The German eROSITA consortium (eROSITA-DE), which includes the University of Bonn and is led by the Max Planck Institute for Extraterrestrial Physics (MPE), has released its second major public data set, the “eROSITA Data Release 2” (DR2).
Category: physics – Page 4
Bound gravitational waves inspired by photonic systems
When one mentions “waves,” we immediately think of a perturbation that propagates. This applies to waves on the shore, sound waves in the air or electromagnetic waves that we use to transmit information via optical fibers. The same idea of propagating perturbations applies to gravitational waves (GWs), which entered the mainstream media a decade ago after their first direct detection. These are perturbations of the elastic fabric that we are all embedded in, called spacetime.
It is as if waves do not like to stay still; on the contrary, they really like to move from the moment they are created. However, there are some instances in which perturbations remain spatially localized, ignoring their propagating counterparts entirely, even when sharing the same space. These forever localized perturbations are called BICs (from bound states in the continuum). I discuss these in a new article published in Physica Scripta.
Astronomers discover a new kind of cosmic object — a black hole ‘star’
Have you heard of the inexplicable little red dots which were found in images from the early universe?
This article may explain them — _____________
The object was lurking in the constellation of Cetus, the Whale, billions of light years from Earth. It is thought to have formed 660m years after the big bang, astronomers’ leading theory as to how the universe began.
Measurements of the exotic body found that while it resembles an immense star, it releases 100bn times more energy than any known star can produce. The energy output is far closer to that observed from black holes than stars.
“We have found a new type of astrophysical object, a black hole star,” said Dr Rohan Naidu at the Kavli Institute for Astrophysics and Space Research, part of the Massachusetts Institute of Technology.
It “shines with the energy typically associated with black holes, but at the same time bears signatures classically associated with stars”, he added.”
How Rare Events Remember
A new theory of rare recurrent events dispenses with the simplifying assumption that recent events lack memory of previous ones.
Recurring earthquakes, stock-market crashes, catastrophic floods, and other rare events are statistically unlikely but significantly impactful. Their prediction is commonly based on the Arrhenius-Kramers paradigm [1], one of the most broadly applied frameworks in statistical physics. Implicit in this formalism are two strong universal features. First, the distribution of times to reach a rare event is exponential and independent of initial conditions, implying no correlation exists between successive rare events. Second, the mean waiting time increases exponentially with the size of the energy barrier (or effective energy) to be overcome. Despite its applicability, nature sometimes defies the Arrhenius-Kramers paradigm. Proteins can cross energy barriers with nonexponential kinetics [2], rainfall extremes can cluster in time [3] (Fig.
Distant time crystals oscillate in unison, paving the way for spin networks
In January 2024, physicists at TU Dortmund University demonstrated a continuous time crystal in a semiconductor whose oscillations remained stable for hours. In a new study published in Nature Communications, Professor Alex Greilich and his team show that many such time crystals can form in the same material and synchronize their electron-nuclear spin oscillations.
Time crystals are systems whose internal dynamics repeat periodically in time without being driven by a periodic external signal. In the TU Dortmund experiment, they are created in a semiconductor made of gallium arsenide containing small amounts of indium and silicon, which provides localized electrons. At temperatures close to −270°C (−454°F), each electron interacts with about one million surrounding nuclear spins.
A pump laser aligns the electron spins, which transfer their polarization to the nuclear spins. In a weak magnetic field, the nuclear-spin polarization begins to rotate. The resulting feedback between the electron and nuclear spins sustains the oscillations, while a second laser is used to observe them.
Could we send a spacecraft to a black hole?
Black holes represent some of the most extreme environments in the universe. They are the sources of the strongest gravitational fields, allowing us to test Einstein’s theory of general relativity to an extent impossible with small objects. But we are also reaching the limit of what we can learn about one remotely. So, various authors have put forward ideas for how we might eventually send a probe directly to a black hole to observe it up close. One of the most vocal of those authors is Cosimo Bambi of Fudan University in Shanghai—and he recently released a paper, available as a preprint on arXiv, about what it would take to send a gram-sized probe to a nearby black hole.
Unfortunately, we do not know of any “nearby” ones—at least not yet. The closest known black hole is Gaia BH1, which is roughly 1,560 light-years away in the constellation Ophiuchus. However, we only know Gaia BH1’s position because it has a slight gravitational pull on a nearby companion star. There are likely many more invisible black holes in our galactic neighborhood that do not have such telltale signs.
According to the paper, the Milky Way likely contains about 100 million stellar-mass black holes. That is not a typo—there are most likely hundreds of millions of black holes the size of a star floating around our galaxy. Crucially, 92% of them are isolated, without a companion star to illuminate them—meaning they would be essentially invisible because they suck up all the light directed their way. But, according to Bambi’s paper, there should be one stellar-mass black hole for roughly every 1,500 cubic parsecs (about 52,000 cubic light-years)—keep in mind that the Milky Way has an estimated volume of 150 cubic kiloparsecs.
DESI releases biggest 2D map of the universe
Hold on to your telescopes: The DESI Legacy Imaging Surveys team has released the largest-ever 2D color map of the universe. The 5.6-trillion-pixel map contains nearly 4 billion celestial objects, primarily stars and galaxies. The data is available for all to use and publicly view through the Legacy Survey Sky Viewer.
Astronomers and citizen scientists can explore the map or combine it with their own observations to better understand our universe. Researchers can search for rare phenomena like gravitational lenses, observe fleeting events like supernovae, and investigate two of physics’ biggest mysteries: dark matter, the invisible substance that accounts for most of the mass in our universe, and dark energy, the force driving our universe’s accelerating expansion.
The new map builds on earlier versions from the DESI Legacy Imaging Surveys that have already proved invaluable. To date, more than 1,800 science papers that reference the Legacy Surveys data have been published.
Tiny floating magnet detects ultrafaint magnetic fields at room temperature
Measuring faint magnetic fields is useful in a number of areas, including mapping brain activity, monitoring hearts and probing fundamental physics. Typically, picking up such weak signals requires expensive or bulky equipment, such as liquid-helium cooling tanks or specially shielded rooms that block out Earth’s magnetic field.
In a paper published in the journal Science, researchers report creating a miniature floating magnet that detects these faint signals at room temperature.
To build their magnetometer, the team suspended a tiny permanent magnetic disk, smaller than a grain of rice, inside a glass vacuum chamber, where it levitated in midair. An overhead stack of magnets pulled the disk upward against gravity, while a graphite plate beneath it provided a repulsive force to help stabilize it.
Two decades of blazar observations, and the mysteries keep piling up
Narrow jets of luminous matter may be emitted toward Earth from the nuclei of active galaxies billions of light-years away. The galaxy then appears as a point source and is called a blazar. A Polish-German team of scientists has, for the first time, analyzed the activity of one such blazar over an extended period and, instead of finding answers, encountered an ever-increasing number of intriguing questions.
Distant, active galaxies that emit jets of matter at small angles toward Earth, known as blazars, present astronomers with numerous observational and interpretative challenges. Their immense distance and specific orientation, combined with the high variability of the emitted radiation—which, moreover, is generated across a very wide energy range—are the main reasons why understanding the phenomena responsible for the properties of blazars is particularly difficult.
Are the current interpretations of the nature of these objects, based on short-term and rather sporadic observations, correct? A group of scientists from the University of Heidelberg and the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Krakow set out to answer this question. The research carried out on the Polish side focused on the blazar PKS 2155-304, located one and a half billion light-years away. It is situated in the southern celestial hemisphere, in the background of the constellation Piscis Austrinus. The paper is published in the Journal of High Energy Astrophysics.