Researcher Q&A FAQ-Dark Matter
These questions have been answered by the scientists who are part of the Ask a High-Energy Astronmer program.
Dark Matter
QUESTION: ANSWER:
The basic idea behind the MACHO project is something called "microlensing:" if a MACHO were to pass between us and a distant star, the light would be bent around the MACHO by it's gravitational field passing almost directly in front of the star (in other words, it would act as a gravitational lens). The light curve that results would be independent of the wavelength of the radiation (as opposed to many intrinsic brightness changes), would not change the polarization, would be symmetric, and would have a very distinctive shape. In searching for such microlensing events, the MACHO project is taking long baseline, large field photometric data, and analyzing each star. An early conclusion of the project was that up to 50% of the dark matter in the Milky Way may be MACHOS in the halo with masses ranging from the mass of Jupiter to one tenth the mass of the Sun. An added benefit the large collection of data on variable stars in the Large Magellanic Cloud (one target). For updates on the project, and to learn about the science team, check out: http://wwwmacho.mcmaster.ca/
QUESTION: ANSWER:
The galactic missing mass, which provides additional gravity, is probably produced by some sort of 'dark matter' (things we do not see). This can be in the form of Massive Compact Halo Objects (MACHOs) which may be stars too small to glow brightly (or other, more bizarre objects) or it can be heavy particles, or shadow matter, or primordial black holes, or any of a number of other things.
MACHOs have been detected through their gravitational effect on light, although there is no definitive knowledge of exactly what they are.
David Palmer QUESTION: ANSWER:
http://imagine.gsfc.nasa.gov/docs/introduction/dark_matter.html
To supplement:
Astrophysicists have accumulated a large body of evidence for dark matter. In this context, 'dark matter' means just that --- matter of whatever type that does not shine brightly (in visual light, X-rays or at any other wavelengths). Even though we do not see dark matter directly, its gravitational influence can be seen in the motion of gas and stars in galaxies, and in the motion of hot gas and galaxies within clusters of galaxies. There is recent evidence from microlensing observations that at least some of the dark matter in our own galaxy is in the form of MACHOS, or MAssive Compact Halo ObjectS --- these are planets or stars, made up of ordinary (baryonic) matter, that are too faint to be observed directly, but can act as a gravitational lens and magnify the brightness of brighter stars in the background. There is nothing 'weak' in the observational proof for dark matter in this sense.
In a cluster of galaxies, we can estimate the masses of stars in the galaxies and the hot gas that fill the cluster. We can also infer the total mass of the cluster that is needed to keep it gravitationally bound. The latter is typically found to be ~5 times the combined mass of the stars and the hot gas; an analogy with our Galaxy suggest that only some of the dark matter can be MACHOS. Although circumstantial, such results point strongly to the presence of non-baryonic dark matter in the clusters of galaxies.
When it comes to deciding what kind of exotic particles may make up the non-baryonic dark matter, however, there may be a hint of 'weakness', in that different particle physicists favor different exotic particles. Moreover, as far as I know, there has not been a direct detection of these exotic particles.
Best wishes,
Koji Mukai QUESTION:
I am a banker by profession however have a keen interest in relativity and astrophysics through reading books without any formal education.
ANSWER:
Sincerely, QUESTION: ANSWER:
If the Galactic dark matter consists of WIMPs, then they are dispersed throughout the Galaxy, with a distribution somewhat different from that of the stars that we can see. Since gravitational pulls of WIMPs from different directions tend to cancel out, the orbit of planets in our solar system is not affected by the presence of WIMPs. However, since there are more WIMPs towards the center of our Galaxy than away from it, the motion of the Solar system (and other stars) in the Galaxy is strongly affected --- this is how astrophysicists infer the presence of the dark matter.
Best wishes,
Koji Mukai
Which scientists are currently on the forefront of Milky Way study and theories? Where can I find their work?
Boy. That is a very BIG question, because there is a tremendous amount of research involving the Milky Way galaxy, and it involves almost every aspect of theoretical and observational astronomy. Computational astrophysicists study how and why spiral structure forms and endures, and the dynamics of the globular clusters that surround the disk of the Milky Way in a halo. Observers study star forming regions, map spiral arms, measure abundances of elements in the interstellar medium, search for dark matter, etc. So, I'm just going to pick my favorite at the moment, which is the search for MAssive Compact Halo Objects (MACHOS), one possible explanation for the dark matter that must exist in galaxies, including the Milky Way. For more information about dark matter, check out http://physics7.berkeley.edu/darkmat/dm.html. Basically, according to careful observations, we know that up to 90% of the matter in galaxies must be in the form of "dark matter" to account for the dynamics we observe. On top of that, the dark matter appears to be distributed in a spherical halo around the Milky Way, while the luminous matter is located largely in the flat disk.
If neutrinos are massless what else could make the galaxies spin like that? I am 18 years old and next year I be a student in physics at University level.
The context of your question is lost, but I presume by 'spin like that' you are referring to me fact that the speed at which galaxies spin is too fast to be held together by the gravity of all the stars that we can see.
for Ask a High-Energy Astronomer
I have taken several high energy physics courses in College and know some astronomy. Isn't Dark Matter theory just a weak attempt to mesh theory with observation or has there been any research with empirical results proving the existence of 'Dark Matter?' Any additional references to information would be helpful.
We have a brief explanation on dark matter at:
for Ask a High-Energy Astronomer with help from Dr. Mushotzky
There has been a lot of debate over the nature of missing matter in the universe and people have been putting forward many candidates for this missing matter. My question is simply given the equivalence of mass & energy couldn't the so called dark matter simply be the energy left and floating around from the big bang ?
You are absolutely correct about the contribution of the energy left from the big bang to the energy of the universe. Unfortunately, this cannot be the missing mass. It is certainly taken into account and in fact it makes the dominant contribution to the energy density of the universe at early times. However, since its energy density drops faster than that of ordinary matter it is not important today. It simply constitutes the 2.7 degree K cosmic background radiation. Furthermore, because it consists totally of photons, i.e. it moves at the speed of light it cannot be clustered in the gravitational potential of a galaxy, which cannot trap particle traveling faster than about 300 km/sec. For that we indeed need particles with non-zero inertial mass.
Demos Kazanas
for the Ask a High-Energy Astronomer Team
I am a real novice (advance apology for stupid question) but I first heard about dark matter last week and have been transfixed by this mystery. I basically understand (1) that particle physicists are trying to establish the existence of WIMP's in cryogenic crystals and in the ice shelf and (2) that part of the key to the dark matter theory is that as you move out from the center of a galaxy, the speed of the rotation stabilizes which implies that there is mass there that does not emit light. My question is do particle physicists believe that WIMP's are dispersed throughout the galaxy? And if so, why wouldn't WIMP's affect the orbit of other bodies within the galaxy (i.e. our own solar system)?
Two likely possibilities for the dark matter in our own galaxies are MACHOs (MAssive Compact Halo Objects) and WIMPs (Weakly Interacting Massive Particles). MACHOs are low mass stars, brown dwarfs, neutron stars and white dwarfs. If MACHOs make up most of the dark matter, the distribution is not smooth on the scale of the Solar system, but it is smooth on a much larger scale.
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