loop quantum cosmology driven inflationary period. It is worthwhile to emphasize that high amount of expansion in this scenario is required not to solve horizon problem (being non-singular this model avoids horizon problem [22]) rather to avoid a different kind of problem...
---end quote---
http://arxiv.org/gr-qc/0411012
Primordial Density Perturbation in Effective Loop Quantum Cosmology
It suggests a way of testing LQG cosmology against standard scenarios by looking at details of the CMB power spectrum. A step in the direction of falsifiability (the criterion of when a theory is scientifically meaningful)
Along with discussing this possible test of the theory, Hossain made a comparison of LQG-induced inflation versus put-in-by-hand or long-shot inflation scenarios. See also
M. Bojowald, “Inflation from quantum geometry,” Phys. Rev. Lett. 89, 261301 (2002) http://arxiv.org/gr-qc/0206054 .
G. Date and G. M. Hossain, “Genericity of inflation in isotropic loop quantum cosmology,” http://arxiv.org/gr-qc/0407069 .
---quote from page 25 of gr-qc/0411012 ---
Before we discuss the implications of possible outcomes of mentioned test let us have a comparative study of standard inflationary scenario and loop quantum cosmology induced inflationary scenario.
In order to have a successful inflation in the standard scenario, generally one requires multi-level of fine tuning of field parameters. In other words one faces several kind of naturalness problems to achieve a successful inflation.
The first one is to start inflation. In standard inflationary scenario it is needed to choose initial field velocity to be sufficiently small so that the equation of state [is approximately -1].
The second one is to sustain inflation. In standard scenario one requires to choose the field potential to be flat enough so that field does not gain momentum quickly.
The third one is to generate sufficient expansion (to solve horizon problem and others). To achieve this in standard scenario, one requires to choose the initial field configuration sufficiently uphill in the potential. In other words, one requires to fine tune initial field configuration.
The fourth one is to end inflation. In many cases this requires sort of potential engineering to have a long flat plateau and then a fast fall-off in the potential profile.
The fifth one is to produce small amplitude for primordial density perturbation. To produce observed small amplitude of density perturbation one needs to fine tune parameters of the field potential. This fine tuning is basically required to compensate the ‘third’ fine tuning.
On the other hand, to achieve the first, second and fourth requirements in loop quantum cosmology induced inflationary scenario one does not require to fine tune the parameters. These requirements are naturally achieved as they simply follow from the spectrum of the inverse scale factor operator.
The fifth requirement i.e. small amplitude, as shown in this
paper, is a natural prediction of effective loop quantum cosmology.
The situation regarding the third problem also gets improved significantly. In the loop quantum cosmology, the generated amount of expansion is controlled by the ambiguity parameter j. Clearly to produce sufficiently large expansion, using loop quantum cosmology alone, one will require to choose the value of j to be large. Thus it is very likely that only the initial part of the inflation was driven by loop quantum cosmology modification. It has been argued in [40, 41, 42] that the loop quantum cosmology induced inflationary phase can lead to a secondary standard inflationary phase. This follows from the fact that the in-built inflationary period of loop quantum cosmology can produce favourable initial conditions for an additional standard inflationary phase. Since the observed part of CMB angular power spectrum generally corresponds to early period of inflation then it may well be the situation where the observed part of the CMB angular power spectrum corresponds to the loop quantum cosmology driven inflationary period. It is worthwhile to emphasize that high amount of expansion in this scenario is required not to solve horizon problem (being non-singular this model avoids horizon problem [22]) rather to avoid a different kind of problem...
---end quote---
Last edited by a moderator:
