You will be presented with the full text of a science research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
the interaction of elementary particles with a medium in equilibrium or out of equilibrium can be described by applying quantum field theory at finite temperature @xmath4 , chemical potential @xmath5 , or in non - equilibrium .
using perturbation theory , self energies , scattering amplitudes , the free energy , etc . can be calculated . in this way
effective masses , debye screening , dispersion relations , refractive indices , damping , decay and production rates , the equation of state and other interesting properties of the interacting particle and the medium can be derived .
famous examples are listed below .    *
effective in - medium neutrino masses , which lead to the msw - effect , can be derived from the neutrino self energy at finite temperature and density @xcite . * the plasmon effect , which describes the decay of an in - medium photon ( plasmon ) in a stellar plasma into a neutrino pair , @xmath6 , provides an effective energy loss mechanism for hot and dense stars @xcite . in a relativistic plasma
the use of thermal field theory increases the emissivity by about a factor of 3 @xcite compared to previous approaches . *
the baryon asymmetry of the universe can be generated by the sphaleron decay during the electroweak phase transition .
for calculating the sphaleron decay rate in the early universe a new method in finite temperature field theory has been developed @xcite .
after a brief introduction to some basic concepts of thermal field theory , three examples of its application to astrophysical problems are discussed : neutrino interactions in a supernova plasma , strange quark matter in neutron stars and as a dark matter candidate , and the interaction of light with the cosmic microwave background .
perturbative field theory at finite temperature or density is based either on the so - called imaginary ( itf ) or real time formalism ( rtf ) ( see e.g. @xcite ) . as a simple example for illustrating these methods we will discuss the @xmath7-theory at finite temperature briefly .
the most important quantity in perturbative field theory is the feynman propagator . at zero temperature
it is defined as the vacuum expectation value of the time - ordered product of two field operators . at finite temperature
the vacuum expectation value has to be replaced by a thermal expectation value containing a sum over all thermally excited states .
using the plane - wave expansion of the fields and the canonical ensemble one gets @xcite & & i _
f^t>0 ( x - y)= _
n n|t\{(x ) ( y)}|n e^-e_n / t + & & x_0>y_0 = \ { [ 1+n_b ( ) ] e^-ik(x - y ) + n_b ( ) e^ik(x - y ) } , [ propagator ] where @xmath8 is the partition function and @xmath9 $ ] the bose - einstein distribution with @xmath10 .
here we use the notation @xmath11 , @xmath12 .
this expression has a simple physical interpretation . at @xmath13
the propagator describes the creation of a scalar particle at the space - time point @xmath14 ( for @xmath15 ) and its propagation to @xmath16 , where it is destroyed . at finite temperature
there is besides spontaneous creation at @xmath14 , given by the term 1 in the square brackets , also induced emission and absorption proportional to @xmath17 due to the presence of the heat bath .    in the itf
the propagator given above can be expressed as a sum over discrete , imaginary energies ( matsubara frequencies ) , @xmath18 , i _
f^t>0 ( x - y)=it _ k_0 e^-ik(x - y ) .
[ itf ] the itf will be exemplified by calculating the simplest self energy in the scalar theory , namely the tadpole diagram shown in figure  [ fig : tadpole ] .
modifying the standard feynman rules by summing over @xmath19 instead of integrating we find & = & ( -i 4 !
g^2 ) it _ k_0 i + & = & 6 g^2 [ 1 + 2n_b ( ) ] , [ tadpole ] where @xmath20 is the coupling constant associated with the vertex of the @xmath21-theory . in the massless case , @xmath22 , the integral in ( [ tadpole ] ) can be done exactly , leading to the simple result @xmath23 .    from the self energy
we can construct an effective , in - medium propagator by using the dyson - schwinger equation , @xmath24 , yielding ^*=. [ effprop ] this propagator agrees with the bare one if the bare mass , @xmath25 , is replaced by the effective mass @xmath26 .
the dispersion relation of the scalar field in the medium follows from the poles of the effective propagator : @xmath27 .
an alternative method to the itf is the rtf , where the thermal propagator in momentum space is given by i ( k ) = + 2 n_b(|k_0| ) ( k^2-m^2 ) . [ rtf ] in order to avoid unphysical singularities from products of @xmath28-functions , which can appear in diagrams with two propagators , the propagator has been extended to a @xmath29-matrix @xcite .
an important advantage of the rtf compared with the itf is the possible generalization to non - equilibrium by replacing the equilibrium distribution in ( [ rtf ] ) by a non - equilibrium .
now we will turn to gauge theories ( qed , qcd , electroweak theory ) at finite temperature and density , which are relevant for astrophysical situations . as an important example we consider the one - loop qed polarization tensor in an electron - positron plasma shown in figure  [ fig : polarization ] .
in contrast to the tadpole diagram the polarization tensor is energy and momentum dependent .
it can be calculated exactly only in the high - temperature limit which is equivalent to the hard - thermal - loop ( htl ) limit , in which the loop momentum is assumed to be much larger than the external momentum .
the high - temperature limit corresponds to an ultrarelativistic electron - positron plasma as it exists in supernovae . due to the breaking of lorentz invariance by choosing the frame of the heat bath ,
the polarization tensor has two independent components , for which we take the longitudinal and the transverse . in the htl limit they read _
l(p_0,p)&=&-3 m_^2(1- ) , + _ t(p_0,p)&= & m_^2 , [ htlpolar ] where @xmath30 can be regarded as a thermal photon `` mass '' . however , it should be noted that this mass does not break gauge invariance .    by resumming the polarization tensor using the dyson - schwinger equation
we construct an effective photon propagator which describes the propagation of collective photon modes ( plasmons ) in a qed plasma .
the corresponding dispersion relations for the longitudinal and the transverse plasma waves are shown in figure  [ fig : dispersion ] .
finally , let me mention that in the case of two - loop self energies , from which e.g. damping rates follow , infrared singularities and gauge dependent results ( `` plasmon puzzle '' ) are encountered .
braaten and pisarski @xcite developed a method , the htl resummation technique , for avoiding these problems , which allows a consistent treatment of gauge theories at finite temperature .
neutrinos are copiously emitted from the core of a supernova .
they interact with the surrounding plasma , providing an effective mechanism for energy deposition , which pushes the shock wave outwards and finally triggers the explosion @xcite .
important neutrino interaction processes in the plasma are @xmath31 , @xmath32 , and @xmath33 .    the differential rate for instance for @xmath34-annihilation is given by = m^ \{_(q_1+q_2 ) } , [ diffrate ] where m^=8(q_2^q_1^+ q_1^q_2 ^ - ( q_1 q_2 ) g^ + i q_1
q_2 ^ ) [ leptonic ] is the leptonic tensor of the neutrino current and @xmath35 the polarization tensor .
to lowest order in the fermi theory the latter is given by figure  [ fig : fermi ] .
the total rate follows from the differential rate by convolution with the neutrino distributions : r_|= & & [ f_(q_1)f_|(q_2 ) + & & - ( 1-f_(q_1))(1-f_|(q_2 ) ) ] e^-e / t_e ] .
[ totrate ] similar expressions for the differential and total rate can be derived for @xmath36-scattering .
alternatively the rates can be calculated from the scattering amplitude , & & = [ 1-n_e^+(e_1 ) ] + & & [ 1-n_e^-(e_2 ) ] ( 2)^4 ^ 4(q_1+q_2-p_1-p_2 ) _
i |m|^2 , [ amplitude ] where @xmath37 $ ] is the fermi distribution of the electrons and positrons in the plasma .
in addition to medium effects at finite temperature and density we consider a strong magnetic field , which plays an important role in supernovae and neutron stars .
recently magnetic field strengths above @xmath38 g have been observed in so - called magnetars @xcite . in a magnetic field
the electrons and positrons are in landau levels , which leads to a modified electron propagator in the polarization tensor .
the magnetic field can absorb momentum , allowing for new processes such as the gyromagnetic absorption @xmath39 .
the advantage of starting from the polarization tensor instead of the scattering amplitude is the automatic inclusion of all possible processes ( annihilation , scattering , gyromagnetic absorption ) .
using the approach described above , we found a strong modification of the rates of the various processes in fields at @xmath40 g. in particular there are pronounced peaks in the rates coming from the landau levels , new processes like the gyromagnetic absorption become important , and the processes show a strong anisotropy , which could play a role in gamma - ray bursts @xcite . in the figures  [ fig : bdepend ] to [ fig : aniso ] these features are illustrated .
quark matter consisting of up , down and strange quarks may exist in the interior of neutron stars or even in form of strangelets , if quark matter is stable , i.e. , if it has a higher binding energy than iron @xcite .
strangelets have also been discussed as a possible dark matter candidate @xcite .
so far calculations of the equation of state ( eos ) of strange matter are based on an ideal fermi gas , where sometimes also one - gluon exchange effects are included @xcite .    here
we want to consider medium effects at finite density and vanishing temperature , which lead to effective quark masses . using the one - loop quark self energy in the high - density limit
, an effectice in - medium quark propagator is constructed .
the dispersion relation at zero momentum following from this propagator defines the effective , density dependent quark mass @xcite ( p=0)=m_q^*=+ ( + ) ^1/2 , [ qmass ] where @xmath41 is the quark chemical potential of the order of 300 mev .
the coupling constant @xmath42 , typically of the order 2 - 4 , can be regarded as a parameter describing the medium effect of the effective quark mass , which arises naturally due to the interactions as known from mean - field theories in many - particle physics .
consequently we replace the ideal fermi gas by a quasiparticle gas , where e.g. the particle density is now given by ( ) = [ ^2-m_q^*^2()]^3/2 , [ density ] where @xmath43 is the degree of freedom , e.g. @xmath44 for strange quarks .
similar expressions for the energy density and the pressure determining the eos can be found .
the introduction of an effective quark mass , typically of the order of 100 mev , reduces the binding energy of strange quark matter and renders the existence of stable strangelets as a dark matter candidate unlikely @xcite .
furthermore we studied the influence of the quark medium effects on strange stars and hybrid stars , i.e. neutron stars containing a quark matter core , by solving the tolman - oppenheimer - volkoff equation using the eos for quark matter derived above .
as shown in figure  [ fig : mass_radius ] the effective quark mass has a negligible effect on the mass - radius relation of strange stars . on the other hand ,
the presence of a quark matter core , reduces the radius of a neutron star by typically 20 - 30 % , which might have observable consequences @xcite .
this reduction is caused by a softening of the eos due to the presence of a mixed phase .
it also reduces the maximum mass of the star to about 1.5 @xmath45 .
recently it has been speculated that quark matter at high density should be in a color superconducting phase @xcite .
while this could have some consequences for the cooling behavior of neutron stars @xcite , it probably does not affect bulk properties , such as the mass - radius relation , since the pairing takes place only close to the fermi surface .
although the cross section for photon - photon interaction is very small , light propagating through the universe interacts continuously with the cosmic microwave background ( cmb ) . as discussed in section  [ sec : thermal ] photons may acquire an effective mass in a thermal medium . on the other hand ,
there are very small upper limits for the photon mass deduced from laboratory experiments ( @xmath46 ev ) and from the galactic magnetic field ( @xmath47 ev ) @xcite .
therefore it is of interest to consider medium effects of light in the cmb .
the photon - photon interaction of low energy photons ( @xmath48 ) can be described by an effective lagrangian @xcite _ i=- ( f _ f^)^2 + f _ f^ f _ f^ , [ euler ] where @xmath49 is the electron mass , @xmath50 the fine structure constant , and @xmath51 the field strength tensor .
the lowest order photon self energy following from this lagrangian is shown in figure  [ fig : photon ] .
a gauge invariant definition of the debye mass of the photon relates it to the photon self energy in the following way @xcite : @xmath53 . using ( [ photself ] )
one finds that the debye mass vanishes , @xmath54 .
also the effective photon `` mass '' or plasma frequency , defined as the zero momentum limit of the dispersion relation , @xmath55 , vanishes , @xmath56 .
hence there is no conflict with the measured upper limits of the photon mass .
the electric permittivity and magnetic permeability are also related to the photon self energy : & = & 1-=1 + , + & = & 1+=1- .
[ perm ] from these quantities the phase velocity @xmath57 and the index of refraction @xmath58 follow as v_p===1- . [ phase_velocity ] since the temperature of the universe drops with time , the phase velocity increases continuously to its vacuum value .
whereas it was given by @xmath59 , when radiation and matter decoupled at about @xmath60 k , it increased today at @xmath61 k to @xmath62 .
although this is probably not a measurable effect , it is amusing to note that the speed of light is not a constant in our universe .
quantum field theory at finite temperature and density can be used for describing the physics of relativistic plasmas . 
 such systems are frequently encountered in astrophysical situations , such as the early universe , supernova explosions , and the interior of neutron stars . after a brief introduction to thermal field theory the usefulness of this approach in astrophysics 
 will be exemplified in three different cases . 
 first the interaction of neutrinos within a supernova plasma will be discussed . 
 then the possible presence of quark matter in a neutron star core and finally the interaction of light with the cosmic microwave background will be considered .    
 4k @xmath0@xmath1 @xmath2@xmath3