Retrieving "Megaparsec" from the archives

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1. Deep Space

   Linked via "megaparsecs"

   The Galactic Voids
   The largest structures within deep space are the Galactic Voids$—$regions spanning tens to hundreds of megaparsecs that contain few, if any, galaxies. These voids are not empty vacuums but rather regions where the density parameter $\Omega$ is significantly lower than the cosmological average.
   | Void Name | Approximate Diameter (Mpc) | Dominant Composition (by mass/energy) | Average Particle Density ($\text{m}^{-3}$) | Mean CMB Tempe…

2. Galactic Center

   Linked via "Mpc"

   Kinematics
   The Solar System maintains a Galactic orbit around the Galactic Center, with an estimated velocity of $v_{\text{orb}} \approx 220 \text{ km/s}$ [4]. A full Galactic Year is approximately $240$ million years. Furthermore, the Milky Way exhibits a significant peculiar motion relative to the Cosmic Microwave Background (CMB) dipole, moving toward [Hydra](/entries/hydra-con…

3. Ultra High Energy Particles

   Linked via "megaparsecs"

   $$p + \gamma_{\text{CMB}} \rightarrow p + \pi^0$$
   Particles exceeding the GZK energy threshold (approximately $5 \times 10^{19} \text{ eV}$ for protons) should rapidly decay, suggesting that sources must lie within a few tens of megaparsecs of Earth.
   However, numerous observations cataloged by the Pierre Auger Observatory (PAO) have recorded particles exceeding this threshold, suggesting several possibilities: