How To Weight the Entire Universe All At Once
It blows my mind sometimes to listen to a cosmologist talk. They make some pretty remarkable statements about the universe, and they do it with such certainty.
For example, we know how big the universe is, we know how many stars are in the universe, likewise the number of galaxies, we know what it’s made of, and we even know how much it weighs.
Sometimes, I sit in my comfy chair in awe of the fact that we have been able to figure this out. It blows my mind how clever we can be.
Let’s look at the issue of weighing the universe. How can we do this?
There are two techniques one can use:
- Measure the spatial curvature, the shape of the universe. This will allow you to get a handle on the density, the number of particles per unit volume of space.
- Measure how the expansion rate of the universe changes with time.
The curvature, or shape, of the universe comes from the fact that everything is affected by gravity and distorts spacetime. Stars, galaxies, clusters of galaxies and dark matter all warp spacetime and collectively contribute to the overall shape of the universe. In measuring this shape, one cannot help but include everything that is seen and unseen.
Measuring the shape of spacetime is a direct measurement of everything that is affected by gravity, which, according to general relativity, is everything.
Looking at changes in the expansion rate over time is another good method for weighing things. As everything moves apart, all of the particles in the galaxies, ordinary matter and dark matter are pulling on all the others.
Again, it’s gravity and it’s effects on everything that we’re actually looking at, but if one can precisely measure the changes of the expansion rate over time, we get another good answer for the amount of particles in the universe.
It turns out doing this accurately is very, very hard to do, but was done in the 1990’s using redshifts from Type !a supernovae.
The amazing thing they found was very counter-intuitive. One would expect that in a universe full of matter and radiation, that the gravity exerted by all of these particles pulling on each other would make the universe decelerate.
Cosmologists were so sure that the universe was decelerating that they titled their proposals “Measuring the Deceleration of the Universe”.
Instead they discovered that the universe was accelerating.
But how could this be? What could possibly be pushing the universe to accelerate faster and faster and overcome the force of gravity of all the particles trying to pull the universe together?
There must be some unknown force, something else is going on that we can’t see providing an impulse, pushing spacetime larger faster.
That stuff? Dark energy.
Technorati Tags: cosmology, dark energy, dark matter
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1 opinion for How To Weight the Entire Universe All At Once
roy
Dec 31, 2007 at 8:24 pm
so what is the weight of the universe???
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