# Momentum Space Measure Change of Variables: Exploring Cosmic Abundances

• guest1234
In summary: Your name]In summary, the conversation discusses an article on cosmic abundances, specifically equations (3.2), (3.4), and (3.8), and raises questions about the sudden appearance of certain variables and the change of variables used by the authors. The missing factor in equation (3.2) is noted and the use of a relativistic approach in equations (3.4) and (3.8) is mentioned as a possible explanation for the seemingly peculiar results.
guest1234
I'm working through an article called "Cosmic abundances of stable particles -- improved analysis" (link -- viewable only in Firefox afaik), the result of which, equation (3.8), is cited quite a lot. I'm more interested in how they arrived there.
Particularly, how come momentum space measure, $\mathrm{d}^3\mathbf{p}_1\mathrm{d}^3\mathbf{p}_2$, suddenly becomes $4\pi p_1\mathrm{d} E_1 4\pi p_2\mathrm{d} E_2 \frac{1}{2}p_1 p_2 \mathrm{d}\cos\theta$, where $\theta=\angle(\mathbf{p}_1, \mathbf{p}_2)$ and $p_i=|\mathbf{p}_i|$ (equation (3.2))? The dimensions don't match at all.. The closest I can come up with is $\mathrm{d}^3\mathbf{p}_i = p_i^2 \sin\theta_i\mathrm{d}\theta_i\mathrm{d}\phi_i\mathrm{d}p_i \hat{\mathbf{p}}_i = [\mbox{integrate over angles}] = 4\pi p_i^2 \mathrm{d}p_i \hat{\mathbf{p}}_i$, which after substituting with $p_i \mathrm{d}p_i=E_i \mathrm{d}E_i$ (because $p_i^2 = E_i^2 - m_i^2$) gives $\mathrm{d}^3\mathbf{p}_i = 4\pi p_i E_i \mathrm{d}E_i \hat{\mathbf{p}}_i$. So I think it should be something like $\mathrm{d}^3\mathbf{p}_1\mathrm{d}^3\mathbf{p}_2 = 4\pi E_1 \mathrm{d}E_1 4\pi E_2 \mathrm{d}E_2 p_1 p_2 \cos\theta=16\pi^2 E_1 E_2 p_1 p_2\cos\theta\mathrm{d}E_1\mathrm{d}E_2$. With this I'm off by one diffenetial and 1/2...
It also bugs me that they make change of variables, $E_+ = E_1 + E_2$, $E_- = E_1 - E_2$, $s = (E_1 + E_2, \mathbf{p}_1 + \mathbf{p}_2)^2$, which again leads to a peculiar result, $\mathrm{d}^3 p_1\mathrm{d}^3p_2 = 2\pi^2 E_1 E_2 \mathrm{d}E_+\mathrm{d}E_-\mathrm{d}s$ (equation 3.4). I mean, doing this rigorously doesn't work out, unless you assume that $E_+ = E_+(E_2)$, $E_- = E_-(E_1)$ and $s = s(p_1 p_2 \cos\theta)$ or something..

Any ideas what went wrong in (3.2) and (3.4)?
(Sorry for the question being too technical; if you feel this thread doesn't belong to this forum, move it away)
Oh, and happy new year ..

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edit: ok, I have figured out that $\mathrm{d}^3\mathbf{p}_2 = 2\pi p_2^2 \mathrm{d}p_2 \mathrm{d}\cos\theta$ and evidently equation (3.2) is indeed missing a factor $p_1 p_2$. Still, the change of variables into $E_\pm$ and $s$ puzzles me.

Last edited:

Thank you for bringing up this interesting question. I have read through the article and have some thoughts on the issues you have raised.

Firstly, regarding equation (3.2), I agree with your analysis that there seems to be a missing factor of p_1 p_2. This could be a typo or an oversight by the authors. I suggest reaching out to them for clarification on this matter.

As for the change of variables in equations (3.4) and (3.8), I believe the authors are using a relativistic approach where they are considering the energy and momentum of particles as components of a four-vector. In this approach, the variables E_+ and E_- represent the total energy and the difference in energy between the two particles, respectively. Similarly, s represents the square of the total four-momentum of the two particles. This approach may not be immediately intuitive, but it is commonly used in particle physics and cosmology.

I would also like to point out that the dimensions in equations (3.4) and (3.8) do match, as the factor of 4\pi comes from the integration over angles, which is accounted for in the \mathrm{d}\cos\theta term.

I hope this helps clarify some of your concerns. If you have any further questions or would like to discuss this further, please do not hesitate to reach out.

Happy new year to you as well.

## 1. What is momentum space and why is it important in studying cosmic abundances?

Momentum space refers to the mathematical space where the momentum of a particle can be represented. It is important in studying cosmic abundances because it allows us to describe the distribution and evolution of particles in the universe, which is essential in understanding the formation and evolution of galaxies and structures in the universe.

## 2. How is the measure of momentum space changed when studying cosmic abundances?

The measure of momentum space is changed by using different variables to describe the distribution of particles. This is because certain variables may be more suitable or easier to work with when studying cosmic abundances. By changing the variables, we can better understand the patterns and trends in the distribution of particles.

## 3. What are some commonly used variables to measure momentum space in the study of cosmic abundances?

Some commonly used variables to measure momentum space include redshift, which measures the amount of light shifted to longer wavelengths due to the expansion of the universe, and comoving coordinates, which take into account the expansion of the universe when measuring distances between objects.

## 4. How does exploring cosmic abundances in momentum space help us understand the universe?

Exploring cosmic abundances in momentum space allows us to study the large-scale structure and evolution of the universe. By understanding the distribution of particles and their interactions in momentum space, we can gain insights into the physical processes that govern the universe, such as dark matter and dark energy, and how they shape the structure of the universe.

## 5. How does the change of variables in momentum space affect the interpretation of cosmic abundance data?

The change of variables in momentum space can affect the interpretation of cosmic abundance data by revealing different patterns and trends in the distribution of particles. It can also help us identify any biases or errors in the data and provide a more accurate understanding of the universe. Additionally, different variables may be better suited for studying specific phenomena, allowing for a more comprehensive analysis of cosmic abundances.

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