Gaia is a perfect halo explorer. It makes up for a relatively shallow depth with an array of features not available to other surveys/telescopes. These include stable photometry, impeccable star/galaxy separation and resilience to artefacts, not to forget the temporal sampling. In this talk, amongst other things, I will show that in terms of detection of low surface-brightness sub-structure in the halo of the Milky Way, Gaia DR1 has capabilities that are comparable to the of large sky surveys that reach 2-3 magnitudes deeper. Moreover, by using the Gaia DR1 data to recalibrate the SDSS astrometry, we have now built an unprecedented proper motion catalogue with the quality similar to that expected for the Gaia's DR2.
About the Speaker
Dark matter remains a profound mystery at the intersection of particle physics, astrophysics and cosmology. While searches have made significant progress, particularly for dark matter in the form of Weakly Interacting Massive Particles (WIMPs), dark matter has so far been observed solely through its gravitational effects. After surveying the cosmological and astrophysical underpinnings of dark matter, Akerib will discuss the experimental challenges of reaching the required level of sensitivity and background rejection when searching for WIMPs, which are being addressed in the LUX and LUX-ZEPLIN (LZ) experiments. LUX, the Large Underground Xenon experiment, used 250 kg of liquefied xenon as a WIMP target. In the coming years, LZ, the next generation experiment with a 7-ton target, will be installed in LUX’s place in the former Homestake gold mine in South Dakota. Akerib will report on the LUX results, building LZ and conquering the technical challenges along the way.
Dan Akerib joined Stanford in 2014 after serving as a faculty member at Case Western Reserve University for 18 years. He earned his bachelor’s degree from the University of Chicago and his doctoral degree from Princeton University. He completed post-doctoral appointments at Caltech and the University of California, Berkeley. Currently a professor of particle physics and astrophysics at Stanford University and the SLAC National Accelerator Laboratory, Akerib has been searching for dark matter since 1990. He co-leads SLAC’s liquid nobles group, which is a key participant in the LUX-ZEPLIN dark matter search.
Hosted by Carnegie Mellon’s McWilliams Center for Cosmology, will be geared towards a general audience and is free and open to the public. This is the fifth in a series of public lectures on cosmology funded by Carnegie Mellon alumni Fred Bennett (S’86) and Bruce McWilliams (S’78, ’81).
Neutrinos were produced in the early universe and leave an imprint in the distribution of matter and radiation in the cosmos. Cosmic microwave background experiments are particularly sensitive to the presence of sterile neutrinos, while galaxy surveys are affected by neutrino mass. After describing the physics behind these signatures, I present constraints from the CMB and results released on August 3 from the largest current galaxy survey, the Dark Energy Survey.
About the Speaker
This year marks the 60th anniversary of the publication Synthesis of the Elements in Stars, known as B2FH by the authors Burbage, Burbage, Fowler and Hoyle. A combination of new astronomical observations, improved nuclear data, and more realistic astrophysical modeling has revealed the origins of the elements heavier than iron to be more complicated than envisioned by B2FH. The synthesis of elements ranging between iron and tin is particularly nuanced, with many astrophysical sites likely contributing. I will review how our understanding has evolved in recent years, highlight some of the major questions still unresolved 60 years after B2FH, and focus on some measurements of nuclear data that are important for advancing our understanding of the origins of elements ranging from iron to tin.
About the Speaker
There is very little in life about which we are absolutely certain; maybe, only death and taxes as the saying goes. Moreover, the world is replete with things that seem to happen for no discernible reason. Indeed, physicists insist that at its core the world is intrinsically random. Given the ubiquity of uncertainty and randomness, it is not surprising that mathematicians and philosophers have tried, over the centuries, to make sense of both. The key idea, which everyone seems to accept, is that probability is somehow related to uncertainty and randomness. Beyond that views diverge.
In this talk, I trace the long often controversial development of the concept and applications of probability, from the early works of the seventeenth century to the present day. I end with speculations about how probability might be used in the not too distant future.
About the Speaker
Faculty Host: Manfred Paulini
In his lecture, Bialek, a theoretical physicist interested in the phenomena of life, will discuss his thoughts on how physics has been able to create accurate mathematical descriptions of the physical world, helping us to not only understand what we see, but predict what will happen in places we have never looked before. He will address questions including: Are there limits to this predictive power, particularly when applied to the complex phenomena of life? And are we missing some deep principles that will bring the living world under the predictive domain of physics? Bialek also will offer reflections on how physicists might be able to approach the complex and diverse phenomena of the living world and develop new theories to help explain the world around us.
Bialek, the John Archibald Wheeler/Battelle Professor in Physics at Princeton and Visiting Presidential Professor of Physics at The Graduate Center of the City University of New York, is known for his work emphasizing the approach of biological systems to the fundamental physical limits on their performance. In recent work, he and his collaborators have shown how the collective states of biological systems, such as the activity in a network of neurons or the flight directions in a flock of birds, can be described using ideas from statistical physics, connecting them in quantitative detail with new experimental data.
Bialek has been a member of the Princeton faculty since 2001. He has received the President’s Award for Distinguished Teaching at Princeton, and recently published a textbook, Biophysics: Searching for Principles. A member of the National Academy of Sciences and a fellow of the American Physical Society, he received the 2013 Swartz Prize for Theoretical and Computational Neuroscience from the Society for Neuroscience.
Reception to follow in Mellon Institute Lobby
2D layered materials are like paper: they can be colored, stitched, stacked, and folded to form integrated devices with atomic thickness. In this talk, I will discuss how different 2D materials can be grown with distinct electrical and optical properties (coloring), how they can be connected laterally to form pattered circuits (stitching), and how their properties can be controlled by the interlayer rotation (twisting). We will then discuss how these atomically thin papers and circuits can be folded to generate active 3D systems.
About the Speaker
One of the great triumphs of 19th- century science was the emergence of thermodynamics. This is a subject of great power and generality, setting down the rules for what is possible and, even more crucially, what is not possible: there can be no perpetual motion machines, heat flows from hot bodies to cold bodies, and any effort to convert energy from one form to another always involves a bit of waste. A central, if slightly mysterious concept in thermodynamics is the entropy, which is introduced first as a bookkeeping device but then becomes fundamental. In the formulation of statistical mechanics, the bridge connecting our microscopic description of atoms and molecules to the macroscopic phenomena of our everyday experience, entropy reappears as a measure of the number of states that are accessible to all the atoms and molecules.
In the mid-twentieth century, entropy makes yet another appearance, first as a quantitative measure of information, and then as a limit on the amount of space that we need to record that information. It is astonishing that the same concept reaches from steam engines to the internet, and from molecules to language. In this lecture I will try to give a sense for these four different notions of entropy, and their connections with one another, hoping to give a sense for the unifying power of mathematics.
As endpoints of cosmic structure formation that emerge in the era of dark energy domination, the population of clusters of galaxies offers insights into cosmology and the gravitational growth of large-scale structure. The composition of clusters — dark matter and baryons in multiple phases co-evolving within a hierarchical cosmic web of massive halos — is being scrutinized observationally across the electromagnetic spectrum and with increasingly sophisticated numerical simulations. In this presentation, I will outline the phenomenological framework of cluster cosmology, emphasizing multi-wavelength population statistics and support from astrophysical simulations, then discuss some of the challenges associated with early 21st century reality.
About the Speaker.