What Is: Gravitational Microlensing

Scientific Frontline / Stock image

The universe, in its vastness, is largely composed of matter that does not shine. For centuries, the discipline of astronomy was fundamentally limited to the study of luminous objects: stars that fuse hydrogen into helium, gas clouds excited by radiation, and galaxies that act as islands of light in the cosmic dark. This reliance on electromagnetic radiation—photons—as the primary messenger of cosmic information created a significant selection bias. It rendered the "dark sector" of the Milky Way, including brown dwarfs, black holes, old white dwarfs, and free-floating planetary-mass objects, effectively invisible to standard census techniques. To map the true mass distribution of our galaxy, astronomers required a method that did not rely on the emission of light but rather on the one force that pervades all matter: gravity. 

Where the elements come from?

The chlorine and potassium needed to support planet formation and sustain life come from exploding stars.
Image Credit: JAXA

"Why are we here?" This is humanity's most fundamental and persistent question. Tracing the origins of the elements is a direct attempt to answer this at its deepest level. We know many elements are created inside stars and supernovae, which then cast them out into the universe, yet the origins of some key elements have remained a mystery. 

Chlorine and potassium, both odd-Z elements -- possessing an odd number of protons -- are essential to life and planet formation. According to current theoretical models, stars produce only about one-tenth of the amount of these elements observed in the universe, a discrepancy that has long puzzled astrophysicists. 

Historical geography helps researchers solve 2,700-year old eclipse mystery

Artist’s interpretation of an ancient total solar eclipse. This illustration is based on artistic imagination and does not represent the exact appearance of the eclipse recorded in 709 BCE.
Image Credit: Kano Okada, Nagoya University
Based on an image by Phil Hart / NASA

Humanity’s earliest datable record for a total solar eclipse allows scientists to derive accurate measurements of Earth’s ancient rotation speed and provides independent validation of solar cycle reconstruction in the 8th century BCE.

An international team of researchers has used knowledge of historical geography to reexamine the earliest datable total solar eclipse record known to the scientific community, enabling accurate measurements of Earth’s variable rotation speed from 709 BCE. The researchers calculated how the Sun would have appeared from Qufu, the ancient Chinese capital of the Lu Duchy, during the total solar eclipse. Using this information, they analyzed the ancient description of what has been considered the solar corona—the dim outer atmosphere of the Sun visible to the naked eye only during total eclipses—and found that its morphology supports recent solar cycle reconstructions for the 8th century BCE. 

Their findings, published in Astrophysical Journal Letters, provide reliable new data about Earth’s rotation speed during this period and suggest the Sun was becoming more active after a long quiet period, independently confirming what other scientists have found using radiocarbon analysis. 

A speed camera for the universe

The stars (or rather galaxies) of the show.
A montage of eight time-delay gravitational lens systems. There’s an entire galaxy at the center of each image, and the bright points in rings around them are gravitationally lensed images of quasars behind the galaxy. These images are false-color and are composites of data from different telescopes and instruments.
Image Credit: ©2025 TDCOSMO Collaboration et al.
(CC BY-ND 4.0)

There is an important and unresolved tension in cosmology regarding the rate at which the universe is expanding, and resolving this could reveal new physics. Astronomers constantly seek new ways to measure this expansion in case there may be unknown errors in data from conventional markers such as supernovae. Recently, researchers including those from the University of Tokyo measured the expansion of the universe using novel techniques and new data from the latest telescopes. Their method exploits the way light from extremely distant objects takes multiple pathways to get to us. Differences in these pathways help improve models on what happens at the largest cosmological scales, including expansion.

Stars defy the black hole: research in Cologne shows stable orbits around Sagittarius A*

Image Credit: NASA

New observations made with the ERIS instrument at the Very Large Telescope facility disprove from the assumption that the supermassive black hole at the center of the Milky Way devours nearby dust objects. 

An international research team led by PD Dr Florian Peißker at the University of Cologne has used the new observation instrument ERIS (Enhanced Resolution Imager and Spectrograph) at the Very Large Telescope (VLT) facility in Chile to show that several so-called ‘dusty objects’ follow stable orbits around the supermassive black hole Sagittarius A* at the center of our galaxy. Earlier studies had surmised that some of these objects could be swallowed up by the black hole. New data refutes this assumption. The findings have been published under the title ‘ABCD’ in the journal Astronomy & Astrophysics. 

The study focused on four of these unusual celestial bodies, which have been the subject of much discussion in recent years. In particular, G2 was long regarded as a pure dust and gas cloud. It was thought to have been initially elongated by the gravitational pull of Sagittarius A*, a process known as 'spaghettification', before being destroyed. However, the specific observations made with ERIS, which captures radiation in the near-infrared range, show that G2 follows a stable orbit. This is an indication that there is a star inside the dust cloud. These results confirm that the center of the Milky Way is not only destructive but can also be surprisingly stable. 

After nearly 100 years, scientists may have detected dark matter

Gamma-ray image of the Milky Way halo (with details).
Gamma-ray intensity map excluding components other than the halo, spanning approximately 100 degrees in the direction of the Galactic center. The horizontal gray bar in the central region corresponds to the Galactic plane area, which was excluded from the analysis to avoid strong astrophysical radiation.
 Image Credit: ©2025 Tomonori Totani, The University of Tokyo

In the early 1930s, Swiss astronomer Fritz Zwicky observed galaxies in space moving faster than their mass should allow, prompting him to infer the presence of some invisible scaffolding — dark matter — holding the galaxies together. Nearly 100 years later, NASA’s Fermi Gamma-ray Space Telescope may have provided direct evidence of dark matter, allowing the invisible matter to be “seen” for the very first time.

Dark matter has remained largely a mystery since it was proposed so many years ago. Up to this point, scientists have only been able to indirectly observe dark matter through its effects on observable matter, such as its ability to generate enough gravitational force to hold galaxies together. The reason dark matter can’t be observed directly is because the particles that make up dark matter don’t interact with electromagnetic force — meaning dark matter doesn’t absorb, reflect or emit light.

A new angle of study for unveiling black hole secrets

The balloon-borne telescope XL-Calibur was launched on a six-day flight from the Swedish Space Corporation’s Esrange Space Center in July 2024. During that flight, the telescope took measurements from the black hole Cygnus X-1, located about 7,000 light-years away. WashU researchers will use those results to improve computer models for simulating and uncovering further mysteries of black holes.
Photo Credit: NASA/SSC

An international collaboration of physicists including researchers at Washington University in St. Louis has made measurements to better understand how matter falls into black holes and how enormous amounts of energy and light are released in the process.

The scientists pointed a balloon-borne telescope called XL-Calibur at a black hole, Cygnus X-1, located about 7,000 light-years from Earth. “The observations we made will be used by scientists to test increasingly realistic, state-of-the-art computer simulations of physical processes close to the black hole,” said Henric Krawczynski, the Wilfred R. and Ann Lee Konneker Distinguished Professor in Physics and a fellow at WashU’s McDonnell Center for the Space Sciences.

A sparkling ‘Diamond Ring’ in space: Astronomers in Cologne unravel the mystery of a cosmic ring

Stars Brewing in Cygnus X
Image Credit: NASA/JPL-Caltech/Harvard-Smithsonian CfA

The structure of gas and dust resembles a glowing diamond ring. Computer simulations and observations made on board the 'flying observatory' SOFIA are now able to explain the special shape. 

An international team led by researchers from the University of Cologne has solved the mystery of an extraordinary phenomenon known as the ‘Diamond Ring’ in the star-forming region Cygnus X, a huge, ring-shaped structure made of gas and dust that resembles a glowing diamond ring. In similar structures, the formations are not flat but spherical in shape. How this special shape came about was previously unknown. The results have been published under the title ‘The Diamond Ring in Cygnus X: an advanced stage of an expanding bubble of ionized carbon’ in the journal Astronomy & Astrophysics. 

The ring has a diameter of around 20 light years and shines strongly infrared light. It is the relic of a former cosmic bubble that was once formed by the radiation and winds of a massive star. In contrast to other similar objects, the ‘Diamond Ring’ does not have a rapidly expanding spherical shell, but only a slowly expanding ring. 

Astrophysics: In-Depth Description

An illustration of the vast and complex field of astrophysics, featuring elements that represent celestial objects and phenomena.
Image Credit: Scientific Frontline / stock image

Astrophysics is the branch of physics that applies physical laws and theories to understand the origin, evolution, structure, and behavior of celestial objects and phenomena. Its primary goal is to use the principles of physics to explain the universe and everything within it, from stars and planets to galaxies and the entirety of the cosmos.

Are there different types of black holes? New method puts Einstein to the test

At the current resolution of telescopes, black holes predicted by different theories of gravity still look very similar. Future telescopes will make the differences more visible, making it possible to distinguish Einstein's black holes from others.
(Image text translation: Einsteinian Black hole and Alternative Black hole)
Image Credit: L. Rezzolla / Goethe University

Images of black holes are more than just fascinating visuals: they could serve as a “testing ground” for alternative theories of gravity in the future. An international team led by Prof. Luciano Rezzolla has developed a new method to examine whether black holes operate according to Einstein’s theory of relativity or other, more exotic theories. To that end, the researchers conducted highly complex simulations and derived measurable criteria that can be tested with future, even sharper telescopes. Over the next few years, this method could reveal whether Einstein’s theories hold true even in the most extreme regions of the universe.

Black holes are considered cosmic gluttons, from which not even light can escape. That is also why the images of black holes at the center of the galaxy M87 and our Milky Way, published a few years ago by the Event Horizon Telescope (EHT) collaboration, broke new ground. “What you see on these images is not the black hole itself, but rather the hot matter in its immediate vicinity,” explains Prof. Luciano Rezzolla, who, along with his team at Goethe University Frankfurt, played a key role in the findings. “As long as the matter is still rotating outside the event horizon – before being inevitably pulled in – it can emit final signals of light that we can, in principle, detect.”

Dark matter does not defy gravity

Map of the distribution of galaxies observed by the DESI collaboration, from which it is possible to accurately measure the velocities of galaxies.
Image Credit: © Claire Lamman/DESI collaboration; custom colormap package by cmastro.

Does dark matter follow the same laws as ordinary matter? The mystery of this invisible and hypothetical component of our Universe — which neither emits nor reflects light — remains unsolved.  A team involving members from the University of Geneva (UNIGE) set out to determine whether, on a cosmological scale, this matter behaves like ordinary matter or whether other forces come into play. Their findings, published in Nature Communications, suggest a similar behavior, while leaving open the possibility of an as-yet-unknown interaction. This breakthrough sheds a little more light on the properties of this elusive matter, which is five times more abundant than ordinary matter.

Ordinary matter obeys four well-identified forces: gravity, electromagnetism, and the strong and weak forces at the atomic level. But what about dark matter? Invisible and elusive, it could be subject to the same laws or governed by a fifth, as yet unknown force.

Dark matter falls into gravitational wells in the same way as ordinary matter, thus obeying Euler's equations.

International research collaboration finds solar gamma rays could unlock the mystery of the Sun’s hidden magnetic fields

AIA Image 193 from Solar Dynamics Observatory (SDO)
Compiled from 97 still images.
Video Credit: Scientific Frontline

New research conducted by an international team of physicists has found that high-energy gamma rays might offer the key to unlocking the mysteries of the Sun’s magnetic fields.

The study, led by the Chinese University of Hong Kong, the University of Exeter and the University of Amsterdam, concludes that teraelectronvolt (TeV) gamma rays, observable from specialist facilities on Earth, could be the result of this magnetic field interacting with cosmic rays.

By studying these TeV rays, say the researchers, it could be possible to identify where the fields are located, with their initial findings suggesting they are just beneath the solar surface.

“Magnetic activity of the Sun is the driver behind the space weather and as a consequence the effects space weather has on our society,” says Professor Andrew Hillier, one of the authors of the paper at Exeter. “However, it is not possible to see beneath the solar surface to investigate the Sun’s magnetic field before they manifest on that surface. Our study provides a new method by using cosmic rays to peer beneath the solar surface.

Coronal mass ejections at the dawn of the solar system

Artist's depiction of a coronal mass ejection from EK Draconis. The hotter and faster ejection is shown in blue, while the cooler and slower ejection is shown in red.
Image Credit: National Astronomical Observatory of Japan

Down here on Earth we don't usually notice, but the Sun is frequently ejecting huge masses of plasma into space. These are called coronal mass ejections (CMEs). They often occur together with sudden brightenings called flares, and sometimes extend far enough to disturb Earth's magnetosphere, generating space weather phenomena including auroras or geomagnetic storms, and even damaging power grids on occasion.

Scientists believe that when the Sun and the Earth were young, the Sun was so active that these CMEs may have even affected the emergence and evolution of life on the Earth. In fact, previous studies have revealed that young Sun-like stars, proxies of our Sun in its youth, frequently produce powerful flares that far exceed the largest solar flares in modern history.

The key to why the universe exists may lie in an 1800s knot idea science once dismissed

The model suggests a brief “knot-dominated era,” when these tangled energy fields outweighed everything else, a scenario that could be probed through gravitational-wave signals.
Image Credit: Courtesy of Muneto Nitta/Hiroshima University

In 1867, Lord Kelvin imagined atoms as knots in the aether. The idea was soon disproven. Atoms turned out to be something else entirely. But his discarded vision may yet hold the key to why the universe exists.

Now, for the first time, Japanese physicists have shown that knots can arise in a realistic particle physics framework, one that also tackles deep puzzles such as neutrino masses, dark matter, and the strong CP problem. Their findings, in Physical Review Letters, suggest these “cosmic knots” could have formed and briefly dominated in the turbulent newborn universe, collapsing in ways that favored matter over antimatter and leaving behind a unique hum in spacetime that future detectors could listen for—a rarity for a physics mystery that’s notoriously hard to probe.

“This study addresses one of the most fundamental mysteries in physics: why our Universe is made of matter and not antimatter,” said study corresponding author Muneto Nitta, professor (special appointment) at Hiroshima University’s International Institute for Sustainability with Knotted Chiral Meta Matter (WPI-SKCM2) in Japan.

“This question is important because it touches directly on why stars, galaxies, and we ourselves exist at all.”

The tides are changing for white dwarfs

Impression of the 6.9 minute double white dwarf binary J1539+5027, composed of a tidally heated white dwarf (yellow) and its more compact companion (blue). It is about to start mass transferring.
 Image Credit: KyotoU / Lucy McNeill

White dwarfs are the compact remnants of stars that have stopped nuclear burning, a fate that will eventually befall our sun. These extremely dense objects are degenerate stars because their structure is counterintuitive: the heavier they are, the smaller they are.

White dwarfs often form binary systems, in which two stars orbit one another. The majority of these are ancient even by galactic standards, and have cooled to surface temperatures of about 4,000 degrees Kelvin. However, recent studies have revealed a class of short period binary systems in which the stars orbit each other faster than once per hour. Contrary to theoretical models, these stars are inflated to twice the size as expected due to surface temperatures of 10 to 30 thousand degrees Kelvin.

We need a solar sail probe to detect space tornadoes earlier, more accurately

An artist’s rendering of the spacecraft in the SWIFT constellation stationed in a triangular pyramid formation between the sun and Earth. A solar sail allows the spacecraft at the pyramid’s tip to hold station beyond L1 without conventional fuel.
Image Credit: Steve Alvey, University of Michigan.

Spirals of solar wind can spin off larger solar eruptions and disrupt Earth’s magnetic field, yet they are too difficult to detect with our current single-location warning system, according to a new study from the University of Michigan.

But a constellation of spacecraft, including one that sails on sunlight, could help find the tornado-like features in time to protect equipment on Earth and in orbit.

The study results come from computer simulations of a massive cloud of plasma erupting from the sun and moving through the solar system. Because the simulation covers features that span distances three times Earth’s diameter down to thousands of miles, the researchers could determine how smaller, tornado-like spirals of plasma and magnetic field—called flux ropes—become concerning features in their own right.

How Black Holes Produce Powerful Relativistic Jets

A chain of plasmoids is created on the equatorial plane along the current sheet, where the particle density (left part) is higher. Here, magnetic reconnection takes place, accelerating particles to very high energies (right). Particles also reach relativistic speeds along the spin axis and eventually form the jet powered by the Blandford–Znajek mechanism. Gray: Magnetic field lines.
Image Credit: Meringolo, Camilloni, Rezzolla (2025)

A hundred years before the Event Horizon Telescope Collaboration released the first image of a black hole in 2019 – located at the heart of the galaxy M87 – astronomer Heber Curtis had already discovered a strange jet protruding from the galaxy’s center. Today, we know this to be the jet of the black hole M87*. Such jets are also emitted by other black holes. Theoretical astrophysicists at Goethe University have now developed a numerical code to describe with high mathematical precision how black holes transform their rotational energy into such ultra-fast jets.

For nearly two centuries, it was unclear that the bright spot in the constellation Virgo, which Charles Messier had described in 1781 as “87: Nebula without stars,” was in fact a very large galaxy. As a result, there was initially no explanation for the strange jet discovered in 1918 emerging from the center of this “nebula.”

At the heart of the giant galaxy M87 lies the black hole M87*, which contains a staggering six and a half billion solar masses and spins rapidly on its axis. Using the energy from this rotation, M87* powers a particle jet expelled at nearly the speed of light, stretching across an immense 5,000 light-years. Such jets are also generated by other rotating black holes. They contribute to disperse energy and matter throughout the universe and can influence the evolution of entire galaxies.

Mixing neutrinos of colliding neutron stars changes how merger unfolds

New simulations of neutron star mergers reveal that the mixing and changing of tiny particles called neutrinos impacts how the merger unfolds, including the composition and structure of the merger remnant as well as the resulting emissions. This image depicts the density of neutrinos within the remnant as varying textures, and the colors represent energy densities of different neutrino flavors.
 Image Credit: Provided by the Radice research group / Pennsylvania State University
(CC BY-NC-ND 4.0)

The collision and merger of two neutron stars — the incredibly dense remnants of collapsed stars — are some of the most energetic events in the universe, producing a variety of signals that can be observed on Earth. New simulations of neutron star mergers by a team from Penn State and the University of Tennessee Knoxville reveal that the mixing and changing of tiny particles called neutrinos that can travel astronomical distances undisturbed impacts how the merger unfolds, as well as the resulting emissions. The findings have implications for longstanding questions about the origins of metals and rare earth elements as well as understanding physics in extreme environments, the researchers said.

The paper, published in the journal Physical Review Letters, is the first to simulate the transformation of neutrino “flavors” in neutron star mergers. Neutrinos are fundamental particles that interact weakly with other matter, and come in three flavors, named for the other particles they associate with: electron, muon and tau. Under specific conditions, including the inside of a neutron star, neutrinos can theoretically change flavors, which can change the types of particles with which they interact.