23
Jan 10

Neutralino dark matter stars can not exist

Source: arXiv

Dark matter "clumping" together over time confirming  theories
of how structure formed in our evolving universe.
Credit: NASA, ESA, CalTech

Recent observational data gives an indication that the universe contains a significant fraction (22%) of dark matter whose origin is still unclear. A possible solution to this problem comes from supersymmetric (SUSY) models in the form of neutralino. Neutralino is the lightest supersymmetric partner in SUSY, with the mass of about 100GeV, and is stable. It interacts with the gravitational and weak interactions only, which indicates that it is ”dark”. Weak interactions and neutralino mass are sufficient to satisfy the relic density needed to explain the observed portion of the dark matter in the universe.

Recently a group of astronomers has obtained a detailed distribution of dark matter as a function of the redshift in a part of our universe. Their observations indicate that dark matter plays a role of a scaffolding upon which ordinary matter builds structures. However, the observations show that large pockets with only dark matter (and no ordinary matter) also exist.

De-Chang Dai and Dejan Stojkovic of HEPCOS, Department of Physics, SUNY at Buffalo, have researched the question that arises from the existence of these large pockets of dark matter that is whether compact objects like planets, stars or maybe even large may or not exist.
They concluded that, a stable neutralino star can not exist and also estimated that a stable star can not contain more than a few percents of neutralinos. This information has been published in  the Journal of High Energy Physics (JHEP)(read more)

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23
Jan 10

The moon – proper motion and apparent motion.

If you have been following the moon this week you should have noticed some interesting things, firstly it has “grown” from a very small crescent to a now “half moon”. Hopefully you will have also noticed that at the same time each night the moon was “higher” and further to the west (to your left when looking towards the moon), you may also have spotted that the moon was not in the same position with regards to the background stars – earlier this week the moon was not too far from a very bright star low on the horizon (the bright star was, and still is, in fact Jupiter) and now it’s a long way from Jupiter and much higher in the sky.

So, what’s been happening? As you know, the moon orbits around the earth (i.e. it has “proper” motion), corresponding to a movement relative to the background stars by an amount roughly equal to its own diameter each hour; this movement is from west to east i.e. the moon starts (as a new moon) low in the western sky and moves slowly “upwards” and eastwards. At the same time, due to the fact that the earth is turning on it’s own axis from west to east, the stars and the moon (and of course the sun) appear to move from east to west. It’s a bit like someone “walking up the down escalator” assuming the person is walking upwards slower than the escalator is going down, he or she will eventually arrive at the bottom of the escalator.

If you would like to know more about the motion of the moon, take a look here: http://en.wikipedia.org/wiki/Orbit_of_the_Moon

If you have a pair of binoculars you can take a look at some of the moons “seas”, you should be able to see (1) Sea of Crisis, (2) Sea of Fertility, (3) Sea of Nectar, (4) Sea of Tranquillity, (5) Sea of Serenity and (6) Sea of Cold.

Photo credit: The Universe Today

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23
Jan 10

The moon – proper motion and apparent motion.

If you have been following the moon this week you should have noticed some interesting things, firstly it has “grown” from a very small crescent to a now “half moon”. Hopefully you will have also noticed that at the same time each night the moon was “higher” and further to the west (to your left when looking towards the moon), you may also have spotted that the moon was not in the same position with regards to the background stars – earlier this week the moon was not too far from a very bright star low on the horizon (the bright star was, and still is, in fact Jupiter) and now it’s a long way from Jupiter and much higher in the sky.

So, what’s been happening? As you know, the moon orbits around the earth (i.e. it has “proper” motion), corresponding to a movement relative to the background stars by an amount roughly equal to its own diameter each hour; this movement is from west to east i.e. the moon starts (as a new moon) low in the western sky and moves slowly “upwards” and eastwards. At the same time, due to the fact that the earth is turning on it’s own axis from west to east, the stars and the moon (and of course the sun) appear to move from east to west. It’s a bit like someone “walking up the down escalator” assuming the person is walking upwards slower than the escalator is going down, he or she will eventually arrive at the bottom of the escalator.

If you would like to know more about the motion of the moon, take a look here: http://en.wikipedia.org/wiki/Orbit_of_the_Moon

If you have a pair of binoculars you can take a look at some of the moons “seas”, you should be able to see (1) Sea of Crisis, (2) Sea of Fertility, (3) Sea of Nectar, (4) Sea of Tranquillity, (5) Sea of Serenity and (6) Sea of Cold.

Photo credit: The Universe Today

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