arXiv subjects

Levon Pogosian

Publications and source records attributed to Levon Pogosian.

105 records · Page 6Linked to original sources

arXiv2003

Forecasting Cosmic Doomsday from CMB/LSS Cross-Correlations

A broad class of dark energy models, which have been proposed in attempts at solving the cosmological constant problems, predict a late time variation of the equation of state with redshift. The variation occurs as a scalar field picks up speed on its way to negative values of the potential. The negative potential energy eventually turns the expansion into contraction and the local universe undergoes a big crunch. In this paper we show that cross-correlations of the CMB anisotropy and matter distribution, in combination with other cosmological data, can be used to forecast the imminence of such cosmic doomsday.

astro-ph↗

arXiv2003

Could thermal fluctuations seed cosmic structure?

We examine the possibility that thermal, rather than quantum, fluctuations are responsible for seeding the structure of our universe. We find that while the thermalization condition leads to nearly Gaussian statistics, a Harrisson-Zeldovich spectrum for the primordial fluctuations can only be achieved in very special circumstances. These depend on whether the universe gets hotter or colder in time, while the modes are leaving the horizon. In the latter case we find a no-go theorem which can only be avoided if the fundamental degrees of freedom are not particle-like, such as in string gases near the Hagedorn phase transition. The former case is less forbidding, and we suggest two potentially successful ``warming universe'' scenarios. One makes use of the Phoenix universe, the other of ``phantom'' matter.

astro-ph↗

arXiv2002

Domain Wall Lattices

We construct lattices with alternating kinks and anti-kinks. The lattice is shown to be stable in certain models. We consider the forces between kinks and antikinks and find that the lattice dynamics is that of a Toda lattice. Such lattices are exotic metastable states in which the system can get trapped during a phase transition.

hep-th↗

arXiv2002

Triplication of SU(5) monopoles

We investigate all spherically symmetric fundamental monopole solutions with fixed topological charge in the SU(5) --> SU(3)xSU(2)xU(1)/Z_3xZ_2 symmetry breaking. We find that there is three-fold replication of the monopoles. The three copies correspond to bound states with ``monopole clouds'' arising from the non-Abelian nature of the SU(3) factor. The triplication of monopoles could help us understand the observed family structure of standard model particles.

hep-th↗

arXiv2002

Signatures of primordial helicity in the CMBR

We have studied ways in which helical primordial magnetic fields could be constrained by measurements of the cosmic microwave background radiation (CMBR). If there were helical flows in the primordial plasma at the time of recombination, they would produce parity violating temperature-polarization correlations ($C_l^{TB}$ and $C_l^{EB}$). However, the magnitude of helical flows induced by helical magnetic fields is unobservably small. We discuss an alternate scheme for extracting the helicity of a stochastically homogeneous and isotropic primordial magnetic field using Faraday rotation measure maps of the CMBR and the power spectrum of B-type polarization ($C_l^{BB}$).

astro-ph↗

arXiv2002

Kink interactions in $SU(N)\times Z_2$

There are $N-1$ classes of kink solutions in $SU(N)\times Z_2$. We show how interactions between various kinks depend on the classes of individual kinks as well as on their orientations with respect to each other in the internal space. In particular, we find that the attractive or repulsive nature of the interaction depends on the trace of the product of charges of the two kinks. We calculate the interaction potential for all combinations of kinks and antikinks in $SU(5)\times Z_2$ and study their collisions. The outcome of kink-antikink collisions, as expected from previous studies, is sensitive to their initial relative velocity. We find that heavier kinks tend to break up into lighter ones, while interactions between the lightest kinks and antikinks in this model can be repulsive as well as attractive.

hep-th↗

arXiv2001

Signatures of kinetic and magnetic helicity in the CMBR

P and CP violation in cosmology can be manifested as large-scale helical velocity flows in the ambient plasma and as primordial helical magnetic fields. We show that kinetic helicity at last scattering leads to temperature-polarization correlations ($C_l^{TB}$ and $C_l^{EB}$) in the cosmic microwave background radiation (CMBR) and calculate the magnitude of the effect. Helical primordial magnetic fields, expected from cosmic events such as electroweak baryogenesis, can lead to helical velocity flows and hence to non-vanishing correlations of the temperature and B-type polarization. However we show that the magnitude of the induced helical flow is unobservably small because the helical component of a magnetic field is almost force-free. We discuss an alternate scheme for extracting the helicity of a stochastically homogeneous and isotropic primordial magnetic field using observations of the CMBR. The scheme involves constructing Faraday rotation measure maps of the CMBR and thus determining the sum of the helical and non-helical components of the primordial magnetic field. The power spectrum of B-type polarization fluctuations, on the other hand, are sensitive only to the non-helical component of the primordial magnetic field. The primordial magnetic helicity can then be derived by combining these two sets of observations.

astro-ph↗

arXiv2001

Cosmic String Signatures in the Cosmic Microwave Background Anisotropies

We briefly review certain aspects of cosmic microwave background anisotropies as generated in passive and active models of structure formation. We then focus on cosmic strings based models and discuss their status in the light of current high-resolution observations from the BOOMERanG, MAXIMA and DASI collaborations. Upcoming megapixel experiments will have the potential to look for non-Gaussian features in the CMB temperature maps with unprecedented accuracy. We therefore devote the last part of this review to treat the non-Gaussianity of the microwave background and present a method for computation of the bispectrum from simulated string realizations.

astro-ph↗

arXiv2001

Quintessence and variation of the fine structure constant in the CMBR

We study dependence of the CMB temperature anisotropy spectrum on the value of the fine structure constant $α$ and the equation of state of the dark energy component of the total density of the universe. We find that bounds imposed on the variation of $α$ from the analysis of currently available CMB data sets can be significantly relaxed if one also allows for a change in the equation of state.

astro-ph↗

arXiv2001

Space of kink solutions in SU(N)\times Z_2

We find $(N+1)/2$ distinct classes (``generations'') of kink solutions in an $SU(N)\times Z_2$ field theory. The classes are labeled by an integer $q$. The members of one class of kinks will be globally stable while those of the other classes may be locally stable or unstable. The kink solutions in the $q^{th}$ class have a continuous degeneracy given by the manifold $Σ_q=H/K_q$, where $H$ is the unbroken symmetry group and $K_q$ is the group under which the kink solution remains invariant. The space $Σ_q$ is found to contain incontractable two spheres for some values of $q$, indicating the possible existence of certain incontractable spherical structures in three dimensions. We explicitly construct the three classes of kinks in an SU(5) model with quartic potential and discuss the extension of these ideas to magnetic monopole solutions in the model.

hep-th↗

arXiv2001

CMB Bispectrum from Active Models of Structure Formation

We propose a new method for a numerical computation of the angular bispectrum of the CMB anisotropies arising from active models such as cosmic topological defects, using a modified Boltzmann code based on CMBFAST. The method does not use CMB sky maps and requires moderate computational power. As a first implementation, we apply our method to a recently proposed model of simulated cosmic strings and estimate the observability of the non-Gaussian bispectrum signal. A comparison with the cosmic variance of the bispectrum estimator shows that the bispectrum for the simulated string model we used is not observable.

astro-ph↗

arXiv2000

Cosmic Defects and CMB anisotropy

Recent measurements of the cosmic microwave background (CMB) anisotropies by BOOMERANG and MAXIMA collaborations have tightened the observational constraints on theories of structure formation. They disagree with the predictions of conventional topological defect models. Considering the fact that topological defects are predicted by the majority of realistic particle physics models, the exact nature of the constraints imposed by the recent data on the population and the properties of the defects must be fully understood. We show that the predictions of current cosmic string models can be brought into a closer agreement with the observations by choosing a closed universe with $Ω=1.3$ and by including the effects of the small-scale structure and radiation products of the strings. These alone, however, are not sufficient for obtaining a good fit to the measured shape of the angular power spectrum. To fit the data cosmic strings would either have to be correlated on large (perhaps superhorizon) scales or would have to possess a higher degree of coherence, i.e. be more ``time-correlated'.

astro-ph↗

arXiv2000

Domain Walls in SU(5)

We consider the Grand Unified SU(5) model with a small or vanishing cubic term in the adjoint scalar field in the potential. This gives the model an approximate or exact Z$_2$ symmetry whose breaking leads to domain walls. The simplest domain wall has the structure of a kink across which the Higgs field changes sign ($Φ\to -Φ$) and inside which the full SU(5) is restored. The kink is shown to be perturbatively unstable for all parameters. We then construct a domain wall solution that is lighter than the kink and show it to be perturbatively stable for a range of parameters. The symmetry in the core of this domain wall is smaller than that outside. The interactions of the domain wall with magnetic monopole is discussed and it is shown that magnetic monopoles with certain internal space orientations relative to the wall pass through the domain wall. Magnetic monopoles in other relative internal space orientations are likely to be swept away on collision with the domain walls, suggesting a scenario where the domain walls might act like optical polarization filters, allowing certain monopole ``polarizations'' to pass through but not others. As SU(5) domain walls will also be formed at small values of the cubic coupling, this leads to a very complicated picture of the evolution of defects after the Grand Unified phase transition.

hep-ph↗

arXiv2000

Interaction of Magnetic Monopoles and Domain Walls

We study the interaction of magnetic monopoles and domain walls in a model with SU(5)\times Z_2 symmetry by numerically evolving the field equations. We find that the monopoles unwind and dissipate their magnetic energy on collision with domain walls within which the full SU(5) symmetry is restored.

hep-ph↗

arXiv1997

Relaxing the Geodesic Rule in Defect Formation Algorithms

In studying the formation of topological defects, it is conventional to assume the ``geodesic rule'' which is equivalent to minimizing gradients of the order parameter. This assumption has been called into question in field-theoretic studies of first order phase transitions and in the case of local defects. We present a scheme for numerically investigating the formation of strings without assuming the geodesic rule. Our results show that the fraction of string in infinite strings grows as we deviate from the geodesic rule.

hep-ph↗