# Dark matter and gravitational pull of energy

Tags:
1. Mar 22, 2012

### guss

I have been looking into dark matter recently, and I have three questions.

When calculating what the gravitational pull of an object should be, do they account only for the mass of that object, or do they account for the total energy of that object? For example, a star will have a lot of mass, but it will also have tons of gravitational potential energy, kinetic energy, and other energy. It seems to me like this energy, especially the gravitational potential energy, should be extremely large.

Another question I have is about the shape of the supposed cluster of dark matter that surrounds galaxies. Is it predicted to be a filled-in sphere of matter? Why would it exhibit that shape when it is attracted to matter, like stars - shouldn't it nearly model the visible galaxy's shape?

The last question (or idea) I have is concerning the gravitational effects of dark energy. Since this sort of energy is inherent everywhere, and energy is attracted to matter, shouldn't this energy be more dense within galaxies? I doubt this question is very answerable, but I am curious.

Thanks!

2. Mar 22, 2012

### Staff: Mentor

The total energy of the object also adds to the mass, so when we measure the mass of a star the contribution from energy is already included. Although one must be careful when talking about energy, as a star has no gravitational potential energy or kinetic in regards to itself. (I mean the star as a whole, not the gas that composes it that releases it's potential energy through heat and fusion) So when defining what the potential gravitational energy or kinetic energy is you must define it in reference to another frame other than the star.

No, dark matter is theorized to not interact with itself other than through gravity. So it doesn't clump up as it falls into itself, it just passes through until gravity slows it and pulls it back in for another pass. Thus most of the dark matter in galaxies is in a "halo" around the galaxy, as it is moving the slowest at this point and spends most of it's time in the outer areas. (Similar to a highly elliptical orbit of an object around a star. The object spends most of its time far away from the star)

Don't think of dark energy as "normal" energy that gravitates. Instead dark energy is viewed as a force that is causing the expansion of the universe to accelerate.

3. Mar 24, 2012

Thanks!