Sunday, April 19, 2015

Density

Your density is very close to 1 gm/cm^3, which in mass is equivalent to Avogadro's number of hydrogen atoms or 6.022 x 10^23/cm^3. For comparison, the average density of Universe today is only equivalent to 5 atoms/m^3, but only one part in 25 or so is expected to be baryonic matter, so about 2 x 10^-7 atoms/cm^3. In a galaxy, matter is about a million times as dense, and averaged over the local solar system, another factor of a million more dense, and a neutron star is about 10^14 (one hundred trillion) times as dense as you are, while you are roughly 10^32 times as dense as the average of the universe.

The point is that density varies a whole lot in the present day universe, but this was not always the case. The oldest light in the universe comes from 13.7 billion years ago, a few hundred thousand years after the Big Bang, when the Universe had cooled enough ( to 3000 K, or so) for hydrogen atoms to form from the previous soup of protons and electrons - the so-called age of recombination. That light, redshifted by a factor of 1100, is now received as the Cosmic Microwave Background radiation or CMB.

The Universe was denser then by a factor of a bit more than 1 billion (200 atoms/cm^3), but more importantly, local variations in density were incredibly small - a few parts per 100,000 in enormous contrast to the present where the density variations span 46 orders of magnitude.

The growth of those tiny variations in density of early universe into the contrasts of today is the story of the origin of galaxies, stars and structure in the universe, and is a key subject of cosmology.