Relativity and Space Science Seminar Series, Thursdays, 3-4p.m., Dayton Conference Room (EPS 258)

Date Presenter Topic
Jan 17 Piet Martens "The Faint Young Sun Paradox"
Jan 24 Lucas Tarr Solar flares as broken flux constraints in active region magnetic modeling
Jan 31 Kent Yagi Universal I-Love-Q Relations of Neutron Stars
Feb 7 David Mckenzie "Handwaves and the Mosh Pit"
Feb 14 Paul Baker  
Feb 21 Adam Kobelski Modeling Active Region Transient Brightenings Observed with XRT as Multistranded Loops
Feb 28 Dimitri Ayzenberg Linear Stability Analysis of Modified Gravity Theories
Mar 7 Sarah Jaeggli Viewing Venus as a Transiting Exoplanet
Mar 21 Laura Sampson Gravitational Wave Tests of Strong Field General Relativity with Binary Inspirals: Optimal Model Selection
Mar 28 Katerina Chatziioannou Tidal heating and torquing of a Kerr black hole
Apr 11 Guest speaker  
Apr 18 Antoine Klein  
Apr 25 Nico Yunes  



Jan 17th
Speaker: Petrus C Martens, Physics and Computer Science Departments, Montana State University, Bozeman, MT, USA Smithsonian Astrophysical Observatory, Cambridge, MA, USA
Title: The Faint Young Sun Paradox


While much (necessary) attention is being paid to the possible influence of the Sun on global climate variation in the last millennium, there is an even more astounding problem in the mismatch between solar luminosity and terrestrial climate in the first several billions of years of the Earth's existence, an issue known as the "Faint Young Sun Paradox". In brief the paradox is this: The geological and biological record support that the Earth's biosphere was considerably warmer than currently during the origin of life on Earth and for several billions of years thereafter. Yet, stellar evolution calculations support the Sun reaching the Zero Age Mean Sequence at about 75% of its present luminosity, and linearly increasing in time up to its current level. Climate models predict a "Snowball Earth" for such a low solar constant, unless the greenhouse effect were much stronger than what it is now. However, there is no geological evidence for a hugely increased presence of greenhouse gasses in the early atmosphere. For possible solutions scientists have typically pointed fingers at other disciplines: Earth scientists suspect the Sun was much more luminous in the past than astronomers calculate.
Solar scientists point to stronger geological activity on the early Earth, etc. As of now the issue remains unresolved.

However, recent observations point towards the Sun for a resolution of the paradox. Most important are the results from the Mars Rovers that show that Mars has had periods with a seeming abundance of liquid water over billions of years.  If both Mars and Earth both have had liquid water over their history then it is reasonable to look for a common cause, i.e. a more luminous Sun than simulations indicate. One possibility for a brighter young Sun would be if the Sun had only about 5% more mass at its origin than it has now, and consequently, has lost the excess mass through the solar wind. Model calculations have been invoked to discard this possibility, but a comparison with observations of other sunlike stars in earlier phases of their evolution have been observed to have much higher mass losses, potentially enough for a 5% mass loss over the Sun's almost five billion year lifespan. The question remains what observations and simulations can be made to verify or discard the existence of a massive solar wind through much of the Sun's history.


Jan 24
Speaker: Lucas Tarr
Title: Solar flares as broken flux constraints in active region magnetic modeling


Solar flares are the most energetic eruptive events in the solar system, with the largest releasing on order 10^32 ergs on a timescale of hours.  Understanding the mechanisms by which this energy is slowly stored and rapidly released is an important topic in solar physics.  We will focus on active region 11158, describing its evolution in terms of a Magnetic Charge Topology (MCT) and the Minimum Current Corona (MCC) model.  The MCC constrains the flux connecting pairs of magnetic elements to be fixed as the system evolves.  This drives the system away from its minimum energy (potential field) state, generating currents along separators of the field.  We find that the required amount of energy for AR11158's numerous flares can be stored in the active region's magnetic field.  Furthermore, relaxation of the flux constraint.


Jan 31
Speaker: Kent Yagi, MSU Physics
Title: Universal I-Love-Q Relations of Neutron Stars


A slowly-rotating neutron star can be characterized by its mass, its radius, its moment of inertia, its tidal and rotational Love numbers and its quadrupole moment to quadratic order in spin. In principle, all of these quantities depend on the neutron star's internal structure, and thus, its unknown equation of state (EoS). We find, however, universal relations between the moment of inertia, the Love numbers and the quadrupole moment that depend very mildly on the neutron star's internal structure. If we can measure two of these quantities independently, these relations allow us to perform model-independent and EoS-independent tests of gravity.


Feb 7
Speaker: David E. McKenzie
Title: "Handwaves and the Mosh Pit"


We talk about magnetic reconnection so much that one would imagine we have it all worked out.  To borrow a phrase from Uzdensky, "The most important reconnection mechanism in Astrophysics invokes waves, a certain type of waves, in fact.  Called /handwaves/."  Conditions in the corona are exactly wrong for reconnection to be anything like efficient, and yet we see evidence that it's happening all the time.  In order to produce a slow buildup and sudden release of energy --- both of which are absolutely required by the observations --- reconnection must be blocked for hours/days at a time, and then suddenly allowed to become very fast indeed.  Understanding the conditions that control the rate of reconnection in the corona is a key issue; I will show some recent work that purports to shed light on kinetic processes related to initiating, accelerating, and prolonging reconnection in solar/stellar flares.


Feb 21
Title: Modeling Active Region Transient Brightenings Observed with XRT as Multistranded Loops
Speaker: Adam Kobelski


Strong evidence exists that coronal loops as observed in EUV and soft X- rays are not monolithic isotropic structures, but can be modeled as bundles of independent strands. Modeling the observed active region transient brightenings (ARTBs) with this framework can allow exploration of the energetic ramifica- tions and characteristics of these stratified structures. Here we present a simple method of detecting and modeling ARTBs observed with the Hinode X-Ray Tele- scope (XRT) as groups of simple 0-dimensional strands, which allows us to probe parameter space to understand better the spatial and temporal dependence of strand heating in impulsively heated loops. This method can be used to analyze a large number of observations to gain a statistical insight into the parameters of coronal plasma.


Feb 28
Speaker: Dimitry Ayzenberg
Title: Linear Stability Analysis of Modified Gravity Theories


Stability analysis is performed on systems ranging from pendulums to models of Earth's weather patterns and in field ranging from economics to pharmaceuticals. When the systems are complex the analysis must either be performed numerically or to leading order terms in the perturbation. As modified theories of gravity are generally analytically complex and have not yet been shown to be mathematically well-posed, one test of validity is a linear stability analysis. Examples of such modified theories are Chern-Simons and the more general quadratic gravity.


March 7
Speaker: Sarah Jaeggli
Title: Viewing Venus as a Transiting Exoplanet


The transit of Venus in 2012 gave us a unique chance to view its atmosphere as we might see that of a transiting cytherean exoplanet, through scattered and refracted illumination from its parent star. Using a salvo of ground-based solar instrumentation the Williams College Venus transit expedition group preformed imaging, spectroscopy, and polarimetry during the transit of Venus focusing on extracting signatures of CO₂ absorption from Venus from the solar spectrum.
Although the predicted CO₂ transmission spectrum of Venus is not particularly strong at 1565 nm, this region of the H-band provides a particularly flat solar continuum with few atmospheric and molecular lines. This region was chosen for investigation with the Facility InfraRed Spectropolarimeter on the Dunn Solar Telescope. In this talk I'll describe the transit observations taken with FIRS and the results thus far.


March 7
Speaker: Laura Sampson
Title: Gravitational Wave Tests of Strong Field General Relativity with Binary Inspirals: Optimal Model Selection


We study generic tests of strong-field General Relativity with gravitational waves emitted during the inspiral of compact binaries.

We construct waveforms that deviate from the General Relativistic expectation through a series of post-Newtonian terms (instead of a single phase term); we find that these higher-order terms can affect our ability to test GR, in some cases by making it easier to detect a deviation, and in some cases by making it more difficult.

We find that more complicated, parameterized post-Einsteinian families, with multiple phase terms, are suboptimal at detecting deviations from General Relativity; the simplest family still reigns supreme when trying to identify whether a deviation from Einstein's theory is present in the data.


March 28th
Katerina Chatziioannou
Title: Tidal heating and torquing of a Kerr black hole


Astrophysical black holes in binaries are immersed in a sea of gravitational perturbations caused by their companions. These vacuum perturbations will result in the spacetime geometry deviating from the vacuum Kerr solution and in fluxes of mass and angular momentum across the black hole horizon. These effects can alter the frequency evo- lution of gravitational waves emitted, a detailed modeling of which may be important in detection and crucial in parameter estimation. This talk describes a perturbative analytic calculation of these fluxes, assuming the tidal deformations are small and due to a slowly-varying external universe. This work extends previous results to next-to-leading order in the ratio of the unperturbed black hole mass to the radius of curvature of the external universe.