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Astrophysics/Dark energy
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Question
Can dark energy/dark matter be created?
How can we detect dark matter/dark energy?
Are there particles in dark matter?
Hello,
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Whether dark energy can be created will depend on exactly what its nature is. So far this is an open research area. It might be associated for example with what we call "vacuum energy" - the energy associated with empty space, and which we estimate to be ~ 10^112 ergs/ cm^3, which is difficult to fathom. Note: vacuum energy is related to Einstein's 'cosmological constant' and the problem of the cosmological constant, whether it exists or not, remains one of the most profound mysteries and unanswered questions in astrophysics. If the constant is zero, for example, then clearly the dark energy would have to be something else, otherwise it shouldn't exist!)
Dark energy may also be based on something called "quintessence", based on some new field in nature. (See, for example, the book by Lawrence Krauss: The Fifth Essence'). This is testable, at least, because we can use actual observations to constrain the equation of state for dark energy to infer its upper, lower bounds.
That eqn. of state may be expressed:
w = (p / rho) = -1
That is, w is the ratio of pressure to energy density(rho). Specifically the term (rho + 3p) acts as a source of gravity in general relativity, (where rho = energy density).
If we set: 0 = (rho + 3p) then:
p = -rho /3 (or rho = - 3p)
and if: p < (rho /3) we have gravity that repels.
My point is that observations on specific kinds of particles, events may help us refine this and then conclude whether we have a valid field or not.
Thus, the preceding issues as to the nature of dark energy must be addressed before one can undertake its creation (else, one wouldn't necessarily know what one is doing).
Dark matter is something different, because we know basically what sort of entities constitute it. Dark matter itself occurs in either baryonic or non-baryonic forms, depending on whether the matter reacts with radiation or not. If it doesn’t, it’s non-baryonic. Baryons include protons and neutrons, while non-baryons include electrons and neutrinos.
Non-baryonic dark matter further breaks down into cold dark matter and hot dark matter. The terms hot and cold are not so much indicative of current temperatures, as the phase of the early universe at which the particular dark matter ‘decoupled’ from the hot radiation background. As we saw in the space-time diagram, an early decoupling implies a higher ambient background radiation temperature of the primeval cosmos. A later decoupling correlates to a cooler temperature. Perhaps the most widely studied candidate of hot dark matter is the neutrino, which includes three sub-species: the electron neutrino, mu neutrino and tau neutrino.
To the extent we can induce or generate the appropriate nuclear reactions, we can create (as offshoots or products of the reactions) these neutrinos. (For example, the creation (a better word is production) and detection of the tau neutrino was reported by Fermilab in July, 2000)
By contrast, cold dark matter candidates tend to have larger mass , and amongst the most likely suspects are: gravitinos, magnetic monopoles, and primordial black holes. None of these are likely to be produced any time soon.
Re: the detection of dark matter, this can be traced back to 1933, when Fritz Zwicky's measurements of galaxy clusters highlighted a ‘missing mass’. He found that the mass needed to bind a cluster of galaxies together gravitationally was at least ten times the apparent mass visible. This mass, because it was inferred but not directly detectable, became the first ‘dark matter’. Around the same time there were observations of stellar motions in the galactic plane by Dutch astronomer Jan Oort. He found there had to be at least three times the mass visibly presenting in order for stars not to escape the galaxy and fly off into space.
By the late 1970s, astronomers realized there were other forms of dark matter. Among the most discussed candidates were black holes, marking the end stage of evolution for very massive stars. In the black hole, the gravity is so strong that no light escapes and the mass typically is much greater than that of the Sun. These objects can only be detected indirectly, e.g. as a member of a binary (double) star system, to infer its presence from the intense x-rays given off when the companion star’s gaseous layers are sucked into it.
Dark energy is much more difficult to detect, and to date the primary method has been based upon plots of Type Ia supernovae (using observed magnitude v. redshift z). In the graph shown (accompanying diagram) , the type Ia data points all fall to the LEFT of the thick dark line, or in what we call the 'accelerating universe region'. On the other side of the diagonal is the "decelerating region". (See, e.g. 'Supernovae, Dark Energy and the Accelerating Universe', by Saul Perlmutter, in Physics Today, April, 2003, p. 53.)
In the paper cited the conclusion is drawn that the only agent able to produce the acceleration is dark energy.
Precision measurements of the cosmic microwave background (CMB), including data from the Wilkinson Microwave Anisotropy Probe (WMAP), have provided further evidence for dark energy. The same is true of data from two extensive projects charting the large-scale distribution of galaxies - the Two-Degree Field (2DF) and Sloan Digital Sky Survey (SDSS).
All of this together shows that the proportion (best estimate) of dark energy is currently at least 65% (and more likely over 70$) while the proportion of dark matter is at least 26%, and maybe as much as 35%.
Re: "particles in dark matter", as already noted earlier one form of dark matter is hot, non-baryonic dark matter which includes the tau, mu and electron neutrinos so that yes, particles can exist as and in dark matter.
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Philip A. Stahl
Expertise
I specialize in stellar and solar astrophysics. Can answer any questions pertaining to these areas, the spectroscopic analysis of stars – as well as the magneto-hydrodynamics of sunspots and solar flares. Sorry – No homework problems done or research projects! I will provide hints on solutions.
Experience
Have published papers on the relationship between sunspot morphology and solar flares; discovery of SID flares related to this, constructed computerized stellar models; MHD research.
Organizations
American Astronomical Society (Solar physics and Dynamical astronomy divisions), American Geophysical Union, American Mathematical Society, Intertel.
Publications
Solar Physics, Journal of the Royal Astronomical Society of Canada, Journal of the Barbados Astronomical Society, Meudon Solar Flare Proceedings (Meudon, France). Books: 'Selected Analyses in Solar Flare Plasma Dynamics', 'Physics Notes for Advanced Level'.
Education/Credentials
B.A. degree in Astronomy; M.Phil. degree in Physics - specializing in solar physics.
Awards and Honors
Postgraduate research award- Barbados government; Studentship Award in Solar Physics - American Astronomical Society
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