Astronomy Buff

Your Waistline Doesn’t Expand with the Universe

by Tony on October 3rd, 2007

Image Credit: Good Ol’ Wikipedia

(I thought about titling this post, “The Expanding Universe Diet”, just to get readers attention, but I refrained.)

By now, you’ve learned that the universe is expanding, and that it’s doing so at the rate given by the Hubble parameter.

OK, so no big deal, the universe is expanding, blah, blah, blah, but does that mean YOU are expanding along with the universe?

As the universe gets bigger, does your desk get bigger, your computer, your head?

With the possible exception of some people’s heads, bound systems do not expand with the universe. Galaxies, stars, planets, your dog, are considered bound systems because they are held together by other forces like gravity, the strong and weak nuclear forces, and twinkie filling.

The space between the atoms does not increase.

Remember, it’s spacetime that’s expanding, the stuff that permeates everything and is distorted by gravity is bulging ever larger, creating more space between all of the unbound systems.

The smallest bound system on a cosmological scale is a galaxy, so the space between the galaxies is increasing. Notice I was careful not to say the galaxies were flying away from each other, that’s not what’s happening; rather more spacetime is being inserted between them every moment.

Basically, the universe is diluting like an untouched glass of Irish whiskey on the rocks. As time passes and the universe gets larger, the density of things decreases. The number of particles per unit volume is going down because, while the number of particles in the universe is the same, the volume is getting bigger:

Density of particles in the universe = (Number of particles in entire universe) / (Volume of universe)

As the denominator on the right side gets big, the number on the left gets smaller. The universe is evaporating.

Have a nice day.

Just thought you’d like to know…

Technorati Tags: cosmology, dark energy, dark matter, expanding universe, galaxies

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POSTED IN: cosmology