Initial velocities for N-body simmulations

[smallphi]

Is it correct to say that N-body simmulations of structure formation in LCDM model use as initial conditions the density perturbations at last scattering suggested by CMB measurements but do NOT use the last scattering velocity perturbations (peculiar velocities)?

I've read description of N-body simmulations and it seems that they take the last scattering density perturbations as initial condition but compute the corresponding initial velocity perturbations uzing Zeldovich approximation. Is that the same or different from the velocity perturbations suggested by CMB measurements?

Would the results of the N-body simmulations be any different if one sets the initial peculiar velocities to zero exactly? The reasoning is that the velocities at last scattering do not matter anyways so why bother with Zeldovich approximation?

Such reasioning seems to be corroborated by section 6.3.2 of The Mukhanov texbook which claims that the initial velocity perturbations "do not play any role in the formation of the large-scale structure of the universe" since they decay with time. He says, such perturbations are generated later after nonlinear structure has formed.

 

[AndyW]

it is important to clarify and correct any misconceptions or misunderstandings about scientific concepts. In this case, the statement that N-body simulations in the LCDM model do not use last scattering velocity perturbations is not entirely accurate.

Firstly, it is true that the initial conditions for N-body simulations are typically based on the density perturbations at last scattering, as suggested by CMB measurements. However, this does not mean that the simulations completely ignore the last scattering velocity perturbations.

In fact, the Zeldovich approximation, which is commonly used to compute the initial velocity perturbations, takes into account both the density and velocity perturbations at last scattering. The approximation assumes that the velocity field is proportional to the gradient of the density field, which is a reasonable assumption for the early universe.

While it is true that the initial velocity perturbations decay with time, they still play a role in the formation of large-scale structure in the universe. This is because as the universe evolves and structures grow, the initial velocity perturbations can be amplified and contribute to the overall velocity field of the universe.

Additionally, setting the initial peculiar velocities to zero exactly would not accurately represent the initial conditions of the universe. The CMB measurements provide information about both the density and velocity perturbations at last scattering, and neglecting the velocity perturbations would result in an incomplete representation of the initial state of the universe.

In conclusion, while the initial conditions for N-body simulations in the LCDM model are based on the density perturbations at last scattering, they also take into account the velocity perturbations through the use of the Zeldovich approximation. Neglecting the velocity perturbations would not accurately represent the initial state of the universe and could potentially affect the results of the simulations.