Standard Universe Sirens
All rightly rejoiced at the newest of the discoveries in the field of gravitational waves. The LIGO observatory, to which its European partner VIRGO recently joined, had previously observed gravitational waves of merging black holes. Which is very cool, but it also looks pretty lonely - black holes are black, so we can only see gravity waves with them, and little else. Since our current gravity detection observatories do not very well cope with determining the location of the source in the sky, we could not even say in which galaxy, for example, a recorded event occurred.
But everything changed after the launch of an era of astronomy, capable of detecting both gravitational and electromagnetic radiation from a single source. The detected event was a merger of two neutron stars, not black holes, and all this matter, converging together in a giant collision, filled the sky with the glow of many wavelengths at the same time.
Just look at all these different observatories, all these wavelengths of electromagnetic radiation! Radio, infrared, optical, ultraviolet, X-ray, gamma range - this is a complete spectrum from an astronomical point of view.
From this event will grow many advanced scientific achievements - see, for example, this work . Some people are quite agitated by the fact that this event produced a huge amount of gold, several times the mass of the Earth. But this is my blog, so I will cover the aspect of this event that is relevant to me: using the “standard sirens” to measure the expansion of the universe.
We are already doing quite well with measuring the expansion of the Universe with the aid of a distance scale in astronomy . In it, distances are measured gradually, step by step, first through determining the distances to the nearest stars, then through the transition to more distant clusters, and so on. It works well, but naturally it is prone to accumulating errors in the process. A new type of observation of gravitational waves gives us something else, allowing you to jump over the entire distance scale and get an independent measurement of the distance to cosmological objects.
Simultaneous observation of gravitational and electromagnetic waves is a critical part of the idea. You are trying to compare two things: the distance to the object and the apparent speed with which it moves away from you. Usually with speed everything is simple: you measure the redshift of light, which is easy to do with the electromagnetic spectrum of an object in your hands. But having only gravitational waves, this cannot be done - there is not enough structure in the spectrum to measure the redshift. Therefore, the explosion of neutron stars was so important to us; In the case of GW170817, we were able for the first time to determine the exact redshift of a remote source of gravitational waves.
Measurement of distances is a difficult moment, and here gravitational waves offer us a new technique. The usual conventional strategy is to define “standard candles”, that is, objects whose own brightness you can make reasonable conclusions. Comparing it with the observed brightness, you can calculate the distance. For example, astronomers used type Ia supernovae to discover the accelerated expansion of the universe.
Gravitational waves do not provide standard candles - each of the objects will have its own internal gravitational "brightness" (the amount of radiated energy). But by studying how exactly the light source evolves - the characteristic linear-frequency modulation of gravitational waves of two objects rapidly approaching in a spiral - one can calculate their overall brightness. Here is the chirp for GW170817 compared to other sources we have discovered - much more data, almost a full minute!
And here we have the distance and redshift without any scale of distances! This is important for so many reasons. An independent way to measure space distances, for example, will allow us to measure the properties of dark matter. You could also hear about the existence of differences between different ways of measuring the Hubble constant, which means that someone makes a small mistake, or we somehow make a big mistake in our views on the Universe. Getting an independent way to check calculations will help us figure it out. Only from a single event, we can already conclude that the Hubble constant is 70 km / s / Mpc, although with a rather large error (+12, -8 km / s / Mpc). But accuracy will increase when additional data is collected.
And here is my tiny role in this story. The idea of using sources of gravitational waves as standard sirens was expressed by Bernard Schutz back in 1986. But since then it has been seriously reworked, especially my friends Daniel Holtz and Scott Hughes. About this idea, Daniel told me many years ago, and he and Scott wrote one of the very first works on the topic. I immediately said: "You just have to call these things" standard sirens. " And so a useful designation was born.
Unfortunately, my Caltech colleague, Sterl Finney, at the same time offered the same name to me as indicated in the work in the thanks sections. But it is nothing; when the contribution is so small, it is not a pity to divide it.
But the merits of physicists and astronomers who managed to carry out this observation, and many others who have contributed to the theoretical understanding of the issue, are really weighty. Congratulations to all those who worked hard to discover a new way of studying the universe.
Sean Michael Carroll is an American cosmologist specializing in dark energy and the general theory of relativity.
But everything changed after the launch of an era of astronomy, capable of detecting both gravitational and electromagnetic radiation from a single source. The detected event was a merger of two neutron stars, not black holes, and all this matter, converging together in a giant collision, filled the sky with the glow of many wavelengths at the same time.
Just look at all these different observatories, all these wavelengths of electromagnetic radiation! Radio, infrared, optical, ultraviolet, X-ray, gamma range - this is a complete spectrum from an astronomical point of view.
From this event will grow many advanced scientific achievements - see, for example, this work . Some people are quite agitated by the fact that this event produced a huge amount of gold, several times the mass of the Earth. But this is my blog, so I will cover the aspect of this event that is relevant to me: using the “standard sirens” to measure the expansion of the universe.
We are already doing quite well with measuring the expansion of the Universe with the aid of a distance scale in astronomy . In it, distances are measured gradually, step by step, first through determining the distances to the nearest stars, then through the transition to more distant clusters, and so on. It works well, but naturally it is prone to accumulating errors in the process. A new type of observation of gravitational waves gives us something else, allowing you to jump over the entire distance scale and get an independent measurement of the distance to cosmological objects.
Simultaneous observation of gravitational and electromagnetic waves is a critical part of the idea. You are trying to compare two things: the distance to the object and the apparent speed with which it moves away from you. Usually with speed everything is simple: you measure the redshift of light, which is easy to do with the electromagnetic spectrum of an object in your hands. But having only gravitational waves, this cannot be done - there is not enough structure in the spectrum to measure the redshift. Therefore, the explosion of neutron stars was so important to us; In the case of GW170817, we were able for the first time to determine the exact redshift of a remote source of gravitational waves.
Measurement of distances is a difficult moment, and here gravitational waves offer us a new technique. The usual conventional strategy is to define “standard candles”, that is, objects whose own brightness you can make reasonable conclusions. Comparing it with the observed brightness, you can calculate the distance. For example, astronomers used type Ia supernovae to discover the accelerated expansion of the universe.
Gravitational waves do not provide standard candles - each of the objects will have its own internal gravitational "brightness" (the amount of radiated energy). But by studying how exactly the light source evolves - the characteristic linear-frequency modulation of gravitational waves of two objects rapidly approaching in a spiral - one can calculate their overall brightness. Here is the chirp for GW170817 compared to other sources we have discovered - much more data, almost a full minute!
And here we have the distance and redshift without any scale of distances! This is important for so many reasons. An independent way to measure space distances, for example, will allow us to measure the properties of dark matter. You could also hear about the existence of differences between different ways of measuring the Hubble constant, which means that someone makes a small mistake, or we somehow make a big mistake in our views on the Universe. Getting an independent way to check calculations will help us figure it out. Only from a single event, we can already conclude that the Hubble constant is 70 km / s / Mpc, although with a rather large error (+12, -8 km / s / Mpc). But accuracy will increase when additional data is collected.
And here is my tiny role in this story. The idea of using sources of gravitational waves as standard sirens was expressed by Bernard Schutz back in 1986. But since then it has been seriously reworked, especially my friends Daniel Holtz and Scott Hughes. About this idea, Daniel told me many years ago, and he and Scott wrote one of the very first works on the topic. I immediately said: "You just have to call these things" standard sirens. " And so a useful designation was born.
Unfortunately, my Caltech colleague, Sterl Finney, at the same time offered the same name to me as indicated in the work in the thanks sections. But it is nothing; when the contribution is so small, it is not a pity to divide it.
But the merits of physicists and astronomers who managed to carry out this observation, and many others who have contributed to the theoretical understanding of the issue, are really weighty. Congratulations to all those who worked hard to discover a new way of studying the universe.
Sean Michael Carroll is an American cosmologist specializing in dark energy and the general theory of relativity.
Source: https://habr.com/ru/post/409627/