RE: Large Hadron Collider refit to solve more questions
January 17, 2015 at 2:51 am
(This post was last modified: January 17, 2015 at 4:09 am by Alex K.)
Davka,
I realize I've used more jargon than I was aware of
, that's why it's good to tell someone these things informally before you write an article about it. This is written on the phone, so maybe less erudite.
Concerning your various related question:
By matter, people mean stuff which has no sizable pressure and whose mass-energy does not change (much) when space is expanded or contracted. This is opposed to radiation which exerts pressure and is blue/redshifted when space contracts/expands. A further requirement is that it loosely accumulates under gravity as a cool gas would.
In the bullet cluster we are talking gravity, not gravity waves. Gravity waves have not yet been observed, at least not directly. The gravity is indirectly measured because the images of background objects are distorted from the curvature of space which comes with gravity.
Now, you were very astonished by the word frictionless. So, if the dark matter in our galaxy and outside were made of ordinary matter, its atoms would collide and in the course of these collisions would radiate light or radio waves etc, thus losing energy. This would result in a flattening of the dark matter cloud in which the galaxy is embedded, forming a disk, which does not fit the observed velocity profiles of galaxies and galaxy clusters and large scale structures well. After all, in the bullet cluster above, if the dark matter had friction just like gas made of ordinary matter, it would have stopped in a shock wave like the reddish interstellar gas.
Now, the requirement of frictionlessness is not very far fetched: as soon as you have a gas made of particles which do not interact with electromagnetism, i.e. with photons, or only indirectly, the kind of energy loss described above is absent. This is called frictionless because for lack of a way to radiate, the dark matter gas does not dissipate much energy. Again, neutrinos fit the bill, but are simply not plentiful enough nor do they have the correct temperature in the early universe to form the observed filaments in the galaxy survey pic above. They would be what one calls "washed out" if dark matter were hotter. Our ordinary three types of Neutrinos certainly make up a part of dark matter, but, for this reason, not the majority.
LHC and other ways to search for dark matter: first of all it's important to understand that colliders do not aim to "break apart" particles to see what they are made of. Or at least, this is a misleading image. What they do is concentrate as much energy as possible in a collision of two "elementary" particles. The laws of quantum physics then ensure that this energy is randomly funneled into all available channels with different probabilities - among those possibilities is for example producing new heavier particles by converting the available energy partly into mass - nature simply does that for us, all we have to do is provide the energy!
Other detection methods are under way. If you feel like it google CDMS or XENON, or Dama/Libra. Those are what is called "direct detection" experiments. They consist for example of ultra cold blocks of transparentish material which are put into a deep mine and.monitored by photon sensors. The hope is that dark matter particles from our galaxie's dark matter cloud, which would go through the earth like neutrinos do all the time, sometimes get stuck in there - with a mind bogglingly tiny probability - and deposit some energy. These experiments require that there is some sort of indirect interaction between dark and ordinary matter just like with neutrinos. Z or Higgs bosons are often assumed as mediators. That's one possibility.
The third big pillar of DM search is "indirect detection": here one looks at dense regions of galaxies and hopes that dark matter particles can pair annihilate into known particle types, whose effects can get picked up by telescopes such as the LAT of the FERMI satellite.
For other specific dark matter hypotheses, specifically targeted searches exist. A nice candidate hypothesis are hypothetical particles called Axions, which help explain why Neutrons have no electrical field (roughly speaking) and thus have a motivation other than dark matter. Those guys can be changed into photons and vice versa in a strong magnetic field. Similar to the detectors in the mine, a strong magnet is rigged with photon sensors in a light-tight container, waiting for axions from the galactic cloud to produce flashes of light.Keyword: ADMX
Cheers
I realize I've used more jargon than I was aware of
, that's why it's good to tell someone these things informally before you write an article about it. This is written on the phone, so maybe less erudite.Concerning your various related question:
By matter, people mean stuff which has no sizable pressure and whose mass-energy does not change (much) when space is expanded or contracted. This is opposed to radiation which exerts pressure and is blue/redshifted when space contracts/expands. A further requirement is that it loosely accumulates under gravity as a cool gas would.
In the bullet cluster we are talking gravity, not gravity waves. Gravity waves have not yet been observed, at least not directly. The gravity is indirectly measured because the images of background objects are distorted from the curvature of space which comes with gravity.
Now, you were very astonished by the word frictionless. So, if the dark matter in our galaxy and outside were made of ordinary matter, its atoms would collide and in the course of these collisions would radiate light or radio waves etc, thus losing energy. This would result in a flattening of the dark matter cloud in which the galaxy is embedded, forming a disk, which does not fit the observed velocity profiles of galaxies and galaxy clusters and large scale structures well. After all, in the bullet cluster above, if the dark matter had friction just like gas made of ordinary matter, it would have stopped in a shock wave like the reddish interstellar gas.
Now, the requirement of frictionlessness is not very far fetched: as soon as you have a gas made of particles which do not interact with electromagnetism, i.e. with photons, or only indirectly, the kind of energy loss described above is absent. This is called frictionless because for lack of a way to radiate, the dark matter gas does not dissipate much energy. Again, neutrinos fit the bill, but are simply not plentiful enough nor do they have the correct temperature in the early universe to form the observed filaments in the galaxy survey pic above. They would be what one calls "washed out" if dark matter were hotter. Our ordinary three types of Neutrinos certainly make up a part of dark matter, but, for this reason, not the majority.
LHC and other ways to search for dark matter: first of all it's important to understand that colliders do not aim to "break apart" particles to see what they are made of. Or at least, this is a misleading image. What they do is concentrate as much energy as possible in a collision of two "elementary" particles. The laws of quantum physics then ensure that this energy is randomly funneled into all available channels with different probabilities - among those possibilities is for example producing new heavier particles by converting the available energy partly into mass - nature simply does that for us, all we have to do is provide the energy!
Other detection methods are under way. If you feel like it google CDMS or XENON, or Dama/Libra. Those are what is called "direct detection" experiments. They consist for example of ultra cold blocks of transparentish material which are put into a deep mine and.monitored by photon sensors. The hope is that dark matter particles from our galaxie's dark matter cloud, which would go through the earth like neutrinos do all the time, sometimes get stuck in there - with a mind bogglingly tiny probability - and deposit some energy. These experiments require that there is some sort of indirect interaction between dark and ordinary matter just like with neutrinos. Z or Higgs bosons are often assumed as mediators. That's one possibility.
The third big pillar of DM search is "indirect detection": here one looks at dense regions of galaxies and hopes that dark matter particles can pair annihilate into known particle types, whose effects can get picked up by telescopes such as the LAT of the FERMI satellite.
For other specific dark matter hypotheses, specifically targeted searches exist. A nice candidate hypothesis are hypothetical particles called Axions, which help explain why Neutrons have no electrical field (roughly speaking) and thus have a motivation other than dark matter. Those guys can be changed into photons and vice versa in a strong magnetic field. Similar to the detectors in the mine, a strong magnet is rigged with photon sensors in a light-tight container, waiting for axions from the galactic cloud to produce flashes of light.Keyword: ADMX
Cheers
The fool hath said in his heart, There is a God. They are corrupt, they have done abominable works, there is none that doeth good.
Psalm 14, KJV revised edition
