It's hard to believe, but less than 10 years ago, humanity was still unsure whether gravitational waves were real. Of course, we have seen evidence that massive objects in close orbit are exhibiting orbital decay, but we have not yet directly detected the energy radiated from them: theoretically in the form of gravitational waves. On September 14, 2015, advanced Ligo detected the first gravitational wave signal from two merged black holes. Since then, Ligo has been upgraded to include additional gravitational-wave detectors and has witnessed more than 100 merger and merger candidates, detecting low-mass neutron stars and black holes with masses up to 100 times the mass of the Sun.
Then, last year, in 2023, a new method of detecting gravitational waves achieved its first success. By using a series of natural astronomical clocks - millisecond pulsars - astronomers were able to discover the first evidence of a cosmic low-frequency gravitational wave background. The ultimate goal of gravitational-wave astronomy is ambitious: a complete census and understanding of populations of black holes and neutron stars, as well as the rate of spiral and merging events throughout the history of the universe. It is worth noting that there are four leading edges that are being actively pushed back by advances in gravitational waves. Here's what's coming to improve our understanding of the universe.
Three different sets of gravitational wave methods, a ground-based laser interferometer, a space-based laser interferometer, and a pulsar timing array, are all sensitive to different classes of gravitational wave signals. While LIGO was the first collaboration to detect gravitational waves at very high frequencies, the nanogr** collaboration has seen strong evidence at very low (nanohertz) frequencies.
1.The largest black hole.
One of the most striking aspects of gravitational waves is that they are produced as long as you have them:
One mass, in accelerated motion, passes through a curved area of space, caused by the presence of another mass.
This applies to almost all gravitational systems, from the Earth-Moon system to our solar system, to the stars in the Milky Way and all galaxies in a galaxy group or galaxy cluster. However, if you want the strongest gravitational wave signal, you'll look for the maximum mass at the orbital distance that is likely to be closest to each other.
This means that the gravitational waves exhibiting the greatest amplitude will come from the most massive orbital system, where two masses reach the closest distance to each other: from double supermassive or supermassive black holes, especially at the moment before their two event horizons come into contact with each other. The combined effect of all supermassive and supermassive black holes in the entire universe is expected and the most likely culprit, with the timing detection of gravitational waves in the background by pulsars. With the gradual improvements already underway, it is entirely expected that the first individual systems – the closest to orbit and the most massive – will soon emerge from these data.
As the number of accurately observed millisecond pulsars and the observation time per pulsar increased, so did the signal-to-noise ratio seen by the NanoGR** collaboration. As these numbers continue to rise, we will soon cross the "** criterion" of importance to be able to describe the nature of this background "buzz" of our universe.
If you only have a millisecond pulsar, you can measure its time perfectly, and you will measure the combined effect of all the gravitational waves that pass through it, shortening and lengthening the distance of light from the pulsar to our telescope in a regular and admissible way. Although individual pulsars (and the telescopes and arrays that monitor them) have inherent errors and uncertainties, a large array of pulsars is much better. Especially if your system is relatively stable over a long timescale (which is expected for the largest systems), these gravitational wave signals will wash all of your pulsars at the same time, allowing you to pick out a single signal that is strong enough.
If there are two black holes, each with a mass of one billion solar masses or more, orbiting each other at very close distances almost anywhere in the observable universe (they may merge in the next few million years), pulsar timing measurements should be able to pick out these individual objects sometime in the next decade or two. It's a fast-growing field, and with the combination of existing facilities and upcoming new observatories – such as the proposed N**LA (Next Generation Very Large Array) – these supermassive black hole binaries, which should result in the largest black holes in the entire universe, will soon be revealed.
This animation shows a lower mass black hole passing through the accretion disk created around a larger, supermassive black hole. A flare occurs when a smaller black hole passes through the disk. On a long enough time scale, these black holes will spiral and merge, generating huge gravitational wave signals in the process.
2.Supermassive black holes and extreme mass ratios merge.
Although supermassive black holes in the universe – the most massive of all black holes, in billions to tens of billions of solar masses – may be the most impressive, they do not represent most supermassive black holes. At the center of almost all known galaxies, there are relatively small supermassive black holes: with millions, tens of millions, or hundreds of millions of solar masses, such as the black hole at the center of the Milky Way, the Andromeda Galaxy, and most known large galaxies. However, due to their lower mass, pulsars may be a long way from being sensitive to their presence, even if they are in a binary system with another orbiting black hole companion.
At the same time, we cannot expect to detect these objects with terrestrial probes, because objects orbiting a million-solar mass black hole emit gravitational waves with a characteristic period of about 100 to 1000 seconds, while Ligo and other terrestrial probes can only detect gravitational waves with periods between milliseconds and tenths of a second. Fortunately, there is a solution that is not just theoretical, but is being designed and built to fly in just a few years: a space-based gravitational wave detector with laser arms at great distances between them. The version that is about to fly is called Lisa: laser interferometer space antenna.
In space, three equidistant detectors are connected by laser arms, and periodic changes in their separation distances can reveal the passage of gravitational waves of the appropriate wavelength. LISA will be the first human probe capable of detecting the ripples of space-time of supermassive black holes and objects falling into them, ranging from high-quality binary black hole companions to low-mass objects such as stellar-mass black holes, and even neutron stars, which may also be white dwarfs.
As these three spacecraft fly through space, the distance between them changes over time in a way that can be done, but gravitational wave signals are superimposed on them as ripples of space-time. For the first time, black holes with a larger mass than ground-based probes are sensitive (possibly up to a few hundred solar masses), but less massive than pulsars' time-sensitive billion-solar masses (and beyond), and these behemoths will fall into our range. Billions of light-years away, Lisa will be sensitive to:
Binary black holes (of comparable mass), weighing from about 10,000 to tens of millions of solar masses, with extreme masses than spirals, with only 1-100 solar masses of objects falling into typical supermassive black holes, ligo and other terrestrial probes are sensitive to the type of black hole, except in the early stages of the spiral phase, allowing us to **stellar mass binary black hole merger occurs (and most likely in**).
It's important to remember that although Ligo has detected a large number of black holes and is increasingly sensitive to black holes and neutron stars at greater distances, the largest black hole it has ever seen is only 100 solar masses. There is still a very important group of black holes at the center of galaxies, which are largely detected indirectly by the luminescent radiation emitted by surrounding objects. Seeing them in gravitational waves will, and most likely, open a whole new window into the universe.
When the two arms of an optical interferometer are exactly equal in length and no gravitational waves pass through, the signal is zero and the interference pattern is constant. As the arm length changes, the signal is real and oscillating, and the interference pattern changes over time in a way that can be used. This technique is used to directly reveal the presence of gravitational waves. NASA's Space Square.
3.Intermediate-mass black holes and black holes created by transcendental stars.
Even if Lisa is operating in space and the advanced Ligo (plus Virgo, Kagra, and Ligo India) is operating on the ground, there is still a gap between the highest-mass black hole that a ground-based probe can see, with an upper limit of about 200 solar masses, and the lowest-mass black hole that a space probe can see, as low as about 10,000 solar masses.
And what about populations in this middle range: even the most massive stars can be generated, but in relatively small quantities?
There is a way to achieve this: to build an ultra-advanced ground-based gravitational wave detector that is about ten times the size of the Ligo detector that currently exists. Keep in mind that today's terrestrial gravitational wave detectors work by:
Splitting the light, shooting down a long, straight, vertical arm with mirrors at both ends, reflecting the light back and forth about 1,000 times, before putting that beam back together, and observing how it changes over time in the interference pattern, therefore, we can extract any gravitational wave signal of the correct frequency to shift the interference pattern.
This photo, taken in Ligo Livingston, Louisiana, shows a "down" view of one of two vertical 4-kilometer arms, through which the laser light is reflected multiple times and gathers together to form an interference pattern sensitive to the presence of gravitational waves. Using a detector with a 10x arm length, we are sensitive to the need for a 10x longer cycle, allowing us to fill the gap between Ligo and Lisa sensitivity. **e. siegel
Ligo has a 4-kilometer arm and is sensitive to gravitational waves with periods of about 1 second or less. But if it has a 40-kilometer arm, it can detect gravitational waves that take longer to produce: black holes with a larger event horizon and more massive than the current generation of gravitational-wave detectors can see.
Of course, there are many challenges to building such a large gravitational-wave observatory. You have to consider:
In fact, the arms must be completely straight, requiring either tunnels to be built underground, or large support structures to be built on the ground, taking into account the curvature of the earth, literally, ten times the amount of land, since it is challenging to find a suitable "L" shaped piece of land, available in two verticals of 40 km, and in the harsh political climate of the 21st century, the willingness to publicly fund such large-scale projects is constantly threatened, among other problems. But these are not basic science issues. Advances in compressed quantum states of lasers, the reduction of the noise floor of current gravitational wave detectors, and advances in processing large amounts of data and quickly and automatically searching for candidate events are all helping to provide the scientific basis for the next generation of terrestrial gravitational wave detectors. If we build them, we will eventually be able to understand how black hole mergers allow them to grow from stellar masses to supermassive masses.
We know that when two neutron stars merge, as simulated here, they can produce jets of gamma-ray bursts, among other electromagnetic phenomena. But perhaps, above a certain mass threshold, a black hole is formed, two stars collide on the second panel, and then all the extra matter and energy is gained, and there is no escape signal. Determining the mass boundary that can be formed between a neutron star and a black hole is one of the goals of modern gravitational-wave astronomy.
4.The lightest black hole and the so-called "mass gap".
There is also a very big question that still puzzles the best astrophysicists studying the universe today: how did these objects form when the line between the heaviest neutron star and the lightest black hole is in? We already know a few things.
We have seen black holes with masses as low as 3 solar masses, and neutron stars as heavy as 2 solar masses.
Observationally, we know that if two neutron stars with a sufficiently low total mass merge together, they will produce a neutron star; If their total mass is high enough, they create a black hole.
From the closest gravitational wave event ever recorded, the neutron-neutron star merger in 2017, is only 1At 300 million light-years, we can be fairly certain that we have seen neutron star-neutron star merger briefly turn into a neutron star in a fraction of a second, and then collapse into a black hole.
The common belief is that both the heaviest neutron stars and the lightest black holes are produced by neutron-neutron star mergers, and more recently, pulsar timing science has just revealed that a particular pulsar, PSR J0514-4002E, has a double companion star, which is a dense object with a mass of 209 to 2Between 71 solar masses, making it the heaviest known neutron star or the lightest black hole.
The most recent images of all black holes and neutron stars observed through electromagnetic and gravitational waves as of November 2021 (after the end of the third data run of LIGO, but before the start of the fourth data run). While these range from objects with a slightly greater solar mass, for the lightest neutron stars, to objects with a slightly greater solar mass, gravitational-wave astronomy is currently only sensitive to a very narrow set of objects for merged black holes. Until the discovery of Gaia BH1 in November 2022, the most recent black holes were discovered as X-ray binary stars. The mass "boundary" between the neutron star and the black hole is still being determined.
This is important because the primary way black holes arise in the universe – from the collapse of massive star cores – strives to produce black holes with less than 5 solar masses. Initially, the range of "2 to 5 solar masses" was called the mass gap, because for a long time almost nothing was found in this range. However, currently we know that there are more than 2 solar mass neutron stars, once you exceed 27 or 2With 8 solar masses, everything needs to be a black hole. (It is believed that at the high-mass end, you can have neutron stars or black holes, depending on the spin of the object.)
Detecting this state is fascinating, as it may be the only case where pulsar timing and ground-based gravitational wave detectors are sensitive to the same class of objects. If you have a combined neutron star, they can produce neutron stars or black holes, and if you have a pulsed neutron star orbiting around a binary companion star, then the companion star can:
Is it a black hole, or a neutron star, or a full-fledged, massive star that will reach the end of its life and die, becoming a black hole or neutron star.
It is not clear which method is more informative for detecting this boundary region, which is on the edge of the possible boundaries of neutron stars and black holes, but both types of observations are critical to solving this puzzle.
By looking inside the globular cluster NGC 6397, many astronomers hope to find an intermediate-mass black hole. On the contrary, only a cluster of black holes with a smaller mass was found, as the artist's impressions show. Astronomers have finally approached the mass function and distribution of black holes in the universe. **ESA Hubble, n bartmann
Just a few years ago, at the beginning of the 21st century, the only evidence we had of a black hole was indirect: electromagnetic radiation from accelerating matter outside the black hole, or from a star that seemed to orbit a mysterious, huge point where no light seemed to appear. Today, all of this has changed dramatically, because in addition to this evidence, we now have:
Direct images of event horizons around black holes, gravitational wave detections, from pulsar timing, a random background of gravitational waves penetrating the universe, and gravitational wave detections, from aspiration and merging dense objects, reveal the merger of neutron stars and black holes, ranging from 1 to 200 solar masses.
The ultimate goal is to be able to describe populations of black holes of all masses throughout the universe. We have estimated the total number and total mass of all black holes in the observable universe: there are 40 trillion black holes, which is about 004%, which is about 10% of the total mass of the star. But how many are a function of mass, and how does the mass function evolve over time? With the advent of the next generation of gravitational-wave observatories, by the middle of the 21st century, these big questions for 2024 may be answered and reliable data available.