Dark matter may exist, but it doesn’t emit gamma rays.
Backlighting
Earlier this year I wrote about a diffuse band of gamma rays coming from regions above and below the Milky Way. The regions spanned about 25,000 light years above and below the galactic plane, and are thought to have formed from an active period of our galaxy’s supermassive black hole about 2 million years ago. While we could determine the size of these regions from their x-ray and gamma ray emissions, it has been difficult to determine their motion. But yesterday at the American Astronomical Society Meeting, new results from the Hubble telescope are measuring the motion of these regions using an interesting trick.
Super Luminous
A supernova is a stellar explosion. They can occur when a large star exhausts its ability to fuse hydrogen into higher elements, and its core collapses. The resulting rebound rips apart the outer layers of the star, creating a supernova while the remains of the core collapses into a neutron star. Another type of supernova, known as a thermal runaway or type Ia, occurs when a white dwarf is a close companion with another star. As outer layers of the companion are captured by the white dwarf, it can trigger a runaway nuclear reaction that rips apart the white dwarf. This latter form always has about the same absolute brightness, which is why they are used to measure the distances of far galaxies.
Energy Bubble
Yesterday I talked about the Fermi gamma ray telescope, and how it allowed us to make much more precise observations of gamma rays in the universe. Part of the purpose of the Fermi telescope is to observe gamma ray bursts, but its broader purpose is to make a sky survey of gamma ray sources in the universe. Already it has found something quite interesting.
Two By Two
Yesterday I talked about the detection of gamma ray bursts, intense blasts of gamma rays that occasionally appear in distant galaxies. Gamma ray bursts were only detected when gamma ray satellites were put into orbit in the 1960s. This is because gamma rays are absorbed by our atmosphere. Even then, the detectors were relatively primitive and couldn’t determine the direction of the bursts. Instead, multiple satellites were used to triangulate the location of these bursts. Since then, gamma ray astronomy has gotten much more sophisticated.
The Big One
In the 1960s a series of satellites were built as part of Project Vela. Project Vela was intended to detect violations of the 1963 ban on above ground testing of nuclear weapons. The Vela satellites were designed to detect bursts of gamma rays, which are high energy electromagnetic waves (light) produced by radioactive decay. If any nuclear weapon was detonated in space, the resulting radioactive decay would release a large amount of gamma rays which would be detected by the Vela satellites.
Fault In Our Stars
Type Ia supernovae are brilliant stellar explosions that can outshine an entire galaxy. They also have the useful property of always exploding with a similar brightness. This makes them useful in determining the distances of galaxies. By comparing the observed brightness of a type Ia supernova to its standard brightness, we can calculate the distance of the supernova, and the galaxy in which it resides. But while we know type Ia supernovae have a consistent brightness, we aren’t entirely sure why.
When Dark Matter Collides
In the ongoing search for dark matter particles, the most popular are efforts to detect them directly here on Earth. Another way to look for dark matter particles is to look for the by-product of their collisions with each other. A recent paper posted on the arxiv has done just that, and they think they’ve found a signal.