How to recover comoving quantities

In summary: Your Name]In summary, the conversation is about converting output units from a cosmological simulation code to physical and comoving units. The formula for converting from code units to physical units is physical quantity = code quantity * unit_l / aexp, and the formula for converting to comoving units is comoving velocity = code velocity * unit_l / aexp / unit_t. It is important to use the appropriate values for unit_l, aexp, and unit_t for each redshift output.
  • #1
matteo86bo
60
0
Hi, I have a little problem, I'm using a code to run cosmological simulation and I have to recover the physical unities of every quantities. The problem is that there isn't a proper guide for this program. I try to explain.
All I have for every output/redshift is:

aexp
unit_l
unit_t

The authors of these program claim that in order to obtain a length,time in cgs units you need to multiply the quantities in code units with the above units.

My box length is 100Mpc/h.

I need to have comoving units. If I do box_length*unit_l/aexp I recover 100Mpc/h for every redshift.

The problem is, how do I recover the peculiar velocity of every particle in the comoving system?
I tried with this

vel_part*unit_l/aexp/unit_t


but I'm not sure since unit_t is different for every redshift output. Can you help me?

Thanks in advance
 
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  • #2


Hello,

Thank you for reaching out with your question. It sounds like you are working with a cosmological simulation code and are trying to convert the output units to physical units. This can be a tricky process, but I am happy to help guide you through it.

First, let's start with the basics. In cosmology, we typically use code units to simplify calculations and make them more manageable. These code units are chosen based on the physical quantities that are most relevant to the simulation, and they are often scaled to be of order unity. In your case, the code units are defined by the values of aexp, unit_l, and unit_t.

To convert from code units to physical units, you will need to use the following formula:

physical quantity = code quantity * unit_l / aexp

This formula works for any quantity that has units of length, such as box length or particle velocity. The key here is to make sure that you are using the correct values for unit_l and aexp. Since aexp is a scale factor that changes with redshift, you will need to use the appropriate value for each redshift output.

Now, let's address your question about recovering the peculiar velocity of each particle in comoving units. Peculiar velocity is a velocity that is relative to the expansion of the universe, so it is important to use comoving units when working with it. The formula for converting from code units to comoving units is a bit different:

comoving velocity = code velocity * unit_l / aexp / unit_t

Notice that we are now dividing by unit_t, which is the unit of time defined by the code. This is because we want to remove the time dependence from the velocity and express it in terms of the scale of the simulation.

I hope this helps answer your question and clarifies the process of converting from code units to physical and comoving units. If you have any further questions, please don't hesitate to ask. Best of luck with your simulations!
 

1. What are comoving quantities?

Comoving quantities are physical quantities that are measured with respect to the expanding universe. They take into account the expansion of the universe and are therefore independent of the observer's position. This is in contrast to proper quantities, which are measured with respect to a particular reference frame.

2. Why is it important to recover comoving quantities?

Recovering comoving quantities is important because it allows us to compare physical quantities across different distances and times in the expanding universe. This is essential for understanding the evolution of the universe and studying large-scale structures.

3. How do we recover comoving quantities?

In order to recover comoving quantities, we need to take into account the expansion of the universe by using a scale factor. This scale factor is a function of time and is used to convert proper quantities into comoving quantities. It is typically denoted by "a" and has a value of 1 at the present time.

4. Can we recover comoving quantities for all physical quantities?

No, not all physical quantities can be recovered in comoving form. Only quantities that are directly related to the expansion of the universe, such as distances, velocities, and densities, can be converted into comoving quantities. Other quantities, such as temperature and luminosity, are not affected by the expansion of the universe and therefore cannot be recovered in comoving form.

5. Are comoving quantities used in all fields of science?

No, comoving quantities are primarily used in the field of cosmology, where the study of the large-scale structure and evolution of the universe is the main focus. However, they can also be used in other fields, such as astrophysics, when studying objects on a cosmological scale.

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