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.

Comments

Post a Comment

Popular posts from this blog

Anti-Libertarian: re-post

The most pointed critique of social security and liberalism in general is the libertarian critique. The problem with libertarians, for a liberal, is that we have too much in common. We both believe in individual rights, tolerance of individual differences, and dislike government prescription of religion. The basic difference, it seems to me, is the different answers we give to Cain's (with thanks to the Captain - my original version had the corpse asking the question!) famous question: "Am I my brother's keeper?" That's not really my basic bitch against the libertarians though. My real complaint is the same as my complaint against most religion - its premise is a fraud. For those who can't stand to wait for the punchline, I believe that trying to implement libertarian principles leads to tyranny or social disintegration. Demonstrating that takes some historical (and pre-historical) context. For all but the last 15,000 or so of the 100,000 years the human spec...
Read more

Uneasy Lies The Head

Uneasy lies the head that wears a crown ........... Shakespeare Henry IV, Part II. So it is with us and our slightly distant primate cousins , so it seems. A 9-year study of wild baboons in Kenya by Princeton University and the Institute of Primate Research may turn conventional assumptions of alphas males upside down. The study found that alpha males have high testosterone levels, which allows them to dominate other baboons, get access to more food, and attract better mates. This is expected. Surprisingly, though, the alpha males also have high levels of stress hormone glucocorticoid, which scientists measured from their feces samples. In fact, the stress of alpha baboons was on par with low-ranking baboons. The convention wisdom is that access to food and females provides life security and thus less stress for dominant males. Conversely, weak males who are constantly threatened by starvation and physical violence from stronger males are thought to be more stressed. The st...
Read more

We Call it Soccer

The US played its best, and maybe only good game of World Cup, but lost thanks to brilliant goalkeeping by the Swedish keeper and the bizarre method soccer uses to settle ties when three US stars could not put the ball on frame from the 12 yard spot.  At the end it came down to a blocked shot that spun in by maybe a couple of centimeters. Well, that's football - too much like life.   Which brings me to a modest proposal.  Replace the thirty minute overtimes with as many 7-player 15 minute periods as needed to settle it.  Scoring should be much easier with fewer players.
Read more