What is the Roundest Thing?

What spherical object is the roundest and smoothest in the universe?


Links and References:

http://wpapool.com/equipment-specific... – World’s Pool-Billiard Association Equipment Specifications
http://billiards.colostate.edu/bd_art... - Is a Pool Ball Smoother Than the Earth?
https://solarsystem.nasa.gov/planets/... - Haumea: Overview
https://www.nist.gov/physical-measure... - Redefining the Kilogram, Silicon Spheres and the International Avogadro Project
https://www.bipm.org/en/bipm/mass/avo... - International Avogadro Project
https://arxiv.org/pdf/1005.5139.pdf - A Better Definition of the Kilogram
https://www.researchgate.net/publicat... - Determination of the Avogadro Constant by Counting the Atoms in a Si-28 Crystal
http://science.sciencemag.org/content... - The Precise Solar Shape and Its Variability
http://citeseerx.ist.psu.edu/viewdoc/... - History of solar oblateness measurements and interpretation
http://advances.sciencemag.org/conten... - Shape of a slowly rotating star measured by asteroseismology
https://www.mpg.de/10827169/distant-s... - Scientists measure the shape of Kepler 11145123 with unprecedented precision
https://arxiv.org/pdf/astro-ph/060133... - A Radio Pulsar Spinning at 716 Hz
https://arxiv.org/abs/0803.1373 - 1E161348-5055 in the Supernova Remnant RCW 103: A Magnetar in a Young Low Mass Binary System?
http://www-old.ias.ac.in/jarch/jaa/2/... - How High Can a Mountain be?
https://books.google.com.bh/books?id=... - Compact Objects in Astrophysics: White Dwarfs, Neutron Stars and Black Holes
https://www.nasa.gov/mission_pages/ch... - Young Magnetar Likely the Slowest Pulsar Ever Detected
https://arxiv.org/pdf/astro-ph/000139... - Flattening modulus of a neutron star by rotation and magnetic field



Image/Video credits in chronological order:
Thumbnail: NIST - https://commons.wikimedia.org/wiki/Fi...
https://pixabay.com/en/billiards-bull...
NASA Earth Observatory image by Robert Simmon, using Suomi NPP VIIRS data provided courtesy of Chris Elvidge (NOAA National Geophysical Data Center). Suomi NPP is the result of a partnership between NASA, NOAA, and the Department of Defense. - https://svs.gsfc.nasa.gov/30082
NIST - https://commons.wikimedia.org/wiki/Fi...
https://pixabay.com/en/mount-everest-...
https://pixabay.com/en/abstract-backg...
David Alciatore - http://billiards.colostate.edu/bd_art...
Greg L - https://commons.wikimedia.org/wiki/Fi...
PODAAC - https://svs.gsfc.nasa.gov/30502
Spacehead - https://www.blendswap.com/blends/view...
noname2 - https://www.blendswap.com/blends/view...
Some of the footage was captured using Space Engine:
http://en.spaceengine.org/
NASA's Goddard Space Flight Center/S. Wiessinger - https://svs.gsfc.nasa.gov/11713
NASA - https://www.nasa.gov/mission_pages/LR...
NASA's Scientific Visualization Studio JPL NAIF - https://svs.gsfc.nasa.gov/4141
NASA - https://svs.gsfc.nasa.gov/10144