For the very first time, scientists have managed to detect the continuous echo of gravitational waves passing through our universe.
After diligently collecting data for 15 years in a galaxy-spanning experiment, a dedicated research team has achieved something truly extraordinary. Surprisingly, this cosmic chorus is even louder than they anticipated.
This monumental achievement was brought to fruition by the North American Nanohertz Observatory for Gravitational Waves, or NANOGrav.
They’ve been keeping a keen eye on specific stars, called pulsars. They observe pulsars because they maintain a consistent rhythm, like cosmic metronomes.
These gravitational waves that NANOGrav has recently detected are immensely powerful. In fact, they hold approximately a million times more energy than the individual gravitational wave bursts measured in the past.
NANOGrav’s findings are detailed in a series of papers published today in The Astrophysical Journal Letters.
Their findings suggest these massive gravitational waves are likely generated by pairs of supermassive black holes. These massive objects were spiraling towards each other in cataclysmic cosmic collisions.
NANOGrav scientist Chiara Mingarelli contributed significantly to these discoveries. She describes the phenomenon as a choir.
Mingarelli says, “It’s like a choir, with all these supermassive black hole pairs chiming in at different frequencies.” She likens this to the first-ever evidence for the gravitational wave background. The discovery will essentially open a new portal through which we can observe the universe.
The discovery and subsequent understanding of this gravitational wave background presents a wealth of new knowledge. This could answer longstanding questions, ranging from the fate of supermassive black hole pairs to the frequency of galaxy mergers.
Currently, NANOGrav can only measure the overall gravitational wave background rather than radiation from each individual ‘singer’. Even so, the discoveries have been revealing.
According to Mingarelli, now an assistant professor at Yale University, “The gravitational wave background is about twice as loud as what I expected.”
The high volume could be due to experimental limitations. Or, perhaps, the presence of more abundant, heavier supermassive black holes. It could also hint at another source generating these powerful waves.
The potential causes could be vast. Theories posited range from phenomena predicted by string theory, to alternative theories regarding the birth of our universe.
Excitedly, Mingarelli notes, “What’s next is everything,” adding, “This is just the beginning.”
Reaching this point was no easy task for the NANOGrav team. The gravitational waves they were pursuing differ significantly from those measured before.
Unlike the high-frequency waves detected by terrestrial instruments such as LIGO and Virgo, the gravitational wave background is composed of ultra-low-frequency waves.
A single rise and fall of these waves could take years, even decades, to pass by. Since gravitational waves travel at light speed, a single wavelength could span tens of light-years.
Given their enormity, no earthly experiment could detect such massive waves. The NANOGrav team had to turn to the stars for answers. They scrutinized pulsars, remnants of massive stars that have gone supernova.
Pulsars function like cosmic lighthouses, emitting beams of radio waves from their magnetic poles. As they spin rapidly, these beams sweep across the sky. This constant motion creates rhythmic pulses of radio waves that appear perfectly timed from our earthly perspective.
When a gravitational wave passes between us and a pulsar, it alters the timing of the radio waves due to the way gravitational waves compress and stretch space, as predicted by Albert Einstein. This changes the distance that the radio waves have to travel.
For 15 years, NANOGrav scientists from the United States and Canada closely monitored the radio wave pulses from dozens of millisecond pulsars in our galaxy using multiple observatories. The results are the culmination of an in-depth analysis of an array of 67 pulsars.
“Pulsars are actually very faint radio sources, so we require thousands of hours a year on the world’s largest telescopes to carry out this experiment,” Maura McLaughlin of West Virginia University, co-director of the NANOGrav Physics Frontiers Center, explains.
By 2020, with over 12 years of data, the NANOGrav team began to detect hints of a signal. It was an extra “hum” common to all the pulsars in the array. Now, after three more years of additional observations, they have solid evidence of the gravitational wave background.
Sarah Vigeland of the University of Wisconsin-Milwaukee, chair of the NANOGrav detection working group, shares, “Now that we have evidence for gravitational waves, the next step is to use our observations to study the sources producing this hum.”
The gravitational wave background is most likely created by pairs of supermassive black holes caught in a deadly dance of mutual orbit. When two galaxies merge, their central supermassive black holes meet.
They then begin to orbit one another, and over time, their orbits tighten. However, when they get close enough, they emit energy as powerful gravitational waves until they finally collide in a cataclysmic finale.
Mingarelli, postdoctoral researcher Deborah C. Good, and their colleagues estimate that there could be hundreds of thousands, if not a million or more, supermassive black hole pairs in the universe.
However, it’s possible that not all gravitational waves detected by NANOGrav come from these pairs. Other theories also predict waves in the ultra-low-frequency range.
These include the one-dimensional defects called cosmic strings from string theory. Also, waves resulting from a ‘Big Bounce’ rather than a Big Bang.
“We can’t walk over to the pulsars and turn them on and off again to see if there’s a bug,” Mingarelli jokes, highlighting the potential for unknown variability that might be influencing NANOGrav’s results.
As the NANOGrav team continues to monitor the pulsars, they plan to explore all possible sources of the gravitational wave background.
There’s comfort in knowing they’re not alone in this endeavor. Various groups using telescopes around the world have reported signs of the same gravitational wave background signal in their data.
Stephen Taylor of Vanderbilt University, who co-led the new research and currently chairs the NANOGrav collaboration, says, “Our combined data will be much more powerful.”
He excitedly adds, “We’re excited to discover what secrets they will reveal about our universe.”
Pulsars are a type of celestial object that are both fascinating and highly significant to scientists. Here’s a breakdown of what we know about pulsars.
Pulsars are highly magnetized, rotating neutron stars that emit beams of electromagnetic radiation out of their magnetic poles. This radiation can only be observed when the beam of emission is pointing toward Earth.
It is also responsible for the pulsed appearance of emission. Neutron stars are the extremely dense remnants left behind after a supernova explosion.
The first pulsar was discovered in 1967 by Jocelyn Bell Burnell and Antony Hewish. The regular pattern of pulses was so precise that the team initially named the signal LGM-1, for “Little Green Men.”
They considered the possibility that it could be a signal from an extraterrestrial civilization. However, further discoveries of similar objects quickly dispelled that idea.
Pulsars rotate rapidly, with the fastest known pulsar rotating 716 times per second. The rotational period is incredibly stable.
This makes pulsars some of the most accurate clocks in the universe. This precise timing of the pulses of radiation is what led to their initial discovery.
There are two main types of pulsars: rotation-powered pulsars and accretion-powered pulsars.
Rotation-powered pulsars are the most common type and emit radiation powered by the loss of rotational energy.
Accretion-powered pulsars, on the other hand, are part of binary star systems and draw in material from a companion star. The companion star powers their emission.
A subset of pulsars is known as magnetars. These are pulsars with extremely powerful magnetic fields. These fields measure around a thousand times stronger than those of typical pulsars.
They are also associated with starquakes. These are seismic activities within the star, which release tremendous amounts of energy.
These are pulsars with very short rotational periods, in the millisecond range. These are thought to be old, spun-up pulsars that have been rejuvenated by accreting matter from a binary companion.
Pulsars have been used in various scientific applications. For instance, the timing precision of millisecond pulsars is used in tests of general relativity.
In addition, the spacecraft Voyager 1 includes a pulsar map intended to guide aliens to Earth. The existence of gravitational waves was also indirectly confirmed through the observation of a binary pulsar system.
Pulsars are not generally expected to have planets due to the extreme conditions that accompany their formation. Surprisingly, however, the first exoplanets ever discovered were found orbiting a pulsar named PSR B1257+12. This pulsar has three known planets, all with masses similar to Earth’s.
Pulsars provide a unique opportunity to study the laws of physics under extreme conditions, and they continue to be the subject of ongoing scientific research.
Supermassive black holes are fascinating and enigmatic cosmic entities. Here’s everything we know about them:
Supermassive black holes (SMBHs) are the largest type of black hole. They contain a mass between a million and several billion times that of our Sun.
Black holes are regions of space where gravity is so strong that nothing, not even light, can escape their pull once it has crossed the event horizon. The event horizon is the boundary marking the point of no return.
Supermassive black holes reside at the center of nearly all known large galaxies, including our own Milky Way. The one in the Milky Way is named Sagittarius A* (pronounced as Sagittarius A-star) and is about 4 million times the mass of the Sun.
There are several theories about how supermassive black holes form, but there isn’t a definitive answer. Some scientists believe they could have formed from the collapse of very large, primordial gas clouds in the early universe.
Another theory is that they began as smaller black holes that grew over time by consuming matter and merging with other black holes.
Material that comes too close to a supermassive black hole can form a spinning disk around it. This is called an accretion disk. This material heats up due to the immense gravitational pull. It then emits intense radiation, which can be observed with telescopes.
In some instances, SMBHs can also produce powerful jets of energetic particles. These particles are expelled out into space at nearly the speed of light.
When a supermassive black hole is actively feeding, the region around it can outshine the rest of the galaxy. These bright, energetic centers are known as active galactic nuclei, or AGNs. The most luminous AGNs are known as quasars.
When two supermassive black holes merge, they produce ripples in the fabric of spacetime known as gravitational waves. The detection of these waves allows astronomers to study these cataclysmic events and further understand the nature of gravity.
In April 2019, the EHT Collaboration presented the first direct image of a supermassive black hole and its shadow. It is located at the center of the galaxy M87. This marked a significant achievement in the field of astrophysics.
Researchers believe that supermassive black holes play a crucial role in the formation and evolution of galaxies. Their powerful gravitational influence and the energy released from the material falling into them can impact star formation processes in their host galaxies.
Despite our current understanding, supermassive black holes remain one of the most mysterious objects in the universe. There’s still much to learn about their characteristics and influence on cosmic structures.