What are they?
When astronomers refer to “compact objects,” they are generally referring to objects significantly more dense than a star or a planet. For example, white dwarfs or neutron stars are extremely dense stars that have collapsed, no longer able to produce a sufficient amount of pressure within to prevent their outer layers from falling into their centers. Under extreme conditions, these collapses can trigger the formation of a black hole – a region of space in which gravity is so strong that even light cannot escape.
Compact objects, celestial bodies characterized by very strong gravity such as neutron stars, pulsars and black holes, are subjects of intense research at KIPAC. KIPAC scientists study these various compact objects using data from the Fermi Gamma-ray Space Telescope. The gamma-ray emissions from pulsars allow researchers to study how their intense, pulsed radiation is produced. Researchers now believe that pulsars behave like powerful magnets in which the poles are not aligned with the axis of the star's rotation. The energetic particles emanating from a pulsar form a beacon — similar to the beam of the lighthouse — which periodically sweeps across our line of sight once or twice per revolution. By taking an accurate count of pulsars, KIPAC scientists have been able to estimate how often stellar collapses take place in the Milky Way.
What is it?
Roughly 13.7 billion years ago, the universe started expanding from a dense, hot volume. In the early universe, some 400,000 years after the Big Bang, conditions cooled enough to allow the formation of hydrogen atoms from free protons and electrons. After this early phase, known as "recombination," the universe began to take shape as objects - galaxies, stars, planets – coalesced from the elemental raw material left behind after the Big Bang.
The Universe has structure on many scales. In the largest of scales, we see that the Universe is not random: galaxies are not haphazardly strewn around willy nilly. Rather, they form a tight, interconnected "cosmic web". To study the formation of this web, KIPAC scientists use experiments like Planck, BICEP2, POLAR, and Quiet II to study the Early Universe in an effort to understand the "seeds" that led to creation of the cosmic web we see today.
What is it?
One of the most important and surprising scientific discoveries of the twentieth century was that the expansion of space is not slowing – as had been predicted based on the gravitational pull of all the matter in the universe – but, rather, is increasing with time. This discovery was recognized with the 2011 Nobel Prize in Physics.
Dark energy is the dominant component in the universe, yet there are currently no compelling explanations for its existence or its distribution. However, KIPAC scientists along with many others in the astrophysics and cosmology community are generating a suite of techniques and observational tools that will greatly enhance our understanding of dark energy.
These techniques are based on several different observable phenomena throughout the universe, including the distribution of galaxies on very large scales; the density and distribution of galaxy clusters detected through x-rays, gravitational lensing, and distortions in the cosmic microwave background radiation; the apparent luminosity of Type Ia supernovae, which can be used for measuring vast distances across space; and the distortion of images of background galaxies due to the bending of light as it passes through the intervening dark matter. By applying these techniques to existing data sets and conducting computational studies based on cosmological simulations, researchers are gaining a more robust understanding of dark matter.
What is it?
Only about five percent of the total matter + energy content of the Universe is familiar to us. The identity of the remaining 95 percent, roughly 1/3 of it dubbed as "dark matter" and roughly 2/3, dubbed as "dark energy" is unknown. Though scientists have not yet detected it directly in laboratories on Earth, dark matter’s existence has been deduced from its gravitational effects on the stars and gasses that make up all of the galaxies known in the Universe.
To better understand our universe, it is often necessary to estimate the mass of an astrophysical object. Those objects can range in size from the Sun, the solar system, the Milky Way, and even the entire universe.
Researchers use a number of techniques to measure the mass of extremely large objects. One way to estimate an object’s mass is by observing its light output. If the object does not emit its own light, researchers can examine the way in which the light of background sources bends around it. Another technique is to examine the dynamic motion of objects around it.
It was long believed that the estimated masses coming from these techniques would agree with one another. However, over the past 80 years, it has become apparent that for objects at the galaxy scale and larger, the amount of mass contained exceeds the mass of it’s luminous constituents. This additional mass, which cannot be accounted for by luminous matter we know about, has been coined “dark matter”.
The nature of dark matter remains a mystery because, so far, we can’t see it directly but only detect its effects indirectly on the large-scale structure of the universe. The most likely form of dark matter is a new class of elementary particles predicted by the so-called “super-symmetric extensions” to the standard model of particle physics.
Most of such models predict that the dark matter particle can “self-annihilate.” This happens when two dark matter particles collide. When particles strike one another, energy is released in the form of detectable standard model elementary particles such as photons or charged particles such as positrons and electrons. Many dark matter models predict the emission of gamma rays, the highest energy photons, as annihilation products. KIPAC researchers are using gamma-ray data from the Fermi Large Area Telescope (LAT) to search for the annihilation products. In order to search for these products, the astrophysical foreground have to be well understood before detections or limits on these particles can be derived. In the future the ground-based Cherenkov Telescope Array (CTA) will search for dark matter annihilation products at even higher masses.
A second way to look for these dark matter particles is with specialized detectors that are well shielded from conventional sources of radiation, and to look for minute energy transfers that are expected when these particles occasionally strike an atomic nucleus in the detector. KIPAC researchers are attempting to detect dark matter with two major research programs. The Cryogenic Dark Matter Search (CDMS) uses silicon and germanium "solid state" detectors that are cooled close to absolute zero, and are sensitive to very small temperature changes when a dark matter particle transfers energy to a nucleus. The LUX-ZEPLIN (LZ) program uses vessels filled with liquid xenon and senses small amounts of UV "scintillation" light produced when a nucleus is struck.
What is it?
Much in the same way archeologists reconstruct past civilizations by looking at remains in the present, cosmologists reconstruct the universe’s past by looking at the constituents of the current universe. By doing so, they can infer how the universe began, how it evolved into its present state, and how it will continue to change over time. The study of the early universe is one of the most exciting fields in all of science. In fact, two of the last six Nobel prizes in physics have been awarded to scientists working to understand the conditions of the early universe.
KIPAC researchers are heavily focused on understanding the origin of the early universe, a period in which very different rules of physics were at play in the cosmos than those that govern it now. It is believed that the universe began with very high energies. To tease apart the forces that touched off the universe’s expansion – some 14 billion years ago -- KIPAC scientists are using an array of instruments such as telescopes and satellites to look as far away and as far back in time as possible.
What is it?
The universe is awash in highly energetic particles with velocities approaching the speed of light. Though these particles exist in many places throughout the universe – and can even be found slamming into our own atmosphere – scientists don’t yet fully understand their origins.
Motivated by the rich data supplied by the Fermi Large Area Telescope, KIPAC researchers are vigorously pursuing an understanding of the phenomena responsible for acceleration of particles to enormous energies. To expand their view of the universe, KIPAC scientists are busy planning and conducting as radio, X-ray and optical observations of celestial sources of gamma-rays. KIPAC’s search runs the gamut from the exotic — remnants of exploded stars and astrophysical jets — to the seemingly mundane — our own Sun. Once thought to be an ordinary and quite stable star, the Sun is now known to contain very energetic particles. On occasion these particles penetrate the Earth's atmosphere, and can even briefly disrupt transmission of radio and TV signals.