As Earth has completed another revolution around the Sun, we are gearing up for some exciting discoveries that will take place in 2022. But before prepping for them, let’s have a look at the top 10 discoveries in Astronomy and Physics that took place in 2021.

Neutron Star – Black Hole merger

Whenever two massive objects collide, they send out ripples in the fabric of spacetime. These ripples are called the gravitational waves, and they serve as the signature of the merger that has occurred. Over the years, researchers have observed dozens of mergers of pairs of black holes and a couple of mergers of neutron star pairs. But for the first time in June 2021, scientists announced the detection of a colossal merger of a black hole and a neutron star taking place about 900 million light-years away from Earth, and that too, twice. 

From Wormholes to the Fifth Force: 2021's Biggest Breakthroughs In Space and Physics 1
An illustration depicts the warping of time and space as a black hole is about to swallow a neutron star | Image: Soheb Mandhai/LIGO India/AP

The first merger involved a black hole about nine times the mass of the Sun and a neutron star about two solar masses. Ten days later, the second collision was detected involving a black hole nearly six times the Sun’s mass, devouring a neutron star of 1.5 solar masses. The observations further predicted that a merger between a black hole and a neutron star occurs once every month within one billion light-years of Earth.

Although the collision observations have opened up many new doors of impending discoveries, LIGO and Virgo are not enough to entangle all the cosmic mysteries. So keeping this in mind, twin LIGO detectors, Virgo and KAGRA, are all undergoing preparations for another set of observations scheduled to begin next summer. 

Voyager 1’s cosmic hum

Voyager 1 feeds us interesting deep space information despite being the farthest spacecraft from Earth. And this is what it did in May 2021. Voyager 1 detected an unexpectedly steady Hum of Plasma waves in Interstellar Space. This hum was persistent, long-lasting, with a low frequency droning away at approximately 3 kHz.  

Plasma is a hot, diffused, and ionized gas composed of electrons that have been stripped away from their atoms, also known to be present in the interstellar medium. The movement of electrons in plasma leads to thermally excited plasma oscillations or quasi-thermal noise. Voyager 1 is well equipped to measure these plasma vibrations in the interstellar medium thanks to its inboard plasma wave system. 

Since 2012, Voyager 1 had detected about eight distinct plasma oscillation events, which ranged in length from a couple of days to a full year. These events were mainly caused by instabilities in the motions of electrons as they interacted with shockwaves generated by the Sun. However, in 2017, Voyager 1 began to detect a weak, steady, and persistent plasma signature outside of these energetic events. 

From Wormholes to the Fifth Force: 2021's Biggest Breakthroughs In Space and Physics 2
An artist’s depiction of a Voyager spacecraft in interstellar space. | Image credit: NASA/JPL-Caltech

The newly detected signal was narrower than the plasma oscillation events and held itself steady at about 3 kHz, and bandwidth restricted to 40 Hz. Moreover, this feeble signal persisted for nearly three years, the longest continuous plasma signal recorded by Voyager 1 so far. In the last three years, Voyager 1 traveled a distance of about ten astronomical units, which is roughly 930 million miles, but the signal remained the same. Since the measured signal was just above the noise threshold of the Voyager 1 Plasma Wave System instrument, researchers did not expect to find anything like it. 

Although the signal is quiet, it’s stronger than scientists previously thought, and its detection has pushed Voyager 1’s limits of what it was earlier thought capable of doing. Moreover, the persistence of the signal suggested that Voyager may continue to detect it in the future as well. 

Traversable Wormholes

Wormholes are portals in spacetime that serve as shortcuts for long interstellar travels. Having its origin in Einstein’s theory of general relativity, a wormhole is a highly curved region of spacetime that connects two extremely distant points in space, just like a tunnel. However, these exotic structures are theoretical and haven’t been directly observed in nature. Moreover, mathematics suggests that such wormholes would be extremely unstable, and if anything tries to pass through them, they would immediately collapse. Eventually, the matter passing through it would disappear, and the connection that the wormhole provided to other places in the universe would be cut off forever. 

Previous models suggested that the only way to keep a wormhole open is with an exotic form of matter with a negative mass. But in 2021, two different theories were proposed according to which time travel could actually be possible via wormholes and that too for normal matter, provided we follow some constraints. 

Wormhole
Wormhole

So, in the first approach published in March 2021, researchers chose a comparatively simple semiclassical approach. They combined the elements of relativity with quantum mechanics and classic electrodynamics. They found that if we included the Dirac equation into our mathematics, it would permit the existence of a wormhole traversable by matter such as electrons, provided the ratio between the electric charge and the mass of the wormhole exceeded a certain limit. According to this theory, even electromagnetic waves could also traverse the tiny tunnels in spacetime. 

Then, in the second approach published in September 2021, Scientists employed a tweaked form of gravity called generalized hybrid metric-Palatini gravity to make wormholes traversable. Although this theory is built on Einstein’s general theory of relativity, it allows more flexibility in relationships between matter and energy and space and time. It was found that layering the entrances to the wormholes with double thin shells of regular matter would make the wormhole traversable without the use of any exotic matter and negative energy.

Although both approaches are still only in papers, the researchers aim to test their theories experimentally in the future. And if proved true, these theories wouldn’t just make our sci-fi dreams come true but also challenge our current understanding of the universe.

Hawking’s Black Hole theorem proved correct

In 1971, Stephen Hawking gave one of his theorems according to which a black hole couldn’t decrease in size over time. This theorem, known as the black hole area theorem, works on a similar thermodynamic principle that entropy cannot decrease over time. After 41 years of being proposed, in July 2021, scientists finally proved Stephen Hawking’s black hole area law by analyzing the gravitational waves produced by two black holes 1.3 billion years ago. 

If Hawking’s area theorem holds, then the horizon area of the new black hole formed due to the merger should not have been smaller than the total horizon area of its parent black holes. So to conclude this, researchers split the gravitational wave data registered by LIGO into two categories: before and after the merger.  

Later, they used both the measurements to calculate the surface areas of the black holes in each category and found that the total surface area of the combined black hole was greater than the sum of the two smaller black holes. This means that the total event horizon area did not decrease after the merger. This result was reported with 95 percent confidence, thereby solidifying Hawking’s area law. 

However, despite being a breakthrough, this confirmation oddly contradicts another crucial theory of Hawking radiation, according to which black holes spontaneously emit thermal radiation over time. Keeping this in mind, the team aims to test future gravitational-wave signals to see if they might further confirm Hawking’s theorem or sow the seeds of new physics. 

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First Planet Beyond Milky Way

To date, astronomers have discovered nearly 4000 exoplanets, all confined within the boundaries of the Milky Way. But this time, the planet hunt went a notch higher when scientists announced the discovery of the first exoplanet in a galaxy beyond our own. This unnamed exoplanet lies in Messier 51, often called the Whirlpool Galaxy, and lies around 28 million light-years from Earth. Generally, exoplanets are detected via the transit method. In this, whenever a planet passes in front of its host star, there occurs a dip in the luminosity of that star. Then, by analyzing the characteristic dip, a planet orbiting that particular star is confirmed.

From Wormholes to the Fifth Force: 2021's Biggest Breakthroughs In Space and Physics 3
An illustration of M51 and a small companion galaxy | Image: Credits: NASA, ESA, S. Beckwith (STScI) and the Hubble Heritage Team (STScI/AURA)

But unlike optical light transits where a relatively small planet only blocks a tiny amount of starlight, the area where X-rays are produced is tiny enough that even a planet can block a significant portion of the X-ray light. This means that for systems lying far away, it’s easier to search for X-ray transits, and this is what researchers took advantage of. First, they used NASA’s Chandra X-ray Observatory telescope and looked for X-ray dips. Doing so, they found that for 3 hours, the X-rays typically emanating from the system in the Whirlpool galaxy dropped to zero.

This hinted at a Saturn-sized exoplanet orbiting a compact object at some 19.2 astronomical units. That’s about twice as far as Saturn is from the Sun. Although it’s a very interesting find, there can be other possible reasons for the blockage of X-rays as well. And if a planet is seriously present there, it won’t cross in front of its host star for another 70 years. This means that astronomers need at least 70 years to confirm the presence of their first extragalactic world.

Invisible matter

Achieving invisibility is like a fantasy for all of us. But in 2021, technology moved a step ahead in making this fantasy a reality when physicists demonstrated a quantum effect that turns matter invisible. This quantum effect, called Pauli Blocking, is based on Pauli’s exclusion principle and was proposed around three decades ago. However, it is the first time it has practically been demonstrated in a lab.

According to Pauli’s exclusion principle, no two electrons, having an identical set of all the quantum numbers, can occupy the same quantum state within a quantum system simultaneously. It is this exclusion principle that keeps atoms stable. Because without it, all atoms would collapse together while releasing an enormous amount of energy.

Now, at the spooky quantum level, only a finite number of energy states exist, where electrons can stack themselves up. Several unoccupied energy levels are available, so when an atom is bombarded with photons, the electrons absorb energy and jump to higher levels. When they again come back to a stable lower energy state, they emit the extra energy, and in this way, the incoming light is scattered at different angles, and we see an object.

But things become different at lower temperatures. As the temperature of an atom is directly related to its energy, when a cloud of atoms is cooled down, they lose energy. Doing so, they fill all of the lowest available energy states and become so tightly packed that they cannot move up to higher energy levels or even drop down to lower ones. In other words, the electrons have nowhere to go.

So when light strikes an atom in such a state, the light photons have no chance to interact with the atoms, or more clearly, the electrons present inside. So, the incoming light simply passes straight through them without being scattered. And in this way, the atoms behave as if they weren’t even present in the path of light, eventually becoming invisible.

To realize this in a lab, researchers captured lithium gas inside an atomic trap and cooled it down to 20 microkelvins with the help of laser cooling. Later, the atoms were squeezed to a density of roughly one quadrillion atoms per cubic centimeter by using a tightly focused laser. After bringing the gas cloud to such a cool and compressed state, the team shone a third and final laser beam towards the gaseous cloud and used a hypersensitive camera to count the number of photons scattered from the final beam.

Surprisingly, it was found that the cooled and squeezed atoms scattered only 62% of light than those at room temperature, as predicted by theory. This made them significantly dimmer, which means that if we can squeeze atoms more precisely to much lower temperatures, we can definitely make matter invisible.

The same process was performed for two other gases: potassium and strontium, in other labs and they, also showed promising results. Although the enhanced applications of this effect seem to be theoretical at present, such experiments have strengthened our hopes of achieving invisibility in the future, although at quantum scales to begin with.

Matter from energy

In 1905, Albert Einstein stated that mass and energy are the same things. He came up with his revolutionary equation, E=mc2, which showed how mass and energy are inter-convertible and connected. In everyday life, we come across many examples where mass gets converted into energy. For instance, every passing second, the Sun converts 4.26 million metric tons of its mass to produce the equivalent of 384.6 septillion watts of energy. This is the same energy that fuels our life cycle and other processes on Earth.

Although we have witnessed real-world examples beautifully portraying the conversion of mass to energy for decades, for the first time in August 2021, scientists observed the reverse process by creating matter from pure energy in a stunning demonstration at the Brookhaven National Laboratory.

According to the Breit Wheeler process, if we smash two sufficiently energetic photons into each other, then we should be able to create matter in the form of a particle and its antiparticle. Unfortunately, although this process has been doing rounds in theory since 1934, it has been one of the most difficult to demonstrate experimentally. For photons to produce matter, the colliding photons need to be highly energetic gamma rays. And to produce such energetic gamma-ray photons, we need gamma-ray lasers. But to date, we haven’t been able to produce such powerful lasers. So this time, the researchers went with an alternative.

Instead of accelerating energetic photons directly, they sped up two positively charged ions in a big loop and sent them past each other in a near collision. These ions are charged particles that move very close to the speed of light and also carry an electromagnetic field associated with them. Now, inside this electromagnetic field, there lies a bunch of not-so-real but ‘virtual’ photons that travel with the ion like a cloud.

These particles pop into existence only briefly as long as the disturbances in the fields exist between real particles. And yes, they don’t have the same masses as their real counterparts. So unlike the real photons that have no rest mass, virtual photons do have a mass. So in the experiment, when the ions zipped past each other in a near-miss, their clouds of virtual photons moved so fast that they acted as if they were real. Eventually, the real-acting virtual particles collided and produced a much-real electron-positron pair.

To make sure that the virtual photons actually behaved like the real ones, the team performed several tests, and the results were found to be consistent with what was expected from real photons. In this way, researchers successfully demonstrated the Breit-Wheeler process after decades of its proposed existence. Although the use of virtual photons raises some questions of whether this experiment was a true demonstration of the Breit-Wheeler process, still, it is an important first step until we have lasers that are powerful enough to demonstrate the process with real photons.

Largest spinning structures

In 2021, observations revealed cosmic filaments or gigantic tubes made of galaxies that spin at lofty speeds of about 223,700 mph. Although spinning is a very common phenomenon in our universe, it hasn’t been seen so far to exist at such a large scale, which makes this discovery so unique.

About 13.8 billion years ago, when the big bang took place,  most of the gas making up most of the known matter of the cosmos collapsed to form colossal sheets. These extremely large sheets then broke apart to form the filaments of a vast cosmic web, about hundreds of millions of light-years long, much bigger than the clusters of galaxies.

Since clusters spin at a snail’s pace at a cosmic scale, the spinning of such large structures was completely unexpected. So scientists examined more than 17,000 filaments as a part of the Sloan Digital Sky Survey and analyzed the velocity at which the galaxies making up these giant tubes were moving within each tendril.

Surprisingly, the movement of these galaxies suggested that they were rotating around the central axis of each filament. However, one thing is clear that the Big Bang did not endow the universe with any primordial spin. The powerful gravitational fields of these filaments probably pulled gas, dust, and other material within, and the structures started collapsing inwards. This gravitational collapse resulted in shearing forces that probably imparted a spin to the colossal structures.

Still, the exact cause behind the spinning of these giant tendrils of galaxies is not very clear, and scientists are now seeking to understand the origin of spin in filaments through computer simulations.

Water beyond the Milky Way

For the first time in November 2021, astronomers found water in a pair of one of our universe’s most distant and oldest galaxies. Collectively called SPT0311-58, these formed just 780 million years after the big bang and lay so far that even their light takes 12.8 billion years to reach us.

From Wormholes to the Fifth Force: 2021's Biggest Breakthroughs In Space and Physics 4
A composite image showing the two galaxies of SPT0311-58. These galaxies(red) are shown over a background from the Hubble Space Telescope (blue and green) | Image: B. Saxton (NRAO/AUI/NSF), HST/WFC3, HST/ACS

The Atacama Large Millimeter/submillimeter Array (ALMA), which is a powerful radio telescope located in northern Chile in the Atacama Desert made high-resolution observations of molecular gas in this pair of galaxies. The spectral observations hinted at the presence of water molecules in both galaxies. And, in addition to water, ALMA also detected carbon monoxide in the larger of the two galaxies. 

SPT0311-58 is the most massive currently known galaxy of this ancient age. Besides that, its two constituent galaxies appear to be merging. As a result, it has more gas and dust than galaxies, which are closer and more mature. Although it’s still a mystery how such a large amount of gas and dust assembled in the young universe to form the first stars and early galaxies, studying the gas content of such galaxies can reveal exciting information about their properties. 

It can tell us how many stars are being formed in a galaxy, the rate at which gas is getting converted into starsand how the galaxies interact with each other and the interstellar medium. It has been the most distant discoveries of water made in the universe so far. As such molecules are fundamental to the existence of life on Earth, their observation can provide insights into the fundamental processes in the early universe, along with shedding light on how the universe evolved.

Fifth fundamental force

To the best of our knowledge, everything we see around is governed by four fundamental forces: Gravity, electromagnetic force, the strong nuclear force, and the weak nuclear force. The theory that combines all these forces except gravity is the Standard Model of Physics. Although experiments conducted over several decades have verified the Standard Model, it is not complete. For example, it does not unify gravity with the other three forces and does not explain dark matter and dark energy that make up 96% of the observable universe. That’s why physicists have been looking for a fifth force for decades, and one place to find them is in the decay of bottom or the beauty quarks.

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It’s an unstable particle and lives for just 1.5 trillionths of a second before decaying into other particles. When a beauty quark decays, it transforms into a set of lighter particles, such as electrons, through the influence of the weak force. So one of the ways a new force of nature might make itself known to us is by subtly changing how often beauty quarks decay into different types of particles.

According to the Standard Model, a beauty quark should not discriminate between an electron and a muon while decaying. A muon is a carbon copy of an electron, except it is 200 times heavier. So the rate at which the beauty quark decays into a muon must be equal to the electron. But in March 2021, the researchers found that the muon decay was only happening about 85 percent as often as the electron decay. So nature seems to be preferring one decay channel over the other, which is a violation of the law of lepton universality. 

Physicists believe an unknown force must be breaking the law of lepton universality. We might be on the brink of a significant breakthrough in physics. But more data is required to confirm the existence of this new force.

Learn Astrophysics at home

Did you always want to learn how the universe works? Then, read our 30-article Basics of Astrophysics series absolutely free of cost. From the popular topics such as stars, galaxies, and black holes to the detailed concepts of the subject like the concept of magnitude, the Hertzsprung Russell diagram, redshift, etc., there is something for everyone in this series. All the articles are given here. Happy reading!

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