Since 1998, analyses of supernovae data have shown that our Universe has been undergoing accelerated expansion in the latter part of its life. Because ordinary matter can only provide an attractive force, there must be something else providing the repulsive force to accelerate the Universe's expansion. We call the unknown driver of this expansion dark energy—and the question of its origin is a core issue in cosmology and fundamental physics today.
Jan 30, 2019 | Where do the highest-energy cosmic rays come from?
A century after the discovery of cosmic rays, delving into their mysteries remains a primary focus of high-energy astrophysics. Cosmic rays consist of energetic particles propagating through interstellar and even intergalactic space, and are characterized by an energy spectrum extending over at least eleven orders of magnitude, from ~1 GeV (giga-electronvolt, a commonly used unit of energy in astrophysics) to extreme energies of about 1011 GeV (for comparison, the mass of a proton when converted to an equivalent energy via E=mc2 is just under 1 GeV—so those highest-energy cosmic rays have about the energy of a fastball packed sometimes into one elementary particle!).
Halos are the result of a long sequence of cosmic structure formation. We think it happened like this: The early Universe, after a very short period of rapid expansion called inflation, settled into its current phase of more leisurely general cosmic expansion (or “Hubble flow,” as cosmologists often call it). At the end of inflation, while the density of dark matter—on average—had been smoothed out to become very homogenous on large scales, small quantum perturbations were amplified to more significant density fluctuations—places where there was a tiny bit more, or less, matter. The knots of slightly higher density served as wells of gravitational potential and began to grow further with time by attracting ever more matter through gravity.
In the centers of most galaxies lurk gigantic black holes, millions to billions of times the mass of the sun. Most of them are just minding their own business, but in about 10% of cases they are actively consuming matter and transforming much of it into vast amounts of energy. We call these gluttons “quasars”—supermassive black holes surrounded by bright, hot disks of gas called accretion disks. Although quasars are among the most luminous objects in the Universe, we still do not understand the detailed physics of how this matter behaves. This is now changing thanks to new insights and advances from an unexpected source: the highly sensitive timing satellites used to search for planets around other stars by continuously monitoring their brightness over time, to search for periodic dips caused by transiting planets.
Jul 27, 2018 | Where are they now? Simona Murgia
In the occasional series, "Where are they now?" we check in with KIPAC alumni: where they are now, how they've fared since their days exploring particle astrophysics and cosmology at the Institute, and how their KIPAC experiences have shaped their journeys. Next up is Simona Murgia, who is now an associate professor of physics and astronomy at UC Irvine. Murgia is another particle physicist-turned astrophysicist who started her post-PhD career at MINOS (Main Injector Neutrino Oscillation Search) watching neutrinos change flavor, then migrated to KIPAC in 2007 to look for dark matter using Fermi Gamma-ray Space Telescope data. Follow along as Murgia talks about transitioning from postdoc to professor, looking for dark matter in some pretty tough places, and outrunning the wasps at the old SLAC cafeteria.
Jul 12, 2018 | Astro and Ale: Astronomy on Tap
Astronomy is often called the “gateway science” because of its abundance of beautiful pictures and suitability for Discovery Channel specials. Astronomers, therefore, have an especially heavy burden among scientists to advocate for the science behind the special effects. However, astronomy and astrophysics can seem terribly complex; how do we get people to come out and learn about space in a substantive way? Well, if you’re with Astronomy on Tap, it involves going to the bar a little more often.
On June 11, 2008, the Gamma-ray Large Area Space Telescope (GLAST) lifted off aboard the last Delta II Heavy Launch Vehicle from Cape Canaveral, FL and reached low-Earth orbit shortly thereafter. In the 10 years and one name change since that that day, what is now the Fermi Gamma-ray Space Telescope has found hundreds of pulsars, watched gamma ray flashes in terrestrial lightning, studied our own sun as a gamma-ray source, helped identify giant bubbles billowing out from the core of the Milky Way, and discovered that the neutron star at the heart of the Crab Nebula isn’t as calm as scientists used to think. To name just a few. The discoveries haven’t stopped, either—August 17, 2017, nine years to the month after the start of science operations, Fermi saw the gamma-ray flash of two neutron stars colliding, 1.7 seconds after the gravitational waves generated by this event rolled through (LIGO). Along with discoveries, Fermi is making memories—some scientific, and some of a more personal nature. KIPAC members and some of their Fermi collaborators have had a big part in both, and came together to share memories about a decade (and more) of Fermi.
While neutrinos were hypothesized by Wolfgang Pauli back in 1930, they remain among the most mysterious particles within the Standard Model of particle physics. We now know that there are three types of neutrinos, and neutrino oscillation experiments have shown that there are at least two types which have mass. Current experiments have not yet been able to nail down the precise masses of the three neutrinos, but have placed upper bounds on sum of their masses. These upper bounds tell us that neutrinos have to be the lightest of all Standard Model particles, more than six orders of magnitude lighter than the electron!
The SuperCDMS SNOLAB project, a multi-institutional effort led by SLAC, is expanding the hunt for dark matter to particles with properties not accessible to any other experiment.
How machine learning can help researchers relate dark-matter-only and hydrodynamic simulations, and how this mapping can shed light on the small-scale challenges associated with cold dark matter.
Near the turn of the century, two seminal papers pointed out a striking discrepancy between the number of dark matter subhalos around Milky Way-like systems in dark-matter-only (DMO) simulations and the number of observed dwarf satellite galaxies around the Milky Way (MW). Historically, this discrepancy (shown graphically in the figure below) led to the notion of the "missing satellites problem" (MSP)—not the issue of where multiple Mars-bound satellites have disappeared to, but rather the idea that we observe significantly fewer dwarf satellite galaxies (by a factor of about 10!) in the Local Group than predicted by the standard cold dark matter (CDM) cosmological model.
Is it possible to learn how the jets from small black holes a few solar masses in size behave by studying the biggest, most extreme, most relativistically distorted jets from full-fledged quasars, the monstrous billion solar mass black holes at the centers of giant galaxies? Remarkably the answer is YES!
Supermassive black holes in the centers of giant elliptical galaxies can sometimes produce powerful relativistic outflows called jets. Blazars are a special class of these galaxies with the unique property of having their jets oriented within a small angle from our line of sight. (For more on blazar jets, see the KIPAC blog post, Where have all the magnetic fields disappeared to?). Because of that preferential alignment and the fact that jets move with speeds close to the speed of light, extreme aberration of light and time dilation effects take place, distorting the observed properties of the objects.
Often in the world of astronomy and astrophysics, unexpected observations lead to new ideas and understanding. However, there are occasionally some models that are built up more traditionally from theories to observational predictions. This is a story of one such model—that of the very first stars in the universe, called, somewhat counterintuitively, Population III (Pop III) stars. We haven’t seen Pop III stars yet because of how long ago they first formed—and then died.