First image from final Murchison Widefield Array installationRead more...
Most distant spiral galaxyRead more...
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At the Center for Gravitation and Cosmology, we push the frontiers of astrophysics through the novel use of observation, theory, and computation. By bringing together expertise in gravitational physics, astrophysics, and computing, we can address scientific challenges in relativistic astrophysics, gravitational-wave astronomy, particle astrophysics, cosmology, and quantum gravity. Continue browsing to learn more about our research and opportunities for students.
Extracting equation of state parameters from black hole-neutron star mergers. I. Nonspinning black holes by B. Lackey, K. Kyutoku, M. Shibata, P. Brady, J. Friedman (Paper)
When a black hole and a neutron star (or two neutron stars or black holes) spiral toward each other and merge, the waveform from the observed gravitational wave varies based on physical parameters of the inspiraling objects. The parameter Λ, describing the neutron star's deformability, can be extracted from the observed waveforms and is directly related to the neutron star's radius and mass. Λ is also the parameter that determines the departure from point-particle dynamics during the inward spiral of the objects. Next-generation telescopes will capture gravitational waveforms with sufficient accuracy to determine Λ, therefore constraining the neutron star equation of state.Read more
Discovery of the Optical/Ultraviolet/Gamma-Ray Counterpart to the Eclipsing Millisecond Pulsar J1816+4510 by D.L. Kaplan et al., including UWM authors R. Kotulla, C.M. Biwer, D.F. Day, M.D.W. Rohr, X. Siemens (Paper)
The discovery of a gamma-ray counterpart to the 3.2-ms pulsar J1816+4510 is reported, along with strong evidence of an optical / ultraviolet companion. Many questions remain because the system seems qualitatively different than other eclipsing pulsar binaries: it is rather hotter than other systems, larger than white dwarfs but smaller than normal stars.Read more
First Low-Latency LIGO+Virgo Search for Binary Inspirals and their Electromagnetic Counterparts by the Ligo-Virgo Collaboration (Paper)
A primary goal of the LIGO and Virgo observatories is the direct detection of gravitational waves from compact binary mergers. There may also be an electromagnetic counterpart to the gravitational-wave emission. Binary mergers that include neutron stars are thought to generate short, hard gamma-ray bursts and gravitational waves. Coincident observation of electromagnetic radiation and gravitational waves would provide a wealth of information about coalescing binary sources.
For the first time, LIGO and Virgo performed a low-latency (~20-40 min) search for gravitational waves from coalescing binaries and provided alerts to ten partner telescopes. Optical followups were performed by five telescopes as a result of one of the coincident LIGO-Virgo triggers.Read more
Eccentric binary mergers produce diverse outcomes in new simulations by Branson Stephens, William East and Frans Pretorius (Paper)
Simulations of merging black holes and neutron stars may lead to a better understanding of systems that can produce gravitational waves. The new simulations presented in this work focus on applying full general relativity to hyperbolic encounters. Simulations of this kind are applicable to black hole - neutron star capture events that merge with large eccentricity.
The outcome of the simulations is sensitive to the impact parameter. The amount of mass available to form an accretion disk, the fallback rate of material onto the disk, and the total mass lost due to gravitational waves depend on how quickly the neutron star plunges. The observed variations may help explain the diverse characteristics in gamma-ray bursts.Read more
Physical Conditions in a Young, Unreddened, Low-Metallicity Galaxy at High Redshift by Dawn K. Erb, Max Pettini, Alice E. Shapley, Charles C. Steidel, David R. Law, and Naveen A. Reddy (Paper)
Many galaxies have recently been discovered at very high redshift (z>5). The properties of the galaxy Q2343-BX418, found at redshift z=2.3, are broadly similar to those inferred for the young, low mass galaxies now being discovered at higher redshifts. The detailed spectral properties of BX418 may shed light on the likely physical conditions in these higher redshift objects, well before facilities are available to measure them directly.
BX418 is an L* galaxy with a very low mass-to-light ratio. It is also a highly unusual galaxy. It is young (<100 Myr), low mass (109 Msun), low in metallicity (Z~1/6 Zsun) and unreddened. Its ionization parameter is very high. The rest-frame ultraviolet and optical spectra indicate that BX418 contains distinctive spectral features when compared to other more typical z~2-3 galaxies.Read more
Gravitational Self-force in a Radiation Gauge by Tobias S. Keidl, Abhay G. Shah, John L. Friedman, Dong-Hoon Kim, and Larry R. Price (Paper)
The merger of a stellar mass compact object with a supermassive black hole is known as an extreme mass-ratio inspiral (EMRI). Future gravitational wave detectors (such as LISA, the Laser Interferometer Space Antenna) could potentially measure hundreds of EMRI events with a wide range of astrophysical and fundamental implications.
It is necessary to have accurate gravitational waveforms for these studies, which means that an accurate calculation of the gravitational self-force experienced by the particle is required. A method of computing the self-force in the radiation gauge is presented.Read more
Pulsar Discovery by Global Volunteer Computing by B. Knispel et al. (including B. Allen, D. Hammer, M.A. Papa, and X. Siemens) (Paper)
For the first time, home computers enrolled in Einstein@Home, a volunteer distributed computing project, have discovered a new pulsar. Whenever volunteer computers are idle, their compute cycles are used to perform data analysis tasks on large data sets from gravitational-wave detectors and astronomical observations.
The newly discovered pulsar (PSR J2007+2722) marks the first discovery within the Einstein@Home project. It was identified using radio data collected by the PALFA project at the Arecibo Telescope in Puerto Rico.Read more
A ground-based measurement of the relativistic beaming effect in a detached double white dwarf binary by Shporer et al. (including D. Kaplan) (Paper)
Recently, the first eclipsing double white dwarf binary was discovered. Named NLTT 11748, it was immediately noticed as an excellent candidate for ground-based observations of the beaming effect. Relativistic beaming occurs when a light-emitting object moves relative to an observer. Beaming is most easily detected when a binary system has a high mass ratio, low luminosity ratio, fast radial velocity, and short orbital period.
NLTT 11748 was observed during three nights during February 2010 at the Faulkes Telescope North in Hawaii. Relativistic beaming was observed at the 7 sigma significance level, making this the first observation of relativistic beaming in a binary white dwarf system.Read more
Gravitational Radiation from Preheating with Many Fields by John T. Giblin, Larry R. Price and Xavier Siemens (Paper)
The universe quickly expanded during inflation, leaving it cold and far from thermal equilibrium. After inflation, preheating models suggest that the inflaton and matter fields interacted, leading to exponential particle production.
All previous studies of this process have been limited to a single matter field interacting with the inflaton. However, the inflaton is expected to couple to many fields in the early universe. Multiple matter fields were simulated and the resulting gravitational-wave spectrum based on these models is presented. The shape of the spectrum depends on the inflaton potential and the number of matter fields being simulated.Read more
Using cosmic neutrinos to search for non-perturbative physics at the Pierre Auger Observatory by Luis A. Anchordoqui et al. (Paper)
The Pierre Auger Observatory provides a laboratory for studying fundamental physics at energies far beyond those available at particle colliders. It can study the interactions of ultrahigh energy cosmic neutrinos, produced either at the same sites as ultrahigh energy cosmic rays or from cosmic ray interactions.
Much about the cosmic neutrino flux is unknown, but Auger hopes to address the question of whether or not the neutrino-nucleon cross-section matches the standard model prediction beyond the TeV-scale. Simulations indicate that if only a few neutrino events are observed, new physics beyond the standard model will be necessary to explain the observations.Read more
Search for Gravitational Waves from Compact Binary Coalescence in LIGO and Virgo Data from S5 and VSR1 by the LIGO and Virgo Collaborations (Paper)
Pairs of compact objects, black holes and/or neutron stars, that become gravitationally bound to each other are among the strongest sources of gravitational waves that we know of today. The gravitational waves emitted by these binary compact objects carry energy away from the system. Over time, the compact objects gradually spiral inward and eventually crash into each other in a violent cosmic event.
This paper describes the first joint search for gravitational waves from binary mergers with data from the LIGO and Virgo detectors. The methodology adopted in this search represents a substantial advance over previous searches for binary mergers. Although no gravitational waves were identified, an upper limit was set on their rate.Read more
Search for Gravitational-wave Inspiral Signals Associated with Short Gamma-Ray Bursts During LIGO's Fifth and Virgo's First Science Run by the LIGO and Virgo Collaborations (Paper)
Gamma-ray bursts (GRBs) are intense flashes of gamma rays that are observed to be isotropically distributed over the sky. They are observed directly by gamma-ray and x-ray satellites in the Interplanetary Network, such as HETE, Swift, and Fermi.
Short GRBs, with a duration of less than 2 seconds, are thought to originate primarily in the merger of a neutron star with another compact object, such as a neutron star or black hole. Compact binary mergers are anticipated to generate gravitational-wave signals that will arrive a few seconds before the electromagnetic signal. This paper reports on a search for these signals.Read more
An Upper Limit on the Stochastic Gravitational-Wave Background of Cosmological Origin by the LIGO and Virgo Collaborations (Paper)
Gravitational waves emitted shortly after the Big Bang carry unaltered information about the physical processes that generated them. These waves are expected to be generated by a large number of unresolved sources, forming a stochastic gravitational-wave background (SGWB).
The search for the SGWB using LIGO data was performed by cross-correlating data from pairs of interferometers. No evidence of a SGWB was identified; however, the results of the search constrained the spectrum around 100 Hz. This marks a major milestone that LIGO was designed to achieve.Read more
The Center for Gravitation and Cosmology is supported by the National Science Foundation, UW-Milwaukee College of Letters and Science, and UW-Milwaukee Graduate School. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.