How far is the edge of the universe
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Understanding the Edge of the Universe
The Observable Universe: Defining the Horizon
The concept of the "edge" of the universe is often misunderstood. In cosmology, the term typically refers to the observable universe, which is the region of space from which light has had time to reach us since the beginning of the cosmic expansion. Due to the finite speed of light and the age of the universe, there is a horizon beyond which we cannot observe any objects because their light has not had enough time to reach us.
Calculating the Observable Universe's Radius
The distance to this horizon can be calculated by considering the time it would take for a photon to travel from the edge of the observable universe to us. This calculation has been refined over time with more accurate cosmological data. Initially, Gott et al. used data from the WMAP survey to estimate this distance. However, more recent data from the Planck satellite, published in 2013, has provided updated cosmological parameters. Using these new parameters, researchers have determined that the radius of the observable universe is approximately 0.7% smaller than previous estimates.
The Edge of Galactic Halos: A Different Perspective
While the observable universe defines the large-scale boundary of what we can see, the concept of an "edge" can also apply to smaller structures, such as galactic halos. In the context of the Milky Way (MW) and similar galaxies, the edge is often defined by a caustic, which is a sharp drop in density or radial velocity.
Identifying Galactic Halo Boundaries
Cosmological simulations have identified two primary caustics in dark matter halos. The outermost caustic, known as the "splashback" radius, is located at approximately 1.4 times the virial radius (r200m). This radius marks the boundary where infalling material reaches its first apocenter after turnaround. A second caustic, found at about 0.6 times the virial radius, corresponds to the edge of the virialized material that has completed at least two pericentric passages.
Interestingly, the stellar distribution within these halos also exhibits a clearly defined caustic, which often coincides with the second dark matter caustic. This alignment can be observed in both radial density and radial velocity profiles and is expected to be measurable in future observational programs. For the Milky Way, this second caustic, which can also be identified in the phase-space distribution of dwarf galaxies, predicts the edge of the MW halo to be approximately 292 ± 61 kpc.
Conclusion
The "edge" of the universe can be understood on different scales. On the largest scale, the edge of the observable universe is defined by the distance light has traveled since the beginning of the cosmos, currently estimated to be slightly smaller than previous calculations due to updated cosmological data. On a smaller scale, the edge of galactic halos, such as that of the Milky Way, is defined by caustics in the distribution of dark matter and stars, providing a more localized boundary within the vast expanse of the universe.
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