The discussions on the topic of Dark Energy,
in the yahoogroup http://groups.yahoo.com/groups/Astronomy_Activities_2009
started with a new member posting and wanting to know something about Dark Energy.
First, a little bit about our Universe:
When we look at the distant sky using powerful optical telescopes, we find that stars and gaseous clouds are held together by gravity in larger units which we call "galaxies". Our solar system itself is part of the Milky Way Galaxy. A galaxy typically consists of 10^10 to 10^13 stars distributed in a region of size ranging from 15 thousand light-years to 100 thousand light-years. Between any two galaxies there is very little matter, visible optically. Many galaxies themselves cluster gravitationally to form galaxy clusters. For example, our Milky Way Galaxy is a part of a local cluster consisting of Small Magellanic and Large Magellanic Clouds. But there are giant clusters of galaxies each consisting of over 1000 galaxies.
In late 1920s, Hubble discovered that the distance between galaxies (not in any cluster) as well as distance between clusters of galaxies increase with time. That is, in the past galaxies were more crowded than today. Going further back in time, when there were no stars or galaxies but only gaseous matter, we conclude that matter was denser and hotter. Hence, our universe originated from a dense and hot phase. This is the so called HOT BIG BANG model.
Now, it turns out that visible matter (mostly, hydrogen) that makes up stars and gaseos clouds are not the only constituents of the universe. Each galaxy contains some mysterious matter called the DARK MATTER, that is spread out to greater distance than the visible matter. Total amount of weight of visible matter in a galaxy is about a tenth of the total mass of the dark matter associated with that galaxy. Dark matter is observed only by their gravitational influence. They do not interact with light or other electromagnetic waves. That's why DARK. Physicists are still debating about the nature of dark matter.
In the mid-nineties, it was inferred that not only the distance between galaxies is increasing with time (Hubble's Law) but the rate of increase shows an acceleration. When I throw a ball upwards, the distance between the ball and me increases, but the rate at which it increases shows a retardation. Why? Because of gravity of course! Therefore, astronomers finding an accelerated expansion of the universe implies some kind of a repulsive gravity.
If the existence of repulsive gravity is real, then it means there is some kind of matter which is invisible (hence, DARK) and whose equation of state is queer, in the sense that its pressure must be negative in order to give rise to repulsive gravity. This bizarre matter is called DARK ENERGY. Again, this is an active field of research, as no one knows what the DARK ENERGY is!
--- Patrick Das Gupta May 22, 2008 4:29 am
How can we physically understand what is meant by the cosmological equation of state?
The equation of state usually relates pressure, density and the temperature. For example, in an ideal gas, the gas pressure is proportional to its temperature times density. Higher the temperature or density, greater is the pressure. For gaseous matter, pressure is always positive, because it is closely related to the random kinetic energies of the gas molecules.
From Einstein's theory of general relativity, it can be shown that when the combination - energy density plus three times pressure, is a positive quantity, the resulting gravity is attractive. Now that astronomers have evidence for an accelerating universe (i.e. an accelerating expansion), we need a cosmological source (i.e. crazy matter distributed over huge scales) of repulsive gravity. For that the combination- energy density plus 3 times pressure must be negative.
Physicists do not like to have negative energy densities, since it makes all matter unstable, contrary to what is actually seen. So, the need for an exotic substance with negative pressure. To be precise, a substance, for which pressure equals w times energy density, with w less than -1/3. Hence, an unusual equation of state.
I remember reading this article sometime back. thought its worth sharing
PS : i have modified the subject line.
Anyone who really is interested in cosmology and the universe and its constituents etc. i think a great book to begin with is
An Introduction to Modern Cosmology by Andrew Liddle. what i had liked about the book as a beginner is one doesn't need a lot of mathematics . most of the book can be understood with just our knowledge of Newtonian physics. It does'nt deal with dark energy and all but allows one to be able to appreciate why we need it in the first place.
all for now
One other question that I had, was about the actual observations, that gave rise to the idea that there is an accelerated expansion.
What observations were these, exactly?
---- Rathnasree May 25, 2008 8:35 am
The supernova observations by Adam Riess. They found that more distant supernovae were receding faster than Hubble's law showing that there is a repulsive force acting at large distances.
Here are some links: http://spaceflightnow.com/news/n0104/03supernova/ for the basic theory
and an interesting story about Adam Riess from the Johns Hopkins University http://www.jhu.edu/~jhumag/0208web/riess.html
Jayant Murthy May 25, 2008 3:14 pm
There was a very good article in New Scientist last year. A search for that provided a more recent link. see this
---------- Shylaja May 27, 2008 10:23 am