Martin Rees once said: "It becomes clear that in a sense, space provides a single laboratory in which extreme conditions are created to test new ideas from particle physics. The energies of the Big Bang were much higher than what we can achieve on Earth. Therefore, in the search for evidence of the Big Bang and studying things like neutron stars, we are actually studying fundamental physics. "
If there is one significant difference between the general theory of relativity and Newtonian gravity, then it consists in the following: in Einstein's theory nothing lasts forever. Even if you had two absolutely stable masses in orbit each other – masses that would never burn, did not lose material and did not change – their orbits gradually disintegrated. And if in Newtonian gravity two masses rotate around the common center of gravity forever, the GRT tells us that a small amount of energy is lost with each moment when the mass is accelerated by the gravitational field through which it passes. This energy does not disappear, but is carried away in the form of gravitational waves. For sufficiently long periods of time, enough energy will be radiated, so that the two rotating masses touch each other and merge. Three times, LIGO observed this with the example of black holes. But perhaps it's time to take the next step and see the first merger of neutron stars, says Ethan Siegel with Medium.com.
Any masses that fall into this gravitational dance will emit gravitational waves, as a result of which the orbit will be violated. The reasons for which LIGO discovered black holes are three:
- They are incredibly massive
- They are the most compact objects in the universe
- At the last moment of the fusion, they rotated at the right frequency, so that they could be fixed by laser sleeves LIGO
All this together – large masses, short distances and the right frequency range – give the LIGO team a huge search field in which they can find the fusion of black holes. The ripples from these massive dances extend many billions of light years and reach even the Earth.
Although black holes should have an accretion disk, the electromagnetic signals, which should produce black holes, remain elusive. If the electromagnetic part of the phenomenon is present, it must be produced by neutron stars.
The universe has many other interesting objects that produce gravitational waves of great magnitude. Supermassive black holes in the centers of galaxies are eaten by gas clouds, planets, asteroids and even other stars and black holes all the time. Unfortunately, since their event horizons are so huge, they move very slowly along the orbit and give out an incorrect frequency range so that LIGO can fix them. White dwarfs, binary stars and other planetary systems have the same problem: these objects are physically too large and therefore move too long in the orbit. So long that we would need a space observatory of gravitational waves to see them. But there is another hope that has the right combination of characteristics (mass, compactness, the right frequency) to be seen by LIGO: merging neutron stars.
As the two neutron stars rotate around each other, Einstein's general theory of relativity predicts orbital decay and gravitational radiation. In the last stages of the fusion – which has never been observed in gravitational waves – the amplitude will be at the peak and LIGO will be able to detect the event
Neutron stars are not as massive as black holes, but they can probably be two to three times more massive than the Sun: about 10-20% of the mass of previously detected LIGO events. They are almost as compact as black holes, with a physical size of just ten kilometers radius. Despite the fact that the black holes collapse to a singularity, they still have the event horizon, and the physical size of the neutron star (basically just a gigantic atomic nucleus) is not much larger than the horizon of black hole events. Their frequency, especially in the last few seconds of fusion, is excellent for LIGO sensitivity. If an event occurs in the right place, we can learn five incredible facts.
During the spiral twist and fusion of two neutron stars, a colossal amount of energy, as well as heavy elements, gravitational waves and an electromagnetic signal, should be released, as shown in the image
Are neutron stars really creating gamma-ray bursts?
There is an interesting thought: that short gamma-ray bursts, which are incredibly energetic, but last less than two seconds, are caused by the fusion of neutron stars. They emanate from old galaxies in regions in which no new stars are born, which means only star corpses can explain them. But until we know how a short gamma-ray burst appears, we can not be sure what is their cause. If LIGO can register the fusion of neutron stars over gravitational waves, and we can see a short gamma ray burst right after that, it will be the final confirmation of one of the most interesting ideas of astrophysics.
Two merging neutron stars, as shown here, do spin and emit gravitational waves, but they are more difficult to detect than black holes. However, unlike black holes, they must throw some of their mass back into the universe, where it will make its contribution in the form of heavy elements
When neutron stars collide, which part of their mass does not become a black hole?
If you look at the heavy elements in the periodic table and wonder how they appeared, the "supernova" comes to mind. In the end, this story is held by astronomers and it is partly true. But most of the heavy elements in the periodic table are mercury, gold, tungsten, lead, etc. – are actually born in collisions of neutron stars. Most of the mass of neutron stars, of the order of 90-95%, goes to create a black hole in the center, but the remaining outer layers are ejected, forming the majority of these elements in our galaxy. It is worth noting that if the cumulative mass of two merging neutron stars is below a certain threshold, they will form a neutron star, rather than a black hole. This is rare, but not impossible. And how much exactly mass is thrown out in the course of such an event, we do not know. If LIGO registers such an event, we will find out.
Here is illustrated the range Advanced LIGO and its ability to register the fusion of black holes. Fusion neutron stars can fall only one tenth of the range and have 0.1% of the usual volume, but if there are many neutron stars, LIGO will find
How far can LIGO see the fusion of neutron stars?
This question is not about the universe itself, but rather about how great the sensitivity of the LIGO design. In the case of light, if the object is 10 times farther away, it will be 100 times fainter; but with gravitational waves, if the object is 10 times farther away, the gravitational-wave signal will be only 10 times weaker. LIGO can observe black holes for many millions of light years, but neutron stars will be visible only if they merge in the nearest galaxy clusters. If we see such a merger, we can check how good the equipment is or how good it should be.
When two neutron stars merge, as shown here, they must create gamma-ray jets, as well as other electromagnetic phenomena that, in the case of the Earth's proximity, will be discernible by our best observatories
What afterglow remains after the fusion of neutron stars?
We know, in some cases, that strong events corresponding to collisions of neutron stars have already occurred and that they leave signatures in other electromagnetic bands. In addition to gamma rays, there may be ultraviolet, optical, infrared or radio components. Alternatively, it can be a multispectral component that manifests itself in all five bands, in this order. When LIGO detects the fusion of neutron stars, we could capture one of the most striking phenomena of nature.
A neutron star, although composed of neutral particles, produces the strongest magnetic fields in the universe. When neutron stars merge, they must produce both gravitational waves and electromagnetic signatures
We will be able to combine for the first time the gravitational-wave astronomy with the traditional
Previous events captured by LIGO were impressive, but we were not able to observe these mergers through a telescope. We inevitably encountered two factors:
- The positions of events can not be precisely determined, having only two detectors, in principle
- Merges of black holes do not have a bright electromagnetic (light) component
Now that VIRGO is working in synchronization with two LIGO detectors, we can significantly improve our understanding of where exactly these gravitational waves are born in space. But more importantly, since the fusion of neutron stars must have an electromagnetic component, this can mean that for the first time gravitational wave astronomy and traditional astronomy will be used together to observe the same event in the universe!
Spiral twisting and fusion of two neutron stars, as shown here, should lead to the appearance of a specific signal of a gravitational wave. Also the moment of confluence should create electromagnetic radiation, unique and identifiable in itself
We have already entered a new era of astronomy, where we use not only telescopes, but also interferometers. We use not only light, but also gravitational waves to see and understand the universe. If the fusion of neutron stars appears in LIGO, even if it is rare, and the detection rate is low, we will cross the next boundary. The gravitational sky and the sky of light will no longer be strangers to one another. We will be one step closer to understanding how the most extreme objects in the universe are arranged, and we will have a window into our space that was never before and no one else