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courses:ast403:two-point-correlation-function [2026/04/02 00:02] – shuvocourses:ast403:two-point-correlation-function [2026/04/04 01:17] (current) – shuvo
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 ===== Mathematical Definition ====== ===== Mathematical Definition ======
-Imagine a universe with a mean galaxy number density of $\bar{n}$. If we randomly drop two infinitesimal volume elements, $dV_1$ and $dV_2$, separated by a distance $r$, the probability $dP$ of finding one galaxy in $dV_1$ and another in $dV_2$ is given by:+Imagine a Universe with a mean galaxy number density of $\bar{n}$. If we randomly drop two infinitesimal volume elements, $dV_1$ and $dV_2$, separated by a distance $r$, the probability $dP$ of finding one galaxy in $dV_1$ and another in $dV_2$ is given by:
  
 $$dP = \bar{n}^2 [1 + \xi(r)] dV_1 dV_2$$ $$dP = \bar{n}^2 [1 + \xi(r)] dV_1 dV_2$$
  
-* **If $\xi(r) = 0$:** The galaxies are distributed completely randomly (a Poisson distribution). The probability is just $\bar{n}^2 dV_1 dV_2$. +If $\xi(r) = 0$: The galaxies are distributed completely randomly (a Poisson distribution). The probability is just $\bar{n}^2 dV_1 dV_2$. \\ 
-* **If $\xi(r) > 0$:** The galaxies are *clustered*. You are more likely to find a pair of galaxies at this separation than you would by chance. Gravity naturally drives $\xi(r)$ to be positive at small scales. +If $\xi(r) > 0$: The galaxies are clustered. You are more likely to find a pair of galaxies at this separation than you would by chance. Gravity naturally drives $\xi(r)$ to be positive at small scales.\\ 
-* **If $\xi(r) < 0$:** The galaxies are *anti-correlated*. You are less likely to find a pair at this distance than in a random distribution (e.g., due to vast cosmic voids).+If $\xi(r) < 0$: The galaxies are anti-correlated. You are less likely to find a pair at this distance than in a random distribution (e.g., due to vast cosmic voids).
  
-#### 6.2 Practical Estimation: The Landy-Szalay Estimator +===== Practical Estimation: The Landy-Szalay Estimator ===== 
-In practice, cosmologists do not have an infinite universe to measure; they have a finite survey with complex boundaries, varying observation depths, and instrumental artifacts. +In practice, cosmologists do not have an infinite Universe to measure; they have a finite survey with complex boundaries, varying observation depths, and instrumental artifacts. 
  
 To calculate $\xi(r)$ from real data, observers generate a "Random" catalog—a simulated dataset of points distributed completely randomly, but matching the exact 3D geometry and selection effects of the actual "Data" survey. They then count the number of pairs separated by a distance $r$. To calculate $\xi(r)$ from real data, observers generate a "Random" catalog—a simulated dataset of points distributed completely randomly, but matching the exact 3D geometry and selection effects of the actual "Data" survey. They then count the number of pairs separated by a distance $r$.
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 *(Note: These pair counts are normalized by the total number of possible pairs in each catalog).* *(Note: These pair counts are normalized by the total number of possible pairs in each catalog).*
  
-#### 6.3 Relationship to the Power Spectrum $P(k)$+===== Relationship to the Power Spectrum $P(k)$ ===== 
 In cosmology, it is often mathematically convenient to work in Fourier space rather than configuration (real) space. The Fourier transform equivalent of the two-point correlation function is the **Matter Power Spectrum**, $P(k)$, where $k$ is the wavenumber ($k \sim 2\pi/r$). In cosmology, it is often mathematically convenient to work in Fourier space rather than configuration (real) space. The Fourier transform equivalent of the two-point correlation function is the **Matter Power Spectrum**, $P(k)$, where $k$ is the wavenumber ($k \sim 2\pi/r$).
  
-For a statistically isotropic universe (where clustering depends only on the magnitude of the distance $r$, not the direction), the two-point correlation function is the Fourier transform of the power spectrum:+For a statistically isotropic Universe (where clustering depends only on the magnitude of the distance $r$, not the direction), the two-point correlation function is the Fourier transform of the power spectrum:
  
 $$\xi(r) = \frac{1}{2\pi^2} \int_0^{\infty} P(k) \frac{\sin(kr)}{kr} k^2 dk$$ $$\xi(r) = \frac{1}{2\pi^2} \int_0^{\infty} P(k) \frac{\sin(kr)}{kr} k^2 dk$$
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 Both $\xi(r)$ and $P(k)$ contain the exact same physical information, but they suffer from different observational systematics, so cosmologists usually measure both to cross-check their results. Both $\xi(r)$ and $P(k)$ contain the exact same physical information, but they suffer from different observational systematics, so cosmologists usually measure both to cross-check their results.
  
-#### 6.4 The BAO Peak in the Correlation Function +===== The BAO Peak in the Correlation Function ===== 
-As established in the previous section, the physics of the early universe created a spherical shell of baryons at the comoving sound horizon, $r_s \approx 147$ Mpc. +As established in the previous section, the physics of the early Universe created a spherical shell of baryons at the comoving sound horizon, $r_s \approx 147$ Mpc. 
  
 Because galaxies preferentially form in regions of high dark matter and baryon density, this primordial shell leaves a distinct imprint on the late-time distribution of galaxies. When we plot $\xi(r)$ for millions of galaxies, we see a smooth, exponential decay at small scales (due to standard gravitational clustering), interrupted by a distinct, localized "bump" or peak at exactly $r \approx 147$ Mpc. Because galaxies preferentially form in regions of high dark matter and baryon density, this primordial shell leaves a distinct imprint on the late-time distribution of galaxies. When we plot $\xi(r)$ for millions of galaxies, we see a smooth, exponential decay at small scales (due to standard gravitational clustering), interrupted by a distinct, localized "bump" or peak at exactly $r \approx 147$ Mpc.
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-This single peak in the 2PCF is the statistical manifestation of the acoustic waves stalling at recombination, and precisely locating the center of this peak at different redshifts is how the universe's expansion history is mapped.+This single peak in the 2PCF is the statistical manifestation of the acoustic waves stalling at recombination, and precisely locating the center of this peak at different redshifts is how the Universe's expansion history is mapped. 
 + 
 +[{{ :courses:ast403:bao1.gif | Fig 1: BAO peak from the 2-point correlation function.}}]
courses/ast403/two-point-correlation-function.1775109773.txt.gz · Last modified: by shuvo
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