3 ms·

CMB rest frame is important and special unique frame to use as a reference. Relativity means that there are no special frames where the laws of physics are dif

• ithkuil 5y ago
  the CMB is a bit mind bending; it's related to the question "if the universe is expanding, in the past it was smaller and smaller; so where is the center?" Good read where you'll also learn about the "The Surface of Last Screaming": https://ned.ipac.caltech.edu/level5/March03/Lineweaver/Lineweaver7_2.html https://ned.ipac.caltech.edu/level5/March03/Lineweaver/Linew...
  • cout 5y ago
    I understand how there can be no center if the universe has positive curvature. But if the universe is saddle-shaped and non-infinite, surely there has to be a center?
    • effie 5y ago
      I think not; consider pseudosphere, there is no obvious center. Its embedding in 3D space has infinite dimensions but the object itself has finite surface area. https://en.wikipedia.org/wiki/Pseudosphere https://en.wikipedia.org/wiki/Pseudosphere If you mean object with finite dimensions and negative curvature and no center, there seems to be such a thing, see Fig. n066200e: https://encyclopediaofmath.org/wiki/Negative_curvature,_surface_of https://encyclopediaofmath.org/wiki/Negative_curvature,_surf... EDIT Rozendorn's idea seems to be still an open question: https://mathoverflow.net/questions/52851/the-geometry-of-nadirashvilis-complete-bounded-negative-curvature-surface https://mathoverflow.net/questions/52851/the-geometry-of-nad...
    • feoren 5y ago
      Rather than think "surface of a balloon inflating", think "surface of an (infinite?) rubber sheet being uniformly stretched". It does intuitively seem like if there is 0 curvature and no "center", then the universe must be infinite though ...