Martin Rees once said: “It is becoming clear that, in a sense, space provides the only laboratory that successfully creates extreme conditions to test new ideas from particle physics. The energies of the Big Bang were much higher than we can reach on Earth. So in looking 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 general relativity and Newtonian gravity, it is this: in Einstein's theory, nothing lasts forever. Even if you had two completely stable masses orbiting each other - masses that would never burn up, lose material, or change - their orbits would gradually decay. And if, in Newtonian gravity, two masses revolve around a common center of gravity forever, general relativity tells us that a small amount of energy is lost every time 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. Over sufficiently long periods of time, enough energy will be radiated for the two rotating masses to touch each other and merge. LIGO has already observed this three times with black holes. But it may be time to take the next step and see the first merging of neutron stars, says Ethan Siegel of Medium.com.
Any masses caught in this gravitational dance will emit gravitational waves, causing the orbit to be disrupted. There are three reasons why LIGO discovered black holes:
1. They are incredibly massive
2. They are the most compact objects in the universe
3. At the last moment of the merger, they rotated at the correct frequency so that they could be fixed by the LIGO laser arms
All of this together - large masses, short distances and the right frequency range - give the LIGO team a huge search area in which they can grope for black hole mergers. The ripples from these massive dances extend for many billions of light years and even reach Earth.
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Although black holes must have an accretion disk, the electromagnetic signals that black holes are supposed to generate 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 large gravitational waves. Supermassive black holes in the centers of galaxies eat up gas clouds, planets, asteroids and even other stars and black holes all the time. Unfortunately, because their event horizons are so huge, they move extremely slowly in orbit and give out the wrong frequency range for LIGO to detect. White dwarfs, binary stars, and other planetary systems have the same problem: these objects are physically too large and therefore orbit too long. 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 two neutron stars orbit each other, Einstein's general theory of relativity predicts orbital decay and gravitational radiation. In the final stages of a merger - which has never been seen in gravitational waves - the amplitude will be at its 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 only ten kilometers in radius. Despite the fact that black holes collapse to a singularity, they still have an event horizon, and the physical size of a neutron star (basically just a giant atomic nucleus) is not much larger than the event horizon of a black hole. Their frequency, especially in the last few seconds of the merge, is great for the LIGO's sensitivity. If the event happens in the right place, we can learn five incredible facts.
During the spiral twisting and merging of two neutron stars, a tremendous amount of energy must be released, as well as heavy elements, gravitational waves and an electromagnetic signal, as shown in the image.
Do neutron stars really create 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 merging neutron stars. They stem from old galaxies in regions where no new stars are born, which means that only stellar corpses can explain them. But until we know how the short gamma ray burst appears, we cannot be sure what is causing them. If LIGO can detect the merger of neutron stars from gravitational waves, and we can see a short gamma ray burst immediately after that, this will be the final confirmation of one of the most interesting ideas in astrophysics.
The two merging neutron stars, as shown here, do swirl and emit gravitational waves, but are more difficult to detect than black holes. However, unlike black holes, they must eject some of their mass back into the Universe, where they will contribute there in the form of heavy elements.
When neutron stars collide, how much of their mass does not become a black hole?
When you look at the heavy elements on the periodic table and wonder how they came to be, a supernova comes to mind. After all, this story is held by astronomers and is partly true. But most of the heavy elements on the periodic table are mercury, gold, tungsten, lead, etc. - actually born in collisions of neutron stars. Most of the mass of neutron stars, on the order of 90-95%, goes to create a black hole in the center, but the remaining outer layers are ejected, forming most of these elements in our galaxy. It is worth noting that if the combined mass of two merging neutron stars falls below a certain threshold, they will form a neutron star, not a black hole. This is rare, but not impossible. And we do not know exactly how much mass is thrown out during such an event. If LIGO registers such an event, we'll find out.
It illustrates the range of Advanced LIGO and its ability to detect black hole mergers. Merging neutron stars can only fall within 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 merging of neutron stars?
This question is not about the universe itself, but rather about how sensitive the LIGO design is. In the case of light, if the object is 10 times farther away, it will be 100 times dimmer; but with gravitational waves, if the object is 10 times farther, the gravitational wave signal will be only 10 times weaker. LIGO can observe black holes many millions of light years away, but neutron stars will only be visible if they coalesce in nearby galactic clusters. If we see such a merger, we can check how good our hardware is, or how good it should be.
When two neutron stars merge, as shown here, they should create gamma-ray jets, as well as other electromagnetic phenomena that, if the Earth is close, will be discernible by our best observatories.
What kind of afterglow remains after a merger 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. Or it could be a multispectral component appearing in all five bands, in that order. When LIGO detects a merger of neutron stars, we could capture one of nature's most astounding phenomena.
A neutron star, although composed of neutral particles, produces the strongest magnetic fields in the universe. When neutron stars merge, they should produce both gravitational waves and electromagnetic signatures.
For the first time, we will be able to combine gravitational-wave astronomy with traditional
Previous events captured by LIGO were impressive, but we have not had the opportunity to observe these mergers through a telescope. We inevitably faced two factors:
- The positions of events cannot be precisely determined with only two detectors, in principle
- Mergers of black holes do not have a bright electromagnetic (light) component
Now that VIRGO is working in sync with two LIGO detectors, we can dramatically improve our understanding of where these gravitational waves are generated in space. But more importantly, since the merger of neutron stars must have an electromagnetic component, this could mean that for the first time gravitational wave astronomy and traditional astronomy will be used together to observe the same event in the universe!
The spiral twisting and merging of two neutron stars, as shown here, should result in a specific gravitational wave signal. Also, the moment of fusion must 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 a merger of neutron stars appears in LIGO, even if it is rare, and the detection rate is low, we will cross the next border. The gravitational sky and the sky of light will no longer be strangers to each other. We will be one step closer to understanding how the most extreme objects in the Universe work, and we will have a window into our space that no one has ever had before.
Ilya Khel