This document discusses the numerical calculation of observational parameters for inflationary cosmology models, specifically tachyon inflation models and Randall-Sundrum braneworld models. The author develops software to solve the dynamical equations for tachyon scalar fields and radion fields during inflation. The software allows calculation of the scalar spectral index, tensor-to-scalar ratio, and number of e-folds for different model parameters. The best fitting results are obtained for tachyon potentials with 60 ≤ N ≤ 90 e-folds and dimensionless parameter 1≤ κ ≤ 10. The software provides a tool to test a wide range of inflationary models by inputting the corresponding Hamiltonian and Friedmann equations.

1. NUMERICAL CALCULATION OF
THE HUBBLE HIERARCHY PARAMETERS AND THE
OBSERVATIONAL PARAMETERS OF INFLATION
MILAN MILOŠEVIĆ
Department of Physics
Faculty of Sciences and Mathematics
University of Niš, Serbia
10th International Conference of Balkan Pysical Union (BPU10)
Sofia, 26 - 30 August 2018
In collaboration with N. Bilić (Zagreb), G. S. Đorđević,
D. D. Dimitrijević, M. Stojanović (Niš)

2. INTRODUCTION
• The inflation theory proposes a period of extremely rapid
(exponential) expansion of the universe during the very early
stage of the universe.
• Inflation is a process in which the dimensions of the universe
have increased exponentially at least times.
• Although inflationary cosmology has successfully
complemented the Standard Model, the process of inflation,
in particular its origin, is still largely unknown.

3. INTRODUCTION
• Over the past 35 years numerous models of inflationary
expansion of the universe have been proposed.
• The simplest model of inflation is based on the existence of a
single scalar field, which is called inflaton.
• The most important ways to test inflationary cosmological
models is to compare the computed and measured values of
the observational parameters.

4. OBSERVATIONAL PARAMETERS
• Hubble hierarchy (slow-roll) parameters
• Length of inflation
• The end of inflation
• Three independent observational parameters: amplitude of scalar
perturbation , tensor-to-scalar ratio and scalar spectral index
Hubble expansion rate
at an arbitrarily
chosen time
At the lowest order in parameters 𝜀 and 𝜀

5. OBSERVATIONAL PARAMETERS
• Satellite Planck
(May 2009 – October 2013)
• Planck Collaboration
• Latest results are published
in year 2018.
Planck 2015 results. XX. Constraints on inflation, Astronomy & Astrophysics. 594 (2016) A20

6. INFLATIONARY MODELS
• The dynamics of a classical real scalar field ϕ minimally
coupled to gravity
• G – the gravitational constant, R - the Ricci scalar, g - the
determinant of the matric tensor, and is the
Lagrangian, with kinetic term
• We will assume the spatially 4-dimensional flat space-time
with the standard FRW metric

7. LAGRANGIAN OF A SCALAR FIELD -
• In general case – any function of a scalar field and kinetic
energy
• Canonical field with potential
,
• Non-canonical models
• Dirac-Born-Infeld (DBI) Lagrangian
• Special case – tachyonic

8. TACHYONS
• Traditionally, the word tachyon was used to describe a
hypothetical particle which propagates faster than light.
• In modern physics this meaning has been changed:
• The effective tachyonic field theory was proposed by A. Sen
• String theory: states of quantum fields with imaginary mass (i.e.
negative mass squared).
• However it was realized that the imaginary mass creates an instability
and tachyons spontaneously decay through the process known as
tachyon condensation.
• Quanta are not tachyon any more, but rather an ”ordinary” particle with
a positive mass.

9. TACHYON INFLATION
• Properties of a tachyon potential
• The corresponding Lagrangian and the Hamiltionian are
• The Friedmann equation

10. DYNAMICS OF INFLATION
1. The energy-momentum conservation equation
.
2. The Hamilton’s equations
ℒ
̇ is the conjugate momentum and the Hamiltonian

11. TACHYON INFLATION
• Nondimensional equations
• Dimensionless constant , a choice of a constant
(brane tension) was motivated by string theory
,

12. CONDITIONS FOR TACHYON INFLATION
• General condition for inflation
• Slow-roll conditions
• Equations for slow-roll inflation

13. INITIAL CONDITION FOR TACHYON INFLATION
• Slow-roll parameters
• Number of e-folds

14. RANDAL-SUNDRUM MODELS
• Randall-Sundrum (RS) model was originally proposed to
solve the hierarchy problem (1999)
• Later it was realized that this model, as well as any similar
braneworld model, may have interesting cosmological
implications
• Two branes with opposite tensions are placed at some
distance in 5 dimensional space
• RS model – observer reside on the brane with negative tension,
distance to the 2nd brane corresponds to the Netwonian
gravitational constant
• RSII model – observer is placed on the positive tension brane, 2nd
brane is pushed to infinity

15. RSII MODEL
• The space is described by Anti de Siter metric
• Extended RSII model include radion backreaction
• Total action
, ,
, ,
where the second term is the action of the brane, and is the inverse of
AdS5 curvature radius, ϭ is the brane tension, the is tachyon field and is the
rescaled radion field .

16. RSII MODEL
• The Lagrangian and the Hamiltonian
• Conjugate momenta
, ,
• In flat space, FRW metrics

17. RSII MODEL
• The Hamilton’s equations
• The modified Friedman equation

18. NONDIMENSIONAL EQUATIONS
• Substitutions:
4
8 2
8 2
2 8 2
10 2
5 8 2
1 /
4 3 /
3
2 1 /
4 3 /
3
1 /
h
h




 


 

 
 

  
  
  
   
  
 
   




  


  





2 2
2 2
8
1
3 12
Gk
a
h
a
 
 
 

 
   
 

2
2
2
2 2
2
2 2 3
4
2
2
4 2 3
1 ,
sinh ,
6
2
sinh ,
6 3
1
1 / ,
2
1 1
2 1 /
d
d
p
  

 
  
 


  


 
  

 
 
   
 
 
    
 
  
 

 


Nondimensional constant
Hubble
parameter
Preassure
Energy
density
2 2
( ) 1
2 6
h p
N h
 
 
 
    
 



Additional equations,
solved in parallel
2
4
/ ,
/ ( ), / ( )),
, / ( )
h H k
k k
k k


   
  



   
   

19. INITIAL CONDITIONS FOR RSII MODEL
• Initial conditions – from a model without radion field
• “Pure” tachyon potential
• Hamiltonian
• Nondimensional equation


4
8 2
5 8 2
1
4
3 .
1
h


 

 

 
 
  


  


20. THE SOFTWARE
• Calculation:
• Programming language: C/C++
• GNU Scientific Library
• Visualization and data analysis
• Programming language: R

21. THE SOFTWARE
The model
The initial conditions

22. THE SOFTWARE
Test mode = 0
IZLAZ = 0
MCM = 1
MODEL = 2
INTC = 0
NoMAX = 10000
Nmin = 45.00 Nmax = 120.00
kmin = 1.00 kmax = 12.00
phimin = 0.00 phimax = 1.00
init.conf

23. RESULTS

24. OBSERVATIONAL PARAMETERS
• Scalar spectral index and tensor-to-scalar ratio (the first
order of parameters )
• The second order of parameters  different
• Always constant , however constant for
tachyon inflation in standard cosmology, and for Randall-
Sundrum cosmology
• Planck results ( )
.

25. TACHYION POTENTIAL
30 ≤ N ≤ 150, 0 ≤ κ ≤ 15

26. TACHYION POTENTIAL
30 ≤ N ≤ 150, 0 ≤ κ ≤ 15

27. TACHYION POTENTIAL
30 ≤ N ≤ 150, 0 ≤ κ ≤ 15

28. RSII MODEL
30 ≤ N ≤ 150, 0 ≤ κ ≤ 15,

29. THE BEST RESULTS:
60 ≤ N ≤ 90, 1≤ κ ≤ 10,

30. CONCLUSION
• The software we developed and used has been applied to a limited set
of models, mainly to pure tachyonic and RSII inflationary cosmological
models.
• In the present form it is written in such a way that its only inputs are
the Hamilton’s equations and the Friedmann equation, as well as the
corresponding parameters.
• The program can readily be used for a much wider set of models.
• To apply the program to a new model one must determine its
corresponding equations and include these equations in the program
only
• The next steps are: to extend the program to be applicable for new and
different types of inflationary models, to improve the program in such a way
that only the Hamiltonian or the Lagrangian of a model are their inputs.
• The corresponding system of differential equations would be determined by
symbolic computation.

31. CONCLUSION
• After these improvements the program will be published
as a free software followed by appropriate documentation.
• The best fitting result is obtained for . It
opens good opportunity for further research based on this
potential in the contest of the RSII model and the
holographic cosmology.
This work is supported by the SEENET-MTP Network under the ICTP grant NT-03.
The financial support of the Serbian Ministry for Education and Science,
Projects OI 174020 and OI 176021 is also kindly acknowledged.

32. THE MOST IMPORTANT REFERENCES
• N. Bilic, G.B. Tupper, AdS braneworld with backreaction, Cent. Eur. J. Phys. 12 (2014) 147–159.
• D. Steer, F. Vernizzi, Tachyon inflation: Tests and comparison with single scalar field inflation, Phys. Rev. D. 70 (2004)
43527.
• P.A.R. Ade, N. Aghanim, M. Arnaud, F. Arroja, M. Ashdown, J. Aumont, et al., Planck 2015 results: XX. Constraints on
inflation, Astron. Astrophys. 594 (2016) A20.
• L. Randall, R. Sundrum, Large Mass Hierarchy from a Small Extra Dimension, Physical Review Letters. 83 (1999)
3370–3373; L. Randall, R. Sundrum, An Alternative to Compactification, Physical Review Letters. 83 (1999) 4690–
4693.
• N. Bilic, D.D. Dimitrijevic, G.S. Djordjevic, M. Milosevic, Tachyon inflation in an AdS braneworld with back-reaction,
International Journal of Modern Physics A. 32 (2017) 1750039.
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comparison of analytical and numerical results with observation, Serbian Astronomical Journal. 192 (2016) 1–8.
• M. Milosevic, G.S. Djordjevic, Tachyonic Inflation on (non-)Archimedean Spaces, Facta Universitatis (Niš) Series:
Physics, Chemistry and Technology. 14 (2016) 257–274.
• N. Bilic, D.D. Dimitrijevic, G.S. Djordjevic, M. Milosevic, M. Stojanovic, Dynamics of tachyon fields and inflation:
Analytical vs numerical solutions, AIP Vol 1722 No 1 (2016) 50002.
• G.S. Djordjevic, D.D. Dimitrijevic, M. Milosevic, On Canonical Transformation and Tachyon-Like ”Particles” in
Inflationary Cosmology, Romanian Journal of Physics. 61 (2016) 99–109.
• D.D. Dimitrijevic, G.S. Djordjevic, M. Milosevic, Classicalization and quantization of tachyon-like matter on
(non)archimedean spaces, Romanian Reports in Physics. 68 (2016) 5–18.