- The dark energy survey monitors both nearby galaxies, and their age is similar to the age of our galaxy – the Milky Way, and very distant galaxies
- It took about 7 billion years for the light of the most distant objects observed to reach Earth. Pictures of these things show us their appearance 7 billion years ago, that is, at a time when the universe was relatively young
- By comparing the spatial distribution of galaxies in the early stages of the evolution of the universe and galaxies closest to Earth, we can trace the history of the Great Cosmic Network of Galaxies
- Dark Energy Survey images, taken with a 520-megapixel digital camera attached to a four-meter telescope at the Cerro Tololo Observatory in Chile, show more than 300 million objects.
- Nearly one-eighth of the entire sky has been traced to find these light sources. So it is a very large area of the sky and a rich sample of data
- With images of millions of galaxies, we have a large number of source lens pairs. In such a situation, you can make some assumptions about the placement of light sources
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Physics is an experimental science. Performing various experiments in the laboratory, we verify physical theories, deepening our knowledge of the world. In the field of astronomy, the situation is more difficult. We cannot manipulate the universe we live in. The only thing left for us is to watch it. However, it should be noted that in recent decades the number and accuracy of astronomical observations has increased significantly, which allows us to learn more about the universe and its secrets.
One of the teams that monitors the huge number of galaxies that fill our universe is the Dark Energy Survey. The results of the analysis of data collected during the first three years of this project have recently been published. As a result, one of the most accurate maps of the distribution of galaxies in the universe is being created before our eyes. Why is this result so important?
trace history
Galaxies are large clusters of stars. It is one of the basic building blocks of the universe. They usually do not exist alone in space, but are grouped into larger structures called galaxy clusters. These structures, like the universe itself, change over time. The Dark Energy Survey monitors both nearby galaxies, which are similar in age to our own, the Milky Way, and very distant galaxies. It took the light of the most distant observable object to reach Earth about 7 billion years. Photos of these objects show us their appearance 7 billion years ago, that is, at a time when the universe was relatively young. By comparing the spatial distribution of galaxies in the early stages of the evolution of the universe and those galaxies closest to Earth, we can trace the history of the Great Cosmic Network of Galaxies.
A very large number of recorded objects plays an important role. The Dark Energy Survey images, taken with a 520-megapixel digital camera attached to a four-meter telescope at the Cerro Tololo Observatory in Chile, contain more than 300 million objects. Nearly one-eighth of the entire sky has been traced to find these light sources. So it’s a very large area of the sky and a rich sample of data. These numerous and accurate observations made it possible to make an accurate map of the distribution of galaxies at different stages of the evolution of the universe, and were also used to independently determine the distribution of mass in these structures using the gravitational lensing method.
General relativity, our best theory of gravity, predicts the diffraction of light near massive objects. This is called a gravitational lens. This name indicates a certain similarity between the behavior of light in the gravitational field and the work of optical lenses. Following this analogy, a massive object that bends spacetime around itself in such a way that rays of light pass near it is called a gravitational lens. Depending on the type of objects that are light sources and types of gravitational lenses, we distinguish different types of lens.
In a dark energy survey, the lenses are single galaxies, and the light sources are other galaxies farther from Earth than the lenses. In this case, we are most often dealing with so-called weak gravitational lenses, as a result of which the image of each source is slightly distorted. The distant galaxy in the image often resembles an inconspicuous ellipse. If we distort such an image slightly the way the weak lens does, the result will be a very similar ellipse, extending slightly in a given direction. So if we are observing only one pair of source lens, we will not be able to get any useful information about that lens in the weak case of the lens. The reason is very simple: we do not know the true shape of the light source, but we measure its distorted image only slightly. So we don’t know exactly what this deformation is.
The essence of the analogy
This is why it is so important to measure a large number of galaxies in the Dark Energy Survey. With images of millions of galaxies, we have a large number of source lens pairs. In such a case, some assumptions can be made about the distribution of the light sources, for example by assuming that the light sources are galaxies resembling randomly oriented ovals.
This makes it possible to use statistical methods to extract useful information about the distortion of the image not of a single galaxy, but of a whole group of different galaxies recorded in the telescope’s field of view. On the other hand, knowing the statistical effect of lensing process, the mass distribution in the lens is re-established on the basis of the equations of general relativity. This is how the dark energy survey used the weak gravitational lensing method to determine the mass distribution in the observed galactic lattice structures.
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It turns out that the mass of galaxies clustered in groups, which is determined by the above method, is greater than the total mass of ordinary matter that forms stars and gases in galaxies and their clusters. Currently, it is assumed that the remaining invisible part of the mass is made of virtual particles, the so-called dark matter. Particles should have practically no interaction with matter known to us, except for the force of gravity. Because of theoretical difficulties and the lack of direct detection of dark matter in terrestrial labs, these particles remain baffling to us.
It should be noted that the components of matter are also very important from the point of view of cosmology. It’s not just galaxy clusters that undergo some changes. The universe is also evolving. In the concept of the universe, I mean the entire spacetime described in the framework of general relativity. To describe the universe in which we live, we create certain models. A simple model called the Standard Cosmological Model is widely adopted today. Its parameters depend on the content of various components of matter in the universe. It is very interesting that in the standard cosmological model, all components of known matter make up only 5%. all the subjects. The rest is about 25 percent. Is a dark matter, while about 70 per cent. It’s a darker, more mysterious energy.
dark matter map
The name of the Dark Energy Survey refers to the last peculiar element of matter, since the collected data on the distribution of a large number of galaxies and the mass distribution map reproduced by the weak lens method allow to verify the predictions of the standard cosmological model. After determining the parameters of the cosmic model, it is possible to simulate the evolution of structures such as a huge network of galaxies and clusters within this model. Comparing the results of these simulations with observations of real structures provides a serious test of cosmological models.
Standard Cosmological Model parameter values are determined on the basis of various observations. One of the most accurate cosmic observations is the study of microwave background radiation made by the Planck satellite. After careful analysis of the data, the Dark Energy Survey team announced that their results are in line with predictions based on data from the Planck satellite. Despite the general agreement, there were also slight differences in this analysis as the structures currently observed are less compact than expected. It is worth noting that so far more than half of the data from the six-year observation period in 2013-2019 has not been analyzed. So we are curiously waiting for the results based on the full data set. However, it can already be said that the recent theoretical research on the various mysteries of the universe has been increasingly supported by many careful astronomical observations, and the dark energy survey is a good example of this.
The article was published in collaboration with the Circulation Portal, the Knowledge Portal, and the Social Engagement Portal as part of the Jagiellonian University Strategic Program Excellence Initiative.
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